Re: [multipathtcp] potential MPTCP proxy charter item

Alan Ford <> Fri, 04 November 2016 11:02 UTC

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From: Alan Ford <>
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Date: Fri, 04 Nov 2016 11:02:02 +0000
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Subject: Re: [multipathtcp] potential MPTCP proxy charter item
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Hi Med,


> On 4 Nov 2016, at 10:12, <> <> wrote:
>> -----Message d'origine-----
>> De : Mirja Kühlewind []
>> Envoyé : vendredi 4 novembre 2016 10:34
>> À : BOUCADAIR Mohamed IMT/OLN; Alan Ford
>> Cc :
>> Objet : Re: [multipathtcp] potential MPTCP proxy charter item
>> Hi all,
>> first, I agree with Alan that such a signal does not need to/should not be
>> part of the MPTCP protocol.
> [Med] Defining another channel to carry the signal "I want to be MPTCP-proxied" outside the MPTCP normal channel will import the same set of hindrances we had with MPTCP design for middlebloxes traversal. The use of MPTCP to convey the "I want to be MPTCP-proxied" leverages on base MPTCP to detect "unfriendly MPTCP" in-path nodes. That's a pragmatic engineering design. 

The MP_CONVERT signal has multiple purposes. When signalled with a target IP address in explicit mode, it is a pure application-layer signal, and is separate from the MPTCP connection entering the MCP.

In implicit mode this is indeed at a slightly different layer, and yes, you are asking the network to do something. But it’s still not a necessary part of the MPTCP protocol to make this happen. This is similar to a transparent HTTP proxy, or to a NAT - neither requires any changes to HTTP or TCP/IP to work.

You could detect MPTCP SYNs and see if your signal is present. If you don’t want to be inspecting every MPTCP SYN in implicit mode, use an existing standard - I suggest RAO. Given this is a controlled environment, you shouldn’t have any deployment concerns here.

If RAO is not appropriate for some reason, I might be persuaded that a flag in the MP_CAPABLE that says “proxies should inspect this” could be added to the base protocol. However, the rest of the protocol - i.e. the MP_CONVERT signal - is application-layer and outside MPTCP.

Ultimately MPTCP Proxying is an application of MPTCP and should be defined as such.