[multipathtcp] draft terminology: MPTCP control block variable SND.NXT

[Nigel Williams <[e-mail]>]

Hi all.

Thanks to Yoshifumi for keeping an eye on jabber during the session, 
though the time-lag did prove to be a bit of an obstacle.

Following on from the point we had regarding 'DS SND.NXT'.

 From the draft [1] v12, p59, Section B.1.2 (variables in the MPTCP 
control block):
"SND.NXT (64 bits):  This is the Data Sequence Number of the next byte 
to be sent.  SND.NXT is used to determine the value of the DSN in the 
DSS option."

This terminology is borrowed from regular single flow TCP, but it's 
meaning does not seem to us to be the same in the MPTCP context. When a 
subflow asks the scheduler for new data to send, the scheduler allocates 
a chunk of the TX socket buffer to the subflow, tells the subflow the 
starting DS value for the chunk and the length of the chunk. The subflow 
then proceeds to send a DS map for the chunk and all the associated data 
before calling back up to the scheduler to get a new chunk allocated.

Each subflow's internal concept of snd.nxt makes sense as in the regular 
single flow TCP case, but at the DS level, packets leave the machine 
with DS-level sequence numbers that jump all over the place (when using 
mappings > 1-MSS on multiple subflows). The interesting variable at the 
DS level is really the largest DS value mapped to a subflow i.e. the 
right most edge of the TX socket buffer which designates the boundary 
between bytes which have been mapped into subflows and bytes which have 
not. This is not analogous to an individual TCP flow's SND.NXT though, 
hence the point of discussion.

cheers,
nigel


[1] http://www.ietf.org/id/draft-ietf-mptcp-multiaddressed-12.txt