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rfc:rfc4821

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).

Abstract

 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.

Mathis & Heffner Standards Track [Page 1] 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

Mathis & Heffner Standards Track [Page 2] 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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

Mathis & Heffner Standards Track [Page 3] 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
 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 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.

Mathis & Heffner Standards Track [Page 4] 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

Mathis & Heffner Standards Track [Page 5] 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
 IPv6.
 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
 Discovery.
 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,

Mathis & Heffner Standards Track [Page 6] 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
    interfaces.
 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
    payload.
    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.

Mathis & Heffner Standards Track [Page 7] 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"
    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 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.

Mathis & Heffner Standards Track [Page 8] 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).

Mathis & Heffner Standards Track [Page 9] 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

Mathis & Heffner Standards Track [Page 10] 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

Mathis & Heffner Standards Track [Page 11] 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
 stored.
 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
 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.

Mathis & Heffner Standards Track [Page 12] 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

Mathis & Heffner Standards Track [Page 13] RFC 4821 Packetization Layer Path MTU Discovery March 2007

 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
 variables:
 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
    deliver.

Mathis & Heffner Standards Track [Page 14] 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
 search_high.
 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).

Mathis & Heffner Standards Track [Page 15] 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.

Mathis & Heffner Standards Track [Page 16] 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
 variables.

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
 search_low.
 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.

Mathis & Heffner Standards Track [Page 17] 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
 conditions:
 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
 small.
 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.

Mathis & Heffner Standards Track [Page 18] 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).

Mathis & Heffner Standards Track [Page 19] 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
 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 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.

Mathis & Heffner Standards Track [Page 20] 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
 [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 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
 situation.
 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

Mathis & Heffner Standards Track [Page 21] 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.

Mathis & Heffner Standards Track [Page 22] 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
 messages.

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-
 overlapping.

Mathis & Heffner Standards Track [Page 23] 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                   <---->
         e
                       .
                       .                (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.

Mathis & Heffner Standards Track [Page 24] 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
 performance.

Mathis & Heffner Standards Track [Page 25] 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.

Mathis & Heffner Standards Track [Page 26] 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.

Mathis & Heffner Standards Track [Page 27] 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
 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
 state.

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.

Mathis & Heffner Standards Track [Page 28] 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.

Mathis & Heffner Standards Track [Page 29] 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:/
                 /www.psc.edu/networking/papers/tcp_friendly.html>.
 [frag-errors]   Heffner, J., "IPv4 Reassembly Errors at High Data
                 Rates", Work in Progress, December 2007.

Mathis & Heffner Standards Track [Page 30] 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
 USA
 Phone: 412-268-3319
 EMail: mathis@psc.edu
 John W. Heffner
 Pittsburgh Supercomputing Center
 4400 Fifth Avenue
 Pittsburgh, PA  15213
 US
 Phone: 412-268-2329
 EMail: jheffner@psc.edu

Mathis & Heffner Standards Track [Page 31] RFC 4821 Packetization Layer Path MTU Discovery March 2007

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Mathis & Heffner Standards Track [Page 32]

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