The document discusses the transport layer of networking, detailing its functions, such as enabling communication between multiple applications and ensuring data delivery reliability. It explains the differences between TCP and UDP protocols, including their applications and characteristics, as well as concepts like port addressing and flow control mechanisms. The document also covers connection management, including the three-way handshake for establishing TCP connections and the processes for error detection and data segmentation.

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Network Fundamentals
Abdelkhalik Elsaid Mosa
abdu.elsaid@yahoo.com
http://abdelkhalik.staff.scuegypt.edu.eg/
Last Update: 10/7/2011
“In God we trust,
all others bring data”
- W. Edwards Deming

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Introduction
• Layer 4, responsible for the overall end-to-end transfer of
application data.
• Transport layer also encompasses these functions:
1. Enables multiple applications to communicate over the
network at the same time on a single device.
2. Ensures that, if required, all the
data is received reliably and
in order by the correct
application.
3. Employs error
handling mechanisms.

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Applications Requirements Vary
• Because different applications have different
requirements, there are multiple Transport layer protocols.

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Data Encapsulation
Transport
Data Link
Physical
Network
Data
Header Data
Presentation
Application
Session
Segment-Datagram
Packet
Bits
Frame
Header Segment / Datagram
Header Packet Trailer
1000000111111000001110000

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TCP and UDP
• UDP
 is a simple, low overhead, connectionless protocol, RFC 768
 Pieces of communication in UDP are called datagrams.
 Datagrams are sent as "best effort“ “Unreliable”.
 Applications that use UDP include:
 DNS,DHCP,SNMP,RIP, TFTP, Online games, Video Streaming, and VoIP.
• TCP
 is a connection-oriented, RFC 793, additional overhead
 Reliable delivery.
 Flow control which prevents overflow
 Applications that use TCP are:
 Web Browsers, E-mail, File Transfers.

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Transport Layer Header

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TCP Header Fields
• Source port : a random value above 1023.
• Destination port: Identifies the destination protocol or application.
• Sequence number: the first byte in the current segment.
Is the SYN flag set or not??
• Acknowledgement number: specifies the next expected byte by
the receiver.
• Window size: determines how many bytes can be sent before
waiting for acknowledgement.
• H.length (Offset): the length of the segment header in bytes.
• Checksum: used for error checking.
• Reserved: Reserved for future use and should be set to zero.

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Port Addressing
• Port Number (L4 Addressing): Uniquely identifies applications.
 Server processes have static port numbers assigned to them.
 Clients dynamically choose a port number for each conversation.
• Socket: is the combination of the port number and the IP
address, Ex: 198.16.23.213:80.
• Socket Pair: consists of the source and destination IP addresses
and port numbers, is also unique and identifies the conversation
between the two hosts.
• IANA is responsible for assigning various addressing standards.

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Types of port numbers
1. Well Known Ports (0 to 1023): are reserved for services and
applications such as HTTP (80), SMTP (25), DNS (53), ... .
2. Registered Ports (1024 to 49151): are assigned to user
processes or applications such as MSN Messenger (1863).
3. Dynamic or Private ‘Ephemeral’ Ports (49152 to 65535):
assigned dynamically to client applications when initiating a
connection.
Well Known UDP Ports
69 - TFTP
520 - RIP
Well Known TCP/UDP Ports
53 - DNS
161 - SNMP
Well Known TCP Ports
Telnet: 23, SMTP:25, HTTP:
80, POP3: 110, HTTPS:443

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Identifying Applications and hosts
• Netstat: examines the open connections on a host
192.168.1.101
172.16.5.5
Destination Port
80
80
Source Port
49890
49888
198.133.219.25
49888
www.cisco.com
66.13.29.25
80
Source Port

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Transport Layer Functions
• TCP/UDP Functions.
1.Segmentation and Reassembly.
2.Conversation Multiplexing using port numbers.
3.Error detection.
• TCP Additional functions
1.Connection Establishment (3-way handshake)
2.Connection Management.
 Reliability (acknowledgements).
 Flow control (Buffering, congestion avoidance, windowing).
 Error correction .
3. Connection Termination (4-way handshake)

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At the source: Segmentation
• divides application data into blocks of data that are of
appropriate size

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At the Destination: TCP in order Reassembly
• The Transport layer reassembles the data before sending it to the
destination application or service.

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At the Destination: UDP Out of order Reassembly
• The Transport layer reassembles the data before sending it to the
destination application or service.

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Conversation Multiplexing
• Multiple Applications can use the network at the same time
where each application is identified by unique port number.

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TCP Connection Establishment (3-way handshake)
• When two hosts communicate using TCP, a connection
(Three-way handshake) is established before data can be
exchanged.

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Connection Management - Reliability
• At the Transport layer the three basic operations of reliability are:
1. Tracking transmitted data: keep track of all the data pieces of
each conversation.
2. Acknowledging received data: The destination sends
acknowledgements to the source for the segments that it
receives.
3. Retransmitting any unacknowledged data: Retransmit any of
data that were not acknowledged by the destination.
If the source does not receive an acknowledgement within a predetermined
amount of time, it retransmits that data to the destination.

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Connection Management - Flow Control
• Flow Control: the management of data flow between devices in
the network.
 is a method of making sure that an excessive amount of data
doesn't overrun the end station.
 Example: imagine that PC A is running at 100 Mbps and PC B is running at
10 Mbps. If PC A sends something to PC B at full speed, 90 percent of the
information will be lost because PC B cannot accept the information at 100
Mbps. This is the reason for flow control.
• Flow control Methods:
1. Buffering
2. Congestion Avoidance
3. Windowing

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Flow Control - Buffering & Congestion Avoidance

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Flow Control - Windowing
• Window Size: The amount of data that a source can transmit
before an acknowledgement must be received

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Flow Control - Windowing
• Flow control assists the reliability of TCP transmission by adjusting
the effective rate of data flow between the two services in the
session.

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Flow Control – Reducing the window size
• If a receiving host has congestion, it may respond to the sending
host with a segment with a reduced window size.

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TCP Connection Termination (4-way Handshake)

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