Request for Comments: 2749
Category: Standards Track
COPS usage for RSVP
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2000). All Rights Reserved.
This document describes usage directives for supporting COPS policy services in RSVP environments.
Table of Contents
1 Introduction....................................................2 2 RSVP values for COPS objects....................................2 2.1 Common Header, client-type...................................2 2.2 Context Object (Context).....................................3 2.3 Client Specific Information (ClientSI).......................4 2.4 Decision Object (Decision)...................................4 3 Operation of COPS for RSVP PEPs.................................6 3.1 RSVP flows...................................................6 3.2 Expected Associations for RSVP Requests......................6 3.3 RSVP's Capacity Admission Control: Commit and Delete.........7 3.4 Policy Control Over PathTear and ResvTear....................7 3.5 PEP Caching COPS Decisions...................................7 3.6 Using Multiple Context Flags in a single query...............8 3.7 RSVP Error Reporting.........................................9 4 Security Considerations.........................................9 5 Illustrative Examples, Using COPS for RSVP......................9 5.1 Unicast Flow Example.........................................9 5.2 Shared Multicast Flows......................................11 6 References.....................................................14 7 Author Information and Acknowledgments.........................15 8 Full Copyright Statement.......................................17
The Common Open Policy Service (COPS) protocol is a query response protocol used to exchange policy information between a network policy server and a set of clients [COPS]. COPS is being developed within the RSVP Admission Policy Working Group (RAP WG) of the IETF, primarily for use as a mechanism for providing policy-based admission control over requests for network resources [RAP].
This document is based on and assumes prior knowledge of the RAP framework [RAP] and the basic COPS [COPS] protocol. It provides specific usage directives for using COPS in outsourcing policy control decisions by RSVP clients (PEPs) to policy servers (PDPs).
Given the COPS protocol design, RSVP directives are mainly limited to RSVP applicability, interoperability and usage guidelines, as well as client specific examples.
2 RSVP values for COPS objects
The usage of several COPS objects is affected when used with the RSVP client type. This section describes these objects and their usage.
2.1 Common Header, client-type
RSVP is COPS client-type 1
2.2 Context Object (Context)
The semantics of the Context object for RSVP is as follows:
R-Type (Request Type Flag)
This context is used when the PEP receives an incoming RSVP message. The PDP may decide to accept or reject the incoming message and may also apply other decision objects to it. If the incoming message is rejected, RSVP should treat it as if it never arrived.
This context is used when the PEP is about to commit local resources to an RSVP flow (admission control). This context applies to Resv messages only. The decision whether to commit local resources is made for the merge of all reservations associated with an RSVP flow (which have arrived on a particular interface, potentially from several RSVP Next-Hops).
Outgoing-Message request (forwarding an outgoing RSVP message)
This context is used when the PEP is about to forward an outgoing RSVP message. The PDP may decide to allow or deny the outgoing message, as well as provide an outgoing policy data object.
M-Type (Message Type)
The M-Type field in the Context Object identifies the applicable RSVP message type. M-Type values are identical to the values used in the "msg type" field in the RSVP header [RSVP].
The following RSVP message types are supported in COPS:
Other message types such as PathTear, ResvTear, and Resv Confirm are not supported. The list of supported message types can only be extended in later versions of RSVP and/or later version of this document.
2.3 Client Specific Information (ClientSI)
All objects that were received in an RSVP message are encapsulated inside the Client Specific Information Object (Signaled ClientSI) sent from the PEP to the remote PDP (see Section 3.1. on multiple flows packed in a single RSVP message).
The PEP and PDP share RSVP state, and the PDP is assumed to implement the same RSVP functional specification as the PEP. In the case where a PDP detects the absence of objects required by [RSVP] it should return an <Error> in the Decision message indicating "Mandatory client-specific info missing". If, on the other hand, the PDP detects the absence of optional RSVP objects that are needed to approve the Request against current policies, the PDP should return a negative <Decision>.
Unlike the Incoming and Outgoing contexts, "Resource Allocation" is not always directly associated with a specific RSVP message. In a multicast session, it may represent the merging of multiple incoming reservations. Therefore, the ClientSI object should specifically contain the SESSION and STYLE objects along with the merged FLOWSPEC, FILTERSPEC list, and SCOPE object (whenever relevant).
2.4 Decision Object (Decision)
COPS provides the PDP with flexible controls over the PEP using RSVP's response to messages. While accepting an RSVP message, PDPs may provide preemption priority, trigger warnings, replace RSVP objects, and much more, using Decision Commands, Flags, and Objects.
Only two commands apply to RSVP
Accept/Allow/Admit an RSVP message or local resource allocation.
Deny/Reject/Remove an RSVP message or local resource allocation.
The only decision flag that applies to RSVP:
If this flag is set, RSVP should schedule a PathErr, in response to a Path message, or a ResvErr (in response of a Resv message).
STATELESS POLICY DATA
This object may include one or more policy elements (as specified for the RSVP Policy Data object [RSVP-EXT]) which are assumed to be well understood by the client's LPDP. The PEP should consider these as an addition to the decision already received from the PDP (it can only add, but cannot override it).
For example, given Policy Elements that specify a flow's preemption priority, these elements may be included in an incoming Resv message or may be provided by the PDP responding to a query.
Stateless objects must be well understood, but not necessarily supported by all PEPs. For example, assuming a standard policy element for preemption priority, it is perfectly legitimate for some PEPs not to support such preemption and to ignore it. The PDP must be careful when using such objects. In particular, it must be prepared for these objects to be ignored by PEPs.
Stateless Policy Data may be returned in decisions and apply individually to each of the contexts flagged in REQ messages. When applied to Incoming, it is assumed to have been received as a POLICY_DATA object in the incoming message. When applied to Resource Allocation it is assumed to have been received on all merged incoming messages. Last, when applied to outgoing messages it is assumed to have been received in all messages contributing to the outgoing message.
The Replacement object may contain multiple RSVP objects to be replaced (from the original RSVP request). Typical replacement is performed on the "Forward Outgoing" request (for instance, replacing outgoing Policy Data), but is not limited, and can also be performed on other contexts (such as "Resources-Allocation Request"). In other cases, replacement of the RSVP FlowSpec object may be useful for controlling resources across a trusted zone (with policy ignorant nodes (PINs). Currently, RSVP clients are only required to allow replacement of three objects: POLICY_DATA, ERROR_SPEC, and FLOWSPEC, but could optionally support replacement of other objects.
RSVP object replacement is performed in the following manner:
If no Replacement Data decision appears in a decision message, all signaled objects are processed as if the PDP was not there. When an object of a certain C-Num appears, it replaces ALL the instances of C-Num objects in the RSVP message. If it appears empty (with a length of 4) it simply removes all instances of C-Num objects without adding anything.
3 Operation of COPS for RSVP PEPs
3.1 RSVP flows
Policy Control is performed per RSVP flow, which is defined by the atomic unit of an RSVP reservation (TC reservation). Reservation styles may also impact the definition of flows; a set of senders which are considered as a single flow for WF reservation are considered as a set of individual flows when FF style is used.
Multiple FF flows may be packed into a single Resv message. A packed message must be unpacked where a separate request is issued for each of the packed flows as if they were individual RSVP messages. Each COPS Request should include the associated POLICY_DATA objects, which are, by default, all POLICY_DATA objects in the packed message. Sophisticated PEPs, capable of looking inside policy objects, may examine the POLICY_DATA or SCOPE object to narrow down the list of associated flows (as an optimization).
Please note that the rules governing Packed RSVP message apply equally to the Incoming as well as the Outgoing REQ context.
3.2 Expected Associations for RSVP Requests
When making a policy decision, the PDP may consider both Resv as well as its matching Path state (associated state). State association is straightforward in the common unicast case since the RSVP flow includes one Path state and one Resv state. In multicast cases this correspondence may be more complicated, as the match may be many-to- many. The COPS protocol assumes that the PDP is RSVP knowledgeable and capable of determining these associations based on the contents of the Client REQ message and especially the ClientSI object.
For example, the PDP should be able to recognize activation and deactivation of RSVP blockade state following discrete events like the arrival of a ResvErr message (activate the blockade state) as well as the change in the outgoing Resv message.
3.3 RSVP's Capacity Admission Control: Commit and Delete
In RSVP, the admission of a new reservation requires both an administrative approval (policy control) and capacity admission control. After being approved by both, and after the reservation was successfully installed, the PEP notifies the remote PDP by sending a report message specifying the Commit type. The Commit type report message signals when billing should effectively begin and performing heavier delayed operations (e.g., debiting a credit card) is permissible by the PDP.
If, instead, a PDP approved reservation fails admission due to lack of resources, the PEP must issue a no-commit report and fold back and send an updated request to its previous state (previously installed reservation). If no state was previously installed, the PEP should issue a delete (DRQ).
3.4 Policy Control Over PathTear and ResvTear
PathTear and ResvTear messages are not controlled by this policy architecture. This relies on two assumptions: First, that MD-5 authentication verifies that the Tear is received from the same node that sent the initial reservation, and second, that it is functionally equivalent to that node holding off refreshes for this reservation. When a ResvTear or PathTear is received at the PEP, all affected states installed on the PDP should either be deleted or updated by the PEP.
3.5 PEP Caching COPS Decisions
Because COPS is a stateful protocol, refreshes for RSVP Path and Resv messages need not be constantly sent to the remote PDP. Once a decision has been returned for a request, the PEP can cache that decision and apply it to future refreshes. When the PEP detects a change in the corresponding Resv or Path message, it should update the PDP with the new request-state. PEPs may continue to use the cached state until receiving the PDP response. This case is very different from initial admission of a flow; given that valid credentials and authentication have already been established, the relatively long RSVP refresh period, and the short PEP-PDP response time, the tradeoff between expedient updates and attack prevention leans toward expediency. However, this is really a PEP choice, and is irrelevant to PDPs.
If the connection is lost between the PEP and the PDP, the cached RSVP state may be retained for the RSVP timeout period to be used for previously admitted flows (but cannot be applied to new or updated state). If the connection can not be reestablished with the PDP or a backup PDP after the timeout period, the PEP is expected to purge all its cached decisions. Without applicable cached decision, the PEP must either reject the flow or resort to its LPDP (if available) for decisions.
Once a connection is reestablished to a new (or the original) PDP the PDP may issue a SSQ request. In this case, the PEP must reissue requests that correspond to the current RSVP state (as if all the state has been updated recently). It should also include in its LPDPDecision the current (cached) decision regarding each such state.
3.6 Using Multiple Context Flags in a single query
RSVP is a store-and-forward control protocol where messages are processed in three distinctive steps (input, resource allocation, and output). Each step requires a separate policy decision as indicated by context flags (see Section 2.2). In many cases, setting multiple context flags for bundling two or three operations together in one request may significantly optimize protocol operations.
The following rules apply for setting multiple Context flags:
a. Multiple context flags can be set only in two generic cases, which
represent a substantial portion of expected COPS transactions, and can be guaranteed not to cause ambiguity.
[Incoming + Allocation + Outgoing]
Multicast with only one Resv message received on the interface
[Incoming + Allocation]
b. Context events are ordered by time since every message must first
be processed as Incoming, then as Resource allocation and only then as Outgoing. When multiple context flags are set, all ClientSI objects included in the request are assumed to be processed according to the latest flag. This rule applies both to the request (REQ) context as well as to the decision (DEC) context.
For example, when combining Incoming + Allocation for an incoming Resv message, the flowspec included in the ClientSI would be the one corresponding to the Resource-Allocation context (TC).
c. Each decision is bound to a context object, which determines which
portion of the request context it applies to. When individual decisions apply to different sub-groups of the context, the PDP should send each group of decision objects encapsulated by the context flags object with the context flags applicable to these objects set (see the examples in Section 5).
3.7 RSVP Error Reporting
RSVP uses the ERROR_SPEC object in PathErr and ResvErr messages to report policy errors. While the contents of the ERROR_SPEC object are defined in [RSVP,RSVP-EXT], the PDP is in the best position to provide its contents (sub-codes). This is performed in the following manner: First, the PEP (RSVP) queries the PDP before sending a PathErr or ResvErr, and then the PDP returns the constructed ERROR_SPEC in the Replacement Data Decision Object.
4 Security Considerations
This document relies on COPS for its signaling and its security. Please refer to section "Security Considerations" in [COPS].
Security for RSVP messages is provided by inter-router MD5 authentication [MD5], assuming a chain-of-trust model. A likely deployment scenario calls for PEPs to be deployed only at the network edge (boundary nodes) while the core of the network (backbone) consists of PIN nodes. In this scenario MD5 trust (authentication) is established between boundary (non-neighboring) PEPs. Such trust can be achieved through internal signing (integrity) of the Policy Data object itself, which is left unmodified as it passes through PIN nodes (see [RSVP-EXT]).
5 Illustrative Examples, Using COPS for RSVP
This section details both typical unicast and multicast scenarios.
5.1 Unicast Flow Example
This section details the steps in using COPS for controlling a Unicast RSVP flow. It details the contents of the COPS messages with respect to Figure 1.
PEP (router) +-----------------+ | | R1 ------------+if1 if2+------------ S1 | | +-----------------+
Figure 1: Unicast Example: a single PEP view
The PEP router has two interfaces (if1, if2). Sender S1 sends to receiver R1.
A Path message arrives from S1:
PEP --> PDP REQ := <Handle A> <Context: in & out, Path> <In-Interface if2> <Out-Interface if1> <ClientSI: all objects in Path message> PDP --> PEP DEC := <Handle A> <Context: in & out, Path> <Decision: Command, Install>
A Resv message arrives from R1:
PEP --> PDP REQ := <Handle B> <Context: in & allocation & out, Resv> <In-Interface if1> <Out-Interface if2> <ClientSI: all objects in Resv message> PDP --> PEP DEC := <Handle B> <Context: in, Resv> <Decision: command, Install> <Context: allocation, Resv> <Decision: command, Install> <Decision: Stateless, Priority=7> <Context: out, Resv> <Decision: command, Install> <Decision: replacement, POLICY-DATA1> PEP --> PDP RPT := <Handle B> <Report type: commit>
Notice that the Decision was split because of the need to specify different decision objects for different context flags.
Time Passes, the PDP changes its decision:
PDP --> PEP DEC := <Handle B> <Context: allocation, Resv> <Decision: command, Install> <Decision: Stateless, Priority=3>
Because the priority is too low, the PEP preempts the flow:
PEP --> PDP DRQ := <Handle B> <Reason Code: Preempted>
Time Passes, the sender S1 ceases to send Path messages:
PEP --> PDP DRQ := <Handle A> <Reason: Timeout>
5.2 Shared Multicast Flows
This section details the steps in using COPS for controlling a multicast RSVP flow. It details the contents of the COPS messages with respect to Figure 2.
PEP (router) +-----------------+ | | R1-------------+ if1 if3 +--------- S1 | | R2----+ | | | | | +--------+ if2 if4 +--------- S2 | | | R3----+ +-----------------+
Figure 2: Multicast example: a single PEP view
Figure 2 shows an RSVP PEP (router) which has two senders (S1, S2) and three receivers (R1, R2, R3) for the same multicast session. Interface if2 is connected to a shared media. In this example, we assume that the multicast membership is already in place. No previous RSVP messages were received, and the first to arrive is a Path message on interface if3 from sender S1:
PEP --> PDP REQ := <Handle A> <Context: in, Path> <In-interface if3> <ClientSI: all objects in incoming Path> PDP --> PEP DEC := <Handle A> <Context: in, Path> <Decision: command, Install>
The PEP consults its forwarding table, and finds two outgoing interface for the path (if1, if2). The exchange below is for interface if1, another exchange would likewise be completed for if2 using the new handle B2.
PEP --> PDP REQ := <Handle B1> <Context: out, Path> <Out-interface if1> <clientSI: all objects in outgoing Path> PDP --> PEP DEC := <Handle B1> <Context: out, Path> <Decision: command, Install> <Decision: Replacement, POLICY-DATA1>
Here, the PDP decided to allow the forwarding of the Path message and provided the appropriate policy-data object for interface if1.
Next, a WF Resv message from receiver R2 arrives on interface if2.
PEP --> PDP REQ := <Handle C> <Context: in & allocation, Resv> <In-interface if2> <ClientSI: all objects in Resv message including RSpec1 > PDP --> PEP DEC := <Handle C> <Context: in, Resv> <Decision: command, Install> <Context: allocation, Resv> <Decision: command, Install> <Decision: Stateless, priority=5> PEP --> PDP RPT := <handle C> <Commit>
Here, the PDP approves the reservation and assigned it preemption priority of 5. The PEP responded with a commit report.
The PEP needs to forward the Resv message upstream toward S1:
PEP --> PDP REQ := <Handle E> <Context: out, Resv> <out-interface if3> <Client info: all objects in outgoing Resv> PDP --> PEP DEC := <Handle E> <Context: out, Resv> <Decision: command, Install> <Decision: replacement, POLICY-DATA2>
Note: The Context object is part of this DEC message even though it may look redundant since the REQ specified only one context flag.
Next, a new WF Resv message from receiver R3 arrives on interface if2 with a higher RSpec (Rspec2). Given two reservations arrived on if2, it cannot perform a request with multiple context flags, and must issue them separately.
The PEP re-issues an updated handle C REQ with a new context object <Context: in , Resv>, and receives a DEC for handle C.
PEP --> PDP REQ := <Handle F> <Context: in , Resv> <In-interface if2> <ClientSI: all objects in Resv message including RSpec2 > PDP --> PEP DEC := <Handle F> <Context: in , Resv> <Decision: command, Install> PEP --> PDP REQ := <Handle G> <Context: allocation, Resv> <In-interface if2> <ClientSI: all objects in merged Resv including RSpec2 > PDP --> PEP DEC := <Handle G> <Context: allocation, Resv> <Decision: command, Install> <Decision: Stateless, Priority=5> PEP --> PDP RPT := <handle G> <Commit>
Given the change in incoming reservations, the PEP needs to forward a new outgoing Resv message upstream toward S1. This repeats exactly the previous interaction of Handle E, except that the ClientSI objects now reflect the merging of two reservations.
If an ResvErr arrives from S1, the PEP maps it to R3 only (because it has a higher flowspec: Rspec2) the following takes place:
PEP --> PDP REQ := <Handle H> <Context: in, ResvErr> <In-interface if3> <ClientSI: all objects in incoming ResvErr> PDP --> PEP DEC := <Handle H> <Context: in, ResvErr> <Decision: command, Install> PEP --> PDP REQ := <Handle I> <Context: out, ResvErr> <Out-interface if2> <ClientSI: all objects in outgoing ResvErr> PDP --> PEP DEC := <Handle I> <Context: out, ResvErr> <Decision: command, Install> <Decision: Replacement, POLICY-DATA3>
When S2 joins the session by sending a Path message, incoming and outgoing Path requests are issued for the new Path. A new outgoing Resv request would be sent to S2.
[RSVP-EXT] Herzog, S., "RSVP Extensions for Policy Control", RFC 2750, January 2000. [RAP] Yavatkar, R., Pendarakis, D. and R. Guerin, "A Framework for Policy Based Admission Control", RFC 2753, January 2000. [COPS] Boyle, J., Cohen, R., Durham, D., Herzog, S., Raja, R. and A. Sastry, "The COPS (Common Open Policy Service) Protocol", RFC 2748, January 2000. [RSVP] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin, "Resource ReSerVation Protocol (RSVP) - Functional Specification", RFC 2205, September 1997.
7 Author Information and Acknowledgments
Special thanks to Andrew Smith and Timothy O'Malley our WG Chairs, Fred Baker, Laura Cunningham, Russell Fenger, Roch Guerin, Ping Pan, and Raj Yavatkar, for their valuable contributions.
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