TCP / IP

Transmission Control Protocol / Internet Protocol

TCP/IP is a connectionless protocol. Information is transfered in packets. Each of these packets is sent through the network individually. There are provisions to open connections to systems. However at some level, information is put into packets, and those packets are treated by the network as completely separate. For example, suppose you want to transfer a 15000 octet file. Most networks can't handle a 15000 octet packet. So the protocols will break this up into something like 30 500-octet packets. Each of these packets will be sent to the other end. At that point, they will be put back together into the 15000-octet file. However while those packets are in transit, the network doesn't know that there is any connection between them. It is perfectly possible that packet 14 will actually arrive before packet 13. It is also possible that somewhere in the network, an error will occur, and a packet won't get through at all. In that case, that packet has to be sent again. In fact, there are two separate protocols involved in doing this. TCP (the transmission control protocol) is responsible for breaking up the message into packets, reassembling them at the other end, resending anything that gets lost, and putting things back in the right order. IP (the internet protocol) is responsible for routing individual packets. It may seem like TCP is doing all the work. And in small networks that is true. However in the Internet, simply getting a packet to its destination can be a complex job. A connection may require the packet to go through several networks at Rutgers, a serial line to the John von Neuman Supercomputer Center, a couple of Ethernets there, a series of 56Kbaud phone lines to another NSFnet site, and more Ethernets on another campus. Keeping track of the routes to all of the destinations and handling incompatibilities among different transport media turns out to be a complex job. Note that the interface between TCP and IP is fairly simple. TCP simply hands IP a packet with a destination. IP doesn't know how this packet relates to any packet before it or after it.