netdev - [PATCH net-next 0/5] mptcp: Improve MPTCP-level window tracking

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Message-Id: <20220504215408.349318-1-mathew.j.martineau@linux.intel.com>
Date:   Wed,  4 May 2022 14:54:03 -0700
From:   Mat Martineau <mathew.j.martineau@...ux.intel.com>
To:     netdev@...r.kernel.org
Cc:     Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        edumazet@...gle.com, matthieu.baerts@...sares.net,
        mptcp@...ts.linux.dev
Subject: [PATCH net-next 0/5] mptcp: Improve MPTCP-level window tracking

This series improves MPTCP receive window compliance with RFC 8684 and
helps increase throughput on high-speed links. Note that patch 3 makes a
change in tcp_output.c

For the details, Paolo says:

I've been chasing bad/unstable performance 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 4/5.

- 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 bumps 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 so
noticeable, still in the current setup the tput changes from
[6-18] Gbps to 19Gbps very stable.

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 | 14 ++++++-----
 net/mptcp/mib.c       |  4 +++
 net/mptcp/mib.h       |  6 +++++
 net/mptcp/options.c   | 58 +++++++++++++++++++++++++++++++++++++------
 net/mptcp/protocol.c  | 32 +++++++++++++++---------
 net/mptcp/protocol.h  |  2 +-
 7 files changed, 90 insertions(+), 28 deletions(-)

base-commit: a37f37a2e7f5ea3ae2a1278f552aa21a8e32c221
-- 
2.36.0