Network Working Group X. Marjou
Internet-Draft A. Sollaud
Intended status: Standards Track France Telecom Orange
Expires: September 5, 2011 March 4, 2011
Application Mechanism for keeping alive the Network Address Translator
(NAT) mappings associated to RTP/RTCP flows.
draft-ietf-avt-app-rtp-keepalive-10
Abstract
This document lists the different mechanisms that enable applications
using Real-time Transport Protocol (RTP) and RTP control protocol
(RTCP) 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.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 5, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. List of Alternatives for Performing RTP Keepalive . . . . . . 5
4.1. Transport Packet of 0-byte . . . . . . . . . . . . . . . . 5
4.2. RTP Packet with Comfort Noise Payload . . . . . . . . . . 5
4.3. RTCP Packets Multiplexed with RTP Packets . . . . . . . . 5
4.4. STUN Indication Packet . . . . . . . . . . . . . . . . . . 6
4.5. RTP Packet with Incorrect Version Number . . . . . . . . . 6
4.6. RTP Packet with Unknown Payload Type . . . . . . . . . . . 6
5. Recommended Solution for Keepalive Mechanism . . . . . . . . . 7
6. Media Format Exceptions . . . . . . . . . . . . . . . . . . . 7
7. Timing and Transport Considerations . . . . . . . . . . . . . 7
8. RTCP Flow Keepalive . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative references . . . . . . . . . . . . . . . . . . . 10
12.2. Informative references . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Documents [RFC4787] and [RFC5382] 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], 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:
o In some RTP usages, such as the Session Inititation Protocol (SIP)
[RFC3550], agents can negotiate a unidirectional media stream by
using the Session Description Protocol (SDP) [RFC4566] "recvonly"
attribute on one agent and "sendonly" on the peer, as defined in
[RFC3264]. [RFC3264] 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 [RFC3264] 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 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 important to note that NAT traversals constraints also usually
require the agents to use Symmetric RTP / RTP Control Protocol (RTCP)
[RFC4961] in addition to RTP keepalive.
This document first states the requirements that must be supported to
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perform RTP keepalives (Section 3). In a second step, the document
reports the different mechanisms to overcome this problem
(Section 4). Section 5 finally states the recommended solution for
RTP keepalive. Section 6 discusses some media format exceptions.
Section 7 adds details about timing and transport considerations.
Section 8 documents how to maintain NAT bindings for RTCP.
This document is not applicable to Interactive Connectivity
Establishment (ICE) [RFC5245] agents. Indeed, the ICE protocol
together with Session Traversal Utilities for NAT (STUN) [RFC5389]
and Traversal Using Relays around NAT (TURN) [RFC5766] 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.
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 mentioned in Section 3.5 of [RFC5405] "It is important to note
that keepalive 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].
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 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:
o 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].
Cons:
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.
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]. RTCP packets
therefore maintain the NAT mappings open as long as the requirements
on parameter selection are fulfilled as discussed in Section 8.
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Note: "on hold" procedures of [RFC3264] do not impact RTCP
transmissions.
Cons:
o Multiplexing RTP and RTCP must be supported by the remote peer.
o Some RTCP monitoring tools expect that RTCP packets are not
multiplexed.
o RTCP must be configured so that Tmin value [RFC3550] is lower or
equal to the Tr interval.
4.4. STUN Indication Packet
The application sends a STUN [RFC5389] Binding Indication packet as
specified in ICE [RFC5245].
Thanks to the RTP validity check, STUN packets will be ignored by the
RTP stack.
Cons:
o 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], the peer should perform a
header validity check, and therefore ignore these types of packet.
Cons:
o Only four version numbers are possible. Using one of them for RTP
keepalive would be wasteful.
o [RFC4566] and [RFC3264] 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.
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The SSRC is the same as for the media for which keepalive is sent.
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] and [RFC3264] mandate not to send media with inactive
and recvonly attributes, however this is mitigated as no real
media is sent with this mechanism.
o [RFC3550] does not preclude examination of received packets by the
peer in an attempt to determine if it is under attack.
o The statement "RTP Packet with Unknown Payload Type" of RFC3550 is
not always observed in real life.
o There is no RTCP reporting for the keepalive packets as RFC3550
mandates to ignore "RTP Packet with Unknown Payload Type".
o Some RTP payload formats do not handle gaps in RTP sequence number
well.
5. Recommended Solution for Keepalive Mechanism
The RECOMMENDED mechanism is the "RTCP packets multiplexed with RTP
packets" (Section 4.3). 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, Section 4.2, Section 4.4, Section 4.5,
Section 4.6) are NOT RECOMMENDED.
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.
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
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SHOULD be configurable to larger values.
For TCP, the recommended Tr value is 7200 seconds.
When using the "RTCP packets multiplexed with RTP packets" solution
for keepalive, Tr MUST comply with the RTCP timing rules of
[RFC3550].
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. RTCP Flow Keepalive
RTCP packets are sent periodically and can thus normally maintain the
NAT mappings open as long as they are sent frequently enough. There
are two conditions for that. First RTCP needs to be used bi-
directionally and in a symmetric fashion, as described in [RFC4961].
Secondly, RTCP needs to be sent frequently enough. However, there
are certain configurations that can break this latter assumption.
There are two factors that need to be considered to ensure that RTCP
is sent frequently enough. First the RTCP bandwidth needs to be
sufficiently large so that transmission will occur more frequently
than the longest acceptable packet transmission interval (Tr). The
worst case RTCP interval (Twc) can be calculated using this formula
by inserting the max value of the following parameters:
o Maximum RTCP packet size (avg_rtcp_size_max)
o Maximum number of participants (members_max)
o RTCP receiver bandwidth (rtcp_bw)
The RTCP bandwidth value to use here is for a worst case, which will
be the receiver proportion when all members are not senders except
one. This can be approximated to be all members. Thus for sessions
where RR and RS values are used, then rtcp_bw shall be set to RR.
For sessions where the [RFC3550] defines proportions of 1/4 for
sender and 3/4 for receivers are used, then rtcp_bw will be 5% of 3/4
of the AS value in bits per second.
Twc = 1.5 / 1.21828 * members_max * rtcp_bw / avg_rtcp_size_max * 8
The second factor is the minimum RTCP interval Tmin defined in
[RFC3550]. Its base value is 5 seconds, but it might also be scaled
to 360 divided by the session bandwidth in kbps. The Extended RTP
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Profile for Real-time Transport Control Protocol (RTCP)-Based
Feedback (RTP/AVPF) [RFC4585] also allows for the setting of a trr-
int parameter which is a minimal RTCP interval for regular RTCP
packets. It is also used as the Tmin value in the regular Td
calculation. An analysis of the algorithm gives that the longest
possible regular RTCP interval possible are:
RTCP_int_max = trr-int * 1.5 + Td * 1.5 / 1.21828
And as long as the there is sufficient bandwidth according to
criteria 1, then this can be simplified by setting Td = trr-int
giving
RTCP_int_max = trr-int * (1.5 + 1.5 / 1.21828) = 2.73123 * trr-int
Thus the requirements on the RTCP parameters are the following for
functioning keepalive:
1. Ensure that sufficient RTCP bandwidth is provided by calculating
Twc and ensure that this is less than or equal to Tr.
2. If AVP or SAVP is used the Tmin value can't be greater that Tr
divided by 1.5 / (e-3/2).
3. If AVPF or SAVPF is to be used trr-min must not be set to a
greater value than Tr / 3.
9. 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] 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.
10. IANA Considerations
None.
11. Acknowledgements
Jonathan Rosenberg provided the major inputs for this draft via the
ICE specification. Magnus Westerlund provided the text for the RTCP
flow keepalive section. In addition, thanks to Alfred E. Heggestad,
Colin Perkins, Dan Wing, Gunnar Hellstrom, Hadriel Kaplan, Randell
Jesup, Remi Denis-Courmont, Robert Sparks, and Steve Casner for their
useful inputs and comments.
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12. References
12.1. Normative references
[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.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
12.2. Informative references
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[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.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
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RFC 4787, January 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
Authors' Addresses
Xavier Marjou
France Telecom Orange
2, avenue Pierre Marzin
Lannion 22307
France
Email: xavier.marjou@orange-ftgroup.com
Aurelien Sollaud
France Telecom Orange
2, avenue Pierre Marzin
Lannion 22307
France
Email: aurelien.sollaud@orange-ftgroup.com
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