CoRE Working Group                                          E. Dijk, Ed.
Internet-Draft                                          Philips Research
Intended status: Informational                            A. Rahman, Ed.
Expires: January 11, 2013               InterDigital Communications, LLC
                                                           July 10, 2012

             Miscellaneous CoAP Group Communication Topics


   This document contains miscellaneous text around the topic of group
   communication for the Constrained Application Protocol (CoAP).  The
   first part contains, for reference, text that was removed from the
   Group Communication for CoAP draft.  The second part describes group
   communication and multicast functionality that may be input to future
   standardization in the CoRE WG.

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   This Internet-Draft will expire on January 11, 2013.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  General Requirements . . . . . . . . . . . . . . . . . . .  4
     2.3.  Security Requirements  . . . . . . . . . . . . . . . . . .  5
   3.  Group Communication Solutions  . . . . . . . . . . . . . . . .  7
     3.1.  IP Multicast Transmission Methods  . . . . . . . . . . . .  7
       3.1.1.  Serial unicast . . . . . . . . . . . . . . . . . . . .  7
       3.1.2.  Unreliable IP Multicast  . . . . . . . . . . . . . . .  7
       3.1.3.  Reliable IP Multicast  . . . . . . . . . . . . . . . .  7
     3.2.  Overlay Multicast  . . . . . . . . . . . . . . . . . . . .  8
     3.3.  CoAP Application Layer Group Management  . . . . . . . . .  9
   4.  DNS-SD Based Group Resource Manipulation . . . . . . . . . . . 12
   5.  Deployment Guidelines  . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Implementation in Target Network Topologies  . . . . . . . 12
       5.2.1.  Single LLN Topology  . . . . . . . . . . . . . . . . . 13
       5.2.2.  Single LLN with Backbone Topology  . . . . . . . . . . 15
       5.2.3.  Multiple LLNs with Backbone Topology . . . . . . . . . 17
       5.2.4.  LLN(s) with Multiple 6LBRs . . . . . . . . . . . . . . 17
       5.2.5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . 17
     5.3.  Implementation Considerations  . . . . . . . . . . . . . . 18
       5.3.1.  MLD Implementation on LLNs and MLD alternatives  . . . 18
       5.3.2.  6LBR Implementation  . . . . . . . . . . . . . . . . . 19
       5.3.3.  Backbone IP Multicast Infrastructure . . . . . . . . . 19
   6.  Miscellaneous Topics . . . . . . . . . . . . . . . . . . . . . 20
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Multicast Listener Discovery (MLD)  . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   This document contains miscellaneous text around the topic of group
   communication for the Constrained Application Protocol, CoAP
   [I-D.ietf-core-coap].  The first part of the document (Section 3)
   contains, for reference, text that was removed from the Group
   Communication for CoAP [I-D.ietf-core-groupcomm] draft and its
   predecessor [I-D.rahman-core-groupcomm].  The second part of the
   document (Section 6) contains text and/or functionality that may be
   considered for inclusion in [I-D.ietf-core-groupcomm] or otherwise
   may be input to future standardization in the CoRE WG.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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.

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

      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:

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      "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).

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

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

   GEN-REQ 6:    Security: see Section 2.3 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

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

   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.

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

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

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   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

   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

   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 [RFC6550].
                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.

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   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.

3.  Group Communication Solutions

   This section includes the text that describes the solutions of IP
   multicast, overlay multicast, and application layer group
   communication which were removed from [I-D.rahman-core-groupcomm]
   version 07 when the text was transferred to

3.1.  IP Multicast Transmission Methods

3.1.1.  Serial unicast

   Even in systems that generally support IP Multicast, there may be
   certain data links (or transports) that don't support IP multicast.
   For those links a serial unicast alternative must be provided.  This
   implies that it should be possible to enumerate the members of a
   group, in order to determine the correct unicast destinations.

3.1.2.  Unreliable IP Multicast

   The CoRE WG charter specified support for non-reliable IP multicast.
   In the current CoAP protocol design [I-D.ietf-core-coap], unreliable
   multicast is realized by the source sending Non-Confirmable messages
   to a multicast IP address.  IP Multicast (using UDP) in itself is
   unreliable, unless specific reliability features are added to it.

3.1.3.  Reliable IP Multicast

   [TBD: This is a difficult problem.  Need to investigate the benefits
   of repeating MGET and MPUT requests (saturation) to get "Pretty Good
   Reliability".  Use the same MID or a new MID for repeated requests?
   Carsten suggests the use of bloom filters to suppress duplicate

   One could argue that non-idempotent operations (POST) cannot be
   supported without a *truly* reliable multicast protocol.  However, is
   this the case?  If a multicast POST request is sent repeatedly with
   the same Message ID (MID), then CoAP nodes that already received it
   once will ignore duplicates.  Sending with Message ID is supported in
   CoAP for Non-Confirmable messages (thus including multicast messages)

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   as per [I-D.ietf-core-coap] section 4.2. ]

   Reliable multicast supports guaranteed delivery of messages to a
   group of nodes.  The following specifies the requirements as was
   proposed originally in version 01 of [I-D.vanderstok-core-bc]:

   o  Validity - If sender sends a message, m, to a group, g, of
      destinations, a path exists between sender and destinations, and
      the sender and destinations are correct, all destinations in g
      eventually receive m.

   o  Integrity - destination receives m at most once from sender and
      only if sender sent m to a group including destination.

   o  Agreement - If a correct destination of g receives m, then all
      correct destinations of g receive m.

   o  Timeliness - For real-time control of devices, there is a known
      constant D such that if m is sent at time t, no correct
      destination receives m after t+D.

   There are various approaches to achieve reliability, such as

   o  Destination node sends response: a destination sends a CoAP
      Response upon multicast Request reception (it SHOULD be a Non-
      Confirmable response).  The source node may retry a request to
      destination nodes that did not respond in time with a CoAP

   o  Route redundancy

   o  Source node transmits multiple times (destinations do not respond)

3.2.  Overlay Multicast

   An alternative group communication solution (to IP Multicast) is an
   "overlay multicast" approach.  We define an overlay multicast as one
   that utilizes an infrastructure based on proxies (rather than an IP
   router based IP multicast backbone) to deliver IP multicast packets
   to end devices.  MLD ([RFC3810]) has been selected as the basis for
   multicast support by the ROLL working group for the RPL routing
   protocol.  Therefore, it is proposed that "IGMP/MLD Proxying"
   [RFC4605] be used as a basis for an overlay multicast solution for

   Specifically, a CoAP proxy [I-D.ietf-core-coap] may also contain an
   MLD Proxy function.  All CoAP devices that want to join a given IP
   multicast group would then send an MLD Join to the CoAP (MLD) proxy.

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   Thereafter, the CoAP (MLD) proxy would be responsible for delivering
   any IP multicast message to the subscribed CoAP devices.  This will
   require modifications to the existing [RFC4605] functionality.

   Note that the CoAP (MLD) proxy may or may not be connected to an
   external IP multicast enabled backbone.  The key function for the
   CoAP (MLD) proxy is to distribute CoAP generated multicast packets
   even in the absence of router support for multicast.

3.3.  CoAP Application Layer Group Management

   Another alternative solution (to IP Multicast and Overlay Multicast)
   is to define CoAP application level group management primitives.
   Thus, CoAP can support group management features without need for any
   underlying IP multicast support.

   Interestingly, such group management primitives could also be offered
   even if there is underlying IP multicast support.  This is useful
   because IP multicast inherently does not support the concept of a
   group with managed members, while a managed group may be required for
   some applications.

   The following group management primitives are in general useful:

   o  discover groups;

   o  query group properties (e.g. related resource descriptions);

   o  create a group;

   o  remove a group;

   o  add a group member;

   o  remove a group member;

   o  enumerate group members;

   o  security and access control primitives.

   In this proposal a (at least one) CoAP Proxy node is responsible for
   group membership management.  A constrained node can specify which
   group it intends to join (or leave) using a CoAP request to the
   appropriate CoAP Proxy.  To Join, the group name will be included in
   optional request header fields (explained below).  These header
   fields will be included in a PUT request to the Proxy.  The Proxy-URI
   is set to the Group Management URI of the Proxy (found previously
   through the "/.well-known/" resource discovery mechanism).  Note that

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   in this solution also CoAP Proxies may exist in a network that are
   not capable of CoAP group operations.

   Group names may be defined as arbitrary strings with a predefined
   maximum length (e.g. 268 characters or the maximum string length in a
   CoAP Option), or as URIs.

   [ TBD: how can a client send a request to a group?  Does it only need
   to know the group name (string or URI) or also an IP multicast
   address?  One way is to send a CoAP request to the CoAP Proxy with a
   group URI directly in the Proxy-URI field.  This avoids having to
   know anything related to IP multicast addresses. ]

   This solution in principle supports both unreliable and reliable
   group communication.  A client would indicate unreliable
   communication by sending a CoAP Non-Confirmable request to the CoAP
   Proxy, or reliable communication by sending a CoAP Confirmable

   It is proposed that CoAP supports two Header Options for group "Join"
   and "Leave".  These Options are Elective so they should be assigned
   an even number.  Assuming the Type for "join" is x (value TBD), the
   Header Options are illustrated by the table in Figure 1:

   | Type | C/E |  Name         | Data type    | Length | Default      |
   |      |     |               |              |        |              |
   |  x   |  E  | Group Join    | String       | 1-270  | ""           |
   |      |     |               |              | B      |              |
   | x+2  |  E  | Group Leave   | String       | 1-270  | ""           |
   |      |     |               |              | B      |              |

            Figure 1: CoAP Header Options for Group Management

   Figure 2 illustrates how a node can join or leave a group using the
   Header Options in a CoAP message:

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |Ver| T |   OC  |     Code      |         Message ID            |
     | delta |length |  Join Group A (ID or URI)
     |   0   |length |  Join Group B (ID or URI)
     |   2   |length |  Leave Group C (ID or URI)

                Figure 2: CoAP Message for Group Management

   Header Fields for the above example:

   Ver: 2-bit unsigned integer for CoAP Version.  Set to 1 by
   implementation as defined by the CoAP specification.

   T: 2-bit unsigned integer for CoAP Transaction Type.  Either '0'
   Confirmation or '1' Non-Confirmable can be used for group "join" or
   "leave" request.

   OC: 4-bit unsigned integer for Option Count.  For this example, the
   value should be "3" since there are three option fields.

   Code: 8-bit unsigned integer to indicate the Method in a Request or a
   Response Code in a Response message.  Any Code can be used so the
   group management can be piggy-backed in either Request or Response

   Message ID: 16-bit value assigned by the source to uniquely identify
   a pair of Request and Response.

   CoAP defines a delta encoding for header options.  The first delta is
   the "Type" for group join in this specific example.  If the type for
   group join is x as illustrated in Figure 2, delta will be x.  In the
   second header option, it is also a group join so the delta is 0.  The
   third header option is a group leave so the delta is 2.

   An alternative solution to using Header Options (explained above) is
   to use designated parameters in the query part of the URI in the
   Proxy-URI field of a POST (TBD: or PUT?) request to a Proxy's group
   management service resource advertized by DNS-SD.  For example, to
   join group1 and leave group2:

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4.  DNS-SD Based Group Resource Manipulation

   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 [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.

5.  Deployment Guidelines

5.1.  Overview

   We recommend to use IP multicast 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 un-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.  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.)

5.2.1.  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.  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 Appendix A).  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.  RPL Multicast Routing

   The RPL routing protocol for LLNs provides support for routing to
   multicast IP destinations (Section 12 of [RFC6550]).  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

   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.  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 [RFC6550] 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 5.3.1.  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
   until retransmission can be stopped as specified by the protocol.

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   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.  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 5.3.1 for more efficient
   substitutes for MLD targeted towards a LLN context.

5.2.2.  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.  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
   their interest in specific multicast IP addresses to the 6LBR.  One

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   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 5.3.1.  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

   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.  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  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

   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 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 or
   to use an MLD alternative suitable for LLNs as described in

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   Section 5.3.1.  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.  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.

5.2.3.  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.

5.2.4.  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? ]

5.2.5.  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 5.3.1.

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5.3.  Implementation Considerations

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

5.3.1.  MLD Implementation on LLNs and MLD alternatives

   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.
   [RFC6636] could be a guideline in this case.

   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 hosts's
   unicast IP address as with ARO, a host "registers" its interest in a
   multicast IPv6 address.  Unlike the ARO, multiple MLO can be used in
   the same ND packet.  A registration period is also defined in the MLO
   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 the regular

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   MLD protocol.

   [ 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 ]

5.3.2.  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.

5.3.3.  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-advanced-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

   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

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   capable router somewhere in the network, while for case 1) this may
   not be the case.

   [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.  Miscellaneous Topics

   This section is a placeholder to add miscellaneous text, topics or
   proposals related to CoAP group communication in future versions of
   this document.

7.  Acknowledgements

   Thanks to all CoRE WG members who participated in the IETF 82
   discussions, which was the trigger to initiate this document.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   Security aspects of group communication for CoAP are discussed in
   [I-D.ietf-core-groupcomm].  The current document contains no new
   proposals yet, for which security considerations have to be analyzed

10.  References

10.1.  Normative References

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

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

   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security

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              Architecture", RFC 3740, March 2004.

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

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

   [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.

   [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.

   [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.

10.2.  Informative References

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

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

              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.

              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.

              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-01 (work in progress),
              March 2012.

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

              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-rahman-core-groupcomm-07 (work in progress),
              October 2011.

              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.

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

Appendix A.  Multicast Listener Discovery (MLD)

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

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   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

   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

   [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.

Authors' Addresses

   Esko Dijk (editor)
   Philips Research


   Akbar Rahman (editor)
   InterDigital Communications, LLC


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