hunting for an MSS option to clamp. The previous code assumed that at least
one more byte of options (such as a TCPOPT_EOL) would follow the MSS
option; now, we allow the MSS option to end on the last byte of the
TCP header.
Packets have been observed "in the wild" with a TCP header length of
'6' (24 bytes.. 20 bytes fixed header, 4 bytes options) with a 4-byte
MSS option exactly filling the 4 bytes of options payload and no
following TCPOPT_EOL.
RFC793 is quite explicit that the EOL byte:
" .. need only be used if the end of the options would not
otherwise coincide with the end of the TCP header."
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