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Malkin / Xylogics, Inc. 2 TFTP Option Extension A. Harkin / Hewlett-Packard Co. 3 Updates: RFC 1350 (STD 33) December 1994 5 TFTP Option Extension 7 Abstract 9 The Trivial File Transfer Protocol [1] is a simple, lock-step, file 10 transfer protocol which allows a client to get or put a file onto a 11 remote host. This document describes a simple extension to TFTP to 12 allow option negotiation prior to the file transfer. 14 Status of this Memo 16 This document is an Internet-Draft. Internet-Drafts are working 17 documents of the Internet Engineering Task Force (IETF), its areas, 18 and its working groups. Note that other groups may also distribute 19 working documents as Internet-Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 To learn the current status of any Internet-Draft, please check the 27 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 28 Directories on ds.internic.net (US East Coast), nic.nordu.net 29 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific 30 Rim). 32 Introduction 34 The option negotiation mechanism proposed in this document is a 35 backward-compatible extension to the TFTP protocol. It allows file 36 transfer options to be negotiated prior to the transfer using a 37 mechanism which is consistent with TFTPs Request Packet format. The 38 mechanism is kept simple by enforcing a request-respond-acknowledge 39 sequence, similar to the lock-step approach taken by TFTP itself. 41 While the option negotiation mechanism is general purpose, in that 42 many types of options may be negotiated, it was created to support 43 the Blocksize option defined in [2]. Additional options are defined 44 in [3]. 46 Packet Formats 48 TFTP options are appended to the Read Request and Write Request 49 packets. A new type of TFTP packet, the Option Acknowledgment 50 (OACK), is used to acknowledge a client's option negotiation request. 51 A new error code, 8, is hereby defined to indicate that a transfer 52 should be terminated due to option negotiation. 54 Options are appended to a TFTP Read Request or Write Request packet 55 as follows: 57 +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+--> 58 | opc |filename| 0 | mode | 0 | opt1 | 0 | value1 | 0 | < 59 +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+--> 61 >-------+---+---~~---+---+ 62 < optN | 0 | valueN | 0 | 63 >-------+---+---~~---+---+ 65 opc 66 The opcode field contains either a 1, for Read Requests, or 2, 67 for Write Requests, as defined in [1]. 69 filename 70 The name of the file to be read or written, as defined in [1]. 71 This is a NULL-terminated field. 73 mode 74 The mode of the file transfer: "netascii", "octet", or "mail", 75 as defined in [1]. This is a NULL-terminated field. 77 opt1 78 The first option, in case-insensitive ASCII (e.g., blksize). 79 This is a NULL-terminated field. 81 value1 82 The value associated with the first option, in case-insensitive 83 ASCII. This is a NULL-terminated field. 85 optN, valueN 86 The final option/value pair. Each NULL-terminated field is 87 specified in case-insensitive ASCII. 89 The options and values are all NULL-terminated, in keeping with the 90 original request format. If multiple options are to be negotiated, 91 they are appended to each other. The order in which options are 92 specified is not significant. The maximum size of a request packet 93 is 512 octets. 95 The OACK packet has the following format: 97 +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+ 98 | opc | opt1 | 0 | value1 | 0 | optN | 0 | valueN | 0 | 99 +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+ 101 opc 102 The opcode field contains a 6, for Option Acknowledgment. 104 opt1 105 The first option acknowledgment, copied from the original 106 request. 108 value1 109 The acknowledged value associated with the first option. If 110 and how this value may differ from the original request is 111 detailed in the specification for the option. 113 optN, valueN 114 The final option/value acknowledgment pair. 116 Negotiation Protocol 118 The client appends options at the end of the Read Request or Write 119 request packet, as shown above. Any number of options may be 120 specified; however, an option may only be specified once. The order 121 of the options is not significant. 123 If the server supports option negotiation, and it recognizes one or 124 more of the options specified in the request packet, the server may 125 respond with an Options Acknowledgment (OACK). Each option the 126 server recognizes, and accepts the value for, is included in the 127 OACK. Some options may allow alternate values to be proposed, but 128 this is an option specific feature. The server must not include in 129 the OACK any option which had not been specifically requested by the 130 client; that is, only the client may initiate option negotiation. 131 Options which the server does not support should be omitted from the 132 OACK; they should not cause an ERROR packet to be generated. If the 133 value of a supported option is invalid, the specification for that 134 option will indicate whether the server should simply omit the option 135 from the OACK, respond with an alternate value, or send an ERROR 136 packet, with error code 8, to terminate the transfer. 138 An option not acknowledged by the server must be ignored by the 139 client and server as if it were never requested. If multiple options 140 were requested, the client must use those options which were 141 acknowledged by the server and must not use those options which were 142 not acknowledged by the server. 144 When the client appends options to the end of a Read Request packet, 145 three possible responses may be returned by the server: 147 OACK - acknowledge of Read Request and the options; 149 DATA - acknowledge of Read Request, but not the options; 151 ERROR - the request has been denied. 153 When the client appends options to the end of a Write Request packet, 154 three possible responses may be returned by the server: 156 OACK - acknowledge of Write Request and the options; 158 ACK - acknowledge of Write Request, but not the options; 160 ERROR - the request has been denied. 162 If a server implementation does not support option negotiation, it 163 will likely ignore any options appended to the client's request. In 164 this case, the server will return a DATA packet for a Read Request 165 and an ACK packet for a Write Request establishing normal TFTP data 166 transfer. In the event that a server returns an error for a request 167 which carries an option, the client may attempt to repeat the request 168 without appending any options. This implementation option would 169 handle servers which consider extraneous data in the request packet 170 to be erroneous. 172 Depending on the original transfer request there are two ways for a 173 client to confirm acceptance of a server's OACK. If the transfer was 174 initiated with a Read Request, then an ACK (with the data block 175 number set to 0) is sent by the client to confirm the values in the 176 server's OACK packet. If the transfer was initiated with a Write 177 Request, then the client begins the transfer with the first DATA 178 packet, using the negotiated values. If the client rejects the OACK, 179 then it sends an ERROR packet, with error code 8, to the server and 180 the transfer is terminated. 182 Once a client acknowledges an OACK, with an appropriate non-error 183 response, that client has agreed to use only the options and values 184 returned by the server. Remember that the server cannot request an 185 option; it can only respond to them. If the client receives an OACK 186 containing an unrequested option, it should respond with an ERROR 187 packet, with error code 8, and terminate the transfer. 189 Examples 191 Read Request 193 client server 194 ------------------------------------------------------- 195 |1|foofile|0|octet|0|blksize|0|1432|0| --> RRQ 196 <-- |6|blksize|0|1432|0| OACK 197 |4|0| --> ACK 198 <-- |3|1| 1432 octets of data | DATA 199 |4|1| --> ACK 200 <-- |3|2| 1432 octets of data | DATA 201 |4|2| --> ACK 202 <-- |3|3|<1432 octets of data | DATA 203 |4|3| --> ACK 205 Write Request 207 client server 208 ------------------------------------------------------- 209 |2|barfile|0|octet|0|blksize|0|2048|0| --> RRQ 210 <-- |6|blksize|0|2048|0| OACK 211 |3|1| 2048 octets of data | --> DATA 212 <-- |4|1| ACK 213 |3|2| 2048 octets of data | --> DATA 214 <-- |4|2| ACK 215 |3|3|<2048 octets of data | --> DATA 216 <-- |4|3| ACK 218 Security Considerations 220 Security issues are not discussed in this memo. 222 References 224 [1] Sollins, K., "The TFTP Protocol (Revision 2)", Request for 225 Comments 1350 (STD 33), October 1992. 227 [2] Malkin, G., Harkin, A., "TFTP Blocksize Option", Internet Draft, 228 draft-ietf-tftpexts-blksize-opt-01.txt, September 1994. 230 [3] Malkin, G., Harkin, A., "TFTP Timeout Interval and Transfer Size 231 Options", Internet Draft, draft-ietf-tftpexts-options-00.txt, 232 December 1994. 234 Authors' Addresses 236 Gary Scott Malkin 237 Xylogics, Inc. 238 53 Third Avenue 239 Burlington, MA 01803 241 Phone: (617) 272-8140 242 EMail: gmalkin@xylogics.com 244 Art Harkin 245 Internet Services Project 246 Information Networks Division 247 19420 Homestead Road MS 43LN 248 Cupertino, CA 95014 250 Phone: (408) 447-3755 251 EMail: ash@cup.hp.com