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Internet checksums are used for IPv4 header checksum, as well as TCP
segment and UDP datagram checksums. Such a checksum represents the
negated sum of adjacent pairs of bytes, using ones' complement
arithmetic.
One property of the Internet checkum is byte order independence [1].
Specifically, the sum of byte-swapped pairs is equal to the result
of byte swapping the sum of those same pairs when not byte-swapped.
So for example if a, b, c, d, y, and z are hexadecimal digits, and
PLUS represents ones' complement addition:
If: ab PLUS cd = yz
Then: ba PLUS dc = zy
For this reason, there is no need to swap the order of bytes in the
checksum value held in a message header, nor the one in the QMAPv4
trailer, in order to operate on them.
In other words, we can determine whether the hardware-computed
checksum matches the one in the message header without any byte
swaps.
(This patch leaves in place all existing type casts.)
[1] https://tools.ietf.org/html/rfc1071
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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 Restructured Text 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.