--- 1/draft-ietf-6man-rfc1981bis-06.txt 2017-05-16 11:13:46.421701613 -0700 +++ 2/draft-ietf-6man-rfc1981bis-07.txt 2017-05-16 11:13:46.465702668 -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: October 9, 2017 J. Mogul +Expires: November 17, 2017 J. Mogul Digital Equipment Corporation R. Hinden, Ed. Check Point Software - April 7, 2017 + May 16, 2017 Path MTU Discovery for IP version 6 - draft-ietf-6man-rfc1981bis-06 + draft-ietf-6man-rfc1981bis-07 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 October 9, 2017. + This Internet-Draft will expire on November 17, 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 @@ -74,68 +74,74 @@ 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 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Normative References . . . . . . . . . . . . . . . . . . 14 9.2. Informative References . . . . . . . . . . . . . . . . . 14 - Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 15 + Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 16 Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 16 B.1. Change History Since RFC1981 . . . . . . . . . . . . . . 17 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 + 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 series of fragments each with a size less than or equal to the PMTU. It is usually preferable that these packets be of the largest size that can successfully traverse the path from the source node to the destination node without the need for IPv6 fragmentation. This packet size is referred to as the Path MTU, and it is equal to the minimum link MTU of all the links in a path. This document defines a standard mechanism for a node to discover the PMTU of an arbitrary path. - IPv6 nodes SHOULD implement Path MTU Discovery in order to discover + IPv6 nodes should implement Path MTU Discovery in order to discover and take advantage of paths with PMTU greater than the IPv6 minimum link MTU [I-D.ietf-6man-rfc2460bis]. A minimal IPv6 implementation (e.g., in a boot ROM) may choose to omit implementation of Path MTU Discovery. - Nodes not implementing Path MTU Discovery MUST use the IPv6 minimum + Nodes not implementing Path MTU Discovery must use the IPv6 minimum link MTU defined in [I-D.ietf-6man-rfc2460bis] as the maximum packet size. In most cases, this will result in the use of smaller packets than necessary, because most paths have a PMTU greater than the IPv6 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. Path MTU Discovery - relies on such messages to determine the MTU of the path. + state is referred to as a black hole connection [RFC2923]. Path MTU + Discovery relies on such messages 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 + 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. host any node that is not a router. upper layer a protocol layer immediately above IPv6. @@ -174,52 +180,53 @@ path MTU the minimum link MTU of all the links in a path between a source node and a destination node. PMTU path MTU Path MTU Discovery process by which a node learns the PMTU of a path EMTU_S Effective MTU for sending, used by upper layer protocols to limit the size of IP packets they - queue for sending [RFC6691]. + queue for sending [RFC6691] [RFC1122]. EMTU_R Effective MTU for receiving, the largest packet - that can be reassembled at the receiver. + that can be reassembled at the receiver + [RFC1122]. flow a sequence of packets sent from a particular source to a particular (unicast or multicast) destination for which the source desires special handling by the intervening routers. 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 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 fragments or change - EMTU_S to cause upper layer to reduce the size of IP packets it - sends. + 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. - The Path MTU Discovery process ends when the 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. + 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. The PMTU of a path may change over time, due to changes in the routing topology. Reductions of the PMTU are detected by Packet Too Big messages. To detect increases in a path's PMTU, a node periodically increases its assumed PMTU. This will almost always result in packets being discarded and Packet Too Big messages being generated, because in most cases the PMTU of the path will not have @@ -231,65 +238,65 @@ 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 to appear to be directly - connected but is in fact more than one hop away. + 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 + 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 packet actually sent by the application) per [ICMPv6]. If a node receives a Packet Too Big message reporting a next-hop MTU - that is less than the IPv6 minimum link MTU, it MUST discard it. A - node MUST NOT reduce its estimate of the Path MTU below the IPv6 - minimum link MTU. + that is less than the IPv6 minimum link MTU, it must discard it. A + node must not reduce its estimate of the Path MTU below the IPv6 + minimum link MTU on receipt of an Packet Too Big message. - When a node receives a Packet Too Big message, it MUST reduce its + When a node receives a Packet Too Big message, it must reduce its estimate of the PMTU for the relevant path, based on the value of the MTU field in the message. The precise behavior of a node in this circumstance is not specified, since different applications may have different requirements, and since different implementation architectures may favor different strategies. - After receiving a Packet Too Big message, a node MUST attempt to - avoid eliciting more such messages in the near future. The node MUST + After receiving a Packet Too Big message, a node must attempt to + avoid eliciting more such messages in the near future. The node must reduce the size of the packets it is sending along the path. Using a PMTU estimate larger than the IPv6 minimum link MTU may continue to - elicit Packet Too Big messages. Since each of these messages (and - the dropped packets they respond to) consume network resources, the - node MUST force the Path MTU Discovery process to end. + elicit Packet Too Big messages. Because each of these messages (and + the dropped packets they respond to) consume network resources, Nodes + using Path MTU Discovery must detect decreases in PMTU as fast as + possible. - Nodes using Path MTU Discovery MUST detect decreases in PMTU as fast - as possible. Nodes MAY detect increases in PMTU, but because doing - so requires sending packets larger than the current estimated PMTU, - and because the likelihood is that the PMTU will not have increased, - this MUST be done at infrequent intervals. An attempt to detect an - increase (by sending a packet larger than the current estimate) MUST - NOT be done less than 5 minutes after a Packet Too Big message has - been received for the given path. The recommended setting for this - timer is twice its minimum value (10 minutes). + Nodes may detect increases in PMTU, but because doing so requires + sending packets larger than the current estimated PMTU, and because + the likelihood is that the PMTU will not have increased, this must be + done at infrequent intervals. An attempt to detect an increase (by + sending a packet larger than the current estimate) must not be done + less than 5 minutes after a Packet Too Big message has been received + for the given path. The recommended setting for this timer is twice + its minimum value (10 minutes). - A node MUST NOT increase its estimate of the Path MTU in response to + A node must not increase its estimate of the Path MTU in response to the contents of a Packet Too Big message. A message purporting to announce an increase in the Path MTU might be a stale packet that has been floating around in the network, a false packet injected as part of a denial-of-service attack, or the result of having multiple paths to the destination, each with a different PMTU. 5. Implementation Issues This section discusses a number of issues related to the implementation of Path MTU Discovery. This is not a specification, @@ -319,21 +326,22 @@ protocol, it becomes hard to share PMTU information between different packetization layers, and the connection-oriented state maintained by some packetization layers may not easily extend to save PMTU information for long periods. It is therefore suggested that the IP layer store PMTU information and that the ICMPv6 layer process received Packet Too Big messages. The packetization layers may respond to changes in the PMTU by changing the size of the messages they send. To support this layering, packetization layers require a way to learn of changes in - the value of MMS_S, the "maximum send transport-message size". + the value of MMS_S, the "maximum send transport-message size" + [RFC1122]. MMS_S is a transport message size calculated by subtracting the size of the IPv6 header (including IPv6 extension headers) from the largest IP packet that can be sent, EMTU_S. MMS_S is limited by a combination of factors, including the PMTU, support for packet fragmentation and reassembly, and the packet reassembly limit (see [I-D.ietf-6man-rfc2460bis] section "Fragment Header"). When source fragmentation is available, EMTU_S is set to EMTU_R, as indicated by the receiver using an upper layer protocol or based on protocol requirements (1500 octets for IPv6). When a message larger than PMTU @@ -348,21 +356,23 @@ fragmentation, see [FRAG]). 5.2. Storing PMTU information Ideally, a PMTU value should be associated with a specific path traversed by packets exchanged between the source and destination nodes. However, in most cases a node will not have enough information to completely and accurately identify such a path. Rather, a node must associate a PMTU value with some local representation of a path. It is left to the implementation to select - the local representation of a path. + the local representation of a path. For nodes with multiple + interfaces, Path MTU information should be maintained for each IPv6 + link. 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 @@ -442,69 +452,63 @@ normal timeout-based retransmission mechanisms would be used to recover from the dropped packets. It is important to understand that the notification of the packetization layer instances using the path about the change in the PMTU is distinct from the notification of a specific instance that a packet has been dropped. The latter should be done as soon as practical (i.e., asynchronously from the point of view of the packetization layer instance), while the former may be delayed until a packetization layer instance wants to create a packet. - Retransmission should be done for only for those packets that are - known to be dropped, as indicated by a Packet Too Big message. 5.3. Purging stale PMTU information 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 + 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. 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. - An upper layer must not retransmit data in response to an increase in - the PMTU estimate, since this increase never comes in response to an - indication of a dropped packet. - 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. 5.4. Packetization layer actions - A packetization layer (e.g., TCP) must track the PMTU for the path(s) + 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. It may be simpler to receive asynchronous notification when the PMTU changes, so that these variables may be also updated. A TCP implementation must also store the Maximum Segment Size (MSS) @@ -512,80 +516,70 @@ largest packet that can be reassembled by the receiver, and must not send any segment larger than this MSS, regardless of the PMTU. The value sent in the TCP MSS option is independent of the PMTU; it is determined by the receiver reassembly limit EMTU_R. This MSS option value is used by the other end of the connection, which may be using an unrelated PMTU value. See [I-D.ietf-6man-rfc2460bis] sections "Packet Size Issues" and "Maximum Upper-Layer Payload Size" for information on selecting a value for the TCP MSS option. - When a Packet Too Big message is received, it implies that a packet - was dropped by the node that sent the ICMPv6 message. It is - sufficient to treat this in the same way as any other dropped - segment, and will be recovered by normal retransmission methods. If - the Path MTU Discovery process requires several steps to find the - PMTU of the full path, this could delay the connection by many round- - trip times. + Reception of a Packet Too Big message implies that a packet was + dropped by the node that sent the ICMPv6 message. A reliable upper + layer protocol will detect this loss by its own means, and recover it + by its normal retransmission methods. The retransmission could + result in delay, depending on the loss detection method used by the + 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 has changed, but only for the - specific connection specified by the Packet Too Big message. The - packet size used in the retransmission should be no larger than the - new PMTU. + 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. - This means that the TCP layer must be able to recognize when a - Packet Too Big notification actually decreases the PMTU that it - has already used to send a packet on the given connection, and - should ignore any other notifications. - - Many TCP implementations incorporate "congestion avoidance" and - "slow-start" algorithms to improve performance [CONG]. Unlike a - retransmission caused by a TCP retransmission timeout, a - retransmission caused by a Packet Too Big message should not change - the congestion window. It should, however, trigger the slow-start - mechanism (i.e., only one segment should be retransmitted until - acknowledgements begin to arrive again). - - TCP performance can be reduced if the sender's maximum window size is - not an exact multiple of the segment size in use (this is not the - congestion window size). + 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 the segment into smaller segments for retransmission. In such a case, the original segment can be fragmented by the IP layer during retransmission. Subsequent segments, when transmitted for the first time, should be no larger than allowed by the Path MTU. Path MTU Discovery for IPv4 [RFC1191] used NFS as an example of a UDP-based application that benefits from PMTU discovery. Since then [RFC7530], states the supported transport layer between NFS and IP must be an IETF standardized transport protocol that is specified to - avoid network congestion; such transports include TCP and the Stream - Control Transmission Protocol (SCTP). In this case, the transport is - itself responsible for determining and using an effective Path MTU, - including implementing PMTU discovery when this is needed. + avoid network congestion; such transports include TCP, Stream Control + Transmission Protocol (SCTP) [RFC4960], and the Datagram Congestion + Control Protocol (DCCP) [RFC4340]. In this case, the transport is + responsible for ensuring that transmitted segments (except probes) + conform to the the Path MTU, including supporting PMTU discovery + probe transmissions as needed. 5.6. Management interface It is suggested that an implementation provide a way for a system utility program to: - Specify that Path MTU Discovery not be done on a given path. - Change the PMTU value associated with a given path. @@ -611,90 +605,117 @@ set its PMTU estimate below the IPv6 minimum link MTU. A sender that falsely reduces to this MTU would observe suboptimal performance. In the second attack, the false message indicates a PMTU larger than reality. If believed, this could cause temporary blockage as the victim sends packets that will be dropped by some router. Within one round-trip time, the node would discover its mistake (receiving Packet Too Big messages from that router), but frequent repetition of this attack could cause lots of packets to be - dropped. A node, however, should never raise its estimate of the - PMTU based on a Packet Too Big message, so should not be - vulnerable to this attack. + dropped. A node, however, must not raise its estimate of the PMTU + based on a Packet Too Big message, so should not be vulnerable to + this attack. + + Both of these attacks can cause a black hole connection, that is, the + TCP three-way handshake completes correctly but the connection hangs + when data is transfered. A malicious party could also cause problems if it could stop a victim from receiving legitimate Packet Too Big messages, but in this case there are simpler denial-of-service attacks available. If ICMPv6 filtering prevents reception of ICMPv6 Packet Too Big messages, the source will not learn the actual path MTU. Packetization Layer Path MTU Discovery [RFC4821] does not rely upon network support for ICMPv6 messages and is therefore considered more - robust than standard PMTUD. It is not susceptible to "black holing" - of ICMPv6 message. See [RFC4890] for recommendations regarding - filtering ICMPv6 messages. + robust than standard PMTUD. It is not susceptible to "black holed" + 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. 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-09 (work in - progress), March 2017. + Specification", draft-ietf-6man-rfc2460bis-11 (work in + progress), April 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 - [CONG] Jacobson, V., "Congestion Avoidance and Control", Proc. - SIGCOMM '88 Symposium on Communications Architectures and - Protocols , August 1988. - [FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology , August 1987. [ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . + [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - + Communication Layers", STD 3, RFC 1122, DOI 10.17487/ + RFC1122, October 1989, + . + [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, . + [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery + for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August + 1996, . + + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ + RFC2119, March 1997, + . + + [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC + 2923, DOI 10.17487/RFC2923, September 2000, + . + + [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram + Congestion Control Protocol (DCCP)", RFC 4340, DOI + 10.17487/RFC4340, March 2006, + . + [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, . [RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/ RFC4890, May 2007, . + [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", + RFC 4960, DOI 10.17487/RFC4960, September 2007, + . + [RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, DOI 10.17487/RFC6691, July 2012, . [RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530, March 2015, . [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, @@ -721,43 +742,52 @@ messages Appendix B. Changes Since RFC 1981 This document is based on RFC1981 has the following changes from RFC1981: o Clarified Section 1 "Introduction" that the purpose of PMTUD is to reduce the need for IPv6 fragmentation. - o Added text to Section 1 "Introduction" and Section 6 "Security - Considerations" about the effects on PMTUD when ICMPv6 messages - are blocked. + o Added text to Section 1 "Introduction" about the effects on PMTUD + when ICMPv6 messages are blocked. + + 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 a short summary to the Section 1 "Introduction" of Packetization Layer Path MTU Discovery ((PLPMTUD) and a reference to RFC4821 that defines it. o Aligned text in Section 2 "Terminology" to match current packetization layer terminology. o Added clarification in Section 4 "Protocol Requirements" that - nodes should validate the payload of ICMP PTB message per RFC4443. + nodes should validate the payload of ICMP PTB message per RFC4443, + and that nodes should detect decreases in PMTU as fast as + possible. o Remove Note from Section 4 "Protocol Requirements" about a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU because this was removed from [I-D.ietf-6man-rfc2460bis]. o Added clarification in Section 5.2 "Storing PMTU information" to discard an ICMPv6 Packet Too Big message if it contains a MTU less than the IPv6 minimum link MTU. + o Added clarification Section 5.2 "Storing PMTU information" that + nodes with multiple interface, Path MTU information should be + stored for each link. + o Removed text in Section 5.2 "Storing PMTU information" about the RH0 routing header because it was deprecated by RFC5095. o Removed text about obsolete security classification from Section 5.2 "Storing PMTU information". o Changed title of Section 5.4 to "Packetization Layer actions" and changed to text in the first paragraph to to generalize this section to cover all packetization layers, not just TCP. @@ -765,33 +795,76 @@ normal packetization layer retransmission methods. o Removed text in Section 5.4 "Packetization Layer actions" that described 4.2 BSD because it is obsolete, and removed reference to 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 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 + 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. + + o Added clarification to Section 4 "Protocol Requirements" + that nodes should detect decreases in PMTU as fast as + possible. + + o Added clarification Section 5.2 "Storing PMTU information" + that nodes with multiple interface, Path MTU information + should be stored for each link. + + o Removed text in Section 5.2 about Retransmission because + it was unneeded. + + o Removed text in Section 5.3 about Retransmission because + it was unneeded. + + o Rewrote text in Section 5.4 "Packetization Layer actions" + regarding reception to make it clearer. + + o Rewrote the text at the end of Section 5.4 to remove + unnecessary details and clarify not change congestion + window. + + o Added references in Section 5.5 for SCTP and added DCCP + (and reference) the list of examples. + + o Added paragraph to Section 5.5 "Security Considerations" + about black hole connections if PTB messages are not + received, and comparison to PLPMTD. + + 07) Editorial changes. + 06) Revised Appendix B "Changes since RFC1981" to have a summary of changes since RFC1981 and a separate subsection with a change history of each Internet Draft. This subsection will be removed when the RFC is published. 06) Editorial changes based on comments received after publishing the -05 draft. 05) Changes based on IETF last call reviews by Gorry Fairhurst, Joe Touch, Susan Hares, Stewart Bryant, Rifaat Shekh-Yusef,