(4.4) Internet Protocols Layered approach to Internet Software 1

Internet Protocols In this section we investigate how messages are transferred over the Internet. 2

4-3 Figure 4.12 Package-shipping example

4-4 Internet Software Layers Application: Constructs message with address Transport: Chops message into packets Network: Handles routing through the Internet Link: Handles actual transmission of packets

4-5 Figure 4.13 The Internet software layers

Application layer: The application layer consists of those software units such as clients and servers that use Internet Communication to carry out there tasks. The application layer uses transport layer to sends and receives messages over the Internet. The responsibility of application layer is to provide an address that is compatible with the transport layer. 6

Application layer: Application layer use the services of the name servers within the Internet to translate mnemonic addresses used by humans into Internet-compatible. 7

Transport layer: The major task of transport layer is to accept messages from the application layer and to ensure that the messages are properly formatted for transmission over the Internet. Transport layer divides long messages into small segments, which are transmitted over the Internet as individual units. 8

Transport layer: This division is necessary because a single long message can obstruct the flow of other messages at the points on the Internet where numerous messages must cross paths. Small segments are interweave at these points, whereas a long message forces others to wait while it passes. Transport layer adds sequence numbers to the small segments it produces so that, the segments can be reassembled at the message’s destination. 9

Transport layer: Then it attaches the destination address to each segment and hands these address segments, known as packets, to the network layer. 10

Network layer: Has the task for forwarding the packets it receives from one network within the Internet to another until they reach their final destinations. Thus, network layer that must deal with the Internet topology. 11

Network layer: The decision is made a follows: If the final destination of the packet is within the current network, the network layer will send the packet there. Otherwise, the network layer will send the packet to a router in the current network through which the packet can be transferred into an adjacent network. 12

Network layer: The network layer appends this address to the packet as an intermediate address and hands the packet to the link layer. 13

Link layer: The link layer has a responsibility of transferring the packet to the intermediate address that was determined by the network layer. Thus the link layer must deal with the communication details particular to the individual network in which the computer resides. If that network is a token ring, the link layer must be wait for possession of the token before transmitting. if the network uses CSMA/CD, the link layer must listen for a silent bus before transmitting. 14

Note : Only the link layer and network layer are involved at intermediate stops. 15

In the opposite side : When the packet transmitted, it is received by the link layer at the computer designated by the local address attached to the message. Then hand it to its network layer where the packet final destination is compared to the current location. Then it hands to its transport layer. 16

In the opposite side : As transport layer receives packets from the network layer, it extracts the underlying message segments and reconstructs the original message according to the sequence numbers that were provided by the transport layer at the message’s origin. Ones the message is assembled, transport layer hands it to the appropriate unit within the application layer. 17

Port number: The units within the application layer should receive an incoming message is an important task of the transport layer. This is handle by assigning unique port numbers to the various units and requiring that the appropriate port number be appended to a message’s address before starting the transmission. 18

Port number: For example: If web browser asked to retrieve the document whose URL is The browser assume that it should contact the HTTP server at via port number 80. But FTP client should communicate with the FTP server through port number 20 and 21 19

4-20 Figure 4.14 Following a message through the Internet

4-21 TCP/IP Protocol Suite Is a collection of protocols used by the Internet to implement the four level communication hierarchy implemented in the Internet. TCP/IP is for Transmission Control Protocol/ Internet Protocol

4-22 Where TCP/IP and UDP Transport Layer – TCP – UDP (User Datagram Protocol) Network Layer IP: – (IPv4) use 32 bits – (IPv6) use 128 bits

4-23 The differences between TCP and UDP The first one: before sending the message as requested by the application layer, a transport layer based on TCP sends its own message to the transport layer at the destination telling it that a message is about to be sent. It then be waits for this message to be acknowledged before starting to sent the application layer’s message.

4-24 The differences between TCP and UDP For this manner, TCP transport layer is said to establish a connection before sending a message. But a transport layer based on UDB does not establish such a connection prior to sending a message. Just send a message to the address and forget about it. UDB is called a connectionless protocol.

4-25 The differences between TCP and UDP The second: TCP transport layers at the origin and destination work together by means of acknowledgments and packet retransmissions to confirm that all segments of a message are successfully transferred to the destination. TCP called a reliable protocol. But UDP is unreliable protocol. Because it does not offer such retransmission services.

4-26 Figure 4.15 Choosing between TCP and UDP

4-27 Security Attacks – Malware (viruses, worms, Trojan horses, spyware, phishing software) – Denial of service – Spam Protection – Firewalls – Spam filters – Proxy Servers – Antivirus software

4-28 Encryption FTPS, HTTPS, SSL Public-key Encryption – Public key: Used to encrypt messages – Private key: Used to decrypt messages Certificates and Digital Signatures

4-29 Figure 4.16 Public-key encryption