Re: Setting Initial Round-trip time


Mike Brescia (brescia@ccv.bbn.com)
01 Nov 86 10:31:54 EST (Sat)


Re: analogies

     ... like having a broken clock which is right twice a day rather than a
     clock which simply runs fast or slow and is never right. I will let you
     pick which analogy to apply!

The analogy I wish to apply would be that neither a broken clock nor a
miscalibrated clock will ever be right if I am trying to count apples or
oranges.

I'd like to attack the assumption that knowing the round trip time will
compensate for the fact that packets are allowed to be dropped in the system.

(Mom & Apple Pie division)

In the current IP model, a packet may be delayed (10 to 100 seconds and more
have been reported), or dropped because of a transmission failure or
congestion at some gateway or packet switch. If a packet is delayed, there
should be no retransmission because the second packet will only be delayed
behind the first. If it is dropped due to transmission failure, the
retransmission should be as soon as possible, so that the end-point hosts see
a minimum disruption. If it is dropped due to congestion, the retransmission
should be only as soon as you know the packet can get through or around the
congestion, otherwise you are only exacerbating it.

If you have arrived at a reasonable round trip time, and you have a packet
which has not been acknowledged after (some factor of) that time, can you
deduce which of the three things has happened to the packet? If you make the
wrong decision, you can make things worse for yourself or the community of
users.

(Blue Sky division)

If the Internet could provide a better guarantee of delivery, as once the
Arpanet did, retransmission would not need to be so widespread, and a good
measure of round trip time would not be so much of a panic. The Internet
model would need to be extended so that the effects of transmission losses and
congestion could be controlled.

    Mike



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