wdiff draft-ietf-idr-bgp4-17.txt draft-ietf-idr-bgp4-18.txt

Network Working Group Y. Rekhter
INTERNET DRAFT Juniper Networks
T. Li
Procket Networks, Inc.
S. Hares
NextHop Technologies, Inc.
Editors

A Border Gateway Protocol 4 (BGP-4)
<draft-ietf-idr-bgp4-17.txt>
<draft-ietf-idr-bgp4-18.txt>

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
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1. Acknowledgments

This

Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document was originally published are to be interpreted as RFC 1267 described in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter.

We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version RFC2119 [RFC2119].

Table of
this document.

We like to explicitly thank Bob Braden for the review Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Definition of the earlier
version commonly used terms . . . . . . . . . . . . . . 4
2. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
3. Summary of this document as well as his constructive Operation . . . . . . . . . . . . . . . . . . . . . 7
3.1 Routes: Advertisement and valuable
comments.

We would also like to thank Bob Hinden, Director for Storage . . . . . . . . . . . . . . 9
3.2 Routing of the
Internet Engineering Steering Group, Information Bases . . . . . . . . . . . . . . . . . . 10
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 11
4.1 Message Header Format . . . . . . . . . . . . . . . . . . . . 11
4.2 OPEN Message Format . . . . . . . . . . . . . . . . . . . . . 12
4.3 UPDATE Message Format . . . . . . . . . . . . . . . . . . . . 14
4.4 KEEPALIVE Message Format . . . . . . . . . . . . . . . . . . 21
4.5 NOTIFICATION Message Format . . . . . . . . . . . . . . . . . 21
5. Path Attributes . . . . . . . . . . . . . . . . . . . . . . . 23
5.1 Path Attribute Usage . . . . . . . . . . . . . . . . . . . . 25
5.1.1 ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.2 AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.3 NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.4 MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . . 28
5.1.5 LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1.6 ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . . 29
5.1.7 AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . . 30
6. BGP Error Handling . . . . . . . . . . . . . . . . . . . . . . 30
6.1 Message Header error handling . . . . . . . . . . . . . . . . 30
6.2 OPEN message error handling . . . . . . . . . . . . . . . . . 31
6.3 UPDATE message error handling . . . . . . . . . . . . . . . . 32
6.4 NOTIFICATION message error handling . . . . . . . . . . . . . 34
6.5 Hold Timer Expired error handling . . . . . . . . . . . . . . 34
6.6 Finite State Machine error handling . . . . . . . . . . . . . 34
6.7 Cease . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6.8 BGP connection collision detection . . . . . . . . . . . . . 35
7. BGP Version Negotiation . . . . . . . . . . . . . . . . . . . 36
8. BGP Finite State machine . . . . . . . . . . . . . . . . . . . 36
8.1 Events for the BGP FSM . . . . . . . . . . . . . . . . . . . 37
8.1.1 Administrative Events . . . . . . . . . . . . . . . . . . 37
8.1.2 Timer Events . . . . . . . . . . . . . . . . . . . . . . . 38
8.1.3 TCP connection based Events . . . . . . . . . . . . . . . . 39
8.1.4 BGP Messages based Events . . . . . . . . . . . . . . . . . 41
8.2 Description of FSM . . . . . . . . . . . . . . . . . . . . . 43
8.2.1 FSM Definition . . . . . . . . . . . . . . . . . . . . . . 43
8.2.1.1 Terms "active" and "passive" . . . . . . . . . . . . . . 43
8.2.1.2 FSM and the team collision detection . . . . . . . . . . . . . . . 44
8.2.2 Finite State Machine . . . . . . . . . . . . . . . . . . . 44
9. UPDATE Message Handling . . . . . . . . . . . . . . . . . . . 57
9.1 Decision Process . . . . . . . . . . . . . . . . . . . . . . 58
9.1.1 Phase 1: Calculation of reviewers he
assembled to review the earlier version (BGP-2) Degree of this document.
This team, consisting Preference . . . . . . . 59
9.1.2 Phase 2: Route Selection . . . . . . . . . . . . . . . . . 60
9.1.2.1 Route Resolvability Condition . . . . . . . . . . . . . . 61
9.1.2.2 Breaking Ties (Phase 2) . . . . . . . . . . . . . . . . . 62
9.1.3 Phase 3: Route Dissemination . . . . . . . . . . . . . . . 64
9.1.4 Overlapping Routes . . . . . . . . . . . . . . . . . . . . 65
9.2 Update-Send Process . . . . . . . . . . . . . . . . . . . . . 66
9.2.1 Controlling Routing Traffic Overhead . . . . . . . . . . . 67
9.2.1.1 Frequency of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination Route Advertisement . . . . . . . . . . . . 67
9.2.1.2 Frequency of toughness, professionalism, and
courtesy.

This updated Route Origination . . . . . . . . . . . . . 68
9.2.2 Efficient Organization of Routing Information . . . . . . . 68
9.2.2.1 Information Reduction . . . . . . . . . . . . . . . . . . 68
9.2.2.2 Aggregating Routing Information . . . . . . . . . . . . . 69
9.3 Route Selection Criteria . . . . . . . . . . . . . . . . . . 72
9.4 Originating BGP routes . . . . . . . . . . . . . . . . . . . 72
10. BGP Timers . . . . . . . . . . . . . . . . . . . . . . . . . 72
Appendix A. Comparison with RFC1771 . . . . . . . . . . . . . . . 73
Appendix B. Comparison with RFC1267 . . . . . . . . . . . . . . . 74
Appendix C. Comparison with RFC 1163 . . . . . . . . . . . . . . 75
Appendix D. Comparison with RFC 1105 . . . . . . . . . . . . . . 75
Appendix E. TCP options that may be used with BGP . . . . . . . . 76
Appendix F. Implementation Recommendations . . . . . . . . . . . 76
Appendix F.1 Multiple Networks Per Message . . . . . . . . . . . 76
Appendix F.2 Reducing route flapping . . . . . . . . . . . . . . 77
Appendix F.3 Path attribute ordering . . . . . . . . . . . . . . 77
Appendix F.4 AS_SET sorting . . . . . . . . . . . . . . . . . . . 77
Appendix F.5 Control over version negotiation . . . . . . . . . . 78
Appendix F.6 Complex AS_PATH aggregation . . . . . . . . . . . . 78
Security Considerations . . . . . . . . . . . . . . . . . . . . . 79
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Authors Information . . . . . . . . . . . . . . . . . . . . . . . 80

Abstract

The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol.

The primary function of the document a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the product list of the IETF IDR
Working Group with Yakov Rekhter and Tony Li as editors. Certain
sections Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of the document borrowed heavily AS connectivity from IDRP [7], which is the
OSI counterpart of BGP. For this credit should
routing loops may be given to the ANSI
X3S3.3 group chaired by Lyman Chapin pruned and to Charles Kunzinger who was
the IDRP editor within that group. We would also like to thank Enke
Chen, Edward Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey
Haas, Dimitry Haskin, John Krawczyk, David LeRoy, Dan Massey, Dan
Pei, Mathew Richardson, John Scudder, John Stewart III, Dave Thaler,
Paul Traina, Russ White, Curtis Villamizar, and Alex Zinin for their
comments.

Many thanks to Sue Hares for her contributions to the document, and
especially for her work on the BGP Finite State Machine.

We would like to specially acknowledge numerous contributions by
Dennis Ferguson.

2. Introduction

The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. It is built on experience gained with EGP as
defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
described in RFC 1092 [2] and RFC 1093 [3].

The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
Autonomous Systems (ASs) that reachability information traverses.
This information is sufficient to construct a graph of AS
connectivity from which routing loops may be pruned and some policy
decisions at some policy decisions at the AS level
may be enforced.

BGP-4 provides a new set of mechanisms for supporting Classless
Inter-Domain Inter-
Domain Routing (CIDR) [8, 9]. [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix and eliminates
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms which allow aggregation of routes, including
aggregation of AS paths.

To characterize

Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy decisions deci-
sions that can (and can not) be enforced using BGP, one must focus on BGP. BGP can support
only the policies conforming to the rule destination-based forwarding
paradigm.

1. Definition of commonly used terms

This section provides definition for terms that have a BGP speaker advertises specific mean-
ing to its peers (other the BGP speakers which it communicates with) in
neighboring ASs only those routes protocol and that it itself uses. This rule
reflects the "hop-by-hop" routing paradigm generally are used throughout the current Internet. Note that some policies cannot be supported by text.

Autonomous System (AS)
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol (IGP) and common metrics to determine how to route pack-
ets within the "hop-by-hop" routing paradigm AS, and thus require techniques such as
source using an inter-AS routing (aka explicit routing) protocol to enforce. For example, BGP
does not enable one AS
determine how to send traffic route packets to other ASs. Since this classic
definition was developed, it has become common for a neighboring single AS intending
that to
use several IGPs and sometimes several sets of metrics within an
AS. The use of the traffic take a different route from that taken by traffic
originating in term Autonomous System here stresses the neighboring AS. On fact
that, even when multiple IGPs and metrics are used, the adminis-
tration of an AS appears to other hand, BGP can support
any policy conforming ASs to the "hop-by-hop" routing paradigm. Since the
current Internet uses only the "hop-by-hop" inter-AS have a single coherent
interior routing paradigm plan and since presents a consistent picture of what
destinations are reachable through it.

BGP can support any policy that conforms to speaker
A router that paradigm, implements BGP.

BGP is highly applicable as an inter-AS routing protocol for the
current Internet. Identifier
A more complete discussion of what policies can and cannot be
enforced with BGP is outside 4-octet unsigned integer indicating the scope BGP Identifier of this document (but refer to the companion document discussing
sender of BGP usage [5]). messages. A given BGP runs over a reliable transport protocol. This eliminates speaker sets the need value of its
BGP Identifier to implement explicit update fragmentation, retransmission,
acknowledgment, and sequencing. Any authentication scheme used by the
transport protocol (e.g., RFC2385 [10]) may be used in addition an IP address assigned to
BGP's own authentication mechanisms. The error notification mechanism
used in BGP assumes that the transport protocol supports a "graceful"
close, i.e., that all outstanding data will be delivered before BGP speaker. The
value of the
connection is closed. BGP uses TCP [4] as its transport protocol. TCP meets BGP's transport
requirements and Identifier is present in virtually all commercial routers determined on startup and
hosts. In the following descriptions is the phrase "transport protocol
connection" can be understood to refer to a TCP connection. BGP uses
TCP port 179
same for establishing its connections.

This document uses every local interface and every BGP peer.

Internal peer
Peer that is in the term `Autonomous System' (AS) throughout. The
classic definition of an same Autonomous System as the local system.

IBGP
Internal BGP (BGP connection between internal peers).

External peer
Peer that is in a set different Autonomous System than the local sys-
tem.

EBGP
External BGP (BGP connection between external peers).

NLRI
Network Layer Reachability Information.

Route
A unit of routers under information that pairs a single technical administration, using an interior gateway protocol
and common metrics to determine how to route packets within set of destinations with the AS,
and using an exterior gateway protocol to determine how to route
packets to other ASs. Since this classic definition was developed, it
has become common for
attributes of a single AS path to use several interior gateway
protocols and sometimes several sets of metrics within an AS. those destinations. The use set of the term Autonomous System here stresses the fact that, even when
multiple IGPs and metrics destina-
tions are used, the administration of an AS
appears to other ASs to have a single coherent interior routing plan
and presents a consistent picture of what destinations systems whose IP addresses are reachable
through it.

The planned use of BGP contained in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented one IP
address prefix carried in a companion
document [5]. This document is the first of a series of documents
planned to explore various aspects of BGP application.

3. Summary Network Layer Reachability Informa-
tion (NLRI) field of Operation

Two systems form a transport protocol connection between one another.
They exchange messages to open and confirm the connection parameters. an UPDATE message. The initial data flow path is the portion informa-
tion reported in the path attributes field of the BGP same UPDATE mes-
sage.

RIB
Routing Information Base.

Adj-RIB-In
The Adj-RIBs-In contain unprocessed routing table information that is
allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require periodic refresh of has
been advertised to the routing table. Therefore, a local BGP speaker must retain the current version of the routes advertised by
all of its peers for the duration of the connection. If the
implementation decides to not store peers.

Loc-RIB
The Loc-RIB contains the routes that have been
received from a peer, but have been filtered out according to
configured local policy, selected by the
local BGP Route Refresh extension [12] may be
used to request speaker's Decision Process.

Adj-RIB-Out
The Adj-RIBs-Out contains the full set of routes from a peer without resetting
the BGP session when the local policy configuration changes.

KEEPALIVE messages may be sent periodically for advertisement to ensure the liveness specific
peers by means of the connection. NOTIFICATION messages are sent in response to errors
or special conditions. If a connection encounters an error condition,
a NOTIFICATION message is sent and the connection is closed.

The hosts executing the Border local speaker's UPDATE messages.

IGP
Interior Gateway Protocol need not be routers.
A non-routing host could exchange routing information with routers
via EGP or even an interior - a routing protocol. That non-routing host
could then use BGP protocol used to exchange
routing information with among routers within a border
router in another single Autonomous System. The implications and
applications of this architecture are Sys-
tem.

Feasible route
A route that is available for further study.

Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links. Similarly, a peer in a
different AS use.

Unfeasible route
A previously advertised feasible route that is referred to no longer available
for use.

2. Acknowledgments

This document was originally published as an external peer, while a peer RFC 1267 in the
same AS may be described as an internal peer. Internal BGP and
external BGP are commonly abbreviated IBGP October 1991,
jointly authored by Kirk Lougheed and EBGP.

If a particular AS has multiple BGP speakers Yakov Rekhter.

We would like to express our thanks to Guy Almes, Len Bosack, and is providing transit
service
Jeffrey C. Honig for other ASs, then care must be taken their contributions to ensure a consistent
view of routing within the AS. A consistent view earlier version
(BGP-1) of the interior
routes of the AS is provided this document.

We would like to specially acknowledge numerous contributions by Den-
nis Ferguson to the interior routing protocol. A
consistent view earlier version of the routes exterior this document.

We like to explicitly thank Bob Braden for the AS can be provided by
having all BGP speakers within the AS maintain direct IBGP
connections with each other. Alternately the interior routing
protocol can pass BGP information among routers within an AS, taking
care not to lose BGP attributes that will be needed by EBGP speakers
if transit connectivity is being provided. For review of the purpose earlier
version (BGP-2) of
discussion, it is assumed that BGP information is passed within an AS
using IBGP. Care must be taken this document as well as his constructive and
valuable comments.

We would also like to ensure that thank Bob Hinden, Director for Routing of the interior routers
have all been updated with transit information before
Internet Engineering Steering Group, and the EBGP
speakers announce team of reviewers he
assembled to other ASs that transit service is being
provided.

3.1 Routes: Advertisement and Storage

For review the purpose earlier version (BGP-2) of this protocol, a route is defined as a unit of
information that pairs a set document.
This team, consisting of destinations Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with the attributes of a
path to those destinations. The set strong combination of destinations are the systems
whose IP addresses are reported in the Network Layer Reachability
Information (NLRI) field toughness, professionalism, and cour-
tesy.

Certain sections of the path document borrowed heavily from IDRP
[IS10747], which is the information reported in
the path attributes field OSI counterpart of BGP. For this credit
should be given to the same UPDATE message.

Routes are advertised between BGP speakers in UPDATE messages.

Routes are stored in the Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, ANSI X3S3.3 group chaired by Lyman Chapin and
to Charles Kunzinger who was the Adj-RIBs-Out. Routes IDRP editor within that will
be advertised group.

We would also like to other BGP speakers must be present thank Benjamin Abarbanel, Enke Chen, Edward
Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
Haskin, John Krawczyk, David LeRoy, Dan Massey, Jonathan Natale, Dan
Pei, Mathew Richardson, John Scudder, John Stewart III, Dave Thaler,
Paul Traina, Russ White, Curtis Villamizar, and Alex Zinin for their
comments.

We would like to specially acknowledge Andrew Lange for his help in
preparing the Adj-RIB-
Out. Routes that will be used by the local BGP speaker must be
present in final version of this document.

Finally, we would like to thank all the Loc-RIB, and members of the next hop IDR Working
Group for each their ideas and support they have given to this document.

3. Summary of these routes
must be resolvable via the local BGP speaker's Routing Table. Routes
that are received from other BGP speakers are present Operation

The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol. It is built on experience gained with EGP as defined in
[RFC904] and EGP usage in the Adj-
RIBs-In.

If NSFNET Backbone as described in
[RFC1092] and [RFC1093].

The primary function of a BGP speaker chooses to advertise the route, it may add speaking system is to or
modify exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the path attributes list of the route before advertising it Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a
peer.

BGP provides mechanisms by graph of AS connectivity from which
routing loops may be pruned and some policy decisions at the AS level
may be enforced.

In the context of this document we assume that a BGP speaker can inform adver-
tises to its peer peers only those routes that it itself uses (in this
context a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate
that is said to "use" a BGP route has been withdrawn from service:

a) the IP prefix that expresses if it is the destination for a previously
advertised most
preferred BGP route can be advertised and is used in forwarding). All other cases are
outside the WITHDRAWN ROUTES field scope of this document.

Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the UPDATE message, thus marking IP
header of the associated route as being
no longer available for use
b) a replacement route with packet. This, in turn, reflects the same NLRI set of policy deci-
sions that can (and can not) be advertised, or

c) the BGP speaker - BGP speaker connection enforced using BGP. Note that some
policies can not be closed, which
implicitly removes from service all routes which supported by the pair of
speakers had advertised destination-based forwarding
paradigm, and thus require techniques such as source routing (aka
explicit routing) to each other.

3.2 Routing Information Bases

The Routing Information Base (RIB) within a be enforced. Such policies can not be enforced
using BGP speaker consists of
three distinct parts:

a) Adj-RIBs-In: The Adj-RIBs-In store routing information either. For example, BGP does not enable one AS to send
traffic to a neighboring AS for forwarding to some destination
(reachable through but) beyond that has
been learned from inbound UPDATE messages. Their contents
represent routes neighboring AS intending that are available as an input to the Decision
Process.

b) Loc-RIB: The Loc-RIB contains the local routing information
traffic take a different route to that the BGP speaker has selected taken by applying its local policies
to the routing information contained traffic originat-
ing in its Adj-RIBs-In.

c) Adj-RIBs-Out: The Adj-RIBs-Out store the information neighboring AS (for that same destination). On the
local other
hand, BGP speaker has selected for advertisement can support any policy conforming to its peers. The
routing information stored in the Adj-RIBs-Out will destination-based
forwarding paradigm.

A more complete discussion of what policies can and can not be carried in
the local
enforced with BGP speaker's UPDATE messages and advertised to its
peers.

In summary, is outside the Adj-RIBs-In contain unprocessed routing information
that has been advertised scope of this document (but refer to
the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local companion document discussing BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes usage [RFC1772]).

BGP-4 provides a new set of mechanisms for advertisement to specific peers by means supporting Classless
Inter-Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms
include support for advertising a set of the local speaker's
UPDATE messages.

Although the conceptual model distinguishes between Adj-RIBs-In, Loc-
RIB, and Adj-RIBs-Out, this neither implies nor requires that destinations as an
implementation must maintain three separate copies of IP prefix
and eliminating the routing
information. The choice concept of implementation (for example, 3 copies network "class" within BGP. BGP-4
also introduces mechanisms which allow aggregation of routes, includ-
ing aggregation of AS paths.

This document uses the information vs 1 copy with pointers) term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is not constrained by the
protocol.

Routing information that a set of routers under
a single technical administration, using an interior gateway protocol
(IGP) and common metrics to determine how to route packets within the router uses
AS, and using an inter-AS routing protocol to forward determine how to route
packets (or to
construct the forwarding table that is used other ASs. Since this classic definition was developed, it
has become common for packet forwarding) is
maintained in the Routing Table. The Routing Table accumulates routes a single AS to directly connected networks, static routes, routes learned from use several IGPs and sometimes
several sets of metrics within an AS. The use of the IGP protocols, term Autonomous
System here stresses the fact that, even when multiple IGPs and routes learned from BGP. Whether or not met-
rics are used, the administration of an AS appears to other ASs to
have a
specific single coherent interior routing plan and presents a consis-
tent picture of what destinations are reachable through it.

The planned use of BGP route should be installed in the Routing Table, and
whether a Internet environment, including such
issues as topology, the interaction between BGP route should override a route to and IGPs, and the same destination
installed by another source is a local
enforcement of routing policy decision, not specified rules is presented in this document. Besides actual packet forwarding, the Routing Table a companion docu-
ment [RFC1772]. This document is used for resolution of the next-hop addresses specified in first of a series of documents
planned to explore various aspects of BGP
updates (see Section 9.1.2).

4. Message Formats application.

BGP uses TCP [RFC793] as its transport protocol. This section describes message formats eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgment, and sequencing. BGP listens on TCP port 179. Any
authentication scheme used by BGP.

Messages are sent over a reliable transport protocol connection. A
message is processed only after it is entirely received. The maximum
message size is 4096 octets. All implementations are required to
support this maximum message size. The smallest message that TCP (e.g., RFC2385 [RFC2385]) may be
sent consists of a
used. The error notification mechanism used in BGP header without assumes that TCP
supports a "graceful" close, i.e., that all outstanding data portion, or 19 octets.

4.1 Message Header Format

Each message has a fixed-size header. There may or may not will be
delivered before the connection is closed.

Two systems form a TCP connection between one another. They exchange
messages to open and confirm the connection parameters.

The initial data flow is the portion following of the header, depending on BGP routing table that is
allowed by the message type. The
layout export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require periodic refresh of these fields is shown below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Marker:

This 16-octet field contains the routing table. To allow local
policy changes to have the correct effect without resetting any BGP
connections, a value that BGP speaker SHOULD either (a) retain the receiver current ver-
sion of the
message can predict. If routes advertised to it by all of its peers for the Type dura-
tion of the message is OPEN, connection, or if
the OPEN message carries no Authentication Information (as an
Optional Parameter), then (b) make use of the Marker must Route Refresh
extension [RFC2918].

KEEPALIVE messages may be all ones.
Otherwise, sent periodically to ensure the value liveness of
the marker can be predicted by some connection. NOTIFICATION messages are sent in response to errors
or special conditions. If a
computation specified as part of the authentication mechanism
(which connection encounters an error condition,
a NOTIFICATION message is specified as part of sent and the Authentication Information)
used. connection is closed.

The Marker can hosts executing BGP need not be used routers. A non-routing host
could exchange routing information with routers via EGP [RFC904] or
even an interior routing protocol. That non-routing host could then
use BGP to detect loss of synchronization
between exchange routing information with a pair of BGP peers, border router in
another Autonomous System. The implications and to authenticate incoming BGP
messages.

Length:

This 2-octet unsigned integer indicates the total length applications of the
message, including the header, this
architecture are for further study.

A peer in octets. Thus, e.g., it allows
one a different AS is referred to locate as an external peer, while a
peer in the transport-level stream the (Marker field
of the) next message. The value of the Length field must always same AS may be at least 19 described as an internal peer. Internal
BGP and no greater than 4096, external BGP are commonly abbreviated IBGP and EBGP.

If a particular AS has multiple BGP speakers and may be further
constrained, depending on the message type. No "padding" of
extra data after the message is allowed, so the Length field providing transit
service for other ASs, then care must have the smallest value required given the rest be taken to ensure a consistent
view of routing within the
message.

Type:

This 1-octet unsigned integer indicates AS. A consistent view of the type code interior
routes of the
message. The following type codes are defined:

1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE

4.2 OPEN Message Format

After a transport protocol connection AS is established, the first
message sent provided by each side is an OPEN message. If the OPEN message IGP used within the AS. For the
purpose of this document, it is
acceptable, assumed that a KEEPALIVE message confirming consistent view of the OPEN is sent back.
Once
routes exterior to the OPEN AS is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may provided by having all BGP speakers
within the AS maintain IBGP with each other. Care must be exchanged.

In addition taken to
ensure that the fixed-size BGP header, interior routers have all been updated with transit
information before the OPEN message contains BGP speakers announce to other ASs that tran-
sit service is being provided.

3.1 Routes: Advertisement and Storage

For the following fields:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version:

This 1-octet unsigned integer indicates the protocol version
number purpose of the message. The current BGP version number this protocol, a route is 4.

My Autonomous System:

This 2-octet unsigned integer indicates the Autonomous System
number of the sender.

Hold Time:

This 2-octet unsigned integer indicates the number defined as a unit of seconds
information that the sender proposes for the value pairs a set of destinations with the Hold Timer. Upon
receipt attributes of an OPEN message, a BGP speaker MUST calculate the
value
path to those destinations. The set of destinations are systems whose
IP addresses are contained in one IP address prefix carried in the Hold Timer by using the smaller
Network Layer Reachability Information (NLRI) field of its configured
Hold Time an UPDATE mes-
sage, and the Hold Time received in path is the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation may reject connections on information reported in the basis path attributes
field of the Hold
Time. The calculated value indicates the maximum number of
seconds that may elapse same UPDATE message.

Routes are advertised between the receipt of successive
KEEPALIVE, and/or BGP speakers in UPDATE messages by messages. Mul-
tiple routes that have the sender.

BGP Identifier:

This 4-octet unsigned integer indicates same path attributes can be advertised in
a single UPDATE message by including multiple prefixes in the BGP Identifier NLRI
field of the sender. A given BGP speaker sets UPDATE message.

Routes are stored in the value of its Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in
Section 3.2.

If a BGP
Identifier speaker chooses to an IP address assigned advertise the route, it may add to that BGP speaker. The
value or
modify the path attributes of the route before advertising it to a
peer.

BGP Identifier is determined on startup and provides mechanisms by which a BGP speaker can inform its peer
that a previously advertised route is the
same no longer available for every local interface and every use.
There are three methods by which a given BGP peer.

Optional Parameters Length:

This 1-octet unsigned integer indicates speaker can indicate
that a route has been withdrawn from service:

a) the total length of IP prefix that expresses the
Optional Parameters field destination for a previously
advertised route can be advertised in octets. If the value of this WITHDRAWN ROUTES field
is zero,
in the UPDATE message, thus marking the associated route as being
no Optional Parameters are present.

Optional Parameters:

This field may contain longer available for use

b) a list replacement route with the same NLRI can be advertised, or

c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair of optional parameters, where
speakers had advertised to each parameter other.

Changing attribute of a route is encoded as accomplished by advertising a <Parameter Type, Parameter
Length, Parameter Value> triplet.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

Parameter Type is
replacement route. The replacement route carries new (changed)
attributes and has the same NLRI as the original route.

3.2 Routing Information Bases

The Routing Information Base (RIB) within a one octet field BGP speaker consists of
three distinct parts:

a) Adj-RIBs-In: The Adj-RIBs-In store routing information that unambiguously
identifies individual parameters. Parameter Length is a one
octet field has
been learned from inbound UPDATE messages received from other BGP
speakers. Their contents represent routes that contains are available as an
input to the length of Decision Process.

b) Loc-RIB: The Loc-RIB contains the Parameter Value
field in octets. Parameter Value is a variable length field local routing information
that is interpreted according to the value of BGP speaker has selected by applying its local policies
to the Parameter
Type field.

This document defines routing information contained in its Adj-RIBs-In. These are
the following Optional Parameters:

a) Authentication Information (Parameter Type 1):

This optional parameter may routes that will be used to authenticate a BGP
peer. The Parameter Value field contains a 1-octet
Authentication Code followed by a variable length
Authentication Data.

0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
| Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Data |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authentication Code:

This 1-octet unsigned integer indicates the
authentication mechanism being used. Whenever an
authentication mechanism is specified local BGP speaker. The next
hop for use within
BGP, three things each of these routes must be included in resolvable via the
specification:

- local BGP
speaker's Routing Table.

c) Adj-RIBs-Out: The Adj-RIBs-Out store the value of information that the Authentication Code which indicates
use of
local BGP speaker has selected for advertisement to its peers. The
routing information stored in the mechanism,
- Adj-RIBs-Out will be carried in
the form local BGP speaker's UPDATE messages and meaning of advertised to its
peers.

In summary, the Authentication Data, Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and
- the algorithm Adj-RIBs-Out organize the routes
for computing values advertisement to specific peers by means of Marker fields.

Note that a separate authentication mechanism may be
used in establishing the transport level connection.

Authentication Data:

Authentication Data is a variable length field that is
interpreted according to local speaker's
UPDATE messages.

Although the value conceptual model distinguishes between Adj-RIBs-In, Loc-
RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the
Authentication Code field. routing
information. The minimum length choice of implementation (for example, 3 copies of
the OPEN message is 29 octets (including
message header).

4.3 UPDATE Message Format

UPDATE messages are used to transfer routing information between BGP
peers. The vs 1 copy with pointers) is not constrained by the
protocol.

Routing information in that the UPDATE packet can be used router uses to forward packets (or to
construct
a graph describing the relationships of forwarding table that is used for packet forwarding) is
maintained in the various Autonomous
Systems. By applying rules Routing Table. The Routing Table accumulates routes
to be discussed, routing information loops
and some other anomalies may be detected and removed from inter-AS
routing.

An UPDATE message is used to advertise feasible routes sharing common
path attribute to a peer, or to withdraw multiple unfeasible directly connected networks, static routes, routes learned from service (see 3.1). An UPDATE message may simultaneously
advertise a feasible route
the IGP protocols, and withdraw multiple unfeasible routes learned from service. The UPDATE message always includes the fixed-size BGP
header, and also includes the other fields as shown below (note, some
of the shown fields may BGP. Whether or not a
specific BGP route should be present in every UPDATE message):

+-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+

Withdrawn Routes Length:

This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field installed in octets. Its value must allow the
length of the Network Layer Reachability Information field to
be determined as specified below.

A value of 0 indicates that no routes are being withdrawn from
service, Routing Table, and that
whether a BGP route should override a route to the WITHDRAWN ROUTES field same destination
installed by another source is a local policy decision, not present specified
in this UPDATE message.

Withdrawn Routes:

This is a variable length field that contains a list of IP
address prefixes for document. Besides actual packet forwarding, the routes that are being withdrawn from
service. Each IP address prefix Routing Table
is encoded as a 2-tuple used for resolution of the
form <length, prefix>, whose fields next-hop addresses specified in BGP
updates (see Section 5.1.3).

4. Message Formats

This section describes message formats used by BGP.

BGP messages are described below:

+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+ sent over a TCP connection. A message is processed
only after it is entirely received. The use and maximum message size is 4096
octets. All implementations are required to support this maximum mes-
sage size. The smallest message that may be sent consists of a BGP
header without a data portion, or 19 octets.

4.1 Message Header Format

Each message has a fixed-size header. There may or may not be a data
portion following the meaning header, depending on the message type. The lay-
out of these fields are as follows:

a) Length:

The is shown below:

Marker:

This 16-octet field is included for compatibility; it MUST be
set to all ones.

Length:

This 2-octet unsigned integer indicates the total length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself,
message, including the header, in octets. Thus, e.g., it allows
one to locate in the TCP stream the (Marker field of zero
octets).

b) Prefix: the) next
message. The Prefix value of the Length field contains an IP address prefix followed by
enough trailing bits to make must always be at least
19 and no greater than 4096, and may be further constrained,
depending on the end message type. No "padding" of extra data after
the message is allowed, so the Length field fall on an
octet boundary. Note that must have the
smallest value required given the rest of trailing bits is
irrelevant.

Total Path Attribute Length: the message.

Type:

This 2-octet 1-octet unsigned integer indicates the total length type code of the
Path Attributes field in octets. Its value must allow
message. This document defines the
length of following type codes:

1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE

[RFC2918] defines one more type code.

4.2 OPEN Message Format

After a TCP is established, the Network Layer Reachability field to be determined
as specified below.

A value of 0 indicates that no Network Layer Reachability
Information field first message sent by each side is present in this UPDATE an
OPEN message.

Path Attributes:

A variable length sequence of path attributes is present in
every UPDATE. Each path attribute If the OPEN message is acceptable, a triple <attribute type,
attribute length, attribute value> of variable length.

Attribute Type KEEPALIVE message
confirming the OPEN is a two-octet field that consists of sent back. Once the
Attribute Flags octet followed by OPEN is confirmed, UPDATE,
KEEPALIVE, and NOTIFICATION messages may be exchanged.

In addition to the Attribute Type Code
octet. fixed-size BGP header, the OPEN message contains
the following fields:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The high-order bit (bit 0)
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version:

This 1-octet unsigned integer indicates the protocol version
number of the Attribute Flags octet message. The current BGP version number is 4.

My Autonomous System:

This 2-octet unsigned integer indicates the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).

The second high-order bit (bit 1) Autonomous System
number of the Attribute Flags octet
is sender.

Hold Time:

This 2-octet unsigned integer indicates the Transitive bit. It defines whether an optional attribute
is transitive (if set to 1) or non-transitive (if set to 0).
For well-known attributes, number of seconds
that the Transitive bit must be set to 1.
(See Section 5 sender proposes for a discussion the value of transitive attributes.)

The third high-order bit (bit 2) the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the Attribute Flags octet
is
value of the Partial bit. It defines whether Hold Timer by using the information
contained smaller of its configured
Hold Time and the Hold Time received in the optional transitive attribute is partial (if
set to 1) OPEN message. The
Hold Time MUST be either zero or complete (if set to 0). For well-known attributes
and for optional non-transitive attributes at least three seconds. An
implementation may reject connections on the Partial bit must
be set to 0.

The fourth high-order bit (bit 3) basis of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).

The lower-order four bits of the Attribute Flags octet are
unused. They must be zero when sent and must be ignored when
received. Hold
Time. The Attribute Type Code octet contains the Attribute Type Code.

Currently defined Attribute Type Codes are discussed in Section
5.

If calculated value indicates the Extended Length bit maximum number of
seconds that may elapse between the Attribute Flags octet is set
to 0, the third octet receipt of successive
KEEPALIVE, and/or UPDATE messages by the Path Attribute contains sender.

BGP Identifier:

This 4-octet unsigned integer indicates the length BGP Identifier of
the attribute data in octets.

If sender. A given BGP speaker sets the Extended Length bit value of the Attribute Flags octet is set its BGP Iden-
tifier to 1, then an IP address assigned to that BGP speaker. The
value of the third BGP Identifier is determined on startup and is the fourth octets of the path
attribute contain
same for every local interface and every BGP peer.

Optional Parameters Length:

This 1-octet unsigned integer indicates the total length of the attribute data
Optional Parameters field in octets.

The remaining octets of the Path Attribute represent If the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following:

a) ORIGIN (Type Code 1):

ORIGIN of this field
is zero, no Optional Parameters are present.

Optional Parameters:

This field may contain a well-known mandatory attribute that defines the
origin list of the path information. The data octet can assume
the following values:

Value Meaning

0 IGP - Network Layer Reachability Information optional parameters, where
each parameter is interior to the originating AS encoded as a <Parameter Type, Parameter
Length, Parameter Value> triplet.

0 1
0 1 EGP - Network Layer Reachability Information
learned via the EGP protocol 2 INCOMPLETE - Network Layer Reachability
Information learned by some other means

Its usage is defined in 5.1.1

b) AS_PATH (Type Code 2):

AS_PATH 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

Parameter Type is a well-known mandatory attribute one octet field that unambiguously identi-
fies individual parameters. Parameter Length is composed
of a sequence one octet
field that contains the length of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.

The path segment type the Parameter Value field in
octets. Parameter Value is a 1-octet long variable length field with that is
interpreted according to the
following values defined:

Value Segment value of the Parameter Type

1 AS_SET: unordered set field.

[RFC2842] defines the Capabilities Optional Parameter.

The minimum length of ASs a route in the
UPDATE OPEN message has traversed

2 AS_SEQUENCE: ordered set of ASs a route is 29 octets (including mes-
sage header).

4.3 UPDATE Message Format

UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE message has traversed

The path segment length is can be used to construct
a 1-octet long field containing graph describing the number relationships of ASs in the path segment value field.

The path segment value field contains one or more AS
numbers, each encoded as a 2-octets long field.

Usage of this attribute is defined in 5.1.2.

c) NEXT_HOP (Type Code 3):

This various Autonomous Sys-
tems. By applying rules to be discussed, routing information loops
and some other anomalies may be detected and removed from inter-AS
routing.

An UPDATE message is a well-known mandatory used to advertise feasible routes sharing common
path attribute that defines to a peer, or to withdraw multiple unfeasible routes
from service (see 3.1). An UPDATE message may simultaneously adver-
tise a feasible route and withdraw multiple unfeasible routes from
service. The UPDATE message always includes the IP
address fixed-size BGP
header, and also includes the other fields as shown below (note, some
of the border router that should shown fields may not be used as present in every UPDATE message):

Withdrawn Routes Length:

This 2-octets unsigned integer indicates the next
hop to total length of
the destinations listed Withdrawn Routes field in octets. Its value must allow the
length of the Network Layer Reachability Information field to
be determined as specified below.

A value of 0 indicates that no routes are being withdrawn from
service, and that the UPDATE message.

Usage of this attribute WITHDRAWN ROUTES field is defined not present in 5.1.3.

d) MULTI_EXIT_DISC (Type Code 4):
this UPDATE message.

Withdrawn Routes:

This is an optional non-transitive attribute a variable length field that is contains a four
octet non-negative integer. The value list of this attribute may
be used by a BGP speaker's decision process to discriminate
among multiple entry points to a neighboring autonomous
system.

Its usage is defined in 5.1.4.

e) LOCAL_PREF (Type Code 5):

LOCAL_PREF is a well-known attribute IP
address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a four octet
non-negative integer. A BGP speaker uses it to inform other
internal peers 2-tuple of the advertising speaker's degree of
preference for an advertised route. Usage of this attribute
is
form <length, prefix>, whose fields are described in 5.1.5.

f) ATOMIC_AGGREGATE (Type Code 6)

ATOMIC_AGGREGATE is a well-known discretionary attribute below:

+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+

The use and the meaning of these fields are as follows:

a) Length:

The Length field indicates the length 0. Usage of this attribute is described in 5.1.6.

g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute bits of the IP
address prefix. A length 6. of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).

b) Prefix:

The attribute Prefix field contains the last AS number that formed the
aggregate route (encoded as 2 octets), an IP address prefix followed by
enough trailing bits to make the IP
address end of the BGP speaker field fall on an
octet boundary. Note that formed the aggregate route
(encoded as 4 octets). value of trailing bits is
irrelevant.

Total Path Attribute Length:

This should be 2-octet unsigned integer indicates the same address as total length of the one used for
Path Attributes field in octets. Its value must allow the BGP Identifier
length of the speaker. Usage
of this attribute is described in 5.1.7. Network Layer Reachability Information:

This variable length field contains a list of IP address
prefixes. The length in octets to be determined
as specified below.

A value of the 0 indicates that no Network Layer Reachability
Information field is not encoded explicitly, but can be
calculated as: present in this UPDATE message Length - 23 - Total message.

Path Attributes Length -
Withdrawn Routes Length

where Attributes:

A variable length sequence of path attributes is present in
every UPDATE message, except for an UPDATE message Length that carries
only the withdrawn routes. Each path attribute is a triple
<attribute type, attribute length, attribute value> of variable
length.

Attribute Type is a two-octet field that consists of the value encoded in
Attribute Flags octet followed by the fixed-
size BGP header, Total Path Attribute Length and Withdrawn
Routes Length are Type Code
octet.

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The high-order bit (bit 0) of the values encoded in Attribute Flags octet is the variable part of
Optional bit. It defines whether the UPDATE message, and 23 attribute is a combined length optional (if
set to 1) or well-known (if set to 0).

The second high-order bit (bit 1) of the fixed-
size BGP header, the Total Path Attribute Length field and Flags octet
is the
Withdrawn Routes Length field.

Reachability information Transitive bit. It defines whether an optional attribute
is encoded as one transitive (if set to 1) or more 2-tuples of non-transitive (if set to 0).
For well-known attributes, the form <length, prefix>, whose fields are described below:

+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+

The use and the meaning of these fields are as follows:

a) Length:

The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates Transitive bit must be set to 1.
(See Section 5 for a prefix that
matches all IP addresses (with prefix, itself, discussion of zero
octets).

b) Prefix: transitive attributes.)

The Prefix field contains IP address prefixes followed by
enough trailing bits to make the end third high-order bit (bit 2) of the field fall on an Attribute Flags octet boundary. Note that
is the value of Partial bit. It defines whether the trailing bits is
irrelevant.

The minimum length of information con-
tained in the UPDATE message optional transitive attribute is 23 octets -- 19 octets
for the fixed header + 2 octets partial (if set
to 1) or complete (if set to 0). For well-known attributes and
for optional non-transitive attributes the Withdrawn Routes Length + 2
octets for Partial bit must be
set to 0.

The fourth high-order bit (bit 3) of the Total Path Attribute Length (the value of Withdrawn
Routes Length Flags octet
is 0 and the value of Total Path Extended Length bit. It defines whether the Attribute
Length is
0).

An UPDATE message can advertise at most one octet (if set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI field 0) or two octets (if set to 1).

The lower-order four bits of the UPDATE
message.

An UPDATE message can list multiple routes to Attribute Flags octet are
unused. They must be withdrawn from
service. Each such route is identified by its destination (expressed
as an IP prefix), which unambiguously identifies zero when sent and must be ignored when
received.

The Attribute Type Code octet contains the route Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.

If the
context Extended Length bit of the BGP speaker - BGP speaker connection to which it has
been previously advertised.

An UPDATE message might advertise only routes Attribute Flags octet is set
to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case 0, the WITHDRAWN ROUTES field need not be
present.

An UPDATE message should not include third octet of the same address prefix in Path Attribute contains the
WITHDRAWN ROUTES and Network Layer Reachability Information fields,
however a BGP speaker MUST be able to process UPDATE messages in this
form. A BGP speaker should treat an UPDATE message length
of this form as if the WITHDRAWN ROUTES doesn't contain attribute data in octets.

If the address prefix.

4.4 KEEPALIVE Message Format

BGP does not use any transport protocol-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause Extended Length bit of the Hold Timer Attribute Flags octet is set
to expire. A reasonable maximum time between KEEPALIVE messages would
be one 1, then the third and the fourth octets of the Hold Time interval. KEEPALIVE messages MUST NOT
be sent more frequently than one per second. An implementation MAY
adjust path
attribute contain the rate at which it sends KEEPALIVE messages as a function length of the Hold Time interval.

If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.

KEEPALIVE message consists attribute data in octets.

The remaining octets of only message header the Path Attribute represent the
attribute value and has a length of
19 octets.

4.5 NOTIFICATION Message Format

A NOTIFICATION message is sent when an error condition is detected. are interpreted according to the Attribute
Flags and the Attribute Type Code. The BGP connection supported Attribute Type
Codes, their attribute values and uses are the following:

a) ORIGIN (Type Code 1):

ORIGIN is closed immediately after sending it.

In addition to a well-known mandatory attribute that defines the fixed-size BGP header,
origin of the NOTIFICATION message
contains path information. The data octet can assume
the following fields:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 values:

Value Meaning

0 IGP - Network Layer Reachability Information
is interior to the originating AS

1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Error Code:

This 1-octet unsigned integer indicates EGP - Network Layer Reachability Information
learned via the type EGP protocol [RFC904]

2 INCOMPLETE - Network Layer Reachability
Information learned by some other means

Usage of
NOTIFICATION. The following Error Codes have been defined:

Error this attribute is defined in 5.1.1.

b) AS_PATH (Type Code Symbolic Name Reference 2):

AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.

The path segment type is a 1-octet long field with the fol-
lowing values defined:

Value Segment Type

1 Message Header Error Section 6.1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed

2 OPEN Message Error Section 6.2

3 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE Message Error Section 6.3

4 Hold Timer Expired Section 6.5

5 Finite State Machine Error Section 6.6

6 Cease Section 6.7

Error subcode:

This message has traversed

The path segment length is a 1-octet unsigned integer provides more specific
information about long field containing
the nature number of ASs (not the reported error. Each Error
Code may have number of octets) in the path
segment value field.

The path segment value field contains one or more Error Subcodes associated with it. If
no appropriate Error Subcode is defined, then AS num-
bers, each encoded as a zero
(Unspecific) value 2-octets long field.

Usage of this attribute is defined in 5.1.2.

c) NEXT_HOP (Type Code 3):

This is a well-known mandatory attribute that defines the IP
address of the border router that should be used for as the next
hop to the destinations listed in the Network Layer Reacha-
bility Information field of the Error Subcode field.

Message Header Error subcodes:

1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.

OPEN Message Error subcodes:

1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - Authentication Failure.
6 - Unacceptable Hold Time. UPDATE Message Error subcodes:

1 - Malformed Attribute List. message.

Usage of this attribute is defined in 5.1.3.

d) MULTI_EXIT_DISC (Type Code 4):

This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may
be used by a BGP speaker's decision process to discriminate
among multiple entry points to a neighboring autonomous sys-
tem.

Usage of this attribute is defined in 5.1.4.

e) LOCAL_PREF (Type Code 5):

LOCAL_PREF is a well-known attribute that is a four octet
non-negative integer. A BGP speaker uses it to inform other
internal peers of the advertising speaker's degree of pref-
erence for an advertised route.

Usage of this attribute is defined in 5.1.5.

f) ATOMIC_AGGREGATE (Type Code 6)

ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0.

Usage of this attribute is defined in 5.1.6.

g) AGGREGATOR (Type Code 7)

AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute. octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). This should be the same address as
the one used for the BGP Identifier of the speaker.

Usage of this attribute is defined in 5.1.7.

Network Layer Reachability Information:

This variable length field contains a list of IP address pre-
fixes. The length in octets of the Network Layer Reachability
Information is not encoded explicitly, but can be calculated
as:

UPDATE message Length - 23 - Total Path Attributes Length -
Withdrawn Routes Length

where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Withdrawn
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the
Withdrawn Routes Length field.

Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:

+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+

The use and the meaning of these fields are as follows:

a) Length:

The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).

b) Prefix:

The Prefix field contains an IP address prefix followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.

The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Withdrawn Routes Length + 2
octets for the Total Path Attribute Length (the value of Withdrawn
Routes Length is 0 and the value of Total Path Attribute Length is
0).

An UPDATE message can advertise at most one set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI field of the UPDATE
message.

An UPDATE message can list multiple routes to be withdrawn from ser-
vice. Each such route is identified by its destination (expressed as
an IP prefix), which unambiguously identifies the route in the con-
text of the BGP speaker - BGP speaker connection to which it has been
previously advertised.

An UPDATE message might advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be
present.

An UPDATE message should not include the same address prefix in the
WITHDRAWN ROUTES and Network Layer Reachability Information fields,
however a BGP speaker MUST be able to process UPDATE messages in this
form. A BGP speaker should treat an UPDATE message of this form as if
the WITHDRAWN ROUTES doesn't contain the address prefix.

4.4 KEEPALIVE Message Format

BGP does not use any TCP-based keep-alive mechanism to determine if
peers are reachable. Instead, KEEPALIVE messages are exchanged
between peers often enough as not to cause the Hold Timer to expire.
A reasonable maximum time between KEEPALIVE messages would be one
third of the Hold Time interval. KEEPALIVE messages MUST NOT be sent
more frequently than one per second. An implementation MAY adjust the
rate at which it sends KEEPALIVE messages as a function of the Hold
Time interval.

If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.

KEEPALIVE message consists of only message header and has a length of
19 octets.

4.5 NOTIFICATION Message Format

A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it.

In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Error Code:

This 1-octet unsigned integer indicates the type of
NOTIFICATION. The following Error Codes have been defined:

Error Code Symbolic Name Reference

1 Message Header Error Section 6.1

2 OPEN Message Error Section 6.2

3 UPDATE Message Error Section 6.3

4 Hold Timer Expired Section 6.5

5 Finite State Machine Error Section 6.6

6 Cease Section 6.7

Error subcode:

This 1-octet unsigned integer provides more specific informa-
tion about the nature of the reported error. Each Error Code
may have one or more Error Subcodes associated with it. If no
appropriate Error Subcode is defined, then a zero (Unspecific)
value is used for the Error Subcode field.

Message Header Error subcodes:

1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.

OPEN Message Error subcodes:

1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - Authentication Failure.
6 - Unacceptable Hold Time.

UPDATE Message Error subcodes:

1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
11 - Malformed AS_PATH.

Data:

This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more
details.

Note that the length of the Data field can be determined from
the message Length field by the formula:

Message Length = 21 + Data Length

The minimum length of the NOTIFICATION message is 21 octets (includ-
ing message header).

5. Path Attributes

This section discusses the path attributes of the UPDATE message.

Path attributes fall into four separate categories:

1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.

Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every
UPDATE message that contains NLRI. Others are discretionary and may
or may not be sent in a particular UPDATE message.

All well-known attributes must be passed along (after proper updat-
ing, if necessary) to other BGP peers.

In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all BGP
implementations support all optional attributes. The handling of an
unrecognized optional attribute is determined by the setting of the
Transitive bit in the attribute flags octet. Paths with unrecognized
transitive optional attributes should be accepted. If a path with
unrecognized transitive optional attribute is accepted and passed
along to other BGP peers, then the unrecognized transitive optional
attribute of that path must be passed along with the path to other
BGP peers with the Partial bit in the Attribute Flags octet set to 1.
If a path with recognized transitive optional attribute is accepted
and passed along to other BGP peers and the Partial bit in the
Attribute Flags octet is set to 1 by some previous AS, it is not set
back to 0 by the current AS. Unrecognized non-transitive optional
attributes must be quietly ignored and not passed along to other BGP
peers.

New transitive optional attributes may be attached to the path by the
originator or by any other BGP speaker in the path. If they are not
attached by the originator, the Partial bit in the Attribute Flags
octet is set to 1. The rules for attaching new non-transitive
optional attributes will depend on the nature of the specific
attribute. The documentation of each new non-transitive optional
attribute will be expected to include such rules. (The description of
the MULTI_EXIT_DISC attribute gives an example.) All optional
attributes (both transitive and non-transitive) may be updated (if
appropriate) by BGP speakers in the path.

The sender of an UPDATE message should order path attributes within
the UPDATE message in ascending order of attribute type. The receiver
of an UPDATE message must be prepared to handle path attributes
within the UPDATE message that are out of order.

The same attribute can not appear more than once within the Path
Attributes field of a particular UPDATE message.

The mandatory category refers to an attribute which must be present
in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary, or discre-
tionary, required, or disallowed in certain contexts.

attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF see section 5.1.5 required
ATOMIC_AGGREGATE see section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary

5.1 Path Attribute Usage

The usage of each BGP path attributes is described in the following
clauses.

5.1.1 ORIGIN

ORIGIN is a well-known mandatory attribute. The ORIGIN attribute
shall be generated by the speaker that originates the associated
routing information. Its value SHOULD NOT be changed by any other
speaker.

5.1.2 AS_PATH

AS_PATH is a well-known mandatory attribute. This attribute identi-
fies the autonomous systems through which routing information carried
in this UPDATE message has passed. The components of this list can be
AS_SETs or AS_SEQUENCEs.

When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify the route's
AS_PATH attribute based on the location of the BGP speaker to which
the route will be sent:

a) When a given BGP speaker advertises the route to an internal
peer, the advertising speaker shall not modify the AS_PATH
attribute associated with the route.

b) When a given BGP speaker advertises the route to an external
peer, then the advertising speaker shall update the AS_PATH
attribute as follows:

1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system shall prepend its own AS number
as the last element of the sequence (put it in the leftmost
position). If the act of prepending will cause an overflow in
the AS_PATH segment, i.e. more than 255 ASs, it shall be legal
to prepend a new segment of type AS_SEQUENCE and prepend its
own AS number to this new segment.

2) if the first path segment of the AS_PATH is of type AS_SET,
the local system shall prepend a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment.

When a BGP speaker originates a route then:

a) the originating speaker shall include its own AS number in a
path segment of type AS_SEQUENCE in the AS_PATH attribute of all
UPDATE messages sent to an external peer. (In this case, the AS
number of the originating speaker's autonomous system will be the
only entry the path segment, and this path segment will be the
only segment in the AS_PATH attribute).

b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to internal peers. (An
empty AS_PATH attribute is one whose length field contains the
value zero).

Whenever the modification of the AS_PATH attribute calls for includ-
ing or prepending the AS number of the local system, the local system
may include/prepend more than one instance of its own AS number in
the AS_PATH attribute. This is controlled via local configuration.

5.1.3 NEXT_HOP

The NEXT_HOP is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next hop to
the destinations listed in the UPDATE message. The NEXT_HOP attribute
is calculated as follows.

1) When sending a message to an internal peer, if the route is not
locally originated the BGP speaker should not modify the NEXT_HOP
attribute, unless it has been explicitly configured to announce
its own IP address as the NEXT_HOP. When announcing a locally
originated route to an internal peer, the BGP speaker should use
as the NEXT_HOP the interface address of the router through which
the announced network is reachable for the speaker; if the route
is directly connected to the speaker, or the interface address of
the router through which the announced network is reachable for
the speaker is the internal peer's address, then the BGP speaker
should use for the NEXT_HOP attribute its own IP address (the
address of the interface that is used to reach the peer).

2) When sending a message to an external peer X, and the peer is
one IP hop away from the speaker:

- Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute
8 - Invalid If the route being announced was learned from an internal
peer or is locally originated, the BGP speaker can use for the
NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 attribute an interface address of the internal peer
router (or the internal router) through which the announced
network is reachable for the speaker, provided that peer X
shares a common subnet with this address. This is a form of
"third party" NEXT_HOP attribute.

- Invalid Network Field.
11 Otherwise, if the route being announced was learned from an
external peer, the speaker can use in the NEXT_HOP attribute an
IP address of any adjacent router (known from the received
NEXT_HOP attribute) that the speaker itself uses for local
route calculation, provided that peer X shares a common subnet
with this address. This is a second form of "third party"
NEXT_HOP attribute.

- Malformed AS_PATH.

Data: Otherwise, if the external peer to which the route is being
advertised shares a common subnet with one of the announcing
router's own interfaces, the router may use the IP address
associated with such an interface in the NEXT_HOP attribute.
This variable-length field is used known as a "first party" NEXT_HOP attribute.

- By default (if none of the above conditions apply), the BGP
speaker should use in the NEXT_HOP attribute the IP address of
the interface that the speaker uses to establish the BGP con-
nection to peer X.

3) When sending a message to an external peer X, and the peer is
multiple IP hops away from the speaker (aka "multihop EBGP"):

- The speaker may be configured to diagnose propagate the reason for NEXT_HOP
attribute. In this case when advertising a route that the NOTIFICATION. The contents
speaker learned from one of its peers, the Data field depend upon NEXT_HOP attribute
of the Error Code and Error Subcode. See Section 6 below for more
details.

Note that advertised route is exactly the length same as the NEXT_HOP
attribute of the Data field can be determined from learned route (the speaker just doesn't modify
the message Length field by NEXT_HOP attribute).

- By default, the formula:

Message Length = 21 + Data Length

The minimum length BGP speaker should use in the NEXT_HOP
attribute the IP address of the NOTIFICATION message interface that the speaker uses
to establish the BGP connection to peer X.

Normally the NEXT_HOP attribute is 21 octets
(including message header).

5. Path Attributes

This section discusses chosen such that the shortest
available path attributes of the UPDATE message.

Path attributes fall into four separate categories:

1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.

Well-known attributes must will be recognized by all taken. A BGP implementations.
Some speaker must be able to support
disabling advertisement of these third party NEXT_HOP attributes are mandatory and to handle
imperfectly bridged media.

A BGP speaker must be included in every
UPDATE message never advertise an address of a peer to that contains NLRI. Others are discretionary and may
or may not be sent in peer
as a particular UPDATE message.

All well-known attributes NEXT_HOP, for a route that the speaker is originating. A BGP
speaker must be passed along (after proper
updating, if necessary) to other never install a route with itself as the next hop.

The NEXT_HOP attribute is used by the BGP peers.

In addition speaker to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected determine the
actual outbound interface and immediate next-hop address that all BGP
implementations support all optional attributes. should
be used to forward transit packets to the associated destinations.

The handling of an
unrecognized optional attribute immediate next-hop address is determined by performing a recur-
sive route lookup operation for the setting IP address in the NEXT_HOP
attribute using the contents of the
Transitive bit Routing Table, selecting one
entry if multiple entries of equal cost exist. The Routing Table
entry which resolves the IP address in the NEXT_HOP attribute will
always specify the outbound interface. If the entry specifies an
attached subnet, but does not specify a next-hop address, then the
address in the NEXT_HOP attribute flags octet. Paths with unrecognized
transitive optional attributes should be accepted. used as the immediate
next-hop address. If a path with
unrecognized transitive the entry also specifies the next-hop address,
this address should be used as the immediate next-hop address for
packet forwarding.

5.1.4 MULTI_EXIT_DISC

The MULTI_EXIT_DISC is an optional non-transitive attribute which may
be used on external (inter-AS) links to discriminate among multiple
exit or entry points to the same neighboring AS. The value of the
MULTI_EXIT_DISC attribute is accepted and passed
along a four octet unsigned number which is
called a metric. All other factors being equal, the exit point with
lower metric should be preferred. If received over EBGP, the
MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other BGP peers, then
speakers within the unrecognized transitive optional same AS. The MULTI_EXIT_DISC attribute of that path must received
from a neighboring AS MUST NOT be passed along with the path propagated to other neighboring
ASs.

A BGP peers with the Partial bit in speaker MUST IMPLEMENT a mechanism based on local configuration
which allows the Attribute Flags octet set MULTI_EXIT_DISC attribute to 1.
If be removed from a path with recognized transitive optional attribute is accepted
and passed along
route. This MAY be done prior to other BGP peers determining the degree of preference
of the route and performing route selection (decision process phases
1 and 2).

An implementation MAY also (based on local configuration) alter the Partial bit in
value of the
Attribute Flags octet is set MULTI_EXIT_DISC attribute received over EBGP. This MAY
be done prior to determining the degree of preference of the route
and performing route selection (decision process phases 1 by some previous AS, it and 2). See
section 9.1.2.2 for necessary restricts on this.

5.1.5 LOCAL_PREF

LOCAL_PREF is not set
back to 0 by the current AS. Unrecognized non-transitive optional
attributes must a well-known attribute that SHALL be quietly ignored and not passed along to other included in all
UPDATE messages that a given BGP
peers.

New transitive optional attributes may be attached speaker sends to the path by the
originator or by any other internal
peers. A BGP speaker in the path. If they are not
attached by the originator, the Partial bit in SHALL calculate the Attribute Flags
octet is set to 1. The rules degree of preference for attaching new non-transitive
optional attributes will depend
each external route based on the nature of locally configured policy, and
include the specific
attribute. The documentation degree of each new non-transitive optional
attribute will be expected preference when advertising a route to include such rules. (The description its
internal peers. The higher degree of
the MULTI_EXIT_DISC attribute gives an example.) All optional
attributes (both transitive and non-transitive) may preference MUST be updated (if
appropriate) by preferred. A
BGP speakers in speaker shall use the path.

The sender degree of an UPDATE message should order path attributes within
the UPDATE message preference learned via LOCAL_PREF
in ascending order of its decision process (see section 9.1.1).

A BGP speaker MUST NOT include this attribute type. The receiver
of an in UPDATE message must be prepared messages that
it sends to handle path attributes
within external peers, except for the case of BGP Confederations
[RFC3065]. If it is contained in an UPDATE message that are out of order.

The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message.

The mandatory category refers to is received
from an external peer, then this attribute which must MUST be present
in both IBGP and EBGP exchanges if NLRI are contained in ignored by the UPDATE
message. Attributes classified as optional
receiving speaker, except for the purpose of the
protocol extension mechanism may be purely discretionary, or
discretionary, required, or disallowed in certain contexts.

attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF disallowed required
ATOMIC_AGGREGATE see section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary

5.1 Path Attribute Usage

The usage case of each BGP path attributes is described in the following
clauses.

5.1.1 ORIGIN

ORIGIN Confederations
[RF3065].

5.1.6 ATOMIC_AGGREGATE

ATOMIC_AGGREGATE is a well-known mandatory discretionary attribute. The ORIGIN attribute
shall be generated by

When a router aggregates several routes for the autonomous system purpose of advertise-
ment to a particular peer, the AS_PATH of the aggregated route nor-
mally includes an AS_SET formed from the set of AS from which the
aggregate was formed. In many cases the network administrator can
determine that originates the
associated routing information. It shall aggregate can safely be included in advertised without the UPDATE
messages
AS_SET and not form route loops.

If an aggregate excludes at least some of all BGP speakers that choose to propagate this
information to other BGP speakers.

5.1.2 AS_PATH

AS_PATH is a well-known mandatory attribute. This attribute
identifies the autonomous systems through which routing information
carried AS numbers present in this UPDATE message has passed. The components
the AS_PATH of this
list can be AS_SETs or AS_SEQUENCEs.

When the routes that are aggregated as a result of dropping
the AS_SET, the aggregated route, when advertised to the peer, SHOULD
include the ATOMIC_AGGREGATE attribute.

A BGP speaker propagates that receives a route which it has learned from
another BGP speaker's UPDATE message, it shall modify with the route's
AS_PATH ATOMIC_AGGREGATE
attribute based on SHOULD NOT remove the location of attribute from the route when propa-
gating it to other speakers.

A BGP speaker to which that receives a route with the ATOMIC_AGGREGATE
attribute MUST NOT make any NLRI of that route will be sent:

a) When a given more specific (as
defined in 9.1.4) when advertising this route to other BGP speakers.

A BGP speaker advertises the that receives a route with the ATOMIC_AGGREGATE
attribute needs to an internal
peer, be cognizant of the fact that the actual path to
destinations, as specified in the advertising speaker shall NLRI of the route, while having the
loop-free property, may not modify be the path specified in the AS_PATH
attribute associated with of the route.

b) When a given

5.1.7 AGGREGATOR

AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.2.2). A
BGP speaker advertises the which performs route to an external
peer, then the advertising speaker shall update aggregation may add the AS_PATH AGGREGATOR
attribute as follows:

1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system which shall prepend contain its own AS number and IP address. The
IP address should be the same as the last element BGP Identifier of the sequence (put it in speaker.

6. BGP Error Handling.

This section describes actions to be taken when errors are detected
while processing BGP messages.

When any of the leftmost
position). If conditions described here are detected, a NOTIFICA-
TION message with the act of prepending will cause an overflow in indicated Error Code, Error Subcode, and Data
fields is sent, and the AS_PATH segment, i.e. more than 255 elements, BGP connection is closed, unless it shall be
legal is
explicitly stated that no NOTIFICATION message is to prepend a new segment of type AS_SEQUENCE be sent and prepend
its own AS number the
BGP connection is not to this new segment.

2) if be closed. If no Error Subcode is specified,
then a zero must be used.

The phrase "the BGP connection is closed" means that the first path segment of TCP connec-
tion has been closed, the AS_PATH is of type AS_SET, associated Adj-RIB-In has been cleared, and
that all resources for that BGP connection have been deallocated.
Entries in the local system shall prepend a new path segment of type
AS_SEQUENCE to Loc-RIB associated with the AS_PATH, including its own AS number in remote peer are marked as
invalid. The fact that
segment.

When a the routes have become invalid is passed to
other BGP speaker originates a route then:

a) peers before the originating speaker shall include its own AS number in a
path segment of type AS_SEQUENCE in routes are deleted from the AS_PATH attribute system.

Unless specified explicitly, the Data field of all
UPDATE messages the NOTIFICATION mes-
sage that is sent to indicate an external peer. (In this case, the AS
number of the originating speaker's autonomous system will be error is empty.

6.1 Message Header error handling.

All errors detected while processing the
only entry Message Header are indicated
by sending the path segment, and this path segment will be NOTIFICATION message with Error Code Message Header
Error. The Error Subcode elaborates on the
only segment in specific nature of the AS_PATH attribute).

b)
error.

The expected value of the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to internal peers. (An
empty AS_PATH attribute is one whose length Marker field contains of the
value zero).

Whenever message header is all
ones. If the modification Marker field of the AS_PATH attribute calls for
including or prepending message header is not as expected,
then a synchronization error has occurred and the AS number of Error Subcode is
set to Connection Not Synchronized.

If the local system, Length field of the local
system may include/prepend more message header is less than one instance of its own AS
number in 19 or greater
than 4096, or if the AS_PATH attribute. This Length field of an OPEN message is controlled via local
configuration.

5.1.3 NEXT_HOP

The NEXT_HOP path attribute defines less than the IP address
minimum length of the border
router that should be used as OPEN message, or if the next hop to Length field of an
UPDATE message is less than the destinations listed
in minimum length of the UPDATE message. The NEXT_HOP attribute is calculated as
follows.

1) When sending message,
or if the Length field of a KEEPALIVE message to an internal peer, the BGP speaker
should is not modify the NEXT_HOP attribute, unless it has been
explicitly configured equal to announce its own IP address as 19, or
if the
NEXT_HOP.

2) When sending Length field of a NOTIFICATION message to an external peer X, and the peer is
one IP hop away from less than the speaker:

- If mini-
mum length of the route being announced was learned from an internal
peer or NOTIFICATION message, then the Error Subcode is locally originated, set
to Bad Message Length. The Data field contains the BGP speaker can use for erroneous Length
field.

If the
NEXT_HOP attribute an interface address Type field of the internal peer
router (or the internal router) through which message header is not recognized, then the announced
network
Error Subcode is reachable for set to Bad Message Type. The Data field contains the speaker, provided that peer X
shares a common subnet
erroneous Type field.

6.2 OPEN message error handling.

All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with this address. This is a form Error Code OPEN Message
Error. The Error Subcode elaborates on the specific nature of
"third party" NEXT_HOP attribute.

- If the route being announced was learned from an external
peer,
error.

If the speaker can use version number contained in the NEXT_HOP attribute an IP
address Version field of any adjacent router (known from the received
NEXT_HOP attribute) that the speaker itself uses for local
route calculation, provided that peer X shares a common subnet
with this address. This
OPEN message is a second form of "third party"
NEXT_HOP attribute.

- If not supported, then the external peer Error Subcode is set to which the route
Unsupported Version Number. The Data field is being advertised
shares a common subnet with one of 2-octets unsigned
integer, which indicates the announcing router's own
interfaces, largest locally supported version number
less than the router may use version the IP address associated with
such an interface remote BGP peer bid (as indicated in the NEXT_HOP attribute. This
received OPEN message), or if the smallest locally supported version
number is known as a
"first party" NEXT_HOP attribute.

- By default (if none of greater than the above conditions apply), version the remote BGP
speaker should use in peer bid, then the NEXT_HOP attribute
smallest locally supported version number.

If the IP address Autonomous System field of the interface that the speaker uses to establish the BGP
session to peer X.

3) When sending a OPEN message to an external peer X, and the peer is
multiple IP hops away from unacceptable,
then the speaker (aka "multihop EBGP"):

- The speaker may be configured Error Subcode is set to propagate the NEXT_HOP
attribute. In this case when advertising a route that Bad Peer AS. The determination of
acceptable Autonomous System numbers is outside the
speaker learned from one scope of its peers, this
protocol.

If the NEXT_HOP attribute Hold Time field of the advertised route OPEN message is exactly the same as unacceptable, then the NEXT_HOP
attribute
Error Subcode MUST be set to Unacceptable Hold Time. An implementa-
tion MUST reject Hold Time values of one or two seconds. An imple-
mentation MAY reject any proposed Hold Time. An implementation which
accepts a Hold Time MUST use the learned route (the speaker just doesn't modify negotiated value for the NEXT_HOP attribute).

- By default, Hold Time.

If the BGP speaker should use in the NEXT_HOP
attribute the IP address Identifier field of the interface that OPEN message is syntactically
incorrect, then the speaker uses Error Subcode is set to establish the Bad BGP session to peer X.

Normally the NEXT_HOP attribute is chosen such Identifier. Syn-
tactic correctness means that the shortest
available path will be taken. A BGP speaker must be able to support
disabling advertisement Identifier field represents a
valid IP host address.

If one of third party NEXT_HOP attributes the Optional Parameters in the OPEN message is not
recognized, then the Error Subcode is set to handle
imperfectly bridged media.

A BGP speaker must never advertise an address Unsupported Optional
Parameters.

If one of a peer to that peer
as a NEXT_HOP, for a route that the speaker Optional Parameters in the OPEN message is originating. A BGP
speaker must never install a route with itself as recognized,
but is malformed, then the next hop. Error Subcode is set to 0 (Unspecific).

6.3 UPDATE message error handling.

All errors detected while processing the UPDATE message are indicated
by sending the NOTIFICATION message with Error Code UPDATE Message
Error. The NEXT_HOP attribute is used by error subcode elaborates on the BGP speaker to determine specific nature of the
actual outbound interface and immediate next-hop address that should
be used to forward transit packets to
error.

Error checking of an UPDATE message begins by examining the associated destinations.
The immediate next-hop address path
attributes. If the Withdrawn Routes Length or Total Attribute Length
is determined by performing a
recursive route lookup operation for too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the IP address in message Length), then the NEXT_HOP Error Subcode is
set to Malformed Attribute List.

If any recognized attribute using has Attribute Flags that conflict with
the contents of Attribute Type Code, then the Routing Table (see Section
9.1.2.2). Error Subcode is set to Attribute
Flags Error. The resolving route will always specify Data field contains the outbound
interface. erroneous attribute (type,
length and value).

If any recognized attribute has Attribute Length that conflicts with
the resolving route specifies expected length (based on the next-hop address,
this address should be used as attribute type code), then the immediate address for packet
forwarding.
Error Subcode is set to Attribute Length Error. The Data field con-
tains the erroneous attribute (type, length and value).

If any of the address in mandatory well-known attributes are not present, then
the NEXT_HOP attribute Error Subcode is directly
resolved through a route set to an attached subnet (such a route will not
specify Missing Well-known Attribute. The Data
field contains the next-hop address), Attribute Type Code of the outbound interface should be taken
from missing well-known
attribute.

If any of the resolving route and mandatory well-known attributes are not recognized,
then the address in Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the NEXT_HOP unrecognized attribute
should be used as (type, length and
value).

If the immediate next-hop address.

5.1.4 MULTI_EXIT_DISC

The MULTI_EXIT_DISC ORIGIN attribute may be used on external (inter-AS)
links to discriminate among multiple exit or entry points to has an undefined value, then the same
neighboring AS. Error Sub-
code is set to Invalid Origin Attribute. The value of Data field contains the MULTI_EXIT_DISC
unrecognized attribute is a four
octet unsigned number which is called a metric. All other factors
being equal, the exit point with lower metric should be preferred. (type, length and value).

If
received over external links, the MULTI_EXIT_DISC NEXT_HOP attribute MAY be
propagated over internal links to other BGP speakers within field is syntactically incorrect, then the same
AS.
Error Subcode is set to Invalid NEXT_HOP Attribute. The MULTI_EXIT_DISC Data field
contains the incorrect attribute received from a neighboring AS MUST
NOT be propagated to other neighboring ASs.

A BGP speaker MUST IMPLEMENT a mechanism based on local configuration
which allows (type, length and value). Syntactic
correctness means that the MULTI_EXIT_DISC NEXT_HOP attribute to be removed from represents a
route. This MAY be done prior to determining valid IP
host address.

The IP address in the degree of preference
of NEXT_HOP must meet the route and performing route selection (decision process phases
1 and 2).

An implementation MAY also (based on local configuration) alter following criteria to be
considered semantically correct:

a) It must not be the
value IP address of the MULTI_EXIT_DISC attribute received over an external
link. If it does so, it shall do so prior to determining receiving speaker

b) In the degree
of preference case of an EBGP where the route and performing route selection (decision
process phases 1 sender and 2).

5.1.5 LOCAL_PREF

LOCAL_PREF is a well-known attribute that SHALL be included receiver are one IP
hop away from each other, either the IP address in all
UPDATE messages that a given BGP speaker sends the NEXT_HOP
must be the sender's IP address (that is used to establish the other internal
peers. A BGP speaker SHALL calculate
connection), or the degree of preference for
each external route based on interface associated with the locally configured policy, NEXT_HOP IP
address must share a common subnet with the receiving BGP speaker.

If the NEXT_HOP attribute is semantically incorrect, the error should
be logged, and
include the degree of preference when advertising a route to its
internal peers. The higher degree of preference MUST should be preferred. A
BGP speaker shall use the degree of preference learned via LOCAL_PREF
in its decision process (see section 9.1.1).

A BGP speaker MUST NOT include ignored. In this case, no NOTIFI-
CATION message should be sent, and connection should not be closed.

The AS_PATH attribute in UPDATE messages that
it sends is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode is set to
Malformed AS_PATH.

If the UPDATE message is received from an external peers, except for peer, the case local
system MAY check whether the leftmost AS in the AS_PATH attribute is
equal to the autonomous system number of BGP Confederations
[13]. the peer than sent the mes-
sage. If it the check determines that this is contained in not the case, the Error
Subcode is set to Malformed AS_PATH.

If an UPDATE message that optional attribute is received from
an external peer, recognized, then the value of this
attribute MUST be ignored by the
receiving speaker, except for is checked. If an error is detected, the case of BGP Confederations [13].

5.1.6 ATOMIC_AGGREGATE

ATOMIC_AGGREGATE attribute is a well-known discretionary attribute.

When a router aggregates several routes for dis-
carded, and the purpose of
advertisement Error Subcode is set to a particular peer, and Optional Attribute Error.
The Data field contains the AS_PATH of attribute (type, length and value).

If any attribute appears more than once in the aggregated
route excludes at least some of UPDATE message, then
the AS numbers present Error Subcode is set to Malformed Attribute List.

The NLRI field in the AS_PATH
of UPDATE message is checked for syntactic valid-
ity. If the routes that are aggregated, field is syntactically incorrect, then the aggregated route, when
advertised Error Subcode
is set to the peer, MUST include the ATOMIC_AGGREGATE attribute.

A BGP speaker that receives Invalid Network Field.

If a route with the ATOMIC_AGGREGATE
attribute MUST NOT remove prefix in the attribute from NLRI field is semantically incorrect (e.g., an
unexpected multicast IP address), an error should be logged locally,
and the route when
propagating it to other speakers.

A BGP speaker prefix should be ignored.

An UPDATE message that receives contains correct path attributes, but no NLRI,
shall be treated as a route with valid UPDATE message.

6.4 NOTIFICATION message error handling.

If a peer sends a NOTIFICATION message, and the ATOMIC_AGGREGATE
attribute MUST NOT make any NLRI receiver of that route more specific (as
defined the mes-
sage detects an error in 9.1.4) when advertising that message, the receiver can not use a
NOTIFICATION message to report this route error back to other BGP speakers.

A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute needs to peer. Any such
error, such as an unrecognized Error Code or Error Subcode, should be cognizant
noticed, logged locally, and brought to the attention of the fact that adminis-
tration of the actual path peer. The means to
destinations, as do this, however, lies outside the
scope of this document.

6.5 Hold Timer Expired error handling.

If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the NLRI Hold
Time field of the route, while having OPEN message, then the
loop-free property, may not NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the path specified in BGP connection
closed.

6.6 Finite State Machine error handling.

Any error detected by the AS_PATH
attribute BGP Finite State Machine (e.g., receipt of the route.

5.1.7 AGGREGATOR

AGGREGATOR is
an optional transitive attribute which unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.

6.7 Cease.

In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be included
in updates which are formed used when a fatal error
indicated by aggregation (see Section 9.2.2.2). this section does exist.

A BGP speaker which performs route aggregation may add support the ability to impose an (locally config-
ured) upper bound on the number of address prefixes the speaker is
willing to accept from a neighbor. When the upper bound is reached,
the AGGREGATOR
attribute which shall contain its own AS number and IP address. The
IP speaker (under control of local configuration) may either (a)
discard new address should be prefixes from the same as neighbor (while maintaining BGP
connection with the neighbor), or (b) terminate the BGP Identifier of connection
with the neighbor. If the speaker.

6. BGP Error Handling.

This section describes actions speaker decides to be taken when errors are detected
while processing terminate its BGP messages.

When any
connection with a neighbor because the number of address prefixes
received from the conditions described here are detected, neighbor exceeds the locally configured upper
bound, then the speaker must send to the neighbor a NOTIFICATION message mes-
sage with the indicated Error Code, Error Subcode,
and Data fields is sent, and the Code Cease.

6.8 BGP connection is closed. collision detection.

If no
Error Subcode is specified, then a zero must be used.

The phrase "the pair of BGP connection is closed" means that the transport
protocol connection has been closed, the associated Adj-RIB-In has
been cleared, and that all resources for that speakers try simultaneously to establish a BGP connection have
been deallocated. Entries in con-
nection to each other, then two parallel connections between this
pair of speakers might well be formed. If the Loc-RIB associated with source IP address used
by one of these connections is the remote
peer are marked same as invalid. The fact that the routes have become
invalid is passed to other BGP peers before the routes are deleted
from destination IP address
used by the system.

Unless specified explicitly, other, and the Data field of destination IP address used by the NOTIFICATION
message that is sent to indicate an error first
connection is empty.

6.1 Message Header error handling.

All errors detected while processing the Message Header are indicated same as the source IP address used by sending the NOTIFICATION message with Error Code Message Header
Error. The Error Subcode elaborates on other, we
refer to this situation as connection collision. Clearly in the specific nature
presence of the
error.

The expected value connection collision, one of these connections must be
closed.

Based on the Marker field value of the message header BGP Identifier a convention is all
ones if the message type established
for detecting which BGP connection is OPEN. to be preserved when a colli-
sion does occur. The expected value of convention is to compare the Marker
field for all other types of BGP messages determined based on the
presence Identifiers of
the Authentication Information Optional Parameter peers involved in the
BGP OPEN message collision and to retain only the actual authentication mechanism (if connection
initiated by the
Authentication Information in BGP speaker with the higher-valued BGP Identifier.

Upon receipt of an OPEN message is present). The
Marker field should be message, the local system must examine all ones of
its connections that are in the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows the OPEN message carried no
authentication information. If BGP
Identifier of the Marker field peer by means outside of the message header protocol. If among
these connections there is not the expected one, then a synchronization error has occurred
and the Error Subcode is set connection to Connection Not Synchronized.

If the Length field of the message header is less than 19 or greater
than 4096, or if the Length field of an OPEN message is less than a remote BGP speaker whose
BGP Identifier equals the
minimum length of one in the OPEN message, or if and this connec-
tion collides with the Length field of an
UPDATE connection over which the OPEN message is less than
received then the minimum length local system performs the following collision reso-
lution procedure:

1. The BGP Identifier of the UPDATE message,
or if local system is compared to the Length field BGP
Identifier of a KEEPALIVE message the remote system (as specified in the OPEN mes-
sage). Comparing BGP Identifiers is not equal to 19, or
if done by treating them as
(4-octet long) unsigned integers.

2. If the Length field value of a NOTIFICATION message the local BGP Identifier is less than the
minimum length of the NOTIFICATION message, then
remote one, the Error Subcode local system closes BGP connection that already
exists (the one that is
set to Bad Message Length. The Data field contains already in the erroneous
Length field.

If OpenConfirm state), and
accepts BGP connection initiated by the Type field of remote system.

3. Otherwise, the message header local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is not recognized, then already in the
Error Subcode
OpenConfirm state).

Unless allowed via configuration, a connection collision with an
existing BGP connection that is set to Bad Message Type. The Data field contains in Established state causes closing
of the
erroneous Type field.

6.2 OPEN message error handling.

All errors newly created connection.

Note that a connection collision can not be detected while processing the OPEN message with connections
that are indicated in Idle, or Connect, or Active states.

Closing the BGP connection (that results from the collision resolu-
tion procedure) is accomplished by sending the NOTIFICATION message
with the Error Code Cease.

7. BGP Version Negotiation

BGP speakers may negotiate the version of the protocol by making mul-
tiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message
Error. The Error, and an Error Subcode elaborates on Unsupported Version
Number, then the specific nature of BGP speaker has available the
error.

If version number it
tried, the version number contained its peer tried, the version number passed
by its peer in the Version field of NOTIFICATION message, and the received
OPEN message is not supported, version numbers that
it supports. If the two peers do support one or more common versions,
then this will allow them to rapidly determine the Error Subcode is set highest common
version. In order to
Unsupported Version Number. The Data field is support BGP version negotiation, future versions
of BGP must retain the format of the OPEN and NOTIFICATION messages.

8. BGP Finite State machine

This section specifies the BGP operation in terms of a 2-octets unsigned
integer, which indicates Finite State
Machine (FSM). The section falls into 2 parts:

1) Description of Events for the largest locally supported version number
less than State machine (section 8.1)
2) Description of the version FSM (section 8.2)

Session Attributes required for each connection are;

1) State
2) Connect Retry timer
3) Hold timer
4) Hold time
5) Keepalive timer

8.1 Events for the remote BGP FSM

8.1.1 Administrative Events

Please note that only Event 1 (manual start) and Event 2 (manual
stop) are mandatory administrative events. All other administrative
events are optional.

Event1: Manual start

Definition: Administrator manually starts peer bid (as indicated in
connection.
Status: Mandatory

Event2: Manual stop

Definition: Local system administrator manually
stops the
received OPEN message), or if peer connection.

Status: Mandatory

Event3: Automatic start

Definition: Local system automatically starts the
BGP connection.

Status: Optional depending on local system

Event4: Manual start with passive TCP establishment

Definition: Administrator manually start the smallest locally supported version
number is greater than peer
connection, but has the version passive flag
enabled. The passive flag indicates
that the remote BGP peer bid, then will listen prior to
establishing the
smallest locally supported version number.

If connection.

Status: Optional depending on local system

Event5: Automatic start with passive TCP establishment

Definition: Local system automatically starts the Autonomous System field of
BGP connection with the OPEN message is unacceptable,
then passive flag
enabled. The passive flag indicates
that the Error Subcode is set peer will listen prior to Bad Peer AS.
establishing a connection.

Status: Optional depending on local system use
of a passive connection.

Event6: Automatic start with bgp_stop_flap option set

Definition: Local system automatically starts the
BGP peer connection with persistent peer
oscillation damping enabled. The determination exact
method of
acceptable Autonomous System numbers damping persistent peer
oscillations is left up to the
implementation. These methods of
damping persistent BGP adjacency
flapping are outside the scope of this
protocol.

If
document.

Status: Optional, used only if the Hold Time field bgp peer has
Enabled a method of damping persistent
BGP peer flapping.

Event7: Auto stop

Definition: Local system automatically stops the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An
implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An
implementation which accepts a
BGP connection.

Status: Optional depending on local system

8.1.2 Timer Events

Event8: Idle hold timer expires

Definition: Idle Hold Time MUST use the negotiated
value for the timer expires. The Idle
Hold Time.

If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode Timer is set to Bad only used when persistent
BGP Identifier.
Syntactic correctness means that the oscillation damping functions are
enabled.

Status: Optional. Used when persistent
BGP Identifier field represents
a valid IP host address.

If one of peer oscillation damping functions
are enabled.

Event9: Connect retry timer expires

Definition: An event triggered by the Optional Parameters in expiration of
the OPEN message is not
recognized, then ConnectRetry timer.

Status: Mandatory

Event10: Hold time expires

Definition: An event generated when the Error Subcode is set HoldTimer
expires.

Status: Mandatory

Event11: Keepalive timer expires

Definition: A periodic event generated due to Unsupported Optional
Parameters.

If one the
expiration of the Optional Parameters in KeepAlive Timer.

Status: Mandatory

Event12: DelayBGP open timer expires

Definition: A timer that delays sending of the OPEN BGP
Open message is recognized,
but is malformed, then the Error Subcode is set to 0 (Unspecific).

If for n seconds after the OPEN message carries Authentication Information (as an
TCP connection has been completed.

Status: Optional Parameter), then the corresponding authentication procedure
is invoked. If the authentication procedure (based on Authentication
Code

8.1.3 TCP Connection based Events

Event13: TCP connection indication & valid remote peer

Definition: Event indicating that TCP connection
request with a valid source IP address and Authentication Data) fails, then the Error Subcode TCP
port, and valid destination IP address
and TCP Port. The definition of
invalid source, and invalid destination
IP address is set left to
Authentication Failure.

6.3 UPDATE message error handling.

All errors detected while processing the UPDATE message are indicated implementation.
BGP's destination port should be port
179 as defined by IANA.

TCP connection request is denoted by sending
the NOTIFICATION message local system receiving a TCP SYN.

Status: Mandatory
Event14: RCV TCP connection indication with Error Code UPDATE Message
Error. The error subcode elaborates on the specific nature of the
error.

Error checking of invalid source or
destination

Definition: TCP connection request received with either
an UPDATE message begins by examining the path
attributes. If the Withdrawn Routes Length invalid source address or Total Attribute Length port
number or an invalid destination
address or port number. BGP destination
port number should be 179 as defined
by IANA.

Again, a TCP connection request is too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the message Length), then the Error Subcode is
set to Malformed Attribute List.

If any recognized attribute has Attribute Flags that conflict
denoted by local system receiving a TCP
SYN with
the Attribute Type Code, then the Error Subcode is set an invalid source port or
destination address or port number.

Status: Mandatory

Event15: TCP connection request sent received an ACK.

Definition: Local system's request to Attribute
Flags Error. The Data field contains the erroneous attribute (type,
length and value).

If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the
Error Subcode is set establish a TCP
connection to Attribute Length Error. The Data field
contains the erroneous attribute (type, length remote side received
an ACK.

The local system's TCP session sent a TCP
SYN, and value).

If any received a TCP SYN, ACK pair of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute.
messages, and Sent a TCP ACK.

Status: Mandatory

Event16: TCP connection confirmed

Definition: The Data
field contains the Attribute Type Code of the missing well-known
attribute.

If any of local system has received a confirmation that
the mandatory well-known attributes are not recognized,
then TCP connection has been established by
the Error Subcode is set to Unrecognized Well-known Attribute. remote site.

The Data field contains the unrecognized attribute (type, length remote peer's TCP engine sent a TCP SYN.
The local peer's TCP engine sent a SYN, ACK
pair, and
value).

If the ORIGIN attribute now has an undefined value, then the Error
Subcode is set to Invalid Origin Attribute. received a final ACK.

Status: Mandatory

Event17: TCP connection fails

Definition: This BGP peer receives a TCP
connection failure notice.

The Data field contains
the unrecognized attribute (type, length and value).

If the NEXT_HOP attribute field remote BGP peer's TCP machine could have
sent a FIN. The local peer would respond
with a FIN-ACK. Another alternative is syntactically incorrect, then that
the
Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field
contains local peer indicated a timeout in the incorrect attribute (type, length
TCP session and value). Syntactic
correctness means that downed the NEXT_HOP attribute represents connection.

Status: Mandatory

8.1.4 BGP Messages based Events

Event18: BGPOpen

Definition: An event indicating that a valid IP
host address. Semantic correctness applies only to the external Open
message has been received.

Status: Mandatory

Event19: BGPOpen with BGP
links, and only when the sender and the receiving speaker are one IP
hop away from each other. To be semantically correct, the IP address
in the NEXT_HOP must not be Delay Open Timer running

Definition: An event indicating that a valid Open
message has been successful
established for a peer that is
currently delaying the IP address sending of the receiving speaker,
and the NEXT_HOP IP address must either be the sender's IP address
(used to establish the an
BGP session), or the interface associated Open message.

Status: Optional

Event20: BGPHeaderErr

Definition: BGP message header is not valid.

Status: Mandatory

Event21: BGPOpenMsgErr

Definition: An BGP Open message has been received
with
the NEXT_HOP IP address must share errors.

Status: Mandatory

Event22: Open collision dump

Definition: An event generated administratively
when a common subnet connection Collision has been
detected while processing an incoming
Open message. This connection has been
selected to disconnected. See section
6.8 for more information on collision
detection.

Event 22 is an administrative could
occur if FSM is implemented as two
linked state machines.

Status: Optional

Event23: NotifMsgVerErr

Definition: An event is generated when a
NOTIFICIATION message with the receiving
BGP speaker. If the NEXT_HOP attribute "version
error" is semantically incorrect, the
error should be logged, received.

Status: Mandatory

Event24: NotifMsg

Definition: An event is generated when a
NOTIFICATION messages is received and
the route should be ignored. In this
case, no NOTIFICATION error code is anything but
"version error".

Status: Mandatory

Event25: KeepAliveMsg

Definition: An event is generated when a KEEPALIVE
message should be sent.

The AS_PATH attribute is checked for syntactic correctness. If the
path received.

Status: Mandatory

Event26: UpdateMsg

Definition: An event is syntactically incorrect, then the Error Subcode generated when a valid
Update message is set to
Malformed AS_PATH.

The information carried by the AS_PATH attribute received.

Status: Mandatory

Event27: UpdateMsgErr

Definition: An event is checked for AS
loops. AS loop detection generated when an invalid
Update message is done by scanning the full AS path (as
specified received.

Status: Mandatory

8.2 Description of FSM

8.2.1 FSM Definition

BGP must maintain a separate FSM for each configured peer, Each BGP
peer paired in the AS_PATH attribute), and checking that the autonomous
system number of the local system does not appear a potential connection unless configured to remain in
the AS path. If idle state, or configured to remain passive, will attempt to to
connect to the autonomous system number appears in other. For the AS path purpose of this discussion, the route may be
stored in active
or connect side of the Adj-RIB-In, but unless TCP connection (the side of a TCP connection
(the side sending the router first TCP SYN packet) is configured to
accept routes with its own autonomous system in the AS path, called outgoing. The
passive or listening side (the sender of the
route shall not be passed to first SYN ACK) is called
an incoming connection. [See section on the terms active and passive
below.]

A BGP Decision Process. Operations of
a router that is configured implementation must connect to accept routes with its own autonomous
system number and listen on TCP port 179 for
incoming connections in addition to trying to connect to peers. For
each incoming connection, a state machine must be instantiated.
There exists a period in which the AS path are outside the scope identity of this document.

If an optional attribute is recognized, then the value peer on the other
end of this
attribute is checked. If an error incoming connection is detected, known but the attribute BGP identifier is
discarded, not
known. During this time, both an incoming and an outgoing connection
for the Error Subcode same configured peering may exist. This is set referred to Optional Attribute Error.
The Data field contains as a
connection collision (see Section x.x, was 6.8).

A BGP implementation will have at most one FSM for each configured
peering plus one FSM for each incoming TCP connection for which the attribute (type, length and value).

If any attribute appears
peer has not yet been identified. Each FSM corresponds to exactly one
TCP connection.

There may be more than once in the UPDATE message, then one connections between a pair of peers if the Error Subcode
connections are configured to use a different pair of IP addresses.
This is set referred to Malformed Attribute List. as multiple "configured peerings" to the same
peer.

8.2.1.1 Terms "active" and "passive"

The NLRI field terms active and passive have been in our vocabulary for almost a
decade and have proven useful. The words active and passive have
slightly different meanings applied to a TCP connection or applied to
a peer. There is only one active side and one passive side to any
one TCP connection per the UPDATE message definition above and the state machine
below. When a BGP speaker is checked for syntactic
validity. If configured active it may end up on
either the field active or passive side of the connection that eventually
gets established. Once the TCP connection is syntactically incorrect, then completed, it doesn't
matter which end was active and which end was passive and the Error
Subcode only
difference is which side of the TCP connection has port number 179.

8.2.1.2 FSM and collision detection

There is set one FSM per BGP connection. Prior to Invalid Network Field.

If determining what peer
a prefix in the NLRI field connection is semantically incorrect (e.g., an
unexpected multicast IP address), an error should associated with there may be logged locally,
and the prefix two connections for a
given peer. There should be ignored.

An UPDATE message that contains correct path attributes, but no NLRI,
shall be treated as a valid UPDATE message.

6.4 NOTIFICATION message error handling.

If a peer sends a NOTIFICATION message, and more than one connection per peer.
The collision detection identifies the case where there is an error in more than
one connection per peer and provides guidance for which connection to
get rid of. When this occurs, the corresponding FSM for the connec-
tion that
message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, closed should be noticed, logged
locally, and brought to the attention disposed of

8.2.2 Finite State Machine

Idle state:

Initially BGP is in the administration of Idle state.

In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. The means In response to do this, however, lies outside the scope of this
document.

6.5 Hold Timer Expired error handling.

If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then
manual start event(Event1) or an automatic start
event(Event3), the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and local system
- initializes all BGP resources,
- sets ConnectRetryCnt (the connect retry counter) to zero
- starts the connect retry timer with initial value,
- initiates a TCP connection to the other BGP peer,
- listens for a connection
closed.

6.6 Finite State Machine error handling.

Any error detected that may be initiated by
the remote BGP Finite State Machine (e.g., receipt of
an unexpected event) peer, and
- changes its state to connect.

An manual stop event (Event2) is indicated by sending ignored in the NOTIFICATION message
with Error Code Finite State Machine Error.

6.7 Cease. Idle state.

In absence of any fatal errors (that are indicated in this section), response to a manual start event with the passive TCP connection
flag (Event 4) or automatic start with the passive TCP connection
flag (Event 5), the local system:
- initializes all BGP peer may choose at any given time resources,
- sets ConnectRetryCnt (the connect retry counter) to close its BGP connection
by sending zero,
- start the NOTIFICATION message connect retry timer with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when initial value,
- listens for a fatal error
indicated by this section does exist.

A BGP speaker connection that may support be initiated by
the ability remote peer, and
- changes its state to impose an (locally
configured) upper bound on the number Active.

The exact value of address prefixes the speaker ConnectRetry timer is willing to accept from a neighbor. When the upper bound is
reached, the speaker (under control of local configuration) may
either (a) discard new address prefixes from the neighbor, or (b)
terminate the BGP peering with the neighbor.
matter, but it should be sufficiently large to allow TCP
initialization.

If the a persistent BGP speaker
decides to terminate its peering peer oscillation damping function is
enabled, two additional events may occur within Idle state:
- Automatic start with a neighbor because bgp_stop_flap set [Event6],
- Idle Hold Timer expired [Event 8].

The method of preventing persistent BGP peer oscillation is
outside the number scope of address prefixes this document.

Any other events [Events 9-27] received from the neighbor exceeds in the locally
configured upper bound, then Idle state,
are noted by the speaker must send to MIB processing as FSM Errors
and the neighbor a
NOTIFICATION message with local peer stays in the Error Code Cease.

6.8 Connection collision detection.

If a pair of BGP speakers try simultaneously to establish a Idle State.

Connect State:

In this state, BGP is waiting for the TCP connection to each other, then two parallel connections between this
pair of speakers might well
be formed. completed.

If the source IP address used
by one of these connections TCP connection succeeds [Event 15 or
Event 16], the local system checks the "Delay Open
Flag". If the delay Open flag is set, the same as local system:
- clears the destination IP address
used by connect retry timer,
- set the other, and BGP open delay timer to the destination IP address used by initial
value.

If the first
connection Delay Open flag is not set, the same as the source IP address used by local system:
- clears the other, we
refer connect retry timer,
- completes BGP initialization
- send an Open message to this situation as connection collision. Clearly in the
presence of connection collision, one of these connections must be
closed.

Based on its peer,
- sets hold timer to a large value, and
- Change the state to Open Sent.

A hold timer value of 4 minutes is suggested.

If the Open Delay timer expires [Event 12] in the connect
state,
- send an Open message to its peer,
- set the BGP Identifier a convention is established
for detecting which BGP connection is hold timer to be preserved when a
collision does occur. The convention is large value, and
- change the state to compare Open Sent.

If the BGP
Identifiers of the peers involved in port receives a TCP connection indication
[Event 13], the collision TCP connection is processed and to retain only
the connection initiated by remains in the BGP speaker with connected state.

If the higher-valued
BGP Identifier.

Upon receipt of TCP connection receives an OPEN message, the local system must examine all of
its connections indication
that are in is invalid or unconfigured. [Event 14]:
- the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows TCP connection is rejected.

If the BGP
Identifier of TCP connection fails (timeout or disconnect)
[Event17], the peer by means outside of local system:
- restarts the protocol. If among
these connections there is a connection connect retry timer,
- continues to listen for a connection that may be
initiated by the remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, peer, and this
connection collides
- changes its state to Active.

If an Open is received with the connection over which the OPEN message BGP Delay Open timer is received then
running [Event 19], the local system performs system:
- clears the connect retry timer (cleared to zero),
- completes the following collision
resolution procedure:

1. The BGP Identifier of initialization,
- Stops and clears the local system is compared BGP Open Delay timer
- Sends an Open message
- Set the hold timer to a large value (4 minutes), and
- changes its state to Open Confirm.

The start events [Event 1, 3-6] are ignored in connect
state.

A manual stop event[Event2], the BGP
Identifier of local system:
- drops the remote system (as specified in TCP connection,
- releases all BGP resources,
- sets ConnectRetryCnt (the connect retry count) to zero
- resets the OPEN
message).

2. If connect retry timer (sets to zero), and
- goes to Idle state.

In response to the value of connect retry timer expired event(Event
9), the local BGP Identifier is less than system:
- Sets the
remote one, MIB FSM error information with connect retry
expired,
- drops the local system closes BGP TCP connection that already
exists (the one that is already in
- restarts the OpenConfirm state), and
accepts connect retry timer
- initiates a TCP connection to the other BGP
peer,
- continues to listen for a connection that may be
initiated by the remote system.

3. Otherwise, BGP peer, and
- stays in Connect state.

In response to any other events [Events 7-8, 10-11, 18, 20-
27] the local system closes newly created BGP connection
(the one associated with system:

- resets the newly received OPEN message), and
continues connect retry timer (sets to use zero),
- drops the existing one (the one that is already in TCP connection,
- release all BGP resources,
- increments the
OpenConfirm state).

Comparing ConnectRetryCnt (connect retry count) by 1,
- [optionally] performs bgp peer oscillation damping, and
- goes to Idle state.

Active State:

In this state BGP Identifiers is done trying to acquire a peer by treating them as (4-octet
long) unsigned integers.

Unless allowed via configuration, listening
for and accepting a connection collision with an
existing BGP connection that is in Established state causes
closing of the newly created TCP connection.

Note that a

A TCP connection collision cannot be detected with
connections that are in Idle, or Connect, succeeds [Event 15 or Active states.

Closing the BGP connection (that results from Event 16], the collision
resolution procedure) is accomplished by sending
local system: process the NOTIFICATION
message with TCP connection flags
- If the Error Code Cease.

7. BGP Version Negotiation. BGP speakers may negotiate delay open flag is set:
o clears the version of connect retry timer,
o completes the protocol by making
multiple attempts to open a BGP connection, starting with initialization, and
o sets the highest
version number each supports. BGP delay Open timer

- If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available delay open flag is not set:
o clears the version number it
tried, connect retry timer,
o completes the version number its peer tried, BGP initialization,
o sends the version number passed
by Open message to it's peer,
o sets its peer in the NOTIFICATION message, hold timer to a large value,
and the version numbers that
it supports. changes its state to OpenSent.

A Hold timer value of 4 minutes is suggested.

If the two peers do support one or more common versions,
then this will allow them to rapidly determine local system receives a valid TCP Indication
[Event 13], the highest common
version. In order to support BGP version negotiation, future versions
of BGP must retain local system processes the format of TCP connection flags.

If the OPEN and NOTIFICATION messages.

8. BGP Finite State machine.

This section specifies BGP operation in terms of local system receives a Finite State
Machine (FSM). Following TCP indication
that is a brief summary and overview of BGP
operations by state as determined by invalid for this FSM.

Initially BGP is in connection [Event 14]:
- the Idle state.

Idle state:

A manual start event is a start event initiated by an operator.
An automatic start event TCP connection is a start event generated by rejected.

If the local system receives a TCP connection
failed [Event 17] (timeout or receives connection
disconnect), the
system.

In this state BGP refuses local system will:
- set TCP disconnect in the MIB reason code,
- restart connect retry timer (with initial value)
- release all incoming BGP connections. No resources are allocated to
- Acknowledge the peer. In response to drop of TCP connection if
TCP disconnect (send a Start
event (manual or automatic), FIN ACK),
- Increment ConnectRetryCnt (connect retry count) by 1, and
- perform the BGP peer oscillation damping process [2].

If the local system has the delay open timer expired [event
12] local system:
- initializes all BGP resources,

- starts clears the ConnectRetry timer, connect retry timer (set to zero),
- initiates a transport connection stops and clears the delay open timer (set to zero)
- completes the other BGP peer, initialization,
- listens for a connection that may be initiated by sends the Open message to it's remote BGP peer, and
- changes sets its hold timer to a large value,
- and set the state to connect.

The exact Open Confirm.

A hold timer value of the ConnectRetry timer 4 minutes is also suggested for this
state transition.

If an Open is a local matter,
but it should be sufficiently large to allow TCP
initialization.

Any other event received in with the IDLE state, BGP delay open timer is ignored.

IdleHold state:

The IdleHold state keeps
running [Event 19], the local system in "Idle" mode until a
certain time period has passed or an operator intervenes
- clears the connect retry timer (cleared to
manually restart zero),
- stops and clears the connection. This "IdleHold timeout"
prevents persistent flapping of a BGP peering session.

Upon entering the Idle Hold state, if the IdleHoldTimer exceeds open delay timer
- completes the local limit BGP initialization,
- stops and clears the "Keep Idle" flag is set.

Upon receiving BGP open delay timer
- sends an Open message
- set its hold timer to a Manual start, large value (4 minutes), and
- changes its state to Open Confirm.

In response the ConnectRetry timer expired event[Event9],
the local system:
- clears the IdleHoldtimer,

- clears "keep Idle" flag

- initializes all BGP resources,

- starts restarts the ConnectRetry timer, connect retry timer (with initial value),
- initiates a transport TCP connection to the other BGP
peer,
- listens Continues to listen for a TCP connection that may be
initiated by the remote BGPPeer, and BGP peer,
- and changes its state to connect.

Upon receiving a IdleHoldtimer expired event, the local system
checks to see that Connect.

The start events [Event1, 3-6] are ignored in the Keep Idle flag is set. If Active
state.

A manual stop event[Event2], the Keep Idle
flag is set, local system:
- Sets the system stays administrative down in the "Idle Hold" state. MIB reason code,
- Sends a Notification with a Cease,
- If any BGP routes exist, delete the routes
- release all BGP resources,
- drops the TCP connection,
- sets ConnectRetryCnt (connect retry count) to zero
- resets the Keep connect retry timer (sets to zero),
- goes to Idle flag is not set, state.

In response to any other event (Events 7-8, 10-11,18, 20-
27), the local system:
- clears stores the IdleHoldtimer, MIB information to indicate appropriate
error [FSM for Events 7-8, 10-11, 18, 20-27]
- and transitions reset the state connect retry timer (sets to Idle.

Getting out of the IdleHoldstate requires either operator
intervention via a manual start or zero),
- release all BGP resources,
- drops the IdleHoldtimer to expire
with TCP connection,
- increments the "Keep Idle" flag ConnectRetryCnt (connect retry count) by one,
- optionally performs BGP peer oscillation damping,
- and goes to be clear.

Any other event received in the IdleHold idle state is ignored.

Connect State:

Open Sent:

In this state, state BGP waits for an Open Message from its peer.

When an OPEN message is waiting received, all fields are checked
for the transport protocol
connection to be completed. correctness. If there are no errors in the transport connection succeeds, OPEN message
[Event 18] the local system:
- clears resets the ConnectRetry timer, BGP Delay timer to zero,
- completes initialization, reset BGP Connect Timer to zero,
- send an Open sends a KEEPALIVE message to its peer, and
- sets a KeepAlive timer (via the text below)
- set sets the Hold timer according to a large value, the negotiated value
(see section 4.2), and
- changes its sets the state to Open Sent.

A hold timer Confirm.

If the negotiated Hold time value of 4 minutes is suggested. zero, then the Hold
and KeepAlive timers are not started. If the transport protocol
value of the Autonomous System field is the same as the
local Autonomous System number, then the connection fails (e.g.,
retransmission timeout), is an
"internal" connection; otherwise, it is an "external"
connection. (This will impact UPDATE processing as
described below.)

If the BGP message header checking [Event20] or OPEN message
check detects an error (see Section 6.2)[Event21], the local system:
- restarts the ConnectRetry timer, sends a NOTIFICATION message with appropriate error
code,
- continues reset the connect retry timer (sets to listen for a connection that may be initiated
by zero),
- if there are any routes associated with the remote BGP peer, and session,
delete these routes
- changes its state to Active.

In response to the ConnectRetry timer expired event, release all BGP resources,
- drop the local
system: TCP connection
- restarts increments the ConnectRetry timer, ConnectRetryCnt (connect retry cout) by 1,
- initiates a transport connection bgp peer oscillation damping process,
- and goes to the other Idle state.

Collision detection mechanisms (section 6.8) need to be
applied when a valid BGP peer,

- continues Open is received [Event 18 or
Event 19]. Please refer to listen section 6.8 for a connection that may be initiated
by the remote details of
the comparison. An administrative collision detect is when
BGP peer, and

- stays implementation determines my means outside the scope of
this document that a connection collision has occurred.

If a connection in Connect state.

The start event (manual or automatic) Open Sent is ignored in determined to be the Connect
state.

In response
connection that must be closed, an administrative collision
detect [Event 22] is signaled to any other event (initiated by the system or
operator), state machine. If such
an administrative collision detect dump [Event 22] is
received in Open Sent, the local system:
- IdleHoldtimer = 2**(ConnectRetryCnt)*60 sets MIB state information to
collision detect closure,
- Increment ConnectRetryCnt by 1, send a NOTIFICATION with a CEASE
- Set resets the connect retry timer to zero,
- Drops TCP connection, timer,
- Releases release all BGP resources, and
- Goes to IdleHoldstate

Active State:

In this state drop the TCP connection,
- increments ConnectRetryCnt (connect rery count) by 1,
- performs any BGP is trying to acquire a peer by listening for oscillation damp process, and accepting a transport protocol connection.
- enters Idle state.

If the transport connection succeeds, a NOTIFICATION message is received with a version
error[Event23], Notification message without version number
[Event 24], the local system:
- clears the ConnectRetry timer,

- completes the initialization,

- sends resets the Open message to it's peer,

- sets its Hold connect retry timer (sets to a large value,

- and changes its state to OpenSent.

A Hold timer value of 4 minutes is suggested.

In response the ConnectRetry timer expired event, the local
system: zero)
- restarts drops the ConnectRetry timer, TCP connection,
- initiates a transport connection to the other releases all BGP peer, resources,
- continues to listen for connection that may be initiated increments the ConnectRetryCnt (connect retry count) by remote BGP peer, 1
- and changes its state to Connect.

If the local system does not allow process any BGP connections with
unconfigured peers, then the local system:

- rejects connections from IP addresses that are not
configured peers, peer oscillation damping,
- and remains in sets the Active state. state to Idle.

The start Start events (initiated by the system or operator) [Event1, 3-6] are ignored in the Active OpenSent
state.

In response to any other

If a manual stop event (initiated by [Event 2] is issued in Open sent
state, the local system:
- Sets administrative down reason in MIB reason,
- sends the system or
operator), Notification with a cease,
- if BGP routes exists, delete the local system: routes,
- IdleHoldtimer = 2**(ConnectRetryCnt)*60 Release all BGP resources,
- Increment ConnectRetryCnt by 1, Drops the TCP connection,
- Set connect set ConnectRetryCnt (connect retry timer count) to zero, and

- Drops TCP connection,
- Releases all BGP resources, resets the Connect Retry timer (set to zero), and
- Goes transitions to IdleHold the Idle state.

Open Sent:

In this state BGP waits for an Open Message from its peer.
When an OPEN message is received, all fields are check for
correctness.

If the BGP message header checking or OPEN
message check detects an error (see Section 6.2), or a
connection collision (see Section 6.8) automatic stop event [Event 7] is issued in Open sent
state, the local system:
- Sets administrative down reason in MIB reason,
- sends the Notification with a NOTIFICATION message cease,
- IdleHoldtimer = 2**(ConnectRetryCnt)*60

- Increment ConnectRetryCnt by 1, if any routes are associated with te BGP session,
delete the routes,
- Set connect retry timer to zero, and release all the BGP resources
- Drops the TCP connection,
- Releases all increments the ConnectRetryCnt (connect retry count) by 1,
- BGP resources, peer oscillation process [2], and
- Goes transitions to IdleHold the Idle state.

If there are no errors in the OPEN message, Hold Timer expires[Event 10], the local system:
- sends a KEEPALIVE message and set Hold timer expired in MIB Error reason code,
- sets send a KeepAlive timer (via the text below) NOTIFICATION message with error code Hold
Timer Expired,
- set reset the Hold connect retry timer according (sets to zero),
- releases all BGP resources,
- drops the negotiated value (see
section 4.2), TCP connection,
- set increments the state ConnectRetryCnt (connect retry count) by 1,
and transitions to Open Confirm.

If the negotiated Hold time value Idle state.

If a TCP indication is received for valid connection
[Event 13] or TCP request aknowledgement [Event 15]
is received, or a TCP connect confirm [Event 16] is
received a second TCP session may be in progress. This
second TCP session is zero, then the Hold Time
timer and KeepAlive timers are not started. If the value of tracked per the Autonomous System field Call Collision
processing (section 6.8) until an OPEN message is the same as the local Autonomous
System number, then the received.

A TCP connection is for an "internal" connection;
otherwise, it invalid port [Event 14] is an "external" connection. (This will impact
UPDATE processing as described below.) ignored.

If a disconnect NOTIFICATION TCP connection failure [Event17], is received from the underlying
transport protocol,
the local system:
- closes the BGP connection,
- restarts the Connect Retry timer,
- and continues to listen for a connection that may be
initiated by the remote BGP peer,
- and goes into Active state.

If the Hold Timer expires,

In response to any other event [Events 8-9, 11-12, 19, 25-27],
the local system:
- send a sends the NOTIFICATION message with error code Hold Timer
Expired, the Error Code Finite
state machine error,
- IdleHoldtimer = 2**(ConnectRetryCnt)*60 resets the connect retry timer (sets to zero),
- Increment releases all BGP resources
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- Set connect retry timer process any bgp peer oscillation damping[2],
- and sets the state to zero, idle.

Open Confirm State:

In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.

If the local system receives a KEEPALIVE message[Event 25],
- restarts the Hold timer, and
- Drops changes its state to Established.

If the local system receives a NOTIFICATION message [Event
23-24] or receives a TCP connection, Disconnect [Event 17] from the
underlying TCP , the local system:
- sets the appropriate MIB information for FSM error,
- resets the connect retry timer (sets the timer to
zero),
- Releases releases all BGP resources, and
- Goes drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count) by 1,
- and sets the state to IdleHold state.

The Start idle.

Any start event (manual and automatic) [Event1, 3-6] is ignored in the
OpenSent OpenConfirm
state.

If a NOTIFICATION message is received with

In response to a version error, manual stop event[Event 2] initiated by
the operator, the local system:
- Closes set Administrative down in MIB Reason code,
- sends the transport connection NOTIFICATION message with Cease,
- if any BGP routes, dete the routes
- Releases releases all BGP resources,
- ConnectRetryCnt = 0,

- Connect retry timer = 0, and
- transition to Idle state.

If any other NOTIFICATION is received, drop the local system:

- IdleHoldtimer = 2**(ConnectRetryCnt)*60 TCP connection,
- Increment sets the ConnectRetryCnt by 1, (connect retry count) to zero
- Set sets the connect retry timer to zero, and
- Drops TCP connection,

- Releases all BGP resources,

- Goes transitions to IdleHold Idle state.

In response to any other event, the Automatic stop event initiated by the
system[Event 7], the local system:
- sets the MIB entry for this peer to administratively
down,
- sends the NOTFICATION NOTIFICATION message with Error Code Finite State
Machine Error,

- IdleHoldtimer = 2**(ConnectRetryCnt)*60

- Increment ConnectRetryCnt by 1, Cease,
- Set connect retry timer reset (set to zero, zero)
- Drops TCP connection, If any BGP routes exist, delete the routes,
- Releases release all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count)
by 1, and
- Goes transitions to IdleHold state.

Open Confirm State

In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.

If the local system receives a KEEPALIVE message, it changes
its state to Established. Idle State.

If the Hold Timer expires before a KEEPALIVE message is
received,
received [Event 10], the local system:
- set the MIB reason to Hold time expired,
- send the NOTIFICATION message with the error code
set to Hold
Timer Time Expired,
- sets IdleHoldTimer = 2**(ConnectRetryCnt)*60
- Increments ConnectRetryCnt by 1,

- Sets resets the connect retry timer (sets the timer to zero, to
zero),
- releases all BGP resources,
- Drop drops the TCP connection,
- Releases all BGP resources, increments the ConnectRetryCnt (connect retry count) by 1,
- and sets the state to Idle.

If the local system receives a KEEPALIVE timer expires
event [Event 11], the system:
- sends a KEEPALIVE message,
- Goes restarts the Keepalive timer, and
- remains in Open Confirmed state.

In the event of TCP establishment [Event 13], or TCP
connection succeeding [Event 15 or Event 16] while in Open
Confirm, the local system needs to track the 2nd
connection.

If a TCP connection is attempted to IdleHoldState. an invalid port [Event
14], the local system will ignore the second connection
attempt.

If an OPEN message is received, all fields are check for
correctness. If the local system receives a NOTIFICATION BGP message header checking [Event20]
or receives
a disconnect NOTIFICATION from the underlying transport
protocol, OPEN message check detects an error (see Section
6.2)[Event21], the local system:
- Sets IdleHold Timer = 2**(ConnectRetryCnt)*60

- Increments ConnectRetryCnt by 1, sends a NOTIFICATION message with appropriate error
code,
- Sets resets the connect retry timer (sets the timer to zero,
zero),
- Drops releases all BGP resources,
- drops the TCP connection,
- Releases all increments the ConnectRetryCnt (connect retry count) by 1,
- runs the BGP resources, peer oscillation damping process [2]
- Goes to IdleHoldstate.

In response and goes to the Stop event initiated by Idle state.

If the system, Open messages is valid [Event 18], the collision
detect function is processed per section 6.8. If this
connection is to be dropped due to call collision, the
local system:
- sends the NOTIFICATION message with Cease,

- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60 the Call Collision cease in the MIB reason
code,
- Increments ConnectRetryCnt by 1, sends a Notification with a Cease
- Sets resets the Connect retry timer (set to zero, zero),
- releases all BGP resources,
- Drops the TCP connection, connection (send TCP FIN),
- Releases all BGP resources, increments the ConnectRetryCnt by 1 (connect retry count), and
- Goes to IdleHoldstate.

In response performs any BGP peer oscillation damping process [2].

If during the processing of another Open message, the BGP
implementation determines my means outside the scope of
this document that a connection collision has occurred and
this connection is to be closed, the local system will
issue a Stop event initiated by call collision dump [Event 22]. When the operator, local
system receives a call collision dump event [Event 22], the
local system:
- sends Sets the MIB FSM variable to indicate collision
detected and dump connection.
- send a NOTIFICATION message with Cease, a CEASE
- deletes all routes associated with connection,
- resets the connect retry timer,
- releases all BGP resources
- sets the ConnectRetryCnt to zero drops all TCP connection,
- sets increments the connect ConnectRetryCnt (connect retry timer to 0 count) by 1,
- transitions to Idle state.

The Start event is ignored in the OpenConfirm and performs any BGP peer oscillation damping, and
- enters Idle state.

In response to any other event, event [Events 8-9, 12, 19, 26-27],
the local system:
- sends a NOTIFICATION with a code of Finite State
Machine Error,
- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60

- Increments ConnectRetryCnt by 1,

- Sets resets the Connect connect retry timer (sets to zero, zero)
- Drops drops the TCP connection,
- Releases releases all BGP resources,
- Goes increments the ConnectRetryCnt (connect retrycount) by 1,
- performs any BGP peer oscillation damping, and
- transitions to IdleHoldstate. Idle state.

Established State:

In the Established state BGP can exchange UPDATE,
NOTFICATION, and KEEPALIVE messages with its peer.

If the local system receives an UPDATE or KEEPALIVE message, it message [Event26],
the local system will:
- process the update packet
- restarts its Hold Timer, timer, if the negotiated Hold Time
value is
non-zero. non-zero, and
- remain in the Established state.

If the local system receives a NOTIFICATION message
[Event23 or Event24] or a disconnect [Event17] from the
underlying transport protocol, TCP, it:
- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60, the appropriate error code in MIB reason code,
- Increments ConnectRetryCnt by 1, if any BGP routes exist, delete all BGP routes,
- Sets resets the Connect connect retry timer (sets to zero, zero),
- Drops releases all the BGP resources,
- drops the TCP connection,
- Releases all BGP resources, increments the ConnectRetryCnt (connect retry count)
by 1, and
- Goes goes to IdleHoldstate. the Idle state.

If the local system receives a Keepalive message
[Event 25], the local system will:
- restarts its Hold Timer, if the negotiated Hold Time
value is non-zero, and
- remain in the Established state.

If the local system receives an UPDATE message, and the
Update message error handling procedure (see Section 6.3) detecs
detects an error [Event27], the local system:
- sends a NOTIFICATION message with Update error,
- resets the connect retry timer (sets to zero),
- drops the TCP connection,
- releases all BGP resources,
- increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP peer oscillation damping,
- and goes to Idle state.

Any start event (Event 1, 3-6) is ignored in the
Established state.

In response to a manual stop event (initiated by an
operator)[Event2], the local sytem:
- sets the Administrative stop in MIB reason code,
- sends the NOTIFICATION message with Cease,
- if BGP routes exist, delete the BGP routes,
- release BGP resources,
- drops TCP connection,
- sets ConnectRetryCnt (connect retry count)
to zero (0),
- resets connect retry timer to zero (0), and
- transitions to the Idle.

In response to an automatic stop event initiated by the
system (automatic) [Event7], the local system:
- sets Administrative Stop in MIB Reason code,
- sends a NOTIFICATION message with Update error,

- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60

- Increments ConnectRetryCnt by 1, Cease,
- Sets resets the Connect connect retry timer (sets to zero, zero)
- Drops the TCP deletes all routes associated with bgp connection,
- Releases releases all BGP resources,
- drops the TCP connection,
- increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP peer oscillation damping, and
- Goes transitions to IdleHoldstate. the idle state.

An example automatic stop event is exceeding the number of
prefixes for a given peer and the local system
automatically disconnecting the peer.

If the Hold timer expires, expires [Event10], the local system:
- sends a NOTIFICATION message with Error Code Hold
Timer Expired,
- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60

- Increments ConnectRetryCnt by 1,

- Sets resets the connect retry timer (sets to zero, zero),
- Drops releases all BGP resources,
- drops the TCP connection,
- Releases all increments the ConnectRetryCnt (connect retry count)
by 1,
- performs any BGP resources, peer oscillation damping,
- Goes and goes to IdleHold Idle state.

If the KeepAlive timer expires, expires [Event11], the local system
sends a KEEPALIVE message, it restarts its KeepAlive timer,
unless the negotiated Hold Time value is zero.

Each time time the local system sends a KEEPALIVE or UPDATE
message, it restarts its KeepAlive timer, unless the
negotiated Hold Time value is zero.

A TCP connection indication [Event 13] received
for a valid port will cause the 2nd connection to be
tracked. A TCP connection indications for
invalid port [Event 14], will be ignored.

In response to a TCP connection succeeds [Event 15
or Event 16], the Stop event initiated by 2nd connection shall be tracked until
it sends an OPEN message.

If a valid Open message [Event 18] is received, it will be
checked to see if it collides (section 6.8) with any other
session. If the system
(automatic), BGP implementation determines that this
connection needs to be terminated, it will process an Call
Collision dump event[Event 22]. If this session needs to be
terminated, the local system: connection will be terminated by:

- sends send a NOTIFICATION with Cease,

- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60 a CEASE
- increments ConnectRetryCnt by 1, deletes all routes associated with connection,
- sets resets the connect retry timer to zero,

- drops the TCP connection,

- releases all BGP resources,

- goes to IdleHold state, and timer,
- deletes all routes.

An example automatic stop event is exceeding the number of
prefixes for a given peer and the local system automatically
disconnecting if any BGP routes, delete the peer.

In response to a stop event initiated by an operator: routes,
- release all resources (including deleting all routes), BGP resources,
- set ConnectRetryCnt to zero (0), drops the TCP connection,
- set connect increments ConnectRetryCnt (connect retry timer to zero (0), count)
by 1,
- and performs any BGP peer oscillation damping,
- transition to the Idle.

The Start event is ignored in and enters the Established state. Idle state

In response to any other event, event [Events 8-9,12, 19-21] the
local system:
- sends a NOTIFICATION message with Error Code Finite
State Machine Error,
- sets IdleHoldtimer = 2**(ConnectRetryCnt)*60

- increments ConnectRetryCnt by 1, deletes all routes associated with BGP connection,
- sets resets the connect retry timer (sets to zero, zero)
- releases all BGP resources,
- drops the TCP connection,
- releases all BGP resources increments the ConnectRetryCnt (connect retry count)
by 1,
- goes to IdleHoldstate, performs any BGP peer oscillation damping, and
- deletes all routes. transitions to Idle.

9. UPDATE Message Handling

An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity valid-
ity as specified in Section 6.3.

If an optional non-transitive attribute is unrecognized, it is
quietly qui-
etly ignored. If an optional transitive attribute is unrecognized,
the Partial bit (the third high-order bit) in the attribute flags
octet is set to 1, and the attribute is retained for propagation to
other BGP speakers.

If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible
propagation propaga-
tion to other BGP speakers.

The information carried by the AS_PATH attribute is checked for AS
loops. AS loop detection is done by scanning the full AS path (as
specified in the AS_PATH attribute), and checking that the autonomous
system number of the local system does not appear in the AS path. If
the autonomous system number appears in the AS path the route may be
stored in the Adj-RIB-In, but unless the router is configured to
accept routes with its own autonomous system in the AS path, the
route shall not be passed to the BGP Decision Process. Operations of
a router that is configured to accept routes with its own autonomous
system number in the AS path are outside the scope of this document.

If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the
previously previ-
ously advertised route is no longer available for use.

If the UPDATE message contains a feasible route, the Adj-RIB-In will
be updated with this route as follows: if the NLRI of the new route
is identical to the one of the route currently stored in the Adj-RIB-
In, then the new route shall replace the older route in the Adj-RIB-
In, thus implicitly withdrawing the older route from service.
Otherwise, Other-
wise, if the Adj-RIB-In has no route with NLRI identical to the new
route, the new route shall be placed in the Adj-RIB-In.

Once the BGP speaker updates the Adj-RIB-In, the speaker shall run
its Decision Process.

9.1 Decision Process

The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIBs-In. The output of the Decision
Process Pro-
cess is the set of routes that will be advertised to all peers; the
selected routes will be stored in the local speaker's Adj-RIB-
Out. Adj-RIB-Out.

The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns either (a)
a non-negative integer denoting the degree of preference for the
route, or (b) a value denoting that this route is ineligible to be
installed in LocRib and will be excluded from the next phase of route
selection.

The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the existence
of other routes, the non-existence of other routes, or the path
attributes of other routes. Route selection then consists of
individual individ-
ual application of the degree of preference function to each feasible
route, followed by the choice of the one with the highest degree of
preference.

The Decision Process operates on routes contained in the Adj-RIB-In,
and is responsible for:

- selection of routes to be used locally by the speaker

- selection of routes to be advertised to other BGP peers

- route aggregation and route information reduction

The Decision Process takes place in three distinct phases, each
triggered trig-
gered by a different event:

a) Phase 1 is responsible for calculating the degree of preference
for each route received from a peer.

b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the Loc-RIB.

c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer,
according to the policies contained in the PIB. Route aggregation
and information reduction can optionally be performed within this
phase.

9.1.1 Phase 1: Calculation of Degree of Preference

The Phase 1 decision function shall be invoked whenever the local BGP
speaker receives from a peer an UPDATE message that advertises a new
route, a replacement route, or withdrawn routes.

The Phase 1 decision function is a separate process which completes
when it has no further work to do.

The Phase 1 decision function shall lock an Adj-RIB-In prior to
operating oper-
ating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it.

For each newly received or replacement feasible route, the local BGP
speaker shall determine a degree of preference as follows:

If the route is learned from an internal peer, either the value of
the LOCAL_PREF attribute shall be taken as the degree of
preference, prefer-
ence, or the local system may compute the degree of preference of
the route based on preconfigured policy information. Note that the
latter (computing the degree of preference based on preconfigured
policy information) may result in formation of persistent routing
loops.

If the route is learned from an external peer, then the local BGP
speaker computes the degree of preference based on preconfigured
policy information. If the return value indicates that the route
is ineligible, the route may not serve as an input to the next
phase of route selection; otherwise the return value is used as
the LOCAL_PREF value in any IBGP readvertisement.

The exact nature of this policy information and the computation
involved is a local matter.

9.1.2 Phase 2: Route Selection

The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process which completes when it
has no further work to do. The Phase 2 process shall consider all
routes that are eligible in the Adj-RIBs-In.

The Phase 2 decision function shall be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function shall
lock all Adj-RIBs-In prior to commencing its function, and shall
unlock them on completion.

If the NEXT_HOP attribute of a BGP route depicts an address that is
not resolvable, or it would become unresolvable if the route was
installed in the routing table the BGP route should be excluded from
the Phase 2 decision function.

It is critical that routers within an AS do not make conflicting
decisions regarding route selection that would cause forwarding loops
to occur.

For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker shall identify the route that has:

a) the highest degree of preference of any route to the same set
of destinations, or

b) is the only route to that destination, or

c) is selected as a result of the Phase 2 tie breaking rules
specified spec-
ified in 9.1.2.2.

The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB. If When the new BGP route is installed in the Routing
Table (as a result of the local policy decision), Rout-
ing Table, care must be taken to ensure that invalid BGP existing routes to the
same destination that are now considered invalid are removed from the
Routing Table. Whether or not the new BGP route replaces an
already existing
non-BGP route in the routing table Routing Table depends on the policy configured
on the BGP speaker.

The local speaker MUST determine the immediate next hop to the next-hop address depicted by from
the NEXT_HOP attribute of the selected route by
performing a best matching route lookup in the Routing Table and
selecting one of the possible paths (if multiple best paths to the
same prefix are available). (see section 5.1.3). If the route to the address depicted by
the NEXT_HOP attribute changes such that
either the immediate next hop or the IGP cost to the NEXT_HOP (if (where
the NEXT_HOP is resolved through an IGP route) changes, route selection Phase 2:
Route Selection should be recalculated as
specified above. performed again.

Notice that even though BGP routes do not have to be installed in the
Routing Table with the immediate next hop(s), implementations must
take care that before any packets are forwarded along a BGP route,
its associated NEXT_HOP address is resolved to the immediate
(directly connected) next-hop address and this address (or multiple
addresses) is finally used for actual packet forwarding.

Unresolvable routes SHALL be removed from the Loc-RIB and the routing
table. However, corresponding unresolvable routes SHOULD be kept in
the Adj-RIBs-In. Adj-RIBs-In (in case they become resolvable).

9.1.2.1 Route Resolvability Condition

As indicated in Section 9.1.2, BGP routers should exclude
unresolvable unresolv-
able routes from the Phase 2 decision. This ensures that only valid
routes are installed in Loc-RIB and the Routing Table.

The route resolvability condition is defined as follows.

1. A route Rte1, referencing only the intermediate network
address, is considered resolvable if the Routing Table contains at
least one resolvable route Rte2 that matches Rte1's intermediate
network address and is not recursively resolved (directly or
indirectly) indi-
rectly) through Rte1. If multiple matching routes are available,
only the longest matching route should be considered.

2. Routes referencing interfaces (with or without intermediate
addresses) are considered resolvable if the state of the
referenced refer-
enced interface is up and IP processing is enabled on this
interface. inter-
face.

BGP routes do not refer to interfaces, but can be resolved through
the routes in the Routing Table that can be of both types. types (those that
specify interfaces or those that do not). IGP routes and routes to
directly connected networks are expected to specify the outbound
interface. Static routes can specify the outbound interface, or the
intermediate address, or both.

Note that a BGP route is considered unresolvable not only in
situations situa-
tions where the router's Routing Table contains no route matching the
BGP route's NEXT_HOP. Mutually recursive routes (routes resolving
each other or themselves), also fail the resolvability check.

It is also important that implementations do not consider feasible
routes that would become unresolvable if they were installed in the
Routing Table even if their NEXT_HOPs are resolvable using the
current cur-
rent contents of the Routing Table (an example of such routes would
be mutually recursive routes). This check ensures that a BGP speaker
does not install in the Routing Table routes that will be removed and
not used by the speaker. Therefore, in addition to local Routing
Table stability, this check also improves behavior of the protocol in
the network.

Whenever a BGP speaker identifies a route that fails the
resolvability resolvabil-
ity check because of mutual recursion, an error message should be
logged.

9.1.2.2 Breaking Ties (Phase 2)

In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all routes with the
same degrees of preference, both those received from internal peers,
and those received from external peers.

The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain
the cost of a path (interior distance) to the address depicted by the
NEXT_HOP attribute of the route, and follow the same route selection
algorithm.

The tie-breaking algorithm begins by considering all equally
preferable prefer-
able routes to the same destination, and then selects routes to be
removed from consideration. The algorithm terminates as soon as only
one route remains in consideration. The criteria must be applied in
the order specified.

Several of the criteria are described using pseudo-code. Note that
the pseudo-code shown was chosen for clarity, not efficiency. It is
not intended to specify any particular implementation. BGP
implementations implemen-
tations MAY use any algorithm which produces the same results as
those described here.

a) Remove from consideration all routes which are not tied for
having the smallest number of AS numbers present in their AS_PATH
attributes. Note, that when counting this number, an AS_SET counts
as 1, no matter how many ASs are in the set, and that, if the
implementation supports [13], then AS numbers present in segments
of type AS_CONFED_SEQUENCE or AS_CONFED_SET are not included in
the count of AS numbers present in the AS_PATH. set.

b) Remove from consideration all routes which are not tied for
having the lowest Origin number in their Origin attribute.

c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS. Routes which
do not have the MULTI_EXIT_DISC attribute are considered to have
the lowest possible MULTI_EXIT_DISC value.

This is also described in the following procedure:

for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration

In the pseudo-code above, MED(n) is a function which returns pseudo-code above, MED(n) is a function which returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the lowest possi-
ble MULTI_EXIT_DISC value, i.e. 0.

Similarly, neighborAS(n) is a function which returns the neighbor
AS from which the route was received. If the route is learned via
IBGP, and the other IBGP speaker didn't originate the route, it is
the neighbor AS from which the other IBGP speaker learned the
route. If the route is learned via IBGP, and the other IBGP
speaker originated the route, it is the local AS.

If a MULTI_EXIT_DISC attribute is removed before re-advertising a
route into IBGP, the MULTI_EXIT_DISC attribute may only be consid-
ered in the comparison of EBGP learned routes, then removed, then
the remaining EBGP learned route may be compared to the remaining
IBGP learned routes, without considering the MULTI_EXIT_DISC
attribute for those EBGP learned routes whose MULTI_EXIT_DISC will
be dropped before advertising to IBGP. Including the
value of route n's
MULTI_EXIT_DISC attribute. If of an EBGP learned route n has no
MULTI_EXIT_DISC attribute, in the function returns comparison with an
IBGP learned route, then dropping the lowest
possible MULTI_EXIT_DISC value, i.e. 0.

Similarly, neighborAS(n) is a function which returns the neighbor
AS from which and adver-
tising the route was received. has been proven to cause route loops.

d) If at least one of the candidate routes was received from an
external peer in a neighboring autonomous system, remove from
consideration con-
sideration all routes which were received from internal peers.

e) Remove from consideration any routes with less-preferred
interior inte-
rior cost. The interior cost of a route is determined by
calculating calcu-
lating the metric to the next hop NEXT_HOP for the route using the Routing
Table. If the next NEXT_HOP hop for a route is reachable, but no cost
can be determined, then this step should be skipped (equivalently,
consider all routes to have equal costs).

This is also described in the following procedure.

for m = all routes still under consideration
for n = all routes in still under consideration
if (cost(n) is better than cost(m))
remove m from consideration

In the pseudo-code above, cost(n) is a function which returns the
cost of the path (interior distance) to the address given in the
NEXT_HOP attribute of the route.

f) Remove from consideration all routes other than the route that
was advertised by the BGP speaker whose BGP Identifier has the
lowest value.

g) Prefer the route received from the lowest neighbor address.

9.1.3 Phase 3: Route Dissemination

The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur:

a) when routes in the Loc-RIB to local destinations have changed

b) when locally generated routes learned by means outside of BGP
have changed

c) when a new BGP speaker - BGP speaker connection has been
established estab-
lished

The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function
shall be blocked from running while the Phase 2 decision function is
in process.

All routes in the Loc-RIB shall be processed into Adj-RIBs-Out
according to configured policy. This policy may exclude a route in
the Loc-RIB from being installed in a particular Adj-RIB-Out. A
route shall not be installed in the Adj-Rib-Out unless the
destination destina-
tion and NEXT_HOP described by this route may be forwarded
appropriately appropri-
ately by the Routing Table. If a route in Loc-RIB is excluded from a
particular Adj-RIB-Out the previously advertised route in that
Adj-RIB-Out Adj-
RIB-Out must be withdrawn from service by means of an UPDATE message
(see 9.2).

Route aggregation and information reduction techniques (see 9.2.2.1)
may optionally be applied.

Any local policy which results in routes being added to an Adj-RIB-
Out without also being added to the local BGP speaker's forwarding
table, is outside the scope of this document.

When the updating of the Adj-RIBs-Out and the Routing Table is
complete, com-
plete, the local BGP speaker shall run the Update-Send process of
9.2.

9.1.4 Overlapping Routes

A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching
multiple mul-
tiple routes. Since BGP encodes NLRI using IP prefixes, overlap will
always exhibit subset relationships. A route describing a smaller
set of destinations (a longer prefix) is said to be more specific
than a route describing a larger set of destinations (a
shorted prefix); shorter pre-
fix); similarly, a route describing a larger set of destinations (a shorter prefix) is
said to be less specific than a route describing a smaller set of destinations (a longer prefix).
destinations.

The precedence relationship effectively decomposes less specific
routes into two parts:

- a set of destinations described only by the less specific route,
and

- a set of destinations described by the overlap of the less
specific spe-
cific and the more specific routes

When overlapping routes are present in the same Adj-RIB-In, the more
specific route shall take precedence, in order from more specific to
least specific.

The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of
destinations desti-
nations described by the overlap will still be reachable using the
less specific route.

If a BGP speaker receives overlapping routes, the Decision Process
MUST consider both routes based on the configured acceptance policy.
If both a less and a more specific route are accepted, then the
Decision Deci-
sion Process MUST either install both the less and the more specific
routes or it MUST aggregate the two routes and install the aggregated
route, provided that both routes have the same value of the NEXT_HOP
attribute.

If a BGP speaker chooses to aggregate, then it MUST SHOULD either include
all AS used to form the aggreagate in an AS_SET or add the
ATOMIC_AGGREGATE attribute to the route. This attribute is now pri-
marily informational. With the elimination of IP routing protocols
that do not support classless routing and the elimination of router
and host implementations that do not support classless routing, there
is no longer a need to deaggregate. Routes SHOULD NOT be de-aggre-
gated. A route that carries ATOMIC_AGGREGATE attribute can not in particular
MUST NOT be de-aggregated. That is, the NLRI of this route can not be
made more specific. Forwarding along such a route does not guarantee
that IP packets will actually traverse only ASs listed in the AS_PATH
attribute of the route.

9.2 Update-Send Process

The Update-Send process is responsible for advertising UPDATE
messages mes-
sages to all peers. For example, it distributes the routes chosen by
the Decision Process to other BGP speakers which may be located in
either the same autonomous system or a neighboring autonomous system.

When a BGP speaker receives an UPDATE message from an internal peer,
the receiving BGP speaker shall not re-distribute the routing
information infor-
mation contained in that UPDATE message to other internal peers,
unless the speaker acts as a BGP Route Reflector [11]. [RFC2796].

As part of Phase 3 of the route selection process, the BGP speaker
has updated its Adj-RIBs-Out. All newly installed routes and all
newly unfeasible routes for which there is no replacement route shall
be advertised to its peers by means of an UPDATE message.

A BGP speaker should not advertise a given feasible BGP route from
its Adj-RIB-Out if it would produce an UPDATE message containing the
same BGP route as was previously advertised.

Any routes in the Loc-RIB marked as unfeasible shall be removed.
Changes to the reachable destinations within its own autonomous
system sys-
tem shall also be advertised in an UPDATE message.

If due to the limits on the maximum size of an UPDATE message (see
Section 4) a single route doesn't fit into the message, the BGP
speaker MUST not advertise the route to its peers and MAY choose to
log an error locally.

9.2.1 Controlling Routing Traffic Overhead

The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE
messages.

9.2.1.1 Frequency of Route Advertisement

The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement and/or with-
drawal of routes to a particular destination from by a single BGP speaker. speaker to a
peer. This rate limiting procedure applies on a per-destination
basis, although the value of MinRouteAdvertisementInterval is set on
a per BGP peer basis.

Two UPDATE messages sent from by a single BGP speaker to a peer that advertise
feasible routes and/or withdrawal of unfeasible routes to some common set of destinations received from
external peers must
set of destinations MUST be separated by at least
MinRouteAdvertisementInterval. MinRouteAdvertise-
mentInterval. Clearly, this can only be achieved precisely by keeping
a separate timer for each common set of destinations. This would be
unwarranted overhead. Any technique which ensures that the interval
between two UPDATE messages sent from a
single BGP speaker to a peer that
advertise feasible routes and/or withdrawal of unfeasible routes to
some common set of destinations received from external peers will be at least
MinRouteAdvertisementInterval, MinRouteAdvertise-
mentInterval, and will also ensure a constant upper bound on the
interval is acceptable.

Since fast convergence is needed within an autonomous system, this
procedure does not apply either
(a) the MinRouteAdvertisementInterval used for routes received from other internal
peers. To avoid long-lived black holes, peers SHOULD
be shorter than the MinRouteAdvertisementInterval used for external
peers, or (b) the procedure does not describe in this section SHOULD NOT apply
to the explicit withdrawal of unfeasible routes (that is,
for routes
whose destinations (expressed as IP prefixes) are listed in the
WITHDRAWN ROUTES field of an UPDATE message). sent to internal peers.

This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected shall SHALL be advertised at the end of
MinRouteAdvertisementInterval. MinRouteAd-
vertisementInterval.

9.2.1.2 Frequency of Route Origination

The parameter MinASOriginationInterval determines the minimum amount
of time that must elapse between successive advertisements of UPDATE
messages that report changes within the advertising BGP speaker's own
autonomous systems.

9.2.1.3 Jitter

To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, Keepalive, and
MinRouteAdvertisementInterval. A given BGP speaker shall apply the
same jitter to each of these quantities regardless of the
destinations to which the updates are being sent; that is, jitter
will not be applied on a "per peer" basis.

The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random
factor which is uniformly distributed in the range from 0.75 to 1.0.

9.2.2 Efficient Organization of Routing Information

Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this
information in an efficient manner.

9.2.2.1 Information Reduction

Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class.

The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following
methods: meth-
ods:

a) Network Layer Reachability Information (NLRI):

Destination IP addresses can be represented as IP address
prefixes. pre-
fixes. In cases where there is a correspondence between the
address structure and the systems under control of an autonomous
system administrator, it will be possible to reduce the size of
the NLRI carried in the UPDATE messages.

b) AS_PATHs:

AS path information can be represented as ordered AS_SEQUENCEs or
unordered AS_SETs. AS_SETs are used in the route aggregation
algorithm algo-
rithm described in 9.2.2.2. They reduce the size of the AS_PATH
information by listing each AS number only once, regardless of how
many times it may have appeared in multiple AS_PATHs that were
aggregated.

An AS_SET implies that the destinations listed in the NLRI can be
reached through paths that traverse at least some of the
constituent con-
stituent autonomous systems. AS_SETs provide sufficient
information informa-
tion to avoid routing information looping; however their use may
prune potentially feasible paths, since such paths are no longer
listed individually as in the form of AS_SEQUENCEs. In practice
this is not likely to be a problem, since once an IP packet
arrives at the edge of a group of autonomous systems, the BGP
speaker at that point is likely to have more detailed path
information infor-
mation and can distinguish individual paths to destinations.

9.2.2.2 Aggregating Routing Information

Aggregation is the process of combining the characteristics of
several sev-
eral different routes in such a way that a single route can be
advertised. adver-
tised. Aggregation can occur as part of the decision process to
reduce the amount of routing information that will be placed in the
Adj-RIBs-Out.

Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information.

Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical:
MULTI_EXIT_DISC, NEXT_HOP.

If the aggregation occurs as part of the update process, routes with different NEXT_HOP values can MULTI_EXIT_DISC attribute SHALL NOT be aggregated when announced through an
external BGP session.
aggregated.

Path attributes that have different type codes can not be aggregated
together. Path attributes of the same type code may be aggregated,
according to the following rules: following rules:

NEXT_HOP:
When aggregating routes that have different NEXT_HOP attribute,
the NEXT_HOP attribute of the aggregated route SHALL identify
an interface on the router that performs the aggregation.

ORIGIN attribute:
If at least one route among routes that are aggregated has ORIGIN ORI-
GIN with the value INCOMPLETE, then the aggregated route must
have the ORIGIN attribute with the value INCOMPLETE. Otherwise, Other-
wise, if at least one route among routes that are aggregated
has ORIGIN with the value EGP, then the aggregated route must
have the origin attribute with the value EGP. In all other case
the value of the ORIGIN attribute of the aggregated route is
IGP.

AS_PATH attribute:
If routes to be aggregated have identical AS_PATH attributes,
then the aggregated route has the same AS_PATH attribute as
each individual route.

For the purpose of aggregating AS_PATH attributes we model each
AS within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to
(e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If
the routes to be aggregated have different AS_PATH attributes,
then the aggregated AS_PATH attribute shall satisfy all of the
following conditions:

- all tuples of type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of
routes to be aggregated.

- all tuples of type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set
(they may appear as either AS_SET or AS_SEQUENCE types).

- for any tuple X of type AS_SEQUENCE in the aggregated
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains
Y, regardless of the type of Y.

- No tuple of type AS_SET with the same value shall appear
more than once in the aggregated AS_PATH.

- Multiple tuples of type AS_SEQUENCE with the same value
may appear in the aggregated AS_PATH only when adjacent to
another tuple of the same type and value.

An implementation may choose any algorithm which conforms to
these rules. At a minimum a conformant implementation shall be
able to perform the following algorithm that meets all of the
above conditions:

- determine the longest leading sequence of tuples (as
defined above) common to all the AS_PATH attributes of the
routes to be aggregated. Make this sequence the leading
sequence of the aggregated AS_PATH attribute.

- set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and
append them to the aggregated AS_PATH attribute.

- if the aggregated AS_PATH has more than one tuple with the
same value (regardless of tuple's type), eliminate all, but
one such tuple by deleting tuples of the type AS_SET from
the aggregated AS_PATH attribute.

- for each pair of adjacent tuples in the aggregated
AS_PATH, if both tuples have the same type, merge them
together, as long as doing so will not cause a segment with
length greater than 255 to be generated.

Appendix 6, F, section 6.8 F.6 presents another algorithm that satisfies satis-
fies the conditions and allows for more complex policy configurations. configu-
rations.

ATOMIC_AGGREGATE:
If at least one of the routes to be aggregated has
ATOMIC_AGGREGATE path attribute, then the aggregated route
shall have this attribute as well.

AGGREGATOR:
All AGGREGATOR attributes of all routes to be aggregated should
be ignored. The BGP speaker performing the route aggregation
may attach a new AGGREGATOR attribute (see Section 5.1.7).

9.3 Route Selection Criteria

Generally speaking, additional rules for comparing routes among
several sev-
eral alternatives are outside the scope of this document. There are
two exceptions:

- If the local AS appears in the AS path of the new route being
considered, then that new route cannot can not be viewed as better than
any other route (provided that the speaker is configured to accept
such routes). If such a route were ever used, a routing loop could
result (see Section 6.3).

- In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid
spontaneous spon-
taneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require
experience, expe-
rience, but the principle is clear.

Care must be taken to ensure that BGP speakers in the same AS do not
make inconsistent decisions.

9.4 Originating BGP routes

A BGP speaker may originate BGP routes by injecting routing
information informa-
tion acquired by some other means (e.g. via an IGP) into BGP. A BGP
speaker that originates BGP routes shall assign the degree of
preference prefer-
ence to these routes by passing them through the Decision Process
(see Section 9.1). These routes may also be distributed to other BGP
speakers within the local AS as part of the update process (see Section Sec-
tion 9.2). The decision whether to distribute non-BGP acquired routes
within an AS via BGP or not depends on the
environment within environment within the AS
(e.g. type of IGP) and should be controlled via configuration.

10 BGP Timers

BGP employs five timers: ConnectRetry (see Section 8), Hold Time (see
Section 4.2), KeepAlive (see Section 8), MinASOriginationInterval
(see Section 9.2.1.2), and MinRouteAdvertisementInterval (see Section
9.2.1.1).

The suggested default value for the ConnectRetry timer is 120 sec-
onds.

The suggested default value for the Hold Time is 90 seconds.

The suggested default value for the KeepAlive timer is 1/3 of the
Hold Time.

The suggested default value for the MinASOriginationInterval is 15
seconds.

The suggested default value for the MinRouteAdvertisementInterval is
30 seconds.

An implementation of BGP MUST allow the Hold Time timer to be config-
urable on a per peer basis, and MAY allow the other timers to be con-
figurable.

To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, KeepAlive, Min-
RouteAdvertisementInterval, and ConnectRetry. A given BGP speaker may
apply the same jitter to each of these quantities regardless of the
destinations to which the updates are being sent; that is, jitter
need not be configured on a "per peer" basis.

The suggested default amount of jitter shall be determined by multi-
plying the base value of the appropriate timer by a random factor
which is uniformly distributed in the AS (e.g. type of IGP) and range from 0.75 to 1.0. A new
random value should be controlled
via configuration. picked each time the timer is set. The range
of the jitter random value MAY be configurable.

Appendix 1. A. Comparison with RFC1771

There are numerous editorial changes (too many to list here).

The following list the technical changes:

Changes to reflect the usages of such features as TCP MD5 [10],
[RFC2385], BGP Route Reflectors [11], [RFC2796], BGP Confederations [13],
[RFC3065], and BGP Route Refresh [12]. [RFC2918].

Clarification on the use of the BGP Identifier in the AGGREGATOR
attribute.

Procedures for imposing an upper bound on the number of prefixes
that a BGP speaker would accept from a peer.

The ability of a BGP speaker to include more than one instance of
its own AS in the AS_PATH attribute for the purpose of inter-AS
traffic engineering.

Clarifications on the various types of NEXT_HOPs.

Clarifications to the use of the ATOMIC_AGGREGATE attribute.

The relationship between the immediate next hop, and the next hop
as specified in the NEXT_HOP path attribute.

Clarifications on the tie-breaking procedures.

Clarifications on the frequency of route advertisements.

Optional Parameter Type 1 (Authentication Information) has been
deprecated.

UPDATE Message Error subcode 7 (AS Routing Loop) has been depre-
cated.

Use of the Marker field for authentication has been deprecated.

Appendix 2. B. Comparison with RFC1267

All the changes listed in Appendix 1, A, plus the following.

BGP-4 is capable of operating in an environment where a set of
reachable reach-
able destinations may be expressed via a single IP prefix. The
concept con-
cept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9].

To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures.

The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise
which AS and which BGP speaker within that AS caused the aggregation.

To insure that Hold Timers are symmetric, the Hold Time is now
negotiated nego-
tiated on a per-connection basis. Hold Times of zero are now
supported. sup-
ported.

Appendix 3. C. Comparison with RFC 1163

All of the changes listed in Appendices 1 A and 2, B, plus the following.

To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and
recovering recov-
ering from collision.

The new document no longer restricts the border router that is passed
in the NEXT_HOP path attribute to be part of the same Autonomous
System Sys-
tem as the BGP Speaker.

New document optimizes and simplifies the exchange of the information
about previously reachable routes.

Appendix 4. D. Comparison with RFC 1105

All of the changes listed in Appendices 1, 2 A, B and 3, C, plus the
following. follow-
ing.

Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD.

The notion of Up/Down/Horizontal relations present in RFC1105 has
been removed from the protocol.

The changes in the message format from RFC1105 are as follows:

1. The Hold Time field has been removed from the BGP header and
added to the OPEN message.

2. The version field has been removed from the BGP header and
added to the OPEN message.

3. The Link Type field has been removed from the OPEN message.

4. The OPEN CONFIRM message has been eliminated and replaced with
implicit confirmation provided by the KEEPALIVE message.

5. The format of the UPDATE message has been changed
significantly. signifi-
cantly. New fields were added to the UPDATE message to support
multiple path attributes.

6. The Marker field has been expanded and its role broadened to
support authentication.

Note that quite often BGP, as specified in RFC 1105, is referred
to as BGP-1, BGP, as specified in RFC 1163, is referred to as
BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
BGP, as specified in this document is referred to as BGP-4.

Appendix 5. E. TCP options that may be used with BGP

If a local system TCP user interface supports TCP PUSH function, then
each BGP message should be transmitted with PUSH flag set. Setting
PUSH flag forces BGP messages to be transmitted promptly to the
receiver.

If a local system TCP user interface supports setting precedence for
TCP connection, then the BGP transport TCP connection used by BGP should be opened with
precedence set to Internetwork Control (110) value (see also
[6]).
[RFC791]).

A local system may protect its BGP sessions connections by using the TCP MD5
Signature Option [10]. [RFC2385].

Appendix 6. F. Implementation Recommendations

This section presents some implementation recommendations.

6.1

Appendix F.1 Multiple Networks Per Message

The BGP protocol allows for multiple address prefixes with the same
path attributes to be specified in one message. Making use of this
capability is highly recommended. With one address prefix per message
there is a substantial increase in overhead in the receiver. Not only
does the system overhead increase due to the reception of multiple
messages, but the overhead of scanning the routing table for updates
to BGP peers and other routing protocols (and sending the associated
messages) is incurred multiple times as well.

One method of building messages containing many address prefixes per
a path attribute set from a routing table that is not organized on a
per path attribute set basis is to build many messages as the routing
table is scanned. As each address prefix is processed, a message for
the associated set of path attributes is allocated, if it does not
exist, and the new address prefix is added to it. If such a message
exists, the new address prefix is just appended to it. If the message
lacks the space to hold the new address prefix, it is transmitted, a
new message is allocated, and the new address prefix is inserted into
the new message. When the entire routing table has been scanned, all
allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the
address prefixes share a common set of path attributes making it
possible pos-
sible to send many address prefixes in one 4096-byte message.

When peering with a BGP implementation that does not compress
multiple multi-
ple address prefixes into one message, it may be necessary to take
steps to reduce the overhead from the flood of data received when a
peer is acquired or a significant network topology change
occurs. One method of doing this is to limit the rate of updates.
This will eliminate the redundant scanning of the routing table to
provide flash updates for BGP peers and other routing protocols. A
disadvantage of this approach is that it increases the propagation
latency of routing information. By choosing a minimum flash update
interval that is not much greater than the time it takes to process
the multiple messages this latency should be minimized. A better
method would be to read all received messages before sending updates.

6.2 Processing Messages on a Stream Protocol

BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not
possible to determine how much data has been received but not yet
processed. change occurs. One
method that can be used in of doing this situation is to first try to read
just limit the message header. For rate of updates. This will elim-
inate the KEEPALIVE message type, this is a
complete message; redundant scanning of the routing table to provide flash
updates for BGP peers and other message types, the header should first be
verified, in particular the total length. If all checks are
successful, the specified length, minus the size routing protocols. A disadvantage of the message
header
this approach is that it increases the amount propagation latency of data left to read. An implementation routing
information. By choosing a minimum flash update interval that
would "hang" is not
much greater than the routing information time it takes to process while trying the multiple messages
this latency should be minimized. A better method would be to read
from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been
received.

6.3
all received messages before sending updates.

Appendix F.2 Reducing route flapping

To avoid excessive route flapping a BGP speaker which needs to
withdraw a destination and send an update about a more specific or
less specific route SHOULD combine them into the same UPDATE message.

6.4 BGP Timers

BGP employs five timers: ConnectRetry, Hold Time, KeepAlive,
MinASOriginationInterval, and MinRouteAdvertisementInterval The
suggested value for the ConnectRetry timer is 120 seconds. The
suggested value for the Hold Time is 90 seconds. The suggested value
for the KeepAlive timer is 1/3 of the Hold Time. The suggested value
for the MinASOriginationInterval is 15 seconds. The suggested value
for the MinRouteAdvertisementInterval is 30 seconds.

An implementation of BGP MUST allow the Hold Time timer to be
configurable, and MAY allow the other timers to be configurable.

6.5 with-
draw a destination and send an update about a more specific or less
specific route SHOULD combine them into the same UPDATE message.

Appendix F.3 Path attribute ordering

Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from
different differ-
ent update messages which are semantically identical. To facilitate
this, it is a useful optimization to order the path attributes
according to type code. This optimization is entirely optional.

6.6

Appendix F.4 AS_SET sorting

Another useful optimization that can be done to simplify this
situation situa-
tion is to sort the AS numbers found in an AS_SET. This optimization
is entirely optional.

6.7

Appendix F.5 Control over version negotiation

Since BGP-4 is capable of carrying aggregated routes which cannot can not be
properly represented in BGP-3, an implementation which supports BGP-4
and another BGP version should provide the capability to only speak
BGP-4 on a per-peer basis.

6.8

Appendix F.6 Complex AS_PATH aggregation

An implementation which chooses to provide a path aggregation
algorithm algo-
rithm which retains significant amounts of path information may wish
to use the following procedure:

For the purpose of aggregating AS_PATH attributes of two routes,
we model each AS as a tuple <type, value>, where "type" identifies
a type of the path segment the AS belongs to (e.g. AS_SEQUENCE,
AS_SET), and "value" is the AS number. Two ASs are said to be the
same if their corresponding <type, value> tuples are the same.

The algorithm to aggregate two AS_PATH attributes works as
follows: fol-
lows:

a) Identify the same ASs (as defined above) within each AS_PATH
attribute that are in the same relative order within both
AS_PATH attributes. Two ASs, X and Y, are said to be in the
same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes.

b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the
same) in both attributes are combined into an AS_SET path
segment seg-
ment that consists of the intervening ASs from both AS_PATH
attributes; this segment is then placed in between the two
consecutive con-
secutive ASs identified in (a) of the aggregated attribute. If
two consecutive ASs identified in (a) immediately follow each
other in one attribute, but do not follow in another, then the
intervening ASs of the latter are combined into an AS_SET path
segment; this segment is then placed in between the two
consecutive consec-
utive ASs identified in (a) of the aggregated attribute.

c) For each pair of adjacent tuples in the aggregated AS_PATH,
if both tuples have the same type, merge them together, as long
as doing so will not cause a segment with length greater than
255 to be generated.

If as a result of the above procedure a given AS number appears
more than once within the aggregated AS_PATH attribute, all, but
the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute.

Security Considerations

BGP supports the ability to authenticate BGP messages by using BGP
authentication. The authentication could be done on a per peer basis.
In addition, BGP supports the ability to authenticate its data stream
by using [10]. [RFC2385]. This authentication could be done on a per peer
basis. Finally, BGP could also use IPSec to authenticate its data
stream. Among the mechanisms mentioned in this paragraph, [10] [RFC2385]
is the most widely deployed.

Normative References

[RFC793] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC793, September 1981.

[RFC791] Postel, J., "Internet Protocol - DARPA Internet Program Pro-
tocol Specification", RFC791, September 1981.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

Non-normative References

[1]

[RFC904] Mills, D., "Exterior Gateway Protocol Formal Specification",
RFC904, April 1984.

[2]

[RFC1092] Rekhter, Y., "EGP and Policy Based Routing in the New
NSFNET Backbone", RFC1092, February 1989.

[3]

[RFC1093] Braun, H-W., "The NSFNET Routing Architecture", RFC1093,
February 1989.

[4] Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC793, September 1981.

[5]

[RFC1772] Rekhter, Y., and P. Gross, "Application of the Border Gateway Gate-
way Protocol in the Internet", RFC1772, March 1995.

[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC791, September 1981.

[7] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol

[RFC1518] Rekhter, Y., Li, T., "An Architecture for Exchange of
Inter-domain Routeing Information among Intermediate Systems to
Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993

[8] IP Address Allo-
cation with CIDR", RFC 1518, September 1993.

[RFC1519] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless Inter-
Domain
Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.

[9] Rekhter, Y.,

[RFC1997] R. Chandra, P. Traina, T. Li, T., "An Architecture for IP Address Allocation
with CIDR", "BGP Communities Attribute",
RFC 1518, September 1993.

[10] 1997, August 1996.

[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC2385, August 1998.

[11]

[RFC2439] C. Villamizar, R. Chandra, R. Govindan, "BGP Route Flap
Damping", RFC2439, November 1998.

[RFC2796] Bates, T., Chandra, R., Chen, E., "BGP Route Reflection -
An Alternative to Full Mesh IBGP", RFC2796, April 2000.

[12]

[RFC2842] R. Chandra, J. Scudder, "Capabilities Advertisement with
BGP-4", RFC2842.

[RFC2858] T. Bates, R. Chandra, D. Katz, Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC2858.

[RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC2918,
September 2000.

[13]

[RFC3065] Traina, P, McPherson, D., Scudder, J., "Autonomous System
Confederations for BGP", RFC3065, February 2001.

[IS10747] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Protocol for Exchange of
Inter-domain Routeing Information among Intermediate Systems to Sup-
port Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993

Editors' Addresses

Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
email: yakov@juniper.net

Tony Li
Procket Networks
1100 Cadillac Ct.
Milpitas, CA 95035
Email: Networks, Inc.

email: tli@procket.com

Susan Hares

NextHop Technologies, Inc.
email: skh@nexthop.com