Internet Engineering Task Force M. Goyal, Ed.
Internet-Draft University of Wisconsin Milwaukee
Intended status: Standards Track E. Baccelli, Ed.
Expires: February 10, 2011 INRIA
P2P. Team
August 9, 2010
Reactive Discovery of Point-to-Point Routes in Low Power and Lossy
Networks
draft-dt-roll-p2p-rpl-02
Abstract
Point to point (P2P) communication between arbitrary IPv6 routers and
hosts in a Low power and Lossy Network (LLN) is a key requirement for
many applications. RPL, the IPv6 Routing Protocol for LLNs,
constrains the LLN topology to a Directed Acyclic Graph (DAG) and
requires the P2P routing to take place along the DAG links. Such P2P
routes may be significantly suboptimal and may lead to traffic
congestion near the DAG root. This document describes a P2P route
discovery mechanism complementary to RPL base functionality. This
mechanism allows an RPL-aware IPv6 router or host to discover and
establish on demand one or more routes to another RPL-aware IPv6
router or host in the LLN such that the discovered routes meet the
specified cost criteria.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 10, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Targeted Use Cases . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Functional Overview . . . . . . . . . . . . . . . . . . . . . 6
5. Propagation of Discovery Messages . . . . . . . . . . . . . . 7
5.1. The Route Discovery Option . . . . . . . . . . . . . . . . 8
5.2. Setting a DIO Carrying a Route Discovery Option . . . . . 9
5.3. Joining a Temporary DAG . . . . . . . . . . . . . . . . . 11
5.4. Processing a DIO Carrying a Route Discovery Option . . . . 11
5.5. Additional Processing of a DIO Carrying a Route
Discovery Option At An Intermediate Router . . . . . . . . 12
5.6. Additional Processing of a DIO Carrying a Route
Discovery Option At The Target Node . . . . . . . . . . . 13
6. Propagation of Discovery Reply Messages . . . . . . . . . . . 13
6.1. The Discovery Reply Object (DRO) . . . . . . . . . . . . . 14
6.1.1. The Source Route Option . . . . . . . . . . . . . . . 16
6.1.2. Processing a DRO At An Intermediate Router . . . . . . 17
6.2. DRO as Acknowledgement for Backward Source Routes . . . . 18
6.3. DRO as Carrier of Forward/Bidirectional Source Routes . . 18
6.4. Establishing Hop-by-hop Routes Via DRO . . . . . . . . . . 18
7. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Authors and Contributors . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
RPL [I-D.ietf-roll-rpl] provides multipoint-to-point (MP2P) routes
from nodes in a Low power and Lossy Network (LLN) to a sink node by
organizing the nodes along a Directed Acyclic Graph (DAG) rooted at
the sink. The nodes determine their position in the DAG so as to
optimize their routing cost to reach the DAG root. A node advertises
its position (the "rank") in the DAG by originating a DODAG
Information Object (DIO) message. The DIO message is sent via link-
local multicast and also includes information such as the DAG root's
identity, the routing metrics/constraints
[I-D.ietf-roll-routing-metrics] and the objective function (OF) in
use. When a node joins the DAG, it determines its own rank in the
DAG based on that advertised by its neighbors and originates its own
DIO message.
RPL enables point-to-multipoint (P2MP) routing from a node to its
descendants in the DAG by allowing a node to send a Destination
Advertisement Object (DAO) upwards along the DAG. The DAO carries
the potentially aggregated information regarding the descendants (and
other local prefixes) reachable through the originating node.
RPL also provides mechanisms for point-to-point (P2P) routing between
any two nodes in the DAG. If the destination is within the source's
"range", the source may directly send packets to the destination.
Otherwise, a packet's path from the source to the destination depends
on the storing/non-storing operation mode of the DAG. In non-storing
mode operation, only the DAG root maintains downward routing
information and hence a packet travels all the way to the DAG root,
which then sends it towards its destination using a source route. In
storing mode operation, if the destination is a DAG descendant and
the source maintains "downwards" routing state about this descendant,
it can forward the packet along this route. Otherwise, the source
sends the packet to a DAG parent, which then applies the same set of
rules to forward the packet further. Thus, a packet travels up the
DAG until it reaches a node that knows of the downwards route to the
destination and then it travels down the DAG towards its destination.
A node may or may not maintain routing state about a descendant
depending on whether its immediate children send it such information
in their DAOs. Thus, in the best case storing mode scenario, the
"upwards" segment of the P2P route between a source and a destination
ends at the first common ancestor of the source and the destination.
In the worst case, the "upwards" segment would extend all the way to
the DAG's root. In both storing and non-storing mode operations, if
the destination did not originate a DAO, the packet will travel all
the way to the DAG's root, where it will be dropped.
The P2P routing functionality available in RPL may be inadequate for
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applications in the home and commercial building domains because of
the following reasons
[I-D.brandt-roll-rpl-applicability-home-building] [RFC5826][RFC5867]:
o The need to maintain routes "proactively", i.e. every possible
destination in the DAG must originate a DAO.
o Depending on the network topology and OF/metrics in use, the
constraint to route only along a DAG may potentially cause
significantly suboptimal P2P routes and severe traffic congestion
near the DAG root.
Clearly, there is a need for a mechanism that provides source-
initiated discovery of P2P routes that are not along an existing DAG.
This document thus describes such a mechanism, complementary to the
basic RPL functionality.
The specified scheme is based on a reactive on-demand approach, which
enables a node to discover one or more "good enough" routes in either
direction between itself and another node in the LLN without any
constraints regarding the existing DAG-membership of the links that
such routes may use. Such routes may be source-routes or hop-by-hop
ones. A complementary functionality, necessary to help decide
whether to initiate a route discovery, is a mechanism to measure the
end-to-end cost of an existing route. Section 7 provides further
details on how such functionality, to be described in a separate
document, can be used to determine the "good enough" criteria for use
in the route discovery mechanism described in this document.
2. Targeted Use Cases
The mechanisms described in this document are intended to be employed
as complementary to RPL in specific scenarios that need point-to-
point (P2P) routes between arbitrary routers.
One target use case, common in a home environment, involves a remote
control (or a motion sensor) that suddenly needs to communicate with
a lamp module, whose network address it knows apriori. In this case,
the source of data (the remote control or the motion sensor) must be
able to discover a route to the destination (the lamp module) "on
demand".
Another target use case, common in a large 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, and thus
routes across this DAG are desirable.
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Targeted use cases also include scenarios where energy or latency
constraints are not satisfied by the P2P routes along a DAG because
they involve traversing many more intermediate 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
[I-D.ietf-roll-terminology] and [I-D.ietf-roll-rpl]. Specifically,
the term node refers to an RPL router or an RPL host as defined in
[I-D.ietf-roll-rpl]. This document introduces the following terms:
Origin Node: The RPL node initiating the route discovery. The origin
node acts as one end point of the routes to be discovered.
Target Node: The RPL node at the other end point of the routes to be
discovered.
Intermediate Router: An RPL router that is neither the origin nor the
target.
Forward Route: A route from the origin node to the target node.
Backward Route: A route from the target node to the origin node.
Bidirectional Route: A route that can be used in both directions:
from the origin node to the target node and vice versa.
Source Route: A complete and ordered list of routers that can be used
by a packet to travel from a source node to a destination node. Such
source routes can be carried by a packet in a proposed Type 4 Routing
Header [I-D.ietf-6man-rpl-routing-header].
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.
Propagation Constraints: The constraints on aggregated routing metric
values, as defined in [I-D.ietf-roll-routing-metrics], that MUST be
satisfied before an intermediate router will process the Route
Discovery Option (defined in this document) contained inside a DODAG
Information Object (DIO).
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Route Constraints: Additional constraints on aggregated routing
metric values, as defined in [I-D.ietf-roll-routing-metrics], that
MUST be satisfied by a discovered route in order to be considered
"good enough".
Good Enough Criteria: The propagation constraints and the route
constraints together constitute the good enough criteria.
4. Functional Overview
This section contains a high level description of the route discovery
mechanism proposed in this document.
The route discovery begins with the origin node generating a
"Discovery" message. The origin node indicates in the message:
o The target node;
o The relevant routing metrics;
o The constraints on how far the Discovery message may travel
(henceforth called the propagation constraints);
o Additional constraints used to determine if a discovered route is
"good enough" (henceforth called the route constraints);
o The direction (forward: from the origin node to the target node;
backward: from the target node to the origin node; or
bidirectional) of the route being discovered;
o The desired number of routes;
o Whether the route is a source-route or a hop-by-hop one.
The Discovery message propagates via IPv6 link-local multicast with a
receiving router discarding the message if it does not satisfy the
propagation constraints or if hop-by-hop routes are desired and the
router cannot store state for such a route. As a copy of the
Discovery message travels towards the target node, it accumulates the
relevant routing metric values as well as the route it takes. When
the target node receives a copy of the Discovery message, it applies
both the propagation constraints and the route constraints to
determine if the discovered route is good enough. Thus, the good
enough discovered routes satisfy both the propagation constraints as
well as the route constraints although the propagation of Discovery
messages is guided by propagation constraints alone. The propagation
constraints and the route constraints together constitute the good
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enough criteria. Using only a subset of the good enough criteria as
the propagation constraints simplifies the operation of intermediate
routers, an important consideration in many LLN application domains.
The route discovery process may result in the discovery of several
good enough routes. This document does not specify how does the
target node select routes among the good enough ones. Example
selection methods include selecting the routes as they are discovered
or selecting the best routes discovered over a certain time period.
If the origin node had requested the discovery of backward source-
routes, the target node caches one or more good enough source-routes
it selects. Additionally, the target node sends one or more
"Discovery Reply" message to the origin node to acknowledge the
discovery of these routes. These acknowledgements allow the origin
node to judge the success of the route discovery.
If the origin node had requested the discovery of "n" forward source-
routes, the target node sends the "n" good enough source-routes it
selects to the origin node in one or more Discovery Reply messages.
If the origin node had requested the discovery of "n" bidirectional
source-routes, the target node caches the "n" good enough source-
routes it identifies and also sends these routes to the origin node
in one or more Discovery Reply messages.
If the origin node had requested the discovery of "n" forward/
backward/bidirectional hop-by-hop routes, the target node sends out a
Discovery Reply message to the origin node for each one of the "n"
good enough routes it selects. The Discovery Reply message travels
towards the origin node along the discovered route. As this message
travels towards the origin node, it establishes appropriate forward/
backward routing state in the routers on the path.
5. Propagation of Discovery Messages
RPL uses DIO message propagation to build a DAG. The DIO message
travels via IPv6 link-local multicast. Each node joining the DAG
determines a rank for itself and ignores the subsequent DIO messages
received from lower (higher in numerical value) ranked neighbors.
Thus, the DIO messages propagate outward from the DAG root rather
than return inward towards the DAG root. The DIO message generation
at a node is further controlled by a trickle timer that allows a node
to avoid generating unnecessary messages [I-D.ietf-roll-trickle].
The link-local multicast based propagation, trickle-controlled
generation and the rank-based poisoning of messages traveling in the
wrong direction (towards the DAG root) provide powerful incentives to
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use the DIO message as the Discovery message and propagate the DIO/
Discovery message by creating a "temporary" DAG. The routing metrics
used for the creation of this temporary DAG SHOULD be same as (or be
a subset of) the routing metrics being used for route discovery.
Similarly, the objective function, used for rank calculation in the
temporary DAG, SHOULD be same as the objective function that
determines the aggregated cost of a route when limited to the routing
metrics being used for temporary DAG creation.
The propagation constraints limit the spread of the temporary DAG.
The temporary DAG restricts the network topology within which the
route discovery takes place. Thus, all the discovered routes lie
within this restricted topology and implicitly satisfy the
propagation constraints. Among the discovered routes, the good
enough routes are the ones that meet the route constraints. Thus,
for successful route discovery, the propagation constraints and the
route constraints MUST be compatible. The division of the overall
good enough criteria between the two sets of constraints is an
implementation specific decision. If desired, an implementation MAY
include all constraints in the set of propagation constraints and
keep the set of route constraints empty.
5.1. The Route Discovery Option
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 = 9 | Option Length | D |H| N | L |O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Target Address |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OCP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of the Route Discovery Option
In order to be used as a Discovery message, a DIO MUST carry a "Route
Discovery" option illustrated in Figure 1. A DIO MUST NOT carry more
than one Route Discovery options. A router MUST ignore the second
and subsequent Route Discovery options carried by a DIO. A Route
Discovery option consists of the following fields:
o Option Type = 0x09 (to be confirmed by IANA).
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o Option Length = 20 or 22 octets depending on whether the OCP field
is included or not.
o D: A 2-bit field that indicates the direction of the desired
routes:
* D = 0x00: Forward;
* D = 0x01: Backward;
* D = 0x02: Bidirectional.
o H: This flag, when set, indicates if hop-by-hop routes are
desired. The flag is cleared if source routes are desired.
o N: A 3-bit unsigned integer indicating the number of routes
desired.
o L: A 2-bit field indicating the minimum "Life Time" of the
temporary DAG, i.e., the minimum duration a router joining the
temporary DAG must maintain its membership in the DAG:
* L = 0x00: Minimum life time is 5 seconds;
* L = 0x01: Minimum life time is 10 seconds;
* L = 0x02: Minimum life time is 1 minute;
* L = 0x03: Minimum life time is 10 minutes.
o O: This flag, when set, indicates that an OCP field is present in
the Route Discovery option.
o Target Address: The IPv6 address of the target node.
o OCP: 16 bit unsigned integer. An optional field, present only if
the O flag is set, This field indicates the objective function
that MAY be used by the target node to compare two good enough
routes.
5.2. Setting a DIO Carrying a Route Discovery Option
A DIO message that carries a Route Discovery option MUST set the Base
Object, described in [I-D.ietf-roll-rpl], in the following manner:
o RPLInstanceID: RPLInstanceID MUST be a local value as described in
Section 4.1 of [I-D.ietf-roll-rpl].
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o Grounded (G) Flag: MUST be cleared since the objective of DAG
formation is the propagation of Route Discovery option. This DAG
is temporary in nature and is not used for routing purpose.
o Destination Advertisement Supported (A) Flag: MUST be cleared for
same reasons as described above.
o Destination Advertisement Trigger (T) Flag: MUST be cleared.
o Mode of Operation (MOP): This document suggests a new value (0x04)
for this field (to be confirmed by IANA).
o DODAGPreference (Prf): TBD
o Destination Advertisement Trigger Sequence Number (DTSN): TBD
o DODAGID: IPv6 address of the origin node.
The other fields in the Base Object are set as per the rules
described in [I-D.ietf-roll-rpl].
The DODAG Configuration option, carried in the DIO message, specifies
the parameters for the trickle timer operation that governs the
generation of DIO messages by the routers joining the temporary DAG.
The future versions of this document will specify the default values
to be used for these parameters. The other fields defined in the
DODAG Configuration option are set as follows:
o The MaxRankIncrease field MUST be set to 0 to disable local repair
of the temporary DAG.
o This document RECOMMENDS a value 1 for the MinHopRankInc field.
o Objective Code Point (OCP): The OCP to be used for temporary DAG
formation. This document RECOMMENDS RPL Objective Function 0, as
defined in [I-D.ietf-roll-of0], for use as the objective function
for the formation of the temporary DAG. The objective function
used for temporary DAG formation SHOULD be compatible with the
objective function to determine the aggregated cost of a
discovered route.
A DIO, that contains a Route Discovery option, MUST specify the
propagation constraints in one or more Metric Container options
placed before the Route Discovery option and the route constraints in
the Metric Container options placed after the Route Discovery option
inside the DIO. The routing metrics being used for temporary DAG
formation SHOULD be same as or a subset of the routing metrics being
used for route discovery. These routing metrics MUST be placed in
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the Metric Container options placed before the Route Discovery
option.
A DIO, carrying a Route Discovery option, MUST NOT carry any Route
Information or Prefix Information options described in
[I-D.ietf-roll-rpl].
5.3. Joining a Temporary DAG
When a node joins a temporary DAG advertized by a DIO carrying the
Route Discovery option, it MUST maintain its membership in the DAG
for the Minimum Life Time duration listed in the Route Discovery
option. Maintaining membership in the DAG implies remembering:
o The RPLInstanceID, the DODAGID and the DODAGVersionNumber for the
temporary DAG;
o The node's rank in the temporary DAG as well as the address of at
least one DAG parent;
o The propagation and the route constraints being used;
o In case of intermediate routers, the values for the routing
metrics, along with the associated source route from the origin
node till this node (carried in a Record Route IPv6 Extension
Header proposed in [I-D.thubert-6man-reverse-routing-header]),
contained in the best DIO (as per the OCP specified in the DODAG
Configuration option) received so far.
Although the main purpose of a temporary DAG's existence is to
facilitate the propagation of the Route Discovery option, the
temporary DAG MAY also be used for the Discovery Reply Object
(defined in Section 6.1 to travel from the target node to the origin
node. Hence, a node in a temporary DAG SHOULD also remember the
address of at least one DAG parent that provides, as per the node's
knowledge, the best end-to-end route back to the origin node. A node
SHOULD delete information about a temporary DAG once the duration of
its membership in the DAG has exceeded the DAG's minimum life time.
5.4. Processing a DIO Carrying a Route Discovery Option
The rules for DIO processing and transmission, described in Section 7
of RPL [I-D.ietf-roll-rpl], apply to DIOs carrying a Route Discovery
option as well except as modified in this document.
The following rules for processing a DIO carrying a Route Discovery
Option apply to both intermediate routers and the target nodes.
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A node MUST discard the DIO with no further processing and optionally
log an error if any of the following conditions are true:
o The node does not support the OCP specified in the DODAG
Configuration option.
o The node does not support one or more of the metrics contained in
the Metric Container options in the DIO.
o The node does not have sufficient information to calculate the
values of these routing metrics.
A node MUST discard the DIO with no further processing and optionally
log an error if any of the following conditions are found to be true
while processing a Route Discovery option contained in the received
DIO:
o The H field is set, i.e. hop-by-hop routes are desired, but the
node cannot participate in a hop-by-hop route.
o The node cannot maintain its membership in the temporary DAG for
the minimum life time duration mentioned in the Route Discovery
option.
5.5. Additional Processing of a DIO Carrying a Route Discovery Option
At An Intermediate Router
After executing the steps listed in Section 5.4, an intermediate
router processes a received DIO carrying a Route Discovery option in
the following manner.
The router updates the routing metric values contained in all the
Metric Container options inside the DIO. The router MUST discard the
DIO with no further processing and optionally log an error if the
aggregated values of the routing metrics do not meet every
propagation constraint listed in the DIO. The router MAY optionally
check the route constraints listed in the DIO and discard the DIO
with no further processing if these constraints are not met.
The router determines if this DIO is the best it has received so far
for this temporary DAG (as per the OCP in the DODAG Configuration
object). If yes, the router makes a copy of the routing metric
values contained in this DIO along with the route travelled by the
DIO so far. The router also resets the trickle timer and, at the
expiry of the timer, generates a new DIO for this temporary DAG
carrying the Route Discovery option, the best metric values it knows
and the source route associated with these values (in a Record Route
IPv6 extension header proposed in
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[I-D.thubert-6man-reverse-routing-header]).
5.6. Additional Processing of a DIO Carrying a Route Discovery Option
At The Target Node
After executing the steps listed in Section 5.4, a target node
processes a received DIO carrying a Route Discovery option in the
following manner.
The target node updates the routing metric values contained in all
the Metric Container options inside the DIO. The target node MUST
discard the DIO with no further processing and optionally log an
error if the aggregated values of the routing metrics do not meet
every propagation and route constraint listed in the Metric Container
options in the DIO.
Otherwise, the target node considers the source route accumulated by
the received DIO as good enough and MAY select it as one of the
discovered routes. This document does not prescribe a particular
method for selecting routes among the good enough ones. Suppose the
Route Discovery option requires the target node to select "n" good
enough routes. The target node may select these "n" routes in any
manner it desires. Example selection methods include selecting the
first "n" good enough routes it discovers or selecting the "n" best
good enough routes (using the OCP specified in the Route Discovery
option to do the comparison) discovered over a certain time period.
If the target node selects at least one good enough route, it MUST
send one or more RPL Control Messages carrying a Discovery Reply
Object (defined in the next section) back to the origin node
(identified by the DODAGID field in the DIO Base Object) as discussed
in the following sections.
A node MUST NOT forward a DIO carrying a Route Discovery option that
lists one of its own addresses as the Target Address.
6. Propagation of Discovery Reply Messages
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6.1. The Discovery Reply Object (DRO)
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 | D |H| N | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DODAGID(*) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Target Address(*) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Figure 2: Format of the Discovery Reply Object (DRO)
This document defines a new RPL Control Message type, the Discovery
Reply Object (DRO) with code 0x04 (to be confirmed by IANA), that
serves the following functions:
o An acknowledgement from the target node to the origin node
regarding the successful discovery of backward source routes;
o Carries one or more forward/bidirectional source routes from the
target to the origin node;
o Establishes a hop-by-hop forward/backward/bidirectional route as
it travels from the target to the origin node.
The format for a Discovery Reply Object (DRO) is shown in Figure 2.
A DRO consists of the following fields:
o RPLInstanceID: The RPLInstanceID of the temporary DAG used for
route discovery.
o Version: The Version of the temporary DAG used for route
discovery.
o D: A 2-bit field that indicates the direction of the discovered
routes:
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* D = 0x00: Forward;
* D = 0x01: Backward;
* D = 0x02: Bidirectional.
This field has the same value as the corresponding field in the
Route Discovery option.
o H: A flag that is set if the Discovery Reply Object is
establishing an hop-by-hop route. If this flag is set, the
Discovery Reply Object also includes:
* The DODAGID and Target Address fields; and
* One Source Route option (defined in Section 6.1.1) that
contains the remaining routers on the hop-by-hop route being
established.
This flag is clear if the Discovery Reply Object carries (or is an
acknowledgement for the discovery of) one or more source routes
contained in the Source Route options.
o N: A 3-bit field that indicates the number of source routes
carried or acknowledged in the Discovery Reply Object. This field
MUST have value 1 if the Discovery Reply Object is establishing a
hop-by-hop route.
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: The DODAGID of the temporary DAG used for route
discovery. The DODAGID also identifies the origin node. This is
an optional field that MUST be present in the Discovery Reply
Object if H flag is set. The RPLInstanceID, the Version and the
DODAGID together uniquely identify the temporary DAG used for
route discovery and can be copied from the Base Object of the DIO
advertizing the temporary DAG.
o Target Address: The IPv6 address of the target node originating
the Discovery Reply Object. This is an optional field that MUST
be present in the Discovery Reply Object if H flag is set.
o Options: The Discovery Reply Object MAY carry up to N Source Route
options (defined in the next section) with each such option
carrying a source route and optionally followed by a Metric
Container option that lists the aggregated values for the routing
metrics for the source route.
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6.1.1. The Source Route Option
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 = 10 | Option Length | Compr | Pad | D | Resvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Address[1..n] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of the Source Route Option
The Source Route option, illustrated in Figure 3, carries a source
route. A Source Route option MAY be a part of the Discovery Reply
Object. When a Source Route option carries a complete source route
between the origin and the target node, it MAY be immediately
followed by a Metric Container option that contains the aggregated
values of the routing metrics for this source route.
A Source Route option consists of the following fields:
o Option Type = 0x0A (to be confirmed by IANA).
o Option Length = Variable, depending on the size of the Addresses
vector.
o Compr: 4-bit unsigned integer indicating the number of prefix
octets that are elided from each address. For example, Compr
value will be 0 if full IPv6 addresses are carried in the
Addresses vector.
o Pad: 4-bit unsigned integer. Number of octets that are used for
padding between Address[n] and the end of the Source Route option.
o D: A 2-bit field that indicates the direction of the source route:
* D = 0x00: Forward, i.e. from the origin node to the target
node;
* D = 0x01: Backward i. e., from the target node to the origin
node;
* D = 0x02: Bidirectional.
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Note that the D field in a Source Route option is independent from
the D field in the DRO containing the Source Route option.
o Resvd: These bits are reserved for future use. These bits MUST be
set to zero on transmission and MUST be ignored on reception.
o Address[1..n]: Vector of addresses, numbered 1 to n. Each vector
element has size (16 - Compr) octets.
Note that the format of the Source Route option is very similar to
that of proposed Type 4 Routing Header
[I-D.ietf-6man-rpl-routing-header].
A common network configuration for an RPL domain is that all routers
within an LLN share a common prefix. The Source Route option uses
the Compr field to allow compaction of the Address[1..n] vector when
all entries share the same prefix as the DODAGID or the Target
Address of the encapsulating Discovery Reply Object. The shared
prefix octets are not carried within the Source Route option and each
entry in Address[1..n] has size (16 - Compr) octets. When Compr is
non-zero, there may exist unused octets between the last entry,
Address[n], and the end of the Source Route option. The Pad field
indicates the number of unused octets that are used for padding.
Note that when Compr is 0, Pad MUST be null and carry a value 0.
The Source Route option MUST NOT specify a path that visits a router
more than once. When generating a Source Route option, the target
node may not know the mapping between IPv6 addresses and routers.
Minimally, the target node MUST ensure that:
o The IPv6 Addresses do not appear more than once;
o The IPv6 addresses of the origin and the target nodes do not
appear in the Address vector.
Multicast addresses MUST NOT appear in a Source Route option.
6.1.2. Processing a DRO At An Intermediate Router
When an intermediate router receives a DRO with a clear H flag, it
MUST forward the DRO to a parent node in the temporary DAG.
When an intermediate router receives a DRO that has H flag set and
contains multiple Source Route options, the router MUST drop the DRO
with no further processing and optionally log an error message.
When an intermediate router receives a DRO that has H flag set and
contains a single Source Route option, the router processes the DRO
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as described in Section 6.4.
6.2. DRO as Acknowledgement for Backward Source Routes
After selecting one or more backward source routes, a target node
MUST send a DRO message to the origin node as an acknowledgement for
the discovered routes. Such an acknowledgement helps the origin node
determine the success of route discovery.
A DRO, serving as an acknowledgement for backward source route
discovery, has its D field set to 0x01 (indicating backward) while
the H flag is cleared (indicating source route). The N field is set
to indicate the number of discovered backward source routes being
acknowledged. Such a DRO message MUST NOT contain any option.
The target node MAY unicast this DRO message to the origin node or it
MAY forward the DRO message to a parent in the temporary DAG. The
target node should take in consideration the minimum life time of the
temporary DAG when deciding to use it to send the DRO to the origin
node.
6.3. DRO as Carrier of Forward/Bidirectional Source Routes
The target node conveys the discovered forward/bidirectional source
routes to the origin node via the Source Route options inside one or
more DRO messages. Such a DRO message MUST have its D field set to
0x00 (if it carries forward routes) or 0x02 (if its carries
bidirectional routes). Also, the H flag MUST be cleared and the N
field MUST indicate the number of Source Route options in the DRO.
Each Source Route option inside the DRO MAY immediately be followed
by a Metric Container option that carries the aggregated values of
the relevant routing metrics for this source route.
The target node MAY unicast this DRO message to the origin node or it
MAY forward the DRO message to a parent in the temporary DAG. The
target node should take in consideration the minimum life time of the
temporary DAG when deciding to use it to send the DRO to the origin
node.
6.4. Establishing Hop-by-hop Routes Via DRO
In order to establish a hop-by-hop route, the target node sends a DRO
message along the discovered route, which is specified in a Source
Route option. The D field in the DRO is set to reflect the direction
of the discovered route. The H bit in the DRO MUST be set and the
DRO MUST include the DODAGID and Target Address fields. The N field
in the DRO MUST be set to 1. The target node forwards the DRO to the
next hop along the discovered route and includes the discovered
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route, excluding itself and the origin node, inside the Source Route
option in backward direction. Thus, the D field in the Source Route
option MUST be 0x01.
A router receiving a DRO message MUST drop the DRO and optionally log
an error if the router cannot establish the hop-by-hop state for the
route or if its own address does not lie as the first element in the
Address vector inside the Source Route option. Otherwise, the router
MUST establish the hop-by-hop state in the direction specified in the
D field in the DRO. The hop-by-hop state in the forward direction
includes the RPLInstanceID, the DODAGID and the target node's
address. The hop-by-hop state in the backward direction includes the
RPLInstanceID, the DODAGID and the origin node's address. After
establishing the hop-by-hop state, the router MUST remove its own
address from the route contained in the Source Route option and
forward the DRO to the next hop (Address[0] in the Source Route
option).
7. Applicability
The route discovery mechanism described in this document may be
invoked by an origin node when no route exists between itself and the
target node or when the existing routes do not satisfy the desired
performance requirements. The mechanism is designed to discover one
or more "good enough" routes in either direction between an origin
and a target node. In some application contexts, the good enough
criteria is intrinsically known. For example, an origin node that
expects a target node to be less than 5 hops away may use "hop-count
< 5" as the good enough criteria. In other application contexts, the
origin node may need to measure the cost of an existing route to the
target node to determine the good enough criteria. For example, an
origin node that measures the total ETX of its along-DAG route to the
target node to be 20 may use "ETX < x*20", where x is a fraction that
the origin node decides, as the good enough criteria. The
functionality required to measure the cost of an existing route
between the origin and the target node will be described in a
separate document. In case, there is no existing route between the
origin and target nodes or the cost measurement for the existing
route fails, the origin node will have to guess the good enough
criteria for the initial route discovery. Once, the initial route
discovery succeeds or fails, the origin node will have a better
estimate for the good enough criteria to be used in the subsequent
route discovery.
This document describes an on-demand discovery mechanism for P2P
routes that is complimentary to the proactive routes offered by RPL
base functionality. The mechanism described in this document may
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result in discovery of better P2P routes than the ones available
along a 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 routing metrics in use and the prevalent
conditions in the network. A network designer may take in
consideration both the benefits (potentially better routes; no need
to maintain routes proactively) and costs (control messages generated
during the route discovery process) when using this mechanism.
8. Security Considerations
TBA
9. IANA Considerations
TBA
10. Authors and Contributors
In addition to the editors, the authors of this document include the
following individuals (listed in alphabetical order).
Anders Brandt, Sigma Designs, Emdrupvej 26A, 1., Copenhagen, Dk-2100,
Denmark. Phone: +45 29609501; Email: abr@sdesigns.dk
Robert Cragie, Gridmerge Ltd, 89 Greenfield Crescent, Wakefieldm WF4
4WA, UK. Phone: +44 1924910888; Email: robert.cragie@gridmerge.com
Jerald Martocci, Johnson Controls, Milwaukee, WI 53202, USA. Phone:
+1 414 524 4010; Email:jerald.p.martocci@jci.com
Charles Perkins, Tellabs Inc., USA. Email:charliep@computer.org
Authors gratefully acknowledge the contributions of Richard Kelsey
and Zach Shelby in the development of this document.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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11.2. Informative References
[I-D.brandt-roll-rpl-applicability-home-building]
Brandt, A., Baccelli, E., and R. Cragie, "Applicability
Statement: The use of RPL in Building and Home
Environments",
draft-brandt-roll-rpl-applicability-home-building-00 (work
in progress), April 2010.
[I-D.ietf-6man-rpl-option]
Hui, J. and J. Vasseur, "RPL Option for Carrying RPL
Information in Data-Plane Datagrams",
draft-ietf-6man-rpl-option-00 (work in progress),
July 2010.
[]
Hui, J., Vasseur, J., and D. Culler, "An IPv6 Routing
Header for Source Routes with RPL",
draft-ietf-6man-rpl-routing-header-00 (work in progress),
July 2010.
[I-D.ietf-roll-of0]
Thubert, P., "RPL Objective Function 0",
draft-ietf-roll-of0-03 (work in progress), July 2010.
[I-D.ietf-roll-routing-metrics]
Vasseur, J., Kim, M., Networks, D., Dejean, N., and D.
Barthel, "Routing Metrics used for Path Calculation in Low
Power and Lossy Networks",
draft-ietf-roll-routing-metrics-08 (work in progress),
July 2010.
[I-D.ietf-roll-rpl]
Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing
Protocol for Low power and Lossy Networks",
draft-ietf-roll-rpl-11 (work in progress), July 2010.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-03 (work in
progress), March 2010.
[I-D.ietf-roll-trickle]
Levis, P., Gnawali, O., Clausen, T., Hui, J., and J. Ko,
"The Trickle Algorithm", draft-ietf-roll-trickle-02 (work
in progress), July 2010.
[]
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Thubert, P., "Reverse Routing Header",
draft-thubert-6man-reverse-routing-header-00 (work in
progress), June 2010.
[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.
Authors' Addresses
Mukul Goyal (editor)
University of Wisconsin Milwaukee
3200 N Cramer St
Milwaukee, WI 53211
USA
Phone: +1 414 2295001
Email: mukul@uwm.edu
Emmanuel Baccelli (editor)
INRIA
Phone: +33-169-335-511
Email: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
P2P Team
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