Network Working Group
Request for Comments: 2090
Category: Experimental
A. Emberson
Lanworks Technologies Inc.
February 1997

TFTP Multicast Option

Status of this Memo

This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited.


The Trivial File Transfer Protocol [1] is a simple, lock-step, file transfer protocol which allows a client to get or put a file onto a remote host.

This document describes a new TFTP option. This new option will allow the multiple clients to receive the same file concurrently through the use of Multicast packets. The TFTP Option Extension mechanism is described in [2].

   Often when similar computers are booting remotely they will each
   download the same image file. By adding multicast into the TFTP
   option  set,  two  or  more  computers  can  download  a  file
   concurrently, thus increasing network efficiency.

This document assumes that the reader is familiar with the terminology and notation of both [1] and [2].

Multicast Option Specification

The TFTP Read Request packet is modified to include the multicast option as follows:

      |  opc=1 | filename | 0 | mode | 0 | multicast | 0 | 0 |


The opcode field contains a 1, for Read Requests, as defined in [1].


The name of the file to be read, as defined in [1]. This is a NULL-terminated field.


The mode of the file transfer: "netascii", "octet", or "mail", as defined in [1]. This is a NULL-terminated field.


      Request  for  multicast  transmission  of  the  file  option,
      "multicast" (case insensitive). This is a NULL-terminated
      field. The value for this option request is a string of zero

If the server is willing to accept the multicast option, it sends an Option Acknowledgment (OACK) to the client including the multicast option, as defined in [2]. The OACK to the client will specify the multicast address and flag to indicate whether that client should send block acknowledgments (ACK).

     |  opc  | multicast | 0 | addr, port, mc | 0 |


      The  opcode  field  contains  the  number  6,  for  Option
      Acknowledgment, as defined in [2].


Acknowledges the multicast option. This is a NULL-terminated field.


The addr field contains the multicast IP address. This field is terminated with a comma.


The port field contains the destination port of the multicast packets. The use of Registered Port number 1758 (tftp-mcast) is recommended. This field is terminated with a comma.


This field will be either 0 or 1, to tell the client whether it is the master client, that is, it is responsible for sending ACKs to the server. This is NULL-terminated field.

Data Transfer

After the OACK is received by the client it will send an ACK for packet zero, as in [2]. With the multicast option being accepted this ACK will indicate to the server that the client wants the first packet. In other words the ACKs may now be seen as a request for the n+1th block of data. This enables each a client to request any block within the file that it may be missing.

To manage the data transfer the server will maintain a list of clients. Typically the oldest client on the list, from here on referred to as the Master Client, will be responsible for sending ACKs. When the master client is finished, the server will send another OACK to the next oldest client, telling it to start sending ACKs. Upon receipt of this OACK the new master client will send an ACK for the block immediately before the first block required to complete its download.

   Any subsequent clients can start receiving blocks of a file during a
   transfer and then request any missing blocks when that client becomes
   the master client. When the current master client is finished, the
   server will notify the next client with an OACK making it the new
   master client. The new master client can start requesting  missed
   packets.  Each  client  must  terminate  the transfer by sending an
   acknowledgment of the last packet or by sending an error message to
   server. This termination can occur even if the client is not the
   master client.

Any subsequent OACKs to a client may have an empty multicast address and port fields, since this information will already be held by that client. In the event a client fails to respond in a timely manner to a OACK enabling it as the master client, the server shall select the next oldest client to be the master client. The server shall reattempt to send a OACK to the non- responding client when the new master client is finished. The server may cease communication with a client after a reasonable number of attempts.

Each transfer will be given a multicast address for use to distribute the data packets. Since there can be multiple servers on a given network or a limited number of addresses available to a given server, it is possible that their might be more than one transfer using a multicast address. To ensure that a client only accepts the correct packets, each transfer must use a unique port on the server. The source IP address and port number will identify the data packets for the transfer. Thus the server must send the unicast OACK packet to the client using the same port as will be used for sending the multicast data packets.

At any point if a client, other than the master client, sends a ACK to the server, the server will respond with another OACK with the mc field holding a value of zero. If this client persists in sending erroneous ACKs, the server may send an error packet to the client, discontinuing the file transfer for that client.

The server may also send unicast packets to a lone client to reduce adverse effects on other machines. As it is possible that machines may be forced to process many extraneous multicast packets when attempting to receive a single multicast address.


           clients                                      server  message
    1  C1  |1|afile|0|octet|0|multicast|0|0| ->                  RRQ
    2                C1 <- |6|multicast|,1758,1|  OACK
    3  C1  |4|0| ->                                              ACK
    4                          M <- |3|1|1| 512 octets of data|  DATA
    5  C1  |4|1| ->                                              ACK
    6                          M <- |3|2|1| 512 octets of data|  DATA
    7  C2  |1|afile|0|octet|0|multicast|0|0| ->                  RRQ
    8                C2 <- |6|multicast|,1758,0|  OACK
    9  C2  |4|0| ->                                              ACK
   10  C1  |4|2| ->                                              ACK
   11                          M <- |3|3|1| 512 octets of data|  DATA
   12  C3  |1|afile|0|octet|0|multicast|0|0| ->                  RRQ
   13                C3 <- |6|multicast|,1758,0|  OACK
   14  C1  |4|3| ->                                              ACK
   15  C2  |4|0| ->                                              ACK
   16              M (except C2) <- |3|4|1| 512 octets of data|  DATA
   17  C1  |4|4| ->                                              ACK
   18                          M <- |3|5|1| 512 octets of data|  DATA
   19  C1  |4|5| ->                                              ACK
   20                          M <- |3|6|1| 100 octets of data|  DATA
   21  C1  |4|6| ->                                              ACK
   22                                   C2 <- |6|multicast|,,1|  OACK
   23  C2  |4|0| ->                                              ACK
   24                          M <- |3|1|1| 512 octets of data|  DATA
   25  C2  |4|1| ->                                              ACK
   26                          M <- |3|2|1| 512 octets of data|  DATA
   27  C2  |4|3| ->                                              ACK
   28                          M <- |3|4|1| 512 octets of data|  DATA
   29  C2  |4|6| ->                                              ACK
   30                                   C3 <- |6|multicast|,,1|  OACK
   31  C3  |4|2| ->                                              ACK
   32                          M <- |3|3|1| 512 octets of data|  DATA
   33  C3  |4|6| ->                                              ACK


         1  request from client 1
         2  option acknowledgment
         3  acknowledgment for option acknowledgment,
            or request for first block of data
         4  first data packet sent to the multicast address
         7  request from client 2
         8  option acknowledgment to client 2,
            send no acknowledgments
         9  OACK acknowledgment from client 2
         15 OACK acknowledgment from client 3
         16 client 2 fails to receive a packet
         21 client 1 acknowledges receipt of the last block,
            telling the server it is done
         23 option acknowledgment to client 2,
            now the master client
         25 client 2 acknowledging with request for first block
         27 client 2 acknowledges with request for missed block
         29 client 2 signals it is finished
         31 client 3 is master client and asks for missing blocks
         33 client 3 signals it is finished


With the use of the multicast and blocksize[3] options TFTP will be capable of fast and efficient downloads of data. Using TFTP with the multicast option will maintain backward compatibility for both clients and servers.

Security Considerations

Security issues are not discussed in this memo.


   [1] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC
       1350, MIT, July 1992.
   [2] Malkin, G., and A. Harkin, "TFTP Option Extension", RFC
       1782, Xylogics, Inc., Hewlett Packard Co., March 1995.
   [3] Malkin, G., and A. Harkin, "TFTP Blocksize Option", RFC
       1783, Xylogics, Inc., Hewlett Packard Co., March 1995.

Author's Address

A. Thomas Emberson
Lanworks Technologies, Inc.
2425 Skymark Avenue
Mississauga, Ontario
Canada L4W 4Y6

   Phone: (905) 238-5528