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Packetization Layer Path MTU Discovery (PLMTUD) For UDP Transports Using Session Traversal Utilities for NAT (STUN)
draft-ietf-tram-stun-pmtud-16

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Document Type
This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Marc Petit-Huguenin , Gonzalo Salgueiro , Felipe Garrido
Last updated 2020-03-20 (Latest revision 2020-03-02)
Replaces draft-petithuguenin-tram-stun-pmtud
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Tolga Asveren
Shepherd write-up Show Last changed 2018-04-20
IESG IESG state I-D Exists
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Responsible AD Magnus Westerlund
Send notices to Gonzalo Camarillo <gonzalo.camarillo@ericsson.com>, Tolga Asveren <tasveren@rbbn.com>
IANA IANA review state Version Changed - Review Needed
draft-ietf-tram-stun-pmtud-16
TRAM                                                   M. Petit-Huguenin
Internet-Draft                                        Impedance Mismatch
Intended status: Standards Track                            G. Salgueiro
Expires: September 3, 2020                                    F. Garrido
                                                                   Cisco
                                                           March 2, 2020

Packetization Layer Path MTU Discovery (PLMTUD) For UDP Transports Using
               Session Traversal Utilities for NAT (STUN)
                     draft-ietf-tram-stun-pmtud-16

Abstract

   The datagram exchanged between two Internet endpoints have to go
   through a series of physical and virtual links that may have
   different limits on the upper size of the datagram they can transmit
   without fragmentation.  Because fragmentation is considered harmful,
   most transports and protocols are designed with a mechanism that
   permits dynamic measurement of the maximum size of a datagram.  This
   mechanism is called Packetization Layer Path MTU Discovery (PLPMTUD).
   But the UDP transport and some of the protocols that use UDP were
   designed without that feature.  The Session Traversal Utilities for
   NAT (STUN) Usage described in this document permits retrofitting an
   existing UDP-based protocol with such a feature.  Similarly, a new
   UDP-based protocol could simply reuse the mechanism described in this
   document.

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 September 3, 2020.

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Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Overview of Operations  . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Probing Mechanisms  . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Simple Probing Mechanism  . . . . . . . . . . . . . . . .   7
       4.1.1.  Sending a Probe Request . . . . . . . . . . . . . . .   7
       4.1.2.  Receiving a Probe Request . . . . . . . . . . . . . .   8
       4.1.3.  Receiving a Probe Response  . . . . . . . . . . . . .   8
     4.2.  Complete Probing Mechanism  . . . . . . . . . . . . . . .   8
       4.2.1.  Sending a Probe Indications and Report Request  . . .   9
       4.2.2.  Receiving an ICMP Packet  . . . . . . . . . . . . . .   9
       4.2.3.  Receiving a Probe Indication and Report Request . . .   9
       4.2.4.  Receiving a Report Response . . . . . . . . . . . . .  10
       4.2.5.  Using Checksums as Packet Identifiers . . . . . . . .  10
       4.2.6.  Using Sequence Numbers as Packet Identifiers  . . . .  11
   5.  Probe Support Signaling Mechanisms  . . . . . . . . . . . . .  12
     5.1.  Explicit Probe Support Signaling Mechanism  . . . . . . .  12
     5.2.  Implicit Probe Support Signaling Mechanism  . . . . . . .  13
   6.  STUN Attributes . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  IDENTIFIERS . . . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  PMTUD-SUPPORTED . . . . . . . . . . . . . . . . . . . . .  13
     6.3.  PADDING . . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  New STUN Methods  . . . . . . . . . . . . . . . . . . . .  14
     8.2.  New STUN Attributes . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Release Notes  . . . . . . . . . . . . . . . . . . .  16
     A.1.  Modifications between draft-ietf-tram-stun-pmtud-16 and

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           draft-ietf-tram-stun-pmtud-15 . . . . . . . . . . . . . .  16
     A.2.  Modifications between draft-ietf-tram-stun-pmtud-15 and
           draft-ietf-tram-stun-pmtud-14 . . . . . . . . . . . . . .  16
     A.3.  Modifications between draft-ietf-tram-stun-pmtud-14 and
           draft-ietf-tram-stun-pmtud-13 . . . . . . . . . . . . . .  17
     A.4.  Modifications between draft-ietf-tram-stun-pmtud-13 and
           draft-ietf-tram-stun-pmtud-12 . . . . . . . . . . . . . .  17
     A.5.  Modifications between draft-ietf-tram-stun-pmtud-12 and
           draft-ietf-tram-stun-pmtud-11 . . . . . . . . . . . . . .  17
     A.6.  Modifications between draft-ietf-tram-stun-pmtud-11 and
           draft-ietf-tram-stun-pmtud-10 . . . . . . . . . . . . . .  17
     A.7.  Modifications between draft-ietf-tram-stun-pmtud-10 and
           draft-ietf-tram-stun-pmtud-09 . . . . . . . . . . . . . .  17
     A.8.  Modifications between draft-ietf-tram-stun-pmtud-09 and
           draft-ietf-tram-stun-pmtud-08 . . . . . . . . . . . . . .  17
     A.9.  Modifications between draft-ietf-tram-stun-pmtud-08 and
           draft-ietf-tram-stun-pmtud-07 . . . . . . . . . . . . . .  17
     A.10. Modifications between draft-ietf-tram-stun-pmtud-07 and
           draft-ietf-tram-stun-pmtud-06 . . . . . . . . . . . . . .  17
     A.11. Modifications between draft-ietf-tram-stun-pmtud-06 and
           draft-ietf-tram-stun-pmtud-05 . . . . . . . . . . . . . .  18
     A.12. Modifications between draft-ietf-tram-stun-pmtud-05 and
           draft-ietf-tram-stun-pmtud-04 . . . . . . . . . . . . . .  18
     A.13. Modifications between draft-ietf-tram-stun-pmtud-04 and
           draft-ietf-tram-stun-pmtud-03 . . . . . . . . . . . . . .  18
     A.14. Modifications between draft-ietf-tram-stun-pmtud-03 and
           draft-ietf-tram-stun-pmtud-02 . . . . . . . . . . . . . .  18
     A.15. Modifications between draft-ietf-tram-stun-pmtud-02 and
           draft-ietf-tram-stun-pmtud-01 . . . . . . . . . . . . . .  19
     A.16. Modifications between draft-ietf-tram-stun-pmtud-01 and
           draft-ietf-tram-stun-pmtud-00 . . . . . . . . . . . . . .  19
     A.17. Modifications between draft-ietf-tram-stun-pmtud-00 and
           draft-petithuguenin-tram-stun-pmtud-01  . . . . . . . . .  19
     A.18. Modifications between draft-petithuguenin-tram-stun-
           pmtud-01 and draft-petithuguenin-tram-stun-pmtud-00 . . .  19
     A.19. Modifications between draft-petithuguenin-tram-stun-
           pmtud-00 and draft-petithuguenin-behave-stun-pmtud-03 . .  19
     A.20. Modifications between draft-petithuguenin-behave-stun-
           pmtud-03 and draft-petithuguenin-behave-stun-pmtud-02 . .  20
     A.21. Modifications between draft-petithuguenin-behave-stun-
           pmtud-02 and draft-petithuguenin-behave-stun-pmtud-01 . .  20
     A.22. Modifications between draft-petithuguenin-behave-stun-
           pmtud-01 and draft-petithuguenin-behave-stun-pmtud-00 . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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1.  Introduction

   The Packetization Layer Path MTU Discovery (PMTUD) specification
   [RFC4821] describes a method to discover the Path MTU, but does not
   describe a practical protocol to do so with UDP.  Many application
   layer protocols based on the transport layer protocol UDP do not
   implement the Path MTU discovery mechanism described in [RFC4821].
   These application layer protocols can make use of the probing
   mechanisms described in this document instead of designing their own
   adhoc extension.  These probing mechanisms are implemented with
   Session Traversal Utilities for NAT (STUN), but their usage is not
   limited to STUN-based protocols.

   The STUN usage defined in this document for Packetization Layer Path
   MTU Discovery (PLPMTUD) between a client and a server permits proper
   measurement of the Path MTU for application layer protocols based on
   the transport layer protocol UDP in the network.  It also simplifies
   troubleshooting and has multiple other applications across a wide
   variety of technologies.

   Complementary techniques can be used to discover additional network
   characteristics, such as the network path (using the STUN Traceroute
   mechanism described in [I-D.martinsen-tram-stuntrace]) and bandwidth
   availability (using the mechanism described in
   [I-D.martinsen-tram-turnbandwidthprobe]).  In addition,
   [I-D.ietf-tsvwg-datagram-plpmtud] provides a robust method for Path
   MTU Discovery for a broader range of protocols and applications.

2.  Overview of Operations

   This section is meant to be informative only and is not intended as a
   substitute for [RFC4821].

   A UDP endpoint that uses this specification to discover the Path MTU
   over UDP and knows that the endpoint it is communicating with also
   supports this specification can choose to use either the Simple
   Probing mechanism (as described in Section 4.1) or the Complete
   Probing mechanism (as described in Section 4.2).  The selection of
   which Probing Mechanism to use is dependent on performance and
   security and complexity trade-offs.

   If the Simple Probing mechanism is chosen, then the client initiates
   Probe transactions, as shown in Figure 1, which decrease in size
   until transactions succeed, indicating that the Path MTU has been
   discovered.  It then uses that information to update the Path MTU.

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                          Client           Server
                            |                 |
                            | Probe Request   |
                            |---------------->|
                            |                 |
                            |  Probe Response |
                            |<----------------|
                            |                 |

                     Figure 1: Simple Probing Example

   If the Complete Probing mechanism (as described in Section 4.2) is
   chosen, then the client sends Probe Indications of various sizes (as
   specified in [RFC4821]) interleaved with UDP packets sent by the UDP
   protocol.  The client then sends a Report Request for the ordered
   list of identifiers for the UDP packets and Probe Indications
   received by the server.  The client then compares the list returned
   in the Report Response with its own list of identifiers for the UDP
   packets and Probe Indications it sent.  The client examines the
   received reports to determine which probes were successful.  When a
   probe succeeds with a larger size than the current PMTU, the PMTU is
   increased.  When the probes indicate the current PMTU is not
   supported the size is decreased.  This mechanism acts to detect that
   traffic is being back holed.

   Because of the possibility of amplification attack, the Complete
   Probing mechanism must be authenticated as specified in Section 5.1.
   Particular care must be taken to prevent amplification when an
   external mechanism is used to trigger the Complete Probing mechanism.

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                        Client              Server
                           | UDP Packet        |
                           |------------------>|
                           |                   |
                           | UDP Packet        |
                           |------------------>|
                           |                   |
                           | Probe Indication  |
                           |------------------>|
                           |                   |
                           | UDP Packet        |
                           |------------------>|
                           |                   |
                           | Probe Indication  |
                           |------------------>|
                           |                   |
                           | Report Request    |
                           |------------------>|
                           |   Report Response |
                           |<------------------|
                           |                   |

                    Figure 2: Complete Probing Example

3.  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.

4.  Probing Mechanisms

   The Probing mechanism is used to discover the Path MTU in one
   direction only: from the client to the server.  Both endpoints MAY
   behave as a client and a server to achieve bi-directional path
   discovery.

   Two Probing mechanisms are described: a Simple Probing mechanism and
   a more complete mechanism that can converge more quickly and find an
   appropriate Path MTU in the presence of congestion.  Additionally,
   the Simple Probing mechanism does not require authentication except
   where used as an implicit signaling mechanism, whereas the complete
   mechanism does.

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   Implementations supporting this specification MUST implement the
   server side of both the Simple Probing mechanism (Section 4.1) and
   the Complete Probing mechanism (Section 4.2).

   Implementations supporting this specification MUST implement the
   client side of the Complete Probing mechanism.  They MAY implement
   the client side of the Simple Probing mechanism.

4.1.  Simple Probing Mechanism

   The Simple Probing mechanism is implemented by sending a Probe
   Request with a PADDING attribute over UDP with the DF bit set in the
   IP header for IPv4 packets and IPv6 packets without the Fragment
   Header included.

   Note: Routers can be configured to clear the DF bit or ignore the DF
   bit which can be difficult or impossible to detect if reassembly
   occurs prior to receiving the packet, rendering PLPMTUD inaccurate.

4.1.1.  Sending a Probe Request

   A client forms a Probe Request by using the Probe Method and
   following the rules in Section 6.1 of [I-D.ietf-tram-stunbis].

   The Probe transaction MUST be authenticated if the Simple Probing
   mechanism is used in conjunction with the Implicit Probing Support
   mechanism described in Section 5.2.  If not, the Probe transaction
   MAY be authenticated.

   The client adds a PADDING attribute with a length that, when added to
   the IP and UDP headers and the other STUN components, is equal to the
   Selected Probe Size, as defined in [RFC4821] Section 7.3.  The
   PADDING bits MUST be set to zero.  The client MUST add the
   FINGERPRINT attribute so the STUN messages are disambiguated from the
   other protocol packets as specified in Section 7 of
   [I-D.ietf-tram-stunbis].

   Then the client sends the Probe Request to the server over UDP with
   the DF bit set for IPv4 packets and IPv6 packets without the Fragment
   Header included.  For the purpose of this transaction, the Rc
   parameter is set to 3 and the initial value for RTO stays at 500 ms
   as specified in Section 6.2.1 of [I-D.ietf-tram-stunbis]

   A client MUST NOT send a probe if it does not have knowledge that the
   server supports this specification.  This is done either by external
   signalling or by a mechanism specific to the UDP protocol to which
   PMTUD capabilities are added or by one of the mechanisms specified in
   Section 5.

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4.1.2.  Receiving a Probe Request

   A server receiving a Probe Request MUST process it as specified in
   [I-D.ietf-tram-stunbis].

   The server then creates a Probe Response.  The server MUST add the
   FINGERPRINT attribute so the STUN messages are disambiguated from the
   other protocol packets as specified in Section 7 of
   [I-D.ietf-tram-stunbis].  The server then sends the response to the
   client.

4.1.3.  Receiving a Probe Response

   A client receiving a Probe Response MUST process it as specified in
   section 6.3.1 of [I-D.ietf-tram-stunbis] and MUST ignore the PADDING
   attribute.  If a response is received this is interpreted as a Probe
   Success, as defined in [RFC4821] Section 7.6.1.  If an ICMP packet
   "Fragmentation needed" or "Packet Too Big" is received then this is
   interpreted as a Probe Failure, as defined in [RFC4821]
   Section 7.6.2.  If the Probe transaction times out, then this is
   interpreted as a Probe Inconclusive, as defined in [RFC4821]
   Section 7.6.4.  Validation SHOULD be performed on the ICMP packet as
   specified in [RFC8085].

4.2.  Complete Probing Mechanism

   The Complete Probing mechanism is implemented by sending one or more
   Probe Indications with a PADDING attribute over UDP with the DF bit
   set in the IP header for IPv4 packets and IPv6 packets without the
   Fragment Header included followed by a Report Request to the same
   server.  A router on the path to the server can reject this
   Indication with an ICMP message or drop it.  The server keeps a
   chronologically ordered list of identifiers for all packets received
   (including retransmitted packets) and sends this list back to the
   client in the Report Response.  The client analyzes this list to find
   which packets were not received.  Because UDP packets do not contain
   an identifier, the Complete Probing mechanism needs a way to identify
   each packet received.

   Some application layer protocols may already have a way of
   identifying each individual UDP packet, in which case these
   identifiers SHOULD be used in the IDENTIFIERS attribute of the Report
   Response.  While there are other possible packet identification
   schemes, this document describes two different ways to identify a
   specific packet when no application layer protocol-specific
   identification mechanism is available.

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   In the first packet identification mechanism, the server computes a
   checksum over each packet received and sends back to the sender the
   list of checksums ordered chronologically.  The client compares this
   list to its own list of checksums.

   In the second packet identification mechanism, the client prepends
   the UDP data with a header that provides a sequence number.  The
   server sends back the chronologically ordered list of sequence
   numbers received that the client then compares with its own list.

4.2.1.  Sending a Probe Indications and Report Request

   A client forms a Probe Indication by using the Probe Method and
   following the rules in [I-D.ietf-tram-stunbis] Section 6.1.  The
   client adds to a Probe Indication a PADDING attribute with a size
   that, when added to the IP and UDP headers and the other STUN
   components, is equal to the Selected Probe Size, as defined in
   [RFC4821] Section 7.3.  The PADDING bits MUST be set to zero.  If the
   authentication mechanism permits it, then the Indication MUST be
   authenticated.  The client MUST add the FINGERPRINT attribute so the
   STUN messages are disambiguated from the other protocol packets.

   Then the client sends a Probe Indication to the server over UDP with
   the DF bit set for IPv4 packets and IPv6 packets without the Fragment
   Header included.

   Then the client forms a Report Request by following the rules in
   [I-D.ietf-tram-stunbis] Section 6.1.  The Report transaction MUST be
   authenticated to prevent amplification attacks.  The client MUST add
   the FINGERPRINT attribute so the STUN messages are disambiguated from
   the other protocol packets.

   Then the client waits half the RTO after sending the last Probe
   Indication and then sends the Report Request to the server over UDP.

4.2.2.  Receiving an ICMP Packet

   If an ICMP packet "Fragmentation needed" or "Packet Too Big" is
   received then this is interpreted as a Probe Failure, as defined in
   [RFC4821] Section 7.5.  Validation SHOULD be performed on the ICMP
   packet as specified in [RFC8085].

4.2.3.  Receiving a Probe Indication and Report Request

   A server supporting this specification will keep the identifiers of
   all packets received in a chronologically ordered list.  The packets
   that are to be associated to a given flow's identifier are selected
   according to Section 5.2 of [RFC4821].  The same identifier can

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   appear multiple times in the list because of retransmissions.  The
   maximum size of this list is calculated such that when the list is
   added to the Report Response, the total size of the packet does not
   exceed the unknown Path MTU, as defined in [I-D.ietf-tram-stunbis]
   Section 6.1.  Older identifiers are removed when new identifiers are
   added to a list that is already full.

   A server receiving a Report Request MUST process it as specified in
   [I-D.ietf-tram-stunbis] and MUST ignore the PADDING attribute.

   The server creates a Report Response and adds an IDENTIFIERS
   attribute that contains the chronologically ordered list of all
   identifiers received so far.  The server MUST add the FINGERPRINT
   attribute.  The server then sends the response to the client.

   The exact content of the IDENTIFIERS attribute depends on what type
   of identifiers have been chosen for the protocol.  Each protocol
   adding PMTUD capabilities as specified by this specification MUST
   describe the format of the contents of the IDENTIFIERS attribute,
   unless it is using one of the formats described in this
   specification.  See Section 6.1 for details about the IDENTIFIERS
   attribute.

4.2.4.  Receiving a Report Response

   A client receiving a Report Response processes it as specified in
   [I-D.ietf-tram-stunbis].  If the response IDENTIFIERS attribute
   contains the identifier of a Probe Indication, then this is
   interpreted as a Probe Success for this probe, as defined in
   [RFC4821] Section 7.5.  If a Probe Indication identifier cannot be
   found in the Report Response, this is interpreted as a Probe Failure,
   as defined in [RFC4821] Section 7.5.  If a Probe Indication
   identifier cannot be found in the Report Response but identifiers for
   other packets sent before or after the Probe Indication can all be
   found, this is interpreted as a Probe Failure as defined in [RFC4821]
   Section 7.5.  If the Report Transaction times out, this is
   interpreted as a Full-Stop Timeout, as defined in [RFC4821]
   Section 3.

4.2.5.  Using Checksums as Packet Identifiers

   When using a checksum as a packet identifier, the client keeps a
   chronologically ordered list of the packets it transmits, along with
   an associated checksum value.  For STUN Probe Indication or Request
   packets, the associated checksum value is the FINGERPRINT value from
   the packet; for other packets a checksum value is computed.  The
   value of the checksum is computed as the CRC-32 of the UDP payload,
   as defined by the Length field of the UDP datagram [RFC4821], XOR'ed

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   with the 32-bit value 0x5354554e.  The 32-bit CRC is the one defined
   in ITU V.42 [[ITU.V42.2002], which has a generator polynomial of x^32
   + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7 + x^5 +
   x^4 + x^2 + x + 1.

   For each STUN Probe Indication or Request, the server retrieves the
   STUN FINGERPRINT value.  For all other packets, the server calculates
   the checksum as described above.  It puts these FINGERPRINT and
   checksum values in a chronologically ordered list that is sent back
   in the Report Response.

   The contents of the IDENTIFIERS attribute is a list of 4 byte
   numbers, each using the same encoding that is used for the contents
   of the FINGERPRINT attribute.

   It could have been possible to use the checksum generated in the UDP
   checksum for this, but this value is generally not accessible to
   applications.  Also, sometimes the checksum is not calculated or is
   off-loaded to network hardware.

4.2.6.  Using Sequence Numbers as Packet Identifiers

   When using sequence numbers, a small header similar to the TURN
   ChannelData header, as defined in Section 11.4 of [RFC5766], is added
   in front of all packets that are not a STUN Probe Indication or
   Request.  The initial sequence number MUST be randomized and is
   monotonically incremented by one for each packet sent.  The most
   significant bit of the sequence number is always 0.  The server
   collects the sequence number of the packets sent, or the 4 first
   bytes of the transaction ID if a STUN Probe Indication or Request is
   sent.  In that case, the most significant bit of the 4 first bytes is
   set to 1.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Channel Number        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                      Sequence number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                       Application Data                        /
   /                                                               /
   |                                                               |
   |                               +-------------------------------+
   |                               |
   +-------------------------------+

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   The Channel Number is always 0xFFFF.  The Length field specifies the
   length in bytes of the sequence number and application data fields.
   The header values are encoded using network order.

   The contents of the IDENTIFIERS attribute is a chronologically
   ordered list of 4 byte numbers, each containing either a sequence
   number, if the packet was not a STUN Probe Indication or Request, or
   the 4 first bytes of the transaction ID, with the most significant
   bit forced to 1, if the packet is a STUN Probe Indication or Request.

5.  Probe Support Signaling Mechanisms

   The PMTUD mechanism described in this document is intended to be used
   by any UDP-based protocols that do not have built-in PMTUD
   capabilities, irrespective of whether those UDP-based protocols are
   STUN-based or not.  So the manner in which a specific protocol
   discovers that it is safe to send PMTUD probes is largely dependent
   on the details of that specific protocol, with the exception of the
   Implicit Mechanism described below, which applies to any protocol.

5.1.  Explicit Probe Support Signaling Mechanism

   Some of these mechanisms can use a separate signalling mechanism (for
   instance, an SDP attribute in an Offer/Answer exchange [RFC3264]), or
   an optional flag that can be set in the protocol that is augmented
   with PMTUD capabilities.  STUN Usages that can benefit from PMTUD
   capabilities can signal in-band that they support probing by
   inserting a PMTUD-SUPPORTED attribute in some STUN methods.  The
   decision of which methods support this attribute is left to each
   specific STUN Usage.

   UDP-based protocols that want to use any of these mechanisms,
   including the PMTUD-SUPPORTED attribute, to signal PMTUD capabilities
   MUST ensure that it cannot be used to launch an amplification attack.

   An amplification attack can be prevented using techniques such as:

   o  Authentication, where the source of the packet and the destination
      share a secret.

   o  3 way handshake with some form of unpredictable cookie.

   o  Make sure that the total size of the traffic potentially generated
      is lower than the size of the request that generated it.

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5.2.  Implicit Probe Support Signaling Mechanism

   As a result of the fact that all endpoints implementing this
   specification are both clients and servers, a Probe Request or
   Indication received by an endpoint acting as a server implicitly
   signals that this server can now act as a client and MAY send a Probe
   Request or Indication to probe the Path MTU in the reverse direction
   toward the former client, that will now be acting as a server.

   The Probe Request or Indication that are used to implicitly signal
   probing support in the reverse direction MUST be authenticated to
   prevent amplification attacks.

6.  STUN Attributes

6.1.  IDENTIFIERS

   The IDENTIFIERS attribute carries a chronologically ordered list of
   UDP packet identifiers.

   While Section 4.2.5 and Section 4.2.6 describe two possible methods
   for acquiring and formatting the identifiers used for this purpose,
   ultimately each protocol has to define how these identifiers are
   acquired and formatted.  Therefore, the contents of the IDENTIFIERS
   attribute is opaque.

6.2.  PMTUD-SUPPORTED

   The PMTUD-SUPPORTED attribute indicates that its sender supports this
   mechanism, as incorporated into the STUN usage or protocol being
   used.  This attribute has no value part and thus the attribute length
   field is 0.

6.3.  PADDING

   The PADDING attribute allows for the entire message to be padded to
   force the STUN message to be divided into IP fragments.  The PADDING
   bits MUST be set to zero.  PADDING can be used in either Binding
   Requests or Binding Responses.

   PADDING MUST NOT be longer than the length that brings the total IP
   datagram size to 64K, minus the IP and UDP headers and the other STUN
   components.  It SHOULD be equal in length to the MTU of the outgoing
   interface, rounded up to an even multiple of four bytes and SHOULD
   ensure a probe does not result in a packet larger than the MTU fo the
   outgoing interface.  STUN messages sent with PADDING are intended to
   test the behavior of UDP fragmentation, therefore they are an

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   exception to the usual rule that STUN messages need to be less than
   the PMTU for the path.

7.  Security Considerations

   The PMTUD mechanism described in this document, when used without the
   signalling mechanism described in Section 5.1, does not introduce any
   specific security considerations beyond those described in [RFC4821]
   and [I-D.ietf-tsvwg-datagram-plpmtud].

   The attacks described in Section 11 of [RFC4821] apply equally to the
   mechanism described in this document.

   The amplification attacks introduced by the signalling mechanism
   described in Section 5.1 can be prevented by using one of the
   techniques described in that section.

   The Simple Probing mechanism may be used without authentication
   because this usage by itself cannot trigger an amplification attack
   as the Probe Response is smaller than the Probe Request except when
   used in conjunction with the Implicit Probing Support Signaling
   mechanism.

8.  IANA Considerations

   This specification defines two new STUN methods and two new STUN
   attributes.

8.1.  New STUN Methods

   IANA is requested to add the following methods to the STUN Method
   Registry:

      0xXXX : Probe

      0xXXX : Report

   See Sections Section 4.1 and Section 4.2 for the semantics of these
   new methods.

8.2.  New STUN Attributes

   IANA is requested to add the following attributes to the STUN Method
   Registry:

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   Comprehension-required range (0x0000-0x7FFF):
   0xXXXX: IDENTIFIERS

   Comprehension-optional range (0x8000-0xFFFF)
   0xXXXX: PMTUD-SUPPORTED

   0x0026: PADDING

   The IDENTIFIERS STUN attribute is defined in Section 6.1, the PMTUD-
   SUPPORTED STUN attribute is defined in Section 6.2; the PADDING STUN
   attribute is defined in Section 6.3.

   NOTE: TO BE DELETED BEFORE PUBLICATION.  PLEASE NOTE THAT THE PADDING
   ATTRIBUTE ENTRY IS REPLACING THE ENTRY MADE BY RFC5780
   (EXPERIMENTAL).  THE SAME VALUE AND NAME ARE USED BUT THE REFERENCE
   SHOULD BE CHANGED TO THIS STANDARDS TRACK DOCUMENT.

9.  References

9.1.  Normative References

   [I-D.ietf-tram-stunbis]
              Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
              D., Mahy, R., and P. Matthews, "Session Traversal
              Utilities for NAT (STUN)", draft-ietf-tram-stunbis-21
              (work in progress), March 2019.

   [ITU.V42.2002]
              International Telecommunications Union, "Error-correcting
              Procedures for DCEs Using Asynchronous-to-Synchronous
              Conversion", ITU-T Recommendation V.42, 2002.

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

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

   [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, <http://www.rfc-editor.org/info/rfc8174>.

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9.2.  Informative References

   [I-D.ietf-tsvwg-datagram-plpmtud]
              Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
              T. Voelker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-12
              (work in progress), December 2019.

   [I-D.martinsen-tram-stuntrace]
              Martinsen, P. and D. Wing, "STUN Traceroute", draft-
              martinsen-tram-stuntrace-01 (work in progress), June 2015.

   [I-D.martinsen-tram-turnbandwidthprobe]
              Martinsen, P., Andersen, T., Salgueiro, G., and M. Petit-
              Huguenin, "Traversal Using Relays around NAT (TURN)
              Bandwidth Probe", draft-martinsen-tram-
              turnbandwidthprobe-00 (work in progress), May 2015.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

Appendix A.  Release Notes

   This section must be removed before publication as an RFC.

A.1.  Modifications between draft-ietf-tram-stun-pmtud-16 and draft-
      ietf-tram-stun-pmtud-15

   o  Modifications to address DISCUSS and COMMENT from IESG review

A.2.  Modifications between draft-ietf-tram-stun-pmtud-15 and draft-
      ietf-tram-stun-pmtud-14

   o  Modifications to address DISCUSS and COMMENT from IESG review

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A.3.  Modifications between draft-ietf-tram-stun-pmtud-14 and draft-
      ietf-tram-stun-pmtud-13

   o  Modifications to address COMMENTS from IESG review

A.4.  Modifications between draft-ietf-tram-stun-pmtud-13 and draft-
      ietf-tram-stun-pmtud-12

   o  Modifications to address nits

A.5.  Modifications between draft-ietf-tram-stun-pmtud-12 and draft-
      ietf-tram-stun-pmtud-11

   o  Modifications following IESG review.  Incorporated RFC5780 PADDING
      attribute (Adam's Discuss) and added IPv6 language (Suresh's
      Discuss).

A.6.  Modifications between draft-ietf-tram-stun-pmtud-11 and draft-
      ietf-tram-stun-pmtud-10

   o  Modifications following IESG review.

A.7.  Modifications between draft-ietf-tram-stun-pmtud-10 and draft-
      ietf-tram-stun-pmtud-09

   o  Modifications following reviews for gen-art (Roni Even) and secdir
      (Carl Wallace).

A.8.  Modifications between draft-ietf-tram-stun-pmtud-09 and draft-
      ietf-tram-stun-pmtud-08

   o  Add 3 ways of preventing amplification attacks.

A.9.  Modifications between draft-ietf-tram-stun-pmtud-08 and draft-
      ietf-tram-stun-pmtud-07

   o  Updates following Spencer's review.

A.10.  Modifications between draft-ietf-tram-stun-pmtud-07 and draft-
       ietf-tram-stun-pmtud-06

   o  Updates following Shepherd review.

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A.11.  Modifications between draft-ietf-tram-stun-pmtud-06 and draft-
       ietf-tram-stun-pmtud-05

   o  Nits.

   o  Restore missing changelog for previous version.

A.12.  Modifications between draft-ietf-tram-stun-pmtud-05 and draft-
       ietf-tram-stun-pmtud-04

   o  Modifications following Brandon Williams review.

A.13.  Modifications between draft-ietf-tram-stun-pmtud-04 and draft-
       ietf-tram-stun-pmtud-03

   o  Modifications following Simon Perreault and Brandon Williams
      reviews.

A.14.  Modifications between draft-ietf-tram-stun-pmtud-03 and draft-
       ietf-tram-stun-pmtud-02

   o  Add new Overview of Operations section with ladder diagrams.

   o  Authentication is mandatory for the Complete Probing mechanism,
      optional for the Simple Probing mechanism.

   o  All the ICE specific text moves to a separate draft to be
      discussed in the ICE WG.

   o  The TURN usage is removed because probing between a TURN server
      and TURN client is not useful.

   o  Any usage of PMTUD-SUPPORTED or other signaling mechanisms
      (formerly knows as discovery mechanisms) must now be
      authenticated.

   o  Both probing mechanisms are MTI in the server, the complete
      probing mechanism is MTI in the client.

   o  Make clear that stopping after 3 retransmission is done by
      changing the STUN parameter.

   o  Define the format of the attributes.

   o  Make clear that the specification is for any UDP protocol that
      does not already have PMTUD capabilities, not just STUN based
      protocols.

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   o  Change the default delay to send the Report Request to 250 ms
      after the last Indication if the RTO is unknown.

   o  Each usage of this specification must the format of the
      IDENTIFIERS attribute contents.

   o  Better define the implicit signaling mechanism.

   o  Extend the Security Consideration section.

   o  Tons of nits.

A.15.  Modifications between draft-ietf-tram-stun-pmtud-02 and draft-
       ietf-tram-stun-pmtud-01

   o  Cleaned up references.

A.16.  Modifications between draft-ietf-tram-stun-pmtud-01 and draft-
       ietf-tram-stun-pmtud-00

   o  Added Security Considerations Section.

   o  Added IANA Considerations Section.

A.17.  Modifications between draft-ietf-tram-stun-pmtud-00 and draft-
       petithuguenin-tram-stun-pmtud-01

   o  Adopted by WG - Text unchanged.

A.18.  Modifications between draft-petithuguenin-tram-stun-pmtud-01 and
       draft-petithuguenin-tram-stun-pmtud-00

   o  Moved some Introduction text to the Probing Mechanism section.

   o  Added cross-reference to the other two STUN troubleshooting
      mechanism drafts.

   o  Updated references.

   o  Added Gonzalo Salgueiro as co-author.

A.19.  Modifications between draft-petithuguenin-tram-stun-pmtud-00 and
       draft-petithuguenin-behave-stun-pmtud-03

   o  General refresh for republication.

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A.20.  Modifications between draft-petithuguenin-behave-stun-pmtud-03
       and draft-petithuguenin-behave-stun-pmtud-02

   o  Changed author address.

   o  Changed the IPR to trust200902.

A.21.  Modifications between draft-petithuguenin-behave-stun-pmtud-02
       and draft-petithuguenin-behave-stun-pmtud-01

   o  Defined checksum and sequential numbers as possible packet
      identifiers.

   o  Updated the reference to RFC 5389

   o  The FINGERPRINT attribute is now mandatory.

   o  Changed the delay between Probe indication and Report request to
      be RTO/2 or 50 milliseconds.

   o  Added ICMP packet processing.

   o  Added Full-Stop Timeout detection.

   o  Stated that Binding request with PMTUD-SUPPORTED does not start
      the PMTUD process if already started.

A.22.  Modifications between draft-petithuguenin-behave-stun-pmtud-01
       and draft-petithuguenin-behave-stun-pmtud-00

   o  Removed the use of modified STUN transaction but shorten the
      retransmission for the simple probing mechanism.

   o  Added a complete probing mechanism.

   o  Removed the PADDING-RECEIVED attribute.

   o  Added release notes.

Acknowledgements

   Thanks to Eilon Yardeni, Geir Sandbakken, Paal-Erik Martinsen,
   Tirumaleswar Reddy, Ram Mohan R, Simon Perreault, Brandon Williams,
   Tolga Asveren, Spencer Dawkins, Carl Wallace, and Roni Even for their
   review comments, suggestions and questions that helped to improve
   this document.

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   Special thanks to Dan Wing, who supported this document since its
   first publication back in 2008.

Authors' Addresses

   Marc Petit-Huguenin
   Impedance Mismatch

   Email: marc@petit-huguenin.org

   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   United States

   Email: gsalguei@cisco.com

   Felipe Garrido
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   United States

   Email: fegarrid@cisco.com

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