Internet DRAFT - draft-awduche-mpls-rsvp-tunnel-applicability
draft-awduche-mpls-rsvp-tunnel-applicability
Internet Engineering Task Force
INTERNET-DRAFT
MPLS Working Group Daniel O. Awduche
Expiration Date: March 2000 UUNET (MCI Worldcom)
Alan Hannan
Xipeng Xiao
Frontier Globalcenter
September, 1999
Applicability Statement for Extensions to RSVP for LSP-Tunnels
draft-awduche-mpls-rsvp-tunnel-applicability-01.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.
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.
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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 may be
available from all label switching router vendors, which allow 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.
This memo describes the applicability of the RSVP-Tunnel
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.
Two fundamental aspects distinguish the RSVP-Tunnel specification [1]
from the original RSVP protocol [3].
The first distinguishing aspect 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. 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-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
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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
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 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 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 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
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The RSVP-Tunnel 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-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 object containing an MPLS label
is inserted into the Resv message. As the Resv message propagates to
the origination node (in the 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. Each node inserts its own LABEL object before propagating
the Resv message upstream. This way, labels are allocated
sequentially all the way from the egress node of the LSP-tunnel to
the origination node. It is when the Resv message reaches the
origination node that 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
and/or for 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
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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 Interior Gateway Protocol (IGP) Shortest
Path First (SPF) algorithms. This feature provides significant
control over the routing function and allows 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. 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-Tunnel 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-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 significant 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
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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 should also guide the amount of bandwidth to
be allocated (if any) to each LSP-tunnel. Policies of this type may
take into consideration traffic statistics derived from the
operational network in addition to 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
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
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-Tunnel
specification is particularly applicable is in traffic engineering in
MPLS based IP networks.
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7.0 Security Considerations
This document does not introduce new security issues. The RSVP-Tunnel
specification adds new opaque objects to RSVP and so 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] 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
141 Caspian Court,
Sunnyvale, CA 94089
Email: alan@globalcenter.net,
Voice: +1 408-543-4891
Xipeng Xiao
Frontier Globalcenter
141 Caspian Court,
Sunnyvale, CA 94089
Email: xipeng@globalcenter.net,
Voice: +1 408-543-4801
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