The Hacker's Encyclopedia 1998

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Article 3084 of comp.mail.uucp: Xref: softwords comp.mail.uucp:3084 news.answers:2184 Path: softwords!news.UVic.CA!ubc-cs!destroyer!uunet!airs!ian From: ian@airs.com (Ian Lance Taylor) Newsgroups: comp.mail.uucp,news.answers Subject: UUCP Internals Frequently Asked Questions Keywords: UUCP, protocol, FAQ Message-ID: <uucp-internals_711880203@airs.com> Date: 23 Jul 92 08:30:11 GMT Expires: 3 Sep 92 08:30:03 GMT Sender: news@airs.com Reply-To: ian@airs.com (Ian Lance Taylor) Followup-To: comp.mail.uucp Lines: 1132 Approved: news-answers-request@MIT.Edu Supersedes: <uucp-internals_709430249@airs.com> Archive-name: uucp-internals Version: $Revision: 1.6 $ Last-modified: $Date: 1992/06/24 23:58:51 $ 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. 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.* files? 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 'x' protocol? What is the 'd' protocol? What is the 'h' 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. 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 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. ------------------------------ 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 transmit the XON and XOFF 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. I do not know how command grades are handled in 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 'Z' and news at grade 'd'. The example above would allow mail 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.* files? 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 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 it opens. E This command takes no arguments. It means that the command should be processed with execve. 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. 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. -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 also 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 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. 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 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, 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 (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 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 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. 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 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 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. 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 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. ------------------------------ 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 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. I do not know why SVR4 chose to implement a new protocol using a new letter, rather than simply making the 'g' protocol adjustable. 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-x Subject: 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, and it is not clear to me that it was ever actually used. Please correct me if I am wrong. ------------------------------ From: UUCP-d Subject: What is the 'd' protocol? This is apparently used for DataKit connections, and relies on a write of zero bytes being read as zero bytes, much as the 'x' protocol does. I don't really know anything else about it. ------------------------------ 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: 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> celit!billd@UCSD.EDU (Bill Davidson) Matthew Farwell <dylan@ibmpcug.co.uk> "Jonathan I. Kamens" <jik@pit-manager.MIT.EDU> "David J. MacKenzie" <djm@eng.umd.edu> david nugent <david@csource.oz.au> Stephen.Page@prg.oxford.ac.uk joey@tessi.UUCP (Joey Pruett) 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 ****************************** -- Ian Taylor | ian@airs.com | First to identify quote wins free e-mail message: ``It takes a man months and months to reconcile himself to a new hat. And just when you're preparing to send it to the jumble sale, he says, `That's rather a nice hat you've got on, where did you get it?' ''