This document discusses OSPF, load balancing, and ECMP. It begins with an overview of OSPF as an intra-AS routing protocol that uses the Dijkstra algorithm to find the shortest path. It then discusses the load balancing problem and how ECMP can provide a solution by equally splitting traffic across multiple equal-cost paths. The benefits of ECMP include load balancing, improved bandwidth utilization, and congestion mitigation. Traditional ECMP has drawbacks, but weighted ECMP can optimize end-to-end delay by distributing traffic based on pre-determined weight ratios.

1. OSPF
Load Balancing
Sharif University of Technology
Department of Computer Engineering
Winter 2016
By: Arash Pourdamghani

2. Arash PourdamghaniWinter 2016
Update: OpenSignal’s New Report
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November 2016
Iran

3. Arash PourdamghaniWinter 2016
Review of OSPF
Stands for Open Shortest Path First
Inside Autonomous Systems(Intra-AS)
Local Link-State(Dijkstra) algorithm
in order to find shortest path
Version 2 -> IPv4 , Version 3 -> IPv6
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4. Arash PourdamghaniWinter 2016
Load Balancing Problem
Optimize Networking!
Layaer-4 switching
NAT-like function
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5. Arash PourdamghaniWinter 2016
Solution: ECMP
Equal-Cost Multiple Routes
Equally split traffic over multiple paths
ECMP is available only for Linux in UNIX world
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http://www.cs.princeton.edu/courses/archive/spring11/cos461/

6. Arash PourdamghaniWinter 2016
Benefits of ECMP
Load balancing
Improve bandwidth utilization
Mitigate congestion
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Source DAG
Each Router has set of next hops
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http://courses.cs.washington.edu/courses/csep561/13au

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ECMP Forwarding
Random Allocation Round-Robin Hash-Threshold
Information Random! Per-packet Header based
Balanced Complete Near-Optimal Partial
Disruption High High Low
TCP-Friendly No No Yes
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9. Arash PourdamghaniWinter 2016
Traditional ECMP drawback
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Link capacity = 1

10. Arash PourdamghaniWinter 2016
Weighted ECMP
Optimize end-to-end delay
Distributes based pre-determined ratios
Use heuristic algorithm
Obtain the near-optimal weight configuration
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Weighted ECMP advantage
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References
J. Moy, (April 1998). "OSPF Version 2". The Internet
Society. OSPFv2. Retrieved 2007-09-28.
J. Kurose, K. Ross, “Computer Network Top-Down
Approach”,6th edition, Pearson Education Inc., 2013
A. Tanenbaum, D. Wetherall, “Computer Networks”,
5th edition, Pearson Education Inc., 2011
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13. Arash PourdamghaniWinter 2016
References(cont’d)
C. Hopps, “Analysis of an Equal-Cost Multi-Path
Algorithm(RFC 2992), The Internet Society”, 2000
J. Zhang, K. Xi, L. Zhang , “Optimizing Network
Performance using Weighted Multipath Routing”,
NYU Tandon School of Engineering
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14. Arash PourdamghaniWinter 2016
Thank You
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