TCP/IP Tutorial and Technical Overview - IBM Redbooks

3.5.1 RARP concept
The reverse address resolution is performed the same way as the ARP address
resolution. The same packet format (see Figure 3-39 on page 121) is used as for
ARP.
An exception is the operation code field that now takes the following values:
3 For the RARP request
4 For the RARP reply
And of course, the physical header of the frame will now indicate RARP as the
higher-level protocol (8035 hex) instead of ARP (0806 hex) or IP (0800 hex) in
the EtherType field.
Some differences arise from the concept of RARP itself:
► ARP only assumes that every host knows the mapping between its own
hardware address and protocol address. RARP requires one or more server
hosts in the network to maintain a database of mappings between hardware
addresses and protocol addresses so that they will be able to reply to
requests from client hosts.
► Due to the size this database can take, part of the server function is usually
implemented outside the adapter's microcode, with optionally a small cache in
the microcode. The microcode part is then only responsible for reception and
transmission of the RARP frames, the RARP mapping itself being taken care
of by server software running as a normal process on the host machine.
► The nature of this database also requires some software to create and update
the database manually.
► If there are multiple RARP servers in the network, the RARP requester only
uses the first RARP reply received on its broadcast RARP request and
discards the others.
3.6 Bootstrap Protocol (BOOTP)
The Bootstrap Protocol (BOOTP) enables a client workstation to initialize with a
minimal IP stack and request its IP address, a gateway address, and the address
of a name server from a BOOTP server. If BOOTP is to be used in your network,
the server and client are usually on the same physical LAN segment. BOOTP
can only be used across bridged segments when source-routing bridges are
being used, or across subnets, if you have a router capable of BOOTP
forwarding.
Chapter 3. Internetworking protocols 125