TCP conversation is an essential part of network traffic. So, before discussing ACLs in detail, it is necessary to discuss TCP conversation. The ACLs control traffic into and out of the network.
Traffic control can be simple or complex. Simple traffic control only permits or denies network traffic based on network address, and complex traffic based on TCP port-based.
TCP Communication
When a host requests data from another host or server, the IP address manages the communication between the source and destination hosts. TCP manages the communication between the source and destination hosts.
When sending an email or downloading a file, TCP is responsible for breaking data into the segment for IP before sending. TCP is also responsible for assembling the data from the segments when data arrives at the destination. The TCP process is just like a conversation in which two hosts on a network agree to pass data between one another.
Transmission Control Protocol provides a connection-oriented and reliable byte stream service. Two applications must establish a TCP connection before exchanging data. It is a full-duplex protocol, where each TCP connection supports a pair of byte streams, each flowing in one direction.
TCP also includes a flow-control mechanism for each byte stream, allowing the receiver to limit data sent from another end. It also implements a congestion-control mechanism. Figure 1 illustrates how a TCP conversation takes place.
TCP segments are marked with flags SYN, ACK, and FIN. The SYN starts (synchronizes) the session, an ACK is an acknowledgment for the received segment, and a FIN finishes the session. A SYN/ACK acknowledges that the transfer is synchronized.
TCP data segments also include the higher-level protocol required to direct the application data to the accurate application. The TCP data segment identifies the port that matches the requested service. Each service has its port number. Some known port numbers are port 80, port 25, port 20, and port 21. Figure- 2 illustrates the range of TCP and UDP ports.
