IP uses four key mechanisms in providing its service: Type of Service, Time to Live, Options, and Header Checksum.
The Options, commonly referred to as IP Options, provide for control functions that are required in some situations but unnecessary for the most common communications. IP Options include provisions for time stamps, security, and special routing.
IP Options may or may not appear in datagrams. They must be implemented by all IP modules (host and gateways). What is optional is their transmission in any particular datagram, not their implementation. In some environments the security option may be required in all datagrams.
The option field is variable in length. There may be zero or more options. IP Options can have one of two formats:
Format 1: A single octet of option-type.
Format 2: An option-type octet, an option-length octet, and the actual option-data octets.
The option-length octet counts the option-type octet, the option-length octet, and the option-data octets.
The option-type octet is viewed as having three fields: a 1-bit copied flag, a 2-bit option class, and a 5-bit option number. These fields form an 8-bit value for the option type field. IP Options are commonly referred to by their 8-bit value.
For a complete list and description of IP Options, refer to RFC 791, Internet Protocol at the following URL: http://www.faqs.org/rfcs/rfc791.html