On Sat, 21 Mar 2015, Steinar H. Gunderson wrote:
On Fri, Mar 20, 2015 at 05:03:16PM -0700, David Lang wrote:
1. If you mark packets as congested if they have ECN and drop them
if they don't, programmers will mark everything ECN (and not slow
transmission) because doing so gives them an advantage over
applications that don't mark their packets with ECN
I'm not sure if this is actually true. Somehow TCP stacks appear to be tricky
enough to mess with that the people who are capable of gaming congestion
control algorithms are also wise enough not to do so. Granted, we are seeing
some mild IW escalation, but you could very well make a TCP that's
dramatically unfair to everything else and deploy that on your CDN, and
somehow we're not seeing that.
It doesn't take deep mucking with the TCP stack. A simple iptables rule to OR a
bit on as it's leaving the box would make the router think that the system has
ECN enabled (or do it on your local gateway if you think it gives you higher
priority over the wider network)
If you start talking about ECN and UDP things are even simpler, there's no need
to go through the OS stack at all, craft your own packets and send the raw
packets
(OK, concession #2, “download accelerators” are doing really bad things with
multiple connections to gain TCP unfairness, but that's on the client side
only, not the server side.)
Based on this, I'm not convinced that people would bulk-mark their packets as
ECN-capable just to get ahead in the queues.
Given the money they will spend and the cargo-cult steps that gamers will do in
the hope of gaining even a slight advantage, I can easily see this happening
It _is_ hard to know when to
drop and when to ECN-mark, though; maybe you could imagine the benefits of
ECN (for the flow itself) to be big enough that you don't actually need to
lower the drop probability (just make the ECN probability a bit higher),
but this is pure unfounded speculation on my behalf.
As I said, there are two possibilities
1. if you mark packets sooner than you would drop them, advantage non-ECN
2. if you mark packets and don't drop them until higher levels, advantage ECN,
and big advantage to fake ECN
David Lang
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