AVT F. Andreasen
Internet-Draft D. Oran
Expires: November 18, 2005 D. Wing
Cisco Systems, Inc.
May 17, 2005
A No-Op Payload Format for RTP
draft-wing-avt-rtp-noop-03
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines an no-op payload format for the Real-time
Transport Protocol (RTP), and a mechanism to request transmission of
an early RTCP report. This can be used to verify RTP connectivity
and to keep Network Address Translator (NAT) bindings and Firewall
pinholes open.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. RTP Payload Format for No-Op . . . . . . . . . . . . . . . . . 4
2.1 Registration . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Use of RTP Header Fields . . . . . . . . . . . . . . . . . 4
2.3 Payload Format . . . . . . . . . . . . . . . . . . . . . . 4
2.4 Sender Operation . . . . . . . . . . . . . . . . . . . . . 5
2.5 Mixer, Translator Operation . . . . . . . . . . . . . . . 5
2.6 Receiver Operation . . . . . . . . . . . . . . . . . . . . 5
2.7 Indication of No-OP Capability using SDP . . . . . . . . . 6
3. Example SDP Offer/Answer . . . . . . . . . . . . . . . . . . . 6
4. MIME Registration . . . . . . . . . . . . . . . . . . . . . . 7
4.1 audio/no-op . . . . . . . . . . . . . . . . . . . . . . . 7
4.2 video/no-op . . . . . . . . . . . . . . . . . . . . . . . 8
4.3 text/no-op . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1 Normative References . . . . . . . . . . . . . . . . . . . 9
8.2 Informational References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
This memo defines a new RTP payload format called "no-op". This
payload behaves like a normal RTP payload, except the RTP packet is
not used to play out media. It is also explicitly designed to
interact constructively with the RTCP feedback profile [6].
This new payload format is useful for:
o media session reception quality assessment, such as at the
beginning of a session;
o keepalives to keep NAT bindings and/or firewall pinholes open when
RTP media traffic is not otherwise being transmitted.
In addition it has a number of uses whose utility is speculative but
for which it is easy pressed into service:
o measurement-based admission control by probing available
bandwidth, and
o synthetic load generation for performance testing and other
minimally-intrusive instrumentation.
When an endpoint mas a media stream marked as 'recvonly' or
'inactive' the endpoint is not supposed to send any media (i.e. RTP
packets). However, to keep a NAT binding alive, the endpoint will
need to send packets over the RTP and RTCP ports. RTP No-Op is
ideally suited to this. In comparison, if one participant in an
audio multicast conference has a 'recvonly' or 'inactive' media
stream yet occasionally sends comfort noise packets in order to keep
its NAT binding open, these comfort noise packets are interpreted as
audio packets by receivers and mixers which can cause undesirable
behavior -- such as selection of the primary speaker or the playout
of comfort noise when no audio should be played.
Unlike Comfort noise [9], which is specific to voice RTP streams, RTP
No-Op is applicable to any kind of RTP stream including video, audio,
realtime text, or any other media type that would benefit from the
capabilities listed above. This gives RTP No-Op an advantage as a
NAT keepalive mechanism. Certain functions and RTP payload types can
use RTP No-Op without re-inventing their own payload-specific NAT
keepalive mechanism -- such as video muting, Clearmode [10], and text
[11].
Some audio codecs have their own 'silence' packets. However, some
codecs only send such 'silence' packets if the noise floor changes;
G.729b [12] is an example of such a codec. RTP No-Op allows the RTP
stack itself, rather than the codec, to send periodic packets as a
keepalive mechanism.
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The key words "MUST", "MUST NOT" "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
2. RTP Payload Format for No-Op
2.1 Registration
The RTP payload format is designated as "no-op" and the MIME types
are "audio/no-op", "video/no-op", and "text/no-op". The default
clock rate is 8000 Hz, but other rates MAY be used. In accordance
with current practice, this payload format does not have a static
payload type number, but uses a RTP payload type number established
dynamically out-of-band, e.g. through SDP [4].
2.2 Use of RTP Header Fields
Timestamp: The RTP timestamp reflects the measurement point for the
current packet. The receiver calculates jitter for RTCP receiver
reports based on all packets with a given timestamp. Note: The
jitter value should primarily be used as a means for comparing the
reception quality between two users or two time-periods, not as an
absolute measure.
Marker bit: The RTP marker bit has no special significance for this
payload type.
2.3 Payload Format
The payload format is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| padding (OPTIONAL) |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The payload contains at least 4 bytes. The first 32 bits are defined
as follows:
bit 0: "R", "Request Early RTCP", is used to request invocation
of RTCP feedback by timely transmission of an RTCP report
(see Section 2.6).
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bits 1-31: Reserved; contents are ignored.
Additional padding bytes MAY be appended up to the ptime value in SDP
(see Section 2.7). These bytes are ignored. Padding may be useful
to generate RTP packets that are the same size as a normal media
payload.
2.4 Sender Operation
As discussed in the introduction, endpoints MUST occasionally send a
packet to their RTP and RTCP peer to keep NAT and firewall bindings
active, even if the media stream is marked 'recvonly' or 'inactive'.
If no other RTP packet has been sent for approximately 30 seconds, an
RTP NoOp packet SHOULD be sent. It is permissible to send a NoOp
packet even for media streams marked 'recvonly' or 'inactive'.
2.5 Mixer, Translator Operation
An RTP mixer or unicast-to-unicast RTP translator SHOULD forward RTP
No-Op payload packets normally. A unicast-to-multicast RTP
translator SHOULD replicate RTP No-Op payload packets normally.
A multicast-to-unicast RTP translator SHOULD NOT replicate an RTP
No-Op packet with the Request Early RTCP bit set unless:
1. all receivers are known to be operating under the bandwidth
limitations rules of [6], and
2. the restriction of applicability to "small groups" in [6] is
observed
Otherwise the sender may be flooded with RTCP reports.
2.6 Receiver Operation
Upon receipt of an RTP packet with the No-Op payload format and the
'Request Early RTCP Report' bit set to 0, the receiver performs
normal RTP receive operations on it -- incrementing the RTP receive
counter, calculating jitter, and so on. The receiver then discards
the packet -- it is not used to play out media.
Upon receipt of an RTP packet with the No-Op payload format and the
'Request Early RTCP Report' bit set to 1, the receiver adjusts
counters as described above and then also performs the following
steps (with reference to the definitions and procedures of the AVPF
profile [6]):
1. ascertains whether the associated RTP session is operating under
the AVPF RTP profile (or one derived from it via combination with
another RTP profile). If not the receiver takes no further
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action on this packet - specifically, if the RTP/AVPF profile (or
one derived from it) is not used the receiver MUST NOT send an
early RTCP report. If so, it continues as follows.
2. generates a feedback "Event" which in turn may trigger the
generation of a "FB message".
3. sends the FB message as an "early RTCP packet" assuming the
bandwidth constraints for feedback messages are satisfied.
4. Otherwise, takes no further action
2.7 Indication of No-OP Capability using SDP
Senders and receivers may indicate support for the No-Op payload
format, for example, by using the Session Description Protocol SDP
[4]. If the payload format is being used for connectivity
verification (e.g. in conjunction with [5]) senders and receivers
MUST advertise the AVPF profile (or a profile used in combination
with it).
The default packetization interval for this payload type is 20ms
(ptime:20) but alternate values can be advertised in SDP using the
ptime attribute value [4].
3. Example SDP Offer/Answer
Offer:
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVPF 0 33
a=rtpmap:0 PCMU/8000
a=rtpmap:33 no-op/8000
m=video 41372 RTP/AVPF 31 36
a=rtpmap:31 H261/90000
a=rtpmap:36 no-op/90000
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Answer:
v=0
o=bob 2808844564 2808844564 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 59174 RTP/AVPF 0 33
a=rtpmap:0 PCMU/8000
a=rtpmap:33 no-op/8000
m=video 59170 RTP/AVPF 32 36
a=rtpmap:31 H261/90000
a=rtpmap:36 no-op/90000
4. MIME Registration
This section registers MIME types for audio/no-op, video/no-op, and
text/no-op.
4.1 audio/no-op
MIME media type name: audio
MIME subtype name: no-op
Required parameters: none
Optional parameters: none
Encoding considerations: This type is only defined for transfer via
RTP [2] and Secure RTP [3].
Security considerations: See Section 5, "Security Considerations", in
this document.
Interoperability considerations: none
Published specification: This document.
Applications which use this media: The "no-op" application subtype is
used to maintain network state or verify network connectivity, when a
more traditional RTP payload type cannot be used.
Additional information:
1. Magic number(s): N/A
2. File extension(s): N/A
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3. Macintosh file type code: N/A
4.2 video/no-op
MIME media type name: video
MIME subtype name: no-op
Required parameters: none
Optional parameters: none
Encoding considerations: This type is only defined for transfer via
RTP [2] and Secure RTP [3].
Security considerations: See Section 5, "Security Considerations", in
this document.
Interoperability considerations: none
Published specification: This document.
Applications which use this media: The "no-op" application subtype is
used to maintain network state or verify network connectivity, when a
more traditional RTP payload type cannot be used.
Additional information:
1. Magic number(s): N/A
2. File extension(s): N/A
3. Macintosh file type code: N/A
4.3 text/no-op
MIME media type name: text
MIME subtype name: no-op
Required parameters: none
Optional parameters: none
Encoding considerations: This type is only defined for transfer via
RTP [2] and Secure RTP [3].
Security considerations: See Section 5, "Security Considerations", in
this document.
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Interoperability considerations: none
Published specification: This document.
Applications which use this media: The "no-op" application subtype is
used to maintain network state or verify network connectivity, when a
more traditional RTP payload type cannot be used.
Additional information:
1. Magic number(s): N/A
2. File extension(s): N/A
3. Macintosh file type code: N/A
5. Security Considerations
Without security of the RTP stream (via SRTP [3], IPsec [8], or other
means), it is possible for an attacker to spoof RTP packets,
including this new packet type. As this new RTP payload type
includes a method to request early transmission of RTCP, this could
be used to cause endpoints to flood the network with RTCP reports.
Thus, the RTCP transmissions MUST NOT exceed the bandwidth
recommendations described in section 6.3 of RFC3550 [2].
6. IANA Considerations
IANA is requested to make MIME type registrations as specified above
in Section 4
7. Acknowledgments
Thanks to Henning Schulzrinne for suggesting using RTCP as a feedback
mechanism.
8. References
8.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[3] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
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[4] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[5] Andreasen, F., "Connectivity Preconditions for Session
Description Protocol Media Streams",
draft-andreasen-mmusic-connectivityprecondition-02 (work in
progress), February 2005.
[6] Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based
Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11 (work in
progress), August 2004.
[7] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Multimedia Session Establishment Protocols",
draft-ietf-mmusic-ice-04 (work in progress), February 2005.
8.2 Informational References
[8] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[9] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[10] Kreuter, R., "RTP Payload Format for a 64 kbit/s Transparent
Call", RFC 4040, April 2005.
[11] Hellstrom, G., "RTP Payload for Text Conversation", RFC 2793,
May 2000.
[12] International Telecommunications Union, "G.729 Annex B",
November 1999,
<http://www.itu.int/ITU-T/publications/recs.html>.
Authors' Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, NJ 08837
USA
Email: fandreas@cisco.com
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David Oran
Cisco Systems, Inc.
7 Ladyslipper Lane
Acton, MA 01720
USA
Email: oran@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
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