1. Chapter 3 Transport Layer
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Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.
Transport Layer

2. Chapter 3: Transport Layer
Our goals:
understand principles behind transport layer services:
multiplexing/demultiplexing
reliable data transfer
flow control
congestion control
learn about transport layer protocols in the Internet:
UDP: connectionless transport
TCP: connection-oriented transport
TCP congestion control
Transport Layer

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
flow control
connection management
3.6 Principles of congestion control
3.7 TCP congestion control
Transport Layer

4. Transport services and protocols
application
transport
network
data link
physical
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
logical end-end transport
application
transport
network
data link
physical
Transport Layer

5. Transport vs. network layer
Household analogy (pg 200):
12 kids sending letters to 12 kids
processes = kids
app messages = letters in envelopes
hosts = houses
transport protocol = Ann and Bill who demux to in-house siblings
network-layer protocol = postal service
network layer: logical communication between hosts
transport layer: logical communication between processes
relies on, enhances, network layer services
Transport Layer

6. Internet transport-layer protocols
reliable, in-order delivery (TCP)
congestion control
flow control
connection setup
connection-oriented
unreliable, unordered delivery: UDP
no-frills extension of “best-effort” IP
connectionless
services not available:
delay guarantees
bandwidth guarantees
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
logical end-end transport
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Transport Layer

7. 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
flow control
connection management
3.6 Principles of congestion control
3.7 TCP congestion control
Transport Layer

8. Multiplexing/demultiplexing
Multiplexing at send host:
Demultiplexing at rcv host:
delivering received segments
to correct socket
gathering data from multiple
sockets, enveloping data with
header (later used for
demultiplexing)
= socket
= process
application
application
application P3 P1 P4 P1 P2
transport
transport
transport
network
network
network
link
link
link
physical
physical
physical
host 3
host 1
host 2
Transport Layer

9. How demultiplexing works
host receives IP datagrams
each datagram has source IP address, destination IP address
each datagram carries 1 transport-layer segment
each segment has source, destination port number
IP addresses & port numbers used to direct segment to appropriate socket
32 bits
source port #
dest port #
other header fields
application
data
(message)
TCP/UDP segment format
Transport Layer

10. different source ports are delivered to the same socket
Connectionless demux
different source ports are delivered to the same socket
Client
IP:B P2
client
IP: A P1 P3
server
IP: C
SP: 6428
DP: 9157
SP: 9157
DP: 6428
DP: 5775
SP: 5775
two-tuple: dest IP/dest port
SP provides “return address”
Transport Layer

11. Connection-oriented demux
TCP socket identified by 4-tuple:
source IP address
source port number
dest IP address
dest port number
recv host 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

12. Connection-oriented demux (cont)
different source ports are delivered to different sockets P1
client
IP: A P4 P5 P6 P2 P1 P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
SP: 9157
SP: 9157
DP: 80
DP: 80
Client
IP:B
server
IP: C
S-IP: A
S-IP: B
D-IP:C
D-IP:C
four-tuple!!
Transport Layer

13. Connection-oriented demux: Threaded Web ServerP1
client
IP: A P4 P2 P1 P3
SP: 5775
DP: 80
S-IP: B
D-IP:C
SP: 9157
SP: 9157
DP: 80
DP: 80
client
IP:B
server
IP: C
S-IP: A
S-IP: B
D-IP:C
D-IP:C
Transport Layer

14. Port Scanning w/ nmap
nmap ( is a tool that scans a host to see what TCP and UDP ports it is currently listening too. You can use this to find out what network services are provided by a server. These services are also “open doors” that may be potential security holes.
Transport Layer

15. 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
flow control
connection management
3.6 Principles of congestion control
3.7 TCP congestion control
Transport Layer

16. 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
Why is there a UDP?
no connection establishment (which can add delay)
simple: no connection state at sender, receiver
small segment header
no congestion control: UDP can blast away as fast as desired
Transport Layer

17. UDP: more often used for streaming multimedia apps other UDP uses
loss tolerant
rate sensitive
other UDP uses
DNS
SNMP
reliable transfer over UDP: add reliability at application layer
application-specific error recovery!
32 bits
source port #
dest port #
Length, in
bytes of UDP
segment,
including
header
length
checksum
Application
data
(message)
UDP segment format
Transport Layer

18. UDP checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
Sender:
treat segment contents as sequence of 16-bit integers
checksum: addition (1’s 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

19. Calculating a 1s Complement Checksum
Note: when adding numbers, a carryout from the most significant bit needs to be added to the result 1 +
wraparound +
sum
Kurose and Ross forgot to say anything about wrapping the carry and adding it to low order bit
“flip the bits”
1s complement
checksum
Transport Layer

20. Internet Checksum Example
sender calculates the
checksum
send the original two 16-bit numbers
plus the checksum to the receiver
receiver calculates
the sum
adds the checksum +
Kurose and Ross forgot to say anything about wrapping the carry and adding it to low order bit
no errors!
Transport Layer

21. Why does UDP Perform Error Checking?
No guarantee that link layer will do it
Ethernet does
UDP is end-to-end error checking, the link layer provides link-to-link error checking
Bit errors may be introduced between the links, e.g., in the router’s memory, which will not be detected by the link layer
NOTE: UDP does nothing to correct errors, it either drops the bad segment, or passes it to the application layer with a warning
Transport Layer

22. 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

23. Principles of Reliable data transfer
top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer

24. Principles of Reliable data transfer
top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer

25. Principles of Reliable data transfer
top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
Transport Layer

26. Reliable data transfer: getting started
rdt_send(): called from above, (e.g., by app.). Passed data to
deliver to receiver upper layer
deliver_data(): called by rdt to deliver data to upper
send
side
receive
side
udt_send(): called by rdt,
to transfer packet over
unreliable channel to receiver
rdt_rcv(): called when packet arrives on rcv-side of channel
Transport Layer

27. Reliable data transfer: getting started
We’ll:
incrementally develop sender, receiver sides of reliable data transfer protocol (rdt)
consider only unidirectional data transfer
but control info will flow on both directions!
use finite state machines (FSM) to specify sender, receiver
event causing state transition
actions taken on state transition
state 1
state: when in this “state” next state uniquely determined by next event
state 2
event
actions
Transport Layer

28. Finite State Machines Finite state machines model behavior
Behavior exists as transitions from state to state
Transitions are initiated by input or events
Transitions are often accompanied by output or actions

29. Elevator Example

30. Pattern Recognizer

31. Even or Odd?

32. Rdt1.0: reliable transfer over a reliable channel
underlying channel perfectly reliable
no bit errors
no loss of packets
separate FSMs for sender, receiver:
sender sends data into underlying channel
receiver read data from underlying channel
Wait for call from above
rdt_send(data)
Wait for call from below
rdt_rcv(packet)
extract (packet,data)
deliver_data(data)
packet = make_pkt(data)
udt_send(packet)
sender
receiver
Transport Layer