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

[Detlef Bosau]

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.


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 
enough.

Is this possible, or do I miss something?

Now to the scenario in detail:


Case 1: Router.


Sender
-----------------------------------------Router-------------------------Receiver
               10 MBps, 100 ms                    300 Bps, 10 ms

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

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.

Sender
-----------------------------------------Splitter-------------------------Receiver
               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 ;-))







Detlef