Chapter 7—packet-switching - Computer & Information Science ...

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1

Chapter 7

packet
-
switching


Chapter 5: data
-
link layer protocols (some
protocols are nearly same for transport layer)


Chapter 6: MAC sublayer (for broadcast
networks).


This chapter: network layer


Telephony networks (i.e., circuit
-
switching) for stream
of real time voice


Packet
-
switching networks for short messages, burst
information, as well as real
-
time applications.

2

Two views of networks


External view: what services provided to transport layer
by a network (i.e. network layer)


Need connection setup or not


What QoSs are provided


Services should be independent from underlying networks so that
transport layer can run over any networks as long as the
networks provide the services


Internal view:


physical topology,



datagram message transfer or virtual circuit information transfer,


addressing and routing
, congestion control.


In this chapter, networks indicate systems to transfer
information from one end to the other. It is reasonable to
consider networks to be network layer here.

3

Comparisons of two views by examples


Broadcast networks and packet
-
switched networks


From external view, both networks provides
transfer of information between users, not too
much different


But from internal view, very different:


A broadcast network (such as LANs) is small,
addressing is simple, frame transferred in one hop so no
routing is needed


In a packet
-
switching network, addressing must
accommodate large
-
scale networks and routing is
necessary.

4

What we will discuss


Network services and internal network operations


Physical view of networks,


Datagram and virtual circuit


Routing


Shortest path algorithms


ATM networks


Traffic management and congestion control (if
have time)

5

Figure 7.1

t
0

t
1

Network

Transfer of a block of info. VS. a sequence of blocks

6

Services provided by network layer


Two basic kinds of services


One view: Transfer of a block of information VS. a sequence of blocks


Second view: connectionless VS. connection
-
oriented.


IP provides connectionless service VS. ATM provides connection
-
oriented service



Other services such as:


Best
-
offer connectionless services


Low
-
delay connectionless service


Connection
-
oriented reliable stream service


Connection
-
oriented transfer of packets with delay and bandwidth guarantees



End
-
to
-
end argument:


Functions should be placed as close as possible to the application since the application is in best
position to determine whether a function is being carried out completely and correctly,


Therefore, network layer should provide
minimum functions

required to meet application
requirements and performance. Leave more functions to upper layers such as transport layer.

7

Figure 7.2

Physical

layer

Data link

layer

Physical

layer

Data link

layer

End system

a

Network

layer

Network

layer

Physical

layer

Data link

layer

Network

layer

Physical

layer

Data link

layer

Network

layer

Transport

layer

Transport

layer

Messages

Messages

Segments

End system

b

Network

service

Network

service


protocol stack: network provides
minimum required

services to transport layer

8

Discussion of connectionless vs. connection
-
oriented


Both connection
-
oriented and connectionless services in the upper layer can be
implemented over a connectionless network (layer) (such as IP), similarly, can
be implemented over a connection
-
oriented network (layer) (such as ATM).


In network layer,
connectionless
service is also called
datagram

service,


connection
-
oriented

service called
virtual
-
circuit

service.

Which is preferred?

From end
-
to
-
end argument, connectionless network (IP) is preferred because

IP makes intermediate nodes as simple as possible and put the burden on end

systems. Moreover, it makes the network grow easily (more scalable).

Why ATM which is connection
-
oriented?

Main reason is QoS because connection
-
oriented network can easily guarantee

bandwidth, delay, etc. Detail motivation behind ATM will be discussed later.

Network layer: connectionless(IP) connection
-
oriented (ATM)

Transport layer: connectionless connection
-
oriented

Thinking:

differences (or implications) of connection
-
oriented in transport layer and in


network layer. What the connection between two ends in transport layer means?

9

Most basic functions of a network (layer)

Routing and forwarding.

Priority and scheduling (to provide QoS)

Congestion control (also in transport layer)

Segmentation

(to deal with different frame sizes of underlying systems)

Alternatively, network (layer) sends error message to let edge system

to do segmentation.

Addressing


1. to deal with different address formats when


interconnecting networks.



2. Hierarchical address for scalability.

10

Figure 7.4



.

.

.

MUX

Network access

Node

Packet network topology
--
Access multiplexer

Multiplexer combines multiple bursty flows

into one aggregated flow.

Access node forwards

packets into
backbone

packet networks
.

11

LAN

Bridge

LAN 1

LAN 2

(a)

(b)

Figure 7.5

Packet network topology
--
LAN


Now nearly every LAN is connected to packet
-
switching network (the
Internet) to share information globally and to make the Internet
extremely large.

LANs were introduced for sharing of resources in a organization.

They are basic building components for Wide Area Networks.

12

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 7.6

Campus network

LANs

extended LANs


subnetworks (by backbone networks)

campus network



university network, finally connect to Internet,


bridges, routers and gateway,… play key
-
role in networks

Backnone network may
be connected by links,
LAN, or ATMs.


Servers provide various services
.

13

Interdomain level

Intradomain level

LAN level

Autonomous system

or domain

Border routers

Border routers

Figure 7.7

Internet service
provider

Intradomain and interdomain levels

Domain: indicate the routers run the same routing protocols.

Autonomous systems: included

domains under a single administrations


Various networks are connected through ISP.


Tremendous small office and home (SOHO)
users connect to the Internet through ISP,
using dynamic IPs.



14

R
A

R
B

R
C

Route
server

NAP

National service provider A

National service provider B

National service provider C

LAN

NAP

NAP

(a)

(b)

Figure 7.8

National service providers are connected by NAPs (national access point)

Route server distributes routing information to routers.

15

Packet
-
switching topology


What a big picture of topology for picket
-
switching
networks!!!



Hierarchical structure, from LANs


campus, university
and organization networks

Internet


ISPs provide backbone networks to connect tremendous
different networks and enormous home PCs


Switches (bridges, routers, gateways): key elements


Various servers provides rich services to users


Domain, intradomain, interdomain for easy administration
and network management.

16

Network

Packet
switch

Transmission
link

Figure 7.9

Simplified picture of switched networks

1.
Transmission lines and packet switches

2.
They provide connectivity between
any

source and destination
dynamically


3.
The resources are allocated when needed, so can be shared among multiple users

4. Information can be transferred in connection
-
oriented or connectionless manner.

17

Brief discussion of switches


Switches are generic term for switching
devices in switched networks


Circuit
-
switches in telephony network


LAN switches in LAN connection, i.e. bridges,
Ethernet switch,…


Packet switches in pack switching networks,
i.e. routers and gateways.

18

Control

1

2

3

N

Line Card

Line Card

Line Card

Line Card

Interconnection

Fabric

Line Card

Line Card

Line Card

Line Card

1

2

3

N

Figure 7.10









Structure of a packet switch/router

Components: input ports, output ports, interconnection fabric, controller

A line card handles several pairs of input/output ports and implements physical layer and data

link layer functions: symbol timing, line coding, framing, physical addressing, error checking,

MAC protocol, data link protocol, and buffering (speed mismatch between lines and fabric).

19

CPU

1

2

3

N

NIC Card

NIC Card

NIC Card

NIC Card

Main Memory

I/O

Bus

Figure 7.11





A PC or workstation can be configured to become a switch

1.Several NICs are inserted into expansion slots,

2. Install routing and other protocols in the computer

Operations:

1.
NIC gets frames from networks and de
-
encapsulate packets

2.
Packets are transferred into memory using I/O bus

3.
CPU performs required routing and protocol processing

4.
Packets are transferred from memory to an appropriate NIC

5. The NIC forms new frame and sends out

20

Three basic resources and bottlenecks in switches


Processing, memory and bus bandwidth


Processing implements protocols, hence processing
capacity places a limit on maximum rate at which
switch can operate


Memory stores packets, hence the amount of memory
determines the rate at which packets are lost, placing
another limit on switch load, moreover memory
bandwidth also place limit on switch rate


I/O bus bandwidth places a limit on total rate at which
information can be transferred between ports.

21

1

2

N

1

2

N

Figure 7.12





Input port demultiplexes incoming packet stream;

packets are routed to output port;

output port multiuplexes outgoing packet stream

Switches/routers play a key role in controlling packet flows, thus

efficiently using network bandwidth and optimizing performance.

22

Network

nodes

Message

Subscriber

B

Subscriber

A

Message

Message

Message

Figure 7.13

Connectionless packet switching

originated from
message switching


message has header with source & destination address



CRC check bit are used to detect errors


Each switch check error, if yes, ask retransmission, if not find next hop.


Message enter into a QUEUE to wait for line free to transmit


Increased utilization of line is at the expense of queuing delay


Loss of message may occur because of insufficient buffer.


End
-
to
-
end error recovery is needed

23

t

t

t

t

Delay

Source

Destination

T

p

Minimum Delay =
3
p

+ 3
T
,

( remember it,will use it later)

Switch 1

Switch 2

Figure 7.14

Delays in message switching

Back

p
: propagation delay

T
: message transmission time

24

Packet switching VS. Message switching

Why packet switching?

1. Long message has more chance to incur error than short packet

2. Long message is not suitable for interactive application because

it causes very long waiting delay on other messages. Moreover

the delay for one message is generally longer than the total delay

of packets the message is divided into.

Examples:
L=1M=10
6

bits over two hops. Bit error rate: p=10
-
6



the probability without error: P
c
=(1
-
p)
L
=(1
-
10
-
6
)
10
6

e
Lp
=
e
-
1


1/3.

So on average, 1/(1/3)=3 tries for 1st hop. Similarly, 3 tries

for 2
nd

hop, Total six tries, 6M.

Suppose divided into 10
5

bit packets, the no
-
error probability

0.9.

So each packet needs 1/0.9=1.1 times on average.

So entire message

Get transmitted over each hop using 1.1M bits transmission.

So total 2.2M bit transmission over two hops.

Example: come soon.

25

Packet 2

Packet 1

Packet 1

Packet 2

Packet 2

Figure 7.15

Datagram packet switching

Operations:
1. Address information is included in header 2. CRC for error recovery


3. switches inspect destination address in header to determine next hop


4. Packets are put in QUEUE to wait for line becoming available


5. Sharing lines among multiple packets, high utilization is at the expense of queue delay


6. Packets travel independently and may along different paths

7.
Route may be detoured,thus bypassing failure and congestion

8. Packets may arrive out of order, resequencing may be required

26

t

t

t

t

3

1

2

3

1

2

3

2

1

3
p

+ 2(
T
/3) first bit received

3
p

+ 3(
T
/3) first bit released

3
p

+ 5 (
T
/3) last bit released

Lp + (L
-
1
)P

first bit received

Lp + LP

first bit released

Lp + LP + (k
-
1
)P

last bit released

where
T = k P

3 hops

L
hops

p

p + P

p + P

Source

Destination

Switch 1

Switch 2

Figure 7.16

Delay in packet switching networks

Assume three packets along the same path and transmitted in succession

27

Delay comparison of message switch vs. packet switch


Support two switches and 3 packets /message


p
: propagation delay,
T
: message transmission time,
P
: packet
transmission time, and
T=3P


Delay in message switching:
3p+3T
see detail


Delay in packet switching:
3p+T+2T/3


Support
L
-
1

switches and
k

packets/message:


T=kP


Delay in message switching:
Lp+LT =Lp+LkP


Delay in packet switching (
k

packets):
Lp+LP+(k
-
1)P


Results: message switching involves additional delay of
(L
-
1)(k
-
1)P.


Note, in the above, we neglect queuing and processing at each
switch.

28

Destination

address

Next hop

1345

2343

2458

3245

0785

3784

7612

1566

Figure 7.16

Routing table in connectionless packet switching

Designing routing table is a key issue in packet
-
switching networks

Which requires: knowledge of network topology and traffic levels.

Moreover, size of table will become very large when network size increase.

29

Example

IP internetworks


Objective of IP is to provide connectionless
transfer service across heterogeneous
networks


Read it through textbook.

30

Virtual
-
circuit packet switching


Establishment of connection between source &
destination prior to the transfer of packets
See figure


Connect request is sent by source and connect
confirm is replied by destination
See figure



Signal exchanges is performed in virtual
-
circuit setup
See figure


All packets from a connection follow the same path.


Admission control could be used to limit load on
networks or specific links


Routing is easier once connection was setup
routing
table

31

Packet

Packet

Figure 7.17

back

Virtual
-
circuit switching

Question: why called
virtual
-
circuit
?

Because resources, e.g., switches, buffers and lines, are shared


by packets from multiple connections, not dedicated to one

specific connection.

32

t

t

t

t

3

1

2

3

1

2

3

2

1

Release

Connect
request

CR

CR

Connect
confirm

CC

CC

Figure 7.19

Back

Beginning latency and delay of packets in virtual
-
circuit packet switching

33

SW
1

SW
2

SW
n

Connect
request

Connect
request

Connect
request

Connect
confirm

Connect
confirm

Figure 7.20



Back

Signaling message exchanges in virtual
-
circuit setup

Parameters such as buffer, bandwidth, delay requirements

were set in every switch along the path during setup

34

Identifier

Output

port

15

15

58

13

13

7

27

12

Next

identifier

44

23

16

34

Entry for packets

with identifier 15

Figure 7.21

Example of virtual
-
circuit routing table for an input port

Each connection is identified by VCI (
virtual
-
circuit identifier
).

VCI may be different along the path for the same connection: the input VCI in a

switch is different from output VCI for the same connection, called local VCIs.

VCI is in the header of a packet, which is much smaller than IP address, as in IP protocol.

Table lookup will find the output port # and output VCI based on input VCI, which is faster.

35

Pro and con about virtual
-
circuit switching


Pro:



Header is shorter, resources allocated during
setup, admission and congestion are easier.


Minimum delay reduces further.
See figure


Con:


Every router needs to maintain state information
about all connections


Once there is a failure, all affected connections
must be set up again.


Example

ATM networks

36

3

1

2

3

1

2

3

2

1

Minimum Delay = 3
p
+
T


t

t

t

t

Source

Destination

Switch 1

Switch 2

Figure 7.22

Cut
-
through packet switching

Forward a packet as soon as the header is received and table lookup is carries out.

Assumptions: no error check, all lines are available.


Back

37

t

t

t

t

3

1

2

3

1

2

3

2

1

3
p

+ 2(
T
/3) first bit received

3
p

+ 3(
T
/3) first bit released

3
p

+ 5 (
T
/3) last bit released

Lp + (L
-
1
)P

first bit received

Lp + LP

first bit released

Lp + LP + (k
-
1
)P

last bit released

where
T = k P

3 hops

L
hops

p

p + P

p + P

Source

Destination

Switch 1

Switch 2

Figure 7.16

Delay in packet switching networks

Assume three packets along the same path and transmitted in succession

38

Example

ATM networks


Connection
-
oriented networks


All information must be divided into fixed
-
length
very small packets called
cell (53 bytes)
. (
Why?)


The setup gives a chance for negotiating parameters
between user requirement and network commitment,
resources are allocated along the path


The connection is defined by a chain of local
identifiers VCIs.


Assume low
-
error rate optical channel, so error
control is done only end
-
to
-
end


Is intended to support a large range of applications