Network Working Group M. Jork
Internet Draft NextPoint Networks
Category: Informational Alia Atlas
Expires: May 2008 British Telecom
L. Fang
Cisco Systems, Inc.
November 3, 2008
LDP IGP Synchronization
draft-ietf-mpls-ldp-igp-sync-03.txt
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Abstract
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
such applications it is not possible to rely on IP forwarding if the
MPLS LSP is not operating appropriately. Blackholing of labeled
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
still be used for IP forwarding, it is not useful for MPLS
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LDP IGP Synchronization November 2008
forwarding, for example, MPLS VPN; BGP route free core; or IP
address carried in the packet is out of the RFC1918 space. This
document describes a mechanism to avoid traffic loss due to this
condition without introducing any protocol changes.
Table of Contents
1. Introduction..................................................2
2. Proposed Solution.............................................3
3. Applicability.................................................4
4. Interaction With TE Tunnels...................................5
5. Security Considerations.......................................5
6. IANA Considerations...........................................5
7. Normative References..........................................6
8. Informational References......................................6
9. Author's Addresses............................................6
10. Acknowledgements.............................................8
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC2119 [RFC
2119].
1. Introduction
LDP [RFC5036] establishes MPLS LSPs along the shortest path to a
destination as determined by IP forwarding. In a common network
design, LDP is used to provide label switched paths throughout the
complete network domain covered by an IGP such as OSPF [RFC2328] or
IS-IS [ISO.10589.1992], i.e. all links in the domain MAY have IGP
as well as LDP adjacencies.
A variety of services a network provider may want to deploy over an
LDP enabled network depend on the availability of edge to edge
label switched paths. In a L2 or L3 VPN scenario for example, a
given PE router relies on the availability of a complete MPLS
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
the other, all the links need to have operational LDP sessions and
the necessary label binding must have been exchanged over those
sessions. If only one link along the IP shortest path is not
covered by an LDP session, a blackhole exists and services
depending on MPLS forwarding will fail. This might be a transient
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or a persistent error condition. Some of the reasons for it could
be
- A configuration error
- An implementation bug
- The link has just come up and has an IGP adjacency but LDP has
either not yet established an adjacency or session or
distributed all the label bindings.
LDP protocol has currently no way to correct the issue, LDP is not
a routing protocol; it can not re-direct traffic to an alternate
IGP path.
2. Proposed Solution
The problem described above exists because LDP is tied to IP
forwarding decisions but no coupling between the IGP and LDP
operational state on a given link exists. If IGP is operational on
a link but LDP is not, a potential network problem exists. So the
solution described by this document is to discourage a link from
being used for IP forwarding as long as LDP is not fully
operational.
This has some similarity to the mechanism specified in [RFC3137]
which allows an OSPF router to advertise that it should not be used
as a transit router. One difference is that [RFC3137] raises the
link costs on all (stub) router links, while the mechanism
described in here applies on a per-link basis.
In detail: when LDP is not "fully operational" (see below) on a
given link, the IGP will advertise the link with maximum cost to
avoid any transit traffic over it if possible. In the case of
OSPF, this cost is LSInfinity (16-bit value 0xFFFF) as proposed in
[RFC3137]. In the case of ISIS, the max metric value is 2^24-2
(0xFFFFFE). Indeed, if a link is configured with 2^24-1 (the
maximum link metric per [RFC3784]) then this link is not advertised
in the topology. It is important to keep the link in the topology
to allow for IP traffic to use the link as a last resort in case of
massive failure.
LDP is considered fully operational on a link when an LDP hello
adjacency exists on it, a suitable associated LDP session (matching
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
exchanged over the session. At the present time, the latter
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LDP IGP Synchronization November 2008
condition can not generally be verified by a router and some
estimated may have to be used. A simple implementation strategy is
to use a configurable hold down timer to allow LDP session
establishment before declaring LDP fully operational. The default
timer is not defined in this document due to the concerns of the
large variations of the LIB table size and the equipment
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
speaker to signal the completion of its initial advertisement
following session establish. When LDP End-of-LIB is implemented, the
configurable hold down timer is no longer needed. The neighbor LDP
session is considered fully operational when the End-of-LIB
notification message is received.
This is typically sufficient to deal with the link when it is being
brought up. LDP protocol extensions to indicate the complete
transmission of all currently available label bindings after a
session has come up are conceivable but not addressed in this
document.
The mechanism described in this document does not entail any
protocol changes and is a local implementation issue.
The problem space and solution specified in this document have also
been discussed in an IEEE Communications Magazine paper [LDP-Fail].
3. Applicability
In general, the proposed procedure is applicable in networks where
the availability of LDP signaled MPLS LSPs and avoidance of
blackholes for MPLS traffic is more important than always choosing
an optimal path for IP forwarded traffic. Note however that non-
optimal IP forwarding only occurs for a short time after a link
comes up or when there is a genuine problem on a link. In the
latter case an implementation should issue network management alerts
to report the error condition and enable the operator to address it.
Example network scenarios that benefit from the mechanism described
here are MPLS VPNs and BGP-free core network designs where traffic
can only be forwarded through the core when LDP forwarding state is
available throughout.
The usefulness of this mechanism also depends on the availability
of alternate paths with sufficient bandwidth in the network should
one link be assigned to the maximum cost due to unavailability of
LDP service over it.
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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
individual peer. So a policy decision has to be made whether the
unavailability of LDP service to one peer should result in the
traffic being diverted away from all the peers on the link.
4. Interaction With TE Tunnels
In some networks, LDP is used in conjunction with RSVP-TE which sets
up traffic-engineered tunnels. The path computation for the TE
tunnels is based on the TE link cost which is flooded by the IGP in
addition to the regular IP link cost. The mechanism described in
this document should only be applied to the IP link cost to prevent
any unnecessary TE tunnel reroutes.
In order to establish LDP LSPs across a TE tunnel, a targeted LDP
session between the tunnel endpoints needs to exist. This presents
a problem very similar to the case of a regular LDP session over a
link (the case discussed so far): when the TE tunnel is used for IP
forwarding, the targeted LDP session needs to be operational to
avoid LDP connectivity problems. Again, raising the IP cost of the
tunnel while there is no operational LDP session will solve the
problem. When there is no IGP adjacency over the tunnel and the
tunnel is not advertised as link into the IGP, this becomes a local
issue of the tunnel headend router.
5. Security Considerations
A DoS attack brings down LDP service on a link or prevents it from
becoming operational on a link could be one of the possibilities
that causes LDP related traffic blackholing. This document does not
address how to prevent LDP session failure. The mechanism described
here is to prevent the link to be used when LDP is not operational
while IGP is. Assigning the IGP cost to maximum on the link where
LDP is failed and IGP is not should not introduce new security
threats. The operation is internal in the router to allow LDP and
IGP to communicate and react. Making many LDP links unavailable,
however, is a security threat which can cause traffic being dropped
due to limited available network capacity. This may be trigged by
operational error or implementation error. They are considered as
general Security issues and should follow the current best security
practice.
6. IANA Considerations
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This document has no actions for IANA.
7. Normative References
[RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
and B. Thomas, "LDP Specification", RFC 5036, October 2007.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
8. Informational References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001.
[RFC3784] Smit, H., Li, T., Intermediate System to Intermediate
System (IS-IS) Extension for Traffic Engineering, June 2004.
[ISO.10589.1992]International Organization for
Standardization,"Intermediate system to intermediate system intra-
domain-routing routine information exchange protocol for use in
conjunction with the protocol for providing the connectionless-mode
Network Service (ISO 8473)", ISO Standard 10589, 1992.
[LDP-Fail] Fang, L., Atlas, A., Chiussi, F., Kompella, K., and
Swallow, G., "LDP Failure Detection and Recovery", IEEE
Communications Magazine, Vol.42, No.10, October 2004.
[LDP End-of-LIB] Asati, R., LDP End-of-LIB, draft-ietf-mpls-ldp-
end-of-lib-01.txt, September 2008.
9. Author's Addresses
Markus Jork
NextPoint Networks
3 Fedral St.
Billerica, MA 01821
USA
Email: mjork@nextpointnetworks.com
Alia Atlas
British Telecom
Email: alia.atlas@bt.com
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LDP IGP Synchronization November 2008
Luyuan Fang
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
USA
Email: lufang@cisco.com
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10. Acknowledgements
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
The authors would like to thank Loa Andersson for his review and
comments, thank Bruno Decraene for his in depth discussion,
comments and helpful suggestions, and thank Dave Ward for his AD
review comments and suggestions.
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