Network Working Group D. Durham, Ed.
Request for Comments: 2748 Intel
Category: Standards Track J. Boyle
Level 3
R. Cohen
Cisco
S. Herzog
IPHighway
R. Rajan
AT&T
A. Sastry
Cisco
January 2000
The COPS (Common Open Policy Service) Protocol
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 Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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].
Abstract
This document describes a simple client/server model for supporting
policy control over QoS signaling protocols. The model does not make
any assumptions about the methods of the policy server, but is based
on the server returning decisions to policy requests. The model is
designed to be extensible so that other kinds of policy clients may
be supported in the future. However, this document makes no claims
that it is the only or the preferred approach for enforcing future
types of policies.
Table Of Contents
1. IntrodUCtion....................................................3
1.1 Basic Model....................................................4
2. The Protocol....................................................6
2.1 Common Header..................................................6
2.2 COPS Specific Object Formats...................................8
2.2.1 Handle Object (Handle).......................................9
2.2.2 Context Object (Context).....................................9
2.2.3 In-Interface Object (IN-Int)................................10
2.2.4 Out-Interface Object (OUT-Int)..............................11
2.2.5 Reason Object (Reason)......................................12
2.2.6 Decision Object (Decision)..................................12
2.2.7 LPDP Decision Object (LPDPDecision).........................14
2.2.8 Error Object (Error)........................................14
2.2.9 Client Specific Information Object (ClientSI)...............15
2.2.10 Keep-Alive Timer Object (KATimer)..........................15
2.2.11 PEP Identification Object (PEPID)..........................16
2.2.12 Report-Type Object (Report-Type)...........................16
2.2.13 PDP Redirect Address (PDPRedirAddr)........................16
2.2.14 Last PDP Address (LastPDPAddr).............................17
2.2.15 Accounting Timer Object (AcctTimer)........................17
2.2.16 Message Integrity Object (Integrity).......................18
2.3 Communication.................................................19
2.4 Client Handle Usage...........................................21
2.5 Synchronization Behavior......................................21
3. Message Content................................................22
3.1 Request (REQ) PEP -> PDP.....................................22
3.2 Decision (DEC) PDP -> PEP....................................24
3.3 Report State (RPT) PEP -> PDP................................25
3.4 Delete Request State (DRQ) PEP -> PDP........................25
3.5 Synchronize State Request (SSQ) PDP -> PEP...................26
3.6 Client-Open (OPN) PEP -> PDP.................................26
3.7 Client-Accept (CAT) PDP -> PEP...............................27
3.8 Client-Close (CC) PEP -> PDP, PDP -> PEP.....................28
3.9 Keep-Alive (KA) PEP -> PDP, PDP -> PEP.......................28
3.10 Synchronize State Complete (SSC) PEP -> PDP..................29
4. Common Operation...............................................29
4.1 Security and Sequence Number Negotiation......................29
4.2 Key Maintenance...............................................31
4.3 PEP Initialization............................................31
4.4 Outsourcing Operations........................................32
4.5 Configuration Operations......................................32
4.6 Keep-Alive Operations.........................................33
4.7 PEP/PDP Close.................................................33
5. Security Considerations........................................33
6. IANA Considerations............................................34
7. References.....................................................35
8. Author Information and Acknowledgments.........................36
9. Full Copyright Statement.......................................38
1. Introduction
This document describes a simple query and response protocol that can
be used to exchange policy information between a policy server
(Policy Decision Point or PDP) and its clients (Policy Enforcement
Points or PEPs). One example of a policy client is an RSVP router
that must exercise policy-based admission control over RSVP usage
[RSVP]. We assume that at least one policy server exists in each
controlled administrative domain. The basic model of interaction
between a policy server and its clients is compatible with the
framework document for policy based admission control [WRK].
A chief objective of this policy control protocol is to begin with a
simple but extensible design. The main characteristics of the COPS
protocol include:
1. The protocol employs a client/server model where the PEP sends
requests, updates, and deletes to the remote PDP and the PDP
returns decisions back to the PEP.
2. The protocol uses TCP as its transport protocol for reliable
exchange of messages between policy clients and a server.
Therefore, no additional mechanisms are necessary for reliable
communication between a server and its clients.
3. The protocol is extensible in that it is designed to leverage
off self-identifying objects and can support diverse client
specific information without requiring modifications to the
COPS protocol itself. The protocol was created for the general
administration, configuration, and enforcement of policies.
4. COPS provides message level security for authentication, replay
protection, and message integrity. COPS can also reuse existing
protocols for security such as IPSEC [IPSEC] or TLS to
authenticate and secure the channel between the PEP and the
PDP.
5. The protocol is stateful in two main ASPects: (1)
Request/Decision state is shared between client and server and
(2) State from various events (Request/Decision pairs) may be
inter-associated. By (1) we mean that requests from the client
PEP are installed or remembered by the remote PDP until they
are eXPlicitly deleted by the PEP. At the same time, Decisions
from the remote PDP can be generated asynchronously at any time
for a currently installed request state. By (2) we mean that
the server may respond to new queries differently because of
previously installed Request/Decision state(s) that are
related.
6. Additionally, the protocol is stateful in that it allows the
server to push configuration information to the client, and
then allows the server to remove such state from the client
when it is no longer applicable.
1.1 Basic Model
+----------------+
Network Node Policy Server
+-----+ COPS +-----+
PEP <-------------------> PDP
+-----+ +-----+
^
-->+-----+
LPDP
+-----+
+----------------+
Figure 1: A COPS illustration.
Figure 1 Illustrates the layout of various policy components in a
typical COPS example (taken from [WRK]). Here, COPS is used to
communicate policy information between a Policy Enforcement Point
(PEP) and a remote Policy Decision Point (PDP) within the context of
a particular type of client. The optional Local Policy Decision Point
(LPDP) can be used by the device to make local policy decisions in
the absence of a PDP.
It is assumed that each participating policy client is functionally
consistent with a PEP [WRK]. The PEP may communicate with a policy
server (herein referred to as a remote PDP [WRK]) to oBTain policy
decisions or directives.
The PEP is responsible for initiating a persistent TCP connection to
a PDP. The PEP uses this TCP connection to send requests to and
receive decisions from the remote PDP. Communication between the PEP
and remote PDP is mainly in the form of a stateful request/decision
exchange, though the remote PDP may occasionally send unsolicited
decisions to the PEP to force changes in previously approved request
states. The PEP also has the capacity to report to the remote PDP
that it has successfully completed performing the PDP"s decision
locally, useful for accounting and monitoring purposes. The PEP is
responsible for notifying the PDP when a request state has changed on
the PEP. Finally, the PEP is responsible for the deletion of any
state that is no longer applicable due to events at the client or
decisions issued by the server.
When the PEP sends a configuration request, it expects the PDP to
continuously send named units of configuration data to the PEP via
decision messages as applicable for the configuration request. When a
unit of named configuration data is successfully installed on the
PEP, the PEP should send a report message to the PDP confirming the
installation. The server may then update or remove the named
configuration information via a new decision message. When the PDP
sends a decision to remove named configuration data from the PEP, the
PEP will delete the specified configuration and send a report message
to the PDP as confirmation.
The policy protocol is designed to communicate self-identifying
objects which contain the data necessary for identifying request
states, establishing the context for a request, identifying the type
of request, referencing previously installed requests, relaying
policy decisions, reporting errors, providing message integrity, and
transferring client specific/namespace information.
To distinguish between different kinds of clients, the type of client
is identified in each message. Different types of clients may have
different client specific data and may require different kinds of
policy decisions. It is expected that each new client-type will have
a corresponding usage draft specifying the specifics of its
interaction with this policy protocol.
The context of each request corresponds to the type of event that
triggered it. The COPS Context object identifies the type of request
and message (if applicable) that triggered a policy event via its
message type and request type fields. COPS identifies three types of
outsourcing events: (1) the arrival of an incoming message (2)
allocation of local resources, and (3) the forwarding of an outgoing
message. Each of these events may require different decisions to be
made. The content of a COPS request/decision message depends on the
context. A fourth type of request is useful for types of clients that
wish to receive configuration information from the PDP. This allows a
PEP to issue a configuration request for a specific named device or
module that requires configuration information to be installed.
The PEP may also have the capability to make a local policy decision
via its Local Policy Decision Point (LPDP) [WRK], however, the PDP
remains the authoritative decision point at all times. This means
that the relevant local decision information must be relayed to the
PDP. That is, the PDP must be granted Access to all relevant
information to make a final policy decision. To facilitate this
functionality, the PEP must send its local decision information to
the remote PDP via an LPDP decision object. The PEP must then abide
by the PDP"s decision as it is absolute.
Finally, fault tolerance is a required capability for this protocol,
particularly due to the fact it is associated with the security and
service management of distributed network devices. Fault tolerance
can be achieved by having both the PEP and remote PDP constantly
verify their connection to each other via keep-alive messages. When a
failure is detected, the PEP must try to reconnect to the remote PDP
or attempt to connect to a backup/alternative PDP. While
disconnected, the PEP should revert to making local decisions. Once a
connection is reestablished, the PEP is expected to notify the PDP of
any deleted state or new events that passed local admission control
after the connection was lost. Additionally, the remote PDP may
request that all the PEP"s internal state be resynchronized (all
previously installed requests are to be reissued). After failure and
before the new connection is fully functional, disruption of service
can be minimized if the PEP caches previously communicated decisions
and continues to use them for some limited amount of time. Sections
2.3 and 2.5 detail COPS mechanisms for achieving reliability.
2. The Protocol
This section describes the message formats and objects exchanged
between the PEP and remote PDP.
2.1 Common Header
Each COPS message consists of the COPS header followed by a number of
typed objects.
0 1 2 3
+--------------+--------------+--------------+--------------+
Version Flags Op Code Client-type
+--------------+--------------+--------------+--------------+
Message Length
+--------------+--------------+--------------+--------------+
Global note: //// implies field is reserved, set to 0.
The fields in the header are:
Version: 4 bits
COPS version number. Current version is 1.
Flags: 4 bits
Defined flag values (all other flags MUST be set to 0):
0x1 Solicited Message Flag Bit
This flag is set when the message is solicited by
another COPS message. This flag is NOT to be set
(value=0) unless otherwise specified in section 3.
Op Code: 8 bits
The COPS operations:
1 = Request (REQ)
2 = Decision (DEC)
3 = Report State (RPT)
4 = Delete Request State (DRQ)
5 = Synchronize State Req (SSQ)
6 = Client-Open (OPN)
7 = Client-Accept (CAT)
8 = Client-Close (CC)
9 = Keep-Alive (KA)
10= Synchronize Complete (SSC)
Client-type: 16 bits
The Client-type identifies the policy client. Interpretation of
all encapsulated objects is relative to the client-type. Client-
types that set the most significant bit in the client-type field
are enterprise specific (these are client-types 0x8000 -
0xFFFF). (See the specific client usage documents for particular
client-type IDs). For KA Messages, the client-type in the header
MUST always be set to 0 as the KA is used for connection
verification (not per client session verification).
Message Length: 32 bits
Size of message in octets, which includes the standard COPS
header and all encapsulated objects. Messages MUST be aligned on
4 octet intervals.
2.2 COPS Specific Object Formats
All the objects follow the same object format; each object consists
of one or more 32-bit words with a four-octet header, using the
following format:
0 1 2 3
+-------------+-------------+-------------+-------------+
Length (octets) C-Num C-Type
+-------------+-------------+-------------+-------------+
// (Object contents) //
+-------------+-------------+-------------+-------------+
The length is a two-octet value that describes the number of octets
(including the header) that compose the object. If the length in
octets does not fall on a 32-bit word boundary, padding MUST be added
to the end of the object so that it is aligned to the next 32-bit
boundary before the object can be sent on the wire. On the receiving
side, a subsequent object boundary can be found by simply rounding up
the previous stated object length to the next 32-bit boundary.
Typically, C-Num identifies the class of information contained in the
object, and the C-Type identifies the subtype or version of the
information contained in the object.
C-num: 8 bits
1 = Handle
2 = Context
3 = In Interface
4 = Out Interface
5 = Reason code
6 = Decision
7 = LPDP Decision
8 = Error
9 = Client Specific Info
10 = Keep-Alive Timer
11 = PEP Identification
12 = Report Type
13 = PDP Redirect Address
14 = Last PDP Address
15 = Accounting Timer
16 = Message Integrity
C-type: 8 bits
Values defined per C-num.
2.2.1 Handle Object (Handle)
The Handle Object encapsulates a unique value that identifies an
installed state. This identification is used by most COPS operations.
A state corresponding to a handle MUST be explicitly deleted when it
is no longer applicable. See Section 2.4 for details.
C-Num = 1
C-Type = 1, Client Handle.
Variable-length field, no implied format other than it is unique from
other client handles from the same PEP (a.k.a. COPS TCP connection)
for a particular client-type. It is always initially chosen by the
PEP and then deleted by the PEP when no longer applicable. The client
handle is used to refer to a request state initiated by a particular
PEP and installed at the PDP for a client-type. A PEP will specify a
client handle in its Request messages, Report messages and Delete
messages sent to the PDP. In all cases, the client handle is used to
uniquely identify a particular PEP"s request for a client-type.
The client handle value is set by the PEP and is opaque to the PDP.
The PDP simply performs a byte-wise comparison on the value in this
object with respect to the handle object values of other currently
installed requests.
2.2.2 Context Object (Context)
Specifies the type of event(s) that triggered the query. Required for
request messages. Admission control, resource allocation, and
forwarding requests are all amenable to client-types that outsource
their decision making facility to the PDP. For applicable client-
types a PEP can also make a request to receive named configuration
information from the PDP. This named configuration data may be in a
form useful for setting system attributes on a PEP, or it may be in
the form of policy rules that are to be directly verified by the PEP.
Multiple flags can be set for the same request. This is only allowed,
however, if the set of client specific information in the combined
request is identical to the client specific information that would be
specified if individual requests were made for each specified flag.
C-num = 2, C-Type = 1
0 1 2 3
+--------------+--------------+--------------+--------------+
R-Type M-Type
+--------------+--------------+--------------+--------------+
R-Type (Request Type Flag)
0x01 = Incoming-Message/Admission Control request
0x02 = Resource-Allocation request
0x04 = Outgoing-Message request
0x08 = Configuration request
M-Type (Message Type)
Client Specific 16 bit values of protocol message types
2.2.3 In-Interface Object (IN-Int)
The In-Interface Object is used to identify the incoming interface on
which a particular request applies and the address where the received
message originated. For flows or messages generated from the PEP"s
local host, the loop back address and ifindex are used.
This Interface object is also used to identify the incoming
(receiving) interface via its ifindex. The ifindex may be used to
differentiate between sub-interfaces and unnumbered interfaces (see
RSVP"s LIH for an example). When SNMP is supported by the PEP, this
ifindex integer MUST correspond to the same integer value for the
interface in the SNMP MIB-II interface index table.
Note: The ifindex specified in the In-Interface is typically relative
to the flow of the underlying protocol messages. The ifindex is the
interface on which the protocol message was received.
C-Num = 3
C-Type = 1, IPv4 Address + Interface
0 1 2 3
+--------------+--------------+--------------+--------------+
IPv4 Address format
+--------------+--------------+--------------+--------------+
ifindex
+--------------+--------------+--------------+--------------+
For this type of the interface object, the IPv4 address specifies the
IP address that the incoming message came from.
C-Type = 2, IPv6 Address + Interface
0 1 2 3
+--------------+--------------+--------------+--------------+
+ +
+ IPv6 Address format +
+ +
+--------------+--------------+--------------+--------------+
ifindex
+--------------+--------------+--------------+--------------+
For this type of the interface object, the IPv6 address specifies the
IP address that the incoming message came from. The ifindex is used
to refer to the MIB-II defined local incoming interface on the PEP as
described above.
2.2.4 Out-Interface Object (OUT-Int)
The Out-Interface is used to identify the outgoing interface to which
a specific request applies and the address for where the forwarded
message is to be sent. For flows or messages destined to the PEP"s
local host, the loop back address and ifindex are used. The Out-
Interface has the same formats as the In-Interface Object.
This Interface object is also used to identify the outgoing
(forwarding) interface via its ifindex. The ifindex may be used to
differentiate between sub-interfaces and unnumbered interfaces (see
RSVP"s LIH for an example). When SNMP is supported by the PEP, this
ifindex integer MUST correspond to the same integer value for the
interface in the SNMP MIB-II interface index table.
Note: The ifindex specified in the Out-Interface is typically
relative to the flow of the underlying protocol messages. The ifindex
is the one on which a protocol message is about to be forwarded.
C-Num = 4
C-Type = 1, IPv4 Address + Interface
Same C-Type format as the In-Interface object. The IPv4 address
specifies the IP address to which the outgoing message is going. The
ifindex is used to refer to the MIB-II defined local outgoing
interface on the PEP.
C-Type = 2, IPv6 Address + Interface
Same C-Type format as the In-Interface object. For this type of the
interface object, the IPv6 address specifies the IP address to which
the outgoing message is going. The ifindex is used to refer to the
MIB-II defined local outgoing interface on the PEP.
2.2.5 Reason Object (Reason)
This object specifies the reason why the request state was deleted.
It appears in the delete request (DRQ) message. The Reason Sub-code
field is reserved for more detailed client-specific reason codes
defined in the corresponding documents.
C-Num = 5, C-Type = 1
0 1 2 3
+--------------+--------------+--------------+--------------+
Reason-Code Reason Sub-code
+--------------+--------------+--------------+--------------+
Reason Code:
1 = Unspecified
2 = Management
3 = Preempted (Another request state takes precedence)
4 = Tear (Used to communicate a signaled state removal)
5 = Timeout (Local state has timed-out)
6 = Route Change (Change invalidates request state)
7 = Insufficient Resources (No local resource available)
8 = PDP"s Directive (PDP decision caused the delete)
9 = Unsupported decision (PDP decision not supported)
10= Synchronize Handle Unknown
11= Transient Handle (stateless event)
12= Malformed Decision (could not recover)
13= Unknown COPS Object from PDP:
Sub-code (octet 2) contains unknown object"s C-Num
and (octet 3) contains unknown object"s C-Type.
2.2.6 Decision Object (Decision)
Decision made by the PDP. Appears in replies. The specific non-
mandatory decision objects required in a decision to a particular
request depend on the type of client.
C-Num = 6
C-Type = 1, Decision Flags (Mandatory)
0 1 2 3
+--------------+--------------+--------------+--------------+
Command-Code Flags
+--------------+--------------+--------------+--------------+
Commands:
0 = NULL Decision (No configuration data available)
1 = Install (Admit request/Install configuration)
2 = Remove (Remove request/Remove configuration)
Flags:
0x01 = Trigger Error (Trigger error message if set)
Note: Trigger Error is applicable to client-types that
are capable of sending error notifications for signaled
messages.
Flag values not applicable to a given context"s R-Type or
client-type MUST be ignored by the PEP.
C-Type = 2, Stateless Data
This type of decision object carries additional stateless
information that can be applied by the PEP locally. It is a
variable length object and its internal format SHOULD be
specified in the relevant COPS extension document for the given
client-type. This object is optional in Decision messages and is
interpreted relative to a given context.
It is expected that even outsourcing PEPs will be able to make
some simple stateless policy decisions locally in their LPDP. As
this set is well known and implemented ubiquitously, PDPs are
aware of it as well (either universally, through configuration,
or using the Client-Open message). The PDP may also include this
information in its decision, and the PEP MUST apply it to the
resource allocation event that generated the request.
C-Type = 3, Replacement Data
This type of decision object carries replacement data that is to
replace existing data in a signaled message. It is a variable
length object and its internal format SHOULD be specified in the
relevant COPS extension document for the given client-type. It is
optional in Decision messages and is interpreted relative to a
given context.
C-Type = 4, Client Specific Decision Data
Additional decision types can be introduced using the Client
Specific Decision Data Object. It is a variable length object and
its internal format SHOULD be specified in the relevant COPS
extension document for the given client-type. It is optional in
Decision messages and is interpreted relative to a given context.
C-Type = 5, Named Decision Data
Named configuration information is encapsulated in this version
of the decision object in response to configuration requests. It
is a variable length object and its internal format SHOULD be
specified in the relevant COPS extension document for the given
client-type. It is optional in Decision messages and is
interpreted relative to both a given context and decision flags.
2.2.7 LPDP Decision Object (LPDPDecision)
Decision made by the PEP"s local policy decision point (LPDP). May
appear in requests. These objects correspond to and are formatted the
same as the client specific decision objects defined above.
C-Num = 7
C-Type = (same C-Type as for Decision objects)
2.2.8 Error Object (Error)
This object is used to identify a particular COPS protocol error.
The error sub-code field contains additional detailed client specific
error codes. The appropriate Error Sub-codes for a particular
client-type SHOULD be specified in the relevant COPS extensions
document.
C-Num = 8, C-Type = 1
0 1 2 3
+--------------+--------------+--------------+--------------+
Error-Code Error Sub-code
+--------------+--------------+--------------+--------------+
Error-Code:
1 = Bad handle
2 = Invalid handle reference
3 = Bad message format (Malformed Message)
4 = Unable to process (server gives up on query)
5 = Mandatory client-specific info missing
6 = Unsupported client-type
7 = Mandatory COPS object missing
8 = Client Failure
9 = Communication Failure
10= Unspecified
11= Shutting down
12= Redirect to Preferred Server
13= Unknown COPS Object:
Sub-code (octet 2) contains unknown object"s C-Num
and (octet 3) contains unknown object"s C-Type.
14= Authentication Failure
15= Authentication Required
2.2.9 Client Specific Information Object (ClientSI)
The various types of this object are required for requests, and used
in reports and opens when required. It contains client-type specific
information.
C-Num = 9,
C-Type = 1, Signaled ClientSI.
Variable-length field. All objects/attributes specific to a client"s
signaling protocol or internal state are encapsulated within one or
more signaled Client Specific Information Objects. The format of the
data encapsulated in the ClientSI object is determined by the
client-type.
C-Type = 2, Named ClientSI.
Variable-length field. Contains named configuration information
useful for relaying specific information about the PEP, a request, or
configured state to the PDP server.
2.2.10 Keep-Alive Timer Object (KATimer)
Times are encoded as 2 octet integer values and are in units of
seconds. The timer value is treated as a delta.
C-Num = 10,
C-Type = 1, Keep-alive timer value
Timer object used to specify the maximum time interval over which a
COPS message MUST be sent or received. The range of finite timeouts
is 1 to 65535 seconds represented as an unsigned two-octet integer.
The value of zero implies infinity.
0 1 2 3
+--------------+--------------+--------------+--------------+
////////////// KA Timer Value
+--------------+--------------+--------------+--------------+
2.2.11 PEP Identification Object (PEPID)
The PEP Identification Object is used to identify the PEP client to
the remote PDP. It is required for Client-Open messages.
C-Num = 11, C-Type = 1
Variable-length field. It is a NULL terminated ASCII string that is
also zero padded to a 32-bit word boundary (so the object length is a
multiple of 4 octets). The PEPID MUST contain an ASCII string that
uniquely identifies the PEP within the policy domain in a manner that
is persistent across PEP reboots. For example, it may be the PEP"s
statically assigned IP address or DNS name. This identifier may
safely be used by a PDP as a handle for identifying the PEP in its
policy rules.
2.2.12 Report-Type Object (Report-Type)
The Type of Report on the request state associated with a handle:
C-Num = 12, C-Type = 1
0 1 2 3
+--------------+--------------+--------------+--------------+
Report-Type /////////////
+--------------+--------------+--------------+--------------+
Report-Type:
1 = Success : Decision was successful at the PEP
2 = Failure : Decision could not be completed by PEP
3 = Accounting: Accounting update for an installed state
2.2.13 PDP Redirect Address (PDPRedirAddr)
A PDP when closing a PEP session for a particular client-type may
optionally use this object to redirect the PEP to the specified PDP
server address and TCP port number:
C-Num = 13,
C-Type = 1, IPv4 Address + TCP Port
0 1 2 3
+--------------+--------------+--------------+--------------+
IPv4 Address format
+--------------+--------------+--------------+--------------+
///////////////////////// TCP Port Number
+-----------------------------+-----------------------------+
C-Type = 2, IPv6 Address + TCP Port
0 1 2 3
+--------------+--------------+--------------+--------------+
+ +
+ IPv6 Address format +
+ +
+--------------+--------------+--------------+--------------+
///////////////////////// TCP Port Number
+-----------------------------+-----------------------------+
2.2.14 Last PDP Address (LastPDPAddr)
When a PEP sends a Client-Open message for a particular client-type
the PEP SHOULD specify the last PDP it has successfully opened
(meaning it received a Client-Accept) since the PEP last rebooted.
If no PDP was used since the last reboot, the PEP will simply not
include this object in the Client-Open message.
C-Num = 14,
C-Type = 1, IPv4 Address (Same format as PDPRedirAddr)
C-Type = 2, IPv6 Address (Same format as PDPRedirAddr)
2.2.15 Accounting Timer Object (AcctTimer)
Times are encoded as 2 octet integer values and are in units of
seconds. The timer value is treated as a delta.
C-Num = 15,
C-Type = 1, Accounting timer value
Optional timer value used to determine the minimum interval between
periodic accounting type reports. It is used by the PDP to describe
to the PEP an acceptable interval between unsolicited accounting
updates via Report messages where applicable. It provides a method
for the PDP to control the amount of accounting traffic seen by the
network. The range of finite time values is 1 to 65535 seconds
represented as an unsigned two-octet integer. A value of zero means
there SHOULD be no unsolicited accounting updates.
0 1 2 3
+--------------+--------------+--------------+--------------+
////////////// ACCT Timer Value
+--------------+--------------+--------------+--------------+
2.2.16 Message Integrity Object (Integrity)
The integrity object includes a sequence number and a message digest
useful for authenticating and validating the integrity of a COPS
message. When used, integrity is provided at the end of a COPS
message as the last COPS object. The digest is then computed over all
of a particular COPS message up to but not including the digest value
itself. The sender of a COPS message will compute and fill in the
digest portion of the Integrity object. The receiver of a COPS
message will then compute a digest over the received message and
verify it matches the digest in the received Integrity object.
C-Num = 16,
C-Type = 1, HMAC digest
The HMAC integrity object employs HMAC (Keyed-Hashing for Message
Authentication) [HMAC] to calculate the message digest based on a key
shared between the PEP and its PDP.
This Integrity object specifies a 32-bit Key ID used to identify a
specific key shared between a particular PEP and its PDP and the
cryptographic algorithm to be used. The Key ID allows for multiple
simultaneous keys to exist on the PEP with corresponding keys on the
PDP for the given PEPID. The key identified by the Key ID was used to
compute the message digest in the Integrity object. All
implementations, at a minimum, MUST support HMAC-MD5-96, which is
HMAC employing the MD5 Message-Digest Algorithm [MD5] truncated to
96-bits to calculate the message digest.
This object also includes a sequence number that is a 32-bit unsigned
integer used to avoid replay attacks. The sequence number is
initiated during an initial Client-Open Client-Accept message
exchange and is then incremented by one each time a new message is
sent over the TCP connection in the same direction. If the sequence
number reaches the value of 0xFFFFFFFF, the next increment will
simply rollover to a value of zero.
The variable length digest is calculated over a COPS message starting
with the COPS Header up to the Integrity Object (which MUST be the
last object in a COPS message) INCLUDING the Integrity object"s
header, Key ID, and Sequence Number. The Keyed Message Digest field
is not included as part of the digest calculation. In the case of
HMAC-MD5-96, HMAC-MD5 will produce a 128-bit digest that is then to
be truncated to 96-bits before being stored in or verified against
the Keyed Message Digest field as specified in [HMAC]. The Keyed
Message Digest MUST be 96-bits when HMAC-MD5-96 is used.
0 1 2 3
+-------------+-------------+-------------+-------------+
Key ID
+-------------+-------------+-------------+-------------+
Sequence Number
+-------------+-------------+-------------+-------------+
+ +
...Keyed Message Digest...
+ +
+-------------+-------------+-------------+-------------+
2.3 Communication
The COPS protocol uses a single persistent TCP connection between the
PEP and a remote PDP. One PDP implementation per server MUST listen
on a well-known TCP port number (COPS=3288 [IANA]). The PEP is
responsible for initiating the TCP connection to a PDP. The location
of the remote PDP can either be configured, or obtained via a service
location mechanism [SRVLOC]. Service discovery is outside the scope
of this protocol, however.
If a single PEP can support multiple client-types, it may send
multiple Client-Open messages, each specifying a particular client-
type to a PDP over one or more TCP connections. Likewise, a PDP
residing at a given address and port number may support one or more
client-types. Given the client-types it supports, a PDP has the
ability to either accept or reject each client-type independently.
If a client-type is rejected, the PDP can redirect the PEP to an
alternative PDP address and TCP port for a given client-type via
COPS. Different TCP port numbers can be used to redirect the PEP to
another PDP implementation running on the same server. Additional
provisions for supporting multiple client-types (perhaps from
independent PDP vendors) on a single remote PDP server are not
provided by the COPS protocol, but, rather, are left to the software
architecture of the given server platform.
It is possible a single PEP may have open connections to multiple
PDPs. This is the case when there are physically different PDPs
supporting different client-types as shown in figure 2.
+----------------+
Network Node Policy Servers
+-----+ COPS Client Type 1 +-----+
<-------------------------> PDP1
+ PEP + COPS Client Type 2 +-----+
<-------------- +-----+
+-----+ ---------- PDP2
^ +-----+
-->+-----+
LPDP
+-----+
+----------------+
Figure 2: Multiple PDPs illustration.
When a TCP connection is torn down or is lost, the PDP is expected to
eventually clean up any outstanding request state related to
request/decision exchanges with the PEP. When the PEP detects a lost
connection due to a timeout condition it SHOULD explicitly send a
Client-Close message for each opened client-type containing an
<Error> object indicating the "Communication Failure" Error-Code.
Additionally, the PEP SHOULD continuously attempt to contact the
primary PDP or, if unsuccessful, any known backup PDPs. Specifically
the PEP SHOULD keep trying all relevant PDPs with which it has been
configured until it can establish a connection. If a PEP is in
communication with a backup PDP and the primary PDP becomes
available, the backup PDP is responsible for redirecting the PEP back
to the primary PDP (via a <Client-Close> message containing a
<PDPRedirAddr> object identifying the primary PDP to use for each
affected client-type). Section 2.5 details synchronization behavior
between PEPs and PDPs.
2.4 Client Handle Usage
The client handle is used to identify a unique request state for a
single PEP per client-type. Client handles are chosen by the PEP and
are opaque to the PDP. The PDP simply uses the request handle to
uniquely identify the request state for a particular Client-Type over
a particular TCP connection and generically tie its decisions to a
corresponding request. Client handles are initiated in request
messages and are then used by subsequent request, decision, and
report messages to reference the same request state. When the PEP is
ready to remove a local request state, it will issue a delete message
to the PDP for the corresponding client handle. A handle MUST be
explicitly deleted by the PEP before it can be used by the PEP to
identify a new request state. Handles referring to different request
states MUST be unique within the context of a particular TCP
connection and client-type.
2.5 Synchronization Behavior
When disconnected from a PDP, the PEP SHOULD revert to making local
decisions. Once a connection is reestablished, the PEP is expected to
notify the PDP of any events that have passed local admission
control. Additionally, the remote PDP may request that all the PEP"s
internal state be resynchronized (all previously installed requests
are to be reissued) by sending a Synchronize State message.
After a failure and before a new connection is fully functional,
disruption of service can be minimized if the PEP caches previously
communicated decisions and continues to use them for some appropriate
length of time. Specific rules for such behavior are to be defined in
the appropriate COPS client-type extension specifications.
A PEP that caches state from a previous exchange with a disconnected
PDP MUST communicate this fact to any PDP with which it is able to
later reconnect. This is accomplished by including the address and
TCP port of the last PDP for which the PEP is still caching state in
the Client-Open message. The <LastPDPAddr> object will only be
included for the last PDP with which the PEP was completely in sync.
If the service interruption was temporary and the PDP still contains
the complete state for the PEP, the PDP may choose not to synchronize
all states. It is still the responsibility of the PEP to update the
PDP of all state changes that occurred during the disruption of
service including any states communicated to the previous PDP that
had been deleted after the connection was lost. These MUST be
explicitly deleted after a connection is reestablished. If the PDP
issues a synchronize request the PEP MUST pass all current states to
the PDP followed by a Synchronize State Complete message (thus
completing the synchronization process). If the PEP crashes and loses
all cached state for a client-type, it will simply not include a
<LastPDPAddr> in its Client-Open message.
3. Message Content
This section describes the basic messages exchanged between a PEP and
a remote PDP as well as their contents. As a convention, object
ordering is expected as shown in the BNF for each COPS message unless
otherwise noted. The Integrity object, if included, MUST always be
the last object in a message. If security is required and a message
was received without a valid Integrity object, the receiver MUST send
a Client-Close message for Client-Type=0 specifying the appropriate
error code.
3.1 Request (REQ) PEP -> PDP
The PEP establishes a request state client handle for which the
remote PDP may maintain state. The remote PDP then uses this handle
to refer to the exchanged information and decisions communicated over
the TCP connection to a particular PEP for a given client-type.
Once a stateful handle is established for a new request, any
subsequent modifications of the request can be made using the REQ
message specifying the previously installed handle. The PEP is
responsible for notifying the PDP whenever its local state changes so
the PDP"s state will be able to accurately mirror the PEP"s state.
The format of the Request message is as follows:
<Request Message> ::= <Common Header>
<Client Handle>
<Context>
[<IN-Int>]
[<OUT-Int>]
[<ClientSI(s)>]
[<LPDPDecision(s)>]
[<Integrity>]
<ClientSI(s)> ::= <ClientSI> <ClientSI(s)> <ClientSI>
<LPDPDecision(s)> ::= <LPDPDecision>
<LPDPDecision(s)> <LPDPDecision>
<LPDPDecision> ::= [<Context>]
<LPDPDecision: Flags>
[<LPDPDecision: Stateless Data>]
[<LPDPDecision: Replacement Data>]
[<LPDPDecision: ClientSI Data>]
[<LPDPDecision: Named Data>]
The context object is used to determine the context within which all
the other objects are to be interpreted. It also is used to determine
the kind of decision to be returned from the policy server. This
decision might be related to admission control, resource allocation,
object forwarding and substitution, or configuration.
The interface objects are used to determine the corresponding
interface on which a signaling protocol message was received or is
about to be sent. They are typically used if the client is
participating along the path of a signaling protocol or if the client
is requesting configuration data for a particular interface.
ClientSI, the client specific information object, holds the client-
type specific data for which a policy decision needs to be made. In
the case of configuration, the Named ClientSI may include named
information about the module, interface, or functionality to be
configured. The ordering of multiple ClientSIs is not important.
Finally, LPDPDecision object holds information regarding the local
decision made by the LPDP.
Malformed Request messages MUST result in the PDP specifying a
Decision message with the appropriate error code.
3.2 Decision (DEC) PDP -> PEP
The PDP responds to the REQ with a DEC message that includes the
associated client handle and one or more decision objects grouped
relative to a Context object and Decision Flags object type pair. If
there was a protocol error an error object is returned instead.
It is required that the first decision message for a new/updated
request will have the solicited message flag set (value = 1) in the
COPS header. This avoids the issue of keeping track of which updated
request (that is, a request reissued for the same handle) a
particular decision corresponds. It is important that, for a given
handle, there be at most one outstanding solicited decision per
request. This essentially means that the PEP SHOULD NOT issue more
than one REQ (for a given handle) before it receives a corresponding
DEC with the solicited message flag set. The PDP MUST always issue
decisions for requests on a particular handle in the order they
arrive and all requests MUST have a corresponding decision.
To avoid deadlock, the PEP can always timeout after issuing a request
that does not receive a decision. It MUST then delete the timed-out
handle, and may try again using a new handle.
The format of the Decision message is as follows:
<Decision Message> ::= <Common Header>
<Client Handle>
<Decision(s)> <Error>
[<Integrity>]
<Decision(s)> ::= <Decision> <Decision(s)> <Decision>
<Decision> ::= <Context>
<Decision: Flags>
[<Decision: Stateless Data>]
[<Decision: Replacement Data>]
[<Decision: ClientSI Data>]
[<Decision: Named Data>]
The Decision message may include either an Error object or one or
more context plus associated decision objects. COPS protocol problems
are reported in the Error object (e.g. an error with the format of
the original request including malformed request messages, unknown
COPS objects in the Request, etc.). The applicable Decision object(s)
depend on the context and the type of client. The only ordering
requirement for decision objects is that the required Decision Flags
object type MUST precede the other Decision object types per context
binding.
3.3 Report State (RPT) PEP -> PDP
The RPT message is used by the PEP to communicate to the PDP its
success or failure in carrying out the PDP"s decision, or to report
an accounting related change in state. The Report-Type specifies the
kind of report and the optional ClientSI can carry additional
information per Client-Type.
For every DEC message containing a configuration context that is
received by a PEP, the PEP MUST generate a corresponding Report State
message with the Solicited Message flag set describing its success or
failure in applying the configuration decision. In addition,
outsourcing decisions from the PDP MAY result in a corresponding
solicited Report State from the PEP depending on the context and the
type of client. RPT messages solicited by decisions for a given
Client Handle MUST set the Solicited Message flag and MUST be sent in
the same order as their corresponding Decision messages were
received. There MUST never be more than one Report State message
generated with the Solicited Message flag set per Decision.
The Report State may also be used to provide periodic updates of
client specific information for accounting and state monitoring
purposes depending on the type of the client. In such cases the
accounting report type should be specified utilizing the appropriate
client specific information object.
<Report State> ::== <Common Header>
<Client Handle>
<Report-Type>
[<ClientSI>]
[<Integrity>]
3.4 Delete Request State (DRQ) PEP -> PDP
When sent from the PEP this message indicates to the remote PDP that
the state identified by the client handle is no longer
available/relevant. This information will then be used by the remote
PDP to initiate the appropriate housekeeping actions. The reason code
object is interpreted with respect to the client-type and signifies
the reason for the removal.
The format of the Delete Request State message is as follows:
<Delete Request> ::= <Common Header>
<Client Handle>
<Reason>
[<Integrity>]
Given the stateful nature of COPS, it is important that when a
request state is finally removed from the PEP, a DRQ message for this
request state is sent to the PDP so the corresponding state may
likewise be removed on the PDP. Request states not explicitly deleted
by the PEP will be maintained by the PDP until either the client
session is closed or the connection is terminated.
Malformed Decision messages MUST trigger a DRQ specifying the
appropriate erroneous reason code (Bad Message Format) and any
associated state on the PEP SHOULD either be removed or re-requested.
If a Decision contained an unknown COPS Decision Object, the PEP MUST
delete its request specifying the Unknown COPS Object reason code
because the PEP will be unable to comply with the information
contained in the unknown object. In any case, after issuing a DRQ,
the PEP may retry the corresponding Request again.
3.5 Synchronize State Request (SSQ) PDP -> PEP
The format of the Synchronize State Query message is as follows:
<Synchronize State> ::= <Common Header>
[<Client Handle>]
[<Integrity>]
This message indicates that the remote PDP wishes the client (which
appears in the common header) to re-send its state. If the optional
Client Handle is present, only the state associated with this handle
is synchronized. If the PEP does not recognize the requested handle,
it MUST immediately send a DRQ message to the PDP for the handle that
was specified in the SSQ message. If no handle is specified in the
SSQ message, all the active client state MUST be synchronized with
the PDP.
The client performs state synchronization by re-issuing request
queries of the specified client-type for the existing state in the
PEP. When synchronization is complete, the PEP MUST issue a
synchronize state complete message to the PDP.
3.6 Client-Open (OPN) PEP -> PDP
The Client-Open message can be used by the PEP to specify to the PDP
the client-types the PEP can support, the last PDP to which the PEP
connected for the given client-type, and/or client specific feature
negotiation. A Client-Open message can be sent to the PDP at any time
and multiple Client-Open messages for the same client-type are
allowed (in case of global state changes).
<Client-Open> ::= <Common Header>
<PEPID>
[<ClientSI>]
[<LastPDPAddr>]
[<Integrity>]
The PEPID is a symbolic, variable length name that uniquely
identifies the specific client to the PDP (see Section 2.2.11).
A named ClientSI object can be included for relaying additional
global information about the PEP to the PDP when required (as
specified in the appropriate extensions document for the client-
type).
The PEP may also provide a Last PDP Address object in its Client-Open
message specifying the last PDP (for the given client-type) for which
it is still caching decisions since its last reboot. A PDP can use
this information to determine the appropriate synchronization
behavior (See section 2.5).
If the PDP receives a malformed Client-Open message it MUST generate
a Client-Close message specifying the appropriate error code.
3.7 Client-Accept (CAT) PDP -> PEP
The Client-Accept message is used to positively respond to the
Client-Open message. This message will return to the PEP a timer
object indicating the maximum time interval between keep-alive
messages. Optionally, a timer specifying the minimum allowed interval
between accounting report messages may be included when applicable.
<Client-Accept> ::= <Common Header>
<KA Timer>
[<ACCT Timer>]
[<Integrity>]
If the PDP refuses the client, it will instead issue a Client-Close
message.
The KA Timer corresponds to maximum acceptable intermediate time
between the generation of messages by the PDP and PEP. The timer
value is determined by the PDP and is specified in seconds. A timer
value of 0 implies no secondary connection verification is necessary.
The optional ACCT Timer allows the PDP to indicate to the PEP that
periodic accounting reports SHOULD NOT exceed the specified timer
interval per client handle. This allows the PDP to control the rate
at which accounting reports are sent by the PEP (when applicable).
In general, accounting type Report messages are sent to the PDP when
determined appropriate by the PEP. The accounting timer merely is
used by the PDP to keep the rate of such updates in check (i.e.
Preventing the PEP from blasting the PDP with accounting reports).
Not including this object implies there are no PDP restrictions on
the rate at which accounting updates are generated.
If the PEP receives a malformed Client-Accept message it MUST
generate a Client-Close message specifying the appropriate error
code.
3.8 Client-Close (CC) PEP -> PDP, PDP -> PEP
The Client-Close message can be issued by either the PDP or PEP to
notify the other that a particular type of client is no longer being
supported.
<Client-Close> ::= <Common Header>
<Error>
[<PDPRedirAddr>]
[<Integrity>]
The Error object is included to describe the reason for the close
(e.g. the requested client-type is not supported by the remote PDP or
client failure).
A PDP MAY optionally include a PDP Redirect Address object in order
to inform the PEP of the alternate PDP it SHOULD use for the client-
type specified in the common header.
3.9 Keep-Alive (KA) PEP -> PDP, PDP -> PEP
The keep-alive message MUST be transmitted by the PEP within the
period defined by the minimum of all KA Timer values specified in all
received CAT messages for the connection. A KA message MUST be
generated randomly between 1/4 and 3/4 of this minimum KA timer
interval. When the PDP receives a keep-alive message from a PEP, it
MUST echo a keep-alive back to the PEP. This message provides
validation for each side that the connection is still functioning
even when there is no other messaging.
Note: The client-type in the header MUST always be set to 0 as the KA
is used for connection verification (not per client session
verification).
<Keep-Alive> ::= <Common Header>
[<Integrity>]
Both client and server MAY assume the TCP connection is insufficient
for the client-type with the minimum time value (specified in the CAT
message) if no communication activity is detected for a period
exceeding the timer period. For the PEP, such detection implies the
remote PDP or connection is down and the PEP SHOULD now attempt to
use an alternative/backup PDP.
3.10 Synchronize State Complete (SSC) PEP -> PDP
The Synchronize State Complete is sent by the PEP to the PDP after
the PDP sends a synchronize state request to the PEP and the PEP has
finished synchronization. It is useful so that the PDP will know when
all the old client state has been successfully re-requested and,
thus, the PEP and PDP are completely synchronized. The Client Handle
object only needs to be included if the corresponding Synchronize
State Message originally referenced a specific handle.
<Synchronize State Complete> ::= <Common Header>
[<Client Handle>]
[<Integrity>]
4. Common Operation
This section describes the typical exchanges between remote PDP
servers and PEP clients.
4.1 Security and Sequence Number Negotiation
COPS message security is negotiated once per connection and covers
all communication over a particular connection. If COPS level
security is required, it MUST be negotiated during the initial
Client-Open/Client-Accept message exchange specifying a Client-Type
of zero (which is reserved for connection level security negotiation
and connection verification).
If a PEP is not configured to use COPS security with a PDP it will
simply send the PDP Client-Open messages for the supported Client-
Types as specified in section 4.3 and will not include the Integrity
object in any COPS messages.
Otherwise, security can be initiated by the PEP if it sends the PDP a
Client-Open message with Client-Type=0 before opening any other
Client-Type. If the PDP receives a Client-Open with a Client-Type=0
after another Client-Type has already been opened successfully it
MUST return a Client-Close message (for Client-Type=0) to that PEP.
This first Client-Open message MUST specify a Client-Type of zero and
MUST provide the PEPID and a COPS Integrity object. This Integrity
object will contain the initial sequence number the PEP requires the
PDP to increment during subsequent communication after the initial
Client-Open/Client-Accept exchange and the Key ID identifying the
algorithm and key used to compute the digest.
Similarly, if the PDP accepts the PEP"s security key and algorithm by
validating the message digest using the identified key, the PDP MUST
send a Client-Accept message with a Client-Type of zero to the PEP
carrying an Integrity object. This Integrity object will contain the
initial sequence number the PDP requires the PEP to increment during
all subsequent communication with the PDP and the Key ID identifying
the key and algorithm used to compute the digest.
If the PEP, from the perspective of a PDP that requires security,
fails or never performs the security negotiation by not sending an
initial Client-Open message with a Client-Type=0 including a valid
Integrity object, the PDP MUST send to the PEP a Client-Close message
with a Client-Type=0 specifying the appropriate error code.
Similarly, if the PDP, from the perspective of a PEP that requires
security, fails the security negotiation by not sending back a
Client-Accept message with a Client-Type=0 including a valid
Integrity object, the PEP MUST send to the PDP a Client-Close message
with a Client-Type=0 specifying the appropriate error code. Such a
Client-Close message need not carry an integrity object (as the
security negotiation did not yet complete).
The security initialization can fail for one of several reasons: 1.
The side receiving the message requires COPS level security but an
Integrity object was not provided (Authentication Required error
code). 2. A COPS Integrity object was provided, but with an
unknown/unacceptable C-Type (Unknown COPS Object error code
specifying the unsupported C-Num and C-Type). 3. The message digest
or Key ID in the provided Integrity object was incorrect and
therefore the message could not be authenticated using the identified
key (Authentication Failure error code).
Once the initial security negotiation is complete, the PEP will know
what sequence numbers the PDP expects and the PDP will know what
sequence numbers the PEP expects. ALL COPS messages must then include
the negotiated Integrity object specifying the correct sequence
number with the appropriate message digest (including the Client-
Open/Client-Accept messages for specific Client-Types). ALL
subsequent messages from the PDP to the PEP MUST result in an
increment of the sequence number provided by the PEP in the Integrity
object of the initial Client-Open message. Likewise, ALL subsequent
messages from the PEP to the PDP MUST result in an increment of the
sequence number provided by the PDP in the Integrity object of the
initial Client-Accept message. Sequence numbers are incremented by
one starting with the corresponding initial sequence number. For
example, if the sequence number specified to the PEP by the PDP in
the initial Client-Accept was
