Data Navigation Protocols
The fundamental network protocol is likely Internet Protocol, which describes how packets will navigate from network to network. It does this by defining the structure of IP addresses. The IP also provides a fragmentation and reassembly function, which means that if a message, or datagram, is too long, an IP packet can be split into smaller chunks for transmission through the network and then put it back together when it gets to its final destination.
What IP does not do is keep track of whether messages actually make it to where they are going, or pace the transmission of messages so that a link does not overfill. IP treats each piece of a message, or Internet datagram, as an independent entity unrelated to any other Internet datagram. The IP must link up with several other protocols to insure reliable end-to-end delivery and retransmission of missing messages.
For instance, the Transmission Control Protocol wraps itself around the IP packet and provides the information needed to see a packet through multiple hops to its destination and determines if all packets made the trip. TCP can figure out which packets were lost and order up replacements. UDP, on the other hand, is the stripped-down version of TCP, moving packets with speed but sacrificing end-to-end reliability.
The File Transfer Protocol mentioned previously operates using TCP. All the data travels reliably over the network, and the transmission is not finished until the packets have all made the trip and been reassembled in order at the destination. FTP's UDP cousin, however, Trivial File Transfer Protocol (TFTP), transmits the packets as a firehose transmits water. It streams, but it has no way to determine by itself if the water is hitting the target.
TCP can detect errors because each packet uses a cyclical redundancy check (CRC), which is like a parity or checksum, to check itself. A checksum is a mathematical mechanism that detects errors in transmission, usually by adding up the numeric value of all the characters transmitted and seeing if the total is the same at both ends of the link.
If IP needs to report errors to the sender, it uses helping protocols from a suite called Internet Control Message Protocol (ICMP). IP includes a facility for limiting the transmission of misdirected messages, a self-destruct mechanism called time-to-live (TTL). Every time a packet passes through a network node that retransmits it, the TTL counter gets decremented, that is, subtracted by one. If the TTL reaches zero before the Internet datagram reaches its destination, the Internet datagram is considered lost, and is destroyed.