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

S. Keshav keshav at uwaterloo.ca
Thu Aug 18 09:15:46 PDT 2005


>> 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

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


More information about the end2end-interest mailing list