[Cerowrt-devel] SQM: tracking some diffserv related internet drafts better

Dave Taht dave.taht at gmail.com
Thu Nov 13 12:26:39 EST 2014

This appears to be close to finalization, or finalized:


And this is complementary:


While wading through all this is tedious, and much of the advice contradictory,
there are a few things that could be done more right in the sqm system
that I'd like to discuss. (feel free to pour a cup of coffee and read
the drafts)

-1) They still think the old style tos imm bit is obsolete. Sigh. Am I
the last person that uses ssh or plays games?

0) Key to this draft is expecting that the AF code points on a single
5-tuple not be re-ordered, which means dumping AF41 into a priority
queue and AF42 into the BE queue is incorrect.

1) SQM only prioritizes a few diffserv codepoints (just the ones for
which I had tools doing classification, like ssh). Doing so with tc
rules is very inefficient presently. I had basically planned on
rolling a new tc and/or iptables filter to "do the right thing" to map
into all 64 codepoints via a simple lookup table (as what is in the
wifi code already), rather than use the existing mechanism... and
as nobody had nailed down the definitions of each one.

That said, I have not measured recently the impact of the extra tc
filters and iptables rules required.

1a) Certainly only doing AF42 in sqm is pretty wrong (that was left
over from my test patches against mosh - mosh ran with AF42 for a
while until they crashed a couple routers with it)

The relevant lines are here:


1b) The cake code presently does it pretty wrong, which is eminately fixable.

1c) And given that the standards are settling, it might be time to
start baking them into a new tc or iptables filter. This would be a
small, interesting project for someone who wants to get their feet wet
writing this sort of thing, and examples abound of how to do it.

2) A lot of these diffserv specs - notably all the AFxx codepoints -
are all about variable drop probability. (Not that this concept has
been proven to work in the real world) We don't do variable drop
probability... and I haven't the slightest clue as to how to do it in
fq_codel. But keeping variable diffserv codepoints in order on the
same 5 tuple seems to be the way things are going. Still I have
trouble folding these ideas into the 3 basic queue system fq_codel
uses, it looks to me as most of the AF codepoints end up in the
current best effort queue, as the priority queue is limited to 30% of
the bandwidth by default.

3) Squashing inbound dscp should still be the default option...

4) My patch set to the wifi code for diffserv support disables the VO
queue almost entirely in favor of punting things to the VI queue
(which can aggregate), but I'm not sure if I handled AFxx

5) So far as I know, no browser implements any of this stuff yet. So
far as I know nobody actually deployed a router that tries to do smart
things with this stuff yet.

6) I really wish there were more codepoints for background traffic than cs1.

Dave Täht


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