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IP Datagram Options and Option Format
(Page 1 of 3)
All IP datagrams must include the
20-byte header, which 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. J
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, which makes the total length of the Options
field in the IP header variable. Each of the options can be either a
single byte long, or multiple bytes in length, depending on how much
information the option needs to convey. When more than one option is
included they are just concatenated together and put into the Options
field as a whole. Since the IP header must be a multiple of 32 bits,
a Padding field is included if the number of bits in all options
together is not a multiple of 32 bits.
IP Option Format
Each IP option has its own subfield
format, generally structured as shown in Table 58
and Figure 87.
For most options, all three subfields are used: Option Type,
Option Length and Option Data. For a few simple options,
however, this complex substructure is not needed. In those cases, the
option type itself communicates all the information required, so the
Option Type field appears alone, while the Option Length
and Option Data subfields are omitted.
Table 58: Internet Protocol Version 4 (IPv4) Option Format
Length: For variable-length options, indicates the size of the
entire option, including all three subfields shown here, in bytes.
0 or Variable
Option Data: For
variable-length options, contains data to be sent as part of the option.