Internet DRAFT - draft-hdesinen-mmusic-oa-bw-attr
draft-hdesinen-mmusic-oa-bw-attr
Network Working Group H. Desineni
Internet-Draft N. Leung
Updates: 3264 (if approved) Qualcomm
Intended status: Standards Track July 18, 2007
Expires: January 19, 2008
Bandwidth attributes in SDP Offer/Answer model
draft-hdesinen-mmusic-oa-bw-attr-00.txt
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Copyright (C) The IETF Trust (2007).
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Abstract
This document defines several SDP bandwidth attributes that can be
used while establishing unicast multimedia sessions with SDP offer/
answer model. The defined SDP attributes can also be used to make
optimized QoS resource allocation decisions over links in the end-to-
end media path.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Asymmetric bandwidth problem . . . . . . . . . . . . . . . . . 6
5. The MSR Bandwidth Modifier . . . . . . . . . . . . . . . . . . 7
5.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Usage Rules . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. ABNF Definition . . . . . . . . . . . . . . . . . . . . . 7
5.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Additional bandwidth attributes for optimized QoS resource
allocation . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. MSPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. PPO . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. RB . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.4. Bit-rate over the last and first hop links . . . . . . . . 9
6.5. Example offer/answer exchange . . . . . . . . . . . . . . 10
7. Obsolete the RFC3890 mandated usage of 'maxprate' with
'TIAS' . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Elastic bit-rate limits . . . . . . . . . . . . . . . . . . . 13
8.1. ERB . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.2. ESB . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. ABNF definitions for new SDP attributes . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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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
and parameters like per packet protocol header overhead enables
better resource allocation over first/last hop links. An offerer
usually does not have a good estimate of 'maxprate' defined in RFC
3890. This is due to the fact that the sender's packetization
behavior is totally outside the control of the stream receiver.
While using variable rate video codecs, the output RTP packet rate of
a sender does not remain constant. Any video bandwidth estimations
based on maximum video packet rate can be misleading. Advanced
packet-switched wide area networks (e.g., 3GPP HSPA) have evolved
towards providing elastic bandwidth capacity limits to their
terminals. Such terminals are allowed to operate in an elastic
bandwidth zone between two bandwidth limits, the guaranteed bitrate
(GBR) and the maximum bitrate (MBR). For example, a video codec can
adapt its video encoding rate in the elastic bandwidth zone enforced
by the network. This document defines two bandwidth attributes which
capture the elastic media stream bandwidth limits enforced by a
network on its terminals.
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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
SDP offer/answer model defined in RFC3264 does not define sufficient
bandwidth attributes that can help optimized resource allocation over
links in the end-to-end the media path. Section 4 captures a use
case which describes the need for additional SDP attributes.
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. Asymmetric bandwidth problem
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.
Wireless wide area networks commonly have some type of QoS
negotiation enabling the sending device to learn the maximum bitrate
available over the first hop link (uplink). Signaling the maximum
media send bitrate to the receiving device can be beneficial to
allocate proper QoS resources over the media path. Such signaling is
especially useful in preventing resource over allocation over last
hop wireless 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 a more restrictive first hop link speed.
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 5.4.
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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
5.4. 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
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[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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6. Additional bandwidth attributes for optimized QoS resource
allocation
Note that 'MSR' attribute alone is not sufficient to capture the end
to end QoS requirement for a unicast media stream. A media sender is
not aware of the actual bit-rate that results over the last hop
downlink of the receiver. Signaling per packet overhead (PPO) at the
receiver's downlink can help a sender estimate the resulting bit-rate
over the receiver's downlink. The attributes defined in the
following section can help both sender and receiver compute the
bitrate that results over first and last hop links. An underlying
assumption is that the media bit-rate (excluding the header overhead)
will not change end to end. There might be changes in the protocol
header overhead as the media traverses different types of networks
(E.g.,IPv4 to IPv6 cloud).
6.1. MSPR
Maximum Send Packet Rate (MSPR) is defined as the maximum RTP packet
rate at a stream sender. This is a send only attribute and it is
signaled by media sender. An intermediate node (Gateway or a SIP
B2BUA) which performs protocol layer translation (e.g., IPv4 to IPv6)
can use this attribute to estimate the total stream bandwidth,
including protocol overhead, that is required in the destination
network.
6.2. PPO
Per Packet Overhead (PPO) is defined as the average per packet
overhead counted in bytes, measured from layer 'X' to the RTP media
layer (including the RTP header), where layer 'X' can be any layer
below RTP. This is a receive only attribute and it is signaled by
media receiver.
6.3. RB
Receive Bit-rate (RB) is defined as the last hop's maximum receiver
bit-rate at layer 'X'. 'RB' will be same as 'AS' if 'X' is IP layer
and there is no compression above IP layer. This is a receive only
attribute and it is signaled by media receiver. According to
RFC3550, the bandwidth attribute "AS" can be used only when IP/UDP/
RTP headers are not compressed. "RB" is useful in environments where
a link in the media path uses header compression.
6.4. Bit-rate over the last and first hop links
Using the attributes defined in the above sections and using the
following formula, a sender can calculate the resulting bit-rate over
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the receiver's last hop link, at the same layer 'X' at which RB was
computed.
br-last-hop is the maximum media bit-rate over the last hop link.
br-last-hop = MSR + (MSPR * PPO)
To prevent exceeding the capacity of the last hop link, a media
sender can choose a combination of MSR and MSPR such that the value
of br-last-hop does not exceed the value of RB signaled by the media
receiver.
(MSR+(MSPR * PPO)) <= RB
A sender may not send at RB due to bandwidth limitations over its
immediate first hop link. Under such circumstances, signaling the
value of br-last-hop can benefit the receiver to allocate only
necessary QoS resources over its downlink. br-last-hop can be
signaled using the attribute 'SB' (Send Bandwidth). A sender has the
freedom to choose a combination of actual media bit-rate and actual
packet rate as long as the resulting value of SB does not exceed the
value of RB. A media sender will choose SB such that SB <= RB. SB
can be calculated using the following formula:
SB = (Actual MSR + Actual packet rate * PPO over the receiver's
downlink)
Note that both SB and RB denote the bit-rate at the same protocol
layer 'X' on the receiver's link as selected by the receiver when
reporting its PPO value. This document recommends stream receivers
to signal the SDP attributes 'RB' and 'PPO' at layer 'X', and TIAS.
It recommends stream senders to signal the attributes 'SB' also at
layer 'X' and 'MSR'.
6.5. Example offer/answer exchange
Alice make the following SDP offer Bob. Alice's downlink has 40 bytes
of per packet protocol overhead and it can support video at a maximum
bit-rate of 140800bps (including protocol overhead). Alice can send
video at a maximum bit rate of 64000bps (excluding protocol
overhead).
m=video <>
b=TIAS:128000 (bps)
b=RB:140800 (bps)
a=PPO:40 (bytes)
b=MSR:64000 (bps)
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Bob sends the following SDP answer in response to the SDP offer from
Alice. Bob's downlink can provide the QoS resources required to
support Alice's video stream. Bob's uplink video stream requires
less QoS resources than offered over Alice's downlink.
m=video <>
b=TIAS:64000 (bps)
b=RB:76800 (bps)
a=PPO:40 (bytes)
b=MSR:64000 (bps)
b=SB:72000; MSPR:25
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7. Obsolete the RFC3890 mandated usage of 'maxprate' with 'TIAS'
Per RFC3890, it is mandatory to signal 'maxprate' with 'TIAS'. The
following are some of the disadvantages of this approach.
An encoder implementation for an application typically only supports
a limited number of packetization rates. For example, a speech
service usually has a fixed frame duration and video encoders may be
optimized for only generating a certain number of video slices per
video frame at certain video frame rates. Without knowledge of these
implementation limitations, a media receiver may easily set a
maxprate value that does not match any of the sender's packetization
rates. This can result in unnecessarily restrictive operation as the
sender has to use a lower packetization rate, thus underutilizing the
bandwidth at the receiver's link. This could even result in no
usable packetization rate if maxprate is set below the encoder's
minimum rate.
Furthermore, the purpose of maxprate is for the receiver to use this
along with the TIAS parameter to limit the total stream bandwidth
(including overhead) the sender would send over the receiver's link.
However, compared to the 'PPO' and 'RB' parameters specified in
sections 6.2 and 6.3 of this document, this approach is unnecessarily
restrictive for achieving a bandwidth limit at the receiver. This
prevents the sender from choosing among all combinations of media bit
rates and packetization rates that would meet the receiver's
bandwidth restrictions.
Due to the above reasons, the mandatory usage of 'maxprate' with
''TIAS' in offer/answer model is obsoleted by this document. Note
that a streaming session is still allowed to use 'maxprate' with
'TIAS' in a declarative manner. (Note: At the time of writing this
document, per the knowledge of the authors, there are no offer/answer
implementations of 'maxprate' with 'TIAS'. There are only
declarative TIAS implementations in the offer/answer model)
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8. Elastic bit-rate limits
A flow is elastic if its source can handle the transport of its data
over varying transport conditions such as variations in available
bandwidth. To enable time-sensitive elastic applications such as
packet-switched multimedia services to make more accurate use of
elastic bandwidth capacity, some type of networks(e.g., 3GPP) are
signalling both minimum guaranteed bandwidth and maximum elastic
bandwidth limits to terminals in their network.
The following parameters (ERB,ESB) enable end-terminals to coordinate
and negotiate the elastic bandwidth limits provided by their
respective networks. ERB and ESB shall always be set to the
bandwidth values granted by the access network. Note that ERB and
ESB are bit-rate values at the same protocol layer 'X' at which PPO
attribute was signaled by the media receiver.
8.1. ERB
ERB: Same as "RB" except that it indicates maximum receivable elastic
throughput including media and overhead to protocol layer X. This is
set based on a maximum elastic throughput limit communicated to the
receiver by its network (Downlink MBR in 3GPP network). Combined
with ERB information a sender can choose what combinations of media
rate and packetization rate could fit through the receiver's "maximum
receivable elastic bandwidth" constraint at layer X. This is a
receive only attribute and hence signaled by only a media receiver.
8.2. ESB
ESB: Same as "SB" except that it indicates maximum sending elastic
throughput including media and overhead at protocol layer X. Layer
'X' is same as the layer at which ERB is signaled. This is set based
on a maximum elastic throughput limit communicated to the sender by
its network (Uplink MBR in 3GPP network). This is a sendonly
attribute and hence signaled by only a media sender. Note that this
attribute is sent only in response to "ERB" attribute received from a
media receiver.
8.3. Example
SDP offer from Alice to Bob: Using ERB, Alice signals the bandwidth
granted by its access network.
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m=video <>
b=TIAS:256000 (bps, supported by the decoder)
b=ERB:190000 (bps, signaled by the access network)
b=RB:140000 (bps, signaled by the access network)
a=PPO:40 (bytes)
b=ESB:100000 (bps, signaled by the access network)
b=SB:72000; MSPR:25
Bob sends the following SDP answer: Using ESB, Bos signals the
bandwidth granted by its access network.
m=video <>
b=ERB:84000 (bps)
b=RB:72000 (bps)
a=PPO:40 (bytes)
b=ESB:190000 (bps)
b=SB:140000; MSPR:25
b=MSR:128000 (bps)
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9. ABNF definitions for new SDP attributes
ABNF definitions for the SDP attributes defined in Section 6 are TBD.
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10. Security Considerations
[TBD]
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11. IANA Considerations
[TBD]
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12. References
12.1. 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 4234, October 2005.
12.2. 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., "Codec Control Messages in the RTP Audio-Visual
Profile with Feedback (AVPF)",
draft-ietf-avt-avpf-ccm-08.txt(Work in Progress) , July 2007.
[12] 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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