Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of reliable
data transfer
3.5 Connection-oriented
transport: TCP




segment structure
reliable data transfer
flow control
connection management
3.6 Principles of
congestion control
3.7 TCP congestion control
Transport Layer
3-1
TCP: Overview

point-to-point:

 one sender, one receiver


 bi-directional data flow
in same connection
reliable:
 all packets delivered
full duplex data:

pipelined:
 W packets are sent and
ACKed
connection-oriented:
 handshaking (exchange
of control msgs)
RFCs: 793, 1122, 1323, 2018, 2581 …
Transport Layer
3-2
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)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
application
data
(variable length)
Transport Layer
3-3
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of reliable
data transfer
3.5 Connection-oriented
transport: TCP




segment structure
reliable data transfer
flow control
connection management
3.6 Principles of
congestion control
3.7 TCP congestion control
Transport Layer
3-4
TCP reliable data transfer

TCP creates rdt
service on top of IP’s
unreliable service
 seq # and acks
 cumulative acks
 duplicate acks
 timers
 retransmissions
 pipelined segments
 sliding window
-> retransmissions are
triggered by:
 timeout events
 duplicate acks
Transport Layer
3-5
TCP seq. #’s and ACKs
Seq. #’s:
 byte stream
“number” of first
byte in segment’s
data
Host A
Host B
ACKs:
 seq # of next byte
expected from
other side
 cumulative ACK
Transport Layer
3-6
TCP: retransmission scenarios
X
loss
Host A
Host B
Seq=92 timeout
Host B
Seq=92 timeout
timeout
Host A
lost ACK scenario
premature timeout
Transport Layer
3-7
TCP retransmission scenarios (more)
timeout
Host A
Host B
X
loss
Cumulative ACK scenario
Transport Layer
3-8
TCP Round Trip Time and Timeout
Q: how to set TCP
timeout value?

longer than RTT
Q: how to estimate RTT?

 but RTT varies


too short:
premature timeout
 unnecessary
retransmissions
too long: slow
reaction to segment
loss

SampleRTT: measured time from
segment transmission until ACK
receipt
SampleRTT will vary, want
estimated RTT “smoother”
 average several recent
measurements, not just
current SampleRTT
Transport Layer
3-9
Example RTT estimation:
RTT: gaia.cs.umass.edu to fantasia.eurecom.fr
350
RTT (milliseconds)
300
250
200
150
100
1
8
15
22
29
36
43
50
57
64
71
78
85
92
99
106
time (seconnds)
SampleRTT
Estimated RTT
Transport Layer
3-10
TCP Round Trip Time and Timeout
EstimatedRTT = (1- α)*EstimatedRTT + α*SampleRTT



Exponential weighted moving average
influence of past sample decreases exponentially fast
typical value: α = 0.125 (RFC6298)
Transport Layer
3-11
TCP Round Trip Time and Timeout

Deviation of SampleRTT :
DevRTT = (1-β)*DevRTT +
β*|SampleRTT-EstimatedRTT|
typically, β = 0.25 (RFC6298)
Then set timeout interval:
TimeoutInterval = EstimatedRTT + 4*DevRTT
Transport Layer
3-12
Example RTT estimation: cont.
Transport Layer
3-13
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of reliable
data transfer

3.5 Connection-oriented
transport: TCP




segment structure
reliable data transfer
flow control
connection management
3.6 Principles of
congestion control
3.7 TCP congestion control
Transport Layer
3-14
TCP Flow Control

flow control
sender won’t overflow
receiver’s buffer by
transmitting too much,
too fast
receive side of TCP
connection has a
receive buffer:


app process may be
slow at reading from
buffer
speed-matching
service: matching the
send rate to the
receiving app’s drain
rate
Transport Layer
3-15
TCP Flow control: how it works



RcvWindow - spare room
in recv buffer
rcvr advertises spare
room by including value
of RcvWindow in
segments
sender limits unACKed
data to RcvWindow
 guarantees receive
buffer doesn’t overflow
Transport Layer
3-16
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of reliable
data transfer
3.5 Connection-oriented
transport: TCP




segment structure
reliable data transfer
flow control
connection management
3.6 Principles of
congestion control
3.7 TCP congestion control
Transport Layer
3-17
TCP Connection Management
Recall: TCP sender, receiver establish
“connection” before exchanging data segments

initialize TCP variables:
 seq. #s
 buffers, flow control info (e.g. RcvWindow)

client: connection initiator

server: contacted by client
Transport Layer
3-18
TCP Connection establishment
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
 server allocates buffers
 specifies server initial
seq. #
Step 3: client receives SYNACK,
replies with ACK segment,
which may contain data
Transport Layer
3-19
TCP Connection termination
Step 1: client end system sends
TCP FIN control segment to
server
Step 2: server receives FIN,
client
server
close
replies with ACK. Closes
connection, sends FIN.
close
Step 3: client receives FIN,
 Enters “timed wait” - will
respond with ACK to
received FINs
Step 4: server, receives ACK.
timed wait
replies with ACK.
closed
Connection closed.
Transport Layer
3-20
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of reliable
data transfer
3.5 Connection-oriented
transport: TCP




segment structure
reliable data transfer
flow control
connection management
3.6 Principles of
congestion control
3.7 TCP congestion control
Transport Layer
3-21
Principles of Congestion Control
Congestion:




informally: “too many sources sending too much data
too fast for network to handle”
different from flow control!
manifestations:
 lost packets (buffer overflow at routers)
 long delays (queueing in router buffers)
Results in unfairness and poor utilization of network
resources:
Resources utilized by dropped packets
Retransmissions
Transport Layer
3-22
Causes/costs of congestion: scenario 1



two senders, two
receivers
one router,
infinite buffers
no retransmission
Host A
Host B
λout
λin : original data
unlimited shared
output link buffers


large delays
when congested
maximum
achievable
throughput
Transport Layer
3-23
Causes/costs of congestion: scenario 2


one router, finite buffers
sender retransmission of timed-out packet
 application-layer input = application-layer output: λin = λout
 transport-layer input includes retransmissions : λ‘in λin
λin : original data
λ'in: original data, plus
λout
retransmitted data
Host B
Host A
finite shared output
link buffers
Transport Layer
3-24
Congestion scenario 2: duplicates

packets may get
dropped at router due
to full buffers
sender times out
prematurely, sending
two copies, both of
which are delivered
R/2
when sending at
R/2, some packets
are retransmissions
including duplicated
that are delivered!
λout

λin
R/2
“costs” of congestion:


more work (retrans) for given “goodput”
unneeded retransmissions: link carries multiple copies of pkt
 decreasing goodput
Transport Layer
3-25
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 (SNA,
DECbit, TCP/IP ECN,
ATM)
 explicit rate sender
should send at
Transport Layer
3-26
Chapter 3 outline
3.1 Transport-layer
services
3.2 Multiplexing and
demultiplexing
3.3 Connectionless
transport: UDP
3.4 Principles of reliable
data transfer
3.5 Connection-oriented
transport: TCP




segment structure
reliable data transfer
flow control
connection management
3.6 Principles of
congestion control
3.7 TCP congestion control
Transport Layer
3-27
TCP Congestion Control: Overview
 end-end control (no network assistance)
 Limit the number of packets in the network to
window W
 Roughly,
rate =
W
RTT
Bytes/sec
 W is dynamic, function of perceived network
congestion
 ACK-clocking mechanism
27
Transport Layer
3-28
TCP Congestion Control: details
How does sender perceive congestion?
 loss event = timeout or 3 duplicate acks
 TCP sender reduces rate (cwnd) after loss event
mechanisms: (RFC5681)




AIMD
slow start
congestion avoidance
fast retransmit
Transport Layer
3-29
TCP AIMD: additive increase, multiplicative
decrease
approach: increase transmission rate (window size),
probing for usable bandwidth, until loss occurs
 additive increase: increase window by 1 every
RTT until loss detected
 multiplicative decrease: cut window in half after
loss
saw tooth
behavior: probing
for bandwidth
cwnd: congestion window size

congestion
window
24 Kbytes
16 Kbytes
8 Kbytes
time
time
Transport Layer
3-30
Slow Start
 “Slow Start” is used to
reach the equilibrium state
 Initially: W = 1 (slow start)
 On each successful ACK:
W=W+1
 Exponential growth of W
each RTT: W = 2 x W
 Enter CA when
W >= ssthresh
receiver
sender
cwnd
1
2
data
segment
ACK
3
4
5
6
7
8
30
Congestion avoidance
 Starts when
W = ssthresh
 On each successful ACK
W = W+ 1/W
 Linear growth of W each RTT
W = W + 1 (additive increase)
Transport Layer
31
3-32
TCP (initial version without loss)
Window
ssthresh
Reached initial
ssthresh value;
switch to CA mode
Time
Slow Start
32
Detecting Packet Loss
 Assumption: loss
10
11
indicates congestion
 Option 1: time-out

Waiting for a time-out can
be long!
12
X
13
14
15
16
17
11
12
12
 Option 2: duplicate ACKs

12
12
How many? At least 3.
Sender
Receiver
35
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-toback
 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
Transport Layer
3-35
Host A
Host B
timeout
X
Figure 3.37 Resending a segment after triple duplicate ACK
Transport Layer
3-36
Fast Retransmit
 Immediately retransmits after 3
dupACKs without waiting for timeout
 Adjusts ssthresh
ssthresh = W/2
 Enter Slow Start (Tahoe)
W=1
36
TCP Congestion Controls
 Tahoe (Jacobson 1988)
Slow Start
 Congestion Avoidance
 Fast Retransmit

 Reno (Jacobson 1990)

Fast Recovery
 SACK
 Vegas (Brakmo & Peterson 1994)

Delay and loss as indicators of congestion
29
Refinement


variable ssthresh
on loss event,
ssthresh is set to
1/2 of W just before
loss event
Transport Layer
3-39
TCP Reno: Fast Recovery
 Objective: prevent `pipe’ from emptying
after fast retransmit
 each dup ACK represents a packet having left
the pipe (successfully received)
 On 3 duplicate ACKs
 Fast retransmit and fast recovery
 On timeout
 Fast retransmit and slow start
46
Done with TCP congestion control mechanisms
What type of pipelining is implemented in TCP?
Recall: Pipelined Protocols
Go-back-N:
 N unacked packets in
pipeline – Window

cumulative acks
 doesn’t ack packet if
there’s a gap

sender has timer for
oldest unacked packet
 if timer expires,
retransmit all unack’ed
packets
Selective Repeat:
 sender can have up to
N unack’ed packets in
pipeline
 rcvr sends individual
ack for each packet
 sender maintains timer
for each unacked
packet
 when timer expires,
retransmit only
unack’ed packet
Transport Layer
3-42
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
Transport Layer
3-43
Why is TCP fair?
AIMD game for 2 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
Transport Layer
3-44
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
connections 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
 new app asks for 11 TCPs,
gets R/2 !
Transport Layer
3-45
Chapter 3: Summary


principles behind transport
layer services:
 multiplexing,
demultiplexing
 reliable data transfer
 flow control
 congestion control
instantiation and
implementation in the
Internet
 UDP
 TCP
Next:
 leaving the network
“edge” (application,
transport layers)
 into the network
“core”
Transport Layer
3-46
TCP: seq # plot
 There is a beautiful way to plot and
visualize the dynamics of TCP behaviour
 Called a “TCP Sequence Number Plot”
 Plot packet events (data and acks) as
points in 2-D space, with time on the
horizontal axis, and sequence number on
the vertical axis
 Example: Consider a 14-packet transfer
63
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
64
TCP: seq n plot
 What can it tell you?
 Everything!!!
65
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
RTT
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
66
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
TCP
Seg.
Size
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
67
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
TCP Connection Duration
68
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
Num
Bytes
Sent
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
69
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
Bytes
+
+
+
+
+
+
Sec
Time
70
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
Access
Network
Bandwidth
(Bytes/Sec)
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
71
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
Sender’s
Flow Control
Window Size
+
+
Time
72
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
TCP
Slow
Start
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
73
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Delayed ACK
+
Time
74
Key: X Data Packet
+ Ack Packet
X
X
X
Packet
Loss
X
X
X
X
X
+
X
X
X
X
+
+
+
+
++
+
+
+
Duplicate
ACK
+
+
Time
75
Cumulative ACK
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
+
X
X
+
X
+ +
+
+
+
X
Retransmit
+
+
+
+
+
+
Time
76
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
+
X
X
+
X
+ +
+
X
+
+
+
+
+
+
+
+
Time
RTO
77
TCP: seq # plot
 What happens when a packet loss occurs?

Consider a 14-packet Web document

For simplicity, consider only a single packet loss
78
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
79
?
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
Time
80
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
X
X
+
X
X
+
+
X
+
+
+
+
+
+
+
+
+
+
+
Time
81
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
82
X
Key: X Data Packet
+ Ack Packet
?X
X
X
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
Time
83
X
X
X
X
X
X
X
X
X
X
+
X
X
+
X
Key: X Data Packet
+ Ack Packet
+ +
+
X
+
+
+
+
+
+
+
+
+
Time
84
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
85
X
X
X
Key: X Data Packet
+ Ack Packet
?
X
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
Time
86
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
+
X
X
++++
X
+
X
+
+
+
+
+
+
+
+
Time
87
X
X
X
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
+
+
+
+
Time
88
Key: X Data Packet
+ Ack Packet
?
X
+
X
+
Time
89
Key: X Data Packet
+ Ack Packet
X
+
+
X
+
+
+
X
+
+
X
X
X
X
X
X
X
++
+
Time
90
TCP: seq # plot
 Main observation:

“Not all packet losses are created equal”
 Losses early in the transfer have a huge
adverse impact on the transfer latency
 Losses near the end of the transfer always
cost at least a retransmit timeout
 Losses in the middle may or may not hurt,
depending on congestion window size at the
time of the loss
91