Subject: Re: [Sigtran] M2PA V6: T7

From: [email-masked]
Date: Thu, 3 Oct 2002 11:23:45 -0400

I disagree. If we accept Howard's proposed modification (that T7 become a 
"per-message" timer), we WOULD fail the link in his example below, because 
message #3 was acknowledged only after 2.7 seconds, which would exceed T7 
(1 second).

The M2PA that is waiting for the ack doesn't know what is the cause of the 
2.7 second delay. It could have been caused by a problem at the peer M2PA, 
which would be a legitimate reason to fail the link. Or it could have been 
caused by the accumulation of normal packet delays or losses in the IP 
network, which would NOT be a legitimate reason to fail the link.

I think we should stick with the text in v6, which follows the letter of 
Q.703:

"A timing mechanism, timer T7, shall be provided which generates an 
indication of excessive delay of acknowledgement if, assuming that there 
are at least one outstanding MSU in the retransmission buffer, no 
new-acknowledgement has been received within a time-out T7."

Barry Nagelberg

"Brian F. G. Bidulock" <[email-masked]>
Sent by: [email-masked]
 09/26/2002 09:30 AM
 Please respond to bidulock

        To:     "May, Howard" <[email-masked]>
        cc:     [email-masked]
        Subject:        Re: [Sigtran] M2PA V6: T7

Howard,

Sounds good to me.

--brian

May, Howard wrote:                                   (Thu, 26 Sep 2002 
13:53:01)
> Excellent.  May I propose the following modification to the spec.
> 
> The per User Data message Timer T7 provides an indication of excessive 
delay
> of acknowledgement.  If the following conditions are true:
> (a) A User Data message is not acknowledged within the span of T7.
> (b) The remote M2PA is not in a Busy condition.
> 
> Howard
> 
> > -----Original Message-----
> > From: Brian F. G. Bidulock [mailto:[email-masked]]
> > Sent: 26 September 2002 13:42
> > To: May, Howard
> > Cc: [email-masked]
> > Subject: Re: [Sigtran] M2PA V6: T7
> > 
> > 
> > Howard,
> > 
> > Well, no.  T7 in Q.703 is not intended to handle acknowledgement
> > delays in the receiver.  The receiver is expected to acknowledge
> > messages immediately upon receipt.  In Q.703 if messages 1, 2
> > and 3 are sent in a group, they are expected to be acknowledged
> > at the same rate at which they were emitted: i.e., no 900 ms
> > delay between acknowledgements.  Also, in the Q.703 queueing model,
> > one-way propagation delays are never greater than about 30ms for
> > the basic method.  In this way, T7 is used as a guard timer.
> > If RTT is variable and does not satisfy the relationship that
> > PD << T7 then you will not get the results expected by the
> > Q.703 model.
> > 
> > For M2PA it might be better, as Jeffrey puts, to view T7 as a
> > per-message timer as the PD << T7 relationship cannot be
> > guaranteed.
> > 
> > --brian
> > 
> > May, Howard wrote:                                 (Thu, 26 
> > Sep 2002 13:25:27)
> > > So in effect there is no bound on the length of time it may take to
> > > acknowledge a message.  My previous example demonstrated 
> > that it took 2.7
> > > seconds to acknowledge Data message three even though T7 is 1s.
> > > 
> > > > -----Original Message-----
> > > > From: Brian F. G. Bidulock [mailto:[email-masked]]
> > > > Sent: 26 September 2002 13:05
> > > > To: May, Howard
> > > > Cc: [email-masked]
> > > > Subject: Re: [Sigtran] M2PA V6: T7
> > > > 
> > > > 
> > > > Howard,
> > > > 
> > > > Close enough.
> > > > 
> > > > --brian
> > > > 
> > > > May, Howard wrote:                                     (Thu, 
> > > > 26 Sep 2002 12:24:59)
> > > > > Hi,
> > > > > 
> > > > > Please can you clarify my understanding of T7 by commenting 
> > > > on the following
> > > > > sequence of events as seen by one end of an M2PA link.
> > > > > 
> > > > > Let us pretend T7 is configured with a value of second.
> > > > > ***********************
> > > > > The M2PA link comes into service.
> > > > > Data Message 1 is transmitted and T7 is started
> > > > > Data Message 2 is transmitted
> > > > > Data Message 3 is transmitted
> > > > > 900 ms passes
> > > > > Data Message 1 is acknowledged and T7 is restarted
> > > > > 900 ms passes
> > > > > Data Message 2 is acknowledged and T7 is restarted
> > > > > 900 ms passes
> > > > > Data Message 3 is acknowledged
> > > > > ************************
> > > > > 
> > > > > Thank you
> > > > > 
> > > > > Howard
> > > > > 
> > > > > _______________________________________________
> > > > > Sigtran mailing list
> > > > > [email-masked]
> > > > > https://www1.ietf.org/mailman/listinfo/sigtran
> > > > 
> > > > -- 
> > > > Brian F. G. Bidulock
> > > > [email-masked]
> > > > http://www.openss7.org/
> > > > 
> > 
> > -- 
> > Brian F. G. Bidulock
> > [email-masked]
> > http://www.openss7.org/
> > 

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/
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