Re: [multipathtcp] potential MPTCP proxy charter item

Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be> Sun, 06 November 2016 21:47 UTC

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To: =?UTF-8?Q?Mirja_K=c3=bchlewind?= <mirja.kuehlewind@tik.ee.ethz.ch>
From: Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be>
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Date: Sun, 6 Nov 2016 22:46:53 +0100
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Subject: Re: [multipathtcp] potential MPTCP proxy charter item
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Mirja,
>
> I really don’t buy our argument why you should develop such a mechanism for MPTCP only compared to a generic solution that could be used by all (existing and future) TCP extensions. MPTCP is not the only extension that could make use of this (tcpinc is another example) and I’m sure there will more mechanisms in future that would benefit from additional option space in the SYN.
>

I'd be very happy if TCPM came up with a solution to this problem that 
works for any TCP extension, but progress in this domain as been very 
slow. On the other hand, the deployment of MPTCP could be an opportunity 
to rethink the way options are negotiated in the SYN and improve that 
for the future without being forced to support 30 years of history of 
TCP stacks.

Tomorrow's hosts will very likely be multihomed and if not, they will 
probably use multiple IPv6 addresses. On such hosts, the ability to 
switch from one address to another will a common requirement. MPTCP 
handles those changes in a clean way.

As a working group, let us imagine that MPTCP becomes the default 
reliable transport protocol by year 202x and that all hosts use MPTCP. 
Having this dream in mind, what could we do today to ensure that MPTCP 
is an clean and evolvable protocol. Here are a few ideas :

- Ensure that the option space in the SYN is large enough to carry the 
options that are required, e.g. to exchange key material for tcpinc

- Why not consider that sending MP_CAPABLE implies other options that 
are negotiated independently today, e.g. :

    - WScale option (would it make sense in 202x to use a 64KB window ?)
    - TCP SACK (would it make sense in 202x to create a connection 
without SACK)
    - Although timestamp option is often negotiated today and required 
by RFC7323 for PAWS, MPTCP does not require PAWS since it uses 64bits DSNs
    - Could we include the TFO negotiation inside the MP_CAPABLE option ?

This would reduce option space in the SYN and simplify the MPTCP stack.




Olivier