Re: [multipathtcp] potential MPTCP proxy charter item

Joe Touch <touch@isi.edu> Wed, 09 November 2016 16:01 UTC

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From: Joe Touch <touch@isi.edu>
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Date: Wed, 9 Nov 2016 08:00:44 -0800
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Subject: Re: [multipathtcp] potential MPTCP proxy charter item
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It might not be for MPTCP, but it's a huge issue for TCP.

Which begs the question of whether MPTCP would then be diverging so far
from TCP that it isn't TCP anymore.

Joe


On 11/9/2016 7:58 AM, mohamed.boucadair@orange.com wrote:
>
> Re-,
>
>  
>
> Thank you for the clarification.
>
>  
>
> Noted. As I don’t have concrete cases where there will be a need to
> pick a particular port, I’m not sure this is a big issue.
>
>  
>
> Cheers,
>
> Med
>
>  
>
> *De :*Joe Touch [mailto:touch@isi.edu]
> *Envoyé :* mercredi 9 novembre 2016 16:50
> *À :* BOUCADAIR Mohamed IMT/OLN
> *Cc :* Mirja Kühlewind; Olivier.Bonaventure@uclouvain.be;
> multipathtcp@ietf.org
> *Objet :* Re: [multipathtcp] potential MPTCP proxy charter item
>
>  
>
>  
>
>  
>
> On 11/9/2016 7:37 AM, mohamed.boucadair@orange.com
> <mailto:mohamed.boucadair@orange.com> wrote:
>
>      
>
>     TCP is asked to connect once. You seem to now expect it to try
>     twice - TCP cannot (it can't pick ports by itself) and user land
>     cannot be expected to or may not be able to.
>
>     [Med] How that is different from establishing many TCP subflows
>     between the same pair of IP address, using distinct source port
>     numbers:
>     https://tools.ietf.org/html/draft-ietf-mptcp-experience-07#section-3.4?
>
>
> The difference is that the ports of the primary flow are no longer
> under your control.
>
> I don't know if there's a workaround for MPTCP, but this is exactly
> why this technique cannot work for TCP in general. Further, if it
> doesn't work for TCP, then MPTCP is no longer TCP in some cases (which
> means it can't be used as such interchangeably).
>
>
>  
>
>    This subflow manager creates N subflows between the same pair of IP
>
>    addresses.  The N subflows are created by the client and differ only
>
>    in the source port selected by the client. 
>
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>   
>
> Since the MPTCP Proxy in UC#2 is tracking TCP connections (TCP/MPTCP
> glue), it is the responsibility of the MPTCP Proxy to select the
> source port to use. This is similar to a NAT function. We are
> leveraging on the TCP BEHAVE recommendations.
>
>  
>
>  It might not be able to use the source port it wants either.
>
> [Med] The only case I see is when all external ports are exhausted.
> This is a very very extremely case.
>
>
> I'm speaking of a case where you want/need to pick both port numbers.
>
> [Med] Acked.
>
>
>
> Joe
>