Internet DRAFT - draft-ietf-idr-bgp-extended-messages
draft-ietf-idr-bgp-extended-messages
Network Working Group R. Bush
Internet-Draft Arrcus & IIJ
Updates: 4271 (if approved) K. Patel
Intended status: Standards Track Arrcus, Inc.
Expires: February 17, 2020 D. Ward
Cisco Systems
August 16, 2019
Extended Message support for BGP
draft-ietf-idr-bgp-extended-messages-36
Abstract
The BGP specification mandates a maximum BGP message size of 4,096
octets. As BGP is extended to support newer AFI/SAFIs and other
features, there is a need to extend the maximum message size beyond
4,096 octets. This document updates the BGP specification RFC4271 by
extending the maximum message size from 4,096 octets to 65,535 octets
for all except the OPEN and KEEPALIVE messages.
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.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 17, 2020.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. BGP Extended Message . . . . . . . . . . . . . . . . . . . . 2
3. Extended Message Capability for BGP . . . . . . . . . . . . . 3
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 4
6. Changes to RFC4271 . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The BGP specification [RFC4271] mandates a maximum BGP message size
of 4,096 octets. As BGP is extended to support newer AFI/SAFIs and
newer capabilities (e.g., BGPsec [RFC8205] and BGP-LS [RFC7752]),
there is a need to extend the maximum message size beyond 4,096
octets. This draft provides an extension to BGP to extend its
message size limit from 4,096 octets to 65,535 octets for all except
the OPEN and KEEPALIVE messages.
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 consists of a BGP KEEPALIVE
which consists of 19 octets.
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3. Extended Message Capability for BGP
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, see
Section 4, BGP Extended Messages.
Peers that wish to use the BGP Extended Message capability MUST
support Error Handling for BGP UPDATE Messages per [RFC7606].
4. Operation
The Extended Message Capability applies to all messages except for
the OPEN and KEEPALIVE messages. The former exception is to 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 Extended Messages to a peer only if the 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 which might be encapsulated
within BGP messages MUST limit the size of their payload to take the
maximum message size into account.
During the years of incremental deployment, speakers that are capable
of Extended Messages should not simply pack as many NLRI in a message
as they can, or otherwise unnecessarily generate UPDATES above the
4,096 octet pre- Extended Message limit, so as not to require
downstream routers to decompose for peers that do not support
Extended Messages. See Section 8.
If a BGP message with a Length greater than 4,096 octets is received
by a BGP listener who has not advertised the Extended Message
Capability, the listener will generate a NOTIFICATION with the Error
Subcode set to Bad Message Length ([RFC4271] Sec 6.1).
A BGP UPDATE will (policy, best path, etc., allowing) typically
propagate throughout the BGP speaking Internet; and hence to BGP
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speakers which may not support Extended Messages. Therefore, an
announcement in an 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 which 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, to present a consistent
external view care must be taken to ensure a consistent view within
the AS. In the context of BGP Extended Messages, a consistent view
can only be guaranteed if all the iBGP speakers advertise the BGP
Extended Message capability. If that is not the case, then the
operator should consider whether the BGP Extended Message capability
should be advertised to external peers or not.
During the incremental deployment of BGP Extended Messages and
[RFC7606] in an iBGP mesh, or with 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 Extended Messages but has
not advertised the BGP Extended Messages capability, presumably due
to configuration, MUST NOT accept an 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 Messages
capability might also genuinely not support Extended Messages. Such
a speaker will follow the error handling procedures of [RFC4271] if
it receives an Extended Message. Similarly, any speaker that treats
an improper Extended Message as a fatal error, MUST follow the error
handling procedures of [RFC4271].
The UPDATE Message Error Handling, as specified in Section 6.3 of
[RFC4271], is unchanged. However, if a NOTIFICATION is to be sent to
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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 Extended Messages.
Future protocol specifications MUST describe how to handle peers
which can only accommodate 4,096 octet messages.
6. Changes to RFC4271
[RFC4271] states "The value of the Length field MUST always be at
least 19 and no greater than 4,096." This document changes the
latter number to 65,535 for all except the OPEN and KEEPALIVE
messages.
[RFC4271] Sec 6.1, specifies raising an error if the length of a
message is over 4,096 octets. For all messages except the OPEN
message, if the receiver has advertised the BGP Extended Messages
Capability, this document raises that limit to 65,535.
7. IANA Considerations
The IANA has made an early allocation for this new BGP Extended
Message Capability referring to this document.
Registry: Capability Codes
Value Description Document
----- ----------------------------------- -------------
6 BGP Extended Message [this draft]
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 which support BGP Extended Messages may act to reduce
the outgoing message, see Section 4, and in doing so cause an attack
by discarding attributes its peer may be expecting. The attributes
eligible under the "attribute discard" must have no effect on route
selection or installation [RFC7606].
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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 which support BGP Extended Messages may 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.
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 which support BGP Extended Messages may incur
resource load (processing, message resizing, etc.) reformatting the
large messages.
9. 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.
10. References
10.1. 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,
<http://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,
<http://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, <http://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,
<http://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, <http://www.rfc-editor.org/info/rfc8174>.
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10.2. Informative References
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<http://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,
<http://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>.
Authors' Addresses
Randy Bush
Arrcus & IIJ
5147 Crystal Springs
Bainbridge Island, Washington 98110
US
Email: randy@psg.com
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Dave Ward
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
US
Email: dward@cisco.com
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