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

Agarwal, Anil Anil.Agarwal at viasat.com
Fri Jan 5 05:25:01 PST 2007

Detlef wrote:
> 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.


Here is a more practical example -

    100 Mbps, 10 us (LAN)      1 Mbps, 300 ms (geo-satellite)

A cwnd of 1 segment of 1500 bytes will achieve roughly 
1500 * 8 / (20 + 120) Mbps
i.e., 85 Mbps, on the LAN segment,
which is much higher than the satellite link rate.

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

Since, the network can support a maximum of 1 Mbps, 
on average, the sender should send 1 segment every 
1500 * 8 / 1000000 seconds
i.e, every 12 ms.
So, stalling the sender using zero window Ack packets is an 
appropriate solution, which does not require any changes to the 
sender TCP stack. The cwnd value may be 1 segment or larger, 
it does not matter.
Anil Agarwal
ViaSat Inc.
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