Network Working Group R. Bush
Internet-Draft IIJ & Arrcus
Updates: 4271 (if approved) K. Patel
Intended status: Standards Track Arrcus, Inc.
Expires: January 3, 2020 D. Ward
Cisco Systems
July 2, 2019
Extended Message support for BGP
draft-ietf-idr-bgp-extended-messages-33
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 January 3, 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 . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 send, receive, and properly
handle, see Section 4, BGP Extended Messages.
A peer which does not advertise this capability MUST NOT send BGP
Extended Messages, and BGP Extended Messages MUST NOT be sent to it.
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 sending and 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 advertised by both peers.
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.
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 MUST treat this as a malformed message, and
MUST generate a NOTIFICATION with the Error Subcode set to Bad
Message Length (see [RFC4271] Sec 6.1).
A BGP UPDATE will (policy, best path, etc., allowing) typically
propagate throughout the BGP speaking Internet; and hence to BGP
speakers which may not support Extended Messages. Therefore, an
announcement in an Extended Message where the size of the attribute
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set plus the NLRI is larger than 4,096 octets may cause lack of
reachability.
A BGP speaker with a mixture of peers some of which have advertised
the BGP Extended Message capability and some which have not, may
receive an UPDATE from one of its capable 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 sender 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).
In an iBGP mesh, if BGP Extended Messages are to be advertized, all
peers MUST advertize the BGP Extended Message capability. This is
not only for consistent internal routing, but also to give a
consistent view to eBGP peers.
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].
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.
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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.
As this draft requires support for [RFC7606] UPDATE error handling,
it inherits the security considerations of [RFC7606].
If a remote attacker 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 a
downgrade attack.
If a remote attacker 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. Worse, [RFC7606] "treat-as-withdraw" may
consistently withdraw announcements causing inconsistent routing.
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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
[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>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<http://www.rfc-editor.org/info/rfc4272>.
[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>.
10.2. Informative References
[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
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Randy Bush
IIJ & Arrcus
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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