RFC 3421 (rfc3421) - Page 4 of 8

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

RFC 3421           Select and Sort Extensions for SLP      November 2002


   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 [RFC 2891].  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
   [RFC 2608] 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.





Zhao, et. al.                 Experimental