MPTCP checksum interop

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

I did some tests with connections with checksums enabled, between the 
export branch and the multipath-tcp.org mptcp_trunk branch (v1 mode).

When the multipath-tcp.org kernel was listening, the connection would 
always fall back to TCP when the first data was sent by the upstream 
kernel. The multipath-tcp.org kernel was the first to fall back and stop 
sending MPTCP headers.

When the upstream kernel was listening, the multipath-tcp.org kernel would 
send corrupt TCP options with the first data packet (the first time a DSS 
option was sent). The upstream kernel would then send TCP RST.

The initial impression is that there are some issues with mptcp_trunk and 
checksums - I did not have time today to look at packet captures between 
two mptcp_trunk kernels.

These results were the same when only one side of the connection had 
checksums enabled, or if both kernels did.

I haven't narrowed down the cause, but will do some more experiments on 
Monday.

--
Mat Martineau
Intel