Network Working Group J. Ash
Request for Comments: 3213 AT&T
Category: Informational M. Girish
Atoga Systems
E. Gray
Sandburst
B. Jamoussi
G. Wright
Nortel Networks Corp.
January 2002
Applicability Statement for CR-LDP
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 (2002). All Rights Reserved.
Abstract
This document discusses the applicability of Constraint-Based LSP
Setup using LDP. It discusses possible network applications,
extensions to Label Distribution Protocol (LDP) required to implement
constraint-based routing, guidelines for deployment and known
limitations of the protocol. This document is a prerequisite to
advancing CR-LDP on the standards track.
1. IntrodUCtion
As the Internet evolves, additional capabilities are required to
ensure proper treatment of data [3], voice, video and other delay
sensitive traffic [4]. MPLS enhances source routing and allows for
certain techniques, used in circuit switching, in IP networks. This
permits a scalable approach to handling these diverse transmission
requirements. CR-LDP [1] is a simple, scalable, open, non-
proprietary, traffic engineering signaling protocol for MPLS IP
networks.
CR-LDP provides mechanisms for establishing eXPlicitly routed Label
Switched Paths (LSPs). These mechanisms are defined as extensions to
LDP [2]. Because LDP is a peer-to-peer protocol based on the
establishment and maintenance of TCP sessions, the following natural
benefits exist:
CR-LDP messages are reliably delivered by the underlying TCP, and
State information associated with explicitly routed LSPs does not
require periodic refresh.
CR-LDP messages are flow controlled (throttled) through TCP.
CR-LDP is defined for the specific purpose of establishing and
maintaining explicitly routed LSPs. Additional optional capabilities
included have minimal impact on system performance and requirements
when not in use for a specific explicitly routed LSP. Optional
capabilities provide for negotiation of LSP services and traffic
management parameters over and above best-effort packet delivery
including bandwidth allocation, setup and holding priorities. CR-LDP
optionally allows these parameters to be dynamically modified without
disruption of the operational (in-service) LSP [4].
CR-LDP allows the specification of a set of parameters to be signaled
along with the LSP setup request. Moreover, the network can be
provisioned with a set of edge traffic conditioning functions (which
could include marking, metering, policing and shaping). This set of
parameters along with the specification of edge conditioning
functions can be shown to be adequate and powerful enough to
describe, characterize and parameterize a wide variety of QoS
scenarios and services including IP differentiated services [5],
integrated services [6], ATM service classes [7], and frame relay
[8].
CR-LDP is designed to adequately support the various media types that
MPLS was designed to support (ATM, FR, Ethernet, PPP, etc.). Hence,
it will work equally well for Multi-service switched networks, router
networks, or hybrid networks.
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 meets their needs.
In particular CR-LDP and RSVP-TE [9] 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. Applicability of RSVP-TE
is described in [10].
2. Applicability of extensions to LDP
To provide support of additional LSP services, CR-LDP extensions are
defined in such a way as to be directly translatable to objects and
messages used in other protocols defined to provide similar services
[9]. Implementations can take advantage of this fact to:
Setup LSPs for provision of an aggregate service associated with
the services being provided via these other protocols.
Directly translate protocol messages to provide services defined
in a non-CR-LDP portion of the network.
Describe, characterize and parameterize a wide variety of QoS
scenarios and services including IP differentiated services,
integrated services, ATM service classes, and frame relay.
Steady state information required for proper maintenance of an LSP
may be as little as 200 bytes or less. It is not unreasonable to
anticipate that CR-LDP implementations may support in excess of one
hundred thousand or one million LSPs switched through a single Label
Switching Router (LSR) under fairly stable conditions.
Because CR-LDP provides for low overhead per LSP - both in terms of
needed state information and control traffic - CR-LDP is applicable
in those portions of the Internet where very large numbers of LSPs
may need to be switched at each LSR. An example of this would be
large backbone networks using MPLS exclusively to transport very
large numbers of traffic streams between a moderately large number of
MPLS edge nodes.
CR-LDP may also be applicable as a mediating service between networks
providing similar service extensions using widely varying signaling
models.
3. Implementation and deployment considerations in relation to LDP
LDP specifies the following label distribution and management modes
(which can be combined in various logical ways described in LDP):
. Downstream On Demand label distribution
. Downstream Unsolicited label distribution
. Independent Label Distribution Control
. Ordered Label Distribution Control
. Conservative Label Retention Mode
. Liberal Label Retention Mode
The applicability of LDP is described in [11].
In networks where only Traffic Engineered LSPs are required, the CR-
LDP implementation and deployment does NOT require all the
functionality defined in the LDP specification. The basic Discovery,
Session, and Notification messages are required. However, CR-LDP
requires one specific combination of the label distribution modes:
. Downstream On Demand Ordered label distribution and
conservative Label Retention Mode
Although CR-LDP is defined as an extension to LDP, support for
Downstream Unsolicited Label Advertisement and Independent Control
modes is not required for support of Strict Explicit Routes. In
addition, implementations of CR-LDP MAY be able to support Loose
Explicit Routes via the use of 'Abstract Nodes' and/or 'Hierarchical
Explicit Routing', without using LDP for hop-by-hop LSP setup.
CR-LDP also includes support for loose explicit routes. Use of this
capability allows the network operator to define an 'explicit path'
through portions of their network with imperfect knowledge of the
entire network topology. Proper use of this capability may also
allow CR-LDP implementations to inter-operate with 'vanilla' LDP
implementations - particularly if it is desired to set up an
explicitly routed LSP for best-effort packet delivery via a loosely
defined path.
Finally, in networks where both Routing Protocol-driven LSPs (a.k.a.
hop-by-hop LSPs) and Traffic Engineered LSPs are required, a single
protocol (LDP, with the extensions defined in CR-LDP) can be used for
both TE and Hop-by-Hop LSPs. New protocols do not have to be
introduced in the network to provide TE-LSP signaling.
4. Limitations
CR-LDP specification only supports point-to-point LSPs. Multi-
point-to-point and point-to-multi-point are for further study (FFS).
CR-LDP specification only supports unidirectional LSP setup. Bi-
directional LSP setup is FFS.
CR-LDP specification only supports a unique label allocation per LSP
setup. Multiple label allocations per LSP setup are FFS.
5. Security Considerations
No additional security issues are introduced in this document. As an
extension to LDP, CR-LDP shares the security concerns associated with
LDP.
6. Acknowledgements
The authors would like to thank the following people for their
careful review of the document and their comments: Loa Andersson,
Peter Ashwood-Smith, Anoop Ghanwani, Juha Heinanen, Jon Weil and
Adrian Farrel.
7. References
[1] Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu, L.,
Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish, M.,
Gray, E., Heinanen, J., Kilty, T. and A. Malis, "Constraint-
based LSP Setup Using LDP", RFC3212, January 2002.
[2] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
Thomas, "LDP Specification", RFC3036, January 2001.
[3] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
McManus, "Requirements for Traffic Engineering Over MPLS", RFC
2702, September 1999.
[4] Ash, B., Lee, Y., Ashwood-Smith, P., Jamoussi, B., Fedyk, D.,
Skalecki, D. and L. Li, "LSP Modification using CR-LDP", RFC
3214, January 2002.
[5] Blake S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.
Weiss, "An Architecture for Differentiated Services", RFC2475,
December 1998.
[6] Shenker, S. and J. Wroclawski, "General Characterization
Parameters for Integrated Service Network Elements", RFC2215,
September 1997.
[7] ATM Forum Traffic Management Specification Version 4.1 (AF-TM-
0121.000), March 1999.
[8] CONGESTION MANAGEMENT FOR THE ISDN FRAME RELAYING BEARER
SERVICE, ITU (CCITT) Recommendation I.370, 1991.
[9] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V. and G.
Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC
3209, December 2001.
[10] Awduche, D., Hannan, A. and X. Xiao, "Applicability Statement
for Extensions to RSVP for LSP-Tunnels", RFC3210, December
2001.
[11] Thomas, B. and E. Gray, "LDP Applicability", RFC3037, January
2001.
8. Author's Addresses
Gerald R. Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748
USA
Phone: 732-420-4578
Fax: 732-368-8659
EMail: gash@att.com
Eric Gray
Sandburst
600 Federal Drive
Andover, MA 01810
Phone: (978) 689-1610
EMail: eric.gray@sandburst.com
Gregory Wright
Nortel Networks Corp.
P O Box 3511 Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1 613 765-7912
EMail: gwright@nortelnetworks.com
M. K. Girish
Atoga Systems
49026 Milmont Drive
Fremont, CA 94538
EMail: muckai@atoga.com
Bilel Jamoussi
Nortel Networks Corp.
600 Technology Park Drive
Billerica, MA 01821
USA
phone: +1 978-288-4506
EMail: Jamoussi@nortelnetworks.com
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