"Hugo Cornwall "The Hacker's handbook"" - читать интересную книгу автора

the requirement for a software facility to re-assign any little-used
key to send the desired 'missing' feature. Typical requirements are
BREAK~ ESC, RETURN (when part of a string as opposed to being the end
of a command) etc. When re-assigning a series of keys, you must make
sure you don't interfere with the essential functioning of the
terminal emulator.
For example, if you designate the sequence ctrl-S to mean 'send a DC1
character to the host', the chances are you will stop the host from
sending anything to you, because ctrl-S is a common command (some-
times called XOF) to call for a pause--incidentally, you can end the
pause by hitting ctrl-Q. Appendix IV gives a list of the full ASCII
implementation and the usual 'special' codes as they apply to
computer-to-computer communications.

File Protocols - When computers are sending large files to each
other, a further layer of protocol, beyond that defining individual
letters, is necessary. For example, if your computer is automatically
saving to disk at regular intervals as the buffer fills up, it is
necessary to be able to tell the host to stop sending for a period,
until the save is complete. On older time-share services, where the
typical terminal is a teletypewriter, the terminal is in constant
danger of being unable mechanically to keep up with the host
computer's output. For this reason, many host computers use one of
two well-known protocols which require the regular exchange of
special control characters for host and user to tell each other all
is well. The two protocols are:

Stop/Start - The receiving computer can at any time send to the host
a Stop (ctrl-S) signal, followed by, when it is ready a Start,
(ctrl-Q).


EOB/ACK - The sending computer divides its file into a blocks (of any
convenient length); after each block is sent, an EOB (End of Block)
character is sent (see ASCII table, Appendix IV). The user's computer
must then respond with a ACK (Acknowledge) character.
These protocols can be used individually, together or not at all.
You may be able to use the 'Show Control Codes' option to check
whether either of the protocols are in use. Alternatively, if you
have hooked on to a service which for no apparent reason, seems to
stop in its tracks, you could try ending an ACK or Start (ctrl-F or
ctrl-S) and see if you can get things moving.

File transmission - All terminal emulators assume you will want to
send, as well as receive, text files. Thus, in addition to the
protocol settings already mentioned, there may be additional ones for
that purpose, e.g. the XMODEM protocol very popular on bulletin
boards. Hackers, of course, usually don't want to place files on
remote computers.....