I didn't notice anyone mention the particular significance of port 9. It
is the "discard" port. I'm guessing the choice of this port isn't an
accident. Rounding down to port 8, while correct if no a=rtcp is
present, may not achieve the intended effect.
I'm curious *why* port 9 is being used in the answer.
Paul
[EMAIL PROTECTED] wrote:
From: "Iliff, Tina" <[EMAIL PROTECTED]>
Hi! I have encountered an application in the "real world" which
provides port 9 in the SDP answer in the 200OK. The SDP m line
indicates RTP/AVP. I recognize that port 9 is acceptable if the SDP m
line indicates BFCP. My question is: Is port 9 acceptable for
RTP/AVP. [...]
P.S. RFC2327 noted a range of 1024-65535 for RTP ports; however,
RFC4566 does not indicate a specific range.
Since RFC 4566 is the latest standard and does not restrict the port
number to a particular range, you should not restrict the port number.
In addition, unless there is a specific functional reason, an
application should never place a restriction on a port number supplied
by the other end, regardless of the standard.
More interesting is RFC 3550, in which the use of even/odd port number
pairs is described for the corresponding RTP and RTCP streams:
11. RTP over Network and Transport Protocols
RTP relies on the underlying protocol(s) to provide demultiplexing of
RTP data and RTCP control streams. For UDP and similar protocols,
RTP SHOULD use an even destination port number and the corresponding
RTCP stream SHOULD use the next higher (odd) destination port number.
For applications that take a single port number as a parameter and
derive the RTP and RTCP port pair from that number, if an odd number
is supplied then the application SHOULD replace that number with the
next lower (even) number to use as the base of the port pair.
So by default, specifying port 9 means that RTP is on port 8 and RTCP
is on port 9. But SDP also allows the RTCP port to be specified
separately. RFC 4566:
5.14. Media Descriptions ("m=")
If non-contiguous ports are used or if they don't follow the
parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:"
attribute MUST be used.
And continuing with RFC 3550:
For applications in which the RTP and RTCP destination port numbers
are specified via explicit, separate parameters (using a signaling
protocol or other means), the application MAY disregard the
restrictions that the port numbers be even/odd and consecutive
although the use of an even/odd port pair is still encouraged.
Dale
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