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.
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
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RFC 3210 Applicability Statement for Extensions December 2001
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.
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
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RFC 3210 Applicability Statement for Extensions December 2001
(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.
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RFC 3210 Applicability Statement for Extensions December 2001
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.
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RFC 3210 Applicability Statement for Extensions December 2001
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.
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.
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]).
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
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RFC 3210 Applicability Statement for Extensions December 2001
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.
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.
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.
[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.
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RFC 3210 Applicability Statement for Extensions December 2001
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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