1. Layered Architecture of Communication Networks: TCP/IP Model.

1. Layered Architecture of Communication Networks: TCP/IP Model

2© Tallal Elshabrawy

Emergence of TCP/IP ARPANET was a research network supported by US DoD

(Department of Defense) interconnecting universities and government installations using telephone leased lines

When satellite and radio networks were added, existing protocols had trouble inter-working the three networks together

TCP/IP Reference Model: The ability to connect multiple networks together in a seamless

way

One of the main objectives of DoD was that communications can survive hardware failures


TCP/IP: Inter-connecting Networks

Ethernet Ethernet

Cellular Network

Satellite Network

ARPANET

SRC

DEST


TCP/IP Reference Model

HTTP SMTP DNS RTP

TCP UDP

IP

Network Interface 1

Network Interface 2

Network Interface 3


Application Layer

Incorporates functions of the top three OSI layers

Example Protocols HTTP (Hypertext Transfer Protocol)

Publishing and receiving HTML pages

SMTP (Simple Mail Transfer Protocol) Sending emails towards a server

DNS (Domain Name System) Helps to map ASCII strings (URLs) to IP addresses

RTP (Real-time Transport Protocol) Delivering audio and video over the Internet


Transport Layer

TCP (Transport Control Protocol) Reliable connection-oriented transfer of a byte stream Applies error recovery and flow control mechanisms

UDP (User Datagram Protocol) Unreliable best-effort connectionless transfer of individual

messages No message sequencing or flow control. Leaves the upper layer to

decide how to deal with these situations Used by applications that require quick delivery and has some

tolerance to information loss


Internet Layer

Handles transfer of datagrams (packets) across multiple networks

Best-effort connectionless packet transfer Best-effort means best-effort. The IP layer attempts to send the

datagrams towards the destination It does not guarantee the transfer would be successful It does not guarantee the datagtams would arrive in sequence

Handles routing and congestion control


Network Interface

It is not really a layer. Rather it defines an interface to underlying networks

TCP/IP assumes that data-link and physical layers are part of the underlying network design

TCP/IP reference model is only concerned with inter-connecting functional networks


TCP/IP vs OSI Reference Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Transport

Internet

Network Interface

OSI Reference

Model

TCP/IP Reference

Model


Example: Web Page Download

Network 2Network 1

1.1

1.2

1.3

1.4

2.1

2.2

2.3

2.4

Assume a host on cleo.guc.edu.eg connected to Network 1 wants to read the news on CNN.com whose server is located on Network 2

Let us simplify the IP addresses to x.y where x is the network id y is the terminal id of a particular network

HOW DOES THE DOWNLOAD OCCUR?

cleo.guc.edu.eg

CNN.com


HTTP

Web browser runs an HTTP client program Web server runs an HTTP server program HTTP Protocol

HTTP client sends its request message: Get … Relevant information associated with Get request

Server’s IP address : 2.3 Filename : index.html Host’s IP address : 1.1

HTTP server sends a status response HTTP server send HTML data content

Get …

200 OK …

HTML Content

www.cnn.com


Establish TCP Connection

Before the HTTP client can send anything, a TCP connection must be established

TCP connection uses a three-way handshake Note that at this phase, we have yet to get the HTTP server involved in

any communications

HTTP Client

TCP Client

Port z

Connection Established

Establish a TCP Connection from port z of 1.1. to port 80 of 2.3

Port 80

TCP Server

TCP Connection Request from 1.1 port z to 2.3 port 80

ACK, TCP Connection Request

from 2.3 port 80 to 1.1 port zACK

It is a logical interface to help the transport layer

identify which application process should be

contacted

Q: What is Port?


HTTP uses the TCP connection to transfer data in both directions

HTTP Client

TCP Client

Port z

Get …

Port 80

TCP Server

TCP Send (80, z, Get)

TCP Send (z, 80, Response)

ACK

HTTP Server

OK, 200

HTML Content

ACK

Get …OK, 200

HTML Content

HTTP Data Transfer using TCP Connection


Interfacing TCP and IP Protocols

IP

Send this datagram to

2.3

Datagram Received

TCP Server

IP

Send this datagram to

1.1

Datagram Received

TCP Client

The role of the IP layer is to attempt its best (best-effort service) to forward datagrams towards their target destination

IP Network

IP Network


Network 2Network 1

1.1

1.2

1.3

1.4

2.1

2.2

2.3

2.4

cleo.guc.edu.eg

CNN.com

1.1 2.3 TCP Data1.1 2.3 TCP Data

Destination Next Hop Interface

1.2 1.2 1.1

1.3 1.3 1.1

2.2 1.4 1.1

2.3 1.4 1.1

2.4 1.4 1.1

Routing Table of cleo.guc.edu.eg

Destination Next Hop Interface

1.1 1.1 1.4

1.2 1.2 1.4

1.3 1.3 1.4

2.2 2.2 2.1

2.3 2.3 2.1

2.4 2.4 2.1

Routing Table of Router C

1.1 2.3 TCP Data1.1 2.3 TCP Data

Router C

IP Routing: Sending Packets from 1.1 to 2.3


IP Routing Terminal 1.1 may only send a packet directly to destinations within the

same network

How would 1.1 know if the destination is in the same network? By looking at the prefix of the IP address

Terminal 2.3 is not on the same network as 1.1

The IP layer of cleo.guc.edu.eg has a routing table that says: If you wish to reach 2.3 then you have to go through router (gateway) 1.4

Router C has multiple interfaces each of which corresponds to an IP address

The routing table of Router C indicates that if you want to reach 2.3 then you will need to send over the interface which has the IP address 2.1


Sending IP Packets over LANs

cleo.guc.edu.egRouter CARP Message:

What is the MAC address of 1.4?

ARP Message:IP Address 1.4 corresponds to MAC address 3f

Router C CNN.com

ARP Message:What is the MAC address of 2.3?

ARP Message:IP Address 2.3 corresponds to MAC address 1g


Sending IP Packets over LAN

To send an IP datagram to IP Address 1.4, cleo.guc.edu.eg puts the IP datagram in an Ethernet frame, and transmits the frame.

However, Ethernet uses different addresses, so-called Media Access Control (MAC) addresses (also called: physical address, hardware address)

Therefore, Argon must first translate the IP address 1.4 into a MAC address.

The translation of addressed is performed via the Address Resolution Protocol (ARP)


HTTP

TCP

IP

EthernetEthernet Ethernet

IP

Router C

1.4

Router C2.3

HTTP

TCP

IP

Ethernet

Cleo.guc.edu.eg

Summary: Sending the TCP Request

Send HTTP Request to CNN.com

Establish a connection to 2.3 at port 80Open TCP connection to 2.3 port 80

Send a datagram (which contains a connection request) to 2.3Send IP datagram to 2.3

Send Ethernet frame to 3f

Send Ethernet frame to 1g

Send IP data-gram to 2.3

Send the datagram to 2.3

Frame is an IP datagram

IP datagram is a TCP segment for port 80

CNN.com

Send the datagram to 1.4

Frame is an IP datagram


User clicks on http://www.nytimes.com/ Ethereal network analyzer captures all frames observed

by its Ethernet NIC Sequence of frames and contents of frame can be

examined in detail down to individual bytes

Internet

How the layers work together

“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003


Ethereal windowsTop Pane

shows frame/packet

sequence

Middle Pane shows

encapsulation for a given frame

Bottom Pane shows hex & text“Communications Networks 2nd Edition”, A. Leon-Garcia, and I. Widjaja, McGraw Hill, 2003


Top pane: frame sequenceDNS

Query

TCP Connection

SetupHTTP

Request & Response



Middle pane: Encapsulation

Ethernet Frame

Ethernet Destination and

Source Addresses

Protocol Type




IP Packet

IP Source and Destination Addresses

Protocol Type

And a lot of other stuff!




TCP Segment

Source and Destination Port

Numbers

HTTP Request

GET


1. Layered Architecture of Communication Networks: TCP/IP Model.

Documents

tallal elshabrawyexample

internetieee globecom

comieee globecom

research network

network idy

tcpip modelieee globecom

information lossieee

hardware failuresieee