[e2e] A simple scenario. (Basically the reason for the sliding window thread ; -))

Joe Touch touch at ISI.EDU
Fri Jan 5 08:38:58 PST 2007

Detlef Bosau wrote:
> Hi.
> When I asked whether wie did sliding window in the Internet, I basically
> had a quite simple scenario in mind and basically I would like a comment
> on this one.
> The basic question is whether the use of a splitter may shorten the RTT
> seen by the sender to that degree, that the appropriate rate cannot be
> achieved by a sliding window protocol even if CWND were set to 1 MSS,
> the sender must hence be stalled from time to time to have the rate slow
> enough.

The window doesn't by itself determine rate; it's ACK clocking that
does. In high BW*delay product nets, the same stalling happens - you
send data, get an ACK, send more, get ACKs of that, etc - and the data
keeps bunching up at the source.

I.e., ACK clocking works only when the data-ACK look experiences a
bottleneck. When it doesn't, things bunch up, and TCP doesn't 'match
rates' at all.

FWIW, the same thing happens when the receiver application doesn't drain
the incoming data fast enough. The receive buffers fill up, and the
sender is stalled. The same thing is happening here.

> So, what should the splitter do?
> 1. stall the sender periodically using zero windo packets?
> 2. don´t care, doesn´t matter?
> 3. ??

Splitters are bad for other reasons, but as you said, let's ignore them
for this discussion..

It seems like the dominant effect is exactly what you expect - the
endpoint (the splitter, really) isn't experiencing the bottleneck, but
it's "application" (the receiver on the modem) is too slow. So you get
bursty 'scheduling' of the sender based on availability of buffers at
the (IMO, real, or at least effective) receiver.


Joe Touch
Sr. Network Engineer, USAF TSAT Space Segment

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