Network Working Group G. Zorn

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.

Zorn, et al. Informational [Page 1]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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

Zorn, et al. Informational [Page 2]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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]

Zorn, et al. Informational [Page 3]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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)

Zorn, et al. Informational [Page 4]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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

Zorn, et al. Informational [Page 5]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 6]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 7]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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

Zorn, et al. Informational [Page 8]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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

Zorn, et al. Informational [Page 9]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 10]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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).

Zorn, et al. Informational [Page 11]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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

Zorn, et al. Informational [Page 12]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 13]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 14]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 15]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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.

Zorn, et al. Informational [Page 16]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

[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.

Zorn, et al. Informational [Page 17]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

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: [email protected]

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: [email protected]

Dory Leifer
Ascend Communications
1678 Broadway
Ann Arbor, MI 48105

Phone: +1 734 747 6152
EMail: [email protected]

Zorn, et al. Informational [Page 18]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

John Shriver
Intel Corporation
28 Crosby Drive
Bedford, MA 01730

Phone: +1 781 687 1329
EMail: [email protected]

Allan Rubens
Ascend Communications
1678 Broadway
Ann Arbor, MI 48105

Phone: +1 313 761 6025
EMail: [email protected]

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

EMail: [email protected]

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

Phone: +1 510 769 6001
EMail: [email protected]

Zorn, et al. Informational [Page 19]

RFC 2868 RADIUS Tunnel Authentication Attributes June 2000

11. Full Copyright Statement

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

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Zorn, et al. Informational [Page 20]