





Internet Engineering Task Force (IETF)                         J. McCann
Request for Comments: 8201                 Digital Equipment Corporation
STD: 87                                                       S. Deering
Obsoletes: 1981                                                  Retired
Category: Standards Track                                       J. Mogul
ISSN: 2070-1721                            Digital Equipment Corporation
                                                          R. Hinden, Ed.
                                                    Check Point Software
                                                               July 2017


                  Path MTU Discovery for IP version 6

Abstract

   This document describes Path MTU Discovery (PMTUD) for IP version 6.
   It is largely derived from RFC 1191, which describes Path MTU
   Discovery for IP version 4.  It obsoletes RFC 1981.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8201.



















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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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Protocol Requirements . . . . . . . . . . . . . . . . . . . .   7
   5.  Implementation Issues . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Layering  . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Storing PMTU Information  . . . . . . . . . . . . . . . .   9
     5.3.  Purging Stale PMTU Information  . . . . . . . . . . . . .  11
     5.4.  Packetization Layer Actions . . . . . . . . . . . . . . .  12
     5.5.  Issues for Other Transport Protocols  . . . . . . . . . .  13
     5.6.  Management Interface  . . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Comparison to RFC 1191 . . . . . . . . . . . . . . .  17
   Appendix B.  Changes Since RFC 1981 . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19





























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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
   and take advantage of paths with PMTU greater than the IPv6 minimum
   link MTU [RFC8200].  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
   link MTU defined in [RFC8200] 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 [RFC2923].  Path MTU
   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].  RFC 4821 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 RFC 1981
   used the "should/must" style language in upper and lower case, this
   document does not cite the RFC 2119 definitions and only uses lower
   case for these words.




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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.
                       Examples are transport protocols such as TCP and
                       UDP, control protocols such as ICMPv6, routing
                       protocols such as OSPF, and internet-layer or
                       lower-layer protocols being "tunneled" over
                       (i.e., encapsulated in) IPv6 such as Internetwork
                       Packet Exchange (IPX), AppleTalk, or IPv6 itself.

   link                a communication facility or medium over which
                       nodes can communicate at the link layer, i.e.,
                       the layer immediately below IPv6.  Examples are
                       Ethernets (simple or bridged); PPP links; X.25,
                       Frame Relay, or ATM networks; and internet-layer
                       or higher-layer "tunnels", such as tunnels over
                       IPv4 or IPv6 itself.

   interface           a node's attachment to a link.

   address             an IPv6-layer identifier for an interface or a
                       set of interfaces.

   packet              an IPv6 header plus payload.  The packet can have
                       a size less than or equal to the PMTU.
                       Alternatively, this can be a larger packet that
                       is fragmented into a series of fragments each
                       with a size less than or equal to the PMTU.

   link MTU            the maximum transmission unit, i.e., maximum
                       packet size in octets, that can be conveyed in
                       one piece over a link.

   path                the set of links traversed by a packet between a
                       source node and a destination node.

   path MTU            the minimum link MTU of all the links in a path
                       between a source node and a destination node.

   PMTU                path MTU.




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   Path MTU Discovery  the 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] [RFC1122].

   EMTU_R              Effective MTU for receiving; the largest packet
                       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 packets or change
   EMTU_S to cause the 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.

   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




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   generated, because in most cases the PMTU of the path will not have
   changed.  Therefore, attempts to detect increases in a path's PMTU
   should be done infrequently.

   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,
   as 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
   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 on receipt of a Packet Too Big message.

   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
   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.  Because each of these messages (and



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   the dropped packets they respond to) consume network resources, 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).

   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 (DoS) 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,
   but rather a set of notes provided as an aid for implementers.

   The issues include:

   -  What layer or layers implement Path MTU Discovery?

   -  How is the PMTU information cached?

   -  How is stale PMTU information removed?

   -  What must transport and higher layers do?

5.1.  Layering

   In the IP architecture, the choice of what size packet to send is
   made by a protocol at a layer above IP.  This memo refers to such a
   protocol as a "packetization protocol".  Packetization protocols are
   usually transport protocols (for example, TCP) but can also be
   higher-layer protocols (for example, protocols built on top of UDP).

   Implementing Path MTU Discovery in the packetization layers
   simplifies some of the inter-layer issues but has several drawbacks:
   the implementation may have to be redone for each packetization
   protocol, it becomes hard to share PMTU information between different



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   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"
   [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
   "Fragment Header", Section 4.5 of [RFC8200]).  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
   is to be transmitted, the source creates fragments, each limited by
   PMTU.  When source fragmentation is not desired, EMTU_S is set to
   PMTU, and the upper-layer protocol is expected to either perform its
   own fragmentation and reassembly or otherwise limit the size of its
   messages accordingly.

   However, packetization layers are encouraged to avoid sending
   messages that will require source fragmentation (for the case against
   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.  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.




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   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
   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.

   If flows [RFC8200] are in use, an implementation could use the flow
   id as the local representation of a path.  Packets sent to a
   particular destination but belonging to different flows may use
   different paths, as with ECMP, in which the choice of path might
   depend on the flow id.  This approach might result in the use of
   optimally sized packets on a per-flow basis, providing finer
   granularity than PMTU values maintained on a per-destination basis.

   For source-routed packets (i.e. packets containing an IPv6 Routing
   header [RFC8200]), the source route may further qualify the local
   representation of a path.

   Initially, the PMTU value for a path is assumed to be the (known) MTU
   of the first-hop link.

   When a Packet Too Big message is received, the node determines which
   path the message applies to based on the contents of the Packet Too
   Big message.  For example, if the destination address is used as the
   local representation of a path, the destination address from the
   original packet would be used to determine which path the message
   applies to.

      Note: if the original packet contained a Routing header, the
      Routing header should be used to determine the location of the
      destination address within the original packet.  If Segments Left
      is equal to zero, the destination address is in the Destination
      Address field in the IPv6 header.  If Segments Left is greater
      than zero, the destination address is the last address
      (Address[n]) in the Routing header.





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   The node then uses the value in the MTU field in the Packet Too Big
   message as a tentative PMTU value or the IPv6 minimum link MTU if
   that is larger, and compares the tentative PMTU to the existing PMTU.
   If the tentative PMTU is less than the existing PMTU estimate, the
   tentative PMTU replaces the existing PMTU as the PMTU value for the
   path.

   The packetization layers must be notified about decreases in the
   PMTU.  Any packetization layer instance (for example, a TCP
   connection) that is actively using the path must be notified if the
   PMTU estimate is decreased.

      Note: even if the Packet Too Big message contains an Original
      Packet Header that refers to a UDP packet, the TCP layer must be
      notified if any of its connections use the given path.

   Also, the instance that sent the packet that elicited the Packet Too
   Big message should be notified that its packet has been dropped, even
   if the PMTU estimate has not changed, so that it may retransmit the
   dropped data.

      Note: An implementation can avoid the use of an asynchronous
      notification mechanism for PMTU decreases by postponing
      notification until the next attempt to send a packet larger than
      the PMTU estimate.  In this approach, when an attempt is made to
      SEND a packet that is larger than the PMTU estimate, the SEND
      function should fail and return a suitable error indication.  This
      approach may be more suitable to a connectionless packetization
      layer (such as one using UDP), which (in some implementations) may
      be hard to "notify" from the ICMPv6 layer.  In this case, the
      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.

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.




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   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), 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 a link with a large MTU that 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.
   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)
   value received from its peer, which represents the EMTU_R, the
   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 Section 5, "Packet Size Issues",
   and Section 8.3, "Maximum Upper-Layer Payload Size", of [RFC8200] for
   information on selecting a value for the TCP MSS option.

   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.





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   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.  The
   packet size used in the retransmission should be no larger than the
   new PMTU.

      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
   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 that 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, 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 Path MTU, including supporting PMTU
   Discovery probe transmissions as needed.










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5.6.  Management Interface

   It is suggested that an implementation provides 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.

   The former can be accomplished by associating a flag with the path;
   when a packet is sent on a path with this flag set, the IP layer does
   not send packets larger than the IPv6 minimum link MTU.

   These features might be used to work around an anomalous situation or
   by a routing protocol implementation that is able to obtain Path MTU
   values.

   The implementation should also provide a way to change the timeout
   period for aging stale PMTU information.

6.  Security Considerations

   This Path MTU Discovery mechanism makes possible two DoS attacks,
   both based on a malicious party sending false Packet Too Big messages
   to a node.

      In the first attack, the false message indicates a PMTU much
      smaller than reality.  In response, the victim node should never
      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, must not raise its estimate of the PMTU
      based on a Packet Too Big message, so it 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 transferred.






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   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 DoS 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-holed"
   connections caused by the filtering of ICMPv6 messages.  See
   [RFC4890] for recommendations regarding filtering ICMPv6 messages.

7.  IANA Considerations

   This document does not require any IANA actions.

8.  References

8.1.  Normative References

   [ICMPv6]   Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <http://www.rfc-editor.org/info/rfc4443>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <http://www.rfc-editor.org/info/rfc8200>.

8.2.  Informative References

   [FRAG]     Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
              In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
              Communications Technology, DOI 10.1145/55483.55524, 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,
              <http://www.rfc-editor.org/info/rfc4861>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <http://www.rfc-editor.org/info/rfc1122>.




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   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <http://www.rfc-editor.org/info/rfc1191>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <http://www.rfc-editor.org/info/rfc2923>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <http://www.rfc-editor.org/info/rfc4340>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC4890]  Davies, E. and J. Mohacsi, "Recommendations for Filtering
              ICMPv6 Messages in Firewalls", RFC 4890,
              DOI 10.17487/RFC4890, May 2007,
              <http://www.rfc-editor.org/info/rfc4890>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <http://www.rfc-editor.org/info/rfc4960>.

   [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, DOI 10.17487/RFC6691, July 2012,
              <http://www.rfc-editor.org/info/rfc6691>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <http://www.rfc-editor.org/info/rfc7530>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <http://www.rfc-editor.org/info/rfc8085>.





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Appendix A.  Comparison to RFC 1191

   RFC 1981 (obsoleted by this document) was based in large part on RFC
   1191, which describes Path MTU Discovery for IPv4.  Certain portions
   of RFC 1191 were not needed in RFC 1981:

   router specification  Packet Too Big messages and corresponding
                         router behavior are defined in [ICMPv6]

   Don't Fragment bit    there is no DF bit in IPv6 packets

   TCP MSS discussion    selecting a value to send in the TCP MSS option
                         is discussed in [RFC8200]

   old-style messages    all Packet Too Big messages report the MTU of
                         the constricting link

   MTU plateau tables    not needed because there are no old-style
                         messages

Appendix B.  Changes Since RFC 1981

   This document is based on RFC 1981 and has the following changes from
   RFC 1981:

   o  Clarified in Section 1, "Introduction", that the purpose of PMTUD
      is to reduce the need for IPv6 fragmentation.

   o  Added text to Section 1, "Introduction", about the effects on
      PMTUD when ICMPv6 messages are blocked.

   o  Added a "Note" to the introduction to document that this
      specification doesn't cite RFC 2119 and only uses lower case
      "should/must" language.  Changed all upper case "should/must" to
      lower case.

   o  Added a short summary to Section 1, "Introduction", about PLPMTUD
      and a reference to RFC 4821 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 messages per RFC
      4443, and that nodes should detect decreases in PMTU as fast as
      possible.





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   o  Removed a "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 [RFC8200].

   o  Added clarification in Section 5.2, "Storing PMTU Information", to
      discard an ICMPv6 Packet Too Big message if it contains an MTU
      less than the IPv6 minimum link MTU.

   o  Added clarification in Section 5.2, "Storing PMTU Information",
      that for nodes with multiple interfaces, Path MTU information
      should be stored for each link.

   o  Removed text in Section 5.2, "Storing PMTU Information", about
      Routing Header type 0 (RH0) because it was deprecated by RFC 5095.

   o  Removed text about obsolete security classification from
      Section 5.2, "Storing PMTU Information".

   o  Changed the title of Section 5.4 to "Packetization Layer Actions"
      and changed the text in the first paragraph to generalize this
      section to cover all packetization layers, not just TCP.

   o  Clarified text in Section 5.4, "Packetization Layer Actions", to
      use 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 a paragraph to Section 6, "Security Considerations", about
      black-hole connections if PTB messages are not received and
      comparison to PLPMTUD.

   o  Updated "Acknowledgements".

   o  Editorial Changes.











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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 Working Group, area directorate reviewers, the IESG, and
   especially Joe Touch and Gorry Fairhurst.

Authors' Addresses

   Jack McCann
   Digital Equipment Corporation


   Stephen E. Deering
   Retired
   Vancouver, British Columbia
   Canada


   Jeffrey Mogul
   Digital Equipment Corporation


   Robert M. Hinden (editor)
   Check Point Software
   959 Skyway Road
   San Carlos, CA  94070
   United States of America

   Email: bob.hinden@gmail.com
















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