Internet DRAFT - draft-rahman-core-groupcomm
draft-rahman-core-groupcomm
CoRE A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E. Dijk, Ed.
Expires: April 14, 2012 Philips Research
October 12, 2011
Group Communication for CoAP
draft-rahman-core-groupcomm-07
Abstract
This is a working document intended to trigger discussion and develop
draft text for the CoAP protocol specification in the area of group
communication. Engineering tradeoffs become more challenging in
constrained environments, therefore group communication is considered
within the context of adjacent topics that may impact or be impacted
by design choices in the subject area. A solution based on IP
multicast is proposed.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 14, 2012.
Copyright Notice
Copyright (c) 2011 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
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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
3. Potential Solutions . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. IP Multicast . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1. Multicast Listener Discovery (MLD) and Multicast
Router Discovery (MRD) . . . . . . . . . . . . . . . . 8
3.3.2. Group URIs and Multicast Addresses . . . . . . . . . . 9
3.3.3. Group Discovery . . . . . . . . . . . . . . . . . . . 9
3.3.4. Group Resource Manipulation . . . . . . . . . . . . . 10
3.3.5. IP Multicast Transmission Methods . . . . . . . . . . 11
3.3.6. Congestion Control . . . . . . . . . . . . . . . . . . 12
3.4. Overlay Multicast . . . . . . . . . . . . . . . . . . . . 13
3.5. CoAP Application Layer Group Management . . . . . . . . . 13
3.6. CoAP Multicast and HTTP Unicast Interworking . . . . . . . 16
3.7. CoAP-Observe for Group Communication . . . . . . . . . . . 18
4. Recommended Solution . . . . . . . . . . . . . . . . . . . . . 18
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2. An Example Protocol Flow . . . . . . . . . . . . . . . . . 19
4.3. Implementation in Target Network Topologies . . . . . . . 22
4.3.1. Single LLN Topology . . . . . . . . . . . . . . . . . 23
4.3.2. Single LLN with Backbone Topology . . . . . . . . . . 25
4.3.3. Multiple LLNs with Backbone Topology . . . . . . . . . 27
4.3.4. LLN(s) with Multiple 6LBRs . . . . . . . . . . . . . . 27
4.3.5. Conclusions . . . . . . . . . . . . . . . . . . . . . 27
4.4. HTTP/CoAP Interworking Aspects . . . . . . . . . . . . . . 28
4.5. Implementation Considerations . . . . . . . . . . . . . . 28
4.5.1. MLD Implementation on LLNs . . . . . . . . . . . . . . 28
4.5.2. 6LBR Implementation . . . . . . . . . . . . . . . . . 29
4.5.3. Backbone IP Multicast Infrastructure . . . . . . . . . 29
5. Security Considerations . . . . . . . . . . . . . . . . . . . 30
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 31
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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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 destination nodes may
consist of an arbitrary mix of constrained and non-constrained nodes.
Sending methods may include serial unicast, multicast, or hybrid
unicast-to-multicast solutions.
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. 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
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floor one of building one", "all lights on floor two of building
one", etc. Groups may also be configured or dynamically formed.
In this draft, we focus and expand discussions on the requirements
pertaining to CoAP "group communication" and "multicast" support
including:
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:
"CoAP supports sending messages to multicast destination
addresses. Such multicast messages MUST be Non-Confirmable.
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 requirement to group communication, using either
(reliable or unreliable) multicast or unicast or combinations
thereof. We assume that all, or a substantial part of, devices
participating in group communication are constrained devices (e.g.
such as Low Power and Lossy Network (LLN) devices).
Machine-to-Machine (M2M) networks may contain groups of nodes that
are highly correlated (e.g. by type or location). For example, all
smart meters in a region may belong to one group, and all light
switches in a building control system belong to another. Group
communication mechanisms can improve efficiency and latency of
communication and reduce bandwidth requirements for a given
application.
In the following sections, we address the issues related to group
communication in detail, with requirements, proposed solutions and
analysis of their impact to the CoAP protocol and implementations.
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3. Potential Solutions
3.1. Overview
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 [STUDY1] 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 [STUDY1] using
metrics such as link stress and level of host complexity [STUDY2].
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.
The approach adopted in this section is to begin with group
communication requirements. This is followed by the solutions of IP
multicast, an overlay multicast solution, and application layer group
communication. Finally additional topics are covered such as group
management and CoAP/HTTP proxies in group communication.
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CoAP CoAP CoAP CoAP
Node 1 Node 2 Proxy 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: CoAP Group Communications
3.2. Requirements
Requirements that a group communication solution in CoRE should
fulfill can be found in existing documents [RFC 5867]
[draft-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. More precise requirements may depend on the chosen
application (area).
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A CoRE group communication solution should (ideally) offer:
REQ 1: Optional Reliability: unreliable group communication, with
preferably reliable group communication as an option.
REQ 2: Efficiency: delivers messages more efficiently than a
"serial unicast only" solution. Also, it should provide a
right balance between group data traffic and control
overhead.
REQ 3: Low latency: deliver a message (preferably) as fast as
possible.
REQ 4: Synchrony: allows near-simultaneous modification of a
resource on all devices in a group, providing to users a
perceived effect of synchrony or simultaneity. It can be
expressed as a time span D such that message m is delivered
to all destinations in a time interval [t,t+D] for
arbitrary t.
REQ 5: Ordering: message ordering in the reliable group
communication mode.
REQ 6: Security: see Section 5 for security requirements for group
communication.
REQ 7: Flexibility: support for one or many source(s), for dense
and sparse networks, for high or low listener density, one
or many group(s), and multi-group membership.
REQ 8: Robust group management: includes functionality to join
groups, leave groups, view group membership, and persistent
group membership in failing node or sleeping node
situations.
REQ 9: Network layer independence: a solution should be specified
independent from specific unicast and/or IP multicast
routing protocols. It should support different routing
protocols and implementations thereof.
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).
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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.
3.3. IP Multicast
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. Therefore, the packet economy and minimal
host complexity of IP multicast make it worth investigating for group
communication in constrained environments.
3.3.1. Multicast Listener Discovery (MLD) and Multicast Router
Discovery (MRD)
The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its
IPv4 pendant IGMP) is today the method of choice used by an (IP
multicast enabled) IPv6 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. It 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 a 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 switch 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. However, it appears that
this protocol is not as commonly implemented in existing products as
MLD is.
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3.3.2. Group URIs and Multicast Addresses
An approach to map group authorities onto IP multicast addresses
using DNS was proposed in [I-D.vanderstok-core-bc]. Examples of
group URI naming (and scoping) for a building control application are
shown below. Group URIs MUST follow the approach of the URI
structure defined in [RFC3986].
URI authority Targeted group
all.bldg6... "all nodes in building 6"
all.west.bldg6... "all nodes in west wing, building 6"
all.floor1.west.bldg6... "all nodes in floor 1, west wing,
etc."
all.bu036.floor1.west.bldg6... "all nodes in office bu036, floor1,
etc."
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 an address allocated
according to [RFC3956], [RFC3306] or [RFC3307].
3.3.3. Group 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].
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 or alternatively
the name _coap._upd.<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.
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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>.
3.3.4. 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
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
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descriptions in DNS (using DNS-SD as in Section 3.3.3 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 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 [ID.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
proxy receiving the request to look up the group membership and
replicate serial unicasts as well as send multicast packets.
3.3.5. IP Multicast Transmission Methods
3.3.5.1. Serial unicast
Even in systems that generally support IP Multicast, there may be
certain data links (or transports) that don't support IP multicast.
For those links a serial unicast alternative must be provided. This
implies that it should be possible to enumerate the members of a
group, in order to determine the correct unicast destinations.
3.3.5.2. Unreliable IP Multicast
The CoRE WG charter specified support for non-reliable IP multicast.
In the current CoAP protocol design [I-D.ietf-core-coap], unreliable
multicast is realized by the source sending Non-Confirmable messages
to a multicast IP address. IP Multicast (using UDP) in itself is
unreliable, unless specific reliability features are added to it.
3.3.5.3. 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
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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
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)
3.3.6. 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.
However, as responses to multicast requests (both MGET or MPUT) are
required in CoAP, using CoAP multicast still may lead to congestion
issues.
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Various means can be implemented to prevent congestion.
[TBD: if an MGET or MPUT request leads to the sending of a CoAP
response by servers, the servers should enforce a random delay within
TIMEOUT before sending their responses. More investigation
required.]
Currently in the CoAP protocol, a MAX_RETRANSMIT value set by default
to 4 is used for retransmission of Confirmable messages. Since CoAP
multicast messages are Non-Confirmable, no retransmissions will occur
in CoAP, making the effective retransmission value 0.
3.4. 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 (Section 3.3.1) 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.
3.5. 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.
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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
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 2:
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+------+-----+---------------+--------------+--------+--------------+
| 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 2: CoAP Header Options for Group Management
Figure 3 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 3: CoAP Message for Group Management
Header Fields for the above example:
Ver: 2-bit unsigned integer for CoAP Version. Set to 1 by
implementation as defined by the CoAP specification.
T: 2-bit unsigned integer for CoAP Transaction Type. Either '0'
Confirmation or '1' Non-Confirmable can be used for group "join" or
"leave" request.
OC: 4-bit unsigned integer for Option Count. For this example, the
value should be "3" since there are three option fields.
Code: 8-bit unsigned integer to indicate the Method in a Request or a
Response Code in a Response message. Any Code can be used so the
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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 3, 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
3.6. 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 4. 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 4: CoAP Multicast and HTTP Unicast Interworking
Note that Figure 4 illustrates the case of IP multicast as the
underlying group communications mechanism. However the overlay
multicast group communication (Section 3.4) or CoAP application group
communication (Section 3.5) can be used as the underlying mechanism
and the principles of the figure would still apply (i.e. CoAP proxy
needs to do interworking between HTTP unicast and CoAP multicast).
A key point in Figure 4 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
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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 3.3.3). The proxy node will
then 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 4 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.
3.7. CoAP-Observe for Group Communication
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 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 in this case for
notifications.
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.
4. Recommended Solution
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4.1. Overview
We recommend that IP multicast as outlined in Section 3.3 be adopted
as the base solution for CoAP Group Communication. This approach re-
uses the IP multicast suite of protocols and can operate on both
constrained and non-constrained network segments. The group
communication can hence work regardless of the underlying networking
technology. 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.
4.2. An Example Protocol Flow
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 5):
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 5: 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 5 is shown in
Figure 6. We assume the following steps occur before the illustrated
flow:
1) Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to
all devices. The CoAP network is formed.
2) Commissioning phase (by applications): The IP multicast address of
the group (Room-A-Lights) has been set in all the Lights. The URI of
the group (Room-A-Lights) has been set in the Light Switch.
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 6: 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.
4.3. 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.)
4.3.1. 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.
4.3.1.1. 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 3.3.1). 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.
4.3.1.2. 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.
4.3.1.3. 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 use link-local RPL routers for their IP
connectivity. Note that the current RPL specification
[I-D.ietf-roll-rpl] considers this case to be out of scope. However,
it was suggested on the ROLL mailing list that RPL could potentially
be run with non-RPL-aware hosts but that it is simply not specified
yet. Such non-RPL hosts can't advertize their IP multicast groups of
interest via RPL DAO messages as defined above. Therefore in that
case MLD can 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 4.5.1.
4.3.1.4. 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
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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.
4.3.1.5. 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 4.5.1 for more efficient
substitutes for MLD targeted towards a LLN context.
4.3.2. 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.
4.3.2.1. 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
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interface. However, this may be too much of a "burden" for
constrained nodes. Light-weight alternatives for MLD are discussed
in Section 4.5.1.
4.3.2.2. 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.
4.3.2.3. 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 4.3.1.3.
4.3.2.4. 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 4.3.2.1.
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 4.3.2.1) 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 4.3.2.1 or
to use an MLD alternative suitable for LLNs as described in
Section 4.5.1. However, following this approach requires possibly an
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extension to Trickle Multicast Forwarding: the protocol should ensure
that MLD-advertized information is somehow communicated to the 6LBR,
possibly over multiple hops. MLD itself supports link-local
communication only.
4.3.2.5. 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.
4.3.3. 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.
4.3.4. 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? ]
4.3.5. 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 4.5.1.
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4.4. HTTP/CoAP Interworking Aspects
The topic of HTTP unicast to CoAP multicast request proxying is
treated in [I-D.castellani-core-http-mapping]. [TBD: only if needed
more information will be added here in the future.]
4.5. Implementation Considerations
In this section various implementation aspects are considered such as
required protocol implementations, additional functionality of the
6LBR and backbone network equipment.
4.5.1. 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.
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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.
[ 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 ]
4.5.2. 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.
4.5.3. 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 & deployed IP (Ethernet/WiFi/...) infrastructure,
to be shared with other IP devices (PCs)
3. Newly designed & deployed IP (Ethernet/WiFi/...) infrastructure,
exclusively used for building control.
Besides physical separation the building control backbone can be
separated from regular (PC) infrastructure by using a different VLAN.
A typical corporate installation will have many LAN switches and/or
routing switches, which pass through IP multicast traffic but on the
other hand do not support acting in the Router role of MLD/IGMP.
Perhaps for case 2) and 3) above it is acceptable to add a MLD/IGMP
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capable router somewhere in the network, while for case 1) this may
not be the case.
[TBD: consider the influence of WiFi based backbone networks. What
if 6LBRs are at the same time also WiFi routers? What if 6LBRs have
an Ethernet connection to legacy WiFI routers? Check if equivalent
with Ethernet backbone.]
5. 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
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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.
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.
6. IANA Considerations
This document makes no request of IANA.
7. Conclusions
Three solutions for enabling CoAP group communications have been
discussed.
Unreliable IP multicast as outlined in Section 3.3 is recommended to
be adopted as the base solution for CoAP Group Communication on LLNs.
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.
8. Acknowledgements
Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo
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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.
9. References
9.1. 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):
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Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, August 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
March 2010.
9.2. Informative References
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-10 (work in
progress), February 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-hc]
Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams in Low Power and Lossy Networks (6LoWPAN)",
draft-ietf-6lowpan-hc-15 (work in progress),
February 2011.
[I-D.ietf-6lowpan-nd]
Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
Discovery Optimization for Low Power and Lossy Networks
(6LoWPAN)", draft-ietf-6lowpan-nd-17 (work in progress),
June 2011.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-07 (work in progress), July 2011.
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[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-07 (work in progress),
July 2011.
[I-D.ietf-core-observe]
Hartke, K. and Z. Shelby, "Observing Resources in CoAP",
draft-ietf-core-observe-02 (work in progress), March 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.vanderstok-core-bc]
Stok, P. and K. Lynn, "CoAP Utilization for Building
Control", draft-vanderstok-core-bc-04 (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-01
(work in progress), July 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-02 (work
in progress), July 2011.
[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.
[ID.goland-http-udp]
Goland, Y., "Multicast and Unicast UDP HTTP Messages",
1999,
<http://tools.ietf.org/html/draft-goland-http-udp-01>.
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[STUDY1] 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>.
[STUDY2] 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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