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

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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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Wireless Sensor Networks

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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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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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TCP end-to-end recovery

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ACK N

1 2 3 4 5 6 7 8 9 10

TCP-DATA N+2

TCP-DATA N

TCP-DATA N+1

TCP-DATA N

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Distributed TCP Caching (DTC)

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  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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Distributed TCP Caching

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2 1

ACK1(SACK(3)‏

ACK1(SACK(2,3)2 ‏

1

ACK4

1 2 3 4 5 6 7 8 9 10

3

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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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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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Disable unnecessary retransmissions

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TCP-DATA N

1 2 3 4 5 6 7 8 9 10

TCP-DATA N TCP-DATA N

TCP-ACK N+1

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TCP‐DATA2TCP‐DATA1

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Cache memory management

DTC

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Node 2 Node 3 Node 1

TCP‐DATA1

TCP‐DATA2

TCP‐DATA1

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Cache memory management

EDTC

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Node 2 Node 3 Node 1

TCP‐DATA1

TCP‐DATA2

TCP‐DATA1

TCP‐DATA1

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TCP ACK losses detection

DTC EDTC

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Node 2 Node 3 Node1

TCP‐ACK3

TCP‐DATA2

Node 2 Node 3

TCP‐ACK3

TCP‐DATA2

TCP‐ACK3

Node 1

TCP‐DATA2

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TCP‐DATA2

TCP‐DATA2

TCP‐DATA2

TCP‐DATA2

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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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Consumed Energy & Transfer Duration

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• 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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DTC vs EDTC with different BER

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•  EDTC is more energy efficient than DTC and TCP in low-lossy networks and high-lossy networks.

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TCP Congestion Windows

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• 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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Maximum Link Layer Attempts

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• The increase of the maximum link layer attempt reduces the end-to-end TCP retransmissions,

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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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