[e2e] Comparing Linux qdiscs in lab conditions (paper)

[Detlef Bosau]

Am 13.02.2013 21:39, schrieb Joe Touch:
>
> FWIW, bufferbloat examples I've seen tend to focus on *upload* out of 
> the home. As you note, download isn't the issue as much.
>
> Try this:
>     - upload a video to youtube (or anywhere)
>     - ping somewhere else

Now, what will happen? The TCP socket will fully utilize the outgoing 
interface's buffer. And the ICMP packet has to enqueue itself at the end 
of the queue.

However, this is not a problem of too much buffering. This is a 
scheduling problem. Actually, VJCC extends TCP's self/_clocking_ /into a 
self/_scheduling_/.

As far as this works at all, it will achieve some kind of "statistically 
similar throughput" for greedy sources and long term flows.

However, from the perspective of the ICMP echo request, the whole issue 
appears as some kind of head of line blocking. (With numerous heads...)
>
> That can happen within a single machine (that was the first case I 
> heard about, and as anticipated the issue was in-kernel buffering) or 
> between two machines (if the home router has too much buffering and 
> too little brains to do proportional sharing).
>

Absolutely. However, this is no issue. What you describe is an IP stack 
that "works as designed".

> Less buffering - either in the OS or the router - sometimes helps 
> reduce delay because the greedy application backs off (due to losses), 
> is scheduled to transmit less often (when on the same machine), or 
> simply loses more packets.

I don't think that the described effect is a buffering issue. The 
problem is that we intertwined
- congestion control
- ressource sharing
- scheduling

into one complex algorithm - and now, we wonder why the whole thing 
works as designed.

And BTW, the congavoid paper only talks about TCP.  I don't remember 
that Van Jacobson or Michael Karels talked about "ping fairness" in this 
paper.... ;-)




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