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Computer Networking: A Top Down Approach �6^th edition �Jim Kurose, Keith Ross�Addison-Wesley�March 2012

Computer Communications �& Networks

CSNC-2413

N/W Architecture & �Application Development

Lec: 3

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Network Architecture

Network communications - a complex task
To deal with this complexity… SIMPLIFY

comm task divided into modules
modules arranged in layers
each layer performs a subset of comm function
Forms a Network Architecture

multiple layers
each layer has one/more Protocols
protocols perform specific comm tasks
provide/obtain services to/from higher/lower layer

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The long-distance carrier system provided in the United States and throughout the world was designed to transmit voice signals over high-capacity transmission links, such as optical fiber, coaxial cable, and microwave. Part of the evolution of these telecommunications networks to digital technology has been the adoption of synchronous TDM transmission structures. In the United States, AT&T developed a hierarchy of TDM structures of various capacities; this structure is used in Canada and Japan as well as the United States. A similar, but unfortunately not identical, hierarchy has been adopted internationally under the auspices of ITU-T.

The basis of the TDM hierarchy (in North America and Japan) is the DS-1 transmission format. The same DS-1 format is used to provide both multiplexed voice and digital data services. Above the DS-1 data rate of 1.544 Mbps, higher-level multiplexing is achieved by interleaving bits from DS-1 inputs. For example, the DS-2 transmission system combines four DS-1 inputs into a 6.312-Mbps stream. Data from the four sources are interleaved 12 bits at a time. Note that 1.544 × 4 = 6.176 Mbps. The remaining capacity is used for framing and control bits.

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Network Architecture

Example of a layered network system

Network Architecture

A structured set of protocols to implement the �communications function

application

transport

network

link

physical

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Internet protocol stack

Application: support applications (network API)

FTP, SMTP, HTTP, DNS, DHCP

Transport: process-process data transfer (ports)

TCP (reliable), UDP (unreliable)

Network: global addressing (IP addr), routing of pkts from src to dest

IPv4, IPv6

Link: data transfer between neighboring network elements

Ethernet, WiFi (802.11), 3G/4G, PPP

Physical: bits “on the wire”

application

transport

network

link

physical

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TCP/IP Model

Some of the protocols & networks in TCP/IP protocol stack…

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Ethernet

WiFi

PPP

3G/4G

HTTP

FTP

SMTP

DNS

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Encapsulation

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Data generated by Appl layer
When being sent, each layer appends its header
When being received, each layer removes its header

Principle of Encapsulation & Decapsulation

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Why Layered Architecture

Network Architecture - layered architecture

provides modularity

changes in one layer do not require changes in other layers
simplifies system maintenance & upgradation

facilitates process of network evolution

allows to change/improve underlying technologies, with increase in application demands

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The long-distance carrier system provided in the United States and throughout the world was designed to transmit voice signals over high-capacity transmission links, such as optical fiber, coaxial cable, and microwave. Part of the evolution of these telecommunications networks to digital technology has been the adoption of synchronous TDM transmission structures. In the United States, AT&T developed a hierarchy of TDM structures of various capacities; this structure is used in Canada and Japan as well as the United States. A similar, but unfortunately not identical, hierarchy has been adopted internationally under the auspices of ITU-T.

The basis of the TDM hierarchy (in North America and Japan) is the DS-1 transmission format. The same DS-1 format is used to provide both multiplexed voice and digital data services. Above the DS-1 data rate of 1.544 Mbps, higher-level multiplexing is achieved by interleaving bits from DS-1 inputs. For example, the DS-2 transmission system combines four DS-1 inputs into a 6.312-Mbps stream. Data from the four sources are interleaved 12 bits at a time. Note that 1.544 × 4 = 6.176 Mbps. The remaining capacity is used for framing and control bits.

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Some network apps

e-mail
web surfing
search engine
P2P file sharing
text messaging
social networking
remote login

streaming stored video�(YouTube, Netflix)
IP radio, TV
multi-user N/W games
voice over IP �(Skype, viber)
real-time video conferencing
…..

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How these appls are created…

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Creating network apps

Write appl programs to run only on end systems

appl comm over N/W
web browser comm with server

No need to write software for network-core devices…

network-core devices function at lower layers

confining appls to end systems� allows rapid appl development & deployment

application

transport

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

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Application architectures

Possible structures of applications:

client-server
peer-to-peer (P2P)

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Client-server architecture

Servers

always-on host
permanent addr (IP & port)
rely on fixed infra-structure
data centers for scaling

Clients

request services from servers
intermittent host connections
dynamic addr (IP & port)
no direct comm with each other

client/server

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P2P architecture

NO always-on server

arbitrary end systems comm directly

peers request services from other �peers, & also provide services
Self Scalability

new peers bring new service demands
also add new service capacity

Complex Management

peers connect intermittently
change IP addrs

min, or no reliance, on infra-structure

peer-peer

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Architectural Challenges

Client Server

Infra-structure intensive
Cost of server hardware, software & access network B/W
System management needs/costs
May become a bottleneck

P2P

Not ISP friendly; require high upload B/W
Security issues
Incentives; users need to volunteer storage, bandwidth & computation resource

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How do Appls comm

Appls within same host

processes usually comm using IPC �(Inter Process Comm)
use Pipes, FIFOs, shared memory

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Applications are processes running on hosts…

Appls in distant hosts

processes comm by�exchanging messages
Use sockets, RPC

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Sockets

Remote processes comm thru sockets…

Socket : “software interface between process & N/W”

Process sends/receives msgs to/from its socket

process reads/writes the socket to receive/send msgs

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Process Address

to receive msgs, process must have identifier
host device has unique �IP address (32 bits)

but many processes may run �on same host

need another level of identifier – Port No (16 bits)

identifier includes both �IP address & port number associated with a process �on a host…

Socket address

IP addr + Port No

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to send HTTP msg to web server gaia.cs.umass.edu…

IP address: 128.119.245.12 port number: 80

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Socket Programming

Allows network applications to communicate across an Internet
Socket API mainly provides �Transport layer service �interface

Stream-sockets for TCP
Datagram-sockets for UDP

Based on client/server �architecture

Socket Interface

TCP

UDP

IP

Network Access

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Client/Server functions

Client / Server may run on same or different hosts
Client makes Request…

sends message to server to perform a task

Server Responds…

performs task & sends back reply

Client

process

Server

process

1. Client sends request

2. Server

handles

request

3. Server sends response

4. Client

handles

response

Resource

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Servers

Servers : long-running application processes (daemons)

typically created at boot-time by OS
run continuously in background
web server, or mail server

Server waits for requests on a well-known port associated with a particular service

Port 7: echo server
Port 23: telnet server
Port 25: mail server
Port 53: DNS server
Port 80: HTTP server

/etc/services �provides list of available services �(Linux machine)

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Clients

Client – appl launched to access some service, mostly on remote system

web browser, ftp client, telnet client, ssh client

Client does not need well known port

usually assigned ephemeral port by kernel
can also be selected by application

Server - need not know client location

Client - needs to know server location�(port + IP address)

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Socket

Socket is an endpoint of bidirectional comm…

identified by socket descriptor

Clients & Servers comm with each other thru sockets

open a socket
write data to socket
read data from socket

App

socket

3

2

1

Dest.

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Socket Address

The pair IP Address + Port -– makes up a “socket-address”

Server

Client

Client socket address

128.2.194.242:3479

Server socket address

208.216.181.15:53

Client IP address

128.2.194.242

Server IP address

208.216.181.15

3479 is an

ephemeral port allocated by kernel (unix)

53 is a well-known �port associated with �DNS servers

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TCP & UDP Ports

TCP and UDP port numbers ranges:

values 0 – 2¹⁶ (65,536 ports)
Internet Assigned Numbers Authority (IANA)

Well Known Ports (0 - 1023)

used by system processes for well known services
HTTP: 80, E-mail: 25, DNS: 53

Registered Ports (1024 - 49151)

used by vendors for common applications
Web Proxy: 8080, IPSec: 1293, Kaaza: 1214

Dynamic or Ephemeral Ports (49152 - 65535)

used by clients
automatically allocated by kernel on temporary basis

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