Computer networks 0. Initial knowledge of Internet and TCP/IP protocol

Internet , ie Internet,Internet. The internet is a world-wide computer network. Connected to the world's countless computing devices, these computing devices for the PC, Linux-based workstations, server servers and so on. These devices can be called host hosts or end Systemsdepending on their role.

The end system is connected by the Communication Link Communication link and the packet Switch packet switch . When sending data, the sending system segments the data and adds the header bytes to each segment. Such formed packets are called packet packet. The packet arrives at the target end of the system and is assembled into the initial data. A packet switch receives a packet that arrives from one of its inbound communications links and forwards the packet from one of its outbound communication links. Packet switches mainly have router router and link layer switch link-layer switch two kinds. A grouping of managers of a series of communication links and packet switches is called through the path of the network route orpath.

-End systems access the Internet through Internet service provider Internet Services Provider,isp . Each ISP is a network of multiple packet switches and multi-segment communication links. ISPs have different levels.

End systems, packet switches, and other Internet components are run to control the information received in the Internet and send a series of protocol protocol. a protocol defines the format and sequence of messages exchanged between two or more communication entities, as well as the actions taken in connection with transmission or reception of messages or other events . The message is similar to the human protocol, but the entity is the hardware or software organization of the equipment in the computer network, all the activities of the Internet, all the remote entities involving two or more communications are subject to the agreement. TCP Transmission Control Protocol protocol and IP Internet protocol Internet Protocol are the two most important protocols in the Internet. The TCP protocol is a connection-oriented communication protocol, and the IP protocol defines the format of packets sent and received in routers and end systems. The main protocol for the Internet is collectively known as the TCP/IP protocol , but not only TCP and two protocols.

Internet standards are developed by IETF, an IETF standard document called RFC. The public Internet generally refers specifically to the Internet. Many private networks, through firewalls, restrict incoming and outgoing network messages, called Intranet intranets.

Applications that provide services over the Internet are called distributed applications Distributed application. These programs run on the end system, writing these applications, need to use the application programming Interface application Programming Interface, the API. The Internet API specifies how the software that runs on one end of the system requests the Internet infrastructure to deliver data to specific destination software that wants to run on the other end of the system.

End system = Host. The host is divided into client clients and server sever. The client program is a program that runs on the client computer, is responsible for making the request, and receives the service from the server-side server program. This is known as client-server mode. peer-to Peer-to-peer, the end system performs both client functions and server functions.

Hierarchical architecture of the protocol.

Network designers organize protocols and network hardware and software that implement these protocols in a layered layer . Each protocol belongs to a layer, and each layer wants to provide service serviceson a layer, that is, the service model of the layer. Each layer provides its services by performing certain actions in that layer, or by using a service that is directly below it. A protocol layer can be implemented by combining software, hardware, or both. Protocol layering has the advantage of conceptualization and structuring.

All protocols for each layer are known as the stack protocolstack. The Internet protocol stack consists of 5 layers: Physical layer, link layer, network layer, Transport layer and application layer .

Application Layer: The application layer is where the network application and its application layer protocols persist. The application layer of the Internet contains many protocols, such as HTTP, to provide request and delivery for Web documents,SMTP, to provide the transmission of e-mail messages,FTP, to provide file transfer between two end systems. Application Layer Protocol Division on multiple end systems, the application usage protocol in one end system is grouped with the application exchange information in the other end system. The grouping of information at the application level is called message message.

Transport Layer: The Transport layer provides services to transfer application beginning between application endpoints. The transport layer has two protocols, TCP and UDP. TCP wants its applications to provide connection-oriented services. This service includes applying beginning to destinations to ensure delivery and flow control. TCP divides long messages into short messages and provides congestion control mechanisms. The UDP protocol provides a non-connected service to its application. This is a service that does not provide unnecessary services, provides no reliability, no traffic control, and no congestion control. Transport layer groupings are called message segment segment.

Network layer: The network layer of the Internet is responsible for moving network layer groupings called Datagram datagram from one host to another. The Internet Transport Layer protocol in the source host submits the transport beginning and destination addresses to the network layer. The network layer of the Internet includes the well-known IP protocol , which defines the various fields in the data hold and how the end systems and routers act on those fields. All Internet components that have a network layer must run the IP protocol. The network layer of the Internet also includes routing protocols that determine routes, which are transmitted from the source to the destination according to the route.

link Layer : The network layer transmits datagrams down to the link layer, and the link layer passes the datagram to the next node along the path, and at the next node, the link layer passes the datagram to the network layer. The services provided by the link layer depend on the specific link layer protocol that is applied to the link, with Ethernet, WiFi, and Point-to-Point protocol PPP. Datagrams need to pass through multiple links from source to destination, so it may be handled by different link layer protocols on different links along the way. The groupings on the link layer are called frame frames.

Physical Layer : The task of the physical layer is to move one of the bits in the frame from one node to the next node. The layer protocol is still related to the link and is further related to the actual transmission of the link. such as the physical layer under the Ethernet protocol has: about twisted-pair copper wire, about coaxial cable, about the fiber and so on. The way to move a bit in each case is different.