Design Considerations for Protocol Extensions
RFC 6709

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Internet Architecture Board (IAB)                           B. Carpenter
Request for Comments: 6709                                 B. Aboba, Ed.
Category: Informational                                      S. Cheshire
ISSN: 2070-1721                                           September 2012

             Design Considerations for Protocol Extensions

Abstract

   This document discusses architectural issues related to the
   extensibility of Internet protocols, with a focus on design
   considerations.  It is intended to assist designers of both base
   protocols and extensions.  Case studies are included.  A companion
   document, RFC 4775 (BCP 125), discusses procedures relating to the
   extensibility of IETF protocols.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6709.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Carpenter, et al.             Informational                     [Page 1]
RFC 6709          Design Considerations for Extensions    September 2012

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................4
   2. Routine and Major Extensions ....................................4
      2.1. What Constitutes a Major Extension? ........................4
      2.2. When is an Extension Routine? ..............................6
   3. Architectural Principles ........................................7
      3.1. Limited Extensibility ......................................7
      3.2. Design for Global Interoperability .........................8
      3.3. Architectural Compatibility ...............................12
      3.4. Protocol Variations .......................................13
      3.5. Testability ...............................................16
      3.6. Protocol Parameter Registration ...........................16
      3.7. Extensions to Critical Protocols ..........................17
   4. Considerations for the Base Protocol ...........................18
      4.1. Version Numbers ...........................................19
      4.2. Reserved Fields ...........................................22
      4.3. Encoding Formats ..........................................23
      4.4. Parameter Space Design ....................................23
      4.5. Cryptographic Agility .....................................26
      4.6. Transport .................................................27
      4.7. Handling of Unknown Extensions ............................28
   5. Security Considerations ........................................29
   6. References .....................................................30
      6.1. Normative References ......................................30
      6.2. Informative References ....................................30
   7. Acknowledgments ................................................35
   8. IAB Members at the Time of Approval ............................35
   Appendix A.  Examples .............................................36
      A.1. Already-Documented Cases ..................................36
      A.2. RADIUS Extensions .........................................36
      A.3. TLS Extensions ............................................39
      A.4. L2TP Extensions ...........................................41

Carpenter, et al.             Informational                     [Page 2]
RFC 6709          Design Considerations for Extensions    September 2012

1.  Introduction

   When developing protocols, IETF Working Groups (WGs) often include
   mechanisms whereby these protocols can be extended in the future.  It
   is often a good principle to design extensibility into protocols; as
   described in "What Makes for a Successful Protocol" [RFC5218], a
   "wildly successful" protocol is one that becomes widely used in ways
   not originally anticipated.  Well-designed extensibility mechanisms