TCP over Multipath Routing in Ad Hoc Networks

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TCP over Multipath Routing in Ad Hoc Networks

• Jiwei Chen, Matthew Azuma
• Tutor Kaixin Xu

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Motivation

• Standard TCP assumes one path
• Problems with Wireless TCP
• Low throughput(high BER)
• Route disconnection and rediscovery
• Interrelation with RTT and RTO
• Multihop wireless
• Hidden terminal optimal window size
• Contention induced loss typically happens before
  congestion loss

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Multipath TCP Pros

• Rich connectivity
• Robustness and resilience
• Full use of network resources
• Load balancing

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Multipath TCP Cons

• Out-of-order problem
• different delays on paths-gt packet reordering-gt
  duplicate ACKs -gt false congestion signals
• Need multipath routing protocol(network support)
• How to spread packets?

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Previous work

• TCP over multipath routing
• Use multipath routing(DSR)
• TCP performance degraded by out-of-order delivery
• Use one primary path while with backup path
• Other transport layer approaches
• Parallel TCP
• Maintain and control a single send buffer for all
  the TCP-v pipes
• Individual congestion control and loss recovery
• Congestion window based data striping

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Our approach

• Take advantage of multiple paths to improve TCP
  throughput
• Routing modifications(DSR) to discover, maintain
  multiple paths
• TCP modifications to spread the packets to
  multiple paths
• Use spreading ratio, more flexible!
• Address out-of-order problem

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Multipath DSR

• Node will send duplicate RREQ
• Receiver will return two best paths
• maximally disjoint -gt min hop number -gt receiving
  time
• Sender will send packets on two paths according
  to TCPs demand
• Keep path view consistent at sender and receiver

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Routing Challenges

• Mobility
• Find new path whenever one path is unavailable?
• Provide more state info about path changes for
  TCP?
• Data packet salvage
• QOS provisioning

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Multipath TCP

• Sender keeps packet path queue
• Sender uses spreading function
• Modified congestion window
• Duplicate TCP control block variables
• RTT variables
• Rtx timers
• Duplicate packet counter

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Multipath TCP diagram
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Out-of-order problem
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Out-of-order problem
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Packet path queue
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Spreading function

• P1Pn 1
• Congestion Avoidance
• Pa percentage for min RTT path
• Pa ((RTTa / RTT1) (RTTa / RTTn))-1
• Pi (RTTa / RTTi) Pa
• Slow Start / Fast Recovery
• Po path in slow start / fast recovery
• Po 1 / (new cwin)
• Pi Pi (old cwin / new cwin)

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Congestion window modifications

• Timeout on path i --gt Slow Start
• cwin (cwin (1 - Pi)) 1
• ssthresh (cwin (1 - (Pi / 2)))
• 3 dup ACK on path i --gt Fast Recovery
• cwin (cwin (1 - (Pi / 2))) 1
• ssthresh (cwin (1 - (Pi / 2)))

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Multiple TCP CB variables
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Where are we now?

• Finished modifications to DSR
• Finished modifications to TCP
• Debugging and testing TCP modifications

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Future work

• Run comparison tests to Backup Multipath Routing
• Eliminate modifications to TCP Receiver
• Try other spreading functions, such as ratio
  according to congestion level, bandwidth
• Mobility issue
• Routing support
• send duplicate packets over multiple paths
• Possibility to separate congestion control and
  reliability control

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Reference

• Haejung Lim, Kaixin Xu, and Mario Gerla, " TCP
  Performance over Multipath Routing in Mobile Ad
  Hoc Networks, " In Proceedings of IEEE ICC 2003.
• H.Y. Hsieh, R. Sivakumar, A Transport Layer
  Approach for Achieving Aggregate Bandwidths on
  Multi-homed Mobile Hosts, ACM MobiCom02, 2002,
  Atlanta, Georgia.