[INET'99] Postel's Style: Lessons in Internet Transfer Protocol Design

Rohit Khare (rohit@fdr.ICS.uci.edu)
Wed, 11 Nov 1998 22:29:02 -0800

Postel's Style: Lessons in Internet Transfer Protocol Design

Tracing the evolution of application-layer transfer protocols -- for
logins (Telnet), files (FTP), e-mail (SMTP), netnews(NNTP), hypertext
(HTTP), and many others -- leads back to the words of Jon Postel. As
editor of three key specifications during the changeover to TCP, he
established a style of Internet protocol specification which persists
tothis day. From the structure of command sequences to the theory of
reply codes, many stylised features of protocol design and
documentation have their origins in Postel's 1983 re-engineering of
Telnet, FTP, and SMTP.

This paper also proposes a framework for discussing Transfer Protocols
(TPs) more broadly. It characterizes successor variations such as
netnews and hypertext transfer by their addressing structure, content
formats, and distribution strategies. Two decades' historical context
also clarifies how several protocols were extended to adapt to new
uses and old options were pruned away. The result is a family tree of
Internet applications 'in the manner of Postel.'

[NOTE: this work is largely derived from an unrefereed column series on
the history of Internet protocol technology which appeared in IEEE
Internet Computing throughout 1998. See http://www.ics.uci.edu/~rohit/ ,
especially /~rohit/IEEE-L7-Jon-NNTP.html ]

Aspects of the Postel Style of Internet Protocol Architecture:

* The hybrid human- and machine-readable style of protocols on the wire
* Reply code theory, originally (xyz) and today (Nxx and extended replies).
* 'liberal in what you accept, conservative in what you generate'
* Security considerations
* Option packages vs. linear version numbering
* Reliance on centrally administered parameter registries (IANA)
* Balancing stateful sessions with 'simple'implementation