1 Chapter Overview TCP/IP DoD model

2 Network Layer Protocols Responsible for end-to-end communications on an internetwork Contrast with data-link layer protocols, which provide communications on the same local area network (LAN)

TCP/IP is everywhere with networks Web pages using TCP/IP Database access can use TCP/IP It started with DOD (tax payer’s $$) In 1973 it is TCP/IP In 1978 they became TCP and IP (two protocols) Mostly done at UC Berkeley and shipped with its version of UNIX 3

DoD model, OSI model and protocols 4

5 Port Numbers and Sockets A port number refers to a specific application or process running on a computer. A socket is a combination of a port number and an IP address. The Internet Assigned Numbers Authority (IANA) assigns well-known port numbers to common Internet applications. The most commonly used port numbers are listed in the Services file on computers running TCP/IP.

Process/Application Apps and Protocols 6 PortProtocolDescription 23TelnetTerminal Emulation (Telephone network) 21FTPAllows file transfers between computers (File Transfer Protocol) 69TFTP Have to know what you want and where it is on the server, no directory browsing, no user authentication (Trivial File Transfer Protocol) 2049NFSAllows remote file systems to be mounted as local (Network File System) 25SMTPUsed to send mail between mail servers (Simple Mail Transfer Protocol) 515LPDUsed for sharing of network printers with TCP/IP (Line Printer Daemon) 161SNMP Collect and manipulates network information (Simple Network Management Protocol) 53DNSResolves FQDN to IP addresses (Domain Name Service) 67BootPUsed by diskless CPs to receive boot file and other info via TFTP DHCP Assigns IP addresses to hosts from a pool. Can send IP address, Subnet mask,Domain Name, Default Gateway, DNS IP, WINS info. (Dynamic Host Configuration Protocol)

Host to host layer protocols Transmission Control Protocol (TCP) User Datagram Protocol (UDP) 7

8 TCP Characteristics TCP is the acronym for Transmission Control Protocol. TCP is Connection oriented Reliable It is used to carry large amounts of data. It provides services that Internet Protocol (IP) lacks. TCP is defined in Request for Comments (RFC) 793.

9 TCP Functions Guaranteed delivery Packet acknowledgment Flow control Error detection

10 Segmentation TCP splits application layer messages into datagram-sized segments and encapsulates each segment with its own header. The collection of segments is called a sequence. The destination system reassembles the segments into the original application layer messages. The segmentation process is completely separate from the network layer fragmentation process.

The TCP Message Format

12 Control Bits Control BitFunction URG Indicates that the segment contains urgent data ACK Indicates that the message acknowledges a previously transmitted segment PSH Indicates that the receiving system should forward the message immediately rather than wait for the rest of the sequence RST Resets the TCP connection and discards the segments received so far SYN Synchronizes the Sequence Number values for both systems FIN Terminates a TCP connection

13 Three-Way Handshake Functions Verify that both computers are operating and ready to receive data Exchange initial sequence numbers (ISNs) Exchange maximum segment sizes (MSSs) Exchange port numbers

14 Three-Way Handshake Messages

15 Transmitting Data Information needed to transmit data: Port number Sequence number MSS (Max segment size)

16 Packet Acknowledgment TCP implements packet acknowledgment by using the Sequence Number and Acknowledgment Number fields. The Sequence Number field specifies the number of bytes transmitted. The Acknowledgment Number field specifies the number of bytes received.

17 Delayed Acknowledgments TCP systems do not have to individually acknowledge every packet they receive. The frequency of acknowledgment is left up to the individual TCP implementation.

18 Positive and Negative Acknowledgments With positive acknowledgment with retransmission, TCP systems acknowledge only the number of bytes they have received correctly. With negative acknowledgment, the computer specifies the information that it has not received correctly. All data beginning with the failed segment is retransmitted. Messages that are not acknowledged are retransmitted.

19 TCP Error Detection TCP provides the only end-to-end error detection for the application layer data. TCP computes a checksum based on The TCP header The application layer information in the TCP Data field A pseudo-header created from some of the fields in the IP header

20 TCP Checksum Value

21 Flow Control Flow control allows a receiving system to control the transmission rate of the sending system. Each computer has a buffer for storing incoming packets. When a computer transmits too quickly, the buffer on the receiving system can fill up, causing packets to be dropped. TCP uses the Window field in its acknowledgment messages to implement flow control. The Window value indicates how much buffer space the receiving system has available. The sending system is permitted to transmit only the number of bytes specified in the Window field.

22 Terminating the Connection

23 UDP Characteristics UDP is the acronym for User Datagram Protocol. UDP is defined in RFC 768. It is a connectionless protocol. It is used primarily for brief request/reply transactions.

24 The UDP Message Format

TCP VS. UDP 25

Port Numbers In computer networking, a port is an application- specific or process-specific software construct serving as a communications endpoint used by Transport Layer protocols, such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Port + IP address  socket 26

Well Known port numbers < 1024 Defined in RFC

IP Layer Protocals IP (Internet Protocol) Encapsulation Addressing Routing Fragmentation Protocol identification 28

29 The IP Datagram Format

ICMP Internet Control Message Protocol Router may use ICMP to send back Destination Unreachable message Or Buffer Full message Over hop limit message A message informing the demise of a IP datagram ping traceroute/tracert 30

ARP and RARP Address Resolution Protocol (ARP) Resolve IP addresses to Mac addresses by broadcast a ARP message arp -a Reverse Address Resolution Protocol (RARP) From MAC to IP, for diskless systems 31

Data Encapsulation The wrapping of protocol info at each layer. Each layer communicate ONLY WITH ITS PEER LAYER 32

Data Encapsulation – Protocol Data Units 33

Data Encapsulation 34