structure of the packet header was examined. To facilitate
the reordering of packets received out of time sequence,
a packet number was introduced. Also, the notion of a variable
length header was developed to improve the utilization
of message bits. Thus, short headers could be used with
short, simple messages, and long headers with long, complex
Elmer Shapiro; Computer
Network Meeting of Oct 9-10, 1967; Nov 1967.
Breaking digital communications into packets gave
network technology something to work with. The key idea of packet
switching is the division of each communication into individual,
equal-sized packets. These packets are then sent
individually to their destination through the network, and the
entire message is reassembled when all the packets arrive. There
are a range of procedures for retransmission of packets that might
get lost in the network.
On the Internet, a typical packet length is about one kilobyte,
or a thousand characters. A large message may be divided into thousands
packets. The beginning of an packet is called the "header" and
records the following information:
- Source. The IP address of
the computer sending the packet.
- Destination. The IP address of the destination computer
- Length. The length of the packet in bytes.
- Number. The total number of packets in the complete
- Sequence. The number of this packet in the whole
list of packets making up this communication.
This data provides the information an Internet router needs
to get packets and messages to their destination. For example,
a destination computer can request the retransmission of missing
packets, since it knows the number of total packets and therefore
can figure out which ones it doesn't have.
For additional reliability, Internet packet headers also contain
an error correction code, which is a number representing a mathematical
of the rest of the packet data. If even a single bit of the packet
is changed in transmission, then recalculation of the error correction
code by a router won't match the code transmitted with the message,
and the packet will be discarded and a request made for retransmission.