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(cvsclient.info)Protocol Notes
Notes on the Protocol
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A number of enhancements are possible. Also see the file TODO in
the CVS source distribution, which has further ideas concerning various
aspects of CVS, some of which impact the protocol. Similarly, the
`http://www.cvshome.org' site, in particular the `Development' pages.
* The `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 `cvs diff'
without arguments.
* The fact that `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.
* The protocol uses an extra network turnaround for protocol
negotiation (`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 `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 `Modified')
or continuation lines (like a generalized `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).
* The scrambling algorithm in the CVS client and server actually
support more characters than those documented in Note: 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 _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
_someone_'s keyboard.
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