rfc491.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                    M. A. Padlipsky
Request for Comments: 491                                    MIT-Multics
NIC: 15356                                                 12 April 1973


                              What Is Free

   In at least three of the RFC's about "mail" and the File Transfer
   Protocol (RFC's 454, 475, 479), something very like the following is
   asserted: "Network mail should be free; i.e., no login or USER
   command should be required."  Unfortunately, "i.e" (=that is) is
   misleading.  It simply does not follow to imply that the only way
   mail can be free is for it not to require a login; explicit login on
   a free account would of course also work.  Indeed, depending upon
   per-Host idiosyncrasies in the Logger / Answering Service / process
   creation environment, an explicit login may well prove to be far more
   natural than an implicit login.  (Even in environments where implicit
   login is easy, surely explicit login is just easy.)  Granted, login
   on a free account requires users to remember the name of the free
   account.  However, this would not be too great a burden to bear if
   there were reasons for preferring an explicit login and if the free
   account had the same name on all Hosts.  Therefore, from the promise
   that Network protocols should not implicitly legislate "unnatural"
   implementations for participating Hosts if it is conveniently
   avoidable, I propose the following formulation:

      Network mail should be free.  Network mail should not require
      users to remember the name of the free account on a given system.
      I.e., it should either be "loginless" or it should take the same
      login everywhere.  But some systems need/want/prefer a login.
      Therefore, USER NETML / PASS NETML should be made to work
      everywhere for free mail.

         Note: "NETML" is fewer than six characters and is upper case
         hence, it should fit in the least common denominator category
         of user identifiers, but it's still long enough not to conflict
         with anybody's initials (in all probability).

   Now, because of the implementation implications this may all sound
   like special pleading, but I claim that another implication of the
   "incorrect" formulation will further show the superiority of an
   explicit login for mail.  For the "loginless" view leads to problems
   in regard to the authentication aspects of login and the accounting
   aspects, by apparently assuming that the sole purpose of login is to
   initiate accounting.  In RFC 475, the problem is exposed when, after
   noting that some systems allow access control to be applied to
   mailboxes, it is asserted that FTP USER command is wrong for access
   control because you'd then be on the free account and a new FTP FROM



Padlipsky                                                       [Page 1]

RFC 491                       What Is Free                 12 April 1973


   command would be right.  (Presumably, FROM would be followed by
   PASS.)  Being reasonably familiar with one of the systems which does
   allow access control on mailboxes, let me point out how it works:
   permissible "principal identifiers" are placed on the "access control
   list" of the mailbox, and when the mailbox is referenced by a process
   the principal identifier of that process must match (explicitly or as
   a member of a class) an entry on the list or access will be
   forbidden.  But the principal identifier is associated with the
   process at login.  Now, it is probably a valid objection to say that
   accounting should be separated from authentification, but it isn't
   always.  So why invent a redundant mechanism based on the assumption
   that it is?

   Another point on authentication via login: it has been argued that
   FTP mail ought to be so cheap that it "can be buried in overhead" by
   the same token, if it's so cheap it shouldn't bother anybody to login
   on his own account if he wants to prove the mail's from himself.

   To be scrupulous, I should close by mentioning the possibility that
   NETML might be repugnant to some Hosts.  If such be the case, then I
   propose that a new FTP FREE command be introduced so that Servers
   need not recognize MAIL as an implicit login.  The reasons here are
   at least twofold: First, it appears that when the "subcommands" to
   MAIL get worked out, some of them will have to precede the MAIL (or
   users will set awfully tired of typing their names, etc.); therefore,
   the list of commands which imply a login grow and grow and Server
   FTP's will have to change and change.  Second, if MAIL implies a
   login, it will be hard in some environments to get the arguments
   across to the process created on behalf of the mailer (and it is not
   a good idea at all to assume that the mailing can be handled by the
   process which is listening on socket 3).  Even introducing a new
   mechanism (and see RFC 451 for my strong feelings against that sort
   of step in general) in FREE seems better than making all the
   assumptions that the loginless alternative does.

   Note that an alternative to this whole line of reasoning would be
   simply to observe that the FTP is internally inconsistent in that it
   acknowledges on the one hand (in the definition of the USER command)
   that some systems may require USER / PASS and then (mis)states on the
   other hand (in the discussion of mail) that they may not.  If this
   abstract point is more satisfying to some readers than the foregoing
   pragmatic argument, well and good.


          [This RFC was put into machine readable form for entry]
     [into the online RFC archives by Helene Morin, Via Genie,12/1999]





Padlipsky                                                       [Page 2]