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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 8, 2009. Abstract This document describes a Session Traversal Utilities for NAT (STUN) usage for discovering the Path MTU between a client and a server. Petit-Huguenin Expires January 8, 2009 [Page 1] Internet-Draft TURN URIs July 2008 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 Petit-Huguenin Expires January 8, 2009 [Page 2] Internet-Draft TURN URIs July 2008 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 Petit-Huguenin Expires January 8, 2009 [Page 3] Internet-Draft TURN URIs July 2008 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. Petit-Huguenin Expires January 8, 2009 [Page 4] Internet-Draft TURN URIs July 2008 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Petit-Huguenin Expires January 8, 2009 [Page 5] Internet-Draft TURN URIs July 2008 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. Petit-Huguenin Expires January 8, 2009 [Page 6] Internet-Draft TURN URIs July 2008 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 Petit-Huguenin Expires January 8, 2009 [Page 7] Internet-Draft TURN URIs July 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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