02 Networking

8/8/2019 02 Networking

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A networking primer

SOFTENG 325

Software Architecture


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SE 325 A Networking Primer 2

The role of a network

End systems (hosts) are connected by a network A network provides a communication service that end systems use

to exchange messages

Amessage is a sequence of bytes

A message is split into chunks called packets; it is packets that are

actually sent across the network

Network

Host Host

Message

MessagePackets


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Networking fundamentals

In general, end systems are not connected directly, but by one ormore switches

Aswitch takes a packet arriving on one of its incoming links andforwards the packet on one of its outgoing links

Aroute (path) is a sequence of communication links and switchesthat a packet traverses from source to destination hosts

Switch

Link


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Circuit switched networks

In circuit switched networks, resources along a path arereserved for the duration of a communication session

Host A

Host B

Each link has n circuits.

A dedicated connection between hostsA and B is set up prior to anycommunication between these hosts.

Each circuit provides a bandwidth of 1/n

for the duration of the connection.


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Packet switched networks

With a packet switched network, resources are not reserved, but used ondemand

Packets are transmitted over each link at the links full transmission rate

A packet switch stores a packet in its entirety before forwarding it on an

outbound link Each packet switch along a path incurs a delay of L/R seconds where

L is the length of the packet

R is the transmission rate of the link

Total delay between two hosts is QL/R where Q is the number of links

Assume a path exists between hosts A and B thatinvolves two switches. The transmission rate of each ofthe three links is 1.536Mbps and the maximum permittedsize of a packet is 64,000 bits. How long does it take totransfer the file of size 640,000 bits?


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Queuing delay with packet

switched networks

If the arrival rate of packets atswitch X exceeds 1.5 Mbps,congestion occurs causing X toqueue packets

Queues are finite in size where a switchs queue is full,incoming packets aredropped

In general, queuing delays are

unpredictable; in practice theyare of the order ofmicroseconds to millisecondsper switch

Queue of

packets at

switch X

10 Mbps links

1.5 Mbps link


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Delivery order in packet

switched networks

Network structures are self-organising in that routing tables storedin switches are periodically updated to take into account congestionand failed switches E.g. the route packets follow between hosts X and Y might change

from [A,B,D,E] to [A,C,F,E]

When transmitting a large message from X to Y, packets mightarrive at Y in a different order to which they sent by X

A

B

C

D

F

E

Host Y

Host X

Host Z


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Efficiency

cenario 1 Assume:

A 1Mbps link

Users alternate betweenactive periods, where they

generate data at a rate of100kbps, and inactive periods

Users are active 10% of thetime

With TDM, the link cansupport only 10 users in orderfor each user to send data at

the rate of 100kbps

With packet switching: With 35 users, the probability

that 11+ are activesimultaneously is 0.0004

With up to 10 simultaneouslyactive users (probability0.9996), the aggregatearrival rate of data at the linkis


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Efficiency Scenario 2

Assume: 10 users One user generates one thousand 1000-bit packets; remaining users remain

quiescent

With TDM and a 1Mbps link with 10 slots per frame, transmission of

the 1M bits takes 10 seconds With packet switching, the full link rate (1Mbps) can be used to send

the 1M bits within 1 second

Circuit switching can underutilise bandwidth.

Give an example of an application where circuit switchingwould be more appropriate than packet switching.

What class of applications would be better served bycircuit switching?


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The Internet protocol

The Internet protocol (IP) is a network protocol that conforms tothe packet-switching model and which moves packets betweenInternet hosts

Key features ofIP:

Each host on the Internet is uniquely identified by an IP address An IP address is a 32 bit number

Packet switches (routers) are special purpose computers that containrouting tables. On receipt of an IP packet, a router uses its routingtable to forward the packet towards its destination

Source IP address DataDestination IP address

Header

Up to 64 Kilobytes


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IP as a communication

service

A communication service such as IP can be described by fourproperties: Performance

Link transmission rate and other performance properties are determined by

the underlying link type(s) of a path Reliability two measures:

Validity, a service is valid if a packet that has been sent will eventually bedelivered to the destination host

Integrity, the message received is identical to the one sent, and nomessages are delivered twice

Ordering

A service provides ordered delivery if packets are delivered in the order inwhich they were sent

Synchronicity A synchronous service is one that can guarantee an upper bound on the

transmission time of a packet

IP is an unreliable service that offers no ordering guarantees.


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Network heterogeneity

Example Range Bandwidth(Mbps)

Latency(ms)

LAN Ethernet 1-2 Km 10-1000 1-10

W AN IProuting Worldwide 0.010-600 100-500

WirelessLAN

WiFi (IEEE802.15.1)

0.15-1.5 Km 2-54 5-20

WirelessWAN

GSM, 3G Worldwide 0.010-2 100-500


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Network heterogeneity

Network heterogeneity is masked by overlaying differentnetworking technologies and protocols with a virtualnetwork layer, IP

Ethernet

LAN WAN

Wireless

LAN

IP


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Protocol stacks

Physical

Link

Network

Transport

ApplicationApplication layer protocols, e.g. HTTP, FTP, DNS

Transport protocols (e.g. UDP and TCP) extendthe network layer with a process-to-process

delivery service

Network protocols (e.g. IP) provide a packetdelivery service between hosts

Despite the IP network layer being unreliable, transport layer protocols can be

developed to provide reliable delivery and other value added services.


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UDP: a simple transport

protocol

UDP (User Datagram Protocol) is a simple Internettransport protocol Beyond basic integrity checking of packets, UDP makes no

attempt to address the (lack of) reliability and ordering ofIP

A UDP packet is sent to a (host/port) pairA host is represented by an IP addressAport at the destination host is used by a process on that host to

listen for incoming packets

A port is a 16 bit number Ports in the range 0 to 1023 are well-known port numbers

Port 80 -- HTTP, port 21 -- FTP, etc.

UDP is an unreliable transport protocol that does not offer ordering guarantees.

A (host, port) pairuniquely identifies

a process on an Internet host.

Source port # Source port # Length Checksum Application data


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Sockets and ports

Asocket is a programmingabstraction which provides anendpoint for communication

Messages sent to a particularInternet address and portnumber can be received onlyby a process whose socket isbound to thatInternetaddress and port number

A socket is associated with atransport protocol typicallyeither TCP or UDP

browser

138.37.94.248

browser

Web server

IP host

Socket

Port

Process

Socket bound to (138.37.94.248), 80)

Port 80


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UDP checksum

The UDP checksum is includedfor error detection

Essence of approach The sender:

Sums the 16 bit wordscomprising the UDP packet

Performs 1s complement onthe sum this is thechecksum value added to theUDP packet

Any overflow is wrappedaround

The receiver adds all 16 bitwords, including thechecksum if the total is nota sequence of 16 1s thepacket is corrupt

0110011001100000

0101010101010101

1000111100001100

01100110011000000101010101010101

1011101110110101

1000111100001100

1011101110110101

0100101011000010

1011010100111101

0100101011000010

1011010100111101

1111111111111111

011001100110000001010101010101011000111100001100

Total of words 1 and 2

Total of words 1, 2 and 3

1s complement of total

Word 1Word 2Word 3

Total of words 1, 2 and 3Checksum value


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UDP

Sender

IP

UDP

Receiver

IP

1. Receiver uses an API to create a UDP

socket, binding it to a particularport.

2. Sender uses an API to create a UDP

socket, binding it to any local port.

3. Sender calls the sockets send( ) operationsupplying the destination IP address,

destination port number and data to send.

4. The UDP layer creates a UDP packet,

filling in the packets fields.

5. The IP layer creates an IP packet and fills

in the source and destination IP address

fields. The IP packet encapsulates the UDP

packet.

6. The IP layer forwards the packet, sending

it to a router.

7. Each router examines the IP packet

header and forwards the packet.

8. On arrival at the destination host, the IPlayer unpacks the IP packet to get the

enclosedUDP packet.

9. The UDP layerplaces the UDP packet in

the queue for the socket associated with the

destination port.

10. The receiver makes a receive( ) call on its

socket and retrieves the packets contents.

IPIP

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4

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6

7 7 7

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10

Host Host

Router

IP

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