LAN Protocols Week 3 Wednesday 9 10 2003 Abdou Illia Fall 2003 1 Learning Objectives Explain how the most used LAN protocols work TCP UDP IP 2 What is a protocol Set of rules that govern data communication over a network Examples of protocols Transmission Control Protocol TCP Internet Protocol IP Simple Mail Transfer Protocol SMTP Examples of protocol suites TCP IP IPX SPX NetBEUI 3 1 Protocols in Windows 98 FigureDialog 2 14 Network Box in Windows Network box Dialog in Windows 98 98 Properties Button 4 TCP IP protocol suite A suite of protocols rather than a protocol Protocol suite that govern data communication on the Internet Designed for use with wide range of network devices Main protocol of most computer operating systems Many troubleshooting and network analysis tools Understood by a large body of network professionals 5 continued 6 2 7 TCP IP and Layered Communications Application Application HTTP HTTPreq req Transport Transport HTTP HTTPreq req TCP H TCP H Internet Internet Data DataLink Link User PC HTTP HTTPreq req TCP H TCP H IP H IP H DL T DL T HTTP HTTPreq req TCP H TCP H IP H IP H DL H DL H Physical Q1 What are Transport layer messages called when TCP is used Q2 What are Internet Network layer messages called when UDP is used 8 How TCP Works Establishes communication sessions between networked computers Provides reliable end to end delivery of data by controlling data flow Sequencing and acknowledging frames 9 3 TCP Bit 0 TCP Segment Source Port Number 16 bits Bit 31 Destination Port Number 16 bits Sequence Number 32 bits Acknowledgment Number 32 bits Header Length Reserved Flag Fields 4 bits 6 bits 6 bits Window Size 16 bits TCP Checksum 16 bits Urgent Pointer 16 bits Options if any Padding Data Field Flag fields are one bit fields They include SYN ACK FIN and RST 10 TCP Segment Source Port Number and Dest Port Number Well Known port number for server applications See slide 27 or Ephemeral port number 1024 and 65535 for client PC applications Sequence Number 32 bit sequential number for the frame Enables TCP to ensure that all frames are received Used to place frames back correct order Acknowledgement Number Number sent in ACK message to acknowledge frame reception If ACK message not sent for a frame frame is retransmitted Header Length Offset Indicates the length of the header Flag Fields Used to identify the kind of TCP segment SYN Synchronization for opening a connection ACK Acknowledgement to acknowledge frame reception FIN to close a connection Used for control RST Reset for an abrupt close FLW Flow control to control transmission flow Window Size Indicates number of bytes to be transmitted before sender receives an ACK of receipt When Window Size is reached Flow control is turned on to stop transmission until ACK is received Checksum 16 bit cyclic redundancy check computed based on values in all fields and placed in the frame by sending node 11 TCP Segment Urgent Pointer Provides a warning to the receiver that urgent data is coming Also points to the end of the urgent data in the sequence Options Can hold additional information and flags about a transmission Padding Used when there is too little or no optional data to complete the required header length which must be divisible by 32 12 4 TCP Time Diagram PC Transport Process Open 1 SYN Open Webserver Transport Process 2 SYN ACK 1 Acknowledgement of 1 3 ACK 2 3 Way Open TCP is a Connection Oriented Protocol 13 TCP Time Diagram Acknowledgements Each correct TCP segment is acknowledged ACK except for pure acknowledgements and resets Sender listens to be sure that each TCP segment is acknowledged If a segment is not acknowledged in a reasonable period of time the sender resends it TCP is a reliable protocol 14 TCP Time Diagram After Opening Data are Transferred PC Transport Process Open 3 Carry HTTP Req Resp 4 1 SYN Open Webserver Transport Process 2 SYN ACK 1 Acknowledgement of 1 3 ACK 2 4 Data HTTP Request 5 ACK 4 6 Data HTTP Response 7 ACK 6 15 5 TCP Time Diagram After Opening Data are Transferred PC Transport Process Webserver Transport Process 8 Data HTTP Request Error 9 Data HTTP Request No ACK so Retransmit Carry HTTP Req Resp 4 10 ACK 9 11 Data HTTP Response 11 ACK 11 Q3 Is TCP a reliable protocol Why Q4 If there is an error in the transmission which Transport process retransmits the damaged segment 16 TCP Time Diagram PC Transport Process Webserver Transport Process 12 FIN Close Close 4 13 ACK 12 14 FIN 15 ACK 14 4 Way Close is Normal 17 UDP Connectionless alternative to TCP that yields a lower overhead but is less reliable Handles and transmits data faster performs no flow control sequencing or acknowledgment Relies only on checksum to ensure reliability 18 6 UDP Datagrams UDP Datagram Bit 0 Bit 31 Sequence Port Number 16 bits Destination Port Number 16 bits UDP Length 16 bits UDP Checksum 16 bits Data Field 19 UDP Datagram PC Transport Process Webserver Transport Process UDP Datagram Connectionless and Unreliable Places a Lighter Load on the Network than TCP Useful Where the Loss of an Occasional Application Message is Not a Serious Problem As in Regular Network Status Messages Or Where There is No Time for Retransmission As in Voice Transmission 20 TCP Versus UDP TCP UDP Layer Transport Transport Message name TCP segment UDP datagram Processing power required Heavyweight Lightweight Reliability Reliable Unreliable Connections Connection oriented Connectionless Q5 What protocol allows the sender to identify the Destination port number 21 7 IP Enables packets to reach a destination on a local or remote network by using IP addresses Used in combination with TCP or UDP 22 IP Bit 0 0100 IP Version 4 Packet Header Version Length 4 bits 4 bits Type of Service 8 bits Bit 31 Total Length 16 bits Identification 16 bits Flags Fragment Offset 13 bits Protocol 8 bits Header Checksum 16 bits 1 ICMP 6 TCP 17 TCP Source IP Address 32 bits Destination IP Address 32 bits Time to Live 8 bits Options if any Padding Data Field 23 IPv4 Packets Version Has value of four 0100 Time to Live TTL Value is set by sender Decremented by one by each router along the way If reaches zero router throws packet away Prevents the endless circulation of misaddressed packets 24 8 IPv4 Packets Protocol Identifies contents of data field 1 ICMP Data Field 6 TCP ICMP TCP or UDP Message 17 UDP IP Header Address Fields 32 bits long of course 25 Other slide 26 Port Numbers Port Number Application Service 20 File Transfer FTP 23 Telnet