Network Working Group                                          M. Mathis
Internet-Draft                                                J. Heffner
Expires: March 2, 2007                                               PSC
                                                         August 29, 2006

                 Packetization Layer Path MTU Discovery

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes a robust method for Path MTU Discovery
   (PMTUD) that relies on TCP or some other Packetization Layer to probe
   an Internet path with progressively larger packets.  This method is
   described as an extension to RFC 1191 and RFC 1981, which specify
   ICMP based Path MTU Discovery for IP versions 4 and 6, respectively.

   The general strategy of the new algorithm is to start with a small
   MTU and search upward, testing successively larger MTUs by probing

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   with single packets.  If a probe is successfully delivered then the
   MTU can be raised.  If the probe is lost, it is treated as an MTU
   limitation and not as a congestion signal.

   Packetization Layer PMTUD (PLPMTUD) introduces some flexibility in
   the implementation of classical Path MTU discovery.  It can be
   configured to perform just ICMP black hole recovery to increase the
   robustness of classical Path MTU Discovery, or at the other extreme,
   all ICMP processing can be disabled and PLPMTUD can completely
   replace classical Path MTU Discovery.

   In the latter configuration, PLPMTUD exactly parallels congestion
   control.  An end-to-end transport protocol adjusts properties of the
   data stream (window size or packet size) while using packet losses to
   deduce the appropriateness of the adjustments.  This technique is
   more philosophically consistent with the end-to-end principle than
   relying on ICMP messages containing transcribed headers of multiple
   protocol layers.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Revision History . . . . . . . . . . . . . . . . . . . . .  5
       1.1.1.  Changes since version -07, July 2006 (IETF 66) . . . .  5
       1.1.2.  Changes since version -06, March 2006 (IETF 65)  . . .  6
       1.1.3.  Changes since version -05, November 2005 (IETF 64) . .  6
       1.1.4.  Changes since version -04, February 2005 (IETF 62) . .  6
       1.1.5.  Changes since version -03, October 2004 (IETF 61)  . .  6
       1.1.6.  Changes since version -02, July 19th 2004 (IETF 60)  .  6
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Accounting for header sizes  . . . . . . . . . . . . . . . 14
     5.2.  Storing PMTU information . . . . . . . . . . . . . . . . . 15
     5.3.  Accounting for IPsec . . . . . . . . . . . . . . . . . . . 16
     5.4.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  Common Packetization Properties  . . . . . . . . . . . . . . . 17
     6.1.  Mechanism to detect loss . . . . . . . . . . . . . . . . . 17
     6.2.  Generating probes  . . . . . . . . . . . . . . . . . . . . 17
   7.  The Probing Method . . . . . . . . . . . . . . . . . . . . . . 18
     7.1.  Packet size ranges . . . . . . . . . . . . . . . . . . . . 18
     7.2.  Selecting initial values . . . . . . . . . . . . . . . . . 19
     7.3.  Selecting probe size . . . . . . . . . . . . . . . . . . . 21
     7.4.  Probing preconditions  . . . . . . . . . . . . . . . . . . 21
     7.5.  Conducting a probe . . . . . . . . . . . . . . . . . . . . 22
     7.6.  Response to probe results  . . . . . . . . . . . . . . . . 22
       7.6.1.  Probe success  . . . . . . . . . . . . . . . . . . . . 22
       7.6.2.  Probe failure  . . . . . . . . . . . . . . . . . . . . 23
       7.6.3.  Probe timeout failure  . . . . . . . . . . . . . . . . 23
       7.6.4.  Probe inconclusive . . . . . . . . . . . . . . . . . . 24
     7.7.  Full stop timeout  . . . . . . . . . . . . . . . . . . . . 24
     7.8.  MTU verification . . . . . . . . . . . . . . . . . . . . . 24
   8.  Host Fragmentation . . . . . . . . . . . . . . . . . . . . . . 25
   9.  Application Probing  . . . . . . . . . . . . . . . . . . . . . 26
   10. Specific Packetization Layers  . . . . . . . . . . . . . . . . 27
     10.1. Probing method using TCP . . . . . . . . . . . . . . . . . 27
     10.2. Probing method using SCTP  . . . . . . . . . . . . . . . . 28
     10.3. Probing method for IP fragmentation  . . . . . . . . . . . 29
     10.4. Probing method using applications  . . . . . . . . . . . . 30
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     13.1. Normative references . . . . . . . . . . . . . . . . . . . 31
     13.2. Informative references . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34

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   Intellectual Property and Copyright Statements . . . . . . . . . . 35

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1.  Introduction

   This document describes a method for Packetization Layer Path MTU
   Discovery (PLPMTUD) which is an extension to existing Path MTU
   Discovery methods described in [RFC1191] and [RFC1981].  In the
   absence of ICMP messages, the proper MTU is determined by starting
   with small packets and probing with successively larger packets.  The
   bulk of the algorithm is implemented above IP, in the transport layer
   (e.g., TCP) or other "Packetization Protocol" that is responsible for
   determining packet boundaries.

   The methods described in this document rely on features of existing
   protocols.  They apply to many transport protocols over IPv4 and
   IPv6.  They do not require cooperation from the lower layers (except
   that they are consistent about what packet sizes are acceptable), or
   from peers.  As the methods apply only to senders, variants in
   implementations will not cause interoperability problems.

   For sake of clarity, we uniformly prefer TCP and IPv6 terminology.
   In the terminology section we also present the analogous IPv4 terms
   and concepts for the IPv6 terminology.  In a few situations we
   describe specific details that are different between IPv4 and IPv6.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document is a product of the Path MTU Discovery (pmtud) working
   group of the IETF and draws heavily on RFC1191 and RFC1981 for
   terminology, ideas, and some of the text.

1.1.  Revision History

   NOTE TO RFC EDITOR: this section to be removed before publication.

   These are all recent substantive changes, in reverse chronological
   order.  This section will be removed prior to publication as an RFC.

   Please send comments and suggestions to  Interim
   drafts and other useful information will be posted at .

1.1.1.  Changes since version -07, July 2006 (IETF 66)

   Last call comments from Gorry Fairhurst, Ivan Beschastnikh, and Mark
   Allman.  Nits and clarifications.

   Changed MAY to SHOULD supress congestion control response on failed

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1.1.2.  Changes since version -06, March 2006 (IETF 65)

   Changed the title to include "Packetization Layer".

   Renamed "Diagnostic Interface" section to "Application Probing" and
   broadened the language to include other uses.

   Clarifications to sections "packet size ranges", "host
   fragmentation", and "probing using applications".

   Language nits.

1.1.3.  Changes since version -05, November 2005 (IETF 64)

   Re-worked probing method sections for TCP and SCTP.  The SCTP section
   reflects the new PAD chunk type, and contains some text from Michael

   Made a number of language clarification and consistency improvements,
   largely from comments by Gorry Fairhurst.

   Added appropriate citations, and removed the last of the "@@" TODO

1.1.4.  Changes since version -04, February 2005 (IETF 62)

   General restructuring and rewriting of some sections based on new
   experience.  Relaxed and generalized a lot of over-specified
   language, for example, the search strategy description.

   Decoupled verification from probing, and relaxed its specification.

   Removed all specified changes to ICMP processing.  We decided this
   was out of scope for this particular document.

   Changed all language to refer to MTU rather than MPS.

1.1.5.  Changes since version -03, October 2004 (IETF 61)

   A number of minor style and grammar edits.

1.1.6.  Changes since version -02, July 19th 2004 (IETF 60)

   Many minor updates throughout the document.

   Added a section describing the interactions between PLPMTUD and

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   congestion control.

   Removed a difficult to implement requirement for future data to

   Added "IP Fragmentation" and "Application protocol" as Packetization

   Clarified interactions between TCP SACK and MTU.

   Updated SCTP section to reflect new probing method using "PAD

   Distilled the protocol specific material into separate subsections
   for each protocol.

   Added a section on common requirements and functions for all
   Packetization Layers.  More accurately characterized the
   "bidirectional" (and other) requirements of the PL protocol.  Updated
   the search strategy in this new section.

   Change "ICMP can't fragment" and "packet too big" to uniformly use
   "ICMP PTB message" everywhere.

   Added Stanislav Shalunov's observation that PLPMTUD parallels
   congestion control.

   Better described the range of interoperability with classical pMTUd
   in the introduction.

   Removed vague language about "not being a protocol" and "excessive

   Slightly redefined flow: the granularity of PLPMTUD within a path.

   Many English NITs and clarifications per Gorry Fairhurst and others.
   Passes strict xml2rfc checking.

   Add a paragraph encouraging interface MTUs that are the optimal for
   the NIC, rather than standard for the media.

   Added a revision history section.

2.  Overview

   Packetization Layer Path MTU Discovery (PLPMTUD) is a method for TCP
   or other Packetization Protocols to dynamically discover the MTU of a

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   path by probing with progressively larger packets.  It is most
   efficient when used in conjunction with the ICMP based Path MTU
   Discovery mechanism as specified in RFC 1191 and RFC 1981, but
   resolves many of the robustness problems of the classical techniques
   since it does not depend on the delivery of ICMP messages.

   This method is applicable to TCP and other transport- or application-
   level protocols that are responsible for choosing packet boundaries
   (e.g., segment sizes) and have an acknowledgment structure that
   delivers to the sender accurate and timely indications of which
   packets were lost.

   The general strategy is for the Packetization Layer to find an
   appropriate Path MTU by probing the path with progressively larger
   packets.  If a probe packet is successfully delivered, then the
   effective Path MTU is raised to the probe size.

   The isolated loss of a probe packet (with or without an ICMP Packet
   Too Big message) is treated as an indication of an MTU limit, and not
   as a congestion indicator.  In this case alone, the Packetization
   Protocol is permitted to retransmit any missing data without
   adjusting the congestion window.

   If there is a timeout or additional packets are lost during the
   probing process, the probe is considered to be inconclusive (e.g.,
   the lost probe does not necessarily indicate that the probe exceeded
   the Path MTU).  Furthermore, the losses are treated like any other
   congestion indication: window or rate adjustments are mandatory per
   the relevant congestion control standards [RFC2914].  Probing can
   resume after a delay which is determined by the nature of the
   detected failure.

   PLPMTUD uses a searching technique to find the Path MTU.  Each
   conclusive probe narrows the MTU search range, either by raising the
   lower limit on a successful probe or lowering the upper limit on a
   failed probe, converging toward the true Path MTU.  For most
   transport layers, the search should be stopped once the range is
   narrow enough that the benefit of a larger effective Path MTU is
   smaller than the search overhead of finding it.

   The most likely (and least serious) probe failure is the link
   experiencing congestion related losses while probing.  In this case
   it is appropriate to retry a probe of the same size as soon as the
   Packetization Layer has fully adapted to the congestion and recovered
   from the losses.  In other cases, additional losses or timeouts
   indicate problems with the link or Packetization Layer.  In these
   situations it is desirable to use longer delays depending on the
   severity of the error.

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   An optional verification process can be used to detect situations
   where raising the MTU raises the packet loss rate.  For example, if a
   link is striped across multiple physical channels with inconsistent
   MTUs, it is possible that a probe will be delivered even if it is too
   large for some of the physical channels.  In such cases, raising the
   Path MTU to the probe size can cause severe packet loss and abysmal
   performance.  After raising the MTU, the new MTU size can be verified
   by monitoring the loss rate.

   PLPMTUD introduces some flexibility in the implementation of
   classical Path MTU discovery, which is subject to protocol failures
   (connection hangs) if ICMP Packet Too Big (PTB) messages are not
   delivered or processed for some reason [RFC2923].  With PLPMTUD,
   classical Path MTU Discovery can include additional consistency
   checks (e.g., validating additional fields in the transcribed header)
   without increasing the risk of connection hangs due to spurious
   failures of the added checks.  Such changes to classical Path MTU
   Discovery are beyond the scope of this document.

   In the limiting case, all ICMP PTB messages might be unconditionally
   ignored, and PLPMTUD can be used as the sole method used to discover
   the Path MTU.  In this configuration, PLPMTUD parallels congestion
   control.  An end-to-end transport protocol adjusts properties of the
   data stream (window size or packet size) while using packet losses to
   deduce the appropriateness of the adjustments.  This technique seems
   to be more philosophically consistent with the end-to-end principle
   of the Internet than relying on ICMP messages containing transcribed
   headers of multiple protocol layers.

   Most of the difficulty in implementing PLPMTUD arises because it
   needs to be implemented in several different places within a single
   node.  In general, each Packetization Protocol needs to have its own
   implementation of PLPMTUD.  Furthermore, the natural mechanism to
   share Path MTU information between concurrent or subsequent
   connections over the same path is a path information cache in the IP
   layer.  The various Packetization Protocols need to have the means to
   access and update the shared cache in the IP layer.  This memo
   describes PLPMTUD in terms of its primary subsystems without fully
   describing how they are assembled into a complete implementation.

   The vast majority of the implementation details described in this
   document are recommendations based on experiences with earlier
   versions of Path MTU Discovery.  These recommendations are motivated
   by a desire to maximize robustness of PLPMTUD in the presence of less
   than ideal network conditions as they exist in the field.

   Section 3 provides a complete glossary of terms.

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   Section 4 describes the details of PLPMTUD that affect
   interoperability with other standards or Internet protocols.

   Section 5 describes how to partition PLPMTUD into layers, and how to
   manage the "path information cache" in the IP layer.

   Section 6 describes the general Packetization Layer properties and
   features needed to implement PLPMTUD.

   Section 7 describes how to use probes to search for the Path MTU.

   Section 8 recommends using IPv4 fragmentation in a configuration that
   mimics IPv6 functionality, to minimize future problems migrating to

   Section 9 describes a programing interface for implementing PLPMTUD
   in applications that choose their own packet boundaries and for tools
   to be able to diagnose path problems that interfere with Path MTU

   Section 10 discusses implementation details for specific protocols,
   including TCP.

3.  Terminology

   We use the following terms in this document:

   IP: Either IPv4 [RFC0791] or IPv6 [RFC2460].

   Node: A device that implements IP.

   Router: A node that forwards IP packets not explicitly addressed to

   Host: Any node that is not a router.

   Upper layer: A protocol layer immediately above IP.  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) IP such as
      IPX, AppleTalk, or IP itself.

   Link: A communication facility or medium over which nodes can
      communicate at the link layer, i.e., the layer immediately below
      IP.  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.  Occasionally we use

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      the slightly more general term "lower layer" for this concept.

   Interface: A node's attachment to a link.

   Address: An IP-layer identifier for an interface or a set of

   Packet: An IP header plus payload.

   MTU: Maximum Transmission Unit, the size in bytes of the largest IP
      packet, including the IP header and payload, that can be
      transmitted on a link or path.  Note that this could more properly
      be called the IP MTU, to be consistent with how other standards
      organizations use the acronym MTU.

   Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet size
      in bytes, that can be conveyed in one piece over a link.  Beware
      that this definition is different from the definition used by
      other standards organizations.

      For IETF documents, link MTU is uniformly defined as the IP MTU
      over the link.  This includes the IP header, but excludes link
      layer headers and other framing which is not part of IP or the IP

      Be aware that other standards organizations generally define link
      MTU to include the link layer headers.

   Path: The set of links traversed by a packet between a source node
      and a destination node.

   Path MTU, or PMTU: The minimum link MTU of all the links in a path
      between a source node and a destination node.

   Classical Path MTU Discovery: Process described in RFC 1191 and RFC
      1981, in which nodes rely on ICMP "Packet Too Big" (PTB) messages
      to learn the MTU of a path.

   Packetization Layer: The layer of the network stack which segments
      data into packets.

   Effective PMTU: The current estimated value for PMTU used by a
      Packetization Layer for segmentation.

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   PLPMTUD: Packetization Layer Path MTU Discovery, the method described
      in this document, which is an extension to classical PMTU

   PTB (Packet Too Big) message: An ICMP message reporting that an IP
      packet is too large to forward.  This is the IPv6 term that
      corresponds to the IPv4 "ICMP Can't fragment" message.

   Flow: A context in which MTU discovery algorithms can be invoked.
      This is naturally an instance of a Packetization Protocol, for
      example, one side of a TCP connection.

   MSS: The TCP Maximum Segment Size [RFC0793], the maximum payload size
      available to the TCP layer.  This is typically the Path MTU minus
      the size of the IP and TCP headers.

   Probe packet: A packet which is being used to test a path for a
      larger MTU.

   Probe size: The size of a packet being used to probe for a larger
      MTU, including IP headers.

   Probe gap: The payload data that will be lost and need to be
      retransmitted if the probe is not delivered.

   Leading window: Any unacknowledged data in a flow at the time a probe
      is sent.

   Trailing window: Any data in a flow sent after a probe, but before
      the probe is acknowledged.

   Search strategy: The heuristics used to choose successive probe sizes
      to converge on the proper Path MTU, as described in section
      Section 7.3.

   Full stop timeout: a timeout where none of the packets transmitted
      after some event are acknowledged by the receiver, including any
      retransmissions.  This is taken as an indication of some failure
      condition in the network, such as a routing change onto a link
      with a smaller MTU.  This is described in more detail in section
      Section 7.7.

4.  Requirements

   All Internet nodes SHOULD implement PLPMTUD in order to discover and
   take advantage of the largest MTU supported along the Internet path.

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   All links MUST enforce their MTU: links that might non-
   deterministically deliver packets that are larger than their rated
   MTU MUST consistently discard such packets.

   All hosts SHOULD use IPv4 fragmentation in a mode that mimics IPv6
   functionality.  All fragmentation SHOULD be done on the host, and all
   IPv4 packets, including fragments, SHOULD have the DF bit set such
   that they will not be fragmented (again) in the network.  See
   Section 8.

   The requirements below only apply to those implementations that
   include PLPMTUD.

   To use PLPMTUD a Packetization Layer MUST have a loss reporting
   mechanism that provides the sender with timely and accurate
   indications of which packets were lost in the network.

   Normal congestion control algorithms MUST remain in effect under all
   conditions except when only an isolated probe packet is detected as
   lost.  In this case alone the normal congestion (window or data rate)
   reduction SHOULD be suppressed.  If any other data loss is detected,
   standard congestion control MUST take place.

   Suppressed congestion control (as above) MUST be rate limited such
   that it occurs less frequently than the worst case loss rate for TCP
   congestion control at a comparable data rate over the same path
   (i.e., less than the "TCP-friendly" loss rate [tcp-friendly]).  This
   SHOULD be enforced by requiring a minimum headway between a
   suppressed congestion adjustment (due to a failed probe) and the next
   attempted probe, which is equal to one round trip time for each
   packet permitted by the congestion window.  Alternatively, this may
   be enforced by not suppressing congestion control if a second probe
   is lost too soon after the first lost probe.  This is discussed
   further in Section 7.6.2.

   Whenever the MTU is raised, the congestion state variables MUST be
   rescaled so as not to raise the window size in bytes (or data rate in
   bytes per seconds).

   Whenever the MTU is reduced (e.g., when processing ICMP PTB messages)
   the congestion state variable SHOULD be rescaled not to raise the
   window size in packets.

   If PLPMTUD updates the MTU for a particular path, all Packetization
   Layer sessions that share the path representation SHOULD be notified
   to make use of the new MTU and make the required congestion control

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   All implementations MUST include mechanisms for applications to
   selectively transmit packets larger than the current effective Path
   MTU (but smaller than the link MTU).  This is necessary to implement
   PLPMTUD within an application (using a connectionless protocol) and
   to implement diagnostic tools that do not rely on the operating
   systems implementation of Path MTU discovery.  See Section 9 for
   further discussion.

   Connectionless protocols and protocols that do not support PLPMTUD
   SHOULD have their own default value for the initial effective path
   MTU, which can be set to a more conservative (smaller) value than the
   initial value used by TCP and other protocols that are well suited to
   PLPMTUD.  Implementation MAY use different heuristics to select the
   initial effective path MTU for each protocol.  There SHOULD be per
   protocol and per route limits on the initial effective path MTU
   (eff_pmtu) and the upper searching limit (search_high).

5.  Layering

   Packetization Layer Path MTU Discovery is most easily implemented by
   splitting its functions between layers.  The IP layer is the best
   place to keep shared state, collect the ICMP messages, track IP
   header sizes and manage MTU information provided by the link layer
   interfaces.  However, the procedures that PLPMTUD uses for probing
   and verification of the Path MTU are very tightly coupled to features
   of the Packetization Layers, such as data recovery and congestion
   control state machines.

   Note that this layering approach is a direct extension of the advice
   in the current PMTUD specifications in RFC 1191 and RFC 1981.

5.1.  Accounting for header sizes

   The way in which PLPMTUD operates across multiple layers requires a
   mechanism for accounting header sizes at all layers between IP and
   the Packetization Layer (inclusive).  When transmitting non-probe
   packets, it is sufficient for the Packetization Layer to ensure an
   upper bound on final IP packet size, so as not to exceed the current
   effective Path MTU.  All Packetization Layers participating in
   classical Path MTU Discovery have this requirement already.  When
   conducting a probe, the Packetization Layer MUST determine the probe
   packet's final size including IP headers.  This requirement is
   specific to PLPMTUD, and satisfying it may require additional inter-
   layer communication in existing implementations.

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5.2.  Storing PMTU information

   This memo uses the concept of a "flow" to define the scope of the
   Path MTU discovery algorithms.  For many implementations, a flow
   would naturally correspond to an instance of each protocol (i.e.,
   each connection or session).  In such implementations, the algorithms
   described in this document are performed within each session for each
   protocol.  The observed PMTU (eff_pmtu in Section 7.1) can optionally
   be shared between different flows with a common path representation.

   Alternatively, PLPMTUD could be implemented such that its complete
   state is associated with the path representations.  Such an
   implementation could use multiple connections or sessions for each
   probe sequence.  This approach is likely to converge much more
   quickly in some environments, such as where an application uses many
   small connections, each of which is too short to complete the Path
   MTU Discovery process.

   Within a single implementation, different protocols can use either of
   these two approaches.  Due to protocol specific differences in
   constraints on generating probes (Section 6.2) and the MTU searching
   algorithm (Section 7.3), it may not be feasible for different
   Packetization Layer protocols to share PLPMTUD state.  This suggests
   that it may be possible for some protocols to share probing state,
   but other protocols can only share observed PMTU.  In this case, the
   different protocols will have different PMTU convergence properties.

   The IP layer is the best place to store cached PMTU values and other
   shared state such as MTU values reported by ICMP PTB messages.
   Ideally, this shared state 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.

   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, and integrates
   nicely with the conceptual model of a host as described in [RFC2461]:
   a PMTU value could be stored with the corresponding entry in the
   destination cache.  Storing the minimum value is suggested since NATs
   and other forms of middle boxes may exhibit differing PMTUs

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   simultaneously at a single IP address.

   Note that network or subnet numbers are not suitable to use as
   representations of a path, because there is not a general mechanism
   to determine the network mask at the remote host.

   If IPv6 flows are in use, an implementation could use the IPv6 flow
   id [RFC2460][RFC1809] 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 MTU values
   maintained on a per-destination basis.

   For source routed packets (i.e., packets containing an IPv6 routing
   header, or IPv4 LSRR or SSRR options), the source route may further
   qualify the local representation of a path.  An implementation could
   use source route information in the local representation of a path.

5.3.  Accounting for IPsec

   This document does not take a stance on the placement of IPsec
   [RFC2401], which logically sits between IP and the Packetization
   Layer.  The PLPMTUD implementation can treat IPsec either as part of
   IP or as part of the Packetization Layer, as long as the accounting
   is consistent within the implementation.  If IPsec is treated as part
   of the IP layer, then each security association to a remote node may
   need to be treated as a separate path.  If IPsec is treated as part
   of the Packetization Layer, the IPsec header size must be included in
   the Packetization Layer's header size calculations.

5.4.  Multicast

   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.

   It is worth noting that classical PMTUD does not work at all as ICMP
   PTB messages are never generated in response to packets with
   multicast destination addresses [RFC1112][RFC2460].

   Minimally, an implementation can maintain a single MTU value to be
   used for all multicast packets originated from the node.  This MTU
   should be sufficiently small that it is expected to be less than the
   path MTU of all paths comprising the multicast tree.  If a path MTU
   of less than the configured multicast MTU is learned via unicast
   means, the multicast MTU may be reduced to this value.  This approach

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   is likely to result in the use of smaller packets than is necessary
   for many paths.

   If the application using multicast gets complete delivery reports
   (unlikely because this requirement has poor scaling properties),
   PLPMTUD could be implemented in multicast protocols so that the
   smallest path MTU learned across a group becomes the effective MTU
   for that group.

6.  Common Packetization Properties

   This section describes general Packetization Layer properties and
   characteristics needed to implement PLPMTUD.  It also describes some
   implementation issues that are common to all Packetization Layers.

6.1.  Mechanism to detect loss

   It is important that the Packetization Layer has a timely and robust
   mechanism for detecting and reporting losses.  PLPMTUD makes MTU
   adjustments on the basis of detected losses.  Any delays or
   inaccuracy in loss notification is likely to result in incorrect MTU
   decisions or slow convergence.  It is important that the mechanism
   can robustly distinguish between the isolated loss of just a probe
   and other losses in the probe's leading and trailing windows.

   It is best if Packetization Protocols use an explicit loss detection
   mechanism such as a SACK scoreboard [RFC3517] or ACK Vector [RFC4340]
   to distinguish real losses from reordered data, although implicit
   mechanisms such as TCP Reno style duplicate acknowledgments counting
   are sufficient.

   PLPMTUD can also be implemented in protocols that rely on timeouts as
   their primary mechanism for loss recovery; however, timeouts should
   be used only when there are no other alternatives.

6.2.  Generating probes

   There are several possible ways to alter Packetization Layers to
   generate probes.  The different techniques incur different overheads
   in three areas: difficulty in generating the probe packet (in terms
   of Packetization Layer implementation complexity and extra data
   motion) possible additional network capacity consumed by the probes
   and the overhead of recovering from failed probes (both network and
   protocol overheads).

   Some protocols might be extended to allow arbitrary padding with
   dummy data.  This greatly simplifies the implementation because the

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   probing can be performed without participation from higher layers and
   if the probe fails, the missing data (the "probe gap") is assured to
   fit within the current MTU when it is retransmitted.  This is
   probably the most appropriate method for protocols that support
   arbitrary length options or multiplexing within the protocol itself.

   Many Packetization Layer protocols can carry pure control messages
   (without any data from higher protocol layers) which can be padded to
   arbitrary lengths.  For example, the SCTP PAD chunk can be used in
   this manner (see Section 10.2).  This approach has the advantage that
   nothing needs to be retransmitted if the probe is lost.

   These techniques do not work for TCP, because there is not a separate
   length field or other mechanism to differentiate between padding and
   real payload data.  With TCP the only approach is to send additional
   payload data in an over-sized segment.  There are at least two
   variants of this approach, discussed in Section 10.1.

   In a few cases, there may be no reasonable mechanisms to generate
   probes within the Packetization Layer protocol itself.  As a last
   resort, it may be possible to rely on an adjunct protocol, such as
   ICMP ECHO ("ping"), to send probe packets.  See Section 10.3 for
   further discussion of this approach.

7.  The Probing Method

   This section describes the details of the MTU probing method,
   including how to send probes and process error indications necessary
   to search for the Path MTU.

7.1.  Packet size ranges

   This document describes the probing method using three state

   search_low: The smallest useful probe size, minus one.  The network
      is expected to be able to deliver packets of size search_low.

   search_high: The greatest useful probe size.  The network is expected
      not to be able to deliver packets of size search_high.

   eff_pmtu: The effective PMTU for this flow.  This is the largest non-
      probe packet permitted by PLPMTUD for the path.

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               search_low          eff_pmtu         search_high
                   |                   |                  |
               non-probe size range
                               probe size range

   Figure 1

   When transmitting non-probes, the Packetization Layer SHOULD create
   packets of size less than or equal to eff_pmtu.

   When transmitting probes, the Packetization Layer MUST select a probe
   size which is larger than search_low and smaller or equal to

   When probing upward, eff_pmtu always equals search_low.  In other
   states, such as initial conditions, after ICMP PTB message processing
   or following PLPMTUD on another flow sharing the same path
   representation, eff_pmtu may be different from search_low.  Normally
   eff_pmtu will be greater than or equal to search_low and less than
   search_high.  It is generally expected but not required that probe
   size will be greater than eff_pmtu.

   For initial conditions when there is no information about the path,
   eff_pmtu may be greater than search_low.  The initial value of
   search_low should be conservatively low, but performance may be
   better if eff_pmtu starts at a higher, less conservative value.  See
   Section 7.2.

   If eff_pmtu is larger than search_low it is explicitly permitted to
   send non-probe packets larger than search_low.  When such a packet is
   acknowledged, it is effectively an "implicit probe" and search_low
   SHOULD be raised to the size of the acknowledged packet.  However, if
   an "implicit probe" is lost, it MUST NOT be treated as a probe
   failure as a true probe would be.  If eff_pmtu is too large, this
   condition will only be detected with ICMP PTB messages or black hole
   discovery (see Section 7.7).

7.2.  Selecting initial values

   The initial value for search_high should be the largest possible
   packet that might be supported by the flow.  This may be limited by
   the local interface MTU, by an explicit protocol mechanism such as
   the TCP MSS option, an intrinsic limit such as the size of a protocol
   length field, or by a configuration option to prevent probing above
   some maximum packet size.  Search_high is likely to be the same as
   the initial path MTU as computed by the classical path MTU discovery

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   It is recommended that search_low be initially set to an MTU size
   that is likely to work over a very wide range of environments.  Given
   today's technologies, a value of 512 bytes is probably safe.  For
   IPv6 flows, a value of 1280 bytes is appropriate.  The initial value
   for search_low SHOULD be configurable.

   Properly functioning Path MTU Discovery is critical to the robust and
   efficient operation of the Internet.  Any major change (as described
   in this document) has the potential to be very disruptive if it
   causes any unexpected changes in protocol behaviors.  The selection
   of the initial value for eff_pmtu determines to what extent a PLPMTUD
   implementation's behavior resembles classical PMTUD in cases where
   the classical method is sufficient.

   A conservative configuration would be to set eff_pmtu to search_high,
   and rely on ICMP PTB messages to set the eff_pmtu down as
   appropriate.  In this configuration classical PMTUD is fully
   functional and PLPMTUD is only invoked to recover from ICMP black
   holes through the procedure described in Section 7.7.

   In some cases where it is known that classical PMTUD is likely to
   fail, (for example, if ICMP PTB messages are administratively
   disabled for security reasons) using a small initial eff_pmtu will
   avoid the costly timeouts required for black hole detection.  The
   trade-off is that using a smaller than necessary initial eff_pmtu
   might cause reduced performance.

   Note that the initial eff_pmtu can be any value in the range
   search_low to search_high.  An initial eff_pmtu of 1400 bytes might
   be a good compromise because it would be safe for nearly all tunnels
   over all common networking gear, and yet close to the optimal MTU for
   the majority of paths in the Internet today.  This might be improved
   by using some statistics of other recent flows: for example the
   initial eff_pmtu for a flow might be set to the median of the probe
   size for all recent successful probes.

   Since the cost of PLPMTUD is dominated by the protocol specific
   overheads of generating and processing probes, it is probably
   desirable for each protocol to have its own heuristics to select the
   initial eff_pmtu.  It is especially important that connectionless
   protocols and other protocols that may not receive clear indications
   of ICMP black holes use conservative (smaller) initial values for
   eff_pmtu, as described in section Section 10.3.

   There SHOULD be per protocol and per route configuration options to
   override initial values for eff_pmtu and other PLPMTUD state

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7.3.  Selecting probe size

   The probe may have a size anywhere in the "probe size range"
   described above.  However, a number of factors affect the selection
   of an appropriate size.  A simple strategy might be to do a binary
   search halving the probe size range with each probe.  However, for
   some protocols such as TCP, failed probes are more expensive than
   successful ones, since data in a failed probe will need to be
   retransmitted.  For such protocols, a strategy using smaller probe
   sizes and "probing up" behaves better.  For many protocols, both at
   and above the Packetization Layer, the benefit of increasing MTU
   sizes may follow a step function such that it is not advantageous to
   probe within certain regions at all.

   As an optimization, it may be appropriate to probe at certain common
   or expected MTU sizes, for example, 1500 bytes for standard Ethernet,
   or 1500 bytes minus header sizes for tunnel protocols.

   Some protocols may use other mechanisms to choose the probe sizes.
   For example, protocols that have certain natural data block sizes
   might simply assemble messages from a number of blocks until the
   total size is smaller than search_high, and larger than search_low
   (if possible).

   Each Packetization Layer must determine when probing has converged,
   that is, when the probe size range is small enough that further
   probing is no longer worth its cost.  When probing has converged, a
   timer should be set.  When the timer expires, search_high should be
   reset to its initial value (described above) so that probing can
   resume.  Thus if the path changes, increasing the Path MTU, then the
   flow will eventually take advantage of it.  The value for this timer
   MUST NOT be less than 5 minutes, and is recommended to be 10 minutes,
   per RFC 1981.

7.4.  Probing preconditions

   Before sending a probe, the flow must at least meet the following
   o  It has no outstanding probes or losses.
   o  If the last probe failed or was inconclusive, then the probe
      timeout has expired (see Section 7.6.2).
   o  The available window is greater than the probe size.
   o  For a protocol using in-band data for probing, enough data is
      available to send the probe.

   For protocols that probe with in-band data, when not enough data is

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   available to probe, the protocol may wish to delay sending non-probes
   in order to accumulate enough data to send a probe.  A delayed
   sending algorithm such as Nagle [RFC0896] should be used to
   appropriately limit the time data is delayed.

   Some protocols may require additional packets after a loss to detect
   it promptly (e.g., TCP loss detection using duplicate
   acknowledgments).  Such a protocol should wait until sufficient data
   and window space is available so that it will be able to transmit
   enough data after the probe to trigger the loss detection mechanism
   in the event of a lost probe.

7.5.  Conducting a probe

   Once a probe size in the appropriate range has been selected, and the
   above preconditions have been met, the Packetization Layer may
   conduct a probe.  To do so, it creates a probe packet such that its
   size, including the outermost IP headers, is equal to the probe size.
   After sending the probe it awaits a response, which will have one of
   the following results:
   Success: The probe is acknowledged as having been received by the
      remote host.

   Failure: A protocol mechanism indicates that the probe was lost, but
      no packets in the leading or trailing window were lost.

   Timeout failure: A protocol mechanism indicates that the probe was
      lost, and no packets in the leading window were lost, but is
      unable to determine if any packets in the trailing window were
      lost.  For example, loss is detected by a timeout, and go-back-n
      retransmission is used.

   Inconclusive: The probe was lost in addition to other packets in the
      leading or trailing windows.

7.6.  Response to probe results

   When a probe has completed, the result should be processed as
   follows, categorized by the probe's result type.

7.6.1.  Probe success

   When the probe is delivered, it is an indication that the Path MTU is
   at least as large as the probe size.  Set search_low to the probe
   size.  If the probe size is larger than the eff_pmtu, raise eff_pmtu
   to the probe size.  The probe size might be smaller than the eff_pmtu
   if the flow has not been using the full MTU of the path because it is

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   subject to some other limitation, such as available data in an
   interactive session.

   Note that if a flow's packets are routed via multiple paths, or over
   a path with a non-deterministic MTU, delivery of a single probe
   packet does not indicate that all packets of that size will be
   delivered.  To be robust in such a case, the Packetization Layer
   should conduct MTU verification as described in Section 7.8.

7.6.2.  Probe failure

   When only the probe is lost, this is treated as an indication that
   the Path MTU is smaller than the probe size.  In this case alone, the
   loss SHOULD NOT be interpreted as congestion signal.

   In the absence of other indications, set search_high to the probe
   size minus one.  The eff_pmtu might be larger than the probe size if
   the flow has not been using the full MTU of the path because it is
   subject to some other limitation, such as available data in an
   interactive session.  If eff_pmtu is larger than the probe size,
   eff_pmtu MUST be reduced to no larger than search_high, and SHOULD be
   reduced to search_low, as the eff_pmtu has been determined to be
   invalid, similar to after a full stop timeout (see Section 7.7).

   If an ICMP PTB message is received matching the probe packet, then
   search_high and eff_pmtu may be set from the MTU value indicated in
   the message.  Note that the ICMP message may be received either
   before or after the protocol loss indication.

   A probe failure event is the one situation under which the
   Packetization Layer is permitted not to treat loss as a congestion
   signal.  Because there is some small risk that suppressing congestion
   control might have unanticipated consequences (even for one isolated
   loss), it is REQUIRED that probe failure events be less frequent than
   the normal period for losses under standard congestion control.
   Specifically, after a probe failure event and suppressed congestion
   control, PLPMTUD MUST NOT probe again until an interval which is
   comparable to the expected interval between congestion control
   events.  See Section 4 for details.  The simplest estimate of the
   interval to the next congestion event is the same number of round
   trips as the current congestion window in packets.

7.6.3.  Probe timeout failure

   If the loss was detected with a timeout and repaired with go-back-n
   retransmission, then congestion window reduction will be necessary.
   The relatively high price of a failed probe in this case may merit a
   longer timeout.  A timeout value of five times the non-timeout

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   failure case (Section 7.6.2) is recommended.

7.6.4.  Probe inconclusive

   The presence of other losses near the loss of the probe may indicate
   that the probe was lost due to congestion rather than because of an
   MTU limitation.  In this case, it is appropriate to update no state,
   and simply probe again when the probing preconditions are met (i.e.,
   when no recent losses have been observed).  At this point, it is
   particularly appropriate to re-probe since the flow's congestion
   window will be at its lowest point, minimizing the probability of
   congestive losses.

7.7.  Full stop timeout

   Under all conditions, a full stop timeout (also known as a
   "persistent timeout" in other documents) should be taken as an
   indication of some significantly disruptive event in the network,
   such as a router failure or a routing change to a path with a smaller
   MTU.  For TCP, this occurs when the R1 timeout threshold described by
   [RFC1122] expires.

   If there is a full stop timeout and there was not an ICMP message
   indicating a reason (PTB, Net unreachable, etc., or the ICMP message
   was ignored for some reason), the suggested first recovery action is
   to treat this as a detected ICMP black hole as defined in [RFC2923].

   The response to a detected black hole depends on the current values
   for search_low and eff_pmtu.  If eff_pmtu is larger than search_low,
   set eff_pmtu to search_low.  Otherwise, set both eff_pmtu and
   search_low to the initial value for search_low.  Upon further
   successive timeouts, search_low and eff_pmtu SHOULD be halved, with a
   lower bound of 68 bytes for IPv4 and 1280 bytes for IPv6.

7.8.  MTU verification

   It is possible for a flow to simultaneously traverse multiple paths,
   but it will only be able to keep a single path representation for the
   flow.  If the paths have different MTUs, storing the minimum MTU of
   all paths in the flow's path representation will result in correct
   behavior.  If ICMP PTB messages are delivered, then classical PMTUD
   will work correctly in this situation.

   If ICMP delivery fails, breaking classical PMTUD, the connection will
   rely solely on PLPMTUD.  However, in this case, PLPMTUD may fail as
   well since its requirement that links MUST NOT deliver packets larger
   than their MTU is violated.  A probe with a size greater than the
   minimum but smaller than the maximum of the Path MTUs may be

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   successful.  However, upon raising the flow's effective PMTU, the
   loss rate will significantly increase.  The flow may still make
   progress, but the resultant loss rate may be unacceptable.  For
   example, when using two-way round-robin striping, 50% of full-sized
   packets would be dropped.

   Striping in this manner is often operationally undesirable (e.g., due
   to packet reordering), and is usually avoided by hashing flows to a
   single path.  However, to increase robustness, an implementation
   should implement some form of MTU verification, such that if
   increasing eff_pmtu results in a sharp increase in loss rate, it will
   fall back to using a lower MTU.

   A recommended strategy would be to save the value of eff_pmtu before
   raising it.  Then, if loss rate rises above a threshold for a period
   of time (e.g., loss rate is higher than 10% over multiple RTO
   intervals), then the new MTU is considered incorrect.  The saved
   value of eff_pmtu can be restored, and search_high reduced in the
   same manner as in a probe failure.  PLPMTUD implementations SHOULD
   implement MTU verification.

8.  Host Fragmentation

   Packetization Layers are encouraged to avoid sending messages that
   will require fragmentation [Kent87] [I-D.heffner-frag-harmful].
   However, entirely preventing fragmentation is not always possible.
   Some Packetization Layers, such as a UDP application outside the
   kernel, may be unable to change the size of messages it sends,
   resulting in datagram sizes that exceed the Path MTU.

   IPv4 permitted such applications to send packets without the DF bit
   set.  Oversized packets without the DF bit set would be fragmented in
   the network or sending host when they encountered a link with a MTU
   smaller than the packet.  In some case, packets could be fragmented
   more than once if there were cascaded links with progressively
   smaller MTUs.  This approach is not recommended.

   It is recommended that IPv4 implementations use a strategy that
   mimics IPv6 functionality.  When an application sends datagrams that
   are larger than the effective Path MTU they should be fragmented to
   the Path MTU in the host IP layer even if they are smaller than the
   link MTU of the first network hop directly attached to the host.  The
   DF bit should be set on the fragments, so they will not be fragmented
   again in the network.  This technique will minimize the likelihood
   that applications will rely on IPv4 fragmentation in a way that
   cannot be implemented in IPv6.  At least one major operating system
   already uses this strategy.  An exception to this rule is if the

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   application indicates that it is sending an oversized packet for
   probing or diagnostic purposes, described in Section 9.

   Since protocols that do not implement PLPMTUD are still subject to
   the black hole problem, it may be desirable to present to these
   protocols a "safe" MTU likely to work on any path (e.g., 1280 bytes).
   Then, allow any protocol implementing PLPMTUD to operate in the full
   range supported by the lower layer.

   Note that IP fragmentation divides data into packets, so it is
   minimally a Packetization Layer.  However, it does not have a
   mechanism to detect lost packets, so it cannot support a native
   implementation of PLPMTUD.  Fragmentation-based PLPMTUD requires an
   adjunct protocol as described in Section 10.3.

9.  Application Probing

   All implementations MUST include a mechanism where applications using
   connectionless protocols can send their own probes.  This is
   necessary to implement PLPMTUD in an application protocol as
   described in Section 10.4 or to implement diagnostic tools for
   debugging problems with PMTUD.  There must be a mechanism that
   permits an application to send datagrams that are larger than
   eff_pmtu, the operating systems estimate of the path MTU, without
   being fragmented.  If these are IPv4 packets, they MUST have the DF
   bit set.

   At this time, most operating systems support two modes for sending
   datagrams: one which silently fragments packets that are too large,
   and another that rejects packets that are too large.  Neither of
   these modes is suitable for implementing PLPMTUD in an application or
   diagnosing problems with path MTU discovery.  A third mode is needed
   where the datagram is sent even if it is larger than the current
   estimate of the path MTU.

   Implementing PLPMTUD in an application also requires a mechanism
   where the application can inform the operating system about the
   outcome of the probe as described in Section 7.6, or directly update
   search_low, search_high and eff_pmtu, described in Section 7.1.

   Diagnostic applications are useful for finding PMTUD problems, such
   as those that might be caused by a buggy router than returns ICMP PTB
   messages with incorrect size information.  Such problems can be most
   quickly located with a tool that can send probes of any specified
   size, and collect and display all returned ICMP PTB messages.

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10.  Specific Packetization Layers

   This section discusses specific implementation details for different
   protocols that can be used as Packetization Layer protocols.  All
   Packetization Layer protocols must consider all of the issues
   discussed in Section 6.  For most protocols, it is self evident how
   to address many of these issues.  It is hoped that the protocols
   described here will be sufficient illustration for implementers to
   adapt other protocols.

10.1.  Probing method using TCP

   TCP has no mechanism to distinguish in-band data from padding.
   Therefore, TCP must generate probes by appropriately segmenting data.
   There are two approaches to segmentation: overlapping and non-

   In the non-overlapping method, data is segmented such that the probe
   and any subsequent segments contain no overlapping data.  If the
   probe is lost, the "probe gap" will be a full probe size minus
   headers.  Data in the probe gap will need to be retransmitted with
   multiple smaller segments.

             TCP sequence number

           t   <---->
           i         <-------->           (probe)
           m                   <---->
                         .                (probe lost)

                     <---->               (probe gap retransmitted)

   Figure 2

   An alternate approach is to send subsequent data overlapping the
   probe such that the probe gap is equal in length to the current MSS.
   In the case of a successful probe, this has added overhead in that it
   will send some data twice, but it will have to retransmit only one
   segment after a lost probe.  When a probe succeeds, there will likely
   be some duplicate acknowledgments generated due to the duplicate data
   sent.  It is important that these duplicate acknowledgments not
   trigger Fast Retransmit.  As such, an implementation using this
   approach SHOULD limit the probe size to three times the current MSS
   (causing at most 2 duplicate acknowledgments), or appropriately

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   adjust its duplicate acknowledgment threshold for data immediately
   after a successful probe.

             TCP sequence number

           t   <---->
           i         <-------->           (probe)
           m               <---->
           e                     <---->

                         .                (probe lost)

                     <---->               (probe gap retransmitted)

   Figure 3

   The choice of which segmentation method to use should be based on
   what is simplest and most efficient for a given TCP implementation.

10.2.  Probing method using SCTP

   In the SCTP protocol [RFC2960], the application writes messages to
   SCTP, which "chunkifies" them into smaller pieces suitable for
   transmission through the network.  Once a message has been
   chunkified, it is assigned a Transmission Sequence Number (TSN).
   Once a TSN have been transmitted, SCTP can not change the chunk size.
   SCTP multi-path support normally requires SCTP to chunkify its
   messages to fit the smallest PMTU of all paths.  Although not
   required, implementations may bundle multiple data chunks together to
   make larger IP packets to send on paths with a larger PMTU.  Note
   that SCTP must independently probe the PMTU on each path to the peer.

   The recommended method for generating probes is to add a chunk
   consisting only of padding to an SCTP message.  The PAD chunk defined
   in [I-D.tuexen-tsvwg-sctp-padding] SHOULD be attached to a minimum
   length HEARTBEAT chunk to build a probe packet.  This method is fully
   compatible with all current SCTP implementations.

   SCTP MAY also probe with a method similar to TCP's described above,
   using inline data.  Using such a method has the advantage that
   successful probes have no additional overhead; however, failed probes
   will require retransmission of data, which may impact flow

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10.3.  Probing method for IP fragmentation

   There are a few protocols and applications that normally send large
   datagrams and rely on IP fragmentation to deliver them.  It has been
   known for a long time that this has some undesirable consequences
   [Kent87].  More recently it has come to light that IPv4 fragmentation
   is not sufficiently robust for general use in today's Internet.  The
   16-bit IP identification field is not large enough to prevent
   frequent mis-associated IP fragments and the TCP and UDP checksums
   are insufficient to prevent the resulting corrupted data from being
   delivered to higher protocol layers [I-D.heffner-frag-harmful].

   As mentioned in Section 8, datagram protocols (such as UDP) might
   rely on IP fragmentation as a Packetization Layer.  However, using IP
   fragmentation to implement PLPMTUD is problematic because the IP
   layer has no mechanism to determine if the packets are ultimately
   delivered to the far node, without direct participation by the

   To support IP fragmentation as a Packetization Layer under an
   unmodified application, we propose to rely on the path MTU sharing
   described in Section 5.2 plus an adjunct protocol to probe the path
   MTU.  There are a number of protocols that might be used for the
   purpose, such as ICMP ECHO and ECHO REPLY, or "traceroute" style UDP
   datagrams that trigger ICMP messages.  Use of ICMP ECHO and ECHO
   REPLY will probe both forward and return paths, so the sender will
   only be able to take advantage of the minimum of the two.  Other
   methods that probe only the forward path are preffered if available.

   All of these approaches have a number of potential robustness
   problems.  The most likely failures are due to losses unrelated to
   MTU (e.g., nodes that discard some protocol types).  These non-MTU-
   related losses can prevent PLPMTUD from raising the MTU, forcing IP
   fragmentation to use a smaller MTU than necessary.  Since these
   failures are not likely to cause interoperability problems they are
   relatively benign.

   However there does exist other more serious failure modes, such as
   might be caused by middle boxes or upper layer routers that choose
   different paths for different protocol types or sessions.  In such
   environments, adjunct protocols may legitimately experience a
   different path MTU than the primary protocol.  If the adjunct
   protocol finds a larger MTU than the primary protocol, PLPMTUD may
   select an MTU that is not usable by the primary protocol.  Although
   this is a potentially serious problem, this sort of situation is
   likely to be viewed as broken by a large number of observers, and
   thus there will be strong motivation to correct it.

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   Since connectionless protocols might not keep enough state to
   effectively diagnose MTU black holes, it would be more robust to err
   on the side of using too small of an initial MTU (e.g., 1kBytes or
   less) prior to probing a path to measure the MTU.  For this reason we
   suggest that IP fragmentation use an initial eff_pmtu which is
   selected as described in Section 7.2, except using a separate global
   control for the default initial eff_mtu for connectionless protocols.

   Connectionless protocols also introduce an additional problem with
   maintaining the path information cache: there are no events
   corresponding to connection establishment and tear-down to use to
   manage the cache itself.  A natural approach would be to keep an
   immutable cache entry for the "default path", which has a eff_pmtu
   that is fixed at the initial value for connectionless protocols.  The
   adjunct path MTU discovery protocol would be invoked once the number
   of fragmented datagrams to any particular destination reaches some
   configurable threshold (e.g., 5 datagrams).  A new path cache entry
   would be created when the adjunct protocol updates eff_pmtu, and
   deleted on the basis of a timer or a Least Recently Used cache
   replacement algorithm.

10.4.  Probing method using applications

   The disadvantages of relying on IP fragmentation and an adjunct
   protocol to perform path MTU discovery can be overcome by
   implementing path MTU discovery within the application itself, using
   the application's own protocol.  The application must have some
   suitable method for generating probes and have an accurate and timely
   mechanism to determine if the probes were lost.

   Ideally the application protocol includes a lightweight echo function
   that confirms message delivery, plus a mechanism for padding the
   messages out to the desired probe size, such that the padding is not
   echoed.  This combination (akin to the SCTP HB plus PAD) is preferred
   because an application can separately measure the MTU of each
   direction on a path with asymmetrical MTUs.

   For protocols that cannot implement PLPMTUD with "echo plus pad"
   there are often alternate methods for generating probes.  For
   example, the protocol may have a variable length echo that
   effectively measures minimum MTU of both the forward and return path,
   or there may be a way to add padding to regular messages carrying
   real application data.  There may also be alternate ways to segment
   application data to generate probes, or as a last resort, it may be
   feasible to extend the protocol with new message types specifically
   to support MTU discovery.

   Note that if it is necessary to add new message types to support

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   PLPMTUD, the most general approach is to add ECHO and PAD messages,
   which permit the the greatest possible latitude in how an application
   specific implementation of PLPMTUD interacts with other applications
   and protocols on the same end system.

   All application probing techniques require the ability to send
   messages that are larger than the current eff_pmtu described in
   Section 9.

11.  Security Considerations

   Under all conditions the PLPMTUD procedures described in this
   document are at least as secure as the current standard Path MTU
   Discovery procedures described in RFC 1191 and RFC 1981.

   Since this algorithm is designed for robust operation without any
   ICMP or other messages from the network, PLPMTUD could be configured
   to ignore all ICMP messages, either globally or on a per application
   basis.  In such a configuration, it cannot be attacked unless the
   attacker can identify and cause probe packets to be lost.  Attacking
   PLPMTUD reduces performance, but not as much as attacking congestion
   control by causing arbitrary packets to be lost.  Such an attacker
   might do far more damage by completely disrupting specific other
   protocols, such as DNS.

   Since packetization protocols may share state with each other, if one
   packetization protocol (particularly an application) were hostile to
   other protocols on the same host, it could harm performance in the
   other protocols by reducing the effective MTU.  If a packetization
   protocol is untrusted, it should not be allowed to write to shared

12.  IANA Considerations


13.  References

13.1.  Normative references

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

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   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

13.2.  Informative references

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC1809]  Partridge, C., "Using the Flow Label Field in IPv6",
              RFC 1809, June 1995.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, September 2000.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC3517]  Blanton, E., Allman, M., Fall, K., and L. Wang, "A
              Conservative Selective Acknowledgment (SACK)-based Loss
              Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

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   [RFC0896]  Nagle, J., "Congestion control in IP/TCP internetworks",
              RFC 896, January 1984.

   [Kent87]   Kent, C. and J. Mogul, "Fragmentation considered harmful",
              Proc. SIGCOMM '87 vol. 17, No. 5, October 1987.

              Mahdavi, J. and S. Floyd, "TCP-Friendly Unicast Rate-Based
              Flow Control", Technical note sent to the end2end-interest
              mailing list , January 1997,

              Heffner, J., "Fragmentation Considered Very Harmful",
              draft-heffner-frag-harmful-01 (work in progress),
              April 2006.

              Tuexen, M. and R. Stewart, "Padding Chunk and Parameter
              for SCTP", draft-tuexen-tsvwg-sctp-padding-00 (work in
              progress), February 2006.

Appendix A.  Acknowledgements

   Many ideas and even some of the text come directly from RFC 1191 and
   RFC 1981.

   Many people made significant contributions to this document,
   including: Randall Stewart for SCTP text, Michael Richardson for
   material from an earlier ID on tunnels that ignore DF, Stanislav
   Shalunov for the idea that pure PLPMTUD parallels congestion control,
   and Matt Zekauskas for maintaining focus during the meetings.  Thanks
   to the early implementors: Kevin Lahey, John Heffner and Rao Shoaib
   who provided concrete feedback on weaknesses in earlier drafts.
   Thanks also to all of the people who made constructive comments in
   the working group meetings and on the mailing list.  I am sure I have
   missed many deserving people.

   Matt Mathis and John Heffner are supported in this work by a grant
   from Cisco Systems, Inc.

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Authors' Addresses

   Matt Mathis
   Pittsburgh Supercomputing Center
   4400 Fifth Avenue
   Pittsburgh, PA  15213

   Phone: 412-268-3319

   John W. Heffner
   Pittsburgh Supercomputing Center
   4400 Fifth Avenue
   Pittsburgh, PA  15213

   Phone: 412-268-2329

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