This document summarizes a presentation on optimizing TCP performance in wireless sensor networks. It describes how TCP can have poor performance in wireless multi-hop networks due to high packet loss. It then presents two proposed optimizations: caching and congestion control to enable local retransmissions, and using multiple TCP connections for spatial reuse. Testbed experiments show that these optimizations improve throughput and reduce energy consumption compared to default TCP, with the best results achieved when combining the optimizations with certain MAC protocols.

1. EWSN 2012
TCP Performance Optimizations for
Wireless Sensor Networks
Philipp Hurni, Ulrich Bürgi, Markus Anwander,
Torsten Braun
Research Group ―Communication and Distributed Systems‖
Institute of Computer Science and Applied Mathematics
University of Bern, Switzerland
braun@iam.unibe.ch, cds.unibe.ch

2. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Overview
> Motivation and Related Work
— Reasons for Poor TCP Performance in Wireless Multi-Hop Networks
— Optimization of TCP in WSNs
> Proposed Optimization Concepts
— Caching and Congestion Control (cctrl) Module including
Channel Activity Monitoring
— Spatial Reuse by Multiple TCP Connections
> Results by Testbed Experiments
— Throughput
— Radio On-Time as Energy Consumption Measure
> Conclusions
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3. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Reasons for Poor TCP Performance in
Wireless Multi-Hop Networks
> Higher bit error rates and packet loss
> Underlying MAC protocols
(exponential back-off, hidden / exposed nodes)
> TCP end-to-end error and congestion control mechanisms
TCP data segment loss TCP acknowledgement loss
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4. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Optimization of TCP in WSNs
> Distributed TCP Caching (Dunkels et al., 2004)
> TCP Support for Sensor Networks (Braun et al., 2007)
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5. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Caching and Congestion Control (cctrl)
Module
> is aware of all TCP packets forwarded by a node by interception of
outbound packets.
> allocates buffer for 2 packets per TCP connection (1 for each direction,
µIP has max. 1 unacknowledged TCP data segment per connection)
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6. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
cctrl Functions
> Caching of
— complete TCP data segments and scheduling of retransmission timer
(RTO = 3 ∙ RTTestimated, RTTestimated = estimated RTT between
intermediate node and destination)
— TCP/IP header for TCP acknowledgements
> Local retransmission of TCP data segment (max. 3 attempts),
when RTO expires prior to TCP acknowledgement reception (a)
> Removal of TCP data segments, if acknowledgement number of TCP
acknowledgement > sequence number of cached TCP data segment
> For retransmitted TCP data segments, for which a TCP
acknowledgement has been received: discard TCP data segment;
regenerate TCP acknowledgement (b)
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7. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Channel Activity Monitoring
> MAC proxy notifies cctrl upon reception of any packet and stores a
timestamp in activity history.
> cctrl continuously calculates channel activity level (= # overheard
packets by MAC proxy during the last time period RTTestimated)
> Observation:
— Channel activity level of most nodes = 0 during long idle periods
— Long idle periods by
– TCP data segment loss at one of the first hops
– TCP acknowledgement loss close to its destination
(i.e. TCP data segment’s source).
> Approach:
— Split RTO into:
– RTO1 = 3 ∙ RTTestimated ∙ 2/3
– RTO2 = 3 ∙ RTTestimated ∙ 1/3
— When RTO1 expires: early retransmission, if channel activity level = 0;
otherwise: retransmission when RTO2 expires.
— Triggers early local retransmissions close to destination
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8. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
MAC Proxy
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9. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Channel Activity Levels with X-MAC
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10. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Long Idle Periods
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11. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Spatial Reuse by Multiple TCP Connections
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12. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Testbed Experiments
> 7 TelosB nodes in different
rooms of a 3 floor building using
U Bern’s Wisebed testbed
> Receiver node 1
> Sender nodes 2-7
> Experiments with different
MAC protocols for 10 minutes,
15 repetitions
> 16 bytes payload
> 79 bytes per TCP data segment
> 63 bytes per TCP
acknowledgement
> Total: approx. 2500 experiments
during > 400 hours
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13. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Throughput
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14. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Radio On-Time as Energy Consumption
Measure
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15. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Throughput with Multiple Connections
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16. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Overall Comparison of Throughput
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17. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Overall Comparison of Energy Consumption
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18. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Cross-Traffic Measurements
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19. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Cross-Traffic Throughput
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20. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Cross-Traffic Comparison
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21. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Conclusions
> Implementation of caching, local TCP data segment
retransmission, and TCP acknowledgement regeneration in
intermediate nodes on top of several WSN MAC protocols
> NullMAC and LPP provide good response to local
retransmission. Results with ContikiMAC are worse.
> cctrl can improve performance and energy efficiency.
> Spatial reuse by multiple TCP connections can further improve
performance.
> Best performance and energy efficiency with cctrl and NullMAC !
→ adaptive MAC protocols (Hurni et al., 2010)
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22. Torsten Braun:TCP Performance Optimizations for Wireless Sensor Networks
Thanks for your attention !
> braun@iam.unibe.ch
> http://cds.unibe.ch
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