The Impact of Load Balancing

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Load Balancing for Routing
Sundar IyerStanford University
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What is Load Balancing? 1Shortest Path Routing
Shortest Path Routing
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What is Load Balancing? .2Routing using
multiple paths
Load Balancing
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Will this always work better? No knowledge of
the OD Matrix
Link from (B, A) is more highly loaded than
with ShortestPath Routing
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Optimal Routing

• MDRP (Gallager)
• Given a network, and a OD traffic matrix D(i,j),
  and delays associated with each link, find the
  split for each D(i,j,k), from source i to
  destination j, along neighbor k, which will
  minimize total expected delay.
• Optimal Routing cannot be achieved using only
  single path routing

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Approaches to Optimal Routing

• Do not change routing, minimal change to
  forwarding
• Randomized Routing
• Change forwarding structure only
• Overlay Approach
• Change both routing and forwarding, and calculate
  unequal cost multiple paths

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Randomized Routing

• Valiants Scheme
• Packet goes from (s,d)
• Two phase routing
• Choose intermediate node r at random
• Route from (s,r) and then (r,d) deterministically
• Analysis for Hypercube, Butterfly and Mesh
  Networks

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LBR Routing (Bak)

• Choose r from s at random
• - Can use bad paths, too much delay
• Choose r from s, such that the distance is
  not greater than f(s)
• F(s) average radius of the network at s
• Can be too weak a restriction (Star Network)
• Can limit capacity to less than the Min cut.
• F(s) does not depend on d
• Incorporate destination
• F(s) dist(s,d) 1 1/max radius

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Overlay Approach - Present Status

• OSPF and IS-IS
• Multipath routing supported
• But only for shortest equal paths
• This enforces a very severe restriction on the
  number of paths available for Load Balancing
• IGRP
• Supports all paths which are less thanc
  (shortest path)

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Overlay Approach - Present Status

• Given multiple paths
• There is nothing which enforces the ratio of the
  forwarding split
• Cisco used to provide
• Flow Caching, which does not enforce any split
• Cisco Express Forwarding
• Per- Destination Load Balancing
• Per- Packet Load Balancing
• Round Robin i.e. equal splits
• Weighted Round Robin is needed to control the
  split ratio

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Overlay Approach

• Link Weight Optimization under equal load sharing
  is NP Hard
• Equal load sharing can reach near optimal only
  for certain topologies
• It is possible to convert MDRP into link weights
  such that shortest paths to a destination d at
  a node are the paths used by MDRP. Wang, Infocom
  01
• Caveat The flow split ratio is not known!

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Questions

• Is it possible to convert MDRP into link weights
  such that shortest paths to a destination d at
  a node are the paths used by MDRP, such that
• the flow split is equal?
• the paths are such that the flow split is in the
  inverse ratio of weights.
• Answer is probably no to both

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Basic Multipath Routing Approach

• Try and Approximate MDRP
• Partition MDRP into two parts
• Find multiple loop free paths between (s,d)
• Split each flow along these paths using
  heuristics
• Heuristics
• Solve difference equations
• For paths with more delay give less weightage

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Problems in Multipath Routing

• Main Problem
• Avoiding Loops
• 3- node Triangle
• Nodes (A,B,C)
• Each node s has two paths to every destination
  d
• Example (s,d) and (s,x,d), where x is the other
  node.
• Packet from A to B, can loop
• A chooses second best path to B (A, C)
• C chooses second best path to B (C, A)
• .. and loop

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Multipath Routing Protocols

• Distance Vector Extensions
• DASM (Garcia, Zaumen)
• Maintain DAG instead of a tree on each router for
  each destination
• Maintains an ordering constraint for every
  destination, valid for all nodes in the network
  to prevent multipath loops
• MDVA (Vutukury)
• Loop Free
• Link State Extensions
• MPDA (Vutukury)
• Loop Free