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Versions: 00 01 02

Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                                  T. Jones
Intended status: Standards Track                  University of Aberdeen
Expires: June 07, 2018                                         M. Tuexen
                                                            I. Ruengeler
                                 Muenster University of Applied Sciences
                                                        December 6, 2017

     Packetization Layer Path MTU Discovery for Datagram Transports
               draft-fairhurst-tsvwg-datagram-plpmtud-02

Abstract

   This document describes a robust method for Path MTU Discovery
   (PMTUD) for datagram Packetization layers.  The method allows a
   Packetization layer (or a datagram application that uses it) to probe
   an network path with progressively larger packets to determine a
   maximum packet size.  The document describes as an extension to RFC
   1191 and RFC 8201, which specify ICMP-based Path MTU Discovery for
   IPv4 and IPv6.  This provides functionally for datagram transports
   that is equivalent to the Packetization layer PMTUD specification for
   TCP, specified in RFC4821.

   When published, this specification updates RFC4821.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 07, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.







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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Features required to provide Datagram PLPMTUD  . . . . . . . .  6
     3.1.  PMTU Probe Packets . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Validation of the current effective PMTU . . . . . . . . .  9
     3.3.  Reduction of the effective PMTU  . . . . . . . . . . . . . 10
   4.  Datagram Packetization Layer PMTUD . . . . . . . . . . . . . . 10
     4.1.  Probing  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Verication and use of PTB messages . . . . . . . . . . . . 11
     4.3.  Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.4.  Constants  . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.5.  Variables  . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.6.  State Machine  . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Specification of Protocol-Specific Methods . . . . . . . . . . 15
     5.1.  DPLPMTUD for UDP and UDP-Lite  . . . . . . . . . . . . . . 16
       5.1.1.  UDP Options  . . . . . . . . . . . . . . . . . . . . . 16
       5.1.2.  UDP Options required for PLPMTUD . . . . . . . . . . . 16
         5.1.2.1.  Echo Request Option  . . . . . . . . . . . . . . . 16
         5.1.2.2.  Echo Response Option . . . . . . . . . . . . . . . 16
       5.1.3.  Sending UDP-Option Probe Packets . . . . . . . . . . . 17
       5.1.4.  Validating the Path with UDP Options . . . . . . . . . 17
       5.1.5.  Handling of PTB Messages by UDP  . . . . . . . . . . . 17
     5.2.  DPLPMTUD for SCTP  . . . . . . . . . . . . . . . . . . . . 17
       5.2.1.  SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . . 17
         5.2.1.1.  Sending SCTP Probe Packets . . . . . . . . . . . . 18
         5.2.1.2.  Validating the Path with SCTP  . . . . . . . . . . 18
         5.2.1.3.  PTB Message Handling by SCTP . . . . . . . . . . . 18
       5.2.2.  DPLPMTUD for SCTP/UDP  . . . . . . . . . . . . . . . . 18
         5.2.2.1.  Sending SCTP/UDP Probe Packets . . . . . . . . . . 18
         5.2.2.2.  Validating the Path with SCTP/UDP  . . . . . . . . 18
         5.2.2.3.  Handling of PTB Messages by SCTP/UDP . . . . . . . 19
       5.2.3.  DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . . 19
         5.2.3.1.  Sending SCTP/DTLS Probe Packets  . . . . . . . . . 19
         5.2.3.2.  Validating the Path with SCTP/DTLS . . . . . . . . 19
         5.2.3.3.  Handling of PTB Messages by SCTP/DTLS  . . . . . . 19
     5.3.  Other IETF Transports  . . . . . . . . . . . . . . . . . . 19
     5.4.  DPLPMTUD by Applications . . . . . . . . . . . . . . . . . 19
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20

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     9.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A. Event-driven state changes . . . . . . . . . . . . . . 22
   Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26

1.  Introduction

   The IETF has specified datagram transport using UDP, SCTP, and DCCP,
   as well as protocols layered on top of these transports (e.g., SCTP/
   UDP, DCCP/UDP).

   Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
   used with any transport that is able to process ICMP Packet Too Big
   (PTB) messages (e.g., [RFC1191] and [RFC8201]). It adjusts the
   effective Path MTU (PMTU), based on reception of ICMP Path too Big
   (PTB) messages to decrease the PMTU when a packet is sent with a size
   larger than the value supported along a path, and a method that from
   time-to-time increases the packet size in attempt to discover an
   increase in the supported PMTU.

   However, Classical PMTUD is subject to protocol failures.  One
   failure arises when traffic using a packet size larger than the
   actual supported PMTU is black-holed (all datagrams sent with this
   size are silently discarded). This could continue to happen when ICMP
   PTB messages are not delivered back to the sender for some reason
   [RFC2923]). For example, ICMP messages are increasingly filtered by
   middleboxes (including firewalls) [RFC4890], and in some cases are
   not correctly processed by tunnel endpoints.

   Another failure could result if a system not on the network path
   sends a PTB that attempts to force the sender to change the effective
   PMTU [RFC8201].  A sender can protect itself from reacting to such
   messages by utilising the quoted packet within the PTB message
   payload to verify that the received PTB message was generated in
   response to a packet that had actually been sent.  However, there are
   situations where a sender is unable to provide this verification
   (e.g., when the PTB message does not include sufficient information,
   often the case for IPv4; or where the information corresponds to an
   encrypted packet). Most routers implement RFC792 [RFC0792], which
   requires them to return only the first 64 bits of the IP payload of
   the packet, whereas RFC1812 [RFC1812] requires routers to return the
   full packet if possible.

   Even when the PTB message includes sufficient bytes of the quoted
   packet, the network layer could lack sufficient context to perform
   verification, because this depends on information about the active
   transport flows at an endpoint node (e.g., the socket/address pairs
   being used, and other protocol header information).

   The term Packetization Layer (PL) has been introduced to describe the
   layer that is responsible for placing data blocks into the payload of
   packets and selecting an appropriate maximum packet size.  This
   function is often performed by a transport protocol, but can also be




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   performed by other encapsulation methods working above the transport.
   PTB verification is more straight forward at the PL or at a higher
   layer.

   In contrast to PMTUD, Packetization Layer Path MTU Discovery
   (PLPMTUD) [RFC4821] does not rely upon reception and verification of
   PTB messages.  It is therefore more robust than Classical PMTUD. This
   has become the recommended approach for implementing PMTU discovery
   with TCP. It uses a general strategy where the PL searches for an
   appropriate PMTU by sending probe packets along the network path with
   a progressively larger packet size.  If a probe packet is
   successfully delivered (as determined by the PL), then the effective
   Path MTU is raised to the size of the successful probe.

   PLPMTUD introduces flexibility in the implementation of PMTU
   discovery.  At one extreme, it can be configured to only perform PTB
   black hole detection and recovery to increase the robustness of
   Classical PMTUD, or at the other extreme, all PTB processing can be
   disabled and PLPMTUD can completely replace Classical PMTUD. PLPMTUD
   can also include additional consistency checks without increasing the
   risk of increased blackholing.

   The UDP-Guidelines [RFC8085] state "an application SHOULD either use
   the path MTU information provided by the IP layer or implement Path
   MTU Discovery (PMTUD)", but does not provide a mechanism for
   discovering the largest size of unfragmented datagram than can be
   used on a path.  PLPMTUD has not currently been specified for UDP,
   while Section 10.2 of [RFC4821] recommends a PLPMTUD probing method
   for SCTP that utilises heartbeat messages as probe packets, but does
   not provide a complete specification.  This document provides the
   details to complete that specification.  Similarly, the method
   defined in this specification could be used with the Datagram
   Congestion Control Protocol (DCCP) [RFC4340] requires implementations
   to support Classical PMTUD and states that a DCCP sender "MUST
   maintain the maximum packet size (MPS) allowed for each active DCCP
   session".  It also defines the current congestion control maximum
   packet size (CCMPS) supported by a path.  This recommends use of
   PMTUD, and suggests use of control packets (DCCP-Sync) as path probe
   packets, because they do not risk application data loss.

   Section 4 of this document presents a set of algorithms for datagram
   protocols to discover a maximum size for the effective PMTU across a
   path.  The methods described rely on features of the PL Section 3 and
   apply to transport protocols over IPv4 and IPv6. It does not require
   cooperation from the lower layers (except that they are consistent
   about which packet sizes are acceptable). A method can utilise ICMP
   PTB messages when received messages are made available to the PL.

   Finally, Section 5 specifies the method for a set of transports, and
   provides information to enables the implementation of PLPMTUD with
   other datagram transports and applications that use datagram
   transports.



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2.  Terminology

   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].

   Other terminology is directly copied from [RFC4821], and the
   definitions in [RFC1122].

   Black-Holed: When the sender is unaware that packets are not
      delivered to the destination endpoint (e.g., when the sender
      transmits packets of a particular size with a previously known
      PMTU, but is unaware of a change to the path that resulted in a
      smaller PMTU).

   Classical Path MTU Discovery: Classical PMTUD is a process described
      in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
      learn the largest size of unfragmented datagram than can be used
      across a path.

   Datagram: A datagram is a transport-layer protocol data unit,
      transmitted in the payload of an IP packet.

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

   EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
      [RFC1122] as "the maximum IP datagram size that may be sent, for a
      particular combination of IP source and destination addresses...".

   EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
      [RFC1122] as the largest datagram size that can be reassembled by
      EMTU_R ("Effective MTU to receive").

   Link: A communication facility or medium over which nodes can
      communicate at the link layer, i.e., a layer below the IP layer.
      Examples are Ethernet LANs and Internet (or higher) layer and
      tunnels.

   Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes of
      the largest IP packet, including the IP header and payload, that
      can be transmitted over a link.  Note that this could more
      properly be called the IP MTU, to be consistent with how other
      standards organizations use the acronym MTU. This includes the IP
      header, but excludes link layer headers and other framing that is
      not part of IP or the IP payload.  Other standards organizations









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      generally define link MTU to include the link layer headers.

   MPS: The Maximum Packet Size (MPS), the largest size of application
      data block that can be sent unfragmented across a path.  In
      PLPMTUD this quantity is derived from Effective PMTU by taking
      into consideration the size of the application and lower protocol
      layer headers, and can be limited by the application protocol.

   Packet: An IP header plus the IP payload.

   Packetization Layer (PL): The layer of the network stack that places
      data into packets and performs transport protocol functions.

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

   Path MTU (PMTU): The minimum of the link MTU of all the links forming
      a path between a source node and a destination node.

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

   Probe packet: A datagram sent with a purposely chosen size (typically
      larger than the current Effective PMTU or MPS) to detect if
      messages of this size can be successfully sent along the end-to-
      end path.

3.  Features required to provide Datagram PLPMTUD

   TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
   All of the requirements in [RFC4821] also apply to use of the
   technique with a datagram PL. Unlike TCP, some datagram PLs require
   additional mechanisms to implement PLPMTUD.

   There are nine requirements for performing the datagram PLPMTUD
   method described in this specification:

   1.  PMTU parameters: A PLPMTUD sender is REQUIRED to provide
       information about the maximum size of packet that can be
       transmitted by the sender on the local link (the Link MTU and MAY
       utilize similar information about the receiver when this is
       supplied (note this could be less than EMTU_R). Some applications
       also have a maximum transport protocol data unit (PDU) size, in
       which case there is no benefit from probing for a size larger
       than this (unless a transport allows multiplexing multiple
       applications PDUs into the same datagram).

   2.  Effective PMTU: A datagram application MUST be able to choose the
       size of datagrams sent to the network, up to the effective PMTU,
       or a smaller value (such as the MPS) derived from this.  This
       value is managed by the PMTUD method.  The effective PMTU
       (specified in Section 1 of [RFC1191]) is equivalent to the EMTU_S
       (specified in [RFC1122]).

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   3.  Probe packets: On request, a PLPMTUD sender is REQUIRED to be
       able to transmit a packet larger than the current effective PMTU
       (but always with a total size less than the link MTU). The method
       can use this as a probe packet.  In IPv4, a probe packet is
       always sent with the Don't Fragment (DF) bit set and without
       network layer endpoint fragmentation.  In IPv6, a probe packet is
       always sent without source fragmentation (as specified in section
       5.4 of [RFC8201]).

   4.  Processing PTB messages: A PLPMTUD sender MAY optionally utilize
       PTB messages received from the network layer to help identify
       when a path does not support the current size of packet probe.
       Any received PTB message SHOULD/MUST be verified before it is
       used to update the PMTU discovery information [RFC8201].  This
       verification confirms that the PTB message was sent in response
       to a packet originating by the sender, and needs to be performed
       before the PMTU discovery method reacts to the PTB message.  When
       the router link MTU is indicated in the PTB message this MAY be
       used by datagram PLPMTUD to reduce the size of a probe, but MUST
       NOT be used increase the effective PMTU ([RFC8201]).

   5.  Reception feedback: The destination PL endpoint is REQUIRED to
       provide a feedback method that indicates when a probe packet has
       been received by the destination endpoint.  The local PL endpoint
       at the sending node is REQUIRED to pass this feedback to the
       sender-side PLPMTUD method.

   6.  Probing and congestion control: The isolated loss of a probe
       packet SHOULD NOT be treated as an indication of congestion and
       its loss does not directly trigger a congestion control reaction
       [RFC4821].

   7.  Probe loss recovery: If the data block carried by a probe message
       needs to be sent reliably, the PL (or layers above) MUST arrange
       retransmission/repair of any resulting loss.  This method MUST be
       robust in the case where probe packets are lost due to other
       reasons (including link transmission error, congestion). The
       PLPMTUD method treats isolated loss of a probe packet (with or
       without an PTB message) as a potential indication of a PMTU limit
       on the path.  The PL MAY retransmit any data included in a lost
       probe packet without adjusting its congestion window [RFC4821].

   8.  Cached effective PMTU: The sender MUST cache the effective PMTU
       value used by an instance of the PL between probes and needs also










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       to consider the disruption that could be incurred by an
       unsuccessful probe - both upon the flow that incurs a probe loss,
       and other flows that experience the effect of additional probe
       traffic.

   9.  Shared effective PMTU state: The PMTU value could also be stored
       with the corresponding entry in the destination cache and used by
       other PL instances.  The specification of PLPMTUD [RFC4821]
       states: "If PLPMTUD updates the MTU for a particular path, all
       Packetization Layer sessions that share the path representation
       (as described in Section 5.2 of [RFC4821]) SHOULD be notified to
       make use of the new MTU and make the required congestion control
       adjustments".  Such methods need to robust to the wide variety of
       underlying network forwarding behaviours.  Section 5.2 of
       [RFC8201] provides guidance on the caching of PMTU information
       and also the relation to IPv6 flow labels.

   In addition the following design principles are stated:

   o  Suitable MPS: The PLPMTUD method SHOULD avoid forcing an
      application to use an arbitrary small MPS (effective PMTU) for
      transmission while the method is searching for the currently
      supported PMTU. Datagram PLs do not necessarily support
      fragmentation of PDUs larger than the PMTU. A reduced MPS can
      adversely impact the performance of a datagram application.

   o  Path validation: The PLPMTUD method MUST be robust to path changes
      that could have occurred since the path characteristics were last
      confirmed.

   o  Datagram reordering: A method MUST be robust to the possibility
      that a flow encounters reordering, or has the traffic (including
      probe packets) is divided over more than one network path.

   o  When to probe: The PLPMTUD method SHOULD determine whether the
      path capacity has increased since it last measured the path.  This
      determines when the path should again be probed.

3.1.  PMTU Probe Packets

   PMTU discovery relies upon the sender being able to generate probe
   messages with a specific size.  TCP is able to generate probe packets
   by choosing to appropriately segment data being sent [RFC4821].

   In contrast, a datagram PL that needs to construct a probe packet has
   to either request an application to send a data block that is larger
   than that generated by an application, or to utilise padding
   functions to extend a datagram beyond the size of the application
   data block.  Protocols that permit exchange of control messages
   (without an application data block) could alternatively prefer to
   generate a probe packet by extending a control message with padding
   data.



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   When the method fails to validate the PMTU for the path, it may be
   required to send a probe packet with a size less than the size of the
   data block generated by an application.  In this case, the PL could
   provide a way to fragment a datagram at the PL, or could instead
   utilise a control packet with padding.

   A receiver needs to be able to distinguish an in-band data block from
   any added padding.  This is needed to ensure that any added padding
   is not passed on to an application at the receiver.

   This results in three possible ways that a sender can create a probe
   packet:

   Probing using appication data: A probe packet that contains a data
      block supplied by an application that matches the size required
      for the probe.  This method requests the application to issue a
      data block of the desired probe size.  If the application/
      transport needs protection from the loss of an unsuccessful probe
      packet, the application/transport needs then to perform transport-
      layer retransmission/repair of the data block (e.g., by
      retransmission after loss is detected or by duplicating the data
      block in a datagram without the padding).

   Probing using appication data and padding data: A probe packet that
      contains a data block supplied by an application that is combined
      with padding to inflate the length of the datagram to the size
      required for the probe.  If the application/transport needs
      protection from the loss of this probe packet, the application/
      transport may perform transport-layer retransmission/repair of the
      data block (e.g., by retransmission after loss is detected or by
      duplicating the data block in a datagram without the padding
      data).

   Probing using padding data: A probe packet that contains only control
      information together with any padding needed to inflate the packet
      to the size required for the probe.  Since these probe packets do
      not carry an application-supplied data block,they do not typically
      require retransmission, although they do still consume network
      capacity and incur endpoint processing.

   A datagram PLPMTUD MAY choose to use only one of these methods to
   simplify the implementation.

3.2.  Validation of the current effective PMTU

   The PL needs a method to determine when probe packets have been
   successfully received end-to-end across a network path.

   Transport protocols can include end-to-end methods that detect and
   report reception of specific datagrams that they send (e.g., DCCP and




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   SCTP provide keep-alive/heartbeat features). When supported, this
   mechanism SHOULD also be used by PLPMTUD to acknowledge reception of
   a probe packet.

   A PL that does not acknowledge data reception (e.g., UDP and UDP-
   Lite) is unable to detect when the packets it sends are discarded
   because their size is greater than the actual PMTUD. These PLs need
   to either rely on an application protocol to detect this, or make use
   of an additional transport method such as UDP-Options [I-D.ietf-
   tsvwg-udp-options].  In addition, they might need to send
   reachability probes (e.g., periodically solicit a response from the
   destination) to determine whether the current effective PMTU is still
   supported by the network path.

   Section Section 4 specifies this function for a set of IETF-specified
   protocols.

3.3.  Reduction of the effective PMTU

   When the current effective PMTU is no longer supported by the network
   path, the transport needs to detect this and reduce the effective
   PMTU.

   o  A PL that sends a datagram larger than the actual PMTU that
      includes no application data block, or one that does not attempt
      to provide any retransmission, can send a new probe packet with an
      updated probe size.

   o  A PL that wishes to resend the application data block, could then
      need to re-fragment the data block to a smaller packet size that
      is expected to traverse the end-to-end path.  This could utilise
      network-layer or PL fragmentation when these are available.  A
      fragmented datagram MUST NOT be used as a probe packet (see
      [RFC8201]).

   A method can additionally utilise PTB messages to detect when the
   actual PMTU supported by a network path is less than the current size
   of datagrams (or probe messages) that are being sent.

4.  Datagram Packetization Layer PMTUD

   This section specifies Datagram PLPMTUD.

   The central idea of PLPMTU discovery is probing by a sender.  Probe
   packets of increasing size are sent to find out the maximum size of a
   user message that is completely transferred across the network path
   from the sender to the destination.








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4.1.  Probing

   The PLPMTUD method utilises a timer to trigger the generation of
   probe packets.  The probe_timer is started each time a probe packet
   is sent to the destination and is cancelled when receipt of the probe
   packet is acknowledged.

   The PROBE_COUNT is initialised to zero when a probe packet is first
   sent with a particular size.  Each time the probe_timer expires, the
   PROBE_COUNT is incremented, and a probe packet of the same size is
   retransmitted.  The maximum number of retransmissions per probing
   size is configured (MAX_PROBES). If the value of the PROBE_COUNT
   reaches MAX_PROBES, probing will be stopped and the last successfully
   probed PMTU is set as the effective PMTU.

   Once probing is completed, the sender continues to use the effective
   PMTU until either a PTB message is received or the PMTU_RAISE_TIMER
   expires.  If the PL is unable to verify reachability to the
   destination endpoint after probing has completed, the method uses a
   REACHABILITY_TIMER to periodically repeat a probe packet for the
   current effective PMTU size, while the PMTU_RAISE_TIMER is running.
   If the resulting probe packet is not acknowledged (i.e.  the
   PROBE_TIMER expires), the method re-starts probing for the PMTU.

4.2.  Verication and use of PTB messages

   XXX A decision on SHOULD/MUST needs to be made XXX

   A node that receives a PTB message from a router or middlebox, SHOULD
   /MUST verify the PTB message.  The node checks the protocol
   information in the quoted payload to verify that the message
   originated from the sending node.  The node also checks that the
   reported MTU size is less than the size used by packet probes.  PTB
   messages are discarded if they fail to pass these checks, or where
   there is insufficient ICMP payload to perform these checks.  The
   checks are intended to provide protection from packets that originate
   from a node that is not on the network path or a node that attempts
   to report a larger MTU than the current probe size.

   PTB messages that have been verified can be utilised by the DPLPMTUD
   algorithm.  A method that utilises these PTB messages can improve
   performance compared to one that relies solely on probing.

4.3.  Timers

   This method utilises three timers:

   PROBE_TIMER: Configured to expire after a period longer than the
      maximum time to receive an acknowledgment to a probe packet.  This
      value MUST be larger than 1 second, and SHOULD be larger than 15
      seconds.  Guidance on selection of the timer value are provide in




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      section 3.1.1 of the UDP Usage Guidelines [RFC8085].

   PMTU_RAISE_TIMER: Configured to the period a sender ought to continue
      use the current effective PMTU, after which it re-commences
      probing for a higher PMTU. This timer has a period of 600 secs, as
      recommended by PLPMTUD [RFC4821].

   REACHABILITY_TIMER: Configured to the period a sender ought to wait
      before confirming the current effective PMTU is still supported.
      This is less than the PMTU_RAISE_TIMER.

      An application that needs to employ keep-alive messages to deliver
      useful service over UDP SHOULD NOT transmit them more frequently
      than once every 15 seconds and SHOULD use longer intervals when
      possible.  DPLPMTUD ought to suspend reachability probes when no
      application data has been sent since the previous probe packet.
      Guidance on selection of the timer value are provide in section
      3.1.1 of the UDP Usage Guidelines[RFC8085].

   An implementation could implement the various timers using a single
   timer process.

4.4.  Constants

   The following constants are defined:

   MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The default
      value of MAX_PROBES is 10.

   MIN_PMTU: The smallest allowed probe packet size.  This value is 1280
      bytes, as specified in [RFC2460].  For IPv4, the minimum value is
      68 bytes.  (An IPv4 routed is required to be able to forward a
      datagram of 68 octets without further fragmentation.  This is the
      combined size of an IPv4 header and the minimum fragment size of 8
      octets.)

   BASE_PMTU: The BASE_PMTU is a considered a size that ought to work in
      most cases.  The size is equal to or larger than the minimum
      permitted and smaller than the maximum allowed.  In the case of
      IPv6, this value is 1280 bytes [RFC2460].  When using IPv4, a size
      of 1200 is RECOMMENDED.

   MAX_PMTU: The MAX_PMTU is the largest size of PMTU that is probed.
      This has to be less than or equal to the minimum of the local MTU
      of the outgoing interface and the destination effective MTU for
      receiving.  An application or PL may reduce this when it knows
      there is no need to send packets above a specific size.

4.5.  Variables

   This method utilises a set of variables:




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   effective PMTU: The effective PMTU is the maximum size of datagram
      that the method has currently determined can be supported along
      the entire path.

   PROBED_SIZE: The PROBED_SIZE is the size of the current probe packet.
      This is a tentative value for the effective PMTU, which is
      awaiting confirmation by an acknowledgment.

   PROBE_COUNT: This is a count of the number of unsuccessful probe
      packets that have been sent with size PROBED_SIZE. The value is
      initialised to zero when a particular size of PROBED_SIZE is first
      attempted.

   PTB_SIZE: The PTB_Size is value returned by a verified PTB message
      indicating the local MTU size of a router along the path.

4.6.  State Machine

   A state machine for Datagram PLPMTUD is depicted in Figure 1. If
   multihoming is supported, a state machine is needed for each active
   path.

































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                                         PROBE_TIMER expiry
                                      (PROBE_COUNT = MAX_PROBES)
                           +-------------+                +--------------+
                        =->| PROBE_START |--------------->|PROBE_DISABLED|
     PROBE_TIMER expiry |  +-------------+                +--------------+
    (PROBE_COUNT =      |     |       |
            MAX_PROBES) -------       |  Connectivity confirmed
                                      v
                     ----------- +------------+ -- PROBE_TIMER expiry
   MAX_PMTU acked    |           | PROBE_BASE |  | (PROBE_COUNT <
   PTB (>= BASE_PMTU)|    -----> +------------+ <-             MAX_PROBES)
     ----------------     |          /\   |  |
     |                    |           |   |  | PTB
     |    PMTU_RAISE_TIMER|           |   |  | (PTB_SIZE < BASE_PMTU)
     |    or reachability |           |   |  |        or
     |     (PROBE_COUNT   |           |   |  |    PROBE_TIMER expiry
     |      = MAX_PROBES) |           |   |  | (PROBE_COUNT = MAX_PROBES)
     |        -------------           |   |   \
     |        |                   PTB |   |    \
     |        |        (< PROBED_SIZE)|   |     \
     |        |                       |   |      ----------------
     |        |                       |   |                     |
     |        |                       |   | Probe               |
     |        |                       |   | acked               |
     v        |                       |   v                     v
   +------------+                +--------------+  Probe +-------------+
   | PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |
   +------------+ MAX_PMTU acked +--------------+  acked +-------------+
    /\    |             or            /\      |
     |    |      PROBE_TIMER expiry    |      |
     |    |(PROBE_COUNT = MAX_PROBES)  |      |
     |    |                            |      |
     ------                            --------
   Reachability probe acked      PROBE_TIMER expiry
    or PROBE_TIMER expiry      (PROBE_COUNT < MAX_PROBES)
   (PROBE_COUNT < MAX_PROBES)            or
                                   Probe acked

   The following states are defined to reflect the probing process:

   PROBE_START: The PROBE_START state is the initial state before
      probing has started.  PLPMTUD is not performed in this state.  The
      state transitions to PROBE_BASE, when a path has been confirmed,
      i.e.  when a sent packet has been acknowledged on this path.  The
      effective PMTU is set to the BASE_PMTU size.  Probing ought to
      start immediately after connection setup to prevent the loss of
      user data.







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   PROBE_BASE: The PROBE_BASE state is the starting point for probing
      with datagram PLPMTUD. It is used to confirm whether the BASE_PMTU
      size is supported by the network path.  On entry, the PROBED_SIZE
      is set to the BASE_PMTU size and the PROBE_COUNT is set to zero.
      A probe packet is sent, and the PROBE_TIMER is started.  The state
      is left when the PROBE_COUNT reaches MAX_PROBES; a PTB message is
      verified, or a probe packet is acknowledged.

   PROBE_SEARCH: The PROBE_SEARCH state is the main probing state.  This
      state is entered either when probing for the BASE_PMTU was
      successful or when there is a successful reachability test in the
      PROBE_ERROR state.  On entry, the effective PMTU is set to the
      last acknowledged PROBED_SIZE.

      On the first probe packet for each probed size, the PROBE_COUNT is
      set to zero.  Each time a probe packet is acknowledged, the
      effective PMTU is set to the PROBED_SIZE, and then the PROBED_SIZE
      is increased.  When a probe packet is not acknowledged within the
      period of the PROBE_TIMER, the PROBE_COUNT is incremented and the
      probe packet is retransmitted.  The state is exited when the
      PROBE_COUNT reaches MAX_PROBES; a PTB message is verified; or a
      probe of size PMTU_MAX is acknowledged.

   PROBE_ERROR: The PROBE_ERROR state represents the case where the
      network path is not known to support an effective PMTU of at least
      the BASE_PMTU size.  It is entered when either a probe of size
      BASE_PMTU has not been acknowledged or a verified PTB message
      indicates a smaller link MTU than the BASE_PMTU. On entry, the
      PROBE_COUNT is set to zero and the PROBED_SIZE is set to the
      MIN_PMTU size, and the effective PMTU is reset to MIN_PMTU size.
      In this state, a probe packet is sent, and the PROBE_TIMER is
      started.  The state transitions to the PROBE_SEARCH state when a
      probe packet is acknowledged.

   PROBE_DONE: The PROBE_DONE state indicates a successful end to a
      probing phase.  Datagram PLPMTUD remains in this state until
      either the PMTU_RAISE_TIMER expires or a PTB message is verified.

      When PLPMTUD uses an unacknowledged PL and is in the PROBE_DONE
      state, a REACHABILITY_TIMER periodically resets the PROBE_COUNT
      and schedules a probe packet with the size of the effective PMTU.
      If the probe packet fails to be acknowledged after MAX_PROBES
      attempts, the method enters the PROBE_BASE state.  When used with
      an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
      probe in this state.

   PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity
      could not be established.  DPLPMTUD MUST NOT probe in this state.

   Appendix Appendix A contains an informative description of key
   events.

5.  Specification of Protocol-Specific Methods

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   This section specifies protocol-specific details for datagram PLPMTUD
   for IETF-specified transports.

5.1.  DPLPMTUD for UDP and UDP-Lite

   The current specifications of UDP [RFC0768] and UDP-LIte [RFC3828] do
   not define a method in the RFC-series that supports PLPMTUD. In
   particular, these transports do not provide the transport layer
   features needed to implement datagram PLPMTUD, and any support for
   Datagram PLPMTUD would therefore need to rely on higher-layer
   protocol features [RFC8085].

5.1.1.  UDP Options

   UDP-Options [I-D.ietf-tsvwg-udp-options] supply the additional
   functionality required to implement datagram PLPMTUD. This enables
   padding to be added to UDP datagrams and can be used to provide
   feedback acknowledgement of received probe packets.

5.1.2.  UDP Options required for PLPMTUD

   This subsection proposes two new UDP-Options that add support for
   requesting a datagram response be sent and to mark this datagram as a
   response to a request.

   XXX << Future versions of the spec may define a parameter in an
   Option to indicate the EMTU_R to the peer.>>

5.1.2.1.  Echo Request Option

   The Echo Request Option allows a sending endpoint to solicit a
   response from a destination endpoint.

   The Echo Request carries a four byte token set by the sender.  This
   token can be set to a value that is likely to be known only to the
   sender (and becomes known to nodes along the end-to-end path). The
   sender can then check the value returned in the response to provide
   additional protection from off-path insertion of data [RFC8085].

        +---------+--------+-----------------+
        | Kind=9  | Len=6  |     Token       |
        +---------+--------+-----------------+
          1 byte    1 byte       4 bytes

5.1.2.2.  Echo Response Option

   The Echo Response Option is generated by the PL in response to
   reception of a previously received Echo Request.  The Token field
   associates the response with the Token value carried in the most
   recently-received Echo Request.  The rate of generation of UDP




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   packets carrying an Echo Response Option MAY be rate-limited.

        +---------+--------+-----------------+
        | Kind=10 | Len=6  |     Token       |
        +---------+--------+-----------------+
          1 byte    1 byte       4 bytes

5.1.3.  Sending UDP-Option Probe Packets

   This method specifies a probe packet that does not carry an
   application data block.  The probe packet consists of a UDP datagram
   header followed by a UDP Option containing the ECHOREQ option, which
   is followed by NOP Options to pad the remainder of the datagram
   payload to the probe size.  NOP padding is used to control the length
   of the probe packet.

   A UDP Option carrying the ECHORES option is used to provide feedback
   when a probe packet is received at the destination endpoint.

5.1.4.  Validating the Path with UDP Options

   Since UDP is an unacknowledged PL, a sender that does not have
   higher-layer information confirming correct delivery of datagrams
   SHOULD implement the REACHABILITY_TIMER to periodically send probe
   packets while in the PROBE_DONE state.

5.1.5.  Handling of PTB Messages by UDP

   Normal ICMP verification MUST be performed as specified in Section
   5.2 of [RFC8085].  This requires that the PL verifies each received
   PTB messages to verify these are received in response to transmitted
   traffic and that the reported LInk MTU is less than the current probe
   size.  A verified PTB message MAY be used as input to the PLPMTUD
   algorithm.

5.2.  DPLPMTUD for SCTP

   Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
   method for SCTP. It recommends the use of the PAD chunk, defined in
   [RFC4820] to be attached to a minimum length HEARTBEAT chunk to build
   a probe packet.  This enables probing without affecting the transfer
   of user messages and without interfering with congestion control.
   This is preferred to using DATA chunks (with padding as required) as
   path probes.

   XXX << Future versions of this specification might define a parameter
   contained in the INIT and INIT ACK chunk to indicate the MTU to the
   peer.  However, multihoming makes this a bit complex, so it might not
   be worth doing.>>

5.2.1.  SCTP/IP4 and SCTP/IPv6

   The base protocol is specified in [RFC4960].


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5.2.1.1.  Sending SCTP Probe Packets

   Probe packets consist of an SCTP common header followed by a
   HEARTBEAT chunk and a PAD chunk.  The PAD chunk is used to control
   the length of the probe packet.  The HEARTBEAT chunk is used to
   trigger the sending of a HEARTBEAT ACK chunk.  The reception of the
   HEARTBEAT ACK chunk acknowledges reception of a successful probe.

   The HEARTBEAT chunk carries a Heartbeat Information parameter which
   should include, besides the information suggested in [RFC4960], the
   probing size, which is the MTU size the complete datagram will add up
   to.  The size of the PAD chunk is therefore computed by reducing the
   probing size by the IPv4 or IPv6 header size, the SCTP common header,
   the HEARTBEAT request and the PAD chunk header.  The payload of the
   PAD chunk contains arbitrary data.

   To avoid fragmentation of retransmitted data, probing starts right
   after the handshake, before data is sent.  Assuming normal behaviour
   (i.e., the PMTU is smaller than or equal to the interface MTU), this
   process will take a few round trip time periods depending on the
   number of PMTU sizes probed.  The Heartbeat timer can be used to
   implement the PROBE_TIMER.

5.2.1.2.  Validating the Path with SCTP

   Since SCTP provides an acknowledged PL, a sender does MUST NOT
   implement the REACHABILITY_TIMER while in the PROBE_DONE state.

5.2.1.3.  PTB Message Handling by SCTP

   Normal ICMP verification MUST be performed as specified in Appendix C
   of [RFC4960].  This requires that the first 8 bytes of the SCTP
   common header are quoted in the payload of the PTB message, which can
   be the case for ICMPv4 and is normally the case for ICMPv6.

   When a PTB mesage has been verified, the router Link MTU indicated in
   the PTB message SHOULD be used with the PLPMTUD algorithm, providing
   that the reported Link MTU is less than the current probe size.

5.2.2.  DPLPMTUD for SCTP/UDP

   The UDP encapsulation of SCTP is specified in [RFC6951].

5.2.2.1.  Sending SCTP/UDP Probe Packets

   Packet probing can be performed as specified in Section 5.2.1.1. The
   maximum payload is reduced by 8 bytes, which has to be considered
   when filling the PAD chunk.

5.2.2.2.  Validating the Path with SCTP/UDP




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   Since SCTP provides an acknowledged PL, a sender does MUST NOT
   implement the REACHABILITY_TIMER while in the PROBE_DONE state.

5.2.2.3.  Handling of PTB Messages by SCTP/UDP

   Normal ICMP verification MUST be performed for PTB messages as
   specified in Appendix C of [RFC4960].  This requires that the first 8
   bytes of the SCTP common header are contained in the PTB message,
   which can be the case for ICMPv4 (but note the UDP header also
   consumes a part of the quoted packet header) and is normally the case
   for ICMPv6. When the verification is completed, the router Link MTU
   size indicated in the PTB message SHOULD be used with the PLPMTUD
   algorithm providing that the reported LInk MTU is less than the
   current probe size.

5.2.3.  DPLPMTUD for SCTP/DTLS

   The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
   specified in [I-D.ietf-tsvwg-sctp-dtls-encaps].  It is used for data
   channels in WebRTC implementations.

5.2.3.1.  Sending SCTP/DTLS Probe Packets

   Packet probing can be done as specified in Section 5.2.1.1.

5.2.3.2.  Validating the Path with SCTP/DTLS

   Since SCTP provides an acknowledged PL, a sender does MUST NOT
   implement the REACHABILITY_TIMER while in the PROBE_DONE state.

5.2.3.3.  Handling of PTB Messages by SCTP/DTLS

   It is not possible to perform normal ICMP verification as specified
   in [RFC4960], since even if the ICMP message payload contains
   sufficient information, the reflected SCTP common header would be
   encrypted.  Therefore it is not possible to process PTB messages at
   the PL.

5.3.  Other IETF Transports

   Quick UDP Internet Connection (QUIC( is a UDP-based transport that
   provides reception feedback [I-D.ietf-quic-transport].

   XXX << This section will be completed in a future revision of this ID
   >>

5.4.  DPLPMTUD by Applications

   Applications that use the Datagram API (e.g., applications built
   directly or indirectly on UDP) can implement DPLPMTUD. Some
   primitives used by DPLPMTUD might not be available via this interface
   (e.g., the ability to access the PMTU cache, or interpret received
   ICMP PTB messages).


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   In addition, it is important that PMTUD is not performed by multiple
   protocol layers.

   XXX << This section will be completed in a future revision of this ID
   >>

6.  Acknowledgements

   This work was partially funded by the European Union's Horizon 2020
   research and innovation programme under grant agreement No.  644334
   (NEAT). The views expressed are solely those of the author(s).

7.  IANA Considerations

   This memo includes no request to IANA.

   XXX << If new UDP Options are specified in this document, a request
   to IANA will be included here.>>

   If there are no requirements for IANA, the section will be removed
   during conversion into an RFC by the RFC Editor.

8.  Security Considerations

   The security considerations for the use of UDP and SCTP are provided
   in the references RFCs.  Security guidance for applications using UDP
   is provided in the UDP-Guidelines [RFC8085].

   PTB messages could potentially be used to cause a node to
   inappropriately reduce the effective PMTU. A node supporting PLPMTUD
   SHOULD/MUST appropriately verify the payload of PTB messages to
   ensure these are received in response to transmitted traffic (i.e., a
   reported error condition that corresponds to a datagram actually sent
   by the path layer.

   XXX Determine if parallel forwarding paths needs to be considred XXX

   A node performing PLPMTUD could experience conflicting information
   about the size of supported probe packets.  This could occur when
   there are multiple paths are concurrently in use and these exhibit a
   different PMTU. If not considered, this could result in data being
   blackholed when the effective PMTU is larger than the smallest PMTU
   across the current paths.

9.  References

9.1.  Normative References

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Internet-Draft draft-ietf-quic-
              transport-04, June 2017.


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   [I-D.ietf-tsvwg-sctp-dtls-encaps]
              Tuexen, M., Stewart, R., Jesup, R. and S. Loreto, "DTLS
              Encapsulation of SCTP Packets", Internet-Draft draft-ietf-
              tsvwg-sctp-dtls-encaps-09, January 2015.

   [I-D.ietf-tsvwg-udp-options]
              Touch, J., "Transport Options for UDP", Internet-Draft
              draft-ietf-tsvwg-udp-options-01, June 2017.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
              10.17487/RFC0768, August 1980, <http://www.rfc-editor.org/
              info/rfc768>.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981, <https://
              www.rfc-editor.org/info/rfc792>.

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

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995, <https://www
              .rfc-editor.org/info/rfc1812>.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.Ed.,
              and G. Fairhurst, Ed., "The Lightweight User Datagram
              Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
              2004, <http://www.rfc-editor.org/info/rfc3828>.

   [RFC4820]  Tuexen, M., Stewart, R. and P. Lei, "Padding Chunk and
              Parameter for the Stream Control Transmission Protocol
              (SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
              <https://www.rfc-editor.org/info/rfc4820>.

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






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   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951, DOI 10.17487/
              RFC6951, May 2013, <https://www.rfc-editor.org/info/
              rfc6951>.

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

   [RFC8201]  McCann, J., Deering, S., Mogul, J. and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017, <https://www.rfc-
              editor.org/info/rfc8201>.

9.2.  Informative References

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

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

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

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

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

Appendix A.  Event-driven state changes

   This appendix contains an informative description of key events:

   Path Setup: When a new path is initiated, the state is set to
      PROBE_START. As soon as the path is confirmed, the state changes
      to PROBE_BASE and the probing mechanism for this path is started.
      A probe packet with the size of the BASE_PMTU is sent.

   Arrival of an Acknowledgment: Depending on the probing state, the
      reaction differs according to Figure 4, which is just a
      simplification of Figure 1 focusing on this event.




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   +--------------+                                    +----------------+
   |  PROBE_START | --3------------------------------->| PROBE_DISABLED |
   +--------------+ --4-----------\                    +----------------+
                                   \
   +--------------+                 \
   | PROBE_ERROR  | ---------------  \
   +--------------+                \  \
                                    \  \
   +--------------+                  \  \              +--------------+
   |  PROBE_BASE  | --1----------     \  ------------> |  PROBE_BASE  |
   +--------------+ --2-----     \     \               +--------------+
                            \     \     \
   +--------------+          \     \     ------------> +--------------+
   | PROBE_SEARCH | --2---    \     -----------------> | PROBE_SEARCH |
   +--------------+ --1---\----\---------------------> +--------------+
                           \    \
   +--------------+         \    \                     +--------------+
   |  PROBE_DONE  |          \    -------------------> |  PROBE_DONE  |
   +--------------+           -----------------------> +--------------+


   Condition 1: The maximum PMTU size has not yet been reached.
   Condition 2: The maximum PMTU size has been reached.  Conition 3:
   Probe Timer expires and PROBE_COUNT = MAX_PROBEs.  Condition 4:
   PROBE_ACK received.

   Probing timeout: The PROBE_COUNT is initialised to zero each time the
      value of PROBED_SIZE is changed.  The PROBE_TIMER is started each
      time a probe packet is sent.  It is stopped when an acknowledgment
      arrives that confirms delivery of a probe packet.  If the probe
      packet is not acknowledged before,the PROBE_TIMER expires, the
      PROBE_ERROR_COUNTER is incremented.  When the PROBE_COUNT equals
      the value MAX_PROBES, the state is changed, otherwise a new probe
      packet of the same size (PROBED_SIZE) is resent.  The state
      transitions are illustrated in Figure 5. This shows a
      simplification of Figure 1 with a focus only on this event.


















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   +--------------+                                    +----------------+
   |  PROBE_START |----------------------------------->| PROBE_DISABLED |
   +--------------+                                    +----------------+

   +--------------+                                    +--------------+
   | PROBE_ERROR  |                 -----------------> | PROBE_ERROR  |
   +--------------+                /                   +--------------+
                                  /
   +--------------+ --2----------/                     +--------------+
   |  PROBE_BASE  | --1------------------------------> |  PROBE_BASE  |
   +--------------+                                    +--------------+

   +--------------+                                    +--------------+
   | PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
   +--------------+ --2---------                       +--------------+
                                \
   +--------------+              \                     +--------------+
   |  PROBE_DONE  |               -------------------> |  PROBE_DONE  |
   +--------------+                                    +--------------+


   Condition 1: The maximum number of probe packets has not been
   reached.  Condition 2: The maximum number of probe packets has been
   reached.

   PMTU raise timer timeout: The path through the network can change
      over time.  It impossible to discover whether a path change has
      increased in the actual PMTU by exchanging packets less than or
      equal to the effective PMTU. This requires PLPMTUD to periodically
      send a probe packet to detect whether a larger PMTU is possible.
      This probe packet is generated by the PMTU_RAISE_TIMER.  When the
      timer expires, probing is restarted with the BASE_PMTU and the
      state is changed to PROBE_BASE.

   Arrival of an ICMP message: The active probing of the path can be
      supported by the arrival of PTB messages sent by routers or
      middleboxes with a link MTU that is smaller than the probe packet
      size.  If the PTB message includes the router link MTU, three
      cases can be distinguished:



      1.  The indicated link MTU in the PTB message is between the
          already probed and effective MTU and the probe that triggered
          the PTB message.

      2.  The indicated link MTU in the PTB message is smaller than the
          effective PMTU.

      3.  The indicated link MTU in the PTB message is equal to the
          BASE_PMTU.



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      In first case, the PROBE_BASE state transitions to the PROBE_ERROR
      state.  In the PROBE_SEARCH state, a new probe packet is sent with
      the sized reported by the PTB message.  Its result is handled
      according to the former events.

      The second case could be a result of a network re-configuration.
      If the reported link MTU in the PTB message is greater than the
      BASE_MTU, the probing starts again with a value of PROBE_BASE.
      Otherwise, the method enters the state PROBE_ERROR.

      In the third case, the maximum possible PMTU has been reached.
      This is probed again, because there could be a link further along
      the path with a still smaller MTU.

      Note: Not all routers include the link MTU size when they send a
      PTB message.  If the PTB message does not indicate the link MTU,
      the probe is handled in the same way as condition 2 of Figure 5.

Appendix B.  Revision Notes

   Note to RFC-Editor: please remove this entire section prior to
   publication.

   Individual draft -00:

   o  Comments and corrections are welcome directly to the authors or
      via the IETF TSVWG working group mailing list.

   o  This update is proposed for WG comments.

   Individual draft -01:

   o  Contains the first representation of the algorithm, showing the
      states and timers

   o  This update is proposed for WG comments.

   Individual draft -02:

   o  Contains updated representation of the algorithm, and textual
      corrections.

   o  The text describing when to set the effective PMTU has not yet
      been verified by the authors

   o  To determine security to off-path-attacks: We need to decide
      whether a received PTB message SHOULD/MUST be verified?  The text
      on how to handle a PTB message indicating a link MTU larger than
      the probe has yet not been verified by the authors

   o  No text currently describes how to handle inconsistent results
      from arbitrary re-routing along different parallel paths


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   o  This update is proposed for WG comments.

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, AB24 3U
   UK

   Email: gorry@erg.abdn.ac.uk


   Tom Jones
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen, AB24 3U
   UK

   Email: tom@erg.abdn.ac.uk


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   Stein fart, 48565
   DE

   Email: tuexen@fh-muenster.de


   Irene Ruengeler
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   Stein fart, 48565
   DE

   Email: i.ruengeler@fh-muenster.de













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