RADIUS tunnel attributes: RADIUS Attributes for Tunnel Protocol Support

Network Working Group                                            G. Zorn
Request for Comments: 2868                           Cisco Systems, Inc.
Updates: RFC 2865                                              D. Leifer
Category: Informational                                        A. Rubens
                                                   Ascend Communications
                                                              J. Shriver
                                                       Intel Corporation
                                                             M. Holdrege
                                                                 ipVerse
                                                               I. Goyret
                                                     Lucent Technologies
                                                               June 2000


             RADIUS Attributes for Tunnel Protocol Support

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document defines a set of RADIUS attributes designed to support
   the provision of compulsory tunneling in dial-up networks.

1.  Motivation

   Many applications of tunneling protocols such as L2TP involve dial-up
   network access.  Some, such as the provision of access to corporate
   intranets via the Internet, are characterized by voluntary tunneling:
   the tunnel is created at the request of the user for a specific
   purpose.  Other applications involve compulsory tunneling: the tunnel
   is created without any action from the user and without allowing the
   user any choice in the matter.  In order to provide this
   functionality, new RADIUS attributes are needed to carry the
   tunneling information from the RADIUS server to the tunnel end
   points; this document defines those attributes.  Specific
   recommendations for, and examples of, the application of these
   attributes for L2TP can be found in RFC 2809.

2.  Specification of Requirements

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [14].

3.  Attributes

   Multiple instances of each of the attributes defined below may be
   included in a single RADIUS packet.  In this case, the attributes to
   be applied to any given tunnel SHOULD all contain the same value in
   their respective Tag fields; otherwise, the Tag field SHOULD NOT be
   used.

   If the RADIUS server returns attributes describing multiple tunnels
   then the tunnels SHOULD be interpreted by the tunnel initiator as
   alternatives and the server SHOULD include an instance of the
   Tunnel-Preference Attribute in the set of Attributes pertaining to
   each alternative tunnel.  Similarly, if the RADIUS client includes
   multiple sets of tunnel Attributes in an Access-Request packet, all
   the Attributes pertaining to a given tunnel SHOULD contain the same
   value in their respective Tag fields and each set SHOULD include an
   appropriately valued instance of the Tunnel-Preference Attribute.

3.1.  Tunnel-Type

   Description

      This Attribute indicates the tunneling protocol(s) to be used (in
      the case of a tunnel initiator) or the the tunneling protocol in
      use (in the case of a tunnel terminator).  It MAY be included in
      Access-Request, Access-Accept and Accounting-Request packets.  If
      the Tunnel-Type Attribute is present in an Access-Request packet
      sent from a tunnel initiator, it SHOULD be taken as a hint to the
      RADIUS server as to the tunnelling protocols supported by the
      tunnel end-point; the RADIUS server MAY ignore the hint, however.
      A tunnel initiator is not required to implement any of these
      tunnel types; if a tunnel initiator receives an Access-Accept
      packet which contains only unknown or unsupported Tunnel-Types,
      the tunnel initiator MUST behave as though an Access-Reject had
      been received instead.

      If the Tunnel-Type Attribute is present in an Access-Request
      packet sent from a tunnel terminator, it SHOULD be taken to
      signify the tunnelling protocol in use.  In this case, if the
      RADIUS server determines that the use of the communicated protocol
      is not authorized, it MAY return an Access-Reject packet.  If a
      tunnel terminator receives an Access-Accept packet which contains
      one or more Tunnel-Type Attributes, none of which represent the
      tunneling protocol in use, the tunnel terminator SHOULD behave as
      though an Access-Reject had been received instead.

   A summary of the Tunnel-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     |     Tag       |     Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Value (cont)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      64 for Tunnel-Type

   Length
      Always 6.

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  Valid values for this field are 0x01 through 0x1F,
      inclusive.  If the Tag field is unused, it MUST be zero (0x00).

   Value
      The Value field is three octets and contains one of the following
      values, indicating the type of tunnel to be started.

   1      Point-to-Point Tunneling Protocol (PPTP) [1]
   2      Layer Two Forwarding (L2F) [2]
   3      Layer Two Tunneling Protocol (L2TP) [3]
   4      Ascend Tunnel Management Protocol (ATMP) [4]
   5      Virtual Tunneling Protocol (VTP)
   6      IP Authentication Header in the Tunnel-mode (AH) [5]
   7      IP-in-IP Encapsulation (IP-IP) [6]
   8      Minimal IP-in-IP Encapsulation (MIN-IP-IP) [7]
   9      IP Encapsulating Security Payload in the Tunnel-mode (ESP) [8]
   10     Generic Route Encapsulation (GRE) [9]
   11     Bay Dial Virtual Services (DVS)
   12     IP-in-IP Tunneling [10]

3.2.  Tunnel-Medium-Type

   Description

      The Tunnel-Medium-Type Attribute indicates which transport medium
      to use when creating a tunnel for those protocols (such as L2TP)
      that can operate over multiple transports.  It MAY be included in
      both Access-Request and Access-Accept packets; if it is present in
      an Access-Request packet, it SHOULD be taken as a hint to the
      RADIUS server as to the tunnel media supported by the tunnel end-
      point.  The RADIUS server MAY ignore the hint, however.

   A summary of the Tunnel-Medium-Type Attribute format is given below.
   The fields are transmitted 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     |      Tag      |    Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      65 for Tunnel-Medium-Type

   Length
      6

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  Valid values for this field are 0x01 through 0x1F,
      inclusive.  If the Tag field is unused, it MUST be zero (0x00).

   Value
      The Value field is three octets and contains one of the values
      listed under "Address Family Numbers" in [14].  For the sake of
      convenience, a relevant excerpt of this list is reproduced below.

   1      IPv4 (IP version 4)
   2      IPv6 (IP version 6)
   3      NSAP
   4      HDLC (8-bit multidrop)
   5      BBN 1822
   6      802 (includes all 802 media plus Ethernet "canonical format")
   7      E.163 (POTS)
   8      E.164 (SMDS, Frame Relay, ATM)
   9      F.69 (Telex)
   10     X.121 (X.25, Frame Relay)
   11     IPX
   12     Appletalk
   13     Decnet IV
   14     Banyan Vines
   15     E.164 with NSAP format subaddress

3.3.  Tunnel-Client-Endpoint

   Description

      This Attribute contains the address of the initiator end of the
      tunnel.  It MAY be included in both Access-Request and Access-
      Accept packets to indicate the address from which a new tunnel is
      to be initiated.  If the Tunnel-Client-Endpoint Attribute is
      included in an Access-Request packet, the RADIUS server should
      take the value as a hint; the server is not obligated to honor the
      hint, however.  This Attribute SHOULD be included in Accounting-
      Request packets which contain Acct-Status-Type attributes with
      values of either Start or Stop, in which case it indicates the
      address from which the tunnel was initiated.  This Attribute,
      along with the Tunnel-Server-Endpoint and Acct-Tunnel-Connection-
      ID attributes, may be used to provide a globally unique means to
      identify a tunnel for accounting and auditing purposes.

   A summary of the Tunnel-Client-Endpoint 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     |       Tag     |    String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      66 for Tunnel-Client-Endpoint.

   Length
      >= 3
   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  If the value of the Tag field is greater than 0x00
      and less than or equal to 0x1F, it SHOULD be interpreted as
      indicating which tunnel (of several alternatives) this attribute
      pertains.  If the Tag field is greater than 0x1F, it SHOULD be
      interpreted as the first byte of the following String field.

   String
      The format of the address represented by the String field depends
      upon the value of the Tunnel-Medium-Type attribute.

      If Tunnel-Medium-Type is IPv4 (1), then this string is either the
      fully qualified domain name (FQDN) of the tunnel client machine,
      or it is a "dotted-decimal" IP address.  Conformant
      implementations MUST support the dotted-decimal format and SHOULD
      support the FQDN format for IP addresses.

      If Tunnel-Medium-Type is IPv6 (2), then this string is either the
      FQDN of the tunnel client machine, or it is a text representation
      of the address in either the preferred or alternate form [17].
      Conformant implementations MUST support the preferred form and
      SHOULD support both the alternate text form and the FQDN format
      for IPv6 addresses.

      If Tunnel-Medium-Type is neither IPv4 nor IPv6, this string is a
      tag referring to configuration data local to the RADIUS client
      that describes the interface and medium-specific address to use.

3.4.  Tunnel-Server-Endpoint

   Description

      This Attribute indicates the address of the server end of the
      tunnel.  The Tunnel-Server-Endpoint Attribute MAY be included (as
      a hint to the RADIUS server) in the Access-Request packet and MUST
      be included in the Access-Accept packet if the initiation of a
      tunnel is desired.  It SHOULD be included in Accounting-Request
      packets which contain Acct-Status-Type attributes with values of
      either Start or Stop and which pertain to a tunneled session.
      This Attribute, along with the Tunnel-Client-Endpoint and Acct-
      Tunnel-Connection-ID Attributes [11], may be used to provide a
      globally unique means to identify a tunnel for accounting and
      auditing purposes.

   A summary of the Tunnel-Server-Endpoint 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     |     Tag       |   String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      67 for Tunnel-Server-Endpoint.

   Length
      >= 3

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  If the value of the Tag field is greater than 0x00
      and less than or equal to 0x1F, it SHOULD be interpreted as
      indicating which tunnel (of several alternatives) this attribute
      pertains.  If the Tag field is greater than 0x1F, it SHOULD be
      interpreted as the first byte of the following String field.

   String
      The format of the address represented by the String field depends
      upon the value of the Tunnel-Medium-Type attribute.

      If Tunnel-Medium-Type is IPv4 (1), then this string is either the
      fully qualified domain name (FQDN) of the tunnel client machine,
      or it is a "dotted-decimal" IP address.  Conformant
      implementations MUST support the dotted-decimal format and SHOULD
      support the FQDN format for IP addresses.

      If Tunnel-Medium-Type is IPv6 (2), then this string is either the
      FQDN of the tunnel client machine, or it is a text representation
      of the address in either the preferred or alternate form [17].
      Conformant implementations MUST support the preferred form and
      SHOULD support both the alternate text form and the FQDN format
      for IPv6 addresses.

      If Tunnel-Medium-Type is not IPv4 or IPv6, this string is a tag
      referring to configuration data local to the RADIUS client that
      describes the interface and medium-specific address to use.

3.5.  Tunnel-Password

   Description

      This Attribute may contain a password to be used to authenticate
      to a remote server.  It may only be included in an Access-Accept
      packet.

   A summary of the Tunnel-Password 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     |     Tag       |   Salt
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Salt (cont)  |   String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      69 for Tunnel-Password

   Length
      >= 5

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  Valid values for this field are 0x01 through 0x1F,
      inclusive.  If the value of the Tag field is greater than 0x00 and
      less than or equal to 0x1F, it SHOULD be interpreted as indicating
      which tunnel (of several alternatives) this attribute pertains;
      otherwise, the Tag field SHOULD be ignored.

   Salt
      The Salt field is two octets in length and is used to ensure the
      uniqueness of the encryption key used to encrypt each instance of
      the Tunnel-Password attribute occurring in a given Access-Accept
      packet.  The most significant bit (leftmost) of the Salt field
      MUST be set (1).  The contents of each Salt field in a given
      Access-Accept packet MUST be unique.

   String
      The plaintext String field consists of three logical sub-fields:
      the Data-Length and Password sub-fields (both of which are
      required), and the optional Padding sub-field.  The Data-Length
      sub-field is one octet in length and contains the length of the
      unencrypted Password sub-field.  The Password sub-field contains
      the actual tunnel password.  If the combined length (in octets) of
      the unencrypted Data-Length and Password sub-fields is not an even
      multiple of 16, then the Padding sub-field MUST be present.  If it
      is present, the length of the Padding sub-field is variable,
      between 1 and 15 octets.  The String field MUST be encrypted as
      follows, prior to transmission:

         Construct a plaintext version of the String field by
         concatenating the Data-Length and Password sub-fields.  If
         necessary, pad the resulting string until its length (in
         octets) is an even multiple of 16.  It is recommended that zero
         octets (0x00) be used for padding.  Call this plaintext P.

         Call the shared secret S, the pseudo-random 128-bit Request
         Authenticator (from the corresponding Access-Request packet) R,
         and the contents of the Salt field A.  Break P into 16 octet
         chunks p(1), p(2)...p(i), where i = len(P)/16.  Call the
         ciphertext blocks c(1), c(2)...c(i) and the final ciphertext C.
         Intermediate values b(1), b(2)...c(i) are required.  Encryption
         is performed in the following manner ('+' indicates
         concatenation):

            b(1) = MD5(S + R + A)    c(1) = p(1) xor b(1)   C = c(1)
            b(2) = MD5(S + c(1))     c(2) = p(2) xor b(2)   C = C + c(2)
                        .                      .
                        .                      .
                        .                      .
            b(i) = MD5(S + c(i-1))   c(i) = p(i) xor b(i)   C = C + c(i)

         The resulting encrypted String field will contain
         c(1)+c(2)+...+c(i).

      On receipt, the process is reversed to yield the plaintext String.

3.6.  Tunnel-Private-Group-ID

   Description

      This Attribute indicates the group ID for a particular tunneled
      session.  The Tunnel-Private-Group-ID Attribute MAY be included in
      the Access-Request packet if the tunnel initiator can pre-
      determine the group resulting from a particular connection and
      SHOULD be included in the Access-Accept packet if this tunnel
      session is to be treated as belonging to a particular private
      group.  Private groups may be used to associate a tunneled session
      with a particular group of users.  For example, it may be used to
      facilitate routing of unregistered IP addresses through a
      particular interface.  It SHOULD be included in Accounting-Request
      packets which contain Acct-Status-Type attributes with values of
      either Start or Stop and which pertain to a tunneled session.

   A summary of the Tunnel-Private-Group-ID 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     |     Tag       |   String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      81 for Tunnel-Private-Group-ID.

   Length
      >= 3

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  If the value of the Tag field is greater than 0x00
      and less than or equal to 0x1F, it SHOULD be interpreted as
      indicating which tunnel (of several alternatives) this attribute
      pertains.  If the Tag field is greater than 0x1F, it SHOULD be
      interpreted as the first byte of the following String field.

   String
      This field must be present.  The group is represented by the
      String field.  There is no restriction on the format of group IDs.

3.7.  Tunnel-Assignment-ID

   Description

      This Attribute is used to indicate to the tunnel initiator the
      particular tunnel to which a session is to be assigned.  Some
      tunneling protocols, such as PPTP and L2TP, allow for sessions
      between the same two tunnel endpoints to be multiplexed over the
      same tunnel and also for a given session to utilize its own
      dedicated tunnel.  This attribute provides a mechanism for RADIUS
      to be used to inform the tunnel initiator (e.g. PAC, LAC) whether
      to assign the session to a multiplexed tunnel or to a separate
      tunnel.  Furthermore, it allows for sessions sharing multiplexed
      tunnels to be assigned to different multiplexed tunnels.

      A particular tunneling implementation may assign differing
      characteristics to particular tunnels.  For example, different
      tunnels may be assigned different QOS parameters.  Such tunnels
      may be used to carry either individual or multiple sessions.  The
      Tunnel-Assignment-ID attribute thus allows the RADIUS server to
      indicate that a particular session is to be assigned to a tunnel
      that provides an appropriate level of service.  It is expected
      that any QOS-related RADIUS tunneling attributes defined in the
      future that accompany this attribute will be associated by the
      tunnel initiator with the ID given by this attribute.  In the
      meantime, any semantic given to a particular ID string is a matter
      left to local configuration in the tunnel initiator.

      The Tunnel-Assignment-ID attribute is of significance only to
      RADIUS and the tunnel initiator.  The ID it specifies is intended
      to be of only local use to RADIUS and the tunnel initiator.  The
      ID assigned by the tunnel initiator is not conveyed to the tunnel
      peer.

      This attribute MAY be included in the Access-Accept.  The tunnel
      initiator receiving this attribute MAY choose to ignore it and
      assign the session to an arbitrary multiplexed or non-multiplexed
      tunnel between the desired endpoints.  This attribute SHOULD also
      be included in Accounting-Request packets which contain Acct-
      Status-Type attributes with values of either Start or Stop and
      which pertain to a tunneled session.

      If a tunnel initiator supports the Tunnel-Assignment-ID Attribute,
      then it should assign a session to a tunnel in the following
      manner:

         If this attribute is present and a tunnel exists between the
         specified endpoints with the specified ID, then the session
         should be assigned to that tunnel.

         If this attribute is present and no tunnel exists between the
         specified endpoints with the specified ID, then a new tunnel
         should be established for the session and the specified ID
         should be associated with the new tunnel.

         If this attribute is not present, then the session is assigned
         to an unnamed tunnel.  If an unnamed tunnel does not yet exist
         between the specified endpoints then it is established and used
         for this and subsequent sessions established without the
         Tunnel-Assignment-ID attribute.  A tunnel initiator MUST NOT
         assign a session for which a Tunnel-Assignment-ID Attribute was
         not specified to a named tunnel (i.e. one that was initiated by
         a session specifying this attribute).

      Note that the same ID may be used to name different tunnels if
      such tunnels are between different endpoints.

   A summary of the Tunnel-Assignment-ID 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     |      Tag      |   String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      82 for Tunnel-Assignment-ID.

   Length
      >= 3

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  If the value of the Tag field is greater than 0x00
      and less than or equal to 0x1F, it SHOULD be interpreted as
      indicating which tunnel (of several alternatives) this attribute
      pertains.  If the Tag field is greater than 0x1F, it SHOULD be
      interpreted as the first byte of the following String field.

   String
      This field must be present.  The tunnel ID is represented by the
      String field.  There is no restriction on the format of the ID.

3.8.  Tunnel-Preference

   Description

      If more than one set of tunneling attributes is returned by the
      RADIUS server to the tunnel initiator, this Attribute SHOULD be
      included in each set to indicate the relative preference assigned
      to each tunnel.  For example, suppose that Attributes describing
      two tunnels are returned by the server, one with a Tunnel-Type of
      PPTP and the other with a Tunnel-Type of L2TP.  If the tunnel
      initiator supports only one of the Tunnel-Types returned, it will
      initiate a tunnel of that type.  If, however, it supports both
      tunnel protocols, it SHOULD use the value of the Tunnel-Preference
      Attribute to decide which tunnel should be started.  The tunnel
      having the numerically lowest value in the Value field of this
      Attribute SHOULD be given the highest preference.  The values
      assigned to two or more instances of the Tunnel-Preference
      Attribute within a given Access-Accept packet MAY be identical.
      In this case, the tunnel initiator SHOULD use locally configured
      metrics to decide which set of attributes to use.  This Attribute
      MAY be included (as a hint to the server) in Access-Request
      packets, but the RADIUS server is not required to honor this hint.

   A summary of the Tunnel-Preference 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     |     Tag       |     Value
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Value (cont)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      83 for Tunnel-Preference

   Length
      Always 6.

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  Valid values for this field are 0x01 through 0x1F,
      inclusive.  If the Tag field is unused, it MUST be zero (0x00).

   Value
      The Value field is three octets in length and indicates the
      preference to be given to the tunnel to which it refers; higher
      preference is given to lower values, with 0x000000 being most
      preferred and 0xFFFFFF least preferred.

3.9.  Tunnel-Client-Auth-ID

   Description

      This Attribute specifies the name used by the tunnel initiator
      during the authentication phase of tunnel establishment.  The
      Tunnel-Client-Auth-ID Attribute MAY be included (as a hint to the
      RADIUS server) in the Access-Request packet, and MUST be included
      in the Access-Accept packet if an authentication name other than
      the default is desired.  This Attribute SHOULD be included in
      Accounting-Request packets which contain Acct-Status-Type
      attributes with values of either Start or Stop and which pertain
      to a tunneled session.

   A summary of the Tunnel-Client-Auth-ID 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     |      Tag      |   String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      90 for Tunnel-Client-Auth-ID.

   Length
      >= 3

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  If the value of the Tag field is greater than 0x00
      and less than or equal to 0x1F, it SHOULD be interpreted as
      indicating which tunnel (of several alternatives) this attribute
      pertains.  If the Tag field is greater than 0x1F, it SHOULD be
      interpreted as the first byte of the following String field.

   String
      This field must be present.  The String field contains the
      authentication name of the tunnel initiator.  The authentication
      name SHOULD be represented in the UTF-8 charset.

3.10.  Tunnel-Server-Auth-ID

   Description

      This Attribute specifies the name used by the tunnel terminator
      during the authentication phase of tunnel establishment.  The
      Tunnel-Client-Auth-ID Attribute MAY be included (as a hint to the
      RADIUS server) in the Access-Request packet, and MUST be included
      in the Access-Accept packet if an authentication name other than
      the default is desired.  This Attribute SHOULD be included in
      Accounting-Request packets which contain Acct-Status-Type
      attributes with values of either Start or Stop and which pertain
      to a tunneled session.

   A summary of the Tunnel-Server-Auth-ID 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     |      Tag      |   String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      91 for Tunnel-Server-Auth-ID.

   Length
      >= 3

   Tag
      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  If the value of the Tag field is greater than 0x00
      and less than or equal to 0x1F, it SHOULD be interpreted as
      indicating which tunnel (of several alternatives) this attribute
      pertains.  If the Tag field is greater than 0x1F, it SHOULD be
      interpreted as the first byte of the following String field.

   String
      This field must be present.  The String field contains the
      authentication name of the tunnel terminator.  The authentication
      name SHOULD be represented in the UTF-8 charset.

4.  Table of Attributes

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

Request Accept Reject Challenge Acct-Request #  Attribute
0+      0+     0      0         0-1          64 Tunnel-Type
0+      0+     0      0         0-1          65 Tunnel-Medium-Type
0+      0+     0      0         0-1          66 Tunnel-Client-Endpoint
0+      0+     0      0         0-1          67 Tunnel-Server-Endpoint
0       0+     0      0         0            69 Tunnel-Password
0+      0+     0      0         0-1          81 Tunnel-Private-Group-ID
0       0+     0      0         0-1          82 Tunnel-Assignment-ID
0+      0+     0      0         0            83 Tunnel-Preference
0+      0+     0      0         0-1          90 Tunnel-Client-Auth-ID
0+      0+     0      0         0-1          91 Tunnel-Server-Auth-ID

   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.

5.  Security Considerations

   The Tunnel-Password Attribute may contain information which should
   only be known to a tunnel endpoint.  However, the method used to hide
   the value of the attribute is such that intervening RADIUS proxies
   will have knowledge of the contents.  For this reason, the Tunnel-
   Password Attribute SHOULD NOT be included in Access-Accept packets
   which may pass through (relatively) untrusted RADIUS proxies.  In
   addition, the Tunnel-Password Attribute SHOULD NOT be returned to an
   unauthenticated client; if the corresponding Access-Request packet
   did not contain a verified instance of the Signature Attribute [15],
   the Access-Accept packet SHOULD NOT contain an instance of the
   Tunnel-Password Attribute.

   Tunnel protocols offer various levels of security, from none (e.g.,
   PPTP) to strong (e.g., IPSec).  Note, however, that in the compulsory
   tunneling case any security measures in place only apply to traffic
   between the tunnel endpoints.  In particular, end-users SHOULD NOT
   rely upon the security of the tunnel to protect their data;
   encryption and/or integrity protection of tunneled traffic MUST NOT
   be considered as a replacement for end-to-end security.

6.  IANA Considerations

   This document defines a number of "magic" numbers to be maintained by
   the IANA.  This section explains the criteria to be used by the IANA
   to assign additional numbers in each of these lists.  The following
   subsections describe the assignment policy for the namespaces defined
   elsewhere in this document.

6.1.  Tunnel-Type Attribute Values

   Values 1-12 of the Tunnel-Type Attribute are defined in Section 5.1;
   the remaining values are available for assignment by the IANA with
   IETF Consensus [16].

6.2.  Tunnel-Medium-Type Attribute Values

   Values 1-15 of the Tunnel-Medium-Type Attribute are defined in
   Section 5.2; the remaining values are available for assignment by the
   IANA with IETF Consensus [16].

7.  References

   [1]  Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W. and
        G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637,
        July 1999.

   [2]  Valencia, A., Littlewood, M. and T. Kolar, T., "Cisco Layer Two
        Forwarding (Protocol) 'L2F'", RFC 2341, May 1998.

   [3]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
        B. Palter, "Layer Two Tunnelling Protocol (L2TP)", RFC 2661,
        August 1999.

   [4]  Hamzeh, K., "Ascend Tunnel Management Protocol - ATMP", RFC
        2107, February 1997.

   [5]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [6]  Perkins, C., "IP Encapsulation within IP", RFC 2003, October
        1996.

   [7]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
        October 1996.

   [8]  Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC
        1827, August 1995.

   [9]  Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
        Encapsulation (GRE)", RFC 1701, October 1994.

   [10] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.

   [11] Zorn, G. and D. Mitton, "RADIUS Accounting Modifications for
        Tunnel Protocol Support", RFC 2867, June 2000.

   [12] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
        Authentication Dial in User Service (RADIUS)", RFC 2865, June
        2000.

   [13] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [14] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
        October 1994.

   [15] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions", RFC
        2869, June 2000.

   [16] Narten, T. and H. Alvestrand, "Guidelines for writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [17] Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.

8.  Acknowledgements

   Thanks to Dave Mitton for pointing out a nasty circular dependency in
   the original Tunnel-Password attribute definition and (in no
   particular order) to Kory Hamzeh, Bertrand Buclin, Andy Valencia,
   Bill Westfield, Kris Michielsen, Gurdeep Singh Pall, Ran Atkinson,
   Aydin Edguer, and Bernard Aboba for useful input and review.

9.  Chair's Address

   The RADIUS 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

10.  Authors' Addresses

   Questions about this memo can also be directed to:

   Glen Zorn
   Cisco Systems, Inc.
   500 108th Avenue N.E., Suite 500
   Bellevue, Washington 98004
   USA

   Phone: +1 425 438 8218
   FAX:   +1 425 438 1848
   EMail: gwz@cisco.com


   Dory Leifer
   Ascend Communications
   1678 Broadway
   Ann Arbor, MI 48105

   Phone:  +1 734 747 6152
   EMail: leifer@del.com
   John Shriver
   Intel Corporation
   28 Crosby Drive
   Bedford, MA  01730

   Phone:  +1 781 687 1329
   EMail: John.Shriver@intel.com


   Allan Rubens
   Ascend Communications
   1678 Broadway
   Ann Arbor, MI 48105

   Phone:  +1 313 761 6025
   EMail: acr@del.com


   Matt Holdrege
   ipVerse
   223 Ximeno Ave.
   Long Beach, CA 90803

   EMail: matt@ipverse.com


   Ignacio Goyret
   Lucent Technologies
   One Ascend Plaza
   1701 Harbor Bay Parkway
   Alameda, CA 94502

   Phone:  +1 510 769 6001
   EMail: igoyret@lucent.com

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