Lab The network simulator ns The network simulator ns Allows us to watch evolution of parameters like cwnd and ssthresh Allows us to watch evolution of parameters like cwnd and ssthresh ns output is in “raw” form; needs processing ns output is in “raw” form; needs processing awk is often very useful for this awk is often very useful for this

Implication of a time-out When a packet is lost, we stop getting new acks When a packet is lost, we stop getting new acks Left edge stops moving to the right Left edge stops moving to the right Dup acks come; suppose no fast retransmit Dup acks come; suppose no fast retransmit Transmitter keeps sending till it exhausts the usable window Transmitter keeps sending till it exhausts the usable window At this point, the transmitter stalls At this point, the transmitter stalls

Implication of a time-out What is happening to the segments already in the pipe? What is happening to the segments already in the pipe? They keep progressing to the receiver, and acks keep getting generated They keep progressing to the receiver, and acks keep getting generated But these are all dup acks But these are all dup acks As the transmitter is not putting new packets into the pipe, eventually the pipe empties out As the transmitter is not putting new packets into the pipe, eventually the pipe empties out

Eventually, pipe is empty

Implication of a time-out If RTO timer value is “appropriately” set, then by the time the timer fires, the pipe will have emptied out If RTO timer value is “appropriately” set, then by the time the timer fires, the pipe will have emptied out This is because RTO is set sufficiently large This is because RTO is set sufficiently large Same situation as when we began Same situation as when we began Hence the need for Slow Start after time-out Hence the need for Slow Start after time-out

Fast Retransmit However, if we do a Fast Retransmit, then the pipe is not empty when we retransmit However, if we do a Fast Retransmit, then the pipe is not empty when we retransmit In particular, if the duplicate ack threshold K is a small fraction of the BDP, the pipe is almost full when the fast retransmit happens In particular, if the duplicate ack threshold K is a small fraction of the BDP, the pipe is almost full when the fast retransmit happens –Hence, no need for Slow Start!

Fast Retransmit Further, K dup acks mean that K packets have left the network Further, K dup acks mean that K packets have left the network By “packet conservation”, can send K more packets! By “packet conservation”, can send K more packets!

Fast recovery Suppose that cwnd = C (packets) Suppose that cwnd = C (packets) This means that the source estimates that the pipe can hold C packets presently This means that the source estimates that the pipe can hold C packets presently Suppose that D dup acks are received Suppose that D dup acks are received This means that D packets have left the pipe This means that D packets have left the pipe –A dup ack is generated as soon as the out-of-order packet reaches the receiver So, D packets can be injected into the network So, D packets can be injected into the network

Fast recovery Observation: Observation: In order to send D packets, need to set In order to send D packets, need to set cwnd = C + D This is as if cwnd was inflated (by 1) for every dup ack received This is as if cwnd was inflated (by 1) for every dup ack received So, upon receiving the K th dup ack So, upon receiving the K th dup ack –ssthresh = (1/2)*cwnd (Usual step in CA) –Retransmit the lost packet

Fast Recovery After Fast Retransmit, set cwnd = ssthresh + K After Fast Retransmit, set cwnd = ssthresh + K Keep increasing cwnd by 1 for every duplicate ack received Keep increasing cwnd by 1 for every duplicate ack received This is the idea of Fast Recovery This is the idea of Fast Recovery Keep transmitting packets whenever allowed by the value of cwnd Keep transmitting packets whenever allowed by the value of cwnd

Fast Recovery When the (fast) retransmitted packet is acknowledged, set cwnd = ssthresh again (cwnd deflated) When the (fast) retransmitted packet is acknowledged, set cwnd = ssthresh again (cwnd deflated) Ready to begin next cycle Ready to begin next cycle Note that we are not beginning in Slow Start Note that we are not beginning in Slow Start Beginning in Congestion Avoidance instead Beginning in Congestion Avoidance instead Initial “slowness” of Slow Start avoided Initial “slowness” of Slow Start avoided Overall speed of data transfer improved Overall speed of data transfer improved

Fast Retransmit & Fast Recovery Reasons for improved performance Reasons for improved performance –Early detection of congestion (triple duplicate acks) and early retransmission –If retransmitted packet was successfully received, next cycle begins in Congestion Avoidance and not Slow Start –However, if retransmission is not successful, then next cycle begins in Slow Start again (no choice)

Fast Retransmit & Fast Recovery SS Loss detection (triple duplicate ack) Fast retransmit cwnd=ssthresh + 3 cwnd inflated for every duplicate ack; keep transmitting if possible Fast recovery Hopefully, retransmission succeeded cwnd= ssthresh Beginning the next cycle in congestion avoidance cwnd=1 Retransmission failed & timeout SS Triple duplicate ack ssthresh=(1/2)*cwnd

Retransmissions Measurements for obtaining RTT RTO calculations (parameters)

RTT measurements Not all transmitted segments are timed Not all transmitted segments are timed One connection is associated with one timer One connection is associated with one timer –If the timer is already in use when a segment S is transmitted, the segment S is not timed cwnd = 1 ack cwnd = 2 ack Segment not timed

RTT measurements RTT: time between sending a byte with a particular sequence number and getting an ack covering that sequence number RTT: time between sending a byte with a particular sequence number and getting an ack covering that sequence number Timing: counting ticks that occur every 500 ms Timing: counting ticks that occur every 500 ms Only segments containing data are timed Only segments containing data are timed –A segment with only an Ack will not be timed

RTT measurements If 1 tick between transmission of data and reception of ack, then RTT is taken to be 500 ms If 1 tick between transmission of data and reception of ack, then RTT is taken to be 500 ms –If 2 ticks, then RTT taken to be 1000 ms What should we do if a packet is retransmitted? What should we do if a packet is retransmitted?

RTT Measurements A packet is transmitted, a timeout occurs, the packet is retransmitted A packet is transmitted, a timeout occurs, the packet is retransmitted Now an ack is received Now an ack is received Ack corresponding to the 1st transmission or the 2nd? Ack corresponding to the 1st transmission or the 2nd? –Retransmission ambiguity problem

RTT Measurements Difficulty: Not sure if the 1 st transmission was lost Difficulty: Not sure if the 1 st transmission was lost Perhaps the RTO was too small and the retransmission happened too soon Perhaps the RTO was too small and the retransmission happened too soon

Karn’s algorithm Upon loss detected by time-out, back off the RTO timer Upon loss detected by time-out, back off the RTO timer –Exponential backoff: double the RTO interval upon a time-out –Continues till an upper limit is reached –Allowing time for congestion to clear out

Karn’s algorithm Ack for a retransmitted packet arrives Ack for a retransmitted packet arrives –No updates of the RTT estimates –No change in RTO values being used currently –Update estimates when ack received for a segment that was not retransmitted

RTT estimates and RTO How long should the time-out interval (RTO) be? How long should the time-out interval (RTO) be? Adaptive Adaptive –Should depend on the round-trip time (RTT) experienced on a connection Therefore, need to keep measuring RTTs Therefore, need to keep measuring RTTs

RTT estimates and RTO Denote the measured RTT value by M Denote the measured RTT value by M [RFC 793] R: “running” estimate of avg RTT [RFC 793] R: “running” estimate of avg RTT –R <---  R + (1-  )M –Smoothed estimator –  = 0.9 (recommended) Round-trip timeout Round-trip timeout –RTO =  R –  recommended  Experience showed that this setting of RTO did not work well (Jacobson) Experience showed that this setting of RTO did not work well (Jacobson)

RTT estimates and RTO Variability in RTT is not adequately captured Variability in RTT is not adequately captured –Leads to unnecessary retransmissions Keep track of the variability in RTT Keep track of the variability in RTT –Calculate RTO based on both mean and mean deviation of RTT A: running estimate of avg RTT A: running estimate of avg RTT –Err = (M - A) –“Error” between measured value and estimate

RTT estimates and RTO Algorithm Algorithm –Update A: A <-- (1 - g) A + g M –g = D: running estimate of deviation D: running estimate of deviation –Update D: D <-- (1 - h) D + h | Err | –h = 0.25 RTO = A + 4D RTO = A + 4D