[GLLUG] Avoid Virgin Media

Dimitrios Siganos dimitris at siganos.org
Fri Dec 21 20:49:55 UTC 2012

On 21/12/12 20:38, Nix wrote:
> On 21 Dec 2012, John Edwards spake thusly:
>> One client had a leased line, 10 Mbit/s on a 100 Mbit/s fibre.
>> Pretty standard, except that instead having their routers queue
>> the traffic over 10 Mbit/s they dropped the packets instead.
> Er, isn't that exactly what you're supposed to do in TCP/IP as a
> congestion indicator? Queueing packets when you get congested is bad,
> bad, bad.

Yes, TCP would expect the router to simply drop packet when there is
congestion, and it has the mechanism to detect that, and it also
throttle itself to ease the situation. However, TCP is used these in
ways it wasn't designed to operate and the ISPs have no choice but to
allow for bad usage of TCP. TCP is not the problem, the applications
that wrongly use TCP are the problem.

For example, on one project I worked on, a wifi sensor was pumping data
to a server over a TCP socket and through a very noisy Wifi environment.
The customer was complaining that the wireless module (that's what I was
working on) was not good because they often lost their TCP connection
and that didn't happen over ethernet.

The problem turned out to be their client/server protocol running over
TCP. They had timeouts as low as 4 seconds between their handshake
stages. If a couple of packets in a row got lost (and in a noisy Wifi
environment that's not unlikely at all), TCP would immediately back off
for a few seconds and their server would timeout and the socket would
get closed.


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