rfc2090 - TFTP Multicast Option
Description: TFTP Multicast Option
Date: February 1997

Internet RFC/STD/FYI/BCP Archives

# RFC2090

[ Index | Search | What's New | Comments | Help ]

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

    Abstract

       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 |
          +--------+----~~----+---+--~~--+---+-----------+---+---+

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

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

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

       multicast
          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
          length.

       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 |
         +-------+-----------+---+-------~~-------+---+

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

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

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

       port
          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.

       mc
          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.

    Example

               clients                                      server  message
               ------------------------------------------------------------
        1  C1  |1|afile|0|octet|0|multicast|0|0| ->                  RRQ
        2                C1 <- |6|multicast|224.100.100.100,1758,1|  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|224.100.100.100,1758,0|  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|224.100.100.100,1758,0|  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

          Comments:
             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

    Conclusion

       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.

    References

       [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
       EMail: tom@lanworks.com

[ Index | Search | What's New | Comments | Help ]