wdiff draft-ietf-radext-rfc3576bis-06.txt draft-ietf-radext-rfc3576bis-07.txt

Network Working Group Murtaza S. Chiba
INTERNET-DRAFT Gopal Dommety
Obsoletes: 3576 Mark Eklund
Category: Informational Cisco Systems, Inc.
<draft-ietf-radext-rfc3576bis-06.txt>
<draft-ietf-radext-rfc3576bis-07.txt> David Mitton
23
27 May 2007 RSA Security, Inc.
Bernard Aboba
Microsoft Corporation

Dynamic Authorization Extensions to Remote Authentication Dial In User
Service (RADIUS)

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.

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This Internet-Draft will expire on December 25, 2007.

Abstract

This document describes a currently deployed extension to the Remote
Authentication Dial In User Service (RADIUS) protocol, allowing
dynamic changes to a user session, as implemented by network access
server products. This includes support for disconnecting users and
changing authorizations applicable to a user session.

Table of Contents

1. Introduction .......................................... 3
1.1 Applicability ................................... 3
1.2 Requirements Language ........................... 4
1.3 Terminology ..................................... 4
2. Overview ............................................. 5
2.1 Disconnect Messages (DM) ........................ 5
2.2 Change-of-Authorization Messages (CoA) .......... 5
2.3 Packet Format ................................... 6
3. Attributes ............................................ 9 10
3.1 Proxy State ..................................... 11 12
3.2 Authorize Only .................................. 12
3.3 State ........................................... 12 13
3.4 Message-Authenticator ........................... 13 14
3.5 Error-Cause ..................................... 14 15
3.6 Table of Attributes ............................. 17 18
4. Diameter Considerations ............................... 20 21
5. IANA Considerations ................................... 22 23
6. Security Considerations ............................... 23 24
6.1 Authorization Issues ............................ 23 24
6.2 Impersonation ................................... 23 25
6.3 IPsec Usage Guidelines .......................... 24 25
6.4 Replay Protection ............................... 27 28
7. Example Traces ........................................ 28 29
8. References ............................................ 28 29
8.1 Normative References ............................ 28 29
8.2 Informative References .......................... 29 30
ACKNOWLEDGMENTS .............................................. 30 31
AUTHORS' ADDRESSES ........................................... 31 32
Appendix A - Changes from RFC 3576 ........................... 32 33
Full Copyright Statement ..................................... 33 35
Intellectual Property ........................................ 33 35

1. Introduction

The RADIUS protocol, defined in [RFC2865], does not support
unsolicited messages sent from the RADIUS server to the Network
Access Server (NAS).

However, there are many instances in which it is desirable for
changes to be made to session characteristics, without requiring the
NAS to initiate the exchange. For example, it may be desirable for
administrators to be able to terminate a user session in progress.
Alternatively, if the user changes authorization level, this may
require that authorization attributes be added/deleted from a user
session.

To overcome these limitations, several vendors have implemented
additional RADIUS commands in order to be able to support unsolicited
messages to be sent from the RADIUS server to the NAS. These extended commands provide
support for Disconnect and Change-of-Authorization (CoA) packets.
Disconnect packets cause a user session to be terminated immediately,
whereas CoA packets modify session authorization attributes such as
data filters.

1.1. Applicability

This protocol is being recommended for publication as an
Informational RFC rather than as a standards-track RFC because of
problems that cannot be fixed without creating incompatibilities with
deployed implementations. This includes security vulnerabilities, as
well as semantic ambiguities resulting from the design of the Change-
of-Authorization (CoA) commands. While fixes are recommended, they
cannot be made mandatory since this would be incompatible with
existing implementations.

Existing implementations of this protocol do not support
authorization checks, so that an ISP sharing a NAS with another ISP
could disconnect or change authorizations for another ISP's users.
In order to remedy this problem, a "Reverse Path Forwarding" check is
recommended. See
described; see Section 6.1. for details.

Existing implementations utilize per-packet authentication and
integrity protection algorithms with known weaknesses [MD5Attack].
To provide stronger per-packet authentication and integrity
protection, the use of IPsec is recommended. See Section 6.3 for
details.

Existing implementations lack replay protection. In order to support
replay detection, it is recommended that an Event-Timestamp Attribute
be added to all packets in situations where IPsec replay protection
is not employed. See Section 6.4 for details.

The approach taken with CoA commands in existing implementations
results in a semantic ambiguity. Existing implementations of the
CoA-Request identify the affected session, as well as supply the
authorization changes. Since RADIUS Attributes included within
existing implementations of the CoA-Request can be used for session
identification or authorization change, it may not be clear which
function a given attribute is serving.

The problem does not exist within the Diameter protocol [RFC3588], in
which server-initiated authorization change is initiated using a Re-
Auth-Request (RAR) command identifying the session via User-Name and
Session-Id AVPs and containing a Re-Auth-Request-Type AVP with value
"AUTHORIZE_ONLY". This results in initiation of a standard
Request/Response sequence where authorization changes are supplied.
As a result, in no command can Diameter AVPs have multiple potential
meanings.

1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

1.3. Terminology

This document frequently uses the following terms:

Dynamic Authorization Client (DAC)
The entity originating Change of Authorization (CoA) Requests or
Disconnect-Requests. While it is possible that the DAC is co-
resident with a RADIUS authentication or accounting server, this
need not necessarily be the case.

Dynamic Authorization Server (DAS)
The entity receiving CoA-Request or Disconnect-Request packets.
The DAS may be a NAS or a RADIUS proxy.

Network Access Server (NAS)
The device providing access to the network.

service
The NAS provides a service to the user, such as IEEE 802 or PPP. Point-
to-Point Protocol (PPP).

session
Each service provided by the NAS to a 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. Overview

This section describes the most commonly implemented features of
Disconnect and Change-of-Authorization (CoA) packets.

2.1. Disconnect Messages (DM)

A Disconnect-Request packet is sent by the RADIUS server Dynamic Authorization
Client in order to terminate a user session on a NAS and discard all
associated session context. The Disconnect-Request packet is sent to
UDP port 3799, and identifies the NAS as well as the user session to
be terminated by inclusion of the identification attributes described
in Section 3.

+----------+ Disconnect-Request +----------+
| | Disconnect-Request | |
| | <-------------------- | Dynamic |
| NAS | | RADIUS Authz |
| | Disconnect-Response | Server Client |
| | ---------------------> | |
+----------+ +----------+

The NAS responds to a Disconnect-Request packet sent by a RADIUS
server Dynamic
Authorization Client with a Disconnect-ACK if all associated session
context is discarded and the user session is no longer connected, or
a Disconnect-NAK, if the NAS was unable to disconnect the session and
discard all associated session context. A Disconnect-ACK MAY contain
the Attribute Acct-Terminate-Cause (49) [RFC2866] with the value set
to 6 for Admin-Reset.

2.2. Change-of-Authorization Messages (CoA)

CoA-Request packets contain information for dynamically changing
session authorizations. Typically this is used to change data
filters. The data filters can be of either the ingress or egress
kind, and are sent in addition to the identification attributes as
described in section 3. The port used, and packet format (described
in Section 2.3), are the same as that for Disconnect-Request packets.

The following attributes MAY be sent in a CoA-Request:

Filter-ID (11) - Indicates the name of a data filter list
to be applied for the session that the
identification attributes map to.

NAS-Filter-Rule (92) - Provides a filter list to be applied
for the session that the identification
attributes map to [RFC4849].

+----------+ CoA-Request +----------+
| | CoA-Request | |
| | <-------------------- | Dynamic |
| NAS | | RADIUS Authz |
| | CoA-Response | Server Client |
| | ---------------------> | |
+----------+ +----------+

The NAS responds to a CoA-Request sent by a RADIUS server Dynamic Authorization
Client with a CoA-
ACK CoA-ACK if the NAS is able to successfully change the
authorizations for the user session, or a CoA-NAK if the Request CoA-Request
is unsuccessful. A NAS MUST respond to a CoA-Request including a
Service-Type Attribute with an unsupported value with a CoA-NAK; an
Error-Cause Attribute with value "Unsupported Service" SHOULD be
included.

2.3. Packet Format

For either Disconnect-Request or CoA-Request packets UDP port 3799 is
used as the destination port. For responses, the source and
destination ports are reversed. Exactly one RADIUS packet is
encapsulated in the UDP Data field.

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

The packet format consists of the fields: Code, Identifier, Length,
Authenticator, and Attributes in Type:Length:Value (TLV) format. All
fields hold the same meaning as those described in RADIUS [RFC2865].
The Authenticator field MUST be calculated in the same way as is
specified for an Accounting-Request in [RFC2866].

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. Packets received with an invalid Code field MUST be
silently discarded. RADIUS codes (decimal) for this extension are
assigned as follows:

40 - Disconnect-Request [RFC3575]
41 - Disconnect-ACK [RFC3575]
42 - Disconnect-NAK [RFC3575]
43 - CoA-Request [RFC3575]
44 - CoA-ACK [RFC3575]
45 - CoA-NAK [RFC3575]

Identifier

The Identifier field is one octet, and aids in matching requests
and replies. RADIUS clients A Dynamic Authorization Server implementing this
specification MUST be capable of detecting a duplicate request if
it has the same
server source IP address, source UDP port and Identifier
within a short span of time.

Unlike RADIUS as defined in [RFC2865], the

The responsibility for retransmission of Disconnect-Request and
CoA-Request packets lies with the RADIUS server. Dynamic Authorization Client.
If after sending these packets, the
RADIUS server Dynamic Authorization Client
does not receive a response, it will retransmit.

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

If the RADIUS server Dynamic Authorization Client is retransmitting a
Disconnect-Request or CoA-Request to the same client Dynamic
Authorization Server as before, and the Attributes haven't
changed, the same Request Authenticator, Identifier and source
port MUST be used. If any Attributes have changed, a new
Authenticator and Identifier MUST be used.

If the Request to a primary proxy Dynamic Authorization Server fails, a
secondary proxy Dynamic Authorization Server must be queried, if available. Issues
available; issues relating to failover algorithms are described in
[RFC3539]. Since this represents a new request, a new Request
Authenticator and Identifier MUST be used. However, where the RADIUS server
Dynamic Authorization Client is sending directly to the client, NAS,
failover typically does not make sense, since Disconnect CoA-Request or CoA
Disconnect-Request packets need to be delivered to the NAS where
the session resides.

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
MUST be treated as padding and ignored on reception. If the
packet is shorter than the Length field indicates, it MUST 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 packets between the RADIUS server Dynamic Authorization Client and client.
the Dynamic Authorization Server.

Request Authenticator

In Request packets, the Authenticator value is a 16 octet MD5
[RFC1321] checksum, called the Request Authenticator. The
Request Authenticator is calculated the same way as for an
Accounting-Request, specified in [RFC2866].

Note that the Request Authenticator of a Disconnect CoA-Request or CoA-
Request
Disconnect-Request cannot be computed the same way as the
Request Authenticator of a RADIUS Access-Request, because there
is no User-Password Attribute in a Disconnect-Request CoA-Request or CoA-Request. Disconnect-
Request.

Response Authenticator

The Authenticator field in a Response packet (e.g. Disconnect-
ACK, Disconnect-NAK, CoA-ACK, or CoA-NAK) is called the
Response Authenticator, and contains a one-way MD5 hash
calculated over a stream of octets consisting of the Code,
Identifier, Length, the Request Authenticator field from the
packet being replied to, and the response Attributes if any,
followed by the shared secret. The resulting 16 octet MD5 hash
value is stored in the Authenticator field of the Response
packet.

Administrative note: As noted in [RFC2865] Section 3, the secret
(password shared between the client Dynamic Authorization Client and the RADIUS server)
Dynamic Authorization Server) SHOULD be at least as large and
unguessable as a well-chosen password.
RADIUS clients The Dynamic Authorization
Server MUST use the source IP address of the RADIUS UDP packet to
decide which shared secret to use, so that requests can be
proxied.

Attributes

In Disconnect and CoA-Request packets, all Attributes attributes are treated
as mandatory. If one or more authorization changes specified in a
CoA-Request cannot be carried out, the NAS MUST send a CoA-NAK. A
NAS MUST respond to a CoA-Request containing one or more
unsupported Attributes or Attribute values with a CoA-NAK; an
Error-Cause Attribute with value 401 (Unsupported Attribute) or
407 (Invalid Attribute Value) MAY be included. A NAS MUST respond
to a Disconnect-Request containing one or more unsupported
Attributes or Attribute values with a Disconnect-NAK; an Error-
Cause Attribute with value 401 (Unsupported Attribute) or 407
(Invalid Attribute Value) MAY be included.

State changes resulting from a CoA-Request MUST be atomic: if the
Request
CoA-Request is successful, the Dynamic Authorization Server MUST
send a CoA-ACK MUST be sent in reply, and all requested authorization changes
MUST be made. If the CoA-Request is unsuccessful, a CoA-NAK MUST
be sent in reply, and the requested authorization changes MUST NOT
be made. Similarly, a state change MUST NOT occur as a result of
an unsuccessful Disconnect-Request; the Dynamic Authorization
Server MUST send a Disconnect-NAK MUST be sent in reply.

Within this specification attributes can be used for
identification, authorization or other purposes. RADIUS Attribute
specifications created after publication of this document SHOULD
state whether an attribute can be included in CoA or Disconnect
messages and if so, which messages it can be included in and
whether it serves as an identification or authorization attribute.

Even if a NAS implements an attribute for use with RADIUS
authentication and accounting, it is possible that it will not
support inclusion of that attribute within Disconnect-Request or CoA-Request and
Disconnect-Request packets, given the difference in attribute
semantics. This is true even for attributes specified as
allowable within Access-Accept packets (such as those defined
within [RFC2865], [RFC2868], [RFC2869], [RFC3162], [RFC3579],
[RFC4372], [RFC4675], [RFC4818] and [RFC4849]).

3. Attributes

In Disconnect-Request and CoA-Request packets, certain attributes are
used to uniquely identify the NAS as well as a user session on the
NAS. All NAS identification attributes included in a Request packet
MUST match in order for a Disconnect-Request or CoA-Request to be
successful; otherwise a Disconnect-NAK or CoA-NAK SHOULD be sent.
For session identification attributes, the User-Name and Acct-
Session-Id Attributes, if included, MUST match in order for a
Disconnect-Request or CoA-Request to be successful; other session
identification attributes SHOULD match. Where a mismatch of session
identification attributes is detected, a Disconnect-NAK or CoA-NAK
SHOULD be sent.

The ability to use NAS or session identification attributes to map to
unique/multiple sessions is beyond the scope of this document.
Identification attributes include NAS and session identification
attributes, as described below.

NAS identification attributes

Attribute # Reference Description
--------- --- --------- -----------
NAS-IP-Address 4 [RFC2865] The IPv4 address of the NAS.
NAS-Identifier 32 [RFC2865] String identifying the NAS.
NAS-IPv6-Address 95 [RFC3162] The IPv6 address of the NAS.

Session identification attributes

Attribute # Reference Description
--------- --- --------- -----------
User-Name 1 [RFC2865] The name of the user
associated with the session.
NAS-Port 5 [RFC2865] The port on which the
session is terminated.
Called-Station-Id 30 [RFC2865] The link address to which
the session is connected.
Calling-Station-Id 31 [RFC2865] The link address from which
the session is connected.
Acct-Session-Id 44 [RFC2866] The identifier uniquely
identifying the session
on the NAS.
Acct-Multi-Session-Id 50 [RFC2866] The identifier uniquely
identifying related sessions.
NAS-Port-Id 87 [RFC2869] String identifying the port
where the session is.
Chargeable-User- 89 [RFC4372] The CUI associated with the
Identity session. Needed where a
privacy NAI is used, because
the User-Name may not be
unique (e.g. "anonymous").

To address security concerns described in Section 6.1, either the
User-Name or Chargeable-User-Identity attribute SHOULD be present in
Disconnect-Request and CoA-Request packets.

Where a Diameter client utilizes the same Session-Id for both
authorization and accounting, inclusion of an Acct-Session-Id
Attribute in a Disconnect-Request or CoA-Request can assist with
Diameter/RADIUS translation, since Diameter RAR and ASR commands
include a Session-Id AVP. An Acct-Session-Id attribute Attribute SHOULD be
included in Disconnect-Request and CoA-Request packets.

Where the

A NAS implementing this specification SHOULD send an Acct-Session-Id
or Acct-Multi-Session-Id Attribute is not within an Access-Request. Where
an Acct-Session-Id or Acct-Multi-Session-Id Attribute is not included
within an Access-Request, the Dynamic Authorizatoin Client will not
know the Acct-Session-Id or Acct-Multi-Session-Id of the session it
is attempting to target, unless it also has access to the accounting
data for that session.

Where an Acct-Session-Id or Acct-Multi-Session-Id Attribute is not
present in a CoA-Request or Disconnect-Request, it is possible that
the the User-Name or Chargeable-User-Identity attributes will not be
sufficient to uniquely identify the session (e.g. if the same user
has multiple sessions on the NAS, or if the privacy NAI is used). As a result, In
this case, session identification MAY be performed by using one or
more of the Acct-Multi-Session-Id, Called-Station-Id, Calling-
Station-Id, Calling-Station-Id, NAS-Port and NAS-Port-Id attributes MAY be used as
additional session identification. NAS-
Port-Id attributes.

To address security concerns described in Section 6.2, one or more of
the NAS-IP-Address or NAS-IPv6-Address Attributes SHOULD be present
in Disconnect-Request and CoA-Request and Disconnect-Request packets; the NAS-Identifier
Attribute MAY be present.

A Disconnect-Request MUST contain only NAS and session identification
attributes. If other attributes are included in a Disconnect-
Request, implementations MUST send a Disconnect-NAK; an Error-Cause
Attribute with value "Unsupported Attribute" MAY be included.

3.1. Proxy State

If there are any Proxy-State attributes in a Disconnect-Request or
CoA-Request received from the server, Dynamic Authorization Client, the forwarding proxy or NAS
Dynamic Authorization Server MUST include those Proxy-State
attributes in its response to the
server. Dynamic Authorization Client.

A forwarding proxy or NAS MUST NOT modify existing Proxy-State,
State, or Class attributes present in the packet. The forwarding
proxy or NAS MUST treat any Proxy-State attributes already in the
packet as opaque data. Its operation MUST NOT depend on the content
of Proxy-State attributes added by previous proxies. The forwarding
proxy MUST NOT modify any other Proxy-State attributes that were in
the packet; it may choose not to forward them, but it MUST NOT change
their contents. If the forwarding proxy omits the Proxy-State
attributes in the request, it MUST attach them to the response before
sending it.

When the proxy forwards a Disconnect or CoA-Request, it MAY add a
Proxy-State Attribute, but it MUST NOT add more than one. If a
Proxy-State Attribute is added to a packet when forwarding the
packet, the Proxy-State Attribute MUST be added after any existing
Proxy-State attributes. The forwarding proxy MUST NOT change the
order of any attributes of the same type, including Proxy-State.
Other attributes can be placed before, after or even between the
Proxy-State attributes.

When the proxy receives a response to a CoA-Request or Disconnect-
Request, it MUST remove its own Proxy-State (the last Proxy- State in
the packet) Attribute before forwarding the response. Since
Disconnect and CoA responses are authenticated on the entire packet
contents, the stripping of the Proxy-State Attribute invalidates the
integrity check - so the proxy needs to recompute it.

3.2. Authorize Only

Support for a CoA-Request including a Service-Type Attribute with
value "Authorize Only" is OPTIONAL on the NAS and RADIUS server. Dynamic
Authorization Client. A Service-Type Attribute MUST NOT be included
within a Disconnect-
Request. Disconnect-Request.

A NAS MUST respond to a CoA-Request including a Service-Type
Attribute with value "Authorize Only" with a CoA-NAK; a CoA-ACK MUST
NOT be sent. If the NAS does not support a Service-Type value of
"Authorize Only" then it MUST respond with a CoA-NAK; an Error-Cause
value of 405 (Unsupported Service) SHOULD be included.

A CoA-Request containing a Service-Type Attribute with value
"Authorize Only" MUST in addition contain only NAS or session
identification attributes, as well as a State Attribute. If other
attributes are included in such a CoA-Request, a CoA-NAK MUST be
sent; an Error-Cause Attribute with value 401 (Unsupported Attribute)
SHOULD be included.

If a CoA-Request packet including a Service-Type value of "Authorize
Only" is successfully processed, the NAS MUST respond with a CoA-NAK
containing a Service-Type Attribute with value "Authorize Only", and
an Error-Cause Attribute with value 507 (Request Initiated). The NAS
then MUST send an Access-Request to the RADIUS server including a
Service-Type Attribute with value "Authorize Only". Only", along with a
State Attribute. This Access-
Request Access-Request SHOULD contain the NAS
identification attributes from the CoA-Request, as well as the
session identification attributes from the CoA-Request legal for inclusion permitted in
an Access-Request as specified
in [RFC2865], [RFC2868], [RFC2869] and [RFC3162]. Access-Request. As noted in [RFC2869] Section 5.19, a Message-Authenticator Message-
Authenticator attribute SHOULD be included in an Access-Request that
does not contain a User-Password, CHAP-Password, ARAP-Password or
EAP-Message Attribute. The RADIUS server then will respond to the
Access-Request with an Access-Accept to (re-)authorize the session or
an Access-Reject to refuse to (re-)authorize it.

3.3. State

The State Attribute is available to be sent by the RADIUS server Dynamic
Authorization Client to the NAS in a CoA-Request packet and MUST be
sent unmodified from the NAS to the RADIUS server Dynamic Authorization Client in a
subsequent ACK or NAK packet.

[RFC2865] Section 5.44 states:

An Access-Request MUST contain either a User-Password or a CHAP-
Password or State. An Access-Request MUST NOT contain both a
User-Password and a CHAP-Password. If future extensions allow
other kinds of authentication information to be conveyed, the
attribute for that can be used in an Access-Request instead of
User-Password or CHAP-Password.

In order to satisfy the requirements of [RFC2865] Section 5.44, an
Access-Request with Service-Type="Authorize-Only" MUST contain a
State attribute.

In order to provide a State attribute to the NAS, a server Dynamic
Authorization Client sending a CoA-Request with a Service-Type value
of "Authorize-Only" MUST include a State Attribute, and the NAS MUST
send the State Attribute unmodified to the RADIUS server in the
resulting Access-Request, if any. A NAS receiving a CoA-Request
containing a Service-Type value of "Authorize-Only" but lacking a
State attribute MUST send a CoA-NAK and SHOULD include an Error-Cause
attribute with value 402 (Missing Attribute).

The State Attribute is also available to be sent by the RADIUS server Dynamic
Authorization Client to the NAS in a CoA-Request that also includes a
Termination-Action Attribute with the value of RADIUS-Request. If
the client NAS performs the Termination-Action by sending a new Access-Request Access-
Request upon termination of the current session, it MUST include the
State Attribute unchanged in that Access-Request. In either usage,
the
client Dynamic Authorization Server MUST NOT interpret the Attribute
locally. A CoA-Request packet must MUST have only zero or one State
Attribute. Usage of the State Attribute is implementation dependent.

3.4. Message-Authenticator

The Message-Authenticator Attribute MAY be used to authenticate and
integrity-protect CoA-Request, CoA-ACK, CoA-NAK, Disconnect-Request,
Disconnect-ACK and Disconnect-NAK packets order to prevent spoofing.

A RADIUS client Dynamic Authorization Server receiving a CoA-Request or Disconnect-Request Disconnect-
Request with a Message-Authenticator Attribute present MUST calculate
the correct value of the Message-Authenticator and silently discard
the packet if it does not match the value sent. A RADIUS server Dynamic
Authorization Client receiving a CoA/Disconnect-ACK or
CoA/Disconnect-NAK with a Message-Authenticator Attribute present
MUST calculate the correct value of the Message-
Authenticator Message-Authenticator and
silently discard the packet if it does not match the value sent.

When a Message-Authenticator Attribute is included within a CoA-
Request or Disconnect-Request, it is calculated as follows:

Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
Request Authenticator, Attributes)

When the HMAC-MD5 message integrity check is calculated the
Request Authenticator field and Message-Authenticator Attribute
should be considered to be sixteen octets of zero. The Message-
Authenticator Attribute is calculated and inserted in the packet
before the Request Authenticator is calculated.

When a Message-Authenticator Attribute is included within a CoA-
ACK, CoA-NAK, Disconnect-ACK or Disconnect-NAK, it is calculated
as follows:

Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
Request Authenticator, Attributes)

When the HMAC-MD5 message integrity check is calculated the
Message-Authenticator Attribute should be considered to be sixteen
octets of zero. The Request Authenticator is taken from the
corresponding CoA/Disconnect-Request. The Message-Authenticator
is calculated and inserted in the packet before the Response
Authenticator is calculated.

3.5. Error-Cause

Description

It is possible that the NAS a Dynamic Authorization Server cannot honor
Disconnect-Request or CoA-Request packets for some reason. The
Error-Cause Attribute provides more detail on the cause of the
problem. It MAY be included within Disconnect-NAK and CoA-NAK and Disconnect-NAK
packets.

A summary of the Error-Cause 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

101 for Error-Cause

Length

Value

The Value field is four octets, containing an integer specifying
the cause of the error. Values 0-199 and 300-399 are reserved.
Values 200-299 represent successful completion, so that these
values may only be sent within Disconnect-ACK or CoA-ACK or Disconnect-ACK packets
and MUST NOT be sent within a Disconnect-NAK CoA-NAK or CoA-NAK. Disconnect-NAK packet.
Values 400-499 represent fatal errors committed by the RADIUS server, Dynamic
Authorization Client, so that they MAY be sent within CoA-NAK or
Disconnect-NAK packets, and MUST NOT be sent within CoA-ACK or
Disconnect-ACK packets. Values 500-599 represent fatal errors
occurring on a NAS or RADIUS
proxy, Dynamic Authorization Server, so that they MAY be
sent within CoA-NAK and Disconnect-NAK packets, and MUST NOT be
sent within CoA-ACK or Disconnect-ACK packets. Error-Cause values
SHOULD be logged by the RADIUS
server. Dynamic Authorization Client. Error-Code
values (expressed in decimal) include:

# Value
--- -----
201 Residual Session Context Removed
202 Invalid EAP Packet (Ignored)
401 Unsupported Attribute
402 Missing Attribute
403 NAS Identification Mismatch
404 Invalid Request
405 Unsupported Service
406 Unsupported Extension
407 Invalid Attribute Value
501 Administratively Prohibited
502 Request Not Routable (Proxy)
503 Session Context Not Found
504 Session Context Not Removable
505 Other Proxy Processing Error
506 Resources Unavailable
507 Request Initiated

"Residual Session Context Removed" is sent in response to a
Disconnect-Request if the user session is no longer active, but
residual session context was found and successfully removed. This
value is only sent within a Disconnect-ACK and MUST NOT be sent
within a CoA-ACK, Disconnect-NAK or CoA-NAK.

"Invalid EAP Packet (Ignored)" is a non-fatal error that MUST NOT
be sent by implementations of this specification.

"Unsupported Attribute" is a fatal error sent if a Request
contains an attribute (such as a Vendor-Specific or EAP-Message
Attribute) that is not supported.

"Missing Attribute" is a fatal error sent if critical attributes
(such as NAS or session identification attributes) are missing
from a Request.

"NAS Identification Mismatch" is a fatal error sent if one or more
NAS identification attributes (see Section 3) do not match the
identity of the NAS receiving the Request.

"Invalid Request" is a fatal error sent if some other aspect of
the Request is invalid, such as if one or more attributes (such as
EAP- Message Attribute(s)) are not formatted properly.

"Unsupported Service" is a fatal error sent if a Service-Type
Attribute included with the Request is sent with an invalid or
unsupported value. This error cannot be sent in response to a
Disconnect-Request.

"Unsupported Extension" is a fatal error sent due to lack of
support for an extension such as Disconnect and/or CoA packets.
This will typically be sent by a proxy receiving an ICMP port
unreachable message after attempting to forward a Request CoA or
Disconnect-Request to the NAS.

"Unsupported

"Invalid Attribute Value" is a fatal error sent if a Request CoA-Request
or Disconnect-Request contains an attribute with an unsupported
value.

"Administratively Prohibited" is a fatal error sent if the NAS is
configured to prohibit honoring of CoA-Request or Disconnect-
Request packets for the specified session.

"Request Not Routable" is a fatal error which MAY be sent by a
RADIUS
proxy and MUST NOT be sent by a NAS. It indicates that the
RADIUS proxy
was unable to determine how to route the Request a CoA or Disconnect-Request
to the NAS. For example, this can occur if the required entries
are not present in the proxy's realm routing table.

"Session Context Not Found" is a fatal error sent if the session
context identified in the Request CoA-Request or Disconnect-Request does
not exist on the NAS.

"Session Context Not Removable" is a fatal error sent in response
to a Disconnect-Request if the NAS was able to locate the session
context, but could not remove it for some reason. It MUST NOT be
sent within a CoA-ACK, CoA-NAK or Disconnect-ACK, only within a
Disconnect-NAK.

"Other Proxy Processing Error" is a fatal error sent in response
to a Request CoA or Disconnect-Request that could not be processed by a
proxy, for reasons other than routing.

"Resources Unavailable" is a fatal error sent when a Request CoA or
Disconnect-Request could not be honored due to lack of available
NAS resources (memory, non- volatile storage, etc.).

"Request Initiated" is a fatal error sent by a NAS in response to
a CoA-
Request CoA-Request including a Service-Type Attribute with a value of
"Authorize Only". It indicates that the CoA-Request has not been
honored, but that the NAS is sending a RADIUS Access-Request
including a Service-Type Attribute with value "Authorize Only" is being sent to
the RADIUS server.

3.6. Table of Attributes

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

Change-of-Authorization Messages

Request ACK NAK # Attribute
0-1 0 0 1 User-Name [Note 1]
0-1 0 0 4 NAS-IP-Address [Note 1]
0-1 0 0 5 NAS-Port [Note 1]
0-1 0 0-1 6 Service-Type
0-1 0 0 7 Framed-Protocol [Note 3]
0-1 0 0 8 Framed-IP-Address [Note 6]
0-1 0 0 9 Framed-IP-Netmask [Note 6]
0-1 0 0 10 Framed-Routing [Note 3]
0+ 0 0 11 Filter-ID [Note 3]
0-1 0 0 12 Framed-MTU [Note 3]
0+ 0 0 13 Framed-Compression [Note 3]
0+ 0 0 14 Login-IP-Host [Note 3]
0-1 0 0 15 Login-Service [Note 3]
0-1 0 0 16 Login-TCP-Port [Note 3]
0+ 0 0 18 Reply-Message [Note 2]
0-1 0 0 19 Callback-Number [Note 3]
0-1 0 0 20 Callback-Id [Note 3]
0+ 0 0 22 Framed-Route [Note 3]
0-1 0 0 23 Framed-IPX-Network [Note 6]
0-1 0-1 0-1 24 State
0+ 0 0 25 Class [Note 3]
0+ 0 0 26 Vendor-Specific [Note 3]
0-1 0 0 27 Session-Timeout [Note 3]
0-1 0 0 28 Idle-Timeout [Note 3]
0-1 0 0 29 Termination-Action [Note 3]
0-1 0 0 30 Called-Station-Id [Note 1]
0-1 0 0 31 Calling-Station-Id [Note 1]
0-1 0 0 32 NAS-Identifier [Note 1]
Request ACK NAK # Attribute
Request ACK NAK # Attribute
0+ 0+ 0+ 33 Proxy-State
0-1 0 0 34 Login-LAT-Service [Note 3]
0-1 0 0 35 Login-LAT-Node [Note 3]
0-1 0 0 36 Login-LAT-Group [Note 3]
0-1 0 0 37 Framed-AppleTalk-Link [Note 3]
0+ 0 0 38 Framed-AppleTalk-Network [Note 3]
0-1 0 0 39 Framed-AppleTalk-Zone [Note 3]
0-1 0 0 44 Acct-Session-Id [Note 1]
0-1 0 0 50 Acct-Multi-Session-Id [Note 1]
Request ACK NAK # Attribute
Request ACK NAK # Attribute
0-1 0-1 0-1 0-1 55 Event-Timestamp
0+ 0 0 56 Egress-VLANID [Note 3]
0-1 0 0 57 Ingress-Filters [Note 3]
0+ 0 0 58 Egress-VLAN-Name [Note 3]
0-1 0 0 59 User-Priority-Table [Note 3]
0-1 0 0 61 NAS-Port-Type [Note 3]
0-1 0 0 62 Port-Limit [Note 3]
0-1 0 0 63 Login-LAT-Port [Note 3]
0+ 0 0 64 Tunnel-Type [Note 5]
0+ 0 0 65 Tunnel-Medium-Type [Note 5]
0+ 0 0 66 Tunnel-Client-Endpoint [Note 5]
0+ 0 0 67 Tunnel-Server-Endpoint [Note 5]
0+ 0 0 69 Tunnel-Password [Note 5]
0-1 0 0 71 ARAP-Features [Note 3]
0-1 0 0 72 ARAP-Zone-Access [Note 3]
0+ 0 0 78 Configuration-Token [Note 3]
0+ 0-1 0 79 EAP-Message [Note 2]
0-1 0-1 0-1 80 Message-Authenticator
0+ 0 0 81 Tunnel-Private-Group-ID [Note 5]
0+ 0 0 82 Tunnel-Assignment-ID [Note 5]
0+ 0 0 83 Tunnel-Preference [Note 5]
0-1 0 0 85 Acct-Interim-Interval [Note 3]
0-1 0 0 87 NAS-Port-Id [Note 1]
0-1 0 0 88 Framed-Pool [Note 6]
0-1 0 0 89 Chargeable-User-Identity [Note 1]
0+ 0 0 90 Tunnel-Client-Auth-ID [Note 5]
0+ 0 0 91 Tunnel-Server-Auth-ID [Note 5]
0-1 0 0 92 NAS-Filter-Rule [Note 3]
0 0 0 94 Originating-Line-Info
0-1 0 0 95 NAS-IPv6-Address [Note 1]
0-1 0 0 96 Framed-Interface-Id [Note 6]
0+ 0 0 97 Framed-IPv6-Prefix [Note 6]
0+ 0 0 98 Login-IPv6-Host [Note 3]
0+ 0 0 99 Framed-IPv6-Route [Note 3]
0-1 0 0 100 Framed-IPv6-Pool [Note 6]
0 0 0+ 101 Error-Cause
0+ 0 0 123 Delegated-IPv6-Prefix [Note 6]
Request ACK NAK # Attribute
Disconnect Messages

Request ACK NAK # Attribute
0-1 0 0 1 User-Name [Note 1]
0-1 0 0 4 NAS-IP-Address [Note 1]
0-1 0 0 5 NAS-Port [Note 1]
0 0 0 6 Service-Type
0 0 0 8 Framed-IP-Address [Note 6]
0+ 0 0 18 Reply-Message [Note 2]
0 0 0 24 State
0+ 0 0 25 Class [Note 4]
0+ 0 0 26 Vendor-Specific
0-1 0 0 30 Called-Station-Id [Note 1]
0-1 0 0 31 Calling-Station-Id [Note 1]
0-1 0 0 32 NAS-Identifier [Note 1]
0+ 0+ 0+ 33 Proxy-State
0-1 0 0 44 Acct-Session-Id [Note 1]
0-1 0-1 0 49 Acct-Terminate-Cause
0-1 0 0 50 Acct-Multi-Session-Id [Note 1]
0-1 0-1 0-1 55 Event-Timestamp
0 0 0 61 NAS-Port-Type
0+ 0-1 0 79 EAP-Message [Note 2]
0-1 0-1 0-1 80 Message-Authenticator
0-1 0 0 87 NAS-Port-Id [Note 1]
0-1 0 0 89 Chargeable-User-Identity [Note 1]
0-1 0 0 95 NAS-IPv6-Address [Note 1]
0 0 0 96 Framed-Interface-Id [Note 6]
0 0 0 97 Framed-IPv6-Prefix [Note 6]
0 0 0 100 Framed-IPv6-Pool [Note 6]
0 0 0+ 101 Error-Cause
Request ACK NAK # Attribute

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.

[Note 1] Where NAS or session identification attributes are included
in Disconnect-Request or CoA-Request packets, they are used for
identification purposes only. These attributes MUST NOT be used for
purposes other than identification (e.g. within CoA-Request packets
to request authorization changes).

[Note 2] The Reply-Message Attribute is used to present a displayable
message to the user. The message is only displayed as a result of a
successful Disconnect-Request or CoA-Request (where a Disconnect-ACK
or CoA-ACK is subsequently sent). Where EAP is used for
authentication, an EAP-Message/Notification-Request Attribute is sent
instead, and Disconnect-ACK or CoA-ACK packets contain an EAP-
Message/Notification-Response Attribute.

[Note 3] When included within a CoA-Request, these attributes
represent an authorization change request. When one of these
attributes is omitted from a CoA-Request, the NAS assumes that the
attribute value is to remain unchanged. Attributes included in a
CoA-Request replace all existing value(s) of the same attribute(s).

[Note 4] When included within a successful Disconnect-Request (where
a Disconnect-ACK is subsequently sent), the Class Attribute SHOULD be
sent unmodified by the client NAS to the RADIUS accounting server in the
Accounting Stop packet. If the Disconnect-Request is unsuccessful,
then the Class Attribute is not processed.

[Note 5] When included within a CoA-Request, these attributes
represent an authorization change request. Where tunnel attribute(s)
are included within a successful CoA-Request, all existing tunnel
attributes are removed and replaced by the new attribute(s).

[Note 6] Where included within a CoA-Request, these attributes
represent a renumbering request. Since these attributes are not used
for session identification, they MUST NOT be included within a
Disconnect-Request. Note that renumbering may not be possible in all
situations. For example, in order to change an IP address on receipt
of a changed Framed-IP-Address address, IPCP re-negotiation could be
required, which is not supported by all PPP implementations.

4. Diameter Considerations

Due to differences in handling change-of-authorization requests in
RADIUS and Diameter, it may be difficult or impossible for a
Diameter/RADIUS gateway to successfully translate a Diameter Re-Auth-
Request (RAR) to a CoA-Request and vice versa. For example, since a
CoA-Request only initiates an authorization change but does not
initiate re-authentication, a RAR command containing a Re-Auth-
Request-Type AVP with value "AUTHORIZE_AUTHENTICATE" cannot be
directly translated to a CoA-Request. A Diameter/RADIUS gateway
receiving a CoA-Request containing authorization changes will need to
translate this into two Diameter exchange. exchanges. First, the
Diameter/RADIUS gateway will issue a RAR command including a Session-
Id AVP and a Re-Auth-Request-Type AVP with value "AUTHORIZE ONLY".
Then the Diameter/RADIUS gateway will respond to the ensuing access
request with a response including the authorization attributes
gleaned from the CoA-Request. For the translation to be possible,
the CoA-Request MUST include a Acct-Session-Id Attribute. If the
Diameter client uses the same Session-Id for both authorization and
accounting, then the Diameter/RADIUS gateway can copy the contents of
the Acct-Session-Id Attribute into the Session-Id AVP; otherwise, it
will need to map the Acct-Session-Id value to an equivalent Session-
Id for use within a RAR command.

To simplify translation between RADIUS and Diameter, a server
compliant with this specification MAY Dynamic
Authorization Clients can include a Service-Type Attribute with value
"Authorize Only" within a CoA-Request. Such a
CoA-Request MUST contain a State Attribute. A NAS supporting the
"Authorize Only" Service-Type within a CoA-Request responds with a
CoA-NAK containing a Service-Type Attribute with value "Authorize
Only", and an Error-Cause Attribute with value "Request Initiated".
The NAS will then send an Access-Request containing a Service-Type
Attribute with a value of "Authorize Only", along with a State
Attribute. CoA-Request, as described in Section 3.2.
A Diameter/RADIUS gateway receiving a CoA-Request containing a
Service-Type with value "Authorize Only" translates this to a RAR
with Re-Auth-Request-Type AVP with value "AUTHORIZE ONLY". The
received RAA is then translated to a CoA-NAK with a Service-Type
value of "Authorize Only". If the Result-Code AVP in the RAA has a
value in the success category, then an Error-Cause Attribute with
value "Request Initiated" is included in the CoA-NAK. If the
Result-Code AVP in the RAA has a value indicating a Protocol Error or
a Transient or Permanent Failure, then an alternate Error-Cause
Attribute is returned as suggested below.

Within Diameter, a server can request that a session be aborted by
sending an Abort-Session-Request (ASR), identifying the session to be
terminated using Session-ID and User-Name AVPs. The ASR command is
translated to a Disconnect-Request containing an Acct-Session-Id and
User-Name attribute. attributes. If the Diameter client utilizes the same
Session-Id in both authorization and accounting, then the value of
the Session-ID AVP may be placed in the Acct-Session-Id attribute;
otherwise the value of the Session-ID AVP will need to be mapped to
an appropriate Acct-Session-Id value. For a Disconnect-Request to
be translatable to an ASR, an Acct-Session-Id attribute MUST be
present. If the Diameter client utilizes the same Session-Id in both
authorization and accounting, then the value of the Acct-Session-Id
may be placed into the Session-ID AVP within the ASR; otherwise the
value of the Acct-Session-Id will need to be mapped to an appropriate
Session-ID value.

An Abort-Session-Answer (ASA) command is sent in response to an ASR
in order to indicate the disposition of the request. A
Diameter/RADIUS gateway receiving a Disconnect-ACK translates this to
an ASA command with a Result-Code AVP of "DIAMETER_SUCCESS". A
Disconnect-NAK received from the server NAS is translated to an ASA command
with a Result-Code AVP which depends on the value of the Error-Cause
Attribute. Suggested translations between Error-Cause Attribute
values and Result-Code AVP values are included below:

# Error-Cause Attribute Value Result-Code AVP
--- --------------------------- ------------------------
201 Residual Session Context DIAMETER_SUCCESS
Removed
202 Invalid EAP Packet DIAMETER_LIMITED_SUCCESS
(Ignored)
401 Unsupported Attribute DIAMETER_AVP_UNSUPPORTED
402 Missing Attribute DIAMETER_MISSING_AVP
403 NAS Identification DIAMETER_REALM_NOT_SERVED
Mismatch
404 Invalid Request DIAMETER_UNABLE_TO_COMPLY
405 Unsupported Service DIAMETER_COMMAND_UNSUPPORTED
406 Unsupported Extension DIAMETER_APPLICATION_UNSUPPORTED
407 Invalid Attribute Value DIAMETER_INVALID_AVP_VALUE
501 Administratively DIAMETER_AUTHORIZATION_REJECTED
Prohibited
502 Request Not Routable (Proxy) DIAMETER_UNABLE_TO_DELIVER
503 Session Context Not Found DIAMETER_UNKNOWN_SESSION_ID
504 Session Context Not DIAMETER_AUTHORIZATION_REJECTED
Removable
505 Other Proxy Processing DIAMETER_UNABLE_TO_COMPLY
Error
506 Resources Unavailable DIAMETER_RESOURCES_EXCEEDED
507 Request Initiated DIAMETER_SUCCESS

Since both the ASR/ASA and Disconnect-Request/Disconnect-
NAK/Disconnect-ACK exchanges involve just a request and response,
inclusion of an "Authorize Only" Service-Type within a Disconnect-
Request is not needed to assist in Diameter/RADIUS translation, and
may make translation more difficult. As a result, as noted in
Section 3.2, the Service-Type Attribute MUST NOT be used within a
Disconnect-Request.

5. IANA Considerations

This document uses the RADIUS [RFC2865] namespace, see
<http://www.iana.org/assignments/radius-types>. In addition to the
allocations already made in [RFC3575] and [RFC3576], this
specification requests allocation of an additional value of the
Error-Cause Attribute (101):

# Value
--- -----
407 Invalid Attribute Value

6. Security Considerations

6.1. Authorization Issues

Where a NAS is shared by multiple providers, it is undesirable for
one provider to be able to send Disconnect-Request or CoA-Requests
affecting the sessions of another provider.

A NAS or RADIUS proxy Dynamic Authorization Server MUST silently discard Disconnect-Request Disconnect-
Request or CoA-Request packets from untrusted sources. By default, In situations
where the Dynamic Authorization Client is co-resident with a RADIUS
authentication or accounting server, a proxy SHOULD MAY perform a "reverse
path forwarding" (RPF) check to verify that a Disconnect-Request or
CoA-Request originates from an authorized RADIUS server. Dynamic Authorization
Client. In addition, it SHOULD be possible to explicitly authorize
additional sources of Disconnect-Request or CoA-
Request CoA-Request packets
relating to certain classes of sessions. For example, a particular
source can be explicitly authorized to send CoA-Request packets
relating to users within a set of realms.

To perform the RPF check, the proxy Dynamic Authorization Server uses the
session identification attributes included in Disconnect-Request or
CoA-Request packets, in order to determine the RADIUS server(s) to
which an equivalent Access-Request could be routed. If the source
address of the Disconnect-Request or CoA-Request is within this set,
then the
Request CoA-Request or Disconnect-Request is forwarded; otherwise it
MUST be silently discarded.

Typically the proxy Dynamic Authorization Server will extract the realm
from the Network Access Identifier [RFC4282] included within the
User-Name or Chargeble-User-Identity Attribute, and determine the
corresponding RADIUS servers in the proxy realm routing tables. The RADIUS servers for that realm are then compared against If the source address
Dynamic Authorization Server maintains long-term session state, it
MAY perform the authorization check based on the session
identification attributes in the CoA-Request. The session
identification attributes can be used to tie a session to a
particular proxy or set of proxies, as with the packet. NAI realm.

Where no RADIUS proxy is present, the RPF check will need to can only be performed by the
NAS itself. if it maintains its own a realm routing table. If the NAS does
not maintain a realm routing table (e.g. it selects forwarding
proxies based on primary/secondary configuration and/or liveness
checks), then an RPF check cannot be performed.

Since authorization to send a Disconnect-Request or CoA-Request is
determined based on the source address and the corresponding shared
secret, the NASes or proxies Dynamic Authorization Server SHOULD configure a different
shared secret for each RADIUS server. Dynamic Authorization Client.

6.2. Impersonation

[RFC2865] Section 3 states:

A RADIUS server MUST 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 RADIUS requests Access-Requests are forwarded by a proxy, the NAS-IP-Address NAS-IP-
Address or NAS-IPv6-Address Attributes will typically not match the
source address observed by the RADIUS server. Since the NAS-Identifier NAS-
Identifier Attribute need not contain an FQDN, this attribute Attribute may not
be resolvable to the source address observed by the RADIUS server,
even when no proxy is present.

As a result, the authenticity check performed by a RADIUS server or
proxy does not verify the correctness of NAS identification
attributes. This makes it possible for a rogue NAS to forge NAS-IP-
Address, NAS-IPv6-Address or NAS-Identifier Attributes within a
RADIUS Access-Request in order to impersonate another NAS. It is
also possible for a rogue NAS to forge session identification attributes such as the Called-Station-Id, Called-
Station-Id, Calling-Station-Id, or Originating-Line-Info [RFC4005].
This could fool the RADIUS server Dynamic Authorization Client into sending Disconnect-Request CoA-
Request or CoA-Request Disconnect-Request packets containing forged session
identification attributes to a NAS targeted by an attacker.

To address these vulnerabilities RADIUS proxies one hop from the NAS
SHOULD check whether NAS identification attributes (see Section 3)
match the packet source
address of packets originating from the NAS. address. Where one or more attributes do not
match, Disconnect-Request or CoA-Request Access-Request packets SHOULD be silently discarded.

Such a check may not always be possible. Since the NAS-Identifier
Attribute need not correspond to an FQDN, it may not be resolvable to
an IP address to be matched against the source address. Also, where
a NAT exists between the RADIUS client and proxy, checking the NAS-
IP-Address or NAS-IPv6-Address Attributes may not be feasible.

6.3. IPsec Usage Guidelines

In addition to security vulnerabilities unique to Disconnect or CoA
packets, the protocol exchanges described in this document are
susceptible to the same vulnerabilities as RADIUS [RFC2865]. It is
RECOMMENDED that IPsec be employed to afford better security.

Implementations of this specification SHOULD support IPsec [RFC4301]
along with IKEv1 [RFC2409] for key management. IPsec ESP [RFC4303]
with non-null transform SHOULD be supported, and IPsec ESP with a
non-null encryption transform and authentication support SHOULD be
used to provide per-packet confidentiality, authentication, integrity
and replay protection. IKE SHOULD be used for key management.

Within RADIUS [RFC2865], a shared secret is used for hiding of
Attributes such as User-Password, as well as in computation of the
Response Authenticator. In RADIUS accounting [RFC2866], the shared
secret is used in computation of both the Request Authenticator and
the Response Authenticator.

Since in RADIUS a shared secret is used to provide confidentiality as
well as integrity protection and authentication, only use of IPsec
ESP with a non-null transform can provide security services
sufficient to substitute for RADIUS application-layer security.
Therefore, where IPsec AH or ESP null is used, it will typically
still be necessary to configure a RADIUS shared secret.

Where RADIUS is run over IPsec ESP with a non-null transform, the
secret shared between the NAS Dynamic Authorization Server and the RADIUS server
Dynamic Authorization Client MAY NOT be configured. In this case, a
shared secret of zero length MUST be assumed. However, a RADIUS server Dynamic
Authorization Client that cannot know whether incoming traffic is
IPsec-protected MUST be configured with a non-null RADIUS shared
secret.

When IPsec ESP is used with RADIUS, per-packet authentication,
integrity and replay protection MUST be used. 3DES-CBC MUST be
supported as an encryption transform and AES-CBC SHOULD be supported.
AES-CBC SHOULD be offered as a preferred encryption transform if
supported. HMAC-SHA1-96 MUST be supported as an authentication
transform. DES-CBC SHOULD NOT be used as the encryption transform.

A typical IPsec policy for an IPsec-capable RADIUS client is
"Initiate IPsec, from me to any destination port UDP 1812". This
IPsec policy causes an IPsec SA to be set up by the RADIUS client
prior to sending a RADIUS Access-Request to a RADIUS traffic. server. If some
RADIUS servers contacted by the RADIUS client do not support IPsec,
then a more granular policy will be required: "Initiate IPsec, from
me to IPsec-Capable-RADIUS-Server, destination port UDP 1812."

For a client Dynamic Authorization Server implementing this specification
the policy would be "Accept IPsec, from any to me, destination port
UDP 3799". This causes the RADIUS client Dynamic Authorization Server to accept
(but not require) use of IPsec. It may not be appropriate to require
IPsec for all RADIUS servers Dynamic Authorization Clients connecting to an IPsec-enabled RADIUS client, IPsec-
enabled Dynamic Authorization Server, since some RADIUS
servers Dynamic
Authorization Clients may not support IPsec.

For an IPsec-capable RADIUS server, a typical IPsec policy is "Accept
IPsec, from any to me, destination port 1812". This causes the
RADIUS server to accept (but not require) use of IPsec. It may not
be appropriate to require IPsec for all RADIUS clients connecting to
an IPsec-enabled RADIUS server, since some RADIUS clients may not
support IPsec.

For servers Dynamic Authorization Clients implementing this specification,
the policy would be "Initiate IPsec, from me to any, destination port
UDP 3799". This causes the RADIUS server Dynamic Authorization Client to initiate
IPsec when sending RADIUS
extension Dynamic Authorization traffic to any RADIUS client. Dynamic
Authorization Server. If some RADIUS clients Dynamic Authorization Servers
contacted by the server Dynamic Authorization Client do not support IPsec,
then a more granular policy will be required, such as "Initiate
IPsec, from me to IPsec-
capable-RADIUS-client, IPsec-capable-Dynamic-Authorization-Server,
destination port UDP 3799".

Where IPsec is used for security, and no RADIUS shared secret is
configured, it is important that the RADIUS client Dynamic Authorization Server and server
Dynamic Authorization Client perform an authorization check. Before
enabling a host to act as a RADIUS
client, Dynamic Authorization Server, the RADIUS server Dynamic
Authorization Client SHOULD check whether the host is authorized to provide network access.
act in that role. Similarly, before enabling a host to act as a RADIUS server,
Dynamic Authorization Client, the RADIUS client Dynamic Authorization Server SHOULD
check whether the host is authorized for that role.

RADIUS servers

Dynamic Authorization Clients can be configured with the IP addresses
(for IKE IKEv1 Aggressive Mode with pre-shared keys) or FQDNs (for
certificate authentication) of RADIUS clients. Dynamic Authorization Servers.
Alternatively, if a separate Certification Authority (CA) exists for RADIUS clients,
Dynamic Authorization Servers, then the
RADIUS server Dynamic Authorization Client
can configure this CA as a trust anchor [RFC3280] for use with IPsec. IKEv1.

Similarly, RADIUS clients Dynamic Authorization Servers can be configured with the
IP addresses (for IKE IKEv1 Aggressive Mode with pre-shared keys) or
FQDNs (for certificate authentication) of RADIUS servers. Dynamic Authorization
Clients. Alternatively, if a separate CA exists for RADIUS servers, Dynamic
Authorization Clients, then the RADIUS client Dynamic Authorization Server can
configure this CA as a trust anchor for use with IPsec. IKEv1.

Since unlike SSL/TLS, IKE IKEv1 does not permit certificate policies to
be set on a per-port basis, certificate policies need to apply to all
uses of IPsec IKEv1 on RADIUS clients Dynamic Authorization Servers and servers. Dynamic
Authorization Clients. In IPsec a deployment supporting only certificate
authentication, a management station initiating an IPsec-protected
telnet session to the RADIUS server Dynamic Authorization Client would need to
obtain a certificate chaining to the RADIUS client Dynamic Authorization Server CA.
Issuing such a certificate migh might not be appropriate if the management
station was not authorized as a RADIUS client. Dynamic Authorization Server.

Where RADIUS clients may Dynamic Authorization Servers obtain their IP address
dynamically (such as an Access Point supporting DHCP), IKEv1 Main
Mode with pre-shared keys [RFC2409] SHOULD NOT be used, since this
requires use of a group pre-
shared pre-shared key; instead, Aggressive Mode
SHOULD be used. Where RADIUS
client Dynamic Authorization Server addresses are
statically assigned either IKEv1 Aggressive Mode or Main Mode MAY be
used. With certificate authentication, IKEv1 Main Mode SHOULD be
used.

Care needs to be taken with IKE IKEv1 Phase 1 Identity Payload selection
in order to enable mapping of identities to pre-shared keys even with
Aggressive Mode. Where the ID_IPV4_ADDR or ID_IPV6_ADDR Identity
Payloads are used and addresses are dynamically assigned, mapping of
identities to keys is not possible, so that group pre-shared keys are
still a practical necessity. As a result, the ID_FQDN identity
payload SHOULD be employed in situations where Aggressive mode is
utilized along with pre-shared keys and IP addresses are dynamically
assigned. This approach also has other advantages, since it allows
the RADIUS server Dynamic Authorization Client and client Dynamic Authorization Server to
configure themselves based on the fully qualified domain name of
their peers.

Note that with IPsec, security services are negotiated at the
granularity of an IPsec SA, so that RADIUS exchanges requiring a set of
security services different from those negotiated with existing IPsec
SAs will need to negotiate a new IPsec SA. Separate IPsec SAs are
also advisable where quality of service considerations dictate
different handling RADIUS conversations. Attempting to apply
different quality of service to connections handled by the same IPsec
SA can result in reordering, and falling outside the replay window.
For a discussion of the issues, see [RFC2983].

6.4. Replay Protection

Where IPsec replay protection is not used, an Event-Timestamp (55)
[RFC2869] Attribute SHOULD be included within CoA-Request and
Disconnect-Request packets, and MAY be included within CoA-ACK, CoA-
NAK, Disconnect-ACK and Disconnect-NAK packets.

When the Event-Timestamp attribute is present, both the NAS Dynamic
Authorization Server and the
RADIUS server Dynamic Authorization Client MUST check
that the Event-Timestamp Attribute is current within an acceptable
time window. If the Event-Timestamp Attribute is not current, then
the packet MUST be silently discarded. This implies the need for
loose time synchronization within the network, which can be achieved
by a variety of means, including SNTP, as described in [RFC4330].
Implementations SHOULD be configurable to discard CoA-Request or
Disconnect-Request packets not containing an Event-Timestamp
attribute.

If the Event-Timestamp Attribute is included, it represents the time
at which the original packet was sent, and therefore it SHOULD NOT be
updated when the packet is retransmitted. If the Event-Timestamp
attribute is not updated, this implies that the Identifier is not
changed in retransmitted packets. As a result, the ability to detect
replay within the time window is dependent on support for duplicate
detection within that same window. As noted in Section 2.3,
duplicate detection is REQUIRED for RADIUS clients Dynamic Authorization Servers
implementing this specification.

The time window used for duplicate detection MUST be the same as the
window used to detect stale Event-Timestamp Attributes. Since the
RADIUS Identifier cannot be repeated within the selected time window,
no more than 256 Requests can be accepted within the time window. As
a result, the chosen time window will depend on the expected maximum
volume of CoA/Disconnect-Requests, so that unnecessary discards can
be avoided. A default time window of 300 seconds should be adequate
in many circumstances.

7. Example Traces

Disconnect Request with User-Name:

0: xxxx xxxx xxxx xxxx xxxx 2801 001c 1b23 .B.....$.-(....#
16: 624c 3543 ceba 55f1 be55 a714 ca5e 0108 bL5C..U..U...^..
32: 6d63 6869 6261

Disconnect Request with Acct-Session-ID:

0: xxxx xxxx xxxx xxxx xxxx 2801 001e ad0d .B..... ~.(.....
16: 8e53 55b6 bd02 a0cb ace6 4e38 77bd 2c0a .SU.......N8w.,.
32: 3930 3233 3435 3637 90234567

Disconnect Request with Framed-IP-Address:

0: xxxx xxxx xxxx xxxx xxxx 2801 001a 0bda .B....."2.(.....
16: 33fe 765b 05f0 fd9c c32a 2f6b 5182 0806 3.v[.....*/kQ...
32: 0a00 0203

8. References

8.1. Normative References

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.

[RFC2865] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.

[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RFC2869] Rigney, C., Willats W. and P. Calhoun, "RADIUS Extensions",
RFC 2869, June 2000.

[RFC3162] Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IPv6", RFC
3162, August 2001.

[RFC3280] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.

[RFC3575] Aboba, B., "IANA Considerations for RADIUS", RFC 3575, July
2003.

[RFC3579] Aboba, B. and P. Calhoun, "RADIUS Support for Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.

[RFC4282] Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The Network
Access Identifier", RFC 4282, December 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.

8.2. Informative References

[MD5Attack]
Dobbertin, H., "The Status of MD5 After a Recent Attack",
CryptoBytes Vol.2 No.2, Summer 1996.

[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.
and I. Goyret, "RADIUS Attributes for Tunnel Protocol
Support", RFC 2868, June 2000.

[RFC2983] Black, D. "Differentiated Services and Tunnels", RFC 2983,
October 2000.

[RFC3539] Aboba, B. and J. Wood, "Authentication, Authorization and
Accounting Transport Profile", RFC 3539, June 2003.

[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B. Aboba,
"Dynamic Authorization Extensions to Remote Authentication
Dial In User Service (RADIUS)", RFC 3576, July 2003.

[RFC4005] Calhoun, P., Zorn, G., Spence, D. and D. Mitton, "Diameter
Network Access Server Application", RFC 4005, August 2005.

[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI", RFC 4330, January 2006.

[RFC4372] Adrangi, F., Lior, A., Korhonen, J. and J. Loughney,
"Chargeable User Identity", RFC 4372, January 2006.

[RFC4675] Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Attributes for
Virtual LAN and Priority Support", RFC 4675, September 2006.

[RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
Attribute", RFC 4818, April 2007.

[RFC4849] Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Filter Rule
Attribute", RFC 4849, April 2007.

[MD5Attack]
Dobbertin, H., "The Status of MD5 After a Recent Attack",
CryptoBytes Vol.2 No.2, Summer 1996.

Acknowledgments

This protocol was first developed and distributed by Ascend
Communications. Example code was distributed in their free server
kit.

The authors would like to acknowledge the valuable suggestions and
feedback from the following people: Avi Lior <avi@bridgewatersystems.com>, Lior, Randy Bush <randy@psg.net>, Bush, Steve Bellovin <smb@research.att.com> Bellovin, Glen Zorn <gwz@cisco.com>, Zorn, Mark Jones <mjones@bridgewatersystems.com>,
Jones, Claudio Lapidus <clapidus@hotmail.com>, Lapidus, Anurag Batta <Anurag_Batta@3com.com>, Batta, Kuntal Chowdhury <chowdury@nortelnetworks.com> Chowdhury, Tim Moore <timmoore@microsoft.com> Moore,
Russ Housley <housley@vigilsec.com> Housley, Joe Salowey <jsalowey@cisco.com> Salowey, Alan DeKok and David Nelson.

Authors' Addresses

Murtaza Chiba
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose CA, 95134

EMail: mchiba@cisco.com
Phone: +1 408 525 7198

Gopal Dommety
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134

EMail: gdommety@cisco.com
Phone: +1 408 525 1404

Mark Eklund
Cisco Systems, Inc.
170 West Tasman Dr.
San Jose, CA 95134

EMail: meklund@cisco.com
Phone: +1 865 671 6255

David Mitton
RSA Security, Inc.
174 Middlesex Turnpike
Bedford, MA 01730

EMail: dmitton@circularnetworks.com

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax: +1 425 936 7329

Appendix A - Changes from RFC 3576

This Appendix lists the major changes between [RFC3576] and this
document. Minor changes, including style, grammar, spelling, and
editorial changes are not mentioned here.

o The term "Dynamic Authorization Client" is used instead of RADIUS
server where it applies to the originator of CoA and Disconnect-
Request packets. Similarly, the term "Dynamic Authorizatin Server"
is used instead of NAS where it applies to the receiver of CoA and
Disconnect-Request packets. Definitions of these terms have been
added (Section 1.3).

o Added requirement for duplicate detection on the RADIUS client Dynamic
Authorization Server (Section 2.3).

o Added Chargeable-User-Identity as a session identification
attribute. Removed Framed-IP-Address, Framed-IPv6-Prefix, Framed-
Interface-Id and NAS-Port-Type attributes as session identification
attributes. Recommended inclusion of Acct-Session-Id or Acct-Multi-
Session-Id attributes in an Access-Request (Section 3).

o Added recommendation that an Acct-Session-Id or Acct-Mult-Session-
Id Attribute be included in an Access-Request (Section 3).

o Added details relating to handling of the Proxy-State Attribute
(Section 3.1).

o Added statement clarification that support for a Service-Type Attribute with
value "Authorize Only" Service-Type is optional on both the NAS and Dynamic
Authorization Client (Section 3.2).

o Added requirements requirement for inclusion of the State Attribute in CoA-
Request packets with including a Service-Type Attribute with a value of
"Authorize Only" (Section 3.3).

o Added clarification on the calculation of the Message-Authenticator
Attribute (Section 3.4).

o An additional Error-Cause Attribute value (407) is allocated for
Invalid Attribute Value (Sections 3.5, 4).

o Updated the CoA-Request Attribute Table to include Filter-Rule,
Delegated-IPv6-Prefix, Egress-VLANID, Ingress-Filters, Egress-VLAN-
Name and User-Priority attributes (Section 3.6).

o Added the Chargeable-User-Identity Attribute to both the CoA-
Request and Disconnect-Request Attribute table (Section 3.6).

o Added note on the use of the CoA-Request for renumbering (Section
3.6).

o Use of the Service-Type Attribute within a Disconnect-Request is
prohibited (Sections 3.2, 3.6, 4). 3.6).

o Added Diameter Considerations (Section 4).

o Changed the text to indicate that the Event-Timestamp Attribute
should not be recalculated on retransmission. The implications for
replay and duplicate detection are discussed (Section 6.4).

o Operation of the RPF check has been clarified. Use of the RPF
check is optional rather than recommended by default (Section 6.1).

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