draft-ietf-sipping-media-policy-dataset: Bandwidth directionality

Worley, Dale R (Dale)

2011-05-09 18:57:47 UTC

Post by Worley, Dale R (Dale)
One problem I'm noticing is that the session-policy XML allows
specifying the bandwidth in each direction, but SDP does not (as far
as I can tell). How should we resolve this? Should we define
directional bandwidth modifiers in SDP? Or should we restrict
bandwidth in a stream based on the directionality attributes of the
stream?
That is, if the sending bandwidth is limited to 10,000 and the
receiving bandwidth is limited to 100,000, then an a=sendrecv stream
would be have b=10000 but an a=recv stream would have b=100000.

That was a misunderstanding on my part. The solution to bandwidth
directionality is that there are two SDPs, an offer and an answer, and
each has a separate b= line. The session-policy's bandwidth limit is
applied to the offer and answer as they are generated (or received).

Detail to remember: An SDP offer or answer describes what the sender
of the SDP wishes to *receive*. (Despite that a standalone SDP
describes what the sender will be sending.)

Detail to remember: The format of the b= line in this case is:
b=AS:<bandwidth in kbits/sec>

Thus, in the case of 10 kbits/sec sending and 100 kbits/sec receiving,
the SDP sent would have "b=AS:10" and the SDP received would have (or
would be modified to have) "b=AS:100" (as session-level attributes).
The session-policy XML would be:

<session-policy>
<max-bw direction="sendonly">10</max-bw>
<max-bw direction="recvonly">100</max-bw>
...
</session-policy>

Dale
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