TCP over Wireless Networks Raj Jain Washington University Saint Louis MO 63131 Jain cse wustl edu These slides are available on line at http www cse wustl edu jain cse574 06 Washington University in St Louis CSE574s 10 1 2006 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 10 2 2006 Raj Jain TCP Key Features 1 2 3 4 5 6 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 Reliable Delivery Segments are buffered at the source until acked Retransmitted if not acked In Order Delivery Destination delivers segments to application only when all previous segments received End to End Semantics Ack Data received at destination Congestion Control Increases load slowly from a low initial start Reduces load if network congested based on segment timeout duplicate acks Congestion Avoidance Explicit Congestion Notification ECN bits in TCP IP headers based on DECbit research Washington University in St Louis CSE574s 10 3 2006 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 10 4 2006 Raj Jain Timeout Calculations Old Method Using only the mean of measured round trip error sampleRTT mean mean a error 0 a 1 timeout d mean d 1 RFC 793 suggested 0 1 to 0 2 for a and 1 3 to 2 0 for d New Method Using mean and standard deviation error sampleRTT mean mean a error 0 a 1 sample dev error dev error sample dev mean dev mean dev b sample dev mean dev 0 b 1 timeout mean c mean dev c 0 The usual values for the constants are a 1 8 b 1 4 c 4 RTT is measured in multiples of a tick 1 tick 500 ms usually RTO is at least 2 ticks Double RTO on repeated timeouts Washington University in St Louis CSE574s 10 5 2006 Raj Jain TCP Congestion Mechanisms Slow Start Fast retransmit and recovery New Reno Selective Acknowledgement Explicit Congestion Notification Washington University in St Louis CSE574s 10 6 2006 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 10 7 2006 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 10 8 Slow Start TIME Congestion Avoidance 2006 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 10 9 2006 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 10 10 2006 Raj Jain FRR Cont 1st Fast Retransmit CWND 2nd Fast Retransmit CWND 2 Timeout CWND 4 CWND 8 Washington University in St Louis Wait for CWND Inflates CSE574s Timeout 10 11 TIME 2006 Raj Jain TCP New Reno Janey Hoe s MS Thesis from MIT Published in SIGCOMM 96 Solution Determine the end of a burst loss Remember the highest segment sent RECOVER Ack RECOVER Partial Ack Ack RECOVER New Ack New Ack Linear increase from 0 5 CWND Partial Ack Retransmit next packet let window inflate Recovers from N losses in N round trips Washington University in St Louis CSE574s 10 12 2006 Raj Jain New Reno Cont 1st Fast Retransmit Receive New ACK Fast Recovery CWND CWND 2 Receive Partial ACK 2nd Fast Retransmit Washington University in St Louis Receive Partial ACK 3rd Fast Retransmit CSE574s 10 13 TIME 2006 Raj Jain Selective Ack RFC 2018 October 1996 Receivers can indicate missing segments Example Using Bytes Ack 500 SACK 1000 1500 2000 2500 Rcvd segment 1 lost 2 rcvd 3 lost 4 rcvd 5 On a timeout ignore all SACK info SACK negotiated at connection setup Used on all duplicate acks 0 499 500 999 Lost Washington University in St Louis 1000 1499 1500 1999 2000 2499 Lost CSE574s 10 14 2006 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 10 15 2006 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
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