Transport LayerLayer #4 (OSI-RM)

Transport Layer

Main function of OSI Transport layer:

Accept data from the Application layer and prepare it for addressing at the Network layer. The Transport layer is responsible for the overall end-to-end transfer of application data.

Transport Layer

Functions of OSI Transport layer:

Segmentation of application data and reassemble these pieces into the various communication streams .

Enables multiple applications to communicate over the network at the same time on a single device.

Ensures that, if required, all the data is received reliably and in order by the correct application.

Employs error handling mechanisms.

Transport Layer services

Two types of transport layer services:

Connection oriented Connection Establishment Reliable Data Transfer Connection Termination

Protocol example: TCP

Connectionless (datagram) Unreliable Data Transfer

Protocol example: UDP

Transmission Control Protocol (TCP)

TCP is a connection-oriented protocol, described in RFC 793.

Functions of TCP:

1. Connection establishment2. Segmentation and reassembly of application data3. Multiplexing of more than one applications4. Error control5. Flow Control6. Connection termination

TCP Header

Connection establishment(Three way handshaking)

Connection establishment

The 3 way handshake role: • Establishes that the destination device is present on the network.

• Verifies that the destination device has an active service and is accepting requests on the destination port number.

• Informs the destination device that the source client intends to establish a communication session on that port number

Connection termination(Four way handshaking)

Segmentation and reassembly of application data

Error Recovery (Reliability)

Error Recovery (Reliability)

TCP Congestion and Flow Control

TCP Congestion and Flow Control

Port addressing

Port numbers – unique application identifiers

• Well known ports (Numbers 0 to 1023) : Assigned by The Internet Assigned Numbers Authority (IANA)

• Registered Ports (Numbers 1024 to 49151)

• Dynamic or Private Ports: Also known as Ephemeral Ports (Numbers 49152 to 65535)

TCP Ports

UDP Ports

TCP and UDP Protocols

Socket

A socket is an IP address, a colon, and a port number.

Example: 128.171.17.13:80

For servers, specifies a specific application on a specific server.

For clients, specifies a specific connection on a specific client.

Multiplexing of more than one applications

User Datagram Protocol (UDP)

UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery. The pieces of communication in UDP are called datagrams. These datagrams are sent as "best effort" by this Transport layer protocol

Functions of UDP:

1. Segmentation and reassembly of application data2. Multiplexing of more than one applications

UDP Header

Applications that use UDP include:

Domain Name System (DNS)Video StreamingVoice over IP (VoIP)

TCP/UDP Comparison

TCP UDP

Connection-oriented delivery Connectionless delivery, faster

Uses windows and ACKs No windows or ACKs

Full header Smaller header, less overhead

Sequencing No sequencing

Provides reliability Relies on app layer protocols for reliability

FTP, HTTP, SMTP, and DNS DNS, TFTP, SNMP, and DHCP