Goals
❒ Understand transport services
Transport Overview and UDP
❍
Multiplexing and Demultiplexing
❍
Reliable data transfer
❍
Flow control
❍
Congestion control
❒ TCP and UDP
Transport Layer
Transport services and protocols
❒ provide logical communication
end systems
❍
send side: breaks app
messages into segments,
passes to network layer
netw ork
data li nk
physi cal
t
or
sp
an
tr
❍
❒
netw ork
data li nk
physi cal
❒
netw ork
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physi cal
netw ork
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physi cal
appli cati o
n
transport
netw ork
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rcv side: reassembles
segments into messages,
passes to app layer
3-2
Transport vs. network layer
netw ork
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physi cal
d
en
den
❒ transport protocols run in
appli cati o
n
transport
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physi cal
l
ca
gi
lo
between app processes
running on different hosts
Transport Layer
3-1
network layer: logical
communication
between hosts
Household analogy:
transport layer:
logical communication
between processes
❒ app messages = letters in
❍
12 kids sending letters to 12
kids
❒ processes = kids
envelopes
❒ hosts = houses
❒ transport protocol = Ann
relies on, enhances,
network layer services
and Bill
❒ network-layer protocol =
postal service
❒ more than one transport
protocol available to apps
❍
Internet: TCP and UDP
Transport Layer
Internet transport-layer protocols
❒
❍
flow control
❍
connection setup
unreliable, unordered
delivery: UDP
❍
❒
no-frills extension of
“best-effort” IP
services not available:
❍
delay guarantees
❍
bandwidth guarantees
netw ork
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physi cal
netw ork
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physi cal
= socket
netw ork
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physi cal
application
netw ork
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physi cal
transport
appli cati o
n
transport
netw ork
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physi cal
network
link
Multiplexing at send host:
gathering data from multiple
sockets, enveloping data with
header (later used for
demultiplexing)
3-5
= process
P3
P1
P1
application
transport
network
P2
P4
application
transport
network
link
link
physical
host 1
Transport Layer
3-4
Multiplexing/demultiplexing
Demultiplexing at rcv host:
delivering received segments
to correct socket
netw ork
data li nk
physi cal
t
or
sp
an
tr
congestion control
d
en
den
❍
appli cati o
n
transport
netw ork
data li nk
physi cal
l
ca
gi
lo
❒
reliable, in-order
delivery (TCP)
Transport Layer
3-3
physical
host 2
physical
host 3
Transport Layer
3-6
1
Connectionless demultiplexing
How demultiplexing works
❒ host receives IP datagrams
❍
❍
❍
each datagram has source
IP address, destination IP
address
32 bits
source port #
each datagram carries 1
transport-layer segment
❒
dest port #
❒
application
data
(message)
❒ host uses IP addresses &
Transport Layer
P2
SP: 6428
DP: 9157
❒
SP: 5775
DP: 6428
Client
IP:B
SP provides “return address”
Transport Layer
P4
SP: 9157
DP: 80
S-IP: A
D-IP:C
P5
server
IP: C
P6
TCP socket identified
by 4-tuple:
❍
source IP address
❍
source port number
❍
dest IP address
❍
dest port number
❒
IP datagrams with
different source IP
addresses and/or
source port numbers
directed to same socket
recv host uses all four
values to direct
segment to appropriate
socket
❒
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 requestTransport Layer 3-10
Connection-oriented demux:
Threaded Web Server
P2
P1P3
P1
P2
P4
SP: 5775
DP: 80
SP: 5775
DP: 80
S-IP: B
D-IP:C
S-IP: B
D-IP:C
SP: 9157
DP: 80
S-IP: B
D-IP:C
3-8
Server host may
support many
simultaneous TCP
sockets:
❍
3-9
Connection-oriented demux
(cont)
client
IP: A
❒
SP: 6428
DP: 5775
server
IP: C
directs UDP segment to
socket with that port
number
Connection-oriented demux
P1
P1
P3
checks destination port
number in segment
Transport Layer
3-7
DatagramSocket serverSocket = new DatagramSocket(6428);
P1
❒
(dest IP address, dest port number)
Connectionless demux (cont)
SP: 9157
DP: 6428
❍
TCP/UDP segment format
port numbers to direct
segment to appropriate
socket
client
IP: A
UDP socket identified by
two-tuple:
When host receives
UDP segment:
❍
DatagramSocket mySocket1 = new
DatagramSocket(99111);
DatagramSocket mySocket2 = new
DatagramSocket(99222);
other header fields
each segment has source,
destination port number
(recall: well-known port
numbers for specific
applications)
❒
Create sockets with port
numbers:
Client
IP:B
Transport Layer 3-11
client
IP: A
SP: 9157
DP: 80
S-IP: A
D-IP:C
server
IP: C
SP: 9157
DP: 80
S-IP: B
D-IP:C
P1P3
Client
IP:B
Transport Layer 3-12
2
UDP: User Datagram Protocol [RFC 768]
❒
❒
“no frills,” “bare bones”
Internet transport protocol
❒ often used for streaming
multimedia apps
“best effort” service, UDP
segments may be:
❍
❍
❒
UDP: more
lost
delivered out of order to
app
❒
connectionless:
❍ no handshaking between
UDP sender, receiver
❍ each UDP segment
handled independently of
others
❍
loss tolerant
❍
rate sensitive
other UDP uses
❍
DNS
❍
SNMP
Length, in
bytes of UDP
segment,
including
header
32 bits
source port #
dest port #
length
checksum
❒ reliable transfer over
UDP: add reliability at
application layer
❍
application-specific
error recovery!
Transport Layer 3-13
Transport Layer 3-14
❒
Goal: detect “errors” (e.g., flipped bits) in
transmitted segment
Receiver:
❒ treat segment contents
❒ compute checksum of
as sequence of 16-bit
integers
received segment
❒ sender puts checksum
value into UDP checksum
field
Note
❍
Sender:
complement sum) of
segment contents
UDP segment format
Internet Checksum Example
UDP checksum
❒ checksum: addition (1’s
Application
data
(message)
❒
When adding numbers, a carryout from the
most significant bit needs to be added to the
result
Example: add two 16-bit integers
❒ check if computed checksum
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
equals checksum field value:
❍
❍
NO - error detected
YES - no error detected.
But maybe errors
nonetheless? More later
….
Transport Layer 3-15
wraparound
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
sum
checksum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
1 0 1 0 0 0 1 0 0 0 1 0 0 Transport
0 0 1Layer
1
3-16
3