Semester 2 Chapter 9 - The TCP/IP Protocol Suite

Semester 2 Chapter 9 - The TCP/IP Protocol Suite By Jack Pol Adelaide Institute of Technical And Further Education South Australia

Lesson Overview • How TCP/IP operates to ensure communication across any set of interconnected networks. • Learn about parts of the TCP/IP protocol stack that support: • File transfer (FTP, TFTP), • e-mail, • Reliable (TCP) and unreliable (UDP) transport layer protocols, • Connectionless datagram (packet) delivery at the network layer, • ICMP provides control and message functions at the network layer, • ARP and RARP. Jack Pol

TCP/IP • Developed by the Defence Advanced Research Projects Agency (DARPA). • Later included with the Berkeley Software Distribution of UNIX. • Now the de facto standard for internetwork communications. • Serves as the transport protocol for the Internet. Jack Pol

9.1.1 The Internet TCP/IP protocols and the OSI model • OSI Layers 7,6 & 5 are rolled into 1 application layer • The Transport layer in both models perform basically the same function • OSI layers 2 & 1 are combined into 1 Network Access layer Jack Pol

9.1.2 TCP/IP protocol stack and the application layer • DNS - Domain Name System • WINS - Windows Internet Naming System • NFS - Network File System (The standard UNIX file system) • POP3 - Post Office Protocol (An e-mail standard) • SMTP - Simple Mail Transfer Protocol • SNMP - Simple Network Management Protocol • FTP - File Transfer Protocol (Upload/Download files from the ‘net) • TFTP - Trivial File Transfer Protocol • HTTP - The Internet standard for web browsing Jack Pol

9.1.2 TCP/IP protocol stack and the application layer • Other Application Layer protocols used for troubleshooting a network include: • Telnet - A standard terminal emulation protocol • PING - Packet InterNet Groper • Traceroute - Traces the path of a packet from the source to the receiver. Jack Pol

9.1.3 TCP/IP protocol stack and the transport layer • The transport layer provides two protocols: • TCP (Transport Control Protocol) • Provides reliable, connection-oriented communications between 2 hosts. • TCP requires more network overhead because data is acknowledged as it is received. • UDP (User Datagram Protocol) • UDP is faster but less reliable, because the recipient does not acknowledge the data as it is received. • Communication reliability is left to the Application Layer Jack Pol

9.1.4 TCP segment format • source port -- the number of the calling port • destination port -- the number of the called port • sequence number -- the number used to ensure correct sequencing of the arriving data • acknowledgment number -- the next expected TCP octet • HLEN -- the number of 32-bit words in the header • reserved -- set to 0 • code bits -- the control functions (e.g. setup and termination of a session) • window -- the number of octets that the sender is willing to accept • checksum -- the calculated checksum of the header and data fields • urgent pointer -- indicates the end of the urgent data • option -- one currently defined: maximum TCP segment size • data -- upper-layer protocol data Jack Pol

9.1.4 UDP segment format • Application layer protocols provide for reliability. • UDP uses no windowing or acknowledgments. • Designed for applications that do not need to put sequences of segments together. • Protocols that use UDP are: • TFTP • SNMP • Network File System (NFS) • Domain Name System (DNS) Jack Pol

9.1.5 TCP and UDP port numbers • Both TCP and UDP use port numbers for communication between hosts. • Port numbers are similar to phone numbers • Transport layer services can be “called” by their port number. • EG When a Host wants to transfer a file using FTP • Uses TCP port 21 to establish and control the connection • Uses TCP port 20 to transfer the data • TCP ports 20 & 21 are called “Well Known Port Numbers” because applications expect to find FTP services on these ports. • Other “Well Known” Port numbers are: • TCP Port 23 - Telnet • TCP Port 25 - SMTP (email) • TCP Port 53 - DNS • TCP Port 80 - HTTP web services • UDP Port 53 - DNS • UDP Port 69 - TFTP • UDP Port 161 - SNMP Request For Comment 1700 defines “Well Known Port Numbers” Jack Pol

9.1.6 TCP three-way handshake/open connection • For TCP to establish a reliable connection between 2 hosts. • Uses a “Three Way Handshake” • Transmits 3 packets before the actual data • The 2 Hosts synchronise their “Initial Sequence Number” (ISN) • Ensures that the communications are assembled in the correct order and no missing packets exist. • The process occurs by • Host 1 sends a SYN packet to Host 2 (1st data packet) • Host 2 ACK the packet from Host 1 (2nd data packet) • Host 2 includes its’ own SYN data for Host 1 (2nd data packet) • Host 1 ACK packet from Host 2 (3rd data packet) • It is call the 3 way handshake because only 3 packets are exchanged • The SYN packet from Host 2 also serves as the acknowledgment to the SYN packet from Host 1 Jack Pol

9.1.7 TCP simple acknowledgment and windowing • TCP “Sliding Windows” control the flow and efficiency of communication • Quite simply, once the window fills with data, the destination host sends an ACK for packets received in that window • The window slides over in order to select new packets. • TCP window sizes can change during the life of the connection so that the window can be Jack Pol

9.2.1TCP/IP and the Internet Layer • The Internet layer of the TCP/IP stack corresponds to the network layer of the OSI model. • Several protocols operate at the TCP/IP Internet layer that corresponds to the OSI network layer: • IP -- provides connectionless, best-effort delivery routing of datagrams; is not concerned with the content of the datagrams; looks for a way to move the datagrams to their destination • ICMP -- provides control and messaging capabilities • ARP -- determines the data link layer address for known IP addresses • RARP -- determines network addresses when data link layer addresses are known Jack Pol

9.2.2The IP datagram • An IP datagram contains an IP header and data, and is surrounded by the Media Access Control (MAC) layer header and MAC layer trailer. One message may be transmitted as a series of datagrams that are reassembled into the message at the receiving location. • VERS -- version number • HLEN -- header length, in 32-bit words • type of service -- how the datagram should be handled • total length -- total length (header + data) • identification, flags, flag offset -- provides fragmentation of datagrams to allow differing MTUs in the internetwork • TTL -- Time-To-Live • protocol -- the upper-layer (Layer 4) protocol sending the datagram • header checksum -- an integrity check on the header • source IP address and destination IP address -- 32-bit IP addresses • IP options -- network testing, debugging, security, and other options Jack Pol

9.2.2The IP datagram • The protocol field determines the Layer 4 protocol being carried within an IP datagram. Although most IP traffic uses TCP, other protocols can also use IP. • Each IP header must identify the destination Layer 4 protocol for the datagram. • Transport layer protocols are numbered, similarly to port numbers. • IP includes the protocol number in the protocol field Jack Pol

9.2.3Internet Control Message Protocol (ICMP) • Provides message control and error reporting services between 2 TCP/IP hosts or between a host server and a gateway to the Internet. • The PING utility uses ICMP messages to check connections between 2 points. • EG: A router receives a packet that it is unable to deliver to its final destination. • The router sends an ICMP unreachable message to the source. • The message might be undeliverable because there is no known route to the destination. • An echo reply is a successful reply to a ping command. • Results could include other ICMP messages, such as host unreachable and timeouts. • How many of you have sent an email with the wrong address and it comes back to you with the message “host unknown”? Jack Pol

9.2.4How ARP works • ARP maps IP addresses to MAC addresses. • The source of the network packet sends the information to the destination. • Unique MAC addresses reference the end points in the exchange. • Networked devices maintain details of MAC and IP addresses of other devices on the network. • This is called an ARP table or cache • Maintained in RAM Jack Pol

9.2.4How ARP works • A computer transmits a packet on the network. • It checks its’ ARP cache for an IP to MAC address mapping • If source finds an appropriate IP to MAC mapping, the source uses the IP and MAC address to encapsulate the data that it is ready to transmit. • Source computer sends the packet directly to the destination. Jack Pol

9.2.4How ARP works • The IP address is listed first, the MAC address next, followed by information about whether the entry is static or dynamic. Jack Pol

9.2.4How ARP works • If a source computer cannot locate an IP to MAC address mapping in its’ ARP table, it must obtain the correct mapping. • Source initiates an ARP request to all hosts to gain the destinations’ MAC address. • The one host that sees that the ARP packet is meant for its’ IP address responds to the ARP request. Jack Pol

How RARP works • RARP is similar to ARP in that computers use it to bind MAC addresses to IP addresses. • RARP is used by Diskless Workstations which have a MAC address burned into their network cards but no IP address. • A clients IP configuration is stored on a RARP server • RARP servers maintain a table of MAC and IP address mapping for RARP clients. • During the boot process, a RARP client will call the RARP server to obtain their IP configuration. Jack Pol

The preceding presentation was put together using the Cisco curriculum and the CCNA Guide to Cisco Networking Fundamentals by Kurt Hudson and Kelly Cannon. Jack Pol

Semester 2 Chapter 9 - The TCP/IP Protocol Suite