[e2e] end2end-interest Digest, Vol 18, Issue 9

Joe Touch touch at ISI.EDU
Thu Aug 18 13:23:42 PDT 2005

Hash: SHA1

S. Keshav wrote:
> Detlef,
>>>The assertion made by ElRakbawy, Klemm and Lindemann is:
>>>Ass.1:   Network contention can be measured by measuring the RTT
>>>variance. A small variance is equivalent to a low degree of contention
>>>and a high variance is equivalent to a high degree of contention.
> ...
>>>Personally, I am in great doubt at this.
> RTT delay is influenced by the following factors:
> 1. Speed of light delay in the path
> 2. Retransmissions in the underlay
> 3. Queues in buffers due to
>     a. self queueing (queueing behind your own packets)
>     b. queueing due to cross traffic
> 4. The service rate of  within a switch fabric in a router
> 5. The size of the packet whose RTT is measured
> Variance in the RTT can be due to variation in any of the above.
> So, if you want to measure contention, you have to do some things cleverly
> at the sender:
>     keep packet size fixed
>     send at a `slow' rate
> and also assume that
>     paths are pinned
>     there are no retransmissions in the underlay

and that the underlay hops have stable RTTs; non-geosync satellites have
varying RTTs

and the points about pinning and retransmissions apply to the link
layers as well as to the network.

> If these hold, then you can link RTT variation to contention.

Yes - but when RTT variance goes up, it means that contention increased
or decreased. It seems more useful to use the first derivative of the
RTT than to use the variance, in that case.


> keshav
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