1. School of Information Technologies
Transport Protocols
Lesson 12
NETS2150/2850
School of Information Technologies

2. Lesson Outcome 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

3. Position of transport layer

4. Transport layer duties

5. Transport services and protocols
application
transport
network
data link
physical
logical end-end transport
provide logical communication between app processes running on different hosts
transport protocols run in end systems (not in ISs)
send side: breaks app messages into segments, passes to network layer
rcv side: reassembles segments into messages, passes to app. layer

6. Transport vs. Network Layer
network layer: logical communication between hosts
transport layer: logical communication between processes
relies on, enhances, network layer services

7. Types of data deliveries

8. TCP & UDP Transmission Control Protocol User Datagram Protocol (UDP)
Connection oriented
RFC 793
User Datagram Protocol (UDP)
Connectionless
RFC 768
Stream Control Transmission Protocol (SCTP) – for VoIP
SCTP
Some protocols in the
TCP/IP protocol suite

9. IP addresses & port numbers
IP addresses versus port numbers

10. Well-known ports Port Protocol Description 7 Echo
   7
Echo
Echoes a received datagram back to the sender
    9
Discard
Discards any datagram that is received
  11
Users
Active users
  13
Daytime
Returns the date and the time
  17
Quote
Returns a quote of the day
  19
Chargen
Returns a string of characters
  20
FTP, Data
File Transfer Protocol (data connection)
  21
FTP, Control
File Transfer Protocol (control connection)
  23
TELNET
Terminal Network
  25
SMTP
Simple Mail Transfer Protocol
  53
DNS
Domain Name Server
  67
BOOTP
Bootstrap Protocol
  79
Finger
  80
HTTP
Hypertext Transfer Protocol
111
RPC
Remote Procedure Call
161
SNMP
Simple Network Management Protocol

11. Internet transport-layer protocols
application
transport
network
data link
physical
reliable, in-order delivery (TCP)
congestion control
flow control
connection setup
unreliable, unordered delivery: UDP
extension of “best-effort” IP
network
data link
physical
network
data link
physical
network
data link
physical
logical end-end transport
network
data link
physical
network
data link
physical
application
transport
network
data link
physical

12. Issues in a Simple Transport Protocol
Multiplexing and Addressing
Flow Control
Connection establishment and termination

13. Socket Address Process-to-process delivery needs two IDs at each end:
- IP address
- Port number

14. Multiplexing/Demultiplexing
gathering data from multiple
sockets, enveloping data with
header (later used for
demultiplexing)
Multiplexing at send host:
delivering received segments
to correct socket/process
Demultiplexing at rcv host:

15. How Demultiplexing Works
source port #
dest port #
32 bits
application
data
(message)
other header fields
TCP/UDP segment format
host receives IP packets
each packet has source IP address, destination IP address
each packet carries 1 transport-layer segment
each segment has source, destination port number
host uses IP addresses & port numbers to direct segment to appropriate socket

16. Connectionless demultiplexing
UDP socket identified by two-tuple:
(dest IP address, dest port number)
When host receives UDP segment:
checks destination port number in segment
directs UDP segment to socket with that port number
IP packets with different source IP addresses and/or source port numbers directed to same socket

17. Connectionless demux (cont)
Eg.: UDP P2 P1 P1 P3
SP: 6428
DP: 9157
SP: 6428
DP: 5775
SP: 9157
SP: 5775
DP: 6428
client
IP: A
DP: 6428
Client
IP:B
server
IP: C
SP provides “return address”

18. 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
E.g.: Web servers have different sockets for each connecting client

19. Connection-oriented demux (cont)
Eg.: TCP P2 P3 P4 P1 P1
SP: 80
DP: 9157
SP: 80
DP: 5775
SP: 9157
SP: 5775
DP: 80
client
IP: A
DP: 80
Client
IP:B
server
IP: C

20. TCP Flow Control receive side of TCP connection has a receive buffer:
sender won’t overflow
receiver’s buffer by
transmitting too much,
too fast
flow control
receive side of TCP connection has a receive buffer:
speed-matching service: matching the send rate to the receiving app’s drain rate
app process may be slow at reading from buffer
J. Kurose and Ross, Computer Networking

21. Connection Establishment and Termination
Allow each end to know the other exists
Negotiation of optional parameters by mutual agreement (3-way handshake)
Triggers allocation of transport entity resources
Only in TCP

22. UDP: User Datagram Protocol [RFC 768]
“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?
fast: 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

23. UDP (2)
often used for streaming multimedia apps
loss tolerant
other UDP uses
DNS (domain name resolution)
SNMP (network management)
Need to perform application-specific error recovery!
32 bits
source port #
dest port #
Length, in
octets,
including
header
length
checksum
Application
data
(message)
UDP segment format

25. TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 point-to-point:
one sender, one receiver
reliable, in-order
send & receive buffers
full duplex data:
bi-directional data flow in same connection
Specifies maximum segment size (MSS)
connection-oriented:
handshaking (exchange of control msgs) init’s sender, receiver state before data exchange
flow controlled:
sender will not overwhelm receiver

26. TCP segment structure source port # dest port # application data
32 bits
application
data
(variable length)
sequence number
acknowledgement number
Receive window
Urg data pnter
checksum F S R P A U
head
len
not
used
Options (variable length)
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)
J. Kurose and Ross, Computer Networking

27. simple Telnet scenario
TCP seq. #’s and ACKs
Seq. #’s:
byte stream “number” of first octet in segment’s data
ACKs:
seq # of next octet expected from other side
cumulative ACK
time
Host A
Host B
Seq=42, ACK=79, data = ‘C’
Seq=79, ACK=43, data = ‘C’
Seq=43, ACK=80
User
types
‘C’
host ACKs
receipt
of echoed
receipt of
‘C’, echoes
back ‘C’
simple Telnet scenario

28. TCP Connection Management
Three way handshake:
Step 1: client host sends TCP SYN segment to server
specifies initial seq #
no data
Step 2: server host receives SYN, replies with SYNACK segment
server allocates buffers
specifies server initial seq. #
Step 3: client receives SYNACK, replies with ACK segment, which may contain data
Recall: TCP sender, receiver establish “connection” before exchanging data segments
initialize TCP variables:
seq. #s
buffers, flow control info (e.g. RcvWindow)

29. TCP Connection Management (cont.)
Closing a connection:
Step 1: client end system sends TCP FIN control segment to server
Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN.
Step 3: client receives FIN, replies with ACK.
Step 4: server, receives ACK. Connection closed.
client
FIN
server
ACK
close
closed
timed wait

30. TCP Connection Management (cont.)
Could be combined (3-way)
Setup Termination

31. Principles of Congestion Control
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!

32. TCP congestion control
There is no explicit feedback from network
Congestion inferred from end-system observed loss, delay
When this happens, TCP reduces its window size
Using CWnd and RcvWnd
More details in NETS3303/3603!

33. Summary
Transport layer establishes the process-to-process communication
TCP uses reliable, connection oriented approach
Used by FTP, HTTP, Telnet apps
UDP uses unreliable, connectionless approach
Used by DNS, SNMP and streaming apps like Realplayer and MS Windows Media Player
Stallings 20.1, 20.2 and 204.