1. Improving Distributed TCP
Caching for Wireless
Sensor Networks
Ahmed Ayadi, Patrick Maillé and David Ros
Institut Telecom / Telecom Bretagne
MedHocNet Workshop, June 25, Juan les Pins 2010

2. Outline
 TCP over wireless sensor networks
 Distributed TCP Caching
 Enhanced Distributed TCP Caching
• Cache memory management
• Disable unnecessary retransmissions
• TCP acknowledgment loss detection
 Simulation results
 Conclusion and Perspectives
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3. Wireless Sensor Networks
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4. Reliable transport protocol for WSNs
 The WSNs are fault-tolerant networks
 The number of sensor network applications grows, so
there is a need to build more powerful hardware
capable of reprogramming or re-tasking sensors.
 New military and hearth applications are sensitive to
losses.
 All these tasks require reliable data transport from the
sink to wireless node and vice versa.
 The most deployed reliable transport protocol model in
the Internet is TCP/IP.
 Running TCP/IP in the sensor network makes it
possible to connect the sensor network directly to IP-
based network infrastructures without proxies.
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5. Problems of TCP deployment in WSNs
 High error rate in wireless networks (interferences,
congestion, collisions, etc),
 End-to-end retransmission of lost packets,
 Length of TCP header (20-60 bytes),
 The number of TCP acknowledgements.
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6. TCP end-to-end recovery
1 2 3 4 5 6 7 8 9 10
TCP-DATA N
TCP-DATA N+1
TCP-DATA N+2
ACK N
TCP-DATA N
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7. Distributed TCP Caching (DTC)
 A DTC node saves the received TCP-DATA segment if
the cache buffer is empty or unlocked.
 A DTC locks a segment if a link layer acknowledgement
is not received.
 DTC uses TCP-SACK to detect packet loss.
 A DTC node measures the round-trip timer (RTT) to the
receiver and sets a retransmission time out (RTO) to
1.5 x RTT.
 A DTC node retransmits a cached segments after a RTO
if it does not receive a TCP-ACK or TCP-SACK.
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8. Distributed TCP Caching
1 2 3 4 5 6 7 8 9 10
1
2
3
ACK1(SACK(3)‫‏‬
ACK1(SACK(2,3)‫‏‬ 2
1
ACK4
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9. Limits of DTC
 DTC does not take into account TCP-ACK losses,
 A DTC node can loss the cached,
 DTC does not smooth the RTT value in intermediate
nodes,
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10. Enhanced Distributed TCP Caching
 New version of DTC for CSMA-CA networks (e.g. IEEE
802.11 and IEEE 802.15.4),
 A EDTC node detects TCP-ACK losses and deletes
already acknowledged TCP segment,
 EDTC disables retransmission of already retransmitted
segments,
 An EDTC node locks received TCP DATA segments and
unlocks them after receiving a LL-ACK,
 EDTC computes smoothed RTO with Paxson and
Allman algorithm.
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11. Disable unnecessary retransmissions
1 2 3 4 5 6 7 8 9 10
TCP-DATA N
TCP-DATA N
TCP-DATA N
TCP-ACK N+1
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12. Cache memory management
DTC TCP‐DATA 2
TCP‐DATA 1
Node 1 Node 2 Node 3
TCP‐DATA 1
TCP‐DATA 2
TCP‐DATA 1
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13. Cache memory management
EDTC TCP‐DATA 1
Node 1 Node 2 Node 3
TCP‐DATA 1
TCP‐DATA 2
TCP‐DATA 1
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14. TCP ACK losses detection
DTC EDTC
Node1 Node 2 Node 3 Node 1 Node 2 Node 3
TCP‐DATA 2
TCP‐DATA 2
TCP‐DATA 2
TCP‐DATA 2
TCP‐ACK 3
TCP‐ACK 3
TCP‐DATA 2
TCP‐DATA 2
TCP‐DATA 2 TCP‐ACK 3
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15. Simulation parameters
Parameters Value
MAC layer IEEE 802.11b RTS/CTS disabled
PHY bitrate 1Mbps
Propagation model FreeSpace Model (PathLoss = 3)
Distance between wireless nodes 200 meters
Routing protocol AODV
Transferred data size 50kBytes
TCP MSS 100 Bytes
TCP advertised window 200 Bytes
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16. Consumed Energy & Transfer Duration
• DTC with the new cache
management (DTC-CM) and
DTC with smoothed RTO (DTC-
SRTO) reduce better the
consumed energy than DTC
• EDTC combines the two
schemes and gives the best
results
• EDTC reduces the transfer
duration and increases the
throughput of the TCP
connection.
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17. DTC vs EDTC with different BER
• EDTC is more energy efficient than DTC and TCP in
low-lossy networks and high-lossy networks.
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18. TCP Congestion Windows
• The increase of the TCP window does not increase
significantly the TCP throughput,
• The use of short TCP window reduces the congestion
and the interference between TCP-ACK and TCP-DATA
messages,
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19. Maximum Link Layer Attempts
• The increase of the maximum link layer attempt
reduces the end-to-end TCP retransmissions,
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20. Conclusion and Perspectives
 EDTC is more energy-efficient than DTC and TCP,
 EDTC increases the chance of TCP to be the reliable
transport protocol for WSNs,
 Compress the TCP header,
 Reduce the number of TCP acknowledgements,
 Adapt the TCP maximum segment size depending on
the loss rate.
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