The client responds with a TCP packet with the ACK flag set.The server receives the packet, creates a TBC (Transmission Control Block) in its memory, and responds with a TCP packet with both the SYN and ACK flags set.The client sends a TCP packet to the server with the SYN flag set.Here is a typical sequence, with the TCP flag SYN meaning “synchronize” and ACK indicating “acknowledgment”: This flow of data is again managed using flags in the TCP headers, as well as counters, which we’ll discuss in more detail in a later section.Įstablishing a connection the traditional wayĪ TCP connection is initiated by the client and goes through a three-way handshake as shown below: Once the communication ends, the connection can be closed using flags in the headers of the TCP packets.Īs TCP is required to handle packets that are out of order, duplicated, or lost, it needs to detect and re-order packets and retransmit packets. This concept of connection is necessary to present the flow of data as a seamless stream on both ends. To do this, TCP establishes a connection between two peers, using flags to establish such a connection. So TCP builds on top of IP to provide a stream-oriented protocol. It also has no concept of “connections” in the sense that the IP stack is not able to logically group sent or received packets according to certain criteria (for example, a logical construct that is a “connection” as seen by the application layer). IP does not handle out-of-order, lost, or duplicate packets. The header contains information such as the originating IP address, the destination IP address, and the protocol used in the payload (such as ICMP, UDP, or TCP). IP packets consist of a header and a payload. IP is about sending packets back and forth. TCP is built on top of IP, or Internet Protocol.
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