Group Communication for CoAP
draft-ietf-core-groupcomm-02

Internet-Draft        Group Communication for CoAP             July 2012

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              March 2010.

   [RFC6550]  Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
              Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
              Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
              Lossy Networks", RFC 6550, March 2012.

   [RFC6636]  Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
              the Internet Group Management Protocol (IGMP) and
              Multicast Listener Discovery (MLD) for Routers in Mobile
              and Wireless Networks", RFC 6636, May 2012.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-10 (work in progress), June 2012.

10.2.  Informative References

   [I-D.cheshire-dnsext-dns-sd]
              Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
              progress), December 2011.

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-14 (work in progress),
              June 2012.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-05 (work in progress), March 2012.

   [I-D.shelby-core-resource-directory]
              Shelby, Z. and S. Krco, "CoRE Resource Directory",
              draft-shelby-core-resource-directory-03 (work in
              progress), May 2012.

   [I-D.vanderstok-core-bc]
              Stok, P. and K. Lynn, "CoAP Utilization for Building
              Control", draft-vanderstok-core-bc-05 (work in progress),
              October 2011.

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   [I-D.lynn-core-discovery-mapping]
              Lynn, K. and Z. Shelby, "CoRE Link-Format to DNS-Based
              Service Discovery Mapping",
              draft-lynn-core-discovery-mapping-01 (work in progress),
              July 2011.

   [I-D.vanderstok-core-dna]
              Stok, P., Lynn, K., and A. Brandt, "CoRE Discovery,
              Naming, and Addressing", draft-vanderstok-core-dna-01
              (work in progress), March 2012.

   [I-D.castellani-core-http-mapping]
              Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Best Practices for HTTP-CoAP Mapping
              Implementation", draft-castellani-core-http-mapping-05
              (work in progress), July 2012.

   [I-D.castellani-core-advanced-http-mapping]
              Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Best Practices for HTTP-CoAP Mapping
              Implementation",
              draft-castellani-core-advanced-http-mapping-00 (work in
              progress), July 2012.

   [I-D.ietf-roll-trickle-mcast]
              Hui, J. and R. Kelsey, "Multicast Forwarding Using
              Trickle", draft-ietf-roll-trickle-mcast-00 (work in
              progress), April 2011.

   [Lao05]    Lao, L., Cui, J., Gerla, M., and D. Maggiorini, "A
              Comparative Study of Multicast Protocols: Top, Bottom, or
              In the Middle?", 2005, <http://www.cs.ucla.edu/NRL/hpi/
              AggMC/papers/comparison_gi_2005.pdf>.

   [Banerjee01]
              Banerjee, B. and B. Bhattacharjee, "A Comparative Study of
              Application Layer Multicast Protocols", 2001, <http://
              wmedia.grnet.gr/P2PBackground/
              a-comparative-study-ofALM.pdf>.

Appendix A.  Multicast Listener Discovery (MLD)

   In order to extend the scope of IP multicast beyond link-local scope,
   an IP multicast routing protocol has to be active in routers on an
   LLN.  To achieve efficient multicast routing (i.e. avoid always
   flooding multicast IP packets), routers have to learn which hosts
   need to receive packets addressed to specific IP multicast

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   destinations.

   The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its
   IPv4 pendant IGMP) is today the method of choice used by an (IP
   multicast enabled) router to discover the presence of multicast
   listeners on directly attached links, and to discover which multicast
   addresses are of interest to those listening nodes.  MLD was
   specifically designed to cope with fairly dynamic situations in which
   multicast listeners may join and leave at any time.

   IGMP/MLD Snooping is a technique implemented in some corporate LAN
   routing/switching devices.  An MLD snooping switch listens to MLD
   State Change Report messages from MLD listeners on attached links.
   Based on this, the switch learns on what LAN segments there is
   interest for what IP multicast traffic.  If the switch receives at
   some point an IP multicast packet, it uses the stored information to
   decide onto which LAN segment(s) to send the packet.  This improves
   network efficiency compared to the regular behavior of forwarding
   every incoming multicast packet onto all LAN segments.  An MLD
   snooping switch may also send out MLD Query messages (which is
   normally done by a device in MLD Router role) if no MLD Router is
   present.

   [RFC6636] discusses optimal tuning of the parameters of MLD for
   routers for mobile and wireless networks.  These guidelines may be
   useful when implementing MLD in LLNs.

Appendix B.  CoAP-Observe Alternative to Group Communication

   The CoAP Observation extension [I-D.ietf-core-observe] can be used as
   a simple (but very limited) alternative for group communication.  A
   group in this case consists of a CoAP server hosting a specific
   resource, plus all CoAP clients observing that resource.  The server
   is the only group member that can send a group message.  It does this
   by modifying the state of a resource under observation and
   subsequently notifying its observers of the change.  Serial unicast
   is used for sending the notifications.  This approach can be a simple
   alternative for networks where IP multicast is not available or too
   expensive.

   The CoAP-Observe approach is unreliable in the sense that, even
   though Confirmable CoAP messages may be used, there are no guarantees
   that an update will be received.  For example, a client may believe
   it is observing a resource while in reality the server rebooted and
   lost its listener state.

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Appendix C.  Change Log

   Changes from ietf-01 to ietf-02:

   o  Rewrote congestion control section based on latest CoAP text
      including Leisure concept (#188)

   o  Updated the CoAP/HTTP interworking section and example use case
      with more details and use of MLD for multicast group joining

   o  Key use cases added (#185)

   o  References to [I-D.vanderstok-core-dna] and
      [I-D.castellani-core-advanced-http-mapping] added

   o  Moved background sections on "MLD" and "CoAP-Observe" to
      Appendices

   o  Removed requirements section (and moved it to
      draft-dijk-core-groupcomm-misc)

   o  Added details for IANA request for group communication multicast
      addresses

   o  Clarified text to distinguish between "link local" and general
      multicast cases

   o  Moved lengthy background section 5 to
      draft-dijk-core-groupcomm-misc and replaced with a summary

   o  Various editorial updates for improved readibility

   o  Changelog added

   Changes from ietf-00 to ietf-01:

   o  Moved CoAP-observe solution section to section 2

   o  Editorial changes

   o  Moved security requirements into requirements section

   o  Changed multicast POST to PUT in example use case

   o  Added CoAP responses in example use case

   Changes from rahman-07 to ietf-00:

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   o  Editorial changes

   o  Use cases section added

   o  CoRE Resource Directory section added

   o  Removed section 3.3.5.  IP Multicast Transmission Methods

   o  Removed section 3.4 Overlay Multicast

   o  Removed section 3.5 CoAP Application Layer Group Management

   o  Clarified section 4.3.1.3 RPL Routers with Non-RPL Hosts case

   o  References added and some normative/informative status changes

Authors' Addresses

   Akbar Rahman (editor)
   InterDigital Communications, LLC

   Email: Akbar.Rahman@InterDigital.com

   Esko Dijk (editor)
   Philips Research

   Email: esko.dijk@philips.com

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