計算機網(wǎng)絡(luò)第五版(英文版)(編輯修改稿)
2025-09-01 16:41 本頁面
【文章內(nèi)容簡介】 Interworking ? Inter vs. inter ? Sub vs. work ? Gateway Problems 4, 5, 7, 8, 9 Network Software ? Protocol layers ? Design Issues for the Layers ? ConnectionOriented vs. Connectionless ? Service Primitives ? Relationship of Services to Protocols Protocol layers (1) ? Protocol layering is the main structuring method used to divide up work functionality. –Each layer municates only by using the one below –Lower layer services are accessed by an interface –At bottom, messages are carried by the medium –Each protocol instance talks virtually to its peer Virtual munication Physical munication Protocol layers (2) ? Each lower layer adds its own header (with control information) to the message to transmit and removes it on receiver ? Layers may also split and join messages, etc. Protocol layers (3) ? Information transmission unit Design Issues for the Layers ? Each layer solves a particular problem but must include mechanisms to address a set of recurring design issues Issue Example mechanisms at different layers Reliability despite failures Codes for error detection/correction (167。 , ) Routing around failures (167。 ) Network growth and evolution Addressing (167。 ) and naming (167。 ) Protocol layering (167。 ) Allocation of resources like bandwidth Multiple access (167。 ) Congestion control (167。 , ) Security against various threats Confidentiality of messages (167。 ) Web security (167。 ) ConnectionOriented vs. Connectionless (1) ? Connectionoriented, must be set up for ongoing use (and torn down after use), ., phone call ? Connectionless, messages are handled separately, ., postal delivery ConnectionOriented vs. Connectionless (2) ? In connectionoriented service – Establishes a connection, uses the connection, and then releases the connection. – When a connection is established, negotiation has to do with getting both sides to agree on some parameters or values to be used during the munication. – In the most cases the bits arrive to receiver in the order they were sent – Message sequences vs. byte streams ConnectionOriented vs. Connectionless (3) ? In connectionless service – Storeandforward switching vs. cutthrough switching – It is possible that the messages arrive to receiver out of order they were sent. – Unreliable connectionless service is called datagram ? Reliable service vs. unreliable service – Must have acknowledgement or need not one Service Primitives (1) ? A service is provided to the layer above as primitives ? Hypothetical example of service primitives that may provide a reliable byte stream (connectionoriented) service Service Primitives (2) ? Hypothetical example of how these primitives may be used for a clientserver interaction Client Server LISTEN (0) ACCEPT RECEIVE SEND (4) DISCONNECT (6) CONNECT (1) SEND RECEIVE DISCONNECT (5) Connect request Accept response Request for data Reply Disconnect Disconnect (2) (3) Relationship of Services to Protocols ? A layer provides a service to the one above [vertical] ? A layer talks to its peer using a protocol [horizontal] Problems: 15, 18 Reference Models ? The OSI Reference Model ? The TCP/IP Reference Model ? Model used for this Course ? Critique of OSI and TCP/IP The OSI Reference Model ? A principled, international standard, seven layer model to connect different systems Provides functions needed by users Converts different representations Manages task dialogs Provides endtoend delivery Sends packets over multiple links Sends frames of information Sends bits
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