GILS Overview

1
XML Encoding Rules (XER)
Rob Bull bull_at_crxnet.com
2
Original Requirements...

• An immediate need (Nov. 1998)
• An abstract specification of search was needed to
  respond to the OpenGIS Consortium RFP for Catalog
  Services - CORBA, COM, Z39.50, SQL, and LDAP)
• XML (eXtensible Markup Language) chosen as the
  notational syntax for the Catalog Services search
  specification

3
Basic Concept of XER

• Rules to convert between XML and BER (Basic
  Encoding Rules) for a protocol specified in ASN.1
  (Abstract Syntax Notation)
• Standardized rules for transforming
• BER encoded structures into equivalent XML
  structures
• XML encoded structures into equivalent BER
  structures

4
Scope...

• ASN.1 is used in a variety of applications,
  protocols, and network environments with various
  encoding rules (e.g., BER, PER, DER)
• XML provides a facility for representing
  character string structures and their markup
  rules
• initial XER focus is Z39.50-1995 (ISO 23950)
  Search and Retrieval protocol
• In the Z39.50 community
• ASN.1 is used with BER
• potentially applicable to LDAP (lightgweight
  directory access protocol) and SNMP (simple
  network management protocol)

5
For example...

• PDU CHOICE searchRequest 22 IMPLICIT
  SearchRequest, searchResponse 23 IMPLICIT
  SearchResponse,
• SearchRequest SEQUENCE resultSetName
  17 IMPLICIT InternationalString, databaseName
  s 18 IMPLICIT SEQUENCE OF DatabaseName,
  query 21 Query
• SearchResponse SEQUENCE
  resultCount 23 IMPLICIT
  INTEGER, numberOfRecordsReturned 24
  IMPLICIT INTEGER, nextResultSetPosition
  25 IMPLICIT INTEGER,
  searchStatus 22 IMPLICIT
  BOOLEAN, resultSetStatus
  26 IMPLICIT INTEGER, records
  Records OPTIONAL,

6
XML Encoding SearchRequest

• lt?xml version'1.0' ?gtltsearch
  xmlns"urnoid1.2.840.10003.2.1"gt
  ltsearchRequestgt ltresultSetNamegt 1
  lt/resultSetNamegt ltdatabaseNamesgt
  xxdefault lt/databaseNamesgt ltquerygt
  query on next slide lt/querygt
  lt/searchRequestgtlt/searchgt

7
XML Encoding Query Detail

• ltquerygt lttype-1gt ltrpnQuerygt
  ltattributeSetgt BIB1 lt/attributeSetgt
  ltrpngt ltrpngt operand next slide
  lt/rpngt ltrpngt operand next slide
  lt/rpngt ltoperatorgt AND lt/operatorgt
  lt/rpngt lt/rpnQuerygt
  lt/type-1gtlt/querygt

8
XML Encoding Operand Detail

• ltoperandgt ltattributeValuegt CreationDate
  lt/attributeValuegt ltattributeValuegt Greater Than
  or Equal lt/attributeValuegt ltattributeValuegt
  Date lt/attributeValuegt lttermgt 1995-01-01
  lt/termgtlt/operandgt

9
XML Encoding SearchResponse

• lt?xml version'1.0' ?gtltsearch
  xmlns"urnoid1.2.840.10003.2.1"gt
  ltsearchResponsegt ltresultCountgt 76
  lt/resultCountgt ltnumberOfRecordsReturnedgt0
  lt/numberOfRecordsReturnedgt
  ltnextResultSetPositiongt 0 lt/nextResultSetPosit
  iongt ltsearchStatusgt lttrue/gtlt/searchStatusgt
  lt/searchResponsegtlt/searchgt

10
Development Approach...

• e-mail list to discuss XER design and issues
• Participants/listeners include
• commercial (large, small)
• desktop software suppliers, telecoms suppliers
• database software suppliers
• Z39.50 and ASN.1 toolkit suppliers and
  consultants
• academic
• research
• Xer syntax and rules have been discussed and a
  draft set of rules created

11
Development Approach...

• Software developed
• Profile for Z39.50 over HTTP discussion paper
• Presentation to ASN.1 committee at Geneva in June
  1999
• has already spawned interest such as ONE-2 project

12
Decisions and Proposals...

• Three categories of decisions and proposals
• decisions where consensus is reached
• proposals that generally affect XER
• proposals that specifically affect XER and Z39.50

13
Consensus Reached...

• The XER generated XML should be readily
  understandable
• XER is generally applicable to ASN.1
• XER requires well-formed XML
• No distinction between explicit, implicit
• encode and decode assumes XER is implicit
• applications need to know ASN.1 and can make BER
  implicit or explicit
• ASN.1 1994 has concept of automatic tags

14
Consensus Reached...

• named values as empty elements, so its not
• ltprotocolVersiongtversion-1, version-2lt/protocolVer
  siongt
• but it is
• ltprotocolVersiongtltversion-1/gtltversion-2/gtlt/protoco
  lVersiongt
• uses XML namespace attribute to identify
  particular XML
• non-validating
• some aspects of the XML record structure can be
  inferred from well-formed values
• beyond the inferred structure various methods
  exist, there is always the base ASN.1
  specification for validation

15
Proposals...

• XML namespace decisions
• adding private PDUs
• how to represent tags for OIDs
• Testbed
• z39.50 client (BER)
• BER/XER gateway (XER)
• XER/BER gateway (XER)
• Z39.50 server (BER)

16
Draft XML Encoding Rules

• 7.1 General rules for encoding7.2 Encoding of a
  boolean data value7.3 Encoding of an integer
  data value7.4 Encoding of an enumerated data
  value7.5 Encoding of a real data value7.6
  Encoding of a bitstring data value7.7 Encoding
  of an octetstring data value7.8 Encoding of a
  null data value7.9 Encoding of a sequence data
  value7.10 Encoding of a sequence-of data
  value7.11 Encoding of a set data value7.12
  Encoding of a set-of data value

17
Draft XML Encoding Rules

• 7.13 Encoding of a choice data value
• 7.14 Encoding of a ASN.1 tagged data value
• 7.15 Encoding of an open type
• 7.16 Encoding of an instance-of data value
• 7.17 Encoding of a data value of the embedded-pdv
  type
• 7.18 Encoding of an object identifier data value
• 7.19 Encoding of a data value of the external
  type
• 7.20 Encoding for data values of the restricted
  character string types
• 7.21 Encoding for data values of the unrestricted
  character string type
• 7.22 Encoding for data values of the any
  character string type
• 7.23 Encoding for data values of the general
  string type
• 7.24 Encoding for data values of the visible
  character string type

18
Profile for Z39.50 over HTTP

• Discussion paper, second draft, April 1999
• Architectural Issues
• Z39.50 is described in ASN.1 but encoding is via
  XER
• requirement for stateful/stateless applications
• stateless - single HTTP request/response sequence
  comprises an entire Z-association
• stateful - a Z-association is maintained across
  multiple TCP/IP sessions
• Z39.50 PDUs will be encapsulated within HTTP
  protocol messages using the Search method

19
Profile for Z39.50 over HTTP

• proposed approaches to mapping HTTP requests and
  responses to Z39.50 requests and responses
• considerations for
• stateful, stateless
• Z39.50 state-machine
• how do we handle trickier services such as
  access control, resource control, segmentation,
  trigger resource control

20
Software Developed so far...

• European Commission, Joint Research Centre,
  Ispra, Italy
• On http//perlz.jrc.it/ a library for handling
  XER encoded packets with Perl is available
• The Z39.50 library contains, as an example, a
  gateway from XER to BER and back
• BER to XML conversion library (Warnock/ Gamiel)
• demonstration utility, software description

21
Opportunities.

• XML developers
• leverage investments in ASN.1-based applications
  already interoperable
• ASN.1 developers
• leverage investments in XML-based applications
  coming up on the Internet

22
Further information...

• XER home page http//asf.gils.net/xer
• with pointers to
• documentation on XER concept
• details on rules defined
• software
• example PDUs
• decisions and proposals
• XER discussion list and the list archive
• Profile for Z39.50 over HTTP
• http//lcweb.loc.gov/z3950/agency/zhttp.html

23
Credits...
This presentation incorporates content from a
presentation to theASN.1 Committee Geneva, June
9, 1999 byEliot Christian, U.S. Geological
Survey and Nick Hibma, E.C. Joint Research
Commission