MPLS Working Group S. Kini (Ed)
Internet Draft W. Lu (Ed)
Updates: 5443 (if approved) Ericsson
Intended Status: Informational September 13, 2010
Expires: March 2011
LDP IGP Synchronization for broadcast networks
draft-ietf-mpls-ldp-igp-sync-bcast-04.txt
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Abstract
RFC 5443 describes a mechanism to achieve LDP IGP Synchronization to
prevent black-holing traffic (e.g. VPN) when an interior gateway
protocol (IGP) is operational on a link but Label Distribution
Protocol (LDP) is not. If this mechanism is applied to broadcast
links that have more than one LDP/IGP peer, the metric increase
procedure can only be applied to the link as a whole but not an
individual peer. When a new LDP peer comes up on a broadcast network,
this can result in loss of traffic through other established peers on
that network. This document describes a mechanism to address that
use-case without dropping traffic. The mechanism does not introduce
any protocol message changes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 7
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Computation of 'cut-edge' . . . . . . . . . . . . . . 9
Appendix B. Sync without support at one end . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
In RFC 5443 ([LDP-IGP-SYNC]), when [LDP] is not fully operational on
a link, the IGP advertises the link with maximum cost to avoid any
transit traffic on the link if possible. When LDP becomes operational
i.e., all the label bindings have been exchanged, the link is
advertised with its correct cost. This tries to ensure that all along
the IGP shortest path, the LDP LSP is available. The mechanisms in
[LDP-IGP-SYNC] have limitations when applied to a broadcast link.
These are described in section 3. A solution is defined in section 4.
2. 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].
3. Problem Statement
On broadcast networks, a router's link-state advertisement (LSA)
contains a single cost to the broadcast network, rather than a
separate cost to each peer on the broadcast network. The operation of
the mechanism in [LDP-IGP-SYNC] is analyzed using the sample topology
of Figure 1 below where routers A, B, C and E are attached to a
common broadcast network. Say all links in that topology have a cost
of 1 except the link A-PE3 that has a cost of 10. The use-case when
router B's link to the broadcast network comes up is analyzed. Before
that link comes up, traffic between PE1 and PE2 flows along the bi-
directional path PE1-A-C-D-PE2 and traffic between PE1 and PE3 flows
along the bi-directional path PE1-A-E-PE3.
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| +---+ +---+
|----| B |-----------|PE2|
| +---+ +---+
+---+ +---+ | |
|PE1|----| A |----| |
+---+ +---+ | |
| | +---+ +---+ |
| |----| C |----| D |----+
| | +---+ +---+
| |
| |
| |
| | +---+
| |----| E |-------------+
| | +---+ |
| | |
| |
| +---+
+---------------------------|PE3|
+---+
Figure 1 LDP IGP Sync on a broadcast network
In one interpretation of the applicability of [LDP-IGP-SYNC] to
broadcast networks, when a new router is discovered on a broadcast
network, that network should avoid transit traffic till LDP becomes
operational between all routers on that network. This can be achieved
by having all the attached routers advertise maximum cost to that
network. This should result in traffic that is being sent via that
broadcast network to be diverted. However, traffic might be
inadvertently diverted to the link that just came up. Till LDP
becomes operational, that traffic will be black-holed. An additional
problem is route churn in the entire network that results in traffic
that should be unaffected taking sub-optimal paths until the high
cost metric is reverted to the normal cost. In Figure 1, when B's
link to the broadcast network comes up and it is discovered by
routers A, C and E, then A, B, C and E can all start advertising
maximum cost to the broadcast network. A will have B as next-hop to
PE2 and will not have a LDP LSP path to PE2 resulting in VPN traffic
from PE1 to PE2 to be black-holed at A. The route churn at A also
results in traffic between PE1 and PE3 to be unnecessarily diverted
to the sub-optimal path PE1-A-PE3 until the maximum cost
advertisement is reverted to the normal cost.
This interpretation has the additional complexity of requiring the
maximum cost advertisement to be reverted by all routers after LDP
peering between all the routers on the broadcast network is
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operational. This is non-trivial and needs co-ordination between all
the routers.
In another alternative interpretation of the applicability of [LDP-
IGP-SYNC] to broadcast networks, only the router whose link to the
broadcast network comes up, advertises maximum cost for that link but
other routers continue to advertise the normal cost. In Figure 1 when
B's link to the broadcast network comes up, it advertises a high cost
to the broadcast network. After the IGP has converged but the LDP
peering A-B is not yet operational, A will have B as the next-hop for
PE2 and will not have a LDP LSP path to PE2. Since A's cost to reach
B not high, A-B-PE2 becomes the shortest path. VPN traffic from PE1
to PE2 will be dropped at A.
4. Solution
The problem described above exists because the link-state database
(LSDB) of the IGP does not describe a link coming up on a broadcast
network with a high bi-directional cost to all other routers on that
broadcast network. A broadcast network is advertised as a pseudo-node
containing a list of routers that the broadcast network is connected
to and the cost of all these links from the pseudo-node to each
router is zero when computing SPF.
The solution proposed below removes the link that is coming up from
the LSDB unless absolutely necessary. Only the router whose link is
coming up plays a role in ensuring this. The other routers on the
broadcast network are not involved. The following text describes this
in more detail.
During the intra-area SPF algorithm execution, an additional
computation is made to detect an alternate path to a directly
connected network that does not have any IGP adjacencies.
If a router has a directly connected network that does not have an
alternate path to reach it, then the interface to that network is a
'cut-edge' in the topology for that router. When a 'cut-edge' goes
down, the network is partitioned into two disjoint sub-graphs. This
property of whether or not an interface is a 'cut-edge' is used when
an IGP adjacency comes up on that interface. The method to determine
whether an interface is a 'cut-edge' is described in Appendix A.
During IGP procedures when the router's first adjacency to the
broadcast network is coming up and the LSA is about to be updated
with a link to the pseudo-node of the broadcast interface, a check is
made whether that interface is a 'cut-edge'. If it is not a 'cut-
edge' then the updating of the LSA with that link to the pseudo-node
is postponed until LDP is operational with all the LDP peers on that
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broadcast interface. After LDP is operational, the LSA is updated
with that link to the pseudo-node (and the LSA is flooded). If the
interface is a 'cut-edge' then the updating of the LSA MUST NOT be
delayed by LDP's operational state. Note that the IGP and LDP
adjacency bring-up procedures are unchanged. The conditional check
whether the interface is a 'cut-edge' must be done just before the
adjacency is about to be reflected in the LSA.
If the IGP is [OSPF], the Router-LSA is not updated with a 'Link Type
2' (link to transit network) for that subnet, until LDP is
operational with all neighboring routers on that subnet.
Similarly, if the IGP is [ISIS], the 'Link State PDU' is updated with
an 'IS Reachability TLV' (or an 'Extended IS Reachability TLV') to
the pseudo-node after LDP is operational with all neighboring routers
on that subnet.
Note that this solution can be introduced in a gradual manner in a
network without any backward compatibility issues.
5. Scope
This document is agnostic to the method that detects LDP to be
operational with a neighbor. It does not define any new method to
detect that LDP is operational. At the time of publishing this
document [LDP-EOL] seems to be the preferred method.
Issues arising out of LDP not being configured on some routers or on
some interfaces are not specific to the method described in this
document and are considered outside the scope of this solution.
6. Applicability
The method described in this document can be easily extended to
point-to-point (p2p) links. However, an implementation may continue
to apply the method described in [LDP-IGP-SYNC] to p2p links but
apply the method described in this document to broadcast networks.
Both methods can co-exist in a network.
The techniques used in this document's solution enable LDP IGP
synchronization in many scenarios where one end of the IGP adjacency
does not support any LDP IGP sync method. This is an optional benefit
and is for further study. Some ways to apply this technique to
achieve that benefit are discussed in Appendix B.
7. Security Considerations
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This document does not introduce any new security considerations
beyond those already described in [LDP-IGP-SYNC].
8. IANA Considerations
This document has no actions for IANA.
9. Conclusions
This document complements [LDP-IGP-SYNC] by providing a solution to
achieve LDP IGP synchronization for broadcast networks. It can also
co-exist with that solution in a network that has a combination of
p2p links and broadcast networks. It can also be introduced into a
network without backward compatibility issues. The solution in this
document can also be used exclusively to achieve LDP IGP
synchronization since this solution applies to both p2p links as well
as broadcast networks.
This solution also has useful properties that can be optionally used
to achieve LDP IGP synchronization when only one end of the IGP
adjacency supports this solution but the other end supports neither
this solution nor the one in [LDP-IGP-SYNC].
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[LDP-IGP-SYNC] Jork, M., et al, "LDP IGP Synchronization", RFC 5443,
March 2009.
[LDP] Andersson, L., et al, "LDP Specification", RFC 5036,
October 2007.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[ISIS] 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.
10.2. Informative References
[LDP-EOL] Asati, R., et al, "Signaling LDP Label Advertisement
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Completion", RFC 5919, June 2010.
11. Acknowledgements
The authors would like to thank Luyuan Fang, Mikael Abrahamsson, Ben
Niven-Jenkins, Bruno Decraene, Jeff Tantsura and Acee Lindem for
their review and useful comments.
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Appendix A. Computation of 'cut-edge'
A 'cut-edge' can be computed during an intra-area SPF run or by using
results of the previous SPF run. If a SPF run was scheduled but is
pending execution, that SPF MUST be executed immediately before any
procedure checks whether an interface is a 'cut-edge'.
An interface is considered a 'cut-edge' if during intra-area SPF
(using Dijkstra's algorithm) there is no alternate path for the
directly connected network. Alternately, lack of connectivity to the
router-id of a directly connected peer via an alternate path as
detected by the last run of SPF can be used. The router-id can be
known during the adjacency bring-up process.
A 'cut-edge' computation should not require any extra SPF runs. It
should not increase the algorithmic complexity of SPF.
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Appendix B. Sync without support at one end
A useful property of the solution described in this document is that
LDP IGP synchronization is achievable in many scenarios where one end
of the IGP adjacency does not support any LDP IGP sync method.
For p2p links (or broadcast links on which the IGP operates in p2p
mode) the applicability is straightforward. An IGP can establish a
p2p adjacency on a p2p link or a broadcast link with the IGP in p2p
mode. When a p2p adjacency comes up, the end of the adjacency that
supports the solution in this document would not advertise the link
to the other router in its LSA unless the edge is a 'cut-edge' or
until LDP becomes operational. Hence neither of the two routers will
have IGP next-hop as the other router unless the link is a 'cut-
edge'. Consider Figure 1 modified such that the broadcast network is
replaced by p2p links between each of A, B, C and E. Say link A-B is
coming up but only A has implemented the solution in this document
whereas B has implemented neither the solution in this document nor
the solution in [LDP-IGP-SYNC]. Since A's LSA does not advertise a
link to B until LDP is operational, B does not have A as next-hop.
After LDP is operational, A advertises the link to B in its LSA.
Hence there is no traffic loss due to LDP LSP not being present.
For broadcast networks the applicability is not straightforward and
should be considered a topic for future study. One way is for the DR
to stop advertising the link in the pseudo-node to the router whose
link is coming up until LDP is operational.
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Authors' Addresses
Sriganesh Kini
Ericsson
300 Holger Way, San Jose, CA 95134
EMail: sriganesh.kini@ericsson.com
Wenhu Lu
Ericsson
300 Holger Way, San Jose, CA 95134
EMail: wenhu.lu@ericsson.com
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