





Network Working Group                                          C. Rigney
Request for Comments: 2138                                    Livingston
Obsoletes: 2058                                                A. Rubens
Category: Standards Track                                          Merit
															  W. Simpson
															  Daydreamer
															  S. Willens
															  Livingston
															  April 1997


		  Remote Authentication Dial In User Service (RADIUS)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes a protocol for carrying authentication,
   authorization, and configuration information between a Network Access
   Server which desires to authenticate its links and a shared
   Authentication Server.

Implementation Note

   This memo documents the RADIUS protocol.  There has been some
   confusion in the assignment of port numbers for this protocol.  The
   early deployment of RADIUS was done using the erroneously chosen port
   number 1645, which conflicts with the "datametrics" service.  The
   officially assigned port number for RADIUS is 1812.

Table of Contents

   1.     Introduction ..........................................    3
	  1.1       Specification of Requirements ...................    4
	  1.2       Terminology .....................................    5
   2.     Operation .............................................    5
	  2.1       Challenge/Response ..............................    7
	  2.2       Interoperation with PAP and CHAP ................    7
	  2.3       Why UDP? ........................................    8
   3.     Packet Format .........................................   10
   4.     Packet Types ..........................................   13
	  4.1       Access-Request ..................................   13



Rigney, et. al.             Standards Track                     [Page 1]

RFC 2138                         RADIUS                       April 1997


	  4.2       Access-Accept ...................................   14
	  4.3       Access-Reject ...................................   15
	  4.4       Access-Challenge ................................   17
   5.     Attributes ............................................   18
	  5.1       User-Name .......................................   21
	  5.2       User-Password ...................................   22
	  5.3       CHAP-Password ...................................   23
	  5.4       NAS-IP-Address ..................................   24
	  5.5       NAS-Port ........................................   25
	  5.6       Service-Type ....................................   26
	  5.7       Framed-Protocol .................................   28
	  5.8       Framed-IP-Address ...............................   29
	  5.9       Framed-IP-Netmask ...............................   29
	  5.10      Framed-Routing ..................................   30
	  5.11      Filter-Id .......................................   31
	  5.12      Framed-MTU ......................................   32
	  5.13      Framed-Compression ..............................   33
	  5.14      Login-IP-Host ...................................   33
	  5.15      Login-Service ...................................   34
	  5.16      Login-TCP-Port ..................................   35
	  5.17      (unassigned) ....................................   36
	  5.18      Reply-Message ...................................   36
	  5.19      Callback-Number .................................   37
	  5.20      Callback-Id .....................................   38
	  5.21      (unassigned) ....................................   38
	  5.22      Framed-Route ....................................   39
	  5.23      Framed-IPX-Network ..............................   40
	  5.24      State ...........................................   40
	  5.25      Class ...........................................   41
	  5.26      Vendor-Specific .................................   42
	  5.27      Session-Timeout .................................   44
	  5.28      Idle-Timeout ....................................   44
	  5.29      Termination-Action ..............................   45
	  5.30      Called-Station-Id ...............................   46
	  5.31      Calling-Station-Id ..............................   47
	  5.32      NAS-Identifier ..................................   48
	  5.33      Proxy-State .....................................   48
	  5.34      Login-LAT-Service ...............................   49
	  5.35      Login-LAT-Node ..................................   50
	  5.36      Login-LAT-Group .................................   51
	  5.37      Framed-AppleTalk-Link ...........................   52
	  5.38      Framed-AppleTalk-Network ........................   53
	  5.39      Framed-AppleTalk-Zone ...........................   54
	  5.40      CHAP-Challenge ..................................   55
	  5.41      NAS-Port-Type ...................................   55
	  5.42      Port-Limit ......................................   56
	  5.43      Login-LAT-Port ..................................   57
	  5.44      Table of Attributes .............................   58



Rigney, et. al.             Standards Track                     [Page 2]

RFC 2138                         RADIUS                       April 1997


   6.     Examples ..............................................   59
	  6.1       User Telnet to Specified Host ...................   60
	  6.2       Framed User Authenticating with CHAP ............   60
	  6.3       User with Challenge-Response card ...............   61
   Security Considerations ......................................   63
   References ...................................................   64
   Acknowledgements .............................................   64
   Chair's Address ..............................................   65
   Author's Addresses ...........................................   65

1.  Introduction

   Managing dispersed serial line and modem pools for large numbers of
   users can create the need for significant administrative support.
   Since modem pools are by definition a link to the outside world, they
   require careful attention to security, authorization and accounting.
   This can be best achieved by managing a single "database" of users,
   which allows for authentication (verifying user name and password) as
   well as configuration information detailing the type of service to
   deliver to the user (for example, SLIP, PPP, telnet, rlogin).

   Key features of RADIUS are:

   Client/Server Model

	  A Network Access Server (NAS) operates as a client of RADIUS.  The
	  client is responsible for passing user information to designated
	  RADIUS servers, and then acting on the response which is returned.

	  RADIUS servers are responsible for receiving user connection
	  requests, authenticating the user, and then returning all
	  configuration information necessary for the client to deliver
	  service to the user.

	  A RADIUS server can act as a proxy client to other RADIUS servers
	  or other kinds of authentication servers.

   Network Security

	  Transactions between the client and RADIUS server are
	  authenticated through the use of a shared secret, which is never
	  sent over the network.  In addition, any user passwords are sent
	  encrypted between the client and RADIUS server, to eliminate the
	  possibility that someone snooping on an unsecure network could
	  determine a user's password.






Rigney, et. al.             Standards Track                     [Page 3]

RFC 2138                         RADIUS                       April 1997


   Flexible Authentication Mechanisms

	  The RADIUS server can support a variety of methods to authenticate
	  a user.  When it is provided with the user name and original
	  password given by the user, it can support PPP PAP or CHAP, UNIX
	  login, and other authentication mechanisms.

   Extensible Protocol

	  All transactions are comprised of variable length Attribute-
	  Length-Value 3-tuples.  New attribute values can be added without
	  disturbing existing implementations of the protocol.

1.1.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
			 definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
			 prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that there
			 may exist valid reasons in particular circumstances to
			 ignore this item, but the full implications must be
			 understood and carefully weighed before choosing a
			 different course.

   MAY       This word, or the adjective "optional", means that this
			 item is one of an allowed set of alternatives.  An
			 implementation which does not include this option MUST be
			 prepared to interoperate with another implementation which
			 does include the option.
















Rigney, et. al.             Standards Track                     [Page 4]

RFC 2138                         RADIUS                       April 1997


1.2.  Terminology

   This document frequently uses the following terms:

   service   The NAS provides a service to the dial-in user, such as PPP
			 or Telnet.

   session   Each service provided by the NAS to a dial-in user
			 constitutes a session, with the beginning of the session
			 defined as the point where service is first provided and
			 the end of the session defined as the point where service
			 is ended.  A user may have multiple sessions in parallel or
			 series if the NAS supports that.

   silently discard
			 This means the implementation discards the packet without
			 further processing.  The implementation SHOULD provide the
			 capability of logging the error, including the contents of
			 the silently discarded packet, and SHOULD record the event
			 in a statistics counter.

2.  Operation

   When a client is configured to use RADIUS, any user of the client
   presents authentication information to the client.  This might be
   with a customizable login prompt, where the user is expected to enter
   their username and password.  Alternatively, the user might use a
   link framing protocol such as the Point-to-Point Protocol (PPP),
   which has authentication packets which carry this information.

   Once the client has obtained such information, it may choose to
   authenticate using RADIUS.  To do so, the client creates an "Access-
   Request" containing such Attributes as the user's name, the user's
   password, the ID of the client and the Port ID which the user is
   accessing.  When a password is present, it is hidden using a method
   based on the RSA Message Digest Algorithm MD5 [1].

   The Access-Request is submitted to the RADIUS server via the network.
   If no response is returned within a length of time, the request is
   re-sent a number of times.  The client can also forward requests to
   an alternate server or servers in the event that the primary server
   is down or unreachable.  An alternate server can be used either after
   a number of tries to the primary server fail, or in a round-robin
   fashion.  Retry and fallback algorithms are the topic of current
   research and are not specified in detail in this document.






Rigney, et. al.             Standards Track                     [Page 5]

RFC 2138                         RADIUS                       April 1997


   Once the RADIUS server receives the request, it validates the sending
   client.  A request from a client for which the RADIUS server does not
   have a shared secret should be silently discarded.  If the client is
   valid, the RADIUS server consults a database of users to find the
   user whose name matches the request.  The user entry in the database
   contains a list of requirements which must be met to allow access for
   the user.  This always includes verification of the password, but can
   also specify the client(s) or port(s) to which the user is allowed
   access.

   The RADIUS server MAY make requests of other servers in order to
   satisfy the request, in which case it acts as a client.

   If any condition is not met, the RADIUS server sends an "Access-
   Reject" response indicating that this user request is invalid.  If
   desired, the server MAY include a text message in the Access-Reject
   which MAY be displayed by the client to the user.  No other
   Attributes are permitted in an Access-Reject.

   If all conditions are met and the RADIUS server wishes to issue a
   challenge to which the user must respond, the RADIUS server sends an
   "Access-Challenge" response.  It MAY include a text message to be
   displayed by the client to the user prompting for a response to the
   challenge, and MAY include a State attribute.  If the client receives
   an Access-Challenge and supports challenge/response it MAY display
   the text message, if any, to the user, and then prompt the user for a
   response.  The client then re-submits its original Access-Request
   with a new request ID, with the User-Password Attribute replaced by
   the response (encrypted), and including the State Attribute from the
   Access-Challenge, if any.  Only 0 or 1 instances of the State
   Attributes should be present in a request.  The server can respond to
   this new Access-Request with either an Access-Accept, an Access-
   Reject, or another Access-Challenge.

   If all conditions are met, the list of configuration values for the
   user are placed into an "Access-Accept" response.  These values
   include the type of service (for example: SLIP, PPP, Login User) and
   all necessary values to deliver the desired service.  For SLIP and
   PPP, this may include values such as IP address, subnet mask, MTU,
   desired compression, and desired packet filter identifiers.  For
   character mode users, this may include values such as desired
   protocol and host.









Rigney, et. al.             Standards Track                     [Page 6]

RFC 2138                         RADIUS                       April 1997


2.1.  Challenge/Response

   In challenge/response authentication, the user is given an
   unpredictable number and challenged to encrypt it and give back the
   result. Authorized users are equipped with special devices such as
   smart cards or software that facilitate calculation of the correct
   response with ease. Unauthorized users, lacking the appropriate
   device or software and lacking knowledge of the secret key necessary
   to emulate such a device or software, can only guess at the response.

   The Access-Challenge packet typically contains a Reply-Message
   including a challenge to be displayed to the user, such as a numeric
   value unlikely ever to be repeated. Typically this is obtained from
   an external server that knows what type of authenticator should be in
   the possession of the authorized user and can therefore choose a
   random or non-repeating pseudorandom number of an appropriate radix
   and length.

   The user then enters the challenge into his device (or software) and
   it calculates a response, which the user enters into the client which
   forwards it to the RADIUS server via a second Access-Request.  If the
   response matches the expected response the RADIUS server replies with
   an Access-Accept, otherwise an Access-Reject.

   Example: The NAS sends an Access-Request packet to the RADIUS Server
   with NAS-Identifier, NAS-Port, User-Name, User-Password (which may
   just be a fixed string like "challenge" or ignored).  The server
   sends back an Access-Challenge packet with State and a Reply-Message
   along the lines of "Challenge 12345678, enter your response at the
   prompt" which the NAS displays.  The NAS prompts for the response and
   sends a NEW Access-Request to the server (with a new ID) with NAS-
   Identifier, NAS-Port, User-Name, User-Password (the response just
   entered by the user, encrypted), and the same State Attribute that
   came with the Access-Challenge.  The server then sends back either an
   Access-Accept or Access-Reject based on whether the response matches
   what it should be, or it can even send another Access-Challenge.

2.2.  Interoperation with PAP and CHAP

   For PAP, the NAS takes the PAP ID and password and sends them in an
   Access-Request packet as the User-Name and User-Password. The NAS MAY
   include the Attributes Service-Type = Framed-User and Framed-Protocol
   = PPP as a hint to the RADIUS server that PPP service is expected.

   For CHAP, the NAS generates a random challenge (preferably 16 octets)
   and sends it to the user, who returns a CHAP response along with a
   CHAP ID and CHAP username.  The NAS then sends an Access-Request
   packet to the RADIUS server with the CHAP username as the User-Name



Rigney, et. al.             Standards Track                     [Page 7]

RFC 2138                         RADIUS                       April 1997


   and with the CHAP ID and CHAP response as the CHAP-Password
   (Attribute 3).  The random challenge can either be included in the
   CHAP-Challenge attribute or, if it is 16 octets long, it can be
   placed in the Request Authenticator field of the Access-Request
   packet.  The NAS MAY include the Attributes Service-Type = Framed-
   User and Framed-Protocol = PPP as a hint to the RADIUS server that
   PPP service is expected.

   The RADIUS server looks up a password based on the User-Name,
   encrypts the challenge using MD5 on the CHAP ID octet, that password,
   and the CHAP challenge (from the CHAP-Challenge attribute if present,
   otherwise from the Request Authenticator), and compares that result
   to the CHAP-Password.  If they match, the server sends back an
   Access-Accept, otherwise it sends back an Access-Reject.

   If the RADIUS server is unable to perform the requested
   authentication it should return an Access-Reject.  For example, CHAP
   requires that the user's password be available in cleartext to the
   server so that it can encrypt the CHAP challenge and compare that to
   the CHAP response.  If the password is not available in cleartext to
   the RADIUS server then the server MUST send an Access-Reject to the
   client.

2.3.  Why UDP?

   A frequently asked question is why RADIUS uses UDP instead of TCP as
   a transport protocol.  UDP was chosen for strictly technical reasons.

   There are a number of issues which must be understood.  RADIUS is a
   transaction based protocol which has several interesting
   characteristics:

   1.   If the request to a primary Authentication server fails, a
		secondary server must be queried.

		 To meet this requirement, a copy of the request must be kept
		 above the transport layer to allow for alternate transmission.
		 This means that retransmission timers are still required.

   2.   The timing requirements of this particular protocol are
		significantly different than TCP provides.

		 At one extreme, RADIUS does not require a "responsive"
		 detection of lost data.  The user is willing to wait several
		 seconds for the authentication to complete.  The generally
		 aggressive TCP retransmission (based on average round trip
		 time) is not required, nor is the acknowledgement overhead of
		 TCP.



Rigney, et. al.             Standards Track                     [Page 8]

RFC 2138                         RADIUS                       April 1997


		 At the other extreme, the user is not willing to wait several
		 minutes for authentication.  Therefore the reliable delivery of
		 TCP data two minutes later is not useful.  The faster use of an
		 alternate server allows the user to gain access before giving
		 up.

   3.   The stateless nature of this protocol simplifies the use of UDP.

		 Clients and servers come and go.  Systems are rebooted, or are
		 power cycled independently.  Generally this does not cause a
		 problem and with creative timeouts and detection of lost TCP
		 connections, code can be written to handle anomalous events.
		 UDP however completely eliminates any of this special handling.
		 Each client and server can open their UDP transport just once
		 and leave it open through all types of failure events on the
		 network.

   4.   UDP simplifies the server implementation.

		 In the earliest implementations of RADIUS, the server was
		 single threaded.  This means that a single request was
		 received, processed, and returned.  This was found to be
		 unmanageable in environments where the back-end security
		 mechanism took real time (1 or more seconds).  The server
		 request queue would fill and in environments where hundreds of
		 people were being authenticated every minute, the request
		 turn-around time increased to longer that users were willing to
		 wait (this was especially severe when a specific lookup in a
		 database or over DNS took 30 or more seconds).  The obvious
		 solution was to make the server multi-threaded.  Achieving this
		 was simple with UDP.  Separate processes were spawned to serve
		 each request and these processes could respond directly to the
		 client NAS with a simple UDP packet to the original transport
		 of the client.

		 It's not all a panacea.  As noted, using UDP requires one thing
		 which is built into TCP: with UDP we must artificially manage
		 retransmission timers to the same server, although they don't
		 require the same attention to timing provided by TCP.  This one
		 penalty is a small price to pay for the advantages of UDP in
		 this protocol.

		 Without TCP we would still probably be using tin cans connected
		 by string.  But for this particular protocol, UDP is a better
		 choice.






Rigney, et. al.             Standards Track                     [Page 9]

RFC 2138                         RADIUS                       April 1997


3.  Packet Format

   Exactly one RADIUS packet is encapsulated in the UDP Data field [2],
   where the UDP Destination Port field indicates 1812 (decimal).

   When a reply is generated, the source and destination ports are
   reversed.

   This memo documents the RADIUS protocol.  There has been some
   confusion in the assignment of port numbers for this protocol.  The
   early deployment of RADIUS was done using the erroneously chosen port
   number 1645, which conflicts with the "datametrics" service.  The
   officially assigned port number for RADIUS is 1812.

   A summary of the RADIUS data format is shown below.  The fields are
   transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                         Authenticator                         |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

Code

   The Code field is one octet, and identifies the type of RADIUS
   packet.  When a packet is received with an invalid Code field, it is
   silently discarded.

   RADIUS Codes (decimal) are assigned as follows:

		1       Access-Request
		2       Access-Accept
		3       Access-Reject
		4       Accounting-Request
		5       Accounting-Response
	   11       Access-Challenge
	   12       Status-Server (experimental)
	   13       Status-Client (experimental)
	  255       Reserved




Rigney, et. al.             Standards Track                    [Page 10]

RFC 2138                         RADIUS                       April 1997


   Codes 4 and 5 are covered in the RADIUS Accounting document [9], and
   are not further mentioned here.  Codes 12 and 13 are reserved for
   possible use, but are not further mentioned here.

Identifier

   The Identifier field is one octet, and aids in matching requests and
   replies.

Length

   The Length field is two octets.  It indicates the length of the
   packet including the Code, Identifier, Length, Authenticator and
   Attribute fields.  Octets outside the range of the Length field
   should be treated as padding and should be ignored on reception.  If
   the packet is shorter than the Length field indicates, it should be
   silently discarded.  The minimum length is 20 and maximum length is
   4096.

Authenticator

   The Authenticator field is sixteen (16) octets.  The most significant
   octet is transmitted first.  This value is used to authenticate the
   reply from the RADIUS server, and is used in the password hiding
   algorithm.

Request Authenticator

	  In Access-Request Packets, the Authenticator value is a 16 octet
	  random number, called the Request Authenticator.  The value SHOULD
	  be unpredictable and unique over the lifetime of a secret (the
	  password shared between the client and the RADIUS server), since
	  repetition of a request value in conjunction with the same secret
	  would permit an attacker to reply with a previously intercepted
	  response.  Since it is expected that the same secret MAY be used
	  to authenticate with servers in disparate geographic regions, the
	  Request Authenticator field SHOULD exhibit global and temporal
	  uniqueness.

	  The Request Authenticator value in an Access-Request packet SHOULD
	  also be unpredictable, lest an attacker trick a server into
	  responding to a predicted future request, and then use the
	  response to masquerade as that server to a future Access-Request.








Rigney, et. al.             Standards Track                    [Page 11]

RFC 2138                         RADIUS                       April 1997


	  Although protocols such as RADIUS are incapable of protecting
	  against theft of an authenticated session via realtime active
	  wiretapping attacks, generation of unique unpredictable requests
	  can protect against a wide range of active attacks against
	  authentication.

	  The NAS and RADIUS server share a secret.  That shared secret
	  followed by the Request Authenticator is put through a one-way MD5
	  hash to create a 16 octet digest value which is xored with the
	  password entered by the user, and the xored result placed in the
	  User-Password attribute in the Access-Request packet.  See the
	  entry for User-Password in the section on Attributes for a more
	  detailed description.

   Response Authenticator

	  The value of the Authenticator field in Access-Accept, Access-
	  Reject, and Access-Challenge packets is called the Response
	  Authenticator, and contains a one-way MD5 hash calculated over a
	  stream of octets consisting of: the RADIUS packet, beginning with
	  the Code field, including the Identifier, the Length, the Request
	  Authenticator field from the Access-Request packet, and the
	  response Attributes, followed by the shared secret.  That is,
	  ResponseAuth = MD5(Code+ID+Length+RequestAuth+Attributes+Secret)
	  where + denotes concatenation.

Administrative Note

   The secret (password shared between the client and the RADIUS server)
   SHOULD be at least as large and unguessable as a well-chosen
   password.  It is preferred that the secret be at least 16 octets.
   This is to ensure a sufficiently large range for the secret to
   provide protection against exhaustive search attacks.  A RADIUS
   server SHOULD use the source IP address of the RADIUS UDP packet to
   decide which shared secret to use, so that RADIUS requests can be
   proxied.

   When using a forwarding proxy, the proxy must be able to alter the
   packet as it passes through in each direction - when the proxy
   forwards the request, the proxy can add a Proxy-State Attribute, and
   when the proxy forwards a response, it removes the Proxy-State
   Attribute. Since Access-Accept and Access-Reject replies are
   authenticated on the entire packet contents, the stripping of the
   Proxy-State attribute would invalidate the signature in the packet -
   so the proxy has to re-sign it.

   Further details of RADIUS proxy implementation are outside the scope
   of this document.



Rigney, et. al.             Standards Track                    [Page 12]

RFC 2138                         RADIUS                       April 1997


Attributes

   Many Attributes may have multiple instances, in such a case the order
   of Attributes of the same Type SHOULD be preserved.  The order of
   Attributes of different Types is not required to be preserved.

   In the section below on "Attributes" where the text refers to which
   packets an attribute is allowed in, only packets with Codes 1, 2, 3
   and 11 and attributes defined in this document are covered in this
   document.  A summary table is provided at the end of the "Attributes"
   section.  To determine which Attributes are allowed in packets with
   codes 4 and 5 refer to the RADIUS Accounting document [9].

4.  Packet Types

   The RADIUS Packet type is determined by the Code field in the first
   octet of the Packet.

4.1.  Access-Request

   Description

	  Access-Request packets are sent to a RADIUS server, and convey
	  information used to determine whether a user is allowed access to
	  a specific NAS, and any special services requested for that user.
	  An implementation wishing to authenticate a user MUST transmit a
	  RADIUS packet with the Code field set to 1 (Access-Request).

	  Upon receipt of an Access-Request from a valid client, an
	  appropriate reply MUST be transmitted.

	  An Access-Request MUST contain a User-Name attribute.  It SHOULD
	  contain either a NAS-IP-Address attribute or NAS-Identifier
	  attribute (or both, although that is not recommended).  It MUST
	  contain either a User-Password attribute or CHAP-Password
	  attribute.  It SHOULD contain a NAS-Port or NAS-Port-Type
	  attribute or both unless the type of access being requested does
	  not involve a port or the NAS does not distinguish among its
	  ports.

	  An Access-Request MAY contain additional attributes as a hint to
	  the server, but the server is not required to honor the hint.

	  When a User-Password is present, it is hidden using a method based
	  on the RSA Message Digest Algorithm MD5 [1].

   A summary of the Access-Request packet format is shown below.  The
   fields are transmitted from left to right.



Rigney, et. al.             Standards Track                    [Page 13]

RFC 2138                         RADIUS                       April 1997


	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Request Authenticator                     |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

	  1 for Access-Request.

   Identifier

	  The Identifier field MUST be changed whenever the content of the
	  Attributes field changes, and whenever a valid reply has been
	  received for a previous request.  For retransmissions, the
	  Identifier MUST remain unchanged.

   Request Authenticator

	  The Request Authenticator value MUST be changed each time a new
	  Identifier is used.

   Attributes

	  The Attribute field is variable in length, and contains the list
	  of Attributes that are required for the type of service, as well
	  as any desired optional Attributes.

4.2.  Access-Accept

   Description

	  Access-Accept packets are sent by the RADIUS server, and provide
	  specific configuration information necessary to begin delivery of
	  service to the user.  If all Attribute values received in an
	  Access-Request are acceptable then the RADIUS implementation MUST
	  transmit a packet with the Code field set to 2 (Access-Accept).







Rigney, et. al.             Standards Track                    [Page 14]

RFC 2138                         RADIUS                       April 1997


	  On reception of an Access-Accept, the Identifier field is matched
	  with a pending Access-Request.  Additionally, the Response
	  Authenticator field MUST contain the correct response for the
	  pending Access-Request.  Invalid packets are silently discarded.

   A summary of the Access-Accept packet format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Response Authenticator                    |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

	  2 for Access-Accept.

   Identifier

	  The Identifier field is a copy of the Identifier field of the
	  Access-Request which caused this Access-Accept.

   Response Authenticator

	  The Response Authenticator value is calculated from the Access-
	  Request value, as described earlier.

   Attributes

	  The Attribute field is variable in length, and contains a list of
	  zero or more Attributes.












Rigney, et. al.             Standards Track                    [Page 15]

RFC 2138                         RADIUS                       April 1997


4.3.  Access-Reject

   Description

	  If any value of the received Attributes is not acceptable, then
	  the RADIUS server MUST transmit a packet with the Code field set
	  to 3 (Access-Reject).  It MAY include one or more Reply-Message
	  Attributes with a text message which the NAS MAY display to the
	  user.

   A summary of the Access-Reject packet format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Response Authenticator                    |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

	  3 for Access-Reject.

   Identifier

	  The Identifier field is a copy of the Identifier field of the
	  Access-Request which caused this Access-Reject.

   Response Authenticator

	  The Response Authenticator value is calculated from the Access-
	  Request value, as described earlier.

   Attributes

	  The Attribute field is variable in length, and contains a list of
	  zero or more Attributes.







Rigney, et. al.             Standards Track                    [Page 16]

RFC 2138                         RADIUS                       April 1997


4.4.  Access-Challenge

   Description

	  If the RADIUS server desires to send the user a challenge
	  requiring a response, then the RADIUS server MUST respond to the
	  Access-Request by transmitting a packet with the Code field set to
	  11 (Access-Challenge).

	  The Attributes field MAY have one or more Reply-Message
	  Attributes, and MAY have a single State Attribute, or none.  No
	  other Attributes are permitted in an Access-Challenge.

	  On receipt of an Access-Challenge, the Identifier field is matched
	  with a pending Access-Request.  Additionally, the Response
	  Authenticator field MUST contain the correct response for the
	  pending Access-Request.  Invalid packets are silently discarded.

	  If the NAS does not support challenge/response, it MUST treat an
	  Access-Challenge as though it had received an Access-Reject
	  instead.

	  If the NAS supports challenge/response, receipt of a valid
	  Access-Challenge indicates that a new Access-Request SHOULD be
	  sent.  The NAS MAY display the text message, if any, to the user,
	  and then prompt the user for a response.  It then sends its
	  original Access-Request with a new request ID and Request
	  Authenticator, with the User-Password Attribute replaced by the
	  user's response (encrypted), and including the State Attribute
	  from the Access-Challenge, if any.  Only 0 or 1 instances of the
	  State Attribute can be present in an Access-Request.

	  A NAS which supports PAP MAY forward the Reply-Message to the
	  dialin client and accept a PAP response which it can use as though
	  the user had entered the response.  If the NAS cannot do so, it
	  should treat the Access-Challenge as though it had received an
	  Access-Reject instead.














Rigney, et. al.             Standards Track                    [Page 17]

RFC 2138                         RADIUS                       April 1997


   A summary of the Access-Challenge packet format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Response Authenticator                    |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

	  11 for Access-Challenge.

   Identifier

	  The Identifier field is a copy of the Identifier field of the
	  Access-Request which caused this Access-Challenge.

   Response Authenticator

	  The Response Authenticator value is calculated from the Access-
	  Request value, as described earlier.

   Attributes

	  The Attributes field is variable in length, and contains a list of
	  zero or more Attributes.

5.  Attributes

   RADIUS Attributes carry the specific authentication, authorization,
   information and configuration details for the request and reply.

   Some Attributes MAY be included more than once.  The effect of this
   is Attribute specific, and is specified in each Attribute
   description.

   The end of the list of Attributes is indicated by the Length of the
   RADIUS packet.





Rigney, et. al.             Standards Track                    [Page 18]

RFC 2138                         RADIUS                       April 1997


   A summary of the Attribute format is shown below.  The fields are
   transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  The Type field is one octet.  Up-to-date values of the RADIUS Type
	  field are specified in the most recent "Assigned Numbers" RFC [3].
	  Values 192-223 are reserved for experimental use, values 224-240
	  are reserved for implementation-specific use, and values 241-255
	  are reserved and should not be used.  This specification concerns
	  the following values:

	  A RADIUS server MAY ignore Attributes with an unknown Type.

	  A RADIUS client MAY ignore Attributes with an unknown Type.

		  1      User-Name
		  2      User-Password
		  3      CHAP-Password
		  4      NAS-IP-Address
		  5      NAS-Port
		  6      Service-Type
		  7      Framed-Protocol
		  8      Framed-IP-Address
		  9      Framed-IP-Netmask
		 10      Framed-Routing
		 11      Filter-Id
		 12      Framed-MTU
		 13      Framed-Compression
		 14      Login-IP-Host
		 15      Login-Service
		 16      Login-TCP-Port
		 17      (unassigned)
		 18      Reply-Message
		 19      Callback-Number
		 20      Callback-Id
		 21      (unassigned)
		 22      Framed-Route
		 23      Framed-IPX-Network
		 24      State
		 25      Class
		 26      Vendor-Specific



Rigney, et. al.             Standards Track                    [Page 19]

RFC 2138                         RADIUS                       April 1997


		 27      Session-Timeout
		 28      Idle-Timeout
		 29      Termination-Action
		 30      Called-Station-Id
		 31      Calling-Station-Id
		 32      NAS-Identifier
		 33      Proxy-State
		 34      Login-LAT-Service
		 35      Login-LAT-Node
		 36      Login-LAT-Group
		 37      Framed-AppleTalk-Link
		 38      Framed-AppleTalk-Network
		 39      Framed-AppleTalk-Zone
		 40-59   (reserved for accounting)
		 60      CHAP-Challenge
		 61      NAS-Port-Type
		 62      Port-Limit
		 63      Login-LAT-Port

   Length

	  The Length field is one octet, and indicates the length of this
	  Attribute including the Type, Length and Value fields.  If an
	  Attribute is received in an Access-Request but with an invalid
	  Length, an Access-Reject SHOULD be transmitted.  If an Attribute
	  is received in an Access-Accept, Access-Reject or Access-Challenge
	  packet with an invalid length, the packet MUST either be treated
	  an Access-Reject or else silently discarded.

   Value

	  The Value field is zero or more octets and contains information
	  specific to the Attribute.  The format and length of the Value
	  field is determined by the Type and Length fields.

	  Note that a "string" in RADIUS does not require termination by an
	  ASCII NUL because the Attribute already has a length field.














Rigney, et. al.             Standards Track                    [Page 20]

RFC 2138                         RADIUS                       April 1997


	  The format of the value field is one of four data types.

	  string    0-253 octets

	  address   32 bit value, most significant octet first.

	  integer   32 bit value, most significant octet first.

	  time      32 bit value, most significant octet first -- seconds
				since 00:00:00 GMT, January 1, 1970.  The standard
				Attributes do not use this data type but it is presented
				here for possible use within Vendor-Specific attributes.


5.1.  User-Name

   Description

	  This Attribute indicates the name of the user to be authenticated.
	  It is only used in Access-Request packets.

   A summary of the User-Name Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  1 for User-Name.

   Length

	  >= 3

   String

	  The String field is one or more octets.  The NAS may limit the
	  maximum length of the User-Name but the ability to handle at least
	  63 octets is recommended.








Rigney, et. al.             Standards Track                    [Page 21]

RFC 2138                         RADIUS                       April 1997


	  The format of the username MAY be one of several forms:

	  monolithic Consisting only of alphanumeric characters.  This
				 simple form might be used to locally manage a NAS.

	  simple    Consisting only of printable ASCII characters.

	  name@fqdn SMTP address.  The Fully Qualified Domain Name (with or
				without trailing dot) indicates the realm in which the
				name part applies.

	  distinguished name
				A name in ASN.1 form used in Public Key authentication
				systems.

5.2.  User-Password

   Description

	  This Attribute indicates the password of the user to be
	  authenticated, or the user's input following an Access-Challenge.
	  It is only used in Access-Request packets.

	  On transmission, the password is hidden.  The password is first
	  padded at the end with nulls to a multiple of 16 octets.  A one-
	  way MD5 hash is calculated over a stream of octets consisting of
	  the shared secret followed by the Request Authenticator.  This
	  value is XORed with the first 16 octet segment of the password and
	  placed in the first 16 octets of the String field of the User-
	  Password Attribute.

	  If the password is longer than 16 characters, a second one-way MD5
	  hash is calculated over a stream of octets consisting of the
	  shared secret followed by the result of the first xor.  That hash
	  is XORed with the second 16 octet segment of the password and
	  placed in the second 16 octets of the String field of the User-
	  Password Attribute.

	  If necessary, this operation is repeated, with each xor result
	  being used along with the shared secret to generate the next hash
	  to xor the next segment of the password, to no more than 128
	  characters.

	  The method is taken from the book "Network Security" by Kaufman,
	  Perlman and Speciner [4] pages 109-110.  A more precise
	  explanation of the method follows:





Rigney, et. al.             Standards Track                    [Page 22]

RFC 2138                         RADIUS                       April 1997


	  Call the shared secret S and the pseudo-random 128-bit Request
	  Authenticator RA.  Break the password into 16-octet chunks p1, p2,
	  etc.  with the last one padded at the end with nulls to a 16-octet
	  boundary.  Call the ciphertext blocks c(1), c(2), etc.  We'll need
	  intermediate values b1, b2, etc.

		 b1 = MD5(S + RA)       c(1) = p1 xor b1
		 b2 = MD5(S + c(1))     c(2) = p2 xor b2
				.                       .
				.                       .
				.                       .
		 bi = MD5(S + c(i-1))   c(i) = pi xor bi

	  The String will contain c(1)+c(2)+...+c(i) where + denotes
	  concatenation.

	  On receipt, the process is reversed to yield the original
	  password.

   A summary of the User-Password Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  2 for User-Password.

   Length

	  At least 18 and no larger than 130.

   String

	  The String field is between 16 and 128 octets long, inclusive.

5.3.  CHAP-Password

   Description

	  This Attribute indicates the response value provided by a PPP
	  Challenge-Handshake Authentication Protocol (CHAP) user in
	  response to the challenge.  It is only used in Access-Request
	  packets.



Rigney, et. al.             Standards Track                    [Page 23]

RFC 2138                         RADIUS                       April 1997


	  The CHAP challenge value is found in the CHAP-Challenge Attribute
	  (60) if present in the packet, otherwise in the Request
	  Authenticator field.

   A summary of the CHAP-Password Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  CHAP Ident   |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  3 for CHAP-Password.

   Length

	  19

   CHAP Ident

	  This field is one octet, and contains the CHAP Identifier from the
	  user's CHAP Response.

   String

	  The String field is 16 octets, and contains the CHAP Response from
	  the user.

5.4.  NAS-IP-Address

   Description

	  This Attribute indicates the identifying IP Address of the NAS
	  which is requesting authentication of the user.  It is only used
	  in Access-Request packets.  Either NAS-IP-Address or NAS-
	  Identifier SHOULD be present in an Access-Request packet.












Rigney, et. al.             Standards Track                    [Page 24]

RFC 2138                         RADIUS                       April 1997


   A summary of the NAS-IP-Address Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  4 for NAS-IP-Address.

   Length

	  6

   Address

	  The Address field is four octets.

5.5.  NAS-Port

   Description

	  This Attribute indicates the physical port number of the NAS which
	  is authenticating the user.  It is only used in Access-Request
	  packets.  Note that this is using "port" in its sense of a
	  physical connection on the NAS, not in the sense of a TCP or UDP
	  port number.  Either NAS-Port or NAS-Port-Type (61) or both SHOULD
	  be present in an Access-Request packet, if the NAS differentiates
	  among its ports.

   A summary of the NAS-Port Attribute format is shown below.  The
   fields are transmitted from left to right.














Rigney, et. al.             Standards Track                    [Page 25]

RFC 2138                         RADIUS                       April 1997


	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  5 for NAS-Port.

   Length

	  6

   Value

	  The Value field is four octets.  Despite the size of the field,
	  values range from 0 to 65535.

5.6.  Service-Type

   Description

	  This Attribute indicates the type of service the user has
	  requested, or the type of service to be provided.  It MAY be used
	  in both Access-Request and Access-Accept packets.  A NAS is not
	  required to implement all of these service types, and MUST treat
	  unknown or unsupported Service-Types as though an Access-Reject
	  had been received instead.

   A summary of the Service-Type Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  6 for Service-Type.





Rigney, et. al.             Standards Track                    [Page 26]

RFC 2138                         RADIUS                       April 1997


   Length

	  6

   Value

	  The Value field is four octets.

	   1      Login
	   2      Framed
	   3      Callback Login
	   4      Callback Framed
	   5      Outbound
	   6      Administrative
	   7      NAS Prompt
	   8      Authenticate Only
	   9      Callback NAS Prompt


	  The service types are defined as follows when used in an Access-
	  Accept.  When used in an Access-Request, they should be considered
	  to be a hint to the RADIUS server that the NAS has reason to
	  believe the user would prefer the kind of service indicated, but
	  the server is not required to honor the hint.

	  Login               The user should be connected to a host.

	  Framed              A Framed Protocol should be started for the
						  User, such as PPP or SLIP.

	  Callback Login      The user should be disconnected and called
						  back, then connected to a host.

	  Callback Framed     The user should be disconnected and called
						  back, then a Framed Protocol should be started
						  for the User, such as PPP or SLIP.

	  Outbound            The user should be granted access to outgoing
						  devices.

	  Administrative      The user should be granted access to the
						  administrative interface to the NAS from which
						  privileged commands can be executed.

	  NAS Prompt          The user should be provided a command prompt
						  on the NAS from which non-privileged commands
						  can be executed.




Rigney, et. al.             Standards Track                    [Page 27]

RFC 2138                         RADIUS                       April 1997


	  Authenticate Only   Only Authentication is requested, and no
						  authorization information needs to be returned
						  in the Access-Accept (typically used by proxy
						  servers rather than the NAS itself).

	  Callback NAS Prompt The user should be disconnected and called
						  back, then provided a command prompt on the
						  NAS from which non-privileged commands can be
						  executed.

5.7.  Framed-Protocol

   Description

	  This Attribute indicates the framing to be used for framed access.
	  It MAY be used in both Access-Request and Access-Accept packets.

   A summary of the Framed-Protocol Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  7 for Framed-Protocol.

   Length

	  6

   Value

	  The Value field is four octets.

	   1      PPP
	   2      SLIP
	   3      AppleTalk Remote Access Protocol (ARAP)
	   4      Gandalf proprietary SingleLink/MultiLink protocol
	   5      Xylogics proprietary IPX/SLIP






Rigney, et. al.             Standards Track                    [Page 28]

RFC 2138                         RADIUS                       April 1997


5.8.  Framed-IP-Address

   Description

	  This Attribute indicates the address to be configured for the
	  user.  It MAY be used in Access-Accept packets.  It MAY be used in
	  an Access-Request packet as a hint by the NAS to the server that
	  it would prefer that address, but the server is not required to
	  honor the hint.

   A summary of the Framed-IP-Address Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  8 for Framed-IP-Address.

   Length

	  6

   Address

	  The Address field is four octets.  The value 0xFFFFFFFF indicates
	  that the NAS should allow the user to select an address (e.g.
	  Negotiated).  The value 0xFFFFFFFE indicates that the NAS should
	  select an address for the user (e.g. Assigned from a pool of
	  addresses kept by the NAS).  Other valid values indicate that the
	  NAS should use that value as the user's IP address.

5.9.  Framed-IP-Netmask

   Description

	  This Attribute indicates the IP netmask to be configured for the
	  user when the user is a router to a network.  It MAY be used in
	  Access-Accept packets.  It MAY be used in an Access-Request packet
	  as a hint by the NAS to the server that it would prefer that
	  netmask, but the server is not required to honor the hint.




Rigney, et. al.             Standards Track                    [Page 29]

RFC 2138                         RADIUS                       April 1997


   A summary of the Framed-IP-Netmask Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  9 for Framed-IP-Netmask.

   Length

	  6

   Address

	  The Address field is four octets specifying the IP netmask of the
	  user.

5.10.  Framed-Routing

   Description

	  This Attribute indicates the routing method for the user, when the
	  user is a router to a network.  It is only used in Access-Accept
	  packets.

   A summary of the Framed-Routing Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  10 for Framed-Routing.





Rigney, et. al.             Standards Track                    [Page 30]

RFC 2138                         RADIUS                       April 1997


   Length

	  6

   Value

	  The Value field is four octets.

	   0      None
	   1      Send routing packets
	   2      Listen for routing packets
	   3      Send and Listen

5.11.  Filter-Id

   Description

	  This Attribute indicates the name of the filter list for this
	  user.  Zero or more Filter-Id attributes MAY be sent in an
	  Access-Accept packet.

	  Identifying a filter list by name allows the filter to be used on
	  different NASes without regard to filter-list implementation
	  details.

   A summary of the Filter-Id Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  11 for Filter-Id.

   Length

	  >= 3










Rigney, et. al.             Standards Track                    [Page 31]

RFC 2138                         RADIUS                       April 1997


   String

	  The String field is one or more octets, and its contents are
	  implementation dependent.  It is intended to be human readable and
	  MUST NOT affect operation of the protocol.  It is recommended that
	  the message contain displayable ASCII characters from the range 32
	  through 126 decimal.

5.12.  Framed-MTU

   Description

	  This Attribute indicates the Maximum Transmission Unit to be
	  configured for the user, when it is not negotiated by some other
	  means (such as PPP).  It is only used in Access-Accept packets.

	  A summary of the Framed-MTU Attribute format is shown below.  The
	  fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  12 for Framed-MTU.

   Length

	  6

   Value

	  The Value field is four octets.  Despite the size of the field,
	  values range from 64 to 65535.












Rigney, et. al.             Standards Track                    [Page 32]

RFC 2138                         RADIUS                       April 1997


5.13.  Framed-Compression

   Description

	  This Attribute indicates a compression protocol to be used for the
	  link.  It MAY be used in Access-Accept packets.  It MAY be used in
	  an Access-Request packet as a hint to the server that the NAS
	  would prefer to use that compression, but the server is not
	  required to honor the hint.

	  More than one compression protocol Attribute MAY be sent.  It is
	  the responsibility of the NAS to apply the proper compression
	  protocol to appropriate link traffic.

   A summary of the Framed-Compression Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  13 for Framed-Compression.

   Length

	  6

   Value

	  The Value field is four octets.

	   0      None
	   1      VJ TCP/IP header compression [5]
	   2      IPX header compression

5.14.  Login-IP-Host

   Description

	  This Attribute indicates the system with which to connect the
	  user, when the Login-Service Attribute is included.  It MAY be
	  used in Access-Accept packets.  It MAY be used in an Access-



Rigney, et. al.             Standards Track                    [Page 33]

RFC 2138                         RADIUS                       April 1997


	  Request packet as a hint to the server that the NAS would prefer
	  to use that host, but the server is not required to honor the
	  hint.

   A summary of the Login-IP-Host Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |            Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Address (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  14 for Login-IP-Host.

   Length

	  6

   Address

	  The Address field is four octets.  The value 0xFFFFFFFF indicates
	  that the NAS SHOULD allow the user to select an address.  The
	  value 0 indicates that the NAS SHOULD select a host to connect the
	  user to.  Other values indicate the address the NAS SHOULD connect
	  the user to.

5.15.  Login-Service

   Description

	  This Attribute indicates the service which should be used to
	  connect the user to the login host.  It is only used in Access-
	  Accept packets.

   A summary of the Login-Service Attribute format is shown below.  The
   fields are transmitted from left to right.










Rigney, et. al.             Standards Track                    [Page 34]

RFC 2138                         RADIUS                       April 1997


	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  15 for Login-Service.

   Length

	  6

   Value

	  The Value field is four octets.

	   0      Telnet
	   1      Rlogin
	   2      TCP Clear
	   3      PortMaster (proprietary)
	   4      LAT

5.16.  Login-TCP-Port

   Description

	  This Attribute indicates the TCP port with which the user is to be
	  connected, when the Login-Service Attribute is also present.  It
	  is only used in Access-Accept packets.

   A summary of the Login-TCP-Port Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  16 for Login-TCP-Port.



Rigney, et. al.             Standards Track                    [Page 35]

RFC 2138                         RADIUS                       April 1997


   Length

	  6

   Value

	  The Value field is four octets.  Despite the size of the field,
	  values range from 0 to 65535.

5.17.  (unassigned)

   Description

	  ATTRIBUTE TYPE 17 HAS NOT BEEN ASSIGNED.

5.18.  Reply-Message

   Description

	  This Attribute indicates text which MAY be displayed to the user.

	  When used in an Access-Accept, it is the success message.

	  When used in an Access-Reject, it is the failure message.  It MAY
	  indicate a dialog message to prompt the user before another
	  Access-Request attempt.

	  When used in an Access-Challenge, it MAY indicate a dialog message
	  to prompt the user for a response.

	  Multiple Reply-Message's MAY be included and if any are displayed,
	  they MUST be displayed in the same order as they appear in the
	  packet.

   A summary of the Reply-Message Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Type

	  18 for Reply-Message.




Rigney, et. al.             Standards Track                    [Page 36]

RFC 2138                         RADIUS                       April 1997


   Length

	  >= 3

   String

	  The String field is one or more octets, and its contents are
	  implementation dependent.  It is intended to be human readable,
	  and MUST NOT affect operation of the protocol.  It is recommended
	  that the message contain displayable ASCII characters from the
	  range 10, 13, and 32 through 126 decimal.  Mechanisms for
	  extension to other character sets are beyond the scope of this
	  specification.

5.19.  Callback-Number

   Description

	  This Attribute indicates a dialing string to be used for callback.
	  It MAY be used in Access-Accept packets.  It MAY be used in an
	  Access-Request packet as a hint to the server that a Callback
	  service is desired, but the server is not required to honor the
	  hint.

   A summary of the Callback-Number Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  19 for Callback-Number.

   Length

	  >= 3











Rigney, et. al.             Standards Track                    [Page 37]

RFC 2138                         RADIUS                       April 1997


   String

	  The String field is one or more octets.  The actual format of the
	  information is site or application specific, and a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.20.  Callback-Id

   Description

	  This Attribute indicates the name of a place to be called, to be
	  interpreted by the NAS.  It MAY be used in Access-Accept packets.

   A summary of the Callback-Id Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  20 for Callback-Id.

   Length

	  >= 3

   String

	  The String field is one or more octets.  The actual format of the
	  information is site or application specific, and a robust
	  implementation SHOULD support the field as undistinguished octets.
	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.21.  (unassigned)

   Description

	  ATTRIBUTE TYPE 21 HAS NOT BEEN ASSIGNED.





Rigney, et. al.             Standards Track                    [Page 38]

RFC 2138                         RADIUS                       April 1997


5.22.  Framed-Route

   Description

	  This Attribute provides routing information to be configured for
	  the user on the NAS.  It is used in the Access-Accept packet and
	  can appear multiple times.

   A summary of the Framed-Route Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  22 for Framed-Route.

   Length

	  >= 3

   String

	  The String field is one or more octets, and its contents are
	  implementation dependent.  It is intended to be human readable and
	  MUST NOT affect operation of the protocol.  It is recommended that
	  the message contain displayable ASCII characters from the range 32
	  through 126 decimal.

	  For IP routes, it SHOULD contain a destination prefix in dotted
	  quad form optionally followed by a slash and a decimal length
	  specifier stating how many high order bits of the prefix should be
	  used.  That is followed by a space, a gateway address in dotted
	  quad form, a space, and one or more metrics separated by spaces.
	  For example, "192.168.1.0/24 192.168.1.1 1 2 -1 3 400". The length
	  specifier may be omitted in which case it should default to 8 bits
	  for class A prefixes, 16 bits for class B prefixes, and 24 bits
	  for class C prefixes.  For example, "192.168.1.0 192.168.1.1 1".

	  Whenever the gateway address is specified as "0.0.0.0" the IP
	  address of the user SHOULD be used as the gateway address.






Rigney, et. al.             Standards Track                    [Page 39]

RFC 2138                         RADIUS                       April 1997


5.23.  Framed-IPX-Network

   Description

	  This Attribute indicates the IPX Network number to be configured
	  for the user.  It is used in Access-Accept packets.

   A summary of the Framed-IPX-Network Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  23 for Framed-IPX-Network.

   Length

	  6

   Value

	  The Value field is four octets.  The value 0xFFFFFFFE indicates
	  that the NAS should select an IPX network for the user (e.g.
	  assigned from a pool of one or more IPX networks kept by the NAS).
	  Other values should be used as the IPX network for the link to the
	  user.

5.24.  State

   Description

	  This Attribute is available to be sent by the server to the client
	  in an Access-Challenge and MUST be sent unmodified from the client
	  to the server in the new Access-Request reply to that challenge,
	  if any.









Rigney, et. al.             Standards Track                    [Page 40]

RFC 2138                         RADIUS                       April 1997


	  This Attribute is available to be sent by the server to the client
	  in an Access-Accept that also includes a Termination-Action
	  Attribute with the value of RADIUS-Request.  If the NAS performs
	  the Termination-Action by sending a new Access-Request upon
	  termination of the current session, it MUST include the State
	  attribute unchanged in that Access-Request.

	  In either usage, no interpretation by the client should be made.
	  A packet may have only one State Attribute.  Usage of the State
	  Attribute is implementation dependent.

   A summary of the State Attribute format is shown below.  The fields
   are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  24 for State.

   Length

	  >= 3

   String

	  The String field is one or more octets.  The actual format of the
	  information is site or application specific, and a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.25.  Class

   Description

	  This Attribute is available to be sent by the server to the client
	  in an Access-Accept and should be sent unmodified by the client to
	  the accounting server as part of the Accounting-Request packet if
	  accounting is supported.  No interpretation by the client should
	  be made.





Rigney, et. al.             Standards Track                    [Page 41]

RFC 2138                         RADIUS                       April 1997


   A summary of the Class Attribute format is shown below.  The fields
   are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  25 for Class.

   Length

	  >= 3

   String

	  The String field is one or more octets.  The actual format of the
	  information is site or application specific, and a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.26.  Vendor-Specific

   Description

	  This Attribute is available to allow vendors to support their own
	  extended Attributes not suitable for general usage.  It MUST not
	  affect the operation of the RADIUS protocol.

	  Servers not equipped to interpret the vendor-specific information
	  sent by a client MUST ignore it (although it may be reported).
	  Clients which do not receive desired vendor-specific information
	  SHOULD make an attempt to operate without it, although they may do
	  so (and report they are doing so) in a degraded mode.

   A summary of the Vendor-Specific Attribute format is shown below.
   The fields are transmitted from left to right.









Rigney, et. al.             Standards Track                    [Page 42]

RFC 2138                         RADIUS                       April 1997


	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length       |            Vendor-Id
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		Vendor-Id (cont)           |  String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  26 for Vendor-Specific.

   Length

	   >= 7

   Vendor-Id

	  The high-order octet is 0 and the low-order 3 octets are the SMI
	  Network Management Private Enterprise Code of the Vendor in
	  network byte order, as defined in the Assigned Numbers RFC [3].

   String

	  The String field is one or more octets.  The actual format of the
	  information is site or application specific, and a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

	  It SHOULD be encoded as a sequence of vendor type / vendor length
	  / value fields, as follows.  The Attribute-Specific field is
	  dependent on the vendor's definition of that attribute.  An
	  example encoding of the Vendor-Specific attribute using this
	  method follows:

	   0                   1                   2                   3
	   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	  |     Type      |  Length       |            Vendor-Id
	  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		   Vendor-Id (cont)           | Vendor type   | Vendor length |
	  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	  |    Attribute-Specific...
	  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-





Rigney, et. al.             Standards Track                    [Page 43]

RFC 2138                         RADIUS                       April 1997


5.27.  Session-Timeout

   Description

	  This Attribute sets the maximum number of seconds of service to be
	  provided to the user before termination of the session or prompt.
	  This Attribute is available to be sent by the server to the client
	  in an Access-Accept or Access-Challenge.

   A summary of the Session-Timeout Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  27 for Session-Timeout.

   Length

	  6

   Value

	  The field is 4 octets, containing a 32-bit unsigned integer with
	  the maximum number of seconds this user should be allowed to
	  remain connected by the NAS.

5.28.  Idle-Timeout

   Description

	  This Attribute sets the maximum number of consecutive seconds of
	  idle connection allowed to the user before termination of the
	  session or prompt.  This Attribute is available to be sent by the
	  server to the client in an Access-Accept or Access-Challenge.









Rigney, et. al.             Standards Track                    [Page 44]

RFC 2138                         RADIUS                       April 1997


   A summary of the Idle-Timeout Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  28 for Idle-Timeout.

   Length

	  6

   Value

	  The field is 4 octets, containing a 32-bit unsigned integer with
	  the maximum number of consecutive seconds of idle time this user
	  should be permitted before being disconnected by the NAS.

5.29.  Termination-Action

   Description

	  This Attribute indicates what action the NAS should take when the
	  specified service is completed.  It is only used in Access-Accept
	  packets.

   A summary of the Termination-Action Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  29 for Termination-Action.




Rigney, et. al.             Standards Track                    [Page 45]

RFC 2138                         RADIUS                       April 1997


   Length

	  6

   Value

	  The Value field is four octets.

	   0      Default
	   1      RADIUS-Request

	  If the Value is set to RADIUS-Request, upon termination of the
	  specified service the NAS MAY send a new Access-Request to the
	  RADIUS server, including the State attribute if any.

5.30.  Called-Station-Id

   Description

   This Attribute allows the NAS to send in the Access-Request packet
   the phone number that the user called, using Dialed Number
   Identification (DNIS) or similar technology.  Note that this may be
   different from the phone number the call comes in on.  It is only
   used in Access-Request packets.

   A summary of the Called-Station-Id Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Type

	  30 for Called-Station-Id.

   Length

	  >= 3

   String

	  The String field is one or more octets, containing the phone
	  number that the user's call came in on.




Rigney, et. al.             Standards Track                    [Page 46]

RFC 2138                         RADIUS                       April 1997


	  The actual format of the information is site or application
	  specific.  Printable ASCII is recommended, but a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.31.  Calling-Station-Id

   Description

	  This Attribute allows the NAS to send in the Access-Request packet
	  the phone number that the call came from, using Automatic Number
	  Identification (ANI) or similar technology.  It is only used in
	  Access-Request packets.

   A summary of the Calling-Station-Id Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  31 for Calling-Station-Id.

   Length

	  >= 3

   String

	  The String field is one or more octets, containing the phone
	  number that the user placed the call from.

	  The actual format of the information is site or application
	  specific.  Printable ASCII is recommended, but a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.







Rigney, et. al.             Standards Track                    [Page 47]

RFC 2138                         RADIUS                       April 1997


5.32.  NAS-Identifier

   Description

	  This Attribute contains a string identifying the NAS originating
	  the Access-Request.  It is only used in Access-Request packets.
	  Either NAS-IP-Address or NAS-Identifier SHOULD be present in an
	  Access-Request packet.

   A summary of the NAS-Identifier Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  32 for NAS-Identifier.

   Length

	  >= 3

   String

	  The String field is one or more octets, and should be unique to
	  the NAS within the scope of the RADIUS server.  For example, a
	  fully qualified domain name would be suitable as a NAS-Identifier.

	  The actual format of the information is site or application
	  specific, and a robust implementation SHOULD support the field as
	  undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.33.  Proxy-State

   Description

	  This Attribute is available to be sent by a proxy server to
	  another server when forwarding an Access-Request and MUST be
	  returned unmodified in the Access-Accept, Access-Reject or
	  Access-Challenge.  This attribute should be removed by the proxy
	  server before the response is forwarded to the NAS.



Rigney, et. al.             Standards Track                    [Page 48]

RFC 2138                         RADIUS                       April 1997


	  Usage of the Proxy-State Attribute is implementation dependent.  A
	  description of its function is outside the scope of this
	  specification.

   A summary of the Proxy-State Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  33 for Proxy-State.

   Length

	  >= 3

   String

	  The String field is one or more octets.  The actual format of the
	  information is site or application specific, and a robust
	  implementation SHOULD support the field as undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.34.  Login-LAT-Service

   Description

	  This Attribute indicates the system with which the user is to be
	  connected by LAT.  It MAY be used in Access-Accept packets, but
	  only when LAT is specified as the Login-Service.  It MAY be used
	  in an Access-Request packet as a hint to the server, but the
	  server is not required to honor the hint.

	  Administrators use the service attribute when dealing with
	  clustered systems, such as a VAX or Alpha cluster. In such an
	  environment several different time sharing hosts share the same
	  resources (disks, printers, etc.), and administrators often
	  configure each to offer access (service) to each of the shared
	  resources. In this case, each host in the cluster advertises its
	  services through LAT broadcasts.




Rigney, et. al.             Standards Track                    [Page 49]

RFC 2138                         RADIUS                       April 1997


	  Sophisticated users often know which service providers (machines)
	  are faster and tend to use a node name when initiating a LAT
	  connection.  Alternately, some administrators want particular
	  users to use certain machines as a primitive form of load
	  balancing (although LAT knows how to do load balancing itself).

   A summary of the Login-LAT-Service Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Type

	  34 for Login-LAT-Service.

   Length

	  >= 3

   String

	  The String field is one or more octets, and contains the identity
	  of the LAT service to use.  The LAT Architecture allows this
	  string to contain $ (dollar), - (hyphen), . (period), _
	  (underscore), numerics, upper and lower case alphabetics, and the
	  ISO Latin-1 character set extension [6].  All LAT string
	  comparisons are case insensitive.

5.35.  Login-LAT-Node

   Description

	  This Attribute indicates the Node with which the user is to be
	  automatically connected by LAT.  It MAY be used in Access-Accept
	  packets, but only when LAT is specified as the Login-Service.  It
	  MAY be used in an Access-Request packet as a hint to the server,
	  but the server is not required to honor the hint.

   A summary of the Login-LAT-Node Attribute format is shown below.  The
   fields are transmitted from left to right.






Rigney, et. al.             Standards Track                    [Page 50]

RFC 2138                         RADIUS                       April 1997


	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  35 for Login-LAT-Node.

   Length

	  >= 3

   String

	  The String field is one or more octets, and contains the identity
	  of the LAT Node to connect the user to.  The LAT Architecture
	  allows this string to contain $ (dollar), - (hyphen), . (period),
	  _ (underscore), numerics, upper and lower case alphabetics, and
	  the ISO Latin-1 character set extension.  All LAT string
	  comparisons are case insensitive.

5.36.  Login-LAT-Group

   Description

	  This Attribute contains a string identifying the LAT group codes
	  which this user is authorized to use.  It MAY be used in Access-
	  Accept packets, but only when LAT is specified as the Login-
	  Service.  It MAY be used in an Access-Request packet as a hint to
	  the server, but the server is not required to honor the hint.

	  LAT supports 256 different group codes, which LAT uses as a form
	  of access rights.  LAT encodes the group codes as a 256 bit
	  bitmap.

	  Administrators can assign one or more of the group code bits at
	  the LAT service provider; it will only accept LAT connections that
	  have these group codes set in the bit map. The administrators
	  assign a bitmap of authorized group codes to each user; LAT gets
	  these from the operating system, and uses these in its requests to
	  the service providers.








Rigney, et. al.             Standards Track                    [Page 51]

RFC 2138                         RADIUS                       April 1997


   A summary of the Login-LAT-Group Attribute format is shown below.
   The fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  36 for Login-LAT-Group.

   Length

	  34

   String

	  The String field is a 32 octet bit map, most significant octet
	  first.  A robust implementation SHOULD support the field as
	  undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.

5.37.  Framed-AppleTalk-Link

   Description

	  This Attribute indicates the AppleTalk network number which should
	  be used for the serial link to the user, which is another
	  AppleTalk router.  It is only used in Access-Accept packets.  It
	  is never used when the user is not another router.

   A summary of the Framed-AppleTalk-Link Attribute format is shown
   below.  The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






Rigney, et. al.             Standards Track                    [Page 52]

RFC 2138                         RADIUS                       April 1997


   Type

	  37 for Framed-AppleTalk-Link.

   Length

	  6

   Value

	  The Value field is four octets.  Despite the size of the field,
	  values range from 0 to 65535.  The special value of 0 indicates
	  that this is an unnumbered serial link.  A value of 1-65535 means
	  that the serial line between the NAS and the user should be
	  assigned that value as an AppleTalk network number.

5.38.  Framed-AppleTalk-Network

   Description

	  This Attribute indicates the AppleTalk Network number which the
	  NAS should probe to allocate an AppleTalk node for the user.  It
	  is only used in Access-Accept packets.  It is never used when the
	  user is another router.  Multiple instances of this Attribute
	  indicate that the NAS may probe using any of the network numbers
	  specified.

   A summary of the Framed-AppleTalk-Network Attribute format is shown
   below.  The fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  38 for Framed-AppleTalk-Network.

   Length

	  6






Rigney, et. al.             Standards Track                    [Page 53]

RFC 2138                         RADIUS                       April 1997


   Value

	  The Value field is four octets.  Despite the size of the field,
	  values range from 0 to 65535.  The special value 0 indicates that
	  the NAS should assign a network for the user, using its default
	  cable range.  A value between 1 and 65535 (inclusive) indicates
	  the AppleTalk Network the NAS should probe to find an address for
	  the user.

5.39.  Framed-AppleTalk-Zone

   Description

	  This Attribute indicates the AppleTalk Default Zone to be used for
	  this user.  It is only used in Access-Accept packets.  Multiple
	  instances of this attribute in the same packet are not allowed.

   A summary of the Framed-AppleTalk-Zone Attribute format is shown
   below.  The fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  39 for Framed-AppleTalk-Zone.

   Length

	  >= 3

   String

	  The name of the Default AppleTalk Zone to be used for this user.
	  A robust implementation SHOULD support the field as
	  undistinguished octets.

	  The codification of the range of allowed usage of this field is
	  outside the scope of this specification.









Rigney, et. al.             Standards Track                    [Page 54]

RFC 2138                         RADIUS                       April 1997


5.40.  CHAP-Challenge

   Description

	  This Attribute contains the CHAP Challenge sent by the NAS to a
	  PPP Challenge-Handshake Authentication Protocol (CHAP) user.  It
	  is only used in Access-Request packets.

	  If the CHAP challenge value is 16 octets long it MAY be placed in
	  the Request Authenticator field instead of using this attribute.

   A summary of the CHAP-Challenge Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |    String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  60 for CHAP-Challenge.

   Length

	  >= 7

   String

	  The String field contains the CHAP Challenge.

5.41.  NAS-Port-Type

   Description

	  This Attribute indicates the type of the physical port of the NAS
	  which is authenticating the user.  It can be used instead of or in
	  addition to the NAS-Port (5) attribute.  It is only used in
	  Access-Request packets.  Either NAS-Port (5) or NAS-Port-Type or
	  both SHOULD be present in an Access-Request packet, if the NAS
	  differentiates among its ports.









Rigney, et. al.             Standards Track                    [Page 55]

RFC 2138                         RADIUS                       April 1997


   A summary of the NAS-Port-Type Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  61 for NAS-Port-Type.

   Length

	  6

   Value

	  The Value field is four octets.  "Virtual" refers to a connection
	  to the NAS via some transport protocol, instead of through a
	  physical port.  For example, if a user telnetted into a NAS to
	  authenticate himself as an Outbound-User, the Access-Request might
	  include NAS-Port-Type = Virtual as a hint to the RADIUS server
	  that the user was not on a physical port.

	  0       Async
	  1       Sync
	  2       ISDN Sync
	  3       ISDN Async V.120
	  4       ISDN Async V.110
	  5       Virtual

5.42.  Port-Limit

   Description

	  This Attribute sets the maximum number of ports to be provided to
	  the user by the NAS.  This Attribute MAY be sent by the server to
	  the client in an Access-Accept packet.  It is intended for use in
	  conjunction with Multilink PPP [7] or similar uses.  It MAY also
	  be sent by the NAS to the server as a hint that that many ports
	  are desired for use, but the server is not required to honor the
	  hint.





Rigney, et. al.             Standards Track                    [Page 56]

RFC 2138                         RADIUS                       April 1997


   A summary of the Port-Limit Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2                   3
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |             Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			  Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

	  62 for Port-Limit.

   Length

	  6

   Value

	  The field is 4 octets, containing a 32-bit unsigned integer with
	  the maximum number of ports this user should be allowed to connect
	  to on the NAS.

5.43.  Login-LAT-Port

   Description

	  This Attribute indicates the Port with which the user is to be
	  connected by LAT.  It MAY be used in Access-Accept packets, but
	  only when LAT is specified as the Login-Service.  It MAY be used
	  in an Access-Request packet as a hint to the server, but the
	  server is not required to honor the hint.

   A summary of the Login-LAT-Port Attribute format is shown below.  The
   fields are transmitted from left to right.

	0                   1                   2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

   Type

	  63 for Login-LAT-Port.




Rigney, et. al.             Standards Track                    [Page 57]

RFC 2138                         RADIUS                       April 1997


   Length

	  >= 3

   String

	  The String field is one or more octets, and contains the identity
	  of the LAT port to use.  The LAT Architecture allows this string
	  to contain $ (dollar), - (hyphen), . (period), _ (underscore),
	  numerics, upper and lower case alphabetics, and the ISO Latin-1
	  character set extension.  All LAT string comparisons are case
	  insensitive.

5.44.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in which kinds of packets, and in what quantity.

   Request   Accept   Reject   Challenge   #    Attribute
   1         0        0        0            1   User-Name
   0-1       0        0        0            2   User-Password [Note 1]
   0-1       0        0        0            3   CHAP-Password [Note 1]
   0-1       0        0        0            4   NAS-IP-Address
   0-1       0        0        0            5   NAS-Port
   0-1       0-1      0        0            6   Service-Type
   0-1       0-1      0        0            7   Framed-Protocol
   0-1       0-1      0        0            8   Framed-IP-Address
   0-1       0-1      0        0            9   Framed-IP-Netmask
   0         0-1      0        0           10   Framed-Routing
   0         0+       0        0           11   Filter-Id
   0         0-1      0        0           12   Framed-MTU
   0+        0+       0        0           13   Framed-Compression
   0+        0+       0        0           14   Login-IP-Host
   0         0-1      0        0           15   Login-Service
   0         0-1      0        0           16   Login-TCP-Port
   0         0+       0+       0+          18   Reply-Message
   0-1       0-1      0        0           19   Callback-Number
   0         0-1      0        0           20   Callback-Id
   0         0+       0        0           22   Framed-Route
   0         0-1      0        0           23   Framed-IPX-Network
   0-1       0-1      0        0-1         24   State
   0         0+       0        0           25   Class
   0+        0+       0        0+          26   Vendor-Specific
   0         0-1      0        0-1         27   Session-Timeout
   0         0-1      0        0-1         28   Idle-Timeout
   0         0-1      0        0           29   Termination-Action
   0-1       0        0        0           30   Called-Station-Id
   0-1       0        0        0           31   Calling-Station-Id



Rigney, et. al.             Standards Track                    [Page 58]

RFC 2138                         RADIUS                       April 1997


   0-1       0        0        0           32   NAS-Identifier
   0+        0+       0+       0+          33   Proxy-State
   0-1       0-1      0        0           34   Login-LAT-Service
   0-1       0-1      0        0           35   Login-LAT-Node
   0-1       0-1      0        0           36   Login-LAT-Group
   0         0-1      0        0           37   Framed-AppleTalk-Link
   0         0+       0        0           38   Framed-AppleTalk-Network
   0         0-1      0        0           39   Framed-AppleTalk-Zone
   0-1       0        0        0           60   CHAP-Challenge
   0-1       0        0        0           61   NAS-Port-Type
   0-1       0-1      0        0           62   Port-Limit
   0-1       0-1      0        0           63   Login-LAT-Port


   Request   Accept   Reject   Challenge   #    Attribute


   [Note 1] An Access-Request MUST contain either a User-Password or a
   CHAP-Password, and MUST NOT contain both.

   The following table defines the meaning of the above table entries.

 0     This attribute MUST NOT be present in packet.
 0+    Zero or more instances of this attribute MAY be present in packet.
 0-1   Zero or one instance of this attribute MAY be present in packet.
 1     Exactly one instance of this attribute MUST be present in packet.

6.  Examples

   A few examples are presented to illustrate the flow of packets and
   use of typical attributes.  These examples are not intended to be
   exhaustive, many others are possible.



















Rigney, et. al.             Standards Track                    [Page 59]

RFC 2138                         RADIUS                       April 1997


6.1.  User Telnet to Specified Host

   The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
   RADIUS Server for a user named nemo logging in on port 3.

	  Code = 1        (Access-Request)
	  ID = 0
	  Length = 56
	  Request Authenticator = {16 octet random number}
	  Attributes:
		  User-Name = "nemo"
		  User-Password = {16 octets of Password padded at end with nulls,
					  XORed with MD5(shared secret|Request Authenticator)}
		  NAS-IP-Address = 192.168.1.16
		  NAS-Port = 3

   The RADIUS server authenticates nemo, and sends an Access-Accept UDP
   packet to the NAS telling it to telnet nemo to host 192.168.1.3.

	  Code = 2        (Access-Accept)
	  ID = 0          (same as in Access-Request)
	  Length = 38
	  Response Authenticator = {16-octet MD-5 checksum of the code (2),
					  id (0), Length (38), the Request Authenticator from
					  above, the attributes in this reply, and the shared
					  secret}
	  Attributes:
		  Service-Type = Login-User
		  Login-Service = Telnet
		  Login-Host = 192.168.1.3

6.2.  Framed User Authenticating with CHAP

   The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
   RADIUS Server for a user named flopsy logging in on port 20 with PPP,
   authenticating using CHAP.  The NAS sends along the Service-Type and
   Framed-Protocol attributes as a hint to the RADIUS server that this
   user is looking for PPP, although the NAS is not required to do so.













Rigney, et. al.             Standards Track                    [Page 60]

RFC 2138                         RADIUS                       April 1997


	  Code = 1        (Access-Request)
	  ID = 1
	  Length = 71
	  Request Authenticator = {16 octet random number also used as
							   CHAP challenge}
	  Attributes:
		  User-Name = "flopsy"
		  CHAP-Password = {1 octet CHAP ID followed by 16 octet
						   CHAP response}
		  NAS-IP-Address = 192.168.1.16
		  NAS-Port = 20
		  Service-Type = Framed-User
		  Framed-Protocol = PPP

   The RADIUS server authenticates flopsy, and sends an Access-Accept
   UDP packet to the NAS telling it to start PPP service and assign an
   address for the user out of its dynamic address pool.

	  Code = 2        (Access-Accept)
	  ID = 1          (same as in Access-Request)
	  Length = 56
	  Response Authenticator = {16-octet MD-5 checksum of the code (2),
					  id (1), Length (56), the Request Authenticator from
					  above, the attributes in this reply, and the shared
					  secret}
	  Attributes:
		  Service-Type = Framed-User
		  Framed-Protocol = PPP
		  Framed-IP-Address = 255.255.255.254
		  Framed-Routing = None
		  Framed-Compression = 1      (VJ TCP/IP Header Compression)
		  Framed-MTU = 1500

6.3.  User with Challenge-Response card

   The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
   RADIUS Server for a user named mopsy logging in on port 7.

	  Code = 1        (Access-Request)
	  ID = 2
	  Length = 57
	  Request Authenticator = {16 octet random number}
	  Attributes:
		  User-Name = "mopsy"
		  User-Password = {16 octets of Password padded at end with nulls,
					  XORed with MD5(shared secret|Request Authenticator)}
		  NAS-IP-Address = 192.168.1.16
		  NAS-Port = 7



Rigney, et. al.             Standards Track                    [Page 61]

RFC 2138                         RADIUS                       April 1997


   The RADIUS server decides to challenge mopsy, sending back a
   challenge string and looking for a response.  The RADIUS server
   therefore and sends an Access-Challenge UDP packet to the NAS.

	  Code = 11       (Access-Challenge}
	  ID = 2          (same as in Access-Request)
	  Length = 78
	  Response Authenticator = {16-octet MD-5 checksum of the code (11),
					  id (2), length (78), the Request Authenticator from
					  above, the attributes in this reply, and the shared
					  secret}
	  Attributes:
		  Reply-Message = "Challenge 32769430.  Enter response at prompt."
		  State =     {Magic Cookie to be returned along with user's response;
					   in this example 8 octets of data}

   The user enters his response, and the NAS send a new Access-Request
   with that response, and includes the State Attribute.

	  Code = 1        (Access-Request)
	  ID = 3          (Note that this changes)
	  Length = 67
	  Request Authenticator = {NEW 16 octet random number}
	  Attributes:
		  User-Name = "mopsy"
		  User-Password = {16 octets of Response padded at end with
					  nulls, XORed with MD5 checksum of shared secret
					  plus above Request Authenticator}
		  NAS-IP-Address = 192.168.1.16
		  NAS-Port = 7
		  State =     {Magic Cookie from Access-Challenge packet, unchanged}

   The Response was incorrect, so the RADIUS server tells the NAS to
   reject the login attempt.

	  Code = 3        (Access-Reject)
	  ID = 3          (same as in Access-Request)
	  Length = 20
	  Response Authenticator = {16-octet MD-5 checksum of the code (3),
					  id (3), length(20), the Request Authenticator from
					  above, the attributes in this reply if any, and the
					  shared secret}
	  Attributes:
			  (none, although a Reply-Message could be sent)







Rigney, et. al.             Standards Track                    [Page 62]

RFC 2138                         RADIUS                       April 1997


Security Considerations

   Security issues are the primary topic of this document.

   In practice, within or associated with each RADIUS server, there is a
   database which associates "user" names with authentication
   information ("secrets").  It is not anticipated that a particular
   named user would be authenticated by multiple methods.  This would
   make the user vulnerable to attacks which negotiate the least secure
   method from among a set.  Instead, for each named user there should
   be an indication of exactly one method used to authenticate that user
   name.  If a user needs to make use of different authentication
   methods under different circumstances, then distinct user names
   SHOULD be employed, each of which identifies exactly one
   authentication method.

   Passwords and other secrets should be stored at the respective ends
   such that access to them is as limited as possible.  Ideally, the
   secrets should only be accessible to the process requiring access in
   order to perform the authentication.

   The secrets should be distributed with a mechanism that limits the
   number of entities that handle (and thus gain knowledge of) the
   secret.  Ideally, no unauthorized person should ever gain knowledge
   of the secrets.  It is possible to achieve this with SNMP Security
   Protocols [8], but such a mechanism is outside the scope of this
   specification.

   Other distribution methods are currently undergoing research and
   experimentation.  The SNMP Security document [8] also has an
   excellent overview of threats to network protocols.




















Rigney, et. al.             Standards Track                    [Page 63]

RFC 2138                         RADIUS                       April 1997


References

   [1]   Rivest, R., and S. Dusse, "The MD5 Message-Digest Algorithm",
		 RFC 1321, MIT Laboratory for Computer Science, RSA Data
		 Security Inc., April 1992.

   [2]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
		 USC/Information Sciences Institute, August 1980.

   [3]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
		 1700, USC/Information Sciences Institute, October 1994.

   [4]   Kaufman, C., Perlman, R., and Speciner, M., "Network Security:
		 Private Communications in a Public World", Prentice Hall, March
		 1995, ISBN 0-13-061466-1.

   [5]   Jacobson, V., "Compressing TCP/IP headers for low-speed serial
		 links", RFC 1144, Lawrence Berkeley Laboratory, February 1990.

   [6]   ISO 8859. International Standard -- Information Processing --
		 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin
		 Alphabet No. 1, ISO 8859-1:1987.
		 <URL:http://www.iso.ch/cate/d16338.html>

   [7]   Sklower, K., Lloyd, B., McGregor, G., and Carr, D., "The PPP
		 Multilink Protocol (MP)", RFC 1717, University of California
		 Berkeley, Lloyd Internetworking, Newbridge Networks
		 Corporation, November 1994.

   [8]   Galvin, J., McCloghrie, K., and Davin, J., "SNMP Security
		 Protocols", RFC 1352, Trusted Information Systems, Inc., Hughes
		 LAN Systems, Inc., MIT Laboratory for Computer Science, July
		 1992.

   [9]   Rigney, C., "RADIUS Accounting", RFC 2139, April 1997.

Acknowledgments

   RADIUS was originally developed by Livingston Enterprises for their
   PortMaster series of Network Access Servers.











Rigney, et. al.             Standards Track                    [Page 64]

RFC 2138                         RADIUS                       April 1997


Chair's Address

   The working group can be contacted via the current chair:

   Carl Rigney
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   Phone: +1 510 426 0770
   EMail: cdr@livingston.com

Authors' Addresses

   Questions about this memo can also be directed to:

   Carl Rigney
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   Phone: +1 510 426 0770
   EMail: cdr@livingston.com

   Allan C. Rubens
   Merit Network, Inc.
   4251 Plymouth Road
   Ann Arbor, Michigan  48105-2785

   EMail: acr@merit.edu

   William Allen Simpson
   Daydreamer
   Computer Systems Consulting Services
   1384 Fontaine
   Madison Heights, Michigan  48071

   EMail: wsimpson@greendragon.com

   Steve Willens
   Livingston Enterprises
   4464 Willow Road
   Pleasanton, California  94588

   EMail: steve@livingston.com






Rigney, et. al.             Standards Track                    [Page 65]


