[PATCH net-next 0/5] mptcp: improve mptcp-level window tracking

[Paolo Abeni <pabeni@redhat.com>]

I've been chasing bad/unstable performances with multiple subflows
on very high speed links.

It looks like the root cause is due to the current mptcp-level
congestion window handling. There are apparently a few different
sub-issues:

- the rcv_wnd is not effectively shared on the tx side, as each
  subflow takes in account only the value received by the underlaying
  TCP connection. This is addressed in patch 1/4

- The mptcp-level offered wnd right edge is currently allowed to shrink.
  Reading section 3.3.4.:

"""
   The receive window is relative to the DATA_ACK.  As in TCP, a
   receiver MUST NOT shrink the right edge of the receive window (i.e.,
   DATA_ACK + receive window).  The receiver will use the data sequence
   number to tell if a packet should be accepted at the connection
   level.
"""

I read the above as we need to reflect window right-edge tracking
on the wire, see patch 3/4.

- The offered window right edge tracking can happen concurrently on
  multiple subflows, but there is no mutex protection. We need an
  additional atomic operation - still patch 3/4

This series additionally bump a few new MIBs to track all the above
(ensure/observe that the suspected races actually take place).

I could not access again the host where the issue was su much
noticeable, still in the current setup the tput changes from
[6-18] Gbps to 19Gbps very stable.

RFC -> v1:
 - added patch 3/5 to address Mat's comment, and rebased the
   following on top of it - I hope Eric may tolerate that, it's
   more an hope than guess ;)

Paolo Abeni (5):
  mptcp: really share subflow snd_wnd
  mptcp: add mib for xmit window sharing
  tcp: allow MPTCP to update the announced window.
  mptcp: never shrink offered window
  mptcp: add more offered MIBs counter.

 include/net/mptcp.h   |  2 +-
 net/ipv4/tcp_output.c | 13 +++++-----
 net/mptcp/mib.c       |  4 +++
 net/mptcp/mib.h       |  6 +++++
 net/mptcp/options.c   | 58 +++++++++++++++++++++++++++++++++++++------
 net/mptcp/protocol.c  | 24 ++++++++++++------
 6 files changed, 84 insertions(+), 23 deletions(-)

-- 
2.35.1