Chapter 3

Transport Layer

Computer
Networking: A
Top Down
Approach
6th edition
Jim Kurose, Keith Ross
Addison-Wesley
March 2012

Transport Layer 3-1

Chapter 3: Transport Layer
our goals:
 understand principles  learn about Internet
behind transport transport layer protocols:
layer services:  UDP: connectionless
 multiplexing, transport
demultiplexing  TCP: connection-oriented
 reliable data transfer reliable transport
 flow control  TCP congestion control
 congestion control

Transport Layer 3-2

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-3

Transport services and protocols
 provide logical application
transport
communication between app network
data link
processes running on physical

different hosts

lo
gi
 transport protocols run in

ca
l
end systems

en
d-
en
 send side: breaks app

d
tra
messages into segments,

ns
po
passes to network layer

rt
 rcv side: reassembles application

segments into messages, transport

network

passes to app layer data link

physical

 more than one transport

protocol available to apps
 Internet: TCP and UDP
Transport Layer 3-4
Transport vs. network layer
 network layer: household analogy:
logical
communication 12 kids in Ann’s house
sending letters to 12 kids in
between hosts Bill’s house:
 transport layer:  hosts = houses

logical  processes = kids

communication  app messages = letters in

envelopes
between processes  transport protocol = Ann
 relies on, enhances, and Bill who demux to in-
network layer house siblings
services  network-layer protocol =
postal service

Transport Layer 3-5

Internet transport-layer protocols
 reliable, in-order application
transport

delivery (TCP) network

data link
physical
 congestion control network
network
data link

lo
data link physical
 flow control

gi
physical

ca
network

l
data link

en
 connection setup physical

d-
en
unreliable, unordered
network

d


tra
data link
physical

ns
delivery: UDP

po
network
data link

rt
 no-frills extension of network
data link
physical

application
“ best-effort” IP physical
network transport
data link network
 services not available: physical data link
physical

 delay guarantees
 bandwidth guarantees

Transport Layer 3-6

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-7

Multiplexing/demultiplexing
multiplexing at sender:
handle data from multiple demultiplexing at receiver:
sockets, add transport header use header info to deliver
(later used for demultiplexing) received segments to correct
socket

application

application P1 P2 application socket

P3 transport P4
process
transport network transport
network link network
link physical link
physical physical

Transport Layer 3-8

How demultiplexing works
 host receives IP datagrams
 each datagram has source IP address, 32 bits
destination IP address
 each datagram carries one transport- source port # dest port #
layer segment
 each segment has source, destination
port number other header fields
 host uses IP addresses & port
numbers to direct segment to
appropriate socket application
data
(payload)

TCP/UDP segment format

Transport Layer 3-9

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-10

UDP: User Datagram Protocol [RFC 768]
 “ no frills,” “ bare bones”  UDP use:
Internet transport protocol  streaming multimedia
 “ best effort” service, UDP apps (loss tolerant, rate
segments may be: sensitive)
 lost  DNS
 delivered out-of-order to  SNMP
app  reliable transfer over
 connectionless:
UDP:
 no handshaking between
UDP sender, receiver  add reliability at
application layer
 each UDP segment
handled independently  application-specific error
of others recovery!

Transport Layer 3-11

UDP: segment header
length, in bytes of
32 bits UDP segment,
source port # dest port # including header

length checksum
why is there a UDP?
 no connection
application establishment (which can
data add delay)
(payload)  simple: no connection
state at sender, receiver
 small header size
UDP segment format
 no congestion control:
UDP can blast away as
fast as desired
Transport Layer 3-12
UDP checksum
Goal: detect “ errors” (e.g., flipped bits) in
transmitted segment
sender: receiver:
 treat segment contents,  compute checksum of
including header fields, received segment
as sequence of 16-bit  check if computed checksum
integers equals checksum field value:
 checksum: addition  NO - error detected
(one’s complement sum)  YES - no error detected.
of segment contents But maybe errors
 sender puts checksum nonetheless? More later ….
value into UDP
checksum field

Transport Layer 3-13

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 3-14

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-15

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-16

TCP: Overview RFCs: 793,1122,1323, 2018, 2581

 point-to-point:  full duplex data:

 one sender, one receiver  bi-directional data flow
 reliable, in-order byte in same connection
steam:  MSS: maximum
segment size
 no “ message boundaries
”  connection-oriented:
 pipelined:  handshaking (exchange
of control msgs) inits
 TCP congestion and flow sender, receiver state
control set window size before data exchange
 flow controlled:
 sender will not
overwhelm receiver
Transport Layer 3-17
 TCP segment structure
32 bits
URG: urgent data counting
(generally not used) source port # dest port #
by bytes
sequence number of data
ACK: ACK #
valid acknowledgement number (not segments!)
head not
PSH: push data now len used
UAP R S F receive window
(generally not used) # bytes
checksum Urg data pointer
rcvr willing
RST, SYN, FIN: to accept
options (variable length)
connection estab
(setup, teardown
commands)
application
Internet data
checksum (variable length)
(as in UDP)

Transport Layer 3-18

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-19

TCP sender events:
data rcvd from app: timeout:
 create segment with  retransmit segment
seq # that caused timeout
 seq # is byte-stream  restart timer
number of first data ack rcvd:
byte in segment  if ack acknowledges
 start timer if not
previously unacked
already running segments
 think of timer as for  update what is known
oldest unacked segment to be ACKed
 expiration interval:  start timer if there are
TimeOutInterval still unacked segments

Transport Layer 3-20

TCP: retransmission scenarios
Host A Host B Host A Host B

SendBase=92
Seq=92, 8 bytes of data Seq=92, 8 bytes of data

Seq=100, 20 bytes of data

timeo

timeo
ACK=100
ut

ut
X
ACK=100
ACK=120

Seq=92, 8 bytes of data Seq=92, 8

SendBase=100 bytes of data
SendBase=120
ACK=100
ACK=120

SendBase=120

lost ACK scenario premature timeout

Transport Layer 3-21
TCP: retransmission scenarios
Host A Host B

Seq=92, 8 bytes of data

Seq=100, 20 bytes of data

ACK=100
timeo

X
ut

ACK=120

Seq=120, 15 bytes of data

cumulative ACK
Transport Layer 3-22
Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-23

TCP flow control
application
application may process
remove data from application
TCP socket buffers ….
TCP socket OS
receiver buffers
… slower than TCP
receiver is delivering
(sender is sending) TCP
code

IP
code
flow control
receiver controls sender, so
sender won’t overflow receiver’s from sender
buffer by transmitting too much,
too fast receiver protocol stack

Transport Layer 3-24

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-25

Agreeing to establish a connection

2-way handshake:
Q: will 2-way handshake
always work in network?
 variable delays
Let’s talk  retransmitted messages (e.g.
ESTAB req_conn(x)) due to message
OK
ESTAB loss
 message reordering
 can’t “ see” other side

choose x
req_conn(x)
ESTAB
acc_conn(x)
ESTAB

Transport Layer 3-26

Agreeing to establish a connection
2-way handshake failure scenarios:

choose x choose x
req_conn(x) req_conn(x)
ESTAB ESTAB
retransmit acc_conn(x) retransmit acc_conn(x)
req_conn(x) req_conn(x)

ESTAB ESTAB
data(x+1) accept
req_conn(x)
retransmit data(x+1)
data(x+1)
connection connection
client x completes server x completes server
client
terminates forgets x terminates forgets x
req_conn(x)

ESTAB ESTAB
data(x+1) accept
half open connection! data(x+1)
(no client!)
Transport Layer 3-27
 TCP 3-way handshake

client state server state

LISTEN LISTEN
choose init seq num, x
send TCP SYN msg
SYNSENT SYNbit=1, Seq=x
choose init seq num, y
send TCP SYNACK
msg, acking SYN SYN RCVD
SYNbit=1, Seq=y
ACKbit=1; ACKnum=x+1
received SYNACK(x)
ESTAB indicates server is live;
send ACK for SYNACK;
this segment may contain ACKbit=1, ACKnum=y+1
client-to-server data
received ACK(y)
indicates client is live
ESTAB

Transport Layer 3-28

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 3-29

TCP: closing a connection
client state server state
ESTAB ESTAB
clientSocket.close()
FIN_WAIT_1 can no longer FINbit=1, seq=x
send but can
receive data CLOSE_WAIT
ACKbit=1; ACKnum=x+1
can still
FIN_WAIT_2 wait for server send data
close

LAST_ACK
FINbit=1, seq=y
TIMED_WAIT can no longer
send data
ACKbit=1; ACKnum=y+1
timed wait
for 2*max CLOSED
segment lifetime

CLOSED

Transport Layer 3-30

Chapter 3 outline
3.1 transport-layer 3.5 connection-oriented
services transport: TCP
3.2 multiplexing and  segment structure
demultiplexing  reliable data transfer
3.3 connectionless  flow control
transport: UDP  connection management
3.4 principles of reliable 3.6 principles of congestion
data transfer control
3.7 TCP congestion control

Transport Layer 3-31

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 3-32