[multipathtcp] Reasons DSS checksum including covered data

[Nigel Williams <[e-mail]>]

Hi all,

Since this is my first post, a quick introduction. My name is Nigel 
Williams and I'm currently working at Swinburne University on our newTCP 
project [1]. As part of this project I am working on an implementation 
of MPTCP for FreeBSD.

After looking through the draft [2] section on Data Sequence Mapping 
(3.3.1), it still isn't clear to me as to why a checksum needs to be 
calculated over the DSS psuedo header plus the data for that mapping. I 
understand why the pseudo header might require a checksum, just not why 
the data covered by the mapping needs it (given that the TCP checksum 
covers the payload in each transmitted packet).

A similar question was raised in a previous thread as part of a 
different discussion [3], but going through the replies I wasn't able to 
find an exact answer:

 >> [GH] I was under the impression that the checksum applies to the packet
 >> only. In my opinion this would make things much simpler. Is there a
 >> specific reason to apply the checksum to the entire bulk rather than
 >> solely the packet? Things would get much simpler if the checksum were
 >> packet-specific.

 >If the DSS-mapping covers multiple packets, the checksum in the 
DSS-option is
 >on the whole payload of these packets plus the pseudo-header.

I might be missing something here, so if anyone has some further detail 
that would be great.

regards,
nigel

[1] caia.swin.edu.au/newtcp
[2] draft-ietf-mptcp-multiaddressed-09
[3] http://www.ietf.org/mail-archive/web/multipathtcp/current/msg01483.html