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doc/dev: document exact CRC parameters for crc32c used in msgr2

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The description of the CRC used by the msgr2 protocol as CRC32-C
can be ambiguous. Document the actual parameters that make up
the CRC that is used.

Signed-off-by: Johannes Cornelis Draaijer <jcdra1@gmail.com>
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datdenkikniet 2025-12-15 22:01:23 +01:00
parent 551ac2ba46
commit 25905e7bd0
1 changed files with 24 additions and 5 deletions
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doc/dev/msgr2.rst
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@ -121,9 +121,10 @@ length and segment alignment fields are zeroed.
The reserved bytes are zeroed.
The preamble checksum is CRC32-C. It covers everything up to
itself (28 bytes) and is calculated and verified irrespective of
the connection mode (i.e. even if the frame is encrypted).
The preamble checksum is CRC32-C (see :ref:`crc`). It covers
everything up to itself (28 bytes) and is calculated and verified
irrespective of the connection mode (i.e. even if the frame is
encrypted).
### msgr2.0-crc mode
@ -152,8 +153,9 @@ back to the user immediately, without making a copy or blocking
until the whole frame is transmitted. Currently this is used only
by the kernel client, see ceph_msg_revoke().
The segment checksum is CRC32-C. For "used" empty segments, it is
set to (__le32)-1. For unused (trailing) segments, it is zeroed.
The segment checksum is CRC32-C (see :ref:`crc`). For "used"
empty segments, it is initialized with (__le32)-1. For unused
(trailing) segments, it is zeroed.
The crcs are calculated just to protect against bit errors.
No authenticity guarantees are provided, unlike in msgr1 which
@ -954,3 +956,20 @@ _____________________________________
session_accepting -> ready [label = "client ident", tooltip = "set connection features"];
ready -> ready [label = "keep alive"];
}
.. _crc:
CRC Calculation
---------------
The CRC algorithm used in this section has the following parameters:
* Width: 32 bits
* Poly: ``0x1EDC6F41``
* Init: ``0`` (for preamble CRC), ``(__le32)-1`` = ``0xFFFFFFFF`` (for segment data CRC)
* Refin: ``true``
* Refout: ``true``
* Xorout: ``0``
* Check: ``0``
* Residue: ``0``