On Wed, 4 May 2022, Mat Martineau wrote:
> 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
I missed a couple of small edits - this should be "patch 1/5"...
>
> - 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
...and this "patch 4/5".
- Mat
>
> 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
>
>
--
Mat Martineau
Intel