Data Communications and Networking

1
Data Communications and Networking

• Chapter 11
• Routing in Switched Networks
• References
• Book Chapters 12.1, 12.3
• Data and Computer Communications, 8th edition
• By William Stallings

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Routing in Packet-Switching Network

• Recall the function of a packet-switching network
• To accept packets from a source station and
  deliver them to a destination station.
• To accomplish this, a path or route through the
  network must be determined.
• In general terms, routing seeks to design routes
  through the network for individual pairs of
  communicating end nodes such that the network is
  used efficiently.
• Routing is one of the most complex and crucial
  design aspects of switched data networks.
• Required characteristics of routing function
• Correctness
• Simplicity
• Robustness the ability to deliver packets via
  some route in the face of localized failures and
  overloads
• Stability
• Fairness
• Optimality
• Efficiency routing involves processing overhead
  transmission overhead

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Performance Criteria

• The selection of a route is generally based on
  some performance criterion.
• The simplest criterion
• Minimum-hop (the least number of nodes)
• A generalization of minimum-hop criterion
• Least-cost routing a cost is associated with
  each link
• E.g., in minimum-hop, each link has a cost of 1
• The route that accumulates the least cost is
  sought.

4
Example Network Configuration
Least-cost path from node 1 to 6
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Decision Time and Place

• Two characteristics of routing decision
• Time and place that the decision is made
• Decision time
• Datagram a routing decision is made individually
  for each packet
• Virtual circuit a routing decision is made at
  the time the virtual circuit is established
• Decision place which node or nodes are
  responsible for the routing decision?
• Distributed routing
• Each node has the responsibility of selecting an
  output link for routing packets as they arrive
• Centralized routing
• Decision is made by some designated node
• Source routing
• Decision is made by the source station

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Network Information Source and Update Timing

• Routing decisions usually are based on knowledge
  of network (not always) topology, traffic load,
  link cost
• Distributed routing
• Nodes use local knowledge
• May collect info from adjacent (directly
  connected) nodes
• May collect info from all nodes on any potential
  route of interest
• Centralized routing
• Collect info from all nodes
• Timing for node information update
• Fixed strategy the info is never updated
• Adaptive strategy the info is regularly updated

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Routing Strategies

• Many routing strategies have evolved in
  packet-switching networks. Four of them will be
  introduced here
• Fixed routing
• Flooding
• Random routing
• Adaptive routing

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Fixed Routing

• A single, permanent route is configured for each
  source-destination (s-d) pair of nodes in the
  network.
• Determine routes using any least-cost algorithm
• The link costs could be based on expected traffic
  or capacity.
• Routes are fixed, or at least only change when
  there is a change in network topology.
• A central routing directory (table) is created,
  which shows for each s-d pair of nodes, the
  identity of the next node on the route.

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Fixed RoutingTables
Using the figure on Slide 4. It is not necessary
to store the complete central routing directory
for each pair of nodes.
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Flooding

• Flooding requires no network info.
• A packet is sent by a source node to every
  neighbor.
• At each node, an incoming packet is forwarded to
  all outgoing links except the incoming link.
• Eventually a number of copies will arrive at
  destination.
• Each packet is uniquely numbered so duplicates
  can be discarded.
• Nodes can remember the identity of those packets
  it has already retransmitted.
• Another technique include a hop count field with
  each packet. Each time a node passes on a packet,
  it decrements the hop count by one. When the
  count reaches zero, the packet is discarded.
• The hop count is sometimes called time-to-live
  (TTL)
• Recall that TTL is also used in IP header which
  has nothing to do with flooding!

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Flooding Example
The TTL value decreases by one after each hop.
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Properties of Flooding

• All possible routes are tried
• A packet will always get through if at least one
  path between source and destination exists.
• At least one copy of the packet will have taken
  the minimum-hop-count route
• Can be used to set up the virtual circuit
• All nodes that are directly or indirectly
  connected to the source node are visited.
• Useful to broadcast information (e.g. routing
  information)
• Disadvantage high traffic load

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Random Routing

• Random routing has the simplicity and robustness
  of flooding, with far less traffic load.
• A node selects only one outgoing path to forward
  the incoming packet
• The outgoing link can be selected at random,
  excluding the link on which the packet arrived.
• A refinement is to select outgoing path based on
  probability calculation
• Like flooding, no network info needed
• Disadvantage route is typically not least-cost
  nor minimum-hop

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Adaptive Routing

• Routing decisions change as conditions on the
  network change
• Failure node or link fails
• Congestion a portion of the network is heavily
  congested
• Requires info about the state of the network
• Drawbacks, compared to fixed routing
• Routing decision is more complex the processing
  burden on nodes increases
• A tradeoff between quality of network info and
  the cost of updating such info
• Advantages
• Improved performance
• Can aid in congestion control, because an
  adaptive routing strategy tends to balance loads.

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Least Cost Algorithms

• Virtually all packet-switching networks base
  their routing decisions on some form of lest-cost
  criterion.
• Can minimize hop with each link cost 1
• Can have link value inversely proportional to
  capacity
• Least-cost routing problem
• Given a network of nodes connected by
  bi-directional links, where each link has a cost
  associated with it in each direction, define the
  cost of a path between two nodes as the sum of
  costs of links traversed. For each pair of nodes,
  find a path with the least cost.
• Two common algorithms
• Dijkstras algorithm
• Bellman-Ford algorithm

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Dijkstras Algorithm Definitions

• Find the shortest paths from a single source node
  to all other nodes, by developing paths in order
  of increasing path length
• N set of nodes in the network
• s source node
• T set of nodes so far incorporated by the
  algorithm (its least-cost path to s is finalized)
• w(i, j) link cost from node i to node j
• w(i, i) 0
• w(i, j) ? if the two nodes are not directly
  connected
• w(i, j) ? 0 if the two nodes are directly
  connected
• L(n) cost of the least-cost path from node s to
  node n that is currently known to the algorithm
• At termination, L(n) is cost of the least-cost
  path from s to n
• Initially, all L(n) are ?

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Dijkstras Algorithm Method

• Step 1 Initialization
• T s the set of nodes so far incorporated
  consists of only the source node
• L(n) w(s, n) for n ? s the initial path
  costs to neighboring nodes are simply the link
  costs
• Step 2 Get Next Node
• Find the neighboring node not in T with the
  least-cost path from s (smallest L(n) )
• Incorporate that node into T
• Also incorporate the edge that is incident on
  that node and a node in T that contributes to the
  path
• Step 3 Update Least-Cost Paths
• L(n) minL(n), L(x) w(x, n) for all n Ï T
• If the latter term is smaller, the path from s to
  n is now updated as the path from s to x extended
  with the edge from x to n

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Dijkstras Algorithm Notes

• Terminate when all nodes have been added to T
• At termination, value L(n) associated with each
  node n is the cost of least-cost path from s to
  n.
• In addition, algorithm defines the least-cost
  path from s to each other node
• One iteration of steps 2 and 3 adds one new node
  to T, and defines the least-cost path from s to
  that node

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Example of Dijkstras Algorithm
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Example of Dijkstras Algorithm
S 1
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Bellman-Ford Algorithm Definitions

• Find the least-cost paths from a given source
  node subject to constraint that the paths contain
  at most one link, then find the shortest paths
  with a constraint of paths of at most two links,
  and so on.
• Finally, this algorithm returns the least-cost
  paths between any pairs of nodes.
• s source node
• w(i, j) link cost from node i to node j
• w(i, i) 0
• w(i, j) ? if the two nodes are not directly
  connected
• w(i, j) ? 0 if the two nodes are directly
  connected
• h maximum number of links in a path at the
  current stage of the algorithm
• Lh(n) cost of the least-cost path from s to n
  under the constraint of no more than h links

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Bellman-Ford Algorithm Method

• Step 1 Initialization
• L0(n) ?, for all n ? s
• Lh(s) 0, for all h
• Step 2 Update
• For each successive h ? 0
• For each n ? s, compute
• Lh1(n)min Lh(n), minjLh(j)w(j,n)
• It means the least-cost path from s to n of
  length h1 is the least-cost path of length h, or
  it is actually a length h1 path, and before it
  reaches n, it passes through j

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Example of Bellman-Ford Algorithm
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Example of Bellman-Ford Algorithm
S 1
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Comparison

• What information needs to be gathered?
• Bellman-ford
• Each node can maintain a set of costs and
  associated paths for every other node and
  exchange this information with its direct
  neighbors from time to time
• Each node can update its costs and paths using
  only the information from its neighbors and
  knowledge of its link costs.
• Dijkstras algorithm
• Each node must have complete topological
  information about the network