wdiff draft-ietf-idr-bgp-prefix-sid-08.txt draft-ietf-idr-bgp-prefix-sid-09.txt

IDR S. Previdi, Ed.
Internet-Draft C. Filsfils
Intended status: Standards Track A. Lindem
Expires: July 6, 9, 2018 Cisco Systems
A. Sreekantiah

H. Gredler
RtBrick Inc.
January 2, 5, 2018

Segment Routing Prefix SID extensions for BGP
draft-ietf-idr-bgp-prefix-sid-08
draft-ietf-idr-bgp-prefix-sid-09

Abstract

Segment Routing (SR) architecture allows a node to steer a packet
flow through any topological path and service chain by leveraging
source routing. The ingress node prepends a an SR header to a packet
containing a set of segment identifiers (SID). Each SID represents a
topological or a service-based instruction. Per-flow state is
maintained only on the ingress node of the SR domain.

This document defines an optional, transitive BGP attribute for
announcing BGP Prefix Segment Identifiers (BGP Prefix-SID)
information.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC 2119 BCP
14 [RFC2119] [RFC8174] when, and only when when, they appear in all upper case. They may also appear in
lower or mixed case
capitals, as English words, without any normative meaning. shown here.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on July 6, 9, 2018.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. BGP-Prefix-SID . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. MPLS BGP Prefix SID . . . . . . . . . . . . . . . . . . . 4
2.2. IPv6 Prefix Segment . . . . . . . . . . . . . . . . . . . 5
3. BGP-Prefix-SID BGP Prefix-SID Attribute . . . . . . . . . . . . . . . . . . 5
3.1. Label-Index TLV . . . . . . . . . . . . . . . . . . . . . 6
3.2. IPv6 SID . . . . . . . . . . . . . . . . . . . . . . . . 6 7
3.3. Originator SRGB TLV . . . . . . . . . . . . . . . . . . . 7
4. Receiving BGP-Prefix-SID BGP Prefix-SID Attribute . . . . . . . . . . . . . 9
4.1. MPLS Dataplane: Labeled Unicast . . . . . . . . . . . . . 9
4.2. IPv6 Dataplane . . . . . . . . . . . . . . . . . . . . . 10
5. Announcing BGP-Prefix-SID Advertising BGP Prefix-SID Attribute . . . . . . . . . . . . . 10
5.1. MPLS Dataplane: Labeled Unicast . . . . . . . . . . . . . 10 11
5.2. IPv6 Dataplane . . . . . . . . . . . . . . . . . . . . . 11
6. Error Handling of BGP-Prefix-SID BGP Prefix-SID Attribute . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Manageability Considerations . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . 14 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

Segment Routing (SR) architecture leverages the source routing
paradigm. A group of inter-connected nodes that use SR forms a an SR
domain. A segment represents either a topological instruction such
as "go to prefix P following shortest path" or a service instruction
(e.g.: "pass through deep packet inspection"). Other types of
segments may be defined in the future.

A segment is identified through a Segment Identifier (SID).
Typically, the ingress node of the SR domain prepends a an SR header
containing segments identifiers (SIDs) to an incoming packet.

As described in [I-D.ietf-spring-segment-routing], when SR is applied
to the MPLS dataplane ([I-D.ietf-spring-segment-routing-mpls]) the
SID consists of a label while when SR is applied to the IPv6
dataplane the SID consists of an IPv6 address.

A BGP-Prefix Segment (and its BGP Prefix-SID), is a BGP segment
attached to a BGP prefix. A BGP Prefix-SID is always a global SID
([I-D.ietf-spring-segment-routing]) within the SR/BGP domain (i.e.,
the set of Autonomous Systems under a common administration and
control and where SR is used) and identifies an instruction to
forward the packet over the ECMP-aware best-path computed by BGP to
the related prefix. The BGP Prefix-SID is the identifier of the BGP
prefix segment. In this document, we always refer to the BGP Segment
by the BGP Prefix-SID.

This document describes the BGP extension to signal the BGP Prefix-
SID. Specifically, this document defines a BGP attribute known as
the BGP Prefix-SID attribute and specifies the rules to originate,
receive, and handle error conditions for the attribute.

The BGP Prefix-SID attribute defined in this document can be attached
to prefixes from AFI/SAFI: AFI/SAFI combinations:

Multiprotocol BGP labeled IPv4/IPv6 Unicast ([RFC8277]).

Multiprotocol BGP ([RFC4760]) unlabeled IPv6 Unicast.

[I-D.ietf-spring-segment-routing-msdc] describes example use cases
where the BGP Prefix-SID is used for the above AFI/SAFI. AFI/SAFI combinations.

It should be noted that:

o A BGP Prefix-SID MAY be global between domains when the
interconnected domains agree on the SID allocation scheme.
Alternatively, when interconnecting domains, the ASBRs of each
domain will have to handle the advertisement of unique SIDs. The
mechanisms for such interconnection are outside the scope of the
protocol extensions defined in this document.

o A BGP Prefix-SID MAY be attached to a prefix. In addition, each
prefix will likely have a different AS_PATH attribute. This
implies that each prefix is advertised individually, reducing the
ability to pack BGP advertisements (when sharing common
attributes).

2. BGP-Prefix-SID

The BGP Prefix-SID attached to a BGP prefix P represents the
instruction "go to Prefix P" along its BGP best path (potentially
ECMP-enabled).

2.1. MPLS BGP Prefix SID

The BGP Prefix-SID is realized on the MPLS dataplane
([I-D.ietf-spring-segment-routing-mpls]) in the following way:

The operator assigns a globally unique "index", label index, L_I, to a
locally sourced prefix of a BGP speaker N which is advertised to
all other BGP speakers in the SR domain.

According to [I-D.ietf-spring-segment-routing], each BGP speaker
is configured with a label block called the Segment Routing Global
Block (SRGB). While [I-D.ietf-spring-segment-routing] recommends
to use the same SRGB across all the nodes within the SR domain,
the SRGB of a node is a local property and could be different on
different speakers. The drawbacks of the use case where BGP
speakers have different SRGBs are documented in
[I-D.ietf-spring-segment-routing] and
[I-D.ietf-spring-segment-routing-msdc].

If traffic-engineering within the SR domain is required, each node
may also be required to advertise topological information and
Peering SID's SIDs for each of its links and peers. This information is
required to perform the explicit path computation and to express any
an explicit path into as a list of SIDs. The advertisement of
topological information and Peer segments (Peer SIDs) is
assumed to be done
through [I-D.ietf-idr-bgpls-segment-routing-epe].

If the BGP speakers are not all configured with the same SRGB, and
if traffic-engineering within the SR domain is required, each node
may be required to advertise its local SRGB in addition to the
topological information.

This documents assumes that BGP-LS is the preferred method for
collecting both topological, peer segments (Peer SIDs) and SRGB information
through [RFC7752],
[I-D.ietf-idr-bgpls-segment-routing-epe] [I-D.ietf-idr-bgpls-segment-routing-epe], and
[I-D.ietf-idr-bgp-ls-segment-routing-ext]. However, as an
optional alternative for the advertisement of the local SRGB
without the topology nor the peer SID's, SIDs, hence without
applicability for TE, the Originator SRGB TLV of the prefix-SID
attribute,
attribute is specified in Section 3.3 of this document.

As defined in [I-D.ietf-spring-segment-routing-mpls], [I-D.ietf-spring-segment-routing], the label index
L_I is an offset in into the SRGB. Each BGP speaker derives its
local MPLS label, L, by adding L_I to the start value of its own
SRGB, and programs L in its MPLS dataplane as its incoming/local
label for the prefix. It should be noted that while SRGBs and
SIDs are advertised using 32-bit values, the derived label is to be
considered as
advertised in the 20 right-most bits. See Section 4.1 for more
details.

The outgoing label for the prefix is found in the NLRI of the
Multiprotocol BGP labeled IPv4/IPv6 Unicast prefix advertisement.
The label index L_I is only used as a hint to derive the local/incoming local/
incoming label.

Section 3.1 of this document specifies the Label-Index TLV of the
BGP Prefix-SID attribute; this TLV can be used to advertise the
label index of for a given prefix.

In order to advertise the label index of a given prefix P and,
optionally, the SRGB, an extension to BGP is needed: the BGP Prefix-
SID attribute. This extension is described in subsequent sections.

2.2. IPv6 Prefix Segment

When SR is used over an IPv6 dataplane, the BGP Prefix-SID consists
of an IPv6 address assigned to the BGP speaker.

3. BGP-Prefix-SID BGP Prefix-SID Attribute

The BGP Prefix-SID attribute is an optional, transitive BGP path
attribute. The attribute type code 40 has been assigned by IANA (see
Section 7).

The BGP Prefix-SID attribute is defined here to be a set of elements
encoded as "Type/Length/Value" tuples (i.e., a set of TLVs). The
following TLVs are defined:

o Label-Index TLV
o IPv6 SID TLV

o Originator SRGB TLV

The Label-Index and Originator SRGB TLVs are used only when SR is
applied to the MPLS dataplane.

The IPv6 SID TLV is used only when SR is applied to the IPv6
dataplane.

3.1. Label-Index TLV

The Label-Index TLV MUST be present in the BGP Prefix-SID attribute
attached to Labeled IPv4/IPv6 unicast prefixes ([RFC8277]) and ([RFC8277]). It will
be ignored when received for other BGP AFI/SAFI combinations. The
Label-Index TLV has the following format:

where:

o Type is 1.

o Length: is 7, the total length in octets of the value portion of
the TLV.

o RESERVED: 8-bit field. MUST be clear on transmission and MUST be
ignored on reception.

o Flags: 16 bits of flags. None are defined by this document. The
flag field MUST be clear on transmission and MUST be ignored on
reception.

o Label Index: 32-bit value representing the index value in the SRGB
space.

3.2. IPv6 SID

The IPv6-SID IPv6 SID TLV MAY be present in the BGP Prefix-SID attribute
attached to MP-BGP unlabeled IPv6 unicast prefixes ([RFC4760]) and ([RFC4760]). It
will be ignored for other BGP AFI/SAFI combinations. The IPv6 SID
TLV has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESERVED | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| IPv6 SID (16 octets) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

o Type is 2.

o Length: is 19, the total length in octets of the value portion of
the TLV.

o RESERVED: 24-bit field for future use. MUST be clear on
transmission and MUST be ignored on reception.

o IPv6 SID: 16 octets.

3.3. Originator SRGB TLV

The Originator SRGB TLV is an optional TLV and has the following
format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRGB 1 (6 octets) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRGB n (6 octets) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

o Type is 3.

o Length is the total length in octets of the value portion of the
TLV: 2 + multiple of 6.

o Flags: 16 bits of flags. None are defined in this document.
Flags MUST be clear on transmission and MUST be ignored on
reception.

o SRGB: 3 octets of base followed by 3 octets of range. Note that
the SRGB field MAY appear multiple times. If the SRGB field
appears multiple times, the SRGB consists of multiple ranges.

The Originator SRGB TLV contains the SRGB of the node originating the
prefix to which the BGP Prefix-SID is attached. The Originator SRGB
TLV MUST NOT be changed during the propagation of the BGP update.

The originator SRGB describes the SRGB of the node where the BGP
Prefix SID is attached. It is used to build segment routing policies
when different SRGBs are used in the fabric, for example
([I-D.ietf-spring-segment-routing-msdc]).

The originator SRGB may only appear on in a BGP Prefix-SID attribute
attached to Labeled IPv4/IPv6 unicast prefixes of SAFI 4 (labeled unicast, [RFC8277]). ([RFC8277]). It will
be ignored when received for other BGP AFI/SAFI combinations.

4. Receiving BGP-Prefix-SID BGP Prefix-SID Attribute

A BGP speaker receiving a BGP Prefix-SID attribute from an EBGP
neighbor residing outside the boundaries of the SR domain, SHOULD MUST
discard the attribute unless it is configured to accept the attribute
from the EBGP neighbor. A BGP speaker MAY log an error for further
analysis when discarding an attribute.

4.1. MPLS Dataplane: Labeled Unicast

A Multiprotocol BGP labeled IPv4/IPv6 Unicast ([RFC8277]) session
type is required.

A BGP speaker MAY may be locally configured with an SRGB=[SRGB_Start,
SRGB_End]. The preferred method for deriving the SRGB is a matter of
local node configuration.

Given a label_index label index L_I, we call L = L_I + SRGB_Start as the derived
label. A BGP Prefix-SID attribute is designated "unacceptable" for a
speaker M if the derived label value L lies outside the SRGB
configured on M. Otherwise the Label Index attribute Label-Index TLV is designated
"acceptable" to speaker M.

The mechanisms through which a given label_index label index value is assigned to
a given prefix are outside the scope of this document. The label-
index value associated with a prefix is locally configured at the BGP
node originating the prefix.

The BGP Prefix-SID attribute MUST contain the Label-Index TLV and MAY
contain the Originator SRGB TLV. A BGP Prefix-SID attribute received
without a Label-Index TLV MUST be considered as "unacceptable" by the
receiving speaker.

If multiple prefixes are received with the same label_index label index value,
all these prefixes MUST have their BGP Prefix-SID attribute
considered as "unacceptable" by the receiving speaker.

When a BGP speaker receives a path from a neighbor with an acceptable
BGP Prefix-SID attribute, it MUST program the derived label as the
local label for the prefix in its MPLS dataplane. In case of any an
error, a BGP speaker MUST follow to the error handling rules
specified in Section 6. A BGP speaker MAY log an error for further
analysis.

When a BGP speaker receives a path from a neighbor with an
unacceptable BGP Prefix-SID attribute or when a BGP speaker receives
a path from a neighbor with a BGP Prefix-SID attribute but is unable
to process it (it does not have the capability or local policy
disables the capability), it MUST treat the path as if it came
without a BGP Prefix-SID attribute. For the purposes of local label
allocation, a BGP speaker MUST assign a local (also called dynamic)
label (non-SRGB) for such a prefix as per classic Multiprotocol BGP
labeled IPv4/IPv6 Unicast ([RFC8277]) operation. A BGP speaker MAY
log an error for further analysis.

The outgoing label is always programmed as per classic Multiprotocol
BGP labeled IPv4/IPv6 Unicast (RFC8277 [RFC8277]) ([RFC8277]) operation.

Specifically, a BGP speaker receiving a prefix with a BGP Prefix-SID
attribute and a label NLRI field of implicit-null Implicit NULL from a neighbor
MUST adhere to standard behavior and program its MPLS dataplane to
pop the top label when forwarding traffic to the prefix. The label
NLRI defines the outbound label that MUST be used by the receiving
node. The Label Index label index gives the information to the receiving node on
which local/incoming label the BGP speaker SHOULD use. assign.

4.2. IPv6 Dataplane

When an SR IPv6 BGP speaker receives a an IPv6 Unicast BGP Update with
a prefix having the BGP Prefix-SID attribute attached, it checks
whether the IPv6 SID TLV is present. If present, then present and the receiver
assumes that chosen as
the originator supports SR on best path, the prefix is installed into the Segment Routing IPv6 dataplane.
dataplane as described in [I-D.ietf-spring-segment-routing].

The Originator SRGB MUST be ignored on reception.

A BGP speaker receiving a

5. Advertising BGP Prefix-SID attribute from an EBGP
neighbor residing outside the boundaries of the SR domain, SHOULD
discard the attribute unless it is configured to accept the attribute
from the EBGP neighbor. A BGP speaker MAY log an error for further
analysis when discarding an attribute.

5. Announcing BGP-Prefix-SID Attribute

The BGP Prefix-SID attribute MAY be attached to labeled BGP prefixes
(IPv4/IPv6) [RFC8277] or to IPv6 unicast prefixes [RFC4760]. In
order to prevent distribution of the BGP Prefix-SID attribute beyond
its intended scope of applicability, attribute filtering SHOULD be
deployed.

5.1. MPLS Dataplane: Labeled Unicast

A BGP speaker that originates advertises a prefix attaches path received from one of its
neighbors SHOULD advertise the BGP Prefix-SID
attribute when received with the path
without modification, as long as the BGP Prefix-SID was acceptable.
If the path did not come with a BGP Prefix-SID attribute, the speaker
MAY attach a BGP Prefix-SID to the path if configured to do so. The
content of the TLVs present in the BGP Prefix-SID is determined by
the configuration.

5.1. MPLS Dataplane: Labeled Unicast

A BGP speaker that originates a prefix attaches the BGP Prefix-SID
attribute when it advertises the prefix to its neighbors via
Multiprotocol BGP labeled IPv4/IPv6 Unicast ([RFC8277]). The value
of the Label-Index label index in the Label-Index TLV is determined by
configuration.

A BGP speaker that originates a BGP Prefix-SID attribute MAY
optionally announce the Originator SRGB TLV along with the mandatory
Label-Index TLV. The content of the Originator SRGB TLV is
determined by the configuration.

Since the Label-index label index value must be unique within an SR domain, by
default an implementation SHOULD NOT advertise the BGP Prefix-SID
attribute outside an Autonomous System unless it is explicitly
configured to do so.

A BGP speaker that advertises a path received from one of its
neighbors SHOULD advertise the Prefix-SID received with the path
without modification, regardless of whether the Prefix-SID was
acceptable. If the path did not come with a Prefix-SID attribute,
the speaker MAY attach a Prefix-SID to the path if configured to do
so. The content of the TLVs present in the Prefix-SID is determined
by the configuration.

In all cases, the label field of the advertised NLRI ([RFC8277],
[RFC4364]) MUST be set to the local/incoming label programmed in the
MPLS dataplane for the given advertised prefix. If the prefix is
associated with one of the BGP speaker's interfaces, this label is the
usual MPLS label (such as the implicit Implicit or explicit Explicit NULL label).

5.2. IPv6 Dataplane

A BGP speaker that originates an IPv6 prefix with the BGP Prefix-SID
attribute,
attribute MAY include the IPv6 SID TLV.

6. Error Handling of BGP-Prefix-SID BGP Prefix-SID Attribute

When a BGP Speaker receives a BGP Update message containing a
malformed or unacceptable BGP Prefix-SID attribute, attribute attached to a
Labeled IPv4/IPv6 unicast prefix [RFC8277], it MUST ignore the
received BGP Prefix-SID attributes and not pass advertise it to other BGP
peers. This is equivalent to the "Attribute discard" action
specified in [RFC7606]. When discarding an attribute, a BGP speaker MAY
SHOULD log an error for further analysis.

If the

When a BGP Speaker receives a BGP Update message containing a
malformed or unacceptable BGP Prefix-SID attribute appears more than once in attached to an BGP
Update message, then, according
unlabeled IPv6 unicast prefix [RFC4760], it MUST treat the
advertisement as a withdrawal. This is equivalent to [RFC7606], all the occurrences of "Treat-as-
withdraw" action specified in [RFC7606]. This action is required
since simply ignoring the BGP Prefix-SID attribute other than would modify the first one SHALL be discarded
installed path and the BGP
Update message SHALL continue to be processed. "Attribute discard" option is not applicable
in this case [RFC7606]. When withdrawing the prefix, a BGP speaker receives an unacceptable Prefix-SID attribute, it
MAY
SHOULD log an error for further analysis.

Consistent with [RFC7606], only the first occurrence of the BGP
Prefix-SID attribute will be considered and subsequent occurrences
will be discarded.

7. IANA Considerations

This document defines a BGP path attribute known as the BGP Prefix-
SID attribute. This document requests IANA to assign an attribute
code type (suggested value: 40) for BGP the BGP Prefix-SID attribute from the
BGP Path Attributes registry.

Currently, IANA temporarily assigned the following:

40 BGP Prefix-SID (TEMPORARY - registered 2015-09-30, expires
2016-09-30) [draft-ietf-idr-bgp-prefix-sid]

This document defines 3 TLVs for the BGP Prefix-SID attribute. These
TLVs need to be registered with IANA. We request IANA to create a
registry for BGP Prefix-SID Attribute TLVs as follows:

Under "Border Gateway Protocol (BGP) Parameters" registry, "BGP
Prefix-SID attribute TLV Types" Reference: draft-ietf-idr-bgp-prefix-sid
Registration Procedure(s): Values 1-254 First Come, First Served,
Value 0 and 255 reserved

Value Type Reference
0 Reserved this document
1 Label-Index this document
2 IPv6 SID this document
3 Originator SRGB this document
4-254 Unassigned
255 Reserved this document

This document also creates a registry for the 16 bits of flags in the
Label-Index TLV. Initially, the registry will be empty. Flag bits
will be allocated First-Come, First Served consistent with the BGP-
SID TLV Types registry.

Finally, this document creates a registry for the 16 bits of flags in
the SRGB Originator TLV. Initially, the registry will be empty.
Flag bits will be allocated First-Come, First Served consistent with
the BGP-SID TLV Types registry.

8. Manageability Considerations

This document defines a BGP attribute to address use cases such as
the one described in [I-D.ietf-spring-segment-routing-msdc]. It is
assumed that advertisement of the BGP Prefix-SID attribute advertisement is
controlled by the operator in order to:

o Prevent undesired origination/advertisement of the BGP Prefix-SID
attribute. By default, a BGP Prefix-SID attribute SHOULD NOT be
originated and
attached to a prefix. The operator MUST be capable
of explicitly enabling prefix and advertised. Hence, BGP Prefix-SID origination.
advertisement SHOULD require explicit enablement.

o Prevent any undesired propagation of the BGP Prefix-SID attribute.
By default, the BGP Prefix-SID is not advertised outside the
boundary of an AS. a single SR/administrative domain which may include
one or more ASes. The propagation to other ASs ASes MUST be
explicitly configured.

The deployment model described in
[I-D.ietf-spring-segment-routing-msdc] assumes multiple Autonomous
Systems (AS) (ASes) under a common administrative domain. For this use
case, the BGP Prefix-SID advertisement is applicable to the inter-AS
context, i.e., EBGP, while it is confined to a single administrative
domain.

9. Security Considerations

This document introduces a BGP attribute (BGP Prefix-SID) which
inherits the security considerations expressed in: [RFC4271] [RFC4271],
[RFC8277], and
[RFC8277]. [I-D.ietf-spring-segment-routing].

It should be noted, noted that, as described in Section 8, that this document
refers to a deployment model where all nodes are under the single
administrative domain. In this context, we assume that the operator
doesn't want to leak outside of the domain any information related to internal prefixes and topology.
topology outside of the administrative domain. The internal
information includes the BGP Prefix-SID. In order to prevent such
leaking, the standard BGP mechanisms (filters) are applied on at the
boundary of the SR SR/administrative domain.

10. Contributors

Keyur Patel
Arrcus, Inc.
US

Email: Keyur@arrcus.com
Saikat Ray
Unaffiliated
US

Email: raysaikat@gmail.com

11. Acknowledgements

The authors would like to thanks thank Satya Mohanty for his contribution to
this document.

The authors would like to thank Alvaro Retana for substantive
comments as part of the Routing AD review.

12. References

12.1. Normative References

[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-14 (work
in progress), December 2017.

[I-D.ietf-spring-segment-routing-mpls]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", draft-ietf-spring-segment-routing-mpls-11
(work in progress), October 2017.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006, <https://www.rfc-
editor.org/info/rfc4271>.

[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007, <https://www.rfc-
editor.org/info/rfc4760>.

[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.

12.2. Informative References

[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Psenak, P., Filsfils, C., Gredler, H., and M.
Chen, "BGP Link-State extensions for Segment Routing",
draft-ietf-idr-bgp-ls-segment-routing-ext-03 (work in
progress), July 2017.

[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Filsfils, C., Patel, K., Ray, S., and J.
Dong, "BGP-LS extensions for Segment Routing BGP Egress
Peer Engineering", draft-ietf-idr-bgpls-segment-routing-
epe-14 (work in progress), December 2017.

[I-D.ietf-spring-segment-routing-msdc]
Filsfils, C., Previdi, S., Mitchell, J., Aries, E., and P.
Lapukhov, "BGP-Prefix Segment in large-scale data
centers", draft-ietf-spring-segment-routing-msdc-08 (work
in progress), December 2017.

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007, <https://www.rfc-
editor.org/info/rfc4760>.

[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016, <https://www.rfc-
editor.org/info/rfc7752>.

Authors' Addresses

Stefano Previdi (editor)
Cisco Systems
IT

Email: stefano@previdi.net
Clarence Filsfils
Cisco Systems
Brussels
Belgium

Email: cfilsfils@cisco.com

Acee Lindem
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95124 95134
301 Midenhall Way
Cary, NC 27513
USA

Email: acee@cisco.com

Arjun Sreekantiah

Email: arjunhrs@gmail.com

Hannes Gredler
RtBrick Inc.

Email: hannes@rtbrick.com