Routing Algorithms and Protocols: Understanding Distance Vector and Link State Methods, Slides of Computer Networks

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Routing

R^ R

R^ R S S S

s

s s

s

s s

s

s s

s R

s

R

Backbone

To internet or wide area network

Organization Servers

Gateway

Departmental Server

Figure 7.

Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks

Metropolitan Area Network (MAN)

RA

RB

RC

Route server

NAP

National service provider A

National service provider B

National service provider C

LAN

NAP NAP

(a)

(b)

Figure 7.

Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks

National ISPs

Network Access Point

Packet 2

Packet 1 Packet 1

Packet 2

Packet 2

Figure 7.

Datagram Packet Switching

Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks

Packet

Packet

Copyright ©2000 The McGraw Hill Companies Leon-Garcia & Widjaja: Communication Networks Figure 7.

Virtual Circuit Network

Identifier Output port

(^15 )

58

13

13

7

27

12

Next identifier

44

23 16

34

Entry for packets with identifier 15

Copyright ©2000 The McGraw Hill Companies (^) Leon-Garcia & Widjaja: Communication Networks Figure 7.

Virtual Circuit Network

Routing Table

Routing Classification

Adaptive Routing

• based on current measurements of traffic and/or topology.

1. centralized
2. isolated
3. distributed

Non-Adaptive Routing

• routing computed in advance and off-line

1. flooding
2. static routing using shortest path algorithms

Flooding

• Pure flooding :: every incoming packet to a

node is sent out on every outgoing line.

• Obvious adjustment – do not send out on arriving link (assuming full-duplex links).
• The routing algorithm can use a hop counter (e.g., TTL) to dampen the flooding.
• Selective flooding :: only send on those lines going “approximately” in the right direction.

Metrics

• Set all link costs to 1.
  • Shortest hop routing.
  • Disregards delay.
• Measure the number of packets queued.
  • Did not work well.
• Timestamp ArrivalTime and DepartTime * and use link-level ACK to compute: Delay = (DepartTime – ArrivalTime) + TransmissionTime + Latency

• Reset after retransmission

Metrics

• Unstable under heavy link load.
• Difficulty with granularity of the links.
• Revised ARPANET routing metric:
• Compress dynamic range of the metric
• Account for link type
• Smooth variation of metric with time:
• Delay transformed into link utilization
• Utilization was averaged with last reported utilization.
• Hard limit set on how much the metric could change per measurement cyle.

Dijkstra’s Shortest Path Algorithm

Initially mark all nodes (except source) with infinite distance. working node = source node Sink node = destination node While the working node is not equal to the sink

1. Mark the working node as permanent.
2. Examine all adjacent nodes in turn If the sum of label on working node plus distance from working node to adjacent node is less than current labeled distance on the adjacent node, this implies a shorter path. Relabel the distance on the adjacent node and label it with the node from which the probe was made.
3. Examine all tentative nodes (not just adjacent nodes) and mark the node with the smallest labeled value as permanent. This node becomes the new working node. Reconstruct the path backwards from sink to source.

Internetwork Routing [Halsall]

Adaptive Routing

Centralized (^) Distributed

Intradomain routing Interdomain routing

Distance Vector routing Link State routing

[IGP] [EGP] [BGP,IDRP]

[RIP] [OSPF,IS-IS,PNNI]

[RCC]

Interior Gateway Protocols (^) Gateway ProtocolsExterior

Isolated

Adaptive Routing

Design Issues:

1. How much overhead is incurred due to gathering the routing information and sending routing packets?
2. What is the time frame (i.e, the frequency) for sending routing packets in support of adaptive routing?
3. What is the complexity of the routing strategy?

Distance Vector Routing

• Historically known as the old ARPANET routing algorithm {or known as Bellman-Ford algorithm }.

Basic idea: each network node maintains a Distance Vector table containing the distance between itself and ALL possible destination nodes.

• Distances are based on a chosen metric and are computed using information from the neighbors’ distance vectors.

Metric: usually hops or delay