Network Working Group F. Templin Internet-Draft Nokia Expires: August 1, 2003 January 31, 2003 MTU Issues in IPv6 Neighbor Discovery draft-templin-ndiscmtu-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 August 1, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document discusses Maximum Transmission Unit (MTU) issues in IPv6 Neighbor Discovery and suggests minor augmentations to the existing specification to rectify the issues. 1. Introduction This document discusses Maximum Transmission Unit (MTU) issues in Neighbor Discovery for IP Version 6 (IPv6) [1]. It argues that the current specification is too restrictive in the use of MTU options, and that per-neighbor MTU values should be maintained in the conceptual Neighbor Cache. It finally proposes minor augmentations to the existing specification to rectify the issues. Templin Expires August 1, 2003 [Page 1] Internet-Draft MTU Issues in IPv6 Neighbor Discovery January 2003 2. Problem Statement ([1], section 4.6.4) defines a Maximum Transmission Unit (MTU) option type. In the current specification, the MTU option is sent only in Router Advertisement messages ([1], section 4.2) and interpreted by receivers as the MTU value for all nodes on the link to use. While this specification provides sufficient mechanism for many of the supported link types in ([1], section 2.2), it may lead to inefficiencies for other types, e.g., for certain variable MTU links. In particular, it may be desirable in some cases for a node to track independent MTU values for different neighbors on a link. For example, on certain multiple-access wireless links the optimal MTU for a particular neighbor may be proportional to the signal to noise ratio sensed at the receiver's MAC layer. In another example, constrained nodes on a link with large MTU may wish to receive smaller packets than more robust nodes. In these and other cases, maintaining independent per-neighbor MTUs for nodes on a link would yield significant efficiency advantages. 3. Proposed Changes In order to support per-neighbor MTUs, the following changes/ augmentations to RFC 2461 are proposed: 1. In ([1], section 4.6.4), add new text allowing the encoding of MTU options in Neighbor Acknowledgement messages. 2. In ([1], section 5.1), add a per-neighbor MTU field (NBR_MTU) in the Neighbor Cache data structure. 3. In ([1], section 6.3.2), add a new host variable, defined as follows: MaxLinkMTU The maximum MTU supported on the link. Default: LinkMTU. 4. In ([1], section 7.2.4), add new text allowing the encoding of MTU options in solicited Neighbor Advertisements." 5. In ([1], section 7.2.5), add new text requiring the value in MTU options received in Neighbor Advertisement messages be written into the NBR_MTU field in the Neighbor Cache entry. 6. In (([1], section 7.2.6), add new text allowing Unsolicited Neighbor Advertisements sent to the unicast address of a neighbor. Templin Expires August 1, 2003 [Page 2] Internet-Draft MTU Issues in IPv6 Neighbor Discovery January 2003 4. Operational Details When nodes implement the changes proposed above, Neighbor Advertisement messages containing MTU options provide a dynamic mechanism for receivers to inform senders of MTU changes. Documents that specify the operation of IPv6 over specific link layers (e.g., Ethernet, FDDI, etc.) provide details for the encoding of MTU options in Neighbor Advertisement messages. Receipt of an MTU option in the initial solicited Neighbor Advertisement provides an indication to the sender that the receiver implements the dynamic MTU mechanism. (Lack thereof conversely provides an indication that the receiver does not implement the dynamic MTU mechanism.) Using these proposed changes, MTU options received in Router Advertisements affect LinkMTU exactly as in the current specification. However, packetization and forwarding layers see an MTU of MaxLinkMTU when they examine the link. The relationship between the three MTU parameters is as follows: LinkMTU <= NbrMTU <= MaxLinkMTU Multicast destinations, and unicast destinations that use a next hop with no MTU support indicated in the Neighbor Cache, see an MTU of LinkMTU. 5. IANA Considerations TBD 6. Security considerations TBD 7. Acknowledgements TBD Normative References [1] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. Informative References Templin Expires August 1, 2003 [Page 3] Internet-Draft MTU Issues in IPv6 Neighbor Discovery January 2003 Author's Address Fred L. Templin Nokia 313 Fairchild Drive Mountain View, CA 94110 US Phone: +1 650 625 2331 EMail: ftemplin@iprg.nokia.com Templin Expires August 1, 2003 [Page 4] Internet-Draft MTU Issues in IPv6 Neighbor Discovery January 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION Templin Expires August 1, 2003 [Page 5] Internet-Draft MTU Issues in IPv6 Neighbor Discovery January 2003 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Templin Expires August 1, 2003 [Page 6]