Subject: [AVT] Re: Comments on draft-ietf-avt-tcrtp-05.txt

From: Stephen Casner <[email-masked]>
Date: Thu, 28 Feb 2002 23:31:58 -0800 (PST)

I have reviewed draft-ietf-avt-tcrtp-05.txt and found it to be
basically clear and well structured although a bit repetitive across
the different sections.  In addition to the comments recently
submitted by other reviewers, I found the following comments, mostly
editorial.

Content-related:

3.3
SOV-TSTAMP: Shouldn't this be 4 bytes for the absolute timestamp and 5
bytes total?  Does that mean the results in the bandwidth examples are
wrong?

SOV-IPID: Both SOV-TSTAMP and SOV-IPID count 1 byte for the extra
flags field.  If IPID and TSTAMP are both present, then the extra byte
gets counted twice.

Editorial:

1.2
"This header compression eliminates" change to
"This header compression reduces"

1.4
"CRTP is only suitable for point-to-point links" change to
"CRTP is only suitable for links with low delay and low loss"

2.3
"It is beneficial to transmit large multiplexed packets between two
points instead of many small packets" change to
"It is beneficial to transmit one large multiplexed packet between two
points instead of many small packets"

2.3.2
"required to reduce the header overhead to a minimum"
There is no minimum.  Perhaps this should say "to the desired level"

3.2 Augment and reorganize these paragraphs as follows:
Original:
  When the expected loss rate in a tunnel is relatively low (less than
  perhaps 5%), the robust operation (described in [ECRTP]) should be
  sufficient to ensure delivery of state changes.

  The optimal value of N will depend on the loss rate in the tunnel.

  A value of N=1 will protect against the loss of a single packet
  within a compressed session at the expense of bandwidth.  A value of
  N=2 will protect against the loss of two packets in a row within a
  compressed session and so on.  Higher values of N have higher
  bandwidth penalties.

Revised:
  When the expected loss rate in a tunnel is relatively low (less than
  perhaps 5%), the robust operation (described in [ECRTP]) should be
  sufficient to ensure delivery of state changes.  This robust
  operation is characterized by a parameter N which means that the
  probability of more than N adjacent packets getting lost on the
  tunnel is small.

  A value of N=1 will protect against the loss of a single packet
  within a compressed session at the expense of bandwidth.  A value of
  N=2 will protect against the loss of two packets in a row within a
  compressed session and so on.  Higher values of N have higher
  bandwidth penalties.

  The optimal value of N will depend on the loss rate in the tunnel.

3.3.4
"the benefits of multiplexing is not as great" change to
"the benefit of multiplexing is not as great"

"benefits of VOIP multiplexing might be" change to
"benefit of VOIP multiplexing might be"

							-- Steve

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Bruce A Thompson <[email-masked]>
Date: Fri, 01 Mar 2002 10:28:07 -0800

>bytes total?  Does that mean the results in the bandwidth examples are

You are right. The absolute time stamp should be 4 bytes. We'll change that in the draft.

This change has a negligible affect on the results of the bandwidth examples because the absolute time stamp is only sent at the beginning of a new talk spurt. In the example, I gave a talk spurt length of 1.5 seconds which is reasonable for typical speech and most VAD implementations. I combined that with a 20 msec packetization rate. This means that the extra overhead for the absolute time stamp will only be sent out every 20 / 1500 packets. That's what makes this factor negligible.

When I changed the SOV-TSTAMP variable in the example of section 3.3.1 from 3 to 5, the effective bandwidth changed from 14.43 kbits/sec to 14.45 kbits/sec.

>flags field.  If IPID and TSTAMP are both present, then the extra byte

Again, this factor makes a negligible difference. IPID and TSTAMP will only be present together at the beginning of a talk spurt. The ratio of packetization rate to talk spurt length makes this inaccuracy negigible.

>points instead of many small packets"

I have already changed this section because of comments from Ram Jagadeesan. This sentence is no longer in section 2.3. Hopefully, you'll find the modified section 2.3 to be more clear.

We will make the rest of your editoral changes below.

        Bruce T

>"benefit of VOIP multiplexing might be"

Bruce Thompson
Cisco Systems
email: [email-masked]
office: (408)527-0446
home office: (408)868-0112
San Jose, CA