UUCP Protocol
=============
The UUCP protocol is a conversation between two UUCP packages. A UUCP
conversation consists of three parts: an initial handshake, a series of
file transfer requests, and a final handshake.
The Initial Handshake
---------------------
Before the initial handshake, the caller will usually have logged in the
called machine and somehow started the UUCP package there. On Unix this
is normally done by setting the shell of the login name used to
`/usr/lib/uucp/uucico'.
All messages in the initial handshake begin with a `^P' (a byte with
the octal value `020円') and end with a null byte (`000円'). A few
systems end these messages with a line feed character (`012円') instead
of a null byte; the examples below assume a null byte is being used.
Some options below are supported by QFT, which stands for Queued File
Transfer, and is (or was) an internal Bell Labs version of UUCP.
Taylor UUCP size negotiation was introduced by Taylor UUCP, and is also
supported by DOS based UUPlus and Amiga based wUUCP and UUCP-1.17.
The initial handshake goes as follows. It is begun by the called
machine.
called: `020円Shere=hostname000円'
 The hostname is the UUCP name of the called machine. Older UUCP
 packages do not output it, and simply send `020円Shere000円'.
caller: `020円Shostname options000円'
 The hostname is the UUCP name of the calling machine. The
 following options may appear (or there may be none):
 `-QSEQ'
 Report sequence number for this conversation. The sequence
 number is stored at both sites, and incremented after each
 call. If there is a sequence number mismatch, something has
 gone wrong (somebody may have broken security by pretending
 to be one of the machines) and the call is denied. If the
 sequence number changes on one of the machines, perhaps
 because of an attempted breakin or because a disk backup was
 restored, the sequence numbers on the two machines must be
 reconciled manually.
 `-xLEVEL'
 Requests the called system to set its debugging level to the
 specified value. This is not supported by all systems.
 `-pGRADE'
 `-vgrade=GRADE'
 Requests the called system to only transfer files of the
 specified grade or higher. This is not supported by all
 systems. Some systems support `-p', some support `-vgrade='.
 UUPlus allows either `-p' or `-v' to be specified on a
 per-system basis in the `SYSTEMS' file (`gradechar' option).
 `-R'
 Indicates that the calling UUCP understands how to restart
 failed file transmissions. Supported only by System V
 Release 4 UUCP, QFT, and Taylor UUCP.
 `-ULIMIT'
 Reports the ulimit value of the calling UUCP. The limit is
 specified as a base 16 number in C notation (e.g.,
 `-U0x1000000'). This number is the number of 512 byte blocks
 in the largest file which the calling UUCP can create. The
 called UUCP may not transfer a file larger than this.
 Supported only by System V Release 4 UUCP, QFT and UUPlus.
 UUPlus reports the lesser of the available disk space on the
 spool directory drive and the ulimit variable in
 `UUPLUS.CFG'. Taylor UUCP understands this option, but does
 not generate it.
 `-N[NUMBER]'
 Indicates that the calling UUCP understands the Taylor UUCP
 size negotiation extension. Not supported by traditional
 UUCP packages. Supported by UUPlus. The optional number is
 a bitmask of features supported by the calling UUCP, and is
 described below.
called: `020円ROK000円'
 There are actually several possible responses.
 `ROK'
 The calling UUCP is acceptable, and the handshake proceeds to
 the protocol negotiation. Some options may also appear; see
 below.
 `ROKN[NUMBER]'
 The calling UUCP is acceptable, it specified `-N', and the
 called UUCP also understands the Taylor UUCP size limiting
 extensions. The optional number is a bitmask of features
 supported by the called UUCP, and is described below.
 `RLCK'
 The called UUCP already has a lock for the calling UUCP,
 which normally indicates the two machines are already
 communicating.
 `RCB'
 The called UUCP will call back. This may be used to avoid
 impostors (but only one machine out of each pair should call
 back, or no conversation will ever begin).
 `RBADSEQ'
 The call sequence number is wrong (see the `-Q' discussion
 above).
 `RLOGIN'
 The calling UUCP is using the wrong login name.
 `RYou are unknown to me'
 The calling UUCP is not known to the called UUCP, and the
 called UUCP does not permit connections from unknown systems.
 Some versions of UUCP just drop the line rather than sending
 this message.
 If the response is `ROK', the following options are supported by
 System V Release 4 UUCP and QFT.
 `-R'
 The called UUCP knows how to restart failed file
 transmissions.
 `-ULIMIT'
 Reports the ulimit value of the called UUCP. The limit is
 specified as a base 16 number in C notation. This number is
 the number of 512 byte blocks in the largest file which the
 called UUCP can create. The calling UUCP may not send a file
 larger than this. Also supported by UUPlus. Taylor UUCP
 understands this option, but does not generate it.
 `-xLEVEL'
 I'm not sure just what this means. It may request the
 calling UUCP to set its debugging level to the specified
 value.
 If the response is not `ROK' (or `ROKN') both sides hang up the
 phone, abandoning the call.
called: `020円Pprotocols000円'
 Note that the called UUCP outputs two strings in a row. The
 protocols string is a list of UUCP protocols supported by the
 caller. Each UUCP protocol has a single character name. These
 protocols are discussed in more detail later in this document.
 For example, the called UUCP might send `020円Pgf000円'.
caller: `020円Uprotocol000円'
 The calling UUCP selects which protocol to use out of the protocols
 offered by the called UUCP. If there are no mutually supported
 protocols, the calling UUCP sends `020円UN000円' and both sides hang
 up the phone. Otherwise the calling UUCP sends something like
 `020円Ug000円'.
Most UUCP packages will consider each locally supported protocol in turn
and select the first one supported by the called UUCP. With some
versions of HDB UUCP, this can be modified by giving a list of protocols
after the device name in the `Devices' file or the `Systems' file. For
example, to select the `e' protocol in `Systems',
 airs Any ACU,e ...
or in Devices,
 ACU,e ttyXX ...
Taylor UUCP provides the `protocol' command which may be used either
for a system or a port. UUPlus allows specification of the protocol
string on a per-system basis in the `SYSTEMS' file.
The optional number following a `-N' sent by the calling system, or an
`ROKN' sent by the called system, is a bitmask of features supported by
the UUCP package. The optional number was introduced in Taylor UUCP
version 1.04. The number is sent as an octal number with a leading
zero. The following bits are currently defined. A missing number
should be taken as `011'.
`01'
 UUCP supports size negotiation.
`02'
 UUCP supports file restart.
`04'
 UUCP supports the `E' command.
`010'
 UUCP requires the file size in the `S' and `R' commands to be in
 base 10. This bit is used by default if no number appears, but
 should not be explicitly sent.
`020'
 UUCP expects a dummy string between the notify field and the size
 field in an `S' command. This is true of SVR4 UUCP. This bit
 should not be used.
After the protocol has been selected and the initial handshake has been
completed, both sides turn on the selected protocol. For some protocols
(notably `g') a further handshake is done at this point.
UUCP Protocol Commands
----------------------
Each protocol supports a method for sending a command to the remote
system. This method is used to transmit a series of commands between
the two UUCP packages. At all times, one package is the master and the
other is the slave. Initially, the calling UUCP is the master.
If a protocol error occurs during the exchange of commands, both sides
move immediately to the final handshake.
The master will send one of five commands: `S', `R', `X', `E', or `H'.
Any file name referred to below is either an absolute file name
beginning with `/', a public directory file name beginning with `~/', a
file name relative to a user's home directory beginning with `~USER/',
or a spool directory file name. File names in the spool directory are
not absolute, but instead are converted to file names within the spool
directory by UUCP. They always begin with `C.' (for a command file
created by `uucp' or `uux'), `D.' (for a data file created by `uucp',
`uux' or by an execution, or received from another system for an
execution), or `X.' (for an execution file created by `uux' or received
from another system).
The S Command
.............
master: `S FROM TO USER -OPTIONS TEMP MODE NOTIFY SIZE'
 The `S' and the `-' are literal characters. This is a request by
 the master to send a file to the slave.
 FROM
 The name of the file to send. If the `C' option does not
 appear in OPTIONS, the master will actually open and send
 this file. Otherwise the file has been copied to the spool
 directory, where it is named TEMP. The slave ignores this
 field unless TO is a directory, in which case the basename of
 FROM will be used as the file name. If FROM is a spool
 directory filename, it must be a data file created for or by
 an execution, and must begin with `D.'.
 TO
 The name to give the file on the slave. If this field names
 a directory the file is placed within that directory with the
 basename of FROM. A name ending in `/' is taken to be a
 directory even if one does not already exist with that name.
 If TO begins with `X.', an execution file will be created on
 the slave. Otherwise, if TO begins with `D.' it names a data
 file to be used by some execution file. Otherwise, TO should
 not be in the spool directory.
 USER
 The name of the user who requested the transfer.
 OPTIONS
 A list of options to control the transfer. The following
 options are defined (all options are single characters):
 `C'
 The file has been copied to the spool directory (the
 master should use TEMP rather than FROM).
 `c'
 The file has not been copied to the spool directory
 (this is the default).
 `d'
 The slave should create directories as necessary (this
 is the default).
 `f'
 The slave should not create directories if necessary,
 but should fail the transfer instead.
 `m'
 The master should send mail to USER when the transfer is
 complete.
 `n'
 The slave should send mail to NOTIFY when the transfer is
 complete.
 TEMP
 If the `C' option appears in OPTIONS, this names the file to
 be sent. Otherwise if FROM is in the spool directory, TEMP
 is the same as FROM. Otherwise TEMP may be a dummy string,
 such as `D.0'. After the transfer has been succesfully
 completed, the master will delete the file TEMP.
 MODE
 This is an octal number giving the mode of the file on the
 master. If the file is not in the spool directory, the slave
 will always create it with mode 0666, except that if (MODE &
 0111) is not zero (the file is executable), the slave will
 create the file with mode 0777. If the file is in the spool
 directory, some UUCP packages will use the algorithm above
 and some will always create the file with mode 0600. This
 field is ignored by UUPlus, since it is meaningless on DOS;
 UUPlus uses 0666 for outgoing files.
 NOTIFY
 This field may not be present, and in any case is only
 meaningful if the `n' option appears in OPTIONS. If the `n'
 option appears, then, when the transfer is successfully
 completed, the slave will send mail to NOTIFY, which must be
 a legal mailing address on the slave. If a SIZE field will
 appear but the `n' option does not appear, NOTIFY will always
 be present, typically as the string `dummy' or simply a pair
 of double quotes.
 SIZE
 This field is only present when doing Taylor UUCP or SVR4
 UUCP size negotiation. It is the size of the file in bytes.
 Taylor UUCP version 1.03 sends the size as a decimal integer,
 while versions 1.04 and up, and all other UUCP packages that
 support size negotiation, send the size in base 16 with a
 leading 0x.
 The slave then responds with an `S' command response.
 `SY START'
 The slave is willing to accept the file, and file transfer
 begins. The START field will only be present when using file
 restart. It specifies the byte offset into the file at which
 to start sending. If this is a new file, START will be 0x0.
 `SN2'
 The slave denies permission to transfer the file. This can
 mean that the destination directory may not be accessed, or
 that no requests are permitted. It implies that the file
 transfer will never succeed.
 `SN4'
 The slave is unable to create the necessary temporary file.
 This implies that the file transfer might succeed later.
 `SN6'
 This is only used by Taylor UUCP size negotiation. It means
 that the slave considers the file too large to transfer at
 the moment, but it may be possible to transfer it at some
 other time.
 `SN7'
 This is only used by Taylor UUCP size negotiation. It means
 that the slave considers the file too large to ever transfer.
 `SN8'
 This is only used by Taylor UUCP. It means that the file was
 already received in a previous conversation. This can happen
 if the receive acknowledgement was lost after it was sent by
 the receiver but before it was received by the sender.
 `SN9'
 This is only used by Taylor UUCP (versions 1.05 and up) and
 UUPlus (versions 2.0 and up). It means that the remote
 system was unable to open another channel (see the discussion
 of the `i' protocol for more information about channels).
 This implies that the file transfer might succeed later.
 `SN10'
 This is reportedly used by SVR4 UUCP to mean that the file
 size is too large.
 If the slave responds with `SY', a file transfer begins. When the
 file transfer is complete, the slave sends a `C' command response.
 `CY'
 The file transfer was successful.
 `CYM'
 The file transfer was successful, and the slave wishes to
 become the master; the master should send an `H' command,
 described below.
 `CN5'
 The temporary file could not be moved into the final
 location. This implies that the file transfer will never
 succeed.
After the `C' command response has been received (in the `SY' case) or
immediately (in an `SN' case) the master will send another command.
The R Command
.............
master: `R FROM TO USER -OPTIONS SIZE'
 The `R' and the `-' are literal characters. This is a request by
 the master to receive a file from the slave. I do not know how
 SVR4 UUCP or QFT implement file transfer restart in this case.
 FROM
 This is the name of the file on the slave which the master
 wishes to receive. It must not be in the spool directory,
 and it may not contain any wildcards.
 TO
 This is the name of the file to create on the master. I do
 not believe that it can be a directory. It may only be in
 the spool directory if this file is being requested to
 support an execution either on the master or on some system
 other than the slave.
 USER
 The name of the user who requested the transfer.
 OPTIONS
 A list of options to control the transfer. The following
 options are defined (all options are single characters):
 `d'
 The master should create directories as necessary (this
 is the default).
 `f'
 The master should not create directories if necessary,
 but should fail the transfer instead.
 `m'
 The master should send mail to USER when the transfer is
 complete.
 SIZE
 This only appears if Taylor UUCP size negotiation is being
 used. It specifies the largest file which the master is
 prepared to accept (when using SVR4 UUCP or QFT, this was
 specified in the `-U' option during the initial handshake).
 The slave then responds with an `R' command response. UUPlus does
 not support `R' requests, and always responds with `RN2'.
 `RY MODE [SIZE]'
 The slave is willing to send the file, and file transfer
 begins. The MODE argument is the octal mode of the file on
 the slave. The master treats this just as the slave does the
 MODE argument in the send command, q.v. I am told that SVR4
 UUCP sends a trailing SIZE argument. For some versions of
 BSD UUCP, the MODE argument may have a trailing `M' character
 (e.g., `RY 0666M'). This means that the slave wishes to
 become the master.
 `RN2'
 The slave is not willing to send the file, either because it
 is not permitted or because the file does not exist. This
 implies that the file request will never succeed.
 `RN6'
 This is only used by Taylor UUCP size negotiation. It means
 that the file is too large to send, either because of the
 size limit specifies by the master or because the slave
 considers it too large. The file transfer might succeed
 later, or it might not (this may be cleared up in a later
 release of Taylor UUCP).
 `RN9'
 This is only used by Taylor UUCP (versions 1.05 and up) and
 FSUUCP (versions 1.5 and up). It means that the remote
 system was unable to open another channel (see the discussion
 of the `i' protocol for more information about channels).
 This implies that the file transfer might succeed later.
 If the slave responds with `RY', a file transfer begins. When the
 file transfer is complete, the master sends a `C' command. The
 slave pretty much ignores this, although it may log it.
 `CY'
 The file transfer was successful.
 `CN5'
 The temporary file could not be moved into the final location.
 After the `C' command response has been sent (in the `RY' case) or
 immediately (in an `RN' case) the master will send another command.
The X Command
.............
master: `X FROM TO USER -OPTIONS'
 The `X' and the `-' are literal characters. This is a request by
 the master to, in essence, execute uucp on the slave. The slave
 should execute `uucp FROM TO'.
 FROM
 This is the name of the file or files on the slave which the
 master wishes to transfer. Any wildcards are expanded on the
 slave. If the master is requesting that the files be
 transferred to itself, the request would normally contain
 wildcard characters, since otherwise an `R' command would
 suffice. The master can also use this command to request
 that the slave transfer files to a third system.
 TO
 This is the name of the file or directory to which the files
 should be transferred. This will normally use a UUCP name.
 For example, if the master wishes to receive the files
 itself, it would use `master!path'.
 USER
 The name of the user who requested the transfer.
 OPTIONS
 A list of options to control the transfer. It is not clear
 which, if any, options are supported by most UUCP packages.
 The slave then responds with an `X' command response. FSUUCP does
 not support `X' requests, and always responds with `XN'.
 `XY'
 The request was accepted, and the appropriate file transfer
 commands have been queued up for later processing.
 `XN'
 The request was denied. No particular reason is given.
 In either case, the master will then send another command.
The E Command
.............
master: `E FROM TO USER -OPTIONS TEMP MODE NOTIFY SIZE COMMAND'
 The `E' command is only supported by Taylor UUCP 1.04 and up. It
 is used to make an execution request without requiring a separate
 `X.*' file. It is only used when the command to be executed
 requires a single input file which is passed to it as standard
 input.
 All the fields have the same meaning as they do for an `S' command,
 except for OPTIONS and COMMAND.
 OPTIONS
 A list of options to control the transfer. The following
 options are defined (all options are single characters):
 `C'
 The file has been copied to the spool directory (the
 master should use TEMP rather than FROM).
 `c'
 The file has not been copied to the spool directory
 (this is the default).
 `N'
 No mail message should be sent, even if the command
 fails. This is the equivalent of the `N' command in an
 `X.*' file.
 `Z'
 A mail message should be sent if the command fails (this
 is generally the default in any case). This is the
 equivalent of the `Z' command in an `X.*' file.
 `R'
 Mail messages about the execution should be sent to the
 address in the NOTIFY field. This is the equivalent of
 the `R' command in an `X.*' file.
 `e'
 The execution should be done with `/bin/sh'. This is the
 equivalent of the `e' command in an `X.*' file.
 COMMAND
 The command which should be executed. This is the equivalent
 of the `C' command in an `X.*' file.
 The slave then responds with an `E' command response. These are
 the same as the `S' command responses, but the initial character is
 `E' rather than `S'.
 If the slave responds with `EY', the file transfer begins. When
 the file transfer is complete, the slave sends a `C' command
 response, just as for the `S' command. After a successful file
 transfer, the slave is responsible for arranging for the command
 to be executed. The transferred file is passed as standard input,
 as though it were named in the `I' and `F' commands of an `X.*'
 file.
 After the `C' command response has been received (in the `EY'
 case) or immediately (in an `EN' case) the master will send another
 command.
The H Command
.............
master: `H'
 This is used by the master to hang up the connection. The slave
 will respond with an `H' command response.
 `HY'
 The slave agrees to hang up the connection. In this case the
 master sends another `HY' command. In some UUCP packages the
 slave will then send a third `HY' command. At this point the
 protocol is shut down, and the final handshake is begun.
 `HN'
 The slave does not agree to hang up. In this case the master
 and the slave exchange roles. The next command will be sent
 by the former slave, which is the new master. The roles may
 be reversed several times during a single connection.
The Final Handshake
-------------------
After the protocol has been shut down, the final handshake is performed.
This handshake has no real purpose, and some UUCP packages simply drop
the connection rather than do it (in fact, some will drop the connection
immediately after both sides agree to hangup, without even closing down
the protocol).
caller: `020円OOOOOO000円'
called: `020円OOOOOOO000円'
That is, the calling UUCP sends six `O' characters and the called UUCP
replies with seven `O' characters. Some UUCP packages always send six
`O' characters.