TCP

TCP ACK generation [RFC 1122, RFC 2581]Event at Receiver

Arrival of in-order segment withexpected seq #. All data up toexpected seq # already ACKed

Arrival of in-order segment withexpected seq #. One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq. # .Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK. Wait up to 500msfor next segment. If no next segment,send ACK

Immediately send single cumulative ACK, ACKing both in-order segments

Immediately send duplicate ACK, indicating seq. # of next expected byte

Immediate send ACK, provided thatsegment startsat lower end of gap

Fast Retransmit• Time-out period often

relatively long:– long delay before resending

lost packet

• Detect lost segments via duplicate ACKs.– Sender often sends many

segments back-to-back– If segment is lost, there will

likely be many duplicate ACKs.

• If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost:– fast retransmit: resend

segment before timer expires

event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } else { increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) { resend segment with sequence number y }

Fast retransmit algorithm:

a duplicate ACK for already ACKed segment

fast retransmit

Zhenhai Duan Computer Science, FSU 5

TCP Round Trip Time and Timeout

Q: how to set TCP timeout value?• longer than RTT

– note: RTT will vary• too short: premature timeout

– unnecessary retransmissions• too long: slow reaction to segment

loss

Q: how to estimate RTT?• SampleRTT: measured time from

segment transmission until ACK receipt– ignore retransmissions, cumulatively

ACKed segments• SampleRTT will vary, want estimated

RTT “smoother”– use several recent measurements, not

just current SampleRTT

Zhenhai Duan Computer Science, FSU 6

TCP Round Trip Time and Timeout

EstimatedRTT = (1-x)*EstimatedRTT + x*SampleRTTExponential weighted moving averageinfluence of given sample decreases exponentially fasttypical value of x: 0.1

Setting the timeout• EstimtedRTT plus “safety margin”• Timeout is doubled every time a segment is timeout• large variation in EstimatedRTT larger safety margin

Timeout = EstimatedRTT + 4*Deviation

Deviation = (1-x)*Deviation + x*|SampleRTT-EstimatedRTT|

TCP TimeoutRTT: gaia.cs.umass.edu to fantasia.eurecom.fr

100

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1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106time (seconnds)

RTT

(mill

isec

onds

)

SampleRTT Estimated RTT

TCP flow/congestion control• Sometimes sender shouldn’t send a pkt whenever

its ready– Receiver not ready (e.g., buffers full)– React to congestion

• Many unACK’ed pkts, may mean long end-end delays, congested networks

• Network itself may provide sender with congestion indication

– Avoid congestion• Sender transmits smoothly to avoid temporary network

overloads

TCP

• To react to these, TCP has only one knob – the size of the send window– Reduce or increase the size of the send window– In our project, the size is fixed

• The size of the send window is determined by two things:– The size of the receiver window the receiver told him in

the TCP segment– His own perception about the level of congestion in the

network

TCP Flow Controlreceiver: explicitly informs

sender of (dynamically changing) amount of free buffer space – RcvWindow field in

TCP segmentsender: keeps the amount of

transmitted, unACKed data less than most recently received RcvWindow

sender won’t overrun

receiver’s buffers bytransmitting too

much, too fast

flow control

receiver buffering

RcvBuffer = size of TCP Receive BufferRcvWindow = amount of spare room in Buffer

What is Congestion?

• Informally: “too many sources sending too much data too fast for network to handle”

• Different from flow control, caused by the network not by the receiver

• How does the sender know whether there is congestion? Manifestations:– Lost packets (buffer overflow at routers)– Long delays (queuing in router buffers)

Causes/costs of congestion

• two senders, two receivers

• one router, infinite buffers

• no retransmission

• large delays when congested

• maximum achievable throughput

unlimited shared output link buffers

Host Ain : original data

Host B

out

Approaches towards congestion control

End-end congestion control:

• no explicit feedback from network

• congestion inferred from end-system observed loss, delay

• approach taken by TCP

Network-assisted congestion control:

• routers provide feedback to end systems– single bit indicating

congestion (SNA, DECbit, TCP/IP ECN, ATM)

– explicit rate sender should send at

Two broad approaches towards congestion control:

TCP Congestion Control

• Idea– Each source determines network capacity for itself– Uses implicit feedback, adaptive congestion

window– ACKs pace transmission (self-clocking)

• Challenge– Determining the available capacity in the first

place– Adjusting to changes in the available capacity

15

Additive Increase/Multiplicative Decrease

• Objective: Adjust to changes in available capacity– A state variable per connection: CongWin

• Limit how much data source has is in transit– MaxWin = MIN(RcvWindow, CongWin)

• Algorithm– Increase CongWin when congestion goes down (no losses)

• Increment CongWin by 1 pkt per RTT (linear increase)– Decrease CongWin when congestion goes up (timeout)

• Divide CongWin by 2 (multiplicative decrease)

Computer Science, FSU 16

TCP Congestion Control• Window-based, implicit, end-end control• Transmission rate limited by congestion window size, Congwin, over

segments:

w segments, each with MSS bytes sent in one RTT:

throughput = w * MSS

RTT Bytes/sec

Congwin

TCP Congestion Control

• two “phases”– slow start– congestion avoidance

• important variables:– Congwin– threshold: defines

threshold between slow start phase and congestion avoidance phase

• “probing” for usable bandwidth: – ideally: transmit as fast as

possible (Congwin as large as possible) without loss

– increase Congwin until loss (congestion)

– loss: decrease Congwin, then begin probing (increasing) again

Why Slow Start?• Objective

– Determine the available capacity in the first place• Idea

– Begin with congestion window = 1 pkt– Double congestion window each RTT

• Increment by 1 packet for each ack• Exponential growth but slower than one blast• Used when

– First starting connection– Connection goes dead waiting for a timeout

TCP Slowstart

• exponential increase (per RTT) in window size (not so slow!)

• loss event: timeout (Tahoe TCP) and/or or three duplicate ACKs (Reno TCP)

initialize: Congwin = 1for (each segment ACKed) Congwin++until (loss event OR CongWin > threshold)

Slowstart algorithmHost A

one segment

RTT

Host B

time

two segments

four segments

TCP Congestion Avoidance

/* slowstart is over */ /* Congwin > threshold */Until (loss event) { every w segments ACKed: Congwin++ }threshold = Congwin/2Congwin = 1perform slowstart

Congestion avoidance

21

TCP Fairness

Fairness goal: if N TCP sessions share same bottleneck link, each should get 1/N of link capacity

TCP connection 1

bottleneckrouter

capacity RTCP connection 2

Why is TCP fair?Two competing sessions:• Additive increase gives slope of 1, as throughput increases• multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughput

Conn

e ctio

n 2

thro

u ghp

ut

congestion avoidance: additive increase

loss: decrease window by factor of 2

However,TCP is not perfectly fair.It biases towardsflows with smallRTT.