Network Working Group F. Templin Internet-Draft Nokia Expires: September 24, 2003 March 26, 2003 Path MTU Support for IPv6-in-IPv4 Tunnels draft-templin-tunnelmtu-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 September 24, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document specifies a means for IPv6-in-IPv4 tunnels to participate in IPv6 path MTU discovery. Also specified is a means for the tunnel decapsulator to inform the encapsulator of appropriate per-neighbor MTU values using IPv6 neighbor discovery. Templin Expires September 24, 2003 [Page 1] Internet-Draft Tunnel MTU March 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Static MTU Determination . . . . . . . . . . . . . . . . . . . . 3 3. Dynamic MTU Determination . . . . . . . . . . . . . . . . . . . 3 4. Additional Notes . . . . . . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. Security considerations . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6 Normative References . . . . . . . . . . . . . . . . . . . . . . 7 Informative References . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . . 9 Templin Expires September 24, 2003 [Page 2] Internet-Draft Tunnel MTU March 2003 1. Introduction IPv6-in-IPv4 tunnels use IPv4 as a link layer for IPv6. The tunnel encapsulator and decapsulator are IPv6 neighbors, but packets inside the tunnel may traverse multiple IPv4 forwarding hops. Thus the MTU of the IPv4 path within the tunnel determines the IPv6 MTU for the tunnel. Packets that are too large to traverse the tunnel must be discarded with an ICMPv6 "packet too big" message [RFC2463] returned to the source, as required by IPv6 Path MTU discovery [RFC1981]. But, reliance on IPv4 path MTU discovery [RFC1191] within the tunnel can lead to black holes that are difficult to diagnose [RFC2923]. Thus, an alternate means for IPv6-in-IPv4 tunnels to participate in IPv6 path MTU discovery is needed and specified in this document. 2. Static MTU Determination IPv6-in-IPv4 tunnels that do not implement a dynamic MTU determination mechanism (see: Section 3) should use an interface MTU, i.e. "LinkMTU" ([RFC2461], section 6.3.2), that is no larger than the smallest known Effective MTU to Receive (EMTU_R) ([RFC1122], section 3.3.2) for all potential decapsulators. The tunnel encapsulator sends all IPv6 packets that are no larger than LinkMTU with the DF bit not set in the encapsulating IPv4 header. Other packets are discarded, with an ICMPv6 "packet too big" message returned to the sender. LinkMTU must be at least 1280 bytes (the minimum IPv6 MTU [RFC2460]) and should otherwise be chosen to minimize IPv4 fragmentation within the tunnel. For example, when all potential decapsulators are known to have an EMTU_R of 10KB and the IPv4 network is known to support large MTUs, the encapsulator may use LinkMTU values as large as 10KB (minus overhead for encapsulation). Conversely, when excessive IPv4 fragmentation is anticipated along some paths, LinkMTU should be set to a smaller value. All IPv6 packets with multicast/anycast destination addresses use the static MTU determination specified above. 3. Dynamic MTU Determination IPv6-in-IPv4 tunnels appear as a single hop at the IPv6 level with (possibly) multiple IPv4 hops occurring inside the tunnel. IPv4 path MTU discovery [RFC1191] uses ICMPv4 "fragmentation needed" messages which may not provide enough information for stateless translation to ICMPv6 ([RFC0792], [RFC1812], section 4.3.2.3). Additionally, ICMPv4 "fragmentation needed" messages can be spoofed, filtered, or not sent at all by some forwarding nodes. Thus, IPv4 path MTU discovery may Templin Expires September 24, 2003 [Page 3] Internet-Draft Tunnel MTU March 2003 result in black holes that are difficult to diagnose [RFC2923]. When RFC 1191 path MTU discovery is deemed inadequate, dynamic tunnel MTU determination may be implemented as follows: 3.1 Interface Initialization The encapsulator sets LinkMTU for the tunnel interface to the MTU of the underlying IPv4 link, minus overhead for IPv6-in-IPv4 encapsulation. If the tunnel is configured over multiple underlying IPv4 links, the largest underlying link MTU is used and locally-generated ICMPv6 "packet too big" messages may result from within the tunnel interface. The encapsulator additionally keeps a link layer cache of per-neighbor MTU values, e.g., as ancillary data in the IPv6 neighbor cache, in the IPv4 path MTU discovery cache, etc. 3.2 Tunnel Endpoint Negotiation When the encapsulator has a large IPv6 packet to send to a decapsulator for which there is no per-neighbor cache state, it performs an initial Neighbor Solicitation/Neighbor Advertisement exchange as in ([RFC2461], sections 7.2.2-7.2.3). If the decapsulator implements the dynamic MTU determination specified herein, it returns a Neighbor Advertisement message containing an MTU option [NDISCMTU] with a value to be used as the initial MTU estimate. If the Neighbor Advertisement message does NOT contain an MTU option, the encapsulator instead uses the static MTU determination specified in Section 2 or an alternate dynamic MTU determination method (e.g., Section 3.5) for this decapsulator. 3.3 Encapsulator Actions The encapsulator sends packets with the DF bit NOT set in the encapsulating IPv4 header with the expectation that the decapsulator will send an unsolicited Neighbor Advertisement message with an MTU option [NDISCMTU] if IPv4 fragmentation occurs inside the tunnel. The encapsulator can probe the tunnel path MTU by null-padding ordinary IPv6 data packets, i.e., by inflating the length field in the IPv4 header ([MECH], section 3.5). When the encapsulator receives an unsolicited Neighbor Advertisement message with an MTU option, it records the value in the per-neighbor MTU cache described above. As in the static case, IPv6 packets that are larger than the tunnel interface LinkMTU are discarded with an ICMPv6 "packet too big" message returned to the sender. In the dynamic case, the encapsulator also performs these actions for IPv6 packts that are larger than the Templin Expires September 24, 2003 [Page 4] Internet-Draft Tunnel MTU March 2003 MTU value for the decapsulator in the neighbor cache. 3.4 Decapsulator Actions The decapsulator monitors fragmented IPv4 packets arriving from the tunnel. If a fragmented packet arrives, the decapsulator sends an unsolicited Neighbor Advertisement message with an MTU option [NDISCMTU] to inform the encapsulator of a new MTU value. The new value is chosen such that subsequent packets from the encapsulator will not incur IPv4 fragmentation. If the IPv4 first fragment does not fully contain the encapsulated IPv6 packet (i.e., if fragmentation was NOT due to padding bytes), the decapsulator also discards the packet and sends an ICMPv6 "packet too big" message to the source. This action is taken even if the decapsulator could correctly reassemble the packet, since IPv6 packets are not permitted to be fragmented by the network. 3.5 Alternate Method When the decapsulator does not implement the above specified scheme, or when the above-specified scheme is not desired, the encapsulator can probe the path MTU to the decapsulator using an alternate method, e.g., as specified below. In this method, the encapsulator sets the DF bit in the IPv4 header of probe packets. Probe packets may be sent either when the encapsulator forwards a large data packet to the decapsulator (i.e., on-demand) or when the path MTU for the decapsulator has not been verified for some time (i.e., periodic). IPv6 Neighbor Solicitation (NS) or ICMPv6 ECHO_REQUEST packets with padding bytes added could be used for this purpose, since successful delivery results in a positive acknowledgement that the probe succeeded vis-a-vis a response from the decapsulator. While probing, the encapsulator maintains a queue of packets that use the decapsulator as the IPv6 next-hop. If the probe succeeds, packets in the queue that are no larger than the probe size are sent to the decapsulator. If the probe fails, packets that are larger than the last known successful probe are dropped and an ICMPv6 "packet too big" message returned to the sender [RFC1981]. The queue should be large enough to buffer the (delay*bandwidth) product for the round-trip time to the decapsulator. When smaller queues are used, loss of packets that are too big for the yet-to-be-determined path MTU may occur with no ICMPv6 "packet too big" message returned. Such loss may occur only in rare instances, but may result in unpredictable behavior in senders that base their adaptation solely on ICMPv6 "packet too big" messages. Templin Expires September 24, 2003 [Page 5] Internet-Draft Tunnel MTU March 2003 4. Additional Notes 1. A "MinMTU" value must be supported by all nodes for multicast/ anycast using the static MTU determination scheme. (The dynamic MTU determination scheme applies only for unicast.) MinMTU is configured such that: 1280 <= MinMTU <= LinkMTU. 2. The specifications above are easily extended to include differentiated service (DS) information to provide accurate MTU estimates when multipath routing is used. 3. To avoid superfluous probing based on counting down/up by small increments, plateau tables (e.g., [RFC1191], section 7) should be used when the actual MTU value is indeterminant. 4. Some forwarding nodes may have broken/non-existent IPv4 fragmentation implementations. Thus, the fragmentation-sensing method described above may be susceptible to black holes along some forwarding paths. The encapsulator can avoid black holes by periodically sending locally-fragmented probe packets to elicit responses from decapsulators. 5. When the fragmentation sensing scheme is used, some implementations may wish to send only probe packets with the DF bit not set and all other packets with the DF bit *set* in order to generate IPv4 "fragmentation needed" messages from the network. This method has the advantage that packets that are too big are discarded before reaching the decapsulator, but is susceptible to the same denial-of-service attacks as for IPv4 path MTU discovery. 5. IANA Considerations N/A 6. Security considerations Security issues are the same as for IPv6 neighbor discovery. Works-in-progress from the IETF SEcuring NEighbor Discovery (SEND) working group may provide future solutions. 7. Acknowledgements Most of the ideas expressed in this document are not new and borrow from earlier mailing list discussions. The fragmentation-sensing method (described in Section 3.1 through Section 3.4) was inspired by a scheme proposed by Charles Lynn on the TCP-IP discussion list in Templin Expires September 24, 2003 [Page 6] Internet-Draft Tunnel MTU March 2003 November 1987. The alternate method (descrbed in Section 3.5) bears close relation to a scheme proposed by Dave Borman on the IPng mailing list in August 1999. Other ideas in the draft may have borrowed to some extent from discussions on the IETF MTU Discovery WG mailing list from November 1989 - February 1995 and discussions on the IETF NGTRANS WG mailing list in August 2002. The author would like to acknowledge certain individuals for helpful discussion on this subject, including Ralph Droms, Tim Gleeson, Jun-ichiro itojun Hagino, Bob Hinden, Christian Huitema, Kevin Lahey, Matt Mathis, Jeff Mogul, Erik Nordmark, Dave Thaler, Lixia Zhang and the members of the Nokia NRC/COM Mountain View team. Normative References [MECH] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-00 (work in progress), February 2003. [NDISCMTU] Templin, F., "MTU Issues in IPv6 Neighbor Discovery", draft-templin-ndiscmtu-00 (work in progress), February 2003. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC1122] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [RFC2463] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 Templin Expires September 24, 2003 [Page 7] Internet-Draft Tunnel MTU March 2003 (IPv6) Specification", RFC 2463, December 1998. Informative References [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, September 2000. 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 September 24, 2003 [Page 8] Internet-Draft Tunnel MTU March 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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