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RFC 3421 - Select and Sort Extensions for the Service Location P


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Network Working Group                                            W. Zhao
Request for Comments: 3421                                H. Schulzrinne
Category: Experimental                               Columbia University
                                                              E. Guttman
                                                        Sun Microsystems
                                                            C. Bisdikian
                                                               W. Jerome
                                                                     IBM
                                                           November 2002

   Select and Sort Extensions for the Service Location Protocol (SLP)

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines two extensions (Select and Sort) for the
   Service Location Protocol (SLP).  These extensions allow a User Agent
   (UA) to request that the Uniform Resource Locator (URL) entries in a
   Service Reply (SrvRply) be limited to the specified number, or be
   sorted according to the specified sort key list.  Using these two
   extensions together can facilitate discovering the best match, such
   as finding a service that has the maximum speed or the minimum load.

1. Introduction

   This document defines two extensions (Select and Sort) for SLP
   [RFC2608].  These extensions allow a UA to request that the URL
   entries in a SrvRply be limited to the specified number, or be sorted
   according to the specified sort key list.  A Directory Agent (DA) or
   Service Agent (SA) that supports the Select and/or Sort extensions
   MUST include the attribute keyword "select-enabled" and/or "sort-
   enabled" in its advertisement (DAAdvert or SAAdvert).  A UA SHOULD
   check these attributes of the contacting DA/SA before it uses the
   Select and/or Sort extensions to query the DA/SA.

   Using the Select extension, a UA can opt for finding just a few (not
   necessarily all) matching services, which is useful if the UA uses a
   low-bandwidth channel.  Using the Sort extension, a UA can ask the
   DA/SA to sort matching URL entries, which helps the UA to choose a
   service from multiple candidates.  Sorting by the server is more
   efficient than sorting by the client since for sorting purposes, the
   former does not need to pass the attributes of matching URLs to the
   client.  Furthermore, using the Select and Sort extensions together
   can facilitate discovering the best match, such as finding a service
   that has the maximum speed or the minimum load, or has a speed
   closest to a reference value.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted according to in BCP 14, RFC 2119
   [RFC2119].

2. Select Extension

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Select Extension ID = 0x4002  |  Next Extension Offset (NEO)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | NEO, cont'd   |      Number of URL Entries    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1. Select Extension

   The format of the Select extension is shown in Figure 1.  A UA uses
   this extension in a Service Request (SrvRqst) to limit the maximum
   number (say, n) of URL entries to be returned.  When a DA/SA receives
   a SrvRqst with a Select extension, it MUST use a Select extension in
   the corresponding SrvRply to indicate the total number (say, m) of
   matching URL entries if the DA/SA supports this extension, otherwise
   the DA/SA MUST set the error code in the corresponding SrvRply to
   OPTION_NOT_UNDERSTOOD [RFC2608].  If n < m, then only the first n
   matching URL entries are returned, else all m matching URL entries
   are returned.  As a special case, a UA may set n to zero to obtain
   the number of matching URL entries without retrieving the entries
   themselves.

   We denote a Select extension as Select(number).  Thus, Select(3)
   means that the corresponding SrvRply can have at most three URL
   entries.

3. Sort Extension

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Sort Extension ID = 0x4003   |  Next Extension Offset (NEO)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | NEO, cont'd   |   length of <sort-key-list>   |<sort-key-list>\
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2. Sort Extension

   The format of the Sort extension is shown in Figure 2.  A UA uses
   this extension in a SrvRqst to request the URL entries in the
   corresponding SrvRply be sorted according to the sort-key-list. The
   sort-key-list is defined using Augmented Backus-Naur Form (ABNF)
   [RFC2234] as follows:

   sort-key-list  = sort-key / sort-key "," sort-key-list
   sort-key       = key-name ":" type ":" ordering [":" ref-value]
   key-name       = attr-tag from Section 5 of RFC 2608
   type           = "s" / "i"
                    ; "s" for string type
                    ; "i" for integer type
   ordering       = "+" / "-"
                    ; "+" for increasing order
                    ; "-" for decreasing order
   ref-value      = intval from Section 5 of RFC 2608

   Each sort-key in the sort-key-list has a key-name, a type specifier,
   an ordering specifier, and an optional reference value.  The key-name
   MUST be a valid attribute name, and its type is explicitly specified.
   Although SLP has five attribute types (integer, string, boolean,
   opaque and keyword), we only consider integer sort and string sort
   since keyword attributes (they have no values) never need to be
   sorted, and boolean and opaque attributes can be sorted as strings if
   needed.  The integer sort uses the integerOrderingMatch rule defined
   in X.520 [X520], whereas the string sort is performed based on
   lexical ordering.  Strings are compared using the rules defined in
   Section 6.4 of RFC 2608.

   Only integer keys may have a reference value, causing the sort to be
   based on the distance to the reference value.  A reference-based
   sort, such as "X:i:+:12", requires the following two steps:

   Step 1. For each matching service, if its attribute X has a value of
           x, then use |x-12| as its metric.

   Step 2. Use the metrics obtained in Step 1 to sort attribute X
           for matching services.

   The SLP sort rules are adapted from the Lightweight Directory Access
   Protocol (LDAP) sort rules defined in RFC 2891 [RFC2891].  Note that
   sort in SLP is a best effort, no sort error will be returned from a
   DA/SA to a UA.

   (1) The sort-key-list is in order of highest to lowest sort key
       precedence (Section 1.1 of RFC 2891).

   (2) Each attribute SHOULD only occur in the sort-key-list once
       (Section 1.1 of RFC 2891).  If an attribute is included in the
       sort-key-list multiple times, only its first occurrence is
       considered, all other occurrences are ignored.

   (3) For a multi-valued attribute, the least value in each entry
       SHOULD be used in sort (Section 2.2 of RFC 2891).

   (4) An entry missing one or more of the sort keys is treated as
       having NULLs for those missing keys (Section 2.2 of RFC 2891).

   (5) NULL is considered as a larger value than all other valid
       values (Section 2.2 of RFC 2891).

   (6) As the attribute type in SLP is not enforced, an attribute may
       have inconsistent values.  For the purpose of sorting,
       inconsistent values may exist only when an attribute is
       sorted as integer.  Inconsistent values SHOULD be treated as
       NULLs.

   When a DA/SA receives a SrvRqst with a Sort extension, it MUST set
   the error code in the corresponding SrvRply to OPTION_NOT_UNDERSTOOD
   [RFC2608] if the DA/SA does not support the Sort extension or cannot
   perform the requested sort.  The DA/SA sets the error code in the
   corresponding SrvRply to zero if it has successfully processed the
   SrvRqst and performed the requested sort.

   We denote a Sort extension as Sort(sort-key-list).  The following
   examples illustrate how to use the Sort extension.

   o Integer sort on speed (decreasing order).

        Sort(speed:i:-)

     [Note] "i" means integer sort, and "-" means decreasing order.

   o Integer sort on load (increasing order) and speed (decreasing
     order).

        Sort(load:i:+,speed:i:-)

     [Note] "+" means increasing order.

   o String sort on model (increasing order).

        Sort(model:s:+)

     [Note] "s" means string sort.

   o Integer sort on speed (increasing order), based on a reference
     value 12.

        Sort(speed:i:+:12)

     [Note] For example, if we have four matching services, with the
     "speed" attribute as 8 (URL1), 10 (URL2), 12 (URL3), and 15 (URL4),
     then the sorted URL list will be "URL3,URL2,URL4,URL1" (based on
     the metric ordering of |12-12| < |12-10| < |12-15| < |12-8|).

4. Using the Select and Sort Extensions Together

   In addition to being used individually, the Select and Sort
   extensions can be used together to facilitate discovering the best
   match, such as finding a service with the maximum speed.  When these
   two extensions appear in the same SrvRqst message, they MUST be
   processed in the order of their presence.  We show some examples
   next.

   o Find the service with the minimum load

        Sort(load:i:+)
        Select(1)

   o Find the three fastest services

        Sort(speed:i:-)
        Select(3)

   o Find the service with the minimum load among the three fastest

        Sort(speed:i:-)
        Select(3)
        Sort(load:i:+)
        Select(1)

   o Find the service that has a speed closest to 12

        Sort(speed:i:+:12)
        Select(1)

5. IANA Considerations

   The Select and Sort extension IDs, 0x4002 and 0x4003, described in
   Section 2 and Section 3, respectively, have been assigned by IANA out
   of the SLP extension space (RFC 2608, Section 9.1) reserved for
   "mandatory to implement" extensions (i.e., the 0x4000-0x7FFF range).

6. Security Considerations

   There are no new security issues beyond those described in RFC 2608.

7. Acknowledgments

   Ira McDonald provided good suggestions.

8. Normative References

   [RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
             Location Protocol, Version 2", RFC 2608, June 1999.

   [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
             requirement levels", BCP 14, RFC 2119, March 1997.

9. Non-normative References

   [X520]    International Telephone Union, "The Directory: Selected
             Attribute Types", X.520, 1997.

   [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", RFC 2234, November 1997.

   [RFC2891] Howes, T., Wahl, M. and A. Anantha, "LDAP Control Extension
             for Server Side Sorting of Search Results", RFC 2891,
             August 2000.

10. Authors' Addresses

   Weibin Zhao
   Department of Computer Science
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY 10027-7003

   EMail: zwb@cs.columbia.edu

   Henning Schulzrinne
   Department of Computer Science
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY 10027-7003

   EMail: hgs@cs.columbia.edu

   Erik Guttman
   Sun Microsystems
   Eichhoelzelstr. 7
   74915 Waibstadt
   Germany

   EMail: Erik.Guttman@sun.com

   Chatschik Bisdikian
   IBM Corp.
   Thomas J. Watson Research Center
   19 Skyline Drive
   Hawthorne, NY 10532, USA

   EMail: bisdik@us.ibm.com

   William F. Jerome
   IBM Corp.
   Thomas J. Watson Research Center
   19 Skyline Drive
   Hawthorne, NY 10532, USA

   EMail: wfj@us.ibm.com

11. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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