IBM Redbook  "TCP/IP Tutorial and Technical Overview'
    Sixth Edition (October 1998)

    by Martin W. Murhammer, Orcun Atakan, Stefan Bretz,
       Larry R. Pugh, Kazunari Suzuki, David H. Wood
    Copyright (C) IBM Corporation, 1999, 1998

Currently reading Chap. 1,
    PDF p. 31.
    (Probably should start at the top.)