Re: [rtcweb] On the meaning of b=

[Randell Jesup <[e-mail]>]

On 11/13/2014 12:20 AM, Justin Uberti wrote:
> Following up on the JSEP discussion on b= from Monday:
>
> Does anyone know whether b= in an offer represents the value the 
> offerer wants to send, or receive? 
> http://tools.ietf.org/html/draft-ietf-mmusic-sdp-mux-attributes-04#section-6.3 
> seems to indicate it's a send declaration, but I was previously under 
> the impression that it represents the amount to be received. RFC 4566 
> is ambiguous on the matter 
> [http://tools.ietf.org/html/rfc4566#section-5.8]

SDP is a description, but really we want to look at Offer-Answer.

In rfc 3264 section 6.1:

    The answerer MAY include a bandwidth attribute for any media stream;
    this indicates the bandwidth that the answerer would like the offerer
    to use when sending media.  The value of zero is allowed, interpreted
    as described in Section 5.


and section 7:

    The offerer SHOULD send media according to the value of any ptime and
    bandwidth attribute in the answer.


So, barring explicit instruction elsewhere for a b= type, those are 
receiver preferences.  This matches something I found out many years ago 
(likely in AVT archives somewhere) after I had presumed they were 
"amount I plan to send" (since I had a config for that, I hooked it up 
to b= as it wasn't clear in the SDP spec alone).  Later when I actually 
started caring about b= (TIAS, etc), I found the error.

-- 
Randell Jesup -- rjesup a t mozilla d o t com