1. Transport layer

2. Classical transport layer
End-to-end
Reliability
retransmission
Flow control vs Congestion control

3. 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

4. Sensor network: traditional viewpoint
Sources to sink communications
Occasional loss is OK

5. New viewpoint Sink to sources communications
Occasional loss is disastrous

6. Reliability in Sensor networks
When reliability is required
Sink to sources
Reprogramming: binary
Reconfiguration: script
Lightweight

7. 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

8. 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

9. 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

10. E2e success rate

11. If multiple reXmission

12. Multi-modal operations
Localize the loss event
Store-and-forward approach

13. Multi-model operations
Packet-forwarding mode
Low error rate
Store-and-forward mode
High error rate

14. 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
Reference
application

15. 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

16. fetch NACK message Loss aggregation Fetch timer Proactive fetch
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

17. 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

18. ESRT: Event-to-sink Reliable Transport

19. 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

20. r vs f
n: the number of source nodes
CSMA-CA
DSR

21. 5 regions
 = r/R, a normalized reliability

23. 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)

24. NC, LR Failure, power down of some nodes
Packet loss due to link errors
f_(i+1) = f_i / _i
Multiplicative increase

25. NC, HR Reliability is overly achieved f_(i+1) = f_i * (1 + 1/_i)/2
Multiplicative
Half the slope

26. C, HR Decrease reporting frequency Energy saving f_(i+1) = f_i / _i
Multiplicative decrease

27. C, LR Worst state Aggressively decrease
k: the number of decision intervals with state (C, LR) including this interval
so, k >= 1

28. OOR
Frequency is left unchanged
f_(i+1) = f_i

29. Congestion detection Local buffer level monitoring
Congestion indication condition
Set the CN bit in the packet header

30. 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?

31. CODA: congestion detection and avoidance

32. Congestion detection Buffer queue length Channel loading
sampling
Report rate/fidelity measurement

33. 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