cs490 chapter 7b, leon-garcia

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

CS490Chapter 7b, Leon-Garcia

Packet Switching 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

Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies Figure 7.2

Physicallayer

Data linklayer

Physicallayer

Data linklayer

End system

Networklayer

Networklayer

Physicallayer

Data linklayer

Networklayer

Physicallayer

Data linklayer

Networklayer

Transportlayer

Transportlayer

MessagesMessages

Segments

End system

Networkservice

Networkservice


3 2 11 2

21

3 2 11 2

21

21

Medium

A B

3 2 11 2

21

C

21

21

2 134 1 2 3 4

End system

End system

Network

1

2

Physical layer entity

Data link layer entity3 Network layer entity

3 Network layer entity

Transport layer entity4

Figure 7.3


Comparison of Virtual Circuit and Datagram Subnets

Issue Datagram Subnet VC Subnet

Addressing Each packet has source and dest address

Packets contain short VC number

State Info Subnet does not hold state info

Each VC requires subnet table space

Routing Packets routed independently

Route chosen on set up. All packets follow this route

Effect of Router Crashes

None, except packets lost during crash

All VCs that pass through this router are terminated

Congestion Control Difficult Easy if enough buffers can be allocated for each VC


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


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


Combinations of Service and Subnet Structure

Upper Layer(Transport Layer)

Datagram Virtual Circuit

Connectionless UDP over IP UDP over IP overATM

Connection-Oriented

TCP over IP ATM AAL1 overATM

Type of Subnet (Network Layer)


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


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


Classification of Routing Algorithms

• Static vs. Dynamic (Adaptive)

• Centralized vs. Distributed

• Decisions for each Packet vs. at Connection Time


1

2

3

4

5

6

A

B

Switch or router

Host

Figure 7.23

Example of a Packet-Switched Network: Topology for Example


1

2

3

4

5

6A

B

CD

1

5

2

37

1

8

54 2

3

6

5

2

Figure 7.24

Virtual Circuit Packet Switching

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.


Incoming Outgoingnode VC node VC A 1 3 2 A 5 3 3 3 2 A 1 3 3 A 5

Incoming Outgoingnode VC node VC 1 2 6 7 1 3 4 4 4 2 6 1 6 7 1 2 6 1 4 2 4 4 1 3

Incoming Outgoingnode VC node VC 3 7 B 8 3 1 B 5 B 5 3 1 B 8 3 7

Incoming Outgoingnode VC node VC C 6 4 3 4 3 C 6

Incoming Outgoingnode VC node VC 2 3 3 2 3 4 5 5 3 2 2 3 5 5 3 4

Incoming Outgoingnode VC node VC 4 5 D 2 D 2 4 5

Node 1

Node 2

Node 3

Node 4

Node 6

Node 5

Figure 7.25

Follow that circuit from A via VC Nos. 1, 2, 7,8 to B in Routers 1, 3, and 6.


2 2 3 3 4 4 5 2 6 3

Node 1

Node 2

Node 3

Node 4

Node 6

Node 5

1 1 2 4 4 4 5 6 6 6

1 3 2 5 3 3 4 3 5 5

Destination Next node

1 1 3 1 4 4 5 5 6 5

1 4 2 2 3 4 4 4 6 6

1 1 2 2 3 3 5 5 6 3




Destination Next nodeDestination Next node

Figure 7.26

Routing Tables for a Datagram Network. Same Topology.


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


0000 0001 0010 0011

0100 0101 0110 0111

1100 1101 1110 1111

1000 1001 1010 1011

R1R2

1

2 5

4

3

00 1 01 3 10 2 11 3

00 3 01 4 10 3 11 5

(a)

0000 0111 1010 1101

0001 0100 1011 1110

0011 0101 1000 1111

0011 0110 1001 1100

R1R2

1

2 5

4

3

0000 1 0111 1 1010 1 … …

0001 4 0100 4 1011 4 … …

(b)

Figure 7.27

Fig. 7.27 Advantage of Hierarchical Routing

b. Non - Hier.

a. Hierarchical -


1

2

3

4

5

6

1

1

2

32

3

5

2

4

Figure 7.28

Fig. 7.28 Sample net with costs. We will use this net for a detailed example on the blackboard

But, first let's finish talking about different types of routing.


1

2

3

4

5

6

1

1

2

2

2

Figure 7.29

Results of Bellman-Ford Algorithm: Shortest path tree for this network.

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.


Shortest Path Routing Approaches

• Distance Vector (original Internet approach, uses only one metric, often hops, uses Bellman-Ford, has count-to-infinity problem, RIP still used in internets)

• Link State (now most common in Internet, uses Dijkstra,can use multiple cost functions, avoids count-to-infinity)


Other Routing Approaches

• Flooding

• Deflection Routing

• Source Routing


(a)

1

2

3

4

5

6

Figure 7.33 - Part 1 of 3

Flooding Routing Algorithm

Send incoming packets on all output ports, except the one it came in on. First step.


1

2

3

4

5

6

(b)


Second Step of Flooding


1

2

3

4

5

6

(c)


Third step of Flooding. Need control to prevent saturation of network. Use "time-to-live" field


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

Figure 7.34

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

busy

Figure 7.35

Routers can do without buffers. Pure switch can be used.

(0,2) wants to send to (1,0), but (0,1) is busy. Deflect to right.


1

2

3

4

5

6

A

B

Source host

Destination host

1,3,6,B

3,6,B 6,B

B

Figure 7.36

Source Routing


48

63

2

1

5 7

Congestion

Figure 7.50


Offered load

Thr

ough

put

Controlled

Uncontrolled

Figure 7.51


Time

Bit

s pe

r se

cond

Peak rate

Average rate

Figure 7.52


Water drains ata constant rate

Leaky bucket

Water pouredirregularly

Figure 7.53


Arrival of a packet at time ta

X’ = X - (ta - LCT)

X’ < 0?

X’ > L?

X = X’ + ILCT = ta

conforming packet

X’ = 0

Nonconformingpacket

X = value of the leaky bucket counterX’ = auxiliary variableLCT = last conformance time

Yes

No

Yes

No

Figure 7.54


I

L+I

Bucketcontent

Time

Time

Packetarrival

Nonconforming

* * * * * * * **

Figure 7.55


Time

MBS

T L I

Figure 7.56


Tagged or dropped

Untagged traffic

Incomingtraffic

Untagged traffic

Leaky bucket 1PCR and CDVT

Leaky bucket 2SCR and MBS

Tagged or dropped

Figure 7.57


Time0 1 2 3

Time0 1 2 3

10 Kbps

Time0 1 2 3

50 Kbps

100 Kbps

(a)

(b)

(c)

Figure 7.58


Incoming

traffic

Shaped

trafficSize N

Packet

Server

Figure 7.59


Incomingtraffic

Shapedtraffic

Size N

Size K

Tokens arriveperiodically

Server

Packet

Token

Figure 7.60


b bytes instantly

t

r bytes per second

Figure 7.61


A(t) = b+rt

R(t) R(t)

No backlog of packets

bR

b R - r

(a)

(b) Buffer occupancy @ 1

0

empty

tt

Figure 7.62


Congestionwindow

10

5

15

20

0

Round-trip times

Slowstart

Congestionavoidance

Congestion occurs

Threshold

Figure 7.63


3 2 11 2

21

3 2 11 2

21

21

Medium

A B

3 2 11 2

21

C

21

21

2 134 1 2 3 4

End system

End system

Network

1

2

Physical layer entity

Data link layer entity3 Network layer entity

3 Network layer entity

Transport layer entity4

Figure 7.3

cs490 chapter 7b, leon-garcia

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