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Versions: (draft-fairhurst-tsvwg-datagram-plpmtud) 00 01 02 03 04 05 06 07 08 09 10

Internet Engineering Task Force                             G. Fairhurst
Internet-Draft                                                  T. Jones
Updates4821 (if approved)                         University of Aberdeen
Intended status: Standards Track                               M. Tuexen
Expires: 7 May 2020                                         I. Ruengeler
                                                              T. Voelker
                                 Muenster University of Applied Sciences
                                                         4 November 2019


     Packetization Layer Path MTU Discovery for Datagram Transports
                  draft-ietf-tsvwg-datagram-plpmtud-09

Abstract

   This document describes a robust method for Path MTU Discovery
   (PMTUD) for datagram Packetization Layers (PLs).  It describes an
   extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
   MTU Discovery for IPv4 and IPv6.  The method allows a PL, or a
   datagram application that uses a PL, to discover whether a network
   path can support the current size of datagram.  This can be used to
   detect and reduce the message size when a sender encounters a network
   black hole (where packets are discarded).  The method can probe a
   network path with progressively larger packets to discover whether
   the maximum packet size can be increased.  This allows a sender to
   determine an appropriate packet size, providing functionally for
   datagram transports that is equivalent to the Packetization Layer
   PMTUD specification for TCP, specified in RFC 4821.

   The document also provides implementation notes for incorporating
   Datagram PMTUD into IETF datagram transports or applications that use
   datagram transports.

   When published, this specification updates RFC 4821.

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 https://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




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   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 7 May 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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 (name-introduction)  . . . . . . . . . . . . . .   4
     1.1.  Classical Path MTU Discovery
           (name-classical-path-mtu-discover)  . . . . . . . . . . .   4
     1.2.  Packetization Layer Path MTU Discovery
           (name-packetization-layer-path-mt)  . . . . . . . . . . .   6
     1.3.  Path MTU Discovery for Datagram Services
           (name-path-mtu-discovery-for-data)  . . . . . . . . . . .   7
   2.  Terminology (name-terminology)  . . . . . . . . . . . . . . .   8
   3.  Features Required to Provide Datagram PLPMTUD
      (name-features-required-to-provid) . . . . . . . . . . . . . .  10
   4.  DPLPMTUD Mechanisms (name-dplpmtud-mechanisms)  . . . . . . .  12
     4.1.  PLPMTU Probe Packets (name-plpmtu-probe-packets)  . . . .  13
     4.2.  Confirmation of Probed Packet Size
           (name-confirmation-of-probed-pack)  . . . . . . . . . . .  14
     4.3.  Detection of Unsupported PLPMTU Size, aka Black Hole
           Detection (name-detection-of-unsupported-pl)  . . . . . .  14
     4.4.  Disabling the Effect of PMTUD
           (name-disabling-the-effect-of-pmt)  . . . . . . . . . . .  15
     4.5.  Response to PTB Messages
           (name-response-to-ptb-messages) . . . . . . . . . . . . .  15
       4.5.1.  Validation of PTB Messages
               (name-validation-of-ptb-messages) . . . . . . . . . .  16
       4.5.2.  Use of PTB Messages
               (name-use-of-ptb-messages)  . . . . . . . . . . . . .  17
   5.  Datagram Packetization Layer PMTUD
      (name-datagram-packetization-laye) . . . . . . . . . . . . . .  18
     5.1.  DPLPMTUD Components (name-dplpmtud-components)  . . . . .  18



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       5.1.1.  Timers (name-timers)  . . . . . . . . . . . . . . . .  19
       5.1.2.  Constants (name-constants)  . . . . . . . . . . . . .  20
       5.1.3.  Variables (name-variables)  . . . . . . . . . . . . .  20
       5.1.4.  Overview of DPLPMTUD Phases
               (name-overview-of-dplpmtud-phases)  . . . . . . . . .  21
     5.2.  State Machine (name-state-machine)  . . . . . . . . . . .  23
     5.3.  Search to Increase the PLPMTU
           (name-search-to-increase-the-plpm)  . . . . . . . . . . .  26
       5.3.1.  Probing for a larger PLPMTU
               (name-probing-for-a-larger-plpmtu)  . . . . . . . . .  26
       5.3.2.  Selection of Probe Sizes
               (name-selection-of-probe-sizes) . . . . . . . . . . .  27
       5.3.3.  Resilience to Inconsistent Path Information
               (name-resilience-to-inconsistent-)  . . . . . . . . .  28
     5.4.  Robustness to Inconsistent Paths
           (name-robustness-to-inconsistent-)  . . . . . . . . . . .  28
   6.  Specification of Protocol-Specific Methods
      (name-specification-of-protocol-s) . . . . . . . . . . . . . .  28
     6.1.  Application support for DPLPMTUD with UDP or UDP-Lite
           (name-application-support-for-dpl)  . . . . . . . . . . .  28
       6.1.1.  Application Request
               (name-application-request)  . . . . . . . . . . . . .  29
       6.1.2.  Application Response
               (name-application-response) . . . . . . . . . . . . .  29
       6.1.3.  Sending Application Probe Packets
               (name-sending-application-probe-p)  . . . . . . . . .  29
       6.1.4.  Initial Connectivity
               (name-initial-connectivity) . . . . . . . . . . . . .  29
       6.1.5.  Validating the Path
               (name-validating-the-path)  . . . . . . . . . . . . .  30
       6.1.6.  Handling of PTB Messages
               (name-handling-of-ptb-messages) . . . . . . . . . . .  30
     6.2.  DPLPMTUD for SCTP (name-dplpmtud-for-sctp)  . . . . . . .  30
       6.2.1.  SCTP/IPv4 and SCTP/IPv6
               (name-sctp-ipv4-and-sctp-ipv6)  . . . . . . . . . . .  30
       6.2.2.  DPLPMTUD for SCTP/UDP
               (name-dplpmtud-for-sctp-udp)  . . . . . . . . . . . .  31
       6.2.3.  DPLPMTUD for SCTP/DTLS
               (name-dplpmtud-for-sctp-dtls) . . . . . . . . . . . .  32
     6.3.  DPLPMTUD for QUIC (name-dplpmtud-for-quic)  . . . . . . .  32
       6.3.1.  Initial Connectivity
               (name-initial-connectivity-5) . . . . . . . . . . . .  33
       6.3.2.  Sending QUIC Probe Packets
               (name-sending-quic-probe-packets) . . . . . . . . . .  33
       6.3.3.  Validating the Path with QUIC
               (name-validating-the-path-with-qu)  . . . . . . . . .  33
       6.3.4.  Handling of PTB Messages by QUIC
               (name-handling-of-ptb-messages-by-q)  . . . . . . . .  34



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   7.  Acknowledgements (name-acknowledgements)  . . . . . . . . . .  34
   8.  IANA Considerations (name-iana-considerations)  . . . . . . .  34
   9.  Security Considerations (name-security-considerations)  . . .  34
   10. References (name-references)  . . . . . . . . . . . . . . . .  35
     10.1.  Normative References (name-normative-references) . . . .  35
     10.2.  Informative References
            (name-informative-references)  . . . . . . . . . . . . .  36
   A.  Revision Notes (name-revision-notes)  . . . . . . . . . . . .  37
   B  Authors' Addresses (name-authors-addresses)  . . . . . . . . .  41

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, QUIC/UDP), and direct datagram transport over the IP
   network layer.  This document describes a robust method for Path MTU
   Discovery (PMTUD) that may be used with these transport protocols (or
   the applications that use their transport service) to discover an
   appropriate size of packet to use across an Internet path.

1.1.  Classical Path MTU Discovery

   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]).  In this document,
   the term PTB message is applied to both IPv4 ICMP Unreachable
   messages (type 3) that carry the error Fragmentation Needed (Type 3,
   Code 4) [RFC0792] and ICMPv6 Packet Too Big messages (Type 2)
   [RFC4443].  When a sender receives a PTB message, it reduces the
   effective MTU to the value reported as the Link MTU in the PTB
   message, and a method that from time-to-time increases the packet
   size in attempt to discover an increase in the supported PMTU.  The
   packets sent with a size larger than the current effective PMTU are
   known as probe packets.

   Packets not intended as probe packets are either fragmented to the
   current effective PMTU, or the attempt to send fails with an error
   code.  Applications are sometimes provided with a primitive to let
   them read the Maximum Packet Size (MPS), derived from the current
   effective PMTU.

   Classical PMTUD is subject to protocol failures.  One failure arises
   when traffic using a packet size larger than the actual PMTU is
   black-holed (all datagrams sent with this size, or larger, are
   discarded).  This could arise when the PTB messages are not delivered
   back to the sender for some reason (see for example [RFC2923]).

   Examples where PTB messages are not delivered include:



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   *  The generation of ICMP messages is usually rate limited.  This
      could result in no PTB messages being generated to the sender (see
      section 2.4 of [RFC4443])

   *  ICMP messages can be filtered by middleboxes (including firewalls)
      [RFC4890].  A stateful firewall could be configured with a policy
      to block incoming ICMP messages, which would prevent reception of
      PTB messages to a sending endpoint behind this firewall.

   *  When the router issuing the ICMP message drops a tunneled packet,
      the resulting ICMP message will be directed to the tunnel ingress.
      This tunnel endpoint is responsible for forwarding the ICMP
      message and also processing the quoted packet within the payload
      field to remove the effect of the tunnel, and return a correctly
      formatted ICMP message to the sender [I-D.ietf-intarea-tunnels].
      Failure to do this prevents the PTB message reaching the original
      sender.

   *  Asymmetry in forwarding can result in there being no return route
      to the original sender, which would prevent an ICMP message being
      delivered to the sender.  This issue can also arise when policy-
      based routing is used, Equal Cost Multipath (ECMP) routing is
      used, or a middlebox acts as an application load balancer.  An
      example is where the path towards the server is chosen by ECMP
      routing depending on bytes in the IP payload.  In this case, when
      a packet sent by the server encounters a problem after the ECMP
      router, then any resulting ICMP message needs to also be directed
      by the ECMP router towards the original sender.

   *  There are additional cases where the next hop destination fails to
      receive a packet because of its size.  This could be due to
      misconfiguration of the layer 2 path between nodes, for instance
      the MTU configured in a layer 2 switch, or misconfiguration of the
      Maximum Receive Unit (MRU).  If the packet is dropped by the link,
      this will not cause a PTB message to be sent to the original
      sender.

   Another failure could result if a node that is not on the network
   path sends a PTB message that attempts to force a sender to change
   the effective PMTU [RFC8201].  A sender can protect itself from
   reacting to such messages by utilising the quoted packet within a PTB
   message payload to validate that the received PTB message was
   generated in response to a packet that had actually originated from
   the sender.  However, there are situations where a sender would be
   unable to provide this validation.  Examples where validation of the
   PTB message is not possible include:





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   *  When a router issuing the ICMP message implements RFC792
      [RFC0792], it is only required to include the first 64 bits of the
      IP payload of the packet within the quoted payload.  There could
      be insufficient bytes remaining for the sender to interpret the
      quoted transport information.

      Note: The recommendation in RFC1812 [RFC1812] is that IPv4 routers
      return a quoted packet with as much of the original datagram as
      possible without the length of the ICMP datagram exceeding 576
      bytes.  IPv6 routers include as much of the invoking packet as
      possible without the ICMPv6 packet exceeding 1280 bytes [RFC4443].

   *  The use of tunnels/encryption can reduce the size of the quoted
      packet returned to the original source address, increasing the
      risk that there could be insufficient bytes remaining for the
      sender to interpret the quoted transport information.

   *  Even when the PTB message includes sufficient bytes of the quoted
      packet, the network layer could lack sufficient context to
      validate the message, because validation 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).

   *  When a packet is encapsulated/tunneled over an encrypted
      transport, the tunnel/encapsulation ingress might have
      insufficient context, or computational power, to reconstruct the
      transport header that would be needed to perform validation.

1.2.  Packetization Layer Path MTU Discovery

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

   In contrast to PMTUD, Packetization Layer Path MTU Discovery
   (PLPMTUD) [RFC4821] does not rely upon reception and validation 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 sends probe packets to search
   for the largest size of unfragmented datagram that can be sent over a
   network path.  Probe packets are sent with a progressively larger
   packet size.  If a probe packet is successfully delivered (as
   determined by the PL), then the PLPMTU is raised to the size of the



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   successful probe.  If no response is received to a probe packet, the
   method reduces the probe size.  The result of probing with the PLPMTU
   is used to set the application MPS.

   PLPMTUD introduces flexibility in the implementation of PMTU
   discovery.  At one extreme, it can be configured to only perform ICMP
   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 (see
   Section 4.5).

   PLPMTUD can also include additional consistency checks without
   increasing the risk that data is lost when probing to discover the
   path MTU.  For example, information available at the PL, or higher
   layers, enables received PTB messages to be validated before being
   utilized.

1.3.  Path MTU Discovery for Datagram Services

   Section 5 of this document presents a set of algorithms for datagram
   protocols to discover the largest size of unfragmented datagram that
   can be sent over a network path.  The method described relies on
   features of the PL described in Section 3 and applies to transport
   protocols operating over IPv4 and IPv6.  It does not require
   cooperation from the lower layers, although it can utilize PTB
   messages when these received messages are made available to the PL.

   The UDP Usage 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
   that can be used on a network path.  Prior to this document, PLPMTUD
   had not been specified for UDP.

   Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
   Stream Control Transport Protocol (SCTP).  SCTP utilizes probe
   packets consisting of a minimal sized HEARTBEAT chunk bundled with a
   PAD chunk as defined in [RFC4820], but RFC4821 does not provide a
   complete specification.  The present document provides the details to
   complete that specification.

   The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
   implementations to support Classical PMTUD and states that a DCCP
   sender "MUST maintain the MPS allowed for each active DCCP session".
   It also defines the current congestion control MPS (CCMPS) supported
   by a network path.  This recommends use of PMTUD, and suggests use of
   control packets (DCCP-Sync) as path probe packets, because they do




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   not risk application data loss.  The method defined in this
   specification could be used with DCCP.

   Section 6 specifies the method for a set of transports, and provides
   information to enable the implementation of PLPMTUD with other
   datagram transports and applications that use datagram transports.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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

   Actual PMTU:  The Actual PMTU is the PMTU of a network path between a
      sender PL and a destination PL, which the DPLPMTUD algorithm seeks
      to determine.

   Black Hole:  A Black Hole is encountered when a sender is unaware
      that packets are not being delivered to the destination end point.
      Two types of Black Hole are relevant to DPLPMTUD:

      Packet Black Hole:  Packets encounter a Packet Black Hole when
                          packets are not delivered to the destination
                          endpoint (e.g., when the sender transmits
                          packets of a particular size with a previously
                          known effective PMTU and they are discarded by
                          the network).

      ICMP Black Hole     An ICMP Black Hole is encountered when the
                          sender is unaware that packets are not
                          delivered to the destination endpoint because
                          PTB messages are not received by the
                          originating PL sender.

   Black holed :  Traffic is black-holed when the sender is unaware that
      packets are not being delivered.  This could be due to a Packet
      Black Hole or an ICMP Black Hole.

   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 that can be used
      across a network path.




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

   Effective PMTU:  The Effective PMTU is the current estimated value
      for PMTU that is used by a PMTUD.  This is equivalent to the
      PLPMTU derived by PLPMTUD.

   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 Link is 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 Link 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.  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 generally define the link MTU to include the link
      layer headers.

   MAX_PMTU:  The MAX_PMTU is the largest size of PLPMTU that DPLPMTUD
      will attempt to use.

   MPS:  The Maximum Packet Size (MPS) is the largest size of
      application data block that can be sent across a network path by a
      PL.  In DPLPMTUD this quantity is derived from the PLPMTU by
      taking into consideration the size of the lower protocol layer
      headers.  Probe packets generated by DPLPMTUD can have a size
      larger than the MPS.

   MIN_PMTU:  The MIN_PMTU is the smallest size of PLPMTU that DPLPMTUD
      will attempt to use.

   Packet:  A Packet is the IP header plus the IP payload.

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



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   Path:  The Path is the set of links and routers traversed by a packet
      between a source node and a destination node by a particular flow.

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

   PTB_SIZE:  The PTB_SIZE is a value reported in a validated PTB
      message that indicates next hop link MTU of a router along the
      path.

   PLPMTU:  The Packetization Layer PMTU is an estimate of the actual
      PMTU provided by the DPLPMTUD algorithm.

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

   Probe packet:  A probe packet is a datagram sent with a purposely
      chosen size (typically the current PLPMTU or larger) to detect if
      packets of this size can be successfully sent end-to-end across
      the network 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 the use of the
   technique with a datagram PL.  Unlike TCP, some datagram PLs require
   additional mechanisms to implement PLPMTUD.

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

   1.  PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
       information about the maximum size of packet that can be
       transmitted by the sender on the local link (the local Link MTU).
       It MAY utilize similar information about the receiver when this
       is supplied (note this could be less than EMTU_R).  This avoids
       implementations trying to send probe packets that can not be
       transmitted by the local link.  Too high of a value could reduce
       the efficiency of the search algorithm.  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.  PLPMTU: A datagram application using a PL not supporting
       fragmentation is REQUIRED to be able to choose the size of



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       datagrams sent to the network, up to the PLPMTU, or a smaller
       value (such as the MPS) derived from this.  This value is managed
       by the DPLPMTUD method.  The PLPMTU (specified as the effective
       PMTU in Section 1 of [RFC1191]) is equivalent to the EMTU_S
       (specified in [RFC1122]).

   3.  Probe packets: On request, a DPLPMTUD sender is REQUIRED to be
       able to transmit a packet larger than the PLMPMTU.  This is used
       to send a probe packet.  In IPv4, a probe packet MUST be sent
       with the Don't Fragment (DF) bit set in the IP header, 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 DPLPMTUD sender MAY optionally utilize
       PTB messages received from the network layer to help identify
       when a network path does not support the current size of probe
       packet.  Any received PTB message MUST be validated before it is
       used to update the PLPMTU discovery information [RFC8201].  This
       validation confirms that the PTB message was sent in response to
       a packet originating by the sender, and needs to be performed
       before the PLPMTU discovery method reacts to the PTB message.  A
       PTB message MUST NOT be used to increase the PLPMTU [RFC8201].

   5.  Reception feedback: The destination PL endpoint is REQUIRED to
       provide a feedback method that indicates to the DPLPMTUD sender
       when a probe packet has been received by the destination PL
       endpoint.  The mechanism needs to be robust to the possibility
       that packets could be significantly delayed along a network path.
       The local PL endpoint at the sending node is REQUIRED to pass
       this feedback to the sender DPLPMTUD method.

   6.  Probe loss recovery: It is RECOMMENDED to use probe packets that
       do not carry any user data.  Most datagram transports permit
       this.  If a probe packet contains user data requiring
       retransmission in case of loss, the PL (or layers above) are
       REQUIRED to arrange any retransmission/repair of any resulting
       loss.  DPLPMTUD is REQUIRED to be robust in the case where probe
       packets are lost due to other reasons (including link
       transmission error, congestion).

   7.  Probing and congestion control: The DPLPMTUD sender treats
       isolated loss of a probe packet (with or without a corresponding
       PTB message) as a potential indication of a PMTU limit for the
       path.  Loss of a probe packet SHOULD NOT be treated as an
       indication of congestion and the loss SHOULD NOT directly trigger
       a congestion control reaction [RFC4821].




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   8.  Shared PLPMTU state: The PLPMTU 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".  Such methods MUST be robust to the
       wide variety of underlying network forwarding behaviors, PLPMTU
       adjustments based on shared PLPMTU values should be incorporated
       in the search algorithms.  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 principles are stated for design of a
   DPLPMTUD method:

   *  MPS: A method is REQUIRED to signal an appropriate MPS to the
      higher layer using the PL.  The value of the MPS can change
      following a change to the path.  It is RECOMMENDED that methods
      avoid forcing an application to use an arbitrary small MPS
      (PLPMTU) for transmission while the method is searching for the
      currently supported PLPMTU.  Datagram PLs do not necessarily
      support fragmentation of PDUs larger than the PLPMTU.  A reduced
      MPS can adversely impact the performance of a datagram
      application.

   *  Path validation: It is RECOMMENDED that methods are robust to path
      changes that could have occurred since the path characteristics
      were last confirmed, and to the possibility of inconsistent path
      information being received.

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

   *  When to probe: It is RECOMMENDED that methods determine whether
      the path has changed since it last measured the path.  This can
      help determine when to probe the path again.

4.  DPLPMTUD Mechanisms

   This section lists the protocol mechanisms used in this
   specification.







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4.1.  PLPMTU Probe Packets

   The DPLPMTUD method relies upon the PL sender being able to generate
   probe packets with a specific size.  TCP is able to generate these
   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
   utilize 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.

   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 listed in order of preference:

   Probing using padding data:  A probe packet that contains only
      control information together with any padding, which is needed to
      be inflated to the size required for the probe packet.  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.

   Probing using application 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 packet.  If the application/transport needs
      protection from the loss of this probe packet, the application/
      transport could 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 application data:  A probe packet that contains a data
      block supplied by an application that matches the size required
      for the probe packet.  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).




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   A PL that uses a probe packet carrying an application data block,
   could need to retransmit this application data block if the probe
   fails.  This could need the PL to re-fragment the data block to a
   smaller packet size that is expected to traverse the end-to-end path
   (which could utilize endpoint network-layer or PL fragmentation when
   these are available).

   DPLPMTUD MAY choose to use only one of these methods to simplify the
   implementation.

   Probe messages sent by a PL MUST contain enough information to
   uniquely identify the probe within Maximum Segment Lifetime, while
   being robust to reordering and replay of probe response and PTB
   messages.

4.2.  Confirmation of Probed Packet Size

   The PL needs a method to determine (confirm) 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
   SCTP provide keep-alive/heartbeat features).  When supported, this
   mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of
   a probe packet.

   A PL that does not acknowledge data reception (e.g., UDP and UDP-
   Lite) is unable itself to detect when the packets that it sends are
   discarded because their size is greater than the actual PMTU.  These
   PLs need to either rely on an application protocol to detect this
   loss.

   Section 6 specifies this function for a set of IETF-specified
   protocols.

4.3.  Detection of Unsupported PLPMTU Size, aka Black Hole Detection

   A PL sender needs to reduce the PLPMTU when it discovers the actual
   PMTU supported by a network path is less than the PLPMTU.  This can
   be triggered when a validated PTB message is received, or by another
   event that indicates the network path no longer sustains the current
   packet size, such as a loss report from the PL, or repeated lack of
   response to probe packets sent to confirm the PLPMTU.  Detection is
   followed by a reduction of the PLPMTU.

   This is performed by sending packet probes of size PLPMTU to verify
   that a network path still supports the last acknowledged PLPMTU size.
   There are two alternative mechanism:



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   *  A PL can rely upon a mechanism implemented within the PL to detect
      excessive loss of data sent with a specific packet size and then
      conclude that this excessive loss could be a result of an invalid
      PMTU (as in PLPMTUD for TCP [RFC4821]).

   *  A PL can use the DPLPMTUD probing mechanism to periodically
      generate probe packets of the size of the current PLPMTU (e.g.,
      using the confirmation timer Section 5.1.1).  A timer tracks
      whether acknowledgments are received.  Successive loss of probes
      is an indication that the current path no longer supports the
      PLPMTU (e.g., when the number of probe packets sent without
      receiving an acknowledgement, PROBE_COUNT, becomes greater than
      MAX_PROBES).

   A PL MAY inhibit sending probe packets when no application data has
   been sent since the previous probe packet.  A PL preferring to use an
   up-to-data PLPMTU once user data is sent again, MAY choose to
   continue PLPMTU discovery for each path.  However, this may result in
   additional packets being sent.

   When the method detects the current PLPMTU is not supported, DPLPMTUD
   sets a lower MPS.  The PL then confirms that the updated PLPMTU can
   be successfully used across the path.  The PL could need 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 use a control packet as the
   packet probe.

4.4.  Disabling the Effect of PMTUD

   A PL implementing this specification MUST suspend network layer
   processing of outgoing packets that enforces a PMTU
   [RFC1191][RFC8201] for each flow utilising DPLPMTUD, and instead use
   DPLPMTUD to control the size of packets that are sent by a flow.
   This removes the need for the network layer to drop or fragment sent
   packets that have a size greater than the PMTU.

4.5.  Response to PTB Messages

   This method requires the DPLPMTUD sender to validate any received PTB
   message before using the PTB information.  The response to a PTB
   message depends on the PTB_SIZE indicated in the PTB message, the
   state of the PLPMTUD state machine, and the IP protocol being used.

   Section 4.5.1 first describes validation for both IPv4 ICMP
   Unreachable messages (type 3) and ICMPv6 Packet Too Big messages,
   both of which are referred to as PTB messages in this document.




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4.5.1.  Validation of PTB Messages

   This section specifies utilization of PTB messages.

   *  A simple implementation MAY ignore received PTB messages and in
      this case the PLPMTU is not updated when a PTB message is
      received.

   *  An implementation that supports PTB messages MUST validate
      messages before they are further processed.

   A PL that receives a PTB message from a router or middlebox, performs
   ICMP validation as specified in Section 5.2 of [RFC8085][RFC8201].
   Because DPLPMTUD operates at the PL, the PL needs to check that each
   received PTB message is received in response to a packet transmitted
   by the endpoint PL performing DPLPMTUD.

   The PL MUST check the protocol information in the quoted packet
   carried in an ICMP PTB message payload to validate the message
   originated from the sending node.  This validation includes
   determining that the combination of the IP addresses, the protocol,
   the source port and destination port match those returned in the
   quoted packet - this is also necessary for the PTB message to be
   passed to the corresponding PL.

   The validation SHOULD utilize information that it is not simple for
   an off-path attacker to determine [RFC8085].  For example, by
   checking the value of a protocol header field known only to the two
   PL endpoints.  A datagram application that uses well-known source and
   destination ports ought to also rely on other information to complete
   this validation.

   These checks are intended to provide protection from packets that
   originate from a node that is not on the network path.  A PTB message
   that does not complete the validation MUST NOT be further utilized by
   the DPLPMTUD method.

   PTB messages that have been validated MAY be utilized by the DPLPMTUD
   algorithm, but MUST NOT be used directly to set the PLPMTU.  A method
   that utilizes these PTB messages can improve the speed at the which
   the algorithm detects an appropriate PLPMTU, compared to one that
   relies solely on probing.  Section 4.5.2 describes this processing.









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4.5.2.  Use of PTB Messages

   A set of checks are intended to provide protection from a router that
   reports an unexpected PTB_SIZE.  The PL also needs to check that the
   indicated PTB_SIZE is less than the size used by probe packets and
   larger than minimum size accepted.

   This section provides a summary of how PTB messages can be utilized.
   This processing depends on the PTB_SIZE and the current value of a
   set of variables:

   MIN_PMTU < PTB_SIZE < BASE_PMTU
      *  A robust PL MAY enter an error state (see Section 5.2) for an
         IPv4 path when the PTB_SIZE reported in the PTB message is
         larger than or equal to 68 bytes and when this is less than the
         BASE_PMTU.

      *  A robust PL MAY enter an error state (see Section 5.2) for an
         IPv6 path when the PTB_SIZE reported in the PTB message is
         larger than or equal to 1280 bytes and when this is less than
         the BASE_PMTU.

   PTB_SIZE = PLPMTU
      *  Completes the search for a larger PLPMTU.

   PTB_SIZE > PROBED_SIZE
      *  Inconsistent network signal.

      *  PTB message ought to be discarded without further processing
         (e. g.  PLPMTU not modified).

      *  The information could be utilized as an input to trigger
         enabling a resilience mode.

   BASE_PMTU <= PTB_SIZE < PLPMTU
      *  Black Hole Detection is triggered and the PLPMTU ought to be
         set to BASE_PMTU.

      *  The PL could use the PTB_SIZE reported in the PTB message to
         initialize a search algorithm.

   PLPMTU < PTB_SIZE < PROBED_SIZE
      *  The PLPMTU continues to be valid, but the last PROBED_SIZE
         searched was larger than the actual PMTU.

      *  The PLPMTU is not updated.





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      *  The PL can use the reported PTB_SIZE from the PTB message as
         the next search point when it resumes the search algorithm.

5.  Datagram Packetization Layer PMTUD

   This section specifies Datagram PLPMTUD (DPLPMTUD).  The method can
   be introduced at various points (as indicated with * in the figure
   below) in the IP protocol stack to discover the PLPMTU so that an
   application can utilize an appropriate MPS for the current network
   path.  DPLPMTUD SHOULD NOT be used by an application if it is already
   used in a lower layer.

     +----------------------+
     |      Application*    |
     +-+-------+----+----+--+
       |       |    |    |
   +---+--+ +--+--+ |  +-+---+
   | QUIC*| |UDPO*| |  |SCTP*|
   +---+--+ +--+--+ |  +--+--+
       |       |    |  |  |
       +-------+--+ |  |  |
                  | |  |  |
                +-+-+--+  |
                | UDP  |  |
                +---+--+  |
                    |     |
     +--------------+-----+-+
     |  Network Interface   |
     +----------------------+

            Figure 1: Examples where DPLPMTUD can be implemented

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

   The following sections identify the components needed for
   implementation, provides an overvoew of the phases of operation, and
   specifies the state machine and search algorithm.

5.1.  DPLPMTUD Components

   This section describes the timers, constants, and variables of
   DPLPMTUD.






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5.1.1.  Timers

   The method utilizes up to three timers:

   PROBE_TIMER:         The PROBE_TIMER is configured to expire after a
                        period longer than the maximum time to receive
                        an acknowledgment to a probe packet.  This value
                        MUST NOT be smaller than 1 second, and SHOULD be
                        larger than 15 seconds.  Guidance on selection
                        of the timer value are provided in section 3.1.1
                        of the UDP Usage Guidelines [RFC8085].

                        If the PL has a path Round Trip Time (RTT)
                        estimate and timely acknowledgements the
                        PROBE_TIMER can be derived from the PL RTT
                        estimate.

   PMTU_RAISE_TIMER:    The PMTU_RAISE_TIMER is configured to the period
                        a sender will continue to use the current
                        PLPMTU, after which it re-enters the Search
                        phase.  This timer has a period of 600 seconds,
                        as recommended by PLPMTUD [RFC4821].

                        DPLPMTUD MAY inhibit sending probe packets when
                        no application data has been sent since the
                        previous probe packet.  A PL preferring to use
                        an up-to-data PMTU once user data is sent again,
                        can choose to continue PMTU discovery for each
                        path.  However, this may result in sending
                        additional packets.

   CONFIRMATION_TIMER:  When an acknowledged PL is used, this timer MUST
                        NOT be used.  For other PLs, the
                        CONFIRMATION_TIMER is configured to the period a
                        PL sender waits before confirming the current
                        PLPMTU is still supported.  This is less than
                        the PMTU_RAISE_TIMER and used to decrease the
                        PLPMTU (e.g., when a black hole is encountered).
                        Confirmation needs to be frequent enough when
                        data is flowing that the sending PL does not
                        black hole extensive amounts of traffic.
                        Guidance on selection of the timer value are
                        provided in section 3.1.1 of the UDP Usage
                        Guidelines [RFC8085].

                        DPLPMTUD MAY inhibit sending probe packets when
                        no application data has been sent since the
                        previous probe packet.  A PL preferring to use



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                        an up-to-data PMTU once user data is sent again,
                        can choose to continue PMTU discovery for each
                        path.  However, this may result in sending
                        additional packets.

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

5.1.2.  Constants

   The following constants are defined:

   MAX_PROBES:  The MAX_PROBES is the maximum value of the PROBE_COUNT
                counter (see Section 5.1.3).  MAX_PROBES represents the
                limit for the number of consecutive probe attempts of
                any size.  The default value of MAX_PROBES is 10.

   MIN_PMTU:    The MIN_PMTU is the smallest allowed probe packet size.
                For IPv6, this value is 1280 bytes, as specified in
                [RFC2460].  For IPv4, the minimum value is 68 bytes.

                Note: An IPv4 router is required to be able to forward a
                datagram of 68 bytes without further fragmentation.
                This is the combined size of an IPv4 header and the
                minimum fragment size of 8 bytes.  In addition,
                receivers are required to be able to reassemble
                fragmented datagrams at least up to 576 bytes, as stated
                in section 3.3.3 of [RFC1122].

   MAX_PMTU:    The MAX_PMTU is the largest size of PLPMTU.  This has to
                be less than or equal to the minimum of the local MTU of
                the outgoing interface and the destination PMTU for
                receiving.  An application, or PL, MAY reduce the
                MAX_PMTU when there is no need to send packets larger
                than a specific size.

   BASE_PMTU:   The BASE_PMTU is a configured size expected to work for
                most paths.  The size is equal to or larger than the
                MIN_PMTU and smaller than the MAX_PMTU.  In the case of
                IPv6, this value is 1280 bytes [RFC2460].  When using
                IPv4, a size of 1200 bytes is RECOMMENDED.

5.1.3.  Variables

   This method utilizes a set of variables:

   PROBED_SIZE:  The PROBED_SIZE is the size of the current probe




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                 packet.  This is a tentative value for the PLPMTU,
                 which is awaiting confirmation by an acknowledgment.

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

   The figure below illustrates the relationship between the packet size
   constants and variables at a point of time when the DPLPMTUD
   algorithm performs path probing to increase the size of the PLPMTU.
   A probe packet has been sent of size PROBED_SIZE.  Once this is
   acknowledged, the PLPMTU will raise to PROBED_SIZE allowing the
   DPLPMTUD algorithm to further increase PROBED_SIZE towards the actual
   PMTU.

        MIN_PMTU                                        MAX_PMTU
          <-------------------------------------------------->
                         |        |     |           |
                         v        |     |           v
                     BASE_PMTU    |     v     Actual PMTU
                                  |  PROBED_SIZE
                                  v
                                PLPMTU

    Figure 2: Relationships between packet size constants and variables

5.1.4.  Overview of DPLPMTUD Phases

   This section provides a high-level informative view of the DPLPMTUD
   method, by describing the movement of the method through several
   phases of operation.  More detail is available in the state machine
   Section 5.2.

















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                         +------+
                +------->| Base |----------------+ Connectivity
                |        +------+                | or BASE_PMTU
                |           |                    | confirmation failed
                |           |                    v
                |           | Connectivity   +-------+
                |           | and BASE_PMTU  | Error |
                |           | confirmed      +-------+
                |           |                    |
                |           v                    | Consistent connectivity
         PLPMTU |       +--------+               | and BASE_PMTU
   confirmation |       | Search |<--------------+ confirmed
         failed |       +--------+
                |          ^  |
                |          |  |
                |    Raise |  | Search
                |    timer |  | algorithm
                |  expired |  | completed
                |          |  |
                |          |  v
                |   +-----------------+
                +---| Search Complete |
                    +-----------------+

                         Figure 3: DPLPMTUD Phases

   Base:             The Base Phase confirms connectivity to the remote
                     peer.  This phase is implicit for a connection-
                     oriented PL (where it can be performed in a PL
                     connection handshake).  A connectionless PL needs
                     to send an acknowledged probe packet to confirm
                     that the remote peer is reachable.  The sender also
                     confirms that BASE_PMTU is supported across the
                     network path.

                     A PL that does not wish to support a path with a
                     PLPMTU less than BASE_PMTU can simplify the phase
                     into a single step by performing the connectivity
                     checks with a probe of the BASE_PMTU size.

                     Once confirmed, DPLPMTUD enters the Search Phase.
                     If this phase fails to confirm, DPLPMTUD enters the
                     Error Phase.

   Search:           The Search Phase utilizes a search algorithm to
                     send probe packets to seek to increase the PLPMTU.
                     The algorithm concludes when it has found a
                     suitable PLPMTU, by entering the Search Complete



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                     Phase.

                     A PL could respond to PTB messages using the PTB to
                     advance or terminate the search, see Section 4.5.

   Search Complete:  The Search Complete Phase is entered when the
                     PLPMTU is supported across the network path.  A PL
                     can use a CONFIRMATION_TIMER to periodically repeat
                     a probe packet for the current PLPMTU size.  If the
                     sender is unable to confirm reachability (e.g., if
                     the CONFIRMATION_TIMER expires) or the PL signals a
                     lack of reachability, DPLPMTUD enters the Base
                     phase.

                     The PMTU_RAISE_TIMER is used to periodically resume
                     the search phase to discover if the PLPMTU can be
                     raised.  Black Hole Detection or receipt of a
                     validated PTB message (see Section 4.5.1) can cause
                     the sender to enter the Base Phase.

   Error:            The Error Phase is entered when there is
                     conflicting or invalid PLPMTU information for the
                     path (e.g. a failure to support the BASE_PMTU) that
                     cause DPLPMTUD to be unable to progress and the
                     PLPMTU is lowered.

                     DPLPMTUD remains in the Error Phase until a
                     consistent view of the path can be discovered and
                     it has also been confirmed that the path supports
                     the BASE_PMTU (or DPLPMTUD is suspended).

   An implementation that only reduces the PLPMTU to a suitable size
   would be sufficient to ensure reliable operation, but can be very
   inefficient when the actual PMTU changes or when the method (for
   whatever reason) makes a suboptimal choice for the PLPMTU.

   A full implementation of DPLPMTUD provides an algorithm enabling the
   DPLPMTUD sender to increase the PLPMTU following a change in the
   characteristics of the path, such as when a link is reconfigured with
   a larger MTU, or when there is a change in the set of links traversed
   by an end-to-end flow (e.g., after a routing or path fail-over
   decision).

5.2.  State Machine

   A state machine for DPLPMTUD is depicted in Figure 4.  If multipath
   or multihoming is supported, a state machine is needed for each path.




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   Note: Some state changes are not shown to simplify the diagram.

      |         |
      | Start   | PL indicates loss
      |         |  of connectivity
      v         v
   +---------------+                                   +---------------+
   |    DISABLED   |                                   |     ERROR     |
   +---------------+               PROBE_TIMER expiry: +---------------+
           | PL indicates     PROBE_COUNT = MAX_PROBES or ^         |
           | connectivity       PTB: PTB_SIZE < BASE_PMTU |         |
           +--------------------+         +---------------+         |
                                |         |                         |
                                v         |         BASE_PMTU Probe |
                             +---------------+            acked     |
                             |      BASE     |----------------------+
                             +---------------+                      |
         Black hole detected or ^ |    ^  ^ Black hole detected or  |
         PTB: PTB_SIZE < PLPMTU | |    |  | PTB: PTB_SIZE < PLPMTU  |
           +--------------------+ |    |  +--------------------+    |
           |                      +----+                       |    |
           |                PROBE_TIMER expiry:                |    |
           |             PROBE_COUNT < MAX_PROBES              |    |
           |                                                   |    |
           |               PMTU_RAISE_TIMER expiry             |    |
           |    +-----------------------------------------+    |    |
           |    |                                         |    |    |
           |    |                                         v    |    v
   +---------------+                                   +---------------+
   |SEARCH_COMPLETE|                                   |   SEARCHING   |
   +---------------+                                   +---------------+
      |    ^    ^                                         |    |    ^
      |    |    |                                         |    |    |
      |    |    +-----------------------------------------+    |    |
      |    |        MAX_PMTU Probe acked or PROBE_TIMER        |    |
      |    |        expiry: PROBE_COUNT = MAX_PROBES or        |    |
      +----+               PTB: PTB_SIZE = PLPMTU              +----+
   CONFIRMATION_TIMER expiry:                        PROBE_TIMER expiry:
   PROBE_COUNT < MAX_PROBES or               PROBE_COUNT < MAX_PROBES or
        PLPMTU Probe acked                           Probe acked or PTB:
                                         PLPMTU < PTB_SIZE < PROBED_SIZE

                Figure 4: State machine for Datagram PLPMTUD


   The following states are defined:





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   DISABLED:         The DISABLED state is the initial state before
                     probing has started.  It is also entered from any
                     other state, when the PL indicates loss of
                     connectivity.  This state is left, once the PL
                     indicates connectivity to the remote PL.

   BASE:             The BASE state is used to confirm that the
                     BASE_PMTU size is supported by the network path and
                     is designed to allow an application to continue
                     working when there are transient reductions in the
                     actual PMTU.  It also seeks to avoid long periods
                     where traffic is black holed while searching for a
                     larger PLPMTU.

                     On entry, the PROBED_SIZE is set to the BASE_PMTU
                     size and the PROBE_COUNT is set to zero.

                     Each time a probe packet is sent, the PROBE_TIMER
                     is started.  The state is exited when the probe
                     packet is acknowledged, and the PL sender enters
                     the SEARCHING state.

                     The state is also left when the PROBE_COUNT reaches
                     MAX_PROBES or a received PTB message is validated.
                     This causes the PL sender to enter the ERROR state.

   SEARCHING:        The SEARCHING state is the main probing state.
                     This state is entered when probing for the
                     BASE_PMTU was successful.

                     The PROBE_COUNT is set to zero when the first probe
                     packet is sent for each probe size.  Each time a
                     probe packet is acknowledged, the PLPMTU is set to
                     the PROBED_SIZE, and then the PROBED_SIZE is
                     increased using the search algorithm.

                     When a probe packet is sent and 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 received PTB
                     message is validated, a probe of size MAX_PMTU is
                     acknowledged, or a black hole is detected.

   SEARCH_COMPLETE:  The SEARCH_COMPLETE state indicates a successful
                     end to the SEARCHING state.  DPLPMTUD remains in
                     this state until either the PMTU_RAISE_TIMER
                     expires, a received PTB message is validated, or a



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                     black hole is detected.

                     When DPLPMTUD uses an unacknowledged PL and is in
                     the SEARCH_COMPLETE state, a CONFIRMATION_TIMER
                     periodically resets the PROBE_COUNT and schedules a
                     probe packet with the size of the PLPMTU.  If the
                     probe packet fails to be acknowledged after
                     MAX_PROBES attempts, the method enters the BASE
                     state.  When used with an acknowledged PL (e.g.,
                     SCTP), DPLPMTUD SHOULD NOT continue to generate
                     PLPMTU probes in this state.

   ERROR:            The ERROR state represents the case where either
                     the network path is not known to support a PLPMTU
                     of at least the BASE_PMTU size or when there is
                     contradictory information about the network path
                     that would otherwise result in excessive variation
                     in the MPS signalled to the higher layer.  The
                     state implements a method to mitigate oscillation
                     in the state-event engine.  It signals a
                     conservative value of the MPS to the higher layer
                     by the PL.  The state is exited when packet probes
                     no longer detect the error or when the PL indicates
                     that connectivity has been lost.

                     Implementations are permitted to enable endpoint
                     fragmentation if the DPLPMTUD is unable to validate
                     MIN_PMTU within PROBE_COUNT probes.  If DPLPMTUD is
                     unable to validate MIN_PMTU the implementation
                     should transition to the DISABLED state.

                     Note: MIN_PMTU may be identical to BASE_PMTU,
                     simplifying the actions in this state.

5.3.  Search to Increase the PLPMTU

   This section describes the algorithms used by DPLPMTUD to search for
   a larger PLPMTU.

5.3.1.  Probing for a larger PLPMTU

   Implementations use a search algorithm across the search range to
   determine whether a larger PLPMTU can be supported across a network
   path.

   The method discovers the search range by confirming the minimum
   PLPMTU and then using the probe method to select a PROBED_SIZE less
   than or equal to MAX_PMTU.  MAX_PMTU is the minimum of the local MTU



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   and EMTU_R (learned from the remote endpoint).  The MAX_PMTU MAY be
   reduced by an application that sets a maximum to the size of
   datagrams it will send.

   The PROBE_COUNT is initialized to zero when a probe packet is first
   sent with a particular size.  A timer is used by the search algorithm
   to trigger the sending of probe packets of size PROBED_SIZE, larger
   than the PLPMTU.  Each probe packet successfully sent to the remote
   peer is confirmed by acknowledgement at the PL, see Section 4.1.

   Each time a probe packet is sent to the destination, the PROBE_TIMER
   is started.  The timer is canceled when the PL receives
   acknowledgment that the probe packet has been successfully sent
   across the path Section 4.1.  This confirms that the PROBED_SIZE is
   supported, and the PROBED_SIZE value is then assigned to the PLPMTU.
   The search algorithm can continue to send subsequent probe packets of
   an increasing size.

   If the timer expires before a probe packet is acknowledged, the probe
   has failed to confirm the PROBED_SIZE.  Each time the PROBE_TIMER
   expires, the PROBE_COUNT is incremented, the PROBE_TIMER is
   reinitialized, and a probe packet of the same size is retransmitted
   (the replicated probe improve the resilience to loss).  The maximum
   number of retransmissions for a particular size is configured
   (MAX_PROBES).  If the value of the PROBE_COUNT reaches MAX_PROBES,
   probing will stop, and the PL sender enters the SEARCH_COMPLETE
   state.

5.3.2.  Selection of Probe Sizes

   The search algorithm needs to determine a minimum useful gain in
   PLPMTU.  It would not be constructive for a PL sender to attempt to
   probe for all sizes.  This would incur unnecessary load on the path
   and has the undesirable effect of slowing the time to reach a more
   optimal MPS.  Implementations SHOULD select the set of probe packet
   sizes to maximize the gain in PLPMTU from each search step.

   Implementations could optimize the search procedure by selecting step
   sizes from a table of common PMTU sizes.  When selecting the
   appropriate next size to search, an implementer ought to also
   consider that there can be common sizes of MPS that applications seek
   to use, and their could be common sizes of MTU used within the
   network.








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5.3.3.  Resilience to Inconsistent Path Information

   A decision to increase the PLPMTU needs to be resilient to the
   possibility that information learned about the network path is
   inconsistent.  A path is inconsistent, when, for example, probe
   packets are lost due to other reasons (i. e. not packet size) or due
   to frequent path changes.  Frequent path changes could occur by
   unexpected "flapping" - where some packets from a flow pass along one
   path, but other packets follow a different path with different
   properties.

   A PL sender is able to detect inconsistency from the sequence of
   PLPMTU probes that it sends or the sequence of PTB messages that it
   receives.  When inconsistent path information is detected, a PL
   sender could use an alternate search mode that clamps the offered MPS
   to a smaller value for a period of time.  This avoids unnecessary
   loss of packets due to MTU limitation.

5.4.  Robustness to Inconsistent Paths

   Some paths could be unable to sustain packets of the BASE_PMTU size.
   To be robust to these paths an implementation could implement the
   Error State.  This allows fallback to a smaller than desired PLPMTU,
   rather than suffer connectivity failure.  This could utilize methods
   such as endpoint IP fragmentation to enable the PL sender to
   communicate using packets smaller than the BASE_PMTU.

6.  Specification of Protocol-Specific Methods

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

   The first subsection provides guidance on how to implement the
   DPLPMTUD method as a part of an application using UDP or UDP-Lite.
   The guidance also applies to other datagram services that do not
   include a specific transport protocol (such as a tunnel
   encapsulation).  The following subsections describe how DPLPMTUD can
   be implemented as a part of the transport service, allowing
   applications using the service to benefit from discovery of the
   PLPMTU without themselves needing to implement this method.

6.1.  Application support for DPLPMTUD with UDP or 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, the UDP transport does not provide the transport layer
   features needed to implement datagram PLPMTUD.




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   The DPLPMTUD method can be implemented as a part of an application
   built directly or indirectly on UDP or UDP-Lite, but relies on
   higher-layer protocol features to implement the method [RFC8085].

   Some primitives used by DPLPMTUD might not be available via the
   Datagram API (e.g., the ability to access the PLPMTU cache, or
   interpret received PTB messages).

   In addition, it is desirable that PMTU discovery is not performed by
   multiple protocol layers.  An application SHOULD avoid using DPLPMTUD
   when the underlying transport system provides this capability.  To
   use common method for managing the PLPMTU has benefits, both in the
   ability to share state between different processes and opportunities
   to coordinate probing.

6.1.1.  Application Request

   An application needs an application-layer protocol mechanism (such as
   a message acknowledgement method) that solicits a response from a
   destination endpoint.  The method SHOULD allow the sender to check
   the value returned in the response to provide additional protection
   from off-path insertion of data [RFC8085], suitable methods include a
   parameter known only to the two endpoints, such as a session ID or
   initialized sequence number.

6.1.2.  Application Response

   An application needs an application-layer protocol mechanism to
   communicate the response from the destination endpoint.  This
   response may indicate successful reception of the probe across the
   path, but could also indicate that some (or all packets) have failed
   to reach the destination.

6.1.3.  Sending Application Probe Packets

   A probe packet that may carry an application data block, but the
   successful transmission of this data is at risk when used for
   probing.  Some applications may prefer to use a probe packet that
   does not carry an application data block to avoid disruption to data
   transfer.

6.1.4.  Initial Connectivity

   An application that does not have other higher-layer information
   confirming connectivity with the remote peer SHOULD implement a
   connectivity mechanism using acknowledged probe packets before
   entering the BASE state.




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6.1.5.  Validating the Path

   An application that does not have other higher-layer information
   confirming correct delivery of datagrams SHOULD implement the
   CONFIRMATION_TIMER to periodically send probe packets while in the
   SEARCH_COMPLETE state.

6.1.6.  Handling of PTB Messages

   An application that is able and wishes to receive PTB messages MUST
   perform ICMP validation as specified in Section 5.2 of [RFC8085].
   This requires that the application to check each received PTB
   messages to validate it is received in response to transmitted
   traffic and that the reported PTB_SIZE is less than the current
   probed size (see Section 4.5.2).  A validated PTB message MAY be used
   as input to the DPLPMTUD algorithm, but MUST NOT be used directly to
   set the PLPMTU.

6.2.  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.

6.2.1.  SCTP/IPv4 and SCTP/IPv6

6.2.1.1.  Initial Connectivity

   The base protocol is specified in [RFC4960].  This provides an
   acknowledged PL.  A sender can therefore enter the BASE state as soon
   as connectivity has been confirmed.

6.2.1.2.  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
   probe size, which is the size of the complete datagram.  The size of
   the PAD chunk is therefore computed by reducing the probing size by



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   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 PL handshake, before data is sent.  Assuming this behavior
   (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.

6.2.1.3.  Validating the Path with SCTP

   Since SCTP provides an acknowledged PL, a sender MUST NOT implement
   the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.

6.2.1.4.  PTB Message Handling by SCTP

   Normal ICMP validation 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 message has been validated, the PTB_SIZE reported in the
   PTB message SHOULD be used with the DPLPMTUD algorithm, providing
   that the reported PTB_SIZE is less than the current probe size (see
   Section 4.5).

6.2.2.  DPLPMTUD for SCTP/UDP

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

6.2.2.1.  Initial Connectivity

   A sender can enter the BASE state as soon as SCTP connectivity has
   been confirmed.

6.2.2.2.  Sending SCTP/UDP Probe Packets

   Packet probing can be performed as specified in Section 6.2.1.2.  The
   maximum payload is reduced by 8 bytes, which has to be considered
   when filling the PAD chunk.

6.2.2.3.  Validating the Path with SCTP/UDP

   Since SCTP provides an acknowledged PL, a sender MUST NOT implement
   the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.




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6.2.2.4.  Handling of PTB Messages by SCTP/UDP

   ICMP validation 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
   validation is completed, the PTB_SIZE indicated in the PTB message
   SHOULD be used with the DPLPMTUD providing that the reported PTB_SIZE
   is less than the current probe size.

6.2.3.  DPLPMTUD for SCTP/DTLS

   The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
   specified in [RFC8261].  It is used for data channels in WebRTC
   implementations.

6.2.3.1.  Initial Connectivity

   A sender can enter the BASE state as soon as SCTP connectivity has
   been confirmed.

6.2.3.2.  Sending SCTP/DTLS Probe Packets

   Packet probing can be done as specified in Section 6.2.1.2.

6.2.3.3.  Validating the Path with SCTP/DTLS

   Since SCTP provides an acknowledged PL, a sender MUST NOT implement
   the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.

6.2.3.4.  Handling of PTB Messages by SCTP/DTLS

   It is not possible to perform ICMP validation 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.

6.3.  DPLPMTUD for QUIC

   Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
   UDP-based transport that provides reception feedback.  The UDP
   payload includes the QUIC packet header, protected payload, and any
   authentication fields.  QUIC depends on a PMTU of at least 1280
   bytes.

   Section 14.1 of [I-D.ietf-quic-transport] describes the path
   considerations when sending QUIC packets.  It recommends the use of



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   PADDING frames to build the probe packet.  Pure probe-only packets
   are constructed with PADDING frames and PING frames to create a
   padding only packet that will elicit an acknowledgement.  Such
   padding only packets enable probing without affecting the transfer of
   other QUIC frames.

   The recommendation for QUIC endpoints implementing DPLPMTUD is that a
   MPS is maintained for each combination of local and remote IP
   addresses [I-D.ietf-quic-transport].  If a QUIC endpoint determines
   that the PMTU between any pair of local and remote IP addresses has
   fallen below an acceptable MPS, it needs to immediately cease sending
   QUIC packets on the affected path.  This could result in termination
   of the connection if an alternative path cannot be found
   [I-D.ietf-quic-transport].

6.3.1.  Initial Connectivity

   The base protocol is specified in [I-D.ietf-quic-transport].  This
   provides an acknowledged PL.  A sender can therefore enter the BASE
   state as soon as connectivity has been confirmed.

6.3.2.  Sending QUIC Probe Packets

   A probe packet consists of a QUIC Header and a payload containing
   PADDING Frames and a PING Frame.  PADDING Frames are a single octet
   (0x00) and several of these can be used to create a probe packet of
   size PROBED_SIZE.  QUIC provides an acknowledged PL, a sender can
   therefore enter the BASE state as soon as connectivity has been
   confirmed.

   The current specification of QUIC sets the following:

   *  BASE_PMTU: 1200.  A QUIC sender needs to pad initial packets to
      1200 bytes to confirm the path can support packets of a useful
      size.

   *  MIN_PMTU: 1200 bytes.  A QUIC sender that determines the PMTU has
      fallen below 1200 bytes MUST immediately stop sending on the
      affected path.

6.3.3.  Validating the Path with QUIC

   QUIC provides an acknowledged PL.  A sender therefore MUST NOT
   implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.







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6.3.4.  Handling of PTB Messages by QUIC

   QUIC operates over the UDP transport, and the guidelines on ICMP
   validation as specified in Section 5.2 of [RFC8085] therefore apply.
   In addition to UDP Port validation QUIC can validate an ICMP message
   by looking for valid Connection IDs in the quoted packet.

7.  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).

8.  IANA Considerations

   This memo includes no request to IANA.

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

9.  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 Usage Guidelines [RFC8085], specifically the
   generation of probe packets is regarded as a "Low Data-Volume
   Application", described in section 3.1.3 of this document.  This
   recommends that sender limits generation of probe packets to an
   average rate lower than one probe per 3 seconds.

   A PL sender needs to ensure that the method used to confirm reception
   of probe packets offers protection from off-path attackers injecting
   packets into the path.  This protection if provided in IETF-defined
   protocols (e.g., TCP, SCTP) using a randomly-initialized sequence
   number.  A description of one way to do this when using UDP is
   provided in section 5.1 of [RFC8085]).

   There are cases where ICMP Packet Too Big (PTB) messages are not
   delivered due to policy, configuration or equipment design (see
   Section 1.1), this method therefore does not rely upon PTB messages
   being received, but is able to utilize these when they are received
   by the sender.  PTB messages could potentially be used to cause a
   node to inappropriately reduce the PLPMTU.  A node supporting
   DPLPMTUD MUST therefore appropriately validate 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, see Section 4.5.1).




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   An on-path attacker, able to create a PTB message could forge PTB
   messages that include a valid quoted IP packet.  Such an attack could
   be used to drive down the PLPMTU.  There are two ways this method can
   be mitigated against such attacks: First, by ensuring that a PL
   sender never reduces the PLPMTU below the base size, solely in
   response to receiving a PTB message.  This is achieved by first
   entering the BASE state when such a message is received.  Second, the
   design does not require processing of PTB messages, a PL sender could
   therefore suspend processing of PTB messages (e.g., in a robustness
   mode after detecting that subsequent probes actually confirm that a
   size larger than the PTB_SIZE is supported by a path).

   Parallel forwarding paths SHOULD be considered.  Section 5.4
   identifies the need for robustness in the method when the path
   information may be inconsistent.

   A node performing DPLPMTUD 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
   black holed when the PLPMTU is larger than the smallest PMTU across
   the current paths.

10.  References

10.1.  Normative References

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-20 (work
              in progress), 23 April 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-20.txt>.

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

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

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





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   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://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, <https://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>.

   [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, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

   [RFC8261]  Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
              "Datagram Transport Layer Security (DTLS) Encapsulation of
              SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
              2017, <https://www.rfc-editor.org/info/rfc8261>.

10.2.  Informative References

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-09 (work in
              progress), 19 July 2018,



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              <http://www.ietf.org/internet-drafts/draft-ietf-intarea-
              tunnels-09.txt>.

   [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,
              <https://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>.

   [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>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://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,
              <https://www.rfc-editor.org/info/rfc4890>.

Appendix A.  Revision Notes

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

   Individual draft -00:





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

   *  This update is proposed for WG comments.

   Individual draft -01:

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

   *  This update is proposed for WG comments.

   Individual draft -02:

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

   *  The text describing when to set the effective PMTU has not yet
      been validated by the authors

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

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

   *  This update is proposed for WG comments.

   Working Group draft -00:

   *  This draft follows a successful adoption call for TSVWG

   *  There is still work to complete, please comment on this draft.

   Working Group draft -01:

   *  This draft includes improved introduction.

   *  The draft is updated to require ICMP validation prior to accepting
      PTB messages - this to be confirmed by WG

   *  Section added to discuss Selection of Probe Size - methods to be
      evaluated and recommendations to be considered

   *  Section added to align with work proposed in the QUIC WG.




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   Working Group draft -02:

   *  The draft was updated based on feedback from the WG, and a
      detailed review by Magnus Westerlund.

   *  The document updates RFC 4821.

   *  Requirements list updated.

   *  Added more explicit discussion of a simpler black-hole detection
      mode.

   *  This draft includes reorganisation of the section on IETF
      protocols.

   *  Added more discussion of implementation within an application.

   *  Added text on flapping paths.

   *  Replaced 'effective MTU' with new term PLPMTU.

   Working Group draft -03:

   *  Updated figures

   *  Added more discussion on blackhole detection

   *  Added figure describing just blackhole detection

   *  Added figure relating MPS sizes

   Working Group draft -04:

   *  Described phases and named these consistently.

   *  Corrected transition from confirmation directly to the search
      phase (Base has been checked).

   *  Redrawn state diagrams.

   *  Renamed BASE_MTU to BASE_PMTU (because it is a base for the PMTU).

   *  Clarified Error state.

   *  Clarified suspending DPLPMTUD.

   *  Verified normative text in requirements section.




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   *  Removed duplicate text.

   *  Changed all text to refer to /packet probe/probe packet/
      /validation/verification/ added term /Probe Confirmation/ and
      clarified BlackHole detection.

   Working Group draft -05:

   *  Updated security considerations.

   *  Feedback after speaking with Joe Touch helped improve UDP-Options
      description.

   Working Group draft -06:

   *  Updated description of ICMP issues in section 1.1

   *  Update to description of QUIC.

   Working group draft -07:

   *  Moved description of the PTB processing method from the PTB
      requirements section.

   *  Clarified what is performed in the PTB validation check.

   *  Updated security consideration to explain PTB security without
      needing to read the rest of the document.

   *  Reformatted state machine diagram

   Working group draft -08:

   *  Moved to rfcxml v3+

   *  Rendered diagrams to svg in html version.

   *  Removed Appendix A.  Event-driven state changes.

   *  Removed section on DPLPMTUD with UDP Options.

   *  Shortened the description of phases.

   Working group draft -09:

   *  Remove final mention of UDP Options

   *  Add Initial Connectivity sections to each PL



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   *  Add to disable outgoing pmtu enforcement of packets

Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen
   AB24 3UE
   United Kingdom

   Email: gorry@erg.abdn.ac.uk


   Tom Jones
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen
   AB24 3UE
   United Kingdom

   Email: tom@erg.abdn.ac.uk


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   Germany

   Email: tuexen@fh-muenster.de


   Irene Ruengeler
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   Germany

   Email: i.ruengeler@fh-muenster.de


   Timo Voelker
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   Germany




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   Email: timo.voelker@fh-muenster.de


















































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