draft-ietf-idr-bgp-flowspec-oid-11.txt   draft-ietf-idr-bgp-flowspec-oid-12.txt 
Network Working Group J. Uttaro Network Working Group J. Uttaro
Internet-Draft AT&T Internet-Draft AT&T
Updates: 5575bis (if approved) J. Alcaide Updates: 5575bis (if approved) J. Alcaide
Intended status: Standards Track C. Filsfils Intended status: Standards Track C. Filsfils
Expires: September 9, 2020 D. Smith Expires: January 9, 2021 D. Smith
Cisco Cisco
P. Mohapatra P. Mohapatra
Sproute Networks Sproute Networks
March 8, 2020 July 8, 2020
Revised Validation Procedure for BGP Flow Specifications Revised Validation Procedure for BGP Flow Specifications
draft-ietf-idr-bgp-flowspec-oid-11 draft-ietf-idr-bgp-flowspec-oid-12
Abstract Abstract
This document describes a modification to the validation procedure This document describes a modification to the validation procedure
defined in [RFC5575bis] for the dissemination of BGP Flow defined for the dissemination of BGP Flow Specifications. The
Specifications. [RFC5575bis] requires that the originator of the dissemination of BGP Flow Specifications requires that the originator
Flow Specification matches the originator of the best-match unicast of the Flow Specification matches the originator of the best-match
route for the destination prefix embedded in the Flow Specification. unicast route for the destination prefix embedded in the Flow
This allows only BGP speakers within the data forwarding path (such Specification. This allows only BGP speakers within the data
as autonomous system border routers) to originate BGP Flow forwarding path (such as autonomous system border routers) to
Specifications. Though it is possible to disseminate such Flow originate BGP Flow Specifications. Though it is possible to
Specifications directly from border routers, it may be operationally disseminate such Flow Specifications directly from border routers, it
cumbersome in an autonomous system with a large number of border may be operationally cumbersome in an autonomous system with a large
routers having complex BGP policies. The modification proposed number of border routers having complex BGP policies. The
herein enables Flow Specifications to be originated from a modification proposed herein enables Flow Specifications to be
centralized BGP route controller. originated from a centralized BGP route controller.
This document updates RFC5575bis.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 9, 2020. This Internet-Draft will expire on January 9, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Requirements Language 1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Introduction 2. Introduction
[RFC5575bis] defined a new BGP [RFC4271] capability that can be used [I-D.ietf-idr-rfc5575bis] defined a new BGP [RFC4271] capability that
to distribute traffic Flow Specifications amongst BGP speakers in can be used to distribute traffic Flow Specifications amongst BGP
support of traffic filtering. The primary intention of [RFC5575bis] speakers in support of traffic filtering. The primary intention of
is to enable downstream autonomous systems to signal traffic [I-D.ietf-idr-rfc5575bis] is to enable downstream autonomous systems
filtering policies to upstream autonomous systems. In this way, to signal traffic filtering policies to upstream autonomous systems.
traffic is filtered closer to the source and the upstream autonomous In this way, traffic is filtered closer to the source and the
system(s) avoid carrying the traffic to the downstream autonomous upstream autonomous system(s) avoid carrying the traffic to the
system only to be discarded. [RFC5575bis] also enables more granular downstream autonomous system only to be discarded. [I-D.ietf-idr-
traffic filtering based upon upper layer protocol information (e.g., rfc5575bis] also enables more granular traffic filtering based upon
protocol port numbers) as opposed to coarse IP destination prefix- upper layer protocol information (e.g., protocol port numbers) as
based filtering. Flow specification NLRIs received from a BGP peer opposed to coarse IP destination prefix-based filtering. Flow
are subject to validity checks before being considered feasible and specification NLRIs received from a BGP peer are subject to validity
subsequently installed within the respective Adj-RIB-In. checks before being considered feasible and subsequently installed
within the respective Adj-RIB-In.
The validation procedure defined within [RFC5575bis] requires that The validation procedure defined within [I-D.ietf-idr-rfc5575bis]
the originator of the Flow Specification NLRI matches the originator requires that the originator of the Flow Specification NLRI matches
of the best-match unicast route for the destination prefix embedded the originator of the best-match unicast route for the destination
in the Flow Specification. This allows only BGP speakers within the prefix embedded in the Flow Specification. This allows only BGP
data forwarding path (such as autonomous system border routers) to speakers within the data forwarding path (such as autonomous system
originate BGP Flow Specification NLRIs. Though it is possible to border routers) to originate BGP Flow Specification NLRIs. Though it
disseminate such Flow Specification NLRIs directly from border is possible to disseminate such Flow Specification NLRIs directly
routers, it may be operationally cumbersome in an autonomous system from border routers, it may be operationally cumbersome in an
with a large number of border routers having complex BGP policies. autonomous system with a large number of border routers having
complex BGP policies.
This document describes a modification to the [RFC5575bis] validation This document describes a modification to the [I-D.ietf-idr-
procedure allowing Flow Specification NLRIs to be originated from a rfc5575bis] validation procedure allowing Flow Specification NLRIs to
centralized BGP route controller within the local autonomous system be originated from a centralized BGP route controller within the
that is not in the data forwarding path. While the proposed local autonomous system that is not in the data forwarding path.
modification cannot be used for inter-domain coordination of traffic While the proposed modification cannot be used for inter-domain
filtering, it greatly simplifies distribution of intra-domain traffic coordination of traffic filtering, it greatly simplifies distribution
filtering policies within an autonomous system which has a large of intra-domain traffic filtering policies within an autonomous
number of border routers having complex BGP policies. By relaxing system which has a large number of border routers having complex BGP
the validation procedure for iBGP, the proposed modification allows policies. By relaxing the validation procedure for iBGP, the
Flow Specifications to be distributed in a standard and scalable proposed modification allows Flow Specifications to be distributed in
manner throughout an autonomous system. a standard and scalable manner throughout an autonomous system.
3. Motivation 3. Motivation
Step (b) of the validation procedure in [RFC5575bis], section 6 is Step (b) of the validation procedure in [I-D.ietf-idr-rfc5575bis],
defined with the underlying assumption that the Flow Specification section 6 is defined with the underlying assumption that the Flow
NLRI traverses the same path, in the inter-domain and intra-domain Specification NLRI traverses the same path, in the inter-domain and
route distribution graph, as that of the longest-match unicast route intra-domain route distribution graph, as that of the longest-match
for the destination prefix embedded in the Flow Specification. unicast route for the destination prefix embedded in the Flow
Specification.
In the case of inter-domain traffic filtering, the Flow Specification In the case of inter-domain traffic filtering, the Flow Specification
originator at the egress border routers of an AS (e.g. RTR-D and originator at the egress border routers of an AS (e.g. RTR-D and
RTR-E of ASN1 in figure 1) matches the eBGP neighbor that advertised RTR-E of ASN1 in figure 1) matches the eBGP neighbor that advertised
the longest match destination prefix (see RTR-F and RTR-G the longest match destination prefix (see RTR-F and RTR-G
respectively in figure 1). Similarly, at the ingress border routers respectively in figure 1). Similarly, at the ingress border routers
of ASN (see RTR-A and RTR-B of ASN1 in figure 1), the Flow of ASN (see RTR-A and RTR-B of ASN1 in figure 1), the Flow
Specification originator matches the egress iBGP border routers that Specification originator matches the egress iBGP border routers that
had advertised the unicast route for the best-match destination had advertised the unicast route for the best-match destination
prefix (see RTR-D and RTR-E respectively in figure 1). This is true prefix (see RTR-D and RTR-E respectively in figure 1). This is true
skipping to change at page 5, line 16 skipping to change at page 5, line 19
plane. In this way, operators can direct border routers within their plane. In this way, operators can direct border routers within their
ASN with specific attack mitigation actions (drop the traffic, ASN with specific attack mitigation actions (drop the traffic,
forward to a clean-pipe center, etc.). forward to a clean-pipe center, etc.).
To originate a Flow Specification NLRI, a central BGP route To originate a Flow Specification NLRI, a central BGP route
controller must set itself as the originator in the Flow controller must set itself as the originator in the Flow
Specification NLRI. This is necessary given the route controller is Specification NLRI. This is necessary given the route controller is
originating the Flow Specification rather than reflecting it, and to originating the Flow Specification rather than reflecting it, and to
avoid the complexity of having to determine the egress border router avoid the complexity of having to determine the egress border router
whose path was chosen as the best in each of the ingress border whose path was chosen as the best in each of the ingress border
routers. Thus, it is necessary to modify step (b) of the routers. Thus, it is necessary to modify step (b) of the [I-D.ietf-
[RFC5575bis] validation procedure such that an iBGP peer that is not idr-rfc5575bis] validation procedure such that an iBGP peer that is
within the data forwarding plane may originate Flow Specification not within the data forwarding plane may originate Flow Specification
NLRIs. NLRIs.
4. Revised Validation Procedure 4. Revised Validation Procedure
4.1. Revision of Route Feasibility 4.1. Revision of Route Feasibility
Step (b) of the validation procedure specified in [RFC5575bis], Step (b) of the validation procedure specified in [I-D.ietf-idr-
section 6 is redefined as follows: rfc5575bis], section 6 is redefined as follows:
a. One of the following conditions MUST hold true: b) One of the following conditions MUST hold true:
1. The originator of the Flow Specification matches the 1. The originator of the Flow Specification matches the
originator of the best-match unicast route for the originator of the best-match unicast route for the destination
destination prefix embedded in the Flow Specification (this prefix embedded in the Flow Specification (This is the unicast
is the unicast route with the longest possible prefix length route with the longest possible prefix length covering the
covering the destination prefix embedded in the Flow destination prefix embedded in the Flow Specification).
Specification).
2. The AS_PATH attribute of the Flow Specification does not 2. The AS_PATH attribute of the Flow Specification does not
contain AS_SET and/or AS_SEQUENCE segments. contain AS_SET and/or AS_SEQUENCE segments.
1. This condition SHOULD be enabled by default. This 1. This condition SHOULD be enabled by default. This default
default behavior should validate an empty AS_PATH. behavior should validate an empty AS_PATH.
2. This condition MAY be disabled by configuration on a BGP 2. This condition MAY be disabled by configuration on a BGP
speaker. speaker.
3. As an exception to this rule, a given AS_PATH with AS_SET 3. As an exception to this rule, a given AS_PATH with AS_SET
and/or AS_SEQUENCE segments MAY be validated by policy. and/or AS_SEQUENCE segments MAY be validated by policy.
Explanation: Explanation:
In this context, an empty AS_PATH means that it does not have In this context, an empty AS_PATH means that it does not have
AS_SET and/or AS_SEQUENCE segments, and local domain means the AS_SET and/or AS_SEQUENCE segments, and local domain means the
local AS [RFC4271] or the local confederation of ASes (in the case local AS [RFC4271] or the local confederation of ASes (in the case
that the local AS belongs to a confederation of ASes [RFC5065]). that the local AS belongs to a confederation of ASes [RFC5065]).
Thus, receiving a Flow Specification with an empty AS_PATH Thus, receiving a Flow Specification with an empty AS_PATH
indicates that the Flow Specification was originated inside the indicates that the Flow Specification was originated inside the
local domain. local domain.
With the above modification to the [RFC5575bis] validation With the above modification to the [I-D.ietf-idr-rfc5575bis]
procedure, a BGP peer within the local domain that is not within validation procedure, a BGP peer within the local domain that is
the data forwarding path can originate a Flow Specification. not within the data forwarding path can originate a Flow
Specification.
Disabling the new condition above (b.2.2) may be a good practice Disabling the new condition above (b.2.2) may be a good practice
when the operator knows with certainty that there is not a Flow when the operator knows with certainty that there is not a Flow
Specification originated inside the local domain. Specification originated inside the local domain.
Also, policy may be useful to validate a specific set of non-empty Also, policy may be useful to validate a specific set of non-empty
AS_PATHs (b.2.3). For example, it could validate a Flow AS_PATHs (b.2.3). For example, it could validate a Flow
Specification whose AS_PATH contains only an AS_SEQUENCE with ASes Specification whose AS_PATH contains only an AS_SEQUENCE with ASes
that are all known to belong to the same administrative domain. that are all known to belong to the same administrative domain.
4.2. Revision of AS_PATH Validation 4.2. Revision of AS_PATH Validation
[RFC5575bis] states: [I-D.ietf-idr-rfc5575bis] states:
o BGP implementations MUST also enforce that the AS_PATH attribute o BGP implementations MUST also enforce that the AS_PATH attribute
of a route received via the External Border Gateway Protocol of a route received via the External Border Gateway Protocol
(eBGP) contains the neighboring AS in the left-most position of (eBGP) contains the neighboring AS in the left-most position of
the AS_PATH attribute. the AS_PATH attribute.
This rule prevents the exchange of BGP Flow Specification NLRIs at This rule prevents the exchange of BGP Flow Specification NLRIs at
Internet exchanges with BGP route servers. Therefore, this document Internet exchanges with BGP route servers. Therefore, this document
also redefines the [RFC5575bis] AS_PATH validation procedure also redefines the [I-D.ietf-idr-rfc5575bis] AS_PATH validation
referenced above as follows: procedure referenced above as follows:
o BGP Flow Specification implementations MUST enforce that the AS in o BGP Flow Specification implementations MUST enforce that the AS in
the left-most position of the AS_PATH attribute of a Flow the left-most position of the AS_PATH attribute of a Flow
Specification route received via the External Border Gateway Specification route received via the External Border Gateway
Protocol (eBGP) matches the AS in the left-most position of the Protocol (eBGP) matches the AS in the left-most position of the
AS_PATH attribute of the best-match unicast route for the AS_PATH attribute of the best-match unicast route for the
destination prefix embedded in the Flow Specification NLRI. destination prefix embedded in the Flow Specification NLRI.
Explanation: Explanation:
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Comparing only the last ASes added is sufficient for eBGP learned Comparing only the last ASes added is sufficient for eBGP learned
Flow Specification NLRIs. Requiring a full AS_PATH match would Flow Specification NLRIs. Requiring a full AS_PATH match would
limit origination of inter-domain Flow Specifications to the limit origination of inter-domain Flow Specifications to the
origin AS of the best-match unicast route for the destination origin AS of the best-match unicast route for the destination
prefix embedded in the Flow Specification only. As such, a full prefix embedded in the Flow Specification only. As such, a full
AS_PATH validity check may prevent transit ASes from originating AS_PATH validity check may prevent transit ASes from originating
inter-domain Flow Specifications, which is not desirable. inter-domain Flow Specifications, which is not desirable.
Redefinition of this AS_PATH validation rule for a Flow Redefinition of this AS_PATH validation rule for a Flow
Specification does not mean that the original rule in [RFC5575bis] Specification does not mean that the original rule in [I-D.ietf-
cannot be enforced as well. Its enforcement remains optional per idr-rfc5575bis] cannot be enforced as well. Its enforcement
[RFC4271] section 6.3. That is, we can enforce the first AS in remains optional per [RFC4271] section 6.3. That is, we can
the AS_PATH to be the same as the neighbor AS for any address- enforce the first AS in the AS_PATH to be the same as the neighbor
family route (including a Flow Specification). AS for any address-family route (including a Flow Specification).
Using the new rule to validate a Flow Specification received from Using the new rule to validate a Flow Specification received from
an Internal Border Gateway Protocol (iBGP) peer is out of the an Internal Border Gateway Protocol (iBGP) peer is out of the
scope of this document. Note that in most scenarios such scope of this document. Note that in most scenarios such
validation would be redundant. validation would be redundant.
Using the new rule to validate a Flow Specification route received Using the new rule to validate a Flow Specification route received
from an External Border Gateway Protocol (eBGP) peer belonging to from an External Border Gateway Protocol (eBGP) peer belonging to
the same local domain (in the case that the local AS belongs to a the same local domain (in the case that the local AS belongs to a
confederation of ASes) is out of the scope of this document. Note confederation of ASes) is out of the scope of this document. Note
that although it's possible, its utility is dubious. that although it's possible, its utility is dubious.
5. Other RFC5575bis Considerations 5. Other RFC5575bis Considerations
This section clarifies some of the terminology and rules referenced This section clarifies some of the terminology and rules referenced
in [RFC5575bis]. Namely: in [I-D.ietf-idr-rfc5575bis]. Namely:
o In the context of this document and [RFC5575bis], AS_PATH o In the context of this document and [I-D.ietf-idr-rfc5575bis],
attribute is defined as the reconstructed AS path information (by AS_PATH attribute is defined as the reconstructed AS path
combining AS_PATH and AS4_PATH attributes, if the BGP speaker is a information (by combining AS_PATH and AS4_PATH attributes, if the
NEW speaker and receives the route from an OLD speaker), according BGP speaker is a NEW speaker and receives the route from an OLD
to section 4.2.3 of [RFC6793]. speaker), according to section 4.2.3 of [RFC6793].
o Support for two-octet AS only implementations is out of the scope o Support for two-octet AS only implementations is out of the scope
of this document (i.e. it's assumed that the BGP speaker supports of this document (i.e. it's assumed that the BGP speaker supports
[RFC6793]). [RFC6793]).
6. Topology Considerations 6. Topology Considerations
[RFC5575bis] indicates that the originator may refer to the [I-D.ietf-idr-rfc5575bis] indicates that the originator may refer to
originator path attribute (ORIGINATOR_ID) or (if the attribute is not the originator path attribute (ORIGINATOR_ID) or (if the attribute is
present) the transport address of the peer from which we received the not present) the transport address of the peer from which we received
update. If the latter applies, a network should be designed so it the update. If the latter applies, a network should be designed so
has a congruent topology. it has a congruent topology.
With the additional second condition (b.2) in the validation With the additional second condition (b.2) in the validation
procedure, non-congruent topologies are supported within the local procedure, non-congruent topologies are supported within the local
domain if the Flow Specification is originated within the local domain if the Flow Specification is originated within the local
domain. domain.
Explanation: Explanation:
Consider the following scenarios without the second condition Consider the following scenarios without the second condition
(b.2) being added to the validation procedure: (b.2) being added to the validation procedure:
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the security characteristics of BGP Flow Specifications remain the security characteristics of BGP Flow Specifications remain
equivalent to the existing security properties of BGP unicast equivalent to the existing security properties of BGP unicast
routing. routing.
BGP updates learned from iBGP peers are trusted so the Traffic Flow BGP updates learned from iBGP peers are trusted so the Traffic Flow
Specifications contained in BGP updates are trusted. Therefore it is Specifications contained in BGP updates are trusted. Therefore it is
not required to validate that the originator of an intra-domain not required to validate that the originator of an intra-domain
Traffic Flow Specification matches the originator of the best-match Traffic Flow Specification matches the originator of the best-match
unicast route for the flow destination prefix. This proposal unicast route for the flow destination prefix. This proposal
continues to enforce the validation Procedure for eBGP learned continues to enforce the validation Procedure for eBGP learned
Traffic Flow Specifications, as per [RFC5575bis] rules. In this way, Traffic Flow Specifications, as per [I-D.ietf-idr-rfc5575bis] rules.
the security properties of [RFC5575bis] are maintained such that an In this way, the security properties of [I-D.ietf-idr-rfc5575bis] are
EBGP peer cannot cause a denial-of-service attack by advertising an maintained such that an EBGP peer cannot cause a denial-of-service
inter-domain Flow Specification for a destination prefix that it does attack by advertising an inter-domain Flow Specification for a
not provide reachability information for. destination prefix that it does not provide reachability information
for.
9. Acknowledgements 9. Acknowledgements
The authors would like to thank Han Nguyen for his direction on this The authors would like to thank Han Nguyen for his direction on this
work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen work as well as Waqas Alam, Keyur Patel, Robert Raszuk, Eric Rosen
and Shyam Sethuram for their review comments. and Shyam Sethuram for their review comments.
10. Normative References 10. Normative References
[I-D.ietf-idr-rfc5575bis] [I-D.ietf-idr-rfc5575bis]
Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M. Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
Bacher, "Dissemination of Flow Specification Rules", Bacher, "Dissemination of Flow Specification Rules",
draft-ietf-idr-rfc5575bis-19 (work in progress), January draft-ietf-idr-rfc5575bis-25 (work in progress), May 2020.
2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006, DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>. <https://www.rfc-editor.org/info/rfc4271>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous [RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, System Confederations for BGP", RFC 5065,
DOI 10.17487/RFC5065, August 2007, DOI 10.17487/RFC5065, August 2007,
<https://www.rfc-editor.org/info/rfc5065>. <https://www.rfc-editor.org/info/rfc5065>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet [RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793, Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012, DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>. <https://www.rfc-editor.org/info/rfc6793>.
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