TCP/IP protocol family--osi model and TCP/IP protocol family

Introduction: Before 1990, the hierarchical model dominated by data communication and networking literature was the Open Systems Interconnection (open system Interconnection,osi) model, and now the TCP/IP protocol family became the dominant commercial architecture.

An OSI seven-layer model

1.1 Physical Layer

The mechanical and electrical specifications that relate to the interface and the transmission media are responsible for moving bits-by- bit from one hop to the next, and that layer cares about the content:

(1) Physical characteristics of interfaces and media: Defines the interface characteristics between the device and the transmitting media, and the type of the transmission media.

(2) The expression of bit: non-zero encoding (1 high 0 low), Manchester code (1 left high right low), differential Manchester code (in case of 0 reverse).

(3) Data rate: The bit rate sent per second.

(4) Synchronization of bits: the sending device and the receiving device use the same bit rate and clock synchronization.

(5) Line configuration: Point-to-point configuration (two devices one link), multipoint configuration (several devices one link).

(6) Physical topology: Star topology (central device), ring topology (connecting to the next device formation ring), bus topology (a public link).

(7) Transmission mode: simplex, half-duplex (two devices can be sent and received, but not at the same time), full duplex.

1.2 Data Link Layer

Converting the physical layer to a reliable link makes the physical layer seem to be error-free to the network layer. The tasks for this layer are as follows:

(1) Group frame: divides the bitstream into a data unit that can be processed.

(2) Physical Address: Additional frame header plus sender and receiver (if not on this network, the receiver is connected to two network device addresses).

(3) Flow control: Prevent the receiver from being overloaded and unable to work.

(4) Error control: After the frame and the end of the implementation, re-transmission of damaged or lost frames, recognition of duplicate frames.

(5) Access control: When multiple devices share the same link, decide which device should have control over the link at any one time.

1.3 Network Layer

Responsible for delivering the packet from the origin to the end point. This layer of tasks includes:

(1) Logical addressing: The additional header includes the receiver and sender logical address.

(2) Routing: Select a route or exchange for data to reach the final destination.

1.4 Transport Layer

Responsible for the delivery of the full message process to the process (the application running on the host) , monitoring the error control and flow control from the source to the endpoint (the data link layer does not control the source-to-endpoint). This layer of tasks includes:

(1) port addressing: The additional header contains the port address.

(2) segment and reload: Use the message segment serial number to reload the message.

(3) Connection control: connection-oriented or non-connection oriented.

(4) Flow control: End-to-end flow control, the data link layer is a single link flow control.

(5) Error control: End-to-end error control, the data link layer is a single link flow control. Error correction is done by retransmission.

1.5 Session Layer

Establish, maintain, and synchronize the interactions between communication systems. This layer of tasks includes:

(1) Dialog control: Allow two process half-duplex or full-duplex communication.

(2) Synchronization: 2000 pages of files, every 100 pages a synchronization point, upload to 523 page crash, only need to start from the No. 501 page to wear on the line.

1.6 Presentation Layer

Consider the syntax and semantics of the information exchanged between the two systems. The tasks for this layer are as follows:

(1) Conversion: Information-bitstream.

(2) Encryption: Encrypt the original information.

(3) Compression: Data compression.

1.7 Application Layer

Enable users to access the network. The tasks for this layer are as follows:

(1) Network virtual Terminal: The software version of the physical terminal, through which the remote host is logged in and operated.

(2) File transfer, access and management.

(3) Mail Service: send and store mail.

(4) Directory services: Access to a wide range of global object and service information.

Two TCP/IP protocol families

2.1 OSI and TCP/IP protocol family correspondence

TCP/IP OSI

Application Layer-Application layer, presentation layer, Session layer

Transport Layer-Transport layer

Network Layer--Network layer

Network interface Layer--physical layer, Data link layer

The current TCP/IP protocol still looks at the network interface layer as the physical layer and the data link layer.

Why merge the OSI above three layers into the application layer.

(1) Some functions of the session layer are already available in some transport layer protocols.

(2) Some corresponding functions in the session layer and presentation layer can be included in the application software for development.

2.2 TCP/IP protocol family

(1) Physical layer: No specific protocol defined, different links have different protocols, communication occurs between two hops or two nodes , the communication unit is bit .

(2) Data link layer: No specific protocol is defined, communication occurs between two hops or two nodes , and communication units are frames .

(3) Network layer: Support Internet Protocol IP, communication is end-to-end communication, the router will not modify the packet source address and destination address, the communication unit is a datagram.

(4) Transport layer: the main support UDP and TCP, communication is end-to-end communication, communication units can be message segments , user Data , etc., depending on the specific protocol.

(5) Application layer: Communication is end-to-end communication, communication unit is a message .

2.3 Four levels of addressing

(1) Physical Address: Included in the frame used by the data link layer, only valid for the link layer , the Ethernet physical address is 6 bytes (48 bits ), including unicast, multicast, and broadcast addresses.

(2) Logical address: Each device has a pair of addresses (logical and physical) on each connection, and the Internet uses 32-bit logical addresses, including unicast, multicast, and broadcast addresses. (3) port address: The label assigned to a process, the port address in TCP/IP is 16 bits .

(4) Specific application addresses: such as e-mail addresses and URLs, the computer translates these addresses into corresponding port addresses and logical addresses.

Note: The physical address changes by hop, and the logical address and port address remain unchanged.