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 08 Washington University in St Louis CSE574s 18 1 2008 Raj Jain Overview TCP Key Features 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 18 2 2008 Raj Jain TCP Key Features 1 Stream Oriented Transmission Multiple application packets may be send in one TCP Segment Maximum Segment Size MSS All acks are byte numbers Segment used in all discussions 2 Reliable Delivery Segments are buffered at the source until acked Retransmitted if not acked 3 In Order Delivery Destination delivers segments to application only when all previous segments received 4 End to End Semantics Ack Data received at destination 5 Congestion Control Increases load slowly from a low initial start Reduces load if network congested based on segment timeout duplicate acks 6 Congestion Avoidance Explicit Congestion Notification ECN bits in TCP IP headers based on DECbit research Washington University in St Louis CSE574s 18 3 2008 Raj Jain TCP Flow Control Cumulative Acks Acks all bytes up to the ack Piggybacked Acks Acks are sent in the reverse packets if possible Delayed Acks Ack delayed in case another segment is received or segment needs to be sent Typically 200 ms Duplicate Acks If an out of order packet is received the previous ack is resent Duplicate acks are not delayed Window Flow Control Throughput Window Round Trip Time Ideal Window Size Round Trip Time Link Capacity Delay bandwidth product TCP sets retransmission timer for only one packet If the ack is not received and the timer expires the packet is assumed lost Washington University in St Louis CSE574s 18 4 2008 Raj Jain TCP Congestion Mechanisms Slow Start Fast retransmit and recovery New Reno Selective Acknowledgement Explicit Congestion Notification Washington University in St Louis CSE574s 18 5 2008 Raj Jain Our Research on TCP Congestion 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 18 6 2008 Raj Jain Slow Start Congestion Window Timeout CWND Segment lost Congestion Avoidance CWND 2 Slow Start Slow Start Washington University in St Louis Wait for Timeout CSE574s 18 7 Slow Start TIME Congestion Avoidance 2008 Raj Jain Slow Start Cont Receiver sends Receive window for flow control Sender maintains a Congestion Window CWND CWND W Receiver Window Set Slow Start Threshold SSThresh 64 kB initially Start with a CWND W of 1 Increase W by 1 after every ack until SSThresh Exponential increase Slow Start W doubles every RTT Increase W by 1 W after every Acks W increases by per RTT Linear increase Congestion Avoidance On a timeout Set SSThresh to half the current window and set window to 1 SSThresh Max 2 0 5W W 1 Washington University in St Louis CSE574s 18 8 2008 Raj Jain Fast Retransmit and Recovery FRR Also known as TCP Reno Ideas Don t have to wait for timeout on a loss Don t reduce to one on single loss Duplicate acks Loss On three duplicate acks Retransmit the lost segment Fast Retransmit Set SSThresh to Max 2 0 5 CWND Reduce CWND to 0 5 CWND of dupacks New ack CWND SSThresh Linear increase Duplicate ack inflate CWND by 1 Send a pkt if allowed Advantage Recovers from loss without a timeout Problem Cannot recover from bursty 3 losses Dupacks are also generated if pkts out of order no loss Washington University in St Louis CSE574s 18 9 2008 Raj Jain FRR Cont 1st Fast Retransmit CWND 2nd Fast Retransmit CWND 2 Timeout CWND 4 CWND 8 TIME Washington University in St Louis Wait for CWND Inflates CSE574s Timeout 18 10 2008 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 18 11 2008 Raj Jain TCP Over Wireless 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 18 12 2008 Raj Jain 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 18 13 2008 Raj Jain Split TCP Solutions Indirect TCP Selective Repeat Protocol SRP Mobile TCP Mobile End Transport Protocol Washington University in St Louis CSE574s 18 14 2008 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 18 15 2008 Raj Jain Selective Repeat Protocol SRP Fixed Host Base Station Mobile Host Two connections Similar to Indirect TCP FH to BS Standard TCP BS to MH Selective repeat protocol on UDP Reference Yavatkar94 Washington University in St Louis CSE574s 18 16 2008 Raj Jain Mobile TCP Asymmetric split
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