TitlePacketization Layer Path MTU Discovery
AuthorM. Mathis, J. Heffner
DateMarch 2007
Format:TXT, HTML
Updated byRFC8899

Network Working Group                                          M. Mathis
Request for Comments: 4821                                    J. Heffner
Category: Standards Track                                            PSC
                                                              March 2007

                 Packetization Layer Path MTU Discovery

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.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Layering . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Accounting for Header Sizes  . . . . . . . . . . . . . . . 10
     5.2.  Storing PMTU Information . . . . . . . . . . . . . . . . . 11
     5.3.  Accounting for IPsec . . . . . . . . . . . . . . . . . . . 12
     5.4.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Common Packetization Properties  . . . . . . . . . . . . . . . 13
     6.1.  Mechanism to Detect Loss . . . . . . . . . . . . . . . . . 13
     6.2.  Generating Probes  . . . . . . . . . . . . . . . . . . . . 13
   7.  The Probing Method . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Packet Size Ranges . . . . . . . . . . . . . . . . . . . . 14
     7.2.  Selecting Initial Values . . . . . . . . . . . . . . . . . 16
     7.3.  Selecting Probe Size . . . . . . . . . . . . . . . . . . . 17
     7.4.  Probing Preconditions  . . . . . . . . . . . . . . . . . . 18
     7.5.  Conducting a Probe . . . . . . . . . . . . . . . . . . . . 18
     7.6.  Response to Probe Results  . . . . . . . . . . . . . . . . 19
       7.6.1.  Probe Success  . . . . . . . . . . . . . . . . . . . . 19
       7.6.2.  Probe Failure  . . . . . . . . . . . . . . . . . . . . 19
       7.6.3.  Probe Timeout Failure  . . . . . . . . . . . . . . . . 20
       7.6.4.  Probe Inconclusive . . . . . . . . . . . . . . . . . . 20
     7.7.  Full-Stop Timeout  . . . . . . . . . . . . . . . . . . . . 20
     7.8.  MTU Verification . . . . . . . . . . . . . . . . . . . . . 21
   8.  Host Fragmentation . . . . . . . . . . . . . . . . . . . . . . 22
   9.  Application Probing  . . . . . . . . . . . . . . . . . . . . . 23
   10. Specific Packetization Layers  . . . . . . . . . . . . . . . . 23
     10.1. Probing Method Using TCP . . . . . . . . . . . . . . . . . 23
     10.2. Probing Method Using SCTP  . . . . . . . . . . . . . . . . 25
     10.3. Probing Method for IP Fragmentation  . . . . . . . . . . . 26
     10.4. Probing Method Using Applications  . . . . . . . . . . . . 27
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 28
     12.2. Informative References . . . . . . . . . . . . . . . . . . 29
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 31

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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.

   This document does not update RFC 1191 or RFC 1981; however, since it
   supports correct operation without ICMP, it implicitly relaxes some
   of the requirements for the algorithms specified in those documents.

   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 which 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 RFC 1191 and RFC 1981 for
   terminology, ideas, and some of the text.

2.  Overview

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

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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 that is determined by the nature of the detected

   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 due to 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.

   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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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.

   Classical Path MTU Discovery 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 be modified to include additional
   consistency checks without increasing the risk of connection hangs
   due to spurious failures of the additional 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 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 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.

   This document does not contain a complete description of an
   implementation.  It only sketches details that do not affect
   interoperability with other implementations and have strong
   externally imposed optimality criteria (e.g., the MTU searching and

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   caching heuristics).  Other details are explicitly included because
   there is an obvious alternative implementation that doesn't work well
   in some (possibly subtle) case.

   Section 3 provides a complete glossary of terms.

   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 programming 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.

   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,

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

      Frame Relay, or Asynchronous Transfer Mode (ATM) networks; and
      Internet (or higher) layer "tunnels", such as tunnels over IPv4 or
      IPv6.  Occasionally we use 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.  Be
      aware 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 that 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 that segments
      data into packets.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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

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

   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 "Fragmentation Needed and DF Set"

   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 that 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 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 7.7.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

4.  Requirements

   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.

   In the distant past, there were a small number of network devices
   that did not enforce MTU, but could not reliably deliver oversized
   packets.  For example, some early bit-wise Ethernet repeaters would
   forward arbitrarily sized packets, but could not do so reliably due
   to finite hardware data clock stability.  This is the only
   requirement that PLPMTUD places on lower layers.  It is important
   that this requirement be explicit to forestall the future
   standardization or deployment of technologies that might be
   incompatible with PLPMTUD.

   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 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.  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).

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   Whenever the MTU is reduced (e.g., when processing ICMP PTB
   messages), the congestion state variable SHOULD be rescaled so as 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 (as described in
   Section 5.2) SHOULD be notified to make use of the new MTU and make
   the required congestion control adjustments.

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

   Implementations MAY use different heuristics to select the initial
   effective Path MTU for each protocol.  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.  There SHOULD be
   per-protocol and per-route limits on the initial effective Path MTU
   (eff_pmtu) and the upper searching limit (search_high).  See
   Section 7.2 for further discussion.

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

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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.

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) MAY 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 SHOULD be used to store the cached PMTU value 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 MAY 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

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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.  Since Network Address Translators (NATs) and
   other forms of middle boxes may exhibit differing PMTUs
   simultaneously at a single IP address, the minimum value SHOULD be

   Network or subnet numbers MUST NOT be used as representations of a
   path, because there is not a general mechanism to determine the
   network mask at the remote host.

   For source-routed packets (i.e., packets containing an IPv6 routing
   header, or IPv4 Loose Source and Record Route (LSRR) or Strict Source
   and Record Route (SSRR) options), the source route MAY further
   qualify the local representation of a path.  An implementation MAY
   use source route information in the local representation of a path.

   If IPv6 flows are in use, an implementation MAY use the 3-tuple of
   the Flow label and the source and destination addresses
   [RFC2460][RFC3697] as the local representation of a path.  Such an
   approach could theoretically 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.

5.3.  Accounting for IPsec

   This document does not take a stance on the placement of IP Security
   (IPsec) [RFC2401], which logically sits between IP and the
   Packetization Layer.  A 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

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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   Minimally, an implementation MAY 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
   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 since this requirement has poor scaling properties),
   PLPMTUD MAY be implemented in multicast protocols such 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 Selective Acknowledgment (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
   NOT be used as the primary mechanism for loss indication unless 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

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   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
   probing can be performed without participation from higher layers and
   if the probe fails, the missing data (the "probe gap") is ensured 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.  Packets of size
      search_high are expected to be too large for the network to

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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

               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 a size less than or equal to eff_pmtu.

   When transmitting probes, the Packetization Layer MUST select a probe
   size that is larger than search_low and smaller than 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).

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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, or by an intrinsic limit such as the size of a
   protocol length field.  In addition, the initial value for
   search_high MAY be limited by a configuration option to prevent
   probing above some maximum size.  Search_high is likely to be the
   same as the initial Path MTU as computed by the classical Path MTU
   Discovery algorithm.

   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 1024 bytes is probably safe enough.
   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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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 10.3.

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

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 that raises the probe
   size in smaller increments might have lower overhead.  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 if possible larger than

   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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

7.4.  Probing Preconditions

   Before sending a probe, the flow MUST meet at least 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.

   In addition, the timely loss detection algorithms in most protocols
   have pre-conditions that SHOULD be satisfied before sending a probe.
   For example, TCP Fast Retransmit is not robust unless there are
   sufficient segments following a probe; that is, the sender SHOULD
   have enough data queued and sufficient receiver window to send the
   probe plus at least Tcprexmtthresh [RFC2760] additional segments.
   This restriction may inhibit probing in some protocol states, such as
   too close to the end of a connection, or when the window is too

   Protocols MAY delay sending non-probes in order to accumulate enough
   data to meet the pre-conditions for probing.  The delayed sending
   algorithm SHOULD use some self-scaling technique to appropriately
   limit the time that the data is delayed.  For example, the returning
   ACKs can be used to prevent the window from falling by more than the
   amount of data needed for the 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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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 whether 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
   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, it 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).

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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 SHOULD ignore 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 that is
   larger than 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 time interval until the next probe.  A time interval that is
   five times the non-timeout failure case (Section 7.6.2) is

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 due to an MTU
   limitation.  In this case, the state variables eff_pmtu, search_low,
   and search_high SHOULD NOT be updated, and the same-sized probe
   SHOULD be attempted again as soon as the probing preconditions are
   met (i.e., once the packetization layer has no outstanding
   unrecovered losses).  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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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 RECOMMENDED first recovery action
   is to treat this as a detected ICMP black hole as defined in

   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 additional
   successive timeouts, search_low and eff_pmtu SHOULD be halved, with a
   lower bound of 68 bytes for IPv4 and 1280 bytes for IPv6.  Even lower
   lower bounds MAY be permitted to support limited operation over links
   with MTUs that are smaller than permitted by the IP specifications.

7.8.  MTU Verification

   It is possible for a flow to simultaneously traverse multiple paths,
   but an implementation 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

   If ICMP delivery fails, breaking classical PMTUD, the connection will
   rely solely on PLPMTUD.  In this case, PLPMTUD may fail as well since
   it assumes a flow traverses a path with a single MTU.  A probe with a
   size greater than the minimum but smaller than the maximum of the
   Path MTUs may be 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 is likely to 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 for other
   reasons (e.g., due to packet reordering) and is usually avoided by
   hashing each flow 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
   retransmission timeout (RTO) intervals), then the new MTU is

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   considered incorrect.  The saved value of eff_pmtu SHOULD 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 SHOULD avoid sending messages that will require
   fragmentation [Kent87] [frag-errors].  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 an 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
   MTU of the first link, 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.  Section 9 describes some exceptions to this rule
   when the application is sending oversized packets for probing or
   diagnostic purposes.

   Since protocols that do not implement PLPMTUD are still subject to
   problems due to ICMP black holes, it may be desirable to limit to
   these protocols to "safe" MTUs likely to work on any path (e.g., 1280
   bytes).  Allow any protocol implementing PLPMTUD to operate over 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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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 that 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
   REQUIRED 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 defective router that 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

10.  Specific Packetization Layers

   All Packetization Layer protocols must consider all of the issues
   discussed in Section 6.  For many protocols, it is straightforward to
   address these issues.  This section discusses specific details for
   implementing PLPMTUD with a couple of protocols.  It is hoped that
   the descriptions here will be sufficient illustration for
   implementers to adapt to additional 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-

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

             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 Stream Control Transmission Protocol (SCTP) [RFC2960], the
   application writes messages to SCTP, which divides the data into
   smaller "chunks" suitable for transmission through the network.  Each
   chunk is assigned a Transmission Sequence Number (TSN).  Once a TSN
   has been transmitted, SCTP cannot change the chunk size.  SCTP multi-
   path support normally requires SCTP to choose a chunk size such that
   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 [RFC4820] SHOULD be attached to a minimum length HEARTBEAT (HB)
   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

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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 [frag-errors].

   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 whether the packets are
   ultimately delivered to the far node, without direct participation by
   the application.

   To support IP fragmentation as a Packetization Layer under an
   unmodified application, an implementation SHOULD 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 preferred 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, other more serious failure modes do exist, 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 incorrect by a large number of observers, and
   thus there will be strong motivation to correct it.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   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., 1 kByte or
   less) prior to probing a path to measure the MTU.  For this reason,
   implementations that use IP fragmentation SHOULD 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 whether 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
   RECOMMENDED 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's, 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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   Note that if it is necessary to add new message types to support
   PLPMTUD, the most general approach is to add ECHO and PAD messages,
   which permit 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 PLPMTUD is designed for robust operation without any ICMP or
   other messages from the network, it can 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 protocols, such as

   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.  References

12.1.  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.

   [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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

   [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.

   [RFC3697]       Rajahalme, J., Conta, A., Carpenter, B., and S.
                   Deering, "IPv6 Flow Label Specification", RFC 3697,
                   March 2004.

   [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.

   [RFC4820]       Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk
                   and Parameter for the Stream Control Transmission
                   Protocol (SCTP)", RFC 4820, March 2007.

12.2.  Informative References

   [RFC2760]       Allman, M., Dawkins, S., Glover, D., Griner, J.,
                   Tran, D., Henderson, T., Heidemann, J., Touch, J.,
                   Kruse, H., Ostermann, S., Scott, K., and J. Semke,
                   "Ongoing TCP Research Related to Satellites",
                   RFC 2760, February 2000.

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

   [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.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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

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

   [tcp-friendly]  Mahdavi, J. and S. Floyd, "TCP-Friendly Unicast Rate-
                   Based Flow Control", Technical note sent to the
                   end2end-interest mailing list , January 1997, <http:/

   [frag-errors]   Heffner, J., "IPv4 Reassembly Errors at High Data
                   Rates", Work in Progress, December 2007.

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

Appendix A.  Acknowledgments

   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 versions.
   Thanks also to all of the people who made constructive comments in
   the working group meetings and on the mailing list.  We are sure we
   have missed many deserving people.

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

Authors' Addresses

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

   Phone: 412-268-3319
   EMail: mathis@psc.edu

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

   Phone: 412-268-2329
   EMail: jheffner@psc.edu

RFC 4821         Packetization Layer Path MTU Discovery       March 2007

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