RFC 906 (rfc906) - Page 2 of 4

Bootstrap loading using TFTP

RFC 906                                                        June 1984


   presuppose the use of any particular data-link level network
   architecture (although the example that we describe below uses
   Ethernet).

Network Protocols used by the Booting System

   To load a file, the booter sends a standard TFTP read request (RRQ)
   packet, containing the name of the file to be loaded.  The file name
   should not assume any operating system dependent naming conventions
   (file names containing only alphanumeric characters should suffice).
   Thereafter, the system receives TFTP DATA packets, and sends TFTP ACK
   and/or ERROR packets, in accordance with the TFTP specification [6].

   TFTP is implemented using the User Datagram Protocol (UDP) [5], which
   is in turn implemented using IP.  Thus, the booter must be able to
   receive IP datagrams containing up to 524 octets (excluding the IP
   header), since TFTP DATA packets can be up to 516 octets long, and
   UDP headers are 8 octets long.  The booting machine is not required
   to respond to incoming TFTP read or write requests.

   We allow for the use of two additional protocols.  These are ARP
   (Address Resolution Protocol) [3], and RARP (Reverse Address
   Resolution Protocol) [1]. The possible use of these protocols is
   described below.  The booter could also use other protocols (such as
   for name lookup), but they should be IP-based, and an internet
   standard.

   The IP datagram containing the initial TFTP RRQ (and all other IP
   datagrams sent by the booter) must of course contain both a source
   internet address and a destination internet address in its IP header.
   It is frequently the case, however, that the booter does not
   initially know its own internet address, but only a lower-level (e.g.
   Ethernet) address.  The Reverse Address Resolution Protocol
   (RARP) [1] may be used by the booter to find its internet address
   (prior to sending the TFTP RRQ).  RARP was motivated by Plummer's
   Address Resolution Protocol (ARP) [3].  Unlike ARP, which is used to
   find the 'hardware' address corresponding to a known higher-level
   protocol (e.g. internet) address, RARP is used to determine a
   higher-level protocol address, given a known hardware address.  RARP
   uses the same packet format as ARP, and like ARP, can be used for a
   wide variety of data-link protocols.

   ARP may also be used.  If the destination internet address is known,
   then an ARP request containing this address may be broadcast, to find
   a corresponding hardware address to which to send the subsequent TFTP
   RRQ.  It may not matter if this request should fail, because the RRQ
   can also be broadcast (at the data-link level).  However, because
   such an ARP request packet also contains the sender's (that is, the


Finlayson