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

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:

• In some RTP usages, such as SIP, agents can negotiate a unidirectional media stream by using the SDP "recvonly" attribute on one agent and "sendonly" on the peer, as defined in RFC 3264 [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.). RFC 3264 directs implementations not to transmit media on the receiving agent. In case the agent receiving the media is located in the private side of a NAT, it will never receive RTP packets from the public peer if the NAT mapping has not been created.
  
  
• Similarly, a bidirectional media stream can be "put on hold". This is accomplished by using the SDP "sendonly" or "inactive" attributes. Again RFC 3264 directs implementations to cease transmission of media in these cases. However, doing so may cause NAT bindings to timeout, and media won't be able to come off hold.
  
  
• In case of audio media, if silence suppression is in use, long periods of silence may cause media transmission to cease sufficiently long for NAT bindings to time out.
  
  
• Some RTP payload formats, such as the payload format for text conversation [RFC4103] (Hellstrom, G. and P. Jones, “RTP Payload for Text Conversation,” June 2005.), may send packets so infrequently that the interval exceeds the NAT binding timeouts.

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:

• Session signaling flows, such as the Session Initiation Protocol (SIP).
  
  
• RTCP flows.

  Recall that [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) recommends a minimum interval of 5 seconds and that "on hold" procedures of [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) do not impact RTCP transmissions. Therefore, when in use, there is always some RTCP flow activity.

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.

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.).

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.

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.

4.1.  UDP Packet of 0-byte

The application sends an empty UDP packet.

Cons:

• This alternative is specific to UDP.

Recommendation:

• This method should not be used for RTP keepalive.

4.2.  DCCP Packet of 0-byte

The application sends an empty DCCP packet.

Cons:

• This alternative is specific to DCCP.

Recommendation:

• This method should not be used for RTP keepalive.

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:

• This alternative is limited to audio formats only.
• Comfort Noise needs to be supported by the remote peer.
• Comfort Noise needs to be signalled in SDP offer/answer.
• The peer is likely to render comfort noise at the other side, so the content of the payload (the noise level) needs to be carefully chosen.

Recommendation:

• This method may be used when the media allows for it.

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:

• Multiplexing RTP and RTCP must be supported by the remote peer.
• Multiplexing RTP and RTCP must be signalled in SDP offer/answer.
• Some RTCP monitoring tools expect that RTCP are not multiplexed.

Recommendation:

• This method must only be used for RTP keepalive when negotiated between agents.

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:

• The sending agent needs support STUN.

Recommendation:

• This method must only be used for sessions between ICE agents, as specified in [DRAFT‑ICE] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.).

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:

• Only four version numbers are possible. Using one of them for RTP keepalive would be wasteful.
• RFC4566 [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) and RFC3264 [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) mandate not to send media with inactive and recvonly attributes, however this is mitigated as no real media is sent with this mechanism.

Recommendation:

• This method should not be used for RTP keepalive.

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:

• RFC4566 [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) and RFC3264 [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) mandate not to send media with inactive and recvonly attributes, however this is mitigated as no real media is sent with this mechanism.

Recommendation:

• This method should be used for RTP keepalive.

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.

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.

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.).

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.

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.

9.  IANA Considerations

This document has no actions for IANA.

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.

11.  References

11.1. Normative references

11.2. Informative references

Authors' Addresses

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