Internet Engineering Task Force E. Chen Internet-Draft Cisco Systems Updates: 4271 (if approved) J. Scudder Intended status: Standards Track Juniper Networks Expires: January 27, 2020 July 26, 2019 Extended Optional Parameters Length for BGP OPEN Message draft-ietf-idr-ext-opt-param-06 Abstract The Optional Parameters in the BGP OPEN message as defined in the base BGP specification are limited to 255 octets due to a one-octet length field. BGP Capabilities are carried in this field and may foreseeably exceed 255 octets in the future, leading to concern about this limitation. In this document we update RFC 4271 by extending the BGP OPEN length field in a backward-compatible manner. The Parameter Length field of individual Optional Parameters is similarly extended. 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 27, 2020. 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 Chen & Scudder Expires January 27, 2020 [Page 1]
Internet-Draft Extended Optional Parameters Length July 2019 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. 1. Introduction The Optional Parameters Length field in the BGP OPEN message is defined in the base BGP specification [RFC4271] as one octet, thus limiting the Optional Parameters field in the OPEN message to 255 octets. As BGP Capabilities [RFC5492] are carried in the Optional Parameters field, and new BGP capabilities continue to be introduced, the limitation is becoming a concern for BGP development. In this document we update [RFC4271] by extending the BGP OPEN length field in a backward-compatible manner. The Parameter Length field of individual Optional Parameters is similarly extended. This is done by using Optional Parameters Length of 255 combined with Optional Parameter Type 255 as a distinguished value pair, which indicates that an extended Optional Parameters Length field follows. In this case the Parameter Length field of the Optional Parameters in the BGP OPEN message is also extended. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2. Protocol Extensions This document reserves Optional Parameter Type code 255 as the "Extended Length" type code. In the event that the length of Optional Parameters in the BGP OPEN message does not exceed 255, the encodings of the base BGP specification [RFC4271] MUST be used without alteration. However, if the length of Optional Parameters is greater than 255, an extended encoding is used. The (non-extended) length field is set to 255, as is the subsequent octet that in the non-extended format would be the first Optional Parameter Type. The subsequent two octets carry the actual length. In addition, the "Parameter Length" field of each Optional Parameter is enlarged to two octets. Other than the larger sizes of the given fields, there is no change to the BGP OPEN message defined in [RFC4271]. Chen & Scudder Expires January 27, 2020 [Page 2]
Internet-Draft Extended Optional Parameters Length July 2019 When receiving an OPEN, a BGP speaker determines the extended encoding is in use if the first Optional Parameter Type field is 255. In this case, the BGP speaker MUST ignore the non-extended Optional Parameters Length field, and must instead rely on the Extended Optional Parameters Length field. Accordingly, when the length of Optional Parameters in the BGP OPEN message is greater than 255, the OPEN message format is modified as follows, repurposing the Optional Parameters Length field as well as the first Optional Parameter Type field to indicate the use of the extended format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+ | Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | My Autonomous System | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hold Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | BGP Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Non-Ext OP Len.|Non-Ext OP Type| Extended Opt. Parm. Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Optional Parameters (variable) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The non-extended Optional Parameters Length field MUST be set to 255 on transmission, and MUST be ignored on receipt once the use of the extended format is determined. The subsequent one-octet field, that in the non-extended format would be the first Optional Parameter Type field, MUST be set to 255 on transmission. On receipt, a value of 255 for this field is the indication that the extended format is in use. In this extended encoding, the subsequent two-octet field, termed the Extended Optional Parameters Length field, is an unsigned integer indicating the total length of the Optional Parameters field in octets. If the value of this field is zero, no Optional Parameters are present (this would never be expected to happen with the extended encoding, however). Likewise, in that situation the Optional Parameters encoding is modified to be the following: Chen & Scudder Expires January 27, 2020 [Page 3]
Internet-Draft Extended Optional Parameters Length July 2019 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Parm. Type | Parameter Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Parameter Value (variable) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The rules for encoding Optional Parameters are unchanged with respect to those given in [RFC4271] other than the extension of the Parameter Length field to be a two-octet unsigned integer. In parsing an OPEN message, a BGP speaker MUST use the value of the one-octet "Optional Parameters Length" field and the value of the octet following it to determine the encoding of the Optional Parameters length, as well as the size of the "Parameter Length" field of the Optional Parameters. If both values are 255, then the four-octet encoding described above is used for the Optional Parameters length. Otherwise the encoding defined in [RFC4271] is used. This encoding is chosen for backward compatibility reasons -- a BGP speaker which has not been upgraded to support this specification may legitimately send Optional Parameters whose length equals exactly 255, thus the Optional Parameters Length field alone is insufficient as an indicator. However, such a speaker would never legitimately send an Optional Parameter whose type code is 255, since that value has been reserved by this specification. 3. Errors If a BGP speaker supporting this specification (a "new speaker") is peering with one which does not (an "old speaker") no interoperability issues arise unless the new speaker needs to encode Optional Parameters whose length exceeds 255. In that case, it will transmit an OPEN message which the old speaker will interpret as containing an Optional Parameter with type code 255. Since by definition the old speaker will not recognize that type code, the old speaker may be expected to close the connection with a NOTIFICATION with an Error Code of OPEN Message Error and an Error Subcode of Unsupported Optional Parameters, according to Section 6.2 of [RFC4271]. Although the above is the most likely error to be sent, it is not impossible that the old speaker might detect some other error first, such as a length error, depending on the details of the implementation. In no case would the peering be expected to Chen & Scudder Expires January 27, 2020 [Page 4]
Internet-Draft Extended Optional Parameters Length July 2019 establish successfully; the only question is which NOTIFICATION would be generated. We note that in any case, such a peering could not succeed, since by definition the extended length encoding would not be used by the new speaker unless the basic encoding was insufficient. Although the Optional Parameter Type code 255 is used in this specification as the indication that the extended encoding is in use, it is not a bonafide Optional Parameter Type in the usual sense, and MUST NOT be used other than as described above. If encountered as an actual Optional Parameter Type code, it MUST be treated as an unrecognized Optional Parameter and handled according to [RFC4271] Section 6.2. 4. IANA Considerations IANA is requested to designate BGP OPEN Optional Parameter Type code 255 as the Extended Length type code. 5. Security Considerations This extension to BGP does not change the underlying security issues inherent in the existing BGP [RFC4272]. 6. Acknowledgements The authors would like to thank Yakov Rekhter and Srihari Sangli for discussing various options to enlarge the Optional Parameters field. We would also like to thank Matthew Bocci, Jakob Heitz, Pradosh Mohapatra, Keyur Patel and Hannes Gredler for their valuable comments. 7. References 7.1. 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>. Chen & Scudder Expires January 27, 2020 [Page 5]
Internet-Draft Extended Optional Parameters Length July 2019 7.2. Informative References [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272, DOI 10.17487/RFC4272, January 2006, <https://www.rfc-editor.org/info/rfc4272>. [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>. Authors' Addresses Enke Chen Cisco Systems Email: enkechen@cisco.com John Scudder Juniper Networks Email: jgs@juniper.net Chen & Scudder Expires January 27, 2020 [Page 6]
