CSCI 3335: COMPUTER NETWORKS
CHAPTER 3
TRANSPORT LAYER
Vamsi Paruchuri
University of Central Arkansas
http://faculty.uca.edu/vparuchuri/3335.htm
Some of the material is adapted from J.F Kurose and K.W. Ross
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
3-2
TCP: Overview

point-to-point:
RFCs: 793, 1122, 1323, 2018, 2581

 one sender, one receiver


 bi-directional data flow
in same connection
 MSS: maximum segment
size
reliable, in-order byte
steam:
 no “message boundaries”
pipelined:

send & receive buffers

socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
connection-oriented:
 handshaking (exchange
of control msgs) inits
sender, receiver state
before data exchange
 TCP congestion and flow
control set window size

full duplex data:
socket
door
flow controlled:
 sender will not
overwhelm receiver
segment
Transport Layer
3-3
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
3-4
TCP segment structure - Quiz
32 bits
source port #
dest port #
sequence number
acknowledgement number
head not
len used U A P R S F
checksum
Receive window
Urg data pnter
Options (variable length)
application
data
(variable length)
Flags: SYN, FIN, RESET,
PUSH, URG, ACK
What is the significance of each field
What is TCP Header size
What is max Receiver Window Size?
Is it large enough?
Which field should be larger “Seq#”
or “Receive window”? Why?
What is the maximum # options?
Which flags are set in first message
in connection set up? Second
message? Third message?
Why are initial Seq # set randomly?
Transport Layer
3-5
TCP Header: Flags (6 bits)

Connection establishment/termination
 SYN – establish; sequence number field
contains valid initial sequence number
 FIN - terminate
RESET - abort connection because one side
received something unexpected
 PUSH - sender invoked push to send
 URG – indicated urgent pointer field is
valid; special data - record boundary
 ACK - indicates Acknowledgement field is
valid

3: Transport Layer
3b-6
TCP Header: ACK flag
ACK flag – if on then acknowledgement
field valid
 Once connection established no reason to
turn off

 Acknowledgment field is always in header so
acknowledgements are free to send along with
data
3: Transport Layer
3b-7
TCP Header: PUSH


Intention: use to indicate not to leave the
data in a TCP buffer waiting for more data
before it is sent
Receiver is supposed to interpret as
deliver to application immediately; most
TCP/IP implementations don’t delay
delivery in the first place though
3: Transport Layer
3b-8
TCP Header: Header Length
32 bits

Header Length (4 bits)
 needed because options field
make header variable length
 Expressed in number of 32
bit words = 4 bytes
 4 bits field => 4 bytes*24 =
60 bytes; 20 bytes of
required header gives 40
bytes possible of options
 Recall UDP header was 8
bytes
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)
application
data
(variable length)
3: Transport Layer
3b-9
Implications of Field Length
32 bits for sequence number (and
acknowledgement); 16 bits for advertised
window size
 Implication for maximum window size?
Window size <= ½ SequenceNumberSpace

 Requirement easily satisfied because receiver
advertised window field is 16 bits
• 232 >> 2* 216
• Even if increase possible advertised window to 231
that would still be ok
3: Transport Layer 3b-10
Implications of Field Length (cont)
 Advertised
Window is 16 bit field =>
maximum window is 64 KB
 Is this enough to fill the pipeline? Not
always
 Pipeline = delay*BW product
 100 ms roundtrip and 100 Mbps => 1.19
MB
3: Transport Layer 3b-11
TCP Header: Common Options


Options used to extend and test TCP
Each option is:
 1 byte of option kind
 1 byte of option length

Examples
 window scale factor: if don’t want to be limited to 216
bytes in receiver advertised window
 timestamp option: if 32 bit sequence number space will
wrap in MSL; add 32 bit timestamp to distinguish
between two segments with the same sequence number
 Maximum Segment Size can be set in SYN packets
3: Transport Layer 3b-12
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
Socket clientSocket = new
Socket("hostname","port
number");

server: contacted by client
Socket connectionSocket =
welcomeSocket.accept();
Three way handshake:
Step 1: client end system
sends TCP SYN control
segment to server
 specifies initial seq #
Step 2: server end system
receives SYN, replies with
SYNACK control segment
 ACKs received SYN
 allocates buffers
 specifies server->
receiver initial seq. #
Step 3: client acknowledges
servers initial seq. #
3: Transport Layer 3b-13
Three-Way Handshake
Active participant
(client)
Passive participant
(server)
3: Transport Layer 3b-14
Connection Establishment
Both data channels opened at once
 Three-way handshake used to agree on a set
of parameters for this communication
channel

 Initial sequence number for both sides (random)
 Receiver advertised window size for both sides
 Optionally, Maximum Segment Size (MSS) for
each side; if not specified MSS of 536 bytes is
assumed to fit into 576 byte datagram
3: Transport Layer 3b-15
Initial Sequence Numbers
Chosen at random in the sequence number
space?
 Well not really randomly; intention of RFC
is for initial sequence numbers to change
over time

 32 bit counter incrementing every 4
microseconds

Vary initial sequence number to avoid
packets that are delayed in network from
being delivered later and interpreted as a
part of a newly established connection (to
avoid reincarnations)
3: Transport Layer 3b-16
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
Q: how receiver handles
out-of-order segments
 A: TCP spec doesn’t
say, - up to
implementor
Host A
User
types
‘C’
Host B
host ACKs
receipt of
‘C’, echoes
back ‘C’
host ACKs
receipt
of echoed
‘C’
simple telnet scenario
Transport Layer
time
3-17
Connection Termination

Each side of the bi-directional connection
may be closed independently
 4 messages: FIN message and ACK of that FIN
in each direction
Each side closes the data channel it can
send on
 One side can be closed and data can
continue to flow in the other direction, but
not usually
 FINs consume sequence numbers like SYNs

3: Transport Layer 3b-18
TCP Connection Management (cont.)
Closing a connection:
client closes socket:
clientSocket.close();
client
close
Step 1: client end system
close
FIN, replies with ACK.
Closes connection, sends
FIN.
timed wait
sends TCP FIN control
segment to server
Step 2: server receives
server
closed
3: Transport Layer 3b-19
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
3-20
TCP 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
Transport Layer
3-21
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
unacked segment)
 expiration interval:
TimeOutInterval
timeout:
 retransmit segment
that caused timeout
 restart timer
Ack rcvd:
 If acknowledges
previously unacked
segments
 update what is known to
be acked
 start timer if there are
outstanding segments
Transport Layer
3-22
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
TCP retransmission scenarios (more)
timeout
Host A
Host B
X
loss
SendBase
= 120
time
Cumulative ACK scenario
Transport Layer
3-24
TCP Round Trip Time and Timeout
Q: how to set TCP
timeout value?

Q: how to estimate RTT?

longer than 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
 ignore retransmissions
SampleRTT will vary, want
estimated RTT “smoother”
 average several recent
measurements, not just
current SampleRTT
Transport Layer
3-25
TCP Round Trip Time and Timeout
EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT



Exponential weighted moving average
influence of past sample decreases exponentially fast
typical value:  = 0.125
Transport Layer
3-26
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
3-27
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
3-28
Host A
Host B
timeout
X
time
Figure 3.37 Resending a segment after triple duplicate ACK
Transport Layer
3-29
TCP Quiz -2





What are “Cumulative Acks”?
What is advantage of having short time outs?
What is advantage of having long time outs?
Describe the method(s) TCP uses to detect packet
losses.
What is Fast Retransmit?
Transport Layer
3-30
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
3-31
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
3-32
Quiz









Why does TCP use time outs?
How does timeout impact the performance of TCP?
What are pros and cons for short (long) timeouts?
How is RTT estimated by TCP?
What is need for "flow control" in TCP?
Describe "flow control" mechanism.
What is the primary cause of congestion?
Mention 3 costs of congestion.
What is difference between flow and congestion
control.
Transport Layer
3-33
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
3-34
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)
a top-10 problem!
Transport Layer
3-35
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
3-36
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
3-37
TCP congestion control:
additive increase,
multiplicative decrease
approach: increase transmission rate (window size),
probing for usable bandwidth, until loss occurs
 additive increase: increase cwnd by 1 MSS every
RTT until loss detected
 multiplicative decrease: cut cwnd 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
3-38
TCP Congestion Control: details

sender limits transmission:
LastByteSent-LastByteAcked
 cwnd

roughly,
rate =

cwnd
RTT
Bytes/sec
cwnd is dynamic, function
of perceived network
congestion
How does sender
perceive congestion?
 loss event = timeout
or 3 duplicate acks
 TCP sender reduces
rate (cwnd) after
loss event
three mechanisms:
 AIMD
 slow start
 conservative after
timeout events
Transport Layer
3-39
TCP Slow Start
when connection
begins, increase rate
exponentially until
first loss event:
Host A
Host B
RTT

 initially cwnd = 1 MSS
 double cwnd every RTT
 done by incrementing
cwnd for every ACK
received

summary: initial rate is
slow but ramps up
exponentially fast
time
Transport Layer
3-40
Refinement: inferring loss


after 3 dup ACKs:
 cwnd is cut in half
 window then grows
linearly
but after timeout event:
 cwnd instead set to 1
MSS;
 window then grows
exponentially
 to a threshold, then
grows linearly
Philosophy:
3 dup ACKs indicates
network capable of
delivering some segments

timeout indicates a
“more alarming”
congestion scenario

Transport Layer
3-41
Refinement
Q: when should the
exponential
increase switch to
linear?
A: when cwnd gets to
1/2 of its value
before timeout.
Implementation:


variable ssthresh
Can you identify different phases?
on loss event, ssthresh is
set to 1/2 of cwnd just
before loss event
Transport Layer
3-42
Connection Timeline
3: Transport Layer 3b-43
Summary: TCP Congestion Control
duplicate ACK
dupACKcount++
L
cwnd = 1 MSS
ssthresh = 64 KB
dupACKcount = 0
slow
start
timeout
ssthresh = cwnd/2
cwnd = 1 MSS
dupACKcount = 0
retransmit missing segment
dupACKcount == 3
ssthresh= cwnd/2
cwnd = ssthresh + 3
retransmit missing segment
New
ACK!
new ACK
cwnd = cwnd+MSS
dupACKcount = 0
transmit new segment(s), as allowed
cwnd > ssthresh
L
timeout
ssthresh = cwnd/2
cwnd = 1 MSS
dupACKcount = 0
retransmit missing segment
timeout
ssthresh = cwnd/2
cwnd = 1
dupACKcount = 0
retransmit missing segment
New
ACK!
new ACK
cwnd = cwnd + MSS (MSS/cwnd)
dupACKcount = 0
transmit new segment(s), as allowed
.
congestion
avoidance
duplicate ACK
dupACKcount++
New
ACK!
New ACK
cwnd = ssthresh
dupACKcount = 0
dupACKcount == 3
ssthresh= cwnd/2
cwnd = ssthresh + 3
retransmit missing segment
fast
recovery
duplicate ACK
cwnd = cwnd + MSS
transmit new segment(s), as allowed
Transport Layer
3-44
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
3-45
Netstat

netstat –a –n
 Shows open connections in various states
 Example:
Active Connections
Proto
TCP
TCP
TCP
UDP
LocalAddr
0.0.0.0:23
192.168.0.100:139
192.168.0.100:1275
127.0.0.1:1070
ForeignAddr
0.0.0.0:0
207.200.89.225:80
128.32.44.96:22
*:*
State
LISTENING
CLOSE_WAIT
ESTABLISHED
Quiz
What are three primary mechanisms of
TCP Congestion Control
 What are the two TCP loss events
 How many packets are transmitted in the
first 4 RTT durations after a TCP
connection is established.

Transport Layer
3-47
Quiz (cont)
Transport Layer
3-48