[Sip-implementors] SDP b= line interpretation

[Yong Xin]

Hi sip-implementors,

Below is an SDP example in RFC 3556:

v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
m=audio 49170 RTP/AVP 0
b=AS:64
b=RS:800
b=RR:2400
m=video 51372 RTP/AVP 31
b=AS:256
b=RS:800
b=RR:2400

Assuming there is an unicast point-to-point session (A-B), above SDP is the offer made by endpoint A, and endpoint B accepts the offer and agrees with the same bandwidth values in the answer. "b=AS" modifier specifies the RTP bandwidth, "b=RS" and "b=RR" modifiers specify the RTCP bandwidth for sender and receiver respectively. In this point-to-point session, both A and B are the sender.

My question is: does the b= line value specify the aggregate bandwidth limit for the whole session (including endpoint A and B) or just the bandwidth limit for local endpoint? Using audio media as an example, how much RTP bandwidth should be allocated on the endpoint A? 64 or 32kbps?

Thanks,
Yong

[sameer momin]

Hello,
As per RFC 4566 for RTP based applications, AS gives the RTP "session
bandwidth" as defined in RFC 3550 section 6.2.

"session bandwidth" as per RFC 3550: for each session, it is assumed that the
data traffic is subject to an aggregate limit called the "session
bandwidth" to be divided among the participants.

"b=" line here specifies the aggregate bandwidth limit for the whole
session.

Regards
Sameer

Post by Yong Xin
Hi sip-implementors,
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
m=audio 49170 RTP/AVP 0
b=AS:64
b=RS:800
b=RR:2400
m=video 51372 RTP/AVP 31
b=AS:256
b=RS:800
b=RR:2400
Assuming there is an unicast point-to-point session (A-B), above SDP is
the offer made by endpoint A, and endpoint B accepts the offer and agrees
with the same bandwidth values in the answer. "b=AS" modifier specifies the
RTP bandwidth, "b=RS" and "b=RR" modifiers specify the RTCP bandwidth for
sender and receiver respectively. In this point-to-point session, both A
and B are the sender.
My question is: does the b= line value specify the aggregate bandwidth
limit for the whole session (including endpoint A and B) or just the
bandwidth limit for local endpoint? Using audio media as an example, how
much RTP bandwidth should be allocated on the endpoint A? 64 or 32kbps?
Thanks,
Yong
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