10.1.2 This page will explain the synchronization process that TCP uses. The process is also called a three-way handshake.
TCP is a connection-oriented protocol. Prior to data transmission, the two communicating hosts go through a synchronization process to establish a virtual connection for each session between hosts. This synchronization process ensures that both sides are ready for data transmission and allows the devices to determine the initial sequence numbers for that session. This process is known as a three-way handshake. This is a three-step process that establishes the virtual connection between the two devices. It is also important to note that the three-way handshake is initiated by a client host. To establish a TCP session, the client host will use the well-known port number of the service it wishes to contact on a server host.
In step one, the initiating host (client) sends a synchronization (SYN flag set) packet to initiate a connection. This indicates that a packet has a valid initial Sequence Number value in this segment for this session of x. The SYN bit set in the header indicates a connection request. The SYN bit is single bit in the code field of the TCP segment header. The Sequence Number is a 32 bit field TCP segment header.
In step two, the other host receives the packet, records the Sequence Number of x from the client, and replies with an acknowledgment (ACK flag set). The ACK control bit set indicates that the Acknowledgment Number field contains a valid acknowledgment value. The ACK flag is a single bit in the code field of the TCP segment header and the Acknowledgment Number is a 32 bit field TCP segment header. Once a connection is established, the ACK flag is set for all segments during the session. The Acknowledgment Number field contains the next sequence number that this host is expecting to receive (x + 1). The Acknowledgment Number of x + 1 means the host has received all bytes up to and including x, and expects to next receive byte x + 1. The host also initiates a return session. This includes a TCP segment with its own initial Sequence Number value of y and with the SYN flag set.
In step three, the initiating host responds with a simple Acknowledgment Number value of y + 1, which is the Sequence Number value of Host B + 1. This indicates that it received the previous acknowledgment and finalizes the connection process for this session.
It is important to understand that initial sequence numbers are used to initiate communication between two devices. They act as reference starting numbers between the two devices. The sequence numbers give each host a way to acknowledge so that the receiver knows the sender is responding to the proper connection request.
The Interactive Media Activity will help students understand synchronization.
The next page will discuss denial of service attacks
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