Routing

1
Routing
Lesson 10 NETS2150/2850 http//www.ug.cs.usyd.edu.
au/nets2150/
School of Information Technologies
2
Lesson Outline

• Know the characteristics of unicast routing
  protocol
• Understand various ways of routing
• Routing Strategies
• Experiment with common routing algorithms
• Dijsktras Algorithm of OSPF
• Bellman-Ford Algorithm of RIP

3
Routing
Goal find good path thru network from source
to destination.

• Graph abstraction for routing algorithms
• graph nodes are routers
• graph edges are physical links
• link cost/weight delay, cost or congestion
  level

• good path
• typically means minimum cost path

4
Routing in Packet Switched Network

• Routing protocol establish mutually consistent
  routing tables in every router
• Characteristics of routing protocol
• Correctness
• Simplicity
• Robustness

• Stability
• Fairness
• Optimality
• Efficiency

network data link physical
1. Send data
2. Receive data
Packet switching network
5
Performance Criteria

• How to select best path?
• Used for selection of path
• Minimum hop
• Least cost

IP routing table Destination Gateway
Netmask Iface 129.78.8.0 0.0.0.0
255.255.252.0 eth0 127.0.0.0 0.0.0.0
255.0.0.0 lo 0.0.0.0
129.78.11.254 0.0.0.0 eth0
6
Example Packet Switched Network
Link cost

• Shortest path VS
• Least-cost path

7
Routing Strategies

• Fixed
• Flooding
• Random
• Adaptive

8
Fixed Routing

• Single permanent path for each source to
  destination pair
• Path fixed, at least until a change in network
  topology

9
Fixed RoutingTables
Network Topology
10
Flooding

• Packet sent by node to every neighbour and the
  neighbour sends to its neighbours
• Incoming packets retransmitted on every link
  except incoming link
• Eventually a number of copies will arrive at
  destination
• Each packet is uniquely numbered so duplicates
  can be discarded
• To bound the flood, max hop count is included in
  the packets

If (Packets hop count max_hop_count) discard!
11
Flooding Example
12
Properties of Flooding

• All possible routes are tried
• Very robust
• At least one packet will have taken minimum hop
  count or best route
• All nodes are visited
• The common approach used to distribute routing
  information, like a nodes link costs to
  neighbours

13
Random Routing

• Node selects one outgoing path for retransmission
  of incoming packet
• Selection can be random or round robin
• Can select outgoing path based on probability
  calculation
• Path is typically not best one
• Used in hot potato routing
• nodes have no buffer to store packets
• packets routed to any path of the lowest delay

14
Adaptive Routing

• Used by almost all packet switching networks
• Routing decisions change as conditions on the
  network change
• Failure
• Congestion
• Requires info about network
• Decisions more complex

15
Adaptive Routing - Advantages

• Improved performance
• Aids congestion control by exercising load
  balancing across different alternative paths
• But, it is complex
• Reacting too quickly can cause oscillation
• Too slowly, may not be relevant

16
Routing Algorithm classification

• Global or decentralised information?
• Global
• all routers have complete topology, link cost
  info
• Known as link state algorithms
• E.g. Dijkstras Algorithm
• Decentralised
• router knows physically-connected neighbours, i.e
  link costs to neighbours
• iterative process of computation, exchange of
  info with neighbours
• Known as distance vector algorithms
• E.g. Bellman-Ford Algorithm

17
Routing Algorithms

• Given network of nodes connected by
  bi-directional links
• Each link has a cost in each direction and may be
  different
• Cost of path between two nodes is sum of costs of
  links traversed
• For each pair of nodes, find a path with the
  least cost

18
Dijkstras Algorithm Definitions

• Find shortest paths from given 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
• 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 least-cost path from node s to
  node n

19
Dijkstras Algorithm

• Step 1 Initialization
• T s Set of nodes so far incorporated consists
  of only source node
• L(n) w(s, n) for n ? s
• Step 2 Get Next Node
• Find neighbouring node, x, not in T with
  least-cost path from s
• Incorporate node x into T
• Step 3 Update Least-Cost Paths
• L(n) minL(n), L(x) w(x, n) for all n Ï T
• Algorithm terminates when all nodes have been
  added to T

20
Dijkstras Algorithm Notes

• At termination, value L(x) associated with each
  node x is cost of least-cost path from s to x.
• One iteration of steps 2 and 3 adds one new node
  to T
• Defines least cost path from s to that node

21
(No Transcript)
22
Results of Example Dijkstras Algorithm
No T L(B) Path L(C) Path L(D) Path L(E) Path L(F) Path
1 A 2 AB 5 A-C 1 AD ? - ? -
2 A,D 2 AB 4 A-D-C 1 AD 3 A-D-E ? -
3 A,D,B 2 AB 4 A-D-C 1 AD 3 A-D-E ? -
4 A,D,B,E 2 AB 4 A-D-C 1 AD 3 A-D-E 5 A-D-E-F
5 A,D,B,E,C 2 AB 4 A-D-C 1 AD 3 A-D-E 5 A-D-E-F
6 A,D,B,E,C,F 2 AB 4 A-D-C 1 AD 3 A-D-E 5 A-D-E-F
23
Bellman-Ford Algorithm Definitions

• Find shortest paths from given node considering
  at most one hop away
• Then, find the shortest paths with a constraint
  of paths of at most two hops. Then 3 hops, and so
  on
• 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 hops (links) in path
• Lh(n) cost of least-cost path from s to n, at
  most h hops away

24
Bellman-Ford Algorithm

• Step 1 Initialization
• Lh(s) 0, for all h
• L0(n) ?, for n ? s
• Step 2 Update
• For each successive h gt 0 and each node n
• If Lh(n) gt minjLh(j) w(j,n) Then
• Lh1(n)minjLh(j)w(j,n)
• Connect n with predecessor node j that achieves
  minimum cost

25
(No Transcript)
26
Results of Example Bellman-Ford Algorithm
hop Lh(B) Path Lh(C) Path Lh(D) Path Lh(E) Path Lh(F) Path
1 2 AB 5 A-C 1 AD ? - ? -
2 2 AB 4 A-D-C 1 AD 3 A-D-E 10 A-C-F
3 2 AB 4 A-D-C 1 AD 3 A-D-E 5 A-D-E-F
4 2 AB 4 A-D-C 1 AD 3 A-D-E 5 A-D-E-F
27
Algorithms Comparison

• Results from two algorithms agree
• Information gathered
• Bellman-Ford
• Each node can maintain set of costs and paths for
  every other node
• Only exchange information with direct neighbours
• Can update costs and paths based on information
  from neighbours and knowledge of link costs
• Used as part of the Routing Information Protocol
  (RIP)
• Dijkstra
• Each node needs complete topology
• Must know link costs of all links in network
• Must exchange information with all other nodes
• Used as part of Open Shortest Path First Protocol
  (OSPF)

28
OSPF Protocol
29
Example ImageStreams TransPort router

• Software spec
• PPP, Cisco HDLC frame relay
• ATM
• PPPoE PPPoA
• Routing procotols
• RIP, RIP II, OSPF, IS-IS BGP4

• Physical Spec
• 533 MHz VIA C3 processor
• 128 MB SDRAM
• Runs on Linux
• Three onboard 10/100 Ethernet ports
• 1 or 2 T1/E1 ports

30
Summary

• Routing is the process of finding a path from a
  source to every destination in the network
• Different routing strategies available
• Common adaptive routing
• Dijsktras algorithm
• Bellman-Ford algorithm
• Read Stallings Section 12.2 and 12.3
• Next Functions of internetwork layer protocol