## Basics of Layer 4 * Provides transparent transfer of data between end hosts. * Provides (or doesn't provide) various services to applications: * TCP Services: * Reliable data transfer. * Error recovery. * Data sequencing. * Flow control * Provides Layer 4 addressing (port numbers). * Port numbers help identify the application Layer protocol being used. * Port numbers provides Session multiplexing. * The following ranges have been designated by IANA (Internet Assigned Numbers Authority). * **Well-known** port numbers: 0 - 1023. * Used for major protocols like HTTP, FTP, etc and are very strictly regulated. * **Registered** port numbers: 1024 - 49151. * Registration is required to use these port numbers. Although it's not as strict as with the well know port range. * **Ephemeral/private/dynamic** port numbers: 49152 - 65535. * Hosts use this range when selecting the random source port. ### Session Multiplexing An example of session multiplexing is a web browser. Each browser tab starts a new session with the external service/server. Different tabs originate their request with a random selected source port that is used by the service/server to know where to reply to. Thus, maintain separate independent sessions per tab. ## TCP (Transmission Control Protocol) * TCP is connection-oriented. * Before actually sending data to the destination host, the two hosts communicate to establish a connection. Once the connection is established, the data exchange begins. * TCP provides reliable communication. * The destination host must acknowledge that it received each TCP segment. * If a segment isn't acknowledged, it is sent again. * TCP provides sequencing. * Sequence numbers in the TCP header allow destination hosts to put segments in the correct order even if they arrive out of order. * TCP provides flow control. * The destination host can tell the source host to increase/decrease the rate that data is sent. ### TCP Header  * Source and destination ports. * Each field is 16 bits in length. * 16 bits = 65535 (2^16) available port numbers. * Sequence number and Acknowledgement number. * Each field is 32 bits in length. * These fields provide sequencing and reliable communication. * Important flag blits (ACK, SYN, FIN) * These three flags are used to establish and terminate connections. * Window Size. * The window size is used for flow control. ### Establishing Connections: Three-Way handshake  ### Terminating Connections: Four-Way handshake  ### TCP: Sequencing / Acknowledgment  * Hosts set a random initial sequence number. * **Forward acknowledgement** is used to indicate the sequence number of the next segment the host expects to receive. ### TCP Retransmission * A host will resend a segment for which it hasn't received an ACK after a set period of time. ### TCP Flow Control: Window Size * Acknowledging every single segment, no matter what size, is inefficient. * The TCP header's **Window Size** field allows more data to be sent before an acknowledgment is required. * A 'sliding window' can be used to dynamically adjust how large the window size is. * The window size is increased as much as possible until a segment is dropped, then the window size backs down to a more reasonable level, and slowly increases again. ### NOTE In all of these examples, very simple sequence numbers were used. In real situations, the sequence numbers get much larger and do not increase by 1 with each message. For the CCNA, just understand the concepts and don't worry about the exact numbers. ## UDP (User Datagram Protocol) * UDP is not connection-oriented. * The sending host does not establish a connection with the destination host before sending data. The data is simply sent. * UDP does not provide reliable communication. * When UDP is used, acknowledgments are not sent for received segments. If a segment is lost, UDP has no mechanism to re-transmit it. Segments are sent 'best-effort'. * UDP does not provide sequencing. * There is no sequence number field in the UDP header. If a segment arrives out of order, UDP has no mechanism to put them back in order. * UDP does not provide flow control. * UDP has no mechanism like TCP's window size to control the flow of data. ### UDP Header  ## Comparing TCP & UDP  * TCP provides more features than UDP, but at the cost of additional overhead. * For applications that require reliable communications (for example downloading a file), TCP is preferred. * For applications like real-time voice and video, UDP is preferred. * There are some applications that use UDP, but provide reliability within the application itself. * Some applications use both TCP & UDP, depending on the situation. ## Port Numbers