[四川大学]计算机网络通信1-2.ppt

Protocol “Layers” Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Organization of air travel a series of steps Organization of air travel: a different view Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below Layered air travel: services Distributed implementation of layer functionality Why layering? Dealing with complex systems: explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system layering considered harmful? Internet protocol stack application: supporting network applications FTP(文件传送),HTTP(Web),SMTP(Email) transport: host-host data transfer (logical communication between processes) TCP, UDP network: routing of datagrams from source to destination (logical communication between hosts) IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” Layering: logical communication Each layer: distributed “entities” (网络实体:end-systems, routers)implement layer functions at each node entities perform actions, exchange messages with peers Layering: logical communication E.g.: transport take data from app add addressing, reliability check info to form “datagram” send datagram to peer wait for peer to acknowledge receipt analogy: post office Layering: physical communication Protocol layering and data Sender: Each layer takes data from above adds header information to create new data unit, and passes new data unit to layer below Receiver: Each layer takes data from below removes header, and puts data up to layer above Inte