[e2e] Why do we need congestion control?

Detlef Bosau detlef.bosau at web.de
Tue Mar 12 14:35:08 PDT 2013

Am 12.03.2013 21:13, schrieb Joe Touch:
> They are complementary:

Perhaps, the German word "komplementär" has a somewhat other connotation 
than "complementary" and perhaps, this confuses me here.
(I have to ask for some patience when my English is not as good as it 
should be, I'm no native speaker.)

>     FEC (including erasure codes) always completes in finite time,
>     but has a nonzero probability it will fail (i.e., that the
>     data won't get through at all)
>     ARQ always gets data through with 100% probability when
>     it completes, but has a nonzero chance of taking an
>     arbitrary long time to do so
> In both cases, you compensate for error by adding delay* (either 
> coding over longer sequences or waiting for feedback).

You cann summarize the whole thing perhaps in that way:

- We can transmit a packet in bounded time, however with a non zero 
probability of failure.
- We can reliably transmit a packet, however with a non zero probability 
of exceeding any bounded time.

> For fast links, ARQ's feedback loop can help.
> For high loss links, FEC's anticipation of loss can help.
> Neither one solves every problem completely, though.

No objection here.
> Joe
> *you can also compensate for delay by tolerating more error, if that's 
> the trade-off you're interested in.

Currently I'm focussed on TCP. So my focus is more the delay introduced 
by reliable transmission.---

Detlef Bosau
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