Re: [multipathtcp] Multipath TCP Address advertisement 2/5 - Reliability

Yoshifumi Nishida <nishida@sfc.wide.ad.jp> Thu, 04 August 2016 06:36 UTC

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From: Yoshifumi Nishida <nishida@sfc.wide.ad.jp>
Date: Wed, 3 Aug 2016 23:36:26 -0700
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Subject: Re: [multipathtcp] Multipath TCP Address advertisement 2/5 - Reliability
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Hi Fabien,

On Wed, Aug 3, 2016 at 8:47 AM, Fabien Duchêne <fabien.duchene@uclouvain.be>
wrote:

> Hello,
>
> As agreed in Berlin during IETF96, I'm sending a series of emails to
> discuss the different contributions proposed
> inhttps://datatracker.ietf.org/doc/draft-duchene-mptcp-add-addr/
>
> This is the part 2/5 : reliability.
>
> In RFC6824, ADD_ADDR options can be attached to segments carrying data
> or pure acknowledgements.
> In practice, notably given the length of ADD_ADDR with IPv6 addresses
> and the HMAC, it is very likely that they will be often sent as pure
> acknowledgements.
> This implies that ADD_ADDR are sent unreliably, which could be
> problematic when the ADD_ADDR is required to allow the establishment of
> additional subflows, as in load balancing scenarios.
> We propose to rely on the "E" (Echo) flag in the ADD_ADDR option.
> This echo flag is used to acknolwedge a received ADD_ADDR by echoing it.
> If the acknowledgement is not received, the ADD_ADDR option will be
> retransmitted up to N times.
>

I have several comments on this. Please let me know if I miss something.

1: Do we really need ADD_ADDR reliability in all cases? An end node might
want to send ADD_ADDR for 'just in case' rather than for "I want you to use
this"
BTW, I'm not sure if the draft tries to replace the current ADD_ADDR or to
propose an additional ADD_ADDR option.

2: The sender cannot be sure whether the info in ADD_ADDR will be used or
not. It depends on the peer's decision.
I might prefer re-transmitting ADD_ADDR up to N times when the sender
doesn't receive MP_JOIN to the address for a certain amount of time.

3: "the receiving host MUST return the exact option.." Although it is MUST,
can we always do this? There might be a situation where there is no option
space or no segment to send.

Thanks,
--
Yoshi