RFC 1235 (rfc1235) - Page 2 of 12
Coherent File Distribution Protocol
Alternative Format: Original Text Document
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
(128.59.16.20) 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.
Overview
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.