freebsd/contrib/cvs/doc/cvsclient.texi

\input texinfo @c -*- texinfo -*-

@setfilename cvsclient.info
@include CVSvn.texi

@node Top
@top CVS Client/Server

This document describes the client/server protocol used by CVS.  It does
not describe how to use or administer client/server CVS; see the regular
CVS manual for that.  This is version @value{CVSVN} of the protocol
specification---@xref{Introduction}, for more on what this version number
means.

@menu
* Introduction::      What is CVS and what is the client/server protocol for?
* Goals::             Basic design decisions, requirements, scope, etc.
* Connection and Authentication::  Various ways to connect to the server
* Password scrambling::  Scrambling used by pserver
* Protocol::          Complete description of the protocol
* Protocol Notes::    Possible enhancements, limitations, etc. of the protocol
@end menu

@node Introduction
@chapter Introduction

CVS is a version control system (with some additional configuration
management functionality).  It maintains a central @dfn{repository}
which stores files (often source code), including past versions,
information about who modified them and when, and so on.  People who
wish to look at or modify those files, known as @dfn{developers}, use
CVS to @dfn{check out} a @dfn{working directory} from the repository, to
@dfn{check in} new versions of files to the repository, and other
operations such as viewing the modification history of a file.  If
developers are connected to the repository by a network, particularly a
slow or flaky one, the most efficient way to use the network is with the
CVS-specific protocol described in this document.

Developers, using the machine on which they store their working
directory, run the CVS @dfn{client} program.  To perform operations
which cannot be done locally, it connects to the CVS @dfn{server}
program, which maintains the repository.  For more information on how
to connect see @ref{Connection and Authentication}.

This document describes the CVS protocol.  Unfortunately, it does not
yet completely document one aspect of the protocol---the detailed
operation of each CVS command and option---and one must look at the CVS
user documentation, @file{cvs.texinfo}, for that information.  The
protocol is non-proprietary (anyone who wants to is encouraged to
implement it) and an implementation, known as CVS, is available under
the GNU Public License.  The CVS distribution, containing this
implementation, @file{cvs.texinfo}, and a copy (possibly more or less up
to date than what you are reading now) of this document,
@file{cvsclient.texi}, can be found at the usual GNU FTP sites, with a
filename such as @file{cvs-@var{version}.tar.gz}.

This is version @value{CVSVN} of the protocol specification.  This
version number is intended only to aid in distinguishing different
versions of this specification.  Although the specification is currently
maintained in conjunction with the CVS implementation, and carries the
same version number, it also intends to document what is involved with
interoperating with other implementations (such as other versions of
CVS); see @ref{Requirements}.  This version number should not be used
by clients or servers to determine what variant of the protocol to
speak; they should instead use the @code{valid-requests} and
@code{Valid-responses} mechanism (@pxref{Protocol}), which is more
flexible.

@node Goals
@chapter Goals

@itemize @bullet
@item
Do not assume any access to the repository other than via this protocol.
It does not depend on NFS, rdist, etc.

@item
Providing a reliable transport is outside this protocol.  The protocol
expects a reliable transport that is transparent (that is, there is no
translation of characters, including characters such as such as
linefeeds or carriage returns), and can transmit all 256 octets (for
example for proper handling of binary files, compression, and
encryption).  The encoding of characters specified by the protocol (the
names of requests and so on) is the invariant ISO 646 character set (a
subset of most popular character sets including ASCII and others).  For
more details on running the protocol over the TCP reliable transport,
see @ref{Connection and Authentication}.

@item
Security and authentication are handled outside this protocol (but see
below about @samp{cvs kserver} and @samp{cvs pserver}).

@item
The protocol makes it possible for updates to be atomic with respect to
checkins; that is if someone commits changes to several files in one cvs
command, then an update by someone else would either get all the
changes, or none of them.  The current @sc{cvs} server can't do this,
but that isn't the protocol's fault.

@item
The protocol is, with a few exceptions, transaction-based.  That is, the
client sends all its requests (without waiting for server responses),
and then waits for the server to send back all responses (without
waiting for further client requests).  This has the advantage of
minimizing network turnarounds and the disadvantage of sometimes
transferring more data than would be necessary if there were a richer
interaction.  Another, more subtle, advantage is that there is no need
for the protocol to provide locking for features such as making checkins
atomic with respect to updates.  Any such locking can be handled
entirely by the server.  A good server implementation (such as the
current @sc{cvs} server) will make sure that it does not have any such
locks in place whenever it is waiting for communication with the client;
this prevents one client on a slow or flaky network from interfering
with the work of others.

@item
It is a general design goal to provide only one way to do a given
operation (where possible).  For example, implementations have no choice
about whether to terminate lines with linefeeds or some other
character(s), and request and response names are case-sensitive.  This
is to enhance interoperability.  If a protocol allows more than one way
to do something, it is all too easy for some implementations to support
only some of them (perhaps accidentally).
@c I vaguely remember reading, probably in an RFC, about the problems
@c that were caused when some people decided that SMTP should accept
@c other line termination (in the message ("DATA")?) than CRLF.  However, I
@c can't seem to track down the reference.
@end itemize

@node Connection and Authentication
@chapter How to Connect to and Authenticate Oneself to the CVS server

Connection and authentication occurs before the CVS protocol itself is
started.  There are several ways to connect.

@table @asis
@item server
If the client has a way to execute commands on the server, and provide
input to the commands and output from them, then it can connect that
way.  This could be the usual rsh (port 514) protocol, Kerberos rsh,
SSH, or any similar mechanism.  The client may allow the user to specify
the name of the server program; the default is @code{cvs}.  It is
invoked with one argument, @code{server}.  Once it invokes the server,
the client proceeds to start the cvs protocol.

@item kserver
The kerberized server listens on a port (in the current implementation,
by having inetd call "cvs kserver") which defaults to 1999.  The client
connects, sends the usual kerberos authentication information, and then
starts the cvs protocol.  Note: port 1999 is officially registered for
another use, and in any event one cannot register more than one port for
CVS, so GSS-API (see below) is recommended instead of kserver as a way
to support kerberos.

@item pserver
The name @dfn{pserver} is somewhat confusing.  It refers to both a
generic framework which allows the CVS protocol to support several
authentication mechanisms, and a name for a specific mechanism which
transfers a username and a cleartext password.  Servers need not support
all mechanisms, and in fact servers will typically want to support only
those mechanisms which meet the relevant security needs.

The pserver server listens on a port (in the current
implementation, by having inetd call "cvs pserver") which defaults to
2401 (this port is officially registered).  The client
connects, and sends the following:

@itemize @bullet
@item
the string @samp{BEGIN AUTH REQUEST}, a linefeed, 
@item
the cvs root, a linefeed,
@item
the username, a linefeed,
@item
the password trivially encoded (see @ref{Password scrambling}), a
linefeed,
@item
the string @samp{END AUTH REQUEST}, and a linefeed.
@end itemize

The client must send the
identical string for cvs root both here and later in the
@code{Root} request of the cvs
protocol itself.  Servers are encouraged to enforce this restriction.
The possible server responses (each of which is followed by a linefeed)
are the following.  Note that although there is a small similarity
between this authentication protocol and the cvs protocol, they are
separate.

@table @code
@item I LOVE YOU
The authentication is successful.  The client proceeds with the cvs
protocol itself.

@item I HATE YOU
The authentication fails.  After sending this response, the server may
close the connection.  It is up to the server to decide whether to give
this response, which is generic, or a more specific response using
@samp{E} and/or @samp{error}.

@item E @var{text}
Provide a message for the user.  After this reponse, the authentication
protocol continues with another response.  Typically the server will
provide a series of @samp{E} responses followed by @samp{error}.
Compatibility note: @sc{cvs} 1.9.10 and older clients will print
@code{unrecognized auth response} and @var{text}, and then exit, upon
receiving this response.

@item error @var{code} @var{text}
The authentication fails.  After sending this response, the server may
close the connection.  The @var{code} is a code describing why it
failed, intended for computer consumption.  The only code currently
defined is @samp{0} which is nonspecific, but clients must silently
treat any unrecognized codes as nonspecific.
The @var{text} should be supplied to the
user.  Compatibility note: @sc{cvs} 1.9.10 and older clients will print
@code{unrecognized auth response} and @var{text}, and then exit, upon
receiving this response.
Note that @var{text} for this response, or the @var{text} in an @code{E}
response, is not designed for machine parsing.  More vigorous use of
@var{code}, or future extensions, will be needed to prove a cleaner
machine-parseable indication of what the error was.
@end table

@c If you are thinking of putting samp or code around BEGIN AUTH REQUEST
@c and friends, watch for overfull hboxes.
If the client wishes to merely authenticate without starting the cvs
protocol, the procedure is the same, except BEGIN AUTH REQUEST is
replaced with BEGIN VERIFICATION REQUEST, END AUTH REQUEST
is replaced with END VERIFICATION REQUEST, and upon receipt of
I LOVE YOU the connection is closed rather than continuing.

Another mechanism is GSSAPI authentication.  GSSAPI is a
generic interface to security services such as kerberos.  GSSAPI is
specified in RFC2078 (GSSAPI version 2) and RFC1508 (GSSAPI version 1);
we are not aware of differences between the two which affect the
protocol in incompatible ways, so we make no attempt to specify one
version or the other.
The procedure here is to start with @samp{BEGIN
GSSAPI REQUEST}.  GSSAPI authentication information is then exchanged
between the client and the server.  Each packet of information consists
of a two byte big endian length, followed by that many bytes of data.
After the GSSAPI authentication is complete, the server continues with
the responses described above (@samp{I LOVE YOU}, etc.).

@item future possibilities
There are a nearly unlimited number of ways to connect and authenticate.
One might want to allow access based on IP address (similar to the usual
rsh protocol but with different/no restrictions on ports < 1024), to
adopt mechanisms such as Pluggable Authentication Modules (PAM), to
allow users to run their own servers under their own usernames without
root access, or any number of other possibilities.  The way to add
future mechanisms, for the most part, should be to continue to use port
2401, but to use different strings in place of @samp{BEGIN AUTH
REQUEST}.
@end table

@node Password scrambling
@chapter Password scrambling algorithm

The pserver authentication protocol, as described in @ref{Connection and
Authentication}, trivially encodes the passwords.  This is only to
prevent inadvertent compromise; it provides no protection against even a
relatively unsophisticated attacker.  For comparison, HTTP Basic
Authentication (as described in RFC2068) uses BASE64 for a similar
purpose.  CVS uses its own algorithm, described here.

The scrambled password starts with @samp{A}, which serves to identify
the scrambling algorithm in use.  After that follows a single octet for
each character in the password, according to a fixed encoding.  The
values are shown here, with the encoded values in decimal.  Control
characters, space, and characters outside the invariant ISO 646
character set are not shown; such characters are not recommended for use
in passwords.  There is a long discussion of character set issues in
@ref{Protocol Notes}.

@example
        0 111           P 125           p  58
! 120   1  52   A  57   Q  55   a 121   q 113
"  53   2  75   B  83   R  54   b 117   r  32
        3 119   C  43   S  66   c 104   s  90
        4  49   D  46   T 124   d 101   t  44
% 109   5  34   E 102   U 126   e 100   u  98
&  72   6  82   F  40   V  59   f  69   v  60
' 108   7  81   G  89   W  47   g  73   w  51
(  70   8  95   H  38   X  92   h  99   x  33
)  64   9  65   I 103   Y  71   i  63   y  97
*  76   : 112   J  45   Z 115   j  94   z  62
+  67   ;  86   K  50           k  93
, 116   < 118   L  42           l  39
-  74   = 110   M 123           m  37
.  68   > 122   N  91           n  61
/  87   ? 105   O  35   _  56   o  48
@end example

@node Protocol
@chapter The CVS client/server protocol

In the following, @samp{\n} refers to a linefeed and @samp{\t} refers to
a horizontal tab; @dfn{requests} are what the client sends and
@dfn{responses} are what the server sends.  In general, the connection is
governed by the client---the server does not send responses without
first receiving requests to do so; see @ref{Response intro} for more
details of this convention.

It is typical, early in the connection, for the client to transmit a
@code{Valid-responses} request, containing all the responses it
supports, followed by a @code{valid-requests} request, which elicits
from the server a @code{Valid-requests} response containing all the
requests it understands.  In this way, the client and server each find
out what the other supports before exchanging large amounts of data
(such as file contents).

@c Hmm, having 3 sections in this menu makes a certain amount of sense
@c but that structure gets lost in the printed manual (not sure about
@c HTML).  Perhaps there is a better way.
@menu

General protocol conventions:

* Entries Lines::                   Transmitting RCS data
* File Modes::                      Read, write, execute, and possibly more...
* Filenames::                       Conventions regarding filenames
* File transmissions::              How file contents are transmitted
* Strings::                         Strings in various requests and responses
* Dates::                           Times and dates

The protocol itself:

* Request intro::                   General conventions relating to requests
* Requests::                        List of requests
* Response intro::                  General conventions relating to responses
* Response pathnames::              The "pathname" in responses
* Responses::                       List of responses
* Text tags::                       More details about the MT response

An example session, and some further observations:

* Example::                         A conversation between client and server
* Requirements::                    Things not to omit from an implementation
* Obsolete::                        Former protocol features
@end menu

@node Entries Lines
@section Entries Lines

Entries lines are transmitted as:

@example
/ @var{name} / @var{version} / @var{conflict} / @var{options} / @var{tag_or_date}
@end example

@var{tag_or_date} is either @samp{T} @var{tag} or @samp{D} @var{date}
or empty.  If it is followed by a slash, anything after the slash
shall be silently ignored.

@var{version} can be empty, or start with @samp{0} or @samp{-}, for no
user file, new user file, or user file to be removed, respectively.

@c FIXME: should distinguish sender and receiver behavior here; the
@c "anything else" and "does not start with" are intended for future
@c expansion, and we should specify a sender behavior.
@var{conflict}, if it starts with @samp{+}, indicates that the file had
conflicts in it.  The rest of @var{conflict} is @samp{=} if the
timestamp matches the file, or anything else if it doesn't.  If
@var{conflict} does not start with a @samp{+}, it is silently ignored.

@var{options} signifies the keyword expansion options (for example
@samp{-ko}).  In an @code{Entry} request, this indicates the options
that were specified with the file from the previous file updating
response (@pxref{Response intro}, for a list of file updating
responses); if the client is specifying the @samp{-k} or @samp{-A}
option to @code{update}, then it is the server which figures out what
overrides what.

@node File Modes
@section File Modes

A mode is any number of repetitions of

@example
@var{mode-type} = @var{data}
@end example

separated by @samp{,}.

@var{mode-type} is an identifier composed of alphanumeric characters.
Currently specified: @samp{u} for user, @samp{g} for group, @samp{o}
for other (see below for discussion of whether these have their POSIX
meaning or are more loose).  Unrecognized values of @var{mode-type}
are silently ignored.

@var{data} consists of any data not containing @samp{,}, @samp{\0} or
@samp{\n}.  For @samp{u}, @samp{g}, and @samp{o} mode types, data
consists of alphanumeric characters, where @samp{r} means read, @samp{w}
means write, @samp{x} means execute, and unrecognized letters are
silently ignored.

The two most obvious ways in which the mode matters are: (1) is it
writeable?  This is used by the developer communication features, and
is implemented even on OS/2 (and could be implemented on DOS), whose
notion of mode is limited to a readonly bit. (2) is it executable?
Unix CVS users need CVS to store this setting (for shell scripts and
the like).  The current CVS implementation on unix does a little bit
more than just maintain these two settings, but it doesn't really have
a nice general facility to store or version control the mode, even on
unix, much less across operating systems with diverse protection
features.  So all the ins and outs of what the mode means across
operating systems haven't really been worked out (e.g. should the VMS
port use ACLs to get POSIX semantics for groups?).

@node Filenames
@section Conventions regarding transmission of file names

In most contexts, @samp{/} is used to separate directory and file
names in filenames, and any use of other conventions (for example,
that the user might type on the command line) is converted to that
form.  The only exceptions might be a few cases in which the server
provides a magic cookie which the client then repeats verbatim, but as
the server has not yet been ported beyond unix, the two rules provide
the same answer (and what to do if future server ports are operating
on a repository like e:/foo or CVS_ROOT:[FOO.BAR] has not been
carefully thought out).

Characters outside the invariant ISO 646 character set should be avoided
in filenames.  This restriction may need to be relaxed to allow for
characters such as @samp{[} and @samp{]} (see above about non-unix
servers); this has not been carefully considered (and currently
implementations probably use whatever character sets that the operating
systems they are running on allow, and/or that users specify).  Of
course the most portable practice is to restrict oneself further, to the
POSIX portable filename character set as specified in POSIX.1.

@node File transmissions
@section File transmissions

File contents (noted below as @var{file transmission}) can be sent in
one of two forms.  The simpler form is a number of bytes, followed by a
linefeed, followed by the specified number of bytes of file contents.
These are the entire contents of the specified file.  Second, if both
client and server support @samp{gzip-file-contents}, a @samp{z} may
precede the length, and the `file contents' sent are actually compressed
with @samp{gzip} (RFC1952/1951) compression.  The length specified is
that of the compressed version of the file.

In neither case are the file content followed by any additional data.
The transmission of a file will end with a linefeed iff that file (or its
compressed form) ends with a linefeed.

The encoding of file contents depends on the value for the @samp{-k}
option.  If the file is binary (as specified by the @samp{-kb} option in
the appropriate place), then it is just a certain number of octets, and
the protocol contributes nothing towards determining the encoding (using
the file name is one widespread, if not universally popular, mechanism).
If the file is text (not binary), then the file is sent as a series of
lines, separated by linefeeds.  If the keyword expansion is set to
something other than @samp{-ko}, then it is expected that the file
conform to the RCS expectations regarding keyword expansion---in
particular, that it is in a character set such as ASCII in which 0x24 is
a dollar sign (@samp{$}).

@node Strings
@section Strings

In various contexts, for example the @code{Argument} request and the
@code{M} response, one transmits what is essentially an arbitrary
string.  Often this will have been supplied by the user (for example,
the @samp{-m} option to the @code{ci} request).  The protocol has no
mechanism to specify the character set of such strings; it would be
fairly safe to stick to the invariant ISO 646 character set but the
existing practice is probably to just transmit whatever the user
specifies, and hope that everyone involved agrees which character set is
in use, or sticks to a common subset.

@node Dates
@section Dates

The protocol contains times and dates in various places.

For the @samp{-D} option to the @code{annotate}, @code{co}, @code{diff},
@code{export}, @code{history}, @code{rdiff}, @code{rtag}, @code{tag},
and @code{update} requests, the server should support two formats:

@example
26 May 1997 13:01:40 -0000  ; @r{RFC 822 as modified by RFC 1123}
5/26/1997 13:01:40 GMT    ; @r{traditional}
@end example

The former format is preferred; the latter however is sent by the CVS
command line client (versions 1.5 through at least 1.9).

For the @samp{-d} option to the @code{log} request, servers should at
least support RFC 822/1123 format.  Clients are encouraged to use this
format too (traditionally the command line CVS client has just passed
along the date format specified by the user, however).

The @code{Mod-time} response and @code{Checkin-time} request use RFC
822/1123 format (see the descriptions of that response and request for
details).

For @code{Notify}, see the description of that request.

@node Request intro
@section Request intro

By convention, requests which begin with a capital letter do not elicit
a response from the server, while all others do -- save one.  The
exception is @samp{gzip-file-contents}.  Unrecognized requests will
always elicit a response from the server, even if that request begins
with a capital letter.

The term @dfn{command} means a request which expects a response (except
@code{valid-requests}).  The general model is that the client transmits
a great number of requests, but nothing happens until the very end when
the client transmits a command.  Although the intention is that
transmitting several commands in one connection should be legal,
existing servers probably have some bugs with some combinations of more
than one command, and so clients may find it necessary to make several
connections in some cases.  This should be thought of as a workaround
rather than a desired attribute of the protocol.

@node Requests
@section Requests

Here are the requests:

@table @code
@item Root @var{pathname} \n
Response expected: no.  Tell the server which @code{CVSROOT} to use.
Note that @var{pathname} is a local directory and @emph{not} a fully
qualified @code{CVSROOT} variable.  @var{pathname} must
already exist; if creating a new root, use the @code{init} request, not
@code{Root}.  @var{pathname} does not include the hostname of the
server, how to access the server, etc.; by the time the CVS protocol is
in use, connection, authentication, etc., are already taken care of.

The @code{Root} request must be sent only once, and it must be sent
before any requests other than @code{Valid-responses},
@code{valid-requests}, @code{UseUnchanged}, @code{Set} or @code{init}.

@item Valid-responses @var{request-list} \n
Response expected: no.
Tell the server what responses the client will accept.
request-list is a space separated list of tokens.

@item valid-requests \n
Response expected: yes.
Ask the server to send back a @code{Valid-requests} response.

@item Directory @var{local-directory} \n
Additional data: @var{repository} \n.  Response expected: no.
Tell the server what directory to use.  The @var{repository} should be a
directory name from a previous server response.  Note that
this both gives a default for @code{Entry} and @code{Modified} and
also for @code{ci} and the other commands; normal usage is to send 
@code{Directory} for each directory in which there will be an
@code{Entry} or @code{Modified}, and then a final @code{Directory}
for the original directory, then the command.
The @var{local-directory} is relative to
the top level at which the command is occurring (i.e. the last
@code{Directory} which is sent before the command);
to indicate that top level, @samp{.} should be sent for
@var{local-directory}.

Here is an example of where a client gets @var{repository} and
@var{local-directory}.  Suppose that there is a module defined by

@example
moddir 1dir
@end example

That is, one can check out @code{moddir} and it will take @code{1dir} in
the repository and check it out to @code{moddir} in the working
directory.  Then an initial check out could proceed like this:

@example
C: Root /home/kingdon/zwork/cvsroot
. . .
C: Argument moddir
C: Directory .
C: /home/kingdon/zwork/cvsroot
C: co
S: Clear-sticky moddir/
S: /home/kingdon/zwork/cvsroot/1dir/
. . .
S: ok
@end example

In this example the response shown is @code{Clear-sticky}, but it could
be another response instead.  Note that it returns two pathnames.
The first one, @file{moddir/}, indicates the working
directory to check out into.  The second one, ending in @file{1dir/},
indicates the directory to pass back to the server in a subsequent
@code{Directory} request.  For example, a subsequent @code{update}
request might look like:

@example
C: Directory moddir
C: /home/kingdon/zwork/cvsroot/1dir
. . .
C: update
@end example

For a given @var{local-directory}, the repository will be the same for
each of the responses, so one can use the repository from whichever
response is most convenient.  Typically a client will store the
repository along with the sources for each @var{local-directory}, use
that same setting whenever operating on that @var{local-directory}, and
not update the setting as long as the @var{local-directory} exists.

A client is free to rename a @var{local-directory} at any time (for
example, in response to an explicit user request).  While it is true
that the server supplies a @var{local-directory} to the client, as noted
above, this is only the default place to put the directory.  Of course,
the various @code{Directory} requests for a single command (for example,
@code{update} or @code{ci} request) should name a particular directory
with the same @var{local-directory}.

Each @code{Directory} request specifies a brand-new
@var{local-directory} and @var{repository}; that is,
@var{local-directory} and @var{repository} are never relative to paths
specified in any previous @code{Directory} request.

Here's a more complex example, in which we request an update of a
working directory which has been checked out from multiple places in the
repository.

@example
C: Argument dir1
C: Directory dir1
C: /home/foo/repos/mod1
. . .
C: Argument dir2
C: Directory dir2
C: /home/foo/repos/mod2
. . .
C: Argument dir3
C: Directory dir3/subdir3
C: /home/foo/repos/mod3
. . .
C: update
@end example

While directories @code{dir1} and @code{dir2} will be handled in similar
fashion to the other examples given above, @code{dir3} is slightly
different from the server's standpoint.  Notice that module @code{mod3}
is actually checked out into @code{dir3/subdir3}, meaning that directory
@code{dir3} is either empty or does not contain data checked out from
this repository.  

The above example will work correctly in @sc{cvs} 1.10.1 and later.  The
server will descend the tree starting from all directories mentioned in
@code{Argument} requests and update those directories specifically
mentioned in @code{Directory} requests.

Previous versions of @sc{cvs} (1.10 and earlier) do not behave the same
way.  While the descent of the tree begins at all directories mentioned
in @code{Argument} requests, descent into subdirectories only occurs if
a directory has been mentioned in a @code{Directory} request.
Therefore, the above example would succeed in updating @code{dir1} and
@code{dir2}, but would skip @code{dir3} because that directory was not
specifically mentioned in a @code{Directory} request.  A functional
version of the above that would run on a 1.10 or earlier server is as
follows:

@example
C: Argument dir1
C: Directory dir1
C: /home/foo/repos/mod1
. . .
C: Argument dir2
C: Directory dir2
C: /home/foo/repos/mod2
. . .
C: Argument dir3
C: Directory dir3
C: /home/foo/repos/.
. . .
C: Directory dir3/subdir3
C: /home/foo/repos/mod3
. . .
C: update
@end example

Note the extra @code{Directory dir3} request.  It might be better to use
@code{Emptydir} as the repository for the @code{dir3} directory, but the
above will certainly work.

One more peculiarity of the 1.10 and earlier protocol is the ordering of
@code{Directory} arguments.  In order for a subdirectory to be
registered correctly for descent by the recursion processor, its parent
must be sent first.  For example, the following would not work to update
@code{dir3/subdir3}:

@example
. . .
C: Argument dir3
C: Directory dir3/subdir3
C: /home/foo/repos/mod3
. . .
C: Directory dir3
C: /home/foo/repos/.
. . .
C: update
@end example

The implementation of the server in 1.10 and earlier writes the
administration files for a given directory at the time of the
@code{Directory} request.  It also tries to register the directory with
its parent to mark it for recursion.  In the above example, at the time
@code{dir3/subdir3} is created, the physical directory for @code{dir3}
will be created on disk, but the administration files will not have been
created.  Therefore, when the server tries to register
@code{dir3/subdir3} for recursion, the operation will silently fail
because the administration files do not yet exist for @code{dir3}.

@item Max-dotdot @var{level} \n
Response expected: no.
Tell the server that @var{level} levels of directories above the
directory which @code{Directory} requests are relative to will be
needed.  For example, if the client is planning to use a
@code{Directory} request for @file{../../foo}, it must send a
@code{Max-dotdot} request with a @var{level} of at least 2.
@code{Max-dotdot} must be sent before the first @code{Directory}
request.

@item Static-directory \n
Response expected: no.  Tell the server that the directory most recently
specified with @code{Directory} should not have
additional files checked out unless explicitly requested.  The client
sends this if the @code{Entries.Static} flag is set, which is controlled
by the @code{Set-static-directory} and @code{Clear-static-directory}
responses.

@item Sticky @var{tagspec} \n
Response expected: no.  Tell the server that the directory most recently
specified with @code{Directory} has a sticky tag or date @var{tagspec}.
The first character of @var{tagspec} is @samp{T} for a tag, @samp{D}
for a date, or some other character supplied by a Set-sticky response
from a previous request to the server.  The remainder of @var{tagspec}
contains the actual tag or date, again as supplied by Set-sticky.

The server should remember @code{Static-directory} and @code{Sticky}
requests for a particular directory; the client need not resend them
each time it sends a @code{Directory} request for a given directory.
However, the server is not obliged to remember them beyond the context
of a single command.

@item Checkin-prog @var{program} \n
Response expected: no.  Tell the server that the directory most recently
specified with @code{Directory} has a checkin program @var{program}.
Such a program would have been previously set with the
@code{Set-checkin-prog} response.

@item Update-prog @var{program} \n
Response expected: no.  Tell the server that the directory most recently
specified with @code{Directory} has an update program @var{program}.
Such a program would have been previously set with the
@code{Set-update-prog} response.

@item Entry @var{entry-line} \n
Response expected: no.  Tell the server what version of a file is on the
local machine.  The name in @var{entry-line} is a name relative to the
directory most recently specified with @code{Directory}.  If the user
is operating on only some files in a directory, @code{Entry} requests
for only those files need be included.  If an @code{Entry} request is
sent without @code{Modified}, @code{Is-modified}, or @code{Unchanged},
it means the file is
lost (does not exist in the working directory).  If both @code{Entry}
and one of @code{Modified}, @code{Is-modified}, or @code{Unchanged} are
sent for the same file, @code{Entry} must be sent first.  For a
given file, one can send @code{Modified}, @code{Is-modified}, or
@code{Unchanged}, but not more than one of these three.

@item Kopt @var{option} \n
This indicates to the server which keyword expansion options to use for
the file specified by the next @code{Modified} or @code{Is-modified}
request (for example @samp{-kb} for a binary file).  This is similar to
@code{Entry}, but is used for a file for which there is no entries line.
Typically this will be a file being added via an @code{add} or
@code{import} request.  The client may not send both @code{Kopt} and
@code{Entry} for the same file.

@item Checkin-time @var{time} \n
For the file specified by the next @code{Modified} request, use
@var{time} as the time of the checkin.  The @var{time} is in the format
specified by RFC822 as modified by RFC1123.  The client may specify any
timezone it chooses; servers will want to convert that to their own
timezone as appropriate.  An example of this format is:

@example
26 May 1997 13:01:40 -0400
@end example

There is no requirement that the client and server clocks be
synchronized.  The client just sends its recommendation for a timestamp
(based on file timestamps or whatever), and the server should just believe
it (this means that the time might be in the future, for example).

Note that this is not a general-purpose way to tell the server about the
timestamp of a file; that would be a separate request (if there are
servers which can maintain timestamp and time of checkin separately).

This request should affect the @code{import} request, and may optionally
affect the @code{ci} request or other relevant requests if any.

@item Modified @var{filename} \n
Response expected: no.  Additional data: mode, \n, file transmission.
Send the server a copy of one locally modified file.  @var{filename} is
a file within the most recent directory sent with @code{Directory}; it
must not contain @samp{/}.  If
the user is operating on only some files in a directory, only those
files need to be included.  This can also be sent without @code{Entry},
if there is no entry for the file.

@item Is-modified @var{filename} \n
Response expected: no.  Additional data: none.  Like @code{Modified},
but used if the server only needs
to know whether the file is modified, not the contents.

The commands which can take @code{Is-modified} instead of
@code{Modified} with no known change in behavior are: @code{admin},
@code{diff} (if and only if two @samp{-r} or @samp{-D} options are
specified), @code{watch-on}, @code{watch-off}, @code{watch-add},
@code{watch-remove}, @code{watchers}, @code{editors},
@code{log}, and @code{annotate}.

For the @code{status} command, one can send @code{Is-modified} but if
the client is using imperfect mechanisms such as timestamps to determine
whether to consider a file modified, then the behavior will be
different.  That is, if one sends @code{Modified}, then the server will
actually compare the contents of the file sent and the one it derives
from to determine whether the file is genuinely modified.  But if one
sends @code{Is-modified}, then the server takes the client's word for
it.  A similar situation exists for @code{tag}, if the @samp{-c} option
is specified.

Commands for which @code{Modified} is necessary are @code{co},
@code{ci}, @code{update}, and @code{import}.

Commands which do not need to inform the server about a working
directory, and thus should not be sending either @code{Modified} or
@code{Is-modified}: @code{rdiff}, @code{rtag}, @code{history},
@code{init}, and @code{release}.

Commands for which further investigation is warranted are:
@code{remove}, @code{add}, and @code{export}.  Pending such
investigation, the more conservative course of action is to stick to
@code{Modified}.

@item Unchanged @var{filename} \n
Response expected: no.  Tell the server that @var{filename} has not been
modified in the checked out directory.  The @var{filename} is
a file within the most recent directory sent with @code{Directory}; it
must not contain @samp{/}.

@item UseUnchanged \n
Response expected: no.  To specify the version of the protocol described
in this document, servers must support this request (although it need
not do anything) and clients must issue it.

@item Notify @var{filename} \n
Response expected: no.
Tell the server that an @code{edit} or @code{unedit} command has taken
place.  The server needs to send a @code{Notified} response, but such
response is deferred until the next time that the server is sending
responses.
The @var{filename} is a file within the most recent directory sent with
@code{Directory}; it must not contain @samp{/}.
Additional data:
@example
@var{notification-type} \t @var{time} \t @var{clienthost} \t
@var{working-dir} \t @var{watches} \n
@end example
where @var{notification-type} is @samp{E} for edit, @samp{U} for
unedit, undefined behavior if @samp{C}, and all other letters should be
silently ignored for future expansion.
@var{time} is the time at which the edit or unedit took place, in a
user-readable format of the client's choice (the server should treat the
time as an opaque string rather than interpreting it).
@c Might be useful to specify a format, but I don't know if we want to
@c specify the status quo (ISO C asctime() format plus timezone) without
@c offering the option of ISO8601 and/or RFC822/1123 (see cvs.texinfo
@c for much much more on date formats).
@var{clienthost} is the name of the host on which the edit or unedit
took place, and @var{working-dir} is the pathname of the working
directory where the edit or unedit took place.  @var{watches} are the
temporary watches, zero or more of the following characters in the
following order: @samp{E} for edit, @samp{U} for unedit, @samp{C} for
commit, and all other letters should be silently ignored for future
expansion.  If @var{notification-type} is @samp{E} the temporary watches
are set; if it is @samp{U} they are cleared.
If @var{watches} is followed by \t then the
\t and the rest of the line should be ignored, for future expansion.

The @var{time}, @var{clienthost}, and @var{working-dir} fields may not
contain the characters @samp{+}, @samp{,}, @samp{>}, @samp{;}, or @samp{=}.

Note that a client may be capable of performing an @code{edit} or
@code{unedit} operation without connecting to the server at that time,
and instead connecting to the server when it is convenient (for example,
when a laptop is on the net again) to send the @code{Notify} requests.
Even if a client is capable of deferring notifications, it should
attempt to send them immediately (one can send @code{Notify} requests
together with a @code{noop} request, for example), unless perhaps if
it can know that a connection would be impossible.

@item Questionable @var{filename} \n
Response expected: no.  Additional data: no.  Tell the server to check
whether @var{filename} should be ignored, and if not, next time the
server sends responses, send (in a @code{M} response) @samp{?} followed
by the directory and filename.  @var{filename} must not contain
@samp{/}; it needs to be a file in the directory named by the most
recent @code{Directory} request.
@c FIXME: the bit about not containing / is true of most of the
@c requests, but isn't documented and should be.

@item Case \n
Response expected: no.  Tell the server that filenames should be matched
in a case-insensitive fashion.  Note that this is not the primary
mechanism for achieving case-insensitivity; for the most part the client
keeps track of the case which the server wants to use and takes care to
always use that case regardless of what the user specifies.  For example
the filenames given in @code{Entry} and @code{Modified} requests for the
same file must match in case regardless of whether the @code{Case}
request is sent.  The latter mechanism is more general (it could also be
used for 8.3 filenames, VMS filenames with more than one @samp{.}, and
any other situation in which there is a predictable mapping between
filenames in the working directory and filenames in the protocol), but
there are some situations it cannot handle (ignore patterns, or
situations where the user specifies a filename and the client does not
know about that file).

@item Argument @var{text} \n
Response expected: no.
Save argument for use in a subsequent command.  Arguments
accumulate until an argument-using command is given, at which point
they are forgotten.

@item Argumentx @var{text} \n
Response expected: no.  Append \n followed by text to the current
argument being saved.

@item Global_option @var{option} \n
Response expected: no.
Transmit one of the global options @samp{-q}, @samp{-Q}, @samp{-l},
@samp{-t}, @samp{-r}, or @samp{-n}.  @var{option} must be one of those
strings, no variations (such as combining of options) are allowed.  For
graceful handling of @code{valid-requests}, it is probably better to
make new global options separate requests, rather than trying to add
them to this request.

@item Gzip-stream @var{level} \n
Response expected: no.
Use zlib (RFC 1950/1951) compression to compress all further communication
between the client and the server.  After this request is sent, all
further communication must be compressed.  All further data received
from the server will also be compressed.  The @var{level} argument
suggests to the server the level of compression that it should apply; it
should be an integer between 1 and 9, inclusive, where a higher number
indicates more compression.

@item Kerberos-encrypt \n
Response expected: no.
Use Kerberos encryption to encrypt all further communication between the
client and the server.  This will only work if the connection was made
over Kerberos in the first place.  If both the @code{Gzip-stream} and
the @code{Kerberos-encrypt} requests are used, the
@code{Kerberos-encrypt} request should be used first.  This will make
the client and server encrypt the compressed data, as opposed to
compressing the encrypted data.  Encrypted data is generally
incompressible.

Note that this request does not fully prevent an attacker from hijacking
the connection, in the sense that it does not prevent hijacking the
connection between the initial authentication and the
@code{Kerberos-encrypt} request.

@item Gssapi-encrypt \n
Response expected: no.
Use GSSAPI encryption to encrypt all further communication between the
client and the server.  This will only work if the connection was made
over GSSAPI in the first place.  See @code{Kerberos-encrypt}, above, for
the relation between @code{Gssapi-encrypt} and @code{Gzip-stream}.

Note that this request does not fully prevent an attacker from hijacking
the connection, in the sense that it does not prevent hijacking the
connection between the initial authentication and the
@code{Gssapi-encrypt} request.

@item Gssapi-authenticate \n
Response expected: no.
Use GSSAPI authentication to authenticate all further communication
between the client and the server.  This will only work if the
connection was made over GSSAPI in the first place.  Encrypted data is
automatically authenticated, so using both @code{Gssapi-authenticate}
and @code{Gssapi-encrypt} has no effect beyond that of
@code{Gssapi-encrypt}.  Unlike encrypted data, it is reasonable to
compress authenticated data.

Note that this request does not fully prevent an attacker from hijacking
the connection, in the sense that it does not prevent hijacking the
connection between the initial authentication and the
@code{Gssapi-authenticate} request.

@item Set @var{variable}=@var{value} \n
Response expected: no.
Set a user variable @var{variable} to @var{value}.

@item expand-modules \n
Response expected: yes.  Expand the modules which are specified in the
arguments.  Returns the data in @code{Module-expansion} responses.  Note
that the server can assume that this is checkout or export, not rtag or
rdiff; the latter do not access the working directory and thus have no
need to expand modules on the client side.

Expand may not be the best word for what this request does.  It does not
necessarily tell you all the files contained in a module, for example.
Basically it is a way of telling you which working directories the
server needs to know about in order to handle a checkout of the
specified modules.

For example, suppose that the server has a module defined by

@example
aliasmodule -a 1dir
@end example

That is, one can check out @code{aliasmodule} and it will take
@code{1dir} in the repository and check it out to @code{1dir} in the
working directory.  Now suppose the client already has this module
checked out and is planning on using the @code{co} request to update it.
Without using @code{expand-modules}, the client would have two bad
choices: it could either send information about @emph{all} working
directories under the current directory, which could be unnecessarily
slow, or it could be ignorant of the fact that @code{aliasmodule} stands
for @code{1dir}, and neglect to send information for @code{1dir}, which
would lead to incorrect operation.
@c Those don't really seem like the only two options.  I mean, what
@c about keeping track of the correspondence from when we first checked
@c out a fresh directory?  Not that the CVS client does this, or that
@c I've really thought about whether it would be a good idea...

With @code{expand-modules}, the client would first ask for the module to
be expanded:

@example
C: Root /home/kingdon/zwork/cvsroot
. . .
C: Argument aliasmodule
C: Directory .
C: /home/kingdon/zwork/cvsroot
C: expand-modules
S: Module-expansion 1dir
S: ok
@end example

and then it knows to check the @file{1dir} directory and send
requests such as @code{Entry} and @code{Modified} for the files in that
directory.

@item ci \n
@itemx diff \n
@itemx tag \n
@itemx status \n
@itemx log \n
@itemx admin \n
@itemx history \n
@itemx watchers \n
@itemx editors \n
@itemx annotate \n
Response expected: yes.  Actually do a cvs command.  This uses any
previous @code{Argument}, @code{Directory}, @code{Entry}, or
@code{Modified} requests, if they have been sent.  The
last @code{Directory} sent specifies the working directory at the time
of the operation.  No provision is made for any input from the user.
This means that @code{ci} must use a @code{-m} argument if it wants to
specify a log message.

@item co \n
Response expected: yes.  Get files from the repository.  This uses any
previous @code{Argument}, @code{Directory}, @code{Entry}, or
@code{Modified} requests, if they have been sent.  Arguments to this
command are module names; the client cannot know what directories they
correspond to except by (1) just sending the @code{co} request, and then
seeing what directory names the server sends back in its responses, and
(2) the @code{expand-modules} request.

@item export \n
Response expected: yes.  Get files from the repository.  This uses any
previous @code{Argument}, @code{Directory}, @code{Entry}, or
@code{Modified} requests, if they have been sent.  Arguments to this
command are module names, as described for the @code{co} request.  The
intention behind this command is that a client can get sources from a
server without storing CVS information about those sources.  That is, a
client probably should not count on being able to take the entries line
returned in the @code{Created} response from an @code{export} request
and send it in a future @code{Entry} request.  Note that the entries
line in the @code{Created} response must indicate whether the file is
binary or text, so the client can create it correctly.

@item rdiff \n
@itemx rtag \n
Response expected: yes.  Actually do a cvs command.  This uses any
previous @code{Argument} requests, if they have been sent.  The client
should not send @code{Directory}, @code{Entry}, or @code{Modified}
requests for this command; they are not used.  Arguments to these
commands are module names, as described for @code{co}.

@item init @var{root-name} \n
Response expected: yes.  If it doesn't already exist, create a @sc{cvs}
repository @var{root-name}.  Note that @var{root-name} is a local
directory and @emph{not} a fully qualified @code{CVSROOT} variable.  The
@code{Root} request need not have been previously sent.

@item update \n
Response expected: yes.  Actually do a @code{cvs update} command.  This
uses any previous @code{Argument}, @code{Directory}, @code{Entry},
or @code{Modified} requests, if they have been sent.  The
last @code{Directory} sent specifies the working directory at the time
of the operation.  The @code{-I} option is not used--files which the
client can decide whether to ignore are not mentioned and the client
sends the @code{Questionable} request for others.

@item import \n
Response expected: yes.  Actually do a @code{cvs import} command.  This
uses any previous @code{Argument}, @code{Directory}, @code{Entry}, or
@code{Modified} requests, if they have been sent.  The
last @code{Directory} sent specifies the working directory at the time
of the operation - unlike most commands, the repository field of each
@code{Directory} request is ignored (it merely must point somewhere
within the root).  The files to be imported are sent in @code{Modified}
requests (files which the client knows should be ignored are not sent;
the server must still process the CVSROOT/cvsignore file unless -I ! is
sent).  A log message must have been specified with a @code{-m}
argument.

@item add \n
Response expected: yes.  Add a file or directory.  This uses any
previous @code{Argument}, @code{Directory}, @code{Entry}, or
@code{Modified} requests, if they have been sent.  The
last @code{Directory} sent specifies the working directory at the time
of the operation.

To add a directory, send the directory to be added using
@code{Directory} and @code{Argument} requests.  For example:

@example
C: Root /u/cvsroot
. . .
C: Argument nsdir
C: Directory nsdir
C: /u/cvsroot/1dir/nsdir
C: Directory .
C: /u/cvsroot/1dir
C: add
S: M Directory /u/cvsroot/1dir/nsdir added to the repository
S: ok
@end example

You will notice that the server does not signal to the client in any
particular way that the directory has been successfully added.  The
client is supposed to just assume that the directory has been added and
update its records accordingly.  Note also that adding a directory is
immediate; it does not wait until a @code{ci} request as files do.

To add a file, send the file to be added using a @code{Modified}
request.  For example:

@example
C: Argument nfile
C: Directory .
C: /u/cvsroot/1dir
C: Modified nfile
C: u=rw,g=r,o=r
C: 6
C: hello
C: add
S: E cvs server: scheduling file `nfile' for addition
S: Mode u=rw,g=r,o=r
S: Checked-in ./
S: /u/cvsroot/1dir/nfile
S: /nfile/0///
S: E cvs server: use 'cvs commit' to add this file permanently
S: ok
@end example

Note that the file has not been added to the repository; the only effect
of a successful @code{add} request, for a file, is to supply the client
with a new entries line containing @samp{0} to indicate an added file.
In fact, the client probably could perform this operation without
contacting the server, although using @code{add} does cause the server
to perform a few more checks.

The client sends a subsequent @code{ci} to actually add the file to the
repository.

Another quirk of the @code{add} request is that with CVS 1.9 and older,
a pathname specified in
an @code{Argument} request cannot contain @samp{/}.  There is no good
reason for this restriction, and in fact more recent CVS servers don't
have it.
But the way to interoperate with the older servers is to ensure that
all @code{Directory} requests for @code{add} (except those used to add
directories, as described above), use @samp{.} for
@var{local-directory}.  Specifying another string for
@var{local-directory} may not get an error, but it will get you strange
@code{Checked-in} responses from the buggy servers.

@item remove \n
Response expected: yes.  Remove a file.  This uses any
previous @code{Argument}, @code{Directory}, @code{Entry}, or
@code{Modified} requests, if they have been sent.  The
last @code{Directory} sent specifies the working directory at the time
of the operation.

Note that this request does not actually do anything to the repository;
the only effect of a successful @code{remove} request is to supply the
client with a new entries line containing @samp{-} to indicate a removed
file.  In fact, the client probably could perform this operation without
contacting the server, although using @code{remove} may cause the server
to perform a few more checks.

The client sends a subsequent @code{ci} request to actually record the
removal in the repository.

@item watch-on \n
@itemx watch-off \n
@itemx watch-add \n
@itemx watch-remove \n
Response expected: yes.  Actually do the @code{cvs watch on}, @code{cvs
watch off}, @code{cvs watch add}, and @code{cvs watch remove} commands,
respectively.  This uses any previous @code{Argument},
@code{Directory}, @code{Entry}, or @code{Modified}
requests, if they have been sent.  The last @code{Directory} sent
specifies the working directory at the time of the operation.

@item release \n
Response expected: yes.  Note that a @code{cvs release} command has
taken place and update the history file accordingly.

@item noop \n
Response expected: yes.  This request is a null command in the sense
that it doesn't do anything, but merely (as with any other requests
expecting a response) sends back any responses pertaining to pending
errors, pending @code{Notified} responses, etc.

@item update-patches \n
Response expected: yes.
This request does not actually do anything.  It is used as a signal that
the server is able to generate patches when given an @code{update}
request.  The client must issue the @code{-u} argument to @code{update}
in order to receive patches.

@item gzip-file-contents @var{level} \n
Response expected: no.  Note that this request does not follow the
response convention stated above.  @code{Gzip-stream} is suggested
instead of @code{gzip-file-contents} as it gives better compression; the
only reason to implement the latter is to provide compression with
@sc{cvs} 1.8 and earlier.  The @code{gzip-file-contents} request asks
the server to compress files it sends to the client using @code{gzip}
(RFC1952/1951) compression, using the specified level of compression.
If this request is not made, the server must not compress files.

This is only a hint to the server.  It may still decide (for example, in
the case of very small files, or files that already appear to be
compressed) not to do the compression.  Compression is indicated by a
@samp{z} preceding the file length.

Availability of this request in the server indicates to the client that
it may compress files sent to the server, regardless of whether the
client actually uses this request.

@item wrapper-sendme-rcsOptions \n
Response expected: yes.
Request that the server transmit mappings from filenames to keyword
expansion modes in @code{Wrapper-rcsOption} responses.

@item @var{other-request} @var{text} \n
Response expected: yes.
Any unrecognized request expects a response, and does not
contain any additional data.  The response will normally be something like
@samp{error  unrecognized request}, but it could be a different error if
a previous request which doesn't expect a response produced an error.
@end table

When the client is done, it drops the connection.

@node Response intro
@section Introduction to Responses

After a command which expects a response, the server sends however many
of the following responses are appropriate.  The server should not send
data at other times (the current implementation may violate this
principle in a few minor places, where the server is printing an error
message and exiting---this should be investigated further).

Any set of responses always ends with @samp{error} or @samp{ok}.  This
indicates that the response is over.

@c "file updating response" and "file update modifying response" are
@c lame terms (mostly because they are so awkward).  Any better ideas?
The responses @code{Checked-in}, @code{New-entry}, @code{Updated},
@code{Created}, @code{Update-existing}, @code{Merged}, and
@code{Patched} are refered to as @dfn{file updating} responses, because
they change the status of a file in the working directory in some way.
The responses @code{Mode}, @code{Mod-time}, and @code{Checksum} are
referred to as @dfn{file update modifying} responses because they modify
the next file updating response.  In no case shall a file update
modifying response apply to a file updating response other than the next
one.  Nor can the same file update modifying response occur twice for
a given file updating response (if servers diagnose this problem, it may
aid in detecting the case where clients send an update modifying
response without following it by a file updating response).

@node Response pathnames
@section The "pathname" in responses

Many of the responses contain something called @var{pathname}.
@c FIXME: should better document when the specified repository needs to
@c end in "/.".
The name is somewhat misleading; it actually indicates a pair of
pathnames.  First, a local directory name
relative to the directory in which the command was given (i.e. the last
@code{Directory} before the command).  Then a linefeed and a repository
name.  Then
a slash and the filename (without a @samp{,v} ending).
For example, for a file @file{i386.mh}
which is in the local directory @file{gas.clean/config} and for which
the repository is @file{/rel/cvsfiles/devo/gas/config}:

@example
gas.clean/config/
/rel/cvsfiles/devo/gas/config/i386.mh
@end example

If the server wants to tell the client to create a directory, then it
merely uses the directory in any response, as described above, and the
client should create the directory if it does not exist.  Note that this
should only be done one directory at a time, in order to permit the
client to correctly store the repository for each directory.  Servers
can use requests such as @code{Clear-sticky},
@code{Clear-static-directory}, or any other requests, to create
directories.
@c FIXME: Need example here of how "repository" needs to be sent for
@c each directory, and cannot be correctly deduced from, say, the most
@c deeply nested directory.

Some server
implementations may poorly distinguish between a directory which should
not exist and a directory which contains no files; in order to refrain
from creating empty directories a client should both send the @samp{-P}
option to @code{update} or @code{co}, and should also detect the case in
which the server asks to create a directory but not any files within it
(in that case the client should remove the directory or refrain from
creating it in the first place).  Note that servers could clean this up
greatly by only telling the client to create directories if the
directory in question should exist, but until servers do this, clients
will need to offer the @samp{-P} behavior described above.

@node Responses
@section Responses

Here are the responses:

@table @code
@item Valid-requests @var{request-list} \n
Indicate what requests the server will accept.  @var{request-list}
is a space separated list of tokens.  If the server supports sending
patches, it will include @samp{update-patches} in this list.  The
@samp{update-patches} request does not actually do anything.

@item Checked-in @var{pathname} \n
Additional data: New Entries line, \n.  This means a file @var{pathname}
has been successfully operated on (checked in, added, etc.).  name in
the Entries line is the same as the last component of @var{pathname}.

@item New-entry @var{pathname} \n
Additional data: New Entries line, \n.  Like @code{Checked-in}, but the
file is not up to date.

@item Updated @var{pathname} \n
Additional data: New Entries line, \n, mode, \n, file transmission.  A
new copy of the file is enclosed.  This is used for a new revision of an
existing file, or for a new file, or for any other case in which the
local (client-side) copy of the file needs to be updated, and after
being updated it will be up to date.  If any directory in pathname does
not exist, create it.  This response is not used if @code{Created} and
@code{Update-existing} are supported.

@item Created @var{pathname} \n
This is just like @code{Updated} and takes the same additional data, but
is used only if no @code{Entry}, @code{Modified}, or
@code{Unchanged} request has been sent for the file in question.  The
distinction between @code{Created} and @code{Update-existing} is so
that the client can give an error message in several cases: (1) there is
a file in the working directory, but not one for which @code{Entry},
@code{Modified}, or @code{Unchanged} was sent (for example, a file which
was ignored, or a file for which @code{Questionable} was sent), (2)
there is a file in the working directory whose name differs from the one
mentioned in @code{Created} in ways that the client is unable to use to
distinguish files.  For example, the client is case-insensitive and the
names differ only in case.

@item Update-existing @var{pathname} \n
This is just like @code{Updated} and takes the same additional data, but
is used only if a @code{Entry}, @code{Modified}, or @code{Unchanged}
request has been sent for the file in question.

This response, or @code{Merged}, indicates that the server has
determined that it is OK to overwrite the previous contents of the file
specified by @var{pathname}.  Provided that the client has correctly
sent @code{Modified} or @code{Is-modified} requests for a modified file,
and the file was not modified while CVS was running, the server can
ensure that a user's modifications are not lost.

@item Merged @var{pathname} \n
This is just like @code{Updated} and takes the same additional data,
with the one difference that after the new copy of the file is enclosed,
it will still not be up to date.  Used for the results of a merge, with
or without conflicts.

It is useful to preserve an copy of what the file looked like before the
merge.  This is basically handled by the server; before sending
@code{Merged} it will send a @code{Copy-file} response.  For example, if
the file is @file{aa} and it derives from revision 1.3, the
@code{Copy-file} response will tell the client to copy @file{aa} to
@file{.#aa.1.3}.  It is up to the client to decide how long to keep this
file around; traditionally clients have left it around forever, thus
letting the user clean it up as desired.  But another answer, such as
until the next commit, might be preferable.

@item Rcs-diff @var{pathname} \n
This is just like @code{Updated} and takes the same additional data,
with the one difference that instead of sending a new copy of the file,
the server sends an RCS change text.  This change text is produced by
@samp{diff -n} (the GNU diff @samp{-a} option may also be used).  The
client must apply this change text to the existing file.  This will only
be used when the client has an exact copy of an earlier revision of a
file.  This response is only used if the @code{update} command is given
the @samp{-u} argument.

@item Patched @var{pathname} \n
This is just like @code{Rcs-diff} and takes the same additional data,
except that it sends a standard patch rather than an RCS change text.
The patch is produced by @samp{diff -c} for @sc{cvs} 1.6 and later (see
POSIX.2 for a description of this format), or @samp{diff -u} for
previous versions of @sc{cvs}; clients are encouraged to accept either
format.  Like @code{Rcs-diff}, this response is only used if the
@code{update} command is given the @samp{-u} argument.

The @code{Patched} response is deprecated in favor of the
@code{Rcs-diff} response.  However, older clients (CVS 1.9 and earlier)
only support @code{Patched}.

@item Mode @var{mode} \n
This @var{mode} applies to the next file mentioned in
@code{Checked-in}.  @code{Mode} is a file update modifying response
as described in @ref{Response intro}.

@item Mod-time @var{time} \n
Set the modification time of the next file sent to @var{time}.
@code{Mod-time} is a file update modifying response
as described in @ref{Response intro}.
The
@var{time} is in the format specified by RFC822 as modified by RFC1123.
The server may specify any timezone it chooses; clients will want to
convert that to their own timezone as appropriate.  An example of this
format is:

@example
26 May 1997 13:01:40 -0400
@end example

There is no requirement that the client and server clocks be
synchronized.  The server just sends its recommendation for a timestamp
(based on its own clock, presumably), and the client should just believe
it (this means that the time might be in the future, for example).

If the server does not send @code{Mod-time} for a given file, the client
should pick a modification time in the usual way (usually, just let the
operating system set the modification time to the time that the CVS
command is running).

@item Checksum @var{checksum}\n
The @var{checksum} applies to the next file sent (that is,
@code{Checksum} is a file update modifying response
as described in @ref{Response intro}).
In the case of
@code{Patched}, the checksum applies to the file after being patched,
not to the patch itself.  The client should compute the checksum itself,
after receiving the file or patch, and signal an error if the checksums
do not match.  The checksum is the 128 bit MD5 checksum represented as
32 hex digits (MD5 is described in RFC1321).
This response is optional, and is only used if the
client supports it (as judged by the @code{Valid-responses} request).

@item Copy-file @var{pathname} \n
Additional data: @var{newname} \n.  Copy file @var{pathname} to
@var{newname} in the same directory where it already is.  This does not
affect @code{CVS/Entries}.

This can optionally be implemented as a rename instead of a copy.  The
only use for it which currently has been identified is prior to a
@code{Merged} response as described under @code{Merged}.  Clients can
probably assume that is how it is being used, if they want to worry
about things like how long to keep the @var{newname} file around.

@item Removed @var{pathname} \n
The file has been removed from the repository (this is the case where
cvs prints @samp{file foobar.c is no longer pertinent}).

@item Remove-entry @var{pathname} \n
The file needs its entry removed from @code{CVS/Entries}, but the file
itself is already gone (this happens in response to a @code{ci} request
which involves committing the removal of a file).

@item Set-static-directory @var{pathname} \n
This instructs the client to set the @code{Entries.Static} flag, which
it should then send back to the server in a @code{Static-directory}
request whenever the directory is operated on.  @var{pathname} ends in a
slash; its purpose is to specify a directory, not a file within a
directory.

@item Clear-static-directory @var{pathname} \n
Like @code{Set-static-directory}, but clear, not set, the flag.

@item Set-sticky @var{pathname} \n
Additional data: @var{tagspec} \n.  Tell the client to set a sticky tag
or date, which should be supplied with the @code{Sticky} request for
future operations.  @var{pathname} ends in a slash; its purpose is to
specify a directory, not a file within a directory.  The client should
store @var{tagspec} and pass it back to the server as-is, to allow for
future expansion.  The first character of @var{tagspec} is @samp{T} for
a tag, @samp{D} for a date, or something else for future expansion.  The
remainder of @var{tagspec} contains the actual tag or date.

@item Clear-sticky @var{pathname} \n
Clear any sticky tag or date set by @code{Set-sticky}.

@item Template @var{pathname} \n
Additional data: file transmission (note: compressed file transmissions
are not supported).  @var{pathname} ends in a slash; its purpose is to
specify a directory, not a file within a directory.  Tell the client to
store the file transmission as the template log message, and then use
that template in the future when prompting the user for a log message.

@item Set-checkin-prog @var{dir} \n
Additional data: @var{prog} \n.  Tell the client to set a checkin
program, which should be supplied with the @code{Checkin-prog} request
for future operations.

@item Set-update-prog @var{dir} \n
Additional data: @var{prog} \n.  Tell the client to set an update
program, which should be supplied with the @code{Update-prog} request
for future operations.

@item Notified @var{pathname} \n
Indicate to the client that the notification for @var{pathname} has been
done.  There should be one such response for every @code{Notify}
request; if there are several @code{Notify} requests for a single file,
the requests should be processed in order; the first @code{Notified}
response pertains to the first @code{Notify} request, etc.

@item Module-expansion @var{pathname} \n
Return a file or directory
which is included in a particular module.  @var{pathname} is relative
to cvsroot, unlike most pathnames in responses.  @var{pathname} should
be used to look and see whether some or all of the module exists on
the client side; it is not necessarily suitable for passing as an
argument to a @code{co} request (for example, if the modules file
contains the @samp{-d} option, it will be the directory specified with
@samp{-d}, not the name of the module).

@item Wrapper-rcsOption @var{pattern} -k '@var{option}' \n
Transmit to the client a filename pattern which implies a certain
keyword expansion mode.  The @var{pattern} is a wildcard pattern (for
example, @samp{*.exe}.  The @var{option} is @samp{b} for binary, and so
on.  Note that although the syntax happens to resemble the syntax in
certain CVS configuration files, it is more constrained; there must be
exactly one space between @var{pattern} and @samp{-k} and exactly one
space between @samp{-k} and @samp{'}, and no string is permitted in
place of @samp{-k} (extensions should be done with new responses, not by
extending this one, for graceful handling of @code{Valid-responses}).

@item M @var{text} \n
A one-line message for the user.
Note that the format of @var{text} is not designed for machine parsing.
Although sometimes scripts and clients will have little choice, the
exact text which is output is subject to vary at the discretion of the
server and the example output given in this document is just that,
example output.  Servers are encouraged to use the @samp{MT} response,
and future versions of this document will hopefully standardize more of
the @samp{MT} tags; see @ref{Text tags}.

@item Mbinary \n
Additional data: file transmission (note: compressed file transmissions
are not supported).  This is like @samp{M}, except the contents of the
file transmission are binary and should be copied to standard output
without translation to local text file conventions.  To transmit a text
file to standard output, servers should use a series of @samp{M} requests.

@item E @var{text} \n
Same as @code{M} but send to stderr not stdout.

@item F \n
@c FIXME: The second sentence, defining "flush", is somewhat off the top
@c of my head.  Is there some text we can steal from ANSI C or someplace
@c which is more carefully thought out?
Flush stderr.  That is, make it possible for the user to see what has
been written to stderr (it is up to the implementation to decide exactly
how far it should go to ensure this).

@item MT @var{tagname} @var{data} \n

This response provides for tagged text.  It is similar to
SGML/HTML/XML in that the data is structured and a naive application
can also make some sense of it without understanding the structure.
The syntax is not SGML-like, however, in order to fit into the CVS
protocol better and (more importantly) to make it easier to parse,
especially in a language like perl or awk.

The @var{tagname} can have several forms.  If it starts with @samp{a}
to @samp{z} or @samp{A} to @samp{Z}, then it represents tagged text.
If the implementation recognizes @var{tagname}, then it may interpret
@var{data} in some particular fashion.  If the implementation does not
recognize @var{tagname}, then it should simply treat @var{data} as
text to be sent to the user (similar to an @samp{M} response).  There
are two tags which are general purpose.  The @samp{text} tag is
similar to an unrecognized tag in that it provides text which will
ordinarily be sent to the user.  The @samp{newline} tag is used
without @var{data} and indicates that a newline will ordinarily be
sent to the user (there is no provision for embedding newlines in the
@var{data} of other tagged text responses).

If @var{tagname} starts with @samp{+} it indicates a start tag and if
it starts with @samp{-} it indicates an end tag.  The remainder of
@var{tagname} should be the same for matching start and end tags, and
tags should be nested (for example one could have tags in the
following order @code{+bold} @code{+italic} @code{text} @code{-italic}
@code{-bold} but not @code{+bold} @code{+italic} @code{text}
@code{-bold} @code{-italic}).  A particular start and end tag may be
documented to constrain the tagged text responses which are valid
between them.

Note that if @var{data} is present there will always be exactly one
space between @var{tagname} and @var{data}; if there is more than one
space, then the spaces beyond the first are part of @var{data}.

Here is an example of some tagged text responses.  Note that there is
a trailing space after @samp{Checking in} and @samp{initial revision:}
and there are two trailing spaces after @samp{<--}.  Such trailing
spaces are, of course, part of @var{data}.

@example
MT +checking-in
MT text Checking in 
MT fname gz.tst
MT text ;
MT newline
MT rcsfile /home/kingdon/zwork/cvsroot/foo/gz.tst,v
MT text   <--  
MT fname gz.tst
MT newline
MT text initial revision: 
MT init-rev 1.1
MT newline
MT text done
MT newline
MT -checking-in
@end example

If the client does not support the @samp{MT} response, the same
responses might be sent as:

@example
M Checking in gz.tst;
M /home/kingdon/zwork/cvsroot/foo/gz.tst,v  <--  gz.tst
M initial revision: 1.1
M done
@end example

For a list of specific tags, see @ref{Text tags}.

@item error @var{errno-code} @samp{ } @var{text} \n
The command completed with an error.  @var{errno-code} is a symbolic
error code (e.g. @code{ENOENT}); if the server doesn't support this
feature, or if it's not appropriate for this particular message, it just
omits the errno-code (in that case there are two spaces after
@samp{error}).  Text is an error message such as that provided by
strerror(), or any other message the server wants to use.
The @var{text} is like the @code{M} response, in the sense that it is
not particularly intended to be machine-parsed; servers may wish to
print an error message with @code{MT} responses, and then issue a
@code{error} response without @var{text} (although it should be noted
that @code{MT} currently has no way of flagging the output as intended
for standard error, the way that the @code{E} response does).

@item ok \n
The command completed successfully.
@end table

@node Text tags
@section Tags for the MT tagged text response

The @code{MT} response, as described in @ref{Responses}, offers a
way for the server to send tagged text to the client.  This section
describes specific tags.  The intention is to update this section as
servers add new tags.

In the following descriptions, @code{text} and @code{newline} tags are
omitted.  Such tags contain information which is intended for users (or
to be discarded), and are subject to change at the whim of the server.
To avoid being vulnerable to such whim, clients should look for the tags
listed here, not @code{text}, @code{newline}, or other tags.

The following tag means to indicate to the user that a file has been
updated.  It is more or less redundant with the @code{Created} and
@code{Update-existing} responses, but we don't try to specify here
whether it occurs in exactly the same circumstances as @code{Created}
and @code{Update-existing}.  The @var{name} is the pathname of the file
being updated relative to the directory in which the command is
occurring (that is, the last @code{Directory} request which is sent
before the command).

@example
MT +updated
MT fname @var{name}
MT -updated
@end example

The @code{importmergecmd} tag is used when doing an import which has
conflicts.  The client can use it to report how to merge in the newly
imported changes.  The @var{count} is the number of conflicts.  The
newly imported changes can be merged by running the following command:
@smallexample
cvs checkout -j @var{tag1} -j @var{tag2} @var{repository}
@end smallexample

@example
MT +importmergecmd
MT conflicts @var{count}
MT mergetag1 @var{tag1}
MT mergetag2 @var{tag2}
MT repository @var{repository}
MT -importmergecmd
@end example

@node Example
@section Example

@c The C:/S: convention is in imitation of RFC1869 (and presumably
@c other RFC's).  In other formatting concerns, we might want to think
@c about whether there is an easy way to provide RFC1543 formatting
@c (without negating the advantages of texinfo), and whether we should
@c use RFC2234 BNF (I fear that would be less clear than
@c what we do now, however).  Plus what about RFC2119 terminology (MUST,
@c SHOULD, &c) or ISO terminology (shall, should, or whatever they are)?
Here is an example; lines are prefixed by @samp{C: } to indicate the
client sends them or @samp{S: } to indicate the server sends them.

The client starts by connecting, sending the root, and completing the
protocol negotiation.  In actual practice the lists of valid responses
and requests would be longer.
@c The reason that we artificially shorten the lists is to avoid phony
@c line breaks.  Any better solutions?
@c Other than that, this exchange is taken verbatim from the data
@c exchanged by CVS (as of Nov 1996).  That is why some of the requests and
@c reponses are not quite what you would pick for pedagogical purposes.

@example
C: Root /u/cvsroot
C: Valid-responses ok error Checked-in M E
C: valid-requests
S: Valid-requests Root Directory Entry Modified Argument Argumentx ci co
S: ok
C: UseUnchanged
@end example

The client wants to check out the @code{supermunger} module into a fresh
working directory.  Therefore it first expands the @code{supermunger}
module; this step would be omitted if the client was operating on a
directory rather than a module.
@c Why does it send Directory here?  The description of expand-modules
@c doesn't really say much of anything about what use, if any, it makes of
@c Directory and similar requests sent previously.

@example
C: Argument supermunger
C: Directory .
C: /u/cvsroot
C: expand-modules
@end example

The server replies that the @code{supermunger} module expands to the
directory @code{supermunger} (the simplest case):

@example
S: Module-expansion supermunger
S: ok
@end example

The client then proceeds to check out the directory.  The fact that it
sends only a single @code{Directory} request which specifies @samp{.}
for the working directory means that there is not already a
@code{supermunger} directory on the client.
@c What is -N doing here?

@example
C: Argument -N
C: Argument supermunger
C: Directory .
C: /u/cvsroot
C: co
@end example

The server replies with the requested files.  In this example, there is
only one file, @file{mungeall.c}.  The @code{Clear-sticky} and
@code{Clear-static-directory} requests are sent by the current
implementation but they have no effect because the default is for those
settings to be clear when a directory is newly created.

@example
S: Clear-sticky supermunger/
S: /u/cvsroot/supermunger/
S: Clear-static-directory supermunger/
S: /u/cvsroot/supermunger/
S: E cvs server: Updating supermunger
S: M U supermunger/mungeall.c
S: Created supermunger/
S: /u/cvsroot/supermunger/mungeall.c
S: /mungeall.c/1.1///
S: u=rw,g=r,o=r
S: 26
S: int mein () @{ abort (); @}
S: ok
@end example

The current client implementation would break the connection here and make a
new connection for the next command.  However, the protocol allows it
to keep the connection open and continue, which is what we show here.

After the user modifies the file and instructs the client to check it
back in.  The client sends arguments to specify the log message and file
to check in:

@example
C: Argument -m
C: Argument Well, you see, it took me hours and hours to find
C: Argumentx this typo and I searched and searched and eventually
C: Argumentx had to ask John for help.
C: Argument mungeall.c
@end example

It also sends information about the contents of the working directory,
including the new contents of the modified file.  Note that the user has
changed into the @file{supermunger} directory before executing this
command; the top level directory is a user-visible concept because the
server should print filenames in @code{M} and @code{E} responses
relative to that directory.
@c We are waving our hands about the order of the requests.  "Directory"
@c and "Argument" can be in any order, but this probably isn't specified
@c very well.

@example
C: Directory .
C: /u/cvsroot/supermunger
C: Entry /mungeall.c/1.1///
C: Modified mungeall.c
C: u=rw,g=r,o=r
C: 26
C: int main () @{ abort (); @}
@end example

And finally, the client issues the checkin command (which makes use of
the data just sent):

@example
C: ci
@end example

And the server tells the client that the checkin succeeded:

@example
S: M Checking in mungeall.c;
S: E /u/cvsroot/supermunger/mungeall.c,v  <--  mungeall.c
S: E new revision: 1.2; previous revision: 1.1
S: E done
S: Mode u=rw,g=r,o=r
S: Checked-in ./
S: /u/cvsroot/supermunger/mungeall.c
S: /mungeall.c/1.2///
S: ok
@end example

@node Requirements
@section Required versus optional parts of the protocol

The following are part of every known implementation of the CVS protocol
(except obsolete, pre-1.5, versions of CVS) and it is considered
reasonable behavior to completely fail to work if you are connected with
an implementation which attempts to not support them.  Requests:
@code{Root}, @code{Valid-responses}, @code{valid-requests},
@code{Directory}, @code{Entry}, @code{Modified}, @code{Unchanged},
@code{Argument}, @code{Argumentx}, @code{ci}, @code{co}, @code{update}.
Responses: @code{ok}, @code{error}, @code{Valid-requests},
@code{Checked-in}, @code{Updated}, @code{Merged}, @code{Removed},
@code{M}, @code{E}.

A server need not implement @code{Repository}, but in order to interoperate
with CVS 1.5 through 1.9 it must claim to implement it (in
@code{Valid-requests}).  The client will not actually send the request.

@node Obsolete
@section Obsolete protocol elements

This section briefly describes protocol elements which are obsolete.
There is no attempt to document them in full detail.

There was a @code{Repository} request which was like @code{Directory}
except it only provided @var{repository}, and the local directory was
assumed to be similarly named.

If the @code{UseUnchanged} request was not sent, there was a @code{Lost}
request which was sent to indicate that a file did not exist in the
working directory, and the meaning of sending @code{Entries} without
@code{Lost} or @code{Modified} was different.  All current clients (CVS
1.5 and later) will send @code{UseUnchanged} if it is supported.

@node Protocol Notes
@chapter Notes on the Protocol

A number of enhancements are possible.  Also see the file @sc{todo} in
the @sc{cvs} source distribution, which has further ideas concerning
various aspects of @sc{cvs}, some of which impact the protocol.
Similarly, the @code{http://www.cyclic.com} site, in particular the
@cite{Development of CVS} page.

@itemize @bullet
@item
The @code{Modified} request could be speeded up by sending diffs rather
than entire files.  The client would need some way to keep the version
of the file which was originally checked out; probably requiring the use
of "cvs edit" in this case is the most sensible course (the "cvs edit"
could be handled by a package like VC for emacs).  This would also allow
local operation of @code{cvs diff} without arguments.

@item
The fact that @code{pserver} requires an extra network turnaround in
order to perform authentication would be nice to avoid.  This relates to
the issue of reporting errors; probably the clean solution is to defer
the error until the client has issued a request which expects a
response.  To some extent this might relate to the next item (in terms
of how easy it is to skip a whole bunch of requests until we get to one
that expects a response).  I know that the kerberos code doesn't wait in
this fashion, but that probably can cause network deadlocks and perhaps
future problems running over a transport which is more transaction
oriented than TCP.  On the other hand I'm not sure it is wise to make
the client conduct a lengthy upload only to find there is an
authentication failure.

@item
The protocol uses an extra network turnaround for protocol negotiation
(@code{valid-requests}).  It might be nice to avoid this by having the
client be able to send requests and tell the server to ignore them if
they are unrecognized (different requests could produce a fatal error if
unrecognized).  To do this there should be a standard syntax for
requests.  For example, perhaps all future requests should be a single
line, with mechanisms analogous to @code{Argumentx}, or several requests
working together, to provide greater amounts of information.  Or there
might be a standard mechanism for counted data (analogous to that used
by @code{Modified}) or continuation lines (like a generalized
@code{Argumentx}).  It would be useful to compare what HTTP is planning
in this area; last I looked they were contemplating something called
Protocol Extension Protocol but I haven't looked at the relevant IETF
documents in any detail.  Obviously, we want something as simple as
possible (but no simpler).

@item
The scrambling algorithm in the CVS client and server actually support
more characters than those documented in @ref{Password scrambling}.
Someday we are going to either have to document them all (but this is
not as easy as it may look, see below), or (gradually and with adequate
process) phase out the support for other characters in the CVS
implementation.  This business of having the feature partly undocumented
isn't a desirable state long-term.

The problem with documenting other characters is that unless we know
what character set is in use, there is no way to make a password
portable from one system to another.  For example, a with a circle on
top might have different encodings in different character sets.

It @emph{almost} works to say that the client picks an arbitrary,
unknown character set (indeed, having the CVS client know what character
set the user has in mind is a hard problem otherwise), and scrambles
according to a certain octet<->octet mapping.  There are two problems
with this.  One is that the protocol has no way to transmit character 10
decimal (linefeed), and the current server and clients have no way to
handle 0 decimal (NUL).  This may cause problems with certain multibyte
character sets, in which octets 10 and 0 will appear in the middle of
other characters.  The other problem, which is more minor and possibly
not worth worrying about, is that someone can type a password on one
system and then go to another system which uses a different encoding for
the same characters, and have their password not work.

The restriction to the ISO646 invariant subset is the best approach for
strings which are not particularly significant to users.  Passwords are
visible enough that this is somewhat doubtful as applied here.  ISO646
does, however, have the virtue (!?) of offending everyone.  It is easy
to say "But the $ is right on people's keyboards!  Surely we can't
forbid that".  From a human factors point of view, that makes quite a
bit of sense.  The contrary argument, of course, is that a with a circle
on top, or some of the characters poorly handled by Unicode, are on
@emph{someone}'s keyboard.

@end itemize

@bye