Get an update when MPTCP fall back to TCP

[VenkateswaranJ]

I want to know at runtime if the MPTCP connection fallback to TCP for some reason. Where can I get that info at the userspace level?

[matttbe]

As discussed on IRC: a userspace application is currently not notified when a fallback is done.

This could eventually be done with CMSG, similar to what is proposed in #78. But then the userspace app should be able to query the status, either via a MPTCP_INFO or a specific one like it is done in mptcp.org kernel with MPTCP_ENABLED or something similar.

As a workaround, it is still possible to use inet diag (or with cmd line: ss) to get info about connections.

[matttbe]

Some ideas from the last meeting we had today:

• another fd to spread the informations?
• use the err queue?
  • so the userspace app would be able to request details from the kernel (e.g. MPTCP_INFO)
  • but it might break apps designed for TCP:
    • can we have a POLLERR event in normal conditions while the connection continue?
    • or only change this behaviour if the userspace app asks for it? setsockopt
• CMSG would not wake up the userspace app:
  • seems not ideal
  • but it should happen after having exchanged msg so the userspace app should have been waken up
  • in theory apps should ignore unknown CMSG