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Traffic Engineering with Traditional IP Routing Protocols Manoj Ganesan 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. 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). 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 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. 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. 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. 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%. 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. Link cost as a function of the load for a link capacity 1 Network-wide cost vs. demand for a proposed AT&T backbone Max link utilization vs. demand with same weights as previous graph 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! 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 AT&T 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. 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.
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