Communication Networks NETW 501 Lecture 3 Layered Architecture

Communication Networks NETW 501 Lecture 3 Layered Architecture of Communication Networks: TCP/IP Model Course Instructor: Dr. -Ing. Maggie Mashaly maggie. ezzat@guc. edu. eg C 3. 220 1

Emergence of TCP/IP Problem: • When satellite and radio networks were added, existing protocols had trouble inter-working the three networks together • ARPANET was a research network supported by US Do. D (Department of Defense) interconnecting universities and government installations using telephone leased lines. • And they had a solution for the problem! Solution: • TCP/IP Reference Model: Ø The ability to connect multiple networks together in a seamless way Ø One of the main objectives of Do. D was that communications can 2

TCP/IP: Inter-connecting Networks Cellular Network ARPANET DEST SRC Ethernet Satellite Network Ethernet 3

TCP/IP vs OSI Reference Model Application Presentation Application Session Transport Network Internet Data Link Network Interface Physical OSI Reference Model TCP/IP Reference Model 4

TCP/IP Reference Model DNS SMTP HTTP RTP Application TCP UDP Transport IP Network Interface 1 Network Interface 2 Internet Network Interface 3 Network Interface 5

Application Layer Application • Incorporates functions of the top three OSI layers • Example Protocols: Ø HTTP (Hypertext Transfer Protocol) › Publishing and receiving HTML pages Transport Internet Network Interface Ø SMTP (Simple Mail Transfer Protocol) › Sending emails towards a server Ø DNS (Domain Name System) › Helps to map ASCII strings (URLs) to IP addresses Ø RTP (Real-time Transport Protocol) › Delivering audio and video over the Internet 6

Transport Layer • TCP (Transport Control Protocol) – Reliable connection-oriented transfer of a byte stream – Applies error recovery and flow control mechanisms Application Transport Internet Network Interface • 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 7

Internet Layer • Handles transfer of datagrams (packets) across multiple networks • Best-effort connectionless packet transfer Application Transport Internet – Best-effort means best-effort. Network Interface 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 datagrams would arrive in sequence • Handles routing and congestion control 8

Network Interface • It is not really a layer. Rather it defines an interface to underlying networks Application Transport Internet • TCP/IP assumes that data-link and physical layers are part of the underlying network design Network Interface • TCP/IP reference model is only concerned with inter-connecting functional networks 9

Example: Web Page Download Network 2 Network 1 cleo. guc. edu. eg 1. 4 2. 1 1. 3 1. 1 1. 2 2. 4 2. 2 2. 3 CNN. com • 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? 10

HTTP www. cnn. c om Get … 200 OK … HTML Content • 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 sends HTML data content 11

Establish TCP Connection Q: What is Port? HTTP Client Establish a TCP Connection from port z of 1. 1. to port 80 of 2. 3 Connection Established Port z It is a logical interface to help the transport layer identify which application process should be contacted Port 80 TCP Connection Request from 1. 1 port z to 2. 3 port 80 TCP Client ACK, TCP Connection Request from 2. 3 port 80 to 1. 1 port z ACK TCP Server • 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 12

HTTP Data Transfer using TCP Connection HTTP Client HTTP Server OK, 200 Get … HTML Content Port z HTML Content Port 80 TCP Send (80, z, Get) TCP Client ACK TCP Send (z, 80, Response) TCP Server ACK • HTTP uses the TCP connection to transfer data in both directions 13

Interfacing TCP and IP Protocols TCP Server TCP Client Send this datagram to 2. 3 Send this datagram to 1. 1 Datagram Received IP IP Network Datagram Received IP • The role of the IP layer is to attempt its best (best-effort service) to forward datagrams towards their target destination 14

IP Routing: Sending Packets from 1. 1 to 2. 3 Network 2 Network 1 cleo. guc. edu. eg 1. 4 1. 3 1. 1 2. 4 Router C 2. 2 1. 2 2. 3 CNN. com Destination Next Hop Interface 1. 2 1. 1 1. 3 1. 1 2. 2 1. 4 1. 1 2. 3 1. 4 1. 1 2. 4 1. 1 Routing Table of cleo. guc. edu. eg Destination Next Hop Interface 1. 1 1. 4 1. 2 1. 4 1. 3 1. 4 2. 2 2. 1 2. 3 2. 1 2. 4 2. 1 Routing Table of Router C 15

IP Routing How does it work? • 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? Ø 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: Ø 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 16

Sending IP Packets over LANs cleo. guc. edu. eg ARP Message: What is the MAC address of 1. 4? Router C ARP Message: IP Address 1. 4 corresponds to MAC address 3 f ARP Message: What is the MAC address of 2. 3? Router C ARP Message: IP Address 2. 3 corresponds to MAC address 1 g CNN. com 17

Sending IP Packets over LAN How does it work? • 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, we must first translate the IP address 1. 4 into a MAC address. • The translation of addresses is performed via the Address Resolution Protocol (ARP) 18

Summary: Sending the TCP Request Cleo. guc. edu. eg CNN. com HTTP TCP IP Frame is an IP datagram Ethernet HTTP Send HTTP Request to CNN. com Establish a connection to 2. 3 at port 80 Open TCP connection to 2. 3 port 80 IP datagram is a TCP segment for port 80 Send IP data-gram to Send a datagram (which contains a connection Send IP datagram to 2. 3 IP 2. 3 request) to 2. 3 Send the datagram to 1. 4 Send Ethernet frame to 3 f Ethernet Router C 1. 4 Frame is an IP datagram Ethernet TCP IP Send the datagram Ethernet to 2. 3 Router C Ethernet frame Send 2. 3 to 1 g 19

How the layers work together? Internet l User clicks on http: //www. nytimes. com/ l Ethereal network analyzer captures all frames observed by its Ethernet NIC l Sequence of frames and contents of frame can be examined in detail down to individual bytes 20

Top Pane shows Ethereal Windows frame/packet sequence Middle Pane shows encapsulation for a given frame Bottom Pane shows hex & text 21

Top Pane: Frame sequence DNS Query TCP Connection Setup HTTP Request & Response 22

Middle Pane: Encapsulation Ethernet Frame Protocol Type Ethernet Destination and Source Addresses 23

Middle Pane: And. Encapsulation a lot of other stuff! IP Packet IP Source and Destination Addresses Protocol Type 24

Middle Pane: Encapsulation TCP Segment Source and Destination Port Numbers GET HTTP Request 25

References • NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy • “Communication Networks 2 nd Edition”, A. Leon-Garcia and I. Widjaja, Mc. Graw Hill, 2013 • “Computer Networks 4 th Edition”, A. S. Tanenbaum, Pearson International 26