Subject: [rtcweb] On the meaning of b=

From: Justin Uberti <[email-masked]>
Date: Wed, 12 Nov 2014 21:20:26 -0800

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].

---

From: Randell Jesup <[email-masked]>
Date: Thu, 13 Nov 2014 11:49:14 -0500

On 11/13/2014 12:20 AM, Justin Uberti wrote:
> Does anyone know whether b= in an offer represents the value the 
> offerer wants to send, or receive? 
> 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

---

From: Justin Uberti <[email-masked]>
Date: Thu, 13 Nov 2014 11:10:03 -0800

+Suhas as FYI

Thanks. That's pretty clear.

On Thu, Nov 13, 2014 at 8:49 AM, Randell Jesup <[email-masked]>
wrote:

>> 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].
>>
>
> (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.
>
> --
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> rtcweb mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/rtcweb
>