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Network Working Group L. Andersson, Ed.
Request for Comments: 5037 Acreo AB
Category: Informational I. Minei, Ed.
Juniper Networks
B. Thomas, Ed.
Cisco Systems, Inc.
October 2007
Experience with the Label Distribution Protocol (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.
Abstract
The purpose of this memo is to document how some of the requirements
specified in RFC 1264 for advancing protocols developed by working
groups within the IETF Routing Area to Draft Standard have been
satisfied by LDP (Label Distribution Protocol). Specifically, this
report documents operational experience with LDP, requirement 5 of
section 5.0 in RFC 1264.
Table of Contents
1. Introduction ....................................................2
2. Operational Experience ..........................................2
2.1. Environment and Duration ...................................2
2.2. Applications and Motivation ................................3
2.3. Protocol Features ..........................................3
2.4. Security Concerns ..........................................4
2.5. Implementations and Inter-Operability ......................4
2.6. Operational Experience .....................................4
3. Security Considerations .........................................5
4. Acknowledgments .................................................5
5. References ......................................................6
5.1. Normative References .......................................6
5.2. Informative References .....................................6
Andersson, et al. Informational [Page 1]
RFC 5037 Experience with the LDP Protocol October 2007
1. Introduction
The purpose of this memo is to document how some of the requirements
specified in [RFC1264] for advancing protocols developed by working
groups within the IETF Routing Area to Draft Standard have been
satisfied by LDP. Specifically, this report documents operational
experience with LDP, requirement 5 of section 5.0 in RFC 1264.
LDP was originally published as [RFC3036] in January 2001. It was
produced by the MPLS Working Group of the IETF and was jointly
authored by Loa Andersson, Paul Doolan, Nancy Feldman, Andre
Fredette, and Bob Thomas. It has since been obsoleted by [RFC5036].
2. Operational Experience
This section discusses operational experience with the protocol. The
information is based on a survey sent to the MPLS Working Group in
October 2004. The questionnaire can be found in the MPLS Working
Group mail archives for October 2004.
11 responses were received, all but 2 requesting confidentiality.
The survey results are summarized to maintain confidentiality. The
networks surveyed span different geographic locations: US, Europe,
and Asia. Both academic and commercial networks responded to the
survey.
2.1. Environment and Duration
The size of the deployments ranges from less than 20 Label Switching
Routers (LSRs) to over 1000 LSRs. Eight out of the 11 deployments
use LDP in the edge and the core, two on the edge only, and one in
the core only.
Sessions exist to peers discovered via both the basic and the
extended discovery mechanisms. In half the cases, more than one
adjacency (and as many as four adjacencies) are maintained per
session. The average number of LDP sessions on an LSR ranges from
under 10 to just over 80. The responses are spread out as follows:
under 10: 4 responses, 20-50: 4 responses, and over 80: 1 response.
In the surveyed networks, the time LDP has been deployed ranges from
under 1 year to over 4 years. The responses are spread out as
follows: under 1 year: 3 responses, 2 years: 2 responses, 3 years: 3
responses, and over 4 years: 3 responses.
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RFC 5037 Experience with the LDP Protocol October 2007
2.2. Applications and Motivation
Nine of the 11 responses list Layer 3 Virtual Private Networks
(L3VPNs) as the application driving the LDP deployment in the
network.
The list of applications is as follows: L3VPNs: 9, pseudowires: 4
current (and one planned deployment), L2VPNs: 4, forwarding based on
labels: 2, and BGP-free core: 1.
There are two major options for label distribution protocols, LDP and
Resource Reservation Protocol-Traffic Engineering (RSVP-TE). One of
the key differences between the two is that RSVP-TE has support for
traffic engineering, while LDP does not. The reasons cited for
picking LDP as the label distribution protocol are:
o The deployment does not require traffic engineering - 6
o Inter-operability concerns if a different protocol is used - 5
o Equipment vendor only supports LDP - 5
o Ease of configuration - 4
o Ease of management - 3
o Scalability concerns with other protocols - 3
o Required for a service offering of the service provider - 1
2.3. Protocol Features
All deployments surveyed use the Downstream Unsolicited Label
Distribution mode. All but one deployment use Liberal Label
retention (one uses conservative).
LSP setup is established with both independent and Ordered Control.
Five of the deployments use both control modes in the same network.
The number of LDP Forwarding Equivalence Classes (FECs) advertised
and LDP routes installed falls in one of two categories: 1) roughly
the same as the number of LSRs in the network and 2) roughly the same
as the number of IGP routes in the network. Of the 8 responses that
were received, 6 were in the first category and 2 in the second.
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RFC 5037 Experience with the LDP Protocol October 2007
2.4. Security Concerns
A security concern was raised by one of the operators with respect to
the lack of a mechanism for securing LDP Hellos.
2.5. Implementations and Inter-Operability
Eight of the 11 responses state that more than one implementation
(and as many as four different ones) are deployed in the same
network.
The consensus is that although implementations differ, no inter-
operability issues exist. The challenges listed by providers running
multiple implementations are:
o Different flexibility in picking for which FECs to advertise
labels.
o Different flexibility in setting transport and LDP router-id
addresses.
o Different default utilization of LDP labels for traffic
resolution. Some vendors use LDP for both VPN and IPv4 traffic
forwarding, while other vendors allow only VPN traffic to
resolve via LDP. The challenge is to restrict the utilization
of LDP labels to VPN traffic in a mixed-vendor environment.
o Understanding the differences in the implementations.
2.6. Operational Experience
In general, operators reported stable implementations and steady
improvement in resiliency to failure and convergence times over the
years. Some operators reported that no issues were found with the
protocol since deploying.
The operational issues reported fall in three categories:
1. Configuration issues. Both the session and adjacency endpoints
must be allowed by the firewall filters. Misconfiguration of
the filters causes sessions to drop (if already established) or
not to establish.
2. Vendor bugs. These include traffic blackholing, unnecessary
label withdrawals and changes, session resets, and problems
migrating from older versions of the technology. Most reports
stated that the problems reported occurred in early versions of
the implementations.
Andersson, et al. Informational [Page 4]
RFC 5037 Experience with the LDP Protocol October 2007
3. Protocol issues.
- The synchronization required between LDP and the IGP was
listed as the main protocol issue. Two issues were
reported: 1) slow convergence, due to the fact that LDP
convergence time is tied to the IGP convergence time, and 2)
traffic blackholing on a link-up event. When an interface
comes up, the LDP session may come up slower than the IGP
session. This results in dropping MPLS traffic for a link-
up event (not a failure but a restoration). This issue is
described in more detail in [LDP-SYNC].
- Silent failures. Failure not being propagated to the head
end of the LSP when setting up LSPs using independent
control.
3. Security Considerations
This document is a survey of experiences from deployment of LDP
implementations; it does not specify any protocol behavior. Thus,
security issues introduced by the document are not discussed.
4. Acknowledgments
The editors would like to thank the operators who participated in the
survey for their valuable input: Shane Amante, Niclas Comstedt, Bruno
Decraene, Mourad Kaddache, Kam Lee Yap, Lei Wang, and Otto Kreiter.
Not all who participated are listed here, due to confidentiality
requests. Those listed have given their consent.
Also, a big thank you to Scott Bradner, who acted as an independent
third party ensuring anonymity of the responses.
The editors would like to thank Rajiv Papneja, Halit Ustundag, and
Loa Andersson for their input to the survey questionnaire.
Andersson, et al. Informational [Page 5]
RFC 5037 Experience with the LDP Protocol October 2007
5. References
5.1. Normative References
[RFC1264] Hinden, R., "Internet Engineering Task Force Internet
Routing Protocol Standardization Criteria", RFC 1264,
October 1991.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
B. Thomas, "LDP Specification", RFC 3036, January 2001.
[RFC3815] Cucchiara, J., Sjostrand, H., and J. Luciani, "Definitions
of Managed Objects for the Multiprotocol Label Switching
(MPLS), Label Distribution Protocol (LDP)", RFC 3815, June
2004.
5.2. Informative References
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[LDP-SYNC] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", Work in Progress, July 2007.
Editors' Addresses
Loa Andersson
Acreo AB
Isafjordsgatan 22
Kista, Sweden
EMail: loa.andersson@acreo.se
loa@pi.se
Ina Minei
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
EMail: ina@juniper.net
Bob Thomas
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
EMail: rhthomas@cisco.com
Andersson, et al. Informational [Page 6]
RFC 5037 Experience with the LDP Protocol October 2007
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