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TCP/IP Guide by Charles M. Kozierok

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IP Datagram Options and Option Format

All IP datagrams must include the standard 20-byte header that contains key information such as the source and destination address of the datagram, fragmentation control parameters, length information, and more. In addition to these invariable fields, the creators of IPv4 included the ability to add options that provide additional flexibility in how IP handles datagrams. Use of these options is, of course, optional. However, all devices that handle IP datagrams must be capable of properly reading and handling them.

The IP datagram may contain zero, one, or more options, so the total length of the Options field in the IP header is variable. Each of the options can be a single byte or multiple bytes in length, ...

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