[AVT] Profile: Silence suppression is allowed by default

[Stephen Casner <casner@acm.org>]

Over the past year we have had a few discussions on the AVT list
regarding silence suppression and comfort noise for various payload
formats.  Sharma Kanchinadham and Anoop Tripathi asked (separately)
for the optional MIME parameters for G723 and G729 audio to be changed
such that the default for "allow comfort noise frames" was NO.  Sharma
also asked for a similar parameter to be added for PCMU to indicate
whether silence suppression was allowed, and for the default to be NO.

I explained that we could not make these changes because silence
suppression has always been an assumed capability for all of the audio
encodings.  It's one of the fundamental reasons for using packets to
carry voice in the first place.  Changing the default now would create
a backwards incompatibility.  Furthermore, a receiver must always be
prepared to recover gracefully when it receives no data because
packets may be lost.

To make this more explicit, I added the following paragraphs at the
beginning of Section 4.1 in the A/V profile:

   Since the ability to suppress silence is one of the primary
   motivations for using packets to transmit voice, the RTP header
   carries both a sequence number and a timestamp to allow a receiver to
   distinguish between lost packets and periods of time when no data was
   transmitted.  Discontiguous transmission (silence suppression) MAY be
   used with any audio payload format.  Receivers MUST accept silence
   suppression unless its use is restricted by signaling specified
   elsewhere.  (Even if the transmitter does not suppress silence, the
   receiver must be prepared to handle periods when no data is present
   since packets may be lost.)

   Some payload formats define a "silence insertion descriptor" or
   "comfort noise" frame to specify parameters for artificial noise that
   may be generated during a period of silence to approximate the
   background noise at the source.  For other payload formats, a generic
   Comfort Noise (CN) payload format is specified in RFC 3389 [9].  When
   the CN payload format is used with another payload format, different
   values in the RTP payload type field distinguish comfort-noise
   packets from those of the selected payload format.

In addition, I added the following first sentence in the four places
where the sentences similar to the second one appear in G723 and G729*:

   Receivers MUST accept comfort noise frames if restriction of their
   use has not been signaled.  The MIME registration for G729 in
   RFC YYYY [7] specifies a parameter that MAY be used with MIME or SDP
   to restrict the use of comfort noise frames.

Are these acceptable?

                                                        -- Steve

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