RFC 8654

RFC 8654: Extended Message Support for BGP 🦋

Okay bestie, let's get the title block out of the way first 💅 This is an Internet Standards Track doc from the IETF (Internet Engineering Task Force) 🌈, so it's the real deal and not just a suggestion. It's dated October 2019, and its ISSN is 2070-1721. The authors are R. Bush (Arrcus & IIJ), K. Patel (Arrcus, Inc.), and D. Ward (Cisco Systems) 👑. It updates RFC 4271 (the BGP-4 spec) and doesn't obsolete anything ✨

Abstract ✨

Okay so the BGP spec (RFC 4271) says a BGP message can be at most 4,096 octets 😭 But BGP keeps picking up new AFIs, SAFIs, and other features, so that limit is lowkey too small now. This doc updates BGP by raising the max message size from 4,096 octets to 65,535 octets for all messages except OPEN and KEEPALIVE 🌸

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 spec [RFC4271] says a BGP message can be at most 4,096 octets 🙈 But BGP keeps getting extended for new AFIs, SAFIs, and other capabilities. Examples are BGPsec [RFC8205] and BGP - Link State (BGP-LS) [RFC7752]. Basically, there's a real need to go past 4,096 octets. This doc gives BGP an extension that raises the message size limit from 4,096 octets to 65,535 octets for all messages except OPEN and KEEPALIVE 💖

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. It's giving main character energy 👑

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]. Non-negotiable bestie 😭

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. Keep an eye on those drops, babe 🔍

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. No sneaking it in, periodt 🙅‍♀️

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. Be careful out there, bestie 🌸

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. 💜

9.2. Informative References 📚

okay so these are the background reads 🧚, not required reading for the spec itself. bestie, the vibes are very "go deeper if you're curious" 💅

[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 gave a huge shoutout to Alvaro Retana for an absolutely amazing review 🔥 and to Enke Chen, Susan Hares, John Scudder, John Levine, and Job Snijders for their input ✨. they also thanked Oliver Borchert and Kyehwan Lee for their implementations and testing 🙌. the people who built and tested the thing deserve all the flowers 💐

Authors' Addresses 🎀

meet the authors, bestie 💖

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