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c084ebd77a
A way for an application to know if an MPTCP connection fell back to TCP is to use getsockopt(MPTCP_INFO) and look for errors. The issue with this technique is that the same errors -- EOPNOTSUPP (IPv4) and ENOPROTOOPT (IPv6) -- are returned if there was a fallback, *or* if the kernel doesn't support this socket option. The userspace then has to look at the kernel version to understand what the errors mean. It is not clean, and it doesn't take into account older kernels where the socket option has been backported. A cleaner way would be to expose this info to the TCP socket level. In case of MPTCP socket where no fallback happened, the socket options for the TCP level will be handled in MPTCP code, in mptcp_getsockopt_sol_tcp(). If not, that will be in TCP code, in do_tcp_getsockopt(). So MPTCP simply has to set the value 1, while TCP has to set 0. If the socket option is not supported, one of these two errors will be reported: - EOPNOTSUPP (95 - Operation not supported) for MPTCP sockets - ENOPROTOOPT (92 - Protocol not available) for TCP sockets, e.g. on the socket received after an 'accept()', when the client didn't request to use MPTCP: this socket will be a TCP one, even if the listen socket was an MPTCP one. With this new option, the kernel can return a clear answer to both "Is this kernel new enough to tell me the fallback status?" and "If it is new enough, is it currently a TCP or MPTCP socket?" questions, while not breaking the previous method. Acked-by: Mat Martineau <martineau@kernel.org> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> Link: https://lore.kernel.org/r/20240509-upstream-net-next-20240509-mptcp-tcp_is_mptcp-v1-1-f846df999202@kernel.org Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.