Lecture 17

November 8 Intra-domain routing

November 13 Internet routing 1

November 15 Internet routing 2

November 20 End-to-end protocols 1

November 22 End-to-end protocols 2

November 27 End-to-end protocols 3

November 29 Exam 4

December 8 Exam 5 and Final

Routing versus Forwarding

Network Number

Interface MAC Address

10 if0 8:0:2b:e4:b:1:2

Network Number Next Hop

10 171.69.245.10

Routing Table:

Forwarding Table:

Why routing protocols?

• Link failures

• New nodes

• Congestion

• Two approaches:– Distance Vector-based on local information– Link State-based on global information

Distance-vector routingDestination Cost Next

Hop

B B

C C

D C

E 1 E

F 1 F

G F

Routing Loops• Example 1

– F detects that link to G has failed– F sets distance to G to infinity and sends update t o A– A sets distance to G to infinity since it uses F to reach G– A receives periodic update from C with 2-hop path to G– A sets distance to G to 3 and sends update to F– F decides it can reach G in 4 hops via A

• Example 2– link from A to E fails– A advertises distance of infinity to E– B and C advertise a distance of 2 to E– B decides it can reach E in 3 hops; advertises this to A– A decides it can read E in 4 hops; advertises this to C– C decides that it can reach E in 5 hops…

Loop-Breaking Heuristics

• Set infinity to 16

• Split horizon

• Split horizon with poison reverse

Router Information Protocol (RIP)

RIP packet

Link State Routing

• Each node establishes a list of directly connected neighbors and cost of each link

• Floods that information in a LSP to all neighbors

• Retransmits LSPs from other nodes- but does not echo to sender

Propagation of LSPs

LSP Information

• ID of sending node

• Link-state of sending node

• Sequence number

• Time to live

Route Calculation

• Each node has enough information to map the network

• Dijkstra’s shorted path algorithm used to compute the routes

Example: Link State Routing

Routing Table CalculationStep Confirmed Tentative Comments

1

2

3

4

Routing Table Calculation

Step Confirmed Tentative Comments

5

6

Routing Table Calculation

Step Confirmed Tentative Comments

7

OSPF

• Authentication

• Hierarchy-Domains and Areas

• Load Balancing

OSPF Header Format

OSPF link-state advertisement

Metrics

• Issues– Number of Hops– Latency– Bandwidth or Capacity– Congestion

• Difficult to assign a scalar cost to such a complex and changing problem

ARPANET 1

• Lowest Cost=Shortest Queue

ARPANET 2

• Delay=(Depart time-Arrival Time)+Transmission Time+Latency

• Reliability incorporated through the Depart Time parameter

• Wide spread of weights-

• Oscillations

ARPANET 3

• Reduce dynamic range of metric

• Averaging

• Hard limit on changes in metric-like the stock market