Network Working Group Q. Zeng Internet-Draft J. Dong Intended status: Standards Track Huawei Technologies Expires: January 5, 2012 July 4, 2011 Maximum Transmission Unit Extended Community for BGP-4 draft-zeng-idr-bgp-mtu-extension-00 Abstract Proper functioning of [RFC1191] path Maximum Transmission Unit (MTU) discovery requires that IP routers have knowledge of the MTU for each link to which they are connected. As MPLS progresses, [RFC3988] specifies some extensions to LDP in support of LDP LSP MTU discovery. For the LSP created using Border Gateway Protocol (BGP) [RFC3107], it does not have the ability to signal the path MTU to the ingress Label Switching Router (LSR). In the absence of this functionality, the MTU for the BGP LSP must be statically configured by network operators or by equivalent off-line mechanisms. This document defines the MTU Extended Community for BGP in support of BGP LSP MTU discovery. 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]. 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 http://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 5, 2012. Copyright Notice Zeng & Dong Expires January 5, 2012 [Page 1] Internet-Draft MTU Extended Community for BGP-4 July 2011 Copyright (c) 2011 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 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. BGP LSP MTU Discovery . . . . . . . . . . . . . . . . . . . . . 3 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. MTU Extended Community . . . . . . . . . . . . . . . . . . 3 2.3. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Applicability Considerations . . . . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . . 5 8.2. Informative References . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 Zeng & Dong Expires January 5, 2012 [Page 2] Internet-Draft MTU Extended Community for BGP-4 July 2011 1. Introduction Proper functioning of [RFC1191] path Maximum Transmission Unit (MTU) discovery requires that IP routers have knowledge of the MTU for each link to which they are connected. As MPLS progresses, [RFC3988] specifies some extensions to LDP in support of LDP LSP MTU discovery. For the LSP created using Border Gateway Protocol (BGP) [RFC3107], it does not have the ability to signal the path MTU to the ingress Label Switching Router (LSR). Without knowledge of the path MTU of the whole BGP LSP, ingress BGP LSRs may transmit packets along that LSP which are either too big or too small, thus these packets may be silently discarded by LSRs or inefficient transmitted. In the absence of this functionality, the MTU for each BGP LSP must be statically configured by network operators or by equivalent off-line mechanisms. This document defines the MTU Extended Community for BGP in support of BGP LSP MTU discovery. 2. BGP LSP MTU Discovery 2.1. Definitions BGP LSP Path MTU: The Path MTU of the LSP from given BGP LSR to a specific prefix. It is carried as a Extended Community with the BGP labeled IPv4 (or IPv6) route. BGP LSR Link MTU: If the two BGP LSRs are directly adjacent, the BGP LSR Link MTU is the MTU of the interface; If the two BGP LSRs are not directly adjacent, the BGP LSR Link MTU is the Path MTU of the underlying tunnel. If there are multiple links between the two BGP LSRs, the BGP LSR Link MTU is the minimum of those link MTUs. 2.2. MTU Extended Community BGP LSP Path MTU is carried in the MTU extended community for BGP-4. The MTU extended community is an optional transitive attribute. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MTU extended community Type | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | MTU Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The MTU extended community type is to be assigned by IANA. The first four octets of the value field should be reserved, and the MTU value Zeng & Dong Expires January 5, 2012 [Page 3] Internet-Draft MTU Extended Community for BGP-4 July 2011 is carried in the following two octets of the value field. 2.3. Signaling The MTU is advertised hop-by-hop from BGP egress LSR to BGP ingress LSR along an BGP LSP. The steps are as follows: A. If BGP speaker A is originator of the labeled IPv4 (or IPv6) route, A sets its BGP LSP Path MTU to the maximal value, advertises the labeled IPv4 (or IPv6) route with the MTU Extended Community to its BGP Peer (its upstream BGP LSR). B. BGP speaker B receives the labeled IPv4 (or IPv6) route with BGP LSP Path MTU from its BGP peer. a) B SHOULD computes the BGP LSR Link MTU to the Next Hop of the received message, then sets its BGP LSP Path MTU to the minimum of the received BGP LSP Path MTU and the BGP LSR Link MTU. b). If B distributes the route with the Next Hop attribute unchanged, it MUST keep the MTU Extended Community unchanged when advertising the message to its upstream BGP LSRs. c). If B would change the Next Hop attribute to itself in the subsequent advertisement, it SHOULD set the MTU Extended Community in the message with its BGP LSP Path MTU obtained through step (a). 3. Applicability Considerations The BGP MTU Extended Community is applicable to the BGP LSP defined in [RFC3107]. 4. IANA Considerations IANA is requested to assign a type and sub-type value for BGP MTU extended community. 5. Security Considerations This extension to BGP does not change the underlying security issues in [RFC4271]. Zeng & Dong Expires January 5, 2012 [Page 4] Internet-Draft MTU Extended Community for BGP-4 July 2011 6. Contributors The following individuals contributed to this document: Haibo Wang Haijun Xu 7. Acknowledgements TBD 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in BGP-4", RFC 3107, May 2001. [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006. [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, February 2006. 8.2. Informative References [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC3988] Black, B. and K. Kompella, "Maximum Transmission Unit Signalling Extensions for the Label Distribution Protocol", RFC 3988, January 2005. [RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364, February 2006. [RFC4659] De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur, "BGP-MPLS IP Virtual Private Network (VPN) Extension for IPv6 VPN", RFC 4659, September 2006. Zeng & Dong Expires January 5, 2012 [Page 5] Internet-Draft MTU Extended Community for BGP-4 July 2011 Authors' Addresses Qing Zeng Huawei Technologies Huawei Building, No.3 Xinxi Rd Beijing 100085 China Email: zengqing@huawei.com Jie Dong Huawei Technologies Huawei Building, No.3 Xinxi Rd Beijing 100085 China Email: jie.dong@huawei.com Zeng & Dong Expires January 5, 2012 [Page 6]