draft-ietf-mpls-ldp-igp-sync-02.txt   draft-ietf-mpls-ldp-igp-sync-03.txt 
Network Working Group M. Jork Network Working Group M. Jork
Internet Draft NextPoint Networks Internet Draft NextPoint Networks
Category: Informational Alia Atlas Category: Informational Alia Atlas
Expires: December 2008 British Telecom Expires: May 2008 British Telecom
L. Fang L. Fang
Cisco Systems, Inc. Cisco Systems, Inc.
June 28, 2008 November 3, 2008
LDP IGP Synchronization LDP IGP Synchronization
draft-ietf-mpls-ldp-igp-sync-02.txt draft-ietf-mpls-ldp-igp-sync-03.txt
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
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aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Abstract Abstract
In certain networks there is a dependency on edge-to-edge LSPs setup In certain networks there is a dependency on edge-to-edge LSPs setup
by LDP, e.g. networks that are used for MPLS VPN applications. For by LDP, e.g. networks that are used for MPLS VPN applications. For
such applications it is not possible to rely on IP forwarding if the such applications it is not possible to rely on IP forwarding if the
MPLS LSP is not operating appropriately. Blackholing of labeled MPLS LSP is not operating appropriately. Blackholing of labeled
traffic can occur in situations where the IGP is operational on a traffic can occur in situations where the IGP is operational on a
link but LDP is not operational on that link. While the link could link but LDP is not operational on that link. While the link could
still be used for IP forwarding, it is not useful for MPLS still be used for IP forwarding, it is not useful for MPLS
LDP IGP Synchronization November 2008
forwarding, for example, MPLS VPN; BGP route free core; or IP forwarding, for example, MPLS VPN; BGP route free core; or IP
address carried in the packet is out of the RFC1918 space. This address carried in the packet is out of the RFC1918 space. This
document describes a mechanism to avoid traffic loss due to this document describes a mechanism to avoid traffic loss due to this
condition without introducing any protocol changes. condition without introducing any protocol changes.
Table of Contents Table of Contents
1. Introduction..................................................2 1. Introduction..................................................2
2. Proposed Solution.............................................3 2. Proposed Solution.............................................3
3. Applicability.................................................4 3. Applicability.................................................4
4. Interaction With TE Tunnels...................................5 4. Interaction With TE Tunnels...................................5
5. Security Considerations.......................................5 5. Security Considerations.......................................5
6. IANA Considerations...........................................5 6. IANA Considerations...........................................5
7. Normative References..........................................6 7. Normative References..........................................6
8. Informational References......................................6 8. Informational References......................................6
9. Author's Addresses............................................6 9. Author's Addresses............................................6
10. Acknowledgements............................................8 10. Acknowledgements.............................................8
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC2119 [RFC this document are to be interpreted as described in RFC2119 [RFC
2119]. 2119].
1. Introduction 1. Introduction
LDP [RFC5036] establishes MPLS LSPs along the shortest path to a LDP [RFC5036] establishes MPLS LSPs along the shortest path to a
destination as determined by IP forwarding. In a common network destination as determined by IP forwarding. In a common network
design, LDP is used to provide label switched paths throughout the design, LDP is used to provide label switched paths throughout the
complete network domain covered by an IGP such as OSPF [RFC2328] or complete network domain covered by an IGP such as OSPF [RFC2328] or
IS-IS [ISO.10589.1992], i.e. all links in the domain have IGP as IS-IS [ISO.10589.1992], i.e. all links in the domain MAY have IGP
well as LDP adjacencies. as well as LDP adjacencies.
A variety of services a network provider may want to deploy over an A variety of services a network provider may want to deploy over an
LDP enabled network depend on the availability of edge to edge LDP enabled network depend on the availability of edge to edge
label switched paths. In a L2 or L3 VPN scenario for example, a label switched paths. In a L2 or L3 VPN scenario for example, a
given PE router relies on the availability of a complete MPLS given PE router relies on the availability of a complete MPLS
forwarding path to the other PE routers for the VPNs it serves. forwarding path to the other PE routers for the VPNs it serves.
This means that along the IP shortest path from one PE router to This means that along the IP shortest path from one PE router to
the other, all the links need to have operational LDP sessions and the other, all the links need to have operational LDP sessions and
the necessary label binding must have been exchanged over those the necessary label binding must have been exchanged over those
sessions. If only one link along the IP shortest path is not sessions. If only one link along the IP shortest path is not
covered by an LDP session, a blackhole exists and services covered by an LDP session, a blackhole exists and services
depending on MPLS forwarding will fail. This might be a transient depending on MPLS forwarding will fail. This might be a transient
LDP IGP Synchronization November 2008
or a persistent error condition. Some of the reasons for it could or a persistent error condition. Some of the reasons for it could
be be
- A configuration error - A configuration error
- An implementation bug - An implementation bug
- The link has just come up and has an IGP adjacency but LDP has - The link has just come up and has an IGP adjacency but LDP has
either not yet established an adjacency or session or either not yet established an adjacency or session or
distributed all the label bindings. distributed all the label bindings.
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This has some similarity to the mechanism specified in [RFC3137] This has some similarity to the mechanism specified in [RFC3137]
which allows an OSPF router to advertise that it should not be used which allows an OSPF router to advertise that it should not be used
as a transit router. One difference is that [RFC3137] raises the as a transit router. One difference is that [RFC3137] raises the
link costs on all (stub) router links, while the mechanism link costs on all (stub) router links, while the mechanism
described in here applies on a per-link basis. described in here applies on a per-link basis.
In detail: when LDP is not "fully operational" (see below) on a In detail: when LDP is not "fully operational" (see below) on a
given link, the IGP will advertise the link with maximum cost to given link, the IGP will advertise the link with maximum cost to
avoid any transit traffic over it if possible. In the case of avoid any transit traffic over it if possible. In the case of
OSPF, this cost is LSInfinity (16-bit value 0xFFFF) as proposed in OSPF, this cost is LSInfinity (16-bit value 0xFFFF) as proposed in
[RFC3137]. In the case of ISIS, the max matrix value is 0xFFFFFE [RFC3137]. In the case of ISIS, the max metric value is 2^24-2
per [RFC 3784]. Note that the link is not just simply removed from (0xFFFFFE). Indeed, if a link is configured with 2^24-1 (the
the topology because LDP depends on the IP reachability to maximum link metric per [RFC3784]) then this link is not advertised
establish its adjacency and session. Also, if there is no other in the topology. It is important to keep the link in the topology
link in the network to reach a particular destination, no to allow for IP traffic to use the link as a last resort in case of
additional harm is done by making this link available for IP massive failure.
forwarding at maximum cost.
LDP is considered fully operational on a link when an LDP hello LDP is considered fully operational on a link when an LDP hello
adjacency exists on it, a suitable associated LDP session (matching adjacency exists on it, a suitable associated LDP session (matching
the LDP Identifier of the hello adjacency) is established to the the LDP Identifier of the hello adjacency) is established to the
peer at the other end of the link and all label bindings have been peer at the other end of the link and all label bindings have been
exchanged over the session. At the present time, the latter exchanged over the session. At the present time, the latter
LDP IGP Synchronization November 2008
condition can not generally be verified by a router and some condition can not generally be verified by a router and some
estimated may have to be used. A simple implementation strategy is estimated may have to be used. A simple implementation strategy is
to use a configurable hold down timer to allow LDP session to use a configurable hold down timer to allow LDP session
establishment before declaring LDP fully operational. The default establishment before declaring LDP fully operational. The default
timer is not defined in this document due to the concerns of the timer is not defined in this document due to the concerns of the
large variations of the LIB table size and the equipment large variations of the LIB table size and the equipment
capabilities. In addition, this is a current work in progress on LDP capabilities. In addition, this is a current work in progress on LDP
End-of-LIB as specified in [LDP End-of-LIB], it enables the LDP End-of-LIB as specified in [LDP End-of-LIB], it enables the LDP
speaker to signal the completion of its initial advertisement speaker to signal the completion of its initial advertisement
following session establish. When LDP End-of-LIB is implemented, the following session establish. When LDP End-of-LIB is implemented, the
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Example network scenarios that benefit from the mechanism described Example network scenarios that benefit from the mechanism described
here are MPLS VPNs and BGP-free core network designs where traffic here are MPLS VPNs and BGP-free core network designs where traffic
can only be forwarded through the core when LDP forwarding state is can only be forwarded through the core when LDP forwarding state is
available throughout. available throughout.
The usefulness of this mechanism also depends on the availability The usefulness of this mechanism also depends on the availability
of alternate paths with sufficient bandwidth in the network should of alternate paths with sufficient bandwidth in the network should
one link be assigned to the maximum cost due to unavailability of one link be assigned to the maximum cost due to unavailability of
LDP service over it. LDP service over it.
LDP IGP Synchronization November 2008
On broadcast links with more than one IGP/LDP peer, the cost-out On broadcast links with more than one IGP/LDP peer, the cost-out
procedure can only be applied to the link as a whole and not an procedure can only be applied to the link as a whole and not an
individual peer. So a policy decision has to be made whether the individual peer. So a policy decision has to be made whether the
unavailability of LDP service to one peer should result in the unavailability of LDP service to one peer should result in the
traffic being diverted away from all the peers on the link. traffic being diverted away from all the peers on the link.
4. Interaction With TE Tunnels 4. Interaction With TE Tunnels
In some networks, LDP is used in conjunction with RSVP-TE which sets In some networks, LDP is used in conjunction with RSVP-TE which sets
up traffic-engineered tunnels. The path computation for the TE up traffic-engineered tunnels. The path computation for the TE
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LDP is failed and IGP is not should not introduce new security LDP is failed and IGP is not should not introduce new security
threats. The operation is internal in the router to allow LDP and threats. The operation is internal in the router to allow LDP and
IGP to communicate and react. Making many LDP links unavailable, IGP to communicate and react. Making many LDP links unavailable,
however, is a security threat which can cause traffic being dropped however, is a security threat which can cause traffic being dropped
due to limited available network capacity. This may be trigged by due to limited available network capacity. This may be trigged by
operational error or implementation error. They are considered as operational error or implementation error. They are considered as
general Security issues and should follow the current best security general Security issues and should follow the current best security
practice. practice.
6. IANA Considerations 6. IANA Considerations
LDP IGP Synchronization November 2008
This document has no actions for IANA. This document has no actions for IANA.
7. Normative References 7. Normative References
[RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A., [RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
and B. Thomas, "LDP Specification", RFC 5036, October 2007. and B. Thomas, "LDP Specification", RFC 5036, October 2007.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
8. Informational References 8. Informational References
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[ISO.10589.1992]International Organization for [ISO.10589.1992]International Organization for
Standardization,"Intermediate system to intermediate system intra- Standardization,"Intermediate system to intermediate system intra-
domain-routing routine information exchange protocol for use in domain-routing routine information exchange protocol for use in
conjunction with the protocol for providing the connectionless-mode conjunction with the protocol for providing the connectionless-mode
Network Service (ISO 8473)", ISO Standard 10589, 1992. Network Service (ISO 8473)", ISO Standard 10589, 1992.
[LDP-Fail] Fang, L., Atlas, A., Chiussi, F., Kompella, K., and [LDP-Fail] Fang, L., Atlas, A., Chiussi, F., Kompella, K., and
Swallow, G., "LDP Failure Detection and Recovery", IEEE Swallow, G., "LDP Failure Detection and Recovery", IEEE
Communications Magazine, Vol.42, No.10, October 2004. Communications Magazine, Vol.42, No.10, October 2004.
[LDP End-of-LIB] Asati, R., LDP End-of-LIB, draft-asati-mpls-ldp- [LDP End-of-LIB] Asati, R., LDP End-of-LIB, draft-ietf-mpls-ldp-
end-of-lib-01.txt, November 2007. end-of-lib-01.txt, September 2008.
9. Author's Addresses 9. Author's Addresses
Markus Jork Markus Jork
NextPoint Networks NextPoint Networks
3 Fedral St. 3 Fedral St.
Billerica, MA 01821 Billerica, MA 01821
USA USA
Email: mjork@nextpointnetworks.com Email: mjork@nextpointnetworks.com
Alia Atlas Alia Atlas
British Telecom British Telecom
Email: alia.atlas@bt.com Email: alia.atlas@bt.com
LDP IGP Synchronization November 2008
Luyuan Fang Luyuan Fang
Cisco Systems, Inc. Cisco Systems, Inc.
300 Beaver Brook Road 300 Beaver Brook Road
Boxborough, MA 01719 Boxborough, MA 01719
USA USA
Email: lufang@cisco.com Email: lufang@cisco.com
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The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
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LDP IGP Synchronization November 2008
10. Acknowledgements 10. Acknowledgements
Funding for the RFC Editor function is provided by the IETF Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA). Administrative Support Activity (IASA).
The authors would like to thank Loa Andersson for his review and The authors would like to thank Loa Andersson for his review and
comments, and thank Bruno Decraene for his in depth discussion, comments, thank Bruno Decraene for his in depth discussion,
comments and helpful suggestions. comments and helpful suggestions, and thank Dave Ward for his AD
review comments and suggestions.
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