Network Working Group X. Marjou
Internet-Draft A. Sollaud
Intended status: BCP France Telecom
Expires: October 10, 2008 April 8, 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. List of Alternatives for Performing RTP Keepalive . . . . . . 5
4.1. UDP Packet of 0-byte . . . . . . . . . . . . . . . . . . . 5
4.2. DCCP Packet of 0-byte . . . . . . . . . . . . . . . . . . 5
4.3. RTP Packet with Comfort Noise Payload . . . . . . . . . . 5
4.4. RTCP Packets Multiplexed with RTP Packets . . . . . . . . 6
4.5. STUN Indication Packet . . . . . . . . . . . . . . . . . . 6
4.6. RTP Packet with Incorrect Version Number . . . . . . . . . 6
4.7. RTP Packet with Unknown Payload Type . . . . . . . . . . . 7
5. Recommended Solution for Keepalive Mechanism . . . . . . . . . 7
6. Media Format Exceptions . . . . . . . . . . . . . . . . . . . 8
6.1. Real-time Text Payload Format Keepalive Mechanism . . . . 8
7. Timing and Transport Considerations . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative references . . . . . . . . . . . . . . . . . . . 9
11.2. Informative references . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
Documents [RFC4787] and [DRAFT-NAT-TCP-REQS] 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], 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:
o 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]. 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.
o 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.
o 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.
o Some RTP payload formats, such as the payload format for text
conversation [RFC4103], 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] in addition to RTP keepalive.
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This document first states the requirements that must be supported to
perform RTP keepalives (Section 3). In a second step, the document
reports the different mechanisms to overcome this problem (Section 4)
and makes recommendations about their use. Section 5 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].
The scope of the draft is also limited to RTP flows. In particular,
this document does not address keepalive activity related to:
o Session signaling flows, such as the Session Initiation Protocol
(SIP).
o RTCP flows.
Recall that [RFC3550] recommends a minimum interval of 5
seconds and that "on hold" procedures of [RFC3264] 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].
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.
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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:
o This alternative is specific to UDP.
Recommendation:
o This method should not be used for RTP keepalive.
4.2. DCCP Packet of 0-byte
The application sends an empty DCCP packet.
Cons:
o This alternative is specific to DCCP.
Recommendation:
o 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].
Cons:
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o This alternative is limited to audio formats only.
o Comfort Noise needs to be supported by the remote peer.
o Comfort Noise needs to be signalled in SDP offer/answer.
o 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:
o 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]. RTCP
packets therefore maintain the NAT mappings open.
Cons:
o Multiplexing RTP and RTCP must be supported by the remote peer.
o Multiplexing RTP and RTCP must be signalled in SDP offer/answer.
o Some RTCP monitoring tools expect that RTCP are not multiplexed.
Recommendation:
o 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] Binding Indication packet
as specified in ICE [DRAFT-ICE].
Thanks to the RTP validity check, STUN packets will be ignored by the
RTP stack.
Cons:
o The sending agent needs support STUN.
Recommendation:
o This method must only be used for sessions between ICE agents, as
specified in [DRAFT-ICE].
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], the peer should perform a
header validity check, and therefore ignore these types of packet.
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Cons:
o Only four version numbers are possible. Using one of them for RTP
keepalive would be wasteful.
o RFC4566 [RFC4566] and RFC3264 [RFC3264] mandate not to send media
with inactive and recvonly attributes, however this is mitigated
as no real media is sent with this mechanism.
Recommendation:
o 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] states
that "a receiver MUST ignore packets with payload types that it does
not understand".
Cons:
o RFC4566 [RFC4566] and RFC3264 [RFC3264] mandate not to send media
with inactive and recvonly attributes, however this is mitigated
as no real media is sent with this mechanism.
Recommendation:
o 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, Section 4.2), or specific to
a media type (Section 4.3). These mechanisms are thus NOT
RECOMMENDED.
Other mechanisms are dependent on the capabilities of the peer
(Section 4.4, Section 4.5). Among these mechanisms, RTCP packets
multiplexed with RTP packets (Section 4.4) is desirable because it
reduces the number of ports used.
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The RECOMMENDED solution is thus the "RTCP packets multiplexed with
RTP packets" (Section 4.4). However, when this mechanism cannot be
negotiated, it is RECOMMENDED to use the fallback "RTP Packet with
Unknown Payload Type" mechanism (Section 4.7) as it will always work.
6. Media Format Exceptions
When a given media format does not allow the keepalive solution
recommended in Section 5, an alternative mechanism SHOULD be defined
in the payload format specification for this media format.
Real-time text payload format [RFC4103] is an example of such a media
format.
6.1. Real-time Text Payload Format Keepalive Mechanism
Real-time text payload format [RFC4103] 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].
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]. 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.
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
[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.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, July 2007.
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.
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[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.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
Authors' Addresses
Xavier Marjou
France Telecom
2, avenue Pierre Marzin
Lannion 22307
France
Email: xavier.marjou@orange-ftgroup.com
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Aurelien Sollaud
France Telecom
2, avenue Pierre Marzin
Lannion 22307
France
Email: aurelien.sollaud@orange-ftgroup.com
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