The TCP/IP Protocol Suite

The figure below shows a comparison of the Open Systems Interconnection (OSI) model and the TCP/IP protocol suite. The TCP/IP set of protocols maps to a four-layer conceptual model: application, transport, Internet and network interface. This model is referred to as the Internet Protocol Suite or the ARPA model. As shown below, each layer in the darker green Internet Protocol Suite corresponds to one or more layers of the white OSI model.



Network Interface
The network interface layer is the equivalent of the OSI physical and data link layers as it defines the host's connection to the network. This layer comprises the hardware and software involved in the interchange of frames between computers. The technologies used can be LAN-based (e.g. Ethernet) or WAN-based (e.g. ISDN)

Internet Layer
The network layer uses a number of protocols to ensure the delivery of packets. These are described below:


IP (Internet Protocol)
IP is the protocol responsible for addressing and routing packets (on the basis of routing algorithms) between networks. It ensures they reach the correct destination network.

ARP
The Address Resolution Protocol (ARP) is responsible for obtaining hardware addresses and matching them to their IP address when the destination computer is on the same network.

ICMP
The Internet Control Management Protocol (ICMP) is used to report errors and send messages about the delivery of a packet. It can also be used to test TCP/IP networks. Two examples of ICMP messages include:
Destination unreachable - used when a router cannot locate the destination
Time exceeded - used when the Time To Live (TTL) of a packet reaches zero
The TTL field in a packet has a maximum value of 255 and this value is reduced by one every time the packet crosses a router. The TTL is eventually reduced to zero if the packet is looping (because of a corrupted routing table) or when congestion causes considerable delays. The router then discards the packet and a warning packet is sent back to the source host.


Transport Layer
The Transport layer provides communication between the source and destination computers, and breaks application layer information into packets. TCP/IP provides two methods of data delivery:
Connection-oriented delivery using TCP
Connectionless delivery using UDP
Application Layer
The Application layer is the layer at which many TCP/IP services (high-level protocols) can be run (such as FTP, HTTP and SMTP). Two application programming interfaces (APIs) are commonly used within the TCP/IP environment: sockets and NetBIOS.

這一篇還講到LAYER(層)的觀念,教科書的解釋果然經典 而且還有圖呢


UNDERSTANDING TCP/IP