Re: [PMOL] SIP Metrics draft: Timers wording

["Daryl Malas" <D.Malas at cablelabs.com>]

Thanks Al.  I will incorporate this and the other changes discussed in
Philly into the draft.  Look for a new revision soon.

--Daryl 

-----Original Message-----
From: pmol-bounces at ietf.org [mailto:pmol-bounces at ietf.org] On Behalf Of
Al Morton
Sent: Tuesday, April 22, 2008 9:30 PM
To: Daryl Malas; pmol at ietf.org; Greg Dowd
Subject: Re: [PMOL] SIP Metrics draft: Timers wording

At 08:04 PM 3/25/2008, Daryl Malas wrote:
>Thank you for the extensive feedback at the IETF conference in Philly 
>regarding the SIP Performance Metrics draft.  During the session, there

>were volunteers to provide me some verbiage for the draft regarding the

>variability of timers relative to the draft.  For those who 
>volunteered, please send me your suggested wording, and I will include 
>it in my next revision of the draft.

I took a shot at this, further suggestions encouraged, Al


      2. Terminology

         ...

         Time Begin (TB), remove, replaced by definition below.

         Time Stop (TS), remove, replaced by definition below.

      3. Time Interval Measurement and Reporting

         Many of the metrics defined in this memo utilize a clock to
assess
         the time interval between two events. This section defines
time-
         related terms and reporting requirements.

         T1 - start time

         This is the time instant (when a request is sent) that begins a
         continuous time interval.  T1 occurs when the designated
request has
         been processed by the SIP application and the first bit of the
         request packet has been sent from the proxy or UA (and is
externally
         observable at some logical or physical interface).

         T1 represents the time at which each request-response test
begins,
         and SHALL be used to designate the time-of-day when a
particular
         measurement was conducted (e.g., The Session Request Delay at
"T1"
         and <some specific UA interface> was measured to be X ms.)

         T4 - end time

         This is the time instant that concludes the continuous time
interval
         begun when the related request is sent.  T4 occurs when the
last bit
         of the designated response is received by the SIP application
at the
         requesting device (and is externally observable at some logical
or
         physical interface).

         Note:  The designations T2 and T3 are reserved for future use
at
         another interface involved in satisfying a request.

         Section 10.1 of [RFC2330] describes time-related issues in
         measurements, and defines the errors that can be attributed to
the
         clocks themselves. These definitions are used in the material
below.

         Time of Day Accuracy

         As defined above, T1 is associated with the start of a request
and
         also serves as the time-of-day stamp associated with a single
         specific measurement. The time offset [RFC2330] is the
difference
         between T1 and a recognized primary source of time, such as UTC
         (offset = T1-UTC).

         When measurement results will be correlated with other results
or
         information using time-of-day stamps, then the time clock that
         supplies T1 SHOULD be synchronized to a primary time source, to
         minimize the time offset.

         Time Interval Accuracy

         The accuracy of the T4-T1 interval is also critical to maintain
and
         report. The relevant definition from [RFC2330] is "skew": the
         difference between time offsets at T1 and T4 is the error for
the
         measurement interval associated with the clock's skew.

         A stable and reasonably accurate clock is needed to make the
time
         interval measurements required by this memo. The clock error
SHOULD
         be constrained to less than +/- 1 ms, implying 1 part per 1000
         frequency accuracy for a 1 second interval.

         There are several other important aspects of clock operation:

         1. Synchronization protocols require some ability to make
adjustments
            to the local clock. However, these adjustments (clock steps
or
            slewing) can cause large errors if they occur during the T1
to T4
            measurement interval. Clock correction SHOULD be suspended
during
            a T1 to T4 measurement interval, unless the time interval
accuracy
            requirement above will be met.

         2. If a free-running clock is used to make the time interval
            measurement, then value of T1 reported SHOULD be derived
from a
            different clock that meets the time of day accuracy
requirements
            described above.

         3. The physical operation of reading time from a clock may be
            constrained by the delay to service the interrupt.
Therefore, the
            accuracy of the time stamp read at T1 or T4 always includes
the
            interrupt delay, and this source of error SHOULD be known
and
            included in the error assessment.




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