Hello,
Please see my comments inline.
Thanks,
Michael.
On Mar 13, 2009, at 5:51 AM, Salvatore Loreto wrote:
<snip>
Hi,
see my comments in line
thanks
Sal
Michael Froman wrote:
<snip>
[Sal] that is exactly the point. Throttle/force/average do not
break or modify in any way the retransmission mechanism.
[MJF] Perfect. I think your statement here is far more clear than
the last sentence of the paragraph you quoted above. More on this
below.
or that, for example, retransmissions modify the time between
forced NOTIFYs by the time taken for the retransmissions to
successfully complete the transaction? An example is the case
when the NOTIFY gets to the subscriber quickly, but the 200OK is
slow to make it back to the notifier. Start with a force value
of 2 seconds. The subscriber thinks the transaction is over and
is now expecting a forced NOTIFY 2 seconds from now. The
notifier has not completed the transaction. The next NOTIFY from
the notifier can happen anytime up to T1 + 2 sec.
I agree that throttle/force/average should not interfere with
the normal retransmission mechanism. However, I don't agree that
waiting for the transaction to complete before starting the
throttle/force/average timer is going to give much benefit (or
always predictable results).
[Sal] While I agree that this behaviour (waiting for the
transaction to complete) can not always give "predictable" results
and that is bad especially if you want use it to track the
location of a UA,
However making "throttle/force/average" timers independent (so to
say) from the the retransmission mechanism can lead to situation
where the timer fires and then you should send out a new NOTIFY
when there is still a retrasmission of a previous one. Any thought
on how to solve this situation without break the retransmission
mechanism?
[MJF] Sending out a new NOTIFY while there is still a
retransmission of a previous NOTIFY is definitely not an optimal
solution, but it does not break the retransmission mechanism. It
might violate a particular package's rules on overlapping NOTIFYs,
but that would be an implementer's obligation to handle correctly.
My primary objection is that the text in the draft seems to specify
exactly one way to deal with when to schedule the next NOTIFY.
Waiting for the response to the NOTIFY before scheduling the time
for the next NOTIFY is an expensive operation with virtually no
benefit under normal operation. If the network is working well, the
NOTIFY/200OK happens effectively immediately and there is no real
value in waiting to start the throttle/force/average timing until
the transaction closes. The timer can start when the NOTIFY goes out.
[Sal] right, but as usual in the ideal situation you do not have
problems. I do think that your suggestion of starting the timer when
the NOTIFY goes out, it a good idea and would improve the mechanism.
However before to change the text I would like to investigate and
discuss deeper the possible side effects.
If the network isn't working well, both sides are receiving and/or
sending messages they'd rather not deal with. Setting the timer
prior to the transaction completing doesn't make the situation
appreciably worse. Because the server cannot accurately determine
the client's view of when the transaction ends during network
misbehavior, this method is guaranteed to produce results confusing
to the user.
[Sal] if the server sends out a new NOTIFY before the previous one
is acknowledged could be worse in some situation, I am thinking of
the cases where a NOTIFY contains a state deltas. In this scenario
(that I think it will be the most common) if a NOTIFY is received
out of order the the Subscriber has to re-subscribe to force a
NOTIFY containing a complete state.
[MJF] Agreed. This is a decision the package implementor can make as
long as the draft doesn't specify the exact method for when to start
the timer for the next NOTIFY. In event packages that do not allow
overlapping NOTIFYs, the implementor would be free to devise a
mechanism for scheduling the next NOTIFY in a way that both satisfies
the event package rules and produces satisfactory throttle/average/
force behavior. Maybe this calls for adding a warning that when
creating the NOTIFY timer mechanism care should be taken to remain
consistent with the supported event package.
I would suggest the following text replace the last paragraph of
4.2.2 (and similar changes to 5.2.2 and 6.3.2):
Retransmissions of NOTIFY requests are not affected by the throttle,
i.e., the throttle only applies to the generation of new
transactions. In other words, the throttle does not in any way
break or modify the normal retransmission mechanism.
This allows the implementor to decide, possibly based on package
considerations and network traffic, how to schedule the time of the
next NOTIFY.
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