TFTP bootstrap: Bootstrap loading using TFTP

Network Working Group                                     Ross Finlayson
Request for Comments: 906                            Stanford University
                                                               June 1984

                      Bootstrap Loading using TFTP


Status of this Memo

   It is often convenient to be able to bootstrap a computer system from
   a communications network.  This RFC proposes the use of the IP TFTP
   protocol for bootstrap loading in this case.

   This RFC specifies a proposed protocol for the ARPA Internet
   community, and requests discussion and suggestions for improvements.

Introduction

   Many computer systems, such as diskless workstations, are
   bootstrapped by loading one or more code files across a network.
   Unfortunately, the protocol used to load these initial files has not
   been standardized - numerous methods have been employed by different
   computer manufacturers. This can make it difficult, for example, for
   an installation to support several different kinds of systems on a
   local-area network.  Each different booting mechanism that is used
   must be supported, for example by implementing a number of servers on
   one or more host machines.  This is in spite of the fact that these
   heterogeneous systems may be able to communicate freely (all using
   the same protocol) once they have been booted.

   We propose that TFTP (Trivial File Transfer Protocol) [6] be used as
   a standard protocol for bootstrap loading.  This protocol is
   well-suited for our purpose, being based on the standard Internet
   Protocol (IP) [4].  It is easily implemented, both in the machines to
   be booted, and in bootstrap servers elsewhere on the net.  (In
   addition, many popular operating systems already support TFTP
   servers.)  The fact that TFTP is a rather slow protocol is not a
   serious concern, due to the fact that it need be used only for the
   primary bootstrap.  A secondary bootstrap could use a faster
   protocol.

   This RFC describes how system to be booted (called the "booter"
   below) would use TFTP to load a desired code file.  It also describes
   an existing implementation (in ROM) for Ethernet.

   Note that we are specifying only the network protocols that would be
   used by the booting system.  We do not attempt to mandate the method
   by which a user actually boots a system (such as the format of a
   command typed at the console).  In addition, our proposal does not
   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
   booter's) internet and hardware addresses, this information is made
   available to the rest of the local subnet, and could be useful for
   routing, for instance.

   If a single destination internet address is not known, then a special
   'broadcast' internet address could be used as the destination address
   in the TFTP RRQ, so that it will be received by all 'local' internet
   hosts.  (At this time, however, no standard for internet broadcasting
   has been officially adopted. [**])

An Example Implementation

   The author has implemented TFTP booting as specified above.  The
   resulting code resides in ROM.  (This implementation is for a
   Motorola 68000 based workstation, booting over an Ethernet.)  A user
   wishing to boot such a machine types a file name, and (optionally)
   the internet address of the workstation, and/or the internet address
   of a server machine from which the file is to be loaded.  The
   bootstrap code proceeds as follows:

      (1) The workstation's Ethernet address is found (by querying the
      Ethernet interface).

      (2) If the internet address of the workstation was not given, then
      a RARP request is broadcast, in order to find it.  If this request
      fails (that is, times out), then the bootstrap fails.

      (3) If the internet address of a server host was given, then
      broadcast an ARP request to try to find a corresponding Ethernet
      address.  If this fails, or if a server internet address was not
      given, then the Ethernet broadcast address is used.

      (4) If the internet address of a server host was not given, then
      we use a special internet address that represents a broadcast on
      the "local subnet", as described in [2].  (This is not an internet
      standard.)

      (5) A TFTP RRQ for the requested file is sent to the Ethernet
      address found in step (3).  The source internet address is that
      found in step (2), and the destination internet address is that
      found in step (4).

   Note that because several TFTP servers may, in general, reply to the
   RRQ, we do not abort if a TFTP ERROR packet is received, because this
   does not preclude the possibility of some other server replying later
   with the first data packet of the requested file.  When the first
   valid TFTP DATA packet is received in response to the RRQ, the source
   internet and Ethernet addresses of this packet are used as the
   destination addresses in subsequent TFTP ACK packets.  Should another
   server later respond with a DATA packet, an ERROR packet is sent back
   in response.

   An implementation of TFTP booting can take up a lot of space if care
   is not taken.  This can be a significant problem if the code is to
   fit in a limited amount of ROM.  However, the implementation
   described above consists of less than 4K bytes of code (not counting
   the Ethernet device driver).

Acknowledgements

   The ideas presented here are the result of discussions with several
   other people, in particular Jeff Mogul.

References

   [1]  Finlayson, R.,  Mann, T.,  Mogul, J.  & Theimer, M.,  "A Reverse
        Address Resolution Protocol", RFC 903  Stanford University,
        June 1984.

   [2]  Mogul, J., "Internet Broadcasting",  Proposed RFC, January 1984.

   [3]  Plummer, D., "An Ethernet Address Resolution Protocol",
        RFC 826,  MIT-LCS, November 1982.

   [4]  Postel, J., ed., "Internet Protocol - DARPA Internet Program
        Protocol Specification", RFC 791, USC/Information Sciences
        Institute, September 1981.

   [5]  Postel, J., "User Datagram Protocol", RFC 768 USC/Information
        Sciences Institute, August 1980.

   [6]  Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, MIT/LCS,
        June 1981.





   [**]  Editor's Note:  While there is no standard for an Internet wide
        broadcast or multicast address, it is strongly recommended that
        the "all ones" local part of the Internet address be used to
        indicate a broadcast in a particular network.  That is, in class
        A network 1 the broadcast address would be 1.255.255.255, in
        class B network 128.1 the broadcast address would be
        128.1.255.255, and in class C network 192.1.1 the broadcast
        address would be 192.1.1.255.