Internet Core Protocols: The Definitive Guide

Name

Protocol Identifier

Synopsis

Identifies the type of higher-level protocol that is embedded within the IP datagram’s data.

Size

Eight bits.

Notes

Remember that IP works only to move datagrams from one host to another, one network at a time. It does not provide much in the way of services to higher-level applications, a function served by TCP and UDP. However, almost every other protocol (including these two transport protocols) uses IP for delivery services.

Normally, the entire higher-level protocol message (including the headers and data) is encapsulated within an IP datagram’s data segment. Once the IP datagram reaches its final destination, the receiving system will read the data segment and pass it on to the appropriate higher-level protocol for further processing. This field provides the destination system with a way to identify the higher-layer protocol for which the embedded message is intended.

Table 2.12 lists the four most common protocols, and their numeric identifiers.

Table 2.12. The Most Common Higher-Level Protocols and Their Numeric Identifiers

Protocol ID	Protocol Type
1	Internet Control Message Protocol (ICMP)
2	Internet Group Management Protocol (IGMP)
6	Transmission Control Protocol (TCP)
17	User Datagram Protocol (UDP)

There are a number of predefined protocol numbers that are registered with the Internet Assigned Numbers Authority (IANA). For a comprehensive list of all the upper-layer Protocol Identifier numbers used by IP, refer to the IANA’s online registry (accessible ...