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2, type of service

Service (service) This very common term in computer networks is undoubtedly a very important concept. In the network architecture, service is adjacent to its upper layers of the network to provide a set of operations, is adjacent to the interface between two layers. As the network hierarchy one-way

dependency, making the network the interface between adjacent layers is one-way: the lower the service provider, the upper is the service user. Expressed in the form of service primitive (primitive), such as library functions or system calls. In order to better discuss the network services, we first explain some terminology. In the network, each layer at least one entity (entity). Entities can be software entities (such as a process) can also be a hardware entity (such as a network card). On different machines on the same floor of the entity, called on other entities (peerentity). N layer service entity to achieve by the use of N + 1 level, while the N layer will have to use N-1 layer of the services provided. N layer to the N +1 layer entities may provide several types of services, such as fast or slow and expensive communications and cheap communication. N +1 layer entities N layer through a service access point (Service Access Point, SAP) to use the services provided by N layers. N layer N + 1 layer of SAP is the N layer can access the service areas. Each SAP has a unique address. IDU by the service data unit (Service Data Unit, SDU), and some control information component. To send SDU, N layer entity SDU can be divided into paragraphs, each with a header after a period as a separate protocol data unit (Protocol Data Unit, PDU) sent, such as "group" is the PDU. PDU header is used to perform them with the level of the same physical layer protocol used to identify which PDU contains data, which contains control information, and provide serial number and other counts. In the network, the lower to the upper services fall into two categories: connection-oriented service (connection-oriented service), and connectionless service (connectionless service). Connection-oriented service is the telephone system service mode of abstraction. Each time a complete data transfer must go through to establish a connection, data

transmission and terminates the connection of three processes. In the data transmission process, the

data packets do not carry the destination address, but use the hyphen. Connection is essentially similar to a pipe, one end of the sender put the data in the pipe, remove the data receiver at the other end. Characterized by the received data and send the data sender in the content and order are identical.

Connectionless service is the postal system in the abstract service model. Each packet with the complete destination address, each packet transmitted in the system independently. Connectionless service can not guarantee the order of packet arrival,reason is that different packets may take a different path to destination, So the first message sent is not necessarily first. Connectionless services are generally not on the error message to restore and re-transmission. In other words, no connection service does not guarantee the reliability of packet transmission. In the computer network, by confirming the general reliability and retransmission (acknowledgement and retransmission) mechanism to achieve. Most connection-oriented services support the recognition of retransmission, but the confirmation and

retransmission to the additional delay. The reliability of some of the less demanding connection-oriented services (such as a digital telephone network) do not support retransmission; because phone users

would rather hear the murmur of conversation with, not like the delay caused by waiting for confirmation. Most connectionless service does not support the recognition of retransmission, it is often no reliable connection transport service is not high.

Title: Computer Network

Author: Lei Chen A, ed

Publishing House: University Press of Xi'an Electronic Science and Technology

Publication Date: 2003-10-1 Words: 479,000 edition: 2 Pages: 315

Price: ￥ 25.00 paper: offset paper ISBN: 9787560605586