ROLL J. Hui
Internet-Draft Cisco
Intended status: Standards Track R. Kelsey
Expires: July 28, 2013 Silicon Labs
January 24, 2013
Multicast Protocol for Low power and Lossy Networks (MPL)
draft-ietf-roll-trickle-mcast-03
Abstract
This document specifies the Multicast Protocol for Low power and
Lossy Networks (MPL) that provides IPv6 multicast forwarding in
constrained networks. MPL avoids the need to construct or maintain
any multicast forwarding topology, disseminating messages to all MPL
forwarders in an MPL domain. MPL uses the Trickle algorithm to
manage message transmissions for both control and data-plane
messages. Different Trickle parameter configurations allow MPL to
trade between dissemination latency and transmission efficiency.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 28, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Information Base Overview . . . . . . . . . . . . . . . . 6
4.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Signaling Overview . . . . . . . . . . . . . . . . . . . . 7
5. MPL Constants . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Multicast Addresses . . . . . . . . . . . . . . . . . . . 9
5.2. Message Types . . . . . . . . . . . . . . . . . . . . . . 9
5.3. MPL Forwarder Parameters . . . . . . . . . . . . . . . . . 9
5.4. Trickle Parameters . . . . . . . . . . . . . . . . . . . . 9
6. Protocol Message Formats . . . . . . . . . . . . . . . . . . . 11
6.1. MPL Option . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. MPL Control Message . . . . . . . . . . . . . . . . . . . 12
6.3. MPL Seed Info . . . . . . . . . . . . . . . . . . . . . . 13
7. Information Base . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Local Interface Set . . . . . . . . . . . . . . . . . . . 15
7.2. Domain Set . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Seed Set . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.4. Buffered Message Set . . . . . . . . . . . . . . . . . . . 15
8. MPL Domains . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. MPL Seed Sequence Numbers . . . . . . . . . . . . . . . . . . 18
10. MPL Data Messages . . . . . . . . . . . . . . . . . . . . . . 19
10.1. MPL Data Message Generation . . . . . . . . . . . . . . . 19
10.2. MPL Data Message Transmission . . . . . . . . . . . . . . 19
10.3. MPL Data Message Processing . . . . . . . . . . . . . . . 20
11. MPL Control Messages . . . . . . . . . . . . . . . . . . . . . 22
11.1. MPL Control Message Generation . . . . . . . . . . . . . . 22
11.2. MPL Control Message Transmission . . . . . . . . . . . . . 22
11.3. MPL Control Message Processing . . . . . . . . . . . . . . 23
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
13.1. MPL Option Type . . . . . . . . . . . . . . . . . . . . . 26
13.2. MPL ICMPv6 Type . . . . . . . . . . . . . . . . . . . . . 26
13.3. Well-known Multicast Addresses . . . . . . . . . . . . . . 26
14. Security Considerations . . . . . . . . . . . . . . . . . . . 27
15. Normative References . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
Low power and Lossy Networks typically operate with strict resource
constraints in communication, computation, memory, and energy. Such
resource constraints may preclude the use of existing IPv6 multicast
routing and forwarding mechanisms. Traditional IP multicast delivery
typically relies on topology maintenance mechanisms to discover and
maintain routes to all subscribers of a multicast group. However,
maintaining such topologies in LLNs is costly and may not be feasible
given the available resources.
Memory constraints may limit devices to maintaining links/routes to
one or a few neighbors. For this reason, the Routing Protocol for
LLNs (RPL) specifies both storing and non-storing modes [RFC6550].
The latter allows RPL routers to maintain only one or a few default
routes towards a LLN Border Router (LBR) and use source routing to
forward messages away from the LBR. For the same reasons, a LLN
device may not be able to maintain a multicast routing topology when
operating with limited memory.
Furthermore, the dynamic properties of wireless networks can make the
cost of maintaining a multicast routing topology prohibitively
expensive. In wireless environments, topology maintenance may
involve selecting a connected dominating set used to forward
multicast messages to all nodes in an administrative domain.
However, existing mechanisms often require two-hop topology
information and the cost of maintaining such information grows
polynomially with network density.
This document specifies the Multicast Protocol for Low power and
Lossy Networks (MPL), which provides IPv6 multicast forwarding in
constrained networks. MPL avoids the need to construct or maintain
any multicast routing topology, disseminating multicast messages to
all MPL forwarders in an MPL domain. By using the Trickle algorithm
[RFC6206], MPL requires only small, constant state for each MPL
device that initiates disseminations. The Trickle algorithm also
allows MPL to be density-aware, allowing the communication rate to
scale logarithmically with density.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The following terms are used throughout this document:
MPL Forwarder - A router that implements this protocol. A MPL
Forwarder is equipped with at least one MPL
Interface.
MPL Interface - An MPL Forwarder's attachment to a
communications medium, over which it transmits
and receives MPL Data Messages and MPL Control
Messages according to this specification. An MPL
Interface is assigned one or more unicast
addresses and is subscribed to one or more MPL
Domain Addresses.
MPL Domain Address - A multicast address that identifies the set of
MPL Interfaces within an MPL Domain. MPL Data
Messages disseminated in an MPL Domain have the
associated MPL Domain Address as their
destination address.
MPL Domain - A scope zone, as defined in [RFC4007], in which
MPL Interfaces subscribe to the same MPL Domain
Address and participate in disseminating MPL Data
Messages.
MPL Data Message - A multicast message that is used to communicate
a multicast payload between MPL Forwarders and
contains an MPL Option in the IPv6 header. A MPL
Data Message has its destination address set to
the MPL Domain Address.
MPL Control Message - A link-local multicast message that is used to
communicate information about recently received
MPL Data Messages to neighboring MPL Forwarders.
MPL Seed - An MPL Forwarder that generates MPL Data
Messages and serves as an entry point into an MPL
Domain.
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3. Applicability Statement
This protocol is an IPv6 multicast forwarding protocol for Low-Power
and Lossy Networks. By implementing a controlled dissemination using
the Trickle algorithm, this protocol is designed for networks that
communicate using low-power and lossy links with widely varying
topologies in both the space and time dimensions.
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4. Protocol Overview
The goal of MPL is to deliver multicast messages to all interfaces
that subscribe to the multicast messages' destination address within
an MPL Domain.
4.1. Information Base Overview
A node records necessary protocol state in the following information
sets:
o The Local Interface Set records the set of local MPL Interfaces
and the unicast addresses assigned to those MPL Interfaces.
o The Domain Set records the set of MPL Domain Addresses and the
local MPL Interfaces that subscribe to those addresses.
o The Seed Set records information about received MPL Data Messages
received from an MPL Seed. The Seed Set maintains the minimum
sequence number that the MPL Forwarder is willing to receive or
has buffered in its Buffered Message Set. MPL uses the Seed Set
and Buffered Message Set to determine when to accept an MPL Data
Message, process its payload, and retransmit it.
o The Buffered Message Set records recently received MPL Data
Messages from an MPL Seed. MPL Data Messages resident in the
Buffered Message Set have sequence numbers that are greater than
or equal to the minimum threshold maintained in the Seed Set. MPL
uses the Buffered Message Set to store MPL Data Messages that may
be transmitted by the MPL Forwarder for forwarding.
4.2. Overview
MPL achieves its goal by implementing a controlled flood that
attempts to disseminate the multicast data message to all interfaces
within an MPL Domain. MPL performs the following tasks to
disseminate a multicast message:
o When having a multicast message to forward into an MPL Domain, the
MPL Seed generates an MPL Data Message that includes the MPL Seed
Identifier, a newly generated sequence number, and the multicast
message. If the multicast destination address is not the MPL
Domain Address, IP-in-IP [RFC2473] is used to encapsulate the
multicast message in the MPL Data Message.
o Upon receiving an MPL Data Message, the MPL Forwarder extracts the
MPL Seed and sequence number and determines whether or not the MPL
Data Message was previously received using the Seed Set and
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Buffered Message Set.
* If the sequence number is less than the lower-bound sequence
number maintained in the Seed Set or a message with the same
sequence number exists within the Buffered Message Set, the MPL
Forwarder marks the MPL Data Message as old.
* Otherwise, the MPL Forwarder marks the MPL Data Message as new.
o For each newly received MPL Data Message, an MPL Forwarder updates
the Seed Set, adds the MPL Data Message into the Buffered Message
Set, processes its payload, and multicasts the MPL Data Message a
number of times on all MPL Interfaces participating in the same
MPL Domain to forward the message.
o Each MPL Forwarder may periodically link-local multicast MPL
Control Messages on MPL Interfaces to communicate information
contained in the MPL Forwarder's Seed Set and Buffered Message
Sets.
o Upon receiving an MPL Control Message, an MPL Forwarder determines
whether there are any new MPL Data Messages that have yet to be
received by the MPL Control Message's source and multicasts those
MPL Data Messages.
MPL's configuration parameters allow two forwarding strategies for
disseminating MPL Data Messages.
Proactive Forwarding - With proactive forwarding, an MPL Forwarder
schedules transmissions of MPL Data Messages using the Trickle
algorithm, without any prior indication that neighboring nodes
have yet to receive the message. After transmitting the MPL Data
Message a limited number of times, the MPL forwarder may terminate
proactive forwarding for the MPL Data Message message.
Reactive Forwarding - With reactive forwarding, an MPL Forwarder
link-local multicasts MPL Control Messages using the Trickle
algorithm [RFC6206]. MPL Forwarders use MPL Control Messages to
discover new MPL Data Messages that have not yet been received.
When discovering that a neighboring MPL Forwarder has not yet
received a new MPL Data Message, the MPL Forwarder schedules those
MPL Data Messages for transmission using the Trickle algorithm.
4.3. Signaling Overview
This protocol generates and processes the following messages:
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MPL Data Message - Generated by an MPL Seed to deliver a multicast
message across an MPL Domain. The MPL Data Message's source is an
address in the Local Interface Set of the MPL Seed that generated
the message and is valid within the MPL Domain. The MPL Data
Message's destination is the MPL Domain Address corresponding to
the MPL Domain. An MPL Data Message contains:
* The Seed Identifier of the MPL Seed that generated the MPL Data
Message.
* The sequence number of the MPL Seed that generated the MPL Data
Message.
* The original multicast message.
MPL Control Message - Generated by an MPL Forwarder to communicate
information contained in the Seed Set and Buffered Message Set to
neighboring MPL Forwarders. An MPL Control Message contains a
list of tuples for each entry in the Seed Set. Each tuple
contains:
* The minimum sequence number maintained in the Seed Set for the
MPL Seed.
* A bit-vector indicating the sequence numbers of MPL Data
Messages resident in the Buffered Message Set for the MPL Seed,
where the first bit represents a sequence number equal to the
minimum threshold maintained in the Seed Set.
* The length of the bit-vector.
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5. MPL Constants
This section describes various program and networking constants used
by MPL.
5.1. Multicast Addresses
MPL makes use of MPL Domain Addresses to identify MPL Interfaces of
an MPL Domain. By default, MPL Forwarders subscribe to the
ALL_MPL_FORWARDERS multicast address with a scope value of 3 (subnet-
local).
For each MPL Domain Address that an MPL Interface subscribes to, the
MPL Interface MUST also subscribe to the MPL Domain Address with a
scope value of 2 (link-local) when reactive forwarding is in use.
MPL Forwarders use the link-scoped MPL Domain Address to communicate
MPL Control Messages to neighboring (i.e. on-link) MPL Forwarders.
5.2. Message Types
MPL defines an IPv6 Option for carrying an MPL Seed Identifier and a
sequence number within an MPL Data Message. The IPv6 Option Type has
value MPL_OPT_TYPE.
MPL defines an ICMPv6 Message (MPL Control Message) for communicating
information contained in its Seed Set and Buffered Message Set to
neighboring MPL Forwarders. The MPL Control Message has ICMPv6 Type
MPL_ICMP_TYPE.
5.3. MPL Forwarder Parameters
PROACTIVE_FORWARDING A boolean value that indicates whether the MPL
Forwarder should schedule MPL Data Message transmissions after
receiving them for the first time.
SEED_SET_LIFETIME The minimum lifetime for an entry in the Seed Set.
5.4. Trickle Parameters
As specified in [RFC6206], a Trickle timer runs for a defined
interval and has three configuration parameters: the minimum interval
size Imin, the maximum interval size Imax, and a redundancy constant
k.
This specification defines a fourth Trickle configuration parameter,
TimerExpirations, which indicates the number of Trickle timer
expiration events that occur before terminating the Trickle
algorithm.
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Each MPL forwarder maintains a separate Trickle parameter set for MPL
Data Message and MPL Control Message transmissions. The Trickle
parameters are listed below:
DATA_MESSAGE_IMIN The minimum Trickle timer interval, as defined in
[RFC6206], for MPL Data Message transmissions.
DATA MESSAGE_IMAX The maximum Trickle timer interval, as defined in
[RFC6206], for MPL Data Message transmissions.
DATA_MESSAGE_K The redundancy constant, as defined in [RFC6206], for
MPL Data Message transmissions.
DATA_MESSAGE_TIMER_EXPIRATIONS The number of Trickle timer
expirations that occur before terminating the Trickle algorithm
for MPL Data Message transmissions.
CONTROL_MESSAGE_IMIN The minimum Trickle timer interval, as defined
in [RFC6206], for MPL Control Message transmissions.
CONTROL_MESSAGE_IMAX The maximum Trickle timer interval, as defined
in [RFC6206], for MPL Control Message transmissions.
CONTROL_MESSAGE_K The redundancy constant, as defined in [RFC6206],
for MPL Control Message transmissions.
CONTROL_MESSAGE_TIMER_EXPIRATIONS The number of Trickle timer
expirations that occur before terminating the Trickle algorithm
for MPL Control Message transmissions.
It is RECOMMENDED that all MPL Forwarder within an MPL Domain use the
same values for the Trickle Parameters above, as specified in
[RFC6206].
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6. Protocol Message Formats
The protocol messages generated and processed by an MPL Forwarder are
described in this section.
6.1. MPL Option
The MPL Option is carried in MPL Data Messages in an IPv6 Hop-by-Hop
Options header, immediately following the IPv6 header. The MPL
Option has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S |M|V| rsv | sequence | seed-id (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type MPL_OPT_TYPE
Opt Data Len Length of the Option Data field in octets.
S 2-bit unsigned integer. Identifies the length of
seed-id. 0 indicates that the seed-id is the IPv6
Source Address and not included in the MPL
Option. 1 indicates that the seed-id is a 16-bit
unsigned integer. 2 indicates that the seed-id is
a 64-bit unsigned integer. 3 indicates that the
seed-id is a 128-bit unsigned integer.
M 1-bit flag. 1 indicates that the value in
sequence is known to be the largest sequence
number that was received from the MPL Seed.
V 1-bit flag. 0 indicates that the MPL Option
conforms to this specification. MPL Options
received in which this flag is 1 MUST be dropped.
rsv 4-bit reserved field. MUST be set to 0 on
transmission and ignored on reception.
sequence 8-bit unsigned integer. Identifies relative
ordering of MPL Data Messages from the MPL Seed
identified by seed-id.
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seed-id Uniquely identifies the MPL Seed that initiated
dissemination of the MPL Data Message. The size
of seed-id is indicated by the S field.
The Option Data (in particular the M flag) of the MPL Option is
updated by MPL Forwarders as the MPL Data Message is forwarded.
Nodes that do not understand the MPL Option MUST discard the MPL Data
Message. Thus, according to [RFC2460] the three high order bits of
the Option Type are set to '011'. The Option Data length is
variable.
The seed-id uniquely identifies an MPL Seed. When seed-id is 128
bits (S=3), the MPL seed MAY use an IPv6 address assigned to one of
its interfaces that is unique within the MPL domain. Managing MPL
Seed Identifiers is not within scope of this document.
The sequence field establishes a total ordering of MPL Data Messages
generated by an MPL Seed for an MPL Domain. The MPL Seed MUST
increment the sequence field's value on each new MPL Data Message
that it generates for an MPL Domain. Implementations MUST follow the
Serial Number Arithmetic as defined in [RFC1982] when incrementing a
sequence value or comparing two sequence values.
Future updates to this specification may define additional fields
following the seed-id field.
6.2. MPL Control Message
An MPL Forwarder uses ICMPv6 messages to communicate information
contained in its Seed Set and Buffered Message Set to neighboring MPL
Forwarders. The MPL Control Message has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. MPL Seed Info[1..n] .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
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Source Address A link-local address assigned to the sending
interface.
Destination Address The link-scoped MPL Domain Address corresponding
to the MPL Domain.
Hop Limit 255
ICMPv6 Fields:
Type MPL_ICMP_TYPE
Code 0
Checksum The ICMP checksum. See [RFC4443].
MPL Seed Info[1..n] List of one or more MPL Seed Info entries.
The MPL Control Message indicates the sequence numbers of MPL Data
Messages that are within the Buffered Message Set. The MPL Control
Message also indicates the sequence numbers of MPL Data Messages that
an MPL Forwarder is willing to receive. The MPL Control Message
allows neighboring MPL Forwarders to determine whether there are any
new MPL Data Messages to exchange.
6.3. MPL Seed Info
An MPL Seed Info encodes the minimum sequence number for the MPL Seed
maintained in the Seed Set. The MPL Seed Info also indicates the
sequence numbers of MPL Data Messages generated by the MPL Seed
within the Buffered Message Set. The MPL Seed Info has the following
format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| min-seqno | bm-len | S | seed-id (0/2/8/16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. buffered-mpl-messages (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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min-seqno 8-bit unsigned integer. The lower-bound sequence
number for the MPL Seed.
bm-len 6-bit unsigned integer. The size of buffered-
mpl-messages in octets.
S 2-bit unsigned integer. Identifies the length of
seed-id. 0 indicates that the seed-id value is
the IPv6 Source Address and not included in the
MPL Seed Info. 1 indicates that the seed-id value
is a 16-bit unsigned integer. 2 indicates that
the seed-id value is a 64-bit unsigned integer. 3
indicates that the seed-id is a 128-bit unsigned
integer.
seed-id Variable-length unsigned integer. Indicates the
MPL Seed associated with this MPL Seed Info.
buffered-mpl-messages Variable-length bit vector. Identifies the
sequence numbers of MPL Data Messages maintained
in the Buffered Message Set for the MPL Seed.
The sequence number is determined by min-seqno +
i, where i is the bit offset within buffered-mpl-
messages.
The MPL Seed Info does not have any octet alignment requirement.
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7. Information Base
7.1. Local Interface Set
The Local Interface Set records the local MPL Interfaces of an MPL
Forwarder. The Local Interface Set consists of Local Interface
Tuples, one per MPL Interface: (AddressSet).
AddressSet - a set of unicast addresses assigned to the MPL
Interface.
7.2. Domain Set
The Domain Set records the MPL Interfaces that subscribe to each MPL
Domain Address. The Domain Set consists of MPL Domain Tuples, one
per MPL Domain: (MPLInterfaceSet).
MPLInterfaceSet - a set of MPL Interfaces that subscribe to the MPL
Domain Address that identifies the MPL Domain.
7.3. Seed Set
The Seed Set records a sliding window used to determine the sequence
numbers of MPL Data Messages that an MPL Forwarder is willing to
accept generated by the MPL Seed. It consists of MPL Seed Tuples:
(SeedID, MinSequence, Lifetime).
SeedID - the identifier for the MPL Seed.
MinSequence - a lower-bound sequence number that represents the
sequence number of the oldest MPL Data Message the MPL Forwarder
is willing to receive or transmit. An MPL Forwarder MUST ignore
any MPL Data Message that has sequence value less than than
MinSequence.
Lifetime - indicates the minimum lifetime of the Seed Set entry. An
MPL Forwarder MUST NOT free a Seed Set entry before its expires.
7.4. Buffered Message Set
The Buffered Message Set records recently received MPL Data Messages
from an MPL Seed. An MPL Forwarder uses the Buffered Message Set to
buffer MPL Data Messages while the MPL Forwarder is forwarding the
MPL Data Messages. The Buffered Message Set consists of Buffered
Message Tuples: (SeedID, SequenceNumber, DataMessage).
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SeedID - the identifier for the MPL Seed that generated the MPL Data
Message.
SequenceNumber - the sequence number for the MPL Data Message.
DataMessage - the MPL Data Message.
All MPL Data Messages within the Buffered Message Set MUST have a
sequence number greater than or equal to MinSequence for the
corresponding SeedID. When increasing MinSequence for an MPL Seed,
the MPL Forwarder MUST delete any MPL Data Messages from the Buffered
Message Set that have sequence numbers less than MinSequence.
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8. MPL Domains
An MPL Domain is a scope zone, as defined in [RFC4007], in which MPL
Interfaces subscribe to the same MPL Domain Address and participate
in disseminating MPL Data Messages.
By default, an MPL Forwarder MUST participate in an MPL Domain
identified by the ALL_MPL_FORWARDERS multicast address with a scope
value of 3 (subnet-local).
An MPL Forwarder MAY participate in additional MPL Domains identified
by other multicast addresses. An MPL Interface MUST subscribe to the
MPL Domain Addresses for the MPL Domains that it participates in.
The allocation of other multicast addresses is out of scope.
For each MPL Domain Address that an MPL Interface subscribes to, the
MPL Interface MUST also subscribe to the same MPL Domain Address with
a scope value of 2 (link-local) when reactive forwarding is in use
(i.e. when communicating MPL Control Messages).
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9. MPL Seed Sequence Numbers
Each MPL Seed maintains a sequence number for each MPL Domain that it
serves. The sequence numbers are included in MPL Data Messages
generated by the MPL Seed. The MPL Seed MUST increment the sequence
number for each MPL Data Message that it generates for an MPL Domain.
Implementations MUST follow the Serial Number Arithmetic as defined
in [RFC1982] when incrementing a sequence value or comparing two
sequence values. This sequence number is used to establish a total
ordering of MPL Data Messages generated by an MPL Seed for an MPL
Domain.
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10. MPL Data Messages
10.1. MPL Data Message Generation
MPL Data Messages are generated by MPL Seeds when they enter the MPL
Domain. All MPL Data messages have the following properties:
o The IPv6 Source Address MUST be an address in the AddressSet of a
corresponding MPL Interface and MUST be valid within the MPL
Domain.
o The IPv6 Destination Address MUST be set to the MPL Domain Address
corresponding to the MPL Domain.
o A MPL Data Message MUST contain an MPL Option in its IPv6 Header
to identify the MPL Seed that generated the message and the
ordering relative to other MPL Data Messages generated by the MPL
Seed.
When the source address is in the AddressList of an MPL Interface
corresponding to the MPL Domain Address and the destination address
is the MPL Domain Address, the application message and the MPL Data
Message MAY be identical. In other words, the MPL Data Message may
contain a single IPv6 header that includes the MPL Option.
Otherwise, IPv6-in-IPv6 encapsulation MUST be used to satisfy the MPL
Data Message requirements listed above [RFC2473]. The complete IPv6-
in-IPv6 message forms an MPL Data Message. The outer IPv6 header
conforms to the MPL Data Message requirements listed above. The
encapsulated IPv6 datagram encodes the multicast data message that is
communicated beyond the MPL Domain.
10.2. MPL Data Message Transmission
An MPL Forwarder manages transmission of MPL Data Messages in the
Buffered Message set using the Trickle algorithm [RFC6206]. An MPL
Forwarder MUST use a separate Trickle timer for each MPL Data Message
that it is actively forwarding. In accordance with Section 5 of RFC
6206 [RFC6206], this document defines the following:
o This document defines a "consistent" transmission as receiving an
MPL Data Message that has the same seed-id and sequence value as
the MPL Data Message managed by the Trickle timer.
o This document defines an "inconsistent" transmission as receiving
an MPL Data Message that has the same seed-id value and the M flag
set, but has a sequence value less than MPL Data Message managed
by the Trickle timer.
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o This document does not define any external "events".
o This document defines MPL Data Messages as Trickle messages.
o The actions outside the Trickle algorithm that the protocol takes
involve managing Seed Set and Buffered Message Set
As specified in [RFC6206], a Trickle timer has three variables: the
current interval size I, a time within the current interval t, and a
counter c. MPL defines a fourth variable, e, which counts the number
of Trickle timer expiration events since the Trickle timer was last
reset.
After DATA_MESSAGE_TIMER_EXPIRATIONS Trickle timer events, the MPL
Forwarder MUST disable the Trickle timer. When a buffered MPL Data
Message does not have an associated Trickle timer, the MPL Forwarder
MAY delete the message from the Buffered Message Set by advancing
MinSequence of the corresponding MPL Seed in the Seed Set. When the
MPL Forwarder no longer buffers any messages for an MPL Seed, the MPL
Forwarder MUST NOT increment MinSequence for that MPL Seed.
When transmitting an MPL Data Message, the MPL Forwarder MUST either
set the M flag to zero or set it to a level that indicates whether or
not the message's sequence number is the largest value that has been
received from the MPL Seed.
10.3. MPL Data Message Processing
Upon receiving an MPL Data Message, the MPL Forwarder first processes
the MPL Option and updates the Trickle timer associated with the MPL
Data Message if one exists.
Upon receiving an MPL Data Message, an MPL Forwarder MUST perform one
of the following actions:
o Accept the message and enter the MPL Data Message in the Buffered
Message Set.
o Accept the message and update the corresponding MinSequence in the
Seed Set to 1 greater than the message's sequence number.
o Discard the message without any change to the MPL Information
Base.
If a Seed Set entry exists for the MPL Seed, the MPL Forwarder MUST
discard the MPL Data Message if its sequence number is less than
MinSequence or exists in the Buffered Message Set.
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If a Seed Set entry does not exist for the MPL Seed, the MPL
Forwarder MUST create a new entry for the MPL Seed before accepting
the MPL Data Message.
If memory is limited, an MPL Forwarder SHOULD reclaim memory
resources by:
o Incrementing MinSequence entries in the Seed Set and deleting MPL
Data Messages in the Buffered Message Set that fall below the
corresponding MinSequence value.
o Deleting other Seed Set entries that have expired and the
corresponding MPL Data Messages in the Buffered Message Set.
If the MPL Forwarder accepts the MPL Data Message, the MPL Forwarder
MUST perform the following actions:
o If PROACTIVE_PROPAGATION is true, the MPL Forwarder MUST
initialize and start a Trickle timer for the MPL Data Message.
o If the MPL Control Message Trickle timer is not running and
CONTROL_MESSAGE_TIMER_EXPIRATIONS is non-zero, the MPL Forwarder
MUST initialize and start the MPL Control Message Trickle timer.
o If the MPL Control Message Trickle timer is running, the MPL
Forwarder MUST reset the MPL Control Message Trickle timer.
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11. MPL Control Messages
11.1. MPL Control Message Generation
An MPL Forwarder generates MPL Control Messages to communicate its
Seed Set and Buffered Message Set to neighboring MPL Forwarders.
Each MPL Control Message is generated according to Section 6.2, with
an MPL Seed Info for each entry in Seed Set. Each MPL Seed Info entry
has the following content:
o S set to the size of the seed-id field in the MPL Seed Info entry.
o min-seqno set to MinSequence of the MPL Seed.
o bm-len set to the size of buffered-mpl-messages in octets.
o seed-id set to the MPL seed identifier.
o buffered-mpl-messages with each bit representing whether or not an
MPL Data Message with the corresponding sequence number exists in
the Buffered Message Set. The i'th bit represents a sequence
number of min-seqno + i. '0' indicates that the corresponding MPL
Data Message does not exist in the Buffered Message Set. '1'
indicates that the corresponding MPL Data Message does exist in
the Buffered Message Set.
11.2. MPL Control Message Transmission
An MPL Forwarder transmits MPL Control Messages using the Trickle
algorithm. A MPL forwarder maintains a single Trickle timer for each
MPL Domain. When CONTROL_MESSAGE_TIMER_EXPIRATIONS is 0, the MPL
Forwarder does not execute the Trickle algorithm and does not
transmit MPL Control Messages. In accordance with Section 5 of RFC
6206 [RFC6206], this document defines the following:
o This document defines a "consistent" transmission as receiving an
MPL Control Message that indicates neither the receiving nor
transmitting node has new MPL Data Message.
o This document defines an "inconsistent" transmission as receiving
an MPL Control Message that indicates either the receiving or
transmitting node has at least one new MPL Data Message to offer.
o This document defines an "event" as increasing MinSequence of any
entry in the Seed Set or adding a message to the Buffered Message
Set.
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o This document defines an MPL Control Message as a Trickle message.
As specified in [RFC6206], a Trickle timer has three variables: the
current interval size I, a time within the current interval t, and a
counter c. MPL defines a fourth variable, e, which counts the number
of Trickle timer expiration events since the Trickle timer was last
reset. After CONTROL_MESSAGE_TIMER_EXPIRATIONS Trickle timer events,
the MPL Forwarder MUST disable the Trickle timer.
11.3. MPL Control Message Processing
An MPL Forwarder processes each MPL Control Message that it receives
to determine if it has any new MPL Data Messages to receive or offer.
An MPL Forwarder determines if a new MPL Data Message has not been
received from a neighboring node if any of the following conditions
hold true:
o The MPL Control Message includes an MPL Seed that does not exist
in the Seed Set.
o The MPL Control Message indicates that the neighbor has an MPL
Data Message in its Buffered Message Set with sequence number
greater than MinSequence (i.e. the i-th bit is set to 1 and min-
seqno + i > MinSequence) and is not included in the MPL
Forwarder's Buffered Message Set.
When an MPL Forwarder determines that it has not yet received an MPL
Data Message buffered by a neighboring device, the MPL Forwarder MUST
reset its Trickle timer associated with MPL Control Message
transmissions. If an MPL Control Message Trickle timer is not
running, the MPL Forwarder MUST initialize and start a new Trickle
timer.
An MPL Forwarder determines if an MPL Data Message in the Buffered
Message Set has not yet been received by a neighboring MPL Forwarder
if any of the following conditions hold true:
o The MPL Control Message does not include an MPL Seed for the MPL
Data Message.
o The MPL Data Message's sequence number is greater than or equal to
min-seqno and not included in the neighbor's Buffered Message Set
(i.e. the MPL Data Message's sequence number does not have a
corresponding bit in buffered-mpl-messages set to 1).
When an MPL Forwarder determines that it has at least one MPL Data
Message in its Buffered Message Set that has not yet been received by
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a neighbor, the MPL Forwarder MUST reset the MPL Control Message
Trickle timer. Additionally, for each of those entries in the
Buffered Message Set, the MPL Forwarder MUST reset the Trickle timer
and reset e to 0. If a Trickle timer is not associated with the MPL
Data Message, the MPL Forwarder MUST initialize and start a new
Trickle timer.
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12. Acknowledgements
The authors would like to acknowledge the helpful comments of Robert
Cragie, Esko Dijk, Ralph Droms, Paul Duffy, Ulrich Herberg, Owen
Kirby, Joseph Reddy, Don Sturek, Dario Tedeschi, and Peter van der
Stok, which greatly improved the document.
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13. IANA Considerations
This document defines one IPv6 Option, a type that must be allocated
from the IPv6 "Destination Options and Hop-by-Hop Options" registry
of [RFC2780].
This document defines one ICMPv6 Message, a type that must be
allocated from the "ICMPv6 "type" Numbers" registry of [RFC4443].
This document registers two well-known multicast addresses from the
IPv6 multicast address space.
13.1. MPL Option Type
IANA is requested to allocate an IPv6 Option Type from the IPv6
"Destination Options and Hop-by-Hop Options" registry of [RFC2780],
as specified in Table 1 below:
+--------------+-----+-----+--------------+-------------+-----------+
| Mnemonic | act | chg | rest | Description | Reference |
+--------------+-----+-----+--------------+-------------+-----------+
| MPL_OPT_TYPE | 01 | 1 | TBD | MPL Option | This |
| | | | (suggested | | Document |
| | | | value 01101) | | |
+--------------+-----+-----+--------------+-------------+-----------+
Table 1: IPv6 Option Type Allocation
13.2. MPL ICMPv6 Type
IANA is requested to allocate an ICMPv6 Type from the "ICMPv6 "type"
Numbers" registry of [RFC4443], as specified in Table 2 below:
+---------------+------+---------------------+---------------+
| Mnemonic | Type | Name | Reference |
+---------------+------+---------------------+---------------+
| MPL_ICMP_TYPE | TBD | MPL Control Message | This Document |
+---------------+------+---------------------+---------------+
Table 2: IPv6 Option Type Allocation
13.3. Well-known Multicast Addresses
IANA is requested to allocate an IPv6 multicast address
"ALL_MPL_FORWARDERS" from the "Variable Scope Multicast Addresses"
sub-registry of the "INTERNET PROTOCOL VERSION 6 MULTICAST ADDRESSES"
registry.
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14. Security Considerations
MPL uses sequence numbers to maintain a total ordering of MPL Data
Messages from an MPL Seed. The use of sequence numbers allows a
denial-of-service attack where an attacker can spoof a message with a
sufficiently large sequence number to: (i) flush messages from the
Buffered Message List and (ii) increase the MinSequence value for an
MPL Seed in the Seed Set. The former side effect allows an attacker
to halt the forwarding process of any MPL Data Messages being
disseminated. The latter side effect allows an attacker to prevent
MPL Forwarders from accepting new MPL Data Messages that an MPL Seed
generates while the sequence number is less than MinSequence.
More generally, the basic ability to inject messages into a Low-power
and Lossy Network can be used as a denial-of-service attack
regardless of what forwarding protocol is used. For these reasons,
Low-power and Lossy Networks typically employ link-layer security
mechanisms to disable an attacker's ability to inject messages.
To prevent attackers from injecting packets through an MPL Forwarder,
the MPL Forwarder MUST NOT accept or forward MPL Data Messages from a
communication interface that does not subscribe to the MPL Domain
Address identified in message's destination address.
MPL uses the Trickle algorithm to manage message transmissions and
the security considerations described in [RFC6206] apply.
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15. Normative References
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers",
BCP 37, RFC 2780, March 2000.
[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
March 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, March 2011.
[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.
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Authors' Addresses
Jonathan W. Hui
Cisco
170 West Tasman Drive
San Jose, California 95134
USA
Phone: +408 424 1547
Email: jonhui@cisco.com
Richard Kelsey
Silicon Labs
25 Thomson Place
Boston, Massachusetts 02210
USA
Phone: +617 951 1225
Email: richard.kelsey@silabs.com
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