1. Introduction to Networks
CSEN 404
Introduction to Networks
Amr El Mougy
Gina Maher
** Slides are attributed to J. F. Kurose

2. Chapter 3 outline Connection-oriented transport: TCP
segment structure
reliable data transfer
flow control
connection management
Principles of congestion control
Transport-layer services
Multiplexing and demultiplexing
Connectionless transport: UDP
Principles of reliable data transfer
Transport Layer

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
logical end-end transport
application
transport
network
data link
physical
Transport Layer

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 sending letters to 12 kids
processes = kids
app messages = letters in envelopes
hosts = houses
transport protocol = Ann and Bill
network-layer protocol = postal service
Transport Layer

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:
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

6. Multiplexing/demultiplexing
Demultiplexing at rcv host:
Multiplexing at send 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 P2 P4 P1
transport
transport
transport
network
network
network
link
link
link
physical
physical
physical
host 3
host 1
host 2
Transport Layer

7. How demultiplexing works
32 bits
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
host uses IP addresses & port numbers to direct segment to appropriate socket
source port #
dest port #
other header fields
application
data
(message)
TCP/UDP segment format
Transport Layer

8. Choosing Port Numbers UDP TCP
Source port:
26145
Dest. port: 80
Src IP: C
Dest. IP: B
UDP
TCP
Source port:
19157
Dest. port: 53
Source port:
7532
Dest. port: 80
Src IP: C
Dest. IP: B
Source port: 53
Dest. port:
19157
Source port:
26145
Dest. port: 80
Src IP: A
Dest. IP: B
Server is always listening on the well-known port number
Client chooses a source port number and inserts it in the header. The destination port number is the well- known port number
In the reply, the server inserts the port number received in the packet in the destination port number
Server is always listening on the well-known port number
Client chooses a source port number and inserts it in the header. The destination port number is the well-known port number
The server creates a socket for the client that is identifiable via the combination of src IP and src port address

9. 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

10. Examples of Transport-layer Protocols used by Applications
Application-Layer Protocol
Underlying Transport Protocol
SMTP
TCP
Remote terminal access
Telnet
Web
HTTP
File transfer
FTP
Remote file server
NFS
Typically UDP
Streaming multimedia
Typically proprietary
UDP or TCP
Internet telephony
Network management
SNMP
Routing protocol
RIP
Name translation
DNS
Transport Layer

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