TCP stands for Transmission Control Protocol and is a connection-oriented, multiservice transport protocol for networks. It is often based on the IP (Internet Protocol), reason for referring to as TCP/IP. TCP itself is defined in RFC 793.
TCP establishes a channel between the two end points of a communication and allows the transmission of data in both directions. The data stream is separated into packets, so called TCP/IP packets, that are sent individually. Each packet can take a different route to the destination. Therefore the packets might arrive in different order. Once at their destination, they will be placed in the correct order and merged.
A TCP/IP packet consists of the head and the actual user data. The head contains meta information about the package, for example, the ports of the transmitter and receiver but also the sequence number of the packet, which is required to restore the correct sequence of packets at the receiver.
Size of the TCP/IP packet
The size of a TCP/IP packet is relatively arbitrary, but must fit into the transfer layer. This quantity is called the MTU for Maximum Transmission Unit. An IP packet itself is specified to be 64KB (65536 bytes) in size (IPv4). For IPv6 even larger packages are available. Since it is mostly transmitted via Ethernet, however, there is the fixed transfer size of 1500 bytes.
As many Internet connections use DSL, additional eight bytes are required for the Point-to-Point Protocol (PPP) between IP and Ethernet, leaving a transfer size of 1492 bytes.
Of these 1492 Bytes, there are 20 bytes for the TCP header, and also 20 bytes for the IP header, resulting in 1452 bytes of user data. These 1452 bytes are called for MSS Maximum Segment Size.
