On the Design of Application Protocols
RFC 3117

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Network Working Group                                            M. Rose
Request for Comments: 3117                  Dover Beach Consulting, Inc.
Category: Informational                                    November 2001

                 On the Design of Application Protocols

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This memo describes the design principles for the Blocks eXtensible
   eXchange Protocol (BXXP).  BXXP is a generic application protocol
   framework for connection-oriented, asynchronous interactions.  The
   framework permits simultaneous and independent exchanges within the
   context of a single application user-identity, supporting both
   textual and binary messages.

Rose                         Informational                      [Page 1]
RFC 3117         On the Design of Application Protocols    November 2001

Table of Contents

   1.  A Problem 19 Years in the Making . . . . . . . . . . . . . . .  3
   2.  You can Solve Any Problem... . . . . . . . . . . . . . . . . .  6
   3.  Protocol Mechanisms  . . . . . . . . . . . . . . . . . . . . .  8
   3.1 Framing  . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.2 Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.3 Reporting  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.4 Asynchrony . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   3.5 Authentication . . . . . . . . . . . . . . . . . . . . . . . . 12
   3.6 Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   3.7 Let's Recap  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Protocol Properties  . . . . . . . . . . . . . . . . . . . . . 14
   4.1 Scalability  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.2 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.3 Simplicity . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.4 Extensibility  . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.5 Robustness . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   5.  The BXXP Framework . . . . . . . . . . . . . . . . . . . . . . 17
   5.1 Framing and Encoding . . . . . . . . . . . . . . . . . . . . . 17
   5.2 Reporting  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.3 Asynchrony . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.4 Authentication . . . . . . . . . . . . . . . . . . . . . . . . 21
   5.5 Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   5.6 Things We Left Out . . . . . . . . . . . . . . . . . . . . . . 21
   5.7 From Framework to Protocol . . . . . . . . . . . . . . . . . . 22
   6.  BXXP is now BEEP . . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 27

Rose                         Informational                      [Page 2]
RFC 3117         On the Design of Application Protocols    November 2001

1. A Problem 19 Years in the Making

   SMTP [1] is close to being the perfect application protocol: it
   solves a large, important problem in a minimalist way.  It's simple
   enough for an entry-level implementation to fit on one or two screens
   of code, and flexible enough to form the basis of very powerful
   product offerings in a robust and competitive market.  Modulo a few
   oddities (e.g., SAML), the design is well conceived and the resulting
   specification is well-written and largely self-contained.  There is
   very little about good application protocol design that you can't
   learn by reading the SMTP specification.

   Unfortunately, there's one little problem: SMTP was originally
   published in 1981 and since that time, a lot of application protocols
   have been designed for the Internet, but there hasn't been a lot of
   reuse going on.  You might expect this if the application protocols
   were all radically different, but this isn't the case: most are
   surprisingly similar in their functional behavior, even though the
   actual details vary considerably.

   In late 1998, as Carl Malamud and I were sitting down to review the
   Blocks architecture, we realized that we needed to have a protocol
   for exchanging Blocks.  The conventional wisdom is that when you need
   an application protocol, there are four ways to proceed:

   1. find an existing exchange protocol that (more or less) does what
      you want;

   2. define an exchange model on top of the world-wide web
      infrastructure that (more or less) does what you want;