Subject: [multipathtcp] sequence number length in DSS option

From: Yoshifumi Nishida <[email-masked]>
Date: Mon, 3 Aug 2015 21:15:00 -0700

Hi,

While I was listening to audio archives, I was wondering if the 4 and 8
octets DSS sequence number discussion has been settled down.
I understand Alan's comment that there is no good way to harmonize on this
between the sender and the receiver in the current spec. Hence,
implementations need to support both modes.

But, what if we have a way to signal that it only supports one mode?
Let's say 8 octet mode is mandatory to be implemented and 4 octet mode is
optional.
I personally think there might be the following two ways for signaling.

1: use new subtype
    create new subtype (say MP_CAPABLE2) and if MP_CAPABLE2 is used instead
of MP_CAPABLE, it indicates that the implementation supports only 8 octets
mode.

2: use new flag in DSS/DataACK option
    allocate one bit in the flag of DSS option (say S flag)
    If sender wants to use 4 octets mode, it first needs to send a DSS
option with S flag and 8 octets seqnum. After it receives DSS with S flag,
it can use DSS with 4 octets seqnum. I think It won't cause any harm, even
if both sides use S flag simultaneously or the sender sets S flag in all
DSS options.

Of course this is useless If all implementors can agree to support both
modes, but just in case.

Thanks,
--
Yoshi

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From: Christoph Paasch <[email-masked]>
Date: Mon, 03 Aug 2015 21:32:23 -0700

Hello Yoshifumi,

> Of course this is useless If all implementors can agree to support both

I do not think it is worth the effort of negotiating support for 4 octet DSS
options, because all current implementations do support both modes.

And if an implementation wants to be interoperable with others, it anyways
will need to implement support for both.

Cheers,
Christoph

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From: Yoshifumi Nishida <[email-masked]>
Date: Fri, 7 Aug 2015 00:16:57 -0700

Hi Christoph,

> will need to implement support for both.

Yep, If all implementations can support both modes, it's totally fine.
I just would like to check if we settled down here. Once a PS spec is
published, it might be painful to change it.

Thanks,
--
Yoshi