Network Working Group M. Jork
Internet Draft NextPoint Networks
Category: Informational Alia Atlas
Expires: June 17, 2009 British Telecom
L. Fang
Cisco Systems, Inc.
December 17, 2008
LDP IGP Synchronization
draft-ietf-mpls-ldp-igp-sync-04.txt
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LDP IGP Synchronization December 2008
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Abstract
In certain networks there is a dependency on edge-to-edge Label
Switched Paths (LSPs) setup by Label Distribution Protocol (LDP),
e.g., networks that are used for MultiProtocol Label Switching
(MPLS) Virtual Private Network (VPN) applications. For such
applications it is not possible to rely on Internet Protocol (IP)
forwarding if the MPLS LSP is not operating appropriately.
Blackholing of labeled traffic can occur in situations where the
Interior Gateway Protocol (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 forwarding, for example,
MPLS VPN; Border Gateway Protocol (BGP) route free core; or IP
address carried in the packet is out of the RFC 1918 [RFC 1918]
space. This document describes a mechanism to avoid traffic loss due
to this condition without introducing any protocol changes.
Table of Contents
1. Introduction..................................................3
2. Proposed Solution.............................................3
3. Applicability.................................................5
4. Interaction with TE Tunnels...................................5
5. Security Considerations.......................................6
6. IANA Considerations...........................................6
7. Normative References..........................................6
8. Informational References......................................6
9. Authors' Addresses............................................7
10. Acknowledgments..............................................7
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].
M. Jork, Alia Atlas, and L. Fang [Page 2]
LDP IGP Synchronization December 2008
1. Introduction
LDP [RFC 5036] 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 Open Shortest
Path First (OSPF) [RFC 2328] or Intermediate system to intermediate
system (IS-IS) [ISO.10589.1992], i.e., all links in the domain 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 layer 2 (L2) or layer 3 (L3) VPN
scenario for example, a given Provider-Edge(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 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 cannot 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.
M. Jork, Alia Atlas, and L. Fang [Page 3]
LDP IGP Synchronization December 2008
This has some similarity to the mechanism specified in [RFC 3137]
which allows an OSPF router to advertise that it should not be used
as a transit router. One difference is that [RFC 3137] 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
[RFC 3137]. 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 [RFC 5305]) 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
condition cannot 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 Label Information Base (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
Failure Recovery].
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LDP IGP Synchronization December 2008
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.
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.
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5. Security Considerations
A DoS attack that 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 prevents the use of the link 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 triggered by operational error or implementation error. They are
considered as general Security issues and should follow the current
best security practice [MPLS-GMPLS-Security].
6. IANA Considerations
This document has no actions for IANA.
7. Normative References
[RFC 5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
and B. Thomas, "LDP Specification", RFC 5036, October 2007.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
1998.
8. Informational References
[RFC 1918] Rekhter, Y., "Address Allocation for Private Internets",
BCP: 5, RFC 1918, February 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC 3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001.
[RFC 5305] Li, T., Smit, H., Intermediate System to Intermediate
System (IS-IS) Extension for Traffic Engineering, October 2008.
M. Jork, Alia Atlas, and L. Fang [Page 6]
LDP IGP Synchronization December 2008
[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 Failure Recovery] 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, work in progress, September 2008.
[MPLS-GMPLS-Security] Fang. L., Ed., "Security Framework for MPLS
and GMPLS Networks", draft-ietf-mpls-mpls-and-gmpls-security-
framework-04.txt, work in progress, November 2008.
9. Authors' 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
Luyuan Fang
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
USA
Email: lufang@cisco.com
Phone: 1 (978) 936-1633
10. Acknowledgments
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
The authors would like to thank Bruno Decraene for his in depth
discussion and comments; thank Dave Ward for his helpful review and
input; and thank Loa Andersson, Ross Callon, Amanda Baber, Francis
Dupont, Donald Eastlake, Russ Housley, Pasi Eronen, Dan Romascanu,
M. Jork, Alia Atlas, and L. Fang [Page 7]
LDP IGP Synchronization December 2008
Bin Mo, Lan Zheng, Bob Thomas, and Dave Ojemann for their review
and comments.
M. Jork, Alia Atlas, and L. Fang [Page 8]