从 FTP 服务器下载多个文件（使用套接字）

默认情况下，FTP 使用STREAM传输模式进行数据传输。在STREAM模式下，文件结束通过关闭数据连接发出信号。因此，在STREAM模式下，每个数据连接只能发送 1 个文件。

要解决这个问题，您必须：

• 发出一个新的PORT/PASV命令来为每个单独的文件建立一个新的数据连接。

• 在传输文件之前发出MODE切换到BLOCK或COMPRESSED传输模式的命令。这两种模式都不会通过关闭数据连接来发出 EOF 信号，而是通过在每个文件末尾的数据连接上发送一个显式标记，从而允许通过单个数据连接传输多个文件。

有关更多详细信息，请阅读官方 FTP 协议规范RFC 959，特别是第 3.3 节“数据连接管理”和第 3.4 节“传输模式”：

3.3.  DATA CONNECTION MANAGEMENT

      Default Data Connection Ports:  All FTP implementations must
      support use of the default data connection ports, and only the
      User-PI may initiate the use of non-default ports.

      Negotiating Non-Default Data Ports:   The User-PI may specify a
      non-default user side data port with the PORT command.  The
      User-PI may request the server side to identify a non-default
      server side data port with the PASV command.  Since a connection
      is defined by the pair of addresses, either of these actions is
      enough to get a different data connection, still it is permitted
      to do both commands to use new ports on both ends of the data
      connection.

      Reuse of the Data Connection:  When using the stream mode of data
      transfer the end of the file must be indicated by closing the
      connection.  This causes a problem if multiple files are to be
      transfered in the session, due to need for TCP to hold the
      connection record for a time out period to guarantee the reliable
      communication.  Thus the connection can not be reopened at once.

         There are two solutions to this problem.  The first is to
         negotiate a non-default port.  The second is to use another
         transfer mode.

         A comment on transfer modes.  The stream transfer mode is
         inherently unreliable, since one can not determine if the
         connection closed prematurely or not.  The other transfer modes
         (Block, Compressed) do not close the connection to indicate the
         end of file.  They have enough FTP encoding that the data
         connection can be parsed to determine the end of the file.
         Thus using these modes one can leave the data connection open
         for multiple file transfers.

   3.4.  TRANSMISSION MODES

      The next consideration in transferring data is choosing the
      appropriate transmission mode.  There are three modes: one which
      formats the data and allows for restart procedures; one which also
      compresses the data for efficient transfer; and one which passes
      the data with little or no processing.  In this last case the mode
      interacts with the structure attribute to determine the type of
      processing.  In the compressed mode, the representation type
      determines the filler byte.

      All data transfers must be completed with an end-of-file (EOF)
      which may be explicitly stated or implied by the closing of the
      data connection.  For files with record structure, all the
      end-of-record markers (EOR) are explicit, including the final one.
      For files transmitted in page structure a "last-page" page type is
      used.

      NOTE:  In the rest of this section, byte means "transfer byte"
      except where explicitly stated otherwise.

      For the purpose of standardized transfer, the sending host will
      translate its internal end of line or end of record denotation
      into the representation prescribed by the transfer mode and file
      structure, and the receiving host will perform the inverse
      translation to its internal denotation.  An IBM Mainframe record
      count field may not be recognized at another host, so the
      end-of-record information may be transferred as a two byte control
      code in Stream mode or as a flagged bit in a Block or Compressed
      mode descriptor.  End-of-line in an ASCII or EBCDIC file with no
      record structure should be indicated by <CRLF> or <NL>,
      respectively.  Since these transformations imply extra work for
      some systems, identical systems transferring non-record structured
      text files might wish to use a binary representation and stream
      mode for the transfer.

      The following transmission modes are defined in FTP:

      3.4.1.  STREAM MODE

         The data is transmitted as a stream of bytes.  There is no
         restriction on the representation type used; record structures
         are allowed.

         In a record structured file EOR and EOF will each be indicated
         by a two-byte control code.  The first byte of the control code
         will be all ones, the escape character.  The second byte will
         have the low order bit on and zeros elsewhere for EOR and the
         second low order bit on for EOF; that is, the byte will have
         value 1 for EOR and value 2 for EOF.  EOR and EOF may be
         indicated together on the last byte transmitted by turning both
         low order bits on (i.e., the value 3).  If a byte of all ones
         was intended to be sent as data, it should be repeated in the
         second byte of the control code.

         If the structure is a file structure, the EOF is indicated by
         the sending host closing the data connection and all bytes are
         data bytes.

      3.4.2.  BLOCK MODE

         The file is transmitted as a series of data blocks preceded by
         one or more header bytes.  The header bytes contain a count
         field, and descriptor code.  The count field indicates the
         total length of the data block in bytes, thus marking the
         beginning of the next data block (there are no filler bits).
         The descriptor code defines:  last block in the file (EOF) last
         block in the record (EOR), restart marker (see the Section on
         Error Recovery and Restart) or suspect data (i.e., the data
         being transferred is suspected of errors and is not reliable).
         This last code is NOT intended for error control within FTP.
         It is motivated by the desire of sites exchanging certain types
         of data (e.g., seismic or weather data) to send and receive all
         the data despite local errors (such as "magnetic tape read
         errors"), but to indicate in the transmission that certain
         portions are suspect).  Record structures are allowed in this
         mode, and any representation type may be used.

         The header consists of the three bytes.  Of the 24 bits of
         header information, the 16 low order bits shall represent byte
         count, and the 8 high order bits shall represent descriptor
         codes as shown below.


         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 marker

         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 marker 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 marker.

         For example, to transmit a six-character marker, 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 |
            +--------+--------+--------+


      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
         escape byte (all zeros) and the second of which contains
         descriptor codes as defined in Block mode.  The descriptor
         codes have the same meaning as in Block mode and apply to the
         succeeding string of bytes.

         Compressed mode is useful for obtaining increased bandwidth on
         very large network transmissions at a little extra CPU cost.
         It can be most effectively used to reduce the size of printer
         files such as those generated by RJE hosts.