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net: Transmit zero UDP checksum as 0xFFFF

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The checksum algorithm used by IPv4, TCP and UDP allows a zero value
to be represented by either 0x0000 and 0xFFFF. But per RFC 768, a zero
UDP checksum must be transmitted as 0xFFFF because 0x0000 is a special
value meaning no checksum.

Substitute 0xFFFF whenever a checksum is computed as zero when
modifying a UDP datagram header. Doing this on IPv4 and TCP checksums
is unnecessary but legal. Add a wrapper for net_checksum_finish() that
makes the substitution.

(We can't just change net_checksum_finish(), as that function is also
used by receivers to verify checksums, and in that case the expected
value is always 0x0000.)

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
This commit is contained in:
Ed Swierk 2017-11-16 06:06:06 -08:00 • committed by Jason Wang
parent ebc2327f07
commit 0dacea92d2
5 changed files with 11 additions and 4 deletions
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6
include/net/checksum.h
View file

@ -33,6 +33,12 @@ net_checksum_add(int len, uint8_t *buf)
return net_checksum_add_cont(len, buf, 0);
}
static inline uint16_t
net_checksum_finish_nozero(uint32_t sum)
{
return net_checksum_finish(sum) ?: 0xFFFF;
}
static inline uint16_t
net_raw_checksum(uint8_t *data, int length)
{