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Network Working GroupX. Marjou
Internet-DraftA. Sollaud
Intended status: BCPFrance Telecom
Expires: October 10, 2008April 08, 2008


Application Mechanism for maintaining alive the Network Address Translator (NAT) mappings associated to RTP flows.
draft-ietf-avt-app-rtp-keepalive-03

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Abstract

This document lists the different mechanisms that enable applications using Real-time Transport Protocol (RTP) to maintain their RTP Network Address Translator (NAT) mappings alive. It also makes a recommendation for a preferred mechanism. This document is not applicable to Interactive Connectivity Establishment (ICE) agents.



Table of Contents

1.  Introduction
2.  Terminology
3.  Requirements
4.  List of Alternatives for Performing RTP Keepalive
    4.1.  UDP Packet of 0-byte
    4.2.  DCCP Packet of 0-byte
    4.3.  RTP Packet with Comfort Noise Payload
    4.4.  RTCP Packets Multiplexed with RTP Packets
    4.5.  STUN Indication Packet
    4.6.  RTP Packet with Incorrect Version Number
    4.7.  RTP Packet with Unknown Payload Type
5.  Recommended Solution for Keepalive Mechanism
6.  Media Format Exceptions
    6.1.  Real-time Text Payload Format Keepalive Mechanism
7.  Timing and Transport Considerations
8.  Security Considerations
9.  IANA Considerations
10.  Acknowledgements
11.  References
    11.1.  Normative references
    11.2.  Informative references
§  Authors' Addresses
§  Intellectual Property and Copyright Statements




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1.  Introduction

Documents [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.) and [DRAFT‑NAT‑TCP‑REQS] (Guha, S., Biswas, K., Ford, B., Francis, P., Sivarkumar, S., and P. Srisuresh, “NAT Behavioral Requirements for TCP,” April 2007.) describe NAT behaviors and point out that two key aspects of NAT are mappings (a.k.a. bindings) and their refreshment. This introduces a derived requirement for applications engaged in a multimedia session involving NAT traversal: they need to generate a minimum of flow activity in order to create NAT mappings and maintain them alive.

When applied to applications using RTP [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), the RTP media stream packets themselves normally fulfill this requirement. However there exist some cases where RTP do not generate a minimum flow activity.

The examples are:

To solve these problems, an agent therefore needs to periodically send keepalive data within the outgoing RTP session of an RTP media stream regardless of whether the media stream is currently inactive, sendonly, recvonly or sendrecv, and regardless of the presence or value of the bandwidth attribute.

It is also important to note that the above examples also require the agents to use symmetric RTP [RFC4961] (Wing, D., “Symmetric RTP / RTP Control Protocol (RTCP),” July 2007.) in addition to RTP keepalive.

This document first states the requirements that must be supported to perform RTP keepalives (Section 3 (Requirements)). In a second step, the document reports the different mechanisms to overcome this problem (Section 4 (List of Alternatives for Performing RTP Keepalive)) and makes recommendations about their use. Section 5 (Recommended Solution for Keepalive Mechanism) finally states the recommended solution for RTP keepalive.

The scope of the draft is limited to non-ICE agents. Indeed, ICE agents need to follow the RTP keepalive mechanism specified in the ICE specification [DRAFT‑ICE] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.).

The scope of the draft is also limited to RTP flows. In particular, this document does not address keepalive activity related to:

Note that if a given media uses a codec that already integrates a keepalive mechanism, no additional keepalive mechanism is required at the RTP level.



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2.  Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).



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3.  Requirements

This section outlines the key requirements that need to be satisfied in order to provide RTP media keepalive.

REQ-1
Some data is sent periodically within the outgoing RTP session for the whole duration of the RTP media stream.
REQ-2
Any type of transport (e.g. UDP, TCP) MUST be supported.
REQ-3
Any media type (e.g. audio, video, text) MUST be supported.
REQ-4
Any media format (e.g. G.711, H.263) MUST be supported.
REQ-5
Session signaling protocols SHOULD not be impacted.
REQ-6
Session description protocols SHOULD not be impacted.
REQ-7
Impacts on existing software SHOULD be minimized.
REQ-8
Remote peer SHOULD not be impacted.
REQ-9
More than one mechanism MAY exist.


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4.  List of Alternatives for Performing RTP Keepalive

This section lists, in no particular order, some alternatives that can be used to perform a keepalive message within RTP media streams.



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4.1.  UDP Packet of 0-byte

The application sends an empty UDP packet.

Cons:

Recommendation:



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4.2.  DCCP Packet of 0-byte

The application sends an empty DCCP packet.

Cons:

Recommendation:



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4.3.  RTP Packet with Comfort Noise Payload

The application sends an RTP packet with a comfort-noise payload [RFC3389] (Zopf, R., “Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN),” September 2002.).

Cons:

Recommendation:



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4.4.  RTCP Packets Multiplexed with RTP Packets

The application sends RTCP packets in the RTP media path itself (i.e. same tuples for both RTP and RTCP packets) [DRAFT‑RTP‑RTCP] (Perkins, C. and M. Magnus, “Multiplexing RTP Data and Control Packets on a Single Port,” August 2007.). RTCP packets therefore maintain the NAT mappings open.

Cons:

Recommendation:



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4.5.  STUN Indication Packet

The application sends a STUN [DRAFT‑STUN] (Rosenberg, J., Matthews, P., Mahy, R., and D. Wing, “Simple Traversal Underneath Network Address Translators (NAT) (STUN),” February 2008.) Binding Indication packet as specified in ICE [DRAFT‑ICE] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.).

Thanks to the RTP validity check, STUN packets will be ignored by the RTP stack.

Cons:

Recommendation:



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4.6.  RTP Packet with Incorrect Version Number

The application sends an RTP packet with an incorrect version number, which value is zero.

Based on RTP specification [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), the peer should perform a header validity check, and therefore ignore these types of packet.

Cons:

Recommendation:



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4.7.  RTP Packet with Unknown Payload Type

The application sends an RTP packet of 0 length with a dynamic payload type that has not been negotiated by the peers (e.g. not negotiated within the SDP offer/answer, and thus not mapped to any media format).

The sequence number is incremented by one for each packet, as it is sent within the same RTP session as the actual media. The timestamp contains the same value a media packet would have at this time. The marker bit is not significant for the keepalive packets and is thus set to zero.

Normally the peer will ignore this packet, as RTP [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) states that "a receiver MUST ignore packets with payload types that it does not understand".

Cons:

Recommendation:



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5.  Recommended Solution for Keepalive Mechanism

Some mechanisms do not meet the requirements as they are either specific to the transport (Section 4.1 (UDP Packet of 0-byte), Section 4.2 (DCCP Packet of 0-byte)), or specific to a media type (Section 4.3 (RTP Packet with Comfort Noise Payload)). These mechanisms are thus NOT RECOMMENDED.

Other mechanisms are dependent on the capabilities of the peer (Section 4.4 (RTCP Packets Multiplexed with RTP Packets), Section 4.5 (STUN Indication Packet)). Among these mechanisms, RTCP packets multiplexed with RTP packets (Section 4.4 (RTCP Packets Multiplexed with RTP Packets)) is desirable because it reduces the number of ports used.

The RECOMMENDED solution is thus the "RTCP packets multiplexed with RTP packets" (Section 4.4 (RTCP Packets Multiplexed with RTP Packets)). However, when this mechanism cannot be negotiated, it is RECOMMENDED to use the fallback "RTP Packet with Unknown Payload Type" mechanism (Section 4.7 (RTP Packet with Unknown Payload Type)) as it will always work.



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6.  Media Format Exceptions

When a given media format does not allow the keepalive solution recommended in Section 5 (Recommended Solution for Keepalive Mechanism), an alternative mechanism SHOULD be defined in the payload format specification for this media format.

Real-time text payload format [RFC4103] (Hellstrom, G. and P. Jones, “RTP Payload for Text Conversation,” June 2005.) is an example of such a media format.



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6.1.  Real-time Text Payload Format Keepalive Mechanism

Real-time text payload format [RFC4103] (Hellstrom, G. and P. Jones, “RTP Payload for Text Conversation,” June 2005.) does not allow to use different payloads within a same RTP session, so the fallback mechanism does not work.

For real-time text, the RECOMMENDED solution is the "RTCP packets multiplexed with RTP packets". When this mechanism cannot be negotiated, it is RECOMMENDED to use an empty T140block containing no data in the same manner as for the idle procedure defined in [RFC4103] (Hellstrom, G. and P. Jones, “RTP Payload for Text Conversation,” June 2005.).



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7.  Timing and Transport Considerations

An application supporting this specification must transmit keepalive packets every Tr seconds during the whole duration of the media session. Tr SHOULD be configurable, and otherwise MUST default to 15 seconds.

When using the "RTCP packets multiplexed with RTP packets" solution for keepalive, Tr MUST comply with the RTCP timing rules of [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.). The fallback "RTP Packet with Unknown Payload Type" solution uses RTP, and thus does no have these RTCP constraints.

Keepalives packets within a particular RTP session MUST use the tuple (source IP address, source TCP/UDP ports, target IP address, target TCP/UDP Port) of the regular RTP packets.

The agent SHOULD only send RTP keepalive when it does not send regular RTP packets.



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8.  Security Considerations

The keepalive packets are sent on the same path as regular RTP media packets. In addition, they do not convey any valuable information. So the mechanism described here does not imply new security issues.



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9.  IANA Considerations

This document has no actions for IANA.



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10.  Acknowledgements

Jonathan Rosenberg provided the major inputs for this draft via the ICE specification. In addition, thanks to Alfred E. Heggestad, Colin Perkins, Dan Wing, Gunnar Hellstrom, and Randell Jesup for their useful inputs and comments.



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11.  References



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11.1. Normative references

[DRAFT-RTP-RTCP] Perkins, C. and M. Magnus, “Multiplexing RTP Data and Control Packets on a Single Port,” draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress), August 2007.
[RFC2119] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” STD 64, RFC 3550, July 2003 (TXT, PS, PDF).
[RFC4961] Wing, D., “Symmetric RTP / RTP Control Protocol (RTCP),” BCP 131, RFC 4961, July 2007 (TXT).


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11.2. Informative references

[DRAFT-ICE] Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[DRAFT-NAT-TCP-REQS] Guha, S., Biswas, K., Ford, B., Francis, P., Sivarkumar, S., and P. Srisuresh, “NAT Behavioral Requirements for TCP,” draft-ietf-behave-tcp-07 (work in progress), April 2007.
[DRAFT-STUN] Rosenberg, J., Matthews, P., Mahy, R., and D. Wing, “Simple Traversal Underneath Network Address Translators (NAT) (STUN),” draft-ietf-behave-rfc3489bis-15 (work in progress), February 2008.
[RFC3264] Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” RFC 3264, June 2002 (TXT).
[RFC3389] Zopf, R., “Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN),” RFC 3389, September 2002 (TXT).
[RFC4103] Hellstrom, G. and P. Jones, “RTP Payload for Text Conversation,” RFC 4103, June 2005 (TXT).
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” RFC 4566, July 2006 (TXT).
[RFC4787] Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” BCP 127, RFC 4787, January 2007 (TXT).


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Authors' Addresses

  Xavier Marjou
  France Telecom
  2, avenue Pierre Marzin
  Lannion 22307
  France
Email:  xavier.marjou@orange-ftgroup.com
  
  Aurelien Sollaud
  France Telecom
  2, avenue Pierre Marzin
  Lannion 22307
  France
Email:  aurelien.sollaud@orange-ftgroup.com


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Full Copyright Statement

Intellectual Property