RFC2090 - TFTP Multicast Option

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　　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

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