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Versions: 00 01 RFC 2229

INTERNET-DRAFT            EXPIRES JANUARY 1998            INTERNET-DRAFT
Network Working Group                                           R. Faith
INTERNET-DRAFT                            U. North Carolina, Chapel Hill
Category: Informational                                        B. Martin
                                                     Miranda Productions
                                                            26 July 1997


                      A Dictionary Server Protocol
                    <draft-rfced-info-faith-01.txt>


Status of this Memo

     This document is an Internet-Draft.  Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its areas,
     and its working groups.  Note that other groups may also distribute
     working documents as Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other docu-
     ments at any time.  It is inappropriate to use Internet- Drafts as
     reference material or to cite them other than as ``work in
     progress.''

     To learn the current status of any Internet-Draft, please check the
     ``1id-abstracts.txt'' listing contained in the Internet- Drafts
     Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net
     (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East
     Coast), or ftp.isi.edu (US West Coast).


Authors' Note

     [[This document has not yet been submitted or accepted as an offi-
     cial RFC.  Two independent server implementations have been com-
     pleted, one at dict://dict.miranda.org:2628 and the other at
     dict://proteus.cs.unc.edu:2628.  This note should be deleted when
     this memo is assigned an RFC number.]]


Abstract

     The Dictionary Server Protocol (DICT) is a TCP transaction based
     query/response protocol that allows a client to access dictionary
     definitions from a set of natural language dictionary databases.





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1.  Introduction

     For many years, the Internet community has relied on the "webster"
     protocol for access to natural language definitions.  The webster
     protocol supports access to a single dictionary and (optionally) to
     a single thesaurus.  In recent years, the number of publicly avail-
     able webster servers on the Internet has dramatically decreased.

     Fortunately, several freely-distributable dictionaries and lexicons
     have recently become available on the Internet.  However, these
     freely-distributable databases are not accessible via a uniform
     interface, and are not accessible from a single site.  They are
     often small and incomplete individually, but would collectively
     provide an interesting and useful database of English words.  Exam-
     ples include the Jargon file [JARGON], the WordNet database [WORD-
     NET], MICRA's version of the 1913 Webster's Revised Unabridged Dic-
     tionary [WEB1913], and the Free Online Dictionary of Computing
     [FOLDOC].  Translating and non-English dictionaries are also becom-
     ing available (for example, the FOLDOC dictionary is being trans-
     lated into Spanish).

     The webster protocol is not suitable for providing access to a
     large number of separate dictionary databases, and extensions to
     the current webster protocol were not felt to be a clean solution
     to the dictionary database problem.

     The DICT protocol is designed to provide access to multiple
     databases.  Word definitions can be requested, the word index can
     be searched (using an easily extended set of algorithms), informa-
     tion about the server can be provided (e.g., which index search
     strategies are supported, or which databases are available), and
     information about a database can be provided (e.g., copyright,
     citation, or distribution information).  Further, the DICT protocol
     has hooks that can be used to restrict access to some or all of the
     databases.


1.1.  Requirements

     In this document, we adopt the convention discussed in Section
     1.3.2 of [RFC1122] of using the capitalized words MUST, REQUIRED,
     SHOULD, RECOMMENDED, MAY, and OPTIONAL to define the significance
     of each particular requirement specified in this document.

     In brief: "MUST" (or "REQUIRED") means that the item is an absolute
     requirement of the specification; "SHOULD" (or "RECOMMENDED") means
     there may exist valid reasons for ignoring this item, but the full
     implications should be understood before doing so; and "MAY" (or



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     "OPTIONAL") means that his item is optional, and may be omitted
     without careful consideration.


2.  Protocol Overview


2.1.  Link Level

     The DICT protocol assumes a reliable data stream such as provided
     by TCP.  When TCP is used, a DICT server listens on port 2628 (typ-
     ically, webster servers listened on port 2627).

     This server is only an interface between programs and the dictio-
     nary databases.  It does not perform any user interaction or pre-
     sentation-level functions.


2.2.  Lexical Tokens

     Commands and replies are composed of characters from the ISO-8859-1
     character set [ISO].  More specifically, using the grammar conven-
     tions from [RFC822]:




























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                                                      ; (  Octal, Decimal.)
          CHAR        =  <any ASCII character>        ; (  0-177,  0.-127.)
          CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)
                          character and DEL>          ; (    177,     127.)
          CR          =  <ASCII CR, carriage return>  ; (     15,      13.)
          LF          =  <ASCII LF, linefeed>         ; (     12,      10.)
          SPACE       =  <ASCII SP, space>            ; (     40,      32.)
          HTAB        =  <ASCII HT, horizontal-tab>   ; (     11,       9.)
          <">         =  <ASCII quote mark>           ; (     42,      34.)
          <'>         =  <ASCII single quote mark>    ; (     47,      39.)
          CRLF        =  CR LF
          WS          =  1*(SPACE / HTAB)

          dqstring    =  <"> *(dqtext/quoted-pair) <">
          dqtext      =  <any CHAR except <">, "\", and CTLs>
          sqstring    =  <'> *(dqtext/quoted-pair) <'>
          sqtext      =  <any CHAR except <'>, "\", and CTLs>
          quoted-pair =  "\" CHAR

          atom        =  1*<any CHAR except SPACE, CTLs, <'>, <">, and "\">
          string      =  *<dqstring / sqstring / quoted-pair>
          word        =  *<atom / string>
          description =  *<word / WS>
          text        =  *<word / WS>



2.3.  Commands

     Commands consist of a command word followed by zero or more parame-
     ters.  Commands with parameters must separate the parameters from
     each other and from the command by one or more space or tab charac-
     ters.  Command lines must be complete with all required parameters,
     and may not contain more than one command.

     Each command line must be terminated by a CRLF.

     The grammar for commands is:


          command     = cmd-word *<WS cmd-param>
          cmd-word    = atom
          cmd-param   = database / strategy / word
          database    = atom
          strategy    = atom


     Commands are not case sensitive.



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     Command lines MUST NOT exceed 1024 characters in length, counting
     all characters including spaces, separators, punctuation, and the
     trailing CRLF.  There is no provision for the continuation of com-
     mand lines.


2.4.  Responses

     Responses are of two kinds, status and textual.


2.4.1.  Status Responses

     Status responses indicate the server's response to the last command
     received from the client.

     Status response lines begin with a 3 digit numeric code which is
     sufficient to distinguish all responses.  Some of these may herald
     the subsequent transmission of text.

     The first digit of the response broadly indicates the success,
     failure, or progress of the previous command (based generally on
     [RFC640,RFC821]):


          1yz - Positive Preliminary reply
          2yz - Positive Completion reply
          3yz - Positive Intermediate reply (not used by DICT)
          4yz - Transient Negative Completion reply
          5yz - Permanent Negative Completion reply


     The next digit in the code indicates the response category:


          x0z - Syntax
          x1z - Information (e.g., help)
          x2z - Connections
          x3z - Authentication
          x4z - Unspecified as yet
          x5z - DICT System (These replies indicate the status of the
                receiver mail system vis-a-vis the requested transfer
                or other DICT system action.)
          x8z - Nonstandard (private implementation) extensions


     The exact response codes that should be expected from each command
     are detailed in the description of that command.



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     Certain status responses contain parameters such as numbers and
     strings.  The number and type of such parameters is fixed for each
     response code to simplify interpretation of the response.  Other
     status responses do not require specific text identifiers.  Parame-
     ter requirements are detailed in the description of relevant com-
     mands.  Except for specifically detailed parameters, the text fol-
     lowing response codes is server-dependent.

     Parameters are separated from the numeric response code and from
     each other by a single space.  All numeric parameters are decimal,
     and may have leading zeros.  All string parameters MUST conform to
     the "atom" or "dqstring" grammar productions.

     If no parameters are present, and the server implementation pro-
     vides no implementation-specific text, then there MAY or MAY NOT be
     a space after the response code.

     Response codes not specified in this standard may be used for any
     installation-specific additional commands also not specified.
     These should be chosen to fit the pattern of x8z specified above.
     The use of unspecified response codes for standard commands is pro-
     hibited.


2.4.2.  General Status Responses

     In response to every command, the following general status
     responses are possible:


          500 Syntax error, command not recognized
          501 Syntax error, illegal parameters
          502 Command not implemented
          503 Command parameter not implemented
          420 Server temporarily unavailable
          421 Server shutting down at operator request



2.4.3.  Text Responses

     Before text is sent a numeric status response line, using a 1yz
     code, will be sent indicating text will follow. Text is sent as a
     series of successive lines of textual matter, each terminated with
     a CRLF.  A single line containing only a period (decimal code 46,
     ".") is sent to indicate the end of the text (i.e., the server will
     send a CRLF at the end of the last line of text, a period, and
     another CRLF).



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     If a line of original text contained a period as the first charac-
     ter of the line, that first period is doubled by the DICT server.
     Therefore, the client must examine the first character of each line
     received.  Those that begin with two periods must have those two
     periods collapsed into one period.  Those that contain only a sin-
     gle period followed by a CRLF indicate the end of the text
     response.

     Following a text response, a 2yz response code will be sent.

     Text lines MUST NOT exceed 1024 characters in length, counting all
     characters including spaces, separators, punctuation, the extra
     initial period (if needed), and the trailing CRLF.

     It is recommended that text use the US-ASCII [ASCII] or ISO-8859-1
     [ISO] character sets, although it is currently beyond the scope of
     this standard to specify encoding for text.  In the future, after
     significant experience with large databases in various languages
     has been gained, and after evaluating the need for character set
     and other encodings (e.g., compressed or BASE64 encoding), standard
     extensions to this protocol should be proposed.  In the mean time,
     private extensions should be used to explore the parameter space to
     determine how best to implement these extensions.



3.  Command and Response Details

     Below, each DICT command and appropriate responses are detailed.
     Each command is shown in upper case for clarity, but the DICT
     server is case-insensitive.

     Except for the AUTH command, every command described in this sec-
     tion MUST be implemented by all DICT servers.


3.1.  Initial Connection

     When a client initially connects to a DICT server, a code 220 is
     sent if the client's IP is allowed to connect:


          220 text capabilities msg-id


     The code 220 is a banner, usually containing host name and DICT
     server version information.




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     The second-to-last sequence of characters in the banner is the
     optional capabilities string, which will allow servers to declare
     support for extensions to the DICT protocol.  The capabilities
     string is defined below:


          capabilities =  ["<" msg-atom *("." msg-atom) ">"]
          msg-atom     =  1*<any CHAR except SPACE, CTLs,
                             "<", ">", ".", and "\">


     Individual capabilities are described by a single msg-atom.  For
     example, the string <html.gzip> might be used to describe a server
     that supports extensions which allow HTML or compressed output.
     Capability names beginning with "x" or "X" are reserved for experi-
     mental extensions, and SHOULD NOT be defined in any future DICT
     protocol specification.

     The last sequence of characters in the banner is a msg-id, similar
     to the format specified in [RFC822].  The simplified description is
     given below:


          msg-id       =  "<" spec ">"            ; Unique message id
          spec         =  local-part "@" domain
          local-part   =  msg-atom *("." msg-atom)
          domain       =  msg-atom *("." msg-atom)


     Note that, in contrast to [RFC822], spaces and quoted pairs are not
     allowed in the msg-id.  This restriction makes the msg-id much eas-
     ier for the client to locate and parse but does not significantly
     decrease any security benefits, since the msg-id may be arbitrarily
     long (as bounded by the response length limits set forth elsewhere
     in this document).

     Note also that the open and close brackets are part of the msg-id
     and should be included in the string that is used to compute the
     MD5 checksum.

     This message id will be used by the client when formulating the
     authentication string used in the AUTH command.

     If the client's IP is not allowed to connect, then a code 530 is
     sent instead:






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          530 Access denied


     Transient failure responses are also possible:


          420 Server temporarily unavailable
          421 Server shutting down at operator request


     For example, response code 420 should be used if the server cannot
     currently fork a server process (or cannot currently obtain other
     resources required to proceed with a usable connection), but
     expects to be able to fork or obtain these resources in the near
     future.

     Response code 421 should be used when the server has been shut down
     at operator request, or when conditions indicate that the ability
     to service more requests in the near future will be impossible.
     This may be used to allow a graceful operator-mediated temporary
     shutdown of a server, or to indicate that a well known server has
     been permanently removed from service (in which case, the text mes-
     sage might provide more information).


3.2.  The DEFINE Command

     DEFINE database word


3.2.1.  Description

     This command will look up the specified word in the specified
     database.  All DICT servers MUST implement this command.

     If the database name is specified with an exclamation point (deci-
     mal code 33, "!"), then all of the databases will be searched until
     a match is found, and all matches in that database will be dis-
     played.  If the database name is specified with a star (decimal
     code 42, "*"), then all of the matches in all available databases
     will be displayed.  In both of these special cases, the databases
     will be searched in the same order as that printed by the "SHOW DB"
     command.

     If the word was not found, then status code 552 is sent.

     If the word was found, then status code 150 is sent, indicating
     that one or more definitions follow.



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     For each definition, status code 151 is sent, followed by the tex-
     tual body of the definition.  The first three space-delimited
     parameters following status code 151 give the word retrieved, the
     name of the database (which is the same as the first column of the
     SHOW DB command), and a short description for the database (which
     is the same as the second column of the SHOW DB command).  The
     short name is suitable for printing as:


          From name:


     before the definition is printed.  This provides source information
     for the user.

     The textual body of each definition is terminated with a CRLF
     period CRLF sequence.

     After all of the definitions have been sent, status code 250 is
     sent.  This command can provide optional timing information (which
     is server dependent and is not intended to be parsable by the
     client).  This additional information is useful when debugging and
     tuning the server.


3.2.2.  Responses


          550 Invalid database, use "SHOW DB" for list of databases
          552 No match
          150 n definitions retrieved - definitions follow
          151 word database name - text follows
          250 ok (optional timing information here)


     Response codes 150 and 151 require special parameters as part of
     their text.  The client can use these parameters to display infor-
     mation on the user's terminal.

     For code 150, parameters 1 indicates the number of definitions
     retrieved.

     For code 151, parameter 1 is the word retrieved, parameter 2 is the
     database name (the first name as shown by "SHOW DB") from which the
     definition has been retrieved, and parameter 3 is the the short
     database description (the second column of the "SHOW DB" command).





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3.2.3.  A Note on Virtual Databases

     The ability to search all of the provided databases using a single
     command is given using the special "*" and "!" databases.

     However, sometimes, a client may want to search over some but not
     all of the databases that a particular server provides.  One alter-
     native is for the client to use the SHOW DB command to obtain a
     list of databases and descriptions, and then (perhaps with the help
     of a human), select a subset of these databases for an interative
     search.  Once this selection has been done once, the results can be
     saved, for example, in a client configuration file.

     Another alternative is for the server to provide "virtual"
     databases which merge several of the regular databases into one.
     For example, a virtual database may be provided which includes all
     of the translating dictionaries, but which does not include regular
     dictionaries or thesauri.  The special "*" and "!" databases can be
     considered as names of virtual databases which provide access to
     all of the databases.  If a server implements virtual databases,
     then the special "*" and "!" databases should probably exclude
     other virtual databases (since they merely provide information
     duplicated in other databases).  If virtual databases are sup-
     ported, they should be listed as a regular database with the SHOW
     DB command (although, since "*" and "!" are required, they need not
     be listed).

     Virtual databases are an implementation-specific detail which has
     absolutely no impact on the DICT protocol.  The DICT protocol views
     virtual and non-virtual databases the same way.

     We mention virtual databases here, however, because they solve a
     problem of database selection which could also have been solved by
     changes in the protocol.  For example, each dictionary could be
     assigned attributes, and the protocol could be extended to specify
     searches over databases with certain attributes.  However, this
     needlessly complicates the parsing and analysis that must be per-
     formed by the implementation.  Further, unless the classification
     system is extremely general, there is a risk that it would restrict
     the types of databases that can be used with the DICT protocol
     (although the protocol has been designed with human-language
     databases in mind, it is applicable to any read-only database
     application, especially those with a single semi-unique alphanu-
     meric key and textual data).







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3.3.  The MATCH Command

     MATCH database strategy word


3.3.1.  Description

     This command searches an index for the dictionary, and reports
     words which were found using a particular strategy.  Not all
     strategies are useful for all dictionaries, and some dictionaries
     may support additional search strategies (e.g., reverse lookup).
     All DICT servers MUST implement the MATCH command, and MUST support
     the "exact" and "prefix" strategies.  These are easy to implement
     and are generally the most useful.  Other strategies are server
     dependent.

     The "exact" strategy matches a word exactly, although different
     servers may treat non-alphanumeric data differently.  We have found
     that a case-insensitive comparison which ignores non-alphanumeric
     characters and which folds whitespace is useful for English-lan-
     guage dictionaries.  Other comparisons may be more appropriate for
     other languages or when using extended character sets.

     The "prefix" strategy is similar to "exact", except that it only
     compares the first part of the word.

     Different servers may implement these algorithms differently.  The
     requirement is that strategies with the names "exact" and "prefix"
     exist so that a simple client can use them.

     Other strategies that might be considered by a server implementor
     are matches based on substring, suffix, regular expressions,
     soundex [KNUTH73], and Levenshtein [PZ85] algorithms.  These last
     two are especially useful for correcting spelling errors.  Other
     useful strategies perform some sort of "reverse" lookup (i.e., by
     searching definitions to find the word that the query suggests).

     If the database name is specified with an exclamation point (deci-
     mal code 33, "!"), then all of the databases will be searched until
     a match is found, and all matches in that database will be dis-
     played.  If the database name is specified with a star (decimal
     code 42, "*"), then all of the matches in all available databases
     will be displayed.  In both of these special cases, the databases
     will be searched in the same order as that printed by the "SHOW DB"
     command.

     If the strategy is specified using a period (decimal code 46, "."),
     then the word will be matched using a server-dependent default



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     strategy, which should be the best strategy available for interac-
     tive spell checking.  This is usually a derivative of the Leven-
     shtein algorithm [PZ85].

     If no matches are found in any of the searched databases, then sta-
     tus code 552 will be returned.

     Otherwise, status code 152 will be returned followed by a list of
     matched words, one per line, in the form:


          database word


     This makes the responses directly useful in a DEFINE command.

     The textual body of the match list is terminated with a CRLF period
     CRLF sequence.

     Following the list, status code 250 is sent, which may include
     server-specific timing and statistical information, as discussed in
     the section on the DEFINE command.


3.3.2.  Responses


          550 Invalid database, use "SHOW DB" for list of databases
          551 Invalid strategy, use "SHOW STRAT" for a list of strategies
          552 No match
          152 n matches found - text follows
          250 ok (optional timing information here)


     Response code 152 requires a special parameter as part of its text.
     Parameter 1 must be the number of matches retrieved.



3.4.  The SHOW Command


3.4.1.  SHOW DB

     SHOW DB
     SHOW DATABASES





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3.4.1.1.  Description

     Displays the list of currently accessible databases, one per line,
     in the form:


          database description


     The textual body of the database list is terminated with a CRLF
     period CRLF sequence.  All DICT servers MUST implement this com-
     mand.

     Note that some databases may be restricted due to client domain or
     lack of user authentication (see the AUTH command).  Information
     about these databases is not available until authentication is per-
     formed.  Until that time, the client will interact with the server
     as if the additional databases did not exist.


3.4.1.2.  Responses


          110 n databases present - text follows
          554 No databases present


     Response code 110 requires a special parameter.  Parameter 1 must
     be the number of databases available to the user.


3.4.2.  SHOW STRAT

     SHOW STRAT
     SHOW STRATEGIES


3.4.2.1.  Description

     Displays the list of currently supported search strategies, one per
     line, in the form:


          strategy description


     The textual body of the strategy list is terminated with a CRLF
     period CRLF sequence.  All DICT servers MUST implement this



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     command.


3.4.2.2.  Responses


          111 n strategies available - text follows
          555 No strategies available


     Response code 111 requires a special parameter.  Parameter 1 must
     be the number of strategies available.


3.4.3.  SHOW INFO

     SHOW INFO database


3.4.3.1.  Description

     Displays the source, copyright, and licensing information about the
     specified database.  The information is free-form text and is suit-
     able for display to the user in the same manner as a definition.
     The textual body of the information is terminated with a CRLF
     period CRLF sequence.  All DICT servers MUST implement this com-
     mand.


3.4.3.2.  Responses


          550 Invalid database, use "SHOW DB" for list of databases
          112 database information follows


     These response codes require no special parameters.



3.4.4.  SHOW SERVER

     SHOW SERVER








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3.4.4.1.  Description

     Displays local server information written by the local administra-
     tor.  This could include information about local databases or
     strategies, or administrative information such as who to contact
     for access to databases requiring authentication.  All DICT servers
     MUST implement this command.


3.4.4.2.  Responses


          114 server information follows


     This response code requires no special parameters.



3.5.  The CLIENT Command

     CLIENT text


3.5.1.  Description

     This command allows the client to provide information about itself
     for possible logging and statistical purposes.  All clients SHOULD
     send this command after connecting to the server.  All DICT servers
     MUST implement this command (note, though, that the server doesn't
     have to do anything with the information provided by the client).


3.5.2.  Responses


          250 ok (optional timing information here)


     This response code requires no special parameters.



3.6.  The STATUS Command

     STATUS





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3.6.1.  Description

     Display some server-specific timing or debugging information.  This
     information may be useful in debugging or tuning a DICT server.
     All DICT servers MUST implement this command (note, though, that
     the text part of the response is not specified and may be omitted).


3.6.2.  Responses


          210 (optional timing and statistical information here)


     This response code requires no special parameters.



3.7.  The HELP Command

     HELP


3.7.1.  Description

     Provides a short summary of commands that are understood by this
     implementation of the DICT server.  The help text will be presented
     as a textual response, terminated by a single period on a line by
     itself.  All DICT servers MUST implement this command.



3.7.2.  Responses


          113 help text follows


     This response code requires no special parameters.


3.8.  The QUIT Command

     QUIT







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3.8.1.  Description

     This command is used by the client to cleanly exit the server.  All
     DICT servers MUST implement this command.



3.8.2.  Responses


          221 Closing Connection


     This response code requires no special parameters.



3.9.  The AUTH Command

     AUTH username authentication-string


3.9.1.  Description

     The client can authenticate itself to the server using a username
     and password.  The authentication-string will be computed as in the
     APOP protocol discussed in [RFC1939].  Briefly, the authentication-
     string is the MD5 checksum of the concatenation of the msg-id
     (obtained from the initial banner) and the "shared secret" that is
     stored in the server and client configuration files.  Since the
     user does not have to type this shared secret when accessing the
     server, the shared secret can be an arbitrarily long passphrase.
     Because of the computational ease of computing the MD5 checksum,
     the shared secret should be significantly longer than a usual pass-
     word.

     Authentication may make more dictionary databases available for the
     current session.  For example, there may be some publicly dis-
     tributable databases available to all users, and other private
     databases available only to authenticated users.  Or, a server may
     require authentication from all users to minimize resource utiliza-
     tion on the server machine.

     Authentication is an optional server capability.  The AUTH command
     MAY be implemented by a DICT server.






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3.9.2.  Responses


          230 Authentication successful
          531 Access denied, use "SHOW INFO" for server information


     These response codes require no special parameters.


4.  Command Pipelining

     All DICT servers MUST be able to accept multiple commands in a sin-
     gle TCP send operation.  Using a single TCP send operation for mul-
     tiple commands can improved DICT performance significantly, espe-
     cially in the face of high latency network links.

     The possible implementation problems for a DICT server which would
     prevent command pipelining are similar to the problems that prevent
     pipelining in an SMTP server.  These problems are discussed in
     detail in [RFC1854], which should be consulted by all DICT server
     implementors.

     The main implication is that a DICT server implementation MUST NOT
     flush or otherwise lose the contents of the TCP input buffer under
     any circumstances whatsoever.

     A DICT client may pipeline several commands and must check the
     responses to each command individually.  If the server has shut
     down, it is possible that all of the commands will not be pro-
     cessed.  For example, a simple DICT client may pipeline a CLIENT,
     DEFINE, and QUIT command sequence as it is connecting to the
     server.  If the server is shut down, the initial response code sent
     by the server may be 420 (temporarily unavailable) instead of 220
     (banner).  In this case, the definition cannot be retrieved, and
     the client should report and error or retry the command.  If the
     server is working, it may be able to send back the banner, defini-
     tion, and termination message in a single TCP send operation.



5.  URL Specification

     The DICT URL scheme is used to refer to definitions or word lists
     available using the DICT protocol:






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     dict://<user>:<passphrase>@<host>:<port>/d:<word>:<database>:<n>
     dict://<user>:<passphrase>@<host>:<port>/m:<word>:<database>:<strat>:<n>


     The "/d" syntax specifies the DEFINE command (see section 3.2),
     whereas the "/m" specifies the MATCH command (section 3.3).

     Some or all of "<user>:<passphrase>@", ":<port>", "<database>",
     "<strat>", and "<n>" may be omitted.

     "<n>" will usually be omitted, but when included, it specifies the
     nth definition or match of a word.  A method for extracting exactly
     this information from the server is not avaiable using the DICT
     protocol.  However, a client using the URL specification could
     obtain all of the definitions or matches, and then select the one
     that is specified.

     If "<user>:<passphrase>@" is omitted, no authentication is done.
     If ":<port>" is omitted, the default port (2628) SHOULD be used.
     If "<database>" is omitted, "!" SHOULD be used (see section 3.2.1).
     If "<strat>" is omitted, "." SHOULD be used (see section 3.3.1).

     Trailing colons may be omitted.  For example, the following URLs
     might specify definitions or matches:


          dict://dict.org/d:shortcake:
          dict://dict.org/d:shortcake:*
          dict://dict.org/d:shortcake:wordnet:
          dict://dict.org/d:shortcake:wordnet:1
          dict://dict.org/d:abcdefgh
          dict://dict.org/d:sun
          dict://dict.org/d:sun::1

          dict://dict.org/m:sun
          dict://dict.org/m:sun::soundex
          dict://dict.org/m:sun:wordnet::1
          dict://dict.org/m:sun::soundex:1
          dict://dict.org/m:sun:::


     See [RFC1738] for the specification of Uniform Resource Locators.









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6.  Extensions

     This protocol was designed so that flat text databases can be used
     with a server after a minimum of analysis and formatting.  Our
     experience is that merely constructing an index for a database may
     be sufficient to make it useful with a DICT server.  The ability to
     serve preformatted text is especially important since freely-avail-
     able databases are often distributed as flat text files without any
     semantic mark-up information (and often contain "ASCII art" which
     precludes the automation of even simple formatting).

     However, given a database with sufficient mark-up information, it
     may be possible to generate output in a variety of different for-
     mats (e.g., simple HTML or more sophisticated SGML).  The specifi-
     cation of formatting is beyond the scope of this document.  The
     requirements for negotiation of format (including character set and
     other encodings) is complex and should be examined over time as
     more experience is gained.  We suggest that the use of different
     formats, as well as other server features, be explored as exten-
     sions to the protocol.


6.1.  Experimental Command Syntax

     Single-letter commands are reserved for debugging and testing,
     SHOULD NOT be defined in any future DICT protocol specification,
     and MUST NOT be used by any client software.

     Commands beginning with the letter "X" are reserved for experimen-
     tal extensions, and SHOULD NOT be defined in any future DICT proto-
     col specification.  Authors of client software should understand
     that these commands are not part of the DICT protocol and may not
     be available on all DICT servers.


6.2.  Experimental Commands and Pipelining

     Experimental commands should be designed so that a client can
     pipeline the experimental commands without knowing if a server sup-
     ports the commands (e.g., instead of using feature negotiation).
     If the server does not support the commands, then a response code
     of 500 will be given, notifying the client that the extension is
     not supported.  Of course, depending on the complexity of the
     extensions added, feature negotiation may be necessary.  To help
     minimize negotiation time, server-supported features may be
     announced in the banner (code 220) using the optional capabilities
     parameter.




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7.  Summary of Response Codes
     Below is a summary of response codes.  A star (*) in the first col-
     umn indicates the response has defined arguments that must be pro-
     vided.

          * 110 n databases present - text follows
          * 111 n strategies available - text follows
            112 database information follows
            113 help text follows
            114 server information follows
          * 150 n definitions retrieved - definitions follow
          * 151 word database name - text follows
          * 152 n matches found - text follows
            210 (optional timing and statistical information here)
          * 220 text msg-id
            221 Closing Connection
            230 Authentication successful
            250 ok (optional timing information here)
            420 Server temporarily unavailable
            421 Server shutting down at operator request
            500 Syntax error, command not recognized
            501 Syntax error, illegal parameters
            502 Command not implemented
            503 Command parameter not implemented
            530 Access denied
            531 Access denied, use "SHOW INFO" for server information
            550 Invalid database, use "SHOW DB" for list of databases
            551 Invalid strategy, use "SHOW STRAT" for a list of strategies
            552 No match
            554 No databases present
            555 No strategies available




8.  Sample Conversations

     Theses are samples of the conversations that might be expected with
     a typical DICT server.  The notation "C:" indicates commands set by
     the client, and "S:" indicates responses sent by the server.  Blank
     lines are included for clarity and do not indicate actual newlines
     in the transaction.


8.1.  Sample 1 - opening connection, HELP, DEFINE, and QUIT commands






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     C: [ client initiates connection ]

     S: 220 dict.org dictd (version 0.9) <27831.860032493@dict.org>



     C: HELP

     S: 113 Help text follows
     S: DEFINE database word            look up word in database
     S: MATCH database strategy word    match word in database using strategy
     S: [ more server-dependent help text ]
     S: .
     S: 250 Command complete



     C: DEFINE ! penguin

     S: 150 1 definitions found: list follows
     S: 151 "penguin" wn "WordNet 1.5" : definition text follows
     S: penguin
     S:   1. n: short-legged flightless birds of cold southern esp. Antarctic
     S:      regions having webbed feet and wings modified as flippers
     S: .
     S: 250 Command complete



     C: DEFINE * shortcacke

     S: 150 2 definitions found: list follows
     S: 151 "shortcake" wn "WordNet 1.5" : text follows
     S: shortcake
     S:   1. n: very short biscuit spread with sweetened fruit and usu.
     S:      whipped cream
     S: .
     S: 151 "Shortcake" web1913 "Webster's Dictionary (1913)" : text follows
     S: Shortcake
     S:    \Short"cake`\, n.
     S:    An unsweetened breakfast cake shortened with butter or lard,
     S:    rolled thin, and baked.
     S: .
     S: 250 Command complete







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     C: DEFINE abcdefgh

     S: 552 No match



     C: quit

     S: 221 Closing connection



8.2.  Sample 2 - SHOW commands, MATCH command


     C: SHOW DB

     S: 110 3 databases present: list follows
     S: wn "WordNet 1.5"
     S: foldoc "Free On-Line Dictionary of Computing"
     S: jargon "Hacker Jargon File"
     S: .
     S: 250 Command complete



     C: SHOW STRAT

     S: 111 5 strategies present: list follows
     S: exact "Match words exactly"
     S: prefix "Match word prefixes"
     S: substring "Match substrings anywhere in word"
     S: regex "Match using regular expressions"
     S: reverse "Match words given definition keywords"
     S: .
     S: 250 Command complete















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     C: MATCH foldoc regex "s.si"

     S: 152 7 matches found: list follows
     S: foldoc Fast SCSI
     S: foldoc SCSI
     S: foldoc SCSI-1
     S: foldoc SCSI-2
     S: foldoc SCSI-3
     S: foldoc Ultra-SCSI
     S: foldoc Wide SCSI
     S: .
     S: 250 Command complete



     C: MATCH wn substring "abcdefgh"

     S: 552 No match



8.3.  Sample 3 - Server downtime


     C: [ client initiates connection ]

     S: 420 Server temporarily unavailable



     C: [ client initiates connection ]

     S: 421 Server shutting down at operator request



8.4.  Sample 4 - Authentication


     C: [ client initiates connection ]

     S: 220 dict.org dictd (version 0.9) <27831.860032493@dict.org>









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     C: SHOW DB

     S: 110 1 database present: list follows
     S: free "Free database"
     S: .
     S: 250 Command complete



     C: AUTH joesmith authentication-string

     S: 230 Authentication successful



     C: SHOW DB

     S: 110 2 databases present: list follows
     S: free "Free database"
     S: licensed "Local licensed database"
     S: .
     S: 250 Command complete



9.  Security Considerations

     This RFC raises no security issues.


10.  References


     [ASCII] US-ASCII. Coded Character Set - 7-Bit American Standard
          Code for Information Interchange. Standard ANSI X3.4-1986,
          ANSI, 1986.


     [FOLDOC] Howe, Denis, ed.  The Free On-Line Dictionary of Comput-
          ing, <URL:http://wombat.doc.ic.ac.uk/>


     [ISO] ISO-8859. International Standard -- Information Processing --
          8-bit Single-Byte Coded Graphic Character Sets -- Part 1:
          Latin alphabet No. 1, ISO 8859-1:1987.






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     [JARGON] The on-line hacker Jargon File, version 4.0.0, 25 JUL
          1996, <URL:http://www.ccil.org/jargon/>


     [KNUTH73] Knuth, Donald E. "The Art of Computer Programming", Vol-
          ume 3: Sorting and Searching (Addison-Wesley Publishing Co.,
          1973, pages 391 and 392). Knuth notes that the soundex method
          was originally described by Margaret K. Odell and Robert C.
          Russell [US Patents 1261167 (1918) and 1435663 (1922)].


     [PZ85] Pollock, Joseph J. and Zamora, Antonio, "Automatic spelling
          correction in scientific and scholarly text," CACM, 27(4):
          Apr. 1985, 358-368.


     [RFC640] Postel, J., "Revised FTP Reply Codes", RFC-640, June,
          1975.


     [RFC821] Postel, J., "Simple Mail Transfer Protocol", RFC-821,
          USC/Information Sciences Institute, August, 1982.


     [RFC822] Crocker, D., "Standard for the Format of ARPA Internet
          Text Messages", RFC-822, Department of Electrical Engineering,
          University of Delaware, August, 1982.


     [RFC977] Kantor, B., Lapsley, P., "Network News Transfer Protocol:
          A Proposed Standard for the Stream-Based Transmission of
          News", RFC-977, U.C. San Diego, U.C. Berkeley, February, 1986.


     [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
          Resource Locators (URL)", RFC-1738, CERN, Xerox PARC, Univer-
          sity of Minnesota, December 1994.


     [RFC1985] Freed, N., and Cargille, A., "SMTP Service Extension for
          Command Pipelining", RFC-1854, Innosoft International, Inc.,
          and Network Working Group, October 1995.


     [RFC1939] Myers, J., Rose, M., "Post Office Protocol - Version 3",
          RFC-1939, Carnegie Mellon/Dover Beach Consulting, May, 1996.





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     [RFC2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Bern-
          ers-Lee, T., "Hypertext Transfer Protocol -- HTTP/1.1",
          RFC-2068, U.C. Irvine, DEC, MIT/LCS, January, 1997.


     [WEB1913] Webster's Revised Unabridged Dictionary (G & C. Merriam
          Co., 1913, edited by Noah Porter).  Online version prepared by
          MICRA, Inc., Plainfield, N.J. and edited by Patrick Cassidy
          <cassidy@micra.com>.  For further information, see
          <URL:ftp://uiarchive.cso.uiuc.edu/pub/etext/guten-
          berg/etext96/pgw*>, and <URL:http://humani-
          ties.uchicago.edu/forms unrest/webster.form.html>


     [WORDNET] Miller, G.A. (1990), ed. WordNet: An On-Line Lexical
          Database. International Journal of Lexicography. Volume 3,
          Number 4.  <URL:http://www.cogsci.princeton.edu/~wn/>


11.  Acknowledgements

     Thanks to Arnt Gulbrandsen and Nicolai Langfeldt for many helpful
     discussions.  Thanks to Bennet Yee, Doug Hoffman, Kevin Martin, and
     Jay Kominek for extensive testing and feedback on the initial
     implementations of the DICT server.  Thanks to Zhong Shao for
     advice and support.

     Thanks to Brian Kanto, Phil Lapsley, and Jon Postel for writing
     exemplary RFCs which were consulted during the preparation of this
     document.


12.  Author's Addresses


          Rickard E. Faith
          EMail: faith@cs.unc.edu (or faith@acm.org)


          Bret Martin
          EMail: bamartin@miranda.org

          The majority of this work was completed while Bret Martin was
          a student at Yale University.





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