The Checksum

Compute and Verify Checksum

The checksum is recomputed with the checksum word zeroed, then verified over the full bytes.

highlighted = computed this step

The checksum covers more than ports

The checksum table sums pseudo-header, UDP header, and payload. Compute mode treats the UDP checksum word as zero.

checksum word index=9\text{checksum word index}=9
Compute UDP checksumCompute mode zeroes the UDP checksum word inside pseudo-header plus UDP bytes.UDP checksum compute - compute zero word 9rowindexvalueword00xc0a8word10x010aword20xc0a8word30x0101word40x0011word50x000aword60x9c40word70x0035word80x000aword90x0000 (zeroed)word100x6869raw sum0x0002885efolded0x8860result0x779f

Compute the checksum

With the checksum word zeroed, the recomputed checksum is 0x779f. That matches the field in the UDP header.

computed checksum=0x779f\text{computed checksum}=0x779f
Compute UDP checksumCompute mode zeroes the UDP checksum word inside pseudo-header plus UDP bytes.UDP checksum compute - compute zero word 9rowindexvalueword00xc0a8word10x010aword20xc0a8word30x0101word40x0011word50x000aword60x9c40word70x0035word80x000aword90x0000 (zeroed)word100x6869raw sum0x0002885efolded0x8860result0x779f

Verify the received datagram

With checksum 0x779f included, the folded sum is 0xffff.

verify fold=0xffff\text{verify fold}=0xffff
Verify UDP checksumVerify mode sums pseudo-header, UDP header, checksum, and payload together.UDP checksum verify - verifyrowindexvalueword00xc0a8word10x010aword20xc0a8word30x0101word40x0011word50x000aword60x9c40word70x0035word80x000aword90x779fword100x6869raw sum0x0002fffdfolded0xffffresult0xffff

Summary

A valid UDP checksum is a recomputed identity over pseudo-header, UDP header, and payload bytes.

valid fold=0xffff\text{valid fold}=0xffff
Verify UDP checksumVerify mode sums pseudo-header, UDP header, checksum, and payload together.UDP checksum verify - verifyrowindexvalueword00xc0a8word10x010aword20xc0a8word30x0101word40x0011word50x000aword60x9c40word70x0035word80x000aword90x779fword100x6869raw sum0x0002fffdfolded0xffffresult0xffff