Network Working Group R. Bush Internet-Draft IIJ & Arrcus Updates: 4271 (if approved) K. Patel Intended status: Standards Track Arrcus, Inc. Expires: January 31, 2020 D. Ward Cisco Systems July 30, 2019 Extended Message support for BGP draft-ietf-idr-bgp-extended-messages-34 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 31, 2020. Bush, et al. Expires January 31, 2020 [Page 1] Internet-Draft Extended Message support for BGP July 2019 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. Bush, et al. Expires January 31, 2020 [Page 2] Internet-Draft Extended Message support for BGP July 2019 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 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). Bush, et al. Expires January 31, 2020 [Page 3] Internet-Draft Extended Message support for BGP July 2019 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 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). 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]. Bush, et al. Expires January 31, 2020 [Page 4] Internet-Draft Extended Message support for BGP July 2019 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]. 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 Bush, et al. Expires January 31, 2020 [Page 5] Internet-Draft Extended Message support for BGP July 2019 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, . [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, . [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February 2009, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Bush, et al. Expires January 31, 2020 [Page 6] Internet-Draft Extended Message support for BGP July 2019 10.2. Informative References [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272, DOI 10.17487/RFC4272, January 2006, . [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, . [RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol Specification", RFC 8205, DOI 10.17487/RFC8205, September 2017, . Authors' Addresses 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 Bush, et al. Expires January 31, 2020 [Page 7]