TCP/IP Tutorial and Technical Overview - IBM Redbooks

Vendor-specific area Optional vendor-specific area. Clients should always
fill the first four bytes with a “magic cookie.” If a
vendor-specific magic cookie is not used, the client
should use 99.130.83.99 followed by an end tag (255)
and set the remaining bytes to zero. The
vendor-specific area can also contain BOOTP Vendor
extensions. These are options that can be passed to
the client at boot time along with its IP address. For
example, the client can also receive the address of a
default router, the address of a domain name server,
and a subnet mask. BOOTP shares the same options
as DHCP, with the exception of several DHCP-specific
options. See RFC 2132 for full details.
After the BOOTP client has processed the reply, it can proceed with the transfer
of the boot file and execute the full boot process. See RFC 906 for the
specification of how this is done with TFTP. In the case of a diskless host, the full
boot process will normally replace the minimal IP protocol stack, loaded from
ROM, and used by BOOTP and TFTP, with a normal IP protocol stack
transferred as part of the boot file and containing the correct customization for
the client.
3.6.1 BOOTP forwarding
The BOOTP client uses the limited broadcast address for BOOTP requests,
which requires the BOOTP server to be on the same subnet as the client.
BOOTP forwarding is a mechanism for routers to forward BOOTP requests
across subnets. It is a configuration option available on most routers. The router
configured to forward BOOTP requests is known as a BOOTP relay agent.
A router will normally discard any datagrams containing illegal source addresses,
such as 0.0.0.0, which is used by a BOOTP client. A router will also generally
discard datagrams with the limited broadcast destination address. However, a
BOOTP relay agent will accept such datagrams from BOOTP clients on port 67.
The process carried out by a BOOTP relay agent on receiving a
BOOTPREQUEST is as follows:
1. When the BOOTP relay agent receives a BOOTPREQUEST, it first checks
the hops field to check the number of hops already completed in order to
decide whether to forward the request. The threshold for the allowable
number of hops is normally configurable.
2. If the relay agent decides to relay the request, it checks the contents of the
router IP address field. If this field is zero, it fills this field with the IP address
of the interface on which the BOOTPREQUEST was received. If this field
already has an IP address of another relay agent, it is not touched.
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