ymqi + 18 -互联网协议.ppt

Chapter 18 Internet Protocols 网际协议 Overview 18.1 Basic Protocol Functions 18.2 Principle of Internetworking 18.3 Connectionless Internetworking 18.4 Internet Protocol 18.5 IPv6 18.1 Basic Protocol Functions Encapsulation 封装 Fragmentation and reassembly分片和重装 Connection control连接控制 Ordered delivery顺序交付 Flow control流量控制 Error control差错控制 Addressing寻址 Multiplexing复用 Transmission services传输服务 Encapsulation 封装 Data usually transferred in blocks Protocol data units (PDUs) Each PDU contains data and control information Addition of control information to data is encapsulation Three categories of control? Address：Of sender and/or receiver Error-detecting code：E.g. FCS Protocol control:Additional information to implement protocol functions Fragmentation分片 Why fragment? Communications network may only accept blocks of up to a certain size More efficient error control: Smaller retransmission Fairer: Prevent station monopolizing medium Smaller buffers Provision of checkpoint and restart/recovery operations PDUS and Fragmentation Reassembly重装 Segmented data must be reassembled into messages分片的数据被重新装配成适合应用层需要的信息 More complex if PDUs out of order Connection Control Logical association, or connection, established between entities Three phases occur? Connection establishment Data transfer Connection termination Ordered Delivery顺序交付 PDUs may arrive out of order Different paths through network PDU order must be maintained Number PDUs sequentially Easy to reorder received PDUs Flow control Performed by receiving entity to limit amount or rate of data sent Stop-and-wait Each PDU must be acknowledged before next sent Credit Amount of data that can be sent without acknowledgment E.g. HDLC sliding-window Error Control Guard against loss or damage Error detection and retransmission Sender inserts error-detecting code in PDU Receiver checks code on incoming PDU If error, discard If transmitter doesn’t get acknowledgment in reasonable time, retransmit Error control is performed at various