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17-TL-ReliableDataTransfer

17-TL-ReliableDataTransfer - Principle of Re s liableData...

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Principles of ReliableData Transfer
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ReliableDelivery Making surethat the packets sent by thesender arecorrectly and reliably received by thereceiver amid network errors, i.e., corrupted/lost packets Can be implemented at LL, NL or TL of the protocol stack. Totally a design choice When and why should this beused? Link Layer Rarely doneover twisted-pair or fiber optic links Usually doneover lossy links (wireless) for performanceimprovement (versus correctness) in P2P links Network/Transport Layers Necessary if theapplication requires thedata to bereliably delivered to thereceiver, e.g., filetransfer
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ReliableDelivery: ServiceModel Reliable, In-order Delivery unreliablechannel ReliableData Transfer Protocol (Sender Side) ReliableData Transfer Protocol (Receiver Side) 1 2 3 4 1 2 3 4 UDT_Send RDT_Receive Deliver_Data RDT_Send Reliable, In-order delivery Typically done when reliability is implemented at the transport layer, e.g., TCP Example application: Filetransfer
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ReliableDelivery: Assumptions We’ll: Consider only unidirectional data transfer A sender sending packets to a receiver Bidirectional communication is a simpleextension, where there are2 sender/receiver pairs Start with simplea protocol and makeit complex as wecontinue
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RDT over UnreliableChannel Unreliablechannel ReliableData Transfer Protocol (Sender Side) ReliableData Transfer Protocol (Receiver Side) 1 2 3 4 1 2 3 4 UDT_Send RDT_Receive Deliver_Data RDT_Send Channel may flip bits in packets/losepackets The received packet may havebeen corrupted during transmission, or dropped at an intermediaterouter due to buffer overflow Thequestion: how to recover fromerrors? ACKs, NACKs, Timeouts… Next
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RDT over UnreliableChannel Two fundamental mechanisms to accomplish reliabledelivery over Unreliable Channels Acknowledgements (ACK), NegativeACK (NACK) Small control packets (header without any data) that a protocol sends back to its peer saying that it has received an earlier packet (positive ACK) or that it has not received a packet (NACK). Sent by the receiver to the sender Timeouts Set by thesender for each transmitted packet If an ACK is received beforethe timer expires, then the packet has made it to the receiver If the timeout occurs, the sender assumes that the packet is lost (corrupted) and retransmits thepacket
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ARQ Thegeneral strategy of using ACKs (NACKs) and timeouts to implement reliabledelivery is called Automatic Repeat reQuest (ARQ) 3 ARQ Mechanisms for ReliableDelivery Stop and Wait Concurrent Logical Channels Sliding Window
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