STINNER Victor added the comment:
http://man7.org/linux/man-pages/man7/tcp.7.html
Sockets API
TCP provides limited support for out-of-band data, in the form of (a
single byte of) urgent data. In Linux this means if the other end
sends newer out-of-band data the older urgent data is inserted as
normal data into the stream (even when SO_OOBINLINE is not set).
This differs from BSD-based stacks.
Linux uses the BSD compatible interpretation of the urgent pointer
field by default. This violates RFC 1122, but is required for
interoperability with other stacks. It can be changed via
/proc/sys/net/ipv4/tcp_stdurg.
It is possible to peek at out-of-band data using the recv(2) MSG_PEEK
flag.
Since version 2.4, Linux supports the use of MSG_TRUNC in the flags
argument of recv(2) (and recvmsg(2)). This flag causes the received
bytes of data to be discarded, rather than passed back in a caller-
supplied buffer. Since Linux 2.4.4, MSG_TRUNC also has this effect
when used in conjunction with MSG_OOB to receive out-of-band data.
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<http://bugs.python.org/issue30844>
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