Network Working Group Vijay Kumar Vasantha(Huawei) Internet Draft Aug 4, 2008 Intended status: Proposed Standard Expires: Feb 5, 2009 IPv6 Path MTU computation using routing protocol. 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 February 5, 2009. Abstract This document describes a mechanism for dynamically computing IPv6 PMTU and the modifications needed in IPv6 to support the solution. Specification of Requirements 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]. Table of Contents Status of This Memo...................................................1 Abstract .............................................................1 Table of Contents.....................................................1 1. Introduction .....................................................2 2. PMTU computation in routing domain ...............................2 2.1 PMTU calculation within a routing protocol area.................3 2.2 PMTU calculation across a routing protocol area.................3 2.3 PMTU calculation across an autonomous system ...................4 2.4 Interoperability with PMTU non-computational routers ...........4 3. Modification in ICMPv6 error message .............................5 3.1 Packet Too Big Message .........................................5 4. Acknowledgments ..................................................5 5. Normative References .............................................5 6. Informative References ...........................................5 7. Authors' Addresses ...............................................5 Full Copyright Statement .............................................6 1. Introduction The current IPv6 PMTU discovery has the following drawbacks, 1. The IPv6 PMTU discovery is done by trial and error method, which can result in inefficient forwarding such as described below and this in turn can result in delay in packet transmission. . Packets may be dropped because of packet too big reason by any intermediate router. . Packets that are very small in size may be forwarded for considerable amount of time resulting in inefficient usage of available bandwidth. 2. The source comes to know about the packet drop only by ICMPv6 packet too big error. But this error packet will have to travel from the problem occurred router to the source of the packet, which consumes considerable amount of bandwidth on all the intermediate links between the originator and the problem occurred node. This document defines a method in which an optimal PTMU is dynamically computed using linked state routing protocols on routers and modification needed in IPv6 to convey PMTU information to the hosts. The method by which PMTU can be calculated for each IPv6 route in an IS-IS routing domain can be seen in [ISISPMTU]. The same computation can be supported by other linked state routing protocols as well and the overhead incurred in doing so is minimal as PMTU computation can be closely associated with route computation. In the remainder of this document, the key words "MUST", "MUST NOT" "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULDNOT", "RECOMMENDED","MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119]. 2. PMTU computation in routing domain Internet is composed of many Autonomous systems and each autonomous system is in turn composed of many areas. The below section describes the overview of PMTU computation at each of these hierarchy in the routing domain. 2.1 PMTU calculation within a routing protocol area -------------- //// \\\\ // \\ | | | AREA K | | | \\ // \\\\ //// -------------- In any routing area all the routers will have complete path to reach all intra-area routes. The MTU to a particular destination along the shortest path tree is considered as the PMTU to that particular destination. Thus all the intra-area routes can be associated with an optimal PMTU value. [ISISPMTU] describes an algorithm to achieve the same and the same algorithm can be used in any other link state IGP. 2.2 PMTU calculation across a routing protocol area ----------- ----------- //// \\\\ //// \\\\ // \\ // \\ | || | | RTA |RTB| RTC | | || | \\ // \\ // \\\\ //// \\\\ //// ----------- ----------- AREA K AREA L As areas are created for hierarchical routing, the inter-area route's path won't be completely visible to intra-area routers. Consider a destination X originated by the router RTC, the intra-area router RTA will know about the destination X only through the area border router RTB and the path information of the destination X on RTA will be confined only till the area border router RTB. As described in sec 2.1 the intra-area router like RTA can compute PMTU for the inter-area destination like X considering only the intra-area path information. But PMTU so computed won't reflect the correct value till destination. Thus for the intra-area router like RTA additional information is needed to compute the correct value of PMTU for inter-area routes. The ABR RTB provides the additional PMTU information from itself to the destination when transferring route between areas and the intra-area router can use the additional information and calculate optimal PMTU. [ISISPMTU] describes an algorithm to achieve the same and the same algorithm can be used in any other link state IGP. 2.3 PMTU calculation across an autonomous system ----------- ----------- //// \\\\ //// \\\\ // \\ // \\ | || | | AS1 |ASBR| AS2 | | || | \\ // \\ // \\\\ //// \\\\ //// ----------- ----------- The problem described in section 2.2 is equally applicable when dealing with inter AS routes. If each AS is operated under a different protocol then intra-AS router cannot know the full path to inter-AS routes, hence the ASBR should support the transformation of PMTU across AS. To support the PMTU computation across the AS all related protocols should support the PMTU computation and PMTU information transfer across ASBR. The method by which this is done in all the protocols is out of scope of this document and the method by which IS-IS supports the calculation and transfers the information across protocol can be seen in [ISISPMTU]. Any change in PMTU of a route should be dynamically detected in routing domain and PMTU should be recomputed. Thus all the routes in a routing domain should be associated with a PMTU value and this PMTU information should be downloaded to forwarding information base. 2.4 Interoperability with PMTU non-computational routers While interacting with PMTU non-computational routers the complete information to compute the PMTU may not be available and the interaction with PMTU non-computational router can occur within an area, across an area and/or across an autonomous system. [ISISPMTU] addresses this problem by carrying out best effort PMTU computation at each hierarchy and the same algorithm can be applied to other link state protocols as well. If the so computed PMTU is not optimal then the originator of the IPv6 packet can know the actual PMTU through the existing ICMPv6 packet too big messages. 3. Modification in ICMPv6 error message 3.1 Packet Too Big Message The router should generate ICMPv6 packet too big error whenever it detects that the size of the packet is greater than the computed PMTU to the destination. The ICMPv6 packet too big error message should convey the computed PMTU to the destination rather than the outgoing interface's MTU value. If a router does not have a computed PMTU value for a destination then the forwarding behavior is unchanged. 4. Acknowledgements The author would like to thank Saravana Kumar and K.L.Srini 5. Normative References [ISISPMTU] Vijay Kumar Vasantha "draft-kumar-isis-path-mtu-00.txt". 6. Informative References [ISO10589] ISO. Intermediate System to Intermediate System Routing Exchange Protocol for Use in Conjunction with the Protocol for Providing the Connectionless-Mode Network Service. ISO 10589, 1992. [RFC1981] McCann, Deering & Mogul RFC 1981 Path MTU Discovery for IP version 6, August 1996 7. Authors' Addresses Vijay Kumar Vasantha Huawei Technologies India Private Limited Bangalore, India - 560008 vijaykumar@huawei.com Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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