--- 1/draft-ietf-6man-rfc1981bis-07.txt 2017-05-27 09:13:10.875994143 -0700 +++ 2/draft-ietf-6man-rfc1981bis-08.txt 2017-05-27 09:13:10.919995184 -0700 @@ -1,23 +1,23 @@ Network Working Group J. McCann Internet-Draft Digital Equipment Corporation Obsoletes: 1981 (if approved) S. Deering Intended status: Standards Track Retired -Expires: November 17, 2017 J. Mogul +Expires: November 28, 2017 J. Mogul Digital Equipment Corporation R. Hinden, Ed. Check Point Software - May 16, 2017 + May 27, 2017 Path MTU Discovery for IP version 6 - draft-ietf-6man-rfc1981bis-07 + draft-ietf-6man-rfc1981bis-08 Abstract This document describes Path MTU Discovery for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC1981. Status of This Memo This Internet-Draft is submitted in full conformance with the @@ -26,21 +26,21 @@ 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 November 17, 2017. + This Internet-Draft will expire on November 28, 2017. Copyright Notice Copyright (c) 2017 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 @@ -67,28 +67,28 @@ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 5 4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 6 5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 7 5.1. Layering . . . . . . . . . . . . . . . . . . . . . . . . 7 5.2. Storing PMTU information . . . . . . . . . . . . . . . . 8 5.3. Purging stale PMTU information . . . . . . . . . . . . . 10 5.4. Packetization layer actions . . . . . . . . . . . . . . . 11 5.5. Issues for other transport protocols . . . . . . . . . . 12 - 5.6. Management interface . . . . . . . . . . . . . . . . . . 13 + 5.6. Management interface . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 - 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 + 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Normative References . . . . . . . . . . . . . . . . . . 14 9.2. Informative References . . . . . . . . . . . . . . . . . 14 - Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 16 + Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 15 Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 16 B.1. Change History Since RFC1981 . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 1. Introduction When one IPv6 node has a large amount of data to send to another node, the data is transmitted in a series of IPv6 packets. These packets can have a size less than or equal to the Path MTU (PMTU). Alternatively, they can be larger packets that are fragmented into a @@ -115,30 +115,31 @@ minimum link MTU. A node sending packets much smaller than the Path MTU allows is wasting network resources and probably getting suboptimal throughput. Nodes implementing Path MTU Discovery and sending packets larger than the IPv6 minimum link MTU are susceptible to problematic connectivity if ICMPv6 [ICMPv6] messages are blocked or not transmitted. For example, this will result in connections that complete the TCP three- way handshake correctly but then hang when data is transferred. This state is referred to as a black hole connection [RFC2923]. Path MTU - Discovery relies on such messages to determine the MTU of the path. + Discovery relies on ICMPv6 Packet Too Big (PTB) to determine the MTU + of the path. An extension to Path MTU Discovery defined in this document can be found in [RFC4821]. RFC4821 defines a method for Packetization Layer Path MTU Discovery (PLPMTUD) designed for use over paths where delivery of ICMPv6 messages to a host is not assured. Note: This document is an update to [RFC1981] that was published prior to [RFC2119] being published. Consequently although RFC1981 - used the "should/must" style language in upper and lower case, the + used the "should/must" style language in upper and lower case, this document does not cite the RFC2119 definitions and only uses lower case for these words. 2. Terminology node a device that implements IPv6. router a node that forwards IPv6 packets not explicitly addressed to itself. @@ -201,26 +202,26 @@ flow id a combination of a source address and a non-zero flow label. 3. Protocol Overview This memo describes a technique to dynamically discover the PMTU of a path. The basic idea is that a source node initially assumes that the PMTU of a path is the (known) MTU of the first hop in the path. If any of the packets sent on that path are too large to be forwarded by some node along the path, that node will discard them and return - ICMPv6 Packet Too Big (PTB) messages. Upon receipt of such a - message, the source node reduces its assumed PMTU for the path based - on the MTU of the constricting hop as reported in the Packet Too Big - message. The decreased PMTU causes the source to send smaller - packets or change EMTU_S to cause upper layer to reduce the size of - IP packets it sends. + ICMPv6 Packet Too Big messages. Upon receipt of such a message, the + source node reduces its assumed PMTU for the path based on the MTU of + the constricting hop as reported in the Packet Too Big message. The + decreased PMTU causes the source to send smaller packets or change + EMTU_S to cause upper layer to reduce the size of IP packets it + sends. The Path MTU Discovery process ends when the source node's estimate of the PMTU is less than or equal to the actual PMTU. Note that several iterations of the packet-sent/Packet-Too-Big-message-received cycle may occur before the Path MTU Discovery process ends, as there may be links with smaller MTUs further along the path. Alternatively, the node may elect to end the discovery process by ceasing to send packets larger than the IPv6 minimum link MTU. @@ -236,24 +237,25 @@ Path MTU Discovery supports multicast as well as unicast destinations. In the case of a multicast destination, copies of a packet may traverse many different paths to many different nodes. Each path may have a different PMTU, and a single multicast packet may result in multiple Packet Too Big messages, each reporting a different next-hop MTU. The minimum PMTU value across the set of paths in use determines the size of subsequent packets sent to the multicast destination. Note that Path MTU Discovery must be performed even in cases where a - node "thinks" a destination is attached to the same link as itself. - In a situation such as when a neighboring router acts as proxy [ND] - for some destination, the destination can appear to be directly - connected but it is in fact more than one hop away. + node "thinks" a destination is attached to the same link as itself, + it might have a PMTU lower than the link MTU. In a situation such as + when a neighboring router acts as proxy [ND] for some destination, + the destination can appear to be directly connected but it is in fact + more than one hop away. 4. Protocol Requirements As discussed in Section 1, IPv6 nodes are not required to implement Path MTU Discovery. The requirements in this section apply only to those implementations that include Path MTU Discovery. Nodes should appropriately validate the payload of ICMPv6 PTB messages to ensure these are received in response to transmitted traffic (i.e., a reported error condition that corresponds to an IPv6 @@ -370,21 +372,21 @@ In the case of a multicast destination address, copies of a packet may traverse many different paths to reach many different nodes. The local representation of the "path" to a multicast destination must represent a potentially large set of paths. Minimally, an implementation could maintain a single PMTU value to be used for all packets originated from the node. This PMTU value would be the minimum PMTU learned across the set of all paths in use by the node. This approach is likely to result in the use of smaller packets than is necessary for many paths. In the case of multipath - routing (e.g., Equal Cost Multipath Routing, ECMP), a set of paths + routing (e.g., Equal Cost Multipath Routing (ECMP) ), a set of paths can exist even for a single source and destination pair. An implementation could use the destination address as the local representation of a path. The PMTU value associated with a destination would be the minimum PMTU learned across the set of all paths in use to that destination. This approach will result in the use of optimally sized packets on a per-destination basis. This approach integrates nicely with the conceptual model of a host as described in [ND]: a PMTU value could be stored with the corresponding entry in the destination cache. @@ -464,47 +466,29 @@ Internetwork topology is dynamic; routes change over time. While the local representation of a path may remain constant, the actual path(s) in use may change. Thus, PMTU information cached by a node can become stale. If the stale PMTU value is too large, this will be discovered almost immediately once a large enough packet is sent on the path. No such mechanism exists for realizing that a stale PMTU value is too small, so an implementation should "age" cached values. When a PMTU value - has not been decreased for a while (on the order of 10 minutes), the - PMTU estimate should be set to the MTU of the first-hop link, and the - packetization layers should be notified of the change. This will - cause the complete Path MTU Discovery process to take place again. + has not been decreased for a while (on the order of 10 minutes), it + should probe to find if a larger PMTU is supported. Note: an implementation should provide a means for changing the timeout duration, including setting it to "infinity". For - example, nodes attached to an FDDI link which is then attached to - the rest of the Internet via a small MTU serial line are never - going to discover a new non-local PMTU, so they should not have to - put up with dropped packets every 10 minutes. - - One approach to implementing PMTU aging is to associate a timestamp - field with a PMTU value. This field is initialized to a "reserved" - value, indicating that the PMTU is equal to the MTU of the first hop - link. Whenever the PMTU is decreased in response to a Packet Too Big - message, the timestamp is set to the current time. - - Once a minute, a timer-driven procedure runs through all cached PMTU - values, and for each PMTU whose timestamp is not "reserved" and is - older than the timeout interval: - - - The PMTU estimate is set to the MTU of the first hop link. - - - The timestamp is set to the "reserved" value. - - - Packetization layers using this path are notified of the increase. + example, nodes attached to a link with a large MTU which is then + attached to the rest of the Internet via a link with a small MTU + are never going to discover a new non-local PMTU, so they should + not have to put up with dropped packets every 10 minutes. 5.4. Packetization layer actions A packetization layer (e.g., TCP) must use the PMTU for the path(s) in use by a connection; it should not send segments that would result in packets larger than the PMTU, except to probe during PMTU discovery (this probe packet must not be fragmented to the PMTU). A simple implementation could ask the IP layer for this value each time it created a new segment, but this could be inefficient. An implementation typically caches other values derived from the PMTU. @@ -531,30 +515,30 @@ upper layer protocol. If the Path MTU Discovery process requires several steps to find the PMTU of the full path, this could finally delay the retransmission by many round-trip times. Alternatively, the retransmission could be done in immediate response to a notification that the Path MTU was decreased, but only for the specific connection specified by the Packet Too Big message, but only based on the message and connection. The packet size used in the retransmission should be no larger than the new PMTU. - Note: A packetization layer must not retransmit in response to - every Packet Too Big message, since a burst of several oversized - segments will give rise to several such messages and hence several - retransmissions of the same data. If the new estimated PMTU is - still wrong, the process repeats, and there is an exponential - growth in the number of superfluous segments sent. - Retransmissions can increase network load in response to - congestion, worsening that congestion. Any packetization layer - that uses retransmission is responsible for congestion control of - its retransmissions. See [RFC8085] for more information. + Note: A packetization layer that determines a probe packet is + lost, needs to adapt the segment size of the retransmission. + Using the reported size in the last Packet Too Big message, + however, can lead to further losses as there might be smaller PMTU + limits at the routers further along the path. This would lead to + loss of all retransmitted segments and therefore cause unnecessary + congestion as well as additional packets to be sent each time a + new router announces a smaller MTU. Any packetization layer that + uses retransmission is therefore also responsible for congestion + control of its retransmissions [RFC8085]. A loss caused by a PMTU probe indicated by the reception of a Packet Too Big message must not be considered as a congestion notification and hence the congestion window may not change. 5.5. Issues for other transport protocols Some transport protocols are not allowed to repacketize when doing a retransmission. That is, once an attempt is made to transmit a segment of a certain size, the transport cannot split the contents of @@ -632,32 +616,37 @@ connections caused by filtering of ICMPv6 message. See [RFC4890] for recommendations regarding filtering ICMPv6 messages. 7. Acknowledgements We would like to acknowledge the authors of and contributors to [RFC1191], from which the majority of this document was derived. We would also like to acknowledge the members of the IPng working group for their careful review and constructive criticisms. + We would also like to acknowledge the contributors to this update of + "Path MTU Discovery for IP version 6". This includes members of the + 6MAN w.g., area directorate reviewers, the IESG, and especially to + Joe Touch and Gorry Fairhurst. + 8. IANA Considerations This document does not have any IANA actions 9. References 9.1. Normative References [I-D.ietf-6man-rfc2460bis] - <>, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) - Specification", draft-ietf-6man-rfc2460bis-11 (work in - progress), April 2017. + Deering, S. and R. Hinden, "Internet Protocol, Version 6 + (IPv6) Specification", draft-ietf-6man-rfc2460bis-13 (work + in progress), May 2017. [ICMPv6] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, DOI 10.17487/RFC4443, March 2006, . 9.2. Informative References [FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", @@ -799,31 +788,43 @@ TP4. o Updated text in Section 5.5 "Issues for other transport protocols" about NFS including adding a current reference to NFS and removing obsolete text. o Added paragraph to Section 6 "Security Considerations" about black hole connections if PTB messages are not received, and comparison to PLPMTD. + o Updated Section 7 "Acknowledgements". + o Editorial Changes. B.1. Change History Since RFC1981 NOTE TO RFC EDITOR: Please remove this subsection prior to RFC Publication + This section describes change history made in each Internet Draft that went into producing this version. The numbers identify the Internet-Draft version in which the change was made. Working Group Internet Drafts + 08) Based on IESG comments, cleaned up text in Section 5.3 + regarding suggested action when PMTU value has not been + decreased recently. + + 08) Revision of Note in Section 5.4 to make text clearer. + + 08) Updated Section 7 "Acknowledgements". + + 08) Editorial Changes. 07) Changes from the IESG Discuss comments from IESG reviews. The changes include: o Added Note to Introduction that document that this document doesn't cite RFC2119 and only uses lower case "should/must" language. Changed all upper case "should/ must" to lower case. o Added references for EMTU_S and EMTU_R.