Networking Basics CCNA 1 Chapter 11

The TCP/IP Transport Layer TCP/IP transport layer includes several protocols –Transmission Control Protocol (TCP) –User Datagram Protocol (UDP) –TCP has more functions –UDP has less overhead Main goal of the transport layer: –Provide the service of taking data from one application process on one computer and delivering that data to the correct application process on another computer

The TCP/IP Transport Layer Comparing transport and internet layers: –Internet layer delivers packets from one computer to another, but is not concerned with which application sent the data or which application on the receiving computer needs the data –If multiple instances of an application are running on the destination computer, the transport layer works to ensure the proper instance gets the data and not the others

The TCP/IP Transport Layer Comparing TCP and UDP

The TCP/IP Transport Layer Flow Control and Windowing When a host sends data using TCP, the receiving host can control how fast each TCP sender sends the data over time – this is called flow control Flow control is used for many reasons –Receiver needs time to process the data –Receiver has finite amount of memory, so if data keeps arriving, it could run out of memory

The TCP/IP Transport Layer Flow Control Through Dynamic Sliding Windows Receiver tells sender how many bytes the sending host can send before it receives an acknowledgement – a value called a window When the sending host has sent an entire window’s worth of data, it must wait for an acknowledgement, slowing its rate of sending data The window size can be increased to allow a faster transfer rate

The TCP/IP Transport Layer Dynamic Windowing

The TCP/IP Transport Layer Flow Control Through Withholding Acknowledgements After a sending host has sent one window worth of bytes, it must wait to send more The receiver can wait to send acknowledgements, which prevents the sender from sending more data This process of withholding acknowledgements is sometimes known as start/stop flow control

The TCP/IP Transport Layer Withholding Acknowledgements

The TCP/IP Transport Layer Establishing and Terminating TCP Connections TCP is a connection-oriented protocol –Allows it to set initial values to the window and to initialize sequence number values TCP uses a process called a three-way handshake to create a new TCP connection –Uses three TCP segments that use two of the TCP flags in the TCP header The synchronize (SYN) flag The acknowledge (ACK) flag

The TCP/IP Transport Layer Three-Way TCP Connection Establishment

The TCP/IP Transport Layer TCP Error Recovery (Reliability) TCP is a reliable protocol (at least in a networking perspective) –Performs error recovery –All data eventually gets to the destination, even if some is initially lost in transport

The TCP/IP Transport Layer TCP Acknowledgements with no Error Recovery Needed

The TCP/IP Transport Layer TCP Error Recovery (Reliability) TCP uses two header fields to signal to other computer as to whether a segment was received –Sequence number Keeps track of all the bytes sent over a TCP connection –Acknowledgement number Lets the sending device know the number of the segment the receiving device expects next Practice of stating the next byte expected to be received is called a forward acknowledgement or expectational acknowledgement

The TCP/IP Transport Layer TCP Error Recovery

The TCP/IP Transport Layer TCP Error Recovery (Reliability) If a segment is lost or destroyed during transmission, the receiving PC requests that it be retransmitted If an acknowledgement is not received by the sending PC within a certain time (a timer has been set), the sending PC resends the segment

The TCP/IP Transport Layer Segmentation, Reassembly, and In-Order Delivery TCP segmentation refers to the process of accepting a large chunk of data from the application protocol and breaking it into pieces that are small enough for transmission –The size of the segment is limited –Ethernet limits frames to 1500 data bytes in the data field of a frame IP and TCP headers are each 20 bytes long Maximum data portion is therefore 1460 bytes

The TCP/IP Transport Layer Segmentation, Reassembly, and In-Order Delivery TCP on the receiving computer reassembles data into its original form The data is put in the correct order –If segments of a file are assembled out-of- order, the file is useless –TCP provides a guarantee of in-order delivery

The TCP/IP Transport Layer Segmentation, Reassembly, and In-Order Delivery Due to IP routing, a TCP receiver can receive data out of order If multiple routes exist between a source and a destination, routers can load-balance over several routes Packets can arrive out of order

The TCP/IP Transport Layer TCP Providing In-Order Delivery

The TCP/IP Transport Layer TCP and UDP Header Reference TCP and UDP use a header to hold information for performing tasks –TCP needs ACK and SYN flags First two fields in TCP and UDP are identical –Both use port numbers to identify application processes TCP has a longer header (20 bytes vs. 8 bytes)

The TCP/IP Transport Layer TCP and UDP Headers

The TCP/IP Transport Layer TCP Header Fields

The TCP/IP Transport Layer TCP Header Fields

The TCP/IP Transport Layer UDP Header Fields

The TCP/IP Transport Layer Identifying Application Processes Using Port Numbers Both TCP and UDP identify the specific application process that sends the data and the application process that needs to receive the data –To make this determination, TCP and UDP use port numbers –Each application uses a different local port number

The TCP/IP Transport Layer Using Port Numbers to Identify the Correct Application Process

The TCP/IP Transport Layer Identifying Application Processes Using Port Numbers In previous slide, the application was assigned a dynamic port number by the host computer –A host typically dynamically allocates port numbers of value 1024 (2 10 ) through 65,535 ( ) –When a host starts a new application process, it allocates a dynamic port number that is not already in use by another process –By each process having its own port number, a PC can have multiple conversations with other PCs (sometimes called multiplexing)

The TCP/IP Transport Layer Identifying Application Processes Using Port Numbers Connection to Servers: Well-Known Ports –Most TCP/IP applications use a client/server model for communications –Servers cannot use dynamic port numbers because clients must know ahead of time what port numbers servers use –Servers must wait and listen on certain port numbers for client requests

The TCP/IP Transport Layer Identifying Application Processes Using Port Numbers Connection to Servers: Well-Known Ports (continued) –For servers to work well, TCP/IP defines one or more well-known ports, each reserved for use by a specific application protocol For example, HTTP connects to a server listening on port 80

The TCP/IP Transport Layer Client Connecting to Well-Known Port of a Web Server (80)

The TCP/IP Transport Layer Popular Applications and Their Well-Known Port Numbers

The TCP/IP Transport Layer Identifying Application Processes Using Port Numbers Comparing Well-Known, Dynamic, and Registered Ports –IANA assigns the values for well-known ports and for registered port numbers Registered ports are assigned to servers that the average end-user can start, such as instant-messaging applications Well-known ports are started by IT staff for applications such as web servers, , FTP servers

The TCP/IP Transport Layer Uses of Port Numbers

The TCP/IP Transport Layer Identifying Application Processes Using Port Numbers Comparing Well-Known, Dynamic, and Registered Ports (continued) –Both TCP and UDP use port numbers –To see the port numbers used on a computers running a Microsoft OS, use the netstat –an command