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CoRE Working Group E. Dijk, Ed.
Internet-Draft Philips Research
Intended status: Informational A. Rahman, Ed.
Expires: April 22, 2013 InterDigital Communications, LLC
October 19, 2012
Miscellaneous CoAP Group Communication Topics
draft-dijk-core-groupcomm-misc-02
Abstract
This document contains miscellaneous text around the topic of group
communication for the Constrained Application Protocol (CoAP). The
first part contains, for reference, text that was removed from the
Group Communication for CoAP draft. The second part describes group
communication and multicast functionality that may be input to future
standardization in the CoRE WG.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 22, 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
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. General Requirements . . . . . . . . . . . . . . . . . . . 4
2.3. Security Requirements . . . . . . . . . . . . . . . . . . 5
3. Group Communication Solutions . . . . . . . . . . . . . . . . 7
3.1. IP Multicast Transmission Methods . . . . . . . . . . . . 7
3.1.1. Serial unicast . . . . . . . . . . . . . . . . . . . . 7
3.1.2. Unreliable IP Multicast . . . . . . . . . . . . . . . 7
3.1.3. Reliable IP Multicast . . . . . . . . . . . . . . . . 7
3.2. Overlay Multicast . . . . . . . . . . . . . . . . . . . . 8
3.3. CoAP Application Layer Group Management . . . . . . . . . 9
4. DNS-SD Based Group Resource Manipulation . . . . . . . . . . . 12
5. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 12
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Implementation in Target Network Topologies . . . . . . . 12
5.2.1. Single LLN Topology . . . . . . . . . . . . . . . . . 13
5.2.2. Single LLN with Backbone Topology . . . . . . . . . . 15
5.2.3. Multiple LLNs with Backbone Topology . . . . . . . . . 17
5.2.4. LLN(s) with Multiple 6LBRs . . . . . . . . . . . . . . 17
5.2.5. Conclusions . . . . . . . . . . . . . . . . . . . . . 17
5.3. Implementation Considerations . . . . . . . . . . . . . . 18
5.3.1. MLD Implementation on LLNs and MLD alternatives . . . 18
5.3.2. 6LBR Implementation . . . . . . . . . . . . . . . . . 19
5.3.3. Backbone IP Multicast Infrastructure . . . . . . . . . 19
6. Miscellaneous Topics . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Multicast Listener Discovery (MLD) . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
This document contains miscellaneous text around the topic of group
communication for the Constrained Application Protocol, CoAP
[I-D.ietf-core-coap]. The first part of the document (Section 3)
contains, for reference, text that was removed from the Group
Communication for CoAP [I-D.ietf-core-groupcomm] draft and its
predecessor [I-D.rahman-core-groupcomm]. The second part of the
document (Section 6) contains text and/or functionality that may be
considered for inclusion in [I-D.ietf-core-groupcomm] or otherwise
may be input to future standardization in the CoRE WG.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Requirements
Requirements that a CoAP group communication solution should fulfill
can be found in existing documents ([RFC5867],
[I-D.ietf-6lowpan-routing-requirements], [I-D.vanderstok-core-bc],
and [I-D.shelby-core-coap-req]). Below, a set of high-level
requirements is listed that a group communication solution should
ideally fulfill. In practice, all these requirements can never be
satisfied at once in an LLN context. Furthermore, different use
cases will have different needs i.e. an elaboration of a subset of
below requirements.
2.1. Background
The requirements for CoAP are documented in
[I-D.shelby-core-coap-req]. In this draft, we focus and expand
discussions on the requirements pertaining to CoAP "group
communication" and "multicast" support as stated in
[I-D.shelby-core-coap-req]:
REQ 9: CoAP will support a non-reliable IP multicast message to be
sent to a group of Devices to manipulate a resource on all the
Devices simultaneously. The use of multicast to query and
advertise descriptions must be supported, along with the support
of unicast responses.
Currently, the CoAP protocol [I-D.ietf-core-coap] supports unreliable
IP multicast using UDP. It defines the unreliable multicast
operation as follows in Section 4.5:
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"CoAP supports sending messages to multicast destination
addresses. Such multicast messages MUST be Non-Confirmable. Some
mechanisms for avoiding congestion from multicast requests are
being considered in [I-D.eggert-core-congestion-control]."
Additional requirements were introduced in [I-D.vanderstok-core-bc]
driven by quality of experience issues in commercial lighting; the
need for large numbers of devices to respond with near simultaneity
to a command (multicast PUT), and for that command to be received
reliably (reliable multicast).
2.2. General Requirements
A CoAP group communication solution should (ideally) meet the
following general requirements:
GEN-REQ 1: Optional Reliability: the application can select
between unreliable group communication and reliable
group communication.
GEN-REQ 2: Efficiency: delivers messages more efficiently than a
"serial unicast" solution. Provides a balance between
group data traffic and control overhead.
GEN-REQ 3: Low latency: deliver a message as quickly as possible.
GEN-REQ 4: Synchrony: allows near-simultaneous modification of a
resource on all devices in a target group, providing a
perceived effect of synchrony or simultaneity. For
example a specified time span D such that a message is
delivered to all destinations in a time interval
[t,t+D].
GEN-REQ 5: Ordering: message ordering may be required for reliable
group communication use cases.
GEN-REQ 6: Security: see Section 2.3 for security requirements for
group communication.
GEN-REQ 7: Flexibility: support for one or many source(s), both
dense and sparse networks, for high or low listener
density, small or large number of groups, and multi-
group membership.
GEN-REQ 8: Robust group management: functionality to join groups,
leave groups, view group membership, and persistent
group membership in failure or sleeping node
situations.
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GEN-REQ 9: Network layer independence: a solution is independent
from specific unicast and/or IP multicast routing
protocols.
GEN-REQ 10: Minimal specification overhead: a group communication
solution should preferably re-use existing/established
(IETF) protocols that are suitable for LLN deployments,
instead of defining new protocols from scratch.
GEN-REQ 11: Minimal implementation overhead: e.g. a solution allows
to re-use existing (software) components that are
already present on constrained nodes such as (typical)
6LoWPAN/CoAP nodes.
GEN-REQ 12: Mixed backbone/LLN topology support: a solution should
work within a single LLN, and in combined LLN/backbone
network topologies, including multi-LLN topologies.
Both the senders and receivers of CoAP group messages
may be attached to different network links or be part
of different LLNs, possibly with routers or switches in
between group members. In addition, different routing
protocols may operate on the LLN and backbone networks.
Preferably a solution also works with existing, common
backbone IP infrastructure (e.g. switches or routers).
GEN-REQ 13: CoAP Proxying support: a CoAP proxy can handle
distribution of a message to a group on behalf of a
(constrained) CoAP client.
GEN-REQ 14: Suitable for operation on LLNs with constrained nodes.
2.3. Security Requirements
Security for group communications at the IP level has been studied
extensively in the IETF MSEC (Multicast Security) WG, and to a lesser
extent in the IRTF SAMRG (Scalable Adaptive Multicast Research
Group). In particular, [RFC3740], [RFC5374] and [RFC4046] are very
instructive. A set of requirements for securing group communications
in CoAP were derived from a study of these previous investigations as
well as understanding of CoAP specific needs. These are listed
below.
A CoAP group communication solution should (ideally) meet the
following security requirements:
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SEC-REQ 1: Group communications data encryption: Important CoAP
group communications shall be encrypted (using a group
key) to preserve confidentiality. It shall also be
possible to send CoAP group communications in the clear
(i.e. unencrypted) for low value data.
SEC-REQ 2: Group communications source data authentication:
Important CoAP group communications shall be
authenticated by verifying the source of the data (i.e.
that it was generated by a given and trusted group
member). It shall also be possible to send
unauthenticated CoAP group communications for low value
data.
SEC-REQ 3: Group communications limited data authentication: Less
important CoAP group communications shall be
authenticated by simply verifying that it originated
from one of the group members (i.e. without explicitly
identifying the source node). This is a weaker
requirement (but simpler to implement) than REQ2. It
shall also be possible to send unauthenticated CoAP
group communications for low value data.
SEC-REQ 4: Group key management: There shall be a secure mechanism
to manage the cryptographic keys (e.g. generation and
distribution) belonging to the group; the state (e.g.
current membership) associated with the keys; and other
security parameters.
SEC-REQ 5: Use of Multicast IPSec: The CoAP protocol
[I-D.ietf-core-coap] allows IPSec to be used as one
option to secure CoAP. If IPSec is used as a way to
security CoAP communications, then multicast IPSec
[RFC5374] should be used for securing CoAP group
communications.
SEC-REQ 6: Independence from underlying routing security: CoAP
group communication security shall not be tied to the
security of underlying routing and distribution
protocols such as PIM [RFC4601] and RPL [RFC6550].
Insecure or inappropriate routing (including IP
multicast routing) may cause loss of data to CoAP but
will not affect the authenticity or secrecy of CoAP
group communications.
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SEC-REQ 7: Interaction with HTTPS: The security scheme for CoAP
group communications shall account for the fact that it
may need to interact with HTTPS (Hypertext Transfer
Protocol Secure) when a transaction involves a node in
the general Internet (non-constrained network)
communicating via a HTTP-CoAP proxy.
3. Group Communication Solutions
This section includes the text that describes the solutions of IP
multicast, overlay multicast, and application layer group
communication which were removed from [I-D.rahman-core-groupcomm]
version 07 when the text was transferred to
[I-D.ietf-core-groupcomm].
3.1. IP Multicast Transmission Methods
3.1.1. Serial unicast
Even in systems that generally support IP Multicast, there may be
certain data links (or transports) that don't support IP multicast.
For those links a serial unicast alternative must be provided. This
implies that it should be possible to enumerate the members of a
group, in order to determine the correct unicast destinations.
3.1.2. Unreliable IP Multicast
The CoRE WG charter specified support for non-reliable IP multicast.
In the current CoAP protocol design [I-D.ietf-core-coap], unreliable
multicast is realized by the source sending Non-Confirmable messages
to a multicast IP address. IP Multicast (using UDP) in itself is
unreliable, unless specific reliability features are added to it.
3.1.3. Reliable IP Multicast
[TBD: This is a difficult problem. Need to investigate the benefits
of repeating MGET and MPUT requests (saturation) to get "Pretty Good
Reliability". Use the same MID or a new MID for repeated requests?
Carsten suggests the use of bloom filters to suppress duplicate
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)
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as per [I-D.ietf-core-coap] section 4.2. ]
Reliable multicast supports guaranteed delivery of messages to a
group of nodes. The following specifies the requirements as was
proposed originally in version 01 of [I-D.vanderstok-core-bc]:
o Validity - If sender sends a message, m, to a group, g, of
destinations, a path exists between sender and destinations, and
the sender and destinations are correct, all destinations in g
eventually receive m.
o Integrity - destination receives m at most once from sender and
only if sender sent m to a group including destination.
o Agreement - If a correct destination of g receives m, then all
correct destinations of g receive m.
o Timeliness - For real-time control of devices, there is a known
constant D such that if m is sent at time t, no correct
destination receives m after t+D.
There are various approaches to achieve reliability, such as
o Destination node sends response: a destination sends a CoAP
Response upon multicast Request reception (it SHOULD be a Non-
Confirmable response). The source node may retry a request to
destination nodes that did not respond in time with a CoAP
response.
o Route redundancy
o Source node transmits multiple times (destinations do not respond)
3.2. Overlay Multicast
An alternative group communication solution (to IP Multicast) is an
"overlay multicast" approach. We define an overlay multicast as one
that utilizes an infrastructure based on proxies (rather than an IP
router based IP multicast backbone) to deliver IP multicast packets
to end devices. MLD ([RFC3810]) has been selected as the basis for
multicast support by the ROLL working group for the RPL routing
protocol. Therefore, it is proposed that "IGMP/MLD Proxying"
[RFC4605] be used as a basis for an overlay multicast solution for
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.
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Thereafter, the CoAP (MLD) proxy would be responsible for delivering
any IP multicast message to the subscribed CoAP devices. This will
require modifications to the existing [RFC4605] functionality.
Note that the CoAP (MLD) proxy may or may not be connected to an
external IP multicast enabled backbone. The key function for the
CoAP (MLD) proxy is to distribute CoAP generated multicast packets
even in the absence of router support for multicast.
3.3. CoAP Application Layer Group Management
Another alternative solution (to IP Multicast and Overlay Multicast)
is to define CoAP application level group management primitives.
Thus, CoAP can support group management features without need for any
underlying IP multicast support.
Interestingly, such group management primitives could also be offered
even if there is underlying IP multicast support. This is useful
because IP multicast inherently does not support the concept of a
group with managed members, while a managed group may be required for
some applications.
The following group management primitives are in general useful:
o discover groups;
o query group properties (e.g. related resource descriptions);
o create a group;
o remove a group;
o add a group member;
o remove a group member;
o enumerate group members;
o security and access control primitives.
In this proposal a (at least one) CoAP Proxy node is responsible for
group membership management. A constrained node can specify which
group it intends to join (or leave) using a CoAP request to the
appropriate CoAP Proxy. To Join, the group name will be included in
optional request header fields (explained below). These header
fields will be included in a PUT request to the Proxy. The Proxy-URI
is set to the Group Management URI of the Proxy (found previously
through the "/.well-known/" resource discovery mechanism). Note that
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in this solution also CoAP Proxies may exist in a network that are
not capable of CoAP group operations.
Group names may be defined as arbitrary strings with a predefined
maximum length (e.g. 268 characters or the maximum string length in a
CoAP Option), or as URIs.
[ TBD: how can a client send a request to a group? Does it only need
to know the group name (string or URI) or also an IP multicast
address? One way is to send a CoAP request to the CoAP Proxy with a
group URI directly in the Proxy-URI field. This avoids having to
know anything related to IP multicast addresses. ]
This solution in principle supports both unreliable and reliable
group communication. A client would indicate unreliable
communication by sending a CoAP Non-Confirmable request to the CoAP
Proxy, or reliable communication by sending a CoAP Confirmable
request.
It is proposed that CoAP supports two Header Options for group "Join"
and "Leave". These Options are Elective so they should be assigned
an even number. Assuming the Type for "join" is x (value TBD), the
Header Options are illustrated by the table in Figure 1:
+------+-----+---------------+--------------+--------+--------------+
| Type | C/E | Name | Data type | Length | Default |
|------+-----+---------------+--------------+--------+--------------+
| | | | | | |
| x | E | Group Join | String | 1-270 | "" |
| | | | | B | |
| x+2 | E | Group Leave | String | 1-270 | "" |
| | | | | B | |
+------+-----|---------------+--------------+--------+--------------+
Figure 1: CoAP Header Options for Group Management
Figure 2 illustrates how a node can join or leave a group using the
Header Options in a CoAP message:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | OC | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delta |length | Join Group A (ID or URI)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |length | Join Group B (ID or URI)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 |length | Leave Group C (ID or URI)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: CoAP Message for Group Management
Header Fields for the above example:
Ver: 2-bit unsigned integer for CoAP Version. Set to 1 by
implementation as defined by the CoAP specification.
T: 2-bit unsigned integer for CoAP Transaction Type. Either '0'
Confirmation or '1' Non-Confirmable can be used for group "join" or
"leave" request.
OC: 4-bit unsigned integer for Option Count. For this example, the
value should be "3" since there are three option fields.
Code: 8-bit unsigned integer to indicate the Method in a Request or a
Response Code in a Response message. Any Code can be used so the
group management can be piggy-backed in either Request or Response
message.
Message ID: 16-bit value assigned by the source to uniquely identify
a pair of Request and Response.
CoAP defines a delta encoding for header options. The first delta is
the "Type" for group join in this specific example. If the type for
group join is x as illustrated in Figure 2, delta will be x. In the
second header option, it is also a group join so the delta is 0. The
third header option is a group leave so the delta is 2.
An alternative solution to using Header Options (explained above) is
to use designated parameters in the query part of the URI in the
Proxy-URI field of a POST (TBD: or PUT?) request to a Proxy's group
management service resource advertized by DNS-SD. For example, to
join group1 and leave group2:
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coap://proxy1.bld2.example.com/groupmgt?j=group1&l=group2
4. DNS-SD Based Group Resource Manipulation
Ideally, all nodes in a given group (defined by its multicast IP
address) must receive the same request with high probability. This
will not be the case if there is diversity in the authority port
(i.e. a diversity of dynamic port addresses across the group) or if
the targeted resource is located at different paths on different
nodes. Extending the definition of group membership to include port
and path discovery is not desirable.
Therefore, some measures must be present to ensure uniformity in port
number and resource name/location within a group.
A first solution in this respect is to couple groups to service
descriptions in DNS (using DNS-SD as in [I-D.vanderstok-core-bc]). A
service description for a multicast group may have a TXT record in
DNS defining a schema X (e.g. "schema=DALI"), which defines by
service standard X (e.g. "DALI") which resources a node supporting X
MUST have. Therefore a multicast source can safely refer to all
resources with corresponding operations as prescribed by standard X.
For port numbers (which can be found using DNS-SD also) the same
holds. Alternatively, only the default CoAP port may be used in all
CoAP multicast requests.
5. Deployment Guidelines
5.1. Overview
We recommend to use IP multicast as the base solution for CoAP Group
Communication, provided that the use case and network characteristics
allow this. It has the advantage that it re-uses the IP multicast
suite of protocols and can operate even if group members are
distributed over both constrained and un-constrained network
segments. Still, this approach may require specifying or
implementing additional IP Multicast functionality in an LLN, in a
backbone network, or in both - this will be evaluated in more detail
in this section.
5.2. Implementation in Target Network Topologies
This section looks in more detail how an IP Multicast based solution
can be deployed onto the various network topologies that we consider
important for group communication use cases. Note that the chosen
solution of IP Multicast for CoAP group communication works mostly
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independently from the underlying network topology and its specific
IP multicast implementation.
Starting from the simplest case of a single LLN topology, we move to
more complex topologies involving a backbone network or multiple
LLNs. With "backbone" we refer here typically to a corporate LAN or
VLAN, which constitutes a single broadcast domain by design. It
could also be an in-home network. A multi-link backbone is also
possible, if there is proper IP multicast routing or forwarding
configured between these links. (The term 6LoWPAN Border Router or
"6LBR" is used here for a border router, though our evaluation is not
necessarily restricted to 6LoWPAN networks.)
5.2.1. Single LLN Topology
The simplest topology is a single LLN, where all the IP multicast
source(s) and destinations are constrained nodes within this same
LLN. Possible implementations of IP multicast routing and group
administration for this topology are listed below.
5.2.1.1. 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 throughout the LLN.
Because conceptually all nodes in the LLN are attached to a single
link, there is in principle no need for nodes to announce their
interest in multicast IP addresses via MLD (see Appendix A). A
multicast message to a specific IP destination, which is delivered to
all 6LoWPAN nodes by the mesh routing algorithm, is accepted by the
IP network layer of that node only if it is listening on that
specific multicast IP address and port.
5.2.1.2. RPL Multicast Routing
The RPL routing protocol for LLNs provides support for routing to
multicast IP destinations (Section 12 of [RFC6550]). Like regular
unicast destinations, multicast destinations are advertised by nodes
using RPL DAO messages. This functionality requires "Storing mode
with multicast support" (Mode Of Operation, MOP is 3) in the RPL
network.
Once all RPL routing tables in the network are populated, any RPL
node can send packets to an IP multicast destination. The RPL
protocol performs distribution of multicast packet both upward
towards the DODAG root and downwards into the DODAG.
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The text in Section 12 of the RPL specification clearly implies that
IP multicast packets are distributed using link-layer unicast
transmissions, looking at the use of the word "copied" in this
section. Specifically in 6LoWPAN networks, this behavior conflicts
with the requirement that IP multicast packets MUST be carried as
link-layer 802.15.4 broadcast frames [RFC4944].
Assuming that link-layer unicast is indeed meant, this approach seems
efficient only in a balanced, sparse tree network topology, or in
situations where the fraction of nodes listening to a specific
multicast IP address is low, or in duty cycled LLNs where link-layer
broadcast is a very expensive operation.
5.2.1.3. RPL Routers with Non-RPL Hosts
Now we consider the case that hosts exist in a RPL network that are
not RPL-aware themselves, but rely on RPL routers for their IP
connectivity beyond link-local scope. Note that the current RPL
specification [RFC6550] leaves this case for future specification
(see Section 16.4). Non-RPL hosts cannot advertise their IP
multicast groups of interest via RPL DAO messages as defined above.
Therefore in that case MLD could be used for such advertisements
(State Change Report messages), with all or a subset of RPL routers
acting in the role of MLD Routers as defined in [RFC3810]. However,
as the MLD protocol is not designed specifically for LLNs it may be a
burden for the constrained RPL router nodes to run the full MLD
protocol. Alternatives are therefore proposed in Section 5.3.1.
5.2.1.4. 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.
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Hosts that receive multicast packets they are not interested in, will
discard multicast packets that are not of interest. Above properties
seem to make Trickle especially efficient for cases where the
multicast listener density is high and the number of distinct
multicast groups relatively low.
5.2.1.5. 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 advertise their interest in specific multicast addresses, or
not. If they do, MLD is a possible option but also protocol-specific
means (as in RPL) is an option. See Section 5.3.1 for more efficient
substitutes for MLD targeted towards a LLN context.
5.2.2. Single LLN with Backbone Topology
A LLN may be connected via a Border Router (e.g. 6LBR) to a backbone
network, on which IP multicast listeners and/or sources may be
present. This section analyzes cases in which IP multicast traffic
needs to flow from/to the backbone, to/from the LLN.
5.2.2.1. 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 learned from previous MLD Reports received on its
backbone link.
Conversely, multicast packets originating on the backbone network
will reach the 6LBR if either the backbone is a single link (LAN/
VLAN) or IPv6 multicast routing is enabled on the backbone. Then,
the 6LBR could simply forward all IP multicast traffic from the
backbone onto the LLN. However, in practice this situation may lead
to overload of the LLN caused by unnecessary multicast traffic.
Therefore the 6LBR SHOULD only forward traffic that one or more nodes
in the LLN have expressed interest in, effectively filtering inbound
LLN multicast traffic.
To realize this "filter", nodes on the LLN may use MLD to announce
their interest in specific multicast IP addresses to the 6LBR. One
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option is for the 6LBR to act in an MLD Router role on its LLN
interface. However, this may be too much of a "burden" for
constrained nodes. Light-weight alternatives for MLD are discussed
in Section 5.3.1.
5.2.2.2. 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.
5.2.2.3. 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 5.2.1.3.
5.2.2.4. 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 5.2.2.1.
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 5.2.2.1) of potentially overloading
the LLN with unwanted backbone IP multicast traffic appears again.
A possible solution to this is (again) to let multicast listeners
advertise their interest using MLD as described in Section 5.2.2.1 or
to use an MLD alternative suitable for LLNs as described in
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Section 5.3.1. However, following this approach requires possibly an
extension to Trickle Multicast Forwarding: the protocol should ensure
that MLD-advertised information is somehow communicated to the 6LBR,
possibly over multiple hops. MLD itself supports link-local
communication only.
5.2.2.5. 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.
5.2.3. Multiple LLNs with Backbone Topology
Now the case of a single backbone network with two or more LLNs
attached to it via 6LBRs is considered. For this case all the
considerations and solutions of the previous section can be applied.
For the specific case that a source on a backbone network has to send
to a very large number of destination located on many LLNs, the use
of IGMP/MLD Proxying [RFC4605] with a leaf IGMP/MLD Proxy located in
each 6LBR may be useful. This method only is defined for a tree
topology backbone network with the IP multicast source at the root of
the tree.
5.2.4. LLN(s) with Multiple 6LBRs
[ TBD: an LLN with multiple 6LBRs may require some additional
consideration. Any need to synchronize mutually on multicast
listener information? ]
5.2.5. Conclusions
For all network topologies that were evaluated, CoAP group
communication can be in principle supported with IP Multicast, making
use of existing protocols. For the case of Trickle Multicast
Forwarding, it appears that an addition to the protocol is required
such that information about multicast listeners can be distributed
towards the 6LBR. Opportunities were identified for an "MLD-like" or
"MLD-lightweight" protocol specifically suitable for LLNs, which
should inter-work with regular MLD on the backbone network. Such MLD
variants are further analyzed in Section 5.3.1.
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5.3. Implementation Considerations
In this section various implementation aspects are considered such as
required protocol implementations, additional functionality of the
6LBR and backbone network equipment.
5.3.1. MLD Implementation on LLNs and MLD alternatives
In previous sections, it was mentioned that the MLDv2 protocol
[RFC3810] may be too costly for use in a LLN. MLD relies on periodic
link-local multicast operations to maintain state. Also it is
optimized to fairly dynamic situations where multicast listeners may
come and go over time. Such dynamic situations are less frequently
found in typical LLN use cases such as building control, where
multicast group membership can remain constant over longer periods of
time (e.g. months) after commissioning.
Hence, a viable strategy is to implement a subset of MLD
functionality in 6LoWPAN nodes which is just enough for the required
functionality. A first option is that 6LoWPAN Routers, like MLD
Snoopers, passively listen to MLD State Change Report messages and
handle the learned ("snooped") IP multicast destinations in the way
defined by the multicast routing protocol they are running (e.g. for
RPL, Routers advertise these destinations using DAO messages).
A second option is to use MLD as-is but adapt the recommended
parameter values such that operation on a LLN becomes more efficient.
[RFC6636] could be a guideline in this case.
A third option is to standardize a new protocol, taking a subset of
MLD functionality into a "MLD for 6LoWPAN" protocol to support
constrained nodes optimally.
A fourth option is now presented, which seems attractive in that it
minimizes standardization, implementation and network communication
overhead all at the same time. This option is to specify a new
Multicast Listener Option (MLO) as an addition to the 6LoWPAN-ND
[I-D.ietf-6lowpan-nd] protocol communication that is anyway ongoing
between a 6LoWPAN host and router(s). This MLO is preferably
designed to be maximally similar to the Address Registration Option
(ARO), which minimizes the need for additional program code on
constrained nodes. With an MLO, instead of registering a hosts's
unicast IP address as with ARO, a host "registers" its interest in a
multicast IPv6 address. Unlike the ARO, multiple MLO can be used in
the same ND packet. A registration period is also defined in the MLO
just like in the ARO. MLO allows a host to persistently register as
a listener to IP multicast traffic and to avoid the overhead of
periodic multicast communication which is required for the regular
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MLD protocol.
[ TBD: consider what aspects are needed/not needed for CoAP/LLN
applications. Will MLDv1 suffice? What to do with options like
'source specific' and include/exclude. Source-specific can also be
dealt with at the destination host by filtering? Do we need limits
on number of records per packet? Do we need a higher MLD reliability
setting - see the parameters in the MLD RFC ]
5.3.2. 6LBR Implementation
To support mixed backbone/LLN scenarios in CoAP group communication,
it is RECOMMENDED that a 6LowPAN Border Router (6LBR) will act in an
MLD Router role on the backbone link. If this is not possible then
the 6LBR SHOULD be configured to act as an MLD Multicast Address
Listener and/or MLD Snooper on the backbone link.
5.3.3. Backbone IP Multicast Infrastructure
For corporate/professional applications, most routing and switching
equipment that is currently on the market is IPv6 capable. For that
reason backbone infrastructure operating IPv4 only is considered out
of scope in this document, at least for the backbone network
segment(s) where IP multicast destinations are present. What is
still in scope is for example an IPv4-only HTTP client that wants to
send a group communication message via a HTTP-CoAP proxy as
considered in [I-D.castellani-core-advanced-http-mapping].
The availability of, and requirements for, IP multicast support may
depend on the specific installation use case. For example, the
following cases may be relevant for new IP based building control
installations:
1. System deployed on existing IP (Ethernet/WiFi/...)
infrastructure, shared with existing IP devices (PCs)
2. Newly designed and deployed IP (Ethernet/WiFi/...)
infrastructure, to be shared with other IP devices (PCs)
3. Newly designed and deployed IP (Ethernet/WiFi/...)
infrastructure, exclusively used for building control.
Besides physical separation the building control backbone can be
separated from regular (PC) infrastructure by using a different VLAN.
A typical corporate installation will have many LAN switches and/or
routing switches, which pass through IP multicast traffic but on the
other hand do not support acting in the Router role of MLD/IGMP.
Perhaps for case 2) and 3) above it is acceptable to add a MLD/IGMP
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capable router somewhere in the network, while for case 1) this may
not be the case.
[TBD: consider the influence of WiFi based backbone networks. What
if 6LBRs are at the same time also WiFi routers? What if 6LBRs have
an Ethernet connection to legacy WiFI routers? Check if equivalent
with Ethernet backbone.]
6. Miscellaneous Topics
This section is a placeholder to add miscellaneous text, topics or
proposals related to CoAP group communication in future versions of
this document.
7. Acknowledgements
Thanks to all CoRE WG members who participated in the IETF 82
discussions, which was the trigger to initiate this document.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
Security aspects of group communication for CoAP are discussed in
[I-D.ietf-core-groupcomm]. The current document contains no new
proposals yet, for which security considerations have to be analyzed
here.
10. References
10.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-12 (work in progress), October 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
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Architecture", RFC 3740, March 2004.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC4046] Baugher, M., Canetti, R., Dondeti, L., and F. Lindholm,
"Multicast Security (MSEC) Group Key Management
Architecture", RFC 4046, April 2005.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, August 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
[RFC6636] Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
the Internet Group Management Protocol (IGMP) and
Multicast Listener Discovery (MLD) for Routers in Mobile
and Wireless Networks", RFC 6636, May 2012.
10.2. Informative References
[I-D.castellani-core-advanced-http-mapping]
Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Best Practices for HTTP-CoAP Mapping
Implementation",
draft-castellani-core-advanced-http-mapping-00 (work in
progress), July 2012.
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[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-nd]
Shelby, Z., Chakrabarti, S., and E. Nordmark, "Neighbor
Discovery Optimization for Low Power and Lossy Networks
(6LoWPAN)", draft-ietf-6lowpan-nd-21 (work in progress),
August 2012.
[I-D.ietf-6lowpan-routing-requirements]
Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
Statement and Requirements for 6LoWPAN Routing",
draft-ietf-6lowpan-routing-requirements-10 (work in
progress), November 2011.
[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-02 (work in progress),
July 2012.
[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Forwarding Using
Trickle", draft-ietf-roll-trickle-mcast-01 (work in
progress), July 2012.
[I-D.rahman-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-rahman-core-groupcomm-07 (work in progress),
October 2011.
[I-D.shelby-core-coap-req]
Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
Kelsey, "CoAP Requirements and Features",
draft-shelby-core-coap-req-02 (work in progress),
October 2010.
[I-D.vanderstok-core-bc]
Stok, P. and K. Lynn, "CoAP Utilization for Building
Control", draft-vanderstok-core-bc-05 (work in progress),
October 2011.
Appendix A. Multicast Listener Discovery (MLD)
In order to extend the scope of IP multicast beyond link-local scope,
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an IP multicast routing protocol has to be active in routers on an
LLN. To achieve efficient multicast routing (i.e. avoid always
flooding multicast IP packets), routers have to learn which hosts
need to receive packets addressed to specific IP multicast
destinations.
The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its
IPv4 pendant IGMP) is today the method of choice used by an (IP
multicast enabled) router to discover the presence of multicast
listeners on directly attached links, and to discover which multicast
addresses are of interest to those listening nodes. MLD was
specifically designed to cope with fairly dynamic situations in which
multicast listeners may join and leave at any time.
IGMP/MLD Snooping is a technique implemented in some corporate LAN
routing/switching devices. An MLD snooping switch listens to MLD
State Change Report messages from MLD listeners on attached links.
Based on this, the switch learns on what LAN segments there is
interest for what IP multicast traffic. If the switch receives at
some point an IP multicast packet, it uses the stored information to
decide onto which LAN segment(s) to send the packet. This improves
network efficiency compared to the regular behavior of forwarding
every incoming multicast packet onto all LAN segments. An MLD
snooping switch may also send out MLD Query messages (which is
normally done by a device in MLD Router role) if no MLD Router is
present.
[RFC6636] discusses optimal tuning of the parameters of MLD for
routers for mobile and wireless networks. These guidelines may be
useful when implementing MLD in LLNs.
Authors' Addresses
Esko Dijk (editor)
Philips Research
Email: esko.dijk@philips.com
Akbar Rahman (editor)
InterDigital Communications, LLC
Email: Akbar.Rahman@InterDigital.com
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