Chapter 4 Notes

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

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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)

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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