draft-ietf-mpls-crldp-applic-00.txt   draft-ietf-mpls-crldp-applic-01.txt 
MPLS Working Group Jerry Ash A new Request for Comments is now available in online RFC libraries.
Internet Draft AT&T
Expiration Date: March 2000
Muckai Girish
SBC Technology Resources Inc.
Eric Gray
Lucent Technologies
Bilel Jamoussi
Gregory Wright
Nortel Networks Corp.
September 1999
Applicability Statement for CR-LDP
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
The list of Internet-Draft Shadow Directories can be accessed at
This Internet-Draft discusses the applicability of Constraint-Based
LSP Setup using LDP [1]. It discusses possible network applications,
extensions to Label Distribution Protocol (LDP) [2] 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.
Jamoussi, et. al. [Page 1]Internet Draft Applicability Statement for CR-LDP September, 1999
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 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 [1]. 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
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.
Jamoussi, et. al. March 2000 [Page 2]Internet Draft Applicability Statement for CR-LDP September, 1999
2. Applicability of extensions to LDP
To provide for 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
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,
Jamoussi, et. al. March 2000 [Page 3]Internet Draft Applicability Statement for CR-LDP September, 1999
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 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 draft. 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 draft and their comments: Loa Andersson, Peter
Ashwood-Smith, Anoop Ghanwani, Juha Heinanen, Jon Weil, and Adrian
Jamoussi, et. al. March 2000 [Page 4]Internet Draft Applicability Statement for CR-LDP September, 1999
7. References
1 B. Jamoussi, et., al., "Constraint-based LSP Setup Using LDP",
work in progress, September 1999.
2 L. Andersson, et., al., "LDP Specification", work in progress,
June 1999.
3 D. Awduche et., al., "Requirements for Traffic Engineering Over
MPLS", RFC 2702, September 1999.
4 G. Ash, et., al.,"LSP Modification using CR-LDP," work in
progress, July 1999.
5 S. Blake et., al., "An Architecture for Differentiated Services",
RFC-2495, December 1998.
6 S. Shenker, J. Wroclawski, "General Characterization Parameters RFC 3213
for Integrated Service Network Elements" RFC-2215
7 ATM Forum Traffic Management Specification Version 4.1 (AF-TM- Title: Applicability Statement for CR-LDP
0121.000), March 1999. Author(s): J. Ash, M. Girish, E. Gray, B. Jamoussi, G. Wright
Status: Informational
Date: January 2002
Mailbox: gash@att.com, eric.gray@sandburst.com,
gwright@nortelnetworks.com, muckai@atoga.com,
Pages: 7
Characters: 14489
Updates/Obsoletes/SeeAlso: None
SERVICE, ITU (CCITT) Recommendation I.370, 1991.
9 D. Awduche, L. Berger, D. Gan, T. Li, G. Swallow, V. Srinivasan, URL: ftp://ftp.rfc-editor.org/in-notes/rfc3213.txt
"Extensions to RSVP for LSP Tunnels," work in progress, September
8. Author's Addresses 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.
Gerald R. Ash M. K. Girish This document is a product of the Multiprotocol Label Switching
AT&T SBC Technology Resources, Inc. Working Group of the IETF.
Room MT E3-3C37 4698 Willow Road,
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Fax: 732-440-6687 mgirish@tri.sbc.com
Email: gash@att.com
Eric W Gray Bilel Jamoussi This memo provides information for the Internet community. It does
Lucent Technologies, Inc. Nortel Networks Corp. not specify an Internet standard of any kind. Distribution of this
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Durham, NH, 03824-0710 Billerica, MA 01821
Phone: +1 603 659 3386 phone: +1 978-288-4506
Ewgray@lucent.com Jamoussi@nortelnetworks.com
Jamoussi, et. al. March 2000 [Page 5]Internet Draft Applicability Statement for CR-LDP September, 1999 This announcement is sent to the IETF list and the RFC-DIST list.
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