OSI Layer-7 network model and TCP/IP layer-4 Network Model
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1. OSI network layered Reference Model
Network Protocol designers should not design a single, huge protocol to provide complete details for all forms of communication, but should divide communication problems into multiple small problems, then a separate protocol is designed for each small problem. This makes it easier to design, analyze, and test each protocol. A major principle of Protocol Division is to ensure that the target system is effective and efficient. To improve efficiency, each Protocol should only pay attention to the communication issues that have not been handled by other protocols. To make the implementation of the master protocol more effective, the protocol should be able to share a specific data structure; the combination of these protocols should be able to handle all possible hardware errors and other exceptions. To ensure the coordination of these Protocols, the Protocol should be designed and developed into a complete, collaborative protocol series (that is, the protocol family), rather than developing each protocol in an isolated way.
In the early days of network history, the International Organization for Standardization (ISO) and the International Telegraph Telephone Advisory Committee (CCITT) jointly published a layer-7 Reference Model for Open System Interconnection. Network processes in a computer operating system include application requests (at the top of the protocol stack) to network media (bottom). The OSI reference model divides functions into seven discrete layers. Figure 2.1 shows the OSI layered model.
┌ ── ─ ┐
│ Application Layer │ Seventh Layer
├ ── ─ ┤
│ Presentation layer │
├ ── ─ ┤
│ Session Layer │
├ ── ─ ┤
│ Transport layer │
├ ── ─ ┤
│ Network layer │
├ ── ─ ┤
│ Data link layer │
├ ── ─ ┤
│ Physical layer │ second layer
└ ── ─ ┘
OSI Layer-7 Reference Model
Layer 7 of the OSI model performs the following operations:
Level 1: Physical Layer
Responsible for encoding the information into current pulses or other signals for online transmission. It consists of the actual interface between the computer and the network medium, can define electrical signals, symbols, line status and clock requirements, data encoding and data transmission connector. Such as the most commonly used RS-232 specification, 10base-t Manchester code and RJ-45 belongs to the first layer. All layers higher than the physical layer communicate with it through predefined interfaces. For example, the Ethernet ancillary unit interface (AUI), a DB-15 connector can be used to connect Layer 1 and Layer 2.
Layer 2: Data Link Layer
Provides reliable data transmission through physical network links. Different data link layers define different network and Protocol features, including physical addressing, network topology, error verification, frame sequence, and traffic control. The physical addressing (corresponding to the network addressing) defines the addressing method of the device at the data link layer. The network topology defines the physical connection mode of the device, such as the bus topology and the ring topology; error verification sends an alarm to the upper-layer protocol when a transmission error occurs. The data frame sequence is reorganized and frames other than the sequence are transmitted. throttling may delay data transmission, in this way, the receiving device will not crash because it receives information that exceeds its processing capability at a certain time point. The data link layer consists of two independent parts: Media Access Control (MAC) and Logical Link Control (LLC ). Mac describes how to schedule, generate, and receive data on a site in a shared media environment. Mac ensures reliable cross-link information transmission, synchronizes data transmission, identifies errors, and controls data flow. Generally, Mac is only important in the shared media environment. Only nodes in the shared media environment can be connected to the same transmission media. The ieee mac rule defines an address to identify multiple devices in the data link layer. The Logical Link Control Sub-layer manages the communication between devices on a single network link. IEEE 802.2 standard defines LLC. LLC supports connectionless services and connection-oriented services. Many domains are defined in the Information frames at the data link layer. These domains allow multiple high-level protocols to share a physical data link.
Layer 3: Network Layer
Establishes connections between source and end points. It generally includes network path finding, and may also include traffic control and error checks. Data transmission between different network segments of the same mac standard generally only involves the data link layer, while data transmission between different MAC standards involves the network layer. For example, an IP router works at the network layer, and thus multiple networks can be interconnected.
Layer 4: Transport Layer
Provides reliable end-to-end network data flow services to the top management. The functions of the transport layer generally include flow control, multi-channel transmission, virtual circuit management, error checksum and recovery. Throttling manages data transmission between devices to ensure that the transmission device does not send data that is larger than the processing capability of the receiving device. Multi-Channel Transmission makes multiple applications Program The data can be transmitted to a physical link. The virtual circuit is established, maintained, and terminated by the transmission layer. Error verification includes various structures established to detect transmission errors; recovery of errors includes the actions taken (such as request data re-transmission) to resolve any errors. Transport Control Protocol (TCP) is a transport layer protocol in the TCP/IP protocol family that provides reliable data transmission.
Layer 5: Session Layer
Establishes, manages, and terminates communication sessions between the presentation layer and the entity. Communication sessions include service requests and service responses that occur between different network application layers. These requests and responses are implemented through the Session Layer Protocol. It also includes creating checkpoints so that a previous status can be returned when communication is interrupted.
Layer 6: Presentation Layer
Multiple functions are provided for data encoding and conversion at the application layer to ensure that information sent at the application layer of one system can be identified by the application layer of another system. The encoding and conversion modes of the presentation layer include public data representation format, performance conversion representation format, public data compression mode, and public data encryption mode.
The common data representation format is the standard image, sound, and video format. By using these standard formats, different types of computer systems can exchange data. Conversion modes exchange information between systems by using different text and data representation, for example, ASCII (American Standard Code for information interchange); standard data compression mode ensures that the compressed data on the original device can be properly decompressed on the target device; the encryption mode ensures that the encrypted data on the original device can be correctly decrypted on the target device.
The presentation layer protocol is generally not associated with a special protocol stack. For example, QuickTime is the video and audio standard of the applet computer, and MPEG is the ISO video compression and encoding standard. Common image formats such as PCX, GIF, and JPEG are different static image compression and encoding standards.
Layer 7: Application Layer
It is closest to the end user's OSI layer, which means that the OSI Application Layer interacts directly with users through application software. Note: The application layer is not composed of actual application software running on the computer, but composed of APIs that provide the application with access to network resources, such applications are beyond the scope of the OSI model. The functions of the application layer generally include identifying communication partners, defining resource availability and synchronous communication. Because the communication partner may be lost, the application layer must define the identification and availability of the communication partner for the Application subroutine that transmits data. When defining resource availability, the application layer must determine whether there are sufficient network resources for communication requests. In synchronous communication, communications between all applications require collaborative operation at the application layer.
OSI application layer protocols include file transmission, access and management protocols (FTAM), file virtual terminal protocol (VIP), and public management system information (CMIP.
2. TCP/IP layered model
The TCP/IP layening model is called the Internet layering model and the Internet Reference Model ).
── ─ ┐ ┌ ── ┬ ── ┐
│ D │ F │ W │ F │ H │ g │ t │ I │ s │ U │
│ N │ I │ H │ t │ o │ e │ R │ M │ s │ its │
│ Layer 4, application layer │ s │ n │ o │ P │ t │ P │ l │ C │ t │ e │
│ G │ I │ P │ H │ n │ P │ n │
│ E │ s │ e │ it │
│ R │ t │
── ─ ┘ └ ── ┴ ── ┘
┌ ── ─ ┐ ┌ ── ─ ┬ ── ─
│ Layer 3, transmission layer │ TCP │ UDP │
└ ── ─ ┘ └ ── ─ ┴ ── ─
┌ ── ─ ┐ ┌ ── ─ ┬ ── ─
│ ICMP │
│ Second layer, inter-network layer │ ── ┘ │
│ IP │
└ ── ─ ┘
┌ ── ─ ┐ ┌ ── ─ ┬ ── ─
│ Layer 1, network interface │ arp/RARP │ others │
└ ── ─ ┘ └ ── ─ ┴ ── ─
TCP/IP layer-4 Reference Model
The TCP/IP protocol is organized into four conceptual layers, with three layers corresponding to the corresponding layers in the ISO Reference Model. The ICP/IP protocol family does not contain the physical layer and data link layer. Therefore, it cannot independently complete the functions of the entire computer network system and must work with many other protocols.
The four protocol layers of the TCP/IP layered model provide the following functions:
Layer 1: Network Interface Layer
This includes protocols used to transmit data from a collaborative IP address on an existing network medium. In fact, TCP/IP standards do not define functions that correspond to the ISO data link layer and physical layer. Instead, it defines a protocol like Address Resolution Protocol (ARP) that provides the data structure of the TCP/IP protocol and interfaces between the actual physical hardware.
Layer 2: inter-network layer
Corresponds to the network layer of the OSI Layer-7 Reference Model. This layer includes IP protocol and RIP Protocol, which are responsible for data packaging, addressing, and routing. It also contains the inter-network Control Message Protocol (ICMP) to provide network diagnostic information.
Layer 3: Transport Layer
Corresponding to the transport layer of the OSI Layer-7 reference model, it provides two end-to-end communication services. Among them, the TCP protocol (Transmission Control Protocol) provides reliable data stream transportation services, and the UDP protocol (use datainprotocol) provides unreliable User Datagram services.
Layer 4: Application Layer
It corresponds to the application layer and expression layer of the OSI Layer-7 Reference Model. The application layer protocols of the Internet include finger, Whois, FTP (file transfer protocol), Gopher, HTTP (Hypertext Transfer Protocol), Telent (Remote Terminal protocol), and SMTP (Simple Mail Transfer Protocol), IRC (Internet Relay Session), NNTP (network news transmission protocol), etc.
PS: ARP and RARP written in ccnp both belong to the network layer. ARP written in the tutorial of Chinese network engineers belongs to the data link layer, I personally think that ARP is a protocol between Layer 2 and Layer 3 in the OSI model, as to which layer of TCP/IP is .... Well, I think it's okay to understand how he works ....
