[multipathtcp] Timestamp option for Multipath TCP

Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be> Thu, 23 March 2017 14:34 UTC

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To: multipathtcp <multipathtcp@ietf.org>
From: Olivier Bonaventure <Olivier.Bonaventure@uclouvain.be>
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Date: Thu, 23 Mar 2017 15:33:57 +0100
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Subject: [multipathtcp] Timestamp option for Multipath TCP
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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 ?