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internet: fix UDP header checksum

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This commit is contained in:
Tommaso Pecorella
2024-02-21 00:35:47 -06:00
parent d60f78c85e
commit 24aad01e9c
1 changed files with 24 additions and 1 deletions
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25
src/internet/model/udp-header.cc
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@@ -189,6 +189,12 @@ UdpHeader::Serialize(Buffer::Iterator start) const
i = start;
i.Next(6);
// RFC 768: If the computed checksum is zero, it is transmitted as all ones
if (checksum == 0)
{
checksum = 0xffff;
}
i.WriteU16(checksum);
}
}
@@ -207,7 +213,24 @@ UdpHeader::Deserialize(Buffer::Iterator start)
m_payloadSize = i.ReadNtohU16() - GetSerializedSize();
m_checksum = i.ReadU16();
if (m_calcChecksum)
// RFC 768: An all zero transmitted checksum value means that the
// transmitter generated no checksum (for debugging or for higher
// level protocols that don't care).
//
// This is common in IPv4, while IPv6 requires UDP to use its checksum.
//
// As strange as it might sound, flipping from 0x0000 to 0xffff does not
// change anything in the verification.
//
// According to RFC 1141, the following holds:
// ~C' = ~(C + (-m) + m') = ~C + (m - m') = ~C + m + ~m'
// If ~C (the original CRC) is zero, m (the CRC field) is zero, and m' is 0xffff,
// then, according to the formula, we have that ~C' is zero.
// I.e., changing the CRC from 0 to 0xffff has no effect on the Rx verification.
//
// Fun fact: if you take an IPv4 header with an Identification field set to zero
// and you change it to 0xffff, the checksum will not change (~_^)
if (m_calcChecksum && m_checksum)
{
uint16_t headerChecksum = CalculateHeaderChecksum(start.GetSize());
i = start;