DN06传输层.pptVIP

2003-10-21 网络与数据通信 第十讲 TCP协议 运输层 （2） 六.可靠数据传输的原理 important in app., transport, link layers top-10 list of important networking topics! characteristics of unreliable channel underneath it will determine complexity of reliable data transfer protocol (rdt) Reliable data transfer: getting started Reliable data transfer: getting started We’ll: incrementally develop sender, receiver sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer but control info will flow on both directions! use finite state machines (FSM) to specify sender, receiver Rdt1.0: reliable transfer over a reliable channel underlying channel perfectly reliable no bit errors no loss of packets separate FSMs for sender, receiver: sender sends data into underlying channel receiver reads data from underlying channel Rdt2.0: channel with bit errors underlying channel may flip bits in packet recall: UDP checksum to detect bit errors the question: how to recover from errors: acknowledgements (ACKs): receiver explicitly tells sender that pkt received OK negative acknowledgements (NAKs): receiver explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK human scenarios using ACKs, NAKs? new mechanisms in rdt2.0 (beyond rdt1.0): error detection receiver feedback: control msgs (ACK,NAK) rcvr-sender rdt2.0: FSM specification rdt2.0: in action (no errors) rdt2.0: in action (error scenario) rdt2.0 has a fatal flaw! What happens if ACK/NAK corrupted? sender doesn’t know what happened at receiver! can’t just retransmit: possible duplicate What to do? sender ACKs/NAKs receiver’s ACK/NAK? What if sender ACK/NAK lost? retransmit, but this might cause retransmission of correctly received pkt! Handling duplicates: sender adds sequence number to each pkt sender retransmits current pkt if ACK/NAK garbled receiver discards (doesn’t deliver up) duplicate pkt rdt2.1: sender, handles garbled ACK/NAKs rdt2.1: receiver, handles garbled ACK/NAKs rdt2.1: discussion