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Internet Engineering Task Force (IETF) M. Goyal, Ed.
Request for Comments: 6997 Univ. of Wisconsin Milwaukee
Category: Experimental E. Baccelli
ISSN: 2070-1721 M. Philipp
INRIA
A. Brandt
Sigma Designs
J. Martocci
Johnson Controls
August 2013
Reactive Discovery of Point-to-Point Routes
in Low-Power and Lossy Networks
Abstract
This document specifies a point-to-point route discovery mechanism,
complementary to the Routing Protocol for Low-power and Lossy
Networks (RPL) core functionality. This mechanism allows an IPv6
router to discover "on demand" routes to one or more IPv6 routers in
a Low-power and Lossy Network (LLN) such that the discovered routes
meet specified metrics constraints.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6997.
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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
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.
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Table of Contents
1. Introduction ....................................................4
2. The Use Cases ...................................................4
3. Terminology .....................................................5
4. Applicability ...................................................6
5. Functional Overview .............................................7
6. P2P Route Discovery Mode of Operation ..........................10
6.1. Setting a P2P Mode DIO ....................................10
7. P2P Route Discovery Option (P2P-RDO) ...........................15
8. The P2P Discovery Reply Object (P2P-DRO) .......................18
8.1. Secure P2P-DRO ............................................20
8.2. Setting a P2P-RDO Carried in a P2P Discovery Reply
Object ....................................................21
9. P2P-RPL Route Discovery by Creating a Temporary DAG ............21
9.1. Joining a Temporary DAG ...................................21
9.2. Trickle Operation for P2P Mode DIOs .......................22
9.3. Processing a P2P Mode DIO .................................24
9.4. Additional Processing of a P2P Mode DIO at an
Intermediate Router .......................................26
9.5. Additional Processing of a P2P Mode DIO at the Target .....27
9.6. Processing a P2P-DRO at an Intermediate Router ............28
9.7. Processing a P2P-DRO at the Origin ........................30
10. The P2P Discovery Reply Object Acknowledgement (P2P-DRO-ACK) ..31
11. Secure P2P-RPL Operation ......................................32
12. Packet Forwarding along a Route Discovered Using P2P-RPL ......33
13. Interoperability with Core RPL ................................34
14. Security Considerations .......................................34
15. IANA Considerations ...........................................36
15.1. Additions to Mode of Operation ...........................36
15.2. Additions to RPL Control Message Options .................36
15.3. Additions to RPL Control Codes ...........................36
16. Known Issues and Future Work ..................................37
17. Acknowledgements ..............................................37
18. References ....................................................38
18.1. Normative References .....................................38
18.2. Informative References ...................................38
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1. Introduction
Targeting Low-power and Lossy Networks (LLNs), the IPv6 Routing
Protocol for LLNs (RPL) [RFC6550] provides paths along a Directed
Acyclic Graph (DAG) rooted at a single router in the network.
Establishment and maintenance of a DAG are performed by routers in
the LLN using Destination-Oriented DAG (DODAG) Information Object
(DIO) messages. When two arbitrary routers (neither of which is the
DAG's root) need to communicate, the data packets are restricted to
travel only along the links in the DAG. Such point-to-point (P2P)
routing functionality may not be sufficient for several home
automation [RFC5826] and building automation [RFC5867] applications,
due to the following reasons:
o The need to pre-establish routes: Each potential destination in
the network must declare itself as such ahead of the time a source
needs to reach it.
o The need to route only along the links in the DAG: A DAG is built
to optimize the routing cost to reach the root. Restricting P2P
routes to use only the in-DAG links may result in significantly
suboptimal routes and severe traffic congestion near the DAG root.
This document describes an extension to core RPL (i.e., the RPL
functionality described in [RFC6550]) that enables an IPv6 router in
the LLN to discover routes to one or more IPv6 routers in the LLN "on
demand". The discovered routes may not be the best available but are
guaranteed to meet the specified routing metric constraints. Thus,
such routes are considered "good enough" from the application's
perspective. This reactive P2P route discovery mechanism is
henceforth referred to as P2P-RPL.
A mechanism to measure the end-to-end cost of an existing route is
specified in [RFC6998]. As discussed in Section 4, measuring the
end-to-end cost of an existing route may help in deciding whether to
initiate the discovery of a better route using P2P-RPL and the metric
constraints to be used for this purpose.
2. The Use Cases
One use case, common in home [RFC5826] and commercial building
[RFC5867] environments, involves a device (say, a remote control)
that suddenly needs to communicate with another device (say, a lamp)
to which it does not already have a route (and whose network address
it knows a priori). In this case, the remote control must be able to
discover a route to the lamp "on demand".
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Another use case, common in a commercial building environment,
involves a large LLN deployment where P2P communication along a
particular DAG among hundreds (or thousands) of routers creates
severe traffic congestion near that DAG's root. In this case, it is
desirable to discover direct routes between various source-
destination pairs that do not pass through the DAG's root.
Other use cases involve scenarios where energy or latency constraints
are not satisfied by the P2P routes along an existing DAG because
they involve traversing many more routers than necessary to reach the
destination.
3. 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].
Additionally, this document uses terminology from [RFC6550] and
[RFC6554]. Further terminology may be found in [ROLL-TERMS]. This
document introduces the following terms:
Origin: The IPv6 router initiating the P2P-RPL route discovery.
Target: The IPv6 router at the other end point of the P2P route(s)
to be discovered. A P2P-RPL route discovery can discover routes
to multiple Targets at the same time.
Intermediate Router: An IPv6 router that is neither the Origin nor a
Target.
Forward direction: The direction from the Origin to the Target.
Reverse direction: The direction from the Target to the Origin.
Forward Route: A route in the Forward direction.
Reverse Route: A route in the Reverse direction.
Bidirectional Route: A route that can be used in both Forward and
Reverse directions.
Ingress-only Interface: A network interface that can only receive
packets.
Egress-only Interface: A network interface that can only send
packets.
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Source Route: A complete and ordered list of routers that can be
used by a packet to travel from a source to a destination node.
Hop-by-hop Route: The route characterized by each router on the
route using its routing table to determine the next hop on the
route.
RPL Security Configuration: The values for the Counter is Time,
Security Algorithm, Key Identifier Mode, and Security Level
fields, as defined in Section 6.1 of [RFC6550], inside the
Security section of a secure RPL control message.
4. Applicability
A route discovery using P2P-RPL may be performed by an Origin when no
route exists between itself and the Target(s) or when the existing
routes do not satisfy the application requirements. P2P-RPL is
designed to discover Hop-by-hop or Source Routes to one or more
Targets such that the discovered routes meet the specified
constraints. In some application contexts, the constraints that the
discovered routes must satisfy are intrinsically known or can be
specified by the application. For example, an Origin that expects
its Targets to be less than 5 hops away may use "hop-count < 5" as
the constraint. In other application contexts, the Origin may need
to measure the cost of the existing route to a Target to determine
the constraints. For example, an Origin that measures the total
expected transmission count (ETX) along its current route to a Target
to be 20 may use "ETX < x*20", where x is a fraction that the Origin
chooses, as the constraint. A mechanism to measure the cost of an
existing route between two IPv6 routers is specified in [RFC6998].
If there is no existing route between the Origin and the Target(s) or
the cost measurement for the existing routes fails, the Origin will
have to guess the constraints to be used in the initial route
discovery. Once the initial route discovery succeeds or fails, the
Origin will have a better estimate for the constraints to be used in
the subsequent route discovery.
P2P-RPL may result in discovery of better P2P routes than those
available along a global DAG designed to optimize routing cost to the
DAG's root. The improvement in route quality depends on a number of
factors, including the network topology, the "distance" between the
Origin and the Target (in terms of the routing metrics in use), and
the prevalent conditions in the network. In general, a P2P-RPL route
may be better than the one along a global DAG if the Origin and the
Target are nearby. Similarly, a P2P-RPL route may not be much better
than the one along a global DAG if the Origin and the Target are far
apart. Note that even when P2P-RPL routes are not much better than
those along a global DAG, P2P-RPL routes may still be able to avoid
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congestion that might occur near the root if the routing takes place
only along a global DAG. In general, the cost associated with a
P2P-RPL route discovery (in terms of the control messages -- mostly
DIOs -- generated) increases with the distance between the Origin and
the Target. However, it is possible to limit the cost of route
discovery by carefully setting the routing constraints, the Trickle
parameters (which govern DIO generation), and the time duration for
which a router maintains its membership in the temporary DAG created
for the route discovery. A network designer may take into
consideration both the benefits (potentially better routes; no need
to maintain routes proactively; avoid congestion near the global
DAG's root) and costs when using P2P-RPL. The latency associated
with a P2P-RPL route discovery again depends on the distance between
the Origin and the Target and on the Trickle parameters.
Like core RPL [RFC6550], P2P-RPL operation requires that links have
bidirectional reachability. For this reason, the routers
participating in a P2P-RPL route discovery must ensure that
o Links that do not have bidirectional reachability do not become
part of the route being discovered; and
o IPv6 addresses belonging to Ingress-only (or Egress-only)
Interfaces do not become part of the route being discovered.
5. Functional Overview
This section contains a high-level description of P2P-RPL.
A P2P-RPL route discovery takes place by forming a DAG rooted at the
Origin. As is the case with core RPL, P2P-RPL uses IPv6 link-local
multicast DIO messages to establish a DAG. However, unlike core RPL,
this DAG is temporary in nature. The routes are discovered and
installed while the DAG is alive. Once the specified duration of
their membership in the DAG is over, the routers leave the DAG, and
hence the DAG ceases to exist. However, the installed routes are
retained for their specified lifetime (which is different than the
specified duration of a router's membership in the DAG) even though
the DAG that caused their installation no longer exists. In P2P-RPL,
the sole purpose of DAG creation is to discover routes to the
Target(s), and DIOs serve as the route discovery messages. Each
router joining the DAG determines a rank for itself in the DAG and
ignores the subsequent DIOs received from lower-ranked (higher in
numerical value) neighbors. Thus, the route discovery messages
propagate away from the Origin rather than return to it. As in core
RPL, DIO generation at a router is controlled by a Trickle timer
[RFC6206], which allows a router to avoid generating unnecessary
messages while providing protection against packet loss. P2P-RPL
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also uses the routing metrics [RFC6551], Objective Functions, and
packet-forwarding framework [RFC6554] [RFC6553] developed for
core RPL.
An Origin may use P2P-RPL to discover routes to one or more Targets
identified by one or more unicast/multicast addresses. P2P-RPL
allows for the discovery of one Hop-by-hop Route or up to four Source
Routes per Target. The discovered routes are guaranteed to meet the
specified routing metric constraints but may not be the best
available. P2P-RPL may fail to discover any route if the specified
routing constraints are overly strict.
The Origin initiates a P2P-RPL route discovery by forming a temporary
DAG rooted at itself. The DIOs used to create the temporary DAG are
identified by a new Mode of Operation (P2P Route Discovery mode,
defined in Section 6). The DIOs listing the P2P Route Discovery mode
as the Mode of Operation are henceforth referred to as the P2P mode
DIOs. A P2P mode DIO always carries exactly one P2P Route Discovery
Option (P2P-RDO, defined in Section 7) in which the Origin specifies
the following information:
o The IPv6 address of a Target. This could be a unicast address or
a multicast address. Any additional Targets may be specified by
including one or more RPL Target options [RFC6550] inside the DIO.
o The nature of the route(s) to be discovered: Hop-by-hop or Source
Routes. This specification allows for the discovery of one
Hop-by-hop Route or up to four Source Routes per Target.
o The desired number of routes (if Source Routes are being
discovered).
o Whether the Target(s) should send P2P Discovery Reply Object
(P2P-DRO) messages (defined in Section 8) back to the Origin on
receiving a DIO message. A P2P-DRO message carries a discovered
Source Route back to the Origin or establishes a Hop-by-hop Route
between the Origin and the Target.
A P2P-RDO also includes the best route from the Origin that the
router, generating the P2P mode DIO, has seen so far.
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A P2P mode DIO MAY also carry:
o One or more Metric Container options to specify:
* The relevant routing metrics.
* The constraints that the discovered route must satisfy. These
constraints also limit how far the DIO messages may travel.
o One or more RPL Target options to specify additional unicast or
multicast Targets.
As the routers join the temporary DAG, they keep track of the best
route(s) (so far from the Origin) they have seen and advertise these
routes, along with the corresponding routing metrics, in their P2P
mode DIOs. A router, including the Target(s), discards a received
P2P mode DIO if the aggregated routing metrics on the route
advertised by the DIO do not satisfy the listed constraints. These
constraints can be used to limit the propagation of P2P mode DIO
messages. A router may also discard a received P2P mode DIO if it
does not wish to be a part of the discovered route due to limited
resources or due to policy reasons.
When a Target receives a P2P mode DIO, it contains inside the P2P-RDO
a complete Source Route from the Origin to this Target. Since the
links in the discovered route have bidirectional reachability
(Section 7), the Target may use the discovered route to reach the
Origin. Thus, a router that provides a particular service in the LLN
(e.g., an outside temperature server) could initiate a P2P-RPL route
discovery listing all its potential clients as Targets, thereby
allowing the clients to discover a Source Route back to the server.
In this case, the Origin (the server) might want to disable the
generation of P2P-DRO messages by the Targets (the clients). If the
Origin has requested that P2P-DRO messages be sent back, the Target
may select the discovered route in the received DIO for further
processing, as described next. This document does not specify a
particular method for the Target to use to select a route for further
processing. Example methods include selecting any route that meets
the constraints or selecting the best route(s) discovered over a
certain time period.
If one or more Source Routes are being discovered, the Target sends
the selected Source Route(s) to the Origin via P2P-DRO messages, with
one P2P-DRO message carrying one discovered route. On receiving a
P2P-DRO message, the Origin stores the discovered route in its
memory. This specification allows the Origin to discover up to four
Source Routes per Target, thereby allowing the Origin to have
sufficient ready-to-use alternatives should one or more of these
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routes fail. If a Hop-by-hop Route is being discovered, the Target
sends a P2P-DRO message containing the selected route to the Origin.
The P2P-DRO message travels back to the Origin along the selected
route, establishing state for the Forward Route in the routers on
the path.
The Target may request that the Origin acknowledge the receipt of a
P2P-DRO message by sending back a P2P-DRO Acknowledgement
(P2P-DRO-ACK) message (defined in Section 10). The Origin unicasts a
P2P-DRO-ACK message to the Target. If the Target does not receive
the requested P2P-DRO-ACK within a certain time interval of sending a
P2P-DRO, it resends the P2P-DRO message (up to a certain number of
times) carrying the same route as before.
The use of Trickle timers to delay the propagation of DIO messages
may cause some nodes to generate these messages even when the desired
routes have already been discovered. In order to preempt the
generation of such unnecessary messages, the Target may set a "Stop"
flag in the P2P-DRO message to let the nodes in the LLN know about
the completion of the route discovery process. The routers receiving
such a P2P-DRO should not generate any more DIOs for this temporary
DAG, nor should they process any received DIOs for this temporary DAG
in the future. However, such routers must still process the P2P-DROs
received for this temporary DAG.
6. P2P Route Discovery Mode of Operation
This section specifies a new RPL Mode of Operation (MOP), P2P Route
Discovery mode (or P2P mode, for short), with value 4. A DIO message
listing P2P mode as the MOP is identified as performing a P2P-RPL
route discovery by creating a temporary DAG. A P2P mode DIO MUST
carry exactly one P2P Route Discovery Option (P2P-RDO, specified in
Section 7).
6.1. Setting a P2P Mode DIO
The Base object in a P2P mode DIO message MUST be set in the
following manner:
o RPLInstanceID: RPLInstanceID MUST be a local value as described in
Section 5.1 of [RFC6550]. The Origin chooses the RPLInstanceID to
be used for a particular route discovery in accordance with the
following rules:
* The Origin SHOULD NOT reuse a RPLInstanceID for a route
discovery if some routers might still maintain membership in
the DAG that the Origin had initiated for the previous route
discovery using this RPLInstanceID. As described in Section 7,
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a router's membership in a DAG created for a P2P-RPL route
discovery lasts for the time duration (say, 't' seconds)
indicated by the L field inside the P2P-RDO. In general, there
is no upper bound on the time duration by when all the routers
have left the DAG created for a P2P-RPL route discovery. In
the specific case where the discovered route must be at most
'n' hops in length, all the routers must have left the DAG
"(n+1)*t" seconds after its initiation by the Origin. In
practice, all the routers should have joined the DAG within 't'
seconds of its initiation (since the route discovery must
complete while the Origin still belongs to the DAG), and hence
all the routers should have left the DAG within "2*t" seconds
of its initiation. Hence, it is usually sufficient that the
Origin wait for twice the duration indicated by the L field
inside the P2P-RDO used for the previous route discovery before
reusing the RPLInstanceID for a new route discovery.
Individual P2P-RPL deployments are encouraged to share their
experience with various RPLInstanceID reuse policies to help
guide the development of a Standards Track version of the
protocol.
* When initiating a new route discovery to a particular Target,
the Origin MUST NOT reuse the RPLInstanceID used in a previous
route discovery to this Target if the state created during the
previous route discovery might still exist in some routers.
Note that it is possible that the previous route discovery did
not succeed yet some routers still ended up creating state.
The Default Lifetime and Lifetime Unit parameters in the DODAG
Configuration Option specify the lifetime of the state that the
routers, including the Origin and the Target, maintain for a
Hop-by-hop or Source Route discovered using P2P-RPL. Suppose
this lifetime is 'X' seconds. As discussed above, any state
created during the previous route discovery was likely created
within "2*t" seconds of its initiation. Hence, it is
sufficient that the Origin lets a time duration equal to
"X+2*t" seconds pass since the initiation of the previous route
discovery before initiating a new route discovery to the same
Target using the same RPLInstanceID.
o Version Number: This field MUST be set to zero. The temporary DAG
used for P2P-RPL route discovery does not exist long enough to
have new versions.
o Grounded (G) Flag: This flag MUST be set to one. Unlike a global
RPL instance, the concept of a floating DAG, used to provide
connectivity within a sub-DAG detached from a grounded DAG, does
not apply to a local RPL instance. Hence, an Origin MUST always
set the G flag to one when initiating a P2P-RPL route discovery.
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Further, item 3 of Section 8.2.2.2 in [RFC6550] does not apply,
and a node MUST NOT initiate a new DAG if it does not have any
parent left in a P2P-RPL DAG.
o Mode of Operation (MOP): This field MUST be set to four,
corresponding to P2P Route Discovery mode.
o Destination Advertisement Trigger Sequence Number (DTSN): This
field MUST be set to zero on transmission and ignored on
reception.
o DODAGPreference (Prf): This field MUST be set to zero (least
preferred).
o DODAGID: This field MUST be set to an IPv6 address of the Origin.
o The other fields in the DIO Base object can be set in the desired
fashion as per the rules described in [RFC6550].
A received P2P mode DIO MUST be discarded if it does not follow the
above-listed rules regarding the RPLInstanceID, Version Number,
G flag, MOP, and Prf fields inside the Base object.
The DODAG Configuration Option inside a P2P mode DIO MUST be set in
the following manner:
o The Origin MUST set the MaxRankIncrease parameter to zero to
disable local repair of the temporary DAG. A received P2P mode
DIO MUST be discarded if the MaxRankIncrease parameter inside the
DODAG Configuration Option is not zero.
o The Origin SHOULD set the Trickle parameters
(DIOIntervalDoublings, DIOIntervalMin, DIORedundancyConstant) as
recommended in Section 9.2.
o The Origin sets the Default Lifetime and Lifetime Unit parameters
to indicate the lifetime of the state that the routers, including
the Origin and the Target(s), maintain for a Hop-by-hop or Source
Route discovered using P2P-RPL.
o The Origin sets the other fields in the DODAG Configuration
Option, including the Objective Code Point (OCP) identifying the
Objective Function, in the desired fashion as per the rules
described in [RFC6550].
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o As discussed in Section 14, P2P-RPL does not distinguish between
the "preinstalled" and "authenticated" security modes described in
[RFC6550]. Consequently, the Origin MUST set the Authentication
Enabled (A) flag to zero. A received P2P mode DIO MUST be
discarded if the A flag inside the DODAG Configuration Option is
not zero.
o An Intermediate Router (or a Target) MUST set various fields in
the DODAG Configuration Option in the outgoing P2P mode DIOs to
the values they had in the incoming P2P mode DIOs for this DAG.
A default DODAG Configuration Option takes effect if a P2P mode DIO
does not carry an explicit one. The default DODAG Configuration
Option has the following parameter values:
o Authentication Enabled: 0
o DIOIntervalMin: 6, which translates to 64 ms as the value for the
Imin parameter in a Trickle operation. This value is roughly one
order of magnitude larger than the typical transmission delay on
IEEE 802.15.4 links and corresponds to the recommendation in
Section 9.2 for well-connected topologies.
o DIORedundancyConstant: 1. See the discussion in Section 9.2.
o MaxRankIncrease: 0 (to disable local repair of the temporary DAG).
o Default Lifetime: 0xFF, to correspond to infinity.
o Lifetime Unit: 0xFFFF, to correspond to infinity.
o Objective Code Point: 0, i.e., OF0 [RFC6552] is the default
Objective Function (OF).
o The remaining parameters have default values as specified in
[RFC6550].
Individual P2P-RPL deployments are encouraged to share their
experience with these default values to help guide the development of
a Standards Track version of the protocol.
The routing metrics and constraints [RFC6551] used in P2P-RPL route
discovery are included in one or more Metric Container options
[RFC6550] inside the P2P mode DIO. Note that a DIO need not include
a Metric Container if OF0 is the Objective Function in effect. In
that case, a P2P mode DIO may still specify an upper limit on the
maximum rank, that a router may have in the temporary DAG, inside
the P2P-RDO.
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A P2P mode DIO:
o MUST carry one (and only one) P2P-RDO. The P2P-RDO allows for the
specification of one unicast or multicast address for the Target.
A received P2P mode DIO MUST be discarded if it does not contain
exactly one P2P-RDO.
o MAY carry one or more RPL Target options to specify additional
unicast/multicast addresses for the Target. If a unicast address
is specified, it MUST be a global address or a unique-local
address.
o MAY carry one or more Metric Container options to specify routing
metrics and constraints.
o MAY carry one or more Route Information Options [RFC6550]. In the
context of P2P-RPL, a Route Information Option advertises to the
Target(s) the Origin's connectivity to the prefix specified in the
option.
o MAY carry one DODAG Configuration Option. If a P2P mode DIO does
not carry an explicit DODAG Configuration Option, the default
DODAG Configuration Option defined in this section is considered
to be in effect.
A RPL option other than those listed above MUST be ignored when found
inside a received P2P mode DIO and MUST NOT be included in the P2P
mode DIOs that the receiving router generates.
In accordance with core RPL, a P2P mode DIO MUST propagate via link-
local multicast. The IPv6 source address in a P2P mode DIO MUST be a
link-local address, and the IPv6 destination address MUST be the
link-local multicast address all-RPL-nodes [RFC6550]. A P2P mode DIO
MUST be transmitted on all interfaces the router has in this RPL
routing domain [RFC6554].
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7. P2P Route Discovery Option (P2P-RDO)
This section defines a new RPL control message option: the P2P Route
Discovery Option (P2P-RDO).
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 = 0x0a | Option Length |R|H| N | Compr | L |MaxRank/NH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| TargetAddr |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address[1..n] |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of the P2P Route Discovery Option (P2P-RDO)
The format of a P2P Route Discovery Option (P2P-RDO) is illustrated
in Figure 1. A P2P mode DIO and a P2P-DRO message (defined in
Section 8) MUST carry exactly one P2P-RDO. A P2P-RDO consists of the
following fields:
o Option Type: 0x0a.
o Option Length: This field is an 8-bit unsigned integer
representing the length in octets of the option, not including the
Option Type and Option Length fields.
o Reply (R): The Origin sets this flag to one to allow the Target(s)
to send P2P-DRO messages back to the Origin. If this flag is set
to zero, a Target MUST NOT generate any P2P-DRO messages.
o Hop-by-hop (H): This flag is valid only if the R flag is set to
one. The Origin sets this flag to one if it desires Hop-by-hop
Routes. The Origin sets this flag to zero if it desires Source
Routes. This specification allows for the establishment of one
Hop-by-hop Route or up to four Source Routes per Target. The
Hop-by-hop Route is established in the Forward direction, i.e.,
from the Origin to the Target. This specification does not allow
for the establishment of Hop-by-hop Routes in the Reverse
direction.
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o Number of Routes (N): This field is valid only if the R flag is
set to one and the H flag is set to zero, i.e., the Targets are
allowed to generate P2P-DRO messages carrying discovered Source
Routes back to the Origin. In this case, the value in the N field
plus one indicates the number of Source Routes that each Target
should convey to the Origin. When Hop-by-hop Routes are being
discovered, the N field MUST be set to zero on transmission and
ignored on reception.
o Compr: This field is a 4-bit unsigned integer indicating the
number of prefix octets that are elided from the Target field and
the Address vector. For example, the Compr value will be zero if
full IPv6 addresses are carried in the Target field and the
Address vector.
o Lifetime (L): This is a 2-bit field that indicates the exact
duration that a router joining the temporary DAG, including the
Origin and the Target(s), MUST maintain its membership in the DAG.
A router MUST leave the temporary DAG once the time elapsed since
it joined reaches the value indicated by this field. The mapping
between the value in this field and the duration of the router's
membership in the temporary DAG is as follows:
* 0x00: 1 second
* 0x01: 4 seconds
* 0x02: 16 seconds
* 0x03: 64 seconds
The Origin sets this field based on its expectation regarding the
time required for the route discovery to complete, which includes
the time required for the DIOs to reach the Target(s) and the
P2P-DROs to travel back to the Origin. The time required for the
DIOs to reach the Target(s) would in turn depend on the Trickle
parameters (Imin and the redundancy constant) as well as the
expected distance (in terms of hops and/or ETX) to the Target(s).
While deciding on the value in this field, the Origin should also
take into account the fact that all routers joining the temporary
DAG would need to stay in the DAG for this much time.
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o MaxRank/NH:
* When a P2P-RDO is included in a P2P mode DIO, this field
indicates the upper limit on the integer portion of the rank
(calculated using the DAGRank() macro defined in [RFC6550])
that a router may have in the temporary DAG being created. An
Intermediate Router MUST NOT join a temporary DAG being created
by a P2P mode DIO if the integer portion of its rank would be
equal to or higher (in numerical value) than the MaxRank limit.
A Target can join the temporary DAG at a rank whose integer
portion is equal to the MaxRank. A router MUST discard a
received P2P mode DIO if the integer part of the advertised
rank equals or exceeds the MaxRank limit. A value of 0 in this
field indicates that the MaxRank is infinity.
* When a P2P-RDO is included in a P2P-DRO message, this field
indicates the index of the next-hop (NH) address inside the
Address vector.
o TargetAddr: This is an IPv6 address of the Target after eliding
Compr number of prefix octets. When the P2P-RDO is included in a
P2P mode DIO, this field may contain a unicast address or a
multicast address. If a unicast address is specified, it MUST be
a global address or a unique-local address. Any additional Target
addresses can be specified by including one or more RPL Target
options [RFC6550] in the DIO. When the P2P-RDO is included in a
P2P-DRO, this field MUST contain a unicast global or unique-local
IPv6 address of the Target generating the P2P-DRO.
o Address[1..n]: This is a vector of IPv6 addresses representing a
complete route so far in the Forward direction:
* Each element in the Address vector has size (16 - Compr) octets
and MUST contain a valid global or unique-local IPv6 address
with the first Compr octets elided.
* The total number of elements inside the Address vector is given
by n = (Option Length - 2 - (16 - Compr))/(16 - Compr).
* The IPv6 address that a router adds to the vector MUST belong
to the interface on which the router received the DIO
containing this P2P-RDO. Further, this interface MUST NOT be
an Ingress-only Interface. This allows the route accumulated
in the Address vector to be a Bidirectional Route that can be
used by a Target to send a P2P-DRO message to the Origin.
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* The Address vector MUST carry the accumulated route in the
Forward direction, i.e., the first element in the Address
vector must contain the IPv6 address of the router next to the
Origin, and so on.
* The Origin and Target addresses MUST NOT be included in the
Address vector.
* A router adding its address to the vector MUST ensure that none
of its addresses already exist in the vector. A Target
specifying a complete route in the Address vector MUST ensure
that the vector does not contain any address more than once.
* The Address vector MUST NOT contain any multicast addresses.
8. The P2P Discovery Reply Object (P2P-DRO)
This section defines two new RPL control message types: the P2P
Discovery Reply Object (P2P-DRO), with code 0x04; and the Secure
P2P-DRO, with code 0x84. A P2P-DRO serves one of the following
functions:
o carries a discovered Source Route from a Target to the Origin;
o establishes a Hop-by-hop Route as it travels from a Target to the
Origin.
A P2P-DRO message can also serve the function of letting the routers
in the LLN know that a P2P-RPL route discovery is complete and no
more DIO messages need to be generated for the corresponding
temporary DAG. A P2P-DRO message MUST carry one (and only one)
P2P-RDO whose TargetAddr field MUST contain a unicast IPv6 address of
the Target that generates the P2P-DRO. A P2P-DRO message MUST travel
from the Target to the Origin via link-local multicast along the
route specified inside the Address vector in the P2P-RDO, as included
in the P2P-DRO. The IPv6 source address in a P2P-DRO message MUST be
a link-local address, and the IPv6 destination address MUST be the
link-local multicast address all-RPL-nodes [RFC6550]. A P2P-DRO
message MUST be transmitted on all interfaces the router has in this
RPL routing domain [RFC6554].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | Version |S|A|Seq| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DODAGID |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Figure 2: Format of the Base P2P Discovery Reply Object (P2P-DRO)
The format of the base P2P Discovery Reply Object (P2P-DRO) is shown
in Figure 2. A base P2P-DRO consists of the following fields:
o RPLInstanceID: This field provides the RPLInstanceID of the
temporary DAG used for route discovery.
o Version: This field provides the Version of the temporary DAG used
for route discovery. Since a temporary DAG always has value zero
for the Version, this field MUST always be set to zero.
o Stop (S): This flag, when set to one by a Target, indicates that
the P2P-RPL route discovery is over. All the routers receiving
such a P2P-DRO, including those not listed in the route carried
inside a P2P-RDO,
* SHOULD NOT process any more DIOs received for this
temporary DAG;
* SHOULD NOT generate any more DIOs for this temporary DAG;
* SHOULD cancel any pending DIO transmissions for this
temporary DAG.
Note that the Stop flag serves to stop further DIO
generation/processing for a P2P-RPL route discovery but does not
affect the processing of P2P-DRO messages at either the Origin or
the Intermediate Routers. In other words, a router (the Origin or
an Intermediate Router) MUST continue to process the P2P-DRO
messages even if an earlier P2P-DRO message (with the same
RPLInstanceID and DODAGID fields) had the Stop flag set to one.
When set to zero, this flag does not imply anything and MUST be
ignored on reception.
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o Ack Required (A): This flag, when set to one by the Target,
indicates that the Origin MUST unicast a P2P-DRO-ACK message
(defined in Section 10) to the Target when it receives the
P2P-DRO.
o Sequence Number (Seq): This 2-bit field indicates the sequence
number for the P2P-DRO. This field is relevant when the A flag is
set to one, i.e., the Target requests an acknowledgement from the
Origin for a received P2P-DRO. The Origin includes the
RPLInstanceID, the DODAGID, and the Sequence Number of the
received P2P-DRO inside the P2P-DRO-ACK message it sends back to
the Target.
o Reserved: These bits are reserved for future use. These bits MUST
be set to zero on transmission and MUST be ignored on reception.
o DODAGID: This field provides the DODAGID of the temporary DAG used
for route discovery. The DODAGID also identifies the Origin. The
RPLInstanceID, the Version, and the DODAGID together uniquely
identify the temporary DAG used for route discovery and can be
copied from the DIO message advertising the temporary DAG.
o Options: The P2P-DRO message:
* MUST carry one (and only one) P2P-RDO that MUST specify a
complete route between the Target and the Origin. A received
P2P-DRO message MUST be discarded if it does not contain
exactly one P2P-RDO.
* MAY carry one or more Metric Container options that contain the
aggregated routing metrics values for the route specified in
the P2P-RDO.
A RPL option other than those listed above MUST be ignored when
found inside a received P2P-DRO message.
8.1. Secure P2P-DRO
A Secure P2P-DRO message follows the format shown in Figure 7 of
[RFC6550], where the base format is the base P2P-DRO shown in
Figure 2.
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8.2. Setting a P2P-RDO Carried in a P2P Discovery Reply Object
A P2P Discovery Reply Object MUST carry one (and only one) P2P-RDO,
which MUST be set as defined in Section 7. Specifically, the
following fields MUST be set as follows:
o Reply (R): This flag MUST be set to zero on transmission and
ignored on reception.
o Hop-by-Hop (H): The H flag in the P2P-RDO included in a P2P-DRO
message MUST have the same value as the H flag in the P2P-RDO
inside the corresponding DIO message.
o Number of Routes (N): This field MUST be set to zero on
transmission and ignored on reception.
o Lifetime (L): This field MUST be set to zero on transmission and
ignored on reception.
o MaxRank/NH: This field indicates the index of the next-hop address
in the Address vector. When a Target generates a P2P-DRO message,
the NH field is set to n = (Option Length - 2 - (16 - Compr))/
(16 - Compr).
o TargetAddr: This field MUST contain a unicast global or unique-
local IPv6 address of the Target generating the P2P-DRO.
o Address[1..n]: The Address vector MUST contain a complete route
between the Origin and the Target such that the first element in
the vector contains the IPv6 address of the router next to the
Origin and the last element contains the IPv6 address of the
router next to the Target.
9. P2P-RPL Route Discovery by Creating a Temporary DAG
This section details the P2P-RPL route discovery operation.
9.1. Joining a Temporary DAG
All the routers participating in a P2P-RPL route discovery, including
the Origin and the Target(s), MUST join the temporary DAG being
created for that purpose. When a router joins a temporary DAG
advertised by a P2P mode DIO, it MUST maintain its membership in the
temporary DAG for the duration indicated by the L field inside the
P2P-RDO. The only purpose of a temporary DAG's existence is to
facilitate the P2P-RPL route discovery process. The temporary DAG
MUST NOT be used to route data packets. In other words, joining a
temporary DAG does not allow a router to provision routing table
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entries listing the router's parents in the temporary DAG as the next
hops (i.e., the last bullet point in Section 3.2.8 of [RFC6550] is
not applicable when the DAG is a temporary DAG created for the
purpose of a P2P-RPL route discovery).
Given the nature of a temporary DAG created for a P2P-RPL route
discovery, this document disallows the solicitation of P2P mode DIOs
using DODAG Information Solicitation (DIS) messages as described in
[RFC6550]. A router participating in a P2P-RPL route discovery MUST
NOT reset its Trickle timer, which controls the transmission of P2P
mode DIOs in response to a multicast DIS. Also, the router MUST NOT
send a P2P mode DIO in response to a unicast DIS. In other words,
the rules in Section 8.3 of [RFC6550] regarding a router's response
to a multicast/unicast DIS are not applicable for P2P mode DIOs.
A router MUST detach from the temporary DAG created for a P2P-RPL
route discovery once the duration of its membership in the DAG has
reached the value indicated by the L field inside the P2P-RDO. After
receiving a P2P-DRO with the Stop flag set to one, a router SHOULD
NOT send or process any more DIOs for this temporary DAG and SHOULD
also cancel any pending DIO transmissions.
9.2. Trickle Operation for P2P Mode DIOs
A RPL router uses a Trickle timer [RFC6206] to control DIO
transmissions. The Trickle control of DIO transmissions provides
quick resolution of any "inconsistency" while avoiding redundant DIO
transmissions. The Trickle algorithm also imparts protection against
loss of DIOs due to inherent lack of reliability in LLNs. When
controlling the transmissions of a P2P mode DIO, a Trickle timer
SHOULD follow the following rules:
o The receipt of a P2P mode DIO that allows the router to advertise
a better route (in terms of the routing metrics and the OF in use)
than before is considered "inconsistent" and hence resets the
Trickle timer. Note that the first receipt of a P2P mode DIO
advertising a particular temporary DAG is always considered an
"inconsistent" event.
o The receipt of a P2P mode DIO from a parent in the temporary DAG
is considered neither "consistent" nor "inconsistent" if it does
not allow the router to advertise a better route than before.
Thus, the receipt of such DIOs has no impact on the Trickle
operation. Note that this document does not impose any
requirements on how a router might choose its parents in the
temporary DAG.
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o The receipt of a P2P mode DIO is considered "consistent" if the
source of the DIO is not a parent in the temporary DAG and either
of the following conditions is true:
* The DIO advertises a better route than the router but does not
allow the router to advertise a better route itself; or
* The DIO advertises a route as good as the route (to be)
advertised by the router.
Note that the Trickle algorithm's DIO suppression rules are in
effect at all times. Hence, a P2P-RPL router may suppress a DIO
transmission even if it has not made any DIO transmissions yet.
o The receipt of a P2P mode DIO that advertises a worse route than
what the router advertises (or would advertise when it gets a
chance to generate its DIO) is considered neither "consistent" nor
"inconsistent", i.e., the receipt of such a DIO has no impact on
the Trickle operation.
o The Imin parameter SHOULD be set taking into account the
connectivity within the network. For highly connected networks, a
small Imin value (on the order of the typical transmission delay
for a DIO) may lead to congestion in the network as a large number
of routers reset their Trickle timers in response to the first
receipt of a DIO from the Origin. These routers would generate
their DIOs within the Imin interval and cause additional routers
to reset their Trickle timers and generate more DIOs. Thus, for
highly connected networks, the Imin parameter SHOULD be set to a
value at least one order of magnitude larger than the typical
transmission delay for a DIO. For sparsely connected networks,
the Imin parameter can be set to a value that is a small multiple
of the typical transmission delay for a DIO. Note that the Imin
value has a direct impact on the time required for a P2P-RPL route
discovery to complete. In general, the time required for a
P2P-RPL route discovery would increase approximately linearly with
the value of the Imin parameter. Since the route discovery must
complete while the Origin still belongs to the temporary DAG
created for that purpose, the Origin should set the time duration
for which a router maintains its membership in the temporary DAG
(indicated by the L field inside the P2P-RDO) to a large enough
value, taking into account the Imin value as well as the expected
distance (in terms of hops and/or ETX) to the Target(s).
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o The Imax parameter SHOULD be set to a large value (several orders
of magnitude higher than the Imin value) and is unlikely to be
critical for P2P-RPL operation. This is because the first receipt
of a P2P mode DIO for a particular temporary DAG is considered an
inconsistent event and would lead to the resetting of the Trickle
timer duration to the Imin value. Given the temporary nature of
the DAGs used in P2P-RPL, the Trickle timer may not get a chance
to increase much.
o The recommended value of redundancy constant "k" is 1. With this
value of "k", a DIO transmission will be suppressed if the router
receives even a single "consistent" DIO during a timer interval.
This setting for the redundancy constant is designed to reduce the
number of messages generated during a route discovery process and
is suitable for environments with low or moderate packet loss
rates. However, this setting may result in an increase in the
time required for the route discovery process to complete. A
higher value for the redundancy constant may be more suitable in
* environments with high packet loss rates; or
* deployments where the time required for the route discovery
process to complete needs to be as small as possible; or
* deployments where specific destinations are reachable only
through specific Intermediate Routers (and hence these
Intermediate Routers should not suppress their DIOs).
A particular deployment should take into account the above-
mentioned factors when deciding on the value of the redundancy
constant.
Individual P2P-RPL deployments are encouraged to share their
experience with these rules to help guide the development of a
Standards Track version of the protocol. Applicability Statements
that specify the use of P2P-RPL MUST provide guidance for setting
Trickle parameters, particularly Imin and the redundancy constant.
9.3. Processing a P2P Mode DIO
The rules for DIO processing and transmission as described in
Section 8 of RPL [RFC6550] apply to P2P mode DIOs as well, except as
modified in this document. In particular, in accordance with
Section 8.2.3 of RPL [RFC6550], a received P2P mode DIO MUST be
discarded if it is malformed, according to the rules specified in
this document and in [RFC6550].
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The following rules for processing a received P2P mode DIO apply to
both Intermediate Routers and the Target.
A router SHOULD discard a received P2P mode DIO with no further
processing if it does not have bidirectional reachability with the
neighbor that generated the received DIO. Note that bidirectional
reachability does not mean that the link must have the same values
for a routing metric in both directions. A router SHOULD calculate
the values of the link-level routing metrics included in the received
DIO, taking into account the metric's value in both Forward and
Reverse directions. Bidirectional reachability along a discovered
route allows the Target to use this route to reach the Origin. In
particular, the P2P-DRO messages travel from the Target to the Origin
along a discovered route.
A router MUST discard a received P2P mode DIO with no further
processing:
o if the DIO advertises INFINITE_RANK as defined in Section 17
of [RFC6550]
o if the integer part of the rank advertised in the DIO equals or
exceeds the MaxRank limit listed in the P2P Route Discovery Option
o if the routing metric values do not satisfy one or more of the
mandatory route constraints listed in the DIO or if the router
cannot evaluate the mandatory route constraints, e.g., if the
router does not support the metrics used in the constraints
o if the router previously received a P2P-DRO message with the same
RPLInstanceID and DODAGID as the received DIO and with the Stop
flag set to one
The router MUST check the Target addresses listed in the P2P-RDO and
any RPL Target options included in the received DIO. If one of its
IPv6 addresses is listed as a Target address or if it belongs to the
multicast group specified as one of the Target addresses, the router
considers itself a Target and processes the received DIO as specified
in Section 9.5. Otherwise, the router considers itself an
Intermediate Router and processes the received DIO as specified in
Section 9.4.
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9.4. Additional Processing of a P2P Mode DIO at an Intermediate Router
An Intermediate Router MUST discard a received P2P mode DIO with no
further processing
o if the DIO is received on an Ingress-only Interface; or
o if the receiving interface does not have a global or unique-local
IPv6 address configured with the address prefix implied by the
Compr field in the P2P-RDO inside the received DIO; or
o if the router cannot uniquely identify the address prefix implied
by the Compr field in the P2P-RDO (this might happen if the
receiving interface has multiple global/unique-local IPv6
addresses, each configured with a different address prefix); or
o if adding its IPv6 address to the route in the Address vector
inside the P2P-RDO would result in the route containing multiple
addresses belonging to this router.
On receiving a P2P mode DIO, an Intermediate Router MUST do the
following. The router MUST determine whether this DIO advertises a
better route than the router itself and whether the receipt of the
DIO would allow the router to advertise a better route than before.
Accordingly, the router SHOULD consider this DIO as
consistent/inconsistent from the Trickle perspective, as described in
Section 9.2. Note that the route comparison in a P2P-RPL route
discovery is performed using the parent selection rules of the OF in
use as specified in Section 14 of RPL [RFC6550]. If the received DIO
would allow the router to advertise a better route, the router MUST
add a unicast IPv6 address of the receiving interface (after eliding
Compr prefix octets) to the route in the Address vector inside the
P2P-RDO and remember this route for inclusion in its future DIOs.
When an Intermediate Router adds an IPv6 address to a route, it MUST
ensure that
o the IPv6 address is a unicast global or unique-local IPv6 address
assigned to the interface on which the DIO containing the route
was received;
o the IPv6 address was configured with the address prefix implied by
the Compr field in the P2P-RDO inside the received DIO.
To improve the diversity of the routes being discovered, an
Intermediate Router SHOULD keep track of multiple routes (as long as
all these routes are the best seen so far), one of which SHOULD be
selected in a uniform random manner for inclusion in the P2P-RDO
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inside the router's next DIO. Note that the route accumulation in a
P2P mode DIO MUST take place even if the Origin does not want any
P2P-DRO messages to be generated (i.e., the R flag inside the P2P-RDO
is set to zero). This is because the Target may still be able to use
the accumulated route as a Source Route to reach the Origin.
9.5. Additional Processing of a P2P Mode DIO at the Target
The Target MAY remember the discovered route contained in the P2P-RDO
in the received DIO for use as a Source Route to reach the Origin.
The lifetime of this Source Route is specified by the Default
Lifetime and Lifetime Unit parameters inside the DODAG Configuration
Option currently in effect. This lifetime can be extended (or
shortened) appropriately, following a hint from an upper-layer
protocol.
If the Reply flag inside the P2P-RDO in the received DIO is set to
one, the Target MUST select one or more discovered routes and send
one or more P2P-DRO messages, carrying one discovered route each,
back to the Origin. If the H flag inside the P2P-RDO is set to one,
the Target needs to select one route and send a P2P-DRO message along
this route back to the Origin. As this P2P-DRO message travels back
to the Origin, the routers on the path establish a hop-by-hop routing
state, thereby establishing a Hop-by-hop Route in the Forward
direction. If the H flag is set to zero, the number of Source Routes
to be selected (and the number of P2P-DRO messages to be sent back)
is given by one plus the value of the N field in the P2P-RDO. The
Target may select the discovered route inside the received DIO as one
or more of the routes that would be carried inside a P2P-DRO message
back to the Origin. This document does not prescribe a particular
method for the Target to select the routes. Example methods include
selecting each route that meets the specified routing constraints
until the desired number of routes has been selected, or selecting
the best routes discovered over a certain time period. If multiple
routes are to be selected, the Target SHOULD avoid selecting routes
that have large segments in common.
If the Target selects the route contained in the P2P-RDO in the
received DIO, it sends a P2P-DRO message back to the Origin
(identified by the DODAGID field in the DIO). The P2P-DRO message
MUST include a P2P-RDO that contains the selected route inside the
Address vector. Various fields inside the P2P-RDO MUST be set as
specified in Section 8.2. The Target MAY set the A flag inside the
P2P-DRO message to one if it desires the Origin to send back a
P2P-DRO-ACK message on receiving the P2P-DRO. In this case, the
Target waits for the duration of P2P_DRO_ACK_WAIT_TIME for the
P2P-DRO-ACK message to arrive. Failure to receive the P2P-DRO-ACK
message within this time duration causes the Target to retransmit the
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P2P-DRO message. The Target MAY retransmit the P2P-DRO message in
this fashion up to MAX_P2P_DRO_RETRANSMISSIONS times. Both
P2P_DRO_ACK_WAIT_TIME and MAX_P2P_DRO_RETRANSMISSIONS are
configurable parameters to be chosen based on the characteristics of
individual deployments. Note that all P2P-DRO transmissions and
retransmissions MUST take place while the Target is still a part of
the temporary DAG created for the route discovery. A Target MUST NOT
transmit a P2P-DRO if it no longer belongs to this DAG.
The Target MAY set the Stop flag inside the P2P-DRO message to one if
o this router is the only Target specified in the corresponding DIO,
i.e., the corresponding DIO specified a unicast address of the
router as the TargetAddr inside the P2P-RDO with no additional
Targets specified via RPL Target options; and
o the Target has already selected the desired number of routes.
The Target MAY include a Metric Container option in the P2P-DRO
message. This Metric Container contains the end-to-end routing
metric values for the route specified in the P2P-RDO. The Target
MUST transmit the P2P-DRO message via a link-local multicast.
A Target MUST NOT forward a P2P mode DIO any further if no other
Targets are to be discovered, i.e., if a unicast IPv6 address (of
this Target) is specified as the TargetAddr inside the P2P-RDO and no
additional Targets are specified via RPL Target options inside the
DIOs for this route discovery. Otherwise, the Target MUST generate
DIOs for this route discovery as an Intermediate Router would.
9.6. Processing a P2P-DRO at an Intermediate Router
If the DODAGID field in the received P2P-DRO does not list a router's
own IPv6 address, the router considers itself an Intermediate Router
and MUST process the received message in the following manner:
o The router MUST discard the received P2P-DRO with no further
processing if it does not belong to the temporary DAG identified
by the RPLInstanceID and the DODAGID fields in the P2P-DRO.
o If the Stop flag inside the received P2P-DRO is set to one, the
router SHOULD NOT send or receive any more DIOs for this temporary
DAG and SHOULD cancel any pending DIO transmissions.
o The router MUST ignore any Metric Container options contained in
the P2P-DRO message.
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o If an Address[NH] element inside the P2P-RDO lists the router's
own unicast IPv6 address, the router is a part of the route
carried in the P2P-RDO. In this case, the router MUST do the
following:
* To prevent loops, the router MUST discard the P2P-DRO message
with no further processing if the Address vector in the P2P-RDO
includes multiple IPv6 addresses assigned to the router's
interfaces.
* If the H flag inside the P2P-RDO is set to one, the router MUST
store the state for the Forward Hop-by-hop Route carried inside
the P2P-RDO. This state consists of:
+ the RPLInstanceID and the DODAGID fields of the P2P-DRO
+ the route's destination, the Target (identified by the
TargetAddr field inside the P2P-RDO)
+ the IPv6 address of the next hop, Address[NH+1] (unless the
NH value equals the number of elements in the Address
vector, in which case the Target itself is the next hop)
This Hop-by-hop routing state MUST expire at the end of the
lifetime specified by the Default Lifetime and Lifetime Unit
parameters inside the DODAG Configuration Option used in P2P
mode DIOs for this route discovery.
* If the router already maintains a Hop-by-hop state listing the
Target as the destination and carrying the same RPLInstanceID
and DODAGID fields as the received P2P-DRO, and the next-hop
information in the state does not match the next hop indicated
in the received P2P-DRO, the router MUST discard the P2P-DRO
message with no further processing. Note that this situation
would occur in the following two cases:
+ When the route listed in the Address vector inside the
P2P-RDO contains a previously undetected loop. In this
case, this rule causes the P2P-DRO messages to be discarded.
+ When a Hop-by-hop Route between the Origin and the Target,
previously established using the same RPLInstanceID and
DODAGID as the route currently being established, still
exists and at least partially overlaps the route currently
being established.
* The router MUST decrement the NH field inside the P2P-RDO and
send the P2P-DRO message further via link-local multicast.
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9.7. Processing a P2P-DRO at the Origin
When a router receives a P2P-DRO message that lists its IPv6 address
in the DODAGID field, the router recognizes itself as the Origin for
the corresponding P2P-RPL route discovery, notes the Target that
originated this message (from the TargetAddr field inside the
P2P-RDO), and processes the message in the following manner:
o The Origin MUST discard the received P2P-DRO with no further
processing if it no longer belongs to the temporary DAG identified
by the RPLInstanceID and the DODAGID fields in the P2P-DRO.
o If the Stop flag inside the received P2P-DRO is set to one, the
Origin SHOULD NOT generate any more DIOs for this temporary DAG
and SHOULD cancel any pending DIO transmissions.
o If the P2P-RDO inside the P2P-DRO has the H flag set to zero, the
Address vector inside the P2P-RDO contains a Source Route to this
Target. The Origin MUST set the lifetime of this Source Route to
the value specified by the Default Lifetime and Lifetime Unit
parameters inside the DODAG Configuration Option in the P2P mode
DIOs used for this route discovery. This lifetime could be
extended (or shortened) appropriately, following a hint from an
upper-layer protocol.
o If the P2P-RDO inside the P2P-DRO has the H flag set to one, the
P2P-DRO message is establishing a Hop-by-hop Route to this Target,
and the Origin MUST store in its memory the state for this
Hop-by-hop Route in the manner described in Section 9.6. This
Hop-by-hop routing state MUST expire at the end of the lifetime
specified by the Default Lifetime and Lifetime Unit parameters
inside the DODAG Configuration Option used in P2P mode DIOs for
this route discovery. A Standards Track version of P2P-RPL may
consider specifying a signaling mechanism that will allow the
Origin to extend (or shorten) the lifetime of a P2P-RPL Hop-by-hop
Route, following a suitable hint from an upper-layer protocol.
o If the received P2P-DRO message contains one or more Metric
Container options, the Origin MAY store the values of the routing
metrics associated with the discovered route in its memory. This
information may be useful in formulating the constraints for any
future P2P-RPL route discovery to this Target.
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o If the A flag is set to one in the received P2P-DRO message, the
Origin MUST generate a P2P-DRO-ACK message as described in
Section 10 and unicast the message to the Target. The Origin MAY
use the route just discovered to send the P2P-DRO-ACK message to
the Target. Section 12 describes how a packet may be forwarded
along a Source/Hop-by-hop Route discovered using P2P-RPL.
10. The P2P Discovery Reply Object Acknowledgement (P2P-DRO-ACK)
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | Version |Seq| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DODAGID |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of the Base P2P Discovery Reply Object
Acknowledgement (P2P-DRO-ACK)
A P2P-DRO message may fail to reach the Origin due to a number of
reasons. Unlike the DIO messages, which benefit from Trickle-
controlled retransmissions, the P2P-DRO messages are prone to loss
due to unreliable packet transmission in LLNs. Since a P2P-DRO
message travels via link-local multicast, it cannot use link-level
acknowledgements to improve the reliability of its transmission.
Also, an Intermediate Router may drop the P2P-DRO message (e.g.,
because of its inability to store the state for the Hop-by-hop Route
that the P2P-DRO is establishing). To protect against the potential
failure of a P2P-DRO message to reach the Origin, the Target MAY
request that the Origin send back a P2P-DRO Acknowledgement
(P2P-DRO-ACK) message on receiving a P2P-DRO message. Failure to
receive such an acknowledgement within the P2P_DRO_ACK_WAIT_TIME
interval of sending the P2P-DRO message forces the Target to resend
the message (as described in Section 9.5).
This section defines two new RPL control message types: the P2P-DRO
Acknowledgement (P2P-DRO-ACK), with code 0x05; and the Secure
P2P-DRO-ACK, with code 0x85. A P2P-DRO-ACK message MUST travel as a
unicast message from the Origin to the Target. The IPv6 source and
destination addresses used in a P2P-DRO-ACK message MUST be global or
unique-local. The format of a base P2P-DRO-ACK message is shown in
Figure 3. Various fields in a P2P-DRO-ACK message MUST have the same
values as the corresponding fields in the P2P-DRO message. The field
marked as "Reserved" MUST be set to zero on transmission and MUST be
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ignored on reception. A Secure P2P-DRO-ACK message follows the
format shown in Figure 7 of [RFC6550], where the base format is the
same as the base P2P-DRO-ACK shown in Figure 3.
11. Secure P2P-RPL Operation
Each RPL control message type, including those defined in this
document, has a secure version. A secure RPL control message is
identified by the value 1 in the most significant bit of the Code
field. Each secure RPL control message contains a Security section
(see Figures 7 and 8 of [RFC6550]) whose contents are described in
Section 6.1 of [RFC6550]. Sections 6.1, 10, and 19 of [RFC6550]
describe core RPL's security apparatus. These sections are
applicable to P2P-RPL's secure operation as well, except as
constrained in this section.
Core RPL allows a router to decide locally on a per-packet basis
whether to use security and, if yes, what Security Configuration (see
definition in Section 3) to use (the only exception being the
requirement to send a Secure DIO in response to a Secure DIS; see
Section 10.2 of [RFC6550]). In contrast, this document requires that
routers participating in a P2P-RPL route discovery follow the
Origin's lead regarding security. The Origin decides whether to use
security, and the particular Security Configuration to be used for
this purpose. All the routers participating in this route discovery
MUST generate only secure control messages if the Origin so decides
and MUST use for this purpose the Security Configuration that the
Origin chose. The Origin MUST NOT set the "Key Identifier Mode"
field inside the chosen Security Configuration to value 1, since this
setting indicates the use of a per-pair key, which is not suitable
for securing messages that travel by (link-local) multicast (e.g.,
DIOs) or that travel over multiple hops (e.g., P2P-DROs). The Origin
MUST use the chosen Security Configuration to secure all the control
messages (DIOs and P2P-DRO-ACKs) it generates.
A router MUST NOT join the temporary DAG being created for a P2P-RPL
route discovery if:
o it receives both secure and unsecure DIOs or Secure DIOs with
different Security Configurations pertaining to this route
discovery (i.e., referring to the same RPLInstanceID and DODAGID
combination) prior to joining; or
o it cannot use the Security Configuration found in the Secure DIOs
pertaining to this route discovery.
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When a router (an Intermediate Router or a Target) joins a temporary
DAG being created using Secure DIOs, it MUST remember the common
Security Configuration used in the received Secure DIOs and MUST use
this configuration to secure all the control messages (DIOs and
P2P-DROs) it generates.
If an Intermediate Router (or a Target) encounters a control message
(a DIO or a P2P-DRO or a P2P-DRO-ACK) pertaining to this route
discovery that is either not secure or does not follow the Security
Configuration the router remembers for this route discovery, the
router MUST enter the "lock down" mode for the remainder of its stay
in this temporary DAG. An Intermediate Router (or a Target) in the
"lock down" mode MUST NOT generate or process any control messages
(irrespective of the Security Configuration used) pertaining to this
route discovery. If the Origin receives a control message (a
P2P-DRO) that does not follow the Security Configuration the Origin
has chosen for this route discovery, it MUST discard the received
message with no further processing.
12. Packet Forwarding along a Route Discovered Using P2P-RPL
An Origin uses the Source Routing Header (SRH) [RFC6554] to send a
packet along a Source Route discovered using P2P-RPL.
Travel along a Hop-by-hop Route, established using P2P-RPL, requires
specifying the RPLInstanceID and the DODAGID (of the temporary DAG
used for the route discovery) to identify the route. This is because
a P2P-RPL route discovery does not use globally unique RPLInstanceID
values, and hence both the RPLInstanceID (a local value assigned by
the Origin) and the DODAGID (an IPv6 address of the Origin) are
required to uniquely identify a P2P-RPL Hop-by-hop Route to a
particular destination.
An Origin includes a RPL option [RFC6553] inside the IPv6 Hop-by-Hop
Options header of a packet to send it along a Hop-by-hop Route
established using P2P-RPL. For this purpose, the Origin MUST set the
DODAGID of the temporary DAG used for the route discovery as the
source IPv6 address of the packet. Further, the Origin MUST specify
inside the RPL option the RPLInstanceID of the temporary DAG used for
the route discovery and set the O flag inside the RPL option to one.
On receiving this packet, an Intermediate Router checks the O flag
and correctly infers the source IPv6 address of the packet as the
DODAGID of the Hop-by-hop Route. The router then uses the DODAGID,
the RPLInstanceID, and the destination address to identify the
routing state to be used to forward the packet further.
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13. Interoperability with Core RPL
This section describes how RPL routers that implement P2P-RPL
interact with RPL routers that do not. In general, P2P-RPL operation
does not affect core RPL operation, and vice versa. However, core
RPL does allow a router to join a DAG as a leaf node even if it does
not understand the Mode of Operation (MOP) used in the DAG. Thus, a
RPL router that does not implement P2P-RPL may conceivably join a
temporary DAG being created for a P2P-RPL route discovery as a leaf
node and maintain its membership even though the DAG no longer
exists. This may impose a drain on the router's memory. However,
such RPL-only leaf nodes do not interfere with P2P-RPL route
discovery, since a leaf node may only generate a DIO advertising an
INFINITE_RANK and all routers implementing P2P-RPL are required to
discard such DIOs. Note that core RPL does not require that a router
join a DAG whose MOP it does not understand. Moreover, RPL routers
in a particular deployment may have strict restrictions on the DAGs
they may join, thereby mitigating the problem.
The P2P-RPL mechanism described in this document works best when all
the RPL routers in the LLN implement P2P-RPL. In general, the
ability to discover routes, as well as the quality of discovered
routes, would deteriorate with the fraction of RPL routers that
implement P2P-RPL.
14. Security Considerations
In general, the security considerations for the operation of P2P-RPL
are similar to those for the operation of RPL (as described in
Section 19 of the RPL specification [RFC6550]). Sections 6.1 and 10
of [RFC6550] describe RPL's security framework, which provides data
confidentiality, authentication, replay protection, and delay
protection services. This security framework can also be used in
P2P-RPL after taking into account the constraints specified in
Section 11. P2P-RPL requires that all routers participating in a
secure route discovery use the Security Configuration chosen by the
Origin. The intention is to avoid compromising the overall security
of a route discovery due to some routers using a weaker Security
Configuration. With the "lock down" mechanism as described in
Section 11 in effect, it is unlikely that an Origin would accept a
route discovered under a Security Configuration other than the one it
intended. Any attempt to use a different Security Configuration
(than the one the Origin intended) is likely to result, in the worst
case, in the failure of the route discovery process. In the best-
case scenario, any such attempt by a rogue router would result in its
neighbors entering the "lock down" mode and acting as firewalls to
allow the route discovery to proceed in the remaining network.
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The RPL specification [RFC6550] describes three modes of security:
unsecured, preinstalled, and authenticated. In the unsecured mode,
secure control messages are not used, and the only available security
is the security provided by the link-layer protocols. In the
preinstalled mode, all the nodes use a preinstalled group key to join
a secure DAG as the "routers" or "hosts", where the term "router"
means a node that is capable of forwarding packets received from its
parents or children in the DAG, and the term "host" refers to nodes
that cannot function as "routers". In the authenticated mode, the
nodes can join a secure DAG as "hosts" using the preinstalled key but
then need to authenticate themselves to a key server to obtain the
key that will allow them to work as "routers". The temporary DAG
created for a P2P-RPL discovery cannot be used for routing packets.
Hence, it is not meaningful to say that a node joins this DAG as a
"router" or a "host" in the sense defined above. Hence, in P2P-RPL,
there is no distinction between the preinstalled and authenticated
modes. A router can join a temporary DAG created for a secure
P2P-RPL route discovery only if it can support the Security
Configuration in use, which also specifies the key in use. It does
not matter whether the key is preinstalled or dynamically acquired.
The router must have the key in use before it can join the DAG being
created for a secure P2P-RPL route discovery.
If a rogue router can support the Security Configuration in use (in
particular, if it knows the key in use), it can join the secure
P2P-RPL route discovery and cause various types of damage. Such a
rogue router could advertise false information in its DIOs in order
to include itself in the discovered route(s). It could generate
bogus P2P-DRO messages carrying bad routes or maliciously modify
genuine P2P-DRO messages it receives. A rogue router acting as the
Origin could launch denial-of-service attacks against the LLN
deployment by initiating fake P2P-RPL route discoveries; in this type
of scenario, RPL's authenticated mode of operation, where a node can
obtain the key to use for a P2P-RPL route discovery only after proper
authentication, would be useful.
Since a P2P-DRO message travels along a Source Route specified inside
the message, some of the security concerns that led to the
deprecation of Type 0 routing headers [RFC5095] may apply. To avoid
the possibility of a P2P-DRO message traveling in a routing loop,
this document requires that each Intermediate Router confirm that the
Source Route listed inside the message does not contain any routing
loop involving itself before the router could forward the message
further. As specified in Section 9.6, this check involves the router
making sure that its IPv6 addresses do not appear multiple times
inside the Source Route with one or more other IPv6 addresses in
between.
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15. IANA Considerations
15.1. Additions to Mode of Operation
This document defines a new Mode of Operation, entitled "P2P Route
Discovery Mode of Operation" (see Section 6), assigned a value of 4
from the "Mode of Operation" space [RFC6550].
+-------+---------------------------------------+---------------+
| Value | Description | Reference |
+-------+---------------------------------------+---------------+
| 4 | P2P Route Discovery Mode of Operation | This document |
+-------+---------------------------------------+---------------+
Mode of Operation
15.2. Additions to RPL Control Message Options
This document defines a new RPL option: "P2P Route Discovery" (see
Section 7), assigned a value of 0x0a from the "RPL Control Message
Options" space [RFC6550].
+-------+---------------------+---------------+
| Value | Meaning | Reference |
+-------+---------------------+---------------+
| 0x0a | P2P Route Discovery | This document |
+-------+---------------------+---------------+
RPL Control Message Options
15.3. Additions to RPL Control Codes
This document defines the following new RPL messages:
o "P2P Discovery Reply Object" (see Section 8), assigned a value of
0x04 from the "RPL Control Codes" space [RFC6550].
o "Secure P2P Discovery Reply Object" (see Section 8.1), assigned a
value of 0x84 from the "RPL Control Codes" space [RFC6550].
o "P2P Discovery Reply Object Acknowledgement" (see Section 10),
assigned a value of 0x05 from the "RPL Control Codes"
space [RFC6550].
o "Secure P2P Discovery Reply Object Acknowledgement" (see
Section 10), assigned a value of 0x85 from the "RPL Control Codes"
space [RFC6550].
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+--------+----------------------------------------+-----------------+
| Code | Description | Reference |
+--------+----------------------------------------+-----------------+
| 0x04 | P2P Discovery Reply Object | This document |
| 0x84 | Secure P2P Discovery Reply Object | This document |
| 0x05 | P2P Discovery Reply Object | This document |
| | Acknowledgement | |
| 0x85 | Secure P2P Discovery Reply Object | This document |
| | Acknowledgement | |
+--------+----------------------------------------+-----------------+
RPL Control Codes
16. Known Issues and Future Work
This document is presented as an Experimental specification to
facilitate P2P-RPL's deployment in LLN scenarios where reactive P2P
route discovery is considered useful or necessary. It is anticipated
that, once sufficient operational experience has been gained, this
specification will be revised to progress it on to the Standards
Track. Experience reports regarding P2P-RPL implementation and
deployment are encouraged, particularly with respect to:
o Secure P2P-RPL operation (Section 11);
o Rules governing Trickle operation (Section 9.2);
o Values in the default DODAG Configuration Option (Section 6.1);
o The RPLInstanceID reuse policy (Section 6.1);
o Utility and implementation complexity of allowing multiple Target
addresses in a P2P-RPL route discovery.
17. Acknowledgements
The authors gratefully acknowledge the contributions of the following
individuals (in alphabetical order) in the development of this
document: Dominique Barthel, Jakob Buron, Cedric Chauvenet, Thomas
Clausen, Robert Cragie, Ralph Droms, Adrian Farrel, Stephen Farrell,
Brian Haberman, Ted Humpal, Richard Kelsey, Phil Levis, Charles
Perkins, Joseph Reddy, Michael Richardson, Zach Shelby, Martin
Stiemerling, Pascal Thubert, Hristo Valev, and JP Vasseur.
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18. References
18.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
March 2012.
18.2. Informative References
[RFC5095] Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095,
December 2007.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[RFC6552] Thubert, P., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)",
RFC 6552, March 2012.
[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553,
March 2012.
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[RFC6998] Goyal, M., Ed., Baccelli, E., Brandt, A., and J. Martocci,
"A Mechanism to Measure the Routing Metrics along a Point-
to-Point Route in a Low-Power and Lossy Network",
RFC 6998, August 2013.
[ROLL-TERMS]
Vasseur, JP., "Terminology in Low power And Lossy
Networks", Work in Progress, March 2013.
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Authors' Addresses
Mukul Goyal (editor)
University of Wisconsin Milwaukee
3200 N. Cramer St.
Milwaukee, WI 53201
USA
Phone: +1-414-229-5001
EMail: mukul@uwm.edu
Emmanuel Baccelli
INRIA
Phone: +33-169-335-511
EMail: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
Matthias Philipp
INRIA
Phone: +33-169-335-511
EMail: matthias-philipp@gmx.de
Anders Brandt
Sigma Designs
Emdrupvej 26A, 1.
Copenhagen, Dk-2100
Denmark
Phone: +45-29609501
EMail: abr@sdesigns.dk
Jerald Martocci
Johnson Controls
507 E. Michigan Street
Milwaukee, WI 53202
USA
Phone: +1-414-524-4010
EMail: jerald.p.martocci@jci.com
Goyal, et al. Experimental [Page 40]