Keywords: SLP, Service Location Protocol, UA, user agent, URL, Uniform Resource Locator, service reply, svrrply
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.
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RFC 3421 Select and Sort Extensions for SLP November 2002
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.
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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.
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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.
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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)
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RFC 3421 Select and Sort Extensions for SLP November 2002
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.
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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
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RFC 3421 Select and Sort Extensions for SLP November 2002
11. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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