Network Working Group                                          R. Hinden
Internet-Draft                                      Check Point Software
Intended status: Experimental                               G. Fairhurst
Expires: 11 March 2022                            University of Aberdeen
                                                        7 September 2021


                IPv6 Minimum Path MTU Hop-by-Hop Option
                     draft-ietf-6man-mtu-option-08

Abstract

   This document specifies a new Hop-by-Hop IPv6 option that is used to
   record the minimum Path MTU along the forward path between a source
   host to a destination host.  The value can then be communicated back
   to the source using the return Path MTU field in the option.

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
   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 11 March 2022.

Copyright Notice

   Copyright (c) 2021 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.




Hinden & Fairhurst        Expires 11 March 2022                 [Page 1]


Internet-Draft               Path MTU Option              September 2021


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Example Operation . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Use of the IPv6 Hop-by-Hop Options Header . . . . . . . .   4
   2.  Motivation and Problem Solved . . . . . . . . . . . . . . . .   5
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   4.  Applicability Statements  . . . . . . . . . . . . . . . . . .   6
   5.  IPv6 Minimum Path MTU Hop-by-Hop Option . . . . . . . . . . .   6
   6.  Router, Host, and Transport Behaviors . . . . . . . . . . . .   8
     6.1.  Router Behavior . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Host OS and Transport Behavior  . . . . . . . . . . . . .   8
       6.2.1.  Host Operating System Behavior  . . . . . . . . . . .   8
       6.2.2.  Transport Behavior  . . . . . . . . . . . . . . . . .   9
         6.2.2.1.  Including the Option in an Outgoing Packet  . . .  10
         6.2.2.2.  Validation of the Packet that includes the
                 Option  . . . . . . . . . . . . . . . . . . . . . .  11
         6.2.2.3.  Receiving the Option  . . . . . . . . . . . . . .  11
         6.2.2.4.  Using the Rtn-PMTU Field  . . . . . . . . . . . .  12
         6.2.2.5.  Detecting Path Changes  . . . . . . . . . . . . .  13
         6.2.2.6.  Detection of Dropping Packets that include the
                 Option  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     8.1.  Router Option Processing  . . . . . . . . . . . . . . . .  14
     8.2.  Network Layer Host Processing . . . . . . . . . . . . . .  14
     8.3.  Validating use of the Option Data . . . . . . . . . . . .  14
     8.4.  Direct use of the Rtn-PMTU Value  . . . . . . . . . . . .  15
     8.5.  Using the Rtn-PMTU Value as a Hint for Probing  . . . . .  15
     8.6.  Impact of Middleboxes . . . . . . . . . . . . . . . . . .  16
   9.  Experiment Goals  . . . . . . . . . . . . . . . . . . . . . .  16
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  17
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   12. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  17
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     13.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   This draft proposes a new IPv6 Hop-by-Hop Option to record the
   minimum of the Maximum Transmission Unit (MTU) along the forward path
   between a source and destination host.  The source host creates a
   packet with this option and fills the Min-PMTU field with the value
   of the MTU for the outbound link that will be used to forward the
   packet towards the destination host.




Hinden & Fairhurst        Expires 11 March 2022                 [Page 2]


Internet-Draft               Path MTU Option              September 2021


   At each subsequent hop where the option is processed, the router
   compares the value of the Min-PMTU Field in the option and the MTU of
   its outgoing link.  If the MTU of the link is less than the Min-PMTU,
   it rewrites the value in the option data with the smaller value.
   When the packet arrives at the destination host, the host can send
   the value of the minimum reported MTU for the path back to the source
   host using the Rtn-PMTU field in the option.  The source host can
   then use this value as input to the method that sets the Path MTU
   (PMTU) used by upper layer protocols.

   This method has the potential to complete discovery of the correct
   value in a single round trip time, even over paths that have
   successive links each configured with a lower MTU.

1.1.  Example Operation

   The figure below illustrates the operation of the method.  In this
   case, the path between the source host and the destination host
   comprises three links, the sender has a link MTU of size MTU-S, the
   link between routers R1 and R2 has an MTU of size 9000 bytes, and the
   final link to the destination has an MTU of size MTU-D.

          +--------+         +----+        +----+         +-------+
          |        |         |    |        |    |         |       |
          | Sender +---------+ R1 +--------+ R2 +-------- + Dest. |
          |        |         |    |        |    |         |       |
          +--------+  MTU-S  +----+  9000B +----+  MTU-D  +-------+


                                  Figure 1

   Three scenarios are described:

   *  Scenario 1, considers all links to have an 9000 byte MTU and the
      method is supported by both routers.  The initial Min-PMTU is not
      modified along the path, and therefore the PMTU is 9000 bytes.

   *  Scenario 2, considers the link to the destination host (MTU-D) to
      have an MTU of 1500 bytes.  This is the smallest MTU, router R2
      updates the Min-PMTU to 1500 bytes and the method correctly
      updates the PMTU to 1500 bytes.  Had there been another smaller
      MTU at a link further along the path that also supports the
      method, the lower MTU would also have been detected.

   *  Scenario 3, considers the case where the router preceding the
      smallest link (R2) does not support the method, and the link to
      the destination host (MTU-D) has an MTU of 1500 bytes.  Therefore,




Hinden & Fairhurst        Expires 11 March 2022                 [Page 3]


Internet-Draft               Path MTU Option              September 2021


      router R2 does not update the Min-PMTU to 1500 bytes.  The method
      then fails to detect the actual PMTU.

   In Scenarios 2 and 3, a lower PMTU would also fail to be detected in
   the case where PMTUD had been used and an ICMPv6 Packet Too Big (PTB)
   message had not been delivered to the sender [RFC8201].

   These scenarios are summarized in the table below.  "H" in R1 and/or
   R2 columns means the router understands the Minimum Path MTU Hop-by-
   Hop option.

         +-+-----+-----+----+----+----------+-----------------------+
         | |MTU-S|MTU-D| R1 | R2 | Rec PMTU | Note                  |
         +-+-----+-----+----+----+----------+-----------------------+
         |1|9000B|9000B| H  | H  |  9000 B  | Endpoints attempt to  |
         |       |     |    |    |          | use a 9000 B PMTU.    |
         +-+-----+-----+----+----+----------+-----------------------+
         |2|9000B|1500B| H  | H  |  1500 B  | Endpoints attempt to  |
         | |     |     |    |    |          | use a 1500 B PMTU.    |
         +-+-----+-----+----+----+----------+-----------------------+
         |3|9000B|1500B| H  | -  |  9000 B  | Endpoints attempt to  |
         | |     |     |    |    |          | use a 9000 B PMTU,    |
         | |     |     |    |    |          | but need to implement |
         | |     |     |    |    |          | a method to fall back |
         | |     |     |    |    |          | to discover and use a |
         | |     |     |    |    |          | 1500 B PMTU.          |
         +-+-----+-----+----+----+----------+-----------------------+


                                  Figure 2

1.2.  Use of the IPv6 Hop-by-Hop Options Header

   IPv6 as specified in [RFC8200] allows nodes to optionally process
   Hop-by-Hop headers.  Specifically from Section 4:

   *  The Hop-by-Hop Options header is not inserted or deleted, but may
      be examined or processed by any node along a packet's delivery
      path, until the packet reaches the node (or each of the set of
      nodes, in the case of multicast) identified in the Destination
      Address field of the IPv6 header.  The Hop-by-Hop Options header,
      when present, must immediately follow the IPv6 header.  Its
      presence is indicated by the value zero in the Next Header field
      of the IPv6 header.

   *  NOTE: While [RFC2460] required that all nodes must examine and
      process the Hop-by-Hop Options header, it is now expected that




Hinden & Fairhurst        Expires 11 March 2022                 [Page 4]


Internet-Draft               Path MTU Option              September 2021


      nodes along a packet's delivery path only examine and process the
      Hop-by-Hop Options header if explicitly configured to do so.

   The Hop-by-Hop Option defined in this document is designed to take
   advantage of this property of how Hop-by-Hop options are processed.
   Nodes that do not support this Option SHOULD ignore them.  This can
   mean that the Min-PMTU value does not account for all links along a
   path.

2.  Motivation and Problem Solved

   The current state of Path MTU Discovery on the Internet is
   problematic.  The mechanisms defined in [RFC8201] are known to not
   work well in all environments.  This fails to work in various cases,
   including when nodes in the middle of the network do not send ICMP
   PTB messages, or rate-limited ICMP messages, or do not have a return
   path to the source host.

   This results in many transport connections being configured to use
   smaller packets (e.g., 1280 bytes) by default and makes it difficult
   to take advantage of paths with a larger PMTU where they do exist.
   Applications that send large packets are forced to use IPv6
   Fragmentation [RFC8200], which can reduce the reliability of Internet
   communication [RFC8900].

   Encapsulations and network-layer tunnels further reduce the payload
   size available for a transport to use.  Also, some use-cases increase
   packet overhead, for example, Network Virtualization Using Generic
   Routing Encapsulation (NVGRE) [RFC7637] encapsulates L2 packets in an
   outer IP header and does not allow IP Fragmentation.

   Sending larger packets can improve host performance, e.g., avoiding
   limits to packet processing by the packet rate.  The potential of
   multi-gigabit Ethernet will only be realized if the packet size is
   increased above 1280 bytes, to avoid exceeding a packet per second
   sending rate that most hosts can process.  For example, the packet
   per second rate required to reach wire speed on a 10G Ethernet link
   with 1280 byte packets is about 977K packets per second (pps), vs.
   139K pps for 9000 byte packets.  A significant difference.

   The purpose of the this draft is to improve the situation by defining
   a mechanism that does not rely on reception of ICMPv6 Packet Too Big
   messages from nodes in the middle of the network.  Instead, this
   provides information to the destination host about the minimum Path
   MTU, and sends this information back to the source host.  This is
   expected to work better than the current RFC8201-based mechanisms.





Hinden & Fairhurst        Expires 11 March 2022                 [Page 5]


Internet-Draft               Path MTU Option              September 2021


3.  Requirements Language

   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.

4.  Applicability Statements

   The Path MTU option is designed for environments where there is
   control over the hosts and nodes that connect them, and where there
   is more than one MTU size in use.  For example in Data Centers and on
   paths between Data Centers, to allow a hosts to better take advantage
   of a path that is able to support a large PMTU.

   The design of the option is sufficiently simple that it can be
   executed on a router's fast path.  A strong pull from router vendors
   customers will be required to create critical mass for this to
   happen.  This could initially be the case for connections within and
   between Data Centers.

   The method could also be useful in other environments, including the
   general Internet, and offers advantage when this Hop-by-Hop Option is
   supported on these paths.  The method is more robust when used to
   probe the path using packets that do not carry application data and
   when also paired with a method such as Packetization Layer PMTUD
   [RFC4821] or Datagram PLPMTUD [RFC8899].

5.  IPv6 Minimum Path MTU Hop-by-Hop Option

   The Minimum Path MTU Hop-by-Hop Option has the following format:



















Hinden & Fairhurst        Expires 11 March 2022                 [Page 6]


Internet-Draft               Path MTU Option              September 2021


    Option    Option    Option
     Type    Data Len   Data
   +--------+--------+--------+--------+---------+-------+-+
   |BBCTTTTT|00000100|     Min-PMTU    |     Rtn-PMTU    |R|
   +--------+--------+--------+--------+---------+-------+-+

     Option Type (see Section 4.2 of [RFC8200]):

     BB     00   Skip over this option and continue processing.

     C       1   Option data can change en route to the packet's final
                 destination.

     TTTTT 10000 Option Type assigned from IANA [IANA-HBH].

     Length:  4  The size of the each value field in Option Data
     field supports PMTU values from 0 to 65,534 octets, the
     maximum size represented by the Path MTU option.


     Min-PMTU: n 16-bits.  The minimum MTU recorded along the path
                 in octets, reflecting the smallest link MTU that
                 the packet experienced along the path.
                 A value less than the IPv6 minimum link
                 MTU [RFC8200] MUST be ignored.

     Rtn-PMTU: n 15-bits.  The returned Path MTU field, carrying the 15
                 most significant bits of the latest received Min-PMTU
                 field for the forward path.  The value zero means that
                 no Reported MTU is being returned.

     R        n  1-bit.  R-Flag.   Set by the source to signal that
                 the destination host should include the received
                 Rtn-PMTU field updated by the reported Min-PMTU value
                 when the destination host is to send a PMTU Option back
                 to the source host.

                                  Figure 3

   NOTE: The encoding of the final two octets (Rtn-PMTU and R-Flag)
   could be implemented by a mask of the latest received Min-PMTU value
   with 0xFFFE, discarding the right-most bit and then performing a
   logical 'OR' with the R-Flag value of the sender.  This encoding fits
   in the minimum-sized HBH Option header.







Hinden & Fairhurst        Expires 11 March 2022                 [Page 7]


Internet-Draft               Path MTU Option              September 2021


6.  Router, Host, and Transport Behaviors

6.1.  Router Behavior

   Routers that are not configured to support Hop-by-Hop Options SHOULD
   ignore this option and SHOULD forward the packet [RFC8200].

   Routers that support Hop-by-Hop Options, but that are not configured
   to support this option SHOULD ignore the option and SHOULD forward
   the packet.

   Routers that support this option SHOULD compare the value of the Min-
   PMTU field with the MTU configured for the outgoing link.  If the MTU
   of the outgoing link is less than the Min-PMTU, the router rewrites
   the Min-PMTU in the Option to use the smaller value.  (The router
   processing is performed without checking the valid range of the Min-
   PMTU or the Rtn-PMTU fields.)

   A router MUST ignore and MUST NOT change the Rtn-PMTU field or the
   R-Flag in the option.

   Discussion:

   *  The design of this option makes it feasible to be implemented
      within the fast path of a router, because the processing
      requirements are minimal.

6.2.  Host OS and Transport Behavior

6.2.1.  Host Operating System Behavior

   The PMTU entry associated with the destination in the IP layer cache
   can be updated using PMTUD after detecting a change using the IPv6
   Minimum Path MTU Hop-by-Hop Option.  This cached value can be used by
   other flows that share the IP cache.

   The value in the host IP layer cache could, for instance, be used by
   PLPMTUD to select an initial PMTU for each flow before a flow
   determines a PMTU for the specific path it is using (e.g., using the
   IPv6 Minimum Path MTU Hop-by-Hop Option and DPLPMTUD).  The cached
   PMTU is only increased by PLPMTUD when the PL determines the path
   actually supports a larger PMTU [RFC4821] [RFC8899].

   When requested to send an IPv6 packet with the Minimum Path MTU
   option, the source host includes the option in an outgoing packet.
   The source host MUST fill the Min-PMTU field with the MTU configured
   for the link over which it will send the packet on the next hop
   towards the destination host.



Hinden & Fairhurst        Expires 11 March 2022                 [Page 8]


Internet-Draft               Path MTU Option              September 2021


   It sets the R Flag if it wishes the remote host to return the
   discovered PMTU value.

   When a host includes the option in a packet it sends, the host SHOULD
   set the Rtn-PMTU field to the previously cached value of the received
   Minimum Path MTU for the flow in the Rtn-PMTU field (see
   Section 6.2.2.3).  If this value is not set (for example, because
   there is no cached reported Min-PMTU value), the Rtn-PMTU field value
   MUST be set to zero.

   The source host MAY request the destination host to return the
   reported Min-PMTU value by setting the R-Flag in the option of an
   outgoing packet.  The R-Flag SHOULD NOT be set when the Minimum Path
   MTU Option was sent solely to provide requested feedback on the
   return Path MTU to avoid each response generating another response.

   The normal sequence of operation of the R-Flag using the terminology
   from the diagram in Figure 1 is:

   1.  Sender sends probe to Dest.  Sender MUST set the R-Flag

   2.  Dest responds by sending a probe including the received Min-PMTU
       as the Rtn-PMTU.  Dest sets R-Flag only if response is desired

   3.  Sender sends response probe back to Dest, MUST NOT set R-Flag.

6.2.2.  Transport Behavior

   This Hop-by-Hop option is intended to be used with a path MTU
   discovery method.

   Section 4.1 of [RFC9000] describes different types of PMTU Probe,
   depending on whether the probe packets carry application data.  When
   the path is expected to support use of the option, the PMTU Probe can
   be sent on packets that include application data, but needs to be
   robust to potential loss of the packet with the possibility that
   retransmission might be needed.  Using a PMTU Probe on packets that
   do not carry application data will avoid the need for loss recovery
   if a router on the path later drops packets that set this option.
   This avoids the transport needing to retransmit a lost packet that
   includes this option.










Hinden & Fairhurst        Expires 11 March 2022                 [Page 9]


Internet-Draft               Path MTU Option              September 2021


6.2.2.1.  Including the Option in an Outgoing Packet

   The upper layer protocol can request the Minimum Path MTU option to
   be included in an outgoing IPv6 packet.  A transport protocol (or
   upper layer protocol) can include this option only on specific
   packets used to test the path.  This option does not need to be
   included in all packets belonging to a flow.

   NOTE: Including this option in a large packet (e.g., one larger than
   the present PMTU) is not likely to be useful, since the large packet
   would itself be dropped by any link along the path with a smaller
   MTU, preventing the Min-PMTU information from reaching the
   destination host.

   Discussion:

   *  In the case of TCP, the option could be included in a packet that
      carries a TCP segment sent after the connection is established.  A
      segment without data could be used, to avoid the need to
      retransmit this data if the probe packet is lost.  The discovered
      value can be used to inform PLPMTUD [RFC4821].

      NOTE: A TCP SYN can also negotiate the Maximum Segment Size (MSS),
      which acts as an upper limit to the packet size that can be sent
      by a TCP sender.  If this option were to be included in a TCP SYN,
      it could increase the probability that the SYN segment is lost
      when routers on the path drop packets with this option (see
      Section 6.2.2.6), which could have an unwanted impact on the
      result of racing options [I-D.ietf-taps-arch] or feature
      negotiation.

   *  The use with datagram transport protocols (e.g., UDP) is harder to
      characterize because applications using datagram transports range
      from very short-lived (low data-volume applications) exchanges, to
      longer (bulk) exchanges of packets between the source and
      destination hosts [RFC8085].

   *  Simple-exchange protocols (i.e., low data-volume applications
      [RFC8085] that only send one or a few packets per transaction),
      might assume that the PMTU is symmetrical.  That is, the PMTU is
      the same in both directions, or at least not smaller for the
      return path.  This optimization does not hold when the paths are
      not symmetric.

   *  A datagram transport can utilise DPLPMTUD [RFC8899].  For example,
      QUIC (see section 14.3 of [RFC9000]), can use DPLPMTUD to
      determine whether the path to a destination will support a desired
      maximum datagram size.  When using the IPv6 MinPMTU HBH option,



Hinden & Fairhurst        Expires 11 March 2022                [Page 10]


Internet-Draft               Path MTU Option              September 2021


      the option could be added to an additional QUIC PMTU Probe that is
      of minimal size (or one no larger than the currently supported
      PMTU size).  Once the return Path MTU value in the Min PMTU HBH
      option has been learned, DPLPMTUD can be triggered to test for a
      larger PLPMTU using an appropriately sized PLPMTU Probe Packet
      (see section 5.3.1 of [RFC8899]).

   *  The use of this option with DNS and DNSSEC over UDP ought to work
      for paths where the PMTU is symmetric.  The DNS server will learn
      the PMTU from the DNS query messages.  If the Rtn-PMTU value is
      smaller, then a large DNSSEC response might be dropped and the
      known problems with PMTUD will then occur.  DNS and DNSSEC over
      transport protocols that can carry the PMTU ought to work.

   *  This method also can be used with Anycast to discover the PMTU of
      the path, but the use needs to be aware that the Anycast binding
      might change.

6.2.2.2.  Validation of the Packet that includes the Option

   An upper layer protocol (e.g., transport endpoint) using this option
   needs to provide protection from data injection attacks by off-path
   devices [RFC8085].  This requires a method to assure that the
   information in the Option Data is provided by a node on the path.
   This validates that the packet forms a part of an existing flow,
   using context available at the upper layer.  For example, a TCP
   connection or UDP application that maintains the related state and
   uses a randomized ephemeral port would provide this basic validation
   to protect from off-path data injection, see Section 5.1 of
   [RFC8085].  IPsec [RFC4301] and TLS [RFC8446] provide greater
   assurance.

   The upper layer discards any received packet when the packet
   validation fails.  When packet validation fails, the upper layer MUST
   also discard the associated Option Data from the minimum Path MTU
   option without further processing.

6.2.2.3.  Receiving the Option

   For a connection-oriented upper layer protocol, caching of the
   received Min-PMTU could be implemented by saving the value in the
   connection context at the transport layer.  A connection-less upper
   layer (e.g., one using UDP), requires the upper layer protocol to
   cache the value for each flow it uses.

   A destination host that receives a Minimum Path MTU Option with the
   R-Flag SHOULD include the Minimum Path MTU option in the next
   outgoing IPv6 packet for the corresponding flow.



Hinden & Fairhurst        Expires 11 March 2022                [Page 11]


Internet-Draft               Path MTU Option              September 2021


   A simple mechanism could only include this option (with the Rtn-PMTU
   field set) the first time this option is received or when it notifies
   a change in the Minimum Path MTU.  This limits the number of packets
   including the option packets that are sent.  However, this does not
   provide robustness to packet loss or recovery after a sender loses
   state.

   Discussion:

   *  Some upper layer protocols send packets less frequently than the
      rate at which the host receives packets.  This provides less
      frequent feedback of the received Rtn-PMTU value.  However, a host
      always sends the most recent Rtn-PMTU value.

6.2.2.4.  Using the Rtn-PMTU Field

   The Rtn-PMTU field provides an indication of the PMTU from on-path
   routers.  It does not necessarily reflect the actual PMTU between the
   sender and destination.  Care therefore needs to be exercised in
   using the Rtn-PMTU value.  Specifically:

   *  The actual PMTU can be lower than the Rtn-PMTU value because Min-
      PMTU field was not updated by a router on the path that did not
      process the option.

   *  The actual PMTU may be lower than the Rtn-PMTU value because the
      there is a layer 2 device with a lower MTU.

   *  The actual PMTU may be larger than the Rtn-PMTU value because of a
      corrupted, delayed or mis-ordered response.  A source host SHOULD
      ignore a Rtn-PMTU value larger than the MTU configured for the
      outgoing link.

   IPv6 requires that every link in the Internet have an MTU of 1280
   octets or greater.  A node MUST ignore a Rtn-PMTU value less than
   1280 octets [RFC8200].

   To avoid unintentional dropping of packets that exceed the actual
   PMTU (e.g., Scenario 3 in Section 1.1), the source host can delay
   increasing the PMTU until a probe packet with the size of the Rtn-
   PMTU value has been successfully acknowledged by the upper layer,
   confirming that the path supports the larger PMTU.  This probing
   increases robustness, but adds one additional path round trip time
   before the PMTU is updated.  This use resembles that of PTB messages
   in section 4.6 of DPLPMTUD [RFC8899] (with the important difference
   that a PTB message can only seek to lower the PMTU, whereas this
   option could trigger a probe packet to seek to increase the PMTU.)




Hinden & Fairhurst        Expires 11 March 2022                [Page 12]


Internet-Draft               Path MTU Option              September 2021


   Section 5.2 of [RFC8201] provides guidance on the caching of PMTU
   information and also the relation to IPv6 flow labels.
   Implementations should consider the impact of Equal Cost Multipath
   (ECMP) [RFC6438].  Specifically, whether a PMTU ought be maintained
   for each transport endpoint, or for each network address.

6.2.2.5.  Detecting Path Changes

   Path characteristics can change and the actual PMTU could increase or
   decrease over time.  For instance, following a path change when
   packets are forwarded over a link with a different MTU than that
   previously used.  To bound the delay in discovering an increase in
   the actual PMTU, a host with a link MTU larger than the current PMTU
   SHOULD periodically send the Minimum Path MTU Option with the R-bit
   set.  DPLPMTUD provides recommendations concerning how this could be
   implemented (see Section 5.3 of [RFC8899]).  Since the option
   consumes less capacity than a full-sized probe packet, there can be
   advantage in using this to detect a change in the path
   characteristics.

6.2.2.6.  Detection of Dropping Packets that include the Option

   There is evidence that some middleboxes drop packets that include
   Hop-by-Hop options.  For example, a firewall might drop a packet that
   carries an unknown extension header or option.  This practice is
   expected to decrease as an option becomes more widely used.  It could
   result in generation of an ICMPv6 message indicating the problem.
   This could be used to (temporarily) suspend use of this option.

   A middlebox that silently discards a packet with this option results
   in dropping of any packet using the option.  This dropping can be
   avoided by appropriate configuration in a controlled environment,
   such as within a data centre, but needs to be considered for Internet
   usage.  Section 6.2 recommends that this option is not used on
   packets where loss might adversely impact performance.

7.  IANA Considerations

   IANA has assigned and registered an IPv6 Hop-by-Hop Option type with
   Temporary status from the "Destination Options and Hop-by-Hop
   Options" registry [IANA-HBH].  This assignment is shown in Section 5.

   IANA is requested to update this registry to point to this document
   and remove the Temporary status.







Hinden & Fairhurst        Expires 11 March 2022                [Page 13]


Internet-Draft               Path MTU Option              September 2021


8.  Security Considerations

   This section discusses the security considerations.  It first reviews
   router option processing.  It then reviews host processing when
   receiving this option at the network layer.  It then considers two
   ways in which the Option Data can be processed, followed by two
   approaches for using the Option Data.  Finally, it discusses
   middlebox implications related to use in the general Internet.

8.1.  Router Option Processing

   This option shares the characteristics of all other IPv6 Hop by Hop
   Options, in that if not supported at line rate it could be used to
   degrade the performance of a router.  This option, while simple, is
   no different to other uses of IPv6 Hop-by-Hop options.

8.2.  Network Layer Host Processing

   A malicious attacker can forge a packet directed at a host that
   carries the minimum Path MTU option.  By design, the fields of this
   IP option can be modified by the network.

   Reception of this packet will require processing as the network stack
   parses the packet before the packet is delivered to the upper layer
   protocol.  This network layer option processing is normally completed
   before any upper layer protocol delivery checks are performed.

   The network layer does not normally have sufficient information to
   validate that the packet carrying an option originated from the
   destination (or an on-path node).  It also does not typically have
   sufficient context to demultiplex the packet to identify the related
   transport flow.  This can mean that any changes resulting from
   reception of the option apply to all flows between a pair of
   endpoints.

   These considerations are no different to other uses of Hop-by-Hop
   options, and this is the use case for PMTUD.  The following section
   describes a mitigation for this attack.

8.3.  Validating use of the Option Data

   Transport protocols should be designed to provide protection from
   data injection attacks by off-path devices and mechanisms should be
   described in the Security Considerations for each transport
   specification (see Section 5.1 of the UDP Guidelines [RFC8085]).  For
   example, a TCP or UDP application that maintains the related state
   and uses a randomized ephemeral port would provide basic protection.
   TLS [RFC8446] or IPsec [RFC4301] provide cryptographic



Hinden & Fairhurst        Expires 11 March 2022                [Page 14]


Internet-Draft               Path MTU Option              September 2021


   authentication.  An upper layer protocol that validates each received
   packet discards any packet when this validation fails.  In this case,
   the host MUST also discard the associated Option Data from the
   minimum Path MTU option without further processing (Section 6.2.2).

   A network node on the path has visibility of all packets it forwards.
   By observing the network packet payload, the node might be able to
   construct a packet that might be validated by the destination host.
   Such a node would also be able to drop or limit the flow in other
   ways that could be potentially more disruptive.  Authenticating the
   packet, for example, using IPsec [RFC4301] or TLS [RFC8446] mitigates
   this attack.

8.4.  Direct use of the Rtn-PMTU Value

   The simplest way to utilize the Rtn-PMTU value is to directly use
   this to update the PMTU.  This approach results in a set of security
   issues when the option carries malicious data:

   *  A direct update of the PMTU using the Rtn-PMTU value could result
      in an attacker inflating or reducing the size of the host PMTU for
      the destination.  Forcing a reduction in the PMTU can decrease the
      efficiency of network use, might increase the number of packets/
      fragments required to send the same volume of payload data, and
      prevents sending an unfragmented datagram larger than the PMTU.
      Increasing the PMTU can result in black-holing (see Section 1.1 of
      [RFC8899]) when the source sends packets larger than the actual
      PMTU.  This persists until the PMTU is next updated.

   *  The method can be used to solicit a response from the destination
      host.  A malicious attacker could forge a packet that cause the
      sender to add the option to a packet sent to the source.  A forged
      value of Rtn-PMTU in the Option Data might also impact the remote
      endpoint, as described in the previous bullet.  This persists
      until a valid minimum Path MTU option is received.  This attack
      could be mitigated by limiting the sending of the minimum Path MTU
      option in reply to incoming packets that carry the option.

8.5.  Using the Rtn-PMTU Value as a Hint for Probing

   Another way to utilize the Rtn-PMTU value is to indirectly trigger a
   probe to determine if the path supports a PMTU of size Rtn-PMTU.
   This approach needs context for the flow, and hence assumes an upper
   layer protocol that validates the packet that carries the option (see
   Section 8.3).  This is the case when used in combination with
   DPLPMTUD [RFC8899].  A set of security considerations result when an
   option carries malicious data:




Hinden & Fairhurst        Expires 11 March 2022                [Page 15]


Internet-Draft               Path MTU Option              September 2021


   *  If the forged packet carries a validated option with a non-zero
      Rtn-PMTU field, the upper layer protocol could utilize the
      information in the Rtn-PMTU field.  A Rtn-PMTU larger than the
      current PMTU can trigger a probe for a new size.

   *  If the forged packet carries a non-zero Min-PMTU field, the upper
      layer protocol would change the cached information about the path
      from the source.  The cached information at the destination host
      will be overwritten when the host receives another packet that
      includes a minimum Path MTU option corresponding to the flow.

   *  Processing of the option could cause a destination host to add the
      minimum Path MTU option to a packet sent to the source host.  This
      option will carry a Rtn-PMTU value that could have been updated by
      the forged packet.  The impact of the source host receiving this
      resembles that discussed previously.

8.6.  Impact of Middleboxes

   There is evidence that some middleboxes drop packets that include
   Hop-by-Hop options.  For example, a firewall might drop a packet that
   carries an unknown extension header or option.  This practice is
   expected to decrease as the option becomes more widely used.  Methods
   to address this are discussed in Section 6.2.2.6.

   When a forged packet cause a packet to be sent including the minimum
   Path MTU option, and the return path does not forward packets with
   this option, the packet will be dropped Section 6.2.2.6.  This attack
   is mitigated by validating the option data before use and by limiting
   the rate of responses generated.  An upper layer could further
   mitigate the impact by responding to a R-Flag by including the option
   in a packet that does not carry application data.

9.  Experiment Goals

   This section describes the experimental goals of this specification.

   A successful deployment of the method depends upon several components
   being implemented and deployed:

   *  Support in the sending node (see Section 6.2.1).  This also
      requires corresponding support in upper layer protocols (see
      Section 6.2.2).

   *  Router support in nodes (see Section 6.1).  The IETF continues to
      provide recommendations on the use of IPv6 Hop-by-Hop options, for
      example Section 2.2.2 of [RFC9099].  This document does not update
      the way router implementations configure support for HBH options.



Hinden & Fairhurst        Expires 11 March 2022                [Page 16]


Internet-Draft               Path MTU Option              September 2021


   *  Support in the receiving node (see Section 6.2.2.3).

   Experience from deployment is an expected input to any decision to
   progress this specification from Experimental to IETF Standards
   Track.  Appropriate inputs might include:

   *  Reports of implementation experience;

   *  Measurements of the number paths where the method can be used;

   *  Measurements showing the benefit realized or the implications of
      using specific methods over specific paths.

10.  Implementation Status

   At the time this document was published there are two known
   implementations of the Path MTU Hop-by-Hop option.  These are:

   *  Wireshark dissector.  This is shipping in production in Wireshark
      version 3.2 [WIRESHARK].

   *  A prototype in the open source version of the FD.io Vector Packet
      Processing (VPP) technology [VPP].  The the time this document was
      published, the source code can be found [VPP_SRC].

11.  Acknowledgments

   A similar mechanism was proposed for IPv4 in 1988 in [RFC1063] by
   Jeff Mogul, C.  Kent, Craig Partridge, and Keith McCloghire.  It was
   later obsoleted in 1990 by [RFC1191] the current deployed approach to
   Path MTU Discovery.

   Helpful comments were received from Tom Herbert, Tom Jones, Fred
   Templin, Ole Troan, Tianran Zhou, Jen Linkova, Brian Carpenter, Peng
   Shuping, Mark Smith, Fernando Gont, and other members of the 6MAN
   working group.

12.  Change log [RFC Editor: Please remove]

   draft-ietf-6man-mtu-option-08, 2021-September-7

   *  Clarifications and editorial changes based on chair review by Ole
      Troan.
   *  Correction and clarifications based on review by Fernando Gont.

   draft-ietf-6man-mtu-option-07, 2021-August-31

   *  Added Experiment Goals section.



Hinden & Fairhurst        Expires 11 March 2022                [Page 17]


Internet-Draft               Path MTU Option              September 2021


   *  Added Implementation Status section.
   *  Updated the IANA Considerations section to point to this document
      and remove Temporary status.
   *  Clarifications and editorial changes based on review by Mark
      Smith.

   draft-ietf-6man-mtu-option-06, 2021-August-7

   *  Transport usage of the mechanism clarified in response to feedback
      and suggestions from Jen Linkova.
   *  Restructured Section 6 to improve readability.
   *  Editorial changes.

   draft-ietf-6man-mtu-option-05, 2021-April-28

   *  Editorial changes.

   draft-ietf-6man-mtu-option-04, 2020-Oct-23

   *  Fixes for typos.

   draft-ietf-6man-mtu-option-03, 2020-Sept-14

   *  Rewrite to make text and terminology more consistent.
   *  Added the notion of validating the packet before use of the HBH
      option data.
   *  Method aligned with the way common APIs send/receive HBH option
      data.
   *  Added reference to DPLPMTUD and clarified upper layer usage.
   *  Completed security considerations section.

   draft-ietf-6man-mtu-option-02, 2020-March-9

   *  Editorial changes to make text and terminology more consistent.
   *  Added reference to DPLPMTUD.

   draft-ietf-6man-mtu-option-01, 2019-September-13

   *  Changes to show IANA assigned code point.
   *  Editorial changes to make text and terminology more consistent.
   *  Added a reference to RFC8200 in Section 2 and a reference to
      RFC6438 in Section 6.2.2.

   draft-ietf-6man-mtu-option-00, 2019-August-9

   *  First 6man w.g. draft version.
   *  Changes to request IANA allocation of code point.
   *  Editorial changes.



Hinden & Fairhurst        Expires 11 March 2022                [Page 18]


Internet-Draft               Path MTU Option              September 2021


   draft-hinden-6man-mtu-option-02, 2019-July-5

   *  Changed option format to also include the Returned PMTU value and
      Return flag and made related text changes in Section 6.2 to
      describe this behavior.
   *  ICMP Packet Too Big messages are no longer used for feedback to
      the source host.
   *  Added to Acknowledgements Section that a similar mechanism was
      proposed for IPv4 in 1988 in [RFC1063].
   *  Editorial changes.

   draft-hinden-6man-mtu-option-01, 2019-March-05

   *  Changed requested status from Standards Track to Experimental to
      allow use of experimental option type (11110) to allow for
      experimentation.  Removed request for IANA Option assignment.
   *  Added Section 2 "Motivation and Problem Solved" section to better
      describe what the purpose of this document is.
   *  Added appendix describing planned experiments and how the results
      will be measured.
   *  Editorial changes.

   draft-hinden-6man-mtu-option-00, 2018-Oct-16

   *  Initial draft.

13.  References

13.1.  Normative References

   [IANA-HBH] "Destination Options and Hop-by-Hop Options",
              <https://www.iana.org/assignments/ipv6-parameters/
              ipv6-parameters.xhtml#ipv6-parameters-2>.

   [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>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.




Hinden & Fairhurst        Expires 11 March 2022                [Page 19]


Internet-Draft               Path MTU Option              September 2021


   [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>.

13.2.  Informative References

   [I-D.ietf-taps-arch]
              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G.,
              Perkins, C., Tiesel, P. S., and C. A. Wood, "An
              Architecture for Transport Services", Work in Progress,
              Internet-Draft, draft-ietf-taps-arch-11, 12 July 2021,
              <https://tools.ietf.org/html/draft-ietf-taps-arch-11>.

   [RFC1063]  Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
              MTU discovery options", RFC 1063, DOI 10.17487/RFC1063,
              July 1988, <https://www.rfc-editor.org/info/rfc1063>.

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

   [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>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [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>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC7637]  Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,
              <https://www.rfc-editor.org/info/rfc7637>.

   [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>.




Hinden & Fairhurst        Expires 11 March 2022                [Page 20]


Internet-Draft               Path MTU Option              September 2021


   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9099]  Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
              "Operational Security Considerations for IPv6 Networks",
              RFC 9099, DOI 10.17487/RFC9099, August 2021,
              <https://www.rfc-editor.org/info/rfc9099>.

   [VPP]      "VPP/What is VPP?",
              <https://wiki.fd.io/view/VPP/What_is_VPP%3F>.

   [VPP_SRC]  "VPP Source", <https://gerrit.fd.io/r/c/vpp/+/21948>.

   [WIRESHARK]
              "Wireshark Network Protocol Analyzer",
              <https://www.wireshark.org>.

Authors' Addresses

   Robert M. Hinden
   Check Point Software
   959 Skyway Road
   San Carlos, CA 94070
   United States of America

   Email: bob.hinden@gmail.com


   Godred Fairhurst
   University of Aberdeen
   School of Engineering, Fraser Noble Building
   Aberdeen



Hinden & Fairhurst        Expires 11 March 2022                [Page 21]


Internet-Draft               Path MTU Option              September 2021


   AB24 3UE
   United Kingdom

   Email: gorry@erg.abdn.ac.uk















































Hinden & Fairhurst        Expires 11 March 2022                [Page 22]