Type field

The type field forms part of the IPv4 (IP version 4) header, and is used to indicate the transport PDU (Protocol Data Unit) that forms the payload of the datagram. This information is used to pass the PDU through the correct SAP (Service Access Point) to the higher layer transport protocol. The values of the type field that indicate the higher layer protocol were listed until 1994 in RFC (Request for Comments) 1700, this has now been superseded by an online database supported by the IANA. Typical examples include 6 for TCP (Transmission Control Protocol) and 17 for UDP (User Datagram Protocol).