Transport layer

Classical transport layer

• End-to-end• Reliability

– retransmission

• Flow control vs Congestion control

Transport layer in sensor network

• Multi-hop “data-centric” transport service– No endpoint address but group or cluster– 1->N (downlink): controlled broadcast– M->1 (uplink): data

aggregation/processing

• Cooperative/collective paradigm– Not competitive

Sensor network: traditional viewpoint

• Sources to sink communications– Occasional loss is OK

New viewpoint• Sink to sources communications

– Occasional loss is disastrous

Reliability in Sensor networks

• When reliability is required– Sink to sources

• Reprogramming: binary• Reconfiguration: script

• Lightweight

Pump slowly, fetch quickly (PSFQ)

• Customizable transport protocol– Different application needs

• Ensure delivery with minimum requirements on routing infrastructure

• Minimum signaling traffic• High error rate

How PSFQ?

• Source paces data slowly (PS)• When sink detects message loss,

quickly performs local recovery (FQ)– Detection: sequence number – Negative ACK

• Assumption: message loss is due to link error rather than congestion

PSFQ

• Negative ACK• Hop by hop error recovery

– Packet loss rate: p– Src and sink are n hops away– Successful e2e delivery: (1-p)^n

E2e success rate

If multiple reXmission

Multi-modal operations

• Localize the loss event– Store-and-forward approach

Multi-model operations

• Packet-forwarding mode– Low error rate

• Store-and-forward mode– High error rate

PSFQ: 3 functions

• Pump: message relaying– Inject message

• File Id, file length, sequence number, TTL

– From user node to every sensor node• Re-tasking (broadcasting)

• Fetch: relay-initiated error recovery

• Report: selective status reporting

Referenceapplication

pump• User node broadcasts a packet every Tmin

until all data segments are sent out– Tmin is for local recovery

• Neighbors receive each segment and decrement TTL by 1

• If TTL > 0 && no gap in sequence number, PSFQ sets a schedule (Tmin..Tmax) to forward the message

• If PSFQ heard the same message 4 times, schedule is canceled

• Tmax can be used to provide a loose statistical delay bound (e2e)– Delay = Tmax * number of hops * number of

fragments

fetch• NACK message

– File id, file length, loss window

• Loss aggregation– Window of lost packets– Bursty loss is possible

• Fetch timer– NACK at every timer expiration– Only one-hop broadcasting

• Proactive fetch– If next packet is not delivered after Tpro

Report • From target node to user node• Report message

– Header: the relaying node id– Payload: a chain of node Ids and seq. numbers

• Source node will trigger by setting report bit in TTL field

• The last hop (final destination) will initiate this reporting

• What if no space to append more info?– Create new message and send it first before

relaying the old one

ESRT: Event-to-sink Reliable Transport

Problem definition• For reliable temporal tracking, the sink must

decide on the event every t units• The sink derives a reliability metric r_i at every

decision interval i• Def1: the observed event reliability, r_i is the

number of received data packets in decision interval i at the sink

• Def2: the desired event reliability, R is the number of data packets required for reliable event detection

• If r_i > R, the event is deemed to be reliably detected

• f: reporting rate

r vs f• n: the number of source nodes

CSMA-CADSR

5 regions

= r/R, a normalized reliability

ESRT

• ESRT identifies the current state S_i– For each interval i: _i, f_i– A congestion detection mechanism

• ESRT derives a new _(i+1) and calculates the updated f_(i+1) for interval i+1 and determines the next state S_(i+1)

NC, LR

• Failure, power down of some nodes• Packet loss due to link errors• f_(i+1) = f_i / _i

– Multiplicative increase

NC, HR

• Reliability is overly achieved• f_(i+1) = f_i * (1 + 1/_i)/2

– Multiplicative– Half the slope

C, HR

• Decrease reporting frequency• Energy saving• f_(i+1) = f_i / _i

– Multiplicative decrease

C, LR• Worst state• Aggressively decrease• k: the number of decision intervals

with state (C, LR) including this interval

so, k >= 1

OOR

• Frequency is left unchanged• f_(i+1) = f_i

Congestion detection

• Local buffer level monitoring• Congestion indication condition

• Set the CN bit in the packet header

Open issue?

• End-to-end reliability is r arrival over R generation

• What if intermediate nodes know this context?– Number of hops– A intermediate node receives some

packets (between r and R)– Does it have to request retransmission?

CODA: congestion detection and avoidance

Congestion detection

• Buffer queue length• Channel loading

– sampling

• Report rate/fidelity measurement

CODA design

• Open-loop hop-by-hop backpressure– Receiver-based detection– Minimum cost sampling– Suppression message

• Closed-loop multi-source regulation– Source sets regulate bit at the event

packets– ACK packets from the sink