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Archive-name: uucp-internals
Version: $Revision: 1.14 $
Last-modified: $Date: 1993/04/22 00:27:10 $
This article was written by Ian Lance Taylor <ian@airs.com> and I may
even update it periodically. Please send me mail about suggestions
or inaccuracies.
This article describes how the various UUCP protocols work, and
discusses some other internal UUCP issues. It does not describe how
to configure UUCP, nor how to solve UUCP connection 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.
This article is in digest format. Some newsreaders will be able to
break it apart into separate articles. Please don't ask me how to do
this, though.
This article answers the following questions. If one of these
questions is posted to comp.mail.uucp, please 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.
Sources
What does "alarm" mean in debugging output?
What are UUCP grades?
What is the format of a UUCP lock file?
What is the format of a UUCP X.* file?
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 'G' protocol?
What is the 'i' protocol?
What is the 'x' protocol?
What is the 'd' protocol?
What is the 'h' protocol?
What is the 'v' protocol?
Thanks
----------------------------------------------------------------------
From: Sources
Subject: Sources
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.
103 Morris Street, Suite A
Sebastopol, CA 95472
It is currently in its tenth edition. The ISBN number is
0-937175-93-5.
Some information is originally due to a Usenet article by Chuck
Wegrzyn. The information on execution files comes partially from
Peter Honeyman. 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. The information on
the 'd' protocol comes from Jonathan Clark, who also supplied
information about QFT. The FSUUCP information comes straight from
Christopher J. Ambler.
Although there are few books about UUCP, there are many about networks
and protocols in general. I recommend two non-technical books which
describe the sorts of things that are available on the network: ``The
Whole Internet,'' by Ed Krol, and ``Zen and the Art of the Internet,''
by Brendan P. Kehoe. Good technical discussions of networking issues
can be found in ``Internetworking with TCP/IP,'' by Douglas E. Comer
and David L. Stevens and in ``Design and Validation of Computer
Protocols'' by Gerard J. Holzmann.
------------------------------
From: alarm
Subject: What does "alarm" mean in debugging output?
The debugging output of many versions of UUCP (but not Taylor UUCP)
will include messages like
alarm 1
or
pkcget: alarm 1
This message means that the UUCP package has timed out while waiting
for some sort of response from the remote system. This normally
indicates some sort of connection problem. For example, the modems
might have lost their connection, or perhaps one of the modems will
not transmit the XON and XOFF characters, or perhaps one side or the
other is dropping characters. It can also mean that the packages
disagree about some aspect of the UUCP protocol, although this is less
common.
Using the information in the rest of this posting, you should be able
to figure out what type of data your UUCP was expecting to receive.
This may give some indication as to exactly what the problem is. It
is difficult to be more specific, since there are many possiblities.
------------------------------
From: UUCP-grades
Subject: What are UUCP grades?
Modern UUCP packages support grades for each command. The grades
generally range from 'A' (the highest) to 'Z' followed by 'a' to 'z'.
Some UUCP packages also support '0' to '9' before 'A'. Some UUCP
packages may permit any ASCII character as a grade.
On Unix, these grades are encoded in the name of the command file. A
command file name generally has the form
C.nnnngssss
where nnnn is the remote system name for which the command is queued,
g is a single character grade, and ssss is a four character sequence
number. For example, a command file created for the system ``airs''
at grade 'Z' might be named
C.airsZ2551
The remote system name will be truncated to seven characters, to
ensure that the command file name will fit in the 14 character file
name limit of the traditional Unix file system. UUCP packages which
have no other means of distinguishing which command files are intended
for which systems thus require all systems they connect to to have
names that are unique in the first seven characters. Some UUCP
packages use a variant of this format which truncates the system name
to six characters. HDB and Taylor UUCP use a different spool
directory format, which allows up to fourteen characters to be used
for each system name.
The sequence number in the command file name may be a decimal integer,
or it may be a hexadecimal integer, or it may contain any alphanumeric
character. Different UUCP packages are different.
FSUUCP (a DOS based UUCP and news package) uses up to 8 characters for
file names in the spool (this is a DOS file name limitation; actually,
with the extension, 11 characters are available, but FSUUCP reserves
that for future use). FSUUCP defaults mail to grade D, and news to
grade N, except that when the grade of incoming mail can be
determined, that grade is preserved if the mail is forwarded to
another system. Mail and news are currently the only 2 types of
transfers supported. The default grades may be changed by editing
the MAIL.RC file for mail, or the FSUUCP.CFG file for news.
I do not know how command grades are handled in other non-Unix UUCP
packages.
Modern UUCP packages allow you to restrict file transfer by grade
depending on the time of day. Typically this is done with a line in
the Systems (or L.sys) file like this:
airs Any/Z,Any2305-0855 ...
This allows grades 'Z' and above to be transferred at any time. Lower
grades may only be transferred at night. I believe that this grade
restriction applies to local commands as well as to remote commands,
but I am not sure. It may only apply if the UUCP package places the
call, not if it is called by the remote system. Taylor UUCP can use
the ``timegrade'' and ``call-timegrade'' commands to achieve the same
effect (and supports the above format when reading Systems or L.sys).
This sort of grade restriction is most useful if you know what grades
are being used at the remote site. The default grades used depend on
the UUCP package. Generally uucp and uux have different defaults. A
particular grade can be specified with the -g option to uucp or uux.
For example, to request execution of rnews on airs with grade 'd', you
might use something like
uux -gd - airs!rnews <article
Uunet queues up mail at grade 'C', but increases the grade based on
the size. News is queued at grade 'd', and file transfers at grade
'n'. The example above would allow mail (below some large size) to be
received at any time, but would only permit news to be transferred at
night.
------------------------------
From: UUCP-lock-file
Subject: What is the format of a UUCP lock file?
This discussion applies only to Unix. I have no idea how UUCP locks
ports on other systems.
UUCP creates files to lock serial ports and systems. On most if not
all systems these same lock files are also used by cu to coordinate
access to serial ports. On some systems getty also uses these lock
files, often under the name uugetty.
The lock file normally contains the process ID of the locking process.
This makes it easy to determine whether a lock is still valid. The
algorithm is to create a temporary file and then link it to the name
that must be locked. If the link fails because a file with that name
already exists, the existing file is read to get the process ID. If
the process still exists, the lock attempt fails. Otherwise the lock
file is deleted and the locking algorithm is retried.
Older UUCP packages put the lock files in the main UUCP spool
directory, /usr/spool/uucp. HDB UUCP generally puts the lock files in
a directory of their own, usually /usr/spool/locks or /etc/locks.
The original UUCP lock file format encodes the process ID as a four
byte binary number. The order of the bytes is host-dependent. HDB
UUCP stores the process ID as a ten byte ASCII decimal number, with a
trailing newline. For example, if process 1570 holds a lock file, it
would contain the eleven characters space, space, space, space, space,
space, one, five, seven, zero, newline. Some versions of UUCP add a
second line indicating which program created the lock (uucp, cu, or
getty/uugetty). I have also seen a third type of UUCP lock file which
does not contain the process ID at all.
The name of the lock file is traditionally "LCK.." followed by the
base name of the device. For example, to lock /dev/ttyd0 the file
LCK..ttyd0 would be created. On SCO Unix, the lock file name is
always forced to lower case even if the device name has upper case
letters.
System V Release 4 UUCP names the lock file using the major and minor
device numbers rather than the device name. The file is named
LK.XXX.YYY.ZZZ, where XXX, YYY and ZZZ are all three digit decimal
numbers. XXX is the major device number of the device holding the
directory holding the device file (e.g., /dev). YYY is the major
device number of the device file itself. ZZZ is the minor device
number of the device file itself. If s holds the result of passing
the device to the stat system call (e.g., stat ("/dev/ttyd0", &s)),
the following line of C code will print out the corresponding lock
file name:
printf ("LK.%03d.%03d.%03d", major (s.st_dev),
major (s.st_rdev), minor (s.st_rdev));
The advantage of this system is that even if there are several links
to the same device, they will all use the same lock file name.
------------------------------
From: X-file
Subject: What is the format of a UUCP X.* file?
UUCP X.* files control program execution. They are created by uux.
They are transferred between computers just like any other file. The
uuxqt daemon reads them to figure out how to execute the job requested
by uux.
An X.* file is simply a text file. The first character of each line
is a command, and the remainder of the line supplies arguments. The
following commands are defined:
C command
This gives the command to execute, including the program and
all arguments. For example,
C rmail ian@airs.com
U user system
This names the user who requested the command, and the system
from which the request came.
I standard-input
This names the file from which standard input is taken. If no
standard input file is given, the standard input will probably
be attached to /dev/null. If the standard input file is not
from the system on which the execution is to occur, it will
also appear in an F command.
O standard-output [ system ]
This names the standard output file. The optional second
argument names the system to which the file should be sent.
If there is no second argument, the file should be created on
the executing system.
F required-file [ filename-to-use ]
The F command can appear multiple times. Each F command names
a file which must exist before the execution can proceed.
This will usually be a file which is transferred from the
system on which uux was executed, but it can also be a file
from the local system or some other system. If the file is
not from the local system, then the command will usually name
a file in the spool directory. If the optional second
argument appears, then the file should be copied to the
execution directory under that name. This is necessary for
any file other than the standard input file. If the standard
input file is not from the local system, it will appear in
both an F command and an I command.
R requestor-address
This is the address to which mail about the job should be
sent. It is relative to the system named in the U command.
If the R command does not appear, then mail is sent to the
user named in the U command.
Z
This command takes no arguments. It means that a mail message
should be sent if the command failed. This is the default
behaviour for most modern UUCP packages, and for them the Z
command does not actually do anything.
N
This command takes no arguments. It means that no mail
message should be sent, even if the command failed.
n
This command takes no arguments. It means that a mail message
should be sent if the command succeeded. Normally a message
is sent only if the command failed.
B
This command takes no arguments. It means that the standard
input should be returned with any error message. This can be
useful in cases where the input would otherwise be lost.
e
This command takes no arguments. It means that the command
should be processed with /bin/sh. For some packages this is
the default anyhow. Most packages will refuse to execute
complex commands or commands containing wildcards, because of
the security holes this opens.
E
This command takes no arguments. It means that the command
should be processed with the execve system call. For some
packages this is the default anyhow.
M status-file
This command means that instead of mailing a message, the
message should be copied to the named file on the system named
by the U command.
# comment
This command is ignored, as is any other unrecognized command.
Here is an example. Given the following command executed on system
test1
uux - test2!cat - test2!~ian/bar !qux '>~/gorp'
(this is only an example, as most UUCP systems will not permit the cat
command to be executed) Taylor UUCP will produce the following X.
file:
U ian test1
F D.test1N003r qux
O /usr/spool/uucppublic test1
F D.test1N003s
I D.test1N003s
C cat - ~ian/bar qux
The standard input will be read into a file and then transferred to
the file D.test1N003s on system test2, and the file qux will be
transferred to D.test1N003r on system test2. When the command is
executed, the latter file will be copied to the execution directory
under the name qux. Note that since the file ~ian/bar is already on
the execution system, no action need be taken for it. The standard
output will be collected in a file, then copied to the directory
/usr/spool/uucppublic on the system test1.
------------------------------
From: UUCP-protocol
Subject: 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). A few systems end
these messages with a line feed character (\012) 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. Some
are supported by FSUUCP, which is a DOS based UUCP and news package.
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. 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. This is
not supported by FSUUCP.
-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 and QFT.
-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 FSUUCP. FSUUCP reports the lesser of the
available disk space on the spool directory drive and the
ulimit variable in FSUUCP.CFG.
-N
Indicates that the calling UUCP understands the Taylor
UUCP size limiting extensions. Supported only by Taylor
UUCP and FSUUCP.
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 also understands the Taylor UUCP size limiting
extensions. Supported only by Taylor UUCP and FSUUCP.
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 FSUUCP.
-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: \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.
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.
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.
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 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 (not supported by FSUUCP).
n
The slave should send mail to NOTIFY when the
transfer is complete (not supported by FSUUCP).
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
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 not used by FSUUCP, since it is
meaningless on DOS.
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 size negotiation,
with Taylor UUCP, SVR4 UUCP, QFT or FSUUCP. It is the
size of the file in bytes. SVR4 UUCP, QFT and FSUUCP send
the size in base 16 as 0x.... while Taylor UUCP sends the
size as a decimal integer (a later version of Taylor UUCP
will probably change to the SVR4 behaviour).
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 and FSUUCP 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 and FSUUCP 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.
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.
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 or FSUUCP 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. FSUUCP does
not support R requests, and always responds with RN2.
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 might
succeed later, or it might 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. 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.
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 protocol).
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.
------------------------------
From: UUCP-g
Subject: 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 to 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 range from 1 to 7, and may be
different in each direction. 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 in reply 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). Most UUCP implementations
that request a packet size larger than 64 bytes can handle any
packet size up to that 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). Below is the routine used by Taylor UUCP; it 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 the last one as a short packet. The
command string is sent with a trailing null byte, to let the receiving
package know when the command is finished. 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.
------------------------------
From: UUCP-f
Subject: 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 typically sleep for a couple of seconds before starting up,
because they 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.
------------------------------
From: UUCP-t
Subject: What is the 't' protocol?
The 't' protocol is intended for use on links which provide reliable
end-to-end connections, such as TCP. 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.
------------------------------
From: UUCP-e
Subject: 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 networks
providing reliable end-to-end connections, such as 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.
------------------------------
From: UUCP-G
Subject: What is the 'G' protocol?
The 'G' protocol is used by SVR4 UUCP. It is identical to the 'g'
protocol, except that it is possible to modify the window and packet
sizes. The SVR4 implementation of the 'g' protocol reportedly is
fixed at a packet size of 64 and a window size of 7. Supposedly SVR4
chose to implement a new protocol using a new letter to avoid any
potential incompatibilities when using different packet or window
sizes.
Most implementations of the 'g' protocol that accept packets larger
than 64 bytes will also accept packets smaller than whatever they
requested in the INITB packet. The SVR4 'G' implementation is an
exception; it will only accept packets of precisely the size it
requests in the INITB packet.
------------------------------
From: UUCP-i
Subject: What is the 'i' protocol?
The 'i' protocol was written by Ian Lance Taylor (who also wrote this
FAQ). It is used by Taylor UUCP version 1.04.
It is a sliding window packet protocol, like the 'g' protocol, but it
supports bidirectional transfers (i.e., file transfers in both
directions simultaneously). It requires an eight bit clear
connection. Several ideas for the protocol were taken from the paper
``A High-Throughput Message Transport System'' by P. Lauder. I don't
know where the paper was published, but the author's e-mail address is
piers@cs.su.oz.au. The 'i' protocol does not adopt his main idea,
which is to dispense with windows entirely. This is because some
links still do require flow control and, more importantly, because
using windows sets a limit to the amount of data which the protocol
must be able to resend upon request. To reduce the costs of window
acknowledgements, the protocol uses a large window and only requires
an ack at the halfway point.
Each packet starts with a six byte header, optionally followed by data
bytes with a four byte checksum. There are currently five defined
packet types (DATA, SYNC, ACK, NAK, SPOS, CLOSE) which are described
below. Although any packet type may include data, any data provided
with an ACK, NAK or CLOSE packet is ignored.
Every DATA, SPOS and CLOSE packet has a sequence number. The sequence
numbers are independent for each side. The first packet sent by each
side is always number 1. Each packet is numbered one greater than the
previous packet, modulo 32.
Every packet has a local channel number and a remote channel number.
For all packets at least one channel number is zero. When a UUCP
command is sent to the remote system, it is assigned a non-zero local
channel number. All packets associated with that UUCP command sent by
the local system are given the selected local channel number. All
associated packets sent by the remote system are given the selected
number as the remote channel number. This permits each UUCP command
to be uniquely identified by the channel number on the originating
system, and therefore each UUCP package can associate all file data
and UUCP command responses with the appropriate command. This is a
requirement for bidirectional UUCP transfers.
The protocol maintains a single global file position, which starts at
0. For each incoming packet, any associated data is considered to
occur at the current file position, and the file position is
incremented by the amount of data contained. The exception is a
packet of type SPOS, which is used to change the file position.
The reason for keeping track of the file position is described below.
The header is as follows:
\007
Every packet begins with ^G.
(PACKET << 3) + LOCCHAN
The five bit packet number combined with the three bit local
channel number. DATA, SPOS and CLOSE packets use the packet
sequence number for the PACKET field. NAK packet types use
the PACKET field for the sequence number to be resent. ACK
and SYNC do not use the PACKET field, and generally leave it
set to 0. Packets which are not associated with a UUCP
command from the local system use a local channel number of 0.
(ACK << 3) + REMCHAN
The five bit packet acknowledgement combined with the three
bit remote channel number. The packet acknowledgement is the
number of the last packet successfully received; it is used by
all packet types. Packets which are not sent in response to a
UUCP command from the remote system use a remote channel
number of 0.
(TYPE << 5) + (CALLER << 4) + LEN1
The three bit packet type combined with the one bit packet
direction combined with the upper four bits of the data
length. The packet direction bit is always 1 for packets sent
by the calling UUCP, and 0 for packets sent by the called
UUCP. This prevents confusion caused by echoed packets.
LEN2
The lower eight bits of the data length. The twelve bits of
data length permit packets ranging in size from 0 to 4095
bytes.
CHECK
The exclusive or of the second through fifth bytes of the
header. This provides an additional check that the header is
valid.
If the data length is non-zero, the packet is immediately followed by
the specified number of data bytes. The data bytes are followed by a
four byte CRC 32 checksum, with the most significant byte first. The
CRC is calculated over the contents of the data field.
The defined packet types are as follows:
0 (DATA)
This is a plain data packet.
1 (SYNC)
SYNC packets are exchanged when the protocol is initialized,
and are described further below. SYNC packets do not carry
sequence numbers (that is, the PACKET field is ignored).
2 (ACK)
This is an acknowledgement packet. Since DATA packets also
carry packet acknowledgements, ACK packets are only used when
one side has no data to send. ACK packets do not carry
sequence numbers.
3 (NAK)
This is a negative acknowledgement. This is sent when a
packet is received incorrectly, and means that the packet
number appearing in the PACKET field must be resent. NAK
packets do not carry sequence numbers (the PACKET field is
already used).
4 (SPOS)
This packet changes the file position. The packet contains
four bytes of data holding the file position, most significant
byte first. The next packet received will be considered to be
at the named file position.
5 (CLOSE)
When the protocol is shut down, each side sends a CLOSE
packet. This packet does have a sequence number, which could
be used to ensure that all packets were correctly received
(this is not needed by UUCP, however, which uses the higher
level H command with an HY response).
When the protocol starts up, both systems send a SYNC packet. The
SYNC packet includes three bytes of data (additional data bytes may be
defined in the future). The first two bytes are the maximum packet
size the remote system should send, most significant byte first. The
third byte is the window size the remote system should use. The
remote system may send packets of any size up to the maximum.
The window size is the number of packets that may be sent before a
packet is acknowledged. There is no requirement that every packet be
acknowledged; any acknowledgement is considered to acknowledge all
packets through the number given. In the current implementation, if
one side has no data to send, it sends an ACK when half the window is
received.
Note that the NAK packet corresponds to the unused 'g' protocol SRJ
packet type, rather than to the RJ packet type. When a NAK is
received, only the named packet should be resent, not any subsequent
packets.
Note that if both sides have data to send, but a packet is lost, it is
perfectly reasonable for one side to continue sending packets, all of
which will acknowledge the last packet correctly received, while the
system whose packet was lost will be unable to send a new packet
because the send window will be full. In this circumstance, neither
side will time out and one side of the communication will be
effectively shut down for a while. Therefore, any system with
outstanding unacknowledged packets should arrange to time out and
resend a packet even if data is being received.
Commands are sent as a sequence of data packets with a non-zero local
channel number. The last data packet for a command includes a
trailing null byte (normally a command will fit in a single data
packet). Files are sent as a sequence of data packets ending with one
of length zero.
The channel numbers permit a more efficient implementation of the UUCP
file send command. Rather than send the command and then wait for the
SY response before sending the file, the file data is sent beginning
immediately after the S command is sent. If an SN response is
received, the file send is aborted, and a final data packet of length
zero is sent to indicate that the channel number may be reused. If an
SY reponse with a file position indicator is received, the file send
adjusts to the file position; this is why the protocol maintains a
global file position.
Note that the use of channel numbers means that each UUCP system may
send commands and file data simultaneously. Moreover, each UUCP
system may send multiple files at the same time, using the channel
number to disambiguate the data. Sending a file before receiving an
acknowledgement for the previous file helps to eliminate the round
trip delays inherent in other UUCP protocols.
------------------------------
From: UUCP-x
Subject: What is the 'x' protocol?
The 'x' protocol is used in Europe (and probably elsewhere) with
machines that contain an builtin X.25 card and can send eight bit data
transparently across X.25 circuits, without interference from the X.28
or X.29 layers. The protocol sends packets of 512 bytes, and relies
on a write of zero bytes being read as zero bytes without stopping
communication. It first appeared in the original System V UUCP
implementation.
------------------------------
From: UUCP-d
Subject: What is the 'd' protocol?
This is apparently used for DataKit muxhost (not RS-232) connections.
No file size is sent. When a file has been completely transferred, a
write of zero bytes is done; this must be read as zero bytes on the
other end.
------------------------------
From: UUCP-h
Subject: What is the 'h' protocol?
This is apparently used in some places with HST modems. It does no
error checking, and is not that different from the 't' protocol. I
don't know the details.
------------------------------
From: UUCP-v
Subject: What is the 'v' protocol?
The 'v' protocol is used by UUPC/extended, a DOS UUCP program. It is
simply a version of the 'g' protocol which supports packets of any
size, and also supports sending packets of different sizes during the
same conversation. There are many 'g' protocol implementations which
support both, but there are also many which do not. Using 'v' ensures
that everything is supported.
------------------------------
From: Thanks
Subject: Thanks
Besides the papers and information acknowledged at the top of this
article, the following people have contributed help, advice,
suggestions and information:
Earle Ake 513-429-6500 <ake@Dayton.SAIC.COM>
cambler@nike.calpoly.edu (Christopher J. Ambler)
jhc@iscp.bellcore.com (Jonathan Clark)
celit!billd@UCSD.EDU (Bill Davidson)
"Drew Derbyshire" <ahd@kew.com>
erik@pdnfido.fidonet.org
Matthew Farwell <dylan@ibmpcug.co.uk>
Romain Kang <romain@pyramid.com>
"Jonathan I. Kamens" <jik@GZA.COM>
"David J. MacKenzie" <djm@eng.umd.edu>
jum@helios.de (Jens-Uwe Mager)
david nugent <david@csource.oz.au>
Stephen.Page@prg.oxford.ac.uk
joey@tessi.UUCP (Joey Pruett)
James Revell <revell@uunet.uu.net>
Larry Rosenman <ler@lerami.lerctr.org>
Rich Salz <rsalz@bbn.com>
kls@ditka.Chicago.COM (Karl Swartz)
Dima Volodin <dvv@hq.demos.su>
jon@console.ais.org (Jon Zeeff)
------------------------------
End of UUCP Internals Frequently Asked Questions
