In the interest of thoroughness in a discussion about IPs, we need to mention User Datagram Protocol (UDP). It also is Transport Layer Protocol, but does not create connections. It is another "send and pray" protocol like IP. Who then would want such a thing? Consider the problem of a database query. You don't need a connection, but you need an answer quickly. If the answer doesn't come back in a reasonable time, ask again.

Both the DNS and Simple Network Management Protocol (SNMP) use UDP. These are both query/response protocols. Figure 32-8 shows the UDP header in all its simplicity. By default, the UDP header is only 8 bytes (octets) long.

Figure 32-8: The UDP header

The port addresses are the same as described for TCP. The length tells how long the segment is in bytes, and the checksum is, as with TCP, a simple sum over the whole segment.