Root initiated routing state in RPL
draft-ietf-roll-dao-projection-07
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| Last updated | 2019-11-03 (Latest revision 2019-05-24) | ||
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draft-ietf-roll-dao-projection-07
ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 6550 (if approved) R.A. Jadhav
Intended status: Standards Track Huawei Tech
Expires: 6 May 2020 M. Gillmore
Itron
3 November 2019
Root initiated routing state in RPL
draft-ietf-roll-dao-projection-07
Abstract
This document proposes a protocol extension to RPL that enables to
install a limited amount of centrally-computed routes in a RPL graph,
enabling loose source routing down a non-storing mode DODAG, or
transversal routes inside the DODAG. As opposed to the classical
route injection in RPL that are injected by the end devices, this
draft enables the Root of the DODAG to projects the routes that are
needed on the nodes where they should be installed.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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 6 May 2020.
Copyright Notice
Copyright (c) 2019 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 (https://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
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and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Subset of a 6LoWPAN Glossary . . . . . . . . . . . . . . 4
2.3. Other Terms . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. References . . . . . . . . . . . . . . . . . . . . . . . 5
3. Extending RFC 6550 . . . . . . . . . . . . . . . . . . . . . 5
4. Identifying a Path . . . . . . . . . . . . . . . . . . . . . 6
5. New RPL Control Messages and Options . . . . . . . . . . . . 7
5.1. New P-DAO Request Control Message . . . . . . . . . . . . 7
5.2. New PDR-ACK Control Message . . . . . . . . . . . . . . . 8
5.3. Route Projection Options . . . . . . . . . . . . . . . . 8
5.4. Sibling Information Option . . . . . . . . . . . . . . . 10
6. Projected DAO . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Non-Storing Mode Projected Route . . . . . . . . . . . . 13
6.2. Storing-Mode Projected Route . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8.1. New RPL Control Codes . . . . . . . . . . . . . . . . . . 17
8.2. Error in Projected Route ICMPv6 Code . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. Normative References . . . . . . . . . . . . . . . . . . . . 18
11. Informative References . . . . . . . . . . . . . . . . . . . 19
Appendix A. Applications . . . . . . . . . . . . . . . . . . . . 20
A.1. Loose Source Routing in Non-storing Mode . . . . . . . . 20
A.2. Transversal Routes in storing and non-storing
modes . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 23
B.1. Using storing mode P-DAO in non-storing mode MOP . . . . 23
B.2. Projecting a storing-mode transversal route . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The "Routing Protocol for Low Power and Lossy Networks" [RFC6550]
(LLN)(RPL) is a generic Distance Vector protocol that is well suited
for application in a variety of low energy Internet of Things (IoT)
networks. RPL forms Destination Oriented Directed Acyclic Graphs
(DODAGs) in which the Root often acts as the Border Router to connect
the RPL domain to the Internet. The Root is responsible to select
the RPL Instance that is used to forward a packet coming from the
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Internet into the RPL domain and set the related RPL information in
the packets.
The 6TiSCH architecture [6TiSCH-ARCHI] leverages RPL for its routing
operation and considers the Deterministic Networking Architecture
[RFC8655] as one possible model whereby the device resources and
capabilities are exposed to an external controller which installs
routing states into the network based on some objective functions
that reside in that external entity.
Based on heuristics of usage, path length, and knowledge of device
capacity and available resources such as battery levels and
reservable buffers, a Path Computation Element ([PCE]) with a global
visibility on the system could install additional P2P routes that are
more optimized for the current needs as expressed by the objective
function.
This draft enables a RPL Root to install and maintain Projected
Routes within its DODAG, along a selected set of nodes that may or
may not include self, for a chosen duration. This potentially
enables routes that are more optimized than those obtained with the
distributed operation of RPL, either in terms of the size of a
source-route header or in terms of path length, which impacts both
the latency and the packet delivery ratio. Projected Routes may be
installed in either Storing and Non-Storing Modes Instances of the
classical RPL operation, resulting in potentially hybrid situations
where the mode of some Projected Routes is different from that of the
other routes in the RPL Instance.
Projected Routes must be used with the parsimony to limit the amount
of state that is installed in each device to fit within its
resources, and to limit the amount of rerouted traffic to fit within
the capabilities of the transmission links. The algorithm used to
compute the paths and the protocol used to learn the topology of the
network and the resources that are available in devices and in the
network are out of scope for this document. Possibly with the
assistance of a Path Computation Element ([PCE]) that could have a
better visibility on the larger system, the Root computes which
segment could be optimized and uses this draft to install the
corresponding Projected Routes.
A Projected Route may be a stand-alone path to a Target or a segment
in a complex Track [6TiSCH-ARCHI] that provides redundant forwarding
solutions to a destination to improve reliability and availability of
the wireless transmissions [RAW-PS].
2. Terminology
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2.1. BCP 14
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 BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Subset of a 6LoWPAN Glossary
This document often uses the following acronyms:
6BBR: 6LoWPAN Backbone Router
6LBR: 6LoWPAN Border Router
6LN: 6LoWPAN Node
6LR: 6LoWPAN Router
DAD: Duplicate Address Detection
DODAG: Destination-Oriented Directed Acyclic Graph
LLN: Low-Power and Lossy Network
NA: Neighbor Advertisement
NCE: Neighbor Cache Entry
ND: Neighbor Discovery
NDP: Neighbor Discovery Protocol
NS: Neighbor Solicitation
RPL: IPv6 Routing Protocol for LLNs [RFC6550]
CMO: Control Message Option
DAO: Destination Advertisement Object
VIO: A Via Information Option, used in Storing Mode P-DAO messages.
SRVIO: A Source-Routed Via Information Option, used in Non-Storing
Mode P-DAO messages.
RPO: A Route Projection Option; it can be a VIO or an SRVIO.
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P-DAO: A Projected DAO is a DAO message sent by the RPL Root to
install a Projected Route.
RTO: RPL Target Option
RAN: RPL-Aware Node
RA: Router Advertisement
RS: Router Solicitation
2.3. Other Terms
Projected Route: A Projected Route is a serial path that is computed
and installed remotely by a RPL Root.
Track: The term Track is used in this document to refer to a complex
path, e.g., a DODAG, that incorporates redundant Projected Routes
towards a destination for increased reliability, high availability
and load balancing.
2.4. References
In this document, readers will encounter terms and concepts that are
discussed in the following documents:
* "Routing Protocol for Low Power and Lossy Networks" [RFC6550], and
* "Terminology in Low power And Lossy Networks" [RFC7102].
3. Extending RFC 6550
This specification introduces two new RPL Control Messages to enable
a RPL Aware Node (RAN) to request the establisment of a path from
self to a Target. A RAN may request the installation of a path by
sending a new P-DAO Request PDR) Message to the Root. The Root
confirms with a new PDR-ACK message back to the requester RAN with a
completion status once it is done installing the path. See
Section 5.1 for more.
Section 6.7 of [RFC6550] specifies Control Message Options (CMO) to
be placed in RPL messages such as the Destination Advertisement
Object (DAO) message. The RPL Target Option (RTO) and the Transit
Information Option (TIO) are such options. In Non-Storing Mode, the
TIO option is used in the DAO message to indicate a parent within a
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DODAG. The TIO applies to the RTOs that immedially preceed it in the
message. Options may be factorized; multiple TIOs may be present to
indicate multiple routes to the one or more contiguous addresses
indicated in the RTOs that immediately precede the TIOs in the RPL
message.
This specification introduces two new CMOs referred to as Route
Projection Options (RPO) to install Projected Routes. One RPO is the
Via Information Option (VIO) and the other is the Source-Routed VIO
(SRVIO). The VIO installs a route on each hop along a Projected
Route (in a fashion analogous to RPL Storing Mode) whereas the SRVIO
installs a source-routing state at the ingress node, which uses that
state to insert a routing header in a fashion similar to Non-Storing
Mode. Like the TIO, the RPOs MUST be preceded by one or more RTOs to
which they apply, and they can be factorized: multiple contiguous
RPOs indicate alternate paths to the Target(s), more in Section 5.3.
This specification also introduces a new CMO to enable a RPL Router
to indicate its siblings to the Root, more in Figure 4.
4. Identifying a Path
It must be noted that RPL has a concept of Instance to represent
different routing topologies but does not have a concept of an
administrative distance, which exists in certain proprietary
implementations to sort out conflicts between multiple sources of
routing information within one routing topology. This draft conforms
the Instance model as follows:
* If the PCE needs to influence a particular Instance to add better
routes in conformance with the routing objectives in that
Instance, it may do so as long as it does not create a loop. A
Projected Route is always preferred over a route that is learned
via RPL. This specification uses the RPL Root as a proxy to the
PCE. If the actual PCE is a separate entity, then a protocol that
is out of scope for this specification is needed to relay the
control elements between the RPL Root and the PCE.
* A PCE that installs a more specific (say, Traffic Engineered) and
possibly complex path (aka a Track) towards a particular Target
MUST use a Local RPL Instance (see section 5 of [RFC6550])
associated to that Target to identify the path. We refer to that
Local RPLInstanceID as TrackID. A projected path is uniquely
identified within the RPL domain by the tuple (Target address,
TrackID). When packet is placed on a Track, a RPL Packet
Information (RPI) is added with the TrackID as RPLInstanceID. The
RPLInstanceID has the 'D' flag set, indicating that the
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destination address in the IPv6 header is the Target that is used
to identify the Track.
* A packet that is routed over a projected path MUST NOT be placed
over a different RPL Instance again. A packet that is placed on a
Global Instance MAY be injected in a Local Instance based on a
network policy and the Local Instance configuration.
A Projected Route is a serial path that may the whole path or a
segment in a complex Track, in which case multiple Projected Routes
are installed with the stuple (Target address, TrackID), and a node
that is present on more than one segment in a Track may be able to
use either of the Projected Routes to forward towards the Target.
The selection of the best route in a Track at forwarding time is out
of scope for this document. [RAW-PS] elaborates on that particular
problem.
5. New RPL Control Messages and Options
5.1. New P-DAO Request Control Message
The PDR is sent to the Root to request a new Path. Exactly one
Target Options MUST be present.
The format of P-DAO Request (PDR) Base Object is as follows:
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 |K|R| Flags | PDRLifetime | PDRSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 1: New P-DAO Request Format
TrackID: 8-bit field indicating the topology Instance associated
with the Track. It is set to zero upon the first request for a
new Track and then to the TrackID once the Track was created, to
either renew it of destroy it.
K: The 'K' flag is set to indicate that the recipient is expected to
send a PDR-ACK back.
R: The 'R' flag is set to indicate that the Requested path should be
redundant.
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PDRLifetime: 8-bit unsigned integer. The requested lifetime for the
Track expressed in Lifetime Units (obtained from the Configuration
option). A PDR with a fresher PDRSequence refreshes the lifetime,
and a PDRLifetime of 0 indicates that the track should be
destroyed.
PDRSequence: 8-bit wrapping sequence number. The PDRSequence obeys
the operation in section 7.2 of [RFC6550]. It is incremented at
each PDR message and echoed in the PDR-ACK by the Root. The
PDRSequence is used to correlate a PDR-ACK message with the PDR
message that triggeted it.
5.2. New PDR-ACK Control Message
The new PDR-ACK is sent as a response to a PDR message with the 'K'
flag set. Its format is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TrackID | PDR-ACK Status| Flags | Track Lifetime|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PDRSequence | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+
Figure 2: New PDR-ACK Control Message Format
TrackID: The RPLInstanceID of the Track that was created. Set to 0
when no Track is created.
PDR-ACK Status: Indicates the completion. A value up to 127 means
acceptance Values of 128 and above are used for rejection codes;
Track Lifetime: Indicates that remaining Lifetime for the Track, 0
if the Track was destroyed or not created.
PDRSequence: 8-bit wrapping sequence number. It is incremented at
each PDR message and echoed in the PDR-ACK.
5.3. Route Projection Options
The RPOs indicate a series of IPv6 addresses that can be compressed
using the method defined in the "6LoWPAN Routing Header" [RFC8138]
specification using the address of the Root found in the DODAGID
field of DIO messages as Compression Reference.
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An RPO indicates a Projected Route that can be a serial Track in full
or a segment of a more complex Track. The Track is identified by a
RPLInstanceID that is either Global or local to the Target of the
Track.
The format of RPOs is as follows:
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 | Option Length |Comp.| Flags | TrackID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Lifetime | Path Sequence | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Via Information option format
Option Type: 0x0A for VIO, 0x0B for SRVIO (to be confirmed by IANA)
Option Length: In bytes; variable, depending on the number of Via
Addresses.
Compression Type: 16-bit unsigned integer. This is the SRH-6LoRH
Type as defined in figure 7 in section 5.1 of [RFC8138] that
corresponds to the compression used for all the Via Addresses.
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TrackID: 8-bit field indicating the topology Instance associated
with the Track.
Path Lifetime: 8-bit unsigned integer. The length of time in
Lifetime Units (obtained from the Configuration option) that the
prefix is valid for route determination. The period starts when a
new Path Sequence is seen. A value of 255 (0xFF) represents
infinity. A value of zero (0x00) indicates a loss of
reachability. A DAO message that contains a Via Information
option with a Path Lifetime of zero for a Target is referred as a
No-Path (for that Target) in this document.
Path Sequence: 8-bit unsigned integer. When a RPL Target option is
issued by the Root of the DODAG (i.e. in a DAO message), that Root
sets the Path Sequence and increments the Path Sequence each time
it issues a RPL Target option with updated information. The
indicated sequence deprecates any state for a given Target that
was learned from a previous sequence and adds to any state that
was learned for that sequence.
Via Address: 2 to 16 bytes, a compressed IPv6 Address. A Via
Address indicates the next hop within the path towards the
destination(s) that is indicated in the Target option that
immediately precede the RPO in the DAO message. Via Addresses are
indicated in the order of the path from the ingress to the egress
nodes. All Via addresses are expressed in the same size as
indicated by the Compression Type.
An RPO MUST contain at least one Via Address, and a Via Address MUST
NOT be present more than once, otherwise the RPO MUST be ignored.
5.4. Sibling Information Option
The Sibling Information Option (SIO) provides indication on siblings
that could be used by the Root to form Projected Routes. The format
of SIOs is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Option Length |Comp.|B| Flags | Opaque |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Step of Rank | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Sibling Address .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Sibling Information Option Format
Option Type: 0x0C (to be confirmed by IANA)
Option Length: In bytes; variable, depending on the number of Via
Addresses.
Compression Type: 16-bit unsigned integer. This is the SRH-6LoRH
Type as defined in figure 7 in section 5.1 of [RFC8138] that
corresponds to the compression used for the Sibling Address.
B: 1-bit flag that is set to indicate that the connectivity to the
sibling is bidirectional and roughly symmetrical. In that case,
only one of the siblings may report the SIO for the hop. If 'B'
is not set then the SIO only indicates connectivity from the
sibling to this node, and does not provide information on the hop
from this node to the sibling.
Opaque: MAY be used to carry information that the node and the Root
understand, e.g., a particular representation of the Link
properties such as a proprietary Link Quality Information for
packets received from the sibling. An industraial Alliance that
uses RPL for a particular use / environment MAY redefine the use
of this field to fit its needs.
Step of Rank: 16-bit unsigned integer. This is the Step of Rank
[RFC6550] as computed by the Objective Function between this node
and the sibling.
Reserved: MUST be set to zero by the sender and MUST be ignored by
the receiver.
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Sibling Address: 2 to 16 bytes, a compressed IPv6 Address. a Via
Address indicates the next hop towards the destination(s) that is
indicated in the Target option that immediately precede the RPO in
the DAO message. Via Addresses are indicated in the order of the
data path from the ingress to the egress nodes. All Via addresses
are expressed in the same size as indicated by the Compression
Type
An SIO MAY be immediately followed by a DAG Metric Container. In
that case the DAG Metric Container provides additional metrics for
the hop from the Sibling to this node.
6. Projected DAO
This draft adds a capability to RPL whereby the Root of a DODAG
projects a route by sending an extended DAO message called a
Projected-DAO (P-DAO) to an arbitrary router in the DODAG, indicating
one or more sequence(s) of routers inside the DODAG via which the
Target(s) indicated in the RPL Target Option(s) (RTO) can be reached.
A P-DAO is sent from a global address of the Root to a global address
of the recipient, and MUST be confirmed by a DAO-ACK, which is sent
back to a global address of the Root.
A P-DAO message MUST contain at least one RTO and at least one RPO
following it. There can be at most one such sequence of RTOs and
then RPOs.
Like a classical DAO message, a P-DAO is processed only if it is
"new" per section 9.2.2. "Generation of DAO Messages" of the RPL
specification [RFC6550]; this is determined using the Path Sequence
information from the RPO as opposed to a TIO. Also, a Path Lifetime
of 0 in an RPO indicates that a route is to be removed.
There are two kinds of operation for the Projected Routes, the
Storing Mode and the Non-Storing Mode.
* The Non-Storing Mode is discussed in Section 6.1. It uses an
SRVIO that carries a list of Via Addresses to be used as a source-
routed path to the Target. The recipient of the P-DAO is the
ingress router of the source-routed path. Upon a Non-Storing Mode
P-DAO, the ingress router installs a source-routed state to the
Target and replies to the Root directly with a DAO-ACK message.
* The Storing Mode is discussed in Section 6.2. It uses a VIO with
one Via Address per consecutive hop, from the ingress to the
egress of the path, including the list of all intermediate routers
in the data path order. The Via Addresses indicate the routers in
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which the routing state to the Target have to be installed via the
next Via Address in the VIO. In normal operations, the P-DAO is
propagated along the chain of Via Routers from the egress router
of the path till the ingress one, which confirms the installation
to the Root with a DAO-ACK message. Note that the Root may be the
ingress and it may be the egress of the path, that it can also be
neither but it cannot be both.
In case of a forwarding error along a Projected Route, an ICMP error
is sent to the Root with a new Code "Error in Projected Route" (See
Section 8.2). The Root can then modify or remove the Projected
Route. The "Error in Projected Route" message has the same format as
the "Destination Unreachable Message", as specified in RFC 4443
[RFC4443]. The portion of the invoking packet that is sent back in
the ICMP message SHOULD record at least up to the routing header if
one is present, and the routing header SHOULD be consumed by this
node so that the destination in the IPv6 header is the next hop that
this node could not reach. if a 6LoWPAN Routing Header (6LoRH)
[RFC8138] is used to carry the IPv6 routing information in the outter
header then that whole 6LoRH information SHOULD be present in the
ICMP message. The sender and exact operation depend on the Mode and
is described in Section 6.1 and Section 6.2 respectively.
6.1. Non-Storing Mode Projected Route
As illustrated in Figure 5, a P-DAO that carries an SRVIO enables the
Root to install a source-routed path towards a Target in any
particular router; with this path information the router can add a
source routed header reflecting the Projected Route to any packet for
which the current destination either is the said Target or can be
reached via the Target.
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------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ | P ^ |
| | DAO | ACK | Loose
o o o o router V | | Source
o o o o o o o o o | P-DAO . Route
o o o o o o o o o o | Source . Path
o o o o o o o o o | Route . From
o o o o o o o o | Path . Root
o o o o o Target V . To
o o o o | Desti-
o o o o | nation
destination V
LLN
Figure 5: Projecting a Non-Storing Route
A route indicated by an SRVIO may be loose, meaning that the node
that owns the next listed Via Address is not necessarily a neighbor.
Without proper loop avoidance mechanisms, the interaction of loose
source routing and other mechanisms may effectively cause loops. In
order to avoid those loops, if the router that installs a Projected
Route does not have a connected route (a direct adjacency) to the
next soure routed hop and fails to locate it as a neighbor or a
neighbor of a neighbor, then it MUST ensure that it has another
Projected Route to the next loose hop under the control of the same
route computation system, otherwise the P-DAO is rejected.
When forwarding a packet to a destination for which the router
determines that routing happens via the Target, the router inserts
the source routing header in the packet to reach the Target. In the
case of a loose source-routed path, there MUST be either a neighbor
that is adjacent to the loose next hop, on which case the packet s
forwarded to that neighbor, or a source-routed path to the loose next
hop; in the latter case, another encapsulation takes place and the
process possibly recurses; otherwise the packet is dropped.
In order to add a source-routing header, the router encapsulates the
packet with an IP-in-IP header and a non-storing mode source routing
header (SRH) [RFC6554]. In the uncompressed form the source of the
packet would be self, the destination would be the first Via Address
in the SRVIO, and the SRH would contain the list of the remaining Via
Addresses and then the Target.
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In practice, the router will normally use the "IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch" [RFC8025]
to compress the RPL artifacts as indicated in [RFC8138]. In that
case, the router indicates self as encapsulator in an IP-in-IP 6LoRH
Header, and places the list of Via Addresses in the order of the VIO
and then the Target in the SRH 6LoRH Header.
In case of a forwarding error along a Source Route path, the node
that fails to forward SHOULD send an ICMP error with a code "Error in
Source Routing Header" back to the source of the packet, as described
in section 11.2.2.3. of [RFC6550]. Upon this message, the
encapsulating node SHOULD stop using the source route path for a
period of time and it SHOULD send an ICMP message with a Code "Error
in Projected Route" to the Root. Failure to follow these steps may
result in packet loss and wasted resources along the source route
path that is broken.
6.2. Storing-Mode Projected Route
As illustrated in Figure 6, the Storing Mode route projection is used
by the Root to install a routing state towards a Target in the
routers along a segment between an ingress and an egress router; this
enables the routers to forward along that segment any packet for
which the next loose hop is the said Target, for Instance a loose
source routed packet for which the next loose hop is the Target, or a
packet for which the router has a routing state to the final
destination via the Target.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ | ^ |
| | DAO | ACK |
o o o o | | |
o o o o o o o o o | ^ | Projected .
o o o o o o o o o o | | DAO | Route .
o o o o o o o o o | ^ | .
o o o o o o o o v | DAO v .
o o LLN o o o |
o o o o o Loose Source Route Path |
o o o o From Root To Destination v
Figure 6: Projecting a route
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In order to install the relevant routing state along the segment
between an ingress and an egress routers, the Root sends a unicast
P-DAO message to the egress router of the routing segment that must
be installed. The P-DAO message contains the ordered list of hops
along the segment as a direct sequence of Via Information options
that are preceded by one or more RPL Target options to which they
relate. Each Via Information option contains a Path Lifetime for
which the state is to be maintained.
The Root sends the P-DAO directly to the egress node of the segment.
In that P-DAO, the destination IP address matches the Via Address in
the last VIO. This is how the egress recognizes its role. In a
similar fashion, the ingress node recognizes its role as it matches
Via Address in the first VIO.
The egress node of the segment is the only node in the path that does
not install a route in response to the P-DAO; it is expected to be
already able to route to the Target(s) on its own. It may either be
the Target, or may have some existing information to reach the
Target(s), such as a connected route or an already installed
Projected Route. If one of the Targets cannot be located, the node
MUST answer to the Root with a negative DAO-ACK listing the Target(s)
that could not be located (suggested status 10 to be confirmed by
IANA).
If the egress node can reach all the Targets, then it forwards the
P-DAO with unchanged content to its loose predecessor in the segment
as indicated in the list of Via Information options, and recursively
the message is propagated unchanged along the sequence of routers
indicated in the P-DAO, but in the reverse order, from egress to
ingress.
The address of the predecessor to be used as destination of the
propagated DAO message is found in the Via Information option the
precedes the one that contain the address of the propagating node,
which is used as source of the packet.
Upon receiving a propagated DAO, an intermediate router as well as
the ingress router install a route towards the DAO Target(s) via its
successor in the P-DAO; the router locates the VIO that contains its
address, and uses as next hop the address found in the Via Address
field in the following VIO. The router MAY install additional routes
towards the addresses that are located in VIOs that are after the
next one, if any, but in case of a conflict or a lack of resource, a
route to a Target installed by the Root has precedence.
The process recurses till the P-DAO is propagated to ingress router
of the segment, which answers with a DAO-ACK to the Root.
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Also, the path indicated in a P-DAO may be loose, in which case the
reachability to the next hop has to be asserted. Each router along
the path indicated in a P-DAO is expected to be able to reach its
successor, either with a connected route (direct neighbor), or by
routing, for Instance following a route installed previously by a DAO
or a P-DAO message. If that route is not connected then a recursive
lookup may take place at packet forwarding time to find the next hop
to reach the Target(s). If it does not and cannot reach the next
router in the P-DAO, the router MUST answer to the Root with a
negative DAO-ACK indicating the successor that is unreachable
(suggested status 11 to be confirmed by IANA).
A Path Lifetime of 0 in a Via Information option is used to clean up
the state. The P-DAO is forwarded as described above, but the DAO is
interpreted as a No-Path DAO and results in cleaning up existing
state as opposed to refreshing an existing one or installing a new
one.
In case of a forwarding error along a Storing Mode Projected Route,
the node that fails to forward SHOULD send an ICMP error with a code
"Error in Projected Route" to the Root. Failure to do so may result
in packet loss and wasted resources along the Projected Route that is
broken.
7. Security Considerations
This draft uses messages that are already present in RPL [RFC6550]
with optional secured versions. The same secured versions may be
used with this draft, and whatever security is deployed for a given
network also applies to the flows in this draft.
TODO: should probably consider how P-DAO messages could be abused by
a) rogue nodes b) via replay of messages c) if use of P-DAO messages
could in fact deal with any threats?
8. IANA Considerations
8.1. New RPL Control Codes
This document extends the IANA registry created by RFC 6550 for RPL
Control Codes as follows:
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+------+--------------------------------------+---------------+
| Code | Description | Reference |
+======+======================================+===============+
| 0x0A | Via Information option | This document |
+------+--------------------------------------+---------------+
| 0x0B | Source-Routed Via Information option | This document |
+------+--------------------------------------+---------------+
Table 1: RPL Control Codes
This document is updating the registry created by RFC 6550 for the
RPL 3-bit Mode of Operation (MOP) as follows:
+-----------+-------------------------------+-----------+
| MOP value | Description | Reference |
+===========+===============================+===========+
| 5 | Non-Storing mode of operation | This |
| | with Projected Routes | document |
+-----------+-------------------------------+-----------+
| 6 | Storing mode of operation | This |
| | with Projected Routes | document |
+-----------+-------------------------------+-----------+
Table 2: DIO Mode of operation
8.2. Error in Projected Route ICMPv6 Code
In some cases RPL will return an ICMPv6 error message when a message
cannot be forwarded along a Projected Route. This ICMPv6 error
message is "Error in Projected Route".
IANA has defined an ICMPv6 "Code" Fields Registry for ICMPv6 Message
Types. ICMPv6 Message Type 1 describes "Destination Unreachable"
codes. This specification requires that a new code is allocated from
the ICMPv6 Code Fields Registry for ICMPv6 Message Type 1, for "Error
in Projected Route", with a suggested code value of 8, to be
confirmed by IANA.
9. Acknowledgments
The authors wish to acknowledge JP Vasseur, James Pylakutty and
Patrick Wetterwald for their contributions to the ideas developed
here.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
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DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., 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,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[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,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.
[RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
RFC 8025, DOI 10.17487/RFC8025, November 2016,
<https://www.rfc-editor.org/info/rfc8025>.
[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
11. Informative References
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
2014, <https://www.rfc-editor.org/info/rfc7102>.
[RFC6997] Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and
J. Martocci, "Reactive Discovery of Point-to-Point Routes
in Low-Power and Lossy Networks", RFC 6997,
DOI 10.17487/RFC6997, August 2013,
<https://www.rfc-editor.org/info/rfc6997>.
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[6TiSCH-ARCHI]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", Work in Progress, Internet-Draft,
draft-ietf-6tisch-architecture-27, 18 October 2019,
<https://tools.ietf.org/html/draft-ietf-6tisch-
architecture-27>.
[RAW-PS] Thubert, P. and G. Papadopoulos, "Reliable and Available
Wireless Problem Statement", Work in Progress, Internet-
Draft, draft-pthubert-raw-problem-statement-04, 23 October
2019, <https://tools.ietf.org/html/draft-pthubert-raw-
problem-statement-04>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
[PCE] IETF, "Path Computation Element", November 2019,
<https://datatracker.ietf.org/doc/charter-ietf-pce/>.
Appendix A. Applications
A.1. Loose Source Routing in Non-storing Mode
A RPL implementation operating in a very constrained LLN typically
uses the Non-Storing Mode of Operation as represented in Figure 7.
In that mode, a RPL node indicates a parent-child relationship to the
Root, using a Destination Advertisement Object (DAO) that is unicast
from the node directly to the Root, and the Root typically builds a
source routed path to a destination down the DODAG by recursively
concatenating this information.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ ^ | |
| | DAO | ACK |
o o o o | | | Strict
o o o o o o o o o | | | Source
o o o o o o o o o o | | | Route
o o o o o o o o o | | |
o o o o o o o o | v v
o o o o
LLN
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Figure 7: RPL non-storing mode of operation
Based on the parent-children relationships expressed in the non-
storing DAO messages,the Root possesses topological information about
the whole network, though this information is limited to the
structure of the DODAG for which it is the destination. A packet
that is generated within the domain will always reach the Root, which
can then apply a source routing information to reach the destination
if the destination is also in the DODAG. Similarly, a packet coming
from the outside of the domain for a destination that is expected to
be in a RPL domain reaches the Root.
It results that the Root, or then some associated centralized
computation engine such as a PCE, can determine the amount of packets
that reach a destination in the RPL domain, and thus the amount of
energy and bandwidth that is wasted for transmission, between itself
and the destination, as well as the risk of fragmentation, any
potential delays because of a paths longer than necessary (shorter
paths exist that would not traverse the Root).
As a network gets deep, the size of the source routing header that
the Root must add to all the downward packets becomes an issue for
nodes that are many hops away. In some use cases, a RPL network
forms long lines and a limited amount of well-Targeted routing state
would allow to make the source routing operation loose as opposed to
strict, and save packet size. Limiting the packet size is directly
beneficial to the energy budget, but, mostly, it reduces the chances
of frame loss and/or packet fragmentation, which is highly
detrimental to the LLN operation. Because the capability to store a
routing state in every node is limited, the decision of which route
is installed where can only be optimized with a global knowledge of
the system, a knowledge that the Root or an associated PCE may
possess by means that are outside of the scope of this specification.
This specification enables to store source-routed or storing mode
state in intermediate routers, which enables to limit the excursion
of the source route headers in deep networks. Once a P-DAO exchange
has taken place for a given Target, if the Root operates in non
storing mode, then it may elide the sequence of routers that is
installed in the network from its source route headers to destination
that are reachable via that Target, and the source route headers
effectively become loose.
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A.2. Transversal Routes in storing and non-storing modes
RPL is optimized for Point-to-Multipoint (P2MP) and Multipoint-to-
Point (MP2P), whereby routes are always installed along the RPL DODAG
respectively from and towards the DODAG Root. Transversal Peer to
Peer (P2P) routes in a RPL network will generally suffer from some
elongated (stretched) path versus the best possible path, since
routing between 2 nodes always happens via a common parent, as
illustrated in Figure 8:
* in non-storing mode, all packets routed within the DODAG flow all
the way up to the Root of the DODAG. If the destination is in the
same DODAG, the Root must encapsulate the packet to place a
Routing Header that has the strict source route information down
the DODAG to the destination. This will be the case even if the
destination is relatively close to the source and the Root is
relatively far off.
* In storing mode, unless the destination is a child of the source,
the packets will follow the default route up the DODAG as well.
If the destination is in the same DODAG, they will eventually
reach a common parent that has a route to the destination; at
worse, the common parent may also be the Root. From that common
parent, the packet will follow a path down the DODAG that is
optimized for the Objective Function that was used to build the
DODAG.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
X
^ v o o
^ o o v o o o o o
^ o o o v o o o o o
^ o o v o o o o o
S o o o D o o o
o o o o
LLN
Figure 8: Routing Stretch between S and D via common parent X
It results that it is often beneficial to enable transversal P2P
routes, either if the RPL route presents a stretch from shortest
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path, or if the new route is engineered with a different objective.
For that reason, earlier work at the IETF introduced the "Reactive
Discovery of Point-to-Point Routes in Low Power and Lossy Networks"
[RFC6997], which specifies a distributed method for establishing
optimized P2P routes. This draft proposes an alternate based on a
centralized route computation.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
|
o o o o
o o o o o o o o o
o o o o o o o o o o
o o o o o o o o o
S>>A>>>B>>C>>>D o o o
o o o o
LLN
Figure 9: Projected Transversal Route
This specification enables to store source-routed or storing mode
state in intermediate routers, which enables to limit the stretch of
a P2P route and maintain the characteristics within a given SLA. An
example of service using this mechanism oculd be a control loop that
would be installed in a network that uses classical RPL for
asynchronous data collection. In that case, the P2P path may be
installed in a different RPL Instance, with a different objective
function.
Appendix B. Examples
B.1. Using storing mode P-DAO in non-storing mode MOP
In non-storing mode, the DAG Root maintains the knowledge of the
whole DODAG topology, so when both the source and the destination of
a packet are in the DODAG, the Root can determine the common parent
that would have been used in storing mode, and thus the list of nodes
in the path between the common parent and the destination. For
Instance in the diagram shown in Figure 10, if the source is node 41
and the destination is node 52, then the common parent is node 22.
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------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| \ \____
/ \ \
o 11 o 12 o 13
/ | / \
o 22 o 23 o 24 o 25
/ \ | \ \
o 31 o 32 o o o 35
/ / | \ | \
o 41 o 42 o o o 45 o 46
| | | | \ |
o 51 o 52 o 53 o o 55 o 56
LLN
Figure 10: Example DODAG forming a logical tree topology
With this draft, the Root can install a storing mode routing states
along a segment that is either from itself to the destination, or
from one or more common parents for a particular source/destination
pair towards that destination (in this particular example, this would
be the segment made of nodes 22, 32, 42).
In the example below, say that there is a lot of traffic to nodes 55
and 56 and the Root decides to reduce the size of routing headers to
those destinations. The Root can first send a DAO to node 45
indicating Target 55 and a Via segment (35, 45), as well as another
DAO to node 46 indicating Target 56 and a Via segment (35, 46). This
will save one entry in the routing header on both sides. The Root
may then send a DAO to node 35 indicating Targets 55 and 56 a Via
segment (13, 24, 35) to fully optimize that path.
Alternatively, the Root may send a DAO to node 45 indicating Target
55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46
indicating Target 56 and a Via segment (13, 24, 35, 46), indicating
the same DAO Sequence.
B.2. Projecting a storing-mode transversal route
In this example, say that a PCE determines that a path must be
installed between node S and node D via routers A, B and C, in order
to serve the needs of a particular application.
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The Root sends a P-DAO with a Target option indicating the
destination D and a sequence Via Information option, one for S, which
is the ingress router of the segment, one for A and then for B, which
are an intermediate routers, and one for C, which is the egress
router.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| P-DAO message to C
o | o o
o o o | o o o o o
o o o | o o o o o o
o o V o o o o o o
S A B C D o o o
o o o o
LLN
Figure 11: P-DAO from Root
Upon reception of the P-DAO, C validates that it can reach D, e.g.
using IPv6 Neighbor Discovery, and if so, propagates the P-DAO
unchanged to B.
B checks that it can reach C and of so, installs a route towards D
via C. Then it propagates the P-DAO to A.
The process recurses till the P-DAO reaches S, the ingress of the
segment, which installs a route to D via A and sends a DAO-ACK to the
Root.
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------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
^ P-DAO-ACK from S
/ o o o
/ o o o o o o o
| o o o o o o o o o
| o o o o o o o o
S A B C D o o o
o o o o
LLN
Figure 12: P-DAO-ACK to Root
As a result, a transversal route is installed that does not need to
follow the DODAG structure.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
|
o o o o
o o o o o o o o o
o o o o o o o o o o
o o o o o o o o o
S>>A>>>B>>C>>>D o o o
o o o o
LLN
Figure 13: Projected Transversal Route
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems, Inc
Building D, 45 Allee des Ormes - BP1200
06254 Mougins - Sophia Antipolis
France
Phone: +33 497 23 26 34
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Email: pthubert@cisco.com
Rahul Arvind Jadhav
Huawei Tech
Kundalahalli Village, Whitefield,
Bangalore 560037
Karnataka
India
Phone: +91-080-49160700
Email: rahul.ietf@gmail.com
Matthew Gillmore
Itron, Inc
Building D, 2111 N Molter Road
Liberty Lake, 99019
United States
Phone: +1.800.635.5461
Email: matthew.gillmore@itron.com
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