Miscellaneous CoAP Group Communication Topics
Author(s): Esko Dijk, Akbar Rahman
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 WG version of Group Communication for CoAP draft. The...
CoRE Working Group E. Dijk, Ed. Internet-Draft Philips Research Intended status: Informational A. Rahman, Ed. Expires: December 19, 2013 InterDigital Communications, LLC June 17, 2013 Miscellaneous CoAP Group Communication Topics draft-dijk-core-groupcomm-misc-04 Abstract 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 WG version of 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. 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 December 19, 2013. Copyright Notice Copyright (c) 2013 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 Dijk & Rahman Expires December 19, 2013 [Page 1] Internet-Draft Miscellaneous CoAP Group Communication June 2013 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Potential Solutions for Group Communication . . . . . . . . . 3 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Background . . . . . . . . . . . . . . . . . . . . . . . 4 4.2. General Requirements . . . . . . . . . . . . . . . . . . 5 4.3. Security Requirements . . . . . . . . . . . . . . . . . . 6 5. Group Communication Solutions . . . . . . . . . . . . . . . . 8 5.1. IP Multicast Transmission Methods . . . . . . . . . . . . 8 5.1.1. Serial unicast . . . . . . . . . . . . . . . . . . . 8 5.1.2. Unreliable IP Multicast . . . . . . . . . . . . . . . 8 5.1.3. Reliable IP Multicast . . . . . . . . . . . . . . . . 8 5.2. Overlay Multicast . . . . . . . . . . . . . . . . . . . . 9 5.3. CoAP Application Layer Group Management . . . . . . . . . 10 6. DNS-SD Based Group Resource Manipulation . . . . . . . . . . 12 7. Group Discovery and Member Discovery . . . . . . . . . . . . 13 7.1. DNS-SD . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. CoRE Resource Directory . . . . . . . . . . . . . . . . . 14 8. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 14 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14 8.2. Implementation in Target Network Topologies . . . . . . . 14 8.2.1. Single LLN Topology . . . . . . . . . . . . . . . . . 15 8.2.2. Single LLN with Backbone Topology . . . . . . . . . . 17 8.2.3. Multiple LLNs with Backbone Topology . . . . . . . . 19 8.2.4. LLN(s) with Multiple 6LBRs . . . . . . . . . . . . . 19 8.2.5. Conclusions . . . . . . . . . . . . . . . . . . . . . 19 8.3. Implementation Considerations . . . . . . . . . . . . . . 19 8.3.1. MLD Implementation on LLNs and MLD alternatives . . . 20 8.3.2. 6LBR Implementation . . . . . . . . . . . . . . . . . 21 8.3.3. Backbone IP Multicast Infrastructure . . . . . . . . 21 9. Miscellaneous Topics . . . . . . . . . . . . . . . . . . . . 22 9.1. CoAP Multicast and HTTP Unicast Interworking . . . . . . 22 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 12. Security Considerations . . . . . . . . . . . . . . . . . . . 24 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 13.1. Normative References . . . . . . . . . . . . . . . . . . 24 13.2. Informative References . . . . . . . . . . . . . . . . . 25 Appendix A. Multicast Listener Discovery (MLD) . . . . . . . . . 27 Appendix B. CoAP-Observe Alternative to Group Communication . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 Dijk & Rahman Expires December 19, 2013 [Page 2] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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 5) 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 9) 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC 2119]. 2. Potential Solutions for Group Communication The classic concept of group communications is that of a single source distributing content to multiple destination recipients that are all part of a group. Before content can be distributed, there is a separate process to form the group. 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 layer 2 (Media Access Control broadcast/multicast) and layer 3 (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 adopted by this draft for CoAP group communication. 3. Use Cases CoAP group communication can be applied in the context of the following use cases: o Discovery of Resource Directory: discovering the local CoRE RD which contains links (URIs) to resources stored on other servers [RFC6690]. Dijk & Rahman Expires December 19, 2013 [Page 3] Internet-Draft Miscellaneous CoAP Group Communication June 2013 o Lighting Control: synchronous operation of a group of IPv6-connected lights (e.g., 6LoWPAN [RFC4944] lights). o Parameter Update: updating parameters/settings simultaneously in a large group of devices in a building/campus control ([I-D.vanderstok-core-bc]) application. o Firmware Update: efficiently updating firmware simultaneously in a large group of devices in a building/campus control ([I-D.vanderstok-core-bc]) application. Here, the use of CoAP group communication could be realized via a multicast extension of CoAP blockwise transfer [I-D.ietf-core-block]. This use case and use of multicast is especially valuable if there are time constraints related to the software update for large groups of devices. o Group Status Report: requesting status information or event reports from a group of devices in a building/campus control application. In this use case, conditional reporting is required: only device that have events to report (as indicated by the request query) respond, others remain silent. This use case requires reliable CoAP group communication, which is currently not in CoRE WG scope. 4. 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. 4.1. 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]: Dijk & Rahman Expires December 19, 2013 [Page 4] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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). 4.2. 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. GEN-REQ 6: Security: see Section 4.3 for security requirements for group communication. Dijk & Rahman Expires December 19, 2013 [Page 5] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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. 4.3. 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. Dijk & Rahman Expires December 19, 2013 [Page 6] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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 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 [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. 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. Dijk & Rahman Expires December 19, 2013 [Page 7] Internet-Draft Miscellaneous CoAP Group Communication June 2013 5. 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 [I-D.ietf-core-groupcomm]. 5.1. IP Multicast Transmission Methods 5.1.1. 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. 5.1.2. 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. 5.1.3. 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 responses. 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) 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 Dijk & Rahman Expires December 19, 2013 [Page 8] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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 response. o Route redundancy o Source node transmits multiple times (destinations do not respond) 5.2. 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 CoAP. 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. 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. Dijk & Rahman Expires December 19, 2013 [Page 9] Internet-Draft Miscellaneous CoAP Group Communication June 2013 5.3. 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 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 Dijk & Rahman Expires December 19, 2013 [Page 10] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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 request. 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: 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 Dijk & Rahman Expires December 19, 2013 [Page 11] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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. 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: coap://proxy1.bld2.example.com/groupmgt?j=group1&l=group2 6. 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. Dijk & Rahman Expires December 19, 2013 [Page 12] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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. 7. Group Discovery and Member Discovery CoAP defines a resource discovery capability, but does not yet specify how to discover groups (e.g. find a group to join or send a multicast message to) or to discover members of a group (e.g. to address selected group members by unicast). These topics are elaborated in more detail in [I-D.vanderstok-core-dna] including examples for using DNS-SD and CoRE Resource Directory. 7.1. 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. Dijk & Rahman Expires December 19, 2013 [Page 13] Internet-Draft Miscellaneous CoAP Group Communication June 2013 Another feature of DNS-SD is the ability to specify service sub-types 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>. 7.2. 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 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. 8. Deployment Guidelines 8.1. 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. 8.2. 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 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 Dijk & Rahman Expires December 19, 2013 [Page 14] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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.) 8.2.1. 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. 126.96.36.199. 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. 188.8.131.52. 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 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. 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]. Dijk & Rahman Expires December 19, 2013 [Page 15] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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. 184.108.40.206. 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 8.3.1. 220.127.116.11. 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. 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. Dijk & Rahman Expires December 19, 2013 [Page 16] Internet-Draft Miscellaneous CoAP Group Communication June 2013 18.104.22.168. 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 8.3.1 for more efficient substitutes for MLD targeted towards a LLN context. 8.2.2. 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. 22.214.171.124. 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 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 8.3.1. Dijk & Rahman Expires December 19, 2013 [Page 17] Internet-Draft Miscellaneous CoAP Group Communication June 2013 126.96.36.199. 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. 188.8.131.52. 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 184.108.40.206. 220.127.116.11. 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 18.104.22.168. 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 22.214.171.124) 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 126.96.36.199 or to use an MLD alternative suitable for LLNs as described in Section 8.3.1. 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. Dijk & Rahman Expires December 19, 2013 [Page 18] Internet-Draft Miscellaneous CoAP Group Communication June 2013 188.8.131.52. 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. 8.2.3. 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. 8.2.4. 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? ] 8.2.5. 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 8.3.1. 8.3. Implementation Considerations In this section various implementation aspects are considered such as required protocol implementations, additional functionality of the 6LBR and backbone network equipment. Dijk & Rahman Expires December 19, 2013 [Page 19] Internet-Draft Miscellaneous CoAP Group Communication June 2013 8.3.1. 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 [RFC6775] 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 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 Dijk & Rahman Expires December 19, 2013 [Page 20] Internet-Draft Miscellaneous CoAP Group Communication June 2013 on number of records per packet? Do we need a higher MLD reliability setting - see the parameters in the MLD RFC ] 8.3.2. 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. 8.3.3. 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 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. Dijk & Rahman Expires December 19, 2013 [Page 21] Internet-Draft Miscellaneous CoAP Group Communication June 2013 [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.] 9. Miscellaneous Topics This section collects miscellaneous text, topics or proposals related to CoAP group communication which do not directly fit into any of the preceding sections. 9.1. CoAP Multicast and HTTP Unicast Interworking CoAP supports operation over UDP multicast, while HTTP does not. For use cases where it is required that CoAP group communication is initiated from an HTTP end-point, it would be advantageous if the HTTP-CoAP Proxy supports mapping of HTTP unicast to CoAP group communication based on IP multicast. One possible way of operation of such HTTP-CoAP Proxy is illustrated in Figure 3. Note that this topic is covered in more detail in [I-D.castellani-core-advanced-http-mapping]. CoAP Mcast CoAP Mcast HTTP-CoAP HTTP Node 1 Rtr1 Node 2 Rtr2 Proxy Node 3 | | | | | | |MLD REQUEST | | | | |(Join Group X) | | | | |--LL-->| | | | | | | |MLD REQUEST | | | | |(Join Group X) | | | | |--LL-->| | | | | | | | HTTP REQUEST | | | | | | (URI to | | | | | | unicast addr) | | | | | |< -----------------| | | | | | | | | | Resolve HTTP Request-Line URI | | | | to Group X multicast address | | | | | | | | CoAP REQUEST (to multicast addr)| | |< ------<---------<-------<------| | | | | | | | | | | | | (optional) CoAP RESPONSE(s) | | | |-------------->| | |-----------------|-------------->| Aggregated | | | | HTTP RESPONSE | Dijk & Rahman Expires December 19, 2013 [Page 22] Internet-Draft Miscellaneous CoAP Group Communication June 2013 | | |------------------>| | | | | Figure 3: CoAP Multicast and HTTP Unicast Interworking Note that Figure 3 illustrates the case of IP multicast as the underlying group communications mechanism. MLD denotes the Multicast Listener Discovery protocol ([RFC3810], Appendix A) and LL denotes a Link-Local multicast. A key point in Figure 3 is that the incoming HTTP Request (from node 3) will carry a Host request-header field that resolves in the general Internet to the proxy node. At the proxy node, this hostname and/or the Request-Line URI will then possibly be mapped (as detailed in [I-D.castellani-core-advanced-http-mapping]) and again resolved (with the CoAP scheme) to an IP multicast address. This may be accomplished, for example, by using DNS or DNS-SD (Section 7). The proxy node will then IP multicast the CoAP Request (corresponding to the received HTTP Request) via multicast routers to the appropriate nodes (i.e. nodes 1 and 2). In terms of the HTTP Response, Figure 3 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-advanced-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 is carried in the HTTP Request-Line (as defined in [I-D.ietf-core-coap] Section 10.2) in a HTTP request sent to the IP address of the Proxy. 10. Acknowledgements Thanks to all CoRE WG members who participated in the IETF 82 discussions, which was the trigger to initiate this document. 11. IANA Considerations Dijk & Rahman Expires December 19, 2013 [Page 23] Internet-Draft Miscellaneous CoAP Group Communication June 2013 This memo includes no request to IANA. 12. 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 here. 13. References 13.1. Normative References [I-D.ietf-core-coap] Shelby, Z., Hartke, K., and C. Bormann, "Constrained Application Protocol (CoAP)", draft-ietf-core-coap-17 (work in progress), May 2013. [RFC 2119] 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", RFC2616, June 1999. [RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security Architecture", RFC3740, March 2004. [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC3810, June 2004. [RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm, "Multicast Security (MSEC) Group Key Management Architecture", RFC4046, April 2005. [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC4601, 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")", RFC4605, August 2006. [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC4944, September 2007. Dijk & Rahman Expires December 19, 2013 [Page 24] Internet-Draft Miscellaneous CoAP Group Communication June 2013 [RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast Extensions to the Security Architecture for the Internet Protocol", RFC5374, November 2008. [RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen, "Building Automation Routing Requirements in Low-Power and Lossy Networks", RFC5867, 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", RFC6550, 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", RFC6636, May 2012. [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC6690, August 2012. [RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC6775, November 2012. 13.2. Informative References [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>. [I-D.castellani-core-advanced-http-mapping] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and E. Dijk, "Best Practices for HTTP-CoAP Mapping Implementation", draft-castellani-core-advanced-http- mapping-01 (work in progress), January 2013. [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] Dijk & Rahman Expires December 19, 2013 [Page 25] Internet-Draft Miscellaneous CoAP Group Communication June 2013 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-core-block] Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP", draft-ietf-core-block-11 (work in progress), March 2013. [I-D.ietf-core-groupcomm] Rahman, A. and E. Dijk, "Group Communication for CoAP", draft-ietf-core-groupcomm-09 (work in progress), May 2013. [I-D.ietf-core-observe] Hartke, K., "Observing Resources in CoAP", draft-ietf- core-observe-08 (work in progress), February 2013. [I-D.ietf-roll-trickle-mcast] Hui, J. and R. Kelsey, "Multicast Protocol for Low power and Lossy Networks (MPL)", draft-ietf-roll-trickle- mcast-04 (work in progress), February 2013. [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-02 (work in progress), October 2012. [I-D.rahman-core-groupcomm] Rahman, A. and E. Dijk, "Group Communication for CoAP", draft-rahman-core-groupcomm-07 (work in progress), October 2011. [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] Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource Directory", draft-shelby-core-resource-directory-05 (work in progress), February 2013. [I-D.vanderstok-core-bc] Dijk & Rahman Expires December 19, 2013 [Page 26] Internet-Draft Miscellaneous CoAP Group Communication June 2013 Stok, P. and K. Lynn, "CoAP Utilization for Building Control", draft-vanderstok-core-bc-05 (work in progress), October 2011. [I-D.vanderstok-core-dna] Stok, P., Lynn, K., and A. Brandt, "CoRE Discovery, Naming, and Addressing", draft-vanderstok-core-dna-02 (work in progress), July 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>. Appendix A. Multicast Listener Discovery (MLD) In order to extend the scope of IP multicast beyond link-local scope, an IP multicast routing protocol has to be active in routers on an LLN. To achieve efficient multicast routing (i.e. avoid always flooding multicast IP packets), routers have to learn which hosts need to receive packets addressed to specific IP multicast destinations. The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its IPv4 pendant IGMP) is today the method of choice used by an (IP multicast enabled) router to discover the presence of multicast listeners on directly attached links, and to discover which multicast addresses are of interest to those listening nodes. MLD was specifically designed to cope with fairly dynamic situations in which multicast listeners may join and leave at any time. IGMP/MLD Snooping is a technique implemented in some corporate LAN routing/switching devices. An MLD snooping switch listens to MLD State Change Report messages from MLD listeners on attached links. Based on this, the switch learns on what LAN segments there is interest for what IP multicast traffic. If the switch receives at some point an IP multicast packet, it uses the stored information to decide onto which LAN segment(s) to send the packet. This improves network efficiency compared to the regular behavior of forwarding every incoming multicast packet onto all LAN segments. An MLD snooping switch may also send out MLD Query messages (which is normally done by a device in MLD Router role) if no MLD Router is present. [RFC6636] discusses optimal tuning of the parameters of MLD for routers for mobile and wireless networks. These guidelines may be useful when implementing MLD in LLNs. Dijk & Rahman Expires December 19, 2013 [Page 27] Internet-Draft Miscellaneous CoAP Group Communication June 2013 Appendix B. CoAP-Observe Alternative to Group Communication The CoAP Observation extension [I-D.ietf-core-observe] can be used as a simple (but very limited) alternative for group communication. A group in this case consists of a CoAP server hosting a specific resource, plus all CoAP clients observing that resource. The server is the only group member that can send a group message. It does this by modifying the state of a resource under observation and subsequently notifying its observers of the change. Serial unicast is used for sending the notifications. This approach can be a simple alternative for networks where IP multicast is not available or too expensive. The CoAP-Observe approach is unreliable in the sense that, even though Confirmable CoAP messages may be used, there are no guarantees that an update will be received. For example, a client may believe it is observing a resource while in reality the server rebooted and lost its listener state. Authors' Addresses Esko Dijk (editor) Philips Research Email: firstname.lastname@example.org Akbar Rahman (editor) InterDigital Communications, LLC Email: Akbar.Rahman@InterDigital.com Dijk & Rahman Expires December 19, 2013 [Page 28]