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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                        November 3, 2008
Expires: May 7, 2009


  Path MTU Discovery Using Session Traversal Utilities for NAT (STUN)
                draft-petithuguenin-behave-stun-pmtud-02

Status of this Memo

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Abstract

   This document describes a Session Traversal Utilities for NAT (STUN)
   usages 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.  Probing Mechanisms . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Simple Probing Mechanism . . . . . . . . . . . . . . . . . . .  4
     4.1.  Sending a Probe Request  . . . . . . . . . . . . . . . . .  4
     4.2.  Receiving a Probe Request  . . . . . . . . . . . . . . . .  4
     4.3.  Receiving a Probe Response . . . . . . . . . . . . . . . .  4
   5.  Complete Probing Mechanism . . . . . . . . . . . . . . . . . .  5
     5.1.  Sending the Probe Indications and Report Request . . . . .  5
     5.2.  Receiving an ICMP packet . . . . . . . . . . . . . . . . .  5
     5.3.  Receiving a Probe Indication and Report Request  . . . . .  5
     5.4.  Receiving a Report Response  . . . . . . . . . . . . . . .  6
     5.5.  Using Checksum as Packet Identifiers . . . . . . . . . . .  6
     5.6.  Using Sequential Numbers as Packet Identifiers . . . . . .  6
   6.  Probe Support Discovery Mechanisms . . . . . . . . . . . . . .  7
     6.1.  Implicit Mechanism . . . . . . . . . . . . . . . . . . . .  7
     6.2.  Probe Support Discovery with TURN  . . . . . . . . . . . .  7
     6.3.  Probe Support Discovery with ICE . . . . . . . . . . . . .  7
   7.  New STUN Method  . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  New STUN Attributes  . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  IDENTIFIERS  . . . . . . . . . . . . . . . . . . . . . . .  8
     8.2.  PMTUD-SUPPORTED  . . . . . . . . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     12.1. Normative References . . . . . . . . . . . . . . . . . . .  8
     12.2. Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Release notes . . . . . . . . . . . . . . . . . . . .  9
     A.1.  Modifications between -02 and -01  . . . . . . . . . . . .  9
     A.2.  Modifications between -01 and -00  . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 11
















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1.  Introduction

   The Packetization Layer Path MTU Discovery specification [RFC4821]
   describes a method to discover the path MTU but does not describe a
   practical protocol to do so with UDP.

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

   Two probing mechanisms are described, a simple probing mechanism and
   a more complete mechanism that can converge quicker.

   The simple 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
   client SHOULD cease retransmissions after 3 missing responses.

   The complete probing mechanism is implemented by sending one or more
   Probe Indication with a PADDING attribute and the DF bit set over UDP
   then 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 time ordered list of identifiers of 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 does not contain an identifier, the complete probing
   mechanism needs a way to identify each packet received.  As example,
   this document describes two different ways to identify a specific
   packet.

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

   In the second packet identifier mechanism, the client adds a
   sequential number in front of each UDP packet sent.  The server sends
   back the ordered list of sequential numbers received that the client
   compares to its own list


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].



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3.  Probing Mechanisms

   A client MUST NOT send a probe if it does not have knowledge that the
   server supports this specification.  This is done by an external
   mechanism specific to each UDP protocol.  Section 6 describes some of
   this mechanisms.

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


4.  Simple Probing Mechanism

4.1.  Sending a Probe Request

   A client forms a Probe Request by following the rules in Section 7.1
   of [RFC5389].  No authentication method is used.  The client adds a
   PADDING [I-D.ietf-behave-nat-behavior-discovery] 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 client MUST add the FINGERPRINT
   attribute.

   Then the client sends the Probe Request to the server over UDP with
   the DF bit set.  The client SHOULD stop retransmitting after 3
   missing responses.

4.2.  Receiving a Probe Request

   A server receiving a Probe Request MUST process it as specified in
   [RFC5389].  The server MUST NOT challenge the client.

   The server then creates a Probe Response.  The server MUST add the
   FINGERPRINT attribute.  The server then sends the response to the
   client.

4.3.  Receiving a Probe Response

   A client receiving a Probe Response MUST process it as specified in
   [RFC5389].  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" is received 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.






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5.  Complete Probing Mechanism

5.1.  Sending the Probe Indications and Report Request

   A client forms a Probe Indication by following the rules in [RFC5389]
   section 7.1.  The client adds to the 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 client MUST add the
   FINGERPRINT attribute.

   Then the client sends the Probe Indication to the server over UDP
   with the DF bit set.

   Then the client forms a Report Request by following the rules in
   [RFC5389] section 7.1.  No authentication method is used.  The client
   MUST add the FINGERPRINT attribute.

   Then the client waits half the RTO if it is known or 50 milliseconds
   after sending the Probe Indication and sends the Report Request to
   the server over UDP.

5.2.  Receiving an ICMP packet

   If an ICMP packet "Fragmentation needed" is received then this is
   interpreted as a Probe Failure as defined in [RFC4821] section 7.5.

5.3.  Receiving a Probe Indication and Report Request

   A server supporting this specification and knowing that the client
   also supports it will keep the identifiers of all packets received in
   a list ordered by receiving time.  The same identifier can appear
   multiple times in the list because of retransmission.  The maximum
   size of this list is calculated so 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 [RFC5389] section 7.1.  Older
   identifiers are removed when new identifiers are added to a list
   already full.

   A server receiving a Report Request MUST process it as specified in
   [RFC5389].  The server MUST NOT challenge the client.

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





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5.4.  Receiving a Report Response

   A client receiving a Report Response processes it as specified in
   [RFC5389].  If the response IDENTIFIERS attribute contains the
   identifier of the Probe Indication, then this is interpreted as a
   Probe Success for this probe as defined in [RFC4821] Section 7.5.  If
   the 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 the Probe Indication identifier cannot be
   found in the Report Response but other packets identifier sent before
   or after the Probe Indication cannot also be found, this is
   interpreted as a Probe Inconclusive as defined in [RFC4821] Section
   7.5.  If the Report Transaction fails in timeout, this is interpreted
   as a Full-Stop Timeout as defined in [RFC4821] Section 3.

5.5.  Using Checksum as Packet Identifiers

   When using checksum as packet identifiers, the client calculate the
   checksum for each packets sent over UDP and keep this checksum in an
   ordered list.  The server does the same thing and send back this list
   in the Report Response.

   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 off-
   loaded to the network card.

5.6.  Using Sequential Numbers as Packet Identifiers

   When using sequential numbers, a small header similar to the TURN
   ChannelData header is added in front of all non-STUN packets.  The
   sequential number is incremented for each packet sent.  The server
   collects the sequence number of the packets sent.

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                       Application Data                        /
   /                                                               /
   |                                                               |
   |                               +-------------------------------+
   |                               |
   +-------------------------------+



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   The Channel Number is always 0xFFFF.


6.  Probe Support Discovery Mechanisms

6.1.  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 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.

6.2.  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 probes 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.

6.3.  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
   probes to the ICE client on reception of a Binding Request with a
   PMTUD-SUPPORTED attributed.  Local candidates receiving Binding
   Request with the PMTUD-SUPPORTED flag must not start PMTUD with the
   remote candidate if already done so.  The ICE client will then use
   the Implicit Mechanism described above to send probes.


7.  New STUN Method

   This specification defines the following new STUN methods:

      0x801 : Probe
      0x802 : Report


8.  New STUN Attributes

   This specification defines the following new STUN attributes:






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      0x4001 : IDENTIFIERS
      0xC001 : PMTUD-SUPPORTED

8.1.  IDENTIFIERS

   The IDENTIFIERS attribute is used in Report Response.  It contains a
   list of UDP packet identifiers.

8.2.  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.


9.  Security Considerations

   TBD


10.  IANA Considerations

   TBD


11.  Acknowledgements

   Thanks to Dan Wing and Eilon Yardeni for their comments, suggestions
   and questions that helped to improve this document.

   This document was written with the xml2rfc tool described in
   [RFC2629].


12.  References

12.1.  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.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.




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   [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.

12.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [I-D.ietf-behave-nat-behavior-discovery]
              MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using STUN", draft-ietf-behave-nat-behavior-discovery-05
              (work in progress), November 2008.


Appendix A.  Release notes

   This section must be removed before publication as an RFC.

A.1.  Modifications between -02 and -01

   o  Replaced the transactions identifiers by packet identifiers
   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.2.  Modifications between -01 and -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.









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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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Full Copyright Statement

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