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Versions: 00 01 02 03 draft-petithuguenin-tram-stun-pmtud

Network Working Group                                  M. Petit-Huguenin
Internet-Draft                                                 8x8, Inc.
Intended status: Standards Track                            July 7, 2008
Expires: January 8, 2009


  Path MTU Discovery Using Session Traversal Utilities for NAT (STUN)
                draft-petithuguenin-behave-stun-pmtud-00

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Abstract

   This document describes a Session Traversal Utilities for NAT (STUN)
   usage for discovering the Path MTU between a client and a server.













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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Probe Mechanism . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Sending a Probe Request . . . . . . . . . . . . . . . . . . 3
     3.2.  Receiving a Probe Request . . . . . . . . . . . . . . . . . 4
     3.3.  Receiving a Probe Response  . . . . . . . . . . . . . . . . 4
   4.  Probe Support Discovery Mechanisms  . . . . . . . . . . . . . . 4
     4.1.  Implicit Mechanism  . . . . . . . . . . . . . . . . . . . . 4
     4.2.  Probe Support Discovery with TURN . . . . . . . . . . . . . 5
     4.3.  Probe Support Discovery with ICE  . . . . . . . . . . . . . 5
   5.  New STUN Method . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  New STUN Attributes . . . . . . . . . . . . . . . . . . . . . . 5
     6.1.  PADDING-RECEIVED  . . . . . . . . . . . . . . . . . . . . . 5
     6.2.  PMTUD-SUPPORTED . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8






























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1.  Introduction

   The Packetization Layer Path MTU Discovery specification [RFC4821]
   describes a method to discover the PMTU but does not describe a
   practical protocol to discover the Path MTU when using UDP.

   This document only describe how the probing mechanism is implemented
   with STUN.  The algorithm to find the Path MTU is described in
   [RFC4821].

   The probing mechanism is implemented by sending a Probe Request with
   a PADDING [I-D.ietf-behave-nat-behavior-discovery] attribute and the
   DF bit set over UDP.  A router on the path to the server can reject
   this request with an ICMP message or drop it.  The STUN
   retransmission algorithm is modified so the third and next
   retransmissions do not include the PADDING attribute, so they are not
   dropped by an intermediate router.  The server responds by indicating
   if the request received contained the PADDING attribute or not.  This
   permits to quickly find if a probe packet if too big for the Path MTU
   or not.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Probe Mechanism

   A client MUST NOT send a Probe Request if it does not have knowledge
   that the server supports this specification.  This is done by an
   external mechanism which is specific to each UDP protocol.  Section 4
   describes some of this mechanisms.

   The probe mechanism is used to measure the Path MTU in one direction
   only, from the client to the server.

3.1.  Sending a Probe Request

   A client forms a Probe Request by following the rules in
   [I-D.ietf-behave-rfc3489bis] section 7.1.  No authentication method
   is used.  The client adds a PADDING
   [I-D.ietf-behave-nat-behavior-discovery] 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.  If the IP address and port tuple used as destination for the



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   Probe Request is also used by another protocol then the client MUST
   add the FINGERPRINT attribute.

   Then the client sends the Probe Request to the server over UDP.  The
   UDP retransmission mechanism described in
   [I-D.ietf-behave-rfc3489bis] section 7.2.1 is modified so that if the
   Probe Request has to be retransmitted three times or more then it is
   stripped of its PADDING attribute before been sent.  The UDP packets
   MUST be sent with the DF bit set.

3.2.  Receiving a Probe Request

   A server receiving a Probe Request MUST process it as specified in
   [I-D.ietf-behave-rfc3489bis].  The server MUST NOT challenge the
   client.

   The server then creates a Probe Response.  If the Probe Request
   contains a PADDING attribute, then a PADDING-RECEIVED attribute is
   added to the response, with a value equals to the size of the PADDING
   attribute received.  If the IP address and port used to send the
   Probe Request is also used by another protocol, then the server MUST
   add the FINGERPRINT attribute.  The server then sends the response to
   the client.

3.3.  Receiving a Probe Response

   A client receiving a Probe Response processes it as specified in
   [I-D.ietf-behave-rfc3489bis].  If the response contains a PADDING-
   RECEIVED attribute, then this is interpreted as a Probe Success as
   defined in [RFC4821] section 7.6.1.  If an ICMP packet "Fragmentation
   needed" is received or if the response does not contain a PADDING-
   RECEIVED attribute, then this is interpreted as a Probe Failure as
   defined in [RFC4821] section 7.6.2.  If the Probe transactions fails
   in timeout, then this is interpreted as a Probe Inconclusive as
   defined in [RFC4821] section 7.6.4.


4.  Probe Support Discovery Mechanisms

4.1.  Implicit Mechanism

   An endpoint acting as a client for the STUN usage described in this
   specification MUST also act as a server for this STUN usage.  This
   means that a server receiving a Probe Request can assumes that it can
   acts as a client to discover the Path MTU to the IP address and port
   from which it received the Probe Request.





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4.2.  Probe Support Discovery with TURN

   A TURN client supporting this STUN usage will add a PMTUD-SUPPORTED
   attribute to the Allocate Request sent to the TURN server.  The TURN
   server can immediately start to send Probe Requests to the TURN
   client on reception of an Allocation Request with a PMTUD-SUPPORTED
   attribute.  The TURN client will then use the Implicit Mechanism
   described above to send probes.

4.3.  Probe Support Discovery with ICE

   An ICE [I-D.ietf-mmusic-ice] client supporting this STUN usage will
   add a PMTUD-SUPPORTED attribute to the Binding Request sent during a
   connectivity check.  The ICE server can immediately start to send
   Probe Requests to the ICE client on reception of a Binding Request
   with a PMTUD-SUPPORTED attributed.  The ICE client will then use the
   Implicit Mechanism described above to send probes.


5.  New STUN Method

   This specification defines one new STUN method:

   Request/Response Transaction
      0x801 : Probe


6.  New STUN Attributes

   This specification defines the following new STUN attributes:

      0x4001 : PADDING-RECEIVED
      0xC002 : PMTUD-SUPPORTED

6.1.  PADDING-RECEIVED

   The PADDING-RECEIVED attribute contains the size of the PADDING
   attribute received.  It is a 16-bit unsigned integer, followed by two
   reserved bytes which MUST be set to 0 on transmission and MUST be
   ignored on reception.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Padding size           |         Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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6.2.  PMTUD-SUPPORTED

   The PMTUD-SUPPORTED attribute is used in STUN usages and extensions
   to signal the support of this specification.  This attribute has no
   content.


7.  Security Considerations

   TBD


8.  IANA Considerations

   TBD


9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-16 (work in progress),
              July 2008.

   [I-D.ietf-behave-nat-behavior-discovery]
              MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using STUN", draft-ietf-behave-nat-behavior-discovery-03
              (work in progress), February 2008.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.










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Author's Address

   Marc Petit-Huguenin
   8x8, Inc.
   3151 Jay Street
   Santa Clara, CA  95054
   US

   Phone: +1 408 654 0875
   Email: marc@8x8.com









































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