TCP and IP

To understand how multiple-packet transmissions are handled, we need to know that the Internet actually uses two protocols, stacked one on top of the other.

The lower level, IP (Internet Protocol), is responsible for labeling individual packets with the source address and destination address of two computers exchanging information over a network. For example, when you access http://www.google.com, the packets you send will have your computer's IP address, such as 192.168.1.101, and the IP address of the www.google.com computer, 152.2.210.81. These addresses work in much the same way that your home address works when someone sends you a letter. The post office can read the address and determine where you are and how best to route the letter to you, much like a router does for Internet traffic.

The upper level, TCP (Transmission Control Protocol), gives you reliability. When two machines negotiate a TCP connection (which they do using IP), the receiver knows to send acknowledgements of the packets it sees back to the sender. If the sender doesn't see an acknowledgement for a packet within some timeout period, it resends that packet. Furthermore, the sender gives each TCP packet a sequence number, which the receiver can use to reassemble packets in case they show up out of order. (This can easily happen if network links go up or down during a connection.)

TCP/IP packets also contain a checksum to enable detection of data corrupted by bad links. (The checksum is computed from the rest of the packet in such a way that if either the rest of the packet or the checksum is corrupted, redoing the computation and comparing is very likely to indicate an error.) So, from the point of view of anyone using TCP/IP and nameservers, it looks like a reliable way to pass streams of bytes between hostname/service-number pairs. People who write network protocols almost never have to think about all the packetizing, packet reassembly, error checking, checksumming, and retransmission that goes on below that level.