Network Working Group M. Chiba

Network Working Group M. Chiba
Request for Comments: 5176 G. Dommety
Obsoletes: 3576 M. Eklund
Category: Informational Cisco Systems, Inc.
D. Mitton
RSA, Security Division of EMC
B. Aboba
Microsoft Corporation
January 2008

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

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.

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.

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Table of Contents

1. Introduction ....................................................2
1.1. Applicability ..............................................3
1.2. Requirements Language ......................................4
1.3. Terminology ................................................4
2. Overview ........................................................4
2.1. Disconnect Messages (DMs) ..................................5
2.2. Change-of-Authorization (CoA) Messages .....................5
2.3. Packet Format ..............................................6
3. Attributes .....................................................10
3.1. Proxy State ...............................................12
3.2. Authorize Only ............................................13
3.3. State .....................................................14
3.4. Message-Authenticator .....................................15
3.5. Error-Cause ...............................................16
3.6. Table of Attributes .......................................20
4. Diameter Considerations ........................................24
5. IANA Considerations ............................................26
6. Security Considerations ........................................26
6.1. Authorization Issues ......................................26
6.2. IPsec Usage Guidelines ....................................27
6.3. Replay Protection .........................................28
7. Example Traces .................................................28
8. References .....................................................29
8.1. Normative References ......................................29
8.2. Informative References ....................................30
9. Acknowledgments ................................................30
Appendix A ........................................................31

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 user session(s) in progress.
Alternatively, if the user changes authorization level, this may
require that authorization attributes be added/deleted from user
session(s).

To overcome these limitations, several vendors have implemented
additional RADIUS commands in order to enable unsolicited messages to
be sent to the NAS. These extended commands provide support for
Disconnect and Change-of-Authorization (CoA) packets. Disconnect

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packets cause user session(s) 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
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 6.2 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 6.3 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 Attribute Value Pairs (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.

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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
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.

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2.1. Disconnect Messages (DMs)

A Disconnect-Request packet is sent by the Dynamic Authorization
Client in order to terminate user session(s) 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(s)
to be terminated by inclusion of the identification attributes
described in Section 3.

+----------+ +----------+
| | Disconnect-Request | |
| | <-------------------- | |
| NAS | | DAC |
| | Disconnect-ACK/NAK | |
| | ---------------------> | |
+----------+ +----------+

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

2.2. Change-of-Authorization (CoA) Messages

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 those 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(s) that the
identification attributes map to.

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

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+----------+ +----------+
| | CoA-Request | |
| | <-------------------- | |
| NAS | | DAC |
| | CoA-ACK/NAK | |
| | ---------------------> | |
+----------+ +----------+

The NAS responds to a CoA-Request sent by a Dynamic Authorization
Client with a CoA-ACK if the NAS is able to successfully change the
authorizations for the user session(s), or a CoA-NAK if the 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 following 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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-

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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. A Dynamic Authorization Server implementing this
specification MUST be capable of detecting a duplicate request if
it has the same source IP address, source UDP port, and Identifier
within a short span of time.

The responsibility for retransmission of Disconnect-Request and
CoA-Request packets lies with the Dynamic Authorization Client.
If after sending these packets, the 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 Dynamic Authorization Client is retransmitting a
Disconnect-Request or CoA-Request to the same 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 Dynamic Authorization Server fails, a
secondary Dynamic Authorization Server must be queried, if
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
Dynamic Authorization Client is sending directly to the NAS,
failover typically does not make sense, since CoA-Request or
Disconnect-Request packets need to be delivered to the NAS where
the session resides.

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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 Dynamic Authorization Client and
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 CoA-Request or
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 CoA-Request or 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 Dynamic Authorization Client and the
Dynamic Authorization Server) SHOULD be at least as large and
unguessable as a well-chosen password. The Dynamic Authorization

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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 CoA-Request and Disconnect-Request packets, all attributes MUST
be 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
CoA-Request is successful for all matching sessions, the NAS MUST
send a CoA-ACK in reply, and all requested authorization changes
MUST be made. If the CoA-Request is unsuccessful for any matching
sessions, the NAS MUST send a CoA-NAK in reply, and the requested
authorization changes MUST NOT be made for any of the matching
sessions. Similarly, a state change MUST NOT occur as a result of
a Disconnect-Request that is unsuccessful with respect to any of
the matching sessions; a NAS MUST send a Disconnect-NAK in reply
if any of the matching sessions cannot be successfully terminated.
A NAS that does not support dynamic authorization changes applying
to multiple sessions MUST send a CoA-NAK or Disconnect-NAK in
reply; an Error-Cause Attribute with value 508 (Multiple Session
Selection Unsupported) SHOULD be included.

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 CoA-Request and
Disconnect-Request packets, given the difference in attribute
semantics. This is true even for attributes specified as

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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 user session(s) on the
NAS. The combination of NAS and session identification attributes
included in a CoA-Request or Disconnect-Request packet MUST match at
least one session in order for a Request to be successful; otherwise
a Disconnect-NAK or CoA-NAK MUST be sent. If all NAS identification
attributes match, and more than one session matches all of the
session identification attributes, then a CoA-Request or Disconnect-
Request MUST apply to all matching sessions.

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 one or
more sessions.
NAS-Port 5 [RFC2865] The port on which a
session is terminated.
Framed-IP-Address 8 [RFC2865] The IPv4 address associated
with a session.
Vendor-Specific 26 [RFC2865] One or more vendor-specific
identification attributes.
Called-Station-Id 30 [RFC2865] The link address to which
a session is connected.
Calling-Station-Id 31 [RFC2865] The link address from which
one or more sessions are
connected.
Acct-Session-Id 44 [RFC2866] The identifier uniquely
identifying a session
on the NAS.

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Acct-Multi-Session-Id 50 [RFC2866] The identifier uniquely
identifying related sessions.
NAS-Port-Id 87 [RFC2869] String identifying the port
where a session is.
Chargeable-User- 89 [RFC4372] The CUI associated with one
Identity or more sessions. Needed
where a privacy Network
Access Identifier (NAI) is
used, since in this case the
User-Name (e.g., "anonymous")
may not identify sessions
belonging to a given user.
Framed-Interface-Id 96 [RFC3162] The IPv6 Interface Identifier
associated with a session,
always sent with
Framed-IPv6-Prefix.
Framed-IPv6-Prefix 97 [RFC3162] The IPv6 prefix associated
with a session, always sent
with Framed-Interface-Id.

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 SHOULD be
included in Disconnect-Request and CoA-Request packets.

A NAS implementing this specification SHOULD send an Acct-Session-Id
or Acct-Multi-Session-Id Attribute within an Access-Request. Where
an Acct-Session-Id or Acct-Multi-Session-Id Attribute is not included
within an Access-Request, the Dynamic Authorization 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 User-Name or Chargeable-User-Identity attributes will not be
sufficient to uniquely identify a single session (e.g., if the same
user has multiple sessions on the NAS, or if the privacy NAI is
used). In this case, if it is desired to identify a single session,
session identification MAY be performed by using one or more of the
Framed-IP-Address, Framed-IPv6-Prefix/Framed-Interface-Id, Called-
Station-Id, Calling-Station-Id, NAS-Port, and NAS-Port-Id attributes.

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To assist RADIUS proxies in routing Request packets to their
destination, one or more of the NAS-IP-Address or NAS-IPv6-Address
attributes SHOULD be present in CoA-Request and Disconnect-Request
packets; the NAS-Identifier Attribute MAY be present. Impersonation
issues with NAS Identification attributes are discussed in [RFC3579],
Section 4.3.7.

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.

The DAC may require access to data from RADIUS authentication or
accounting packets. It uses this data to compose compliant CoA-
Request or Disconnect-Request packets. For example, as described in
Section 3.3, a CoA-Request packet containing a Service-Type Attribute
with a value of "Authorize Only" is required to contain a State
Attribute. The NAS will subsequently transmit this attribute to the
RADIUS server in an Access-Request. In order for the DAC to include
a State Attribute that the RADIUS server will subsequently accept,
some coordination between the two parties may be required.

This coordination can be achieved in multiple ways. The DAC may be
co-located with a RADIUS server, in which case it is presumed to have
access to the necessary data. The RADIUS server may also store that
information in a common database. The DAC can then be separated from
the RADIUS server, so long as it has access to that common database.

Where the DAC is not co-located with a RADIUS server, and does not
have access to a common database, the DAC SHOULD send CoA-Request or
Disconnect-Request packets to a RADIUS server acting as a proxy,
rather than sending them directly to the NAS.

A RADIUS server receiving a CoA-Request or Disconnect-Request packet
from the DAC MAY then add or update attributes (such as adding NAS or
session identification attributes or appending a State Attribute),
prior to forwarding the packet. Having CoA/Disconnect-Requests
forwarded by a RADIUS server can also enable upstream RADIUS proxies
to perform a Reverse Path Forwarding (RPF) check (see Section 6.1).

3.1. Proxy State

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

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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-Request 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 Attribute (the last
Proxy-State in the packet) 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 MUST recompute it.

3.2. Authorize Only

To simplify translation between RADIUS and Diameter, Dynamic
Authorization Clients can include a Service-Type Attribute with value
"Authorize Only" within a CoA-Request; see Section 4 for details on
Diameter considerations. Support for a CoA-Request including a
Service-Type Attribute with value "Authorize Only" is OPTIONAL on the
NAS and Dynamic Authorization Client. A Service-Type Attribute MUST
NOT be included within a 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
Attribute with a 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

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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", along with a
State Attribute. This Access-Request SHOULD contain the NAS
identification attributes from the CoA-Request, as well as the
session identification attributes from the CoA-Request permitted in
an Access-Request; it also MAY contain other attributes permitted in
an Access-Request.

As noted in [RFC2869], Section 5.19, a 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 Dynamic
Authorization Client to the NAS in a CoA-Request packet and MUST be
sent unmodified from the NAS to the 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 Attribute with value "Authorize
Only" MUST contain a State Attribute.

In order to provide a State Attribute to the NAS, a Dynamic
Authorization Client sending a CoA-Request with a Service-Type
Attribute with a 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

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receiving a CoA-Request containing a Service-Type Attribute with a
value of "Authorize Only" but lacking a State Attribute MUST send a
CoA-NAK and SHOULD include an Error-Cause Attribute with a value of
402 (Missing Attribute).

The State Attribute is also available to be sent by the 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 NAS performs the Termination-Action by sending a new Access-
Request upon termination of the current session, it MUST include the
State Attribute unchanged in that Access-Request. In either usage,
the Dynamic Authorization Server MUST NOT interpret the Attribute
locally. A CoA-Request packet 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 in order to prevent
spoofing.

A Dynamic Authorization Server receiving a CoA-Request or
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
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 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
MUST each 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.

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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 MUST 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 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 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

6

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

Chiba, et al. Informational [Page 16]

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values may only be sent within CoA-ACK or Disconnect-ACK packets
and MUST NOT be sent within a CoA-NAK or Disconnect-NAK packet.
Values 400-499 represent fatal errors committed by the 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 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 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
508 Multiple Session Selection Unsupported

"Residual Session Context Removed" is sent in response to a
Disconnect-Request if one or more user sessions are 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.

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"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 CoA-Request or
Disconnect-Request to the NAS.

"Invalid Attribute Value" is a fatal error sent if a 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 that MAY be sent by a
proxy and MUST NOT be sent by a NAS. It indicates that the proxy
was unable to determine how to route a CoA-Request 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 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 CoA or Disconnect-Request that could not be processed by a
proxy, for reasons other than routing.

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

"Resources Unavailable" is a fatal error sent when a 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 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 one or more RADIUS Access-
Requests including a Service-Type Attribute with value "Authorize
Only" to the RADIUS server.

"Multiple Session Selection Unsupported" is a fatal error sent by
a NAS in response to a CoA-Request or Disconnect-Request whose
session identification attributes match multiple sessions, where
the NAS does not support Requests applying to multiple sessions.

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

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 (Notes 1, 6)
0-1 0 0 9 Framed-IP-Netmask (Note 3)
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 3)
0-1 0-1 0-1 24 State
0+ 0 0 25 Class (Note 3)
0+ 0 0 26 Vendor-Specific (Note 7)
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)
Request ACK NAK # Attribute

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Request ACK NAK # Attribute
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 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)
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 3)
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 (Notes 1, 6)
0+ 0 0 97 Framed-IPv6-Prefix (Notes 1, 6)
0+ 0 0 98 Login-IPv6-Host (Note 3)
0+ 0 0 99 Framed-IPv6-Route (Note 3)
Request ACK NAK # Attribute

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Request ACK NAK # Attribute
0-1 0 0 100 Framed-IPv6-Pool (Note 3)
0 0 0+ 101 Error-Cause
0+ 0 0 123 Delegated-IPv6-Prefix (Note 3)
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 1)
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 (Note 7)
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 1)
0 0 0 97 Framed-IPv6-Prefix (Note 1)
0 0 0+ 101 Error-Cause
Request ACK NAK # Attribute

The following 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.

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(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 Extension Authentication
Protocol (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 values 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 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 attributes
are included within a successful CoA-Request, all existing tunnel
attributes are removed and replaced by the new attribute(s).

(Note 6) Since the Framed-IP-Address, Framed-IPv6-Prefix, and
Framed-Interface-Id attributes are used for session identification,
renumbering cannot be accomplished by including values of these
attributes within a CoA-Request. Instead, a CoA-Request including a
Service-Type Attribute with a value of "Authorize Only" is sent; new
values can be supplied in an Access-Accept sent in response to the
ensuing Access-Request. Note that renumbering will not be possible
in all situations. For example, in order to change an IP address,
IPCP or IPv6CP re-negotiation could be required, which is not
supported by all PPP implementations.

(Note 7) Within Disconnect-Request packets, Vendor-Specific
Attributes (VSAs) MAY be used for session identification. Within
CoA-Request packets, VSAs MAY be used for either session
identification or authorization change. However, the same Attribute
MUST NOT be used for both purposes simultaneously.

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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 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. To enable translation, the CoA-Request
SHOULD 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.

Where an Acct-Session-Id Attribute is not present in a CoA-Request or
Disconnect-Request, a Diameter/RADIUS gateway will either need to
determine the appropriate Acct-Session-Id or, if it cannot do so, it
can send a CoA-NAK or Disconnect-NAK in reply, possibly including an
Error-Cause Attribute with a value of 508 (Multiple Session Selection
Unsupported).

To simplify translation between RADIUS and Diameter, Dynamic
Authorization Clients can include a Service-Type Attribute with value
"Authorize Only" within a CoA-Request, as described in Section 3.2.
A Diameter/RADIUS gateway receiving a CoA-Request containing a
Service-Type Attribute with a 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
Attribute with value "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

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translated to a Disconnect-Request containing Acct-Session-Id and
User-Name 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 Attribute. To enable translation of a
Disconnect-Request to an ASR, an Acct-Session-Id Attribute SHOULD be
present.

If the Diameter client utilizes the same Session-Id in both
authorization and accounting, then the value of the Acct-Session-Id
Attribute may be placed into the Session-ID AVP within the ASR;
otherwise the value of the Acct-Session-Id Attribute will need to be
mapped to an appropriate Session-ID AVP.

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 NAS is translated to an ASA command
with a Result-Code AVP that 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

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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 allocates additional values of the Error-Cause
Attribute (101):

# Value
--- -----
407 Invalid Attribute Value
508 Multiple Session Selection Unsupported

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-Requests or CoA-Requests
affecting the sessions of another provider.

A Dynamic Authorization Server MUST silently discard Disconnect-
Request or CoA-Request packets from untrusted sources. In situations
where the Dynamic Authorization Client is co-resident with a RADIUS
authentication or accounting server, a proxy MAY perform a "reverse
path forwarding" (RPF) check to verify that a Disconnect-Request or
CoA-Request originates from an authorized Dynamic Authorization
Client. In addition, it SHOULD be possible to explicitly authorize
additional sources of Disconnect-Request or 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 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 CoA-Request or Disconnect-Request is forwarded; otherwise it
MUST be silently discarded.

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Typically, the Dynamic Authorization Server will extract the realm
from the Network Access Identifier [RFC4282] included within the
User-Name or Chargeable-User-Identity Attribute, and determine the
corresponding RADIUS servers in the realm routing tables. If the
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 NAI realm.

Where no proxy is present, the RPF check can only be performed by the
NAS 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 Dynamic Authorization Server SHOULD configure a different
shared secret for each Dynamic Authorization Client.

6.2. 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,
utilizing the profile described in [RFC3579], Section 4.2.

For Dynamic Authorization Servers implementing this specification,
the IPsec policy would be "Require IPsec, from any to me, destination
port UDP 3799". This causes the Dynamic Authorization Server to
require use of IPsec. If some Dynamic Authorization Clients do not
support IPsec, then a more granular policy will be required: "Require
IPsec, from IPsec-Capable-DAC to me".

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

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

6.3. 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 Dynamic
Authorization Server and the 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 Simple Network Time Protocol (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 Dynamic Authorization Servers
implementing this specification.

The time window used for duplicate detection MUST be the same as the
window used to detect a stale Event-Timestamp Attribute. 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

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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.

[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.

[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.

Chiba, et al. Informational [Page 29]

RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

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.

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

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

[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.

9. 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 valuable suggestions and
feedback from Avi Lior, Randy Bush, Steve Bellovin, Glen Zorn, Mark
Jones, Claudio Lapidus, Anurag Batta, Kuntal Chowdhury, Tim Moore,
Russ Housley, Joe Salowey, Alan DeKok, and David Nelson.

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

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

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

o Clarified expected behavior when session identification attributes
match more than one session (Sections 2.3, 3, 3.5, 4).

o Added Chargeable-User-Identity as a session identification
attribute. Removed NAS-Port-Type as a session identification
attribute (Section 3).

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

o Added discussion of scenarios in which the "Dynamic Authorization
Client" and RADIUS server are not co-located (Section 3).

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

o Added clarification that support for a Service-Type Attribute with
value "Authorize Only" is optional on both the NAS and Dynamic
Authorization Client (Section 3.2). Use of the Service-Type
Attribute within a Disconnect-Request is prohibited (Sections 3.2,
3.6).

o Added requirement for inclusion of the State Attribute in CoA-
Request packets 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 Additional Error-Cause Attribute values are allocated for Invalid
Attribute Value (407) and Multiple Session Selection
Identification (508) (Sections 3.5, 4).

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

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 Use of Vendor-Specific Attributes (VSAs) for session
identification and authorization change has been clarified
(Section 3.6).

o Added Note 6 on the use of the CoA-Request for renumbering, and
Note 7 on the use of Vendor-Specific attributes (Section 3.6).

o Added Diameter Considerations (Section 4).

o Event-Timestamp Attribute should not be recalculated on
retransmission. The implications for replay and duplicate
detection are discussed (Section 6.3).

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

o Text on impersonation (included in [RFC3579], Section 4.3.7) and
IPsec operation (included in [RFC3579], Section 4.2) has been
removed, and is now referenced.

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

Authors' Addresses

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

EMail: [email protected]
Phone: +1 408 525 7198

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

EMail: [email protected]
Phone: +1 408 525 1404

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

EMail: [email protected]
Phone: +1 865 671 6255

David Mitton
RSA, Security Division of EMC
174 Middlesex Turnpike
Bedford, MA 01730

EMail: [email protected]

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: [email protected]
Phone: +1 425 706 6605
Fax: +1 425 936 7329

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RFC 5176 Dynamic Authorization Extensions to RADIUS January 2008

Full Copyright Statement

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Chiba, et al. Informational [Page 34]