📄 rfc783.4up.ps
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(acknowledgment packet, in this special case the block number will be) s5 140 M(zero. \(Normally, since an acknowledgment packet is acknowledging a data) s5 114 M(packet, the acknowledgment packet will contain the block number of the) s5 88 M(data packet being acknowledged.\) If the reply is an error packet, then) s5 49 M( 6) s_R%Page: (8) 8%BeginPageSetup_S% N-up sub-page 4/4278 -794 translate0.490859 dup scale/pagenum 8 def/fname (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/rfc783.txt) def/fdir (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/) def/ftail (rfc783.txt) def% User defined strings:/fmodstr (Thu Oct 15 00:00:00 1992) def/pagenumstr (8) def/user_header_p true def/user_header_left_str (RFC783) def/user_header_center_str (RFC.net) def/user_header_right_str (Page 8 of 20) def%%EndPageSetupdo_header5 725 M(the request has been denied.) s5 673 M( In order to create a connection, each end of the connection chooses a) s5 647 M(TID for itself, to be used for the duration of that connection. The) s5 621 M(TID's chosen for a connection should be randomly chosen, so that the) s5 595 M(probability that the same number is chosen twice in immediate succession) s5 569 M(is very low. Every packet has associated with it the two TID's of the) s5 543 M(ends of the connection, the source TID and the destination TID. These) s5 517 M(TID's are handed to the supporting UDP \(or other datagram protocol\) as) s5 491 M(the source and destination ports. A requesting host chooses its source) s5 465 M(TID as described above, and sends its initial request to the known TID) s5 439 M(69 decimal \(105 octal\) on the serving host. The response to the) s5 413 M(request, under normal operation, uses a TID chosen by the server as its) s5 387 M(source TID and the TID chosen for the previous message by the requestor) s5 361 M(as its destination TID. The two chosen TID's are then used for the) s5 335 M(remainder of the transfer. ) s5 296 M( As an example, the following shows the steps used to establish a) s5 270 M(connection to write a file. Note that WRQ, ACK, and DATA are the names) s5 244 M(of the write request, acknowledgment, and data types of packets) s5 218 M(respectively. The appendix contains a similar example for reading a) s5 192 M(file.) s5 153 M( 1. Host A sends a "WRQ" to host B with source= A's TID,) s5 140 M( destination= 69.) s5 101 M( 2. Host B sends a "ACK" \(with block number= 0\) to host A with) s5 88 M( source= B's TID, destination= A's TID.) s5 49 M( 7) s_R_RS%%Page: (9-12) 3%%BeginPageSetup_S24 818 translate%Page: (9) 9%BeginPageSetup_S% N-up sub-page 1/40 -392 translate0.490859 dup scale/pagenum 9 def/fname (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/rfc783.txt) def/fdir (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/) def/ftail (rfc783.txt) def% User defined strings:/fmodstr (Thu Oct 15 00:00:00 1992) def/pagenumstr (9) def/user_header_p true def/user_header_left_str (RFC783) def/user_header_center_str (RFC.net) def/user_header_right_str (Page 9 of 20) def%%EndPageSetupdo_header5 725 M(At this point the connection has been established and the first data) s5 699 M(packet can be sent by Host A with a sequence number of 1. In the next) s5 673 M(step, and in all succeeding steps, the hosts should make sure that the) s5 647 M(source TID matches the value that was agreed on in steps 1 and 2. If a) s5 621 M(source TID does not match, the packet should be discarded as erroneously) s5 595 M(sent from somewhere else. An error packet should be sent to the source) s5 569 M(of the incorrect packet, while not disturbing the transfer.) s5 543 M(This can be done only if the TFTP in fact receives a packet with an) s5 517 M(incorrect TID. If the supporting protocols do not allow it, this) s5 491 M(particular error condition will not arise.) s5 426 M( The following example demonstrates a correct operation of the protocol) s5 400 M(in which the above situation can occur. Host A sends a request to host) s5 374 M(B. Somewhere in the network, the request packet is duplicated, and as a) s5 348 M(result two acknowledgments are returned to host A, with different TID's) s5 322 M(chosen on host B in response to the two requests. When the first) s5 296 M(response arrives, host A continues the connection. When the second) s5 270 M(response to the request arrives, it should be rejected, but there is no) s5 244 M(reason to terminate the first connection. Therefore, if different TID's) s5 218 M(are chosen for the two connections on host B and host A checks the) s5 192 M(source TID's of the messages it receives, the first connection can be) s5 166 M(maintained while the second is rejected by returning an error packet.) s5 75 M( 8) s_R%Page: (10) 10%BeginPageSetup_S% N-up sub-page 2/4278 -392 translate0.490859 dup scale/pagenum 10 def/fname (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/rfc783.txt) def/fdir (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/) def/ftail (rfc783.txt) def% User defined strings:/fmodstr (Thu Oct 15 00:00:00 1992) def/pagenumstr (10) def/user_header_p true def/user_header_left_str (RFC783) def/user_header_center_str (RFC.net) def/user_header_right_str (Page 10 of 20) def%%EndPageSetupdo_header5 725 M(5. TFTP Packets) s5 699 M( TFTP supports five types of packets, all of which have been mentioned) s5 673 M(above:) s5 634 M( opcode operation) s5 621 M( 1 Read request \(RRQ\)) s5 608 M( 2 Write request \(WRQ\)) s5 595 M( 3 Data \(DATA\)) s5 582 M( 4 Acknowledgment \(ACK\)) s5 569 M( 5 Error \(ERROR\)) s5 530 M(The TFTP header of a packet contains the opcode associated with that) s5 504 M(packet.) s5 478 M( Figure 5-1: RRQ/WRQ packet) s5 413 M( 2 bytes string 1 byte string 1 byte) s5 400 M( ------------------------------------------------) s5 387 M( | Opcode | Filename | 0 | Mode | 0 |) s5 374 M( ------------------------------------------------) s5 322 M( RRQ and WRQ packets \(opcodes 1 and 2 respectively\) have the format) s5 296 M(shown in Figure 5-1. The file name is a sequence of bytes in netascii) s5 270 M(terminated by a zero byte. The mode field contains the string) s5 244 M("netascii", "octet", or "mail" \(or any comibnation of upper and lower) s5 218 M(case, such as "NETASCII", NetAscii", etc.\) in netascii indicating the) s5 192 M(three modes defined in the protocol. A host which receives netascii) s5 166 M(mode data must translate the data to its own format. Octet mode is used) s5 140 M(to transfer a file that is in the 8-bit format of the machine from which) s5 114 M(the file is being transferred. It is assumed that each type of machine) s5 88 M(has a single 8-bit format that is more common, and that that format is) s5 49 M( 9) s_R%Page: (11) 11%BeginPageSetup_S% N-up sub-page 3/40 -794 translate0.490859 dup scale/pagenum 11 def/fname (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/rfc783.txt) def/fdir (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/) def/ftail (rfc783.txt) def% User defined strings:/fmodstr (Thu Oct 15 00:00:00 1992) def/pagenumstr (11) def/user_header_p true def/user_header_left_str (RFC783) def/user_header_center_str (RFC.net) def/user_header_right_str (Page 11 of 20) def%%EndPageSetupdo_header5 725 M(chosen. For example, on a DEC-20, a 36 bit machine, this is four 8-bit) s5 699 M(bytes to a word with four bits of breakage. If a host receives a octet) s5 673 M(file and then returns it, the returned file must be identical to the) s5 647 M(original. Mail mode uses the name of a mail recipient in place of a) s5 621 M(file and must begin with a WRQ. Otherwise it is identical to netascii) s5 595 M(mode. The mail recipient string should be of the form "username" or) s5 569 M("username@hostname". If the second form is used, it allows the option) s5 543 M(of mail forwarding by a relay computer.) s5 491 M( The discussion above assumes that both the sender and recipient are) s5 465 M(operating in the same mode, but there is no reason that this has to be) s5 439 M(the case. For example, one might build a storage server. There is no) s5 413 M(reason that such a machine needs to translate netascii into its own form) s5 387 M(of text. Rather, the sender might send files in netascii, but the) s5 361 M(storage server might simply store them without translation in 8-bit) s5 335 M(format. Another such situation is a problem that currently exists on) s5 309 M(DEC-20 systems. Neither netascii nor octet accesses all the bits in a) s5 283 M(word. One might create a special mode for such a machine which read all) s5 257 M(the bits in a word, but in which the receiver stored the information in) s5 231 M(8-bit format. When such a file is retrieved from the storage site, it) s5 205 M(must be restored to its original form to be useful, so the reverse mode) s5 179 M(must also be implemented. The user site will have to remember some) s5 153 M(information to achieve this. In both of these examples, the request) s5 127 M(packets would specify octet mode to the foreign host, but the local host) s5 101 M(would be in some other mode. No such machine or application specific) s5 75 M(modes have been specified in TFTP, but one would be compatible with this) s5 36 M( 10) s_R%Page: (12) 12%BeginPageSetup_S% N-up sub-page 4/4278 -794 translate0.490859 dup scale/pagenum 12 def/fname (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/rfc783.txt) def/fdir (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/) def/ftail (rfc783.txt) def% User defined strings:/fmodstr (Thu Oct 15 00:00:00 1992) def/pagenumstr (12) def/user_header_p true def/user_header_left_str (RFC783) def/user_header_center_str (RFC.net) def/user_header_right_str (Page 12 of 20) def%%EndPageSetupdo_header5 725 M(specification.) s5 673 M( It is also possible to define other modes for cooperating pairs of) s5 647 M(hosts, although this must be done with care. There is no requirement) s5 621 M(that any other hosts implement these. There is no central authority) s5 595 M(that will define these modes or assign them names.) s5 569 M( Figure 5-2: DATA packet) s5 504 M( 2 bytes 2 bytes n bytes) s5 491 M( ----------------------------------) s5 478 M( | Opcode | Block # | Data |) s5 465 M( ----------------------------------) s5 413 M( Data is actually transferred in DATA packets depicted in Figure 5-2.) s5 387 M(DATA packets \(opcode = 3\) have a block number and data field. The block) s5 361 M(numbers on data packets begin with one and increase by one for each new) s5 335 M(block of data. This restriction allows the program to use a single) s5 309 M(number to discriminate between new packets and duplicates. The data) s5 283 M(field is from zero to 512 bytes long. If it is 512 bytes long, the) s5 257 M(block is not the last block of data; if it is from zero to 511 bytes) s5 231 M(long, it signals the end of the transfer. \(See the section on Normal) s5 205 M(Termination for details.\)) s5 153 M( All packets other than those used for termination are acknowledged) s5 127 M(individually unless a timeout occurs. Sending a DATA packet is an) s5 101 M(acknowledgment for the ACK packet of the previous DATA packet. The WRQ) s5 75 M(and DATA packets are acknowledged by ACK or ERROR packets, while RRQ and) s5 36 M( 11) s_R_RS%%Page: (13-16) 4%%BeginPageSetup_S24 818 translate%Page: (13) 13%BeginPageSetup_S% N-up sub-page 1/40 -392 translate0.490859 dup scale/pagenum 13 def/fname (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/rfc783.txt) def/fdir (/usr/local/share/doc/rfc/Mirrors/ftp.isi.edu/in-notes/) def/ftail (rfc783.txt) def% User defined strings:/fmodstr (Thu Oct 15 00:00:00 1992) def/pagenumstr (13) def/user_header_p true def/user_header_left_str (RFC783) def/user_header_center_str (RFC.net) def/user_header_right_str (Page 13 of 20) def%%EndPageSetupdo_header5 725 M( Figure 5-3: ACK packet) s5 660 M( 2 bytes 2 bytes) s5 647 M( ---------------------) s⌨️ 快捷键说明
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