Subject: Re: Comments on draft-ietf-ippm-ipdv.05

From: Al Morton <[email-masked]>
Date: Thu, 07 Dec 2000 21:01:54 -0500

IPPM folks,

There has been a pause in the discussion on delay variation
since Phil Chimento offered to begin a serious re-write.  
I'd like to offer my support of Mike Pierce's position, that measuring 
a set of delays and determining their variation provides 
information beyond what is available with the current metric
(the difference in delay between consecutive packet pairs).
I've got the impression that Phil will be attempting to include
both these metrics in the text of the new draft 
(is that right, Phil?).

Assuming both these metrics are described in ipdv-06, it may help
to capture the advantages of each, providing guidance to future
readers. Much of this material could be distilled from the recent 
messages. Since my past efforts have been mostly associated with
ITU-T and T1A1, I'll change hats and highlight what I consider to 
be two key advantages of the current ipdv metric:

Ruediger Geib's message reminds us that the pair-wise delay differences
can be calculated without synchronized measurement clocks.  When all the
code and the wires come together (implementation time), it can be very
comforting to know that this measurement will be useful even if others
must be discarded due to clock synchronization problems.

Another point for ipdv is its similarity to the calculation of
interarrival jitter measurement in RTCP reports.  RFC 1889 (RTP)
gives the calculation of interarrival jitter in section 6.3.1, with
a sample implementation in an appendix.  Although there are some 
differences in method (RTCP interarrival jitter uses order of arrival, 
as opposed to sending sequence with ipdv) there should be a favorable 
comparison between a "smoothed jitter" computed using ipdv singletons
and the RTCP reports of jitter in many circumstances (If many packets 
were re-ordered, the results would probably not agree). It seems 
valuable to have a metric that can be related to measurements
made by user's endpoints.

Shortly, the ITU-T will be having discussions on performance parameters for
delay variation.  I'm convinced that both metrics should be included
in the appropriate Recommendations there as well.

Regards,
Al Morton

---

From: Al Morton <[email-masked]>
Date: Tue, 12 Dec 2000 01:32:16 -0500

At 10:16 AM 11/15/00 +0100, Geib, Ruediger wrote:
>Hi IPPMers (and to spare [5] from being deleted ;-)
>
>As "unknown reference" of above draft I'd like to point out my view on
Mike's comments:
<snip>
>
>Finally I'd like to ask Mike to provide access to the following references
quoted by him:
>Y.1540 and Y.1541.
>Distributiuon of these documents to the IPPM WG was promised during the
last meeting. I however never saw any information on the list or on the
archive.
Ruediger's request for a copy of Draft ITU-T Rec Y.1541
can't be satisfied by the ITU directly, but since T1A1 
discusses this as well, the 11/2000 version is posted as below:

ftp://ftp.t1.org/pub//t1a1/t1a1.3/0a130560.doc
ftp://ftp.t1.org/pub//t1a1/t1a1.3/0a130560.ps

Note that draft Y.1541 is very much a work-in-progress,
especially some of the values in Table 1, and the 
parameter definition for delay variation.

I'm sure that a near-final version of I.380 -> Y.1540 exists
on T1A1's site, and when I find it I'll post that URL, too.

Al