Transmission Control Protocol

Header Format

Ports -- multiplexing
Sequence numbers (and acks)
Flags

Urgent (useless)
Acknowledgement
Push
Reset
SYN
FIN

Windows -- how much to send
Checksum -- data integrity
Urgent Pointer (still useless)
Options

Maximum Segment Size
Window Scale (cf. RFC1323)
Selective Acknowledgment (SACK -- cf. RFC2018)

Congestion Control Elements

Historically these have been underspecified. RFC2001 (Jan 1997) specified them for the Standards Track:

Slow Start
Congestion Avoidance
Fast Retransmit
Fast Recovery