TCP/IP Layers Overview

TCP/IP Layers

• The TCP/IP model has four layers: Application, Transport, Internet, and Network Interface.
• The Application Layer defines the applications used for processing requests and the ports/sockets utilized.
• The Transport Layer defines the type of connection between hosts and how acknowledgments are handled.
• The Internet Layer defines protocols for addressing and routing data packets.
• The Network Interface Layer defines how the host connects to the network.
• TCP/IP protocol developed in 1978, predates the OSI model.
• The layers of TCP/IP correlate to seven layers of the OSI model.
• Table 6-1 shows the TCP/IP model layers and their purposes.
• Each layer's purpose is described in detail in the respective layer notes.

Port Number Assignments

• Ports are used by applications for network connections.
• Port numbers are assigned by ICANN (Internet Corporation for Assigned Names and Numbers).
• Ports 1-1023 are "well-known" ports. Ports 1024-49151 are registered ports. And ports 49152-65535 are private ports.
• Table 6-2 illustrates the port number assignments.
• Table 6-3 displays common applications and their corresponding port numbers.

Transport Layer

• Transport Layer protocols are critical for network connection establishment, data delivery, and session termination.
• Two main protocols within this layer are TCP and UDP.
• TCP (Transmission Control Protocol) is a connection-oriented protocol. It establishes the connection, manages data transfer, and terminates the connection. It includes error checking and recovery of lost data, along with specifying termination procedures.
• UDP (User Datagram Protocol) is a connectionless protocol. Data packets are sent without prior connection establishment, and without verifying delivery. UDP is helpful for applications like streaming media.

TCP Connection Handshake

• Setting up a TCP connection involves a three-step handshake: SYN, SYN+ACK, and ACK packets.
• These packets are exchanged between two hosts (illustrated in Figures 6-8 and 6-9).
• The first packet sent is SYN from host A to B. The return packet SYN+ACK acknowledges host A's packet. Finally, host A acknowledges host B's SYN+ACK packet with ACK.
• Figure 6-10 shows a TCP packet transmission captured using a protocol analyzer.

TCP Connection Termination

• The TCP connection is terminated using a 4-step process
• Host A sends a FIN (finish) packet to host B, indicating completion of data transmission.
• Host B acknowledges the reception of the FIN packet with an ACK packet
• Host B then sends a FIN packet to Host A.
• Host A acknowledges Host B's FIN packet with an ACK.

UDP

• UDP (User Datagram Protocol) is a connectionless protocol that doesn't require a dedicated connection or packet acknowledgment before sending.
• UDP packets are transported without establishing a connection and without verifying delivery.

The Internet Layer

• The Internet Layer defines protocols for data packet addressing and routing within a TCP/IP network.
• Key Protocols in this layer include IP, ARP, ICMP, and IGMP.

IP (Internet Protocol)

• IP (Internet Protocol) is responsible for addressing data packets for delivery across an IP network.
• An IP address has a network portion (like an area code) and a host portion (like a local exchange number). The network portion directs the data to the correct network and the host portion identifies the destination computer within that network.

ARP (Address Resolution Protocol)

• ARP (Address Resolution Protocol) is used to translate IP addresses into hardware addresses (MAC addresses).
• ARP requests are sent out on a network to locate the hardware address associated with a specific IP address. The destination host replies with its MAC address in an ARP reply.

ICMP (Internet Control Message Protocol)

• ICMP (Internet Control Message Protocol) controls data flow, handles errors, and provides diagnostics in a network.
• ICMP provides a mechanism for reporting errors and diagnostics in IP networks, including 'source-quench' for requesting a slowdown of data transfer, and 'ping'.

IGMP (Internet Group Management Protocol)

• IGMP (Internet Group Management Protocol) is used for multicasting, enabling a host to send data to multiple destination hosts simultaneously.
• Multicast addresses are reserved for IGMP.

Network Interface Layer

• The Network Interface Layer controls the physical connection between a host and its network (e.g., Ethernet, Token-Ring, Frame Relay, ATM).
• The TCP/IP protocol is not tied to any specific networking technology, making it adaptable to various technologies.