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CS 372 – introduction to computer networks*
Tuesday July 13
* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 3, slide: 1
TCP: Overview
 point-to-point:
 one sender, one receiver
 reliable, in-order byte
stream:

no “message boundaries”
 pipelined:
 TCP congestion and flow
control set window size
 send & receive buffers
socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
RFCs: 793, 1122, 1323, 2018, 2581
 full duplex data:
 bi-directional data flow
in same connection
 MSS: maximum segment
size
 connection-oriented:
 handshaking (exchange
of control msgs) init’s
sender, receiver state
before data exchange
 flow controlled:
 sender will not
socket
door
overwhelm receiver
segment
Chapter 3, slide: 2
TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pnter
Options (variable length,
padded to 32 bits)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
application
data
(variable length)
Chapter 3, slide: 3
TCP Connection Establishment
Three way handshake:
Step 1: client host sends TCP
SYN segment to server
 specifies initial seq #
 no data
Step 2: server host receives
SYN, replies with SYNACK
segment
client
server
Connection
request
Connection
granted
ACK
server allocates buffers
 specifies server initial
seq. #
Step 3: client receives SYNACK,
replies with
ACK segment,
In is
Java,
this is equivalent
to:
In Java, this
equivalent
to:
which may contain data

Socket connectionSocket
= new welcomeSocket.accept();
Socket clientSocket
= new Socket("hostname","port#);
Chapter 3, slide: 4
TCP Connection: tear down
Closing a connection:
Step 1: client:
client
server
close
 closes socket
clientSocket.close();
 sends TCP FIN control
closing
segment to server
Step 2: server:
 receives FIN,
 replies with ACK
 closes connection,
 sends FIN.
Chapter 3, slide: 5
TCP Connection: tear down
Closing a connection:
Step 3: client:
client
server
close
 receives FIN,
 replies with ACK.
closing
 enters “timed wait” - keep
Step 4: server:
 receives ACK
 connection closed.
timed wait
responding with ACKs to
received FINs
closed
closed
TCP: a reliable data transfer
 TCP creates rdt
service on top of IP’s
unreliable service
 Pipelined segments
 Cumulative acks
 TCP uses single
retransmission timer
 Retransmissions are
triggered by:


timeout events
duplicate acks
 Initially consider
simplified TCP sender:


ignore duplicate acks
ignore flow control,
congestion control
Chapter 3, slide: 7
TCP sender events:
data rcvd from app:
 Create segment with
seq #
 seq # is byte-stream
number of first data
byte in segment
 start timer if not
already running (think
of timer as for oldest
unACK’ed segment)
 expiration interval:
TimeOutInterval
timeout:
 retransmit segment
that caused timeout
 restart timer
Ack rcvd:
 If acknowledges
previously unACK’ed
segments


update what is known to
be ACK’ed
start timer if there are
outstanding segments
Chapter 3, slide: 8
TCP seq. #’s and ACKs
Seq. #’s:
 byte stream
“number” of first
byte in segment’s
data
ACKs:
 seq # of next byte
expected from
other side
 cumulative ACK
Host A
User
types
‘C’
Host B
host ACKs
receipt of
‘C’, echoes
back ‘C’
host ACKs
receipt
of echoed
‘C’
simple telnet scenario
time
Chapter 3, slide: 9
TCP: retransmission scenarios
Host A
X
loss
Sendbase
= 100
SendBase
= 120
SendBase
= 100
time
SendBase
= 120
lost ACK scenario
Host B
Seq=92 timeout
Host B
Seq=92 timeout
timeout
Host A
time
premature timeout
Chapter 3, slide: 10
TCP retransmission scenarios (more)
timeout
Host A
Host B
X
loss
SendBase
= 120
time
Cumulative ACK scenario
Chapter 3, slide: 11
TCP ACK generation
[RFC 1122, RFC 2581]
Event at Receiver
TCP Receiver action
Arrival of in-order segment with
expected seq #. All data up to
expected seq # already ACKed
Delayed ACK. Wait up to 500ms
for next segment. If no next segment,
send ACK
Arrival of in-order segment with
expected seq #. One other
segment has ACK pending
Immediately send single cumulative
ACK, ACKing both in-order segments
Arrival of out-of-order segment
higher-than-expect seq. # .
Gap detected
Immediately send duplicate ACK,
indicating seq. # of next expected byte
Arrival of segment that
partially or completely fills gap
Immediate send ACK, provided that
segment starts at lower end of gap
Chapter 3, slide: 12
Fast Retransmit
client
 Suppose: Packet 0 gets lost


Q: when retrans. of Packet 0 will
happen?? Why happens at that time?
A: typically at t1 ; we think it is lost
when timer expires
 Can we do better??
server
Timer is
set at t0
Think of what means to receive many
duplicate ACKs


it means Packet 0 is lost
Why wait till timeout since we know
packet 0 is lost
=> Fast retransmit
=> better perfermance
 Why 3 dup ACK, not just 1 or 2


Think of what happens when pkt0
arrives after pkt1 (delayed, not lost)
Think of what happens when pkt0
arrives after pkt1 & 2, etc.
Timer
expires
at t1
Chapter 3, slide: 13
Fast Retransmit: recap
 Receipt of duplicate
ACKs indicate lost of
segments


Sender often sends
many segments back-toback
If segment is lost,
there will likely be many
duplicate ACKs.
This is how TCP works:
 If sender receives 3 ACKs
for the same data, it
supposes that segment after
ACK’ed data was lost:
 fast retransmit:


resend segment before
timer expires
better performance
Chapter 3, slide: 14
TCP Flow Control (cont.)
flow control
 receive side of TCP
connection has a
receive buffer:
sender won’t overflow
receiver’s buffer by
transmitting too much,
too fast
 speed-matching
 app process may be
service: matching the
send rate to the
receiving app’s drain
rate
slow at reading from
buffer
Chapter 3, slide: 15
TCP Flow Control
 TCP uses sliding window for flow control
 Receiver specifies window size (window
advertisement )
Specifies how many bytes in the data stream can
be sent
 Carried in segment along with ACK

 Sender can transmit any number of bytes
 any size segment between last acknowledged byte
and within advertised window size
Chapter 3, slide: 16
TCP Flow control: how it works
 Rcvr advertises spare
room by including value
of RcvWindow in
segment header
(suppose TCP receiver
discards out-of-order
segments)
 unused buffer space:
 Sender limits unACKed
data to RcvWindow

guarantees receive
buffer doesn’t overflow
= rwnd
= RcvBuffer-[LastByteRcvd LastByteRead]
Chapter 3, slide: 17
Sliding window problem
 Under some circumstances, sliding window can result
in transmission of many small segments



If receiving application consumes a few data bytes, receiver
will advertise a small window
Sender will immediately send small segment to fill window
Inefficient in processing time and network bandwidth
• Why?
 Solutions:
 Receiver delays advertising new window
 Sender delays sending data when window is small
Chapter 3, slide: 18
Review questions
Problem:




Host A
Host B
TCP connection between A and B
B received upto 248 bytes
A sends back-to-back 2
segments to B with 40 and 60
bytes
B ACKs every pkt it receives
Q1: Seq# in 1st and 2nd seg. from A
to B ?
Q2: Spse: 1st seg. gets to B first.
What is seq# in 1st ACK?
Chapter 3, slide: 19
Review questions
Problem:




Host A
Host B
TCP connection between A and B
B received upto 248 bytes
A sends back-to-back 2
segments to B with 40 and 60
bytes
B ACKs every pkt it receives
Q1: Seq# in 2nd seg. from A to B ?
Q2: Spse: 1st seg. gets to B first.
What is seq# in 1st ACK?
Q3: Spse: 2nd seg. gets to B first.
What is seq# in 1st ACK? And in
2nd ACK?
Chapter 3, slide: 20
Review questions
Problem:




Host A
Host B
TCP connection between A and B
B received upto 248 bytes
A sends back-to-back 2
segments to B with 40 and 60
bytes
B ACKs every pkt it receives
Q1: Seq# in 2nd seg. from A to B ?
Q2: Spse: 1st seg. gets to B first.
What is seq# in 1st ACK?
Q3: Spse: 2nd seg. gets to B first.
What is seq# in 1st ACK? And in
2nd ACK?
Chapter 3, slide: 21
Chapter 3 outline
 1 Transport-layer
services
 2 Multiplexing and
demultiplexing
 3 Connectionless
transport: UDP
 4 Principles of reliable
data transfer
 5 Connection-oriented
transport: TCP
 6 Principles of
congestion control
 7 TCP congestion control
Chapter 3, slide: 22
Principles of Congestion Control
 cause:
end systems are sending too much data too fast for
network/routers to handle
 manifestations:


lost/dropped packets (buffer overflow at routers)
long delays (queueing in router buffers)
 different from flow control!
 a top-10 problem!
Chapter 3, slide: 23
Causes/costs of congestion: scenario 1
Host A
 two senders, two
receivers
 one router,
infinite buffers
 no retransmission
Host B
lout
lin : original data
unlimited shared
output link buffers
 large delays
when congested
Chapter 3, slide: 24
Causes/costs of congestion: scenario 2
 one router, finite buffers
 sender retransmission of lost packet
Host A
Host B
lin : original
data
l'in : original data, plus
retransmitted data
lout
finite shared output
link buffers
Chapter 3, slide: 25
Causes/costs of congestion: scenario 2
 Case (a):
l
=
l in (no retransmission)
in
 Case (b): “perfect” retransmission only when loss:
l > lout
in
 Case (c): retransmission of delayed (not lost) packet makes
l
in
larger (than perfect case) for same
R/2
R/2
lout
R/2
lin
a.
R/2
lout
lout
lout
R/3
lin
b.
R/2
R/4
lin
R/2
c.
Chapter 3, slide: 26
Approaches towards congestion control
Two broad 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
• ECN (explicit congestion
notification)

explicit rate at which
sender should send
• ICMP (internet control
messaging protocol)
Chapter 3, slide: 27
Chapter 3 outline
 1 Transport-layer
services
 2 Multiplexing and
demultiplexing
 3 Connectionless
transport: UDP
 4 Principles of reliable
data transfer
 5 Connection-oriented
transport: TCP
 6 Principles of
congestion control
 7 TCP congestion control
Chapter 3, slide: 28
TCP congestion control
 Keep in mind:
 Too slow: under-utilization => waste of network resources by
not using them!
 Too fast: over-utilization => waste of network resources by
congesting them!
 Challenge is then:
 Not too slow, nor too fast!!
 Approach:
 Increase slowly the sending rates to probe for usable
bandwidth
 Decrease the sending rates when congestion is observed
=> Additive-increase, multiplicative decrease (AIMD)
Chapter 3, slide: 29
TCP congestion control: AIMD
Additive-increase, multiplicative decrease (AIMD)
(also called “congestion avoidance”)


additive increase: increase CongWin by 1 MSS every RTT until
loss detected
multiplicative decrease: cut CongWin in half after loss
congestion window size
congestion
window
24 Kbytes
16 Kbytes
Saw tooth
behavior: probing
for bandwidth
8 Kbytes
time
time
Chapter 3, slide: 30
TCP Congestion Control: details
 sender limits transmission:
LastByteSent-LastByteAcked
 CongWin
 Roughly,
rate =
How does sender perceive
congestion?
 loss event
 timeout or
 3 duplicate ACKs
 TCP sender reduces rate
CongWin
Bytes/sec
RTT
 CongWin is dynamic, function
of perceived network
congestion
(CongWin) after loss event
Improvements:
 AIMD, any problem??

Think of the start of
connections
 Solution: start a little
faster, and then slow down
=>“slow-start”
Chapter 3, slide: 31
TCP Slow Start
 When connection begins,
TCP addresses this via
CongWin = 1 MSS
Slow-Start mechanism
 Example: MSS = 500 Bytes  When connection begins,
RTT = 200 msec
increase rate
 initial rate = 20 kbps
exponentially fast
 available bandwidth may
be >> MSS/RTT

desirable to quickly ramp
up to respectable rate
 When loss of packet
occurs (indicates that
connection reaches up
there), then slow down
Chapter 3, slide: 32
TCP Slow Start (more)
How it is done
Host A
begins, increase rate
exponentially until
first loss event:


RTT
 When connection
Host B
double CongWin every
RTT
done by incrementing
CongWin for every ACK
received
 Summary: initial rate
is slow but ramps up
exponentially fast
time
Chapter 3, slide: 33
Refinement: TCP Tahoe
Question:
When should exponential increase (Slow-Start) switch to linear (AIMD)?
Here is how it works:
 Define a variable, called Threshold
 Start “Slow-Start”
 When CongWin =Threshold:
Switch to AIMD (linear)
 At loss event:
 Set Threshold = 1/2 CongWin
 Start over with CongWin = 1

Question: What should Threshold be set to at first??
Answer: start Slow-Start until packet loss occurs
• When loss occurs, set Threshold = ½ current CongWin
Chapter 3, slide: 34
More refinement: TCP Reno
Loss event: Timeout
vs. dup ACKs
 3 dup ACKs: fast retransmit
client
server
Timer is
set at t0
Timer
expires
at t1
Chapter 3, slide: 35
More refinement: TCP Reno
Loss event: Timeout
vs. dup ACKs
 3 dup ACKs: fast retransmit
 Timeout: retransmit
client
server
Timer is
set at t0
Any difference (think
congestion) ??
 3 dup ACKs indicate network still
capable of delivering some
segments after a loss
 Timeout indicates a “more”
alarming congestion scenario
Timer
expires
at t1
Chapter 3, slide: 36
More refinement: TCP Reno
TCP Reno treats “3 dup ACKs” different from “timeout”
How does TCP Reno
work?
 After 3 dup ACKs:


CongWin is cut in half
congestion avoidance
(window grows linearly)
 But after timeout event:


CongWin instead set
to 1 MSS;
Slow-Start (window
grows exponentially)
Chapter 3, slide: 37
Summary: TCP Congestion Control
 When CongWin is below Threshold, sender in
slow-start phase, window grows exponentially.
 When CongWin is above Threshold, sender is in
congestion-avoidance phase, window grows linearly.
 When a triple duplicate ACK occurs, Threshold
set to CongWin/2 and CongWin set to
Threshold.
 When timeout occurs, Threshold set to
CongWin/2 and CongWin is set to 1 MSS.
Chapter 3, slide: 38
Average throughput of TCP
Avg. throughout as a function of window size W and RTT?


Ignore Slow-Start
Let W, the window size when loss occurs, be constant
 When window is W, throughput is ??
throughput(high) = W/RTT
 Just after loss, window drops to W/2, throughput is ??
throughput(low) = W/(2RTT).
 Throughput then increases linearly from W/(2RTT) to
W/RTT
 Hence, average throughout = 0.75 W/RTT
Chapter 3, slide: 39
TCP Fairness
Fairness goal: if K TCP sessions share same
bottleneck link of bandwidth R, each should have
average rate of R/K
TCP connection 1
TCP
connection 2
bottleneck
router
capacity R
Chapter 3, slide: 40
Fairness (more)
Fairness and UDP
 Multimedia apps often
do not use TCP

do not want rate
throttled by congestion
control
 Instead use UDP:
 pump audio/video at
constant rate, tolerate
packet loss
Fairness and parallel TCP
connections
 nothing prevents app from
opening parallel coneccion
between 2 hosts.
 Web browsers do this
 Example: link of rate R
supporting 9 connections;


new app asks for 1 TCP, gets
rate R/10
other new app asks for 11
TCPs, gets R/2 !
Chapter 3, slide: 41
Question?
Again assume two competing sessions only:
 Additive increase gives slope of 1, as throughout increases
 Constant/equal decrease instead of multiplicative decrease!!!
(Call this scheme: AIED)
TCP connection 1
TCP
connection 2
bottleneck
router
capacity R
Chapter 3, slide: 42
Question?
Again assume two competing sessions only:
 Additive increase gives slope of 1, as throughout increases
 Constant/equal decrease instead of multiplicative decrease!!!
(Call this scheme: AIED)
R
equal bandwidth share
Question:
 How would (R1, R2)
(R1, R2)
vary when AIED is
used instead of AIMD?
 Is it fair? Does it converge
to equal share?
Connection 1 throughput R
Chapter 3, slide: 43
Why is TCP fair?
Two competing sessions:
 Additive increase gives slope of 1, as throughout increases
 multiplicative decrease decreases throughput proportionally
R
equal bandwidth share
loss: decrease window by factor of 2
congestion avoidance: additive increase
loss: decrease window by factor of 2
congestion avoidance: additive increase
Connection 1 throughput R
Chapter 3, slide: 44