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 The opcode field contains a 1, for Read Requests, as defined in [1].
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RFC 2090 TFTP Multicast Option February 1997
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 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.
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RFC 2090 TFTP Multicast Option February 1997
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.
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RFC 2090 TFTP Multicast Option February 1997
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
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RFC 2090 TFTP Multicast Option February 1997
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.
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RFC 2090 TFTP Multicast Option February 1997
Author's Address
A. Thomas Emberson Lanworks Technologies, Inc. 2425 Skymark Avenue Mississauga, Ontario Canada L4W 4Y6