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.
Abstract
This document defines requirements for protocols used by Network
Access Servers (NAS).
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [KEYWORDS].
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].
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]
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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.
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.
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
required.
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].
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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
reception windows wrap under load, unique session identifier ranges
that are so small that they wrap within the lifetime of potential
long sessions, counter values that cannot accommodate reasonable
current and future bandwidth usage, and computational processes with
high overhead that must be performed frequently.
In order to operationally support large loads, load balancing and
fail-over to multiple AAA servers will be required. The AAA protocol
MUST provide for NAS's to balance individual AAA requests between two
or more AAA servers. The load balancing mechanism SHOULD be built in
to the AAA protocol itself.
The AAA protocol MUST be able to detect a failure of the transport
protocol to deliver a message or messages within a known and
controllable time period, so it can engage retransmission or server
fail-over processes. The reliability and robustness of
authentication requests MUST be predictable and configurable.
The AAA protocol design MUST NOT introduce a single point of failure
during the AAA process. The AAA protocol MUST allow any sessions
between a NAS and a given AAA server to fail-over to a secondary
server without loss of state information. This fail-over mechanism
SHOULD be built in to the AAA protocol itself.
NAS's operated by one authority provide network access services for
clients operated by another authority, to network destinations
operated by yet another authority. This type of arrangement is of
growing importance; for example, dial roaming is now a nearly
ubiquitous service. Therefore, the AAA protocol MUST support AAA
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services that travel between multiple domains of authority. The AAA
protocol MUST NOT use a model that assumes a single domain of
authority.
The AAA protocol MUST NOT dictate particular business models for the
relationship between the administrative domains. The AAA protocol
MUST support proxy, and in addition SHOULD support other multi-domain
relationships such as brokering and referral.
The AAA protocol MUST also meet the protocol requirements specified
in [ROAMING-REQUIREMENTS].
Years of operational experience with AAA protocols and NAS's has
proven that the Attribute-Value protocol model is an optimal
representation of AAA data. The protocol SHOULD use an Attribute-
Value representation for AAA data. This document will assume such a
model. Even if the AAA protocol does not use this as an on-the-wire
data representation, Attribute-Value can serve as abstraction for
discussing AAA information.
Experience has also shown that attribute space tends to run out
quickly. In order to provide room for expansion in the attribute
space, the AAA protocol MUST support a minimum of 64K Attributes (16
bits), each with a minimum length of 64K (16 bits).
The AAA protocol MUST support simple attribute data types, including
integer, enumeration, text string, IP address, and date/time. The
AAA protocol MUST also provide some support for complex structured
data types. Wherever IP addresses are carried within the AAA
protocol, the protocol MUST support both IPv4 and IPv6 [IPV6]
addresses. Wherever text information is carried within the AAA
protocol, the protocol MUST comply with the IETF Policy on Character
Sets and Languages [RFC 2277].
At a minimum, the AAA protocol MUST support, or be easily extended to
support, the set of attributes supported by RADIUS [RADIUS] and
RADIUS Accounting [RADIUS-ACCOUNTING]. If the base AAA protocol does
not support this complete set of attributes, then an extension to
that protocol MUST be defined which supports this set.
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NAS and AAA development is always progressing. In order to prevent
the AAA protocol from being a limiting factor in NAS and AAA Server
development, the AAA protocol MUST provide a built-in extensibility
mechanism, which MUST include a means for adding new standard
attribute extensions. This MUST include a method for registering or
requesting extensions through IANA, so that long-term working group
involvement is not required to create new attribute types. Ideally,
the AAA protocol SHOULD separate specification of the transport from
specification of the attributes.
The AAA protocol MUST include a means for individual vendors to add
value through vendor-specific attributes and SHOULD include support
for vendor-specific data types.
It is poor security practice for a NAS to communicate with an AAA
server that is not trusted, and vice versa. The AAA protocol MUST
provide mutual authentication between AAA server and NAS.
At a minimum, the AAA protocol SHOULD support use of a secret shared
pairwise between each NAS and AAA server to mutually verify identity.
This is intended for small-scale deployments. The protocol MAY
provide stronger mutual security techniques.
AAA server/NAS identity verification based solely on shared secrets
can be difficult to deploy properly at large scale, and it can be
tempting for NAS operators to use a single shared secret (that rarely
changes) across all NAS's. This can lead to an easy compromise of
the secret. Therefore, the AAA protocol MUST also support mutual
verification of identity using a public-key infrastructure that
supports expiration and revocation of keys.
Some attributes are more operationally sensitive than others. Also,
in a multi-domain scenario, attributes may be inserted by servers
from different administrative domains. Therefore, the AAA protocol
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MUST support selective encryption of attributes on an attribute-by-
attribute basis, even within the same message. This requirement
applies equally to Authentication, Authorization, and Accounting
data.
End users who are requesting network access through a NAS will
present various types of credentials. It is the purpose of the AAA
protocol to transport these credentials between the NAS and the AAA
server.
The AAA protocol MUST support transport of credentials from the AAA
server to the NAS, between the User and the NAS, and between the NAS
and the AAA server.
The AAA protocol MUST support re-authentication at any time during
the course of a session, initiated from either the NAS or the AAA
server. This is a requirement of CHAP [CHAP].
The AAA protocol MUST be able to support multi-phase authentication
methods, including but not limited to support for:
- Text prompting from the NAS to the user
- A series of binary challenges and responses of arbitrary length
- An authentication failure reason to be transmitted from the NAS
to the user
- Callback to a pre-determined phone number
Security protocol development is going on constantly as new threats
are identified and better cracking methods are developed. Today's
secure authentication methods may be proven insecure tomorrow. The
AAA protocol MUST provide some support for addition of new
authentication credential types.
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Many authentication protocols are defined within the framework of
PPP. The AAA protocol MUST be able to act as an intermediary
protocol between the authenticate and the authenticator for the
following authentication protocols:
- PPP Password Authentication Protocol [PPP]
- PPP Challenge Handshake Authentication Protocol [CHAP]
- PPP Extensible Authentication Protocol [EAP]
The following are common types of credentials used for user
identification. The AAA protocol MUST be able to carry the following
types of identity credentials:
- A user name in the form of a Network Access Identifier [NAI].
- An Extensible Authentication Protocol [EAP] Identity Request
Type packet.
- Telephony dialing information such as Dialed Number
Identification Service (DNIS) and Caller ID.
If a particular type of authentication credential is not needed for a
particular user session, the AAA protocol MUST NOT require that dummy
credentials be filled in. That is, the AAA protocol MUST support
authorization by identification or assertion only.
The following are common types of credentials used for
authentication. The AAA protocol MUST be able to carry the following
types of authenticating credentials at a minimum:
- A secret or password.
- A response to a challenge presented by the NAS to the user
- A one-time password
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- An X.509 digital certificate [X.509]
- A Kerberos v5 ticket [KERBEROS]
Where passwords are used as authentication credentials, the AAA
protocol MUST provide a secure means of hiding the password from
intermediates in the AAA conversation. Where challenge/response
mechanisms are used, the AAA protocol MUST also prevent against
replay attacks.
In all cases, the protocol MUST specify that authorization data sent
from the NAS to the AAA server is to be regarded as information or
"hints", and not directives. The AAA protocol MUST be designed so
that the AAA server makes all final authorization decisions and does
not depend on a certain state being expected by the NAS.
The AAA protocol MUST support dynamic re-authorization at any time
during a user session. This re-authorization may be initiated in
either direction. This dynamic re-authorization capability MUST
include the capability to request a NAS to disconnect a user on
demand.
Resource Management MUST be supported on demand by the NAS or AAA
Server at any time during the course of a user session. This would
be the ability for the NAS to allocate and deallocate shared
resources from a AAA server servicing multiple NASes. These
resources may include, but are not limited to; IP addresses,
concurrent usage limits, port usage limits, and tunnel limits. This
capability should have error detection and synchronization features
that will recover state after network and system failures. This may
be accomplished by session information timeouts and explicit interim
status and disconnect messages. There should not be any dependencies
on the Accounting message stream, as per current practices.
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This feature is primarily intended for NAS-local network resources.
In a proxy or multi-domain environment, resource information should
only be retained by the server doing the allocation, and perhaps it's
backups. Authorization resources in remote domains should use the
dynamic authorization features to change and revoke authorization
status.
The AAA protocol serves as a primary means of gathering data used for
making Policy decisions for network access. Therefore, the AAA
protocol MUST allow network operators to make policy decisions based
on the following parameters:
- Time/day restrictions. The AAA protocol MUST be able to
provide an unambiguous time stamp, NAS time zone indication,
and date indication to the AAA server in the Authorization
information.
- Location restrictions: The AAA protocol MUST be able to
provide an unambiguous location code that reflects the
geographic location of the NAS. Note that this is not the same
type of thing as either the dialing or dialed station.
- Dialing restrictions: The AAA protocol MUST be able to provide
accurate dialed and dialing station indications.
- Concurrent login limitations: The AAA protocol MUST allow an
AAA Server to limit concurrent logins by a particular user or
group of users. This mechanism does not need to be explicitly
built into the AAA protocol, but the AAA protocol must provide
sufficient authorization information for an AAA server to make
that determination through an out-of-band mechanism.
The AAA protocol is used to enforce policy at the NAS. Essentially,
on granting of access, a particular access profile is applied to the
user's session. The AAA protocol MUST at a minimum provide a means
of applying profiles containing the following types of information:
- IP Address assignment: The AAA protocol MUST provide a means of
assigning an IPv4 or IPv6 address to an incoming user.
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- Protocol Filter application: The AAA protocol MUST provide a
means of applying IP protocol filters to user sessions. Two
different methods MUST be supported.
First, the AAA protocol MUST provide a means of selecting a
protocol filter by reference to an identifier, with the details
of the filter action being specified out of band. The AAA
protocol SHOULD define this out-of-band reference mechanism.
Second, the AAA protocol MUST provide a means of passing a
protocol filter by value. This means explicit passing of
pass/block information by address range, TCP/UDP port number,
and IP protocol number at a minimum.
- Compulsory Tunneling: The AAA protocol MUST provide a means of
directing a NAS to build a tunnel or tunnels to a specified
end- point. It MUST support creation of multiple simultaneous
tunnels in a specified order. The protocol MUST allow, at a
minimum, specification of the tunnel endpoints, tunneling
protocol type, underlying tunnel media type, and tunnel
authentication credentials (if required by the tunnel type).
The AAA protocol MUST support at least the creation of tunnels
using the L2TP [L2TP], ESP [ESP], and AH [AH] protocols. The
protocol MUST provide means of adding new tunnel types as they
are standardized.
- Routing: The AAA protocol MUST provide a means of assigning a
particular static route to an incoming user session.
- Expirations/timeouts: The AAA protocol MUST provide a means of
communication session expiration information to a NAS. Types
of expirations that MUST be supported are: total session time,
idle time, total bytes transmitted, and total bytes received.
- Quality of Service: The AAA protocol MUST provide a means for
supplying Quality of Service parameters to the NAS for
individual user sessions.
The AAA protocol is a means for network operators to perform
management of network resources. The AAA protocol MUST provide a
means of collecting resource state information, and controlling
resource allocation for the following types of network resources.
- Network bandwidth usage per session, including multilink
sessions.
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- Access port usage, including concurrent usage and usage pools.
- Connect time.
- IP Addresses and pools.
- Compulsory tunnel limits.
When an AAA protocol passes credentials that will be used to
authenticate compulsory tunnels, the AAA protocol MUST provide a
means of securing the credentials from end-to-end of the AAA
conversation. The AAA protocol MUST also provide protection against
replay attacks in this situation.
The accounting and auditing functions of the AAA protocol are used
for network planning, resource management, policy decisions, and
other functions that require accurate knowledge of the state of the
NAS. NAS operators need to be able to engineer their network usage
measurement systems to a predictable level of accuracy. Therefore,
an AAA protocol MUST provide a means of guaranteed delivery of
accounting information between the NAS and the AAA Server(s).
NAS operators often require a real time view onto the status of
sessions served by a NAS. Therefore, the AAA protocol MUST support
real-time delivery of accounting and auditing information. In this
context, real time is defined as accounting information delivery
beginning within one second of the triggering event.
The AAA protocol SHOULD also support delivery of stored accounting
and auditing information in batches (non-real time).
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There may be delays associated with the delivery of accounting
information. The NAS operator will desire to know the time an event
actually occurred, rather than simply the time when notification of
the event was received. Therefore, the AAA protocol MUST carry an
unambiguous time stamp associated with each accounting event. This
time stamp MUST be unambiguous with regard to time zone. Note that
this assumes that the NAS has access to a reliable time source.
At a minimum, the AAA protocol MUST support delivery of accounting
information triggered by the following events:
- Start of a user session
- End of a user session
- Expiration of a predetermined repeating time interval during a
user session. The AAA protocol MUST provide a means for the
AAA server to request that a NAS use a certain interval
accounting time.
- Dynamic re-authorization during a user session (e.g., new
resources being delivered to the user)
- Dynamic re-authentication during a user session
NAS operators need to maintain an accurate view onto the status of
sessions served by a NAS, even through failure of an AAA server.
Therefore, the AAA protocol MUST support a means of requesting
current session state and accounting from the NAS on demand.
At a minimum, the AAA protocol MUST support delivery of the following
types of accounting/auditing data:
- All parameters used to authenticate a session.
- Details of the authorization profile that was applied to the
session.
- The duration of the session.
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- The cumulative number of bytes sent by the user during the
session.
- The cumulative number of bytes received by the user during the
session.
- The cumulative number of packets sent by the user during the
session.
- The cumulative number of packets received by the user during
the session.
- Details of the access protocol used during the session (port
type, connect speeds, etc.)
Note that accounting and auditing data are operationally sensitive
information. The AAA protocol MUST provide a means to assure end-
to-end integrity of this data. The AAA protocol SHOULD provide a
means of assuring the end-to-end confidentiality of this data.
Network operators use accounting data for network planning, resource
management, and other business-critical functions that require
confidence in the correctness of this data. The AAA protocol SHOULD
provide a mechanism to ensure that the source of accounting data
cannot easily repudiate this data after transmission.
Many of the requirements in this document first took form in Glen
Zorn's, "Yet Another Authentication Protocol (YAAP)" document, for
which grateful acknowledgment is made.
See above for security requirements for the NAS AAA protocol.
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Where an AAA architecture spans multiple domains of authority, AAA
information may need to cross trust boundaries. In this situation, a
NAS might operate as a shared device that services multiple
administrative domains. Network operators are advised take this into
consideration when deploying NAS's and AAA Servers.
This document does not directly specify any IANA considerations.
However, the following recommendations are made:
Future development and extension of an AAA protocol will be made much
easier if new attributes and values can be requested or registered
directly through IANA, rather than through an IETF Standardization
process.
The AAA protocol might use enumerated values for some attributes,
which enumerate already-defined IANA types (such as protocol number).
In these cases, the AAA protocol SHOULD use the IANA assigned numbers
as the enumerated values.
[AH] Kent, S. and R. Atkinson, "IP Authentication
Header (AH)", RFC 2402, November 1998.
[CHAP] Simpson, J., "PPP Challenge Handshake
Authentication Protocol (CHAP)", RFC 1994,
August 1996.
[CONGEST] Floyd, S., "Congestion Control Principles",
RFC 2914, Sept. 2000.
[EAP] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284,
March 1998.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating
Security Payload (ESP)", RFC 2406, November
1998.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC
2119, March 1997.
[KERBEROS] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510,
September 1993.
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[IPV6] Deering, S. and R. Hinden, "Internet
Protocol, Version 6 (IPv6) Specification",
RFC 2460, December 1998.
[L2TP] Townsley, W., Valencia, A., Rubens, A., Pall,
G., Zorn, G. and B. Plater, "Layer Two
Tunneling Protocol (L2TP)", RFC 2661, August
1999.
[NAI] Aboba, B. and M. Beadles, "The Network Access
Identifier", RFC 2486, January 1999.
[NAS-MODEL] Mitton, D. and M. Beadles, "Network Access
Server Requirements Next Generation
(NASREQNG) NAS Model", RFC 2881, July 2000.
[NAS-EXT] Mitton, D., "Network Access Servers
Requirements: Extended RADIUS Practices", RFC
2882, July 2000.
[PPP] Simpson, W., "The Point-to-Point Protocol
(PPP)", STD 51, RFC 1661, July 1994.
[PPPOE] Mamakos, L., Lidl, K., Evarts, J., Carrel,
D., Simone, D. and R. Wheeler, "A Method for
Transmitting PPP Over Ethernet (PPPoE)", RFC
2516, February 1999.
[ROUTING-REQUIREMENTS] Baker, F., "Requirements for IP Version 4
Routers", RFC 1812, June 1995.
[TELNET] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC 2277] Alvestrand, H., "IETF Policy on Character
Sets and Languages", BCP 18, RFC 2277,
January 1998.
[X.509] ITU-T Recommendation X.509 (1997 E):
Information Technology - Open Systems
Interconnection - The Directory:
Authentication Framework, June 1997.
[RADIUS] Rigney, C., Rubens. A., Simpson, W. and S.
Willens, "Remote Authentication Dial In User
Service (RADIUS)", RFC 2138, April 1997.
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[RADIUS-ACCOUNTING] Rigney, C., "RADIUS Accounting", RFC 2139,
April 1997.
[ROAMING-REQUIREMENTS] Aboba, B. and G. Zorn, "Criteria for
Evaluating Roaming Protocols", RFC 2477,
January 1999.
Mark Anthony Beadles
SmartPipes, Inc.
565 Metro Place South Suite 300
Dublin, OH 43017
Phone: 614-923-6200
David Mitton
Nortel Networks
880 Technology Park Drive
Billerica, MA 01821
Phone: 978-288-4570
EMail: dmitton@nortelnetworks.com
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