congestion control 2, cc2Sols.pdf, question on page 43

congestion control 2, cc2Sols.pdf, question on page 43

par Tiannan Sha,
Number of replies: 2

Hi,

I found this question very confusing. Maybe I misunderstood the bufferbloat problem. But as far as I understand, as long as my routers have big buffers, there won't be any packet loss due to congestion control? The packets arrived later will just wait in the buffers and won't get dropped? I don't think ECN has any impact on whether there are packets dropping due to congestion?

Thanks,

Tiannan

In reply to Tiannan Sha

Re: congestion control 2, cc2Sols.pdf, question on page 43

par Marc Fabien Egli,
Hello,

A router can set the ECN flag of packets if it detects too much load in its buffer. This means that the packet would not be dropped but it would still inform about congestion in the router. Thus, the sender will slow down and avoid overloading the buffer which would result in packet loss due to congestion.

Marc
In reply to Tiannan Sha

Re: congestion control 2, cc2Sols.pdf, question on page 43

par Jean-Yves Le Boudec,

Even a large buffer is never large enough to prevent packet drop. 

Unless ECN is used, congestion control needs packet losses to be activated. 

Consider for example a buffer with output speed = 1 Gb/s, and assume it is used by 2'000 TCP connections used for streaming video at 1Mb/s each. The total incoming rate is 2 Gb/s, which is 1Gb/s more than the outgoing rate; therefore the buffer fills by 1 Gb every second. Say that the buffer size is 1 GB (which is large for a network buffer). After 8 seconds the buffer is full and will start to drop packets. 

JYLB