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Network Working Group                                       R. DenenbergRequest for Comments: 2056                           Library of CongressCategory: Standards Track                                       J. Kunze                                 University of California, San Francisco                                                                D. Lynch                                          SilverPlatter Information Ltd.                                                                 Editors                                                           November 1996Uniform Resource Locators for Z39.50Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.1. Introduction   Z39.50 is an information retrieval protocol that does not fit neatly   into a retrieval model designed primarily around the stateless fetch   of data.  Instead, it models a general user inquiry as a session-   oriented, multi-step task, any step of which may be suspended   temporarily while the server requests additional parameters from the   client before continuing.  Some, none, or all of these client/server   interactions may require participation of the client user, depending   only on the client software (the protocol itself makes no such   requirements).   On the other hand, retrieval of "well-known" data may be performed in   a single step, that is, with a degenerate Z39.50 session consisting   of exactly one protocol search request and response.  Besides the   basic search sub-service, there are several ancillary sub-services   (e.g., Scan, Result Set Delete).  Among the functions covered by   combinations of the sub-services, two core functions emerge as   appropriately handled by two separate URL schemes:  the Session URL   and the Retrieval URL.   Using two schemes instead of one makes a critical distinction between   a Z39.50 Session URL, which opens a client session initialized for   interactive use by the user, and a Z39.50 Retrieval URL, which opens   and closes a client session to retrieve a specific information item.   Making this distinction at the scheme level allows the user interface   to reflect it on to the user, without requiring the user interface toDenenberg, et. al.          Standards Track                     [Page 1]

RFC 2056                    URLs for Z39.50                November 1996   parse otherwise opaque parts of the URL (consistent with current   practice).2. Some Basic Concepts   This section briefly describes the usage of Z39.50-specific   terminology within the URL definitions below: specifically, the terms   database, elementset, recordsyntax, and docid.   The Z39.50 protocol specifies various information retrieval   operations, the two most basic of which are Search and Present. In a   Search operation a client provides search criteria and indicates a   database (or several databases) on the server to search.  The   essential result of a Search operation is that a result set is   created at the server, consisting of pointers to the selected   database records.   Z39.50 models database records, abstract database records, and   retrieval records.  A database record is a unit of information in a   database, represented in a data structure local to the server.  An   abstract database record is an abstract representation of that   information, where the client and server share a common understanding   of the representation.  This allows logical elements to be addressed   and selected for transfer, via an element set specification, or, as   used below, an "elementset".  A retrieval record is the set of   selected elements packaged in an exportable structure, by the   application of a "recordsyntax".   Thus a Search operation results in entries pointing to database   records; via a Present operation, a client requests a retrieval   record, corresponding to a database record, corresponding to an entry   in the result set. The client indicates the composition and format of   the retrieval record by specifying an elementset and recordsyntax,   respectively.   A special case of a Z39.50 search is a "known-item" search, when a   client intends that a search identify a single, known database   record, or "document" (for purposes of illustration, assume that a   database record corresponds to a document), and further, the client   knows an identifier for the document that can be used to effect this   known-item search.  In this case, this identifier is often referred   to as a document identifier, or "docid".Denenberg, et. al.          Standards Track                     [Page 2]

RFC 2056                    URLs for Z39.50                November 19963. The Z39.50 Session URL   The Z39.50 Session URL may be informally described as providing the   mechanism to switch the user to a Z39.50 client application.   -  Host is required.   -  Port is optional, and defaults to 210.   -  All other parameters are optional.   -  The Z39.50 client will start a session to the specified host/port      (alternatively, it need not explicitly start a session, but may      instead utilize an already open session to the same host/port).   -  A database must be included if docid is included.   -  If docid is included, the client will perform the specified search      (in the same manner as for the retrieval URL, specified below).   -  If docid is not included, and other parameters (besides host/port)      are specified, the client may use those parameters as "hints".      Various clients may choose to treat them as requirements, or as      preferences, or ignore them.   -  In any case (whether a search is performed or not), the client      will leave the Z39.50 session open for the user, to do      retrievals, new searches, etc.  (This is the main distinction      from the Retrieval URL which leaves it up to the client whether      or not to keep the Z39.50 session open.)4. The Z39.50 Retrieval URL   The Z39.50 Retrieval URL is intended to allow a Z39.50 session to be   used as a transparent transfer mechanism to retrieve a specific   information object.  A Z39.50 client uses information in the URL to   formulate a Search Request.  The server's Search Response indicates   how many records match the Request.  If the number of matching   records does not equal one, the retrieval is considered unsuccessful,   and the client application's behavior is not defined.  If the number   of matching records equals one, the server may have included the   desired record in the Search Response.  If not, the client requests   transmission of the record with a Present Request.  After the client   has received the specified record it may close the Z39.50 session   immediately, or keep it open for subsequent retrievals.   -  Host is required.   -  Port is optional, and defaults to 210.   -  A database is required.   -  The meaning of a retrieval URL with no docid is undefined.   -  The docid is placed into a type-1 query, as the single term, in      the general format (tag 45), using the Bib-1 attribute set, with      a Use attribute value of docid, and a structure attribute of URx.      The docid string is server-defined and completely opaque to the      client.Denenberg, et. al.          Standards Track                     [Page 3]

RFC 2056                    URLs for Z39.50                November 1996   - If element set name (esn) is not specified, it is the client's      choice.  If esn is specified, it should be used either in the      Search request for the value of small- and/or medium-      set-element-set-names or in a Present request following a      Search.  These terms and their use are defined within the Z39.50      Standard [2].   -  If record syntax (rs) is not specified, it is the client's choice.      If one or more record syntaxes are specified, the client should      select one (preferably the first in the list that it supports)      and use it in a Search or Present request as the value of      PreferredRecordSyntax.5. BNF for Z39.50 URLs   The Z39.50 Session and Retrieval URLs follow the Common Internet   Scheme Syntax as defined inRFC 1738, "Uniform Resource Locators   (URL)" [1].  In the definition, literals are quoted with "", optional   elements are enclosed in [brackets], "|" is used to designate   alternatives, and elements may be preceded with <n>* to designate n   or more repetitions of the following element; n defaults to 0.z39.50url      = zscheme "://" host [":" port]                      ["/" [database *["+" database]                             ["?" docid]]                           [";esn=" elementset]                           [";rs=" recordsyntax *[ "+" recordsyntax]]]zscheme        = "z39.50r" | "z39.50s"database       = uchardocid          = ucharelementset     = ucharrecordsyntax   = uchar   Future extensions to these URLs will be of the form of   [;keyword=value].   The following definitions are fromRFC 1738. Between the Internet   Draft version andRFC 1738 two relevant changes were made: '=' was   moved from the <extra> character class to <reserved>, and <national>   was removed from the alternatives in <unreserved>. Neither <national>   nor <punctuation> is referred to in this document nor inRFC 1738.Denenberg, et. al.          Standards Track                     [Page 4]

RFC 2056                    URLs for Z39.50                November 1996lowalpha       = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |                 "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |                 "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |                 "y" | "z"hialpha        = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |                 "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |                 "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"alpha          = lowalpha | hialphadigit          = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |                 "8" | "9"safe           = "$" | "-" | "_" | "." | "+"extra          = "!" | "*" | "'" | "(" | ")" | ","national       = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"punctuation    = "<" | ">" | "#" | "%" | <">reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "="hex            = digit | "A" | "B" | "C" | "D" | "E" | "F" |                 "a" | "b" | "c" | "d" | "e" | "f"escape         = "%" hex hexunreserved     = alpha | digit | safe | extrauchar          = unreserved | escapexchar          = unreserved | reserved | escapedigits         = 1*digit6. Security Considerations   The two Z39.50 URL schemes are subject to the same security   implications as the general URL scheme [1], so the usual precautions   apply.  This means, for example, that a locator might no longer point   to the object that was originally intended.  It also means that it   may be possible to construct a URL so that an attempt to perform a   harmless idempotent operation such as the retrieval of an object will   in fact cause a possibly damaging remote operation to occur.7. Acknowledgements   The Z39.50 Implementors Group contributed the substance of this   document.8. References   [1] Berners-Lee, T., Masinter, L., McCahill, M. (editors), "Uniform       Resource Locators (URL)",RFC 1738, December 1994.ftp://ds.internic.net/rfc/rfc1738.txt   [2] ANSI/NISO Z39.50-1995, "ANSI Z39.50: Information Retrieval       Service and Protocol", 1995.ftp://ftp.loc.gov/pub/z3950/Denenberg, et. al.          Standards Track                     [Page 5]

RFC 2056                    URLs for Z39.50                November 1996   [3] ANSI/NISO Z39.50-1992, "ANSI Z39.50: Information Retrieval       Service and Protocol", 1992.ftp://ftp.cni.org/pub/NISO/docs/Z39.50-1992/www/Z39.50.toc.html       (also available in hard copy from Omnicom Information Service,       115 Park St., SE, Vienna, VA  22180).9. Editors' Addresses   Ray Denenberg   Library of Congress   Collections Services   Network Development/MSO   Washington DC 20540   Phone: (202) 707-5795   Fax:   (202) 707-0115   EMail: ray@rden.loc.gov   John A. Kunze   Center for Knowledge Management   University of California, San Francisco   530 Parnassus Ave, Box 0840   San Francisco, CA  94143-0840   Phone: (415) 502-6660   Fax:   (415) 476-4653   EMail: jak@ckm.ucsf.edu   Denis Lynch   SilverPlatter Information Ltd.   10 Barely Mow Passage   Chiswick, London W4 4PH   U.K.   Voice: +44 (0)181-995-8242   Fax:   +44 (0)181-995-5159   EMail: DenisL@SilverPlatter.comDenenberg, et. al.          Standards Track                     [Page 6]

RFC 2056                    URLs for Z39.50                November 1996Appendix. Examples of Z39.50 URLs   A basic Z39.50 session URL that a client might use to open a   connection to the MELVYL union catalog "cat" at the University of   California is        z39.50s://melvyl.ucop.edu/cat   A URL that would open the MELVYL magazine database just long enough   to fetch an article from volume 30, number 19 of a hypothetical   periodical might look like        z39.50r://melvyl.ucop.edu/mags?elecworld.v30.n19   As a final example, here is another retrieval URL that a client could   use to request a full record (element set "f") in the MARC syntax   from a hypothetical database called TMF at CNIDR:        z39.50r://cnidr.org:2100/tmf?bkirch_rules__a1;esn=f;rs=marc   As in the previous example, the part of the string after the `?' is   determined by the server.  In this example, the server is running on   non-standard port 2100.Denenberg, et. al.          Standards Track                     [Page 7]