Historically, t_maxseg was the value that store a value of:
min(PeerOfferedMSS, MTU - minimum protocol header)
With T/TCP there appeared t_maxopd. Later, T/TCP was removed,
but t_maxopd stayed in place. After some permutations, the
t_maxopd stores min(PeerOfferedMSS, MTU - minimum protocol header)
t_maxseg stores (t_maxopd - optlen)
Note, that it is impossible to store (t_maxopd - optlen), since optlen
is calculated per-packet. Really, the following invariant is held:
t_maxseg == t_maxopd - TCPOLEN_TSTAMP_APPA
It was violated only in very rare cases, like manually set MSS via
TCP_MAXSEG setsockopt(). Btw, the setsockopt is broken now.
So, in crucial code t_maxopd took the place of t_maxseg. The t_maxseg
is still in use, but in code that can recover from deviations of the
value: cwnd or ssthreash calculations, statistics.
The patch removes t_maxopd, and restores old notion to t_maxseg - bare
offered MSS (or MTU product), that doesn't take option length into