Chapter 3
Transport Layer
Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
Transport Layer 3-1
Chapter 3: Transport Layer
our goals:
understand
principles behind
transport layer
services:
multiplexing,
demultiplexing
reliable data transfer
flow control
learn about Internet
transport layer protocols:
UDP: connectionless
transport
TCP: connection-oriented
reliable transport
Transport Layer 3-2
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 3-3
Transport services and protocols
provide logical communication
between app processes
running on different hosts
transport protocols run in
end systems
send side: breaks app
messages into segments,
passes to network layer
rcv side: reassembles
segments into messages,
passes to app layer
more than one transport
protocol available to apps
Internet: TCP and UDP
application
transport
network
data link
physical
application
transport
network
data link
physical
Transport Layer 3-4
Transport vs. network layer
network layer: logical
communication
between hosts
transport layer:
logical
communication
between processes
relies on, enhances,
network layer
services
household analogy:
12 kids in Anns house sending
letters to 12 kids in Bills
house:
hosts = houses
processes = kids
app messages = letters in
envelopes
transport protocol = Ann
and Bill who demux to inhouse siblings
network-layer protocol =
postal service
Transport Layer 3-5
Internet transport-layer protocols
reliable, in-order
delivery (TCP)
congestion control
flow control
connection setup
unreliable, unordered
delivery: UDP
no-frills extension of
best-effort IP
services not available:
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
delay guarantees
bandwidth guarantees
Transport Layer 3-6
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 3-7
Multiplexing/demultiplexing
multiplexing at sender:
handle data from multiple
sockets, add transport header
(later used for demultiplexing)
demultiplexing at receiver:
use header info to deliver
received segments to correct
socket
application
application
P1
P2
application
P3
transport
P4
transport
network
transport
link
network
network
link
physical
physical
socket
process
link
physical
Transport Layer 3-8
How demultiplexing works
host receives IP datagrams
32 bits
each datagram has source IP
address, destination IP
address
each datagram carries one
transport-layer segment
each segment has source,
destination port number
source port #
dest port #
other header fields
application
data
(payload)
host uses IP addresses &
port numbers to direct
segment to appropriate
socket
TCP/UDP segment format
Transport Layer 3-9
Connectionless demultiplexing
recall: created socket has
host-local port #:
DatagramSocket mySocket1
= new DatagramSocket(12534);
when host receives UDP
segment:
checks destination port #
in segment
directs UDP segment to
socket with that port #
recall: when creating
datagram to send into
UDP socket, must specify
destination IP address
destination port #
IP datagrams with same
dest. port #, but different
source IP addresses
and/or source port
numbers will be directed
to same socket at dest
Transport Layer 3-10
Connectionless demux: example
DatagramSocket
mySocket2 = new
DatagramSocket
(9157);
DatagramSocket
serverSocket = new
DatagramSocket
(6428);
application
application
DatagramSocket
mySocket1 = new
DatagramSocket
(5775);
application
P1
P3
P4
transport
transport
transport
network
network
network
link
link
physical
link
physical
physical
source port: 6428
dest port: 9157
source port: ?
dest port: ?
source port: ?
dest port: ?
source port: 9157
dest port: 6428
Transport Layer 3-11
Connection-oriented demux
TCP socket identified
by 4-tuple:
source IP address
source port number
dest IP address
dest port number
demux: receiver uses
all four values to direct
segment to appropriate
socket
server host may support
many simultaneous TCP
sockets:
each socket identified by
its own 4-tuple
web servers have
different sockets for
each connecting client
non-persistent HTTP will
have different socket for
each request
Transport Layer 3-12
Connection-oriented demux: example
application
application
P4
P5
application
P6
P3
P3
P2
transport
transport
network
network
link
link
physical
physical
transport
network
link
server: IP
address B
source IP,port: B,80
dest IP,port: A,9157
host: IP
address A
physical
host: IP
address C
source IP,port: C,5775
dest IP,port: B,80
source IP,port: A,9157
dest IP, port: B,80
source IP,port: C,9157
dest IP,port: B,80
three segments, all destined to IP address: B,
dest port: 80 are demultiplexed to different sockets
Transport Layer 3-13
Connection-oriented demux: example
threaded server
application
application
P3
application
P4
P3
P2
transport
transport
network
network
link
link
physical
physical
host: IP
address A
transport
network
link
server: IP
address B
source IP,port: B,80
dest IP,port: A,9157
source IP,port: A,9157
dest IP, port: B,80
physical
source IP,port: C,5775
dest IP,port: B,80
host: IP
address C
source IP,port: C,9157
dest IP,port: B,80
Transport Layer 3-14
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 3-15
UDP: User Datagram Protocol [RFC 768]
no frills, bare bones
Internet transport
protocol
best effort service, UDP
segments may be:
lost
delivered out-of-order
to app
connectionless:
no handshaking
between UDP sender,
receiver
each UDP segment
handled independently
of others
UDP use:
streaming multimedia
apps (loss tolerant, rate
sensitive)
DNS
SNMP
reliable transfer over
UDP:
add reliability at
application layer
application-specific error
recovery!
Transport Layer 3-16
UDP: segment header
32 bits
source port #
dest port #
length
checksum
length, in bytes of
UDP segment,
including header
why is there a UDP?
application
data
(payload)
UDP segment format
no connection
establishment (which can
add delay)
simple: no connection
state at sender, receiver
small header size
no congestion control:
UDP can blast away as
fast as desired
Transport Layer 3-17
UDP checksum
Goal: detect errors (e.g., flipped bits) in transmitted
segment
sender:
treat segment contents,
including header fields,
as sequence of 16-bit
integers
checksum: addition
(ones complement sum)
of segment contents
sender puts checksum
value into UDP
checksum field
receiver:
compute checksum of
received segment
check if computed
checksum equals checksum
field value:
NO - error detected
YES - no error detected.
But maybe errors
nonetheless? More later
.
Transport Layer 3-18
Internet checksum: example
example: add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
Note: when adding numbers, a carryout from the most
significant bit needs to be added to the result
Transport Layer 3-19
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
10
TCP: Overview
RFCs: 793,1122,1323, 2018, 2581
point-to-point:
one sender, one receiver
bi-directional data flow
in same connection
MSS: maximum segment
size
reliable, in-order byte
stream:
no message boundaries
pipelined:
full duplex data:
connection-oriented:
handshaking (exchange
of control msgs) inits
sender, receiver state
before data exchange
TCP congestion and
flow control set window
size
flow controlled:
sender will not
overwhelm receiver
Transport Layer 3-21
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
UAP R S F
len used
checksum
counting
by bytes
of data
(not segments!)
receive window
Urg data pointer
options (variable length)
# bytes
rcvr willing
to accept
application
data
(variable length)
Transport Layer 3-22
11
TCP seq. numbers, ACKs
sequence numbers:
byte stream number of
first byte in segments
data
acknowledgements:
seq # of next byte
expected from other side
cumulative ACK
Q: how receiver handles
out-of-order segments
A: TCP spec doesnt say,
- up to implementor
outgoing segment from sender
source port #
dest port #
sequence number
acknowledgement number
rwnd
checksum
urg pointer
window size
sender sequence number space
sent
ACKed
sent, not- usable not
yet ACKed but not usable
(in-flight) yet sent
incoming segment to sender
source port #
dest port #
sequence number
acknowledgement number
rwnd
A
checksum
urg pointer
Transport Layer 3-23
TCP seq. numbers, ACKs
Host B
Host A
User
types
C
host ACKs
receipt
of echoed
C
Seq=42, ACK=79, data = C
Seq=79, ACK=43, data = C
host ACKs
receipt of
C, echoes
back C
Seq=43, ACK=80
simple telnet scenario
Transport Layer 3-24
12
TCP round trip time, 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, timeout
EstimatedRTT = (1- )*EstimatedRTT + *SampleRTT
exponential weighted moving average
influence of past sample decreases exponentially fast
typical value: = 0.125
RTT: gaia.cs.umass.edu to fantasia.eurecom.fr
350
RTT: gaia.cs.umass.edu to fantasia.eurecom.fr
RTT (milliseconds)
RTT (milliseconds)
300
250
200
sampleRTT
150
EstimatedRTT
100
1
15
22
29
36
43
50
57
64
71
time (seconnds)
time (seconds)
SampleRTT
Estimated RTT
78
85
92
99
106
Transport Layer 3-26
13
TCP round trip time, timeout
timeout interval: EstimatedRTT plus safety margin
large variation in EstimatedRTT -> larger safety margin
estimate SampleRTT deviation from EstimatedRTT:
DevRTT = (1-)*DevRTT +
*|SampleRTT-EstimatedRTT|
(typically, = 0.25)
TimeoutInterval = EstimatedRTT + 4*DevRTT
estimated RTT
safety margin
Transport Layer 3-27
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 3-28
14
TCP reliable data transfer
TCP creates rdt service
on top of IPs unreliable
service
pipelined segments
cumulative acks
single retransmission
timer
Lets initially consider
simplified TCP sender:
ignore duplicate acks
ignore flow control,
congestion control
retransmissions
triggered by:
timeout events
duplicate acks
Transport Layer 3-29
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 ack acknowledges
previously unacked
segments
update what is known
to be ACKed
start timer if there are
still unacked segments
Transport Layer 3-30
15
TCP sender (simplified)
L
NextSeqNum = InitialSeqNum
SendBase = InitialSeqNum
wait
for
event
data received from application above
create segment, seq. #: NextSeqNum
pass segment to IP (i.e., send)
NextSeqNum = NextSeqNum + length(data)
if (timer currently not running)
start timer
timeout
retransmit not-yet-acked segment
with smallest seq. #
start timer
ACK received, with ACK field value y
if (y > SendBase) {
SendBase = y
/* SendBase1: last cumulatively ACKed byte */
if (there are currently not-yet-acked segments)
start timer
else stop timer
}
Transport Layer 3-31
TCP: retransmission scenarios
Host B
Host A
Host B
Host A
SendBase=92
ACK=100
Seq=92, 8 bytes of data
timeout
timeout
Seq=92, 8 bytes of data
Seq=100, 20 bytes of data
ACK=100
ACK=120
Seq=92, 8 bytes of data
SendBase=100
ACK=100
Seq=92, 8
bytes of data
SendBase=120
ACK=120
SendBase=120
lost ACK scenario
premature timeout
Transport Layer 3-32
16
TCP: retransmission scenarios
Host B
Host A
Seq=92, 8 bytes of data
timeout
Seq=100, 20 bytes of data
ACK=100
ACK=120
Seq=120, 15 bytes of data
cumulative ACK
Transport Layer 3-33
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
immediately send ACK, provided that
segment starts at lower end of gap
Transport Layer 3-34
17
TCP 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 backto-back
if segment is lost, there
will likely be many
duplicate ACKs.
TCP fast retransmit
if sender receives 3
ACKs for same data
(triple
(triple duplicate
duplicate ACKs),
ACKs),
resend unacked
segment with smallest
seq #
likely that unacked
segment lost, so dont
wait for timeout
Transport Layer 3-35
TCP fast retransmit
Host B
Host A
Seq=92, 8 bytes of data
Seq=100, 20 bytes of data
X
timeout
ACK=100
ACK=100
ACK=100
ACK=100
Seq=100, 20 bytes of data
fast retransmit after sender
receipt of triple duplicate ACK
Transport Layer 3-36
18
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 flow control
application may
remove data from
TCP socket buffers .
slower than TCP
receiver is delivering
(sender is sending)
application
process
application
TCP
code
IP
code
flow control
receiver controls sender, so
sender wont overflow receivers
buffer by transmitting too much,
too fast
OS
TCP socket
receiver buffers
from sender
receiver protocol stack
Transport Layer 3-38
19
TCP flow control
receiver advertises free
buffer space by including
rwnd value in TCP header
of receiver-to-sender
segments
to application process
RcvBuffer
RcvBuffer size set via
socket options (typical default
is 4096 bytes)
many operating systems
autoadjust RcvBuffer
rwnd
buffered data
free buffer space
TCP segment payloads
sender limits amount of
unacked (in-flight) data to
receivers rwnd value
guarantees receive buffer
will not overflow
receiver-side buffering
Transport Layer 3-39
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 3-40
20
Connection Management
before exchanging data, sender/receiver handshake:
agree to establish connection (each knowing the other willing
to establish connection)
agree on connection parameters
application
application
connection state: ESTAB
connection variables:
seq # client-to-server
server-to-client
rcvBuffer size
at server,client
connection state: ESTAB
connection Variables:
seq # client-to-server
server-to-client
rcvBuffer size
at server,client
network
network
Socket clientSocket =
newSocket("hostname","port
number");
Socket connectionSocket =
welcomeSocket.accept();
Transport Layer 3-41
TCP 3-way handshake
client state
server state
LISTEN
LISTEN
choose init seq num, x
send TCP SYN msg
SYNSENT
ESTAB
received SYNACK(x)
indicates server is live;
send ACK for SYNACK;
this segment may contain
client-to-server data
SYNbit=1, Seq=x
choose init seq num, y
send TCP SYNACK
SYN RCVD
msg, acking SYN
SYNbit=1, Seq=y
ACKbit=1; ACKnum=x+1
ACKbit=1, ACKnum=y+1
received ACK(y)
indicates client is live
ESTAB
Transport Layer 3-42
21
TCP 3-way handshake: FSM
closed
Socket connectionSocket =
welcomeSocket.accept();
L
SYN(x)
SYNACK(seq=y,ACKnum=x+1)
create new socket for
communication back to client
listen
Socket clientSocket =
newSocket("hostname","port
number");
SYN(seq=x)
SYN
sent
SYN
rcvd
SYNACK(seq=y,ACKnum=x+1)
ACK(ACKnum=y+1)
ESTAB
ACK(ACKnum=y+1)
L
Transport Layer 3-43
TCP: closing a connection
client, server each close their side of connection
send TCP segment with FIN bit = 1
respond to received FIN with ACK
on receiving FIN, ACK can be combined with own FIN
simultaneous FIN exchanges can be handled
Transport Layer 3-44
22
TCP: closing a connection
client state
server state
ESTAB
ESTAB
clientSocket.close()
FIN_WAIT_1
FIN_WAIT_2
can no longer
send but can
receive data
FINbit=1, seq=x
CLOSE_WAIT
ACKbit=1; ACKnum=x+1
wait for server
close
LAST_ACK
FINbit=1, seq=y
TIMED_WAIT
timed wait
for 2*max
segment lifetime
can still
send data
can no longer
send data
ACKbit=1; ACKnum=y+1
CLOSED
CLOSED
Transport Layer 3-45
Chapter 3: summary
principles behind
transport layer services:
multiplexing,
demultiplexing
reliable data transfer
flow control
congestion control
instantiation,
implementation in the
Internet
next:
leaving the
network edge
(application,
transport layers)
into the network
core
UDP
TCP
Transport Layer 3-46
23
