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Versions: 00 01 draft-shelby-core-resource-directory

CoRE Working Group                                            C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                             March 7, 2011
Expires: September 8, 2011


                      CoRE Simple Server Discovery
             draft-bormann-core-simple-server-discovery-00

Abstract

   CoRE defines a mechanism for resource discovery based on Web linking.
   Many applications also need a simple form of discovery for the
   servers carrying these resources.  This specification shows a simple
   way to extend the link-based resource discovery into a basic form of
   server discovery.

   The current version -00 of this document is just an initial draft
   that is intended to spark discussion.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 8, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Discovery Servers  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  CoAP Server Discovery  . . . . . . . . . . . . . . . . . . . .  5
   4.  Finding a Candidate CoAP Server Discovery Server . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
































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1.  Introduction

   CoRE defines a mechanism for resource discovery based on Web linking
   [RFC5988] [I-D.ietf-core-link-format].  Many applications also need a
   simple form of discovery for the servers carrying these resources.

   More sophisticated CoRE server discovery mechanisms have been
   proposed [I-D.brandt-coap-subnet-discovery].  The present
   specification is not intended as a competing protocol but shows a
   very simple way to extend the link-based resource discovery into a
   basic form of server discovery.  It is an open question whether
   different applications need different discovery solutions or whether
   there can be a "scalable" solution that covers both simple and
   complex scenarios.

   The protocol as designed here has been prototyped in the SAHARA
   project at TZI in just a few lines of code.  The current version -00
   of this document serves as an initial draft that is intended to spark
   discussion.  Not all aspects of the protocol as specified are part of
   the current prototype.  We expect to update the specification both
   based on WG feedback and as we gain experience with the prototype.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.  (See
   [RFC2119].)

   The term "byte" is used in its now customary sense as a synonym for
   "octet".

   The terminology from [I-D.ietf-core-coap] applies.

   In addition:

   CoAP Server Discovery:  This protocol.

   CoAP Server Discovery Server (CSDS):  A server for this protocol,
      which interacts with other CoAP servers, collects resource
      discovery information from them and integrates it into larger
      resource discovery information sets.

   Candidate CSDS:  An IP address that might or might not be useful for
      conversion to a CSDS URI.






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2.  Discovery Servers

   This specification defines a simple form of server discovery that
   makes use of CoAP Server Discovery Servers (CSDS), which are
   addressed simply using the CoAP protocol [I-D.ietf-core-coap].

   The assumption is that there is a way to find one or more CSDSs (see
   Section 4 for a number of such ways).  New CoAP servers that want to
   provide discoverable services can make themselves known at the CSDSs.
   CoAP clients can ask the CSDSs for a resource directory in the usual
   way, which will include both information about the discovery server's
   own resources and information about other servers that made
   themselves known to the discovery servers.






































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3.  CoAP Server Discovery

   Simple CoAP Server Discovery makes use of a simple mapping from a
   server's IP address to a default discovery URI: The default discovery
   URI is created from the server IP address, the CoAP default port
   [I-D.ietf-core-coap], and the absolute path "/.well-known/core"
   [I-D.ietf-core-link-format].

   A CoAP server that wants to make itself discoverable occasionally
   sends a POST request to the default discovery URI of any Candidate
   CSDS that it finds.

   The body of the POST request is either

   o  empty, in which case the CoAP Server Discovery Server is
      encouraged by this POST request to perform GET requests at the
      requesting server's default discovery URI.

   or

   o  a link-format document, which indicates the specific services that
      the requesting server wants to make known to the CSDS.

   The CSDS integrates the information it received this way into its
   resource directory.  It MAY make the information available to further
   CSDSs, if it can ensure that a loop does not form.  The protocol used
   between CSDSs to ensure loop-free operation is outside the scope of
   this document.























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4.  Finding a Candidate CoAP Server Discovery Server

   CoAP servers that want to contact a CSDS can obtain candidate IP
   addresses for such servers (Candidate CSDS) in a number of ways.

   In a 6LoWPAN, good candidates can be taken from:

   o  specific static configuration (e.g., anycast addresses), if any,

   o  the ABRO option of 6LoWPAN-ND [I-D.ietf-6lowpan-nd],

   o  other ND options that happen to point to servers (such as RDNSS),

   o  DHCPv6 options that might be defined later.

   In networks with more inexpensive use of multicast, the Candidate
   CSDS may be a well-known multicast address, i.e.  CSDS are found by
   simply sending POST requests to that well-known multicast address
   (details TBD).

   As some of these sources are just (more or less educated) guesses,
   CoAP servers MUST make use of any error messages to very strictly
   rate-limit requests to Candidate CSDSs that don't work out.  E.g., an
   ICMP Destination Unreachable message (and, in particular, the port
   unreachable code for this message) may indicate the lack of a CoAP
   server on the candidate host, or a CoAP error response code such as
   4.05 "Method Not Allowed" may indicate unwillingness of a CoAP server
   to act as a CSDS.























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5.  IANA Considerations

   This document has no actions for IANA.
















































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6.  Security Considerations

   (None so far; this section will certainly grow as additional security
   considerations beyond those listed in the base specifications become
   known.)














































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7.  Acknowledgements

   The concept for this document was inspired by Zach Shelby et al.'s
   CoAP discovery node that was available in various CoAP interop
   events.  The current implementation was performed by the students of
   the SAHARA project, including Bengt Kohrt, Julian Kornberger, Henning
   Mueller, and Christian Thedieck.  Philip Nguyen read an early draft
   of this document (but all errors are mine).  Anders Brandt's draft
   [I-D.brandt-coap-subnet-discovery] is a fine piece of work and
   certainly motivated me to finally write this up.









































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8.  References

8.1.  Normative References

   [I-D.ietf-6lowpan-nd]
              Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
              Discovery Optimization for Low-power and Lossy Networks",
              draft-ietf-6lowpan-nd-15 (work in progress),
              December 2010.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-04 (work in progress), January 2011.

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-02 (work in progress),
              December 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

8.2.  Informative References

   [I-D.brandt-coap-subnet-discovery]
              Brandt, A., "Discovery of CoAP servers across subnets",
              draft-brandt-coap-subnet-discovery-00 (work in progress),
              March 2011.




















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Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Fax:   +49-421-218-7000
   Email: cabo@tzi.org








































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