Negotiating Human Language in Real-Time Communications
draft-ietf-slim-negotiating-human-language-03
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| Last updated | 2016-07-21 | ||
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draft-ietf-slim-negotiating-human-language-03
CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E. Dijk, Ed.
Expires: January 11, 2013 Philips Research
July 10, 2012
Group Communication for CoAP
draft-ietf-core-groupcomm-02
Abstract
CoAP is a RESTful transfer protocol for constrained devices. It is
anticipated that constrained devices will often naturally operate in
groups (e.g. in a building automation scenario all lights in a given
room may need to be switched on/off as a group). This document
defines how the CoAP protocol should be used in a group communication
context. An approach for using CoAP on top of IP multicast is
detailed for both constrained and un-constrained networks. Also,
various use causes and corresponding protocol flows are provided to
illustrate important concepts. Finally, guidance is provided for
deployment in various 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 January 11, 2013.
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
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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
described in the Simplified BSD License.
Table of Contents
1. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Potential Solutions for Group Communication . . . . . . . 4
3. IP Multicast Based Group Communication . . . . . . . . . . . . 4
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Group URIs and IP Multicast Addresses . . . . . . . . . . 5
3.3. Group Discovery and Member Discovery . . . . . . . . . . . 6
3.3.1. DNS-SD . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3.2. CoRE Resource Directory . . . . . . . . . . . . . . . 6
3.4. Group Resource Manipulation . . . . . . . . . . . . . . . 7
3.5. Congestion Control . . . . . . . . . . . . . . . . . . . . 8
3.6. CoAP Multicast and HTTP Unicast Interworking . . . . . . . 8
4. Use Cases and Corresponding Protocol Flows . . . . . . . . . . 10
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Network Configuration . . . . . . . . . . . . . . . . . . 11
4.3. Discovery of Resource Directory . . . . . . . . . . . . . 13
4.4. Lighting Control . . . . . . . . . . . . . . . . . . . . . 14
5. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 18
5.1. Target Network Topologies . . . . . . . . . . . . . . . . 18
5.2. Multicast Routing . . . . . . . . . . . . . . . . . . . . 18
5.3. Use of the Multicast Listener Discovery (MLD) protocol . . 18
5.4. 6LoWPAN-Specific Guidelines . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Multicast Listener Discovery (MLD) . . . . . . . . . 22
Appendix B. CoAP-Observe Alternative to Group Communication . . . 23
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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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].
This document assumes readers are familiar with the terms and
concepts that are used in [I-D.ietf-core-coap]. In addition, this
document defines the following terminology:
Group Communication
A source node sends a single message which is delivered to
multiple destination nodes, where all destinations are identified
to belong to a specific group. The source node may or may not be
part of the group. The underlying mechanism for group
communication is assumed to be multicast based. The network where
the group communication takes place can be either a constrained or
a regular (un-constrained) network
Multicast
Sending a message to multiple destination nodes simultaneously.
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)
Low power and Lossy Network (LLN): A type of constrained network
where the devices are interconnected by a variety of low power,
lossy links such as IEEE 802.15.4, Bluetooth, WiFi, wired or low
power power-line communication links.
2. Introduction
2.1. Background
The Constrained Application Protocol (CoAP) is an application
protocol (analogous to HTTP) for resource constrained devices
operating in an IP network [I-D.ietf-core-coap]. Constrained devices
can be large in number, but are often highly correlated to each other
(e.g. by type or location). For example, all the light switches in a
building may belong to one group and all the thermostats may belong
to another group. Groups may be composed by function. For example,
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the group "all lights in building one" may consist of the groups "all
lights on floor one of building one", "all lights on floor two of
building one", etc. Groups may be preconfigured or dynamically
formed. If information needs to be sent to or received from a group
of devices, group communication mechanisms can improve efficiency and
latency of communication and reduce bandwidth requirements for a
given application. HTTP does not support any equivalent
functionality to CoAP group communication.
2.2. Scope
In this draft, we address the issues related to CoAP group
communication in detail, with use cases, recommended approaches and
analysis of the impact to the CoAP protocol and to implementations.
The guiding principle is to apply wherever possible existing IETF
protocols to achieve group communication functionality. In many
cases the contribution of this document lies in explaining how
existing mechanisms may be used to together fulfill CoAP group
communication needs for specific use cases.
2.3. Potential Solutions for Group Communication
The classic concept of group communications is that of a single
source distributing content to multiple destination recipients that
are all part of a group. Before content can be distributed, there is
a separate process to form the group. The source may be either a
member or non-member of the group.
Group communication solutions have evolved from "bottom" to "top",
i.e., from layer 2 (Media Access Control broadcast/multicast) and
layer 3 (IP multicast) to application layer group communication, also
referred to as application layer multicast. A study published in
2005 [Lao05] identified new solutions in the "middle" (referred to as
overlay multicast) that utilize an infrastructure based on proxies.
Each of these classes of solutions may be compared [Lao05] using
metrics such as link stress and level of host complexity
[Banerjee01]. The results show for a realistic internet topology
that IP Multicast is the most resource-efficient, with the downside
being that it requires the most effort to deploy in the
infrastructure. IP Multicast is the solution adopted by this draft
for CoAP group communication.
3. IP Multicast Based Group Communication
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3.1. Introduction
IP Multicast routing 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 routing is not widely
deployed on the general Internet. However, IP Multicast is very
popular in specific deployments such as in enterprise networks (e.g.
for video conferencing), smart home networks (e.g. UPnP/mDNS) and
carrier IPTV deployments. The packet economy and minimal host
complexity of IP multicast make it attractive for group communication
in constrained environments. Therefore IP multicast is the
recommended underlying mechanism for CoAP group communications, and
the approach assumed in this document.
To achieve IP multicast beyond a subnet, an IP multicast routing
protocol needs to be active on routers. The RPL protocol [RFC6550]
for example is able to route multicast traffic in constrained LLNs.
While PIM-SM [RFC4601] is often used for multicast routing in un-
constrained networks.
IP multicast can also be run in a Link-Local (LL) scope. This means
that there is no routing involved and the IP multicast message is
only sent and received in the local subnet.
3.2. Group URIs and IP Multicast Addresses
A group of CoAP nodes can be addressed using its IP multicast
addresses or a group URI ([I-D.vanderstok-core-dna]) which can be
mapped to a site-local or global multicast IP address via DNS
resolution. A CoAP node can become a group member by listening for
CoAP messages on the corresponding IP multicast address. Group URIs
MUST follow the URI syntax [RFC3986]. Examples of hierarchical group
naming (and scoping) for a building control application are shown
below.
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"
Reverse mapping (from IP multicast address to group authority) is
supported using the reverse DNS resolution technique
([I-D.vanderstok-core-dna]).
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3.3. 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). These topics
are elaborated in more detail in [I-D.vanderstok-core-dna] including
examples for using DNS-SD and CoRE Resource Directory.
3.3.1. DNS-SD
DNS-based Service Discovery [I-D.cheshire-dnsext-dns-sd] defines a
conventional way to configure DNS PTR, SRV, and TXT records to enable
enumeration of services, such as services offered by CoAP nodes, or
enumeration of all CoAP nodes, within specified subdomains. A
service is specified by a name of the form
<Instance>.<ServiceType>.<Domain>, where the service type for CoAP
nodes is _coap._udp and the domain is a DNS domain name that
identifies a group as in the examples above. For each CoAP end-point
in a group, a PTR record with the name _coap._udp and/or a PTR record
with the name _coap._udp.<Domain> is defined and it points to an SRV
record having the <Instance>.<ServiceType>.<Domain> name.
All CoAP nodes in a given subdomain may be enumerated by sending a
query for PTR records named _coap._udp to the authoritative DNS
server for that zone. A list of SRV records is returned. Each SRV
record contains the port and host name (AAAA record) of a CoAP node.
The IP address of the node is obtained by resolving the host name.
DNS-SD also specifies an optional TXT record, having the same name as
the SRV record, which can contain "key=value" attributes. This can
be used to store information about the device, e.g. schema=DALI,
type=switch, group=lighting.bldg6, etc.
Another feature of DNS-SD is the ability to specify service subtypes
using PTR records. For example, one could represent all the CoAP
groups in a subdomain by PTR records with the name
_group._sub._coap._udp or alternatively
_group._sub._coap._udp.<Domain>.
3.3.2. CoRE Resource Directory
CoRE Resource Directory [I-D.shelby-core-resource-directory] defines
the concept of a Resource Directory (RD) server where CoAP servers
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can register their resources offered and CoAP clients can discover
these resources by querying the RD server. RD syntax can be mapped
to DNS-SD syntax and vice versa [I-D.lynn-core-discovery-mapping],
such that the above approach can be reused for group discovery and
group member discovery.
Specifically, the Domain (d) parameter can be set to the group URI by
an end-point registering to the RD. If an end-point wants to join
multiple groups, it has to repeat the registration process for each
group it wants to join.
3.4. Group Resource Manipulation
Group communications SHALL only be used for idempotent messages (i.e.
CoAP GET, PUT, DELETE). Group communications SHALL NOT be used for
non-idempotent messages (i.e. CoAP POST). The CoAP messages that
are sent via group communications SHALL be Non-Confirmable. A
unicast response MAY be sent back to answer the group request (e.g.
response "2.05 Content" to a group GET request) taking into account
the security and congestion control rules defined in
[I-D.ietf-core-coap].
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 solution is to
impose the following restrictions:
o All CoAP multicast requests MUST be sent either to the default
CoAP port (i.e. default Uri-Port as defined in
[I-D.ietf-core-coap]), or to a port number obtained via a service
discovery lookup operation being a valid CoAP port for the
targeted multicast group.
o All CoAP multicast requests SHOULD operate only on URIs (links)
which were retreived either from a "/.well-known/core" lookup on
at least one group member node, or from equivalent service
discovery lookup.
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3.5. Congestion Control
Multicast CoAP requests may result in a multitude of replies from
different nodes, potentially causing congestion. Therefore sending
multicast requests should be conservatively controlled.
CoAP reduces multicast-specific congestion risks through the
following measures:
o A server MAY choose not to respond to a multicast request if
there's nothing useful to respond (e.g. error or empty response).
o A server SHOULD limit the support for multicast requests to
specific resources where multicast operation is required.
o A multicast request MUST be Non-Confirmable.
o A server does not respond immediately to a multicast request, but
SHOULD first wait for a time that is randomly picked within a
predetermined time interval called the Leisure.
o A server SHOULD NOT accept multicast requests that can not be
authenticated.
Additional guidelines to reduce congestion risks are:
o A server in an LLN should only support multicast GET for resources
that are small i.e. where the payload of the response fits into a
single link-layer frame.
o A server can minimize the payload length in response to a
multicast GET on "/.well-known/core" by using hierarchy in
arranging link descriptions for the response. An example of this
is given in Section 5 of [I-D.ietf-core-link-format].
o Preferably IP multicast with link-local scope should be used,
rather than global or site-local.
o The Hop Limit field in the IPv6 packet should be chosen as low as
possible (if the CoAP/IP stack allows setting of this value. TBD
- discuss whether this guideline is relevant/realistic in CoAP
context)
3.6. CoAP Multicast and HTTP Unicast Interworking
CoAP supports operation over UDP multicast, while HTTP does not. For
use cases where it is required that CoAP group communication is
initiated from an HTTP end-point, it would be advantageous if the
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HTTP-CoAP Proxy supports mapping of HTTP unicast to CoAP group
communication based on IP multicast. One possible way of operation
of such HTTP-CoAP Proxy is illustrated in Figure 1. Note that this
topic is covered in more detail in
[I-D.castellani-core-advanced-http-mapping].
CoAP Mcast CoAP Mcast HTTP-CoAP HTTP
Node 1 Rtr1 Node 2 Rtr2 Proxy Node 3
| | | | | |
|MLD REQUEST | | | |
|(Join Group X) | | | |
|--LL-->| | | | |
| | |MLD REQUEST | |
| | |(Join Group X) | |
| | |--LL-->| | |
| | | | | HTTP REQUEST |
| | | | | (URI to |
| | | | | unicast addr) |
| | | | |< -----------------|
| | | | | |
| | | Resolve HTTP Request-Line URI |
| | | to Group X multicast address |
| | | | | |
| CoAP REQUEST (to multicast addr)| |
|< ------<---------< ------<------| |
| | | | | |
| | | |
| (optional) CoAP RESPONSE(s) | |
| |------------- >| |
|-----------------|-------------->| |
| | | HTTP RESPONSE |
| | |----------------- >|
| | | |
Figure 1: CoAP Multicast and HTTP Unicast Interworking
Note that Figure 1 illustrates the case of IP multicast as the
underlying group communications mechanism. MLD denotes the Multicast
Listener Discovery protocol ([RFC3810], Appendix A) and LL denotes a
Link-Local multicast.
A key point in Figure 1 is that the incoming HTTP Request (from node
3) will carry a Host request-header field that resolves in the
general Internet to the proxy node. At the proxy node, this hostname
and/or the Request-Line URI will then possibly be mapped (as detailed
in [I-D.castellani-core-http-mapping]) and again resolved (with the
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CoAP scheme) to an IP multicast address. This may be accomplished,
for example, by using DNS or DNS-SD (Section 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 1 illustrates that it will be
generated by the proxy node based on aggregated responses of the CoAP
nodes and sent back to the client in the general Internet that sent
the HTTP Request (i.e. node 1). In
[I-D.castellani-core-advanced-http-mapping] the HTTP Response that
the Proxy may use to aggregate multiple CoAP responses is described
in more detail. So in terms of overall operation, the CoAP proxy can
be considered to be a "non-transparent" proxy according to [RFC2616].
Specifically, [RFC2616] states that a "non-transparent proxy is a
proxy that modifies the request or response in order to provide some
added service to the user agent, such as group annotation services,
media type transformation, protocol reduction or anonymity
filtering."
An alternative to the above is using a Forward Proxy. In this case,
the CoAP request URI is carried in the HTTP Request-Line (as defined
in [I-D.ietf-core-coap] Section 8) in a HTTP request sent to the IP
address of the Proxy.
4. Use Cases and Corresponding Protocol Flows
4.1. Introduction
The use of CoAP group communication is shown in the context of the
following use cases and corresponding protocol flows:
o Discovery of Resource Directory: discovering the local CoAP RD
which contains links (URIs) to resources stored on other servers
[I-D.ietf-core-link-format].
o Lighting Control: synchronous operation of a group of 6LoWPAN
[RFC4944] IPv6-connected lights
o Parameter Update: updating parameters/settings simultaneously in a
large group of devices in a building/campus control
([I-D.vanderstok-core-bc]) application --- TBD
o Firmware Update: efficiently updating firmware simultaneously in a
large group of devices in a building/campus control
([I-D.vanderstok-core-bc]) application --- TBD suggests a
multicast extension of core-block.
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o Group Status Report: requesting status information or event
reports from a group of devices in a building/campus control
application --- TBD, may require reliable group communication to
be feasible.
4.2. Network Configuration
We assume the following network configuration for all the use cases
as shown in Figure 2:
o 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.
o Light-1 and the Light Switch are connected to a router (Rtr-1)
which is also a CoAP Proxy, a CoAP Resource Directory (RD) and a
6LoWPAN Border Router (6LBR).
o Light-2 and the Light-3 are connected to another router (Rtr-2)
which is also a CoAP Proxy, a CoAP RD and a 6LBR.
o The routers are connected to 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 (Appendix A) for the following use cases to
work.
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Network
Backbone
|
################################################ |
# Room-A # |
# ********************** # |
# ** LoWPAN-1 (subnet-1) ** # |
# * * # |
# * +----------+ * # |
# * | Light |-------+ * # |
# * | Switch | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-1 |-----------------------------|
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-1 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
# # |
# ********************** # |
# ** LoWPAN-2 (subnet-2) ** # |
# * * # |
# * +----------+ * # |
# * | Light-2 |-------+ * # |
# * | | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-2 |-----------------------------|
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-3 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
################################################# |
|
+--------+ |
| DNS |------------------|
| Server |
+--------+
Figure 2: Network Topology of a Large Room (Room-A)
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4.3. Discovery of Resource Directory
The protocol flow for discovery of a RD for the given network (of
Figure 2) is shown in Figure 3:
o The fixture for Light-2 is installed and powered on for the first
time.
o Light-2 will then search for the local RD (RD-2) by sending out a
GET request (for the "/.well-known/core" resource) via a LL IP
multicast message. In this case, the group is assumed to include
all nodes in the subnet.
o This LL IP multicast message will then go to each node in
subnet-2. However, only Rtr-2 (RD-2) will respond because the GET
is qualified by the query string "?rt=core-rd".
o Note that the flow is shown only for Light-2 for clarity. Similar
flows will happen for Light-1, Light-3 and the Light Switch when
they are first powered on.
The RD may also be discovered by other means such as by assuming a
default location (e.g. on a 6LBR), using DHCP, etc. However, these
approaches do not invoke CoAP group communication.
For other discovery use cases such as discovering local CoAP servers,
services or resources group communication can be used in a similar
fashion as in the above use case.
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Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (RD-1) (RD-2) Backbone
| | | | | | |
| | | | | | |
********************************** | | |
* Light-2 is installed * | | |
* and powers on for first time * | | |
********************************** | | |
| | | | | | |
| | | | | | |
| | COAP NON (GET | |
| | /.well-known/core?rt=core-rd) | |
| |--------LL-------------------------------->| |
| | | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
| | COAP NON (Response | |
| | 2.05 Content | |
| | </rd>; rt="core-rd; ins="Primary")| |
| |<------------------------------------------| |
| | | | | | |
| | | | | | |
Figure 3: Resource Directory Discovery via Multicast Message
4.4. Lighting Control
The protocol flow for a building automation lighting control scenario
for the network (Figure 2) is shown in sequence in Figure 4,
Figure 5, and Figure 6. We assume the following steps occur before
the illustrated flow:
o 1) Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to
all devices. The CoAP network is formed.
o 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.
o 3) The indicated MLD Report messages are link-local multicast. In
each LoWPAN, it is assumed that a multicast routing protocol in
6LRs will then propagate the Join information contained in the MLD
Report over multiple hops to the 6LBR.
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Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (CoAP (CoAP Backbone
| | | | Proxy) Proxy) |
| | | | | | |
| | | | | | |
| MLD Report: Join | | | | |
| Group (Room-A-Lights) | | | |
|------------------------------------------>| | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | |-------------------->|
| | | | | | |
| | MLD Report: Join | | | |
| | Group (Room-A-Lights) | | |
| |------------------------------------------>| |
| | | | | | |
| | | MLD Report: Join | | |
| | | Group (Room-A-Lights) | |
| | |------------------------------->| |
| | | | | | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | | |--------->|
| | | | | | |
| | | | | | |
Figure 4: Joining Lighting Groups
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Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (CoAP (CoAP Backbone
| | | | Proxy) Proxy) |
| | | | | | |
| | *********************** | |
| | * User flips on * | |
| | * light switch to * | |
| | * turn on all the * | |
| | * lights in Room A * | |
| | *********************** | |
| | | | | | |
| | | | | | |
| | | COAP NON (PUT | | |
| | | Proxy-URI | | |
| | | URI for Room-A-Lights |
| | | Payload=turn on lights) |
| | | |--------->| | |
| | | | | | |
| | | | | | |
| | | | Request DNS resolution of |
| | | | URI for Room-A-Lights |
| | | | |-------------------->|
| | | | | | |
| | | | | | |
| | | | DNS returns: AAAA |
| | | | Group (Room-A-Lights) |
| | | | IPv6 multicast address |
| | | | |<--------------------|
| | | | | | |
| | | | | | |
| | | COAP NON (Put |
| | | | URI Path |
| | | | Payload=turn on lights)|
| | | | Destination IP Address = |
| | | | IP multicast address |
| | | | for Group (Room-A-Lights)|
| | | | Originating IP Address = |
| | | | RTR-1 |
| | | | |-------------------->|
|<------------------------------------------| | |
| | | | | | |
| | | | | |<---------|
| |<---------|<-------------------------------| |
| | | | | | |
| | | | | | |
Figure 5: Sending Lighting Control Multicast Message
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Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (CoAP (CoAP Backbone
| | | | Proxy) Proxy) |
| | | | | | |
*********************** | | | |
* Lights in Room-A * | | | |
* turn on (nearly * | | | |
* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
| | | | | | |
| COAP NON (Response | | | |
| Success) | | | |
|------------------------------------------>| | |
| | | | | | |
| | | | | | |
| COAP NON (Response | | | |
| Success ) | | | |
| |------------------------------->| | |
| | | | | | |
| | | | | | |
| | COAP NON (Response | | |
| | Success) | | |
| | |-------------------->| | |
| | | | | | |
| | | ****************************** |
| | | * Rtr-1 as CoAP Proxy * |
| | | * processes all responses * |
| | | * to multicast message * |
| | | * and formulates one * |
| | | * consolidated response * |
| | | * to originator * |
| | | ****************************** |
| | | | | | |
| | | COAP NON (Response | |
| | | Success) | |
| | | |<---------| | |
| | | | | | |
Figure 6: Sending Lighting Control Response to Multicast Message
NOTE: In the last step of Figure 6, instead of a single consolidated
response the CoAP Proxy Rtr-1 could also return multiple individual
CoAP responses, similar to the case that a CoAP client sends a CoAP
multicast request directly. The format of a consolidated response is
currently not defined in [I-D.ietf-core-coap].
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5. Deployment Guidelines
This section provides some guidelines how an IP Multicast based
solution for CoAP group communication can be deployed in various
network configurations.
5.1. Target Network Topologies
CoAP group communication can be deployed in various network
topologies. First, the target network may be a regular IP network,
or a LLN such as e.g. a 6LoWPAN network, or consist of mixed
constrained/unconstrained network segments. Second, it may be a
single subnet only or multi-subnet; e.g. multiple 6LoWPAN networks
joined by a single backbone LAN. Third, a wireless network segment
may have all nodes reachable in a single IP hop, or it may require
multiple IP hops for some pairs of nodes to reach eachother.
Each topology may pose different requirements on the configuration of
routers and protocol(s), in order to enable efficient CoAP group
communication.
5.2. Multicast Routing
If a network (segment) requires multiple IP hops to reach certain
nodes, a multicast routing protocol is required to propagate
multicast UDP packets to these nodes. Examples of routing protocols
specifically for LLNs, able to route multicast, are RPL (Section 12
of [RFC6550]) and Trickle Multicast Forwarding
[I-D.ietf-roll-trickle-mcast].
5.3. Use of the Multicast Listener Discovery (MLD) protocol
CoAP nodes that are IP hosts (not routers) are unaware of the
specific multicast routing protocol being used. When such a host
needs to join a specific (CoAP) multicast group, it usually requires
a way to signal to the multicast routers which multicast traffic it
wants to receive. For efficient multicast routing (i.e. avoid always
flooding multicast IP packets), routers must know which hosts need to
receive packets addressed to specific IP multicast destinations.
The Multicast Listener Discovery (MLD) protocol ([RFC3810],
Appendix A) is the standard IPv6 method to achieve this. [RFC6636]
discusses tuning of MLD for mobile and wireless networks. These
guidelines may be useful when implementing MLD in LLNs.
Alternatively, to avoid the addition of MLD in LLN deployments, all
nodes can be configured as multicast routers.
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5.4. 6LoWPAN-Specific Guidelines
To support multi-LoWPAN scenarios for 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 (Appendix A) on the backbone link.
To avoid that backbone IP multicast traffic needlessly congests
6LoWPAN network segments, it is RECOMMENDED that a filtering means is
implemented to block IP multicast traffic from 6LoWPAN segments where
none of the 6LoWPAN nodes listen to this traffic. Possible means
are:
o Filtering in 6LBRs based on information from the routing protocol.
This allows a 6LBR to only forward multicast traffic onto the
LoWPAN, for which it is known that there exists at least one
listener on the LoWPAN.
o Filtering in 6LBRs based on MLD reports. Similar as previous but
based directly on MLD reports from 6LoWPAN nodes. This only works
in a single-IP-hop 6LoWPAN network such as a mesh-under routing
network.
o Filtering in 6LBRs based on settings. Filtering tables with
blacklists/whitelists can be configured in the 6LBR by system
administration for all 6LBRs or configured on a per-6LBR basis.
o Filtering in router(s) that provide access to 6LoWPAN network
segments. For example, in an access router/bridge that connects a
regular intranet LAN to a building control IPv6 backbone. This
backbone connects multiple 6LoWPAN segments.
6. Security Considerations
TBD
7. IANA Considerations
A request is made to IANA for reserving a range of IP addresses for
"CoAP group communication" for:
o IPv4 link-local scope multicast.
o IPv6 link-local scope multicast.
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o IPv4 general multicast.
o IPv6 general multicast.
8. Conclusions
IP multicast as outlined in Section 3 is recommended to be adopted as
the base solution for CoAP Group Communication for situations where
the use case and network characteristics allow use of IP multicast.
This approach requires no standards changes to the IP multicast suite
of protocols and it provides interoperability with IP multicast group
communication on un-constrained backbone networks.
9. Acknowledgements
Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo
Castellani, Guang Lu, Salvatore Loreto, Kerry Lynn, Dale Seed, Zach
Shelby, Peter van der Stok, and Juan Carlos Zuniga for their helpful
comments and discussions that have helped shape this document.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 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.
Rahman "humintlang" to
reference RFC 5646
o Added clarifying text for (possible) re-use of existing 'lang'
attribute saying that the registration would be updated to reflect
different semantics for multiple values for interactive versus
non-interactive media.
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o Added clarifying text for (possible) new attribute "humintlang" to
attempt to better describe the role of language tags in media in
an offer and an answer.
10.13. Changes from draft-gellens-...-00 to -01
o Changed name of (possible) new attribute from 'humlang" to
"humintlang"
o Added discussion of silly state (language not appropriate for
media type)
o Added Voice Carry Over example
o Added mention of multilingual people and multiple languages
o Minor text clarifications
11. Contributors
Gunnar Hellstrom deserves special mention for his reviews,
assistance, and especially for contributing the core text in
Appendix A.
12. Acknowledgments
Many thanks to Bernard Aboba, Harald Alvestrand, Flemming Andreasen,
Francois Audet, Eric Burger, Keith Drage, Doug Ewell, Christian
Groves, Andrew Hutton, Hadriel Kaplan, Ari Keranen, John Klensin,
Paul Kyzivat, John Levine, Alexey Melnikov, James Polk, Pete Resnick,
Peter Saint-Andre, and Dale Worley for reviews, corrections,
suggestions, and participating in in-person and email discussions.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <http://www.rfc-editor.org/info/rfc5646>.
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13.2. Informational References
[I-D.iab-privacy-considerations]
Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", draft-iab-privacy-
considerations-09 (work in progress), May 2013.
[I-D.ietf-slim-multilangcontent]
Tomkinson, N. and N. Borenstein, "Multiple Language
Content Type", draft-ietf-slim-multilangcontent-02 (work
in progress), July 2016.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840,
DOI 10.17487/RFC3840, August 2004,
<http://www.rfc-editor.org/info/rfc3840>.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, DOI 10.17487/RFC3841, August 2004,
<http://www.rfc-editor.org/info/rfc3841>.
Appendix A. Historic Alternative Proposal: Caller-prefs
The decision to base the proposal at the media negotiation level, and
specifically to use SDP, came after significant debate and
discussion. It is possible to meet the objectives using a variety of
mechanisms, but none are perfect. Using SDP means dealing with the
complexity of SDP, and leaves out real-time session protocols that do
not use SDP. The major alternative proposal was to use SIP. Using
SIP leaves out non-SIP session protocols, but more fundamentally,
would occur at a different layer than the media negotiation. This
results in a more fragile solution since the media modality and
language would be negotiated using SIP, and then the specific media
formats (which inherently include the modality) would be negotiated
at a different level (typically SDP, especially in the emergency
calling cases), making it easier to have mismatches (such as where
the media modality negotiated in SIP don't match what was negotiated
using SDP).
An alternative proposal was to use the SIP-level Caller Preferences
mechanism from RFC 3840 [RFC3840] and RFC 3841 [RFC3841].
The Caller-prefs mechanism includes a priority system; this would
allow different combinations of media and languages to be assigned
different priorities. The evaluation and decisions on what to do
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with the call can be done either by proxies along the call path, or
by the addressed UA. Evaluation of alternatives for routing is
described in RFC 3841 [RFC3841].
A.1. Use of Caller Preferences Without Additions
The following would be possible without adding any new registered
tags:
Potential callers and recipients MAY include in the Contact field in
their SIP registrations media and language tags according to the
joint capabilities of the UA and the human user according to RFC 3840
[RFC3840].
The most relevant media capability tags are "video", "text" and
"audio". Each tag represents a capability to use the media in two-
way communication.
Language capabilities are declared with a comma-separated list of
languages that can be used in the call as parameters to the tag
"language=".
This is an example of how it is used in a SIP REGISTER:
REGISTER user@example.net
Contact: <sip:user1@example.net> audio; video; text;
language="en,es,ase"
Including this information in SIP REGISTER allows proxies to act on
the information. For the problem set addressed by this document, it
is not anticipated that proxies will do so using registration data.
Further, there are classes of devices (such as cellular mobile
phones) that are not anticipated to include this information in their
registrations. Hence, use in registration is OPTIONAL.
In a call, a list of acceptable media and language combinations is
declared, and a priority assigned to each combination.
This is done by the Accept-Contact header field, which defines
different combinations of media and languages and assigns priorities
for completing the call with the SIP URI represented by that Contact.
A priority is assigned to each set as a so-called "q-value" which
ranges from 1 (most preferred) to 0 (least preferred).
Using the Accept-Contact header field in INVITE requests and
responses allows these capabilities to be expressed and used during
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call set-up. Clients SHOULD include this information in INVITE
requests and responses.
Example:
Accept-Contact: *; text; language="en"; q=0.2
Accept-Contact: *; video; language="ase"; q=0.8
This example shows the highest preference expressed by the caller is
to use video with American Sign Language (language code "ase"). As a
fallback, it is acceptable to get the call connected with only
English text used for human communication. Other media may of course
be connected as well, without expectation that it will be usable by
the caller for interactive communications (but may still be helpful
to the caller).
This system satisfies all the needs described in the previous
chapters, except that language specifications do not make any
distinction between spoken and written language, and that the need
for directionality in the specification cannot be fulfilled.
To some degree the lack of media specification between speech and
text in language tags can be compensated by only specifying the
important medium in the Accept-Contact field.
Thus, a user who wants to use English mainly for text would specify:
Accept-Contact: *;text;language="en";q=1.0
While a user who wants to use English mainly for speech but accept it
for text would specify:
Accept-Contact: *;audio;language="en";q=0.8
Accept-Contact: *;text;language="en";q=0.2
However, a user who would like to talk, but receive text back has no
way to do it with the existing specification.
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A.2. Additional Caller Preferences for Asymmetric Needs
In order to be able to specify asymmetric preferences, there are two
possibilities. Either new language tags in the style of the
humintlang parameters described above for SDP could be registered, or
additional media tags describing the asymmetry could be registered.
A.2.1. Caller Preferences for Asymmetric Modality Needs
The following new media tags should be defined:
speech-receive
speech-send
text-receive
text-send
sign-send
sign-receive
A user who prefers to talk and get text in return in English would
register the following (if including this information in registration
data):
REGISTER user@example.net
Contact: <sip:user1@example.net> audio;text;speech-send;text-
receive;language="en"
At call time, a user who prefers to talk and get text in return in
English would set the Accept-Contact header field to:
Accept-Contact: *; audio; text; speech-receive; text-send;
language="en";q=0.8
Accept-Contact: *; text; language="en"; q=0.2
Note that the directions specified here are as viewed from the callee
side to match what the callee has registered.
A bridge arranged for invoking a relay service specifically arranged
for captioned telephony would register the following for supporting
calling users:
REGISTER ct@ctrelay.net
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Contact: <sip:ct1@ctreley.net> audio; text; speech-receive;
text-send; language="en"
A bridge arranged for invoking a relay service specifically arranged
for captioned telephony would register the following for supporting
called users:
REGISTER ct@ctrelay.net
Contact: <sip:ct2@ctreley.net> audio; text; speech-send; text-
receive; language="en"
At call time, these alternatives are included in the list of possible
outcome of the call routing by the SIP proxies and the proper relay
service is invoked.
A.2.2. Caller Preferences for Asymmetric Language Tags
An alternative is to register new language tags for the purpose of
asymmetric language usage.
Instead of using "language=", six new language tags would be
registered:
humintlang-text-recv
humintlang-text-send
humintlang-speech-recv
humintlang-speech-send
humintlang-sign-recv
humintlang-sign-send
These language tags would be used instead of the regular
bidirectional language tags, and users with bidirectional
capabilities SHOULD specify values for both directions. Services
specifically arranged for supporting users with asymmetric needs
SHOULD specify only the asymmetry they support.
Author's Address
Randall Gellens
Core Technology Consulting
Email: rg+ietf@randy.pensive.org
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