BGP extended messages: Extended Message Support for BGP
Lets BGP routers exchange messages up to 65,535 octets instead of 4,096, once both sides agree.
Bush, R., Patel, K., and D. Ward, "Extended Message Support for BGP", RFC 8654, DOI 10.17487/RFC8654, October 2019, <https://www.rfc-editor.org/info/rfc8654>.
@misc{rfc8654,
series = {Request for Comments},
number = 8654,
howpublished = {RFC 8654},
publisher = {RFC Editor},
doi = {10.17487/RFC8654},
url = {https://www.rfc-editor.org/info/rfc8654},
author = {R. Bush and K. Patel and D. Ward},
title = {{Extended Message Support for BGP}},
pagetotal = 7,
year = 2019,
month = oct,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Bush
Request for Comments: 8654 Arrcus & IIJ
Updates: 4271 K. Patel
Category: Standards Track Arrcus, Inc.
ISSN: 2070-1721 D. Ward
Cisco Systems
October 2019
Extended Message Support for BGP
Abstract
The BGP specification (RFC 4271) mandates a maximum BGP message size
of 4,096 octets. As BGP is extended to support new Address Family
Identifiers (AFIs), Subsequent AFIs (SAFIs), and other features,
there is a need to extend the maximum message size beyond 4,096
octets. This document updates the BGP specification by extending the
maximum message size from 4,096 octets to 65,535 octets for all
messages except for OPEN and KEEPALIVE messages.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8654.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. BGP Extended Message
3. BGP Extended Message Capability
4. Operation
5. Error Handling
6. Changes to RFC 4271
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
The BGP specification [RFC4271] mandates a maximum BGP message size
of 4,096 octets. As BGP is extended to support new AFIs, SAFIs, and
other capabilities (e.g., BGPsec [RFC8205] and BGP - Link State (BGP-
LS) [RFC7752]), there is a need to extend the maximum message size
beyond 4,096 octets. This document provides an extension to BGP to
extend the message size limit from 4,096 octets to 65,535 octets for
all messages except for OPEN and KEEPALIVE messages.
1.1. 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. BGP Extended Message
A BGP message over 4,096 octets in length is a BGP Extended Message.
BGP Extended Messages have a maximum message size of 65,535 octets.
The smallest message that may be sent is a BGP KEEPALIVE, which
consists of 19 octets.
3. BGP Extended Message Capability
The BGP Extended Message Capability is a new BGP capability [RFC5492]
defined with Capability Code 6 and Capability Length 0.
To advertise the BGP Extended Message Capability to a peer, a BGP
speaker uses BGP Capabilities Advertisement [RFC5492]. By
advertising the BGP Extended Message Capability to a peer, a BGP
speaker conveys that it is able to receive and properly handle BGP
Extended Messages (see Section 4).
Peers that wish to use the BGP Extended Message Capability MUST
support error handling for BGP UPDATE messages per [RFC7606].
4. Operation
The BGP Extended Message Capability applies to all messages except
for OPEN and KEEPALIVE messages. These exceptions reduce the
complexity of providing backward compatibility.
A BGP speaker that is capable of receiving BGP Extended Messages
SHOULD advertise the BGP Extended Message Capability to its peers
using BGP Capabilities Advertisement [RFC5492]. A BGP speaker MAY
send BGP Extended Messages to a peer only if the BGP Extended Message
Capability was received from that peer.
An implementation that advertises the BGP Extended Message Capability
MUST be capable of receiving a message with a length up to and
including 65,535 octets.
Applications generating information that might be encapsulated within
BGP messages MUST limit the size of their payload to take the maximum
message size into account.
If a BGP message with a length greater than 4,096 octets is received
by a BGP listener who has not advertised the BGP Extended Message
Capability, the listener will generate a NOTIFICATION with the Error
Subcode set to Bad Message Length ([RFC4271], Section 6.1).
A BGP UPDATE will (if allowed by policy, best path, etc.) typically
propagate throughout the BGP-speaking Internet and hence to BGP
speakers that may not support BGP Extended Messages. Therefore, an
announcement in a BGP Extended Message where the size of the
attribute set plus the NLRI is larger than 4,096 octets may cause
lack of reachability.
A BGP speaker that has advertised the BGP Extended Message Capability
to its peers may receive an UPDATE from one of its peers that
produces an ongoing announcement that is larger than 4,096 octets.
When propagating that UPDATE onward to a neighbor that has not
advertised the BGP Extended Message Capability, the speaker SHOULD
try to reduce the outgoing message size by removing attributes
eligible under the "attribute discard" approach of [RFC7606]. If the
message is still too big, then it must not be sent to the neighbor
([RFC4271], Section 9.2). Additionally, if the NLRI was previously
advertised to that peer, it must be withdrawn from service
([RFC4271], Section 9.1.3).
If an Autonomous System (AS) has multiple internal BGP speakers and
also has multiple external BGP neighbors, care must be taken to
ensure a consistent view within the AS in order to present a
consistent external view. In the context of BGP Extended Messages, a
consistent view can only be guaranteed if all the Internal BGP (iBGP)
speakers advertise the BGP Extended Message Capability. If that is
not the case, then the operator should consider whether or not the
BGP Extended Message Capability should be advertised to external
peers.
During the incremental deployment of BGP Extended Messages and use of
the "attribute discard" approach of [RFC7606] in an iBGP mesh or with
External BGP (eBGP) peers, the operator should monitor any routes
dropped and any discarded attributes.
5. Error Handling
A BGP speaker that has the ability to use BGP Extended Messages but
has not advertised the BGP Extended Message Capability, presumably
due to configuration, MUST NOT accept a BGP Extended Message. A
speaker MUST NOT implement a more liberal policy accepting BGP
Extended Messages.
A BGP speaker that does not advertise the BGP Extended Message
Capability might also genuinely not support BGP Extended Messages.
Such a speaker will follow the error-handling procedures of [RFC4271]
if it receives a BGP Extended Message. Similarly, any speaker that
treats an improper BGP Extended Message as a fatal error MUST follow
the error-handling procedures of [RFC4271].
Error handling for UPDATE messages, as specified in Section 6.3 of
[RFC4271], is unchanged. However, if a NOTIFICATION is to be sent to
a BGP speaker that has not advertised the BGP Extended Message
Capability, the size of the message MUST NOT exceed 4,096 octets.
It is RECOMMENDED that BGP protocol developers and implementers are
conservative in their application and use of BGP Extended Messages.
Future protocol specifications MUST describe how to handle peers that
can only accommodate 4,096 octet messages.
6. Changes to RFC 4271
[RFC4271] states "The value of the Length field MUST always be at
least 19 and no greater than 4096." This document changes the latter
number to 65,535 for all messages except for OPEN and KEEPALIVE
messages.
Section 6.1 of [RFC4271] specifies raising an error if the length of
a message is over 4,096 octets. For all messages except for OPEN and
KEEPALIVE messages, if the receiver has advertised the BGP Extended
Message Capability, this document raises that limit to 65,535.
7. IANA Considerations
IANA has made the following allocation in the "Capability Codes"
registry:
+-------+----------------------+-----------+
| Value | Description | Reference |
+=======+======================+===========+
| 6 | BGP Extended Message | RFC 8654 |
+-------+----------------------+-----------+
Table 1: Addition to "Capability Codes"
Registry
8. Security Considerations
This extension to BGP does not change BGP's underlying security
issues [RFC4272].
Due to increased memory requirements for buffering, there may be
increased exposure to resource exhaustion, intentional or
unintentional.
If a remote speaker is able to craft a large BGP Extended Message to
send on a path where one or more peers do not support BGP Extended
Messages, peers that support BGP Extended Messages may:
* act to reduce the outgoing message (see Section 4) and, in doing
so, cause an attack by discarding attributes one or more of its
peers may be expecting. The attributes eligible under the
"attribute discard" approach must have no effect on route
selection or installation [RFC7606].
* act to reduce the outgoing message (see Section 4) and, in doing
so, allow a downgrade attack. This would only affect the
attacker's message, where 'downgrade' has questionable meaning.
* incur resource load (processing, message resizing, etc.) when
reformatting the large messages.
9. References
9.1. Normative References
[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>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[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>.
9.2. Informative References
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[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>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
Acknowledgments
The authors thank Alvaro Retana for an amazing review; Enke Chen,
Susan Hares, John Scudder, John Levine, and Job Snijders for their
input; and Oliver Borchert and Kyehwan Lee for their implementations
and testing.
Authors' Addresses
Randy Bush
Arrcus & IIJ
5147 Crystal Springs
Bainbridge Island, WA 98110
United States of America
Email: randy@psg.com
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Dave Ward
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
United States of America
Email: dward@cisco.com
AI ELI5
On this page
§1Why 4,096 Octets Is Not Enough
RFC 4271 requires a maximum BGP message size of 4,096 octets. As BGP is extended to carry new address families (AFIs), subsequent address families (SAFIs) and other features, for example BGPsec (RFC 8205) and BGP-LS (RFC 7752), that limit becomes too small.
This RFC extends the limit from 4,096 octets to 65,535 octets for all messages except OPEN and KEEPALIVE. A message longer than 4,096 octets is called a BGP Extended Message.
BGP routers talk using messages, and the old rule said a message could not be longer than one small page. As routers had more to say, one page was not enough.
§2The Extended Message Capability
Support is announced with a new BGP capability, the BGP Extended Message Capability, defined with Capability Code 6 and Capability Length 0. A speaker advertises it using the Capabilities Advertisement mechanism of RFC 5492. Advertising it tells the peer that the speaker can receive and properly handle Extended Messages.
A speaker that can receive Extended Messages SHOULD advertise the capability. It MAY send Extended Messages to a peer only if it received the capability from that peer. Anyone who advertises it MUST be able to receive a message up to and including 65,535 octets. Peers that want to use it MUST also support the UPDATE error handling of RFC 7606.
Before sending a long letter, a router checks that the other router has said, 'I can read long letters.' If it has not said so, the router sticks to short letters.
§3Sending and Propagating Large Messages
The capability applies to all messages except OPEN and KEEPALIVE; these exceptions reduce the complexity of backward compatibility. The smallest message is still a KEEPALIVE of 19 octets. Applications that put information into BGP messages MUST limit their payload with the maximum message size in mind.
An UPDATE usually spreads across many networks, including to speakers that do not support Extended Messages, so an announcement whose attribute set plus NLRI exceeds 4,096 octets may cause lack of reachability. When a speaker must pass such an UPDATE on to a neighbor that has not advertised the capability, it SHOULD first try to shrink it by removing attributes eligible under the 'attribute discard' approach of RFC 7606. If it is still too big, it must not be sent, and if that prefix was advertised to the peer before, it must be withdrawn.
Inside an AS, a consistent external view can only be guaranteed if all iBGP speakers advertise the capability. Otherwise the operator should consider whether to advertise it to external peers, and should monitor dropped routes and discarded attributes during rollout.
If a long letter has to pass through a router that can only read short ones, the sender first crosses out the unimportant lines. If it is still too long, it is not sent at all.
| Message | Maximum length |
|---|---|
| OPEN | 4,096 octets |
| KEEPALIVE | 4,096 octets |
| Other messages, peer did not advertise the capability | 4,096 octets |
| Other messages, peer advertised the capability THIS RFC | 65,535 octets |
§4Error Handling
A speaker that could use Extended Messages but did not advertise the capability, for example because of configuration, MUST NOT accept one, and MUST NOT adopt a more liberal policy. A listener that has not advertised the capability and receives a message longer than 4,096 octets generates a NOTIFICATION with the error subcode Bad Message Length.
UPDATE error handling from Section 6.3 of RFC 4271 is unchanged. However, a NOTIFICATION sent to a speaker that has not advertised the capability MUST NOT exceed 4,096 octets.
The RFC also recommends that implementers be conservative in using Extended Messages, and requires future protocol specifications to say how to handle peers that can only take 4,096-octet messages.
If a router did not say it could read long letters, it must refuse them, and any complaint sent back to it has to stay short too.
§5Changes to RFC 4271
RFC 4271 says the value of the Length field MUST always be at least 19 and no greater than 4096. This document changes the upper number to 65,535 for all messages except OPEN and KEEPALIVE.
Likewise, Section 6.1 of RFC 4271 requires an error when a message is over 4,096 octets. For every message except OPEN and KEEPALIVE, that limit rises to 65,535 when the receiver has advertised the capability. IANA assigned value 6, 'BGP Extended Message', in the Capability Codes registry.
The old rulebook said 'no longer than one page'. This RFC adds a note: 'unless both sides agreed to longer ones'.
§6Security Considerations
The extension does not change BGP's underlying security issues (see RFC 4272). Because larger messages need more buffer memory, there may be more exposure to resource exhaustion, intentional or not.
If a remote speaker crafts a large message on a path where some peers lack support, a speaker that shrinks the message by discarding attributes could break something a peer expected, or enable a downgrade attack, which would only affect the attacker's own message. Reformatting large messages also costs processing. The discardable attributes must have no effect on route selection or installation.
Bigger letters need bigger mailboxes, so someone could try to flood a router with huge letters. Cutting down a letter on its way can also drop a detail someone was waiting for.