Explicit Notification121Explicit NotificationExplicit Notification SchemesGeneral Philosophy• Approximate Ideal TCP behavior: Ideally, the TCP sender should simply retransmit a packet lost due to transmission errors, without taking any congestion control actions122• 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 packetExplicit Notification Schemes• Motivated by the Explicit Congestion Notification (ECN)proposals [Floyd94]123Variations proposed in literature differ in• who sends explicit notification• how they know to send the explicit notification• what the sender does on receiving the notificationExplicit NotificationSpace Communication Protocol Standards-Transport Protocol (SCPS-TP)Satellitewireless124Ground stationTCP destinationsSpace 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 packets125destinations of recent error-free TCP packets– destinations are cached • The TCP destinations tag acks with corruption-experienced bit• TCP sender, after receiving an ack with corruption-experienced bit, does not back off until it receives an ack without that bit (even if timeout or fast retransmit occurs)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 dupacksequence number with 126station compares the dupacksequence 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 windowMH FHBS4 3 2 1 134wirelessRecordhole at 2111 1Dupack with ELN setExplicit 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 127•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 statePartial 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)128intermediate host (typically, base station)• Normal TCP cumulative ack needed by the sender for reliability purposesPartial 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 errors129–loss assumed to be due to wireless errors3736Partial ack37Cumulative ackVarious Schemes• Link-layer retransmissions• Split connection approach• TCP-Aware link layer•TCP-Unaware approximation of TCP-aware link 130•TCP-Unaware approximation of TCP-aware link layer• Explicit notification• Receiver-based discrimination• Sender-based discriminationReceiver-Based Discrimination 131Receiver-Based Discrimination SchemeReceiver-Based Scheme [Biaz98Asset]• MH is TCP receiver• Receiver uses a heuristic to guess cause of packet loss132loss• 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 windowReceiver-Based Scheme• Packet loss due to congestionFH MHBS1012 11133FH MHBS111012TCongestion lossReceiver-Based Scheme• Packet loss due to transmission errorFH MHBS1012 11134FH MHBS101112Error loss2 TReceiver-Based Scheme• Receiver uses the inter-arrival time between consecutively received packets to guess the cause of a packet loss135cause 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 senderReceiver-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 station136–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 inter-packet delays variableReceiver-Based Scheme : Advantages• Can be implemented without modifying the base station (an “end-to-end” scheme)137• May be used despite encryption, or if data & acks traverse different pathsVarious Schemes• Link-layer retransmissions• Split connection approach• TCP-Aware link layer•TCP-Unaware approximation of TCP-aware link 138•TCP-Unaware approximation of TCP-aware link layer• Explicit notification• Receiver-based discrimination• Sender-based discriminationSender-Based Discrimination 139Sender-Based Discrimination SchemeSender-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 window140reduce 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)Heuristics for Congestion Avoidancethroughputkneecliff141loadloadRTTkneeHeuristics for Congestion Avoidance• Define condition C as a function of congestion window size and observed RTTs•Condition C evaluated when a new RTT is calculated142•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 CHeuristics for Congestion AvoidanceSome proposals• Normalized Delay Gradient [jain89]r = [RTT(i)-RTT(i-1)] / [RTT(i)+RTT(i-1)]143w = [W(i)-W(i-1)] /