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

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Date:   Wed, 4 May 2022 15:05:45 -0700 (PDT)
From:   Mat Martineau <mathew.j.martineau@...ux.intel.com>
To:     netdev@...r.kernel.org
cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        edumazet@...gle.com, matthieu.baerts@...sares.net,
        mptcp@...ts.linux.dev
Subject: Re: [PATCH net-next 0/5] mptcp: Improve MPTCP-level window
 tracking


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