A. Kulkarni, Ed.
Common Open Policy Service (COPS)
Over Transport Layer Security (TLS)
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2005).
This document describes how to use Transport Layer Security (TLS) to secure Common Open Policy Service (COPS) connections over the Internet.
This document also updates RFC 2748 by modifying the contents of the Client-Accept message.
Table Of Contents
1. Introduction ....................................................2 2. COPS Over TLS ...................................................3 3. Separate Ports versus Upward Negotiation ........................3 4. COPS/TLS Objects and Error codes ................................4 4.1. The TLS Message Integrity Object (Integrity-TLS) ...........4 4.2. Error Codes ................................................4 5. COPS/TLS Secure Connection Initiation ...........................5 5.1. PEP Initiated Security Negotiation .........................5 5.2. PDP Initiated Security Negotiation .........................6 6. Connection Closure ..............................................7 6.1. PEP System Behavior ........................................7 6.2. PDP System Behavior ........................................8 7. Endpoint Identification and Access Control ......................8 7.1. PEP Identity ...............................................9 7.2. PDP Identity ...............................................9 8. Cipher Suite Requirements ......................................10 9. Backward Compatibility .........................................10 10. IANA Considerations ...........................................10 11. Security Considerations .......................................11 12. Acknowledgements ..............................................11 13. References ....................................................12 13.1. Normative References .....................................12 13.2. Informative References ...................................12
COPS [RFC2748] was designed to distribute clear-text policy information from a centralized Policy Decision Point (PDP) to a set of Policy Enforcement Points (PEP) in the Internet. COPS provides its own security mechanisms to protect the per-hop integrity of the deployed policy. However, the use of COPS for sensitive applications (e.g., some types of security policy distribution) requires additional security measures, such as data confidentiality. This is because some organizations find it necessary to hide some or all of their security policies, e.g., because policy distribution to devices such as mobile platforms can cross domain boundaries.
TLS [RFC2246] was designed to provide channel-oriented security. TLS standardizes SSL and may be used with any connection-oriented service. TLS provides mechanisms for both one- and two-way authentication, dynamic session keying, and data stream privacy and integrity.
This document describes how to use COPS over TLS. "COPS over TLS" is abbreviated COPS/TLS.
COPS - Common Open Policy Service. See [RFC2748].
COPS/TCP - A plain-vanilla implementation of COPS.
COPS/TLS - A secure implementation of COPS using TLS.
PDP - Policy Decision Point. Also referred to as the Policy Server.
PEP - Policy Enforcement Point. Also referred to as the Policy
Client. See [RFC2753].
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 RFC 2119 [RFC2119].
2. COPS Over TLS
COPS/TLS is very simple: use COPS over TLS similar to how you would use COPS over TCP (COPS/TCP). Apart from a specific procedure used to initialize the connection, there is no difference between COPS/TLS and COPS/TCP.
3. Separate Ports versus Upward Negotiation
There are two ways in which insecure and secure versions of the same protocol can be run simultaneously.
In the first method, the secure version of the protocol is also allocated a well-known port. This strategy of having well-known port numbers for both, the secure and insecure versions, is known as 'Separate Ports'. The clients requiring security can simply connect to the well-known secure port. This method is easy to implement, with no modifications needed to existing insecure implementations. The disadvantage, however, is that it doesn't scale well, because a new port is required for each secure implementation. More problems with this approach have been listed in [RFC2595].
The second method is known as 'Upward Negotiation'. In this method, the secure and insecure versions of the protocol run on the same port. The client connects to the server, both discover each others' capabilities, and start security negotiations if desired. This method usually requires some changes to the protocol being secured.
In view of the many issues with the Separate Ports approach, the authors have decided to use the Upward Negotiation method for COPS/TLS.
4. COPS/TLS Objects and Error codes
This section describes the COPS objects and error codes needed to support COPS/TLS.
4.1. The TLS Message Integrity Object (Integrity-TLS)
The TLS Integrity object is used by the PDP and the PEP to start the TLS negotiation. This object should be included only in the Client- Open or Client-Accept messages. It MUST NOT be included in any other COPS message.
0 1 2 3 +----------+----------+----------+----------+ | Length (Octets) | C-Num=16 | C-Type=2 | +----------+----------+----------+----------+ | //////// | Flags | +----------+----------+----------+----------+ Note: //// implies the field is reserved, set to 0, and should be ignored on receipt.
Flags: 16 bits
0x01 = StartTLS
This flag indicates that the sender of the message wishes to initiate a TLS handshake.
The Client-Type of any message containing this object MUST be 0. Client-Type 0 is used to negotiate COPS connection level security and must only be used during the connection establishment phase. Please refer to section 4.1 of [RFC2748] for more details.
4.2. Error Codes
This section uses the error codes described in section 2.2.8 (Error Object) of [RFC2748].
Error Code: 13= Unknown COPS Object:
Sub-code (octet 2) contains the unknown object's C-Num, and (octet 3) contains unknown object's C-Type. If the PEP or PDP does not support TLS, the C-Num specified MUST be 16 and the C-Type MUST be 2. This demonstrates that the TLS version of the Integrity object is not known.
This error code MUST be used by either PEP or PDP to indicate a security-related connection closure if it cannot support a TLS connection for the COPS protocol.
If the PDP wishes to negotiate a different security mechanism than requested by the PEP in the Client-Open, it MUST send the following error code:
Error Code: 15= Authentication Required
Where the Sub-code (octet 2) contains the C-Num=16 value for the Integrity Object and (octet 3) MUST specify the PDP required/preferred Integrity object C-Type. If the server does not support any form of COPS-Security, it MUST set the Sub-code (octet 2) to 16 and (octet 3) to zero instead, signifying that no type of the Integrity object is supported.
5. COPS/TLS Secure Connection Initiation
Security negotiation may be initiated by either the PDP or the PEP. The PEP can initiate a negotiation via a Client-Open message, while a PDP can initiate a negotiation via a Client-Accept message.
Once the TLS connection is established, all COPS data MUST be sent as TLS "application data".
5.1. PEP Initiated Security Negotiation
A PEP MAY initiate a TLS security negotiation with a PDP using the Client-Open message. To do this, the Client-Open message MUST have a Client-Type of 0 and MUST include the Integrity-TLS object.
Upon receiving the Client-Open message, the PDP SHOULD respond with a Client-Accept message containing the Integrity-TLS object.
Note that in order to carry the Integrity-TLS object, the contents of the Client-Accept message defined in section 3.7 of [RFC2748] need not change, except that the C-Type of the integrity object contained there-in should now be C-Type=2. For Example:
<Client-Accept> ::= <Common Header>
<KA Timer> [<ACCT Timer>] [<Integrity (C-Num=16, C-Type=2)>]
Note also that this new format of the Client-Accept message does not replace or obsolete the existing Client-Accept message format, which can continue to be used for non-secure COPS session negotiations.
Upon receiving the appropriate Client-Accept message, the PEP SHOULD initiate the TLS handshake.
The message exchange is as follows:
C: Client-Open (Client-Type = 0, Integrity-TLS) S: Client-Accept (Client-Type = 0, Integrity-TLS) <TLS handshake> C/S: <...further messages...>
In case the PDP does not wish to open a secure connection with the PEP, it MUST reply with a Client-Close message and close the connection. The Client-Close message MUST include the error code 15= Authentication required, with the Sub-code (octet 2) set to 16 for the Integrity object's C-Num, and (octet 3) set to the C-Type corresponding to the server's preferred Integrity type, or zero for no security.
A PEP requiring the Integrity-TLS object in a Client-Accept message MUST close the connection if the Integrity-TLS object is missing. The ensuing Client-Close message MUST include the error code 15= Authentication required, with the Sub-code (octet 2) containing the required Integrity object's C-Num=16, and (octet 3) containing the required Integrity object's C-Type=2.
5.2. PDP Initiated Security Negotiation
The PEP initially opens a TCP connection with the PDP on the standard COPS port and sends a Client-Open message. This Client-Open message MUST have a Client-Type of 0.
The PDP SHOULD then reply with a Client-Accept message. In order to signal the PEP to start the TLS handshake, the PDP MUST include the Integrity-TLS object in the Client-Accept message.
Upon receiving the Client-Accept message with the Integrity-TLS object, the PEP SHOULD initiate the TLS handshake. If for any reason the PEP cannot initiate the handshake, it MUST close the connection.
The message exchange is as follows:
C: Client-Open (Client-Type = 0) S: Client-Accept (Client-Type = 0, Integrity-TLS) <TLS handshake> C/S: <...further messages...>
After receiving the Client-Accept, the PEP MUST NOT send any messages until the TLS handshake is complete. Upon receiving any message from the PEP before the TLS handshake starts, the PDP MUST issue a Client-Close message with an error code 15= Authentication Required.
A PDP wishing to negotiate security with a PEP having an existing non-secure connection MUST send a Client-Close with the error code 15= Authentication required, with the Sub-code (octet 2) containing the required Integrity object's C-Num=16, and (octet 3) containing the required Integrity object's C-Type=2, and then wait for the PEP to reconnect. Upon receiving the Client-Open message, it SHOULD use the Client-Accept message to initiate security negotiation.
6. Connection Closure
TLS provides facilities to securely close its connections. Reception of a valid closure alert assures an implementation that no further data will arrive on that connection. The TLS specification requires TLS implementations to initiate a closure alert exchange before closing a connection. It also permits TLS implementations to close connections without waiting to receive closure alerts from the peer, provided they send their own first. A connection closed in this way is known as an "incomplete close". TLS allows implementations to reuse the session in this case, but COPS/TLS makes no use of this capability.
A connection closed without first sending a closure alert is known as a "premature close". Note that a premature close does not call into question the security of the data already received, but simply indicates that subsequent data might have been truncated. Because TLS is oblivious to COPS message boundaries, it is necessary to examine the COPS data itself (specifically the Message header) to determine whether truncation occurred.
6.1. PEP System Behavior
PEP implementations MUST treat premature closes as errors and any data received as potentially truncated. The COPS protocol allows the PEP system to find out whether truncation took place. A PEP system detecting an incomplete close SHOULD recover gracefully.
PEP systems SHOULD send a closure alert before closing the connection. PEPs unprepared to receive any more data MAY choose not to wait for the PDP system's closure alert and simply close the connection, thus generating an incomplete close on the PDP side.
6.2. PDP System Behavior
COPS permits a PEP to close the connection at any time, and requires PDPs to recover gracefully. In particular, PDPs SHOULD be prepared to receive an incomplete close from the PEP, since a PEP often shuts down for operational reasons unrelated to the transfer of policy information between the PEP and PDP.
Implementation note: The PDP ordinarily expects to be able to signal the end of data by closing the connection. However, the PEP may have already sent the closure alert and dropped the connection.
PDP systems MUST attempt to initiate an exchange of closure alerts with the PEP system before closing the connection. PDP systems MAY close the connection after sending the closure alert, thus generating an incomplete close on the PEP side.
7. Endpoint Identification and Access Control
All PEP implementations of COPS/TLS MUST support an access control mechanism to identify authorized PDPs. This requirement provides a level of assurance that the policy arriving at the PEP is actually valid. PEP deployments SHOULD require the use of this access control mechanism for operation of COPS over TLS. When access control is enabled, the PEP implementation MUST NOT initiate COPS/TLS connections to systems not authorized as PDPs by the access control mechanism.
Similarly, PDP COPS/TLS implementations MUST support an access control mechanism permitting them to restrict their services to authorized PEP systems only. However, deployments MAY choose not to use an access control mechanism at the PDP, as organizations might not consider the types of policy being deployed as sensitive, and therefore do not need to incur the expense of managing credentials for the PEP systems. If access controls are used, however, the PDP implementation MUST terminate COPS/TLS connections from unauthorized PEP systems and log an error if an auditable logging mechanism is present.
Implementations of COPS/TLS MUST use X.509 v3 certificates conforming to [RFC3280] to identify PDP and PEP systems. COPS/TLS systems MUST perform certificate verification processing conforming to [RFC3280].
If a subjectAltName extension of type dNSName or iPAddress is present in the PDP's certificate, it MUST be used as the PDP identity. If both types are present, dNSName SHOULD be used as the PDP identity. If neither type is present, the most specific Common Name field in the Subject field of the certificate SHOULD be used.
Matching is performed using the matching rules specified by [RFC3280]. If more than one identity of a given type is present in the certificate (e.g., more than one dNSName in the subjectAltName certificate extension), a match in any one of the provided identities is acceptable. Generally, the COPS system uses the first name for matching, except as noted below in the IP address checking requirements.
7.1. PEP Identity
When PEP systems are not access controlled, the PDP does not need external knowledge of what the PEP's identity ought to be and so checks are neither possible nor necessary. In this case, there is no requirement for PEP systems to register with a certificate authority, and COPS over TLS uses one-way authentication, of the PDP to the PEP.
When PEP systems are access controlled, PEPs MUST be the subjects of end entity certificates [RFC3280]. In this case, COPS over TLS uses two-way authentication, and the PDP MUST perform the same identity checks for the PEPs as described above for the PDP.
When access controls are in effect at the PDP, PDP implementations MUST have a mechanism to securely acquire the trust anchor for each authorized Certification Authority (CA) that issues certificates to supported PEPs.
7.2. PDP Identity
Generally, COPS/TLS requests are generated by the PEP consulting bootstrap policy information that identifies PDPs that the PEP is authorized to connect to. This policy provides the PEP with the hostname or IP address of the PDP. How this bootstrap policy information arrives at the PEP is outside the scope of this document. However, all PEP implementations MUST provide a mechanism to securely deliver or configure the bootstrap policy.
All PEP implementations MUST be able to securely acquire the trust anchor for each authorized Certification Authority (CA) that issues PDP certificates. Also, the PEPs MUST support a mechanism to securely acquire an access control list (ACL) or filter identifying the set of authorized PDPs associated with each CA. Deployments must take care to avoid circular dependencies in accessing trust anchors and ACLs. At a minimum, trust anchors and ACLs may be installed manually.
PEP deployments that participate in multiple domains, such as those on mobile platforms, MAY use different CAs and access control lists in each domain.
If the PDP hostname or IP address is available via the bootstrap policy, the PEP MUST check it against the PDP's identity as presented in the PDP's TLS Certificate message.
In some cases, the bootstrap policy will identify the authorized PDP only by an IP address of the PDP system. In this case, the subjectAltName MUST be present in the certificate, and it MUST include an iPAddress format matching the expected name of the policy server.
If the hostname of the PDP does not match the identity in the certificate, a PEP on a user-oriented system MUST either notify the user (PEP systems MAY afford the user the opportunity to continue with the connection in any case) or terminate the connection with a bad certificate error. PEPs on unattended systems MUST log the error to an appropriate audit log (if available) and MUST terminate the connection with a bad certificate error. Unattended PEP systems MAY provide a configuration setting that disables this check, but then MUST provide a setting that enables it.
8. Cipher Suite Requirements
Implementations MUST support the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher suite. All other cipher suites are optional.
9. Backward Compatibility
The PEP and PDP SHOULD be backward compatible with peers that have not been modified to support COPS/TLS. They SHOULD handle errors generated in response to the Integrity-TLS object.
10. IANA Considerations
The IANA has added the following C-Num, C-Type combination for the Integrity-TLS object to the registry at http://www.iana.org/assignments/cops-parameters: 0x10 0x02 Message Integrity, Integrity-TLS [RFC4261]
For Client-Type 0, the IANA has added the following Flags value for the Integrity-TLS object:
0x01 = StartTLS
Further, for Client-Type 0, the IANA has added the following text for Error Sub-Codes:
Error Code: 15
Octet 2: C-Num of the Integrity object
Octet 3: C-Type of the supported/preferred Integrity object or
Error-Code Error-SubCode Description Octet 2 Octet 3 --------------------------------------------------- 15 16 0 No security 15 16 2 Integrity-TLS supported/preferred
Further values for the Flags field and the reserved field can only be assigned by IETF Consensus rule, as defined in [RFC2434].
11. Security Considerations
A COPS PDP and PEP MUST check the results of the TLS negotiation to see whether an acceptable degree of authentication and privacy have been achieved. If the negotiation has resulted in unacceptable algorithms or key lengths, either side MAY choose to terminate the connection.
A man-in-the-middle attack can be launched by deleting the Integrity-TLS object or altering the Client-Open or Client-Accept messages. If security is required, the PEP and PDP bootstrap policy must specify this, and PEP and PDP implementations should reject Client-Open or Client-Accept messages that fail to include an Integrity-TLS object.
This document freely plagiarizes and adapts Eric Rescorla's similar document [RFC2818] that specifies how HTTP runs over TLS.
Discussions with David Durham, Scott Hahn, and Ylian Sainte-Hillaire also lead to improvements in this document.
The authors wish to thank Uri Blumenthal for doing a thorough security review of the document.
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2748] Durham, D., Boyle, J., Cohen, R., Herzog, S., Rajan, R., and A. Sastry, "The COPS (Common Open Policy Service) Protocol", RFC 2748, January 2000. [RFC2753] Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework for Policy-based Admission Control", RFC 2753, January 2000. [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.
13.2. Informative References
Amol Kulkarni Intel Corporation 2111 N.E. 25th Avenue Hillsboro, OR 97214 USA EMail: firstname.lastname@example.org Jesse R. Walker Intel Corporation 2111 N.E. 25th Avenue Hillsboro, OR 97214 USA EMail: email@example.com
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