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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Dynamic Host Configuration Protocol (DHCP)
                     9  DHCP Messaging, Message Types and Formats

Previous Topic/Section
DHCP Message Format
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1
2
34
Next Page
Summary Of DHCP Options / BOOTP Vendor Information Fields
Next Topic/Section

DHCP Options, Option Format and "Option Overloading"
(Page 2 of 4)

Options and Option Format

DHCP maintains, formalizes and further extends the idea of using the Vend field to carry general-purpose parameters. Instead of being called vendor information extensions, or vendor information fields, these fields are now called simply DHCP options. Similarly, the Vend field has been renamed the Options field, reflecting its new role as a way of conveying vendor-independent options between client and server.

Keeping with the desire to maintain compatibility between BOOTP and DHCP, the DHCP Options field is in most ways the same as the vendor-independent interpretation of the BOOTP Vend field introduced by RFC 1048. The first four bytes of the field still carry the “magic cookie” value “99.130.83.99” to identify the information as vendor-independent option fields. The rest of the Option field consists of one or more subfields, each of which has a “type, length, value” (TLV-encoded) substructure as in BOOTP. The main differences between BOOTP vendor information fields and DHCP options are the field names and the fact that the DHCP Options field is variable in length where the BOOTP Vend field is fixed at 64 bytes. The structure of the DHCP Options field as a whole is shown in Figure 268; the subfield names of each option are described in Table 190.


Figure 268: DHCP Options Field Format

The format of the DHCP Options field is, unsurprisingly, very similar to that of the BOOTP Vendor-Specific Area, as shown in Figure 257. The Options field begins with the same four-byte “magic cookie” and then contains a number of variable-length option fields. Each option has the format described in Table 190.

 



Table 190: DHCP Option Format

Subfield Name

Size (bytes)

Description

Code

1

Option Code: A single octet that specifies the option type.

Len

1

Option Length: The number of bytes in this particular option. This does not include the two bytes for the Code and Len fields.

Data

Variable

Option Data: The data being sent, which has a length indicated by the Len subfield, and which is interpreted based on the Code subfield.


All of the DHCP options follow the format of Table 190, except for two special cases, again the same as BOOTP. A Code value of 0 is used as a pad, when subfields need to be aligned on word boundaries; it contains no information. The value 255 is used to mark the end of the vendor information fields. Both of these codes contain no actual data, so to save space, when either is used just the single Code value is included; the Len and Data fields are omitted. A device seeing a Code value of 0 just skips it as filler; a device seeing a Code value of 255 knows it has reached the end of the fields in this Options field.


Previous Topic/Section
DHCP Message Format
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Pages in Current Topic/Section
1
2
34
Next Page
Summary Of DHCP Options / BOOTP Vendor Information Fields
Next Topic/Section

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