Marwan Al-Namari Week 5

Responsible for delivering packets between endpoints over multiple links Physical Link Network Transport Application

Store-and-forward packet switching » Connectionless service – datagrams » Connection-oriented service – virtual circuits » Comparison of virtual-circuits and datagrams »

Hosts send packets into the network; packets are forwarded by routers ISP’s equipment

UDP : User Datagram Protocol. Packet is forwarded using destination address inside it Different packets may take different paths ISP’s equipment A’s table (initially) A’s table (later) C’s Table E’s Table

TCP: Transmission Control Protocol. Packet is forwarded along a virtual circuit using tag inside it Virtual circuit (VC) is set up ahead of time ISP’s equipment A’s table C’s Table E’s Table

Almost everything on the Internet involves two programmes talking to each one another. e.g. browsing a web site Connecting a programme on one computer to another programme on another computer is like making a telephone call. The server must know the phone number of the receiver and the receiver must be around to pick up the phone. In the TCP/IP world a phone number is called a SOCKET composed of three parts: IP address of the receiver, receiving programmes port number and protocol being used

socket = protocol ip address : port number e.g. TCP : 80 UDP :53

UDP provides a connectionless datagram service that offers unreliable, best- effort delivery of data transmitted in messages. This means that neither the arrival of datagrams nor the correct sequencing of delivered packets is guaranteed. UDP does not recover from lost data through retransmission. UDP is used by applications that do not require an acknowledgement of receipt of data and that typically transmit small amounts of data at one time. NetBIOS name service, NetBIOS datagram service, and SNMP are services and applications that use UDP. UDP is defined in RFC 768. E.g. When Net Send * is used; messages go out on UDP port 138. The following table describes the key fields in the UDP header.

FieldFunction Source PortUDP port of sending host. Destination Port UDP port of destination host. UDP Checksum Verifies the integrity of the UDP header and the UDP data.

UDP Port NumberDescription 53Domain Name System (DNS) Name Queries 69Trivial File Transfer Protocol (TFTP) 137NetBIOS name service 138NetBIOS datagram service 161SNMP