Network Working Group J. Walker
Request for Comments: 4261 A. Kulkarni, Ed.
Updates: 2748 Intel Corp.
Category: Standards Track December 2005 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 (C) The Internet Society (2005).
This document describes how to use Transport Layer Security (TLS) to
secure Common Open Policy Service (COPS) connections over the
This document also updates RFC 2748 by modifying the contents of the
Table Of Contents
1. Introduction ....................................................22. COPS Over TLS ...................................................33. Separate Ports versus Upward Negotiation ........................34. COPS/TLS Objects and Error codes ................................44.1. The TLS Message Integrity Object (Integrity-TLS) ...........44.2. Error Codes ................................................45. COPS/TLS Secure Connection Initiation ...........................55.1. PEP Initiated Security Negotiation .........................55.2. PDP Initiated Security Negotiation .........................66. Connection Closure ..............................................76.1. PEP System Behavior ........................................76.2. PDP System Behavior ........................................87. Endpoint Identification and Access Control ......................87.1. PEP Identity ...............................................97.2. PDP Identity ...............................................98. Cipher Suite Requirements ......................................109. Backward Compatibility .........................................1010. IANA Considerations ...........................................1011. Security Considerations .......................................1112. Acknowledgements ..............................................1113. References ....................................................1213.1. Normative References .....................................1213.2. Informative References ...................................121. Introduction
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
This document describes how to use COPS over TLS. "COPS over TLS" is
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
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
4. COPS/TLS Objects and Error codes
This section describes the COPS objects and error codes needed to
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
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
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: 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>
[<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)
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
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)
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
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
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
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
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
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
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
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
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
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 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
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
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.
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