Re: [RRG] Are we solving the wrong problem?

[Brian E Carpenter <brian.e.carpenter@gmail.com>]

On 2008-03-06 04:58, Mark Handley wrote:
> On Wed, Mar 5, 2008 at 3:42 PM, marcelo bagnulo braun
> <marcelo@it.uc3m.es> wrote:
>> Hi Pekka,
>>
>>  El 05/03/2008, a las 6:39, Pekka Nikander escribió:
>>
>>
>>  ...
>>
>>  >
>>  > The biggest differences between your thinking and HIP seems to be
>>  > that HIP is implemented below TCP, so that it works also for UDP.  I
>>  > think it matches better with the IP semantics, as a single IP
>>  > address is (today) typically associated with a number of co-located
>>  > TCP and UDP end-points.
>>  >
>>  > Then there is also work specific to TCP, e.g Christian Huitema's
>>  > eTCP (or whatever it was called).
>>  >
>>  > I'm glad to see that more people come to the roughly same
>>  > conclusions, independently.  :-)
>>  >
>>
>>  right, but i think that what is missing in all these proposals is to
>>  understand the interaction with congestion control, which may be
>>  critical if they get deployed.
> 
> Absolutely.  The stable load-balancing properties come out of using
> multiple links simultaneously in such a way that congestion on one
> path causes more bytes to be sent the other way.  But crucially it
> must not move all the traffic the other way, or the system is unstable
> and can oscillate backwards and forwards between the multiple paths.
> This is the reason why I think you can solve the problem at L4 without
> changing the apps (or at the application layer, but only if you change
> the apps), but I don't think you get the same properties if you solve
> the problem below layer 4.

You certainly won't; but layer 4 already has to deal with unannounced
changes in layer 3 today, doesn't it? One question is whether a
hypothetical new pattern of layer 3 behaviour would require another
round of layer 4 algorithm R&D.

I have difficulty seeing how we can generically solve the problems
before us at layer 4, given that layer 4 is *not* the waist of the
hourglass, despite the arguments Jonathan makes in
draft-rosenberg-internet-waist-hourglass-00.txt. Making layer 4
aware of layer 3 behaviour, on the other hand, seems very
desirable to me. (To give a caricatural example, it would be
absurd if every time SCTP changed addresses, a layer 3 man-and-encap
scheme mapped them back to the same locators.)

    Brian


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