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

Detlef Bosau detlef.bosau at web.de
Fri Jan 5 02:48:20 PST 2007


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.

So, I write it down once again, perhaps making my question more clear.

The parameters are examples, so please don´t kill me whether they don´t 
are "that typical".

Basic scenario:

Sender------(some Internet path) -----Router---(link)--------Receiver

The router may be replaced by a splitter, see below,

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 

Is this possible, or do I miss something?

Now to the scenario in detail:

Case 1: Router.

               10 MBps, 100 ms                    300 Bps, 10 ms

Baiscally the link behind the router has a "slow dialin-modem bandwidth" 

Imagine a 12000 bit packet traverling from Sender to Receiver.
What´s the RTT then? Let´s have a look:
Sender-Router: 1.2 ms serialization latency + 100 ms transport latency =
101.2 ms
Router-Receiver: 40 s serialization latency + 10 ms tranport latency =
30.01 s
Sender-Receiver: 40.1112 ms.

If there is one packet in transit in each direction, i.e. the line is
full in both directions, we would roughly have CWND/RTT = 2*12000 bit /80 s
= 300 bit/s and anything is fine.

Now lets replace the router by a splitter:

Case 2: Splitter.

               10 MBps, 100 ms                                     300
Bps, 10 ms                        (Bandwidth, latency)

If the Splitter is doing "dumb spoofing", i.e. any packets are
acknowledged immediately as they are received, the sender would see a
round trip time of about 200 ms. So even in the stop´n waite case, i.e.
CWND = 1*12000 bit, the throughput sender/splitter is
12000 bit / 200 ms = 60 bit / ms = 60 kbit/s. Which is obvioulsly to
fast for the 300 bps modem line to carry.

So, what should the splitter do?

1. stall the sender periodically using zero windo packets?
2. don´t care, doesn´t matter?
3. ??

(let´s ignore my own stupid ideas on this one for the moment ;-))


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