Re: [Bloat] sigcomm wifi

[Michael Welzl <michawe@ifi.uio.no>]

On 21. aug. 2014, at 08:52, Eggert, Lars wrote:

> On 2014-8-21, at 0:05, Jim Gettys <jg@freedesktop.org> wrote:
>> ​And what kinds of AP's?  All the 1G guarantees you is that your bottleneck is in the wifi hop, and they can suffer as badly as anything else (particularly consumer home routers).
>> 
>> The reason why 802.11 works ok at IETF and NANOG is that:
>>  o) they use Cisco enterprise AP's, which are not badly over buffered.

I'd like to better understand this particular bloat problem:

100s of senders try to send at the same time. They can't all do that, so their cards retry a fixed number of times (10 or something, I don't remember, probably configurable), for which they need to have a buffer.

Say, the buffer is too big. Say, we make it smaller. Then an 802.11 sender trying to get its time slot in a crowded network will have to drop a packet, requiring the TCP sender to retransmit the packet instead. The TCP sender will think it's congestion (not entirely wrong) and reduce its window (not entirely wrong either). How appropriate TCP's cwnd reduction is probably depends on how "true" the notion of congestion is ... i.e. if I can buffer only one packet and just don't get to send it, or it gets a CRC error ("collides" in the air), then that can be seen as a pure matter of luck. Then I provoke a sender reaction that's like the old story of TCP mis-interpreting random losses as a sign of congestion. I think in most practical systems this old story is now a myth because wireless equipment will try to buffer data for a relatively long time instead of exhibiting sporadic random drops to upper layers. That is, in principle, a good thing - but buffering too much has of course all the problems that we know. Not an easy trade-off at all I think.

I have two questions: 1) is my characterization roughly correct?
2) have people investigated the downsides (negative effect on TCP) of buffering *too little* in wireless equipment? (I suspect so?)  Finding where "too little" begins could give us a better idea of what the ideal buffer length should really be.

Cheers,
Michael