Chapter 4.1 – The OSI Transport Layer The Transport layer prepares application data for transport over the network and processes

network data for use by applications Roles of the Transport Layer

Tracking the individual communication between applications on the source and dest. Host Segmenting data and managing each piece Reassembling the segments into streams of application data Identifying the different applications

Purpose Enabling Applications on Devices to Communicate Transport layer moves data between applications on devices in the network

Separating Multiple Communications Transport layer segments the data and manages the separation of data for different

applications. Multiple applications running on a device receive the correct data.

Segmentation Transport layer divides the data into segments that are easier to manage and transport Provides the means to both send and receive data while running mult. Apps concurrently on

a computer Each piece of information flowing is known as a conversation Transport layer adds to the piece a header containing binary data Enable different transport layer protocols to perform different functions

Controlling the Conversations Transport Layer Services

Segmentation and Reassembly Segmentation allows multiplexing – multiple applications can use the network at the

same time Data segmentation facilitates data carriage by the lower netowkr layers Error checking can be performed on the data in the segment to check if the segment

was changed during transmission Divides app. Data into blocks of data that are appropriate size. At dest. Transport layer reassembles the data before sending to dest. App/service

Conversation Multiplexing Apps/Services running on host assigned a port Some Protocols provide:

Connection-oriented conversations Reliability delivery Ordered data reconstruction Flow control

Transport Layer Services Establishing a Session ensures the application is ready to receive the data Reliable Delivery means lost segments are resent so the data is received complete Same order delivery ensures data is delivered sequentially as it was sent Flow Control manages data delivery if there is congestion on the host

Supporting Reliable Communication

Transport Layer Protocols Three basic operations of reliability:

Tracking transmitted data Acknowledging received data Retransmitting any unacknowledged data

To support reliability operations, more control data is exchanged between hosts Determining the Need for Reliability

Databases, we pages, e-mail require all sent data arrive at dest. In original condition Additional network overhead considered to be required

Other apps (video stream) does not require network overhead TCP and UDP

UDP provides for low overhead data delivery Data pieces called datagrams sent as “best effort” UDP Applications

DNS Video Streaming VoIP

Each segment has 8 bytes of overhead in the header TCP incurs additional overhead

Each segment has 20 bytes of overhead in the header TCP Applications

Web Browsers E-Mail File Transfers

Port Addressing Network layer IP address + port number = “socket” Data for different applications is directed to the correct application because each application

has a unique port number The Internet Assigned Numbers Authority (IANA) assigns port numbers Well Known Ports (0 – 1023) are reserved for services and applications

TCP Ports 21 FTP 23 Telnet 25 SMTP 80 HTTP 110 POP3 194 Internet Relay Chat (IRC) 443 Secure HTTP (HTTPS)

UDP Ports 69 TFTP 520 RIP

TCP/UDP Common Ports 53 DNS 161 SNMP 631 AOL IM

Registered Ports (1024 – 49151) are assigned to user processes or applications Dynamic / Private Ports (29152 – 65535) usually assigned dynamically to client applications

when initiating a connection

Netstat – important network utility that can be used to verify TCP connections Segmentation and Reassembly – Divide and Conquer

TCP and UDP Segment differently TCP Header provides for:

Source & destination (ports Sequencing for same order delivery Acknowledgement of received segments Flow control and congestion management

UDP Source and destination (ports)

The TCP Protocol Communicating with Reliability