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Versions: (draft-rahman-core-groupcomm) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 RFC 7390

CoRE Working Group                                        A. Rahman, Ed.
Internet-Draft                          InterDigital Communications, LLC
Intended status: Informational                              E. Dijk, Ed.
Expires: September 10, 2012                             Philips Research
                                                           March 9, 2012


                      Group Communication for CoAP
                      draft-ietf-core-groupcomm-01

Abstract

   This is a working document intended to develop draft text for the
   CoAP protocol specification in the area of group communication.  A
   solution based on IP multicast is proposed and detailed.  Also,
   guidance is provided for deployment in various constrained network
   topologies.

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 10, 2012.

Copyright Notice

   Copyright (c) 2012 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
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Problem Statement and Scope  . . . . . . . . . . . . . . .  5
     2.3.  Potential Solutions for Group Communication  . . . . . . .  5
     2.4.  CoAP-Observe Alternative to Group Communication  . . . . .  6
   3.  Use Cases and Requirements . . . . . . . . . . . . . . . . . .  7
     3.1.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.1.  Background . . . . . . . . . . . . . . . . . . . . . .  7
       3.2.2.  General Requirements . . . . . . . . . . . . . . . . .  8
       3.2.3.  Security Requirements  . . . . . . . . . . . . . . . . 10
   4.  IP Multicast Solution  . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  Multicast Listener Discovery (MLD) & Multicast Router
           Discovery (MRD)  . . . . . . . . . . . . . . . . . . . . . 11
     4.3.  Group URIs and IP Multicast Addresses  . . . . . . . . . . 12
     4.4.  Group Discovery and Member Discovery . . . . . . . . . . . 13
       4.4.1.  DNS-SD . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.4.2.  CoRE Resource Directory  . . . . . . . . . . . . . . . 13
     4.5.  Group Resource Manipulation  . . . . . . . . . . . . . . . 14
     4.6.  Congestion Control . . . . . . . . . . . . . . . . . . . . 15
     4.7.  CoAP Multicast and HTTP Unicast Interworking . . . . . . . 15
   5.  Deployment Guidelines  . . . . . . . . . . . . . . . . . . . . 17
     5.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.2.  Example Lighting Use Case  . . . . . . . . . . . . . . . . 17
     5.3.  Implementation in Target Network Topologies  . . . . . . . 22
       5.3.1.  Single LLN Topology  . . . . . . . . . . . . . . . . . 23
       5.3.2.  Single LLN with Backbone Topology  . . . . . . . . . . 25
       5.3.3.  Multiple LLNs with Backbone Topology . . . . . . . . . 27
       5.3.4.  LLN(s) with Multiple 6LBRs . . . . . . . . . . . . . . 27
       5.3.5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . 27
     5.4.  Implementation Considerations  . . . . . . . . . . . . . . 28
       5.4.1.  MLD Implementation on LLNs . . . . . . . . . . . . . . 28
       5.4.2.  6LBR Implementation  . . . . . . . . . . . . . . . . . 29
       5.4.3.  Backbone IP Multicast Infrastructure . . . . . . . . . 29
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 30
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     10.2. Informative References . . . . . . . . . . . . . . . . . . 32



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   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34


















































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1.  Conventions and 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 [RFC2119].

   The following are definitions of specific terminology used in this
   draft.

   Group Communication: A source node sends a message to more than one
   destination node, where all destinations are identified to belong to
   a specific group.  The set of source nodes and/or the set of
   destination nodes may consist of an arbitrary mix of constrained and
   non-constrained nodes.

   Multicast: Sending a message to multiple receiving nodes
   simultaneously.  Typically, this is done as part of a group
   communication process.  There are various options to implement
   multicast including layer 2 (Media Access Control) or layer 3 (IP)
   mechanisms.

   IP Multicast: A specific multicast solution based on the use of IP
   multicast addresses as defined in "IANA Guidelines for IPv4 Multicast
   Address Assignments" [RFC5771] and "IP Version 6 Addressing
   Architecture" [RFC4291].

   Low power and Lossy Network (LLN): LLNs are made up of constrained
   devices.  These devices may be interconnected by a variety of links,
   such as IEEE 802.15.4, Bluetooth, WiFi, wired or low power power-line
   communication links.


2.  Introduction

2.1.  Background

   The Constrained Application Protocol (CoAP) is an application
   protocol (analogous to HTTP) for resource constrained devices
   operating in an IP network [I-D.ietf-core-coap].  Constrained devices
   can be large in number, but are often highly correlated to each other
   (e.g. by type or location).  For example, all the light switches in a
   building may belong to one group and all the thermostats belong to
   another group.  All the smart meters in the same region can belong to
   a group as well.  Groups may be composed by function; for example,
   the group "all lights in building one" may consist of the groups "all
   lights on floor one of building one", "all lights on floor two of
   building one", etc.  Groups may be preconfigured or dynamically
   formed.  If information needs to be sent to or received from a group



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   of devices, group communication mechanisms can improve efficiency and
   latency of communication and reduce bandwidth requirements for a
   given application.

2.2.  Problem Statement and Scope

   In this draft, we address the issues related to group communication
   in detail, with requirements, use cases, proposed solutions and
   analysis of their impact to the CoAP protocol and to implementations.
   We assume that all, or a substantial part of, CoAP devices
   participating in group communication are constrained devices (e.g.
   Low Power and Lossy Network (LLN) devices).  The guiding principle is
   to apply wherever possible existing IETF protocols to achieve group
   communication functionality.  In many cases the contribution of this
   document lies in explaining how existing mechanisms may be used to
   together fulfill CoAP group communication needs for specific use
   cases.

2.3.  Potential Solutions for Group Communication

   The classic concept of group communications is that of a single
   source distributing content to multiple recipients that are all part
   of a group, as shown in the example sequence diagram in Figure 1.
   Also shown there is the pre-requisite step of forming the group
   before content can be distributed to it.  The source may be either a
   member or non-member of the group.

   Group communication solutions have evolved from "bottom" to "top",
   i.e., from the network layer (IP multicast) to application layer
   group communication, also referred to as application layer multicast.
   A study published in 2005 [Lao05] identified new solutions in the
   "middle" (referred to as overlay multicast) that utilize an
   infrastructure based on proxies.

   Each of these classes of solutions may be compared [Lao05] using
   metrics such as link stress and level of host complexity
   [Banerjee01].  The results show for a realistic internet topology
   that IP Multicast is the most resource-efficient, with the downside
   being that it requires the most effort to deploy in the
   infrastructure.  IP Multicast is the solution recommended by this
   draft with detailed analysis and guidance for this choice being
   provided in the following sections.









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                                             Group
          Node 1           Node 2            Coordinator       Node 3
            |                 |               |                   |
            |   REQUEST       |               |                   |
            |  (Join Group X) |               |                   |
            |-----------------|------------- >|                   |
            |   RESPONSE      |               |                   |
            |< ---------------|---------------|                   |
            |                 |               |                   |
            |                 |    REQUEST    |                   |
            |                 |(Join Group X) |                   |
            |                 |------------- >|                   |
            |                 |   RESPONSE    |                   |
            |                 |< -------------|                   |
            |                 |               |     REQUEST       |
            |                 |               |    (Send to       |
            |                 |               |     Group X )     |
            |                 |               |< -----------------|
            |                 |               |                   |
            |                 |          Map to                   |
            |                 |      Group X addresses            |
            |                 |               |                   |
            |   REQUEST (to multicast addr)   |                   |
            |< ---------------|< -------------|                   |
            |                 |               |                   |
            |     (optional) RESPONSE         |                   |
            |                 |------------- >|                   |
            |-----------------|-------------->|                   |
            |                 |               |       RESPONSE    |
            |                 |               |----------------- >|
            |                 |               |                   |


               Figure 1: Example Group Communication Concept

2.4.  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 in that case 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.




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   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.


3.  Use Cases and Requirements

3.1.  Use Cases

   The use of CoAP group communication is shown in the context of
   several use cases.  The following use cases are identified at this
   point:

   o  Lighting Control: synchronous operation of a group of 6LoWPAN
      IPv6-connected lights

   o  Discovery: discovering CoAP devices and the Resource and Services
      they offer

   o  Parameter Update: updating parameters/settings simultaneously in a
      large group of devices in a building/campus control
      ([I-D.vanderstok-core-bc]) application

   In a future version of this document, more use cases should be added
   and described in more detail.

3.2.  Requirements

   Requirements that a CoAP group communication solution should fulfill
   can be found in existing documents ([RFC5867],
   [I-D.ietf-6lowpan-routing-requirements], [I-D.vanderstok-core-bc],
   and [I-D.shelby-core-coap-req]).  Below, a set of high-level
   requirements is listed that a group communication solution should
   ideally fulfill.  In practice, all these requirements can never be
   satisfied at once in an LLN context.  Furthermore, different use
   cases will have different needs i.e. an elaboration of a subset of
   below requirements.

3.2.1.  Background

   The requirements for CoAP are documented in
   [I-D.shelby-core-coap-req].  In this draft, we focus and expand
   discussions on the requirements pertaining to CoAP "group
   communication" and "multicast" support as stated in
   [I-D.shelby-core-coap-req]:




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      REQ 9: CoAP will support a non-reliable IP multicast message to be
      sent to a group of Devices to manipulate a resource on all the
      Devices simultaneously.  The use of multicast to query and
      advertise descriptions must be supported, along with the support
      of unicast responses.

   Currently, the CoAP protocol [I-D.ietf-core-coap] supports unreliable
   IP multicast using UDP.  It defines the unreliable multicast
   operation as follows in Section 4.5:

      "CoAP supports sending messages to multicast destination
      addresses.  Such multicast messages MUST be Non-Confirmable.  Some
      mechanisms for avoiding congestion from multicast requests are
      being considered in [I-D.eggert-core-congestion-control]."

   Additional requirements were introduced in [I-D.vanderstok-core-bc]
   driven by quality of experience issues in commercial lighting; the
   need for large numbers of devices to respond with near simultaneity
   to a command (multicast PUT), and for that command to be received
   reliably (reliable multicast).

3.2.2.  General Requirements

   A CoAP group communication solution should (ideally) meet the
   following general requirements:

   GEN-REQ 1:    Optional Reliability: the application can select
                 between unreliable group communication and reliable
                 group communication.

   GEN-REQ 2:    Efficiency: delivers messages more efficiently than a
                 "serial unicast" solution.  Provides a balance between
                 group data traffic and control overhead.

   GEN-REQ 3:    Low latency: deliver a message as quickly as possible.

   GEN-REQ 4:    Synchrony: allows near-simultaneous modification of a
                 resource on all devices in a target group, providing a
                 perceived effect of synchrony or simultaneity.  For
                 example a specified time span D such that a message is
                 delivered to all destinations in a time interval
                 [t,t+D].

   GEN-REQ 5:    Ordering: message ordering may be required for reliable
                 group communication use cases.






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   GEN-REQ 6:    Security: see Section 6 for security requirements for
                 group communication.

   GEN-REQ 7:    Flexibility: support for one or many source(s), both
                 dense and sparse networks, for high or low listener
                 density, small or large number of groups, and multi-
                 group membership.

   GEN-REQ 8:    Robust group management: functionality to join groups,
                 leave groups, view group membership, and persistent
                 group membership in failure or sleeping node
                 situations.

   GEN-REQ 9:    Network layer independence: a solution is independent
                 from specific unicast and/or IP multicast routing
                 protocols.

   GEN-REQ 10:   Minimal specification overhead: a group communication
                 solution should preferably re-use existing/established
                 (IETF) protocols that are suitable for LLN deployments,
                 instead of defining new protocols from scratch.

   GEN-REQ 11:   Minimal implementation overhead: e.g. a solution allows
                 to re-use existing (software) components that are
                 already present on constrained nodes such as (typical)
                 6LoWPAN/CoAP nodes.

   GEN-REQ 12:   Mixed backbone/LLN topology support: a solution should
                 work within a single LLN, and in combined LLN/backbone
                 network topologies, including multi-LLN topologies.
                 Both the senders and receivers of CoAP group messages
                 may be attached to different network links or be part
                 of different LLNs, possibly with routers or switches in
                 between group members.  In addition, different routing
                 protocols may operate on the LLN and backbone networks.
                 Preferably a solution also works with existing, common
                 backbone IP infrastructure (e.g. switches or routers).

   GEN-REQ 13:   CoAP Proxying support: a CoAP proxy can handle
                 distribution of a message to a group on behalf of a
                 (constrained) CoAP client.

   GEN-REQ 14:   Suitable for operation on LLNs with constrained nodes.








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3.2.3.  Security Requirements

   Security for group communications at the IP level has been studied
   extensively in the IETF MSEC (Multicast Security) WG, and to a lesser
   extent in the IRTF SAMRG (Scalable Adaptive Multicast Research
   Group).  In particular, [RFC3740], [RFC5374] and [RFC4046] are very
   instructive.  A set of requirements for securing group communications
   in CoAP were derived from a study of these previous investigations as
   well as understanding of CoAP specific needs.  These are listed
   below.

   A CoAP group communication solution should (ideally) meet the
   following security requirements:

   SEC-REQ 1:   Group communications data encryption: Important CoAP
                group communications shall be encrypted (using a group
                key) to preserve confidentiality.  It shall also be
                possible to send CoAP group communications in the clear
                (i.e. unencrypted) for low value data.

   SEC-REQ 2:   Group communications source data authentication:
                Important CoAP group communications shall be
                authenticated by verifying the source of the data (i.e.
                that it was generated by a given and trusted group
                member).  It shall also be possible to send
                unauthenticated CoAP group communications for low value
                data.

   SEC-REQ 3:   Group communications limited data authentication: Less
                important CoAP group communications shall be
                authenticated by simply verifying that it originated
                from one of the group members (i.e. without explicitly
                identifying the source node).  This is a weaker
                requirement (but simpler to implement) than REQ2.  It
                shall also be possible to send unauthenticated CoAP
                group communications for low value data.

   SEC-REQ 4:   Group key management: There shall be a secure mechanism
                to manage the cryptographic keys (e.g. generation and
                distribution) belonging to the group; the state (e.g.
                current membership) associated with the keys; and other
                security parameters.

   SEC-REQ 5:   Use of Multicast IPSec: The CoAP protocol
                [I-D.ietf-core-coap] allows IPSec to be used as one
                option to secure CoAP.  If IPSec is used as a way to
                security CoAP communications, then multicast IPSec
                [RFC5374] should be used for securing CoAP group



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                communications.

   SEC-REQ 6:   Independence from underlying routing security: CoAP
                group communication security shall not be tied to the
                security of underlying routing and distribution
                protocols such as PIM [RFC4601] and RPL
                [I-D.ietf-roll-rpl].  Insecure or inappropriate routing
                (including IP multicast routing) may cause loss of data
                to CoAP but will not affect the authenticity or secrecy
                of CoAP group communications.

   SEC-REQ 7:   Interaction with HTTPS: The security scheme for CoAP
                group communications shall account for the fact that it
                may need to interact with HTTPS (Hypertext Transfer
                Protocol Secure) when a transaction involves a node in
                the general Internet (non-constrained network)
                communicating via a HTTP-CoAP proxy.


4.  IP Multicast Solution

4.1.  Introduction

   IP Multicast protocols have been evolving for decades, resulting in
   proposed standards such as Protocol Independent Multicast - Sparse
   Mode (PIM-SM) [RFC4601].  Yet, due to various technical and marketing
   reasons, IP Multicast is not widely deployed on the general Internet.
   However, IP Multicast is popular in specific deployments such as in
   enterprise networks (e.g. for video conferencing or general IP
   multicast PC applications within a single LAN broadcast domain) and
   carrier IPTV deployments.  The packet economy and minimal host
   complexity of IP multicast make it attractive for group communication
   in constrained environments.  IP multicast is the recommended
   solution for CoAP group communications.

4.2.  Multicast Listener Discovery (MLD) & Multicast Router Discovery
      (MRD)

   In order to extend the scope of IP multicast beyond link-local, 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 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



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   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 an MLD Router) if no MLD router is present.

   The Multicast Router Discovery (MRD) protocol [RFC4286] defines a way
   to discover multicast routers, for the purpose of using this
   information by IGMP/MLD snooping devices.

   [I-D.ietf-multimob-igmp-mld-tuning] 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.

4.3.  Group URIs and IP Multicast Addresses

   An approach to map group authorities onto IP multicast addresses
   using DNS was proposed in [I-D.vanderstok-core-bc].  Based on this,
   examples of group URI naming (and scoping) for a building control
   application are shown below.  Group URIs MUST follow the URI syntax
   defined in [RFC3986].

     URI authority                  Targeted group
     all.bldg6.example.com          "all nodes in building 6"
     all.west.bldg6.example.com     "all nodes in west wing, building 6"
     all.floor1.west.bldg6.examp... "all nodes in floor 1, west wing,
                                     building 6"
     all.bu036.floor1.west.bldg6... "all nodes in office bu036, floor1,
                                     west wing, building 6"

   The authority portion of the URI is used to identify a node (or
   group) and the resulting DNS name is bound to a unicast or multicast
   IP address.  Each example group URI shown above can be mapped to a
   unique multicast IP address.  This may be a site-local or global
   address allocated according to [RFC3956], [RFC3306] or [RFC3307].






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4.4.  Group Discovery and Member Discovery

   CoAP defines a resource discovery capability but, in the absence of a
   standardized group communication infrastructure, it is limited to
   link-local scope IP multicast; examples may be found in
   [I-D.ietf-core-link-format].  A service discovery capability is
   required to extend discovery to other subnets and scale beyond a
   certain point, as originally proposed in [I-D.vanderstok-core-bc].
   Discovery includes both discovering groups (e.g. find a group to join
   or send a multicast message to) and discovering members of a group
   (e.g. to address selected group members by unicast).

4.4.1.  DNS-SD

   DNS-based Service Discovery [I-D.cheshire-dnsext-dns-sd] defines a
   conventional way to configure DNS PTR, SRV, and TXT records to enable
   enumeration of services, such as services offered by CoAP nodes, or
   enumeration of all CoAP nodes, within specified subdomains.  A
   service is specified by a name of the form
   <Instance>.<ServiceType>.<Domain>, where the service type for CoAP
   nodes is _coap._udp and the domain is a DNS domain name that
   identifies a group as in the examples above.  For each CoAP end-point
   in a group, a PTR record with the name _coap._udp and/or a PTR record
   with the name _coap._udp.<Domain> is defined and it points to an SRV
   record having the <Instance>.<ServiceType>.<Domain> name.

   All CoAP nodes in a given subdomain may be enumerated by sending a
   query for PTR records named _coap._udp to the authoritative DNS
   server for that zone.  A list of SRV records is returned.  Each SRV
   record contains the port and host name (AAAA record) of a CoAP node.
   The IP address of the node is obtained by resolving the host name.
   DNS-SD also specifies an optional TXT record, having the same name as
   the SRV record, which can contain "key=value" attributes.  This can
   be used to store information about the device, e.g. schema=DALI,
   type=switch, group=lighting.bldg6, etc.

   Another feature of DNS-SD is the ability to specify service subtypes
   using PTR records.  For example, one could represent all the CoAP
   groups in a subdomain by PTR records with the name
   _group._sub._coap._udp or alternatively
   _group._sub._coap._udp.<Domain>.

4.4.2.  CoRE Resource Directory

   CoRE Resource Directory [I-D.shelby-core-resource-directory] defines
   the concept of a Resource Directory (RD) server where CoAP servers
   can register their resources offered and CoAP clients can discover
   these resources by querying the RD server.  RD syntax can be mapped



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   to DNS-SD syntax and vice versa [I-D.lynn-core-discovery-mapping],
   such that the above approach can be reused for group discovery and
   group member discovery.

   Specifically, the Domain (d) parameter can be set to the group URI by
   an end-point registering to the RD.  If an end-point wants to join
   multiple groups, it has to repeat the registration process for each
   group it wants to join.

4.5.  Group Resource Manipulation

   Group communications shall only be used for idempotent messages (i.e.
   CoAP GET, PUT, DELETE).  Group communications shall NOT be used for
   non-idempotent messages (i.e.  CoAP POST).  The CoAP messages that
   are sent via group communications shall only be of the Non-
   Confirmable type.  A response may be sent back to the group message
   (.e.g "Response 2.01" to a group GET request).  This will typically
   require a CoAP proxy in the message processing path to process the
   multiple responses.  See also Section 5.2.

   Ideally, all nodes in a given group (defined by its multicast IP
   address) must receive the same request with high probability.  This
   will not be the case if there is diversity in the authority port
   (i.e. a diversity of dynamic port addresses across the group) or if
   the targeted resource is located at different paths on different
   nodes.  Extending the definition of group membership to include port
   and path discovery is not desirable.

   Therefore, some measures must be present to ensure uniformity in port
   number and resource name/location within a group.

   A first solution in this respect is to couple groups to service
   descriptions in DNS (using DNS-SD as in Section 4.4 and
   [I-D.vanderstok-core-bc]).  A service description for a multicast
   group may have a TXT record in DNS defining a schema X (e.g.
   "schema=DALI"), which defines by service standard X (e.g.  "DALI")
   which resources a node supporting X MUST have.  Therefore a multicast
   source can safely refer to all resources with corresponding
   operations as prescribed by standard X. For port numbers (which can
   be found using DNS-SD also) the same holds.  Alternatively, only the
   default CoAP port may be used in all CoAP multicast requests.

   A second solution is to impose the following restrictions, e.g. for
   groups not found using, or advertised in, DNS-SD:

   o  All CoAP multicast requests MUST be sent to the well-known CoAP
      port.




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   o  All CoAP multicast requests SHOULD operate on /.well-known/core
      URIs

4.6.  Congestion Control

   CoAP requests may be multicast, resulting a multitude of replies from
   different nodes, potentially causing congestion.
   [I-D.eggert-core-congestion-control] suggests to conservatively
   control sending multicast requests.

   CoAP already addresses the congestion problem to some extent by
   requiring all multicast CoAP requests to be Non-Confirmable.  In CoAP
   a MAX_RETRANSMIT value set by default to 4 is used for retransmission
   of Confirmable messages, but since CoAP multicast messages are Non-
   Confirmable their effective retransmission value is 0.  However, as
   responses to multicast requests SHOULD be sent
   ([I-D.ietf-core-coap]), using CoAP multicast still may lead to
   congestion issues.

   Various means can be implemented to prevent congestion.  For a
   multicast request that leads to the sending of a response by a
   server, CoAP currently recommends a required random delay, within a
   specified TIMEOUT period, before the server can send the response.
   In order to cope with the different requirements for TIMEOUT imposed
   by different use cases and network topologies, one recommended
   approach is to define a CoAP Option via which a CoAP client can
   indicate a preference for TIMEOUT for a specific response.  This
   Option proposal will be done in a separate draft.

4.7.  CoAP Multicast and HTTP Unicast Interworking

   Within the constrained network, CoAP runs over UDP for which IP
   multicast is supported.  In a non-constrained network (i.e. general
   Internet), HTTP over TCP is used for which IP multicast is not
   supported.  Therefore a CoAP/HTTP Proxy node that supports group
   communication needs to have functionalities to support interworking
   of unicast and multicast.  One possible way of operation of the Proxy
   is illustrated in Figure 2.  Note that this topic is covered in more
   detail in [I-D.castellani-core-http-mapping].












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           CoAP             CoAP           CoAP/HTTP            HTTP
          Node 1           Node 2            Proxy             Node 3
            |                 |               |                   |
            |   REQUEST       |               |                   |
            |  (Join Group X) |               |                   |
            |-----------------|------------- >|                   |
            |   RESPONSE      |               |                   |
            |< ---------------|---------------|                   |
            |                 |               |                   |
            |                 |    REQUEST    |                   |
            |                 | (Join Group X)|                   |
            |                 |------------- >|                   |
            |                 |   RESPONSE    |                   |
            |                 |< -------------|                   |
            |                 |               |                   |
            |                 |               |                   |
            |                 |               |  HTTP REQUEST     |
            |                 |               |    (URI to        |
            |                 |               |   unicast addr)   |
            |                 |               |< -----------------|
            |                 |               |                   |
            |                 |          Map URI                  |
            |                 |     to Group X multicast address  |
            |                 |               |                   |
            |   REQUEST (to multicast addr)   |                   |
            |< ---------------|< -------------|                   |
            |                 |               |                   |
            |                 |               |                   |
            |     (optional) RESPONSE         |                   |
            |                 |------------- >|                   |
            |-----------------|-------------->|                   |
            |                 |               |  HTTP RESPONSE    |
            |                 |               |----------------- >|
            |                 |               |                   |


          Figure 2: CoAP Multicast and HTTP Unicast Interworking

   Note that Figure 2 illustrates the case of IP multicast as the
   underlying group communications mechanism.

   A key point in Figure 2 is that the incoming HTTP Request (from node
   3) will carry a URI (with the HTTP scheme) that resolves in the
   general Internet to the proxy node.  At the proxy node, the URI will
   then possibly be mapped (as detailed in
   [I-D.castellani-core-http-mapping]) and again resolved (with the CoAP
   scheme) to an IP multicast destination.  This may be accomplished,
   for example, by using DNS-SD (Section 4.4).  The proxy node will then



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   IP multicast the CoAP Request (corresponding to the received HTTP
   Request) to the appropriate nodes (i.e. nodes 1 and 2).

   In terms of the HTTP Response, Figure 2 illustrates that it will be
   generated by the proxy node based on aggregated responses of the CoAP
   nodes and sent back to the client in the general Internet that sent
   the HTTP Request (i.e. node 1).  In
   [I-D.castellani-core-http-mapping] the HTTP Response that the Proxy
   may use to aggregate multiple CoAP responses is described in more
   detail.  So in terms of overall operation, the CoAP proxy can be
   considered to be a "non-transparent" proxy according to [RFC2616].
   Specifically, [RFC2616] states that a "non-transparent proxy is a
   proxy that modifies the request or response in order to provide some
   added service to the user agent, such as group annotation services,
   media type transformation, protocol reduction or anonymity
   filtering."

   An alternative to the above is using a Forward Proxy.  In this case,
   the CoAP request URI could be carried in the HTTP Request Line (as
   defined in [I-D.ietf-core-coap] Section 8) in a HTTP request sent to
   the IP address of the Proxy.


5.  Deployment Guidelines

5.1.  Overview

   We recommend to use IP multicast as outlined in Section 4 as the base
   solution for CoAP Group Communication, provided that the use case and
   network characteristics allow this.  It has the advantage that it re-
   uses the IP multicast suite of protocols and can operate even if
   group members are distributed over both constrained and non-
   constrained network segments.  Still, this approach may require
   specifying or implementing additional IP Multicast functionality in
   an LLN, in a backbone network, or in both - this will be evaluated in
   more detail in this section.

5.2.  Example Lighting Use Case

   We first present an example use case to illustrate the overall steps
   in an IP Multicast based CoAP Group Communication solution.  We
   assume the following network configuration for this example (see
   Figure 3):

   1) A large room (Room-A) with three lights (Light-1, Light-2,
   Light-3) controlled by a Light Switch.  The devices are organized
   into two 6LoWPAN subnets.




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   2) Light-1 and the Light Switch are connected to a router (Rtr-1)
   which is also a CoAP Proxy and a 6LoWPAN Border Router (6LBR).

   3) Light-2 and the Light-3 are connected to another router (Rtr-2)
   which is also a CoAP Proxy and a 6LBR.

   4) The routers are connected to a an IPv6 network backbone which is
   also multicast enabled.  In the general case, this means the network
   backbone and 6LBRs support a PIM based multicast routing protocol,
   and MLD for forming groups.  In a limited case, if the network
   backbone is one link, then the routers only have to support MLD-
   snooping for the example use case to work.







































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                                                                 Network
                                                                Backbone
                                                                       |
      ################################################                 |
      #                                       Room-A #                 |
      #         **********************               #                 |
      #       **     LoWPAN-1         **             #                 |
      #     *                            *           #                 |
      #    *     +----------+             *          #                 |
      #   *      |  Light   |-------+      *         #                 |
      #  *       |  Switch  |       |       *        #                 |
      #  *       +----------+  +---------+  *        #                 |
      #  *                     |  Rtr-1  |-----------------------------|
      #  *                     +---------+  *        #                 |
      #  *       +----------+        |      *        #                 |
      #   *      |  Light-1 |--------+     *         #                 |
      #    *     +----------+             *          #                 |
      #     *                            *           #                 |
      #       **                      **             #                 |
      #         **********************               #                 |
      #                                              #                 |
      #                                              #                 |
      #        **********************                #                 |
      #       **     LoWPAN-2         **             #                 |
      #     *                            *           #                 |
      #    *     +----------+             *          #                 |
      #   *      |  Light-2 |-------+      *         #                 |
      #  *       |          |       |       *        #                 |
      #  *       +----------+  +---------+  *        #                 |
      #  *                     |  Rtr-2  |-----------------------------|
      #  *                     +---------+  *        #                 |
      #  *       +----------+        |      *        #                 |
      #   *      |  Light-3 |--------+     *         #                 |
      #    *     +----------+             *          #                 |
      #     *                            *           #                 |
      #       **                      **             #                 |
      #         **********************               #                 |
      #                                              #                 |
     #################################################                 |
                                                                       |
                                           +--------+                  |
                                           |  DNS   |------------------|
                                           | Server |
                                           +--------+


            Figure 3: Network Topology of a Large Room (Room-A)




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   The corresponding protocol flow for an IP Multicast based CoAP Group
   Communication solution for the network shown in Figure 3 is shown in
   sequence in Figure 4, Figure 5, and Figure 6.  We assume the
   following steps occur before the illustrated flow:

   1) Startup phase: 6LoWPANs are formed.  IPv6 addresses assigned to
   all devices.  The CoAP network is formed.

   2) Commissioning phase (by applications): The IP multicast address of
   the group (Room-A-Lights) has been set in all the Lights.  The URI of
   the group (Room-A-Lights) has been set in the Light Switch.

   The indicated MLD Report messages are link-local multicast.  In each
   LoWPAN, it is assumed that a multicast routing protocol in 6LRs will
   propagate the Join information over multiple hops to the 6LBR.


                                    Light      Rtr-1     Rtr-2   Network
   Light-1   Light-2    Light-3     Switch    (CoAP     (CoAP   Backbone
    |          |          |          |         Proxy)    Proxy)       |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    | MLD Report: Join    |          |          |          |          |
    | Group (Room-A-Lights)          |          |          |          |
    |------------------------------------------>|          |          |
    |          |          |          |          |MLD Report: Join     |
    |          |          |          |          |Group (Room-A-Lights)|
    |          |          |          |          |-------------------->|
    |          |          |          |          |          |          |
    |          | MLD Report: Join    |          |          |          |
    |          | Group (Room-A-Lights)          |          |          |
    |          |------------------------------------------>|          |
    |          |          |          |          |          |          |
    |          |          | MLD Report: Join    |          |          |
    |          |          | Group (Room-A-Lights)          |          |
    |          |          |------------------------------->|          |
    |          |          |          |          |          |          |
    |          |          |          |          |MLD Report: Join     |
    |          |          |          |          |Group (Room-A-Lights)|
    |          |          |          |          |          |--------->|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |


                 Figure 4: Joining Groups in a Large Room






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                                    Light      Rtr-1     Rtr-2   Network
   Light-1   Light-2    Light-3     Switch    (CoAP     (CoAP   Backbone
    |          |          |          |         Proxy)    Proxy)       |
    |          |          |          |          |          |          |
    |          |          ***********************          |          |
    |          |          *   User flips on     *          |          |
    |          |          *   light switch to   *          |          |
    |          |          *   turn on all the   *          |          |
    |          |          *   lights in Room A  *          |          |
    |          |          ***********************          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          | COAP NON (PUT       |          |          |
    |          |          |          (Proxy-URI |          |          |
    |          |          |          (URI for Room-A-Lights))         |
    |          |          |          turn on lights)       |          |
    |          |          |          |--------->|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |     Request DNS resolution of  |
    |          |          |          |     URI for Room-A-Lights      |
    |          |          |          |          |-------------------->|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |     DNS returns: AAAA          |
    |          |          |          |     Group (Room-A-Lights)      |
    |          |          |          |     IPv6 multicast address     |
    |          |          |          |          |<--------------------|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          | COAP NON (Put                  |
    |          |          |          |          (URI Path)            |
    |          |          |          |          turn on lights)       |
    |          |          |          |  * Destination IP Address =    |
    |          |          |          |       IP multicast address     |
    |          |          |          |       for Group (Room-A-Lights)|
    |          |          |          |  * Originating IP Address =    |
    |          |          |          |        RTR-1                   |
    |          |          |          |          |-------------------->|
    |<------------------------------------------|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |<---------|
    |          |<---------|<-------------------------------|          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |


            Figure 5: Sending Multicast Message in a Large Room



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                                    Light      Rtr-1     Rtr-2   Network
   Light-1   Light-2    Light-3     Switch    (CoAP     (CoAP   Backbone
    |          |          |          |         Proxy)    Proxy)       |
    |          |          |          |          |          |          |
    ***********************          |          |          |          |
    *   Lights in Room-A  *          |          |          |          |
    *   turn on (nearly   *          |          |          |          |
    *   simultaneously)   *          |          |          |          |
    ***********************          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |     COAP NON (Response         |          |          |          |
    |                (Success))      |          |          |          |
    |------------------------------------------>|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          COAP NON (Response    |          |          |          |
    |                    (Success))  |          |          |          |
    |          |------------------------------->|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |        COAP NON (Response      |          |          |
    |          |                  (Success))    |          |          |
    |          |          |-------------------->|          |          |
    |          |          |          |          |          |          |
    |          |          |          ******************************   |
    |          |          |          *  Rtr-1 as CoAP Proxy       *   |
    |          |          |          *  processes all reponses    *   |
    |          |          |          *  to multicast message      *   |
    |          |          |          *  and formulates one        *   |
    |          |          |          *  consolidated response     *   |
    |          |          |          *  to originator             *   |
    |          |          |          ******************************   |
    |          |          |          |          |          |          |
    |          |          |      COAP NON (Response                   |
    |          |          |           (Success))                      |
    |          |          |          |<---------|          |          |
    |          |          |          |          |          |          |


         Figure 6: Sending Response to Mullticast in a Large Room

5.3.  Implementation in Target Network Topologies

   This section looks in more detail how an IP Multicast based solution
   can be deployed onto the various network topologies that we consider
   important for group communication use cases.  Note that the chosen
   solution of IP Multicast for CoAP group communication works mostly



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   independently from the underlying network topology and its specific
   IP multicast implementation.

   Starting from the simplest case of a single LLN topology, we move to
   more complex topologies involving a backbone network or multiple
   LLNs.  With "backbone" we refer here typically to a corporate LAN or
   VLAN, which constitutes a single broadcast domain by design.  It
   could also be an in-home network.  A multi-link backbone is also
   possible, if there is proper IP multicast routing or forwarding
   configured between these links.  (The term 6LoWPAN Border Router or
   "6LBR" is used here for a border router, though our evaluation is not
   necessarily restricted to 6LoWPAN networks.)

5.3.1.  Single LLN Topology

   The simplest topology is a single LLN, where all the IP multicast
   source(s) and destinations are constrained nodes within this same
   LLN.  Possible implementations of IP multicast routing and group
   administration for this topology are listed below.

5.3.1.1.  Mesh-Under Multicast Routing

   The LLN may be set up in either a mesh-under or a route-over
   configuration.  In the former case, the mesh routing protocol should
   take care of routing IP multicast messages throughout the LLN.

   Because conceptually all nodes in the LLN are attached to a single
   link, there is in principle no need for nodes to announce their
   interest in multicast IP addresses via MLD (see Section 4.2).  A
   multicast message to a specific IP destination, which is delivered to
   all 6LoWPAN nodes by the mesh routing algorithm, is accepted by the
   IP network layer of that node only if it is listening on that
   specific multicast IP address and port.

5.3.1.2.  RPL Multicast Routing

   The RPL routing protocol for LLNs provides support for routing to
   multicast IP destinations (Section 12 of [I-D.ietf-roll-rpl]).  Like
   regular unicast destinations, multicast destinations are advertised
   by nodes using RPL DAO messages.  This functionality requires
   "Storing mode with multicast support" (Mode Of Operation, MOP is 3)
   in the RPL network.

   Once all RPL routing tables in the network are populated, any RPL
   node can send packets to an IP multicast destination.  The RPL
   protocol performs distribution of multicast packet both upward
   towards the DODAG root and downwards into the DODAG.




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   The text in Section 12 of the RPL specification clearly implies that
   IP multicast packets are distributed using link-layer unicast
   transmissions, looking at the use of the word "copied" in this
   section.  Specifically in 6LoWPAN networks, this behavior conflicts
   with the requirement that IP multicast packets MUST be carried as
   link-layer 802.15.4 broadcast frames [RFC4944].

   Assuming that link-layer unicast is indeed meant, this approach seems
   efficient only in a balanced, sparse tree network topology, or in
   situations where the fraction of nodes listening to a specific
   multicast IP address is low, or in duty cycled LLNs where link-layer
   broadcast is a very expensive operation.

5.3.1.3.  RPL Routers with Non-RPL Hosts

   Now we consider the case that hosts exist in a RPL network that are
   not RPL-aware themselves, but rely on RPL routers for their IP
   connectivity beyond link-local scope.  Note that the current RPL
   specification [I-D.ietf-roll-rpl] leaves this case for future
   specification (see Section 16.4).  Non-RPL hosts cannot advertise
   their IP multicast groups of interest via RPL DAO messages as defined
   above.  Therefore in that case MLD could be used for such
   advertisements (State Change Report messages), with all or a subset
   of RPL routers acting in the role of MLD Routers as defined in
   [RFC3810].  However, as the MLD protocol is not designed specifically
   for LLNs it may be a burden for the constrained RPL router nodes to
   run the full MLD protocol.  Alternatives are therefore proposed in
   Section 5.4.1.

5.3.1.4.  Trickle Multicast Forwarding

   Trickle Multicast Forwarding [I-D.ietf-roll-trickle-mcast] is an IP
   multicast routing protocol suitable for LLNs, that uses the Trickle
   algorithm as a basis.  It is a simple protocol in the sense that no
   topology maintenance is required.  It can deal especially well with
   situations where the node density is a-priori unknown.

   Nodes from anywhere in the LLN can be the multicast source, and nodes
   anywhere in the LLN can be multicast destinations.

   Using Trickle Multicast Forwarding it is not required for IP
   multicast destinations (listeners) to announce their interest in a
   specific multicast IP address, e.g. by means of MLD.  Instead, all
   multicast IP packets regardless of IP destination address are stored
   and forwarded by all routers.  Because forwarding is always done by
   multicast, both hosts and routers will be able to receive all
   multicast IP packets.  Routers that receive multicast packets they
   are not interested in, will only buffer these for a limited time



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   until retransmission can be stopped as specified by the protocol.
   Hosts that receive multicast packets they are not interested in, will
   discard multicast packets that are not of interest.  Above properties
   seem to make Trickle especially efficient for cases where the
   multicast listener density is high and the number of distinct
   multicast groups relatively low.

5.3.1.5.  Other Route-Over Methods

   Other known IP multicast routing methods may be used, for example
   flooding or other to be defined methods suitable for LLNs.  An
   important design consideration here is whether multicast listeners
   need to advertise their interest in specific multicast addresses, or
   not.  If they do, MLD is a possible option but also protocol-specific
   means (as in RPL) is an option.  See Section 5.4.1 for more efficient
   substitutes for MLD targeted towards a LLN context.

5.3.2.  Single LLN with Backbone Topology

   A LLN may be connected via a Border Router (e.g. 6LBR) to a backbone
   network, on which IP multicast listeners and/or sources may be
   present.  This section analyzes cases in which IP multicast traffic
   needs to flow from/to the backbone, to/from the LLN.

5.3.2.1.  Mesh-Under Multicast Routing

   Because in a mesh routing network conceptually all nodes in the LLN
   are attached to a single link, a multicast IP packet originating in
   the LLN is typically delivered by the mesh routing algorithm to the
   6LBR as well, although there is no guaranteed delivery.  The 6LBR may
   be configured to accept all IP multicast traffic from the LLN and
   then may forward such packets onto its backbone link.  Alternatively,
   the 6LBR may act in an MLD Router or MLD Snooper role on its backbone
   link and decide whether to forward a multicast packet or not based on
   information learned from previous MLD Reports received on its
   backbone link.

   Conversely, multicast packets originating on the backbone network
   will reach the 6LBR if either the backbone is a single link (LAN/
   VLAN) or IPv6 multicast routing is enabled on the backbone.  Then,
   the 6LBR could simply forward all IP multicast traffic from the
   backbone onto the LLN.  However, in practice this situation may lead
   to overload of the LLN caused by unnecessary multicast traffic.
   Therefore the 6LBR SHOULD only forward traffic that one or more nodes
   in the LLN have expressed interest in, effectively filtering inbound
   LLN multicast traffic.

   To realize this "filter", nodes on the LLN may use MLD to announce



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   their interest in specific multicast IP addresses to the 6LBR.  One
   option is for the 6LBR to act in an MLD Router role on its LLN
   interface.  However, this may be too much of a "burden" for
   constrained nodes.  Light-weight alternatives for MLD are discussed
   in Section 5.4.1.

5.3.2.2.  RPL Multicast Routing

   For RPL routing within the 6LoWPAN, we first consider the case of an
   IP multicast source on the backbone network with one or more IP
   multicast listeners on the RPL LLN.  Typically, the 6LBR would be the
   root of a DODAG so that the 6LBR can easily forward the IP multicast
   packet received on its backbone interface to the right RPL nodes in
   the LLN down along this DODAG (based on previously DAO-advertized
   destinations).

   Second, a multicast source may be in the RPL LLN and listeners may be
   both on the LLN and on the backbone.  For this case RPL defines that
   the multicast packet will propagate both up and down the DODAG,
   eventually reaching the DODAG root (typically a 6LBR) from which the
   packet can be routed onto the backbone in a manner specified in the
   previous section.

5.3.2.3.  RPL Routers with Non-RPL Hosts

   For the case that a RPL LLN contains non-RPL hosts, the solutions
   from the previous section can be used if in addition RPL routers
   implement MLD or "MLD like" functionality similar to as described in
   Section 5.3.1.3.

5.3.2.4.  Trickle Multicast Forwarding

   First, we consider the case of an IP multicast source node on the LLN
   (where all 6LRs support Trickle Multicast Forwarding) and IP
   multicast listeners that may be on the LLN and on the backbone.  As
   Trickle will eventually deliver multicast packets also to a 6LBR,
   which acts as a Trickle Multicast router as well, the 6LBR can then
   forward onto the backbone in the ways described earlier in
   Section 5.3.2.1.

   Second, for the case of an IP multicast source on the backbone and
   multicast listeners on both backbone and/or LLN, the 6LBR needs to
   forward multicast traffic from the backbone onto the LLN.  Here, the
   aforementioned problem (Section 5.3.2.1) of potentially overloading
   the LLN with unwanted backbone IP multicast traffic appears again.

   A possible solution to this is (again) to let multicast listeners
   advertise their interest using MLD as described in Section 5.3.2.1 or



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   to use an MLD alternative suitable for LLNs as described in
   Section 5.4.1.  However, following this approach requires possibly an
   extension to Trickle Multicast Forwarding: the protocol should ensure
   that MLD-advertised information is somehow communicated to the 6LBR,
   possibly over multiple hops.  MLD itself supports link-local
   communication only.

5.3.2.5.  Other Route-Over Methods

   For other multicast routing methods used on the LLN, there are
   similar considerations to the ones in sections above: the strong need
   to filter IP multicast traffic coming into the LLN, the need for
   reporting multicast listener interest (e.g. with MLD or a to-be-
   defined MLD alternative) by constrained (6LoWPAN) nodes, and the need
   for LLN-internal routing as identified in the previous section such
   that the MLD communicated information can reach the 6LBR to be used
   there in multicast traffic filtering decisions.

5.3.3.  Multiple LLNs with Backbone Topology

   Now the case of a single backbone network with two or more LLNs
   attached to it via 6LBRs is considered.  For this case all the
   considerations and solutions of the previous section can be applied.

   For the specific case that a source on a backbone network has to send
   to a very large number of destination located on many LLNs, the use
   of IGMP/MLD Proxying [RFC4605] with a leaf IGMP/MLD Proxy located in
   each 6LBR may be useful.  This method only is defined for a tree
   topology backbone network with the IP multicast source at the root of
   the tree.

5.3.4.  LLN(s) with Multiple 6LBRs

   [ TBD: an LLN with multiple 6LBRs may require some additional
   consideration.  Any need to synchronize mutually on multicast
   listener information? ]

5.3.5.  Conclusions

   For all network topologies that were evaluated, CoAP group
   communication can be in principle supported with IP Multicast, making
   use of existing protocols.  For the case of Trickle Multicast
   Forwarding, it appears that an addition to the protocol is required
   such that information about multicast listeners can be distributed
   towards the 6LBR.  Opportunities were identified for an "MLD-like" or
   "MLD-lightweight" protocol specifically suitable for LLNs, which
   should inter-work with regular MLD on the backbone network.  Such MLD
   variants are further analyzed in Section 5.4.1.



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5.4.  Implementation Considerations

   In this section various implementation aspects are considered such as
   required protocol implementations, additional functionality of the
   6LBR and backbone network equipment.

5.4.1.  MLD Implementation on LLNs

   In previous sections, it was mentioned that the MLDv2 protocol
   [RFC3810] may be too costly for use in a LLN.  MLD relies on periodic
   link-local multicast operations to maintain state.  Also it is
   optimized to fairly dynamic situations where multicast listeners may
   come and go over time.  Such dynamic situations are less frequently
   found in typical LLN use cases such as building control, where
   multicast group membership can remain constant over longer periods of
   time (e.g. months) after commissioning.

   Hence, a viable strategy is to implement a subset of MLD
   functionality in 6LoWPAN nodes which is just enough for the required
   functionality.  A first option is that 6LoWPAN Routers, like MLD
   Snoopers, passively listen to MLD State Change Report messages and
   handle the learned ("snooped") IP multicast destinations in the way
   defined by the multicast routing protocol they are running (e.g. for
   RPL, Routers advertise these destinations using DAO messages).

   A second option is to use MLD as-is but adapt the recommended
   parameter values such that operation on a LLN becomes more efficient.

   A third option is to standardize a new protocol, taking a subset of
   MLD functionality into a "MLD for 6LoWPAN" protocol to support
   constrained nodes optimally.

   A fourth option is now presented, which seems attractive in that it
   minimizes standardization, implementation and network communication
   overhead all at the same time.  This option is to specify a new
   Multicast Listener Option (MLO) as an addition to the 6LoWPAN-ND
   [I-D.ietf-6lowpan-nd] protocol communication that is anyway ongoing
   between a 6LoWPAN host and router(s).  This MLO is preferably
   designed to be maximally similar to the Address Registration Option
   (ARO), which minimizes the need for additional program code on
   constrained nodes.  With an MLO, instead of registering a unicast IP
   address, a host "registers" its interest in a multicast IP address.
   Unlike ARO, multiple MLO can be used in the same ND packet.  A
   registration period is also defined just like in the ARO.  MLO allows
   a host to persistently register as a listener to IP multicast traffic
   and to avoid the overhead of periodic multicast communication which
   is required for full MLD.




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   [ TBD: consider what aspects are needed/not needed for CoAP/LLN
   applications.  Will MLDv1 suffice?  What to do with options like
   'source specific' and include/exclude.  Source-specific can also be
   dealt with at the destination host by filtering?  Do we need limits
   on number of records per packet?  Do we need a higher MLD reliability
   setting - see the parameters in the MLD RFC ]

5.4.2.  6LBR Implementation

   To support mixed backbone/LLN scenarios in CoAP group communication,
   it is RECOMMENDED that a 6LowPAN Border Router (6LBR) will act in an
   MLD Router role on the backbone link.  If this is not possible then
   the 6LBR SHOULD be configured to act as an MLD Multicast Address
   Listener and/or MLD Snooper on the backbone link.

5.4.3.  Backbone IP Multicast Infrastructure

   For corporate/professional applications, most routing and switching
   equipment that is currently on the market is IPv6 capable.  For that
   reason backbone infrastructure operating IPv4 only is considered out
   of scope in this document, at least for the backbone network
   segment(s) where IP multicast destinations are present.  What is
   still in scope is for example an IPv4-only HTTP client that wants to
   send a group communication message via a HTTP-CoAP proxy as
   considered in [I-D.castellani-core-http-mapping].

   The availability of, and requirements for, IP multicast support may
   depend on the specific installation use case.  For example, the
   following cases may be relevant for new IP based building control
   installations:

   1.  System deployed on existing IP (Ethernet/WiFi/...)
       infrastructure, shared with existing IP devices (PCs)

   2.  Newly designed and deployed IP (Ethernet/WiFi/...)
       infrastructure, to be shared with other IP devices (PCs)

   3.  Newly designed and deployed IP (Ethernet/WiFi/...)
       infrastructure, exclusively used for building control.

   Besides physical separation the building control backbone can be
   separated from regular (PC) infrastructure by using a different VLAN.
   A typical corporate installation will have many LAN switches and/or
   routing switches, which pass through IP multicast traffic but on the
   other hand do not support acting in the Router role of MLD/IGMP.
   Perhaps for case 2) and 3) above it is acceptable to add a MLD/IGMP
   capable router somewhere in the network, while for case 1) this may
   not be the case.



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   [TBD: consider the influence of WiFi based backbone networks.  What
   if 6LBRs are at the same time also WiFi routers?  What if 6LBRs have
   an Ethernet connection to legacy WiFI routers?  Check if equivalent
   with Ethernet backbone.]


6.  Security Considerations

   TBD


7.  IANA Considerations

   This document makes no request of IANA.


8.  Conclusions

   IP multicast as outlined in Section 4 is recommended to be adopted as
   the base solution for CoAP Group Communication for situations where
   the use case and network characteristics allow use of IP multicast.
   This approach requires no standards changes to the IP multicast suite
   of protocols and it provides interoperability with IP multicast group
   communication on unconstrained backbone networks.

   The proposals for group communication described in this draft should
   be considered for incorporation into the overall CoAP protocol
   specification.


9.  Acknowledgements

   Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo
   Castellani, Guang Lu, Salvatore Loreto, Kerry Lynn, Dale Seed, Zach
   Shelby, Peter van der Stok, and Juan Carlos Zuniga for their helpful
   comments and discussions that have helped shape this document.


10.  References

10.1.  Normative References

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.



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   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, August 2002.

   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
              Addresses", RFC 3307, August 2002.

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
              Architecture", RFC 3740, March 2004.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address",
              RFC 3956, November 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4046]  Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
              "Multicast Security (MSEC) Group Key Management
              Architecture", RFC 4046, April 2005.

   [RFC4286]  Haberman, B. and J. Martin, "Multicast Router Discovery",
              RFC 4286, December 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, August 2006.

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

   [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
              Extensions to the Security Architecture for the Internet
              Protocol", RFC 5374, November 2008.

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for



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              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              March 2010.

   [RFC5867]  Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low-Power and
              Lossy Networks", RFC 5867, June 2010.

   [I-D.ietf-core-coap]
              Frank, B., Bormann, C., Hartke, K., and Z. Shelby,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

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.eggert-core-congestion-control]
              Eggert, L., "Congestion Control for the Constrained
              Application Protocol (CoAP)",
              draft-eggert-core-congestion-control-01 (work in
              progress), January 2011.

   [I-D.ietf-6lowpan-routing-requirements]
              Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for 6LoWPAN Routing",
              draft-ietf-6lowpan-routing-requirements-10 (work in
              progress), November 2011.

   [I-D.ietf-6lowpan-hc]
              Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams in Low Power and Lossy Networks (6LoWPAN)",
              draft-ietf-6lowpan-hc-15 (work in progress),
              February 2011.

   [I-D.ietf-6lowpan-nd]
              Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
              Discovery Optimization for Low Power and Lossy Networks
              (6LoWPAN)", draft-ietf-6lowpan-nd-18 (work in progress),
              October 2011.

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-11 (work in progress),
              January 2012.




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   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-04 (work in progress),
              February 2012.

   [I-D.shelby-core-coap-req]
              Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
              Kelsey, "CoAP Requirements and Features",
              draft-shelby-core-coap-req-02 (work in progress),
              October 2010.

   [I-D.shelby-core-resource-directory]
              Krco, S. and Z. Shelby, "CoRE Resource Directory",
              draft-shelby-core-resource-directory-02 (work in
              progress), October 2011.

   [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.

   [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.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-02
              (work in progress), October 2011.

   [I-D.ietf-roll-rpl]
              Brandt, A., Vasseur, J., Hui, J., Pister, K., Thubert, P.,
              Levis, P., Struik, R., Kelsey, R., Clausen, T., and T.
              Winter, "RPL: IPv6 Routing Protocol for Low power and
              Lossy Networks", draft-ietf-roll-rpl-19 (work in
              progress), March 2011.

   [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.

   [I-D.ietf-multimob-igmp-mld-tuning]
              Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
              IGMP and MLD for Routers in Mobile and Wireless Networks",



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              draft-ietf-multimob-igmp-mld-tuning-05 (work in progress),
              March 2012.

   [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>.


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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