Internet Draft J. Manner
Expires: December 2006 M. Stiemerling
H. Tschofenig
June 2006
Authorization for NSIS Signaling Layer Protocols
<draft-manner-nsis-nslp-auth-01.txt>
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Signaling layer protocols in the NSIS working group may rely on GIST
to handle authorization. Still, in certain cases, the signaling layer
protocol may require separate authorization to be performed when a
node receives a request for a certain kind of service or resources.
This draft presents a generic model and object formats for session
authorization within the NSIS Signaling Layer Protocols. The goal of
session authorization is to allow the exchange of information between
network elements in order to authorize the use of resources for a
service and to coordinate actions between the signaling and
transport planes.
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Table of Contents
1 Conventions used in this document ............................ 2
2 Introduction ................................................. 3
3 Session Authorization Object ................................. 3
3.1 Session Authorization Object format ........................ 4
3.2 Session Authorization Attributes ........................... 5
3.2.1 Authorizing Entity Identifier ............................ 6
3.2.2 Session Identifier ....................................... 7
3.2.3 Source Address ........................................... 8
3.2.4 Destination .............................................. 9
3.2.5 Start time ............................................... 10
3.2.6 End time ................................................. 10
3.2.7 Resources Authorized ..................................... 11
3.2.8 Authentication data ...................................... 12
4 Integrity of the AUTH_SESSION policy element ................. 12
4.1 Shared symmetric keys ...................................... 12
4.1.1 Operational Setting using shared symmetric keys .......... 13
4.2 Kerberos ................................................... 13
4.3 Public Key ................................................. 14
4.3.1 Operational Setting for public key based authentication .. 14
4.3.1.1 X.509 V3 digital certificates .......................... 14
4.3.1.2 PGP digital certificates ............................... 15
5 Framework .................................................... 16
5.1 The Coupled Model .......................................... 16
5.2 The associated model with one policy server ................ 16
5.3 The associated model with two policy servers ............... 17
5.4 The non-associated model ................................... 17
6 Message Processing Rules ..................................... 18
6.1 Generation of the AUTH_SESSION by the authorizing entity .. 18
6.2 Processing within the QoS NSLP ............................. 18
6.2.1 Message Generation ....................................... 18
6.2.2 Message Reception ........................................ 19
6.2.3 Authorization (QNE/PDP) .................................. 19
6.2.4 Error Signaling .......................................... 20
6.3 Processing with the NAT/FW NSLP ............................ 20
6.3.1 Message Generation ....................................... 20
6.3.2 Message Reception ........................................ 20
6.3.3 Authorization (Router/PDP) ............................... 21
6.3.4 Error Signaling .......................................... 21
7 IANA Considerations .......................................... 21
8 Security Considerations ...................................... 22
9 Acknowledgements ............................................. 22
10 References .................................................. 22
10.1 Normative References ...................................... 22
10.2 Informative References .................................... 23
1. Conventions used in this document
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 BCP 14, RFC 2119
[RFC-2119].
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The term "NSLP node" (NN) is used to refer to an NSIS node running an
NSLP protocol that can make use of the authorization object discussed
in this document. Currently, this node would run either the QoS or
the NAT/FW NSLP service.
2. Introduction
The NSIS working group is specifying a suite of protocols for the
next generation in Internet signaling. The design is based on a
generalized transport protocol for signaling applications, the
General Internet Signaling Transport (GIST) [GIST], and various kinds
of signaling applications. Two signaling applications and their NSIS
Signaling Layer Protocols (NSLP) have been designed, a Quality of
Service application (QoS NSLP) [QoS NSLP] and a NAT/firewall
application (NAT/FW) [NATFW NSLP].
The security architecture is based on a chain-of-trust model, where
each GIST hop may chose the appropriate security protocol, taking
into account the signaling application requirements. This model is
appropriate for a number of different use cases, and allows the
signaling applications to leave the handling of security to GIST.
Yet, in order to allow for finer-grain per-session admission control,
it is necessary to provide a mechanism for ensuring that the use of
resources by a host has been properly authorized before allowing the
signaling application to commit the resource request, e.g., a QoS
reservation or mappings for NAT traversal. In order to meet this
requirement,there must be information in the NSLP message which may
be used to verify the validity of the request. This can be done by
providing the host with a session authorization policy element which
is inserted into the message and verified by the network.
This document describes a generic NSLP layer session authorization
policy object (AUTH_SESSION) used to convey authorization information
for the request. The requesting host inserts its authorization
information into the NSLP message to allow verification of the
network resource request. Network elements verify the request and
then process the resource reservation message based on admission
policy. This work is based on RFC 3520 [RFC3520] and RFC 3521
[RFC3521].
3. Session Authorization Object
This section presents a new NSLP layer object called session
authorization (AUTH_SESSION). The AUTH_SESSION object can be used in
the currently specified and future NSLP protocols.
The authorization attributes follow the format and specification
given in RFC3520 [RFC3520].
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3.1. Session Authorization Object format
The AUTH_SESSION object contains a list of fields which describe the
session, along with other attributes. The object header follows the
generic NSLP object header.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|B|r|r| Type |r|r|r|r| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ +
// Session Authorization Attribute List //
+ +
+---------------------------------------------------------------+
The value for the Type field comes from GIST object type space. The
Length field is given in units of 32 bit words and measures the
length of the Value component of the TLV object (i.e. it does not
include the standard header).
The bits marked 'A' and 'B' are extensibility flags, and used to
signal the desired treatment for objects whose treatment has not been
defined in the protocol specification (i.e. whose Type field is
unknown at the receiver). The following four categories of object
have been identified, and are described here.
AB=00 ("Mandatory"): If the object is not understood, the entire
message containing it MUST be rejected with a "Object Type Error"
message with subcode 1 ("Unrecognised Object"). In the NATFW NSLP
case it MUST be rejected with an error response of class 'Protocol
error' (0x3) with error code 'Unknown object present' (0x06).
AB=01 ("Ignore"): If the object is not understood, it MUST be deleted
and the rest of the message processed as usual.
AB=10 ("Forward"): If the object is not understood, it MUST be
retained unchanged in any message forwarded as a result of message
processing, but not stored locally.
AB=11 ("Refresh"): If the object is not understood, it should be
incorporated into the locally stored signaling application state for
this flow/session, forwarded in any resulting message, and also used
in any refresh or repair message which is generated locally. In the
NATFW NSLP this combination AB=11 MUST NOT be used and an error
response of class 'Protocol error' (0x3) with error code 'Invalid
Flag-Field combination' (0x09) MUST be generated.
The remaining bits marked 'r' are reserved. The extensibility flags
follow the definition in the GIST specification. The AUTH_SESSION
object defines in this specification MUST have the AB-bits set to
"10". An NN may use the authorization information if it is configured
to do so, but may also just skip the object.
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Type: 0x0a (TBD by IANA)
Length: Variable
Session Authorization Attribute List: variable length
The session authorization attribute list is a collection of
objects which describes the session and provides other information
necessary to verify the resource reservation request. An initial
set of valid objects is described in Section 3.2.
3.2. Session Authorization Attributes
A session authorization attribute may contain a variety of
information and has both an attribute type and subtype. The attribute
itself MUST be a multiple of 4 octets in length, and any attributes
that are not a multiple of 4 octets long MUST be padded to a 4-octet
boundary. All padding bytes MUST have a value of zero.
+--------+--------+--------+--------+
| Length | X-Type |SubType |
+--------+--------+--------+--------+
| Value ...
+--------+--------+--------+--------+
Length: 16 bits
The length field is two octets and indicates the actual length of
the attribute (including Length, X-Type and SubType fields) in
number of octets. The length does NOT include any bytes padding to
the value field to make the attribute a multiple of 4 octets long.
X-Type: 8 bits
Session authorization attribute type (X-Type) field is one octet.
IANA acts as a registry for X-Types as described in Section 7,
IANA Considerations. Initially, the registry contains the
following X-Types:
1 AUTH_ENT_ID The unique identifier of the entity which
authorized the session.
2 SESSION_ID Unique identifier for this session.
3 SOURCE_ADDR Address specification for the session
originator.
4 DEST_ADDR Address specification for the session
end-point.
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5 START_TIME The starting time for the session.
6 END_TIME The end time for the session.
7 RESOURCES The resources which the user is authorized
to request.
8 AUTHENTICATION_DATA Authentication data of the session
authorization policy element.
SubType: 8 bits
Session authorization attribute sub-type is one octet in length.
The value of the SubType depends on the X-Type.
Value: variable length
The attribute specific information.
3.2.1. Authorizing Entity Identifier
AUTH_ENT_ID is used to identify the entity which authorized the
initial service request and generated the session authorization
policy element. The AUTH_ENT_ID may be represented in various
formats, and the SubType is used to define the format for the ID. The
format for AUTH_ENT_ID is as follows:
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: Length of the attribute, which MUST be > 4.
X-Type: AUTH_ENT_ID
SubType:
The following sub-types for AUTH_ENT_ID are defined. IANA acts as
a registry for AUTH_ENT_ID sub-types as described in Section 7,
IANA Considerations. Initially, the registry contains the
following sub-types of AUTH_ENT_ID:
1 IPV4_ADDRESS IPv4 address represented in 32 bits
2 IPV6_ADDRESS IPv6 address represented in 128 bits
3 FQDN Fully Qualified Domain Name as defined in
RFC 1034 as an ASCII string.
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4 ASCII_DN X.500 Distinguished name as defined in RFC
2253 as an ASCII string.
5 UNICODE_DN X.500 Distinguished name as defined in RFC
2253 as a UTF-8 string.
6 URI Universal Resource Identifier, as defined
in RFC 2396.
7 KRB_PRINCIPAL Fully Qualified Kerberos Principal name
represented by the ASCII string of a
principal followed by the @ realm name as
defined in RFC 1510 (e.g.,
johndoe@nowhere).
8 X509_V3_CERT The Distinguished Name of the subject of
the certificate as defined in RFC 2253 as a
UTF-8 string.
9 PGP_CERT The PGP digital certificate of the
authorizing entity as defined in RFC 2440.
OctetString: Contains the authorizing entity identifier.
3.2.2. Session Identifier
SESSION_ID is a unique identifier used by the authorizing entity to
identify the request. It may be used for a number of purposes,
including replay detection, or to correlate this request to a policy
decision entry made by the authorizing entity. In this specification,
the SESSION_ID is the opaque 128-bit SID value allocated by the NSLP
for this particular session.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: 128 bits
X-Type: SESSION_ID
SubType: No subtypes for SESSION_ID are currently defined; this field
MUST be set to zero.
OctetString
Contains the session identifier.
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3.2.3. Source Address
SOURCE_ADDR is used to identify the source address specification of
the authorized session. This X-Type may be useful in some scenarios
to make sure the resource request has been authorized for that
particular source address and/or port.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: Length of the attribute, which MUST be > 4.
X-Type: SOURCE_ADDR
SubType:
The following sub types for SOURCE_ADDR are defined. IANA acts as
a registry for SOURCE_ADDR sub-types as described in Section 7,
IANA Considerations. Initially, the registry contains the
following sub types for SOURCE_ADDR:
1 IPV4_ADDRESS IPv4 address represented in 32 bits
2 IPV6_ADDRESS IPv6 address represented in 128 bits
3 UDP_PORT_LIST list of UDP port specifications,
represented as 16 bits per list entry.
4 TCP_PORT_LIST list of TCP port specifications,
represented as 16 bits per list entry.
5 SPI Security Parameter Index represented in 32
bits
OctetString: The OctetString contains the source address information.
In scenarios where a source address is required (see Section 5), at
least one of the subtypes 1 or 2 MUST be included in every Session
Authorization Data Policy Element. Multiple SOURCE_ADDR attributes
MAY be included if multiple addresses have been authorized. The
source address of the request (e.g., a QoS NSLP RESERVE) MUST match
one of the SOURCE_ADDR attributes contained in this Session
Authorization Data Policy Element.
At most, one instance of subtype 3 MAY be included in every Session
Authorization Data Policy Element. At most, one instance of subtype 4
MAY be included in every Session Authorization Data Policy Element.
Inclusion of a subtype 3 attribute does not prevent inclusion of a
subtype 4 attribute (i.e., both UDP and TCP ports may be authorized).
If no PORT attributes are specified, then all ports are considered
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valid; otherwise, only the specified ports are authorized for use.
Every source address and port list must be included in a separate
SOURCE_ADDR attribute.
3.2.4. Destination
DEST_ADDR is used to identify the destination address of the
authorized session. This X-Type may be useful in some scenarios to
make sure the resource request has been authorized for that
particular destination address and/or port.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: Length of the attribute, which MUST be > 4.
X-Type: DEST_ADDR
SubType:
The following sub types for DEST_ADDR are defined. IANA acts as a
registry for DEST_ADDR sub-types as described in Section 7, IANA
Considerations. Initially, the registry contains the following
sub types for DEST_ADDR:
1 IPV4_ADDRESS IPv4 address represented in 32 bits
2 IPV6_ADDRESS IPv6 address represented in 128 bits
3 UDP_PORT_LIST list of UDP port specifications,
represented as 16 bits per list entry.
4 TCP_PORT_LIST list of TCP port specifications,
represented as 16 bits per list entry.
5 SPI Security Parameter Index represented in 32
bits
OctetString: The OctetString contains the destination address
specification.
In scenarios where a destination address is required (see Section 5),
at least one of the subtypes 1 or 2 MUST be included in every Session
Authorization Data Policy Element. Multiple DEST_ADDR attributes MAY
be included if multiple addresses have been authorized. The
destination address field of the resource reservation datagram (e.g.,
RSVP PATH) MUST match one of the DEST_ADDR attributes contained in
this Session Authorization Data Policy Element.
At most, one instance of subtype 3 MAY be included in every Session
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Authorization Data Policy Element. At most, one instance of subtype 4
MAY be included in every Session Authorization Data Policy Element.
Inclusion of a subtype 3 attribute does not prevent inclusion of a
subtype 4 attribute (i.e., both UDP and TCP ports may be authorized).
If no PORT attributes are specified, then all ports are considered
valid; otherwise, only the specified ports are authorized for use.
Every destination address and port list must be included in a
separate DEST_ADDR attribute.
3.2.5. Start time
START_TIME is used to identify the start time of the authorized
session and can be used to prevent replay attacks. If the
AUTH_SESSION policy element is presented in a resource request, the
network SHOULD reject the request if it is not received within a few
seconds of the start time specified.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: Length of the attribute, which MUST be > 4.
X-Type: START_TIME
SubType:
The following sub types for START_TIME are defined. IANA acts as
a registry for START_TIME sub-types as described in Section 7,
IANA Considerations. Initially, the registry contains the
following sub types for START_TIME:
1 NTP_TIMESTAMP NTP Timestamp Format as defined in
RFC 1305.
OctetString: The OctetString contains the start time.
3.2.6. End time
END_TIME is used to identify the end time of the authorized session
and can be used to limit the amount of time that resources are
authorized for use (e.g., in prepaid session scenarios).
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
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Length: Length of the attribute, which MUST be > 4.
X-Type: END_TIME
SubType:
The following sub types for END_TIME are defined. IANA acts as a
registry for END_TIME sub-types as described in Section 7, IANA
Considerations. Initially, the registry contains the following sub
types for END_TIME:
1 NTP_TIMESTAMP NTP Timestamp Format as defined in
RFC 1305.
OctetString: The OctetString contains the end time.
3.2.7. Resources Authorized
RESOURCES is used to define the characteristics of the authorized
session. This X-Type may be useful in some scenarios to specify the
specific resources authorized to ensure the request fits the
authorized specifications.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: Length of the attribute, which MUST be > 4.
X-Type: RESOURCES
SubType:
The following sub-types for RESOURCES are defined. IANA acts as a
registry for RESOURCES sub-types as described in Section 7, IANA
Considerations. Initially, the registry contains the following
sub types for RESOURCES:
1 BANDWIDTH Maximum bandwidth (kbps) authorized.
2 FLOW_SPEC Flow spec specification as defined in RFC 2205.
3 SDP SDP Media Descriptor as defined in RFC 2327.
4 DSCP Differentiated services codepoint as defined in
RFC 2474.
OctetString: The OctetString contains the resources specification.
In scenarios where a resource specification is required (see Section
5), at least one of the subtypes 1 through 4 (inclusive) MUST be
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included in every Session Authorization Data Policy Element. Multiple
RESOURCE attributes MAY be included if multiple types of resources
have been authorized (e.g., DSCP and BANDWIDTH).
3.2.8. Authentication data
The AUTHENTICATION_DATA attribute contains the authentication data of
the AUTH_SESSION policy element and signs all the data in the policy
element up to the AUTHENTICATION_DATA. If the AUTHENTICATION_DATA
attribute has been included in the AUTH_SESSION policy element, it
MUST be the last attribute in the list. The algorithm used to compute
the authentication data depends on the AUTH_ENT_ID SubType field. See
Section 4 entitled Integrity of the AUTH_SESSION policy element.
A summary of AUTHENTICATION_DATA attribute format is described below.
+-------+-------+-------+-------+
| Length |X-Type |SubType|
+-------+-------+-------+-------+
| OctetString ...
+-------+-------+-------+-------+
Length: Length of the attribute, which MUST be > 4.
X-Type: AUTHENTICATION_DATA
SubType: No sub types for AUTHENTICATION_DATA are currently defined.
This field MUST be set to 0.
OctetString: The OctetString contains the authentication data of the
AUTH_SESSION.
4. Integrity of the AUTH_SESSION policy element
This section describes how to ensure the integrity of the policy
element is preserved.
4.1. Shared symmetric keys
In shared symmetric key environments, the AUTH_ENT_ID MUST be of
subtypes: IPV4_ADDRESS, IPV6_ADDRESS, FQDN, ASCII_DN, UNICODE_DN or
URI. An example AUTH_SESSION object is shown below.
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+--------------+--------------+--------------+--------------+
|1000| Type = AUTH_SESSION |0000| Object length |
+--------------+--------------+--------------+--------------+
| Length |SESSION_ID | zero |
+--------------+--------------+--------------+--------------+
| OctetString (The session identifier 128-bits) ...
+--------------+--------------+--------------+--------------+
| Length | AUTH_ENT_ID | IPV4_ADDRESS |
+--------------+--------------+--------------+--------------+
| OctetString (The authorizing entity's Identifier) |
+--------------+--------------+--------------+--------------+
| Length |AUTH DATA. | zero |
+--------------+--------------+--------------+--------------+
| KEY_ID |
+--------------+--------------+--------------+--------------+
| OctetString (Authentication data) ...
+--------------+--------------+--------------+--------------+
4.1.1. Operational Setting using shared symmetric keys
This assumes both the Authorizing Entity and the Network router/PDP
are provisioned with shared symmetric keys and with policies
detailing which algorithm to be used for computing the authentication
data along with the expected length of the authentication data for
that particular algorithm.
Key maintenance is outside the scope of this document, but
AUTH_SESSION implementations MUST at least provide the ability to
manually configure keys and their parameters. The key used to produce
the authentication data is identified by the AUTH_ENT_ID field. Since
multiple keys may be configured for a particular AUTH_ENT_ID value,
the first 32 bits of the AUTH_DATA field MUST be a key ID to be used
to identify the appropriate key. Each key must also be configured
with lifetime parameters for the time period within which it is valid
as well as an associated cryptographic algorithm parameter specifying
the algorithm to be used with the key. At a minimum, all AUTH_SESSION
implementations MUST support the HMAC-MD5-128 [RFC-2104], [RFC-1321]
cryptographic algorithm for computing the authentication data.
It is good practice to regularly change keys. Keys MUST be
configurable such that their lifetimes overlap allowing smooth
transitions between keys. At the midpoint of the lifetime overlap
between two keys, senders should transition from using the current
key to the next/longer-lived key. Meanwhile, receivers simply accept
any identified key received within its configured lifetime and reject
those that are not.
4.2. Kerberos
RFC 3520 provides a mechanism to secure the authorization token using
Kerberos. Kerberos, however, has not seen deployment in this context
and is not well applicable for this particular usage scenario. Hence,
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Kerberos support will not be provided by this specification.
4.3. Public Key
In a public key environment, the AUTH_ENT_ID MUST be of the subtypes:
X509_V3_CERT or PGP_CERT. The authentication data is used for
authenticating the authorizing entity. An example of the public key
AUTH_SESSION policy element is shown below.
+--------------+--------------+--------------+--------------+
|1000| Type = AUTH_SESSION |0000| Object length |
+--------------+--------------+--------------+--------------+
| Length |SESSION_ID | zero |
+--------------+--------------+--------------+--------------+
| OctetString (The session identifier 128-bits) ...
+--------------+--------------+--------------+--------------+
| Length | AUTH_ENT_ID | PGP_CERT |
+--------------+--------------+--------------+--------------+
| OctetString (Authorizing entity Digital Certificate) ...
+--------------+--------------+--------------+--------------+
| Length |AUTH DATA. | zero |
+--------------+--------------+--------------+--------------+
| OctetString (Authentication data) ...
+--------------+--------------+--------------+--------------+
4.3.1. Operational Setting for public key based authentication
Public key based authentication assumes the following:
- Authorizing entities have a pair of keys (private key and
public key).
- Private key is secured with the authorizing entity.
- Public keys are stored in digital certificates and a trusted
party, certificate authority (CA) issues these digital
certificates.
- The verifier (PDP or router) has the ability to verify the
digital certificate.
Authorizing entity uses its private key to generate
AUTHENTICATION_DATA. Authenticators (router, PDP) use the
authorizing entity's public key (stored in the digital certificate)
to verify and authenticate the policy element.
4.3.1.1. X.509 V3 digital certificates
When the AUTH_ENT_ID is of type X509_V3_CERT, AUTHENTICATION_DATA
MUST be generated following these steps:
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- A Signed-data is constructed as defined in Section 5 of CMS
[RFC-3369]. A digest is computed on the content (as specified in
Section 6.1) with a signer-specific message-digest algorithm. The
certificates field contains the chain of authorizing entity's
X.509 V3 digital certificates. The certificate revocation list is
defined in the crls field. The digest output is digitally signed
following Section 8 of RFC 3447, using the signer's private key.
When the AUTH_ENT_ID is of type X509_V3_CERT, verification MUST be
done following these steps:
- Parse the X.509 V3 certificate to extract the distinguished name
of the issuer of the certificate.
- Certification Path Validation is performed as defined in Section 6
of RFC 3280.
- Parse through the Certificate Revocation list to verify that the
received certificate is not listed.
- Once the X.509 V3 certificate is validated, the public key of the
authorizing entity can be extracted from the certificate.
- Extract the digest algorithm and the length of the digested data
by parsing the CMS signed-data.
- The recipient independently computes the message digest. This
message digest and the signer's public key are used to verify the
signature value.
This verification ensures integrity, non-repudiation and data origin.
4.3.1.2. PGP digital certificates
When the AUTH_ENT_ID is of type PGP_CERT, AUTHENTICATION_DATA MUST be
generated following these steps:
- AUTHENTICATION_DATA contains a Signature Packet as defined in
Section 5.2.3 of RFC 2440. In summary:
- Compute the hash of all data in the AUTH_SESSION policy element
up to the AUTHENTICATION_DATA.
- The hash output is digitally signed following Section 8 of
RFC 3447, using the signer's private key.
When the AUTH_ENT_ID is of type PGP_CERT, verification MUST be done
following these steps:
- Validate the certificate.
- Once the PGP certificate is validated, the public key of the
authorizing entity can be extracted from the certificate.
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- Extract the hash algorithm and the length of the hashed data by
parsing the PGP signature packet.
- The recipient independently computes the message digest. This
message digest and the signer's public key are used to verify the
signature value.
This verification ensures integrity, non-repudiation and data origin.
5. Framework
[RFC-3521] describes a framework in which the AUTH_SESSION policy
element may be utilized to transport information required for
authorizing resource reservation for media flows. [RFC-3521]
introduces 4 different models:
1- the coupled model
2- the associated model with one policy server
3- the associated model with two policy servers
4- the non-associated model.
The fields that are required in an AUTH SESSION policy element
dependent on which of the models is used.
5.1. The Coupled Model
In the coupled model, the only information that MUST be included in
the policy element is the SESSION_ID; it is used by the Authorizing
Entity to correlate the resource reservation request with the media
authorized during session set up. Since the End Host is assumed to be
untrusted, the Policy Server SHOULD take measures to ensure that the
integrity of the SESSION_ID is preserved in transit; the exact
mechanisms to be used and the format of the SESSION_ID are
implementation dependent.
5.2. The associated model with one policy server
In this model, the contents of the AUTH_SESSION policy element MUST
include:
- A session identifier - SESSION_ID. This is information that the
authorizing entity can use to correlate the resource request with
the media authorized during session set up.
- The identity of the authorizing entity - AUTH_ENT_ID. This
information is used by an NN to determine which
authorizing entity (Policy Server) should be used to solicit
resource policy decisions.
In some environments, an NN may have no means for determining if the
identity refers to a legitimate Policy Server within its domain. In
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order to protect against redirection of authorization requests to a
bogus authorizing entity, the AUTH_SESSION MUST also include:
AUTHENTICATION_DATA. This authentication data is calculated over
all other fields of the AUTH_SESSION policy element.
5.3. The associated model with two policy servers
The content of the AUTH_SESSION Policy Element is identical to the
associated model with one policy server.
5.4. The non-associated model
In this model, the AUTH_SESSION MUST contain sufficient information
to allow the Policy Server to make resource policy decisions
autonomously from the authorizing entity. The policy element is
created using information about the session by the authorizing
entity. The information in the AUTH_SESSION policy element MUST
include:
- Calling party IP address or Identity (e.g., FQDN) - SOURCE_ADDR
X-TYPE
- Called party IP address or Identity (e.g., FQDN) - DEST_ADDR
X-TYPE
- The characteristics of (each of) the media stream(s) authorized
for this session - RESOURCES X-TYPE
- The authorization lifetime - START_TIME X-TYPE
- The identity of the authorizing entity to allow for validation of
the token in shared symmetric key and Kerberos schemes -
AUTH_ENT_ID X-TYPE
- The credentials of the authorizing entity in a public-key
scheme - AUTH_ENT_ID X-TYPE
- Authentication data used to prevent tampering with the
AUTH_SESSION policy element - AUTHENTICATION_DATA
Furthermore, the AUTH_SESSION policy element MAY contain:
- The lifetime of (each of) the media stream(s) - END_TIME X-TYPE
- Calling party port number - SOURCE_ADDR X-TYPE
- Called party port number - DEST_ADDR X-TYPE
All AUTH_SESSION fields MUST match with the resource request. If a
field does not match, the request SHOULD be denied.
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6. Message Processing Rules
This section discusses the message processing related to the
AUTH_SESSION object. We describe the details of the QoS NSLP and
NAT/FW NSLP. New NSLP protocols should use the same logic in making
use of the AUTH_SESSION object.
6.1. Generation of the AUTH_SESSION by the authorizing entity
1. Generate the AUTH_SESSION policy element with the appropriate
contents as specified in Section 5.
2. If authentication is needed, the entire AUTH_SESSION policy
element is constructed, excluding the length, type and subtype fields
of the AUTH_SESSION field. Note that the message MUST include either
a START_TIME or a SESSION_ID (See Section 9), to prevent replay
attacks. The output of the authentication algorithm, plus
appropriate header information, is appended to the AUTH_SESSION
policy element.
6.2. Processing within the QoS NSLP
The AUTH_SESSION object may be used with QoS NSLP QUERY and RESERVE
messages to authorize the query operation for network resources, and
a resource reservation request, respectively.
Moreover, the AUTH_SESSION object may also be used with RESPONSE
messages in order to indicate that the authorizing entity changed the
original request. For example, the session start or end times may
have been modified, or the client may have requested authorization
for all ports, but the authorizing entity only allowed the use of
certain ports.
If the QNI receives a RESPONSE message with an AUTH_SESSION object,
the QNI MUST inspect the AUTH_SESSION object to see what
authentication attribute was changed by an authorizing entity. The
QNI SHOULD also silently accept AUTH_SESSION objects in RESPONSE
message which do not indicate any change to the original
authorization request.
6.2.1. Message Generation
A QoS NSLP message is created as specified in [QoS NSLP].
1. The policy element received from the authorizing entity MUST be
copied without modification into the AUTH_SESSION object.
2. The AUTH_SESSION object (containing the policy element) is
inserted in the NSLP message in the appropriate place.
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6.2.2. Message Reception
The QoS NSLP message is processed as specified in [QOS NSLP] with
following modifications.
1. If the QNE is policy aware then it SHOULD use the Diameter QoS
application or the RADIUS QoS protocol to communicate with the PDP.
To construct the AAA message it is necessary to extract the
AUTH_SESSION object and the QoS related objects from the QoS NSLP
message and to craft the respective RADIUS or Diameter message. The
message processing and object format is described in the respective
RADIUS or Diameter QoS protocol, respectively. If the QNE is policy
unaware then it ignores the policy data objects and continues
processing the NSLP message.
2. If the response from the PDP is negative the request must be
rejected. A negative response in RADIUS is an Access-Reject and in
Diameter is based on the 'DIAMETER_SUCCESS' value in the Result-Code
AVP of the QoS-Authz-Answer (QAA) message. The QNE must contruct and
send a RESPONSE message with the status of authorization failure as
specified in [QoS NSLP].
3. Continue processing the NSIS message.
6.2.3. Authorization (QNE/PDP)
1. Retrieve the policy element from the AUTH_SESSION object. Check
the
PE type field and return an error if the identity type is not
supported.
2. Verify the message integrity.
- Shared symmetric key authentication: The QNE/PDP uses the
AUTH_ENT_ID field to consult a table keyed by that field.
The table should identify the cryptographic authentication
algorithm to be used along with the expected length of the
authentication data and the shared symmetric key for the
authorizing entity. Verify that the indicated length of the
authentication data is consistent with the configured table
entry and validate the authentication data.
- Public Key: Validate the certificate chain against the trusted
Certificate Authority (CA) and validate the message signature
using the public key.
- Kerberos based usage is not provided by this document.
3. Once the identity of the authorizing entity and the validity of
the service request has been established, the authorizing
router/PDP MUST then consult its authorization policy in order to
determine whether or not the specific request is authorized (e.g.,
based on available credits, information in the subscriber's
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database). To the extent to which these access control decisions
require supplementary information, routers/PDPs MUST ensure that
supplementary information is obtained securely.
4. Verify the requested resources do not exceed the authorized QoS.
6.2.4. Error Signaling
When the PDP (e.g., a RADIUS or Diameter server) fails to verify the
policy element then the appropriate actions described the respective
AAA document need to be taken.
The QNE node MUST return a RESPONSE message with the INFO_SPEC error
code Authorization Failure as defined in the QoS NSLP specification.
The QNE MAY include an INFO_SPEC Object Value Info to indicate which
AUTH_SESSION attribute created the error.
6.3. Processing with the NAT/FW NSLP
6.3.1. Message Generation
A NAT/FW NSLP message is created as specified in [NATFW NSLP].
1. The policy element received from the authorizing entity MUST be
copied without modification into the AUTH_SESSION object.
2. The AUTH_SESSION object (containing the policy element) is
inserted in the NATFW NSLP message in the appropriate place.
6.3.2. Message Reception
The NAT/FW NSLP message is processed as specified in [NATFW NSLP]
with following modifications.
1. If the router is policy aware then it SHOULD use the Diameter
application or the RADIUS protocol to communicate with the PDP. To
construct the AAA message it is necessary to extract the
AUTH_SESSION element and the NATFW policy rule related objects
from the NSLP message and to craft the respective RADIUS or
Diameter message. The message processing and object format is
described in the respective RADIUS or Diameter protocols,
respectively. If the router is policy unaware then it ignores the
policy data objects and continues processing the NSLP message.
2. Reject the message if the response from the PDP is negative. A
negative response in RADIUS is an Access-Reject and in Diameter is
based on the 'DIAMETER_SUCCESS' value in the Result-Code AVP.
3. Continue processing the NSIS message.
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6.3.3. Authorization (Router/PDP)
1. Retrieve the AUTH_SESSION object and the policy element. Check the
PE
type field and return an error if the identity type is not
supported.
2. Verify the message integrity.
- Shared symmetric key authentication: The Network router/PDP uses
the AUTH_ENT_ID field to consult a table keyed by that field. The
table should identify the cryptographic authentication algorithm
to be used along with the expected length of the authentication
data and the shared symmetric key for the authorizing entity.
Verify that the indicated length of the authentication data is
consistent with the configured table entry and validate the
authentication data.
- Public Key: Validate the certificate chain against the trusted
Certificate Authority (CA) and validate the message signature
using the public key.
- Kerberos based usage is not provided by this document.
3. Once the identity of the authorizing entity and the validity of
the service request has been established, the authorizing
router/PDP MUST then consult its authorization policy in order to
deter mine whether or not the specific request is authorized. To
the extent to which these access control decisions require
supplementary information, routers/PDPs MUST ensure that
supplementary information is obtained securely.
6.3.4. Error Signaling
When the PDP (e.g., a RADIUS or Diameter server) fails to verify the
AUTH_SESSION element then the appropriate actions described the
respective AAA document need to be taken. The NATFW NSLP node MUST
return an error message of class 'Permanent failure' (0x5) with error
code 'Authorization failed' (0x02).
7. IANA Considerations
This specification makes the following request to IANA:
- Assign a new object value for the AUTH_SESSION object from the GIST
object value space.
- All AUTH_SESSION object internal values and numbers should be taken
from the allocations already done for RFC 3520 [RFC 3520].
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8. Security Considerations
This document describes a mechanism for session authorization to
prevent theft of service. There are three types of security issues to
consider: protectiong against replay attacks, integrity of the
AUTH_SESSION object, and the choice of the authentication algorithms
and keys.
The first issue, replay attacks, MUST be prevented. In the non-
associated model, the AUTH_SESSION object MUST include a START_TIME
field and the Policy Servers MUST support NTP to ensure proper clock
synchronization. Failure to ensure proper clock synchronization will
allow replay attacks since the clocks of the different network
entities may not be in synch. The start time is used to verify that
the request is not being replayed at a later time. In all other
models, the SESSION_ID is used by the Policy Server to ensure that
the resource request successfully correlates with records of an
authorized session. If a AUTH_SESSION object is replayed, it MUST be
detected by the policy server (using internal algorithms) and the
request MUST be rejected.
The second issue, the integrity of the policy element, is preserved
in untrusted environments by including the AUTHENTICATION_DATA
attribute. Therefore, this attribute MUST always be included.
In environments where shared symmetric keys are possible, they should
be used in order to keep the AUTH_SESSION policy element size to a
strict minimum, e.g., when wireless links are used. A secondary
option would be PKI authentication, which provides a high level of
security and good scalability. However, it requires the presence of
credentials in the AUTH_SESSION policy element which impacts its
size.
9. Acknowledgements
This document is based on the RFC 3520 [RFC3520] and credit therefore
goes to the authors of RFC 3520, namely Louis-Nicolas Hamer, Brett
Kosinski, Bill Gage and Hugh Shieh.
10. References
10.1. Normative References
[GIST] Schulzrinne, H., and R. Hancock, "GIST: General Internet
Messaging Protocol for Signaling", Work in Progress.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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[NATFW NSLP] Stiemerling, M., et al., "NAT/Firewall NSIS Signaling
Layer Protocol", Work in Progress.
[QOS NSLP] Manner, J., et al., "NSLP for Quality-of-Service
signalling", Work in Progress.
[RFC4080] Hancock, R., "Next Steps in Signaling: Framework", RFC
4080, December 2004.
[RFC4081] Tschofenig, H. and D. Kroeselberg, "Security Threats for
NSIS", RFC 4081, October 2004.
10.2. Informative References
[RFC-1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC-2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February 1997.
[RFC-3369] Housley, R., "Cryptographic Message Syntax", RFC 3369,
August 2002.
[RFC3520] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh,
"Session Authorization Policy Element", RFC 3520, April 2003.
[RFC3521] Hamer, L-N., Gage, B., and H. Shieh, "Framework for
Session Set-up with Media Authorization", RFC 3521, April 2003.
Authors' Addresses
Jukka Manner
Telecommunications Software and Multimedia Laboratory
Department of Computer Science and Engineering
Helsinki University of Technology
P.O. Box 5400
Espoo, FIN-02015
Finland
Email: jmanner@tml.hut.fi
Martin Stiemerling
Network Laboratories, NEC Europe Ltd.
Kurfuersten-Anlage 36
Heidelberg 69115
Germany
Phone: +49 (0) 6221 905 11 13
Email: stiemerling@netlab.nec.de
URI: http://www.stiemerling.org
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
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Germany
Email: Hannes.Tschofenig@siemens.com
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