A subflow closed after an error is never re-created, even once the path
is back (#440).
When TCP gives up on an established subflow we created, after a
retransmission or keepalive timeout, the in-kernel PM now creates a new
one to the same addresses, while another subflow can still send data.
Its SYN retries space out the attempts. Their number is set by a new
limit, subflow-retries, like TCP_SYNCNT: the maximum, 127, by default,
about 4 hours, and 0 disables it.
Patch 1 fixes a selftest tool issue seen with the new test. Patches 2
and 3 add the new limit, patch 4 the retry, patches 5 and 6 the
selftests.
I used an LLM to help write and review the code, the tests and the
commit messages of this series. I checked and tested every change
myself.
Known issue: in about 1 run in 20, more often on debug kernels, the
first new test fails. Once the path is back, the re-created subflow
joins, but the server resets it right away with an MP_RST, after
DSSNotMatching, and the PAWS counters are high on both sides. It looks
like an existing issue on the receive path, triggered by the two-way
transfer with losses rather than by the retry itself, but I have not
found the cause yet: it does not show up with tcpdump or tracepoints
running. Any hint is welcome.
Changes in v12, after the netdev Sashiko review of v11: when no other
subflow can send data, only the re-created subflows still connecting are
closed, a joined one is fully established soon; a subflow in CLOSE_WAIT
no longer keeps them open either; the doc says the initial subflow is
not re-created if the peer does not accept joins to its initial address;
small test fixes; and the patches are rebased on the current export
branch.
Changes in v11, after the Sashiko review of v10: a subflow is only
re-created while another subflow can send data, so the close timeout
works as before, and all the code re-arming or stopping it is gone; the
re-created subflows not established yet are closed from the worker, or
when the MPTCP socket is shut down or closed, never while iterating on
the subflows; the path-back test is gone, and the third test checks the
only subflow is not re-created.
Changes in v10, after the Sashiko review of v9: when the close timeout
expires, the connection is kept if a re-created subflow connected in the
meantime, since an active join does not stop the timer; the re-created
subflows still connecting are closed once the msk is no longer
established and no other subflow is left; a new test lets the path come
back before the close timeout; and the patches are rebased on the
current export branch, without the Assisted-by tag, removed upstream.
Changes in v9, after the Sashiko reviews of v8 and more checks: every
timeout is retried, whatever ICMP error TCP reports for it, and only
resets, local aborts and MPTCP protocol errors are excluded; if no other
subflow can send data, the close timeout still applies, the new subflow
no longer keeps the connection open for all its SYN retries; the
re-created subflows closed with the msk have their data in the msk
backlog cleaned, since they can get established in between; the doc says
this is a single connection attempt, ended by an ICMP error, and that 0
disables it; the close test blocks both paths, and its server reads all
the data first; the tests read the counters before blocking a path; and
a new test checks the close timeout with only one subflow.
Changes in v8, after the Sashiko reviews of v7 and Matthieu's comments:
the default number of SYN retries is the maximum, 127; the re-created
subflows still connecting are closed before the write shutdown
disconnects them; the doc is more precise; and a separate test closes
the connection while a subflow is being re-created.
Changes in v7, after the netdev Sashiko reviews of v5 and v6, and
Abdul's tests: only a path error, a retransmission or keepalive
timeout, makes the PM re-create a subflow, not a reset, so a peer
cannot make it loop; the flags are bools under the msk socket lock;
mptcp.rst documents tcp_retries2 and the keepalive needed by a subflow
that only receives data; and small fixes in the tests.
Changes in v6, after the Sashiko reviews (netdev Sashiko on v4, which
also applied to v5, and sashiko.dev on v5): only subflows that were
established are re-created, so the PM still moves on to the next
endpoints as before; removed remote addresses and the local ID 0 are
handled too; the policy checks the maximum value, so the parse_limit()
patch is gone; and the test uses the net/lib.sh helpers. Details in
each patch.
v11: https://lore.kernel.org/mptcp/20261026125345.3019372-1-anilkaushikwireless@gmail.com/
v10: https://lore.kernel.org/mptcp/20261026063657.2955783-1-anilkaushikwireless@gmail.com/
v9: https://lore.kernel.org/mptcp/20261025134909.2794368-1-anilkaushikwireless@gmail.com/
v8: https://lore.kernel.org/mptcp/20261022151130.2285198-1-anilkaushikwireless@gmail.com/
v7: https://lore.kernel.org/mptcp/20261020031711.1896660-1-anilkaushikwireless@gmail.com/
v6: https://lore.kernel.org/mptcp/20261018041748.1655711-1-anilkaushikwireless@gmail.com/
v5: https://lore.kernel.org/mptcp/20261016160926.1155262-1-anilkaushikwireless@gmail.com/
v4: https://lore.kernel.org/mptcp/20261016055010.980819-1-anilkaushikwireless@gmail.com/
v3: https://lore.kernel.org/mptcp/20261014143349.768138-1-anilkaushikwireless@gmail.com/
v2: https://lore.kernel.org/mptcp/20261013025653.523755-1-anilkaushikwireless@gmail.com/
v1: https://lore.kernel.org/mptcp/20261012171740.464517-1-anilkaushikwireless@gmail.com/
Anil Kaushik (6):
selftests: mptcp: connect: retry write() on EAGAIN
netlink: specs: mptcp: add the subflow-retries attribute
mptcp: pm: in-kernel: add the subflow-retries limit
mptcp: pm: in-kernel: retry subflows closed after an error
selftests: mptcp: pm_nl_ctl: support the subflow-retries limit
selftests: mptcp: join: validate subflow retry after link failure
Documentation/netlink/specs/mptcp_pm.yaml | 13 +
Documentation/networking/mptcp.rst | 19 +
include/uapi/linux/mptcp.h | 2 +-
include/uapi/linux/mptcp_pm.h | 1 +
net/mptcp/mptcp_pm_gen.c | 10 +-
net/mptcp/mptcp_pm_gen.h | 4 +-
net/mptcp/pm.c | 39 ++
net/mptcp/pm_kernel.c | 160 +++++++-
net/mptcp/protocol.c | 78 +++-
net/mptcp/protocol.h | 51 +++
net/mptcp/sockopt.c | 2 +
.../selftests/net/mptcp/mptcp_connect.c | 18 +
.../testing/selftests/net/mptcp/mptcp_join.sh | 342 +++++++++++++++++-
.../testing/selftests/net/mptcp/pm_netlink.sh | 53 ++-
tools/testing/selftests/net/mptcp/pm_nl_ctl.c | 34 +-
15 files changed, 798 insertions(+), 28 deletions(-)
--
2.25.1