TCP over Wireless Networks Raj Jain Professor of Computer Science and Engineering Washington University in Saint Louis Saint Louis MO 63130 Audio Video recordings of this lecture are available at http www cse wustl edu jain cse574 10 Washington University in St Louis CSE574s 20 1 2010 Raj Jain Overview TCP Congestion Mechanisms Our Initial Research on TCP Congestion TCP Over Wireless Issues and Solutions TCP over Satellite Our research on TCP over Satellite and Wireless Washington University in St Louis CSE574s 20 2 2010 Raj Jain Research on Congestion Control 1Mbps 1Mbps 1Mbps Time 6 minutes 1Mbps 10Mbps 1Mbps Time 6 hours Bit in header Early 1980s Digital Equipment Corporation DEC introduced Ethernet products Noticed that throughput goes down with a higher speed link in middle because no congestion mechanisms in TCP Results 1 Timeout Congestion Reduce the TCP window to one on a timeout Jain 1986 2 Routers should set a bit when congested DECbit Jain Ramakrishnan Chiu 1988 3 Introduced the term Congestion Avoidance 4 Additive increase and multiplicative decrease AIMD principle Chiu and Jain 1989 There were presented to IETF in 1986 Slow start based on Timeout and AIMD Van Jacobson 1988 Washington University in St Louis CSE574s 20 3 2010 Raj Jain AIMD Principle Additive Increase Multiplicative Decrease W1 W2 Capacity Efficiency C W1 W2 Fairness W1 W2 to W1 DW W2 DW Linear increase 45 line W1 W2 to kW1 kW2 Multiplicative decrease line through origin W2 Efficient W1 Fair Capacity Ref D Chiu and Raj Jain Analysis of the Increase Decrease Algorithms for Congestion Avoidance in Computer Networks Journal of Computer Networks and ISDN Vol 17 No 1 June 1989 pp 1 14 http www cse wustl edu jain papers cong av htm Washington University in St Louis CSE574s 20 4 2010 Raj Jain Slow Start Congestion Control Window Flow Control Avoids receiver overrun Need congestion control to avoid network overrun The sender maintains two windows Credits from the receiver Congestion window from the network Congestion window is always less than the receiver window Starts with a congestion window CWND of 1 segment one max segment size Do not disturb existing connections too much Increase CWND by 1 MSS every time an ack is received Washington University in St Louis CSE574s 20 5 2010 Raj Jain Slow Start Cont If segments lost remember slow start threshold SSThresh to CWND 2 Set CWND to 1 MSS Increment by 1 per ack until SSthresh Increment by 1 MSS CWND per ack afterwards Congestion Window CWND Timeout SSThresh Receiver Window Idle Interval 1 Washington University in St Louis CSE574s 20 6 Time 2010 Raj Jain Slow Start Cont At the beginning SSThresh Receiver window After a long idle period exceeding one round trip time reset the congestion window to one Exponential growth phase is also known as Slow start phase The linear growth phase is known as congestion avoidance phase Washington University in St Louis CSE574s 20 7 2010 Raj Jain Fast Recovery Optional implemented in TCP Reno Earlier version was TCP Tahoe Duplicate Ack indicates a lost out of order segment On receiving 3 duplicate acks Enter Fast Recovery mode Retransmit missing segment Set SSTHRESH CWND 2 Set CWND SSTHRESH 3 MSS Every subsequent duplicate ack CWND CWND 1MSS Washington University in St Louis CSE574s 20 8 2010 Raj Jain Problems of Current TCP TCP cannot distinguish wireless errors from congestion Frequent errors Frequent window reductions Low throughput On CDMA Overload Errors Otherwise no relationship Sender Receiver Error or Congestion Duplicate acks Reduced window Washington University in St Louis CSE574s 20 9 2010 Raj Jain TCP Over Wireless 1 2 3 4 5 6 Link Layer Mechanisms Split TCP Solutions TCP Aware Link Layer Protocols Explicit Notification Schemes TCP Over Satellite Our Results for Satellite and Wireless Networks Washington University in St Louis CSE574s 20 10 2010 Raj Jain 1 Link Layer Mechanisms Forward Error Correction FEC Reduces loss due to errors Reduced link throughput even if no errors Automatic Repeat Request ARQ Link layer retransmission and acknowledgement No reduction in throughput if no errors Reduced throughput and increased delay at link layer May cause congestion May increase variance of RTT Increased RTO Receiver 1 May cause head of line blocking Receiver 2 Adaptive Link layer strategies Dynamically vary FEC code retransmission limit frame size Washington University in St Louis CSE574s 20 11 2010 Raj Jain 2 Split TCP Solutions Indirect TCP Selective Repeat Protocol SRP Mobile TCP Mobile End Transport Protocol Washington University in St Louis CSE574s 20 12 2010 Raj Jain Indirect TCP Fixed Host Base Station Mobile Host Two TCP connections Fixed host to Base Base to Mobile Independent flow control on two connections Packets buffered in the base Ack at sender MH has received Violates TCP s end to end semantics BS retains hard state BS failure loss of data On handoff stored packets must be sent to new BS Does not work if connection not bi directional E g satellites Reference Bakre95 Bakre 97 Washington University in St Louis CSE574s 20 13 2010 Raj Jain 3 TCP Aware Link Layer Protocols Snoop Protocol WTCP Delayed DupAcks Protocol SCPS TP Washington University in St Louis CSE574s 20 14 2010 Raj Jain Snoop Protocol Split connection and link level retransmission Base monitors returning acks Retransmits on duplicate acks and drops the duplicate ack Advantages Only soft state at BS Only BS modified No changes to FH or MH If wireless link delay is less than 4 packets 3 duplicate acks will not happen and a simple link level retransmission without dropping duplicate ack will also work Disadvantages Does not work with encrypted packets Does not work on asymmetric paths Ref Balakrishnan95 Washington University in St Louis CSE574s 20 15 2010 Raj Jain 4 Explicit Notification Schemes Explicit Loss Notification Explicit Loss Notification 2 Explicit Bad State Notification Partial Ack Protocols Washington University in St Louis CSE574s 20 16 2010 Raj Jain Explicit Loss Notification Works with Mobile host sources First link on the path is wireless BS keeps track of missing packets from mobile When DupAcks is received BS sets ELN bit in the DupAcks When mobile receives the DupAcks with ELN bit it does not back off Simply retransmits Reference Balakrishnan98 Washington University in St Louis CSE574s 20 17 2010 Raj Jain 5 TCP Over Satellite IETF TCPSAT Satellite Transport Protocol STP Early Acks ACKprime Washington University in St Louis CSE574s
View Full Document
Unlocking...