RFC 2749
COPS usage for RSVP
RFC 2749: COPS usage for RSVP 💖
bestie, this is RFC 2749 🎀 with the title COPS usage for RSVP, from the Network Working Group, dated January 2000 🌸. It's a Standards Track document, so it's an Internet standards track protocol spec 🔥. The authors are S. Herzog, Ed. (IPHighway), J. Boyle (Level3), R. Cohen (Cisco), D. Durham (Intel), R. Rajan (AT&T), and A. Sastry (Cisco) 👑. The header has no Obsoletes or Updates line, so it doesn't list anything it replaces or changes ✨.
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.
Abstract 🦋
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.......................................171. Introduction 🌈
okay so the Common Open Policy Service (COPS) protocol is a query response protocol, and its whole job is swapping policy information between a network policy server and a set of clients [COPS] 💅. It's being built inside the RSVP Admission Policy Working Group (RAP WG) of the IETF, mainly as a way to do policy-based admission control over requests for network resources [RAP] ✨.
this document is based on the RAP framework [RAP] and the basic COPS [COPS] protocol, so it assumes you already know those 🧚. It gives specific usage directions for using COPS to hand off policy control decisions from RSVP clients (PEPs) to policy servers (PDPs) 💜.
given how COPS is designed, the RSVP directions are mostly about RSVP applicability, interoperability and usage guidelines, plus client specific examples 🍓.
2. RSVP values for COPS objects 🎀
several COPS objects change their behaviour when the client type is RSVP, so this section explains those objects and how they get used 🫶.
2.1. Common Header, client-type 🔥
RSVP is COPS client-type 1
2.2. Context Object (Context) 💖
the Context object means something specific for RSVP, and here's how it works 👑.
R-Type (Request Type Flag)
Incoming-Message request 📥
this context gets 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 🙈.
Resource-Allocation request 🪙
this context gets 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 gets 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:
PathResvPathErrResvErr
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 get wrapped up inside the Client Specific Information Object (Signaled ClientSI) and 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 gives the PDP flexible controls over the PEP, based on how RSVP responds 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 🎯.
DECISION COMMANDS
only two commands apply to RSVP 🙌.
Install ✅
Positive Response: Accept/Allow/Admit an RSVP message or local resource allocation.
Remove ❌
Negative Response: Deny/Reject/Remove an RSVP message or local resource allocation.
DECISION FLAGS
the only decision flag that applies to RSVP is this one 🚩.
Trigger Error ⚠️
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 📚.
REPLACEMENT DATA
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 ✨
okay so when the PDP is making a policy decision, it can look at both the Resv and its matching Path state (the associated state) 👀. In the common unicast case this is super straightforward, because an RSVP flow has one Path state and one Resv state, done 💖. Multicast is where it gets messy, since the match can be many-to-many 🌀. The COPS protocol assumes the PDP is RSVP knowledgeable 🧠 and can work out these associations from the contents of the Client REQ message, especially the ClientSI object 🎀.
For example, the PDP should be able to recognize when RSVP blockade state gets activated or deactivated after discrete events, like the arrival of a ResvErr message (that activates the blockade state) 🚫, and also when the outgoing Resv message changes 🔄. bestie needs to be on top of this one 💅.
3.3. RSVP's Capacity Admission Control: Commit and Delete 💸
Okay this part is giving admission control realness 😤. In RSVP, admitting a new reservation needs BOTH an administrative approval (policy control) AND capacity admission control 🫶. Once it's approved by both, and the reservation is successfully installed, the PEP tells the remote PDP by sending a report message with the Commit type 🎉. The Commit report is the signal for when billing should effectively begin, and it's the moment the PDP is allowed to do heavier delayed stuff, like debiting a credit card 💳✨.
Now, if a PDP-approved reservation fails admission because there aren't enough resources 😭, the PEP MUST issue a no-commit report and fold back, then send an updated request to its previous state (the 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, MD-5 authentication verifies that the Tear came from the same node that sent the initial reservation. Second, it's functionally equivalent to that node holding off refreshes for this reservation 💭. When a ResvTear or PathTear arrives 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 stateful, Path and Resv refreshes don't need to be constantly sent to the remote PDP 🙌. Once a decision comes back for a request, the PEP can cache it and apply it to future refreshes 💜. When the PEP notices a change in the corresponding Resv or Path message, it SHOULD update the PDP with the new request-state 🔄. PEPs MAY keep using the cached state until they receive the PDP's response ⏳. This case is very different from initial admission of a flow. Since valid credentials and authentication are already established, the RSVP refresh period is relatively long, and the PEP-PDP response time is short, the tradeoff between expedient updates and attack prevention leans toward expediency 🏃♀️💨. But honestly this is really a PEP choice and is irrelevant to PDPs 🤷♀️.
If the connection between the PEP and the PDP is lost, the cached RSVP state MAY be kept for the RSVP timeout period, to be used for previously admitted flows (but it can't be applied to new or updated state) 🥺. If the connection can't be reestablished with the PDP or a backup PDP after the timeout period, the PEP is expected to purge all its cached decisions 🧽. Without an applicable cached decision, the PEP MUST either reject the flow or fall back to its LPDP (if available) for decisions 🚪.
Once a connection is reestablished with a new (or the original) PDP, the PDP MAY issue an SSQ request 📨. In this case, the PEP MUST reissue requests that correspond to the current RSVP state (as if all the state had been updated recently) 🔁. It SHOULD also include, in its LPDPDecision, the current (cached) decision for each such state 🎀.
3.6. Using Multiple Context Flags in a single query 🧩
RSVP is a store-and-forward control protocol where messages get processed in three distinct steps: input, resource allocation, and output 📬. Each step needs its own policy decision, indicated by context flags (see Section 2.2) 🚩. In lots of cases, setting multiple context flags to bundle two or three operations into one request can seriously optimize things ⚡.
The following rules apply when setting multiple Context flags:
- Multiple context flags can be set ONLY in two generic cases. These represent a big chunk of expected COPS transactions and can be guaranteed not to cause ambiguity 💯.
Unicast FF:
[Incoming + Allocation + Outgoing]Multicast with only one Resv message received on the interface:
[Incoming + Allocation]- 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 to the request (REQ) context AND the decision (DEC) context 🫶.
For example, when combining Incoming + Allocation for an incoming Resv message, the flowspec included in the ClientSI is the one corresponding to the Resource-Allocation context (TC) 🌊.
- 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 those 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 🙈. The contents of the ERROR_SPEC object are defined in [RSVP,RSVP-EXT], but the PDP is in the best position to provide its contents (sub-codes) 🧠✨. It works like this: 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 the 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 walks through the steps for using COPS to control 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 viewThe 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, and 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, and the sender S1 stops sending Path messages 💤:
PEP --> PDP DRQ := <Handle A>
<Reason: Timeout>5.2 Shared Multicast Flows ✨
okay so this is where it gets spicy 🌶️ we're doing multicast, which means one session with a whole crowd of receivers. This section walks through the COPS messages for one multicast RSVP flow, with Figure 2 as our map 🗺️
PEP (router)
+-----------------+
| |
R1-------------+ if1 if3 +--------- S1
| |
R2----+ | |
| | |
+--------+ if2 if4 +--------- S2
| | |
R3----+ +-----------------+
Figure 2: Multicast example: a single PEP viewFigure 2 is a PEP (router) with two senders (S1, S2) and three receivers (R1, R2, R3) all in the same multicast session 🦋 Interface if2 is hooked up to a shared media. We're assuming the multicast membership is already set up. No RSVP messages have come in yet, and the first one to show up 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 checks its forwarding table and finds two outgoing interfaces for the path (if1, if2). The exchange below is for if1. The same exchange would then happen 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>So the PDP said yes to forwarding the Path message and handed over the policy-data object for if1 ✨
Next up, a WF Resv message from receiver R2 lands 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>The PDP approves the reservation and gives it a preemption priority of 5 👑 The PEP answers with a commit report, and we're giving it a whole standing ovation 👏
Now the PEP has to forward that 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 still in this DEC message even though it might look redundant, since the REQ only specified one context flag.
Next, a new WF Resv message from receiver R3 arrives on if2, and this one asks for a higher RSpec (Rspec2) 😤 Since two reservations are now coming in on if2, the PEP can't bundle several context flags into one request. It has to send them one at a time 🍓
The PEP sends a fresh request with a new context object <Context: in , Resv>, and gets back a DEC for it:
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>Since the incoming reservations changed, the PEP has to send a new outgoing Resv upstream toward S1 🔁 This is basically the same back-and-forth as Handle E, except the ClientSI objects now include both merged reservations 🧚
If an ResvErr comes in from S1, the PEP only maps it to R3, because R3 has the higher flowspec (Rspec2). Here's how that goes down 💔
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>And when S2 joins the session by sending a Path message, the PEP sends incoming and outgoing Path requests for that new Path. A new outgoing Resv request goes to S2 too 🌈
6 References 📚
[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, and to Fred Baker, Laura Cunningham, Russell Fenger, Roch Guerin, Ping Pan, and Raj Yavatkar, for their valuable contributions 🌸
Jim Boyle
Level 3 Communications
1025 Eldorado Boulevard
Broomfield, CO 80021
Phone: 720.888.1192
EMail: jboyle@Level3.net
Ron Cohen
CISCO Systems
4 Maskit St.
Herzeliya Pituach 46766 Israel
Phone: 972.9.9700064
EMail: ronc@cisco.com
David Durham
Intel
2111 NE 25th Avenue
Hillsboro, OR 97124
Phone: 503.264.6232
EMail: David.Durham@intel.com
Raju Rajan
AT&T Labs Research
180 Park Ave., P.O. Box 971
Florham Park, NJ 07932
Phone: 973.360.7229
EMail: raju@research.att.com
Shai Herzog
IPHighway, Inc.
55 New York Avenue
Framingham, MA 01701
Phone: 508.620.1141
EMail: herzog@iphighway.com
Arun Sastry
Cisco Systems
4 The Square
Stockley Park
Uxbridge, Middlesex UB11 1BN
UK
Phone: +44-208-756-8693
EMail: asastry@cisco.com8 Full Copyright Statement ⚖️
Copyright (C) The Internet Society (2000). All Rights Reserved.
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The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.
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Acknowledgement 💜
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