Support for RSVP in Layer 3 VPNs
draft-davie-tsvwg-rsvp-l3vpn-01.txt

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Abstract

RFC 4364 defines an approach to building provider-provisioned Layer 3 VPNs. It may be desirable to use RSVP to perform admission control on the links between CE and PE routers. This document specifies procedures by which RSVP messages travelling from CE to CE across an L3VPN may be appropriately handled by PE routers so that admission control can be performed on PE-CE links. Optionally, admission control across the provider's backbone may also be supported.

Requirements Language

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Table of Contents

1.  Introduction
    1.1.  Terminology
2.  Problem Statement
    2.1.  Model of Operation
3.  Admission Control on PE-CE Links
    3.1.  Path Message Processing at Ingress PE
    3.2.  Path Message Processing at Egress PE
    3.3.  Resv Processing at Egress PE
    3.4.  Resv Processing at Ingress PE
    3.5.  Other RSVP Messages
4.  Admission Control in Provider's Backbone
5.  Inter-AS operation
    5.1.  Inter-AS Option A
    5.2.  Inter-AS Option B
    5.3.  Inter-AS Option C
6.  Operation with RSVP disabled
7.  Support for CE-CE RSVP-TE
8.  Object Definitions
    8.1.  VPN_Label Object
    8.2.  VRF_ID Object
9.  IANA Considerations
10.  Security Considerations
11.  Acknowledgments
Appendix A.  Alternatives Considered
Appendix A.1.  GMPLS UNI approach
Appendix A.2.  VRF label approach
Appendix A.3.  VRF label plus VRF address approach
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

[RFC4364] (Rosen, E. and Y. Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs),” February 2006.) defines a Layer 3 VPN service known as BGP/MPLS VPNs. [RFC2205] (Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, “Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification,” September 1997.) defines the Resource Reservation Protocol (RSVP) which may be used to perform admission control as part of the Integrated Services (int-serv) architecture [RFC1633] (Braden, B., Clark, D., and S. Shenker, “Integrated Services in the Internet Architecture: an Overview,” June 1994.)[RFC2210] (Wroclawski, J., “The Use of RSVP with IETF Integrated Services,” September 1997.).

Customers of a layer 3 VPN service may run RSVP for the purposes of admission control in their own networks. Since the links between Provider Edge (PE) and Customer Edge (CE) routers in a layer 3 VPN may often be resource constrained, it may be desirable to be able to perform admission control over those links. In order to perform admission control using RSVP in such an environment, it is necessary that RSVP control messages, such as Path messages and Resv messages, are appropriately handled by the PE routers. This presents a number of challenges in the context of BGP/MPLS VPNs:

• RSVP Path message processing depends on routers recognizing the router alert option in the IP header. However, packets traversing the backbone of a BGP/MPLS VPN are MPLS encapsulated and thus the router alert option is not normally visible to the egress PE.
• BGP/MPLS VPNs support non-unique addressing of customer networks. Thus a PE at the ingress or egress of the provider backbone may be called upon to process Path messages from different customer VPNs with non-unique destination addresses.
• A PE at the ingress of the provider's backbone may receive Resv messages corresponding to different customer VPNs from other PEs, and needs to be able to associate those Resv messages with the appropriate customer VPNs.

This document describes a set of procedures to overcome these challenges and thus to enable admission control using RSVP over the PE-CE links. We note that similar techniques may be applicable to other protocols used for admission control such as NSIS [RFC4080] (Hancock, R., Karagiannis, G., Loughney, J., and S. Van den Bosch, “Next Steps in Signaling (NSIS): Framework,” June 2005.).

Additionally, it may be desirable to perform admission control over the provider's backbone on behalf of one or more L3VPN customers. Core (P) routers in a BGP/MPLS VPN do not have forwarding entries for customer routes, and thus cannot natively process RSVP messages for customer flows. Also the core is a shared resource that carries traffic for many customers, so issues of resource allocation among customers and trust (or lack thereof) must also be addressed. This draft also specifies procedures for supporting such a scenario.

This draft deals with establishing reservations for unicast flows only. Because the support of multicast traffic in BGP/MPLS VPNs is still evolving, and raises additional challenges for admission control, we leave the support of multicast flows for further study at this point.

1.1.  Terminology

This document draws freely on the terminology defined in [RFC2205] (Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, “Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification,” September 1997.) and [RFC4364] (Rosen, E. and Y. Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs),” February 2006.). For convenience, we provide a few brief definitions here:

• CE (Customer Edge) Router: Router at the edge of a customer site that attaches to the network of the VPN provider.
• PE (Provider Edge) Router: Router at the edge of the service provider's network that attaches to one or more customer sites.
• VPN Label: An MPLS label associated with a route to a customer prefix in a VPN (also called a VPN route label).
• VRF: VPN Routing and Forwarding Table. A PE typically has multiple VRFs, enabling it to be connected to CEs that are in different VPNs.

2.  Problem Statement

The problem space of this document is the support of admission control between customer sites when the customer subscribes to a BGP/MPLS VPN. We subdivide the problem into (a) the problem of admission control on the PE-CE links (in both directions), and (b) the problem of admission control across the provider's backbone.

For the PE-CE link subproblem, the most basic challenge is that RSVP control messages contain IP addresses that are drawn from the customer's address space, and PEs must be able to deal with traffic from many customers who may have non-unique (or overlapping) address spaces. Thus, it is essential that a PE be able in all cases to identify the correct VPN context in which to process an RSVP control message. Much of this draft deals with this issue.

For the case of making reservations across the provider backbone, we observe that BGP/MPLS VPNs do not create any per-customer forwarding state in the P (provider core) routers. Thus, in order to make reservations on behalf of customer-specified flows, it is clearly necessary to make some sort of aggregated reservation from PE-PE and then map individual, customer-specific reservations onto an aggregate reservation. That is similar to the problem tackled in [RFC3175] (Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie, “Aggregation of RSVP for IPv4 and IPv6 Reservations,” September 2001.) and [RFC4804] (Le Faucheur, F., “Aggregation of Resource ReSerVation Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels,” February 2007.), with the additional complications of handling customer-specific addressing associated with BGP/MPLS VPNs.

Finally, we note that RSVP Path messages are normally addressed to the destination of a session, and contain the router alert IP option. Routers along the path to the destination that are configured to process RSVP messages must detect the presence of the router alert option to allow them to intercept Path messages. However, the egress PEs of a network supporting BGP/MPLS VPNs receive packets destined for customer sites as MPLS-encapsulated packets, and normally forward based only on examination of the MPLS label. Hence, a Path message would typically be forwarded without examination of the IP options and would therefore not receive appropriate processing at the PE. This problem of recognizing and processing Path messages is also discussed below.

2.1.  Model of Operation

Figure 1 illustrates the basic model of operation with which this document is concerned.

                      --------------------------
                     /       Provider           \
        |----|      |         Backbone           |      |----|
Sender->| CE1|  |-----|                       |-----|   |CE2 |->Receiver
        |    |--|     |   |---|     |---|     |     |---|    |
        |----|  |     |   | P |     | P |     |     |   |----|
                | PE1 |---|   |-----|   |-----| PE2 |
                |     |   |   |     |   |     |     |
                |     |   |---|     |---|     |     |
                |-----|                       |-----|
                    |                            |
                     \                          /
                      --------------------------

Figure 1. Model of Operation for RSVP-based admission control over MPLS/BGP VPN

To establish a unidirectional reservation for a point-to-point flow from Sender to Receiver that takes account of resource availability on the CE-PE and PE-CE links only, the following steps must take place:

1. Sender sends a Path message to an IP address of the Receiver.
2. Path message is processed by CE1 using normal RSVP procedures and forwarded towards the Receiver along the link CE1-PE1.
3. PE1 processes Path message and forwards towards the Receiver across the provider backbone.
4. PE2 processes Path message and forwards towards the Receiver along link PE2-CE2.
5. CE2 processes Path message using normal RSVP procedures and forwards towards Receiver.
6. Receiver sends Resv message to CE2.
7. CE2 sends Resv message to PE2.
8. PE2 processes Resv message (including performing admission control on link PE2-CE2) and sends Resv to PE1.
9. PE1 processes Resv message and sends Resv to CE1.
10. CE1 processes Resv using normal RSVP procedures, performs admission control on the link CE1-PE1 and sends Resv message to Sender if successful.

In each of the steps involving Resv messages (6 through 10) the node sending the Resv uses the previously established Path state to determine the "RSVP Previous Hop (PHOP)" and sends a Resv message to that address. We note that establishing that Path state correctly at PEs is one of the challenges posed by the BGP/MPLS environment.

3.  Admission Control on PE-CE Links

In the following sections we trace through the steps outlined in Section 2.1 (Model of Operation) and expand on the details for those steps where standard RSVP procedures need to be extended or modified to support the BGP/MPLS VPN environment. For all the remaining steps described in the preceding section, standard RSVP processing rules apply.

3.1.  Path Message Processing at Ingress PE

When a Path message arrives at the ingress PE (step 3 of Section 2.1 (Model of Operation)) the PE needs to establish suitable Path state and forward the Path message on to the egress PE. In the following paragraphs we described the steps taken by the ingress PE.

The Path message is addressed to the eventual destination (the receiver at the remote customer site) and carries the IP Router Alert option, in accordance with [RFC2205] (Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, “Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification,” September 1997.). The ingress PE must recognize the router alert, intercept these messages and process them as RSVP signalling messages.

As noted above, there is an issue in recognizing Path messages as they arrive at the egress PE (PE 2 in Figure 1). Since standard Path messages carry the router alert IP option, one possible approach would be to use the MPLS router alert label [RFC3032] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.) when sending a Path message from ingress PE to egress PE. However this may suffer from problems of backwards compatibility with existing deployed hardware that may not process the Router Alert label. The preferred approach proposed here is to address the Path messages sent by the ingress PE directly to the egress PE; that is, rather than using the ultimate receiver's destination address as the destination address of the Path message, we use the loopback address of the egress PE as the destination address of the Path message. This approach has the advantage that it does not require any new data plane capabilities for the egress PE beyond those of a standard BGP/MPLS VPN PE. Details of the processing of this message at the egress PE are described below. The approach of addressing a Path message directly to an RSVP next hop that is not the next IP hop is already used in other environments such as those of [RFC4206] (Kompella, K. and Y. Rekhter, “Label Switched Paths (LSP) Hierarchy with Generalized Multi-Protocol Label Switching (GMPLS) Traffic Engineering (TE),” October 2005.) and [RFC4804] (Le Faucheur, F., “Aggregation of Resource ReSerVation Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels,” February 2007.).

The details of operation at the ingress PE are as follows. When the ingress PE (PE1 in Figure 1) receives a Path message from CE1 that is addressed to the receiver, the VRF that is associated with the incoming interface is identified, just as for normal data path operations. The Path state for the session is stored, and is associated with that VRF, so that potentially overlapping addresses among different VPNs do not appear to belong to the same session. The destination address of the receiver is looked up in the appropriate VRF, and the BGP Next-Hop for that destination is identified. That next-hop is the egress PE (PE2 in Figure 1). The VPN label for that destination is obtained and placed in a new RSVP object (VPN_LABEL, defined below.) A new Path message is constructed with a destination address equal to the address of the egress PE identified above. This new Path message will contain all the objects from the original Path message, plus the VPN_LABEL object. Note that the SESSION object contains the ultimate (customer) destination address of the flow, while the IP header for the message contains the address of the egress PE. The RSVP_HOP object in the Path message contains an IP address of the ingress PE. The Path message also contains a new identifier that will be echoed by the egress PE inside the Resv message, thereby allowing the ingress PE to identify the correct VRF in which to process the Resv. The VRF_ID object that serves this function is defined below, and is used to carry a locally significant VRF identifier. The VRF identifier needs to be meaningful only to the PE that creates this object.

3.2.  Path Message Processing at Egress PE

When a Path message arrives at the egress PE, it is addressed to the PE itself, and is handed to RSVP for processing. The router needs to

a.

Determine the egress VRF for this flow, and how to forward a Path message on towards the correct CE and ultimate destination;

b.

Store the information received in the Path message (including the VRF_ID Object);

c.

Construct a suitable Path message with the correct destination address and forward it.

For step a, we can imagine the router containing an RSVP module and a forwarding module (this division is for exposition only; there is no intention to specify the internal implementation here). The RSVP module extracts the MPLS label contained in the VPN_LABEL object, and the destination IP address contained in the SESSION object, and passes them to the normal forwarding module for MPLS-encapsulated packets. The forwarding module returns to RSVP the outgoing interface information, including the egress VRF, that would have been used had a packet with that MPLS label and IP address been received. (Note that in many cases the MPLS label alone is all that is needed to determine the forwarding information for the packet, but in some cases it is necessary to pop the label and examine the IP address; hence both are passed to the forwarding module.)

Step b proceeds as follows. Note that [RFC2205] (Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, “Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification,” September 1997.) identifies the fields in the SESSION object to define a session, specifically the destination address, protocol and destination port. In this draft, we can consider the identity of the egress VRF that was determined in step a also to be part of the session definition. The identity of this egress VRF is therefore stored with the Path state to facilitate processing of Resv messages for this session.

Now the RSVP module can construct a Path message which differs from the Path it received in the following ways:

a.

Its destination address is the IP address extracted from the SESSION Object;

b.

It does not contain the VPN_LABEL Object or the VRF_ID Object.

c.

The RSVP_HOP Object contains the IP address of the outgoing interface of the egress PE and an LIH, as per normal RSVP processing.

The router then sends the Path message on towards its destination over the interface identified above.

3.3.  Resv Processing at Egress PE

When a receiver at the customer site originates a Resv message for the session, normal RSVP procedures apply until the Resv, making its way back towards the sender, arrives at the "egress" PE (it is "egress" with respect to the direction of data flow, i.e. PE2 in figure 1). On arriving at PE2, the SESSION and FILTER_SPEC objects in the Resv, and the VRF in which the Resv was received, are used to find the matching Path state stored previously. At this stage, admission control can be performed on the PE-CE link.

Assuming admission control is successful, the PE constructs a Resv message to send to the RSVP HOP stored in the Path state, i.e., the ingress PE (PE1 in Figure 1). It includes the VRF_ID object that was obtained from the Path message as described above. The Resv message is addressed to the ingress PE and sent.

If admission control is not successful, a ResvError message is sent towards the receiver as per normal RSVP processing.

3.4.  Resv Processing at Ingress PE

Upon receiving a Resv message at the ingress PE (with respect to data flow, i.e. PE1 in Figure 1), the PE extracts the VRF identifier from VRF_ID object and determines which VRF the session is associated with. It is now possible to locate the appropriate Path state for the reservation, and generate a Resv message to send to the appropriate CE. Since we assume in this section that admission control over the Provider’s backbone is not needed, the ingress PE does not perform any admission control for this reservation.

3.5.  Other RSVP Messages

Processing of PathError, PathTear, ResvError, ResvTear and ResvConf messages is generally straightforward and follows the rules of [RFC2205] (Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, “Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification,” September 1997.). However, for PathTear, ResvError, and ResvConf messages travelling from an ingress PE to an egress PE, these additional rules must be observed:

• The VRF_ID and VPN_LABEL objects must be included in the message;
• The message must be directly addressed to the appropriate PE, without using the IP Router Alert option;
• The VPN_LABEL must be used to determine how to process and forward the message, in the same way that it is used to process and forward Path messages, as described in Section 3.2 (Path Message Processing at Egress PE).

For ResvTear and PathError messages sent from an egress PE to an ingress PE, the following rules must be observed:

• The appropriate VRF_ID object must be included in the message;
• The message must be directly addressed to the appropriate ingress PE;
• The VRF_ID must be used to to determine how to process and forward the message, in the same way that it is used to process and forward Resv messages, as described in Section 3.4 (Resv Processing at Ingress PE).

4.  Admission Control in Provider's Backbone

The preceding section outlines how per-customer reservations can be made over the PE-CE links. This may be sufficient in many situations where the backbone is well engineered with ample capacity and there is no need to perform any sort of admission control in the backbone. However, in some cases where excess capacity cannot be relied upon (e.g., during failures or unanticipated periods of overload) it may be desirable to be able to perform admission control in the backbone on behalf of customer traffic.

Because of the fact that routes to customer addresses are not present in the P routers, along with the concerns of scalability that would arise if per-customer reservations were allowed in the P routers, it is clearly necessary to map the per-customer reservations described in the preceding section onto some sort of aggregate reservations. Furthermore, customer data packets need to be tunneled across the provider backbone just as in normal BGP/MPLS VPN operation.

Given these considerations, a feasible way to achieve the objective of admission control in the backbone is to use the ideas described in [RFC4804] (Le Faucheur, F., “Aggregation of Resource ReSerVation Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels,” February 2007.). MPLS-TE tunnels can be established between PEs as a means to perform aggregate admission control in the backbone.

An MPLS-TE tunnel from an ingress PE to an egress PE can be thought of as a virtual link of a certain capacity. The main change to the procedures described above is that when a Resv is received at the ingress PE, an admission control decision can be performed by checking whether sufficient capacity of that virtual link remains available to admit the new customer reservation. We note also that [RFC4804] (Le Faucheur, F., “Aggregation of Resource ReSerVation Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels,” February 2007.) uses the IF_ID RSVP_HOP object to identify the tunnel across the backbone, rather than the simple RSVP_HOP object described in Section 3.1 (Path Message Processing at Ingress PE). The procedures of [RFC4804] (Le Faucheur, F., “Aggregation of Resource ReSerVation Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels,” February 2007.) should be followed here as well.

To achieve effective admission control in the backbone, there needs to be some way to separate the data plane traffic that has a reservation from that which does not. We assume that packets that are subject to admission control on the core will be given a particular MPLS EXP value, and that no other packets will be allowed to enter the core with this value unless they have passed admission control. Some fraction of link resources will be allocated to queues on core links for packets bearing that EXP value, and the MPLS-TE tunnels will use that resource pool to make their constraint-based routing and admission control decisions. This is all consistent with the principles of aggregate RSVP reservations described in [RFC3175] (Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie, “Aggregation of RSVP for IPv4 and IPv6 Reservations,” September 2001.).

5.  Inter-AS operation

[RFC4364] (Rosen, E. and Y. Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs),” February 2006.) defines three modes of inter-AS operation for MPLS/BGP VPNs, referred to as options A, B and C. In the following sections we describe how the scheme described above can operate in each inter-AS environment.

5.1.  Inter-AS Option A

Option A is quite straightforward. Each ASBR operates like a PE, and the ASBR-ASBR links can be viewed as PE-CE links in terms of admission control. If the procedures defined in Section 3 (Admission Control on PE-CE Links) are enabled on both ASBRs, then CAC may be performed on the inter-ASBR links. In addition, the operator of each AS can independently decide whether or not to perform CAC across his backbone.

5.2.  Inter-AS Option B

To support inter-AS Option B, we require some additional processing of RSVP messages on the ASBRs. Recall that in option B, the VPN label is swapped by each ASBR as a packet goes from one AS to another. Thus, the VPN_LABEL that is placed in a Path message at an ingress PE will be the VPN Label that was advertised by the egress ASBR for the session.

In this scenario, we require ASBRs to store the Path and Resv state (even though the ASBRs may or may not be required to perform admission control). An ASBR that receives a Path with the VPN_LABEL object will look up that label in its forwarding table and find the outgoing label and place that label in the VPN_LABEL object before sending the Path on to the next ASBR. Thus a Path message will make its way from ingress PE, through some number of ASBRs, to an egress PE, with the VPN_LABEL object getting modified at each ASBR.

Now consider the process for getting messages back to the correct VRF in the reverse direction. Because the VRF_ID is locally significant to the PE that first inserted it (and only to that PE) an ASBR cannot simply pass that object along unmodified. Instead the ASBR should create its own locally significant "pseudo-VRF_ID" (pseudo since the ASBR doesn't have VRFs in Option B). The function of that "VRF_ID" is simply to ensure that when the ASBR receives a Resv coming upstream, it can look up the VRF_ID received in the Resv and figure out what PE (or upstream ASBR) to send the Resv to, and what VRF_ID to insert in the Resv it sends. This approach ensures that a Resv will be forwarded back to the correct ingress PE and will arrive there with the appropriate VRF_ID to be processed appropriately.

Note that this approach allows (but does not require) admission control on any segment of the end-end path. (For example, one could do CAC only on CE-PE links, or on ASBR-ASBR links as well, on some of the P cores but not others, etc.)

5.3.  Inter-AS Option C

Option C is also quite straightforward, because there exists an LSP directly from ingress PE to egress PE. In this case, there is no significant difference in operation from the single AS case described in Section 3 (Admission Control on PE-CE Links). Furthermore, if it is desired to provide admission control from PE to PE, it can be done by building an inter-AS TE tunnel and then using the procedures described in Section 4 (Admission Control in Provider's Backbone).

6.  Operation with RSVP disabled

It is often the case that RSVP will not be enabled on the PE-CE links. In such an environment, a customer may reasonably expect that RSVP messages sent into the L3 VPN network should be forwarded just like any other IP datagrams. This transparency is useful when the customer wishes to use RSVP within his own sites or perhaps to perform admission control on the CE-PE links (in CE->PE direction only), without involvement of the PEs. For this reason, a PE SHOULD NOT discard or modify RSVP messages sent towards it from a CE when RSVP is not enabled on the PE-CE links. Similarly a PE SHOULD NOT discard or modify RSVP messages which are destined for one of its attached CEs, even when RSVP is not enabled on those links. Note that the presence of the router alert option in some RSVP messages may cause them to be forwarded outside of the normal forwarding path, but that the guidance of this paragraph still applies in that case. Note also that this guidance applies regardless of whether RSVP-TE is used in some, all, or none of the L3VPN network.

7.  Support for CE-CE RSVP-TE

[I‑D.kumaki‑l3vpn‑e2e‑rsvp‑te‑reqts] (Kumaki, K., “Requirements for supporting Customer RSVP and RSVP-TE Over a BGP/MPLS IP-VPN,” February 2008.) describes a set of requirements for the establishment for CE-CE MPLS LSPs across networks offering an L3VPN service. The requirements specified in that draft are similar to those addressed by this document, in that both address the issue of handling RSVP requests from customers in a VPN context. It is possible that the solution described here could be adapted to meet the requirements of [I‑D.kumaki‑l3vpn‑e2e‑rsvp‑te‑reqts] (Kumaki, K., “Requirements for supporting Customer RSVP and RSVP-TE Over a BGP/MPLS IP-VPN,” February 2008.). A later version of this draft will examine this possibility in detail.

8.  Object Definitions

8.1.  VPN_Label Object

The usage of the VPN_LABEL Object is described in Section 3.1 (Path Message Processing at Ingress PE) and Section 3.2 (Path Message Processing at Egress PE). The VPN_LABEL object should appear in all RSVP messages that contain a SESSION object and are sent from ingress PE to egress PE. The object MUST NOT be included in any RSVP messages that are sent outside of the provider's backbone (except in the inter-AS option B and C cases, as described above, when it may appear on inter-AS links). The format of the object is as follows:

         VPN_LABEL object: Class = TBA, C-Type = 1

           +-------------+-------------+-------------+-------------+
           |  Reserved(12 bits) |  Label (20 bits)                 |
           +-------------+-------------+-------------+-------------+

The Reserved bits must be set to zero on transmission and ignored on receipt.

8.2.  VRF_ID Object

The usage of the VRF_ID Object is described in Section 3 (Admission Control on PE-CE Links). The VRF_ID object is a locally significant opaque value. The object is inserted into RSVP messages that carry a SESSION object, and that travel between the Ingress and Egress PEs. It MUST NOT be included in any RSVP messages that are sent outside of the provider's backbone (except in the inter-AS option B and C cases, as described above, when it may appear on inter-AS links). The format of the object is as follows:

         VRF_ID object: Class = TBA, C-Type = 1

           +-------------+-------------+-------------+-------------+
           |          VRF_ID (32 bits)                             |
           +-------------+-------------+-------------+-------------+

9.  IANA Considerations

This document requires IANA assignment of two new RSVP Class Numbers to accommodate the new objects described in Section 8 (Object Definitions). These should be assigned from the range 0x11bbbbbb, so that they will be ignored but forwarded by routers that do not understand them.

10.  Security Considerations

[RFC4364] (Rosen, E. and Y. Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs),” February 2006.) addresses the security considerations of BGP/MPLS VPNs in general. General RSVP security considerations are addressed in [RFC2205] (Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, “Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification,” September 1997.). To ensure the integrity of RSVP, the RSVP Authentication mechanisms defined in [RFC2747] (Baker, F., Lindell, B., and M. Talwar, “RSVP Cryptographic Authentication,” January 2000.) and [RFC3097] (Braden, R. and L. Zhang, “RSVP Cryptographic Authentication -- Updated Message Type Value,” April 2001.)may be used. These protect RSVP message integrity hop-by-hop and provide node authentication as well as replay protection, thereby protecting against corruption and spoofing of RSVP messages. [I‑D.behringer‑tsvwg‑rsvp‑security‑groupkeying] (Behringer, M. and F. Faucheur, “Applicability of Keying Methods for RSVP Security,” November 2007.) discusses applicability of various keying approaches for RSVP Authentication. We note that the RSVP signaling in MPLS VPN is likely to spread over multiple administrative domains (e.g. the service provider operating the VPN service, and the customers of the service). Therefore the considerations in [I‑D.behringer‑tsvwg‑rsvp‑security‑groupkeying] (Behringer, M. and F. Faucheur, “Applicability of Keying Methods for RSVP Security,” November 2007.) about inter-domain issues are likely to apply.

Beyond those general issues, two specific issues are introduced by this document: resource usage on PEs, and resource usage in the provider backbone. We discuss these in turn.

A customer who makes resource reservations on the CE-PE links for his sites is only competing for link resources with himself, as in standard RSVP, at least in the common case where each CE-PE link is dedicated to a single customer. Thus, from the perspective of the CE-PE links, this draft does not introduce any new security issues. However, because a PE typically serves multiple customers, there is also the possibility that a customer might attempt to use excessive computational resources on a PE (CPU cycles, memory etc.) by sending large numbers of RSVP messages to a PE. In the extreme this could represent a form of denial-of-service attack. In order to prevent such an attack, a PE should have mechanisms to limit the fraction of its processing resources that can be consumed by any one CE or by the set of CEs of a given customer. For example, a PE might implement a form of rate limiting on RSVP messages that it receives from each CE.

The second concern arises only when the service provider chooses to offer resource reservation across the backbone, as described in Section 4 (Admission Control in Provider's Backbone). In this case, the concern may be that a single customer might attempt to reserve a large fraction of backbone capacity, perhaps with a co-ordinated effort from several different CEs, thus denying service to other customers using the same backbone. [RFC4804] (Le Faucheur, F., “Aggregation of Resource ReSerVation Protocol (RSVP) Reservations over MPLS TE/DS-TE Tunnels,” February 2007.) provides some guidance on the security issues when RSVP reservations are aggregated onto MPLS tunnels, which are applicable to the situation described here. We note that a provider may use local policy to limit the amount of resources that can be reserved by a given customer from a particular PE, and that a policy server could be used to control the resource usage of a given customer across multiple PEs if desired.

11.  Acknowledgments

Thanks to Ashwini Dahiya, Prashant Srinivas, Manu Pathak, Yakov Rekhter and Eric Rosen for their many contributions to solving the problems described in this draft.

Appendix A.  Alternatives Considered

At this stage a number of alternatives to the approach described above have been considered. We document some of the approaches considered here to assist future discussion. None of these has been shown to improve upon the approach described above, and the first two seem to have significant drawbacks relative to the approach described above.

Appendix A.1.  GMPLS UNI approach

[RFC4208] (Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter, “Generalized Multiprotocol Label Switching (GMPLS) User-Network Interface (UNI): Resource ReserVation Protocol-Traffic Engineering (RSVP-TE) Support for the Overlay Model,” October 2005.) defines the GMPLS UNI. In Section 7 the operation of the GMPLS UNI in a VPN context is briefly described. This is somewhat similar to the problem tackled in the current document. The main difference is that the GMPLS UNI is primarily aimed at the problem of allowing a CE device to request the establishment of an LSP across the network on the other side of the UNI. Hence the procedures in [RFC4208] (Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter, “Generalized Multiprotocol Label Switching (GMPLS) User-Network Interface (UNI): Resource ReserVation Protocol-Traffic Engineering (RSVP-TE) Support for the Overlay Model,” October 2005.) would lead to the establishment of an LSP across the VPN provider's network for every RSVP request received, which is not desired in this case.

To the extent possible, the approach described in this document is consistent with [RFC4208] (Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter, “Generalized Multiprotocol Label Switching (GMPLS) User-Network Interface (UNI): Resource ReserVation Protocol-Traffic Engineering (RSVP-TE) Support for the Overlay Model,” October 2005.), while filling in more of the details and avoiding the problem noted above.

Appendix A.2.  VRF label approach

Another approach to solving the problems described here involves the use of label switching to ensure that Path, Resv, and other RSVP messages are directed to the appropriate VRF. One challenge with such an approach is that [RFC4364] (Rosen, E. and Y. Rekhter, “BGP/MPLS IP Virtual Private Networks (VPNs),” February 2006.) does not require labels to be allocated for VRFs, only for customer prefixes, and that there is no simple, existing method for advertising the fact that a label is bound to a VRF. If, for example, an ingress PE sent a Path message labelled with a VPN label that was advertised by the egress PE for the prefix that matches the destination address in the Path, there is a risk that the egress PE would simply label-switch the Path directly on to the CE without performing RSVP processing.

A second challenge with this approach is that an IP address needs to be associated with a VRF and used as the PHOP address for the Path message sent from ingress PE to egress PE. That address must be reachable from the egress PE, and exist in the VRF at the ingress PE. Such an address is not always available in today's deployments, so this represents at least a change to existing deployment practices.

Appendix A.3.  VRF label plus VRF address approach

It is possible to create an approach based on that described in the previous section which addresses the main challenges of that approach. The basic approach has two parts: (a) define a new BGP Extended Community to tag a route (and its associated MPLS label) as pointing to a VRF; (b) allocate a "dummy" address to each VRF, specifically to be used for routing RSVP messages. The dummy address (which could be anything, e.g. a loopback of the associated PE) would be used as a PHOP for Path messages and would serve as the destination for Resv messages but would not be imported into VRFs of any other PE.

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses

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