Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools

Problem, Formatting or Query -  Send Feedback

Was this page helpful?-10+1


Network Working Group J. Ioannidis Request for Comments: 1235 G. Maguire, Jr.

                                                   Columbia University
                                        Department of Computer Science
                                                             June 1991
              The Coherent File Distribution Protocol

Status of this Memo

 This memo describes the Coherent File Distribution Protocol (CFDP).
 This is an Experimental Protocol for the Internet community.
 Discussion and suggestions for improvement are requested.  Please
 refer to the current edition of the "IAB Official Protocol Standards"
 for the standardization state and status of this protocol.
 Distribution of this memo is unlimited.


 The Coherent File Distribution Protocol (CFDP) has been designed to
 speed up one-to-many file transfer operations that exhibit traffic
 coherence on media with broadcast capability.  Examples of such
 coherent file transfers are identical diskless workstations booting
 simultaneously, software upgrades being distributed to more than one
 machines at a site, a certain "object" (bitmap, graph, plain text,
 etc.) that is being discussed in a real-time electronic conference or
 class being sent to all participants, and so on.
 In all these cases, we have a limited number of servers, usually only
 one, and <n> clients (where <n> can be large) that are being sent the
 same file.  If these files are sent via multiple one-to-one
 transfers, the load on both the server and the network is greatly
 increased, as the same data are sent <n> times.
 We propose a file distribution protocol that takes advantage of the
 broadcast nature of the communications medium (e.g., fiber, ethernet,
 packet radio) to drastically reduce the time needed for file transfer
 and the impact on the file server and the network.  While this
 protocol was developed to allow the simultaneous booting of diskless
 workstations over our experimental packet-radio network, it can be
 used in any situation where coherent transfers take place.
 CFDP was originally designed as a back-end protocol; a front-end
 interface (to convert file names and requests for them to file
 handles) is still needed, but a number of existing protocols can be
 adapted to use with CFDP.  Two such reference applications have been
 developed; one is for diskless booting of workstations, a simplified

Ioannidis & Maguire, Jr. [Page 1] RFC 1235 CFDP June 1991

 BOOTP [3] daemon (which we call sbootpd) and a simple, TFTP-like
 front end (which we call vtftp).  In addition, our CFDP server has
 been extended to provide this front-end interface.  We do not
 consider this front-end part of the CFDP protocol, however, we
 present it in this document to provide a complete example.
 The two clients and the CFDP server are available as reference
 implementations for anonymous ftp from the site CS.COLUMBIA.EDU
 ( in directory pub/cfdp/.  Also, a companion document
 ("BOOTP extensions to support CFDP") lists the "vendor extensions"
 for BOOTP (a-la RFC-1084 [4]) that apply here.


 CFDP is implemented as a protocol on top of UDP [5], but it can be
 implemented on top of any protocol that supports broadcast datagrams.
 Moreover, when IP multicast [6] implementations become more
 widespread, it would make more sense to use a multicast address to
 distribute CFDP packets, in order to reduce the overhead of non-
 participating machines.
 A CFDP client that wants to receive a file first contacts a server to
 acquire a "ticket" for the file in question.  This server could be a
 suitably modified BOOTP server, the equivalent of the tftpd daemon,
 etc. The server responds with a 32-bit ticket that will be used in
 the actual file transfers, the block size sent with each packet
 (which we shall call "BLKSZ" from now on), and the size (in bytes) of
 the file being transferred ("FILSZ").  BLKSZ should be a power of
 two.  A good value for BLKSZ is 512. This way the total packet size
 (IPheader+UDPheader+CFDPheader+data=20+8+12+512=552), is kept well
 under the magic number 576, the minimum MTU for IP networks [7].
 Note that this choice of BLKSZ supports transfers of files that are
 up to 32 Mbytes in size.  At this point, the client should allocate
 enough buffer space (in memory, or on disk) so that received packets
 can be placed directly where they belong, in a way similar to the
 NetBLT protocol [8].
 It is assumed that the CFDP server will also be informed about the
 ticket so that it can respond to requests.  This can be done, for
 example, by having the CFDP server and the ticket server keep the
 table of ticket-to-filename mappings in shared memory, or having the
 CFDP server listening on a socket for this information.  To reduce
 overhead, it is recommended that the CFDP server be the same process
 as the front-end (ticket) server.
 After the client has received the ticket for the file, it starts
 listening for (broadcast) packets with the same ticket, that may
 exist due to an in-progress transfer of the same file.  If it cannot

Ioannidis & Maguire, Jr. [Page 2] RFC 1235 CFDP June 1991

 detect any traffic, it sends to the CFDP server a request to start
 transmitting the whole file.  The server then sends the entire file
 in small, equal-sized packets consisting of the ticket, the packet
 sequence number, the actual length of data in this packet (equal to
 BLKSZ, except for the last packet in the transfer), a 32-bit
 checksum, and the BLKSZ bytes of data.  Upon receipt of each packet,
 the client checksums it, marks the corresponding block as received
 and places its contents in the appropriate place in the local file.
 If the client does not receive any packets within a timeout period,
 it sends to the CFDP server a request indicating which packets it has
 not yet received, and then goes back to the receiving mode.  This
 process is repeated until the client has received all blocks of the
 The CFDP server accepts requests for an entire file ("full" file
 requests, "FULREQ"s), or requests for a set of BLKSZ blocks
 ("partial" file requests, "PARREQ"s).  In the first case, the server
 subsequently broadcasts the entire file, whereas in the second it
 only broadcasts the blocks requested.  If a FULREQ or a PARREQ
 arrives while a transfer (of the same file) is in progress, the
 requests are ignored.  When the server has sent all the requested
 packets, it returns to its idle state.
 The CFDP server listens for requests on UDP/IP port "cfdpsrv". The
 clients accept packets on UDP/IP port "cfdpcln" (both to be defined
 by the site administrator), and this is the destination of the
 server's broadcasts.  Those two port numbers are sent to the client
 with the initial handshake packet, along with the ticket.  If the
 minimal ticket server is implemented as described later in this
 document, it is recommended (for interoperability reasons) that it
 listens for requests on UDP/IP port 120 ("cfdptkt").
 Let us now examine the protocol in more detail.

Protocol Specification

Initial Handshake (not strictly part of the protocol):

 The client must acquire a ticket for the file it wishes to transfer,
 and the CFDP server should be informed of the ticket/filename
 mapping.  Again, this can be done inside a BOOTP server, a modified
 TFTP server, etc., or it can be part of the CFDP server itself.  We
 present here a suggested protocol for this phase.
 The client sends a "Request Ticket" (REQTKT) request to the CFDP
 Ticket server, using UDP port "cfdptkt".  If the address of the
 server is unknown, the packet can be sent to the local broadcast
 address.  Figure 1 shows the format of this packet.

Ioannidis & Maguire, Jr. [Page 3] RFC 1235 CFDP June 1991

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |      'R'      |      'Q'      |      'T'      |      'K'      |
    |                                                               |
    /                                                               /
    \     Filename, null-terminated, up to 512 octets               \
    /                                                               /
    |                                                               |
                     Fig. 1: "ReQuest TicKet" packet.
 The filename is limited to 512 octets.  This should not cause a
 problem in most, if not all, cases.
 The ticket server replies with a "This is Your Ticket" (TIYT) packet
 containing the ticket.  Figure 2 shows the format of this packet.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |      'T'      |      'I'      |      'Y'      |      'T'      |
    |                           "ticket"                            |
    |                       BLKSZ (by default 512)                  |
    |                             FILSZ                             |
    |            IP address of CFDP server (network order)          |
    |   client UDP port# (cfdpcln)  |   server UDP port# (cfdpsrv)  |
                  Fig. 2: "This Is Your Ticket" packet.
 The reply is sent to the UDP port that the RQTK request came from.
 The IP address of the CFDP server is provided because the original
 handshake server is not necessarily on the same machine as the ticket
 server, let alone the same process.  Similarly, the cfdpcln and
 cfdpsrv port numbers (in network order) are communicated to the
 client.  If the client does not use this ticket server, but rather
 uses BOOTP or something else, that other server should be responsible
 for providing the values of cfdpcln and cfdpsrv.  The ticket server
 also communicates this ticket/filename/filesize to the real CFDP
 server.  It is recommended that the ticket requests be handled by the

Ioannidis & Maguire, Jr. [Page 4] RFC 1235 CFDP June 1991

 regular CFDP server, in which case informing the CFDP server of the
 ticket/filename binding is trivial (as it is internal to the
 Once the client has received the ticket for the filename it has
 requested, the file distribution can proceed.

Client Protocol:

 Once the ticket has been established, the client starts listening for
 broadcast packets on the cfdpcln/udp port that have the same "ticket"
 as the one it is interested in.  In the state diagram below, the
 client is in the CLSTART state.  If the client can detect no packets
 with that ticket within a specified timeout period, "TOUT-1", it
 assumes that no transfer is in progress.  It then sends a FULREQ
 packet (see discussion above) to the CFDP server, asking it to start
 transmitting the file, and goes back to the CLSTART state (so that it
 can time out again if the FULREQ packet is lost).  Figure 3 shows the
 format of the FULREQ packet.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                           "ticket"                            |
    |                           checksum                            |
    |      'F'      |       0       |         length == 0           |
                Fig. 3: FULREQ (FULl file REQuest) packet.
 When the first packet arrives, the client moves to the RXING state
 and starts processing packets.  Figure 4 shows the format of a data

Ioannidis & Maguire, Jr. [Page 5] RFC 1235 CFDP June 1991

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                           "ticket"                            |
    |                           checksum                            |
    |          block number         |          data length          |
    |                                                               |
    /                                                               /
    \      up to BLKSZ octets of data                               \
    /                                                               /
    |                                                               |
                           Fig. 4: Data Packet
 The format is self-explanatory.  "Block number" the offset (in
 multiples of BLKSZ) from the beginning of the file, data length is
 always BLKSZ except for the very last packet, where it can be less
 than that, and the rest is data.
 As each packet arrives, the client verifies the checksum and places
 the data in the appropriate position in the file.  While the file is
 incomplete and packets keep arriving, the client stays in the RXING
 state, processing them.  If the client does not receive any packets
 within a specified period of time, "TOUT-2", it times out and moves
 to the INCMPLT state.  There, it determines which packets have not
 yet been received and transmits a PARREQ request to the server.  This
 request consists of as many block numbers as will fit in the data
 area of a data packet.  If one such request is not enough to request
 all missing packets, more will be requested when the server has
 finished sending this batch and the client times out.  Also, if the
 client has sent a PARREQ and has not received any data packets within
 a timeout period, "TOUT-3", it retransmits the same PARREQ.  Figure 5
 shows the format of the PARtial REQuest packet.

Ioannidis & Maguire, Jr. [Page 6] RFC 1235 CFDP June 1991

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |                           "ticket"                            |
    |                           checksum                            |
    |      'P'      |       0       |      data length (2*N)        |
    |           Block #0            |           Block #1            |
    |           Block #2            |           Block #3            |
    /                                                               /
    \      data  (block numbers requested)                          \
    /                                                               /
    |           Block #N-2          |           Block #N-1          |
              Fig. 5: PARREQ (PARtial file REQuest) packet.
 When all packets have been received the client enters the CLEND state
 and stops listening.
 Figure 6 summarizes the client's operations in a state diagram.

Ioannidis & Maguire, Jr. [Page 7] RFC 1235 CFDP June 1991

                         |  CLSTART  |
                         |           | <---.
                         |   send    |     | timeout TOUT-1
                         |  FULREQ   | ----'
                         |           |
           received packet     | received packet
    .-----------------------.  |
    |                       V  V
   +---------+             +---------+
   | INCMPLT |             |  RXING  |
   |         |   timeout   |         | <---.
   |  send   |<------------| process |     | received packet
   | PARREQ  |    TOUT-2   | packet  | ----'
   |         |             |         |
   +---------+             +---------+
      ^   |                     |
      |   |                     |finished
      `---'                     |
     timeout                    V
      TOUT-3               +---------+
                           |  CLEND  |
              Fig. 6: Client State Transition Diagram

Server Protocol:

 As described above, the CFDP server accepts two kinds of requests: a
 request for a full file transfer, "FULREQ", and a request for a
 partial (some blocks only) file transfer, "PARREQ".  For the first,
 it is instructed to start sending out the contents of a file.  For
 the second, it will only send out the requested blocks.  The server
 should know at all times which files correspond to which "tickets",
 and handle them appropriately.  Note that this may run into
 implementation limits on some Unix systems (e.g., on older systems, a
 process could only have 20 files open at any one time), but that
 should not normally pose a problem.
 The server is initially in the SIDLE state, idling (see diagram
 below).  When it receives a FULREQ packet, it goes to the FULSND
 state, whence it broadcasts the entire contents of the file whose
 ticket was specified in the FULREQ packet.  When it is done, it goes
 back to the SIDLE state. When it receives a PARREQ packet, it goes to
 the PARSND state and broadcasts the blocks specified in the PARREQ
 packet. When it has finished processing the block request, it goes

Ioannidis & Maguire, Jr. [Page 8] RFC 1235 CFDP June 1991

 once again back to the SIDLE state.
                   receive    +-------+    receive
              .---------------| SIDLE |---------------.
              |    FULREQ     +-------+     PARREQ    |
              |                 ^   ^                 |
              |                 |   |                 |
              V                 |   |                 V
          +--------+            |   |            +--------+
          | FULSND |            |   |            | PARSND |
          |        |    done    |   |    done    |        |
          |  send  |------------'   `------------|  send  |
          | entire |                             | req'ed |
          |  file  |                             | blocks |
          +--------+                             +--------+
              Fig. 7: Server State Transition Diagram

Packet Formats

 The structure of the packets has been already described.  In all
 packet formats, numbers are assumed to be in network order ("big-
 endian"), including the ticket and the checksum.
 The checksum is the two's complement of the unsigned 32-bit sum with
 no end-around-carry (to facilitate implementation) of the rest of the
 packet.  Thus, to compute the checksum, the sender sets that field to
 zero and adds the contents of the packet including the header.  The
 it takes the two's complement of that sum and uses it as the
 checksum.  Similarly, the receiver just adds the entire contents of
 the packet, ignoring overflows, and the result should be zero.

Tuneable Parameters: Packet Size, Delays and Timeouts

 It is recommended that the packet size be less than the minimum MTU
 on the connected network where the file transfers are taking place.
 We want this so that there be no fragmentation; one UDP packet should
 correspond to one hardware packet.  It is further recommended that
 the packet size be a power of two, so that offsets into the file can
 be computed from the block number by a simple logical shift
 operation.  Also, it is usually the case that page-aligned transfers
 are faster on machines with a paged address space.  Small packet
 sizes are inefficient, since the header will be a larger fraction of
 the packet, and packets larger than the MTU will be fragmented.  A
 good selection for BLKSZ is 512 or 1024. Using that BLKSZ, one can
 transfer files up to 32MB or 64MB respectively (since the limit is
 the 16-bit packet sequence number).  This is adequate for all but
 copying complete disks, and it allows twice as many packets to be

Ioannidis & Maguire, Jr. [Page 9] RFC 1235 CFDP June 1991

 requested in a PARREQ request than if the sequence number were 32
 bits.  If larger files must be transferred, they could be treated as
 multiple logical files, each with a size of 32MB (or 64MB).
 Since most UDP/IP implementations do not buffer enough UDP datagrams,
 the server should not transmit packets faster than its clients can
 consume them.  Since this is a one-to-many transfer, it is not
 desirable to use flow-control to ensure that the server does not
 overrun the clients.  Rather, we insert a small delay between packets
 transmitted.  A good estimate of the proper delay between two
 successive packets is twice the amount of time it takes for the
 interface to transmit a packet.  On Unix implementations, the ping
 program can be used to provide an estimate of this, by specifying the
 same packet length on the command line as the expected CFDP packet
 length (usually 524 bytes).
 The timeouts for the client are harder to compute. While there is a
 provision for the three timeouts (TOUT-1, TOUT-2 and TOUT-3) to be
 different, there is no compelling reason not to make them the same.
 Experimentally, we have determined that a timeout of 6-8 times the
 transfer time for a packet works best.  A timeout of less than that
 runs the risk of mistaking a transient network problem for a timeout,
 and more than that delays the transfer too much.


 To summarize, here is the timeline of a sample file distribution
 using CFDP to three clients.  Here we request a file with eight
 blocks.  States are capitalized, requests are preceded with a '<'
 sign, replies are followed by a '>' sign, block numbers are preceded
 with a '#' sign, and actions are in parentheses:


IDLE everybody idle

           CLSTART                                CL1 wants a file
           <TKRQ                                  requests ticket

TIYT> server replies

           (timeout)                              listens for traffic
           <FULREQ                                full request

#0 RXING CL1 starts receiving

           (rx 0)

#1 (rx 1) CLSTART CL2 decides to join


#2 (rx 2) SRV still sending TIYT> responds to TKRQ #3 (rx 3) (listens) CL2 listens

                       RXING                      found traffic

Ioannidis & Maguire, Jr. [Page 10] RFC 1235 CFDP June 1991

#4 (rx 4) (rx 4) CLSTART CL3 joins in


#5 (missed) (rx 5) CL1 missed a packet TIYT> (listens) #6 (rx 6) (rx 6) RXING CL3 found traffic

#7 (rx 7) (rx 7) (rx 7) Server finished IDLE

           (wait)      (wait)        (wait)       CL1 managed to
           (timeout)   (wait)        (wait)       timeout
           <PARREQ[5]  (timeout)     (timeout)    CL1 blockrequests...

#5 (rx 5) <PARREQ[0123] <PARREQ[0123456] ignored by SRV

           CLEND                                  CL1 has all packets

IDLE (wait) (wait) CL2+3 missed #5

                       (timeout)     (timeout)
                       <PARREQ[0123] <PARREQ[0123456] CL2's req gets

#0 (rx 0) (rx 0) through, CL3 ignored #1 (rx 1) (rx 1) moving along #2 (rx 2) (rx 2) #3 (rx 3) (rx 3) IDLE CLEND (wait) CL2 finished


#4 (rx 4) #5 (rx 5) #5 (rx 6) IDLE CLEND CL3 finished


 [1] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, MIT, June
 [2] Finlayson, R., "Bootstrap Loading Using TFTP", RFC 906, Stanford,
     June 1984.
 [3] Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC 951,
     Stanford and SUN Microsystems, September 1985.
 [4] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1084,
     USC/Information Sciences Institute, December 1988.
 [5] Postel, J., "User Datagram Protocol", RFC 768, USC/Information
     Sciences Institute, August 1980.
 [6] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
     Stanford University, August 1989.

Ioannidis & Maguire, Jr. [Page 11] RFC 1235 CFDP June 1991

 [7] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
     Specification", RFC 791, DARPA, September 1981.
 [8] Clark, D., Lambert, M., and L. Zhang, "NETBLT: A Bulk Data
     Transfer Protocol", RFC 998, MIT, March 1987.

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 John Ioannidis
 Columbia University
 Department of Computer Science
 450 Computer Science
 New York, NY 10027
 Gerald Q. Maguire, Jr.
 Columbia University
 Department of Computer Science
 450 Computer Science
 New York, NY 10027
 Phone:  (212) 854-2736

Ioannidis & Maguire, Jr. [Page 12]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1235.txt · Last modified: 1991/06/20 22:54 (external edit)