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Re: [multipathtcp] Timestamp option for Multipath TCP

Alan Ford <alan.ford@gmail.com> Wed, 29 March 2017 03:16 UTC

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From: Alan Ford <alan.ford@gmail.com>
Date: Wed, 29 Mar 2017 03:16:35 +0000
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To: Olivier.Bonaventure@uclouvain.be, multipathtcp <multipathtcp@ietf.org>
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Subject: Re: [multipathtcp] Timestamp option for Multipath TCP
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Hi Olivier,

Are these really mutually exclusive options? Relaxing the 7323 requirement
seems the right thing to do anyway; adding an option is a nice extra for
people that need TS for measurements other than PAWS.

But we could in theory put timestamp negotiation in MP_CAPABLE so we define
what it means for one end to require timestamps, and then use regular TCP
timestamps - no need for it to be a MPTCP option.

Or we could say that MP_CAPABLE + Timestamp = TS required. Then that makes
it negotiable; 6824bis would be changing the semantics of the presence of
today's TS option.

Regards,
Alan

On Thu, 23 Mar 2017 at 09:34, Olivier Bonaventure <
Olivier.Bonaventure@uclouvain.be> wrote:

> Hello,
>
>
> The standard timestamp option for TCP is defined in RFC7323. It is
> widely implemented and used. It has two main objectives :
>
> - timing measurements
> - protection agains wrapped sequence numbers
>
>
> Given the importance of the second utilisation, RFC7323 has mandated the
> presence of a timestamp option in each segment once negotiated in the
> three-way handshake. For Multipath TCP, the DSN option already protects
> us from PAWS problems and it should not be mandatory to include
> timestamps in each packet sent over a Multipath TCP connection. Given
> the length of the timestamp and DSN options these two options already
> consume a large number of bytes and could limit the number of SACK
> blocks that can be placed inside acknowledgements.
>
> I will only be able to attend the Chicago meeting remotely, but I would
> like to start a discussion on the utilisation of timestamps by Multipath
> TCP on the list. I see two possibilities, but there are possibly more :
>
> 1. Ask for a revision of RFC7323 that allows timestamp options to be
> optional in packets of Multipath TCP connections
>
> 2. Define a new Multipath TCP option to carry timestamps. The
> utilisation of this option would be included in Multipath TCP and thus
> no option besides MP_CAPABLE would need to be included in the SYN.
>
> Do you have any opinion on these two possibilities ?
>
>
> Thanks
>
>
>
> Olivier
>
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