Internet Domain Routing S. Chakrabarti Internet-Draft IP Infusion - An Access Company Intended status: Standards Track March 2008 Expires: September 2, 2008 A proposal for modification of BGP 4-octet AS number usage draft-chakrabarti-idr-rfc4893-mod-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 2, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract RFC 4893 defines BGP support for four-octet AS number space. This document proposes clarification texts for RFC 4893 for clear understanding of the transition behavior between existing implementations with two-octet AS numbers and the new BGP implementations with four-octet AS numbers. This document also proposes an addition of notification message and clearly defines the processing of "My AS Number" field in the BGP OPEN message for better interoperability during the transition phase of two-octet and four- Chakrabarti Expires September 2, 2008 [Page 1] Internet-Draft RFC 4893 Calrification March 2008 octet compliant BGP speakers. Table of Contents 1. Requirements notation . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Clarification issue-I . . . . . . . . . . . . . . . . . . . . . 3 4.1. Proposal for new text . . . . . . . . . . . . . . . . . . . 4 5. Clarification - issue-2 . . . . . . . . . . . . . . . . . . . . 4 5.1. Proposal for new text . . . . . . . . . . . . . . . . . . . 5 6. Clarification - issue-3 . . . . . . . . . . . . . . . . . . . . 5 6.1. Proposal for change in protocol . . . . . . . . . . . . . . 6 7. Calrification issue-4 . . . . . . . . . . . . . . . . . . . . . 6 8. Proposal for a NOTIFICATION message . . . . . . . . . . . . . . 6 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 11. Normative References . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 8 Chakrabarti Expires September 2, 2008 [Page 2] Internet-Draft RFC 4893 Calrification March 2008 1. Requirements notation 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 [3]. 2. Introduction RFC 4893[1] defines the extensions to BGP in order to use 4-byte autonomous system (AS) number and it also describes the behavior of BGP speakers with 4-byte AS numbers and the existing BGP speakers with 2-byte AS numbers for ease of transitions. However, the specification requires more clarity in handling the AS numbers in OPEN and UPDATE messages between the 2-byte AS number speakers and 4-byte AS number speakers. Without the clear understanding of handling of these messages the existing and new implementations of BGP speakers may fail to interoperate or may degrade routing services over the Internet. This document is initiated based on some questions raised during an implementation of RFC 4893. Thus the goal of this document is to point out the areas of clarification required in the 4-byte AS number specification[1]. Besides the clarification text, it also proposes a notification message and clearly defines the processing of "MY AS Number" field in BGP[2] when 4-byte AS number capability message is present. 3. Terminology OLD BGP Speaker: A BGP speaker which is RFC 4271[2] compliant and does not implement 4byte extension to the AS number as defined in RFC 4893. NEW BGP Speaker: A BGP speaker which implements the 4-byte AS number support as defined in RFC 4893. 4. Clarification issue-I RFC4893 is unclear about the processing of "My AS Number" field in the OPEN message[2]. Section 3 mentions about the capability message for 4byte ASN support: "The Capability that is used by a BGP speaker to convey to its BGP peer the 4-octet Autonomous System number capability, also carries the 4-octet Autonomous System number of the speaker in the Capability Value field of the Capability Optional Parameter. The Capability Length field of the Capability is set to Chakrabarti Expires September 2, 2008 [Page 3] Internet-Draft RFC 4893 Calrification March 2008 4. " and "We denote this special AS number as AS_TRANS for ease of description in the rest of this specification. This AS number is also placed in the "My Autonomous System" field of the OPEN message originated by a NEW BGP speaker, if the speaker does not have a (globally unique) 2-octet AS number." The questions are : 1) When 4-byte AS number capability message is present and the receiver is able to process the capability message, should it ignore the AS number field in the OPEN message? [ note: 2-byte mappable As Numbered BGP speaker may send 4-byte AS capability support] 4.1. Proposal for new text A separate section on handling OPEN message would be very useful. A suggested text is below. Processing and sending OPEN message: 1) Sending OPEN message:If the BGP speaker has a 2byte AS number or 2-byte mappable 4-byte AS number, it uses the 2 byte ASN in the "My AS number" field of OPEN message. If the BGP speaker has a 4-byte non-mappable AS number, then it uses AS_TRANS in "My AS Number" field of OPEN message. 2) Receiving OPEN message: If a NEW BGP speaker receives a OPEN message with extended AS number capability, then it uses the 4-byte AS number from the extended AS capability message and may disregard the value in the "My AS number" field in the OPEN message. If there is no extended AS capability is present and the OPEN messge "My AS number" field contains AS_TRANS, then the NEW BGP implementation sends a notification message to the peer and closes connection. An OLD BGP speaker-implementation is not aware of the extended AS number capability; it processes the OPEN message as per RFC 4271. 5. Clarification - issue-2 Currently, in section 4.2.1 RFC 4893 states: "Note that peering between a NEW BGP speaker and an OLD one is possible only if the NEW BGP speaker has a 2-octet AS number. However, this document does not assume that an Autonomous System with NEW speakers has to have a globally unique 2-octet AS number - AS_TRANS could be used instead (even if a multiple Autonomous System would use it)." Chakrabarti Expires September 2, 2008 [Page 4] Internet-Draft RFC 4893 Calrification March 2008 R1 R2 R3 R4 o-------------------o-------------------o----------------------o OLD NEW OLD NEW (50) (77777) (100) (65666) A scenario with OLD and NEW BGP speakers In the above scenario, if both R2 and R4 peer with R3, R3 configuration may assume that R4 and R2 are part of same AS. This may cause R3 to make undesirable routing decision. Some clarification/recommendation is required in this case. 5.1. Proposal for new text Note that peering between a NEW BGP speaker and an OLD one is possible only if the NEW BGP speaker has a 2-octet AS number or a 2-octet mappable extended AS number. However, this document does not assume that an Autonomous System with NEW speakers has to have a globally unique 2-octet AS number - AS_TRANS could be used instead;careful considerations are required such that it does not affect the routing path of the traffic due to some local policy on AS number at the OLD BGP speaker. During transition to NEW BGP speaker from an OLD BGP speaker, the above scenario should be avoided. 6. Clarification - issue-3 Section 3 of RFC4893 states: "NEW BGP speakers carry AS path information expressed in terms of 4-octet Autonomous Systems numbers by using the existing AS_PATH attribute, except that each AS number in this attribute is encoded not as a 2-octet, but as a 4-octet entity." R1 R2 R3 R4 o-------------------o-------------------o----------------------o NEW NEW OLD NEW (77777) (65666) (100) (200) 2nd scenario with OLD and NEW BGP speakers According to the current specification, R1 will send AS_PATH with 4-byte AS numbers to R2. Since R2 is peering with an OLD BGP speaker, it will make the conversion of 4-byte AS_PATH attributes to 2-byte AS_PATH attributes and pass them to R3 along with AS4_PATH attributes. Chakrabarti Expires September 2, 2008 [Page 5] Internet-Draft RFC 4893 Calrification March 2008 Since OLD and NEW BGP speakers will exist in the network for a long time, it might be clean to use 4-byte numbers in AS4_PATH attributes only and corresponding value AS_TRANS in AS_PATH attribute even when two NEW BGP peers with non-mappable 4-byte AS number exchange information. It also simplifies the NEW BGP speaker implementation and processing of AS_PATH. This simplifies the NEW BGP implementation and saves the extra time in processing an UPDATE message. 6.1. Proposal for change in protocol A NEW BGP speaker with 4-byte AS number always includes AS4_PATH attribute containing the extended 4-byte AS number. If the AS number is 2-byte mappable, then it adds the corresponding 2-byte mapped AS number in the AS_PATH attribute, otherwise it uses AS_TRANS as the AS number in the corresponding AS_PATH attribute. Thus the NEW BGP speaker will always have AS4_PATH and a corresponding AS_PATH attribute. Following a complete transition to 4-byte AS numbered systems, AS_PATH may be replaced by AS4_PATH by turning a configuration knob on each system. Thus a NEW BGP implementation may consider providing a configuration knob which disables AS_PATH attribute sending and processing. 7. Calrification issue-4 Minor nit: "truly 4-octet" should be defined as a quantity higher than 65535. Should the NEW BGP speaker send a NOTIFICATION message when it receives a OPEN message with AS_TRANS but without any corresponding capability message ? Note that although AS_TRANS(23456) is a reserved number now, it is still possible to receive a OPEN message with AS_TRANS value from an OLD BGP speaker or from a ill-behaving NEW BGP speaker. 8. Proposal for a NOTIFICATION message When two BGP speakers correspond with each other by sending AS_TRANS value in the 'My AS number' field, then the OPEN message MUST contain the 4-octet AS number capability option. If the 4-octet capability is missing in OPEN message where the 'My AS Number' field contains AS_TRANS value, a NEW BGP speaker-receiver SHOULD send a notification with code=2, subcode=2 [bad peer AS] to the sender of the OPEN message. If an OLD BGP speaker receives a OPEN message with AS_TRANS value in Chakrabarti Expires September 2, 2008 [Page 6] Internet-Draft RFC 4893 Calrification March 2008 the 'My AS number' field it should treat it normally as per RFC 4271 and local policy. 9. IANA Considerations This document has no actions for IANA. 10. Acknowledgements 11. Normative References [1] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS Number Space", RFC 4893, May 2007. [2] Rekhter, Y., Li, T., and S. Hares, "Border Gateway Protocol 4", RFC 4271, January 2006. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Author's Address Samita Chakrabarti IP Infusion - An Access Company 125 S. Market Street San Jose USA Email: samitac@ipinfusion.com Chakrabarti Expires September 2, 2008 [Page 7] Internet-Draft RFC 4893 Calrification March 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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