Internet Draft Jesse Walker
Expiration: February 2005 Amol Kulkarni, Ed.
File: draft-ietf-rap-cops-tls-08.txt Intel Corp.
COPS Over TLS
Last Updated: August 16, 2004
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC2026].
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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 [RFC2119].
Abstract
This memo describes how to use TLS to secure COPS connections over
the Internet.
Please send comments on this document to the rap@ops.ietf.org
mailing list.
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Table Of Contents
Glossary..........................................................3
1 Introduction...................................................3
2 COPS Over TLS..................................................3
3 Separate Ports versus Upward Negotiation.......................3
4 COPS/TLS Objects and Error codes...............................4
4.1 The StartTLS ClientSI Object..................................4
4.2 Error Codes...................................................4
5 COPS/TLS Secure Connection Initiation..........................4
5.1 PDP Initiated Security Negotiation............................4
5.2 PEP Initiated Security Negotiation............................5
6 Connection Closure.............................................6
6.1 PEP System Behavior...........................................6
6.2 PDP System Behavior...........................................6
7 Port Number....................................................7
8 Endpoint Identification and Access Control.....................7
8.1 PDP Identity..................................................8
8.2 PEP Identity..................................................8
9 Backward Compatibility.........................................9
10 IANA Considerations............................................9
11 Security Considerations........................................9
12 Acknowledgements...............................................9
13 References....................................................10
13.1 Normative References........................................10
13.2 Informative References......................................10
14 Author Addresses.............................................10
15 Intellectual Property Statement..............................11
16 Full Copyright Statement......................................11
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Glossary
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. See [RFC2753].
PEP - Policy Enforcement Point. Also referred to as the Policy
Client. See [RFC2753].
1 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
such as some types of security policy distribution requires
additional security measures, such as data privacy. 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.
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, with a new
port required for each secure implementation. More problems with
this approach have been listed in [RFC2595].
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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.
COPS/TLS uses the Upward Negotiation method to secure COPS messages.
4 COPS/TLS Objects and Error codes
This section describes the COPS objects and error codes needed to
support COPS/TLS.
4.1 The StartTLS ClientSI Object
The StartTLS ClientSI object is used by the PDP and the PEP to start
the TLS negotiation. This object can be included either in the
ClientAccept message, or a Request message. Also, the ClientType of
any message containing this ClientSI object MUST be 0.
0 1 2 3
+----------+----------+----------+----------+
| Length (Octets) | C-Num=9 | C-Type=1 |
+----------+----------+----------+----------+
| //////// | Flags |
+----------+----------+----------+----------+
Flags:
1 = TLS
4.2 Error Codes
This section adds to the error codes described in section 2.2.8
(Error Object) of [RFC2748].
Error Code: 16 = TLS Required
This error code should be used by either PEP or PDP if they require
security but the other side doesn't support it.
5 COPS/TLS Secure Connection Initiation
Security negotiation may be initiated either by the PDP or the PEP.
The PDP can initiate a security negotiation either via a
ClientAccept or a ClientClose message, while a PEP can initiate a
negotiation via a Request message.
Once the TLS connection is established, all COPS data MUST be sent
as TLS "application data".
5.1 PDP Initiated Security Negotiation
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The PEP initially opens a TCP connection with the PDP on the
standard COPS port and sends a ClientOpen message. This ClientOpen
message MUST have a ClientType of 0.
The PDP then replies with a ClientAccept. In order to signal the PEP
to start the TLS handshake, the PDP MUST include the StartTLS
ClientSI object in the ClientAccept message.
Note that in order to carry the StartTLS ClientSI object, the
contents of the ClientAccept message defined in section 3.7 of
[RFC2748] need to change to the following:
<Client-Accept> ::= <Common Header>
<KA Timer>
[<ACCT Timer>]
[<ClientSI>]
[<Integrity>]
Upon receiving the ClientAccept message with the StartTLS ClientSI
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: ClientOpen (ClientType = 0)
S: ClientAccept (ClientType = 0, StartTLS)
<TLS handshake>
C/S: <...further messages...>
Until the TLS handshake is complete the PEP MUST NOT send any
messages other than ClientClose and KeepAlive. Upon receiving any
other message, a PDP expecting a TLS negotiation MUST issue a
ClientClose message with an error code of 16.
5.2 PEP Initiated Security Negotiation
If a PEP wishes to use TLS on an existing non-secure COPS
connection, it MUST issue a Request message with a ClientType of 0.
The StartTLS ClientSI object MUST be included in the request.
In response, the PDP SHOULD send a Decision message containing the
appropriate Command-Code (1 = Install/Accept, 2 = Remove/Reject) in
the Decision Flags object.
If the request is accepted, the PEP MUST start the TLS handshake.
After the TLS handshake is complete, the PDP MUST synchronize state
with the PEP.
The message exchange is as follows:
<...existing COPS/TCP connection...>
C: Request (ClientType = 0, StartTLS)
S: Decision (ClientType = 0, Install)
<TLS handshake>
S: Synchronize
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If the PEP's TLS request is rejected by the PDP, the PEP MAY choose
to continue using the non-secure connection. Else, it MUST close the
connection by sending a ClientClose message with an error code of
16.
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 MUST 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 end of data by closing the connection. However, the PEP
may have already sent the closure alert and dropped the
connection.
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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 Port Number
The first data a PDP expects to receive from the PEP is a Client-
Open message. The first data a TLS server (and hence a COPS/TLS PDP)
expects to receive is the ClientHello. Consequently, COPS/TLS runs
over a separate port in order to distinguish it from COPS alone.
When COPS/TLS runs over a TCP/IP connection, the TCP port is any
non-well-known port of the PDP's choice. This port MUST be
communicated to the COPS/TCP PDP running on the well-known COPS TCP
port. The PEP may use any TCP port. This does not preclude COPS/TLS
from running over another transport. TLS only presumes a reliable
connection-oriented data stream.
8 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 implementations 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, implementations 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 of the types is present, the most specific
Common Name field in the Subject field of the certificate SHOULD be
used.
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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 name 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.
8.1 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 signing
certificates of authorized Certificate Authorities that issue PDP
certificates. Also, the PEPs MUST support a mechanism to securely
acquire an access control list or filter identifying the CA's set of
authorized PDPs.
PEP implementations 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 iPAdress 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 which enables it.
8.2 PEP Identity
When PEP systems are not access controlled, the PDP need have no
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
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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 PKI clients in
the sense of [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 signing certificates
of the Certificate Authorities issuing certificates to any of the
PEPs they support.
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 StartTLS ClientSI object.
In case a PEP does not start the TLS handshake upon receiving the
StartTLS ClientSI object, the PDP MUST close the connection.
10 IANA Considerations
The IANA shall add the following Error-Code to the cops-parameters
document:
Error-Code: 16
Description: TLS Required
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 StartTLS
ClientSI object from the ClientAccept or Request messages. To
prevent this, the PEP and PDP MUST use the Integrity object as
defined in [RFC2748].
A downgrade attack against a PEP requesting TLS negotiation is
possible by modifying the PDP's Decision message flag to 'Remove'.
Again, this can be avoided by using the Integrity object as defined
in [RFC2748].
12 Acknowledgements
This document freely plagiarizes and adapts Eric Rescorla's similar
document [RFC2818] that specifies how HTTP runs over TLS.
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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 References
13.1 Normative References
[RFC2026] Bradner, S., "The Internet Standards Process - Revision
3", RFC 2026, October 1996
[RFC2119] Bradner, S., "Key Words for use in RFCs to indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2748] Durham, D., Boyle, J., Cohen, R., Herzog, R., Rajan,
R., Sastry, A., "The COPS (Common Open Policy Service) Protocol",
RFC 2748, January 2000.
[RFC3280] Housley, R., Ford, W., Polk, W., Solo, D., "Internet
X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile ", RFC 3280, April 2002.
[RFC2246] Dierks, T., Allen, C., "The TLS Protocol", RFC 2246,
January 1999.
13.2 Informative References
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207, February 2002.
14 Author Addresses
Jesse R. Walker
Intel Corporation
2111 N.E. 25th Avenue
Hillsboro, OR 97214
USA
jesse.walker[at]intel.com
Amol Kulkarni
Intel Corporation
JF3-206
2111 N.E. 25th Avenue
Hillsboro, OR 97214
USA
amol.kulkarni[at]intel.com
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15 Intellectual Property Statement
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