Re: [multipathtcp] Questions on https://tools.ietf.org/html/draft-boucadair-mptcp-plain-mode-09

Yoshifumi Nishida <nishida@sfc.wide.ad.jp> Tue, 15 November 2016 14:35 UTC

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From: Yoshifumi Nishida <nishida@sfc.wide.ad.jp>
Date: Tue, 15 Nov 2016 06:35:23 -0800
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Subject: Re: [multipathtcp] Questions on https://tools.ietf.org/html/draft-boucadair-mptcp-plain-mode-09
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Hi Med,
Sorry for my slow response. I put comments in lines.

On Sun, Nov 13, 2016 at 11:20 PM,  <mohamed.boucadair@orange.com>; wrote:
> Hi Yoshi,
>
> Please see inline.
>
> Cheers,
> Med
>
>> -----Message d'origine-----
>> De : Yoshifumi Nishida [mailto:nishida@sfc.wide.ad.jp]
>> Envoyé : jeudi 10 novembre 2016 20:21
>> À : BOUCADAIR Mohamed IMT/OLN
>> Cc : philip.eardley@bt.com; multipathtcp@ietf.org
>> Objet : Re: [multipathtcp] Questions on https://tools.ietf.org/html/draft-
>> boucadair-mptcp-plain-mode-09
>>
>> Hi Med,
>>
>> Thanks for the clarification. So, in my understanding,
>> off-path/on-path concept is like the followings.  (please point out if
>> I miss something)
>>
>> on-path (implicit mode) ... The client doesn't need to know the
>> presence of MCP. The adpp just specifies the server address for the
>> end point of mptcp connection.
>
> [Med] Yes, this is only for the initial subflow. Subsequent subflows will use an address of the proxy.
>
>>
>> off-path (explicit mode) ... The client needs to know the presence of
>> MCP. The app needs to use MCP's address for the destination. It also
>> need to get MCP's address (via DHCP or something else) and tells mptcp
>> stack to put it in MP_CONVERT option (via API?)
>
> [Med] this may be one way to implement it, but this is not my favorite one. I suggest the following:
> * The application does not need to know about the on-off/path modes.
> * This is handled at the MPTCP service level: if an address is configured via DHCP (or other means), it will behave in the Explicit mode.
> * MP_CONVERT option(s) are inserted by the MPTCP server accordingly.

Hmm. OK. But, DHCP is outside of mptcp. How the info from DHCP will be
delivered to mptcp without using API?

>> In this case, I think the off-path clients need to know that they are
>> off-path nodes and need to rely on MCP *before* communication starts.
>
> [Med] This is typically an information that is provided during network attachment, etc. For example,
> * a cellular device can retrieve the MCP information during the PDP context activation by means of a dedicated PCO Information Element.
> * a CPE will retrieve this information when it bootstraps.
>
> Note that  the client needs DNS information and other important service elements before to make use of a service. That's not particular to MPTCP.
>
>> But, I'm not very sure how they get such knowledge.
>
> [Med] Same as for DNS and other services (SBC, etc.). Do you foresee any particular issue?

i might miss something here. but, please let me explain a bit more.
BTW, I am looking at the figure 1 in the draft.
Let's say when H1 got address via DHCP, it found a MCP address in the
option. H1 doesn't know RM supports mptcp or not.
So, it use MCP as destination for mptcp connection. But, when RM
happens to be a mptcp node, this will be suboptimal.

To avoid this, I guess H1 needs to go into on-path mode (ignore MCP
info and use RM for destination) for this RM.
But, this might mean H1 will need to change modes based on
destinations and looks tricky.

--
Yoshi