Switching and Routing
Outline
Routing overview
Store-and-Forward switches
Virtual circuits vs. Datagram
switching
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Internet Structure
Large corporation
“Consumer ” ISP
Peering
point
Backbone service provider
“ Consumer” ISP
Large corporation
Peering
point
“Consumer”ISP
Small
corporation
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Routing Overview
• Up to now we have considered applications and the transport
layer and assumed the network knows how to get packets from
host A to host B
• We’re now moving down to the network layer of the protocol
stack
• Routing is the task of determining how to get packets from host A
to host B through a series of hops
• We will consider routing from two perspectives
– Intra-domain
• OSPF, RIP
– Inter-domain
• BGP
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Scalable Networks
• Switch
– forwards packets from input port to output port
– port selected based on address in packet header
T3
T3
STS-1
Input
ports
Switch
T3
T3
STS-1
Output
ports
• Advantages
– cover large geographic area (tolerate latency)
– support large numbers of hosts (scalable bandwidth)
• How does the switch decide which port to place packets?
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General Routing Methods
• Datagram or connectionless
– Make sure each packet has enough info to enable switching decision
• Virtual circuit or connection-oriented
– Circuit is constructed before packets are sent
• Source routing
– Less common but simple
– All data about route is provided by source host
• Assume all end nodes have addresses
• Assume ports on switches are either have a number or name
of switch to which it is attached
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Source Routing
0 Sw itch 1
3
0
1
3
2 Sw itch 2
2
3 0 1
3
1
1
2
1 3 0
0
Host A
0 1 3
1
0 Sw itch 3
3
2
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Host B
6
Virtual Circuit Switching
• Explicit connection setup (and tear-down) phase
• Subsequence packets follow same circuit
• Sometimes called connection-oriented model
0 Switch 1
1
3
2
3
5
• Analogy:
phone call
• Each switch
maintains a VC
table
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2 Switch 2
1
0
Host A
7
1
Virtual circuit identifier
CS 640
0 Switch 3
3
4
2
Host B
7
Datagram Switching
• No connection setup phase
• Each packet forwarded independently
• Sometimes called connectionless model
Host D
• Analogy: postal
system
• Each switch
maintains a
forwarding
(routing) table
Host E
0 Switch 1
3
Host C
Host F
1
2 Switch 2
2
3
1
0
Host A
Host G
1
0 Switch 3 Host B
3
2
Host H
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Virtual Circuit Model
• Typically wait full RTT for connection setup before
sending first data packet.
• While the connection request contains the full address for
destination, each data packet contains only a small
identifier, making the per-packet header overhead small.
• If a switch or a link in a connection fails, the connection is
broken and a new one needs to be established.
• Connection setup provides an opportunity to reserve
resources.
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Datagram Model
• There is no round trip time delay waitint for connection
setup; a host can send data as soon as it is ready.
• Source host has no way of knowing if the network is
capable of delivering a packet or if the destination host is
even up.
• Since packets are treated independently , it is possible to
route around link and node failures.
• Since every packet must carry the full address of the
destination, the overhead per packet is higher than for the
connection-oriented model.
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Scalable Routing
• Forwarding vs Routing
– forwarding: to select an output port based on destination address
and routing table
– routing: process by which routing table is built
• Network as a Graph
A
6
1
3
4
C
2
1
B
9
E
F
1
D
• Problem: Find lowest cost path between two nodes
• Factors
– static: topology
– dynamic: load
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