2016.6.29 Computer Network Review Essentials Fourth chapter of the service provided by the Web layer

1. The network layer provides two types of services:

(1) The telecommunication network uses the virtual circuit service:

* * Using the connection-oriented communication mode, so that the telecommunications network to the user (in fact, the telephone) to provide reliable transmission of services;

* * When two computers to communicate, you should also establish a connection (but in packet switching is to establish a virtual circuit VC), in order to ensure that both sides of the communication needs to fill in the complete destination host address, but need to fill out this virtual circuit number. Thus reducing the cost of grouping;

(2) Network layer Way: The network layer to provide simple and flexible, no connection, to do their best to deliver the datagram service;

* * The network layer does not provide a commitment to quality of service, that is, the transmission of the packet may be error, loss, repetition, disorder (that is, not in order to reach the end), of course, there is no guarantee of packet delivery period;

* * Because the transmission network does not provide end-to-end reliable transmission service, this makes the router in the network can do relatively simple, and inexpensive;

* * If the communication between processes in the host (that is, the end system) needs to be reliable, then the transport layer in the network host is responsible (including error handling, traffic control, etc.);

(3) Comparison of virtual circuit service and datagram service:

* * Virtual Circuit services:

① idea: Reliable communication should be ensured by the network;

Establishment of a ② connection: must have;

③ endpoint Address: Used only in the connection establishment phase, each group uses a short virtual circuit number;

④ Packet Forwarding: packets belonging to the same virtual circuit are forwarded according to the same route;

⑤ When the node fails: all the virtual circuits that pass through the failed nodes do not work;

⑥ The order of grouping: always arrive at the end according to the sending order;

* * Datagram Service:

① idea: Reliable communication should be guaranteed by the user host;

Establishment of a ② connection: not required;

③ End Address: Each group has a complete address of the end;

④ packet Forwarding: Each packet is routed independently of the selected route;

⑤ When a node fails: a faulted node may lose its grouping, and some routes may change;

⑥ The order of grouping: the time to reach the end, not necessarily in the order of delivery;

2016.6.29 Computer Network Review Essentials Fourth chapter of the service provided by the Web layer