RSVP-TE Applicability: Applicability Statement for Extensions to RSVP for LSP-Tunnels

Network Working Group                                         D. Awduche
Request for Comments: 3210                                Movaz Networks
Category: Informational                                       A.  Hannan
                                                             Routingloop
                                                                 X. Xiao
                                                                Photuris
                                                           December 2001


     Applicability Statement for Extensions to RSVP for LSP-Tunnels

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This memo discusses the applicability of "Extensions to RSVP
   (Resource ReSerVation Protocol) for LSP Tunnels".  It highlights the
   protocol's principles of operation and describes the network context
   for which it was designed.  Guidelines for deployment are offered and
   known protocol limitations are indicated.  This document is intended
   to accompany the submission of "Extensions to RSVP for LSP Tunnels"
   onto the Internet standards track.

1.0 Introduction

   Service providers and users have indicated that there is a great need
   for traffic engineering capabilities in IP networks.  These traffic
   engineering capabilities can be based on Multiprotocol Label
   Switching (MPLS) and can be implemented on label switching routers
   (LSRs) from different vendors that interoperate using a common
   signaling and label distribution protocol.  A description of the
   requirements for traffic engineering in MPLS based IP networks can be
   found in [2].  There is, therefore, a requirement for an open, non-
   proprietary, standards based signaling and label distribution
   protocol for the MPLS traffic engineering application that will allow
   label switching routers from different vendors to interoperate.

   The "Extensions to RSVP for LSP tunnels" (RSVP-TE) specification [1]
   was developed by the IETF MPLS working group to address this
   requirement.  RSVP-TE is a composition of several related proposals
   submitted to the IETF MPLS working group.  It contains all the
   necessary objects, packet formats, and procedures required to
   establish and maintain explicit label switched paths (LSPs).
   Explicit LSPs are foundational to the traffic engineering application
   in MPLS based IP networks.  Besides the traffic engineering
   application, the RSVP-TE specification may have other uses within the
   Internet.

   This memo describes the applicability of the RSVP-TE specifications
   [1].  The protocol's principles of operation are highlighted, the
   network context for which it was developed is described, guidelines
   for deployment are offered, and known protocol limitations are
   indicated.

   This applicability statement concerns only the use of RSVP to set up
   unicast LSP-tunnels.  It is noted that not all of the features
   described in RFC2205 [3] are required to support the instantiation
   and maintenance of LSP-tunnels.  Aspects related to the support of
   other features and capabilities of RSVP by an implementation that
   also supports LSP-tunnels are beyond the scope of this document.
   However, support of such additional features and capabilities should
   not introduce new security vulnerabilities in environments that only
   use RSVP to set up LSP-tunnels.

   This applicability statement does not preclude the use of other
   signaling and label distribution protocols for the traffic
   engineering application in MPLS based networks.  Service providers
   are free to deploy whatever signaling protocol that meets their
   needs.

   In particular, CR-LDP [6] and RSVP-TE [1] are two signaling protocols
   that perform similar functions in MPLS networks.  There is currently
   no consensus on which protocol is technically superior.  Therefore,
   network administrators should make a choice between the two based
   upon their needs and particular situation.

2.0 Technical Overview of Extensions to RSVP for LSP Tunnels

   The RSVP-TE specification extends the original RSVP protocol by
   giving it new capabilities that support the following functions in an
   MPLS domain:

     (1) downstream-on-demand label distribution
     (2) instantiation of explicit label switched paths
     (3) allocation of network resources (e.g., bandwidth) to
         explicit LSPs
     (4) rerouting of established LSP-tunnels in a smooth fashion
         using the concept of make-before-break

     (5) tracking of the actual route traversed by an LSP-tunnel
     (6) diagnostics on LSP-tunnels
     (7) the concept of nodal abstraction
     (8) preemption options that are administratively controllable

   The RSVP-TE specification introduces several new RSVP objects,
   including the LABEL-REQUEST object, the RECORD-ROUTE object, the
   LABEL object, the EXPLICIT-ROUTE object, and new SESSION objects.
   New error messages are defined to provide notification of exception
   conditions.  All of the new objects defined in RSVP-TE are optional
   with respect to the RSVP protocol, except the LABEL-REQUEST and LABEL
   objects, which are both mandatory for the establishment of LSP-
   tunnels.

   Two fundamental aspects distinguish the RSVP-TE specification [1]
   from the original RSVP protocol [3].

   The first distinguishing aspect is the fact that the RSVP-TE
   specification [1] is intended for use by label switching routers (as
   well as hosts) to establish and maintain LSP-tunnels and to reserve
   network resources for such LSP-tunnels.  The original RSVP
   specification [3], on the other hand, was intended for use by hosts
   to request and reserve network resources for micro-flows.

   The second distinguishing aspect is the fact that the RSVP-TE
   specification generalizes the concept of "RSVP flow." The RSVP-TE
   specification essentially allows an RSVP session to consist of an
   arbitrary aggregation of traffic (based on local policies) between
   the originating node of an LSP-tunnel and the egress node of the
   tunnel.  To be definite, in the original RSVP protocol [3], a session
   was defined as a data flow with a particular destination and
   transport layer protocol.  In the RSVP-TE specification, however, a
   session is implicitly defined as the set of packets that are assigned
   the same MPLS label value at the originating node of an LSP-tunnel.
   The assignment of labels to packets can be based on various criteria,
   and may even encompass all packets (or subsets thereof) between the
   endpoints of the LSP-tunnel.  Because traffic is aggregated, the
   number of LSP-tunnels (hence the number of RSVP sessions) does not
   increase proportionally with the number of flows in the network.
   Therefore, the RSVP-TE specification [1] addresses a major scaling
   issue with the original RSVP protocol [3], namely the large amount of
   system resources that would otherwise be required to manage
   reservations and maintain state for potentially thousands or even
   millions of RSVP sessions at the micro-flow granularity.

   The reader is referred to [1] for a technical description of the
   RSVP-TE protocol specification.

3.0 Applicability of Extensions to RSVP for LSP Tunnels

   Use of RSVP-TE is appropriate in contexts where it is useful to
   establish and maintain explicit label switched paths in an MPLS
   network.  LSP-tunnels may be instantiated for measurement purposes
   and/or for routing control purposes.  They may also be instantiated
   for other administrative reasons.

   For the measurement application, an LSP-tunnel can be used to capture
   various path statistics between its endpoints.  This can be
   accomplished by associating various performance management and fault
   management functions with an LSP-tunnel, such as packet and byte
   counters.  For example, an LSP-tunnel can be instantiated, with or
   without bandwidth allocation, solely for the purpose of monitoring
   traffic flow statistics between two label switching routers.

   For the routing control application, LSP-tunnels can be used to
   forward subsets of traffic through paths that are independent of
   routes computed by conventional Interior Gateway Protocol (IGP)
   Shortest Path First (SPF) algorithms.  This feature introduces
   significant flexibility into the routing function and allows policies
   to be implemented that can result in the performance optimization of
   operational networks.  For example, using LSP-tunnels, traffic can be
   routed away from congested network resources onto relatively
   underutilized ones.  More generally, load balancing policies can be
   actualized that increase the effective capacity of the network.

   To further enhance the control application, RSVP-TE may be augmented
   with an ancillary constraint-based routing entity.  This entity may
   compute explicit routes based on certain traffic attributes, while
   taking network constraints into account.  Additionally, IGP link
   state advertisements may be extended to propagate new topology state
   information.  This information can be used by the constraint-based
   routing entity to compute feasible routes.  Furthermore, the IGP
   routing algorithm may itself be enhanced to take pre-established
   LSP-tunnels into consideration while building the routing table.  All
   these augmentations are useful, but not mandatory.  In fact, the
   RSVP-TE specification may be deployed in certain contexts without any
   of these additional components.

   The capability to monitor point to point traffic statistics between
   two routers and the capability to control the forwarding paths of
   subsets of traffic through a given network topology together make the
   RSVP-TE specifications applicable and useful for traffic engineering
   within service provider networks.

   These capabilities also make the RSVP-TE applicable, in some
   contexts, as a component of an MPLS based VPN provisioning framework.

   It is significant that the MPLS architecture [4] states clearly that
   no single label distribution protocol is assumed for the MPLS
   technology.  Therefore, as noted in the introduction, this
   applicability statement does not (and should not be construed to)
   prevent a label switching router from implementing other signaling
   and label distribution protocols that also support establishment of
   explicit LSPs and traffic engineering in MPLS networks.

4.0 Deployment and Policy Considerations

   When deploying RSVP-TE, there should be well defined administrative
   policies governing the selection of nodes that will serve as
   endpoints for LSP-tunnels.  Furthermore, when devising a virtual
   topology for LSP-tunnels, special consideration should be given to
   the tradeoff between the operational complexity associated with a
   large number of LSP-tunnels and the control granularity that large
   numbers of LSP-tunnels allow.  Stated otherwise, a large number of
   LSP-tunnels allows greater control over the distribution of traffic
   across the network, but increases network operational complexity.  In
   large networks, it may be advisable to start with a simple LSP-tunnel
   virtual topology and then introduce additional complexity based on
   observed or anticipated traffic flow patterns.

   Administrative policies may also guide the amount of bandwidth to be
   allocated (if any) to each LSP-tunnel.  Policies of this type may
   take into consideration empirical traffic statistics derived from the
   operational network in addition to other factors.

5.0 Limitations

   The RSVP-TE specification supports only unicast LSP-tunnels.
   Multicast LSP-tunnels are not supported.

   The RSVP-TE specification supports only unidirectional LSP-tunnels.
   Bidirectional LSP-tunnels are not supported.

   The soft state nature of RSVP remains a source of concern because of
   the need to generate refresh messages periodically to maintain the
   state of established LSP-tunnels.  This issue is addressed in several
   proposals that have been submitted to the RSVP working group (see
   e.g. [5]).

6.0 Conclusion

   The applicability of the "Extensions to RSVP for LSP Tunnels"
   specification has been discussed in this document.  The specification
   introduced several enhancements to the RSVP protocol, which make it
   applicable in contexts in which the original RSVP protocol would have
   been inappropriate.  One context in which the RSVP-TE specification
   is particularly applicable is in traffic engineering in MPLS based IP
   networks.

7.0 Security Considerations

   This document does not introduce new security issues.  The RSVP-TE
   specification adds new opaque objects to RSVP.  Therefore, the
   security considerations pertaining to the original RSVP protocol
   remain relevant.  When deployed in service provider networks, it is
   mandatory to ensure that only authorized entities are permitted to
   initiate establishment of LSP-tunnels.

8.0 Acknowledgments

   The authors gratefully acknowledge the useful comments received from
   the following individuals during initial review of this memo in the
   MPLS WG mailing list: Eric Gray, John Renwick, and George Swallow.

9.0 References

   [1]   Awduche, D., Berger, L., Gan, D., Li, T., Swallow, G. and V.
         Srinivasan, "RSVP-TE: Extensions to RSVP for LSP Tunnels," RFC
         3209, December 2001.

   [2]   Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
         McManus, "Requirements for Traffic Engineering Over MPLS," RFC
         2702, September 1999.

   [3]   Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
         "Resource ReSerVation Protocol (RSVP) -- Version 1, Functional
         Specification", RFC 2205, September 1997.

   [4]   Rosen, E., Viswanathan, A. and R. Callon, "A Proposed
         Architecture for MPLS", RFC 3031, January 2001.

   [5]   Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F. and S.
         Molendini, "RSVP Refresh Overhead Reduction Extensions", RFC
         2961, April 2001.

   [6]   Jamoussi, B. et al, "Constraint-Based LSP Setup using LDP,"
         Work in Progress.

10.0 Authors' Addresses

   Daniel O. Awduche
   Movaz Networks
   7926 Jones Branch Drive, Suite 615
   McLean, VA 22102

   EMail: awduche@movaz.com
   Voice: +1 703-298-5291

   Alan Hannan
   RoutingLoop
   112 Falkirk Court
   Sunnyvale, CA 94087

   EMail: alan@routingloop.com
   Voice: +1 408 666-2326

   XiPeng Xiao
   Photuris Inc.
   2025 Stierlin Ct.
   Mountain View, CA 94043

   EMail: xxiao@photuris.com
   Voice: +1 650-919-3215

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