Internet Engineering Task Force
INTERNET-DRAFT
MPLS Working Group Daniel O. Awduche
Expiration Date: January 2000 UUNET (MCI Worldcom)
Alan Hannan
Xipeng Xiao
Frontier Globalcenter
July, 1999
Applicability Statement for Extensions to RSVP for LSP-Tunnels
draft-awduche-mpls-rsvp-tunnel-applicability-00.txt
Status of this Memo
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Abstract
This memo discusses the applicability of "Extensions to RSVP for LSP
Tunnels" [1]. It highlights the protocol's principles of operation
and describes the network context for which it was designed. It
offers guidelines for deployment and indicates known protocol
limitations. This document is intended to accompany the submission of
"Extensions to RSVP for LSP Tunnels" onto the Internet standards
track.
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1.0 Introduction
Service providers and users have expressed a strong need for traffic
engineering capabilities in IP networks (based on Multiprotocol Label
Switching -MPLS-) that 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 be
available from all label switching router vendors, allowing such
devices to interoperate.
The "Extensions to RSVP for LSP tunnels" (RSVP-Tunnel) specification
[1] was developed by the IETF MPLS working group to address this
requirement. RSVP-Tunnel 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-Tunnel specification may have other uses within the Internet.
Two fundamental aspects distinguish the RSVP-Tunnel specification [1]
from the original RSVP protocol [3].
The first is the fact that the RSVP-Tunnel 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; whereas the original RSVP specification [3] 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-Tunnel
specification generalizes the concept of "RSVP flow." The RSVP-Tunnel
specification essentially allows an RSVP session to consist of an
arbitrary aggregation of traffic (based on local policies) between
the origination 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-Tunnel specification, however,
a session is implicitly defined as the set of packets that are
assigned the same MPLS label value at the origination 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
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sessions) does not increase proportionally with the number of flows
in the network. Therefore, the RSVP-Tunnel 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.
This applicability statement does not preclude the use of other
signaling and label distribution protocols for the traffic
engineering application in MPLS based IP networks. Service providers
are free to deploy whatever signaling protocol that meets their
needs.
2.0 Technical Overview of Extensions to RSVP for LSP Tunnels
The RSVP-Tunnel specification extends the original RSVP protocol with
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 (such as 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 node
abstraction, and (8) preemption options that are administratively
controllable.
The RSVP-Tunnel specification introduces several new RSVP objects,
including: LABEL-REQUEST object, RECORD-ROUTE object, LABEL object,
EXPLICIT-ROUTE object, and new SESSION objects. New error messages
are defined to provide notification of exception conditions. All the
new objects defined in RSVP-Tunnel 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.
Informally, establishment of an LSP-tunnel proceeds in the following
way: First, the origination node of the LSP-tunnel creates an RSVP
Path message and inserts a LABEL-REQUEST object into it. Optionally,
an EXPLICIT-ROUTE object, a RECORD-ROUTE object, and a
SESSION_ATTRIBUTE object may also be inserted into the path message.
The LABEL-REQUEST object indicates that a label binding is requested;
the EXPLICIT-ROUTE object depicts the explicit route for the LSP-
tunnel as a sequence of abstract nodes; the RECORD-ROUTE object
specifies that a path vector record of the route traversed is
required; finally, the SESSION_ATTRIBUTE object is used for session
identification and diagnosis. When the Path message reaches the
egress node of the LSP-tunnel, a Resv message is created and a LABEL
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object containing an MPLS label is inserted into the Resv message. As
the Resv message propagates to the origination node, in reverse
direction along the path traversed by the Path message, each node
uses the MPLS label in the LABEL object from its downstream neighbor
as outgoing label for the LSP-tunnel, and inserts its own LABEL
object before propagating the Resv message upstream. In this way,
labels are allocated sequentially all the way from the egress node of
the LSP-tunnel to the origination node, whence the LSP-tunnel becomes
established.
3.0 Applicability of Extensions to RSVP for LSP Tunnels
Use of RSVP-Tunnel 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,
or for control purposes, or for both.
For the measurement application, an LSP-tunnel can be used to capture
various path statistics between its endpoints. For example, an LSP-
tunnel can be instantiated, with or without bandwidth allocation,
purely for the purpose of monitoring traffic flow statistics between
two label switching routers.
For the control application, LSP-tunnels can be used to forward
subsets of traffic through paths that are independent of routes
computed by conventional IGP SPF algorithms. This provides
significant control over the routing function, allowing policies to
be implemented that 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-Tunnel may be
augmented with an ancillary constraint-based routing entity that
computes 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, which 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. Indeed, the RSVP-Tunnel
specification may be deployed in certain contexts without any of
these additional components.
The capability to monitor point to point traffic statistics between
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two routers and the capability to control the forwarding paths of
subsets of traffic through a given network topology together make the
RSVP-Tunnel specifications applicable and useful for traffic
engineering within service provider networks.
These capabilities also make the RSVP-Tunnel applicable, in some
contexts, as a component of an MPLS based VPN provisioning framework.
It is to be noted that the MPLS architecture [4] states clearly that
no single label distribution protocol is assumed for the MPLS
technology. Therefore, 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-Tunnel, there should be well defined
administrative policies governing the selection of nodes that will
serve as endpoints for LSP-tunnels. Furthermore, in 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 should also guide the amount of bandwidth
that should be allocated (if any) to each LSP-tunnel. Such policies
may take into account traffic statistics derived from the operational
network as well as other factors.
5.0 Limitations
The RSVP-Tunnel specification supports only unicast LSP-tunnels.
Multicast LSP-tunnels are not supported.
The RSVP-Tunnel 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
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state of established LSP-tunnels. This issue is addressed in several
proposals that have been submitted to the RSVP working group (see
e.g. [6]).
6.0 Conclusion
This document discussed the applicability of the "Extensions to RSVP
for LSP Tunnels" specification. The specification introduced several
enhancements to the RSVP protocol, making it applicable in contexts
in which the original RSVP protocol would have been inappropriate.
One context in which the RSVP-Tunnel 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-Tunnel
specification adds new opaque objects to RSVP, hence 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 References
[1] D. Awduche, L. Berger, D. Gan, T. Li, G. Swallow,
V. Srinivasan, "Extensions to RSVP for LSP Tunnels,"
Work in Progress.
[2] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, J. McManus,
"Requirements for Traffic Engineering Over MPLS,"
Work in Progress.
[3] Braden, R. et al., "Resource ReSerVation Protocol (RSVP) --
Version 1, Functional Specification", RFC 2205, September 1997.
[4] E. Rosen, A. Viswanathan, R. Callon, "A Proposed Architecture
for MPLS", Work in Progress.
[5] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow,
A. Viswanathan, "A Framework for Multiprotocol Label
Switching", Work in Progress.
[6] L. Berger, D. Gan, G. Swallow, "RSVP Refresh Reduction
Extensions," Work in Progress.
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10.0 AUTHORS' ADDRESSES
Daniel O. Awduche
UUNET (MCI Worldcom)
3060 Williams Drive
Fairfax, VA 22031
Email: awduche@uu.net
Voice: +1 703-208-5277
Alan Hannan
Frontier Globalcenter
1154 E. Arques Ave.
Sunnyvale, CA 94086
Email: alan@globalcenter.net,
Voice: +1 408-328-4891
Xipeng Xiao
Frontier Globalcenter
1154 E. Arques Ave.
Sunnyvale, CA 94086
Email: xipeng@globalcenter.net,
Voice: +1 408-328-480
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