Network Fundamentals: Ch4 - Transport Layer

Suez Canal University – Faculty of Computers & Informatics - Cisco Local Academy

Network Fundamentals

CH4: TRANSPORT LAYER

Abdelkhalik Elsaid [email protected]

http://abdelkhalik.staff.scuegypt.edu.eg/

Last Update: 10/7/2011

“In God we trust, all others bring data” - W. Edwards Deming

mailto:[email protected]




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.


Applications Requirements Vary

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


Data Encapsulation

TransportTransport

Data LinkData Link

PhysicalPhysical

NetworkNetwork

DataData

Header DataHeader Data

Presentation

Application

Session

Segment-Datagram

Packet

Bits

Frame

Header Segment / DatagramHeader Segment / Datagram

Header Packet TrailerHeader Packet Trailer

10000001111110000011100001000000111111000001110000


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.


Transport Layer Header


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.


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.


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 Ports69 - TFTP520 - RIP

Well Known TCP/UDP Ports53 - DNS

161 - SNMP

Well Known TCP PortsTelnet: 23, SMTP:25, HTTP: 80, POP3: 110, HTTPS:443


Identifying Applications and hosts

• Netstat: examines the open connections on a host

192.168.1.101

172.16.5.5

Destination Port8080

Source Port

49890

49888

198.133.219.2549888

www.cisco.com

66.13.29.25

80

Source Port


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)


At the source: Segmentation

• divides application data into blocks of data that are of appropriate size


At the Destination: TCP in order Reassembly

• The Transport layer reassembles the data before sending it to the destination application or service.


At the Destination: UDP Out of order Reassembly

• The Transport layer reassembles the data before sending it to the destination application or service.


Conversation Multiplexing

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


TCP Connection Establishment (3-way handshake)

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


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.


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. Buffering2. Congestion Avoidance3. Windowing


Flow Control - Buffering & Congestion Avoidance


Flow Control - Windowing

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


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.


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.


TCP Connection Termination (4-way Handshake)


Thank You..

Technology

Network Fundamentals: Ch4 - Transport Layer