Explicit Notification 121 Explicit Notification Schemes General Philosophy Approximate Ideal TCP behavior Ideally the TCP sender should simply retransmit a packet lost due to transmission errors without taking any congestion control actions A wireless node somehow determines that packets are lost due to errors and informs the sender using an explicit notification Sender on receiving the notification does not reduce congestion window but retransmits lost packet 122 Explicit Notification Schemes Motivated by the Explicit Congestion Notification ECN proposals Floyd94 Variations proposed in literature differ in who sends explicit notification how they know to send the explicit notification what the sender does on receiving the notification 123 Explicit Notification Space Communication Protocol Standards Transport Protocol SCPS TP Satellite wireless Ground station TCP destinations 124 Space Communication Protocol Standards Transport Protocol SCPS TP The receiving ground station keeps track of how many packets with errors are received their checksums failed When the error rate exceeds a threshold the ground station sends corruption experienced messages to destinations of recent error free TCP packets destinations are cached The TCP destinations tag acks with corruptionexperienced bit TCP sender after receiving an ack with corruptionexperienced bit does not back off until it receives an ack without that bit even if timeout or fast retransmit occurs 125 Explicit Loss Notification Balakrishnan98 when MH is the TCP sender Wireless link first on the path from sender to receiver The base station keeps track of holes in the packet sequence received from the sender When a dupack is received from the receiver the base station compares the dupack sequence number with the recorded holes if there is a match an ELN bit is set in the dupack When sender receives dupack with ELN set it retransmits packet but does not reduce congestion Record window hole at 2 MH 4 3 wireless 2 1 1 BS 4 1 3 1 1 Dupack with ELN set FH 1 126 Explicit Bad State Notification Bakshi97 when MH is TCP receiver Base station attempts to deliver packets to the MH using a link layer retransmission scheme If packet cannot be delivered using a small number of retransmissions BS sends a Explicit Bad State Notification EBSN message to TCP sender When TCP sender receives EBSN it resets its timer timeout delayed when wireless channel in bad state 127 Partial Ack Protocols Cobb95 Biaz97 Send two types of acknowledgements A partial acknowledgement informs the sender that a packet was received by an intermediate host typically base station Normal TCP cumulative ack needed by the sender for reliability purposes 128 Partial Ack Protocols When a packet for which a partial ack is received is detected to be lost the sender does not reduce its congestion window loss assumed to be due to wireless errors 37 37 Partial ack 36 Cumulative ack 129 Various Schemes Link layer retransmissions Split connection approach TCP Aware link layer TCP Unaware approximation of TCP aware link layer Explicit notification Receiver based discrimination Sender based discrimination 130 Receiver Based Discrimination Scheme 131 Receiver Based Scheme Biaz98Asset MH is TCP receiver Receiver uses a heuristic to guess cause of packet loss When receiver believes that packet loss is due to errors it sends a notification to the TCP sender TCP sender on receiving the notification retransmits the lost packet but does not reduce congestion window 132 Receiver Based Scheme Packet loss due to congestion 12 FH 11 10 BS MH T 12 FH BS 10 MH 11 Congestion loss 133 Receiver Based Scheme Packet loss due to transmission error 12 FH 11 10 BS MH 2T 12 FH BS 11 Error loss 10 MH 134 Receiver Based Scheme Receiver uses the inter arrival time between consecutively received packets to guess the cause of a packet loss On determining a packet loss as being due to errors the receiver may tag corresponding dupacks with an ELN bit or send an explicit notification to sender 135 Receiver Based Scheme Disadvantages Limited applicability The slowest link on the path must be the last wireless hop to ensure some queuing will occur at the base station The queueing delays for all packets at the base station should be somewhat uniform multiple connections on the link will make interpacket delays variable 136 Receiver Based Scheme Advantages Can be implemented without modifying the base station an end to end scheme May be used despite encryption or if data acks traverse different paths 137 Various Schemes Link layer retransmissions Split connection approach TCP Aware link layer TCP Unaware approximation of TCP aware link layer Explicit notification Receiver based discrimination Sender based discrimination 138 Sender Based Discrimination Scheme 139 Sender Based Discrimination Scheme Biaz98ic3n Biaz99techrep Sender can attempt to determine cause of a packet loss If packet loss determined to be due to errors do not reduce congestion window Sender can only use statistics based on round trip times window sizes and loss pattern unless network provides more information example explicit loss notification 140 Heuristics for Congestion Avoidance throughput cliff knee RTT load load 141 Heuristics for Congestion Avoidance Define condition C as a function of congestion window size and observed RTTs Condition C evaluated when a new RTT is calculated condition C typically evaluates to 2 or 3 possible values for now assume 2 values TRUE or FALSE If C True reduce congestion window Several proposals for condition C 142 Heuristics for Congestion Avoidance Some proposals Normalized Delay Gradient jain89 r RTT i RTT i 1 RTT i RTT i 1 w W i W i 1 W i W i 1 Condition C r w 0 143 Heuristics for Congestion Avoidance Some proposals Normalized Throughput Gradient Wang91 Throughput gradient TG i T i T i 1 W i W i 1 Normalized Throughout Gradient NTG TG i TG 1 Condition C NTG 0 5 144 Heuristics for Congestion Avoidance Some proposals TCP Vegas Brakmo94 expected throughput ET W i RTTmin actual throughput AT W i RTT i Condition C ET AT beta 145 Sender Based Heuristics Record latest value evaluated for condition C When a packet loss is detected if last evaluation of C is TRUE assume packet loss is due to congestion else assume that packet loss is due to transmission errors If packet loss determined to be due to errors do not reduce congestion window 146 Sender Based Heuristics Disadvantage Does not work quite well enough as yet Reason