Traffic Engineering with Traditional IP Routing Protocols

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Traffic Engineering with Traditional IP Routing
Protocols
Manoj Ganesan
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Aim of the Paper

• Provides an overview of working with the
  traditional IP routing protocols.
• No modification to the routing protocols or the
  routers themselves.
• How to adapt link weights, based on a
  network-wide view of the traffic and topology
  within a domain.
• Summarizing the results of techniques for
  optimizing OSPF/IS-IS weights to the prevailing
  traffic.

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Introduction

• In some sense, IP networks manage themselves.
• Adjusting sending rate depending on bandwidth
• Routers compute new paths
• However these mechanisms do ensure efficiency.
• Eg Under-utilized links.
• The focus of this paper would be traffic within a
  single AS (company, ISP, etc).

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Intradomain Traffic Engineering

• Path selection based on Static Link Weights.
• Limitations of static link weights, at the outset
  
• Limited routing scenarios.
• No adaption of link weights, basically.
• Expensive extensions have been proposed.
• Can modify static link weights to do the job

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A simple example.

• Initial configuration 3 units of load on (u,t)
• Local change to the weight of the congested link,
  increased to 2. 2.5 units of load on (w,t).
• Global optimization of the link weights. Most
  optimal solution.

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Advantages of using traditional OSPF

• Process of arriving at a good set of weights is
  handled externally from the routers.
• Modification of link weights is performed on a
  relatively coarse time scale.
• Centralized approach for setting routing
  parameters. Has the following advantages
• Protocol stability.
• Low protocol overhead.
• Use link weights to express routing config
• Compatibility with traditional shortest path
  IGPs.
• Concise representation.

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Traffic Engineering Framework.

• Measure
• Should have a timely and accurate view of the
  current state of the network.
• Estimate of the volume of traffic between each
  pair of routers.
• Model
• Control
• Appropriate commands to affected routers.
• Router updates its link-state database floods
  the new value of the rest of the network.
• Each router computes new shortest paths.
• No frequent changes to link weights.

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Performance properties

• Quantifying how well we can engineer traffic flow
  using traditional OSPF/IS-IS routing protocols.
• Link Utilization.
• Comparing solutions with OPT routing, and simple
  configurations like InvCapOSPF and UnitOSPF.
• Returning back to our original example.
• UnitOSPF and InvCapOSPF, utilization 150
  (u,v).
• Last diagram, utilization for u,v 100.

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Performance comparison with a network wide
objective

• Advanced OSPF, comes closest to OPT routing (only
  3 worse utilization than OPT)
• Minimizing max link utilization might be too
  specific and localized.
• Unavoidable congested links.
• No penalty to solutions that force traffic to
  traverse very long paths.
• Advances OSPF has an additional objective.
• The cost of using a link increases with the
  utilization, with an explosive growth as the
  utilization exceeds 100
• Network wide cost of a routing solution is then
  the sum of all link costs.

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Link cost as a function of the load for a link
capacity 1
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Network-wide cost vs. demand for a proposed ATT
backbone
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Max link utilization vs. demand with same weights
as previous graph
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Changing traffic demands

• Optimizing the weights for a single topology and
  traffic matrix is not sufficient.
• Robustness was tested by changing traffic
  matrices, fluctuations, errors, etc.
• Previous weight settings (for the original TM)
  continued to perform well.
• Optimizing weights across traffic matrices!

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Few changes to Link Weights

• Changes to link weights are necessary in response
  to large shifts in traffic and certain router or
  link failures.
• Fortunately, changing even a single link weight
  is often effective.
• For an operational ATT topology, increasing a
  single weight from 1024 to 1025 could reduce max
  utilization by 8.
• Also, existing IGP weights continued to do well
  even after a single link failure.

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Conclusions

• An overview of how to modify existing IP
  protocols to work efficiently in case of traffic
  fluctuations.
• This approach treats traffic engineering as a
  networks operation task, rather than a
  responsibility of the underlying routing
  protocol.
• As mentioned before, modifying traditional
  protocols have many advantages over proposed
  traffic engineering extensions to these protocols.