Application Mechanism for keeping alive the Network Address Translator (NAT) mappings associated to RTP flows.
draft-ietf-avt-app-rtp-keepalive-08

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.

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on December 20, 2010.

Copyright Notice

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Table of Contents

1.  Introduction
2.  Terminology
3.  Requirements
4.  List of Alternatives for Performing RTP Keepalive
    4.1.  Transport Packet of 0-byte
    4.2.  RTP Packet with Comfort Noise Payload
    4.3.  RTCP Packets Multiplexed with RTP Packets
    4.4.  STUN Indication Packet
    4.5.  RTP Packet with Incorrect Version Number
    4.6.  RTP Packet with Unknown Payload Type
5.  Recommended Solution for Keepalive Mechanism
6.  Media Format Exceptions
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

1.  Introduction

Documents [RFC4787] (Audet, F. and C. Jennings, “Network Address Translation (NAT) Behavioral Requirements for Unicast UDP,” January 2007.) and [RFC5382] (Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, “NAT Behavioral Requirements for TCP,” October 2008.) describe Network Address Translator (NAT) behaviors and point out that two key aspects of NAT are mappings (a.k.a. bindings) and keeping them refreshed. 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.

When applied to applications using the real-time transport protocol (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 does not generate the minimum required flow activity.

The examples are:

• In some RTP usages, such as the Session Inititation Protocol (SIP) [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), agents can negotiate a unidirectional media stream by using the Session Description Protocol (SDP) [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) "recvonly" attribute on one agent and "sendonly" on the peer, as defined in [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.). [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) 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 [RFC3264] (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) 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.
  
  
• 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 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)). Section 5 (Recommended Solution for Keepalive Mechanism) finally states the recommended solution for RTP keepalive.

This document is not applicable to Interactive Connectivity Establishment (ICE) [RFC5245] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” April 2010.) agents. Indeed, the ICE protocol together with Simple Traversal of User Datagram Protocol (STUN) [RFC5389] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for NAT (STUN),” October 2008.) and Traversal Using Relays around NAT (TURN) [RFC5761] (Perkins, C. and M. Westerlund, “Multiplexing RTP Data and Control Packets on a Single Port,” April 2010.) solve the overall Network Address Translator (NAT) traversal mechanism of media streams. In the context of RTP media streams, some agents may not require all ICE functionalities and may only need a keepalive mechanism. This document thus applies to such agents, and does not apply to agents implementing ICE.

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).
  
  
• RTP Control Protocol (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.

As mentionned in Section 3.5 of [RFC5405] (Eggert, L. and G. Fairhurst, “Unicast UDP Usage Guidelines for Application Designers,” November 2008.) "It is important to note that keep-alive messages are NOT RECOMMENDED for general use -- they are unnecessary for many applications and can consume significant amounts of system and network resources."

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

Impacts on existing software SHOULD be minimized.

REQ-7

Remote peer SHOULD NOT be impacted.

REQ-8

The support for RTP keepalive SHOULD be described in the SDP.

REQ-9

The solution SHOULD cover the integration with RTCP.

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.  Transport Packet of 0-byte

The application sends an empty transport packet (e.g. UDP packet, DCCP packet).

Cons:

• This alternative is specific to each transport protocol.

4.2.  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.

4.3.  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) [RFC5761] (Perkins, C. and M. Westerlund, “Multiplexing RTP Data and Control Packets on a Single Port,” April 2010.). RTCP packets therefore maintain the NAT mappings open.

Cons:

• Multiplexing RTP and RTCP must be supported by the remote peer.
• Some RTCP monitoring tools expect that RTCP are not multiplexed.

4.4.  STUN Indication Packet

The application sends a STUN [RFC5389] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for NAT (STUN),” October 2008.) Binding Indication packet as specified in ICE [RFC5245] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” April 2010.).

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

Cons:

• The sending agent needs to support STUN.

4.5.  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] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) and [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.

4.6.  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.

The SSRC is the same than one one of the media for which keepalive is sent.

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] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) and [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.
• [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) does not preclude examination of received packets by the peer in an attempt to determine if it is under attack.
• The statement "RTP Packet with Unknown Payload Type" of RFC3550 is not always observed in real life.

5.  Recommended Solution for Keepalive Mechanism

The RECOMMENDED mechanism is the "RTCP packets multiplexed with RTP packets" (Section 4.3 (RTCP Packets Multiplexed with RTP Packets)). This mechanism is desirable because it reduces the number of ports when RTP and RTCP are used. It also has the advantage of taking into account RTCP aspects, which is not the case of other mechanisms.

Other mechanisms (Section 4.1 (Transport Packet of 0-byte), Section 4.2 (RTP Packet with Comfort Noise Payload), Section 4.4 (STUN Indication Packet), Section 4.5 (RTP Packet with Incorrect Version Number), Section 4.6 (RTP Packet with Unknown Payload Type)) are NOT RECOMMENDED.

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.

7.  Timing and Transport Considerations

An application supporting this specification MUST transmit either keepalive packets or media packets at least once every Tr seconds during the whole duration of the media session.

Tr has different value according to the transport protocol

For UDP, the minimum RECOMMENDED Tr value is 15 seconds, and Tr SHOULD be configurable to larger values.

For TCP, [TODO].

For DCCP, [TODO].

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.).

Keepalive 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 RTP keepalive packets are sent on the same path as regular RTP media packets and may be perceived as an attack by a peer. However, [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) mandates a peer to "ignore packets with payload types that it does not understand". A peer that does not understand the keepalive message will thus appropriately drop the received packets.

9.  IANA Considerations

None.

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, Hadriel Kaplan, Magnus Westerlund, Randell Jesup, Remi Denis-Courmont, Robert Sparks, and Steve Casner for their useful inputs and comments.

11.  References

11.1. Normative references

11.2. Informative references

Authors' Addresses