FastStart and payload size

Paul Long plong at SMITHMICRO.COM
Fri Mar 10 15:45:17 EST 2000


(Worries me, too. I don't like Fast Connect. "Half-baked" comes to mind.
Apologies to whoever the authors are...)

Exactly, the caller has to make a series of educated guesses regarding
outgoing caps, based on the implementor's experience and common sense.

(Wait a minute. Let me get on my high horse... okay, I'm on... :-)
These fields have always indicated _maximum_ framing. For example, if I
indicate that I can receive packets containing as many as 20 frames per
packet (for G.711, a frame happens to be 1ms in duration), that means you
can send me packets containing 1, 20, 10, 2, 15, 8, etc., frames. Mix and
match. Put a different number of frames in each successive packet if you
want to. It doesn't matter. The _only_ constraint is that no packet may
contain more than 20 frames. It has never meant that you can only send me 20
frames per packet for each and every packet, which for G.711 would be 20ms
packets all the time. However, several vendors apparently have
"extra-normative" technical constraints that they must deal with regarding
packet size, i.e., DSPs that require being fed chunks of fixed-size audio,
e.g., 20ms packets. But there is no way in H.245 and therefore H.323 to
indicate a fixed-size packet. These vendors should either "fix" their DSPs
or perform re-framing between transport and decoder. In our software-only
implementation, we have a FIFO into which we shove all incoming audio
frames. Once frames are installed in the FIFO, it doesn't matter where the
packet boundary was. The decoder just processes frames in the order they
were received. I'm sure that there are really good technical reasons why
some implementations require fixed-size audio packets. Based on our perhaps
too simplistic implementation, however, this has always puzzled me.

Paul Long
Smith Micro Software, Inc.

-----Original Message-----
From: Francois Audet [mailto:audet at NORTELNETWORKS.COM]
Sent: Friday, March 10, 2000 1:55 PM
Subject: Re: FastStart and payload size

Hum... That kind of worries me.

It means that the burden is on the originating side to "guess" what
the terminator is likely to support.

I also agree that practically speaking, implementations normally
support multiples of 10 ms as the payload size (with the notable
exeption of Netmeeting).

However, technically speaking, if you advertize 30 ms as the maximum
payload size you can support, it implies that in order to comply with
the H.323 specification, you would support any value between 0 ms
(whatever it means) and 30 ms. I don't think many implementations could
do this. But that is a separate issue.

In any case, if we all understand that the originator is responsible
for accurately describing a decent subset of the capabilities it can
support, we probably should describe this in the spec because I
am convinced that most people will simply put the maximum payload
size they can support...

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