1. A Scalable Multipath Algorithm in Hierarchical MPLS Networks
The Need for Multipath
Objectives
What is multipath?
Multipaths are a set of disjoint paths (or LSPs in MPLS).
Two LSPs are said to be disjoint if they share no link on the path.
Set up multiple disjoint LSPs that span multiple OSPF* areas in IP/MPLS network without any path computation servers.
(*) We assume OSPF is used as the routing protocol and RSVP-TE is used as the MPLS signaling.
Fig. LSPs that span multiple areas.
Fig. Example of multipath (disjoint LSPs).
Area １ E１
Disjoint ＬＳＰs
Share no link on each path. C E３
ABR
ABR
Backbone Area C C C
ABR
ABR
Area ２ C E２ E C C E
LSP：Label Switched Path
Why multi-area?
One solution to overcome the scalability issue of the routing protocol.
By dividing the routing domain into multiple areas, we can reduce the total volume of routing protocol messages in the network.
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Why is it important?
Key technology for the IP traffic engineering.
Used for LSP protection/restoration and load balancing.
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Main Issues
In multi-area IP networks, no router has the information on the entire network topology
⇒Need some mechanisms for setting up disjoint LSPs that span multiple areas and require routers in each area to cooperate to set up such LSPs.
Fig. Function block of an MPLS router
TE Control
Path Finding
RSVP-TE
Signaling
Path finding algorithm
Need to compute the route of LSPs per area.
How to find a pair of disjoint paths in each area.
Routing protocol
Need some extensions to ABRs (Area Border Routers).
What information should routers advertise.
Signaling protocol
If disjoint paths can be found in each area, how to stitch
those paths.
Topology
Database
OSPF
Routing & Flooding
Forwarding
T. Miyamura , T. Kurimoto, M. Aoki
Tél/Fax: /2042
NTT Network Service Systems Laboratories, NTT Corporation
Logo 1
Logo 2

2. Path Finding Algorithm
Our Proposal
IETF solution
Kompella[13] proposed a mechanism for the multi-area traffic engineering (TE) at IETF.
Their concern is mainly on the mechanism for setting up TE paths, not disjoint LSPs.
They didn’t consider the case where multiple ABRs(Area Border Routers) exist in the same area.
Fig. Problem of IETF solution.
Two LSPs shares the same ABRs E１ C
Area １
ABR C C
Secondary LSP
ABR
Backbone Area
ABR C
ABR
Area ２ C
Our proposal
Take into consideration the case where multiple ABRs exist in the sama area.
Can construct disjoint LSPs that share no ABR (if possible) in multiarea networks.
Path calculation is performed by the head node in each area on the path. (e.g. E1 in Area 1 and ABR1/2 in Backbone).
Primary LSP C E２
Fig. Our proposal
Sending PATH message for the secondary LSP to ABR3
Flooding the backbone: extended Summary-LSA
{to e1: cost = 20 via node C1}
{to e1:cost = 45 via node C2} E１ C
The key of our proposal
Path finding algorithm, called extended-KSP
Coordination among ABRs that exist in the same area.
Area １
ABR1 C
Sending PATH message to
ABR2 and ABR1 for primary and secondary LSPs, respectively C
ABR2
Backbone Area
ABR3 C
ABR4
Area ２
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Sending PATH message for the primary LSP to ABR4 C１ E２
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Flooding the backbone: Summary-LSA
{to e1: cost = 20 via node C1}
Path Finding Algorithm
Motivation
Each ABR calculates the shortest path independently. Two LSPs from different nodes may share the same link.
⇒Need for the algorithm that finds two disjoint paths from different two nodes to the same node/different two nodes.
Fig. Enlargement of Area 2.
Second shortest path from ABR3 to egress e1.
(cost = 45)
Outline of the extended KSP (K-shortest path)
Path finding algorithm based on the KSP algorithm.
Extends the KSP algorithm to compute the path from different two nodes to the same node/different two nodes.
Consider the case where disjoint paths from ABR3 and ABR4 to node E2.
1) Compute the shortest path from ABR4 to E2.
2) Relabeling the cost on the above path, compute the shortest path from ABR3 to E2 based on the newly assigned cost.
Area 2
ABR 3 15 C2 15 C3 30 15 10
ABR 4 E2 10 C1 10
Shortest path from ABR3 to egress e1.
(cost = 20)
Shortest path from ABR4 to egress e1.
(cost = 20)

3. Extension of Routing Protocol Performance Evaluation
OSPF Messages
How to select one from two or more ABRs in the same area?
Decide primary and secondary ABRs among ABRs in the same area. A primary ABR is always chosen as the ABR for the primary LSP.
A secondary ABR calculate the extended-KSP and flood the information on disjoint paths.
How to flood the information on disjoint paths.
Basically, use the normal summary-LSAs (Link State Advertisement).
Except for the representation of TOS value.
Encode the information on disjoint paths as ‘1101’, which is not used in ordinary OSPF.
Fig. OSPF message (LSA)
LS age
Options
3 or 4
Link State ID
Advertising Router
LS sequence number
LS checksum
length
Network Mask
metric
TOS
TOS metric
Some state change of the topology occurs
Decide primary and secondary ABRs
Primary
Secondary
Compute the SPF algorithm
Compute the E-KSP algorithm
Flood the information about the shortest path as Summary-LSAs
Flood the information about the second-shortest path as extended Summary-LSAs
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Performance Evaluation
N nodes
Aim of the simulation
Evaluate the performance of our algorithm in terms of the protocol overhead, efficiency, and reliability.
N nodes
Backbone area
Simulation Models
Network is consisted of 1 backbone and 4 areas. Each area has 2 ABRs.
The cost of horizontal or vertical links is set to 2, and that of diagonal links is set to 1.
Number of nodes equal to M2+4(N2-1)
Compare the performance with Flat, which is an ideal algorithm.
In Flat, each node has the information on the
entire network and calculates disjoint paths
using the KSP algorithm.
M nodes
Area Border Router （ABR）
Label Edge Router （LER）
M nodes
Label Switching Router
（LSR）

4. Result 1 -Protocol Overhead Result 2 –Reliability and Efficiency
First, we investigate the protocol overhead for various numbers of routers in the network.
Here, we investigate the performance of path finding algorithms in terms of reliability and efficiency.
Simulation results (Fig. A)
Fig. A shows the message size of routing protocol for our algorithm and ordinary OSPF for various number of destinations.
A message in our algorithm is about 40% longer than one in ordinary OSPF.
Simulation results (Fig. B)
Fig.B Shows the total volume of routing protocol messages in the network for various network size.
For large-size network (N>10), our algorithm can reduce the total volume of messages by almost two orders of magnitude in comparison with Flat.
Simulation models
500 LSPs were randomly constructed between edge routers.
We used the followings as the performance evaluation index.
-Reliability：Number of overlapped nodes/links.
-Efficiency：The sum of link cost on all LSPs.
We compared the performance of our algorithm with Flat. Note that Flat corresponds to the almost ideal solution.
Simulation results (Fig.C)
The total length of LSPs in our algorithm is only 10% longer than that of Flat.
As for reliability, our algorithm performs better than Flat and reduces the number of overlapped links/nodes by about 17%.
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Fig.A: Comparison of the proposed algorithm and OSPF in terms of the size of a Summary-LSA
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Fig.C: Comparison of the proposed algorithm and OSPF in terms of the size of a Summary-LSA
Fig.B: Comparison of the proposed algorithm and Flat in terms of the total volume of LSAs
Concluding Remarks
Concluding remarks
We proposed a mechanism for constructing multiple LSPs that span multiple OSPF area. Our approach does not require any path computation server or additional hardware.
We also presented extension of MPLS signaling and routing protocol.
We investigated the performance of our algorithm in terms of the protocol overhead, efficiency and efficiency.
The simulation results shows our algorithm can construct end-to-end disjoint LSPs in multiarea networks at the cost of moderate increase in protocol overhead.
Future work
Simulations for various network topology.
More detailed evaluation of the protocol overhead including the MPLS signaling.
Extend our algorithm to support TE (Traffic Engineering) LSPs.
Implementation of our algorithm.