Transport layer • Congestion control • Reliable transport – Data from sensors/ from sink should be reliably transferred • (De)multiplexing – Many applications may exist, transport layer should (de)multiplex from/to network/application layer
Reliable Multi-Segment Transport (RMST) Protocol • Has two modes of operation: – Non-caching mode • Reliability is provided using source and destination only – Caching mode • Nodes on the path are involved in providing reliability • Packets in a data flow are labelled by a unique sequence number
RMST • Non-caching mode – Packet losses are detected by the sink and requested from the source in an end-to-end fashion through NACK packets • requires no involvement from intermediate nodes => less processing and storage
RMST • Caching mode – Some sensors on the path cache the packets • They may detect packet losses and can resend lost packets • reliable segments are created between intermediate nodes with caching capabilities
Pump Slowly, Fetch Quickly (PSFQ) Protocol • Slow data distribution (Pump slowly) • Quick error recovery (Fetch quickly) • NACK based
PSFQ • Data dostribution – Two timers are used: Tmin and Tmax – A node transmits packets every Tmin – A node relays packets at random time (Tmin, Tmax) – Random time reduces redundant broadcasts
PSFQ • Error recovery is done by sending NACK packets whenever a packet lost is detected • A node sends NACK every Tr < Tmax time.
PSFQ • PSFQ may not work if the last packet is lost • To solve this, PSFQ uses Tpro timer – If a node does not receive a packet for Tpro time, the node sends NACK packet.
Congestion Detection and Avoidance Protocol (CODA) • Aim is to detect and avoid congestion – If a node detects the congestion, it sends a suppression message towards the source. • The nodes on the path reduce the packet rate or drop.
CODA
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Lecture 5 (Transport).ppt