Criteria for Evaluating Network Access Server Protocols
RFC 3169

Document Type RFC - Informational (September 2001; No errata)
Authors Mark Beadles  , David Mitton 
Last updated 2013-03-02
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Network Working Group                                         M. Beadles
Request for Comments: 3169                              SmartPipes, Inc.
Category: Informational                                        D. Mitton
                                                         Nortel Networks
                                                          September 2001

        Criteria for Evaluating Network Access Server 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.


   This document defines requirements for protocols used by Network
   Access Servers (NAS).

1.  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [KEYWORDS].

2.  Introduction

   This document defines requirements for protocols used by Network
   Access Servers (NAS).  Protocols used by NAS's may be divided into
   four spaces:  Access protocols, Network protocols, AAA protocols, and
   Device Management protocols.  The primary focus of this document is
   on AAA protocols.

   The reference model of a NAS used by this document, and the analysis
   of the functions of a NAS which led to the development of these
   requirements, may be found in [NAS-MODEL].

3.  Access Protocol Requirements

   There are three basic types of access protocols used by NAS's.  First
   are the traditional telephony-based access protocols, which interface
   to the NAS via a modem or terminal adapter or similar device.  These
   protocols typically support asynchronous or synchronous PPP [PPP]

Beadles & Mitton             Informational                      [Page 1]
RFC 3169         Criteria for Evaluating NAS Protocols    September 2001

   carried over a telephony protocol.  Second are broadband pseudo-
   telephony access protocols, which are carried over xDSL or cable
   modems, for example.  These protocols typically support an
   encapsulation method such as PPP over Ethernet [PPPOE].  Finally are
   the virtual access protocols used by NAS's that terminate tunnels.
   One example of this type of protocol is L2TP [L2TP].

   It is a central assumption of the NAS model used here that a NAS
   accepts multiple point-to-point links via one of the above access
   protocols.  Therefore, at a minimum, any NAS access protocol MUST be
   able to carry PPP.  The exception to this requirement is for NAS's
   that support legacy text login methods such as telnet [TELNET],
   rlogin, or LAT.  Only these access protocols are exempt from the
   requirement to support PPP.

4.  Network Protocol Requirements

   The network protocols supported by a NAS depend entirely on the kind
   of network to which a NAS is providing access.  This document does
   not impose any additional requirements on network protocols beyond
   the protocol specifications themselves.  For example, if a NAS that
   serves a routed network includes internet routing functionality, then
   that NAS must adhere to [ROUTING-REQUIREMENTS], but there are no
   additional protocol requirements imposed by virtue of the device
   being a NAS.

5.  AAA Protocol Requirements

5.1.  General protocol characteristics

   There are certain general characteristics that any AAA protocol used
   by NAS's must meet.  Note that the transport requirements for
   authentication/authorization are not necessarily the same as those
   for accounting/auditing.  An AAA protocol suite MAY use the same
   transport and protocol for both functions, but this is not strictly

5.1.1.  Transport requirements  Transport independence

   The design of the AAA protocol MUST be transport independent.
   Existing infrastructures use UDP-based protocols [RADIUS], gateways
   to new protocols must be practical to encourage migration.  The
   design MUST comply with congestion control recommendations in RFC
   2914 [CONGEST].

Beadles & Mitton             Informational                      [Page 2]
RFC 3169         Criteria for Evaluating NAS Protocols    September 2001  Scalability

   Very large scale NAS's that serve up to thousands of simultaneous
   sessions are now being deployed.  And a single server system may
   service a large number of ports.  This means that, in the extreme,
   there may be an almost constant exchange of many small packets
   between the NASes and the AAA server.  An AAA protocol transport
   SHOULD support being optimized for a long-term exchange of small
   packets in a stream between a pair of hosts.

   The protocol MUST be designed to support a large number of ports,
   clients, and concurrent sessions.  Examples of poor design would
   include message identifiers which values are so small that queues and
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