RFC 959 October 1985 File Transfer Protocol Block Header +----------------+----------------+----------------+ | Descriptor | Byte Count | | 8 bits | 16 bits | +----------------+----------------+----------------+ The descriptor codes are indicated by bit flags in the descriptor byte. Four codes have been assigned, where each code number is the decimal value of the corresponding bit in the byte. Code Meaning
128 End of data block is EOR 64 End of data block is EOF 32 Suspected errors in data block 16 Data block is a restart er With this encoding, more than one descriptor coded condition may exist for a particular block. As many bits as necessary may be flagged. The restart er is embedded in the data stream as an integral number of 8-bit bytes representing printable characters in the language being used over the control connection (e.g., default--NVT-ASCII). <SP> (Space, in the appropriate language) must not be used WITHIN a restart er. For example, to transmit a six-character er, the following would be sent: +--------+--------+--------+ |Descrptr| Byte count | |code= 16| = 6 | +--------+--------+--------+ +--------+--------+--------+ | Marker | Marker | Marker | | 8 bits | 8 bits | 8 bits | +--------+--------+--------+ +--------+--------+--------+ | Marker | Marker | Marker | | 8 bits | 8 bits | 8 bits | +--------+--------+--------+ Postel & Reynolds [Page 22]
RFC 959 October 1985 File Transfer Protocol 3.4.3. COMPRESSED MODE There are three kinds of information to be sent: regular data, sent in a byte string; compressed data, consisting of replications or filler; and control information, sent in a two-byte escape sequence. If n>0 bytes (up to 127) of regular data are sent, these n bytes are preceded by a byte with the left-most bit set to 0 and the right-most 7 bits containing the number n. Byte string: 1 7 8 8 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |0| n | | d(1) | ... | d(n) | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ ^ ^ |---n bytes---| of data String of n data bytes d(1),..., d(n) Count n must be positive. To compress a string of n replications of the data byte d, the following 2 bytes are sent: Replicated Byte: 2 6 8 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |1 0| n | | d | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ A string of n filler bytes can be compressed into a single byte, where the filler byte varies with the representation type. If the type is ASCII or EBCDIC the filler byte is <SP> (Space, ASCII code 32, EBCDIC code 64). If the type is Image or Local byte the filler is a zero byte. Filler String: 2 6 +-+-+-+-+-+-+-+-+ |1 1| n | +-+-+-+-+-+-+-+-+ The escape sequence is a double byte, the first of which is the Postel & Reynolds
FTP的命令还是比较多的，可以查查FTP的标准文档当你需要传输文件的时候，才临时使用PASV/PORT等命令建立数据连接
File Transfer Protocol
Block Header +----------------+----------------+----------------+
| Descriptor | Byte Count |
| 8 bits | 16 bits |
+----------------+----------------+----------------+
The descriptor codes are indicated by bit flags in the
descriptor byte. Four codes have been assigned, where each
code number is the decimal value of the corresponding bit in
the byte. Code Meaning
128 End of data block is EOR
64 End of data block is EOF
32 Suspected errors in data block
16 Data block is a restart er With this encoding, more than one descriptor coded condition
may exist for a particular block. As many bits as necessary
may be flagged. The restart er is embedded in the data stream as an
integral number of 8-bit bytes representing printable
characters in the language being used over the control
connection (e.g., default--NVT-ASCII). <SP> (Space, in the
appropriate language) must not be used WITHIN a restart er. For example, to transmit a six-character er, the following
would be sent: +--------+--------+--------+
|Descrptr| Byte count |
|code= 16| = 6 |
+--------+--------+--------+ +--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+ +--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
Postel & Reynolds [Page 22]
RFC 959 October 1985
File Transfer Protocol
3.4.3. COMPRESSED MODE There are three kinds of information to be sent: regular data,
sent in a byte string; compressed data, consisting of
replications or filler; and control information, sent in a
two-byte escape sequence. If n>0 bytes (up to 127) of regular
data are sent, these n bytes are preceded by a byte with the
left-most bit set to 0 and the right-most 7 bits containing the
number n. Byte string: 1 7 8 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|0| n | | d(1) | ... | d(n) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
^ ^
|---n bytes---|
of data String of n data bytes d(1),..., d(n)
Count n must be positive. To compress a string of n replications of the data byte d, the
following 2 bytes are sent: Replicated Byte: 2 6 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|1 0| n | | d |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ A string of n filler bytes can be compressed into a single
byte, where the filler byte varies with the representation
type. If the type is ASCII or EBCDIC the filler byte is <SP>
(Space, ASCII code 32, EBCDIC code 64). If the type is Image
or Local byte the filler is a zero byte. Filler String: 2 6
+-+-+-+-+-+-+-+-+
|1 1| n |
+-+-+-+-+-+-+-+-+ The escape sequence is a double byte, the first of which is the
Postel & Reynolds