LabLab

The network simulator The network simulator nsns

Allows us to watch evolution of parameters Allows us to watch evolution of parameters like like cwnd cwnd and and ssthreshssthresh

ns output is in “raw” form; ns output is in “raw” form; needs needs processingprocessing

awk awk is often very useful for thisis often very useful for this

Implication of a time-outImplication of a time-out

When a packet is lost, we stop getting When a packet is lost, we stop getting new new acksacks

Left edge stops moving to the rightLeft edge stops moving to the right

Dup acks come; suppose no fast retransmitDup acks come; suppose no fast retransmit

Transmitter keeps sending till it exhausts Transmitter keeps sending till it exhausts the usable windowthe usable window

At this point, the transmitter stallsAt this point, the transmitter stalls

Implication of a time-outImplication of a time-out What is happening to the segments What is happening to the segments already already

in the pipe?in the pipe?

They keep progressing to the receiver, and They keep progressing to the receiver, and acks keep getting generatedacks keep getting generated

But these are all dup acksBut these are all dup acks

As the transmitter is not putting new As the transmitter is not putting new packets into the pipe, eventually the pipe packets into the pipe, eventually the pipe empties outempties out

Eventually, pipe is empty

Implication of a time-outImplication of a time-out

If If RTO timer value is “appropriately” set, RTO timer value is “appropriately” set, then by the time the timer fires, then by the time the timer fires, the pipe will the pipe will have emptied outhave emptied out

This is because RTO is set sufficiently largeThis is because RTO is set sufficiently large

Same situation as when we beganSame situation as when we began

Hence the need for Hence the need for Slow Start Slow Start after time-outafter time-out

Fast RetransmitFast Retransmit

However, if we do a Fast Retransmit, However, if we do a Fast Retransmit, then the pipe is then the pipe is notnot empty when we empty when we retransmitretransmit

In particular, if the duplicate ack In particular, if the duplicate ack threshold threshold KK is a small fraction of the is a small fraction of the BDPBDP, the pipe is almost full when the , the pipe is almost full when the fast retransmit happensfast retransmit happens

– Hence, no need for Slow Start!Hence, no need for Slow Start!

Fast RetransmitFast Retransmit

Further, Further, KK dup acks mean that dup acks mean that KK packets packets have left the networkhave left the network

By “By “packet conservationpacket conservation”, can send ”, can send KK more packets!more packets!

Fast recoveryFast recovery Suppose that Suppose that cwndcwnd = = C (packets)C (packets)

This means that the source estimates that the This means that the source estimates that the pipe can hold C packets presentlypipe can hold C packets presently

Suppose that D Suppose that D dup acksdup acks are receivedare received

This means that D packets have left the pipeThis means that D packets have left the pipe– A A dup ack dup ack is generated as soon as the out-of-order is generated as soon as the out-of-order

packet reaches the receiverpacket reaches the receiver

So, D packets can be injected into the network So, D packets can be injected into the network

Fast recoveryFast recovery Observation:Observation:

In order to send D packets, need to set In order to send D packets, need to set

cwndcwnd = = C + DC + D

This is as if This is as if cwnd cwnd was was inflated inflated (by 1) for every (by 1) for every dup ack receiveddup ack received

So, upon receiving the So, upon receiving the KKth th dup ackdup ack– ssthresh = (1/2)*cwnd ssthresh = (1/2)*cwnd (Usual step in CA)(Usual step in CA)– Retransmit the lost packetRetransmit the lost packet

Fast RecoveryFast Recovery After Fast Retransmit, set After Fast Retransmit, set cwndcwnd = =

ssthreshssthresh + + KK

Keep increasing Keep increasing cwndcwnd by 1 for every by 1 for every duplicate ack receivedduplicate ack received

This is the idea ofThis is the idea of Fast RecoveryFast Recovery

Keep transmitting packets whenever Keep transmitting packets whenever allowed by the value ofallowed by the value of cwndcwnd

Fast RecoveryFast Recovery When the (fast) retransmitted packet is When the (fast) retransmitted packet is

acknowledged, set acknowledged, set cwnd = ssthreshcwnd = ssthresh again again (cwnd (cwnd deflateddeflated))

Ready to begin next cycleReady to begin next cycle

Note that we are not beginning in Slow StartNote that we are not beginning in Slow Start

Beginning in Congestion Avoidance insteadBeginning in Congestion Avoidance instead

Initial “slowness” of Slow Start avoidedInitial “slowness” of Slow Start avoided

Overall speed of data transfer improvedOverall speed of data transfer improved

Fast Retransmit & Fast Fast Retransmit & Fast RecoveryRecovery

Reasons for improved performanceReasons for improved performance

– Early detection of congestion (triple duplicate Early detection of congestion (triple duplicate acks) and early retransmissionacks) and early retransmission

– If retransmitted packet was successfully If retransmitted packet was successfully received, next cycle begins in Congestion received, next cycle begins in Congestion Avoidance and not Slow StartAvoidance and not Slow Start

– However, if retransmission is not successful, However, if retransmission is not successful, then next cycle begins in Slow Start again (no then next cycle begins in Slow Start again (no choice)choice)

Fast Retransmit & Fast Fast Retransmit & Fast RecoveryRecovery

SS Loss detection(triple duplicate ack)

Fast retransmitcwnd=ssthresh + 3

cwnd inflated for everyduplicate ack; keep transmitting if possible

Fast recovery

Hopefully, retransmission succeeded

cwnd=ssthresh

Beginning the nextcycle in congestionavoidance

cwnd=1

Retransmissionfailed & timeout

SS

Triple duplicateack

ssthresh=(1/2)*cwnd

RetransmissionsRetransmissions

Measurementsfor obtaining RTT

RTO calculations(parameters)

RTT measurementsRTT measurements Not all transmitted segments are timedNot all transmitted segments are timed

One connection is associated with one timerOne connection is associated with one timer– If the timer is already in use when a segment S is If the timer is already in use when a segment S is

transmitted, the segment S is not timedtransmitted, the segment S is not timed

cwnd = 1

ack

cwnd = 2

ack

Segmentnot timed

RTT measurementsRTT measurements

RTTRTT: : time between sending a byte with a time between sending a byte with a particular sequence number and getting an particular sequence number and getting an ack covering that sequence numberack covering that sequence number

Timing: counting ticks that occur every 500 Timing: counting ticks that occur every 500 msms

Only segments containing data are timedOnly segments containing data are timed

– A segment with only an Ack will not be timedA segment with only an Ack will not be timed

RTT measurementsRTT measurements

If 1 tick between transmission of data and If 1 tick between transmission of data and reception of ack, then RTT is taken to be reception of ack, then RTT is taken to be 500 ms500 ms

– If 2 ticks, then RTT taken to be 1000 msIf 2 ticks, then RTT taken to be 1000 ms

What should we do if a packet is What should we do if a packet is retransmitted?retransmitted?

RTT MeasurementsRTT Measurements

A packet is transmitted, a timeout occurs, A packet is transmitted, a timeout occurs, the packet is retransmittedthe packet is retransmitted

Now an ack is receivedNow an ack is received

Ack corresponding to the 1st transmission Ack corresponding to the 1st transmission or the 2nd?or the 2nd?

– Retransmission ambiguity problemRetransmission ambiguity problem

RTT MeasurementsRTT Measurements

Difficulty: Not sure if the 1Difficulty: Not sure if the 1stst transmission transmission was lostwas lost

Perhaps the RTO was too small and the Perhaps the RTO was too small and the retransmission happened too soonretransmission happened too soon

Karn’s algorithmKarn’s algorithm

Upon loss detected by time-out, back off Upon loss detected by time-out, back off the RTO timerthe RTO timer

– Exponential backoff: double the RTO interval Exponential backoff: double the RTO interval upon a time-outupon a time-out

– Continues till an upper limit is reachedContinues till an upper limit is reached

– Allowing time for congestion to clear outAllowing time for congestion to clear out

Karn’s algorithmKarn’s algorithm

Ack for a retransmitted packet arrivesAck for a retransmitted packet arrives

– No updates of the RTT estimatesNo updates of the RTT estimates

– No change in RTO values being used currentlyNo change in RTO values being used currently

– Update estimates when ack received for a Update estimates when ack received for a segment that was not retransmittedsegment that was not retransmitted

RTT estimates and RTORTT estimates and RTO

How long should the time-out interval (RTO) How long should the time-out interval (RTO) be?be?

AdaptiveAdaptive

– Should depend on the round-trip time (RTT) Should depend on the round-trip time (RTT) experienced on a connectionexperienced on a connection

Therefore, need to keep measuring RTTsTherefore, need to keep measuring RTTs

RTT estimates and RTORTT estimates and RTO Denote the measured RTT value by MDenote the measured RTT value by M

[RFC 793] R: “running” estimate of avg RTT[RFC 793] R: “running” estimate of avg RTT – R <--- R <--- R + (1-R + (1-)M)M– Smoothed estimatorSmoothed estimator = 0.9= 0.9 (recommended)(recommended)

Round-trip timeoutRound-trip timeout– RTO = RTO = RR recommendedrecommended

Experience showed that this setting of RTO did not Experience showed that this setting of RTO did not work well (Jacobson)work well (Jacobson)

RTT estimates and RTORTT estimates and RTO

Variability in RTT is not adequately Variability in RTT is not adequately capturedcaptured– Leads to unnecessary retransmissionsLeads to unnecessary retransmissions

Keep track of the variability in RTTKeep track of the variability in RTT– Calculate RTO based on both mean and mean Calculate RTO based on both mean and mean

deviation of RTTdeviation of RTT

A: running estimate of avg RTTA: running estimate of avg RTT– Err = (M - A)Err = (M - A)– ““Error” between measured value and estimateError” between measured value and estimate

RTT estimates and RTORTT estimates and RTO

AlgorithmAlgorithm– Update A:Update A: A <-- (1 - g) A + g MA <-- (1 - g) A + g M– g = 0.125g = 0.125

D: running estimate of deviationD: running estimate of deviation– Update D:Update D: D <-- (1 - h) D + h | Err |D <-- (1 - h) D + h | Err |– h = 0.25h = 0.25

RTO = A + 4DRTO = A + 4D