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CS490
Chapter 7b, Leon-Garcia
Packet Switching Networks
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Today’s Outline
• 7.3 Datagrams vs. Virtual Circuits (a little more)
• Plus Definition of ATM
• 7.4 Routing in Packet Networks
– Distance Vector, Link State, Flooding, Deflection
Routing, Source Routing
• 7.5 Shortest Path Algorithms
– Bellman Ford Algorithm
– Construction of Routing Table and Updates
– (On the blackboard)
– We will not cover Dijkstra's algorithm in detail
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Messages
Messages
Segments
Transport
layer
Transport
layer
Network
service
Network
service
Network
layer
Network
layer
Network
layer
End system Data link
layer
a
Data link
layer
Data link
layer
Data link End system
layer
b
Physical
layer
Physical
layer
Physical
layer
Physical
layer
Network
layer
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.2
C
1 2
3
2 1
End system
b
End system
a
4 3 2 1
1 2
3
2 1
1 2
3
B
2
1
2 1
1 2 3 4
Medium
A
Network
1
Physical layer entity
3
2
Network layer entity
4
Transport layer entity
Network layer entity
Data link layer entity
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.3
Comparison of Virtual Circuit and Datagram Subnets
Issue
Datagram Subnet VC Subnet
Addressing
Each packet has
source and dest
address
Subnet does not
hold state info
Packets routed
independently
State Info
Packets contain
short VC number
Each VC requires
subnet table space
Routing
Route chosen on set
up. All packets
follow this route
Effect of Router
None, except
All VCs that pass
Crashes
packets lost during through this router
crash
are terminated
Congestion Control Difficult
Easy if enough
buffers can be
allocated for each
VC
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
IP Internet Protocol (Network Layer)
• Actually most information on IP is in Chapter 8 on TCP/IP
• Here we should just know that IP is a datagram service,
packets are routed independently of one another
• It is not connection-oriented at the network layer, but can
be at the transport layer above
• The IP packet has a header of 20-60 bytes including source
and destination addresses, CRC, and various option and
control fields. Details in 8.2.
• The total length of a packet, including info, can be up to
65K bytes, but transit of Ethernet LANs often limits to
1500 bytes
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Asynchronous Transfer Mode Definition
• We will skip 7.6 as far as the exam is concerned, but here
is a concise definition of ATM (p 483)
• Connection oriented in network layer
• Short (48 info bytes) fixed length packets called “cells”
• Cells contain short (5 byte) headers that point to
connections
• ATM uses fast hardware switches up to 10,000 ports with
up to 150Mbps each
• ATM has some of the best features of circuit switching and
packet switching. Asychronous = no master clock
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Combinations of Service and Subnet Structure
Upper Layer
(Transport Layer)
Type of Subnet (Network Layer)
Datagram
Virtual Circuit
Connectionless
UDP over IP
UDP over IP over
ATM
ConnectionOriented
TCP over IP
ATM AAL1 over
ATM
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
7.4 Routing in Packet Switched Networks
• Net = Routers (or Switches) and links
• Routing involves
– Setting up routing tables
– Forwarding packets
• Routing Algorithm tries to set up “best” routes
– minimize hops or
– minimize delay or
– maximize bandwidth or ...
• The Routing Algorithm needs global info about net
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Goals of Routing Algorithm
•
•
•
•
•
Rapid and Accurate Delivery of Packets
Adapt to Failure of Node or Link
Adapt to Change in Traffic Loads
Determine Connectivity of Network
Low Overhead
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Classification of Routing Algorithms
• Static vs. Dynamic (Adaptive)
• Centralized vs. Distributed
• Decisions for each Packet vs. at Connection
Time
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Example of a Packet-Switched
Network: Topology for Example
1
3
6
A
4
B
Host
2
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5
Switch or router
Leon-Garcia & Widjaja: Communication Networks
Figure 7.23
Virtual Circuit Packet Switching
1
A
1
2
3
5
7
3
1
4
8
6
B
5
2
4
3
C
2
6
5
5
2
D
Note: VC numbers change at each router.
Route on top (thin line) changes from 1 to
2 to 7 to 8. Next slide has routing tables.
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.24
Follow that circuit from A via VC Nos. 1, 2, 7,8 to B in
Routers 1, 3, and 6.
Node 3
Node 1
Incoming
node VC
A 1
A 5
3
2
3
3
Outgoing
node VC
3
2
3
3
A 1
A 5
Incoming
node VC
1
2
1
3
4
2
6
7
6
1
4
4
Outgoing
node VC
6
7
4
4
6
1
1
2
4
2
1
3
Node 6
Incoming
node VC
3
7
3
1
B
5
B
8
Outgoing
node VC
B
8
B 5
3
1
3
7
Node 4
Node 2
Incoming
node VC
C
6
4
3
Outgoing
node VC
4
3
C 6
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Incoming
node VC
2
3
3
4
3
2
5
5
Outgoing
node VC
3
2
5
5
2
3
3
4
Leon-Garcia & Widjaja: Communication Networks
Node 5
Incoming
node VC
4
5
D
2
Outgoing
node VC
D
2
4
5
Figure 7.25
Routing Tables for a Datagram Network. Same Topology.
Node 1
Destination
Next node
2
2
3
3
4
4
5
2
6
3
Node 2
Destination
Next node
1
1
3
1
4
4
5
5
6
5
Copyright ©2000 The McGraw Hill Companies
Node 3
Destination
Next node
1
2
4
5
6
1
4
4
6
6
Destination
1
2
3
5
6
Node 6
Destination
Next node
1
3
2
5
3
3
4
3
5
5
Node 4
Next node
1
2
3
5
3
Leon-Garcia & Widjaja: Communication Networks
Destination
1
2
3
4
6
Node 5
Next node
4
2
4
4
6
Figure 7.26
Hierarchical Addresses in the Internet
• Actually the book covers TCP/IP together in Chapter 8
• Here (p 488) it points out that routing is simplified if hosts
within a domain have the same prefix (network address).
• Then routers outside the domain only have to examine (and
store) the prefix
• Thus IP addresses are always divided into a network
address and a host address. (Usually there are three levels,
often: network address, LAN address, host address)
• See Fig 7.27
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
(a)
Fig. 7.27 Advantage of Hierarchical Routing
0000
0001
0010
0011
1
0100
0101
0110
0111
4
3
a. Hierarchical -
R2
R1
5
2
1000
1001
1010
1011
00
01
10
11
1
3
2
3
00
01
10
11
1100
1101
1110
1111
3
4
3
5
(b)
0000
0111
1010
1101
1
0001
0100
1011
1110
4
3
b. Non - Hier.
R2
R1
5
2
0011
0110
1001
1100
Copyright ©2000 The McGraw Hill Companies
0000
0111
1010
…
1
1
1
…
0001
0100
1011
…
4
4
4
…
Leon-Garcia & Widjaja: Communication Networks
0011
0101
1000
1111
Figure 7.27
Fig. 7.28 Sample net with costs. We will use this net
for a detailed example on the blackboard
1
2
1
3
6
5
2
3
4
1
2
3
2
4
5
But, first let's finish talking about different types of routing.
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.28
Results of Bellman-Ford Algorithm:
Shortest path tree for this network.
1
2
1
3
6
2
1
2
4
2
5
Our bird's eye view of the net allows us to easily see that this is the
lowest cost solution, but it's not so easy for the routers to do this
automatically. They only have information measured by other routers
to use.
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.29
Shortest Path Routing Approaches
• Distance Vector (original Internet approach, uses only one
metric, often hops, uses Bellman-Ford, has count-toinfinity problem, RIP still used in internets)
• Link State (now most common in Internet, uses
Dijkstra,can use multiple cost functions, avoids count-toinfinity)
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Other Routing Approaches
• Flooding
• Deflection Routing
• Source Routing
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Flooding Routing Algorithm
(a)
1
3
6
4
2
5
Send incoming packets on all output ports, except the
one it came in on. First step.
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.33 - Part 1 of 3
Second Step of Flooding
(b)
1
3
6
4
2
5
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.33 - Part 2 of 3
Third step of Flooding. Need control to prevent
saturation of network. Use "time-to-live" field
(c)
1
3
6
4
2
5
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.33 - Part 3 of 3
Hot Potato or Deflection Routing
0,0
0,1
0,2
0,3
1,0
1,1
1,2
1,3
2,0
2,1
2,2
2,3
3,0
3,1
3,2
3,3
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.34
Routers can do without buffers. Pure switch can be used.
busy
0,0
0,1
0,2
0,3
1,0
1,1
1,2
1,3
2,0
2,1
2,2
2,3
3,0
3,1
3,2
3,3
(0,2) wants to send to (1,0), but (0,1) is busy. Deflect to right.
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.35
Source Routing
3,6,B
6,B
1,3,6,B
1
3
6
B
A
4
B
Source host
2
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5
Destination host
Leon-Garcia & Widjaja: Communication Networks
Figure 7.36
Congestion
3
6
1
4
8
2
5
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.50
Throughput
Controlled
Uncontrolled
Offered load
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.51
Bits per second
Peak rate
Average rate
Time
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.52
Water poured
irregularly
Leaky bucket
Water drains at
a constant rate
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.53
Arrival of a packet at time ta
X’ = X - (ta - LCT)
X’ < 0?
No
Nonconforming
packet
Yes
Yes
X’ = 0
X’ > L?
No
X = X’ + I
LCT = ta
conforming packet
Copyright ©2000 The McGraw Hill Companies
X = value of the leaky bucket counter
X’ = auxiliary variable
LCT = last conformance time
Leon-Garcia & Widjaja: Communication Networks
Figure 7.54
Nonconforming
Packet
arrival
Time
L+I
Bucket
content
I
* *
*
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*
*
** *
Leon-Garcia & Widjaja: Communication Networks
*
Time
Figure 7.55
MBS
T
Copyright ©2000 The McGraw Hill Companies
L
I
Leon-Garcia & Widjaja: Communication Networks
Time
Figure 7.56
Incoming
traffic
Leaky bucket 1
PCR and CDVT
Tagged or
dropped
Untagged
traffic
Leaky bucket 2
SCR and MBS
Tagged or
dropped
Untagged traffic
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.57
10 Kbps
(a)
0
1
2
3
Time
1
2
3
Time
1
2
3
Time
50 Kbps
(b)
0
100 Kbps
(c)
0
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.58
Incoming
traffic
Shaped
traffic
Size N
Server
Packet
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.59
Tokens arrive
periodically
Size K
Token
Incoming
traffic
Shaped
traffic
Size N
Server
Packet
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.60
b bytes
instantly
r bytes per
second
t
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.61
(a)
A(t) = b+rt
No backlog
of packets
R(t)
(b)
R(t)
b
R-r
Buffer
occupancy
@1
0
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b
R
empty
t
Leon-Garcia & Widjaja: Communication Networks
t
Figure 7.62
20
Congestion
avoidance
Congestion occurs
15
Congestion
window
Threshold
10
5
Slow
start
0
Round-trip times
Copyright ©2000 The McGraw Hill Companies
Leon-Garcia & Widjaja: Communication Networks
Figure 7.63
C
1 2
3
2 1
End system
b
End system
a
4 3 2 1
1 2
3
2 1
1 2
3
B
2
1
2 1
1 2 3 4
Medium
A
Network
1
Physical layer entity
3
2
Network layer entity
4
Transport layer entity
Network layer entity
Data link layer entity
Copyright ©2000 The McGraw Hill Companies
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Leon-Garcia & Widjaja: Communication Networks
Figure 7.3