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rfc:rfc1981

Network Working Group J. McCann Request for Comments: 1981 Digital Equipment Corporation Category: Standards Track S. Deering

                                                            Xerox PARC
                                                              J. Mogul
                                         Digital Equipment Corporation
                                                           August 1996
                Path MTU Discovery for IP version 6

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

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.

Table of Contents

 1. Introduction.................................................2
 2. Terminology..................................................2
 3. Protocol overview............................................3
 4. Protocol Requirements........................................4
 5. Implementation Issues........................................5
 5.1. Layering...................................................5
 5.2. Storing PMTU information...................................6
 5.3. Purging stale PMTU information.............................8
 5.4. TCP layer actions..........................................9
 5.5. Issues for other transport protocols......................11
 5.6. Management interface......................................12
 6. Security Considerations.....................................12
 Acknowledgements...............................................13
 Appendix A - Comparison to RFC 1191............................14
 References.....................................................14
 Authors' Addresses.............................................15

McCann, Deering & Mogul Standards Track [Page 1] RFC 1981 Path MTU Discovery for IPv6 August 1996

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.  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.  This packet size is referred to as the Path MTU
 (PMTU), and it is equal to the minimum link MTU of all the links in a
 path.  IPv6 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 [IPv6-SPEC].  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 use the IPv6 minimum link
 MTU defined in [IPv6-SPEC] 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.

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 ICMP, routing protocols such as OSPF,
               and internet or lower-layer protocols being "tunneled"
               over (i.e., encapsulated in) IPv6 such as 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 (or higher) layer "tunnels",
               such as tunnels over IPv4 or IPv6 itself.
 interface   - a node's attachment to a link.

McCann, Deering & Mogul Standards Track [Page 2] RFC 1981 Path MTU Discovery for IPv6 August 1996

 address     - an IPv6-layer identifier for an interface or a set of
               interfaces.
 packet      - an IPv6 header plus payload.
 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
 Path MTU
 Discovery   - process by which a node learns the PMTU of a path
 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 [ICMPv6].  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 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.
 Alternatively, the node may elect to end the discovery process by
 ceasing to send packets larger than the IPv6 minimum link MTU.

McCann, Deering & Mogul Standards Track [Page 3] RFC 1981 Path MTU Discovery for IPv6 August 1996

 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
 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.
 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.

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.
 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.  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.
 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,

McCann, Deering & Mogul Standards Track [Page 4] RFC 1981 Path MTU Discovery for IPv6 August 1996

 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 reduce its estimate of the Path MTU below the IPv6
 minimum link MTU.
    Note: A node may receive a Packet Too Big message reporting a
    next-hop MTU that is less than the IPv6 minimum link MTU.  In that
    case, the node is not required to reduce the size of subsequent
    packets sent on the path to less than the IPv6 minimun link MTU,
    but rather must include a Fragment header in those packets [IPv6-
    SPEC].
 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,
 but rather a set of notes provided as an aid for implementors.
 The issues include:
  1. What layer or layers implement Path MTU Discovery?
  1. How is the PMTU information cached?
  1. How is stale PMTU information removed?
  1. 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).

McCann, Deering & Mogul Standards Track [Page 5] RFC 1981 Path MTU Discovery for IPv6 August 1996

 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
 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 ICMP 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
 MMS_S is derived from the Path MTU by subtracting the size of the
 IPv6 header plus space reserved by the IP layer for additional
 headers (if any).
 It is possible that a packetization layer, perhaps a UDP application
 outside the kernel, is unable to change the size of messages it
 sends.  This may result in a packet size that exceeds the Path MTU.
 To accommodate such situations, IPv6 defines a mechanism that allows
 large payloads to be divided into fragments, with each fragment sent
 in a separate packet (see [IPv6-SPEC] section "Fragment Header").
 However, packetization layers are encouraged to avoid sending
 messages that will require 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.
 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 in
 fact 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.

McCann, Deering & Mogul Standards Track [Page 6] RFC 1981 Path MTU Discovery for IPv6 August 1996

 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.  The set of paths in use to a
 particular destination is expected to be small, in many cases
 consisting of a single path.  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 [IPv6-SPEC] 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, with the choice of path depending on the flow id.
 This approach will 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 [IPv6-SPEC]), the source route may further qualify the local
 representation of a path.  In particular, a packet containing a type
 0 Routing header in which all bits in the Strict/Loose Bit Map are
 equal to 1 contains a complete path specification.  An implementation
 could use source route information in the local representation of a
 path.
    Note: Some paths may be further distinguished by different
    security classifications.  The details of such classifications are
    beyond the scope of this memo.
 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.

McCann, Deering & Mogul Standards Track [Page 7] RFC 1981 Path MTU Discovery for IPv6 August 1996

 The node then uses the value in the MTU field in the Packet Too Big
 message as a tentative PMTU value, 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 ICMP 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.
 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.

McCann, Deering & Mogul Standards Track [Page 8] RFC 1981 Path MTU Discovery for IPv6 August 1996

 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.
    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:
  1. The PMTU estimate is set to the MTU of the first hop link.
  1. The timestamp is set to the "reserved" value.
  1. Packetization layers using this path are notified of the increase.

5.4. TCP layer actions

 The TCP layer must track 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.  A simple implementation could ask the IP layer
 for this value each time it created a new segment, but this could be
 inefficient.  Moreover, TCP implementations that follow the "slow-
 start" congestion-avoidance algorithm [CONG] typically calculate and
 cache several other values derived from the PMTU.  It may be simpler
 to receive asynchronous notification when the PMTU changes, so that
 these variables may be updated.

McCann, Deering & Mogul Standards Track [Page 9] RFC 1981 Path MTU Discovery for IPv6 August 1996

 A TCP implementation must also store the MSS value received from its
 peer, and must not send any segment larger than this MSS, regardless
 of the PMTU.  In 4.xBSD-derived implementations, this may require
 adding an additional field to the TCP state record.
 The value sent in the TCP MSS option is independent of the PMTU.
 This MSS option value is used by the other end of the connection,
 which may be using an unrelated PMTU value.  See [IPv6-SPEC] 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 ICMP message.  It is sufficient
 to treat this as any other dropped segment, and wait until the
 retransmission timer expires to cause retransmission of the segment.
 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.
 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.
    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.
    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, which is always a multiple of the segment

McCann, Deering & Mogul Standards Track [Page 10] RFC 1981 Path MTU Discovery for IPv6 August 1996

 size).  In many systems (such as those derived from 4.2BSD), the
 segment size is often set to 1024 octets, and the maximum window size
 (the "send space") is usually a multiple of 1024 octets, so the
 proper relationship holds by default.  If Path MTU Discovery is used,
 however, the segment size may not be a submultiple of the send space,
 and it may change during a connection; this means that the TCP layer
 may need to change the transmission window size when Path MTU
 Discovery changes the PMTU value.  The maximum window size should be
 set to the greatest multiple of the segment size that is less than or
 equal to the sender's buffer space size.

5.5. Issues for other transport protocols

 Some transport protocols (such as ISO TP4 [ISOTP]) 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.
 The Sun Network File System (NFS) uses a Remote Procedure Call (RPC)
 protocol [RPC] that, when used over UDP, in many cases will generate
 payloads that must be fragmented even for the first-hop link.  This
 might improve performance in certain cases, but it is known to cause
 reliability and performance problems, especially when the client and
 server are separated by routers.
 It is recommended that NFS implementations use Path MTU Discovery
 whenever routers are involved.  Most NFS implementations allow the
 RPC datagram size to be changed at mount-time (indirectly, by
 changing the effective file system block size), but might require
 some modification to support changes later on.
 Also, since a single NFS operation cannot be split across several UDP
 datagrams, certain operations (primarily, those operating on file
 names and directories) require a minimum payload size that if sent in
 a single packet would exceed the PMTU.  NFS implementations should
 not reduce the payload size below this threshold, even if Path MTU
 Discovery suggests a lower value.  In this case the payload will be
 fragmented by the IP layer.

McCann, Deering & Mogul Standards Track [Page 11] RFC 1981 Path MTU Discovery for IPv6 August 1996

5.6. Management interface

 It is suggested that an implementation provide a way for a system
 utility program to:
  1. Specify that Path MTU Discovery not be done on a given path.
  1. 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 denial-of-
 service 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.  This should not entirely stop data flow, since the
 victim node should never set its PMTU estimate below the IPv6 minimum
 link MTU.  It will, however, result in 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.
 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.

McCann, Deering & Mogul Standards Track [Page 12] RFC 1981 Path MTU Discovery for IPv6 August 1996

Acknowledgements

 We would like to acknowledge the authors of and contributors to
 [RFC-1191], 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.

McCann, Deering & Mogul Standards Track [Page 13] RFC 1981 Path MTU Discovery for IPv6 August 1996

Appendix A - Comparison to RFC 1191

 This document is based in large part on RFC 1191, which describes
 Path MTU Discovery for IPv4.  Certain portions of RFC 1191 were not
 needed in this document:
 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 [IPv6-SPEC]
 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

References

 [CONG]      Van Jacobson.  Congestion Avoidance and Control.  Proc.
             SIGCOMM '88 Symposium on Communications Architectures and
             Protocols, pages 314-329.  Stanford, CA, August, 1988.
 [FRAG]      C. Kent and J. Mogul.  Fragmentation Considered Harmful.
             In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
             Communications Technology.  August, 1987.
 [ICMPv6]    Conta, A., and S. Deering, "Internet Control Message
             Protocol (ICMPv6) for the Internet Protocol Version 6
             (IPv6) Specification", RFC 1885, December 1995.
 [IPv6-SPEC] Deering, S., and R. Hinden, "Internet Protocol, Version
             6 (IPv6) Specification", RFC 1883, December 1995.
 [ISOTP]     ISO.  ISO Transport Protocol Specification: ISO DP 8073.
             RFC 905, SRI Network Information Center, April, 1984.
 [ND]        Narten, T., Nordmark, E., and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", Work in Progress.
 [RFC-1191]  Mogul, J., and S. Deering, "Path MTU Discovery",
             RFC 1191, November 1990.

McCann, Deering & Mogul Standards Track [Page 14] RFC 1981 Path MTU Discovery for IPv6 August 1996

 [RPC]       Sun Microsystems, Inc., "RPC: Remote Procedure Call
             Protocol", RFC 1057, SRI Network Information Center,
             June, 1988.

Authors' Addresses

 Jack McCann
 Digital Equipment Corporation
 110 Spitbrook Road, ZKO3-3/U14
 Nashua, NH 03062
 Phone: +1 603 881 2608
 Fax:   +1 603 881 0120
 Email: mccann@zk3.dec.com
 Stephen E. Deering
 Xerox Palo Alto Research Center
 3333 Coyote Hill Road
 Palo Alto, CA 94304
 Phone: +1 415 812 4839
 Fax:   +1 415 812 4471
 EMail: deering@parc.xerox.com
 Jeffrey Mogul
 Digital Equipment Corporation Western Research Laboratory
 250 University Avenue
 Palo Alto, CA 94301
 Phone: +1 415 617 3304
 EMail: mogul@pa.dec.com

McCann, Deering & Mogul Standards Track [Page 15]

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