Network Working Group H. Desineni
Internet-Draft N. Leung
Intended status: Standards Track Qualcomm
Expires: August 27, 2007 February 23, 2007
'MSR' Bandwidth modifier in SDP Offer/Answer model
draft-hdesinen-mmusic-oa-send-bw-attr-02.txt
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Copyright (C) The IETF Trust (2007).
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Abstract
This document defines a new SDP bandwidth modifier that can be used
to specify the maximum media bitrate in 'send' direction of a stream
in SDP offer/answer model.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Use case . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. The MSR Bandwidth Modifier . . . . . . . . . . . . . . . . . . 7
5.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Usage Rules . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. ABNF Definition . . . . . . . . . . . . . . . . . . . . . 7
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
RFC 3264 defines the SDP Offer/answer model for unicast multimedia
sessions where information from both participants is needed for the
complete view of the session. According to RFC 3264, bandwidth
attribute present in the SDP offer signals the desired bandwidth that
the offerer would like to receive. Hence, bandwidth attributes "AS"
and "TIAS" defined in RFC 3550 and RFC 3890 respectively can only be
used as receive-only attributes in SDP offer/answer model. A stream
receiver (Offerer or Answerer) signals the value of "TIAS" based on
maximum media bitrate it desires to receive. The stream sender
(Answerer or Offerer) may send media at a maximum bitrate much lower
than "TIAS". Knowing the sender's (smaller) maximum media bitrate
enables better resource allocation.
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2. Conventions
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].
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3. Background
Section 5.8 of RFC 4566 defines the syntax for "b=" bandwidth
attribute. It also defines the bandwidth type "AS" as application's
concept of maximum bandwidth.
According to section 6.2 of RFC 3550, for RTP based applications,
"AS" includes the header overhead due to lower layer transport and
network protocols (e.g., UDP, IP ) and does not include the overhead
due to link level headers.
Section 6.2 of RFC 3890 defines the bandwidth type "TIAS". In the
context of RTP transport, "TIAS" provides the bandwidth for RTP
payload which includes payload format header and payload data.
"TIAS" does not include any other layer overhead including RTP layer.
According to RFC3264, any bandwidth attribute present in an SDP offer
signals the desired bandwidth that the offerer would like to receive.
Hence, bandwidth attributes "AS" and "TIAS" defined in RFC 3550 and
RFC 3890 respectively can only be used as receive-only attributes in
SDP offer/answer model.
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4. Use case
In SDP offer/answer model, the answerer's send bitrate for the stream
in answerer->offerer direction depends on the value of "AS" or "TIAS"
signaled in the SDP offer. The answerer may send media at a bit-rate
much lower than "AS" or "TIAS" signaled by the offerer. This could
be due to several reasons including QoS resource limitation over the
answerer's first hop link. Similarly, offerer's send bitrate for the
stream in offerer->answerer direction may be much lower than "AS" or
"TIAS" signaled in the SDP answer.
Cellular links commonly have some type of QoS negotiation enabling
the cellular device to learn the maximum bitrate available over the
first hop link. Signaling the maximum media send bitrate to the
receiving device can be beneficial to allocate proper QoS resources
over the last hop link. This is especially beneficial in preventing
resource overallocation when the last hop is a cellular link.
Bandwidth inequality over first and last hop links can also arise
when the offerer and answerer are attached to different types of
networks. E.g., Offerer is a cellular device and answerer is a PC
phone with wireline connection.
The 'MSR' bandwidth modifier defined in the following section can be
used to specify the maximum media bitrate in 'send' direction of a
stream. For an example usage, see Section 6.
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5. The MSR Bandwidth Modifier
5.1. Definition
MSR bandwidth modifier is defined as shown below:
b=MSR: <bandwidth-value> ; see [7] for ABNF definition.
The Maximum Send Rate (MSR) bandwidth modifier specifies the maximum
media bitrate in 'send' direction of a stream in SDP offer/answer
model. 'MSR' has an integer bit-rate value in bits per second. A
fractional bandwidth value SHALL always be rounded up to the next
integer. The bandwidth value is the maximum bit-rate without
counting the protocol header bytes from IP or other transport layers
like TCP,UDP and RTP.
5.2. Usage Rules
'MSR' bandwidth modifier is intended to be used only at the SDP media
level. It MUST not be used with a 'recvonly' stream.
5.3. ABNF Definition
The bandwidth modifier:
MSR-bandwidth-def = "b" "=" "MSR" ":" bandwidth-value CRLF
bandwidth-value = 1*DIGIT
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6. Example
Alice offers a receive only stream to Bob with maximum receive media
bitrate ("TIAS") 128000bps (H.263 profile 0 level 45). Bob responds
with SDP answer by marking the stream as sendonly. Note that the SDP
answer from Bob does not include any bandwidth attribute as it
contains a sendonly stream.
[Offer from Alice]
m=video 34564 RTP/AVP 96
a=rtpmap:96 h263-1998/90000
a=fmtp:96 profile=0; level=45;
b=TIAS:128000
a=maxprate:20
a=recvonly
[Answer from Bob]
m=video 46236 RTP/AVP 96
a=rtpmap:96 h263-1998/90000
a=fmtp:96 profile=0;level=45;
a=sendonly
Due to QoS limitation on its first hop link, Bob can send media at
maximum bitrate of 64000bps only. Unaware of Bob's maximum media
bitrate, Alice overallocates QoS resources on its access (last hop)
link. Signaling Bob's maximum media bitrate in the SDP answer can
help Alice reserve correct QoS resources in its access (last hop)
link. The complete offer/answer example with proposed bandwidth
attribute 'MSR' is shown below.
[Offer from Alice]
m=video 34564 RTP/AVP 96
a=rtpmap:96 h263-1998/90000
a=fmtp:96 profile=0; level=45;
b=TIAS:128000
a=maxprate:20
a=recvonly
[Answer from Bob]
m=video 46236 RTP/AVP 96
a=rtpmap:96 h263-1998/90000
a=fmtp:96 profile=0;level=45;
b=MSR:64000
a=sendonly
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7. Security Considerations
[TBD]
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8. IANA Considerations
[TBD]
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9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Schulzrinne, H., "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, March 1997.
[3] Rosenberg, J., "An Offer/Answer Model with Session Description
Protocol (SDP)", RFC 3264, June 2002.
[4] Narten, T., "Guidelines for Writing an IANA Considerations
Section in RFCs", RFC 2434, October 1998.
[5] Handley, M., "SDP: Session Description Protocol", RFC 4566,
July 2006.
[6] Westerlund, M., "A Transport Independent Bandwidth Modifier for
the Session Description Protocol (SDP)", RFC 3890,
September 2004.
[7] Crocker, C., "Augmented BNF for Syntax Specifications: ABNF",
RFC 2234, November 1997.
9.2. Informative References
[8] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", STD 65, RFC 3551, July 2003.
[9] Schulzrinne, H., "Real Time Streaming Protocol (RTSP)",
RFC 2326, April 1998.
[10] Johnston, A., "SDP Offer/Answer Examples", RFC 4317,
December 2005.
[11] Westerlund, M., "How to Write an RTP Payload Format",
draft-ietf-avt-rtp-howto-00.txt(Work in Progress) , May 2006.
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Authors' Addresses
Harikishan Desineni
Qualcomm
5775 Morehouse Drive
San Diego, CA 92126
USA
Phone: +1 858 845 8996
Email: hd@qualcomm.com
URI: http://www.qualcomm.com
Nikolai Leung
Qualcomm
7710 Takoma Ave
Takoma Park, MD 20912
USA
Phone: +1 858 845 3333
Email: nleung@qualcomm.com
URI: http://www.qualcomm.com
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