Group Communication for CoAP
draft-ietf-core-groupcomm-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7390.
|
|
|---|---|---|---|
| Authors | Akbar Rahman , Esko Dijk | ||
| Last updated | 2012-01-10 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 7390 (Experimental) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-core-groupcomm-00
CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E. Dijk, Ed.
Expires: July 13, 2012 Philips Research
January 10, 2012
Group Communication for CoAP
draft-ietf-core-groupcomm-00
Abstract
This is a working document intended to develop draft text for the
CoAP protocol specification in the area of group communication. A
solution based on IP multicast is proposed and detailed. Also,
guidance is provided for deployment in various constrained network
topologies.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 13, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
Table of Contents
1. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Problem Statement and Scope . . . . . . . . . . . . . . . 4
2.3. Potential Solutions for Group Communication . . . . . . . 5
3. Use Cases and Requirements . . . . . . . . . . . . . . . . . . 6
3.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
4. IP Multicast Solution . . . . . . . . . . . . . . . . . . . . 8
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Multicast Listener Discovery (MLD) and Multicast
Router Discovery (MRD) . . . . . . . . . . . . . . . . . . 9
4.3. Group URIs and IP Multicast Addresses . . . . . . . . . . 10
4.4. Group Discovery and Member Discovery . . . . . . . . . . . 10
4.4.1. DNS-SD . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4.2. CoRE Resource Directory . . . . . . . . . . . . . . . 11
4.5. Group Resource Manipulation . . . . . . . . . . . . . . . 11
4.6. Congestion Control . . . . . . . . . . . . . . . . . . . . 13
4.7. CoAP Multicast and HTTP Unicast Interworking . . . . . . . 13
5. CoAP-Observe Solution . . . . . . . . . . . . . . . . . . . . 15
6. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 15
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. Example Lighting Use Case . . . . . . . . . . . . . . . . 16
6.3. Implementation in Target Network Topologies . . . . . . . 19
6.3.1. Single LLN Topology . . . . . . . . . . . . . . . . . 20
6.3.2. Single LLN with Backbone Topology . . . . . . . . . . 22
6.3.3. Multiple LLNs with Backbone Topology . . . . . . . . . 24
6.3.4. LLN(s) with Multiple 6LBRs . . . . . . . . . . . . . . 24
6.3.5. Conclusions . . . . . . . . . . . . . . . . . . . . . 24
6.4. Implementation Considerations . . . . . . . . . . . . . . 25
6.4.1. MLD Implementation on LLNs . . . . . . . . . . . . . . 25
6.4.2. 6LBR Implementation . . . . . . . . . . . . . . . . . 26
6.4.3. Backbone IP Multicast Infrastructure . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 28
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
11.1. Normative References . . . . . . . . . . . . . . . . . . . 29
11.2. Informative References . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The following are definitions of specific terminology used in this
draft.
Group Communication: A source node sends a message to more than one
destination node, where all destinations are identified to belong to
a specific group. The set of source nodes and/or the set of
destination nodes may consist of an arbitrary mix of constrained and
non-constrained nodes.
Multicast: Sending a message to multiple receiving nodes
simultaneously. Typically, this is done as part of a group
communication process. There are various options to implement
multicast including layer 2 (Media Access Control) or layer 3 (IP)
mechanisms.
IP Multicast: A specific multicast solution based on the use of IP
multicast addresses as defined in "IANA Guidelines for IPv4 Multicast
Address Assignments" [RFC5771] and "IP Version 6 Addressing
Architecture" [RFC4291].
Low power and Lossy Network (LLN): LLNs are made up of constrained
devices. These devices may be interconnected by a variety of links,
such as IEEE 802.15.4, Bluetooth, WiFi, wired or low-power powerline
communication links.
2. Introduction
2.1. Background
The CoRE working group is chartered to design and standardize a
Constrained Application Protocol (CoAP) for resource constrained
devices and networks [I-D.ietf-core-coap]. The requirements for CoAP
are documented in [I-D.shelby-core-coap-req].
Constrained devices can be large in number, but highly correlated to
each other (e.g. by type or location). For example, all the light
switches in a building may belong to one group and all the
thermostats belong to another group. All the smart meters in the
same region can belong to a group as well. Groups may be composed by
function; for example, the group "all lights in building one" may
consist of the groups "all lights on floor one of building one", "all
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lights on floor two of building one", etc. Groups may be
preconfigured or dynamically formed. If information needs to be sent
to or received from a group of devices, group communication
mechanisms can improve efficiency and latency of communication and
reduce bandwidth requirements for a given application.
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]:
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).
2.2. Problem Statement and Scope
In this draft, we expand the scope from unreliable IP multicast in
the current CoAP spec to group communication, using either (reliable
or unreliable) multicast or unicast or combinations thereof. We
assume that all, or a substantial part of, CoAP devices participating
in group communication are constrained devices (e.g. Low Power and
Lossy Network (LLN) devices).
In the following sections, we address the issues related to group
communication in detail, with requirements, use cases, proposed
solutions and analysis of their impact to the CoAP protocol and to
implementations. The guiding principle is to apply wherever possible
existing CoAP mechanisms to achieve group communication
functionality. In many cases the contribution of this document lies
in explaining how existing mechanisms may be used to fulfill group
communication needs for specific use cases.
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2.3. Potential Solutions for Group Communication
The classic concept of group communications is that of a single
source distributing content to multiple recipients that are all part
of a group, as shown in the example sequence diagram in Figure 1.
Also shown there is the pre-requisite step of forming the group
before content can be distributed to it.
Group communication solutions have evolved from "bottom" to "top",
i.e., from the network layer (IP multicast) to application layer
group communication, also referred to as application layer multicast.
A study published in 2005 [Lao05] identified new solutions in the
"middle" (referred to as overlay multicast) that utilize an
infrastructure based on proxies.
Each of these classes of solutions may be compared [Lao05] using
metrics such as link stress and level of host complexity
[Banerjee01]. The results show for a realistic internet topology
that IP Multicast is most resource-efficient, with the only downside
being that it requires most effort to deploy in the infrastructure.
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Group
Node 1 Node 2 Coordinator Node 3
| | | |
| REQUEST | | |
| (Join Group X) | | |
|-----------------|------------- >| |
| RESPONSE | | |
|< ---------------|---------------| |
| | | |
| | REQUEST | |
| |(Join Group X) | |
| |------------- >| |
| | RESPONSE | |
| |< -------------| |
| | | REQUEST |
| | | (Send to |
| | | Group X ) |
| | |< -----------------|
| | | |
| | Map to |
| | Group X addresses |
| | | |
| REQUEST (to multicast addr) | |
|< ---------------|< -------------| |
| | | |
| (optional) RESPONSE | |
| |------------- >| |
|-----------------|-------------->| |
| | | RESPONSE |
| | |----------------- >|
| | | |
Figure 1: Example Group Communication Concept
3. Use Cases and Requirements
3.1. Use Cases
The use of CoAP group communication is shown in the context of
several use cases. The following use cases are identified at this
point:
o Lighting Control: synchronous operation of a group of 6LoWPAN
IPv6-connected lights.
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o Discovery: discovering CoAP devices and the Resource and Services
they offer.
o Parameter Update: updating parameters/settings simultaneously in a
large group of devices in a building/campus control
([I-D.vanderstok-core-bc]) application.
In a future version of this document, more use cases should be added
and described in more detail.
3.2. Requirements
Requirements that a group communication solution in CoRE should
fulfill can be found in existing documents ([RFC5867],
[I-D.ietf-6lowpan-routing-requirements], [I-D.vanderstok-core-bc],
[I-D.shelby-core-coap-req]). Below, a set of high-level requirements
is listed that a group communication solution in CoRE 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.
A CoRE group communication solution should (ideally) offer:
REQ 1: Optional Reliability: the application can select between
unreliable group communication and reliable group
communication.
REQ 2: Efficiency: delivers messages more efficiently than a
"serial unicast" solution. Provides a balance between
group data traffic and control overhead.
REQ 3: Low latency: deliver a message as quickly as possible.
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 timespan D such that a message is delivered to
all destinations in a time interval [t,t+D].
REQ 5: Ordering: message ordering may be required for reliable
group communication use cases.
REQ 6: Security: see Section 7 for security requirements for group
communication.
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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.
REQ 8: Robust group management: functionality to join groups,
leave groups, view group membership, and persistent group
membership in failure or sleeping node situations.
REQ 9: Network layer independence: a solution is independent from
specific unicast and/or IP multicast routing protocols.
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.
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.
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).
REQ 13: CoAP Proxying support: a CoAP proxy can handle distribution
of a message to a group on behalf of a (constrained) CoAP
client.
REQ 14: Suitable for operation on LLNs with constrained nodes.
4. IP Multicast Solution
4.1. Introduction
Because CoAP supports sending requests to an multicast IP destination
address, IP Multicast is an obvious candidate for a group
communication solution.
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IP Multicast protocols have been evolving for decades, resulting in
proposed standards such as Protocol Independent Multicast - Sparse
Mode (PIM-SM) [RFC4601]. Yet, due to various technical and marketing
reasons, IP Multicast is not widely deployed on the general Internet.
However, IP Multicast is popular in specific deployments such as in
enterprise networks (e.g. for video conferencing or general IP
multicast PC applications within a single LAN broadcast domain) and
carrier IPTV deployments. The packet economy and minimal host
complexity of IP multicast make it attractive for group communication
in constrained environments.
4.2. Multicast Listener Discovery (MLD) and Multicast Router Discovery
(MRD)
In order to extend the scope of IP multicast beyond link-local, an IP
multicast routing protocol has to be active in routers on an LLN. To
achieve efficient multicast routing (i.e. avoid always flooding
multicast IP packets), routers have to learn which hosts need to
receive packets addressed to specific IP multicast destinations.
The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its
IPv4 pendant IGMP) is today the method of choice used by an (IP
multicast enabled) router to discover the presence of multicast
listeners on directly attached links, and to discover which multicast
addresses are of interest to those listening nodes. MLD was
specifically designed to cope with fairly dynamic situations in which
multicast listeners may join and leave at any time.
IGMP/MLD Snooping is a technique implemented in some corporate LAN
routing/switching devices. An MLD snooping switch listens to MLD
State Change Report messages from MLD listeners on attached links.
Based on this, the switch learns on what LAN segments there is
interest for what IP multicast traffic. If the switch receives at
some point an IP multicast packet, it uses the stored information to
decide onto which LAN segment(s) to send the packet. This improves
network efficiency compared to the regular behavior of forwarding
every incoming multicast packet onto all LAN segments. An MLD
snooping switch may also send out MLD Query messages (which is
normally done by an MLD Router) if no MLD router is present.
The Multicast Router Discovery (MRD) protocol [RFC4286] defines a way
to discover multicast routers, for the purpose of using this
information by IGMP/MLD snooping devices.
[I-D.ietf-multimob-igmp-mld-tuning] discusses optimal tuning of the
parameters of MLD for routers for mobile and wireless networks.
These guidelines may be useful when implementing MLD in LLNs.
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4.3. Group URIs and IP Multicast Addresses
An approach to map group authorities onto IP multicast addresses
using DNS was proposed in [I-D.vanderstok-core-bc]. Based on this,
examples of group URI naming (and scoping) for a building control
application are shown below. Group URIs MUST follow the URI syntax
defined in [RFC3986].
URI authority Targeted group
all.bldg6.example.com "all nodes in building 6"
all.west.bldg6.example.com "all nodes in west wing, building 6"
all.floor1.west.bldg6.examp... "all nodes in floor 1, west wing,
building 6"
all.bu036.floor1.west.bldg6... "all nodes in office bu036, floor1,
west wing, building 6"
The authority portion of the URI is used to identify a node (or
group) and the resulting DNS name is bound to a unicast or multicast
IP address. Each example group URI shown above can be mapped to a
unique multicast IP address. This may be a site-local or global
address allocated according to [RFC3956], [RFC3306] or [RFC3307].
4.4. Group Discovery and Member Discovery
CoAP defines a resource discovery capability but, in the absence of a
standardized group communication infrastructure, it is limited to
link-local scope IP multicast; examples may be found in
[I-D.ietf-core-link-format]. A service discovery capability is
required to extend discovery to other subnets and scale beyond a
certain point, as originally proposed in [I-D.vanderstok-core-bc].
Discovery includes both discovering groups (e.g. find a group to join
or send a multicast message to) and discovering members of a group
(e.g. to address selected group members by unicast).
4.4.1. DNS-SD
DNS-based Service Discovery [I-D.cheshire-dnsext-dns-sd] defines a
conventional way to configure DNS PTR, SRV, and TXT records to enable
enumeration of services, such as services offered by CoAP nodes, or
enumeration of all CoAP nodes, within specified subdomains. A
service is specified by a name of the form
<Instance>.<ServiceType>.<Domain>, where the service type for CoAP
nodes is _coap._udp and the domain is a DNS domain name that
identifies a group as in the examples above. For each CoAP end-point
in a group, a PTR record with the name _coap._udp and/or a PTR record
with the name _coap._udp.<Domain> is defined and it points to an SRV
record having the <Instance>.<ServiceType>.<Domain> name.
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All CoAP nodes in a given subdomain may be enumerated by sending a
query for PTR records named _coap._udp to the authoritative DNS
server for that zone. A list of SRV records is returned. Each SRV
record contains the port and host name (AAAA record) of a CoAP node.
The IP address of the node is obtained by resolving the host name.
DNS-SD also specifies an optional TXT record, having the same name as
the SRV record, which can contain "key=value" attributes. This can
be used to store information about the device, e.g. schema=DALI,
type=switch, group=lighting.bldg6, etc.
Another feature of DNS-SD is the ability to specify service subtypes
using PTR records. For example, one could represent all the CoAP
groups in a subdomain by PTR records with the name
_group._sub._coap._udp or alternatively
_group._sub._coap._udp.<Domain>.
4.4.2. CoRE Resource Directory
CoRE Resource Directory [I-D.shelby-core-resource-directory] defines
the concept of a Resource Directory (RD) server where CoAP servers
can register their resources offered and CoAP clients can discover
these resources by querying the RD server. RD syntax can be mapped
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
groupmember discovery.
Specifically, the Domain (d) parameter can be set to the group URI by
an end-point registering to the RD. If an end-point wants to join
multiple groups, it has to repeat the registration process for each
group it wants to join.
4.5. Group Resource Manipulation
At least two forms of group resource manipulation must be supported.
The first is push (multicast PUT or MPUT for short) as e.g. "turn off
all the lights simultaneously". Logically, this is similar to
publishing a value to multiple subscribers. The second operation is
pull (multicast GET or MGET), which is essential for discovery during
commissioning and can be illustrated by the example "return all the
resources on all CoAP servers advertized by their .well-known/core
URI". MGET to an "all-nodes" or "all-CoAP-nodes" multicast IP
address should perhaps be limited in scope to link-local multicast
for scaling [TBD: and possibly for security reasons, e.g. DoS
attacks].
Conceptually, the result of a multicast GET or PUT should be the same
as if the client had unicast them serially (that is, a set of {URI,
representation} tuples). Practically, there are major benefits to
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avoiding serial unicast in favor of a multicast CoAP GET/PUT
solution:
- packet economy on constrained networks
- M2M resource discovery (solves the "chicken-and-egg" problem)
- apparent simultaneity of events (e.g. in lighting applications)
- average lower latency per event (e.g. in lighting applications)
Ideally, all nodes in a given group (defined by its multicast IP
address) must receive the same request with high probability. This
will not be the case if there is diversity in the authority port
(i.e. a diversity of dynamic port addresses across the group) or if
the targeted resource is located at different paths on different
nodes. Extending the definition of group membership to include port
and path discovery is not desirable.
Therefore, some measures must be present to ensure uniformity in port
number and resource name/location within a group.
A first solution in this respect is to couple groups to service
descriptions in DNS (using DNS-SD as in Section 4.4 and
[I-D.vanderstok-core-bc]). A service description for a multicast
group may have a TXT record in DNS defining a schema X (e.g.
"schema=DALI"), which defines by service standard X (e.g. "DALI")
which resources a node supporting X MUST have. Therefore a multicast
source can safely refer to all resources with corresponding
operations as prescribed by standard X. For port numbers (which can
be found using DNS-SD also) the same holds. Alternatively, only the
default CoAP port may be used in all CoAP multicast requests.
A second solution is to impose the following restrictions, e.g. for
groups not found using, or advertized in, DNS-SD:
o All CoAP multicast requests MUST be sent to the well-known CoAP
port.
o All CoAP multicast requests SHOULD operate on /.well-known/core
URIs
One question is whether the application (or middleboxes) need to be
aware that a request is intended for a group. A separate scheme as
proposed by [I-D.goland-http-udp] might be useful (e.g. "corem" vs.
"core"). To the extent that group membership might be implemented as
a series of IP multicast, serial unicast, or some combination, having
a distinct scheme for group operations might be a useful signal for a
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proxy receiving the request to look up the group membership and
replicate serial unicasts as well as send multicast packets.
4.6. Congestion Control
CoAP requests may be multicast, resulting a multitude of replies from
different nodes, potentially causing congestion.
[I-D.eggert-core-congestion-control] suggests to conservatively
control sending multicast requests.
CoAP already addresses the congestion problem to some extent by
requiring all multicast CoAP requests to be Non-Confirmable. In CoAP
a MAX_RETRANSMIT value set by default to 4 is used for retransmission
of Confirmable messages, but since CoAP multicast messages are Non-
Confirmable their effective retransmission value is 0. However, as
responses to multicast requests (both MGET or MPUT) SHOULD be sent
([I-D.ietf-core-coap]), using CoAP multicast still may lead to
congestion issues.
Various means can be implemented to prevent congestion.
For an MGET or MPUT request that leads to the sending of a CoAP
response by a server, the CoRE WG currently considers a required
random delay, within a specified TIMEOUT period, before the server
can send the response. In order to cope with the different
requirements for TIMEOUT imposed by different use cases and network
topologies, one recommended approach is to define a CoAP Option via
which a CoAP client can indicate a preference for TIMEOUT for a
specific response. This Option proposal will be done in a separate
draft.
4.7. CoAP Multicast and HTTP Unicast Interworking
Within the constrained network, CoAP runs over UDP for which IP
multicast is supported. In a non-constrained network (i.e. general
Internet), HTTP over TCP is used for which IP multicast is not
supported. Therefore a CoAP/HTTP Proxy node that supports group
communication needs to have functionalities to support interworking
of unicast and multicast. One possible way of operation of the Proxy
is illustrated in Figure 2. Note that this topic is covered in more
detail in [I-D.castellani-core-http-mapping].
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CoAP CoAP CoAP/HTTP HTTP
Node 1 Node 2 Proxy Node 3
| | | |
| REQUEST | | |
| (Join Group X) | | |
|-----------------|------------- >| |
| RESPONSE | | |
|< ---------------|---------------| |
| | | |
| | REQUEST | |
| | (Join Group X)| |
| |------------- >| |
| | RESPONSE | |
| |< -------------| |
| | | |
| | | |
| | | HTTP REQUEST |
| | | (URI to |
| | | unicast addr) |
| | |< -----------------|
| | | |
| | Map URI |
| | to Group X multicast address |
| | | |
| REQUEST (to multicast addr) | |
|< ---------------|< -------------| |
| | | |
| | | |
| (optional) RESPONSE | |
| |------------- >| |
|-----------------|-------------->| |
| | | HTTP RESPONSE |
| | |----------------- >|
| | | |
Figure 2: CoAP Multicast and HTTP Unicast Interworking
Note that Figure 2 illustrates the case of IP multicast as the
underlying group communications mechanism.
A key point in Figure 2 is that the incoming HTTP Request (from node
3) will carry a URI (with the HTTP scheme) that resolves in the
general Internet to the proxy node. At the proxy node, the URI will
then possibly be mapped (as detailed in
[I-D.castellani-core-http-mapping]) and again resolved (with the CoAP
scheme) to an IP multicast destination. This may be accomplished,
for example, by using DNS-SD (Section 4.4). The proxy node will then
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IP multicast the CoAP Request (corresponding to the received HTTP
Request) to the appropriate nodes (i.e. nodes 1 and 2).
In terms of the HTTP Response, Figure 2 illustrates that it will be
generated by the proxy node based on aggregated responses of the CoAP
nodes and sent back to the client in the general Internet that sent
the HTTP Request (i.e. node 1). In
[I-D.castellani-core-http-mapping] the HTTP Response that the Proxy
may use to aggregate multiple CoAP responses is described in more
detail. So in terms of overall operation, the CoAP proxy can be
considered to be a "non-transparent" proxy according to [RFC2616].
Specifically, [RFC2616] states that a "non-transparent proxy is a
proxy that modifies the request or response in order to provide some
added service to the user agent, such as group annotation services,
media type transformation, protocol reduction or anonymity
filtering."
An alternative to the above is using a Forward Proxy. In this case,
the CoAP request URI could be carried in the HTTP Request Line (as
defined in [I-D.ietf-core-coap] Section 8) in a HTTP request sent to
the IP address of the Proxy.
5. CoAP-Observe Solution
The CoAP Observation extension [I-D.ietf-core-observe] can be
directly used for group communication. A group then consists of a
CoAP server hosting a specific resource, plus all CoAP clients
observing that resource. The server is in that case the only group
member that can send a group message. It does this by modifying the
state of a resource under observation and subsequently notifying its
observers of the change. Serial unicast is used for sending the
notifications. This approach can be beneficial for group
communication in networks where IP multicast is not available, too
unreliable, or too expensive.
Group communication 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.
6. Deployment Guidelines
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6.1. Overview
We recommend to use IP multicast as outlined in Section 4 as the base
solution for CoAP Group Communication, provided that the use case and
network characteristics allow this. It has the advantage that it re-
uses the IP multicast suite of protocols and can operate even if
groupmembers are distributed over both constrained and non-
constrained network segments. Still, this approach may require
specifying or implementing additional IP Multicast functionality in
an LLN, in a backbone network, or in both - this will be evaluated in
more detail in this section.
6.2. Example Lighting Use Case
We first present an example use case to illustrate the overall steps
in an IP Multicast based CoAP Group Communication solution. We
assume the following network configuration for this example (see
Figure 3):
1) A large room (Room-A) with three lights (Light-1, Light-2,
Light-3) controlled by a Light Switch. The devices are organized
into two 6LoWPAN subnets.
2) Light-1 and the Light Switch are connected to a router (Rtr-1)
which is also a CoAP Proxy and a 6LoWPAN Border Router (6LBR).
3) Light-2 and the Light-3 are connected to another router (Rtr-2)
which is also a CoAP Proxy and a 6LBR.
4) The routers are connected to a an IPv6 network backbone which is
also multicast enabled. In the general case, this means the network
backbone and 6LBRs support a PIM based multicast routing protocol,
and MLD for forming groups. In a limited case, if the network
backbone is one link, then the routers only have to support MLD-
snooping for the example use case to work.
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Network
Backbone
|
################################################ |
# Room-A # |
# ********************** # |
# ** LoWPAN-1 ** # |
# * * # |
# * +----------+ * # |
# * | Light |-------+ * # |
# * | Switch | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-1 |-----------------------------|
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-1 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
# # |
# ********************** # |
# ** LoWPAN-2 ** # |
# * * # |
# * +----------+ * # |
# * | Light-2 |-------+ * # |
# * | | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-2 |-----------------------------|
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-3 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
################################################# |
|
+--------+ |
| DNS |------------------|
| Server |
+--------+
Figure 3: Network Topology of a Large Room (Room-A)
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The corresponding protocol flow for an IP Multicast based CoAP Group
Communication solution for the network shown in Figure 3 is shown in
Figure 4. We assume the following steps occur before the illustrated
flow:
1) Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to
all devices. The CoAP network is formed.
2) Commissioning phase (by applications): The IP multicast address of
the group (Room-A-Lights) has been set in all the Lights. The URI of
the group (Room-A-Lights) has been set in the Light Switch.
Light Network
Light-1 Light-2 Light-3 Switch Rtr-1 Rtr-2 Backbone
| | | | | | |
| | | | | | |
| MLD Report: Join | | | | |
| Group (Room-A-Lights) | | | |
|------------------------------------------>| | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | |-------------------->|
| | | | | | |
| | MLD Report: Join | | | |
| | Group (Room-A-Lights) | | |
| |------------------------------------------>| |
| | | | | | |
| | | MLD Report: Join | | |
| | | Group (Room-A-Lights) | |
| | |------------------------------->| |
| | | | | | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | | |--------->|
| | | | | | |
| | *********************** | |
| | * User flips on * | |
| | * light switch to * | |
| | * turn on all the * | |
| | * lights in Room A * | |
| | *********************** | |
| | | | | | |
| | | | | | |
| | | COAP NON (POST | | |
| | | (Proxy-URI | | |
| | | (URI for Room-A-Lights)) |
| | | turn on lights) | |
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| | | |--------->| | |
| | | | | | |
| | | | | | |
| | | | Request DNS resolution of |
| | | | URI for Room-A-Lights |
| | | | |-------------------->|
| | | | | | |
| | | | | | |
| | | | DNS returns: AAAA |
| | | | Group (Room-A-Lights) |
| | | | IPv6 multicast address |
| | | | |<--------------------|
| | | | | | |
| | | | | | |
| | | | COAP NON (POST |
| | | | (URI Path) |
| | | | turn on lights) |
| | | | with IP multicast address|
| | | | for Group (Room-A-Lights)|
| | | | |-------------------->|
|<------------------------------------------| | |
| | | | | | |
| | | | | |<---------|
| |<---------|<-------------------------------| |
| | | | | | |
*********************** | | | |
* Lights in Room-A * | | | |
* turn on (nearly * | | | |
* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
Figure 4: Turning on Lights in a Large Room (Room-A)
The indicated MLD Report messages are link-local multicast. In each
LoWPAN, it is assumed that a multicast routing protocol in 6LRs will
propagate the Join information over multiple hops to the 6LBR.
6.3. Implementation in Target Network Topologies
This section looks in more detail how an IP Multicast based solution
can be deployed onto the various network topologies that we consider
important for group communication use cases. Note that the chosen
solution of IP Multicast for CoAP group communication works mostly
independently from the underlying network topology and its specific
IP multicast implementation.
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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.)
6.3.1. Single LLN Topology
The simplest topology is a single LLN, where all the IP multicast
source(s) and destinations are constrained nodes within this same
LLN. Possible implementations of IP multicast routing and group
administration for this topology are listed below.
6.3.1.1. Mesh-Under Multicast Routing
The LLN may be set up in either a mesh-under or a route-over
configuration. In the former case, the mesh routing protocol should
take care of routing IP multicast messages througout the LLN.
Because conceptually all nodes in the LLN are attached to a single
link, there is in principle no need for nodes to announce their
interest in multicast IP addresses via MLD (see Section 4.2). A
multicast message to a specific IP destination, which is delivered to
all 6LoWPAN nodes by the mesh routing algorithm, is accepted by the
IP network layer of that node only if it is listening on that
specific multicast IP address and port.
6.3.1.2. RPL Multicast Routing
The RPL routing protocol for LLNs provides support for routing to
multicast IP destinations (Section 12 of [I-D.ietf-roll-rpl]). Like
regular unicast destinations, multicast destinations are advertized
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
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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.
6.3.1.3. RPL Routers with Non-RPL Hosts
Now we consider the case that hosts exist in a RPL network that are
not RPL-aware themselves, but rely on RPL routers for their IP
connectivity beyond link-local scope. Note that the current RPL
specification [I-D.ietf-roll-rpl] leaves this case for future
specification (see Section 16.4). Non-RPL hosts can't advertize
their IP multicast groups of interest via RPL DAO messages as defined
above. Therefore in that case MLD could be used for such
advertizements (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 6.4.1.
6.3.1.4. Trickle Multicast Forwarding
Trickle Multicast Forwarding [I-D.ietf-roll-trickle-mcast] is an IP
multicast routing protocol suitable for LLNs, that uses the Trickle
algorithm as a basis. It is a simple protocol in the sense that no
topology maintenance is required. It can deal especially well with
situations where the node density is a-priori unknown.
Nodes from anywhere in the LLN can be the multicast source, and nodes
anywhere in the LLN can be multicast destinations.
Using Trickle Multicast Forwarding it is not required for IP
multicast destinations (listeners) to announce their interest in a
specific multicast IP address, e.g. by means of MLD. Instead, all
multicast IP packets regardless of IP destination address are stored
and forwarded by all routers. Because forwarding is always done by
multicast, both hosts and routers will be able to receive all
multicast IP packets. Routers that receive multicast packets they
are not interested in, will only buffer these for a limited time
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
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seem to make Trickle especially efficient for cases where the
multicast listener density is high and the number of distinct
multicast groups relatively low.
6.3.1.5. Other Route-Over Methods
Other known IP multicast routing methods may be used, for example
flooding or other to be defined methods suitable for LLNs. An
important design consideration here is whether multicast listeners
need to advertize 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 6.4.1 for more efficient
substitutes for MLD targeted towards a LLN context.
6.3.2. Single LLN with Backbone Topology
A LLN may be connected via a Border Router (e.g. 6LBR) to a backbone
network, on which IP multicast listeners and/or sources may be
present. This section analyzes cases in which IP multicast traffic
needs to flow from/to the backbone, to/from the LLN.
6.3.2.1. Mesh-Under Multicast Routing
Because in a mesh routing network conceptually all nodes in the LLN
are attached to a single link, a multicast IP packet originating in
the LLN is typically delivered by the mesh routing algorithm to the
6LBR as well, although there is no guaranteed delivery. The 6LBR may
be configured to accept all IP multicast traffic from the LLN and
then may forward such packets onto its backbone link. Alternatively,
the 6LBR may act in an MLD Router or MLD Snooper role on its backbone
link and decide whether to forward a multicast packet or not based on
information learnt 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
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constrained nodes. Light-weight alternatives for MLD are discussed
in Section 6.4.1.
6.3.2.2. RPL Multicast Routing
For RPL routing within the 6LoWPAN, we first consider the case of an
IP multicast source on the backbone network with one or more IP
multicast listeners on the RPL LLN. Typically, the 6LBR would be the
root of a DODAG so that the 6LBR can easily forward the IP multicast
packet received on its backbone interface to the right RPL nodes in
the LLN down along this DODAG (based on previously DAO-advertized
destinations).
Second, a multicast source may be in the RPL LLN and listeners may be
both on the LLN and on the backbone. For this case RPL defines that
the multicast packet will propagate both up and down the DODAG,
eventually reaching the DODAG root (typically a 6LBR) from which the
packet can be routed onto the backbone in a manner specified in the
previous section.
6.3.2.3. RPL Routers with Non-RPL Hosts
For the case that a RPL LLN contains non-RPL hosts, the solutions
from the previous section can be used if in addition RPL routers
implement MLD or "MLD like" functionality similar to as described in
Section 6.3.1.3.
6.3.2.4. Trickle Multicast Forwarding
First, we consider the case of an IP multicast source node on the LLN
(where all 6LRs support Trickle Multicast Forwarding) and IP
multicast listeners that may be on the LLN and on the backbone. As
Trickle will eventually deliver multicast packets also to a 6LBR,
which acts as a Trickle Multicast router as well, the 6LBR can then
forward onto the backbone in the ways described earlier in
Section 6.3.2.1.
Second, for the case of an IP multicast source on the backbone and
multicast listeners on both backbone and/or LLN, the 6LBR needs to
forward multicast traffic from the backbone onto the LLN. Here, the
aforementioned problem (Section 6.3.2.1) of potentially overloading
the LLN with unwanted backbone IP multicast traffic appears again.
A possible solution to this is (again) to let multicast listeners
advertize their interest using MLD as described in Section 6.3.2.1 or
to use an MLD alternative suitable for LLNs as described in
Section 6.4.1. However, following this approach requires possibly an
extension to Trickle Multicast Forwarding: the protocol should ensure
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that MLD-advertized information is somehow communicated to the 6LBR,
possibly over multiple hops. MLD itself supports link-local
communication only.
6.3.2.5. Other Route-Over Methods
For other multicast routing methods used on the LLN, there are
similar considerations to the ones in sections above: the strong need
to filter IP multicast traffic coming into the LLN, the need for
reporting multicast listener interest (e.g. with MLD or a to-be-
defined MLD alternative) by constrained (6LoWPAN) nodes, and the need
for LLN-internal routing as identified in the previous section such
that the MLD communicated information can reach the 6LBR to be used
there in multicast traffic filtering decisions.
6.3.3. Multiple LLNs with Backbone Topology
Now the case of a single backbone network with two or more LLNs
attached to it via 6LBRs is considered. For this case all the
considerations and solutions of the previous section can be applied.
For the specific case that a source on a backbone network has to send
to a very large number of destination located on many LLNs, the use
of IGMP/MLD Proxying [RFC4605] with a leaf IGMP/MLD Proxy located in
each 6LBR may be useful. This method only is defined for a tree
topology backbone network with the IP multicast source at the root of
the tree.
6.3.4. LLN(s) with Multiple 6LBRs
[ TBD: an LLN with multiple 6LBRs may require some additional
consideration. Any need to synchronize mutually on multicast
listener information? ]
6.3.5. Conclusions
For all network topologies that were evaluated, CoAP group
communication can be in principle supported with IP Multicast, making
use of existing protocols. For the case of Trickle Multicast
Forwarding, it appears that an addition to the protocol is required
such that information about multicast listeners can be distributed
towards the 6LBR. Opportunities were identified for an "MLD-like" or
"MLD-lightweight" protocol specifically suitable for LLNs, which
should interwork with regular MLD on the backbone network. Such MLD
variants are further analyzed in Section 6.4.1.
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6.4. Implementation Considerations
In this section various implementation aspects are considered such as
required protocol implementations, additional functionality of the
6LBR and backbone network equipment.
6.4.1. MLD Implementation on LLNs
In previous sections, it was mentioned that the MLDv2 protocol
[RFC3810] may be too costly for use in a LLN. MLD relies on periodic
link-local multicast operations to maintain state. Also it is
optimized to fairly dynamic situations where multicast listeners may
come and go over time. Such dynamic situations are less frequently
found in typical LLN use cases such as building control, where
multicast group membership can remain constant over longer periods of
time (e.g. months) after commissioning.
Hence, a viable strategy is to implement a subset of MLD
functionality in 6LoWPAN nodes which is just enough for the required
functionality. A first option is that 6LoWPAN Routers, like MLD
Snoopers, passively listen to MLD State Change Report messages and
handle the learnt ("snooped") IP multicast destinations in the way
defined by the multicast routing protocol they are running (e.g. for
RPL, Routers advertize these destinations using DAO messages).
A second option is to use MLD as-is but adapt the recommended
parameter values such that operation on a LLN becomes more efficient.
A third option is to standardize a new protocol, taking a subset of
MLD functionality into a "MLD for 6LoWPAN" protocol to support
constrained nodes optimally.
A fourth option is now presented, which seems attractive in that it
minimizes standardization, implementation and network communication
overhead all at the same time. This option is to specify a new
Multicast Listener Option (MLO) as an addition to the 6LoWPAN-ND
[I-D.ietf-6lowpan-nd] protocol communication that is anyway ongoing
between a 6LoWPAN host and router(s). This MLO is preferably
designed to be maximally similar to the Address Registration Option
(ARO), which minimizes the need for additional program code on
constrained nodes. With an MLO, instead of registering a unicast IP
address, a host "registers" its interest in a multicast IP address.
Unlike ARO, multiple MLO can be used in the same ND packet. A
registration period is also defined just like in the ARO. MLO allows
a host to persistently register as a listener to IP multicast traffic
and to avoid the overhead of periodic multicast communication which
is required for full MLD.
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[ TBD: consider what aspects are needed/not needed for CoAP/LLN
applications. Will MLDv1 suffice? What to do with options like
'source specific' and include/exclude. Source-specific can also be
dealt with at the destination host by filtering? Do we need limits
on number of records per packet? Do we need a higher MLD reliability
setting - see the parameters in the MLD RFC ]
6.4.2. 6LBR Implementation
To support mixed backbone/LLN scenarios in CoAP group communication,
it is RECOMMENDED that a 6LowPAN Border Router (6LBR) will act in an
MLD Router role on the backbone link. If this is not possible then
the 6LBR SHOULD be configured to act as an MLD Multicast Address
Listener and/or MLD Snooper on the backbone link.
6.4.3. Backbone IP Multicast Infrastructure
For corporate/professional applications, most routing and switching
equipment that is currently on the market is IPv6 capable. For that
reason backbone infrastructure operating IPv4 only is considered out
of scope in this document, at least for the backbone network
segment(s) where IP multicast destinations are present. What is
still in scope is for example an IPv4-only HTTP client that wants to
send a group communication message via a HTTP-CoAP proxy as
considered in [I-D.castellani-core-http-mapping].
The availability of, and requirements for, IP multicast support may
depend on the specific installation use case. For example, the
following cases may be relevant for new IP based building control
installations:
1. System deployed on existing IP (Ethernet/WiFi/...)
infrastructure, shared with existing IP devices (PCs)
2. Newly designed and deployed IP (Ethernet/WiFi/...)
infrastructure, to be shared with other IP devices (PCs)
3. Newly designed and deployed IP (Ethernet/WiFi/...)
infrastructure, exclusively used for building control.
Besides physical separation the building control backbone can be
separated from regular (PC) infrastructure by using a different VLAN.
A typical corporate installation will have many LAN switches and/or
routing switches, which pass through IP multicast traffic but on the
other hand do not support acting in the Router role of MLD/IGMP.
Perhaps for case 2) and 3) above it is acceptable to add a MLD/IGMP
capable router somewhere in the network, while for case 1) this may
not be the case.
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[TBD: consider the influence of WiFi based backbone networks. What
if 6LBRs are at the same time also WiFi routers? What if 6LBRs have
an Ethernet connection to legacy WiFI routers? Check if equivalent
with Ethernet backbone.]
7. Security Considerations
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.
Note that some of the requirements are marked optional. This means
that, depending on the use case, these may be required or not. For
this purpose each use case can be associated to a security profile as
specified in [I-D.garcia-core-security]. The security profile
prescribes what requirements should be taken into account for this
profile. A mapping of these requirements to these profiles has not
yet been done.
REQ1- 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.
REQ2- 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.
REQ3- 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.
REQ4- 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.
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REQ5- 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.
REQ6- Independence from underlying routing security: CoAP group
communication security shall not be tied to the security of
underlying routing and distribution protocols such as PIM [RFC4601]
and RPL [I-D.ietf-roll-rpl]. Insecure or inappropriate routing
(including IP multicast routing) may cause loss of data to CoAP but
will not affect the authenticity or secrecy of CoAP group
communications.
REQ7- 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.
8. IANA Considerations
This document makes no request of IANA.
9. Conclusions
IP multicast as outlined in Section 4 is recommended to be adopted as
the base solution for CoAP Group Communication on LLNs, for
situations where the use case and network characteristics allow use
of IP multicast. This approach requires no standards changes to the
IP multicast suite of protocols and it provides interoperability with
IP multicast group communication on unconstrained backbone networks.
The proposals for group communication described in this draft should
be considered for incorporation into the overall CoAP protocol
specification.
10. Acknowledgements
Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo
Castellani, Guang Lu, Salvatore Loreto, Kerry Lynn, Dale Seed, Zach
Shelby, Peter van der Stok, and Juan Carlos Zuniga for their helpful
comments and discussions that have helped shape this document.
11. References
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11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3306] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", RFC 3306, August 2002.
[RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC 3307, August 2002.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, March 2004.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3956] Savola, P. and B. Haberman, "Embedding the Rendezvous
Point (RP) Address in an IPv6 Multicast Address",
RFC 3956, November 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Multicast Security (MSEC) Group Key Management
Architecture", RFC 4046, April 2005.
[RFC4286] Haberman, B. and J. Martin, "Multicast Router Discovery",
RFC 4286, December 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, August 2006.
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[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
March 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-08 (work in progress), October 2011.
11.2. Informative References
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
progress), December 2011.
[I-D.eggert-core-congestion-control]
Eggert, L., "Congestion Control for the Constrained
Application Protocol (CoAP)",
draft-eggert-core-congestion-control-01 (work in
progress), January 2011.
[I-D.ietf-6lowpan-routing-requirements]
Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for 6LoWPAN Routing",
draft-ietf-6lowpan-routing-requirements-10 (work in
progress), November 2011.
[I-D.ietf-6lowpan-hc]
Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams in Low Power and Lossy Networks (6LoWPAN)",
draft-ietf-6lowpan-hc-15 (work in progress),
February 2011.
[I-D.ietf-6lowpan-nd]
Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
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Discovery Optimization for Low Power and Lossy Networks
(6LoWPAN)", draft-ietf-6lowpan-nd-18 (work in progress),
October 2011.
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-09 (work in progress),
November 2011.
[I-D.ietf-core-observe]
Hartke, K. and Z. Shelby, "Observing Resources in CoAP",
draft-ietf-core-observe-03 (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. and S. Krco, "CoRE Resource Directory",
draft-shelby-core-resource-directory-02 (work in
progress), October 2011.
[I-D.vanderstok-core-bc]
Stok, P. and K. Lynn, "CoAP Utilization for Building
Control", draft-vanderstok-core-bc-05 (work in progress),
October 2011.
[I-D.lynn-core-discovery-mapping]
Lynn, K. and Z. Shelby, "CoRE Link-Format to DNS-Based
Service Discovery Mapping",
draft-lynn-core-discovery-mapping-01 (work in progress),
July 2011.
[I-D.castellani-core-http-mapping]
Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Best practices for HTTP-CoAP mapping
implementation", draft-castellani-core-http-mapping-02
(work in progress), October 2011.
[I-D.garcia-core-security]
Garcia-Morchon, O., Keoh, S., Kumar, S., Hummen, R., and
R. Struik, "Security Considerations in the IP-based
Internet of Things", draft-garcia-core-security-03 (work
in progress), October 2011.
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[I-D.ietf-roll-rpl]
Winter, T., Thubert, P., Brandt, A., Clausen, T., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., and J.
Vasseur, "RPL: IPv6 Routing Protocol for Low power and
Lossy Networks", draft-ietf-roll-rpl-19 (work in
progress), March 2011.
[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Forwarding Using
Trickle", draft-ietf-roll-trickle-mcast-00 (work in
progress), April 2011.
[I-D.ietf-multimob-igmp-mld-tuning]
Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
IGMP and MLD for Routers in Mobile and Wireless Networks",
draft-ietf-multimob-igmp-mld-tuning-02 (work in progress),
October 2011.
[I-D.goland-http-udp]
Goland, Y., "Multicast and Unicast UDP HTTP Messages",
1999,
<http://tools.ietf.org/html/draft-goland-http-udp-01>.
[Lao05] Lao, L., Cui, J., Gerla, M., and D. Maggiorini, "A
Comparative Study of Multicast Protocols: Top, Bottom, or
In the Middle?", 2005, <http://www.cs.ucla.edu/NRL/hpi/
AggMC/papers/comparison_gi_2005.pdf>.
[Banerjee01]
Banerjee, B. and B. Bhattacharjee, "A Comparative Study of
Application Layer Multicast Protocols", 2001, <http://
wmedia.grnet.gr/P2PBackground/
a-comparative-study-ofALM.pdf>.
Authors' Addresses
Akbar Rahman (editor)
InterDigital Communications, LLC
Email: Akbar.Rahman@InterDigital.com
Esko Dijk (editor)
Philips Research
Email: esko.dijk@philips.com
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