[e2e] What's wrong with this picture?
lachlan.andrew at gmail.com
Fri Sep 11 14:41:24 PDT 2009
2009/9/11 David P. Reed <dpreed at reed.com>:
> I appreciate the your explanation, Dimitrios, is an attempt to explain why
> this might happen at layer 2. However, the service I am using is offered as
> an IP (layer 3) service. It is a *requirement* that layer 3 drop packets
> to signal congestion buildup.
No, IP is claimed to run over a "best effort" network. That means
that the router *may* discard packets, but doesn't mean that it
*must*. If the delay is less than the IP lifetime (3 minutes?) then
the router is within spec (from the E2E point of view). The dominance
of IP was exactly that it doesn't place heavy "requirements" on the
forwarding behaviour of the routers.
> Otherwise, the binding of this particular layer 2 transport (with elastic 10
> second queues) is something that is just WRONG to claim as an high-speed
> Internet service. (except for the case where the other end is 10 light
> seconds away).
No, it is not "WRONG" to claim a high bit-rate service is high-speed,
even if it has high latency. High-speed is not equivalent to
low-delay. However, I agree that it is a very bad design to have such
> The entire congestion control mechanism (and approximate fairness mechanism)
> of the Internet works on the assumption that congestion is signaled to those
> who are congesting the network - so they can back off, which they will in
> fact do.
If the networks have changed, we should change the assumptions that TCP makes.
In the old days, VJ changed TCP so that it would run over congested
un-reliable networks. If TCP is now being asked to run over congested
reliable networks, shouldn't we update TCP? There are many methods
which use delay as an indicator of congestion, as well as using loss.
(Should I plug Steven Low's FAST here?) We don't need anything very
fine-tuned in a case like this; just something very basic.
Of course, fixing TCP to work over any IP connection (as it was
intended) does not mean that the underlying networks should not be
optimised. As Lloyd said, we already have recommendations.
> I don't know who designed this system, but I can say that any serious ISP
> should NOT buy such equipment if it functions this way.
I agree, but that shouldn't stop the E2E principle from being applied,
and trying to design E2E protocols to cope with it.
Lachlan Andrew Centre for Advanced Internet Architectures (CAIA)
Swinburne University of Technology, Melbourne, Australia
Ph +61 3 9214 4837
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