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Here is a FAQ posting that I have worked up.  It is intended to answer
most questions about how UUCP works, what the UUCP protocols are,
etc., in nauseating detail.  It does not answer such useful questions
as how to configure UUCP, how to make an X.25 PAD not trap ^P, etc.,
and other questions for which I am not necessarily qualified to
provide answers.  Let this posting serve as a challenge to other
people to produce more FAQ answers, which can either be folded into
this posting, or, more likely, form a separate posting.

This is the first trial of this posting.  I'll consider any comments,
make a relatively final version, and start posting it every month.
Enjoy!

Frequently Asked Questions about UUCP internals
Last updated 1992-02-25

This article was written by Ian Lance Taylor <[e-mail]> and I may
even update it periodically.  Send me mail about suggestions or
inaccuracies.

This article describes how the various UUCP protocols work.  It does
not describe how to configure UUCP, nor how to solve UUCP problems,
nor how to deal with UUCP mail.  There are currently no FAQ postings
on any of these topics, and I do not plan to write any.

If you haven't read the news.announce.newusers articles, read them.

I took a lot of the information from Jamie E. Hanrahan's paper in the
Fall 1990 DECUS Symposium, and from Managing UUCP and Usenet by Tim
O'Reilly and Grace Todino (with contributions by several other
people).  The latter includes most of the former, and is published by
        O'Reilly & Associates, Inc.
        632 Petaluma Avenua
        Sebastopol, CA 95472
It is currently in its tenth edition.  The ISBN number is
0-937175-48-X.

Some information is originally due to a Usenet article by Chuck
Wegrzyn.  The information on the 'g' protocol comes partially from a
paper by G.L. Chesson of Bell Laboratories, partially from Jamie E.
Hanrahan's paper, and partially from source code by John Gilmore.  The
information on the 'f' protocol comes from the source code by Piet
Berteema.  The information on the 't' protocol comes from the source
code by Rick Adams.  The information on the 'e' protocol comes from a
Usenet article by Matthias Urlichs.

This article answers the following questions.  If one of these
questions is posted to comp.mail.uucp, you are encouraged to send mail
to the poster referring her or him to this FAQ.  There is no reason to
post a followup, as most of us know the answer already.

*) What is the UUCP protocol?
*) What is the 'g' protocol?
*) What is the 'f' protocol?
*) What is the 't' protocol?
*) What is the 'e' protocol?
*) What is the 'x' protocol?

*) What is the 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.

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) and end with a null byte (\000).

The initial handshake goes as follows.  It is begun by the called
machine.

called: \020Shere=hostname\000
    The hostname is the UUCP name of the called machine.  Older UUCP
    packages do not output it, and simply send \020Shere\000.

caller: \020Shostname options\000
    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.
        -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=.
        -R
            Indicates that the calling UUCP understands how to restart
            failed file transmissions.  Supported only by System V
            Release 4 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.
        -N
            Indicates that the calling UUCP understands the Taylor
            UUCP size limiting extensions.  Supported only by Taylor
            UUCP.

called: \020ROK\000
    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
            The calling UUCP is acceptable, it specified -N, and the
            called UUCP understands the Taylor UUCP size limiting
            extensions.  Supported only by Taylor UUCP.
        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.
    If the response is ROK, the following options are supported by
    System V Release 4 UUCP.
        -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.
        -xLEVEL
            I'm not sure about this one.  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: \020Pprotocols\000
    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 \020Pgf\000.

caller: \020Uprotocol\000
    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 \020UN\000 and both
    sides hang up the phone.  Otherwise the calling UUCP sends
    something like \020Ug\000.

At this point the initial handshake has been completed, and both sides
turn on the selected protocol.  For some protocols (notably 'g') a
further handshake is done at this point.

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 four commands: S, R, X or H.

Any file name referred to below is either an absolute pathname
beginning with "/", a public directory pathname beginning with "~/", a
pathname relative to a user's home directory beginning with "~USER/",
or a spool directory file name.  File names in the spool directory are
not pathnames, but instead are converted to pathnames 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).

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 (a name ending in '/' is taken to name a
            directory even if one does not already exist with that
            name) the file is placed within that directory with the
            basename of FROM.  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 may 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 is a dummy
            string, normally "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
            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.
        NOTIFY
            This field is only present if the n option appears in
            OPTIONS.  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 be the
            string "dummy" or simply a pair of double quotes.
        SIZE
            This field is only present when doing size negotiation,
            either with Taylor UUCP or SVR4 UUCP.  It is the size of
            the file in bytes.  SVR4 UUCP sends the size in base 16 as
            0x.... while Taylor UUCP sends the size as a decimal
            integer.

    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
            SVR4 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 may 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.

    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.
        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.

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 implements 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, this was
            specified in the -U option during the initial handshake).

    The slave then responds with an R command response.
        RY MODE
            The slave is willing to send the file, and file transfer
            begins.  MODE 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.
        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 may
            succeed later, or it may not (this will be cleared up in a
            later release of Taylor UUCP).

    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.

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.  It will often contain
            wildcard characters, since otherwise an R command will
            normally suffice (however, this command can also be used
            to request the transfer of a file on the slave to a third
            system).  Any wildcards should be expanded on the slave.
        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.
        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.

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.

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 protocl).

caller: \020OOOOOO\000
called: \020OOOOOOO\000

That is, the calling UUCP sends six O's and the called UUCP replies
with seven O's.  Some UUCP packages always send six O's.

*) What is the 'g' protocol?

The 'g' protocol is a packet based flow controlled error correcting
protocol that requires an eight bit clear connection.  It is the
original UUCP protocol, and is supported by all UUCP implementations.
Many implementations of it are only able to support small window and
packet sizes, specifically a window size of 3 and a packet size of 64
bytes, but the protocol itself can support up to a window size of 7
and a packet size of 4096 bytes.  Complaints about the inefficiency of
the 'g' protocol generally refer to specific implementations, rather
than the correctly implemented protocol.

The 'g' protocol was originally designed for general packet drivers,
and thus contains some features that are not used by UUCP, including
an alternate data channel and the ability to renegotiate packet and
window sizes during the communication session.

The 'g' protocol is spoofed by many Telebit modems.  When spoofing is
in effect, each Telebit modem uses the 'g' protocol to communicate
with the attached computer, but the data between the modems is sent
using a Telebit proprietary error correcting protocol.  This allows
for very high throughput over the Telebit connection, which, because
it is half-duplex, would not normally be able to handle the 'g'
protocol very well at all.

This discussion of the 'g' protocol explains how it works, but does
not discuss useful error handling techniques.  Some discussion of this
can be found in Jamie E. Hanrahan's paper, cited above.

All 'g' protocol communication is done with packets.  Each packet
begins with a six byte header.  Control packets consist only of the
header.  Data packets contain additional data.

The header is as follows:

    \020
        Every packet begins with a ^P.
    k (1 <= k <= 9)
        The k value is always 9 for a control packet.  For a data
        packet, the k value indicates how must data follows the six
        byte header.  The amount of data is 2 ** (k + 4), where **
        indicates exponentiation.  Thus a k value of 1 means 32 data
        bytes and a k value of 8 means 4096 data bytes.  The k value
        for a data packet must be between 1 and 8 inclusive.
    checksum low byte
    checksum high byte
        The checksum value is described below.
    control byte
        The control packet indicates the type of packet, and is
        described below.
    xor byte
        This byte is the xor of k, the checksum low byte, the checksum
        high byte and the control byte (i.e. the second, third, fourth
        and fifth header bytes).  It is used to ensure that the header
        data is valid.

The control byte in the header is composed of three bit fields,
referred to here as TT (two bits), XXX (three bits) and YYY (three
bits).  The control is TTXXXYYY, or (TT << 6) + (XXX << 3) + YYY.

The TT field takes on the following values:
    0
        This is a control packet.  In this case the k byte in the
        header must be 9.  The XXX field indicates the type of control
        packet; these types are described below.
    1
        This is an alternate data channel packet.  This is not used by
        UUCP.
    2
        This is a data packet, and the entire contents of the attached
        data field (whose length is given by the k byte in the header)
        are valid.  The XXX and YYY fields are described below.
    3
        This is a short data packet.  Let the length of the data field
        (as given by the k byte in the header) be L.  Let the first
        byte in the data field be B1.  If B1 is less than 128 (if the
        most significant bit of B1 is 0), then there are L - B1 valid
        bytes of data in the data field, beginning with the second
        byte.  If B1 >= 128, let B2 be the second byte in the data
        field.  Then there are L - ((B1 & 0x7f) + (B2 << 7)) valid
        bytes of data in the data field, beginning with the third
        byte.  In all cases L bytes of data are sent (and all data
        bytes participate in the checksum calculation) but some of the
        trailing bytes may be dropped by the receiver.   The XXX and
        YYY fields are described below.

In a data packet (short or not) the XXX field gives the sequence
number of the packet.  Thus sequence numbers can range from 0 to 7,
inclusive.  The YYY field gives the sequence number of the last
correctly received packet.

Each communication direction uses a window which indicates how many
unacknowledged packets may be transmitted before waiting for an
acknowledgement (the window may be different in each direction).  The
window may range from 1 to 7. For example, if the window is 3 and the
last packet acknowledged was packet number 6, packet numbers 7, 0 and
1 may be sent but the sender must wait for an acknowledgement before
sending packet number 2.  This acknowledgement could come as the YYY
field of a data packet or as the YYY field of a RJ or RR control
packet (described below).

Each packet must be transmitted in order (the sender may not skip
sequence numbers).  Each packet must be acknowledged, and each packet
must be acknowledged in order.

In a control packet, the XXX field takes on the following values:
    1 CLOSE
        The connection should be closed immediately.  This is
        typically sent when one side has seen too many errors and
        wants to give up.  It is also sent when shutting down the
        protocol.  If an unexpected CLOSE packet is received, a CLOSE
        packet should be sent and the 'g' protocol should halt,
        causing UUCP to enter the final handshake.
    2 RJ or NAK
        The last packet was not received correctly.  The YYY field
        contains the sequence number of the last correctly received
        packet.
    3 SRJ
        Selective reject.  The YYY field contains the sequence number
        of a packet that was not received correctly, and should be
        retransmitted.  This is not used by UUCP, and most
        implementations will not recognize it.
    4 RR or ACK
        Packet acknowledgement.  The YYY field contains the sequence
        number of the last correctly received packet.
    5 INITC
        Third initialization packet.  The YYY field contains the
        maximum window size to use.
    6 INITB
        Second initialization packet.  The YYY field contains the
        packet size to use.  It requests a size of 2 ** (YYY + 5);
        note that this is not the same coding used for the k byte in
        the packet header (it is 1 less).  Some UUCP implementations
        can handle any packet size up to that specified; some can only
        handled exactly the size specified.
    7 INITA
        First initialization packet.  The YYY field contains the
        maximum window size to use.

The checksum of a control packet is simply 0xaaaa - the control byte.

The checksum of a data packet is 0xaaaa - (CHECK ^ the control byte),
where ^ denotes exclusive or, and CHECK is the result of the following
routine as run on the contents of the data field (every byte in the
data field participates in the checksum, even for a short data
packet).  This is the routine used by Taylor UUCP, and is a slightly
modified version of a routine which John Gilmore patched from G.L.
Chesson's original paper.  The z argument points to the data and the c
argument indicates how much data there is.

int
igchecksum (z, c)
     register const char *z;
     register int c;
{
  register unsigned int ichk1, ichk2;

  ichk1 = 0xffff;
  ichk2 = 0;

  do
    {
      register unsigned int b;

      /* Rotate ichk1 left.  */
      if ((ichk1 & 0x8000) == 0)
        ichk1 <<= 1;
      else
        {
          ichk1 <<= 1;
          ++ichk1;
        }

      /* Add the next character to ichk1.  */
      b = *z++ & 0xff;
      ichk1 += b;

      /* Add ichk1 xor the character position in the buffer counting from
         the back to ichk2.  */
      ichk2 += ichk1 ^ c;

      /* If the character was zero, or adding it to ichk1 caused an
         overflow, xor ichk2 to ichk1.  */
      if (b == 0 || (ichk1 & 0xffff) < b)
        ichk1 ^= ichk2;
    }
  while (--c > 0);

  return ichk1 & 0xffff;
}

When the 'g' protocol is started, the calling UUCP sends an INITA
control packet with the window size it wishes the called UUCP to use.
The called UUCP responds with an INITA packet with the window size it
wishes the calling UUCP to use.  Pairs of INITB and INITC packets are
then similarly exchanged.  When these exchanges are completed, the
protocol is considered to have been started.

When a UUCP package transmits a command, it sends one or more data
packets.  All the data packets will normally be complete, although
some UUCP packages may send a the last one as a short packet.  The
UUCP package receiving the command will know when the command has
finished because it will be terminated by a null byte sent as data.
Some UUCP packages require the last byte of the last packet sent to be
null, even if the command ends earlier in the packet.  Some packages
may require all the trailing bytes in the last packet to be null, but
I have not confirmed this.

When a UUCP package sends a file, it will send a sequence of data
packets.  The end of the file is signalled by a short data packet
containing zero valid bytes (it will normally be preceeded by a short
data packet containing the last few bytes in the file).

Note that the sequence numbers cover the entire communication session,
including both command and file data.

When the protocol is shut down, each UUCP package sends a CLOSE
control packet.

*) What is the 'f' protocol?

The 'f' protocol is a seven bit protocol which checksums an entire
file at a time.  It only uses the characters between \040 and \176
(ASCII space and ~) inclusive as well as the carriage return
character.  It can be very efficient for transferring text only data,
but it is very inefficient at transferring eight bit data (such as
compressed news).  It is not flow controlled, and the checksum is
fairly insecure over large files, so using it over a serial connection
requires handshaking (XON/XOFF can be used) and error correcting
modems.  Some people think it should not be used even under those
circumstances.

I believe the 'f' protocol originated in BSD versions of UUCP.  It was
originally intended for transmission over X.25 PAD links.

The 'f' protocol has no startup or finish protocol.  However, both
sides should sleep for a couple of seconds before starting up, because
typically they will switch the terminal into XON/XOFF mode and want to
allow the changes to settle before beginning transmission.

When a UUCP package transmits a command, it simply sends a string
terminated by a carriage return.

When a UUCP package transmits a file, each byte b of the file is
translated according to the following table:

       0 <= b <=  037: 0172, b + 0100 (0100 to 0137)
     040 <= b <= 0171:       b        ( 040 to 0171)
    0172 <= b <= 0177: 0173, b - 0100 ( 072 to  077)
    0200 <= b <= 0237: 0174, b - 0100 (0100 to 0137)
    0240 <= b <= 0371: 0175, b - 0200 ( 040 to 0171)
    0372 <= b <= 0377: 0176, b - 0300 ( 072 to  077)

That is, a byte between \040 and \171 inclusive is transmitted as is,
and all other bytes are prefixed and modified as shown.

When all the file data is sent, a seven byte sequence is sent: two
bytes of \176 followed by four ASCII bytes of the checksum as printed
in base 16 followed by a carriage return.  For example, if the
checksum was 0x1234, this would be sent: "\176\1761234\r".

The checksum is initialized to 0xffff.  For each byte that is sent it
is modified as follows (where b is the byte before it has been
transformed as described above):

      /* Rotate the checksum left.  */
      if ((ichk & 0x8000) == 0)
        ichk <<= 1;
      else
        {
          ichk <<= 1;
          ++ichk;
        }

      /* Add the next byte into the checksum.  */
      ichk += b;

When the receiving UUCP sees the checksum, it compares it against its
own calculated checksum and replies with a single character followed
by a carriage return.
    G
        The file was received correctly.
    R
        The checksum did not match, and the file should be resent from
        the beginning.
    Q
        The checksum did not match, but too many retries have occurred
        and the communication session should be abandoned.

The sending UUCP checks the returned character and acts accordingly.

*) What is the 't' protocol?

The 't' protocol is intended for TCP links.  It does no error checking
or flow control, and requires an eight bit clear channel.

I believe the 't' protocol originated in BSD versions of UUCP.

When a UUCP package transmits a command, it first gets the length of
the command string, C.  It then sends ((C / 512) + 1) * 512 bytes (the
smallest multiple of 512 which can hold C bytes plus a null byte)
consisting of the command string itself followed by trailing null
bytes.

When a UUCP package sends a file, it sends it in blocks.  Each block
contains at most 1024 bytes of data.  Each block consists of four
bytes containing the amount of data in binary (most significant byte
first, the same format as used by the Unix function htonl) followed by
that amount of data.  The end of the file is signalled by a block
containing zero bytes of data.

*) What is the 'e' protocol?

The 'e' protocol is similar to the 't' protocol.  It does no flow
control or error checking and is intended for use over TCP.

The 'e' protocol originated in versions of HDB UUCP.

When a UUCP package transmits a command, it simply sends the command
as an ASCII string terminated by a null byte.

When a UUCP package transmits a file, it sends the complete size of
the file as an ASCII decimal number.  The ASCII string is padded out
to 20 bytes with null bytes (i.e. if the file is 1000 bytes long, it
sends "1000\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0").  It then sends the
entire file.

*) What is the 'x' protocol?

I believe that the 'x' protocol was intended for use over X.25 virtual
circuits.  It relies on a write of zero bytes being read as zero bytes
without stopping communication.  I have heard that it does not work
correctly.  If someone would care to fill this in more, I would be
grateful.
--
Ian Taylor                  [e-mail]                  uunet!airs!ian
First person to identify this quote wins a free e-mail message:
``The will to be stupid is a very powerful force.''