Root initiated routing state in RPL
draft-ietf-roll-dao-projection-12
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| Last updated | 2020-09-21 | ||
| Replaces | draft-thubert-roll-dao-projection | ||
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draft-ietf-roll-dao-projection-12
ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 6550 (if approved) R.A. Jadhav
Intended status: Standards Track Huawei Tech
Expires: 25 March 2021 M. Gillmore
Itron
21 September 2020
Root initiated routing state in RPL
draft-ietf-roll-dao-projection-12
Abstract
This document 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 or resilient than those
obtained with the classical 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 March 2021.
Copyright Notice
Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2.2. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Other Terms . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. References . . . . . . . . . . . . . . . . . . . . . . . 6
3. Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . . 6
4. Identifying a Path . . . . . . . . . . . . . . . . . . . . . 7
5. New RPL Control Messages and Options . . . . . . . . . . . . 8
5.1. New P-DAO Request Control Message . . . . . . . . . . . . 8
5.2. New PDR-ACK Control Message . . . . . . . . . . . . . . . 9
5.3. Route Projection Options . . . . . . . . . . . . . . . . 10
5.4. Sibling Information Option . . . . . . . . . . . . . . . 13
6. Projected DAO . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Requesting a Track . . . . . . . . . . . . . . . . . . . 16
6.2. Routing over a Track . . . . . . . . . . . . . . . . . . 16
6.3. Non-Storing Mode Projected Route . . . . . . . . . . . . 17
6.4. Storing-Mode Projected Route . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8.1. New RPL Control Codes . . . . . . . . . . . . . . . . . . 21
8.2. New RPL Control Message Options . . . . . . . . . . . . . 21
8.3. SubRegistry for the Projected DAO Request Flags . . . . . 22
8.4. SubRegistry for the PDR-ACK Flags . . . . . . . . . . . . 22
8.5. Subregistry for the PDR-ACK Acceptance Status Values . . 22
8.6. Subregistry for the PDR-ACK Rejection Status Values . . . 23
8.7. SubRegistry for the Route Projection Options Flags . . . 23
8.8. SubRegistry for the Sibling Information Option Flags . . 24
8.9. Error in Projected Route ICMPv6 Code . . . . . . . . . . 24
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
10. Normative References . . . . . . . . . . . . . . . . . . . . 24
11. Informative References . . . . . . . . . . . . . . . . . . . 25
Appendix A. Applications . . . . . . . . . . . . . . . . . . . . 26
A.1. Loose Source Routing . . . . . . . . . . . . . . . . . . 27
A.2. Transversal Routes . . . . . . . . . . . . . . . . . . . 28
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 30
B.1. Using Storing Mode P-DAO in Non-Storing Mode MOP . . . . 30
B.2. Projecting a storing-mode transversal route . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction
RPL, the "Routing Protocol for Low Power and Lossy Networks" [RPL]
(LLNs), 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
Internet into the RPL domain and set the related RPL information in
the packets. The "6TiSCH Architecture" [6TiSCH-ARCHI] uses RPL for
its routing operations.
The 6TiSCH Architecture also leverages the "Deterministic Networking
Architecture" [RFC8655] centralized 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. With DetNet
and 6TiSCH, the component of the controller that is responsible of
computing routes is called a Path Computation Element ([PCE]).
Based on heuristics of usage, path length, and knowledge of device
capacity and available resources such as battery levels and
reservable buffers, a PCE with a global visibility on the system can
compute P2P routes that are more optimized for the current needs as
expressed by the objective function.
This draft proposes protocol extensions to RPL that enable the Root
to install a limited amount of centrally-computed routes in a RPL
graph, on behalf of a PCE that may be collocated or separated from
the Root. Those extensions enable loose source routing down and
transversal routes inside the main DODAG running a base RPL Instance.
This specification expects that the base RPL Instance is operated in
RPL Non-Storing Mode of Operation (MOP) to sustain the exchanges with
the Root. In that Mode, the Root has enough information to build a
basic DODAG topology based on parents and children, but lacks the
knowledge of siblings. This document adds the capability for nodes
to advertise sibling information in order to improve the topological
awareness of the Root.
As opposed to the classical RPL operations where routes are injected
by the Target nodes, the protocol extensions enable the Root of a
DODAG to project the routes that are needed onto the nodes where they
should be installed. This specification uses the term Projected
Route to refer to those routes.
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A Projected Route may be installed in either Storing and Non-Storing
Mode, potentially resulting in hybrid situations where the Mode of
the Projected Route is different from that of the main RPL Instance.
A Projected Route may be a stand-alone end-to-end path to a Target or
a Segment in a more complex forwarding graph called a Track.
The concept of a Track was introduced in the 6TiSCH architecture, as
a complex path to a Target destination with redundant forwarding
solutions along the way. A node at the ingress of more than one
Segment in a Track may use any combination of those Segments to
forward a packet towards the Target.
The "Reliable and Available Wireless (RAW) Architecture/Framework"
[RAW-ARCHI] enables a dynamic path selection within the Track to
increase the transmission diversity and combat diverse causes of
packet loss.
To that effect, RAW defines the Path Selection Engine (PSE) as a
complement of the PCE operating in the dataplane. The PSE controls
the use of the Packet ARQ, Replication, Elimination, and Overhearing
(PAREO) functions over the Track segments.
While the time scale at which the PCE (re)computes the Track can be
long, for an operation based on long-term statistical metrics to
perform global optimizations at the scale of the whole network, the
PSE makes forwarding decision at the time scale of one or a small
collection of packets, using a knowledge that is changing rapidly but
limited in scope of the Track itself. This way, the PSE can provide
a dynamic balance between the reliability and availability
requirements of the flows and the need to conserve energy and
spectrum.
Projected Routes must be used with the parsimony to limit the amount
of state that is installed in each device to fit within the device
resources, and to maintain the amount of rerouted traffic within the
capabilities of the transmission links. The methods used to learn
the node capabilities and the resources that are available in the
devices and in the network are out of scope for this document.
This specification uses the RPL Root as a proxy to the PCE. The PCE
may be collocated with the Root, or may reside in an external
Controller. In that case, the PCE exchanges control messages with
the Root over a Southbound API, that is out of scope for this
specification. The algorithm to compute the paths and the protocol
used by an external PCE to obtain the topology of the network from
the Root are also out of scope.
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2. Terminology
2.1. Requirements Language
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. Glossary
This document often uses the following acronyms:
CMO: Control Message Option
DAO: Destination Advertisement Object
DAG: Directed Acyclic Graph
DODAG: Destination-Oriented Directed Acyclic Graph; A DAG with only
one vertice (i.e., node) that has no outgoing edge (i.e., link)
LLN: Low-Power and Lossy Network
MOP: RPL Mode of Operation
P-DAO: Projected DAO
PDR: P-DAO Request
RAN: RPL-Aware Node (either a RPL Router or a RPL-Aware Leaf)
RAL: RPL-Aware Leaf
RPI: RPL Packet Information
RPL: IPv6 Routing Protocol for LLNs [RPL]
RPO: A Route Projection Option; it can be a VIO or an SRVIO.
RTO: RPL Target Option
RUL: RPL-Unaware Leaf
SIO: RPL Sibling Information Option
SRVIO: A Source-Routed Via Information Option, used in Non-Storing
Mode P-DAO messages.
SubDAG: A DODAG rooted at a node which is a child of that node and a
subset of a larger DAG
TIO: RPL Transit Information Option
VIO: A Via Information Option, used in Storing Mode P-DAO messages.
2.3. Other Terms
Projected Route: A Projected Route is a serial path that is
computed, installed and maintained remotely by a RPL Root.
Projected DAO: A DAO message used to install a Projected Route.
Track: A complex path with redundant Segments to a destination.
TrackID: A RPL Local InstanceID with the 'D' bit set. The TrackId
is associated with a Target address that is the Track destination.
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2.4. References
In this document, readers will encounter terms and concepts that are
discussed in the "Routing Protocol for Low Power and Lossy Networks"
[RPL] and "Terminology in Low power And Lossy Networks" [RFC7102].
3. Updating RFC 6550
This specification introduces two new RPL Control Messages to enable
a RPL Aware Node (RAN) to request the establisment of a Track from
self to a Target. The RAN makes its request 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, see Section 5.1 for more.
Section 6.7 of [RPL] specifies the RPL 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
inform the root of the parent-child relationships within the DODAG,
and the Root has a full knowledge of the DODAG structure. The TIO
applies to the RTOs that preceed it immediately 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 encapsulate a packet with an IPv6 Routing Header in a
fashion similar to RPL Non-Storing Mode.
Like the TIO, the RPOs MUST be preceded by exactly one RTO to which
they apply, and SRVIOs MAY be factorized, though VIOs MUST NOT be.
Factorized contiguous SRVIOs indicate alternate paths to the Target,
more in Section 5.3.
This specification also introduces a new CMO to enable a RAN to
advertise a selection of its candidate neighbors as siblings to the
Root, using a new Sibling Information Option (SIO) as specified in
Section 5.4.
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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.
* The PCE may use P-DAOs to install a specific (say, Traffic
Engineered) and possibly complex path, that we refer to as a
Track, towards a particular Target. In that case it MUST use a
Local RPL Instance (see section 5 of [RPL]) associated to that
Target to identify the Track.
We refer to the local RPLInstanceID as TrackID. The TrackID MUST
be unique for a particular Target IPv6 address. The Track is
uniquely identified within the RPL domain by the tuple (Target
address, TrackID) where the TrackID is always represented with the
'D' flag set.
The Track where a packet is placed is signaled by a RPL Packet
Information (RPI) (see [USEofRPLinfo]) in the outer chain of IPv6
Headers. The RPI contains the TrackID as RPLInstanceID and the
'D' flag is set to indicate that the destination address in the
IPv6 header is the Target that is used to identify the Track, more
in Section 6.2.
* The PCE may also install a projected Route as a complement to the
main DODAG, e.g., using the Storing-Mode Mode along a Source-
Routed path in order to enable loose source routing and reduce the
Routing Header. In that case, the global RPLInstanceID of the
main DODAG is signaled in place of the TrackId on the P-DAO, and
the RPI in the packet indicates the global RPLInstanceID, more in
Appendix A.1.
* A packet that is routed over the RPL Instance associated to a
Track MUST NOT be placed over a different RPL Instance again.
Conversely, 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.
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A Projected Route is a serial path that may represent the end-to-end
route or only a Segment in a complex Track, in which case multiple
Projected Routes are installed with the same tuple (Target address,
TrackID) and a different Segment ID each.
All properties of a Track operations are inherited form the main
instance that is used to install the Track. For instance, the use of
compression per [RFC8138] is determined by whether it is used in the
main instance, e.g., by setting the "T" flag [TURN-ON_RFC8138] in the
RPL configuration option.
5. New RPL Control Messages and Options
5.1. New P-DAO Request Control Message
The P-DAO Request (PDR) message is sent to the Root to request a new
that the PCE establishes a new a projected route from self ot the
Target indicated in the Target Option as a full path of a collection
of Segments in a Track. Exactly one Target Option MUST be present,
more in Section 6.1.
The RPL Control Code for the PDR is 0x09, to be confirmed by IANA.
The format of 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TrackID |K|R| Flags | ReqLifetime | PDRSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 1: New P-DAO Request Format
TrackID: 8-bit field indicating the RPLInstanceID 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.
Flags: Reserved. The Flags field MUST initialized to zero by the
sender and MUST be ignored by the receiver
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ReqLifetime: 8-bit unsigned integer.
The requested lifetime for the Track expressed in Lifetime Units
(obtained from the DODAG 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, obeying the operation
in section 7.2 of [RPL].
The PDRSequence is used to correlate a PDR-ACK message with the
PDR message that triggeted it. It is incremented at each PDR
message and echoed in the PDR-ACK by the Root.
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. The RPL Control Code for the PDR-ACK is 0x0A, to be
confirmed by IANA. 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 | Flags | Track Lifetime| PDRSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PDR-ACK Status| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+
Figure 2: New PDR-ACK Control Message Format
TrackID: The RPLInstanceID of the Track that was created. The value
of 0x00 is used to when no Track was created.
Flags: Reserved. The Flags field MUST initialized to zero by the
sender and MUST be ignored by the receiver
Track Lifetime: Indicates that remaining Lifetime for the Track,
expressed in Lifetime Units; a value of zero (0x00) indicates that
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.
PDR-ACK Status: 8-bit field indicating the completion.
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The PDR-ACK Status is substructured as indicated in Figure 3:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E|R| Value |
+-+-+-+-+-+-+-+-+
Figure 3: PDR-ACK status Format
E: 1-bit flag. Set to indicate a rejection. When not set, a
value of 0 indicates Success/Unqualified acceptance and other
values indicate "not an outright rejection".
R: 1-bit flag. Reserved, MUST be set to 0 by the sender and
ignored by the receiver.
Status Value: 6-bit unsigned integer. Values depending on the
setting of the 'E' flag as indicated respectively in Table 4
and Table 5.
Reserved: The Reserved field MUST initialized to zero by the sender
and MUST be ignored by the receiver
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.
An RPO indicates a Projected Route that can be a serial Track in full
or a Segment of a more complex Track. In Non-Storing Mode, multiple
RPO may be placed after a same Target Option to reflect different
Segments originated at this node. The Track is identified by a
TrackID that is a Local RPLInstanceID to the Target of the Track.
The format of RPOs 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 |C| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TrackID | SegmentID |Segm. Sequence | Seg. Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Route Projection Option format (uncompressed form)
Option Type: 0x0B for VIO, 0x0C for SRVIO (to be confirmed by IANA)
Option Length: In bytes; variable, depending on the number of Via
Addresses.
C: 1-bit flag. Set to indicate that the following Via Addresses are
expressed as one or more SRH-6LoRH as defined in section 5.1 of
[RFC8138]. Figure 4 illustrates the case where the "C" flag is
not set, meaning that the Via Addresses are expressed in 128 bits.
Flags: Reserved. The Flags field MUST initialized to zero by the
sender and MUST be ignored by the receiver
Reserved: The Reserved field MUST initialized to zero by the sender
and MUST be ignored by the receiver
TrackID: 8-bit field indicating the topology Instance associated
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with the Track. This field carries either a TrackID, such that
the tuple (Target Address, TrackID) forms a unique ID of the Track
in the RPL domain, or the glocal InstanceID of the main DODAG, in
which case the RPO adds a route to the main DODAG as an individual
Segment.
SegmentID: 8-bit field that identifies a Segment within a Track or
the main DODAG as indicated by the TrackId field. A Value of 0 is
used to signal a serial path, i.e., made of a single segment.
Segment Sequence: 8-bit unsigned integer. The Segment Sequence
obeys the operation in section 7.2 of [RPL] and the lollipop
starts at 255. When the Root of the DODAG needs to refresh or
update a Segment in a Track, it increments the Segment Sequence
individually for that Segment. The Segment information indicated
in the RTO deprecates any state for the Segment indicated by the
SegmentID within the indicated Track and sets up the new
information. A RTO with a Segment Sequence that is not as fresh
as the current one is ignored. a RTO for a given target with the
same (TrackID, SegmentID, Segment Sequence) indicates a retry; it
MUST NOT change the Segment and MUST be propagated or answered as
the first copy.
Segment Lifetime: 8-bit unsigned integer. The length of time in
Lifetime Units (obtained from the Configuration option) that the
Segment is usable. The period starts when a new Segment 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 Segment Lifetime of zero
for a Target is referred as a No-Path (for that Target) in this
document.
Via Address: The collection of Via Addresses along one Segment,
indicated in the order of the path from the ingress to the egress
nodes. If the "C" flag is set, the fields Via Address 1 .. Via
Address n in Figure 4 are replaced by one or more of the headers
illustrated in Fig. 6 of [RFC8138]. In the case of a VIO, or if
[RFC8138] is turned off, then the Root MUST use only one SRH-
6LoRH, and the compression is the same for all addresses. If
[RFC8138] is turned on, then the Root SHOULD optimize the size of
the SRVIO; in that case, more than one SRH-6LoRH are needed if the
compression of the addresses change inside the Segment and
different SRH-6LoRH Types are used.
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.
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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:
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|D|Flags| Opaque |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Step of Rank | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Sibling DODAGID (if 'D' flag not set) .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Sibling Address .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Sibling Information Option Format
Option Type: 0x0D (to be confirmed by IANA)
Option Length: In bytes; variable, depending on the number of Via
Addresses.
Compression Type: 3-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.
Reserved for Flags: MUST be set to zero by the sender and MUST be
ignored by the receiver.
B: 1-bit flag that is set to indicate that the connectivity to the
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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.
D: 1-bit flag that is set to indicate that sibling belongs to the
same DODAG. When not set, the Sibling DODAGID is indicated.
Flags: Reserved. The Flags field MUST initialized to zero by the
sender and MUST be ignored by the receiver
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
[RPL] as computed by the Objective Function between this node and
the sibling.
Reserved: The Reserved field MUST initialized to zero by the sender
and MUST be ignored by the receiver
Sibling DODAGID: 2 to 16 bytes, the DODAGID of the sibling in a
[RFC8138] compressed form as indicated by the Compression Type
field. This field is present when the 'D' flag is not set.
Sibling Address: 2 to 16 bytes, the IPv6 Address of the sibling in a
[RFC8138] compressed form as indicated by the Compression Type
field.
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 one or more extended DAO message called
Projected-DAO (P-DAO) messages 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.
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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 exactly one RTO and either one VIO or
one or more SRVIOs following it. There can be at most one such
sequence of RTOs and then RPOs.
Like a classical DAO message, a P-DAO causes a change of state only
if it is "new" per section 9.2.2. "Generation of DAO Messages" of
the RPL specification [RPL]; this is determined using the Segment
Sequence information from the RPO as opposed to the Path Sequence
from a TIO. Also, a Segment Lifetime of 0 in an RPO indicates that
the projected route associated to the Segment 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 6.3. 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 6.4. 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
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 8.9). 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)
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[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 6.3 and Section 6.4 respectively.
6.1. Requesting a Track
A Node is free to ask the Root for a new Track with a PDR message,
for a duration indicated in a Requested Lifetime field. Upon that
Request, the Root install the necessary Segments and answers with a
PDR-ACK indicated the granted Track Lifetime. When the Track
Lifetime returned in the PDR-ACK is close to elapse, the resquesting
Node needs to resend a PDR using the TrackID in the PDR-ACK to get
the lifetime of the Track prolonged, else the Track will time out and
the Root will tear down the whole structure.
The Segment Lifetime in the P-DAO messages does not need to be
aligned to the Requested Lifetime in the PDR, or between P-DAO
messages for different Segments. The Root may use shorter lifetimes
for the Segments and renew them faster than the Track is, or longer
lifetimes in which case it will need to tear down the Segments if the
Track is not renewed.
The Root is free to install which ever Segments it wants, and change
them overtime, to serve the Track as needed, without notifying the
resquesting Node. If the Track fails and cannot be reestablished,
the Root notifies the resquesting Node asynchronously with a PDR-ACK
with a Track Lifetime of 0, indicating that the Track has failed, and
a PDR-ACK Status indicating the reason of the fault.
All the Segments MUST be of a same mode, either Storing or Non-
Storing. All the Segments MUST be created with the same TrackId and
Target in the P-DAO.
6.2. Routing over a Track
Sending a packet over a Track implies the addition of a RPI to
indicate the Track, in association with the IPv6 destination. In
case of a Non-Storing Mode Projected Route, a Source Routing Header
is needed as well.
The Destination IPv6 Address of a packet that is place in a Track
MUST be that of the Target of Track. The outer header of the packet
MUST contain an RPI that indicates the TrackId as RPL Instance ID.
If the Track Ingress is the originator of the packet and the Track
Egress (i.e., the Target) is the destination of the packet, there is
no need of an encapsulation. Else, i.e., if the Track Ingress is
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forwarding a packet into the Track, or if the the final destination
is reached via is not the Target, but reached over the Track via the
Track Egress, then an IP-in-IP encapsulation is needed.
6.3. Non-Storing Mode Projected Route
As illustrated in Figure 6, 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.
------+---------
| 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 6: 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.
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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
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.
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 is 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 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 [RPL]. 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.
6.4. Storing-Mode Projected Route
As illustrated in Figure 7, 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.
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------+---------
| 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 7: Projecting a route
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 Segment 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).
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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.
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 Segment 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.
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7. Security Considerations
This draft uses messages that are already present in RPL [RPL] 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 Subregistry created by RFC 6550 for
RPL Control Codes as indicated in Table 1:
+======+=============================+===============+
| Code | Description | Reference |
+======+=============================+===============+
| 0x09 | Projected DAO Request (PDR) | This document |
+------+-----------------------------+---------------+
| 0x0A | PDR-ACK | This document |
+------+-----------------------------+---------------+
Table 1: New RPL Control Codes
8.2. New RPL Control Message Options
This document extends the IANA Subregistry created by RFC 6550 for
RPL Control Message Options as indicated in Table 2:
+=======+======================================+===============+
| Value | Meaning | Reference |
+=======+======================================+===============+
| 0x0B | Via Information option | This document |
+-------+--------------------------------------+---------------+
| 0x0C | Source-Routed Via Information option | This document |
+-------+--------------------------------------+---------------+
| 0x0D | Sibling Information option | This document |
+-------+--------------------------------------+---------------+
Table 2: RPL Control Message Options
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8.3. SubRegistry for the Projected DAO Request Flags
IANA is required to create a registry for the 8-bit Projected DAO
Request (PDR) Flags field. Each bit is tracked with the following
qualities:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Reference
Registration procedure is "Standards Action" [RFC8126]. The initial
allocation is as indicated in Table 3:
+============+========================+===============+
| Bit number | Capability description | Reference |
+============+========================+===============+
| 0 | PDR-ACK request (K) | This document |
+------------+------------------------+---------------+
| 1 | Requested path should | This document |
| | be redundant (R) | |
+------------+------------------------+---------------+
Table 3: Initial PDR Flags
8.4. SubRegistry for the PDR-ACK Flags
IANA is required to create an subregistry for the 8-bit PDR-ACK Flags
field. Each bit is tracked with the following qualities:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Reference
Registration procedure is "Standards Action" [RFC8126]. No bit is
currently defined for the PDR-ACK Flags.
8.5. Subregistry for the PDR-ACK Acceptance Status Values
IANA is requested to create a Subregistry for the PDR-ACK Acceptance
Status values.
* Possible values are 6-bit unsigned integers (0..63).
* Registration procedure is "Standards Action" [RFC8126].
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* Initial allocation is as indicated in Table 4:
+-------+------------------------+---------------+
| Value | Meaning | Reference |
+-------+------------------------+---------------+
| 0 | Unqualified acceptance | This document |
+-------+------------------------+---------------+
Table 4: Acceptance values of the PDR-ACK Status
8.6. Subregistry for the PDR-ACK Rejection Status Values
IANA is requested to create a Subregistry for the PDR-ACK Rejection
Status values.
* Possible values are 6-bit unsigned integers (0..63).
* Registration procedure is "Standards Action" [RFC8126].
* Initial allocation is as indicated in Table 5:
+-------+-----------------------+---------------+
| Value | Meaning | Reference |
+-------+-----------------------+---------------+
| 0 | Unqualified rejection | This document |
+-------+-----------------------+---------------+
Table 5: Rejection values of the PDR-ACK Status
8.7. SubRegistry for the Route Projection Options Flags
IANA is requested to create a Subregistry for the 5-bit Route
Projection Options (RPO) Flags field. Each bit is tracked with the
following qualities:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Reference
Registration procedure is "Standards Action" [RFC8126]. No bit is
currently defined for the Route Projection Options (RPO) Flags.
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8.8. SubRegistry for the Sibling Information Option Flags
IANA is required to create a registry for the 5-bit Sibling
Information Option (SIO) Flags field. Each bit is tracked with the
following qualities:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Reference
Registration procedure is "Standards Action" [RFC8126]. The initial
allocation is as indicated in Table 6:
+============+===================================+===============+
| Bit number | Capability description | Reference |
+============+===================================+===============+
| 0 | Connectivity is bidirectional (B) | This document |
+------------+-----------------------------------+---------------+
Table 6: Initial SIO Flags
8.9. 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, Remy Liubing, 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,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Thubert, et al. Expires 25 March 2021 [Page 24]
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[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>.
[RPL] 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>.
[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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>.
[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-29, 27 August 2020,
<https://tools.ietf.org/html/draft-ietf-6tisch-
architecture-29>.
Thubert, et al. Expires 25 March 2021 [Page 25]
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[RAW-ARCHI]
Thubert, P., Papadopoulos, G., and R. Buddenberg,
"Reliable and Available Wireless Architecture/Framework",
Work in Progress, Internet-Draft, draft-pthubert-raw-
architecture-04, 6 July 2020,
<https://tools.ietf.org/html/draft-pthubert-raw-
architecture-04>.
[TURN-ON_RFC8138]
Thubert, P. and L. Zhao, "Configuration option for RFC
8138", Work in Progress, Internet-Draft, draft-thubert-
roll-turnon-rfc8138-03, 8 July 2019,
<https://tools.ietf.org/html/draft-thubert-roll-turnon-
rfc8138-03>.
[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>.
[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>.
[USEofRPLinfo]
Robles, I., Richardson, M., and P. Thubert, "Using RPI
Option Type, Routing Header for Source Routes and IPv6-in-
IPv6 encapsulation in the RPL Data Plane", Work in
Progress, Internet-Draft, draft-ietf-roll-useofrplinfo-40,
25 June 2020, <https://tools.ietf.org/html/draft-ietf-
roll-useofrplinfo-40>.
[PCE] IETF, "Path Computation Element",
<https://datatracker.ietf.org/doc/charter-ietf-pce/>.
Appendix A. Applications
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A.1. Loose Source Routing
A RPL implementation operating in a very constrained LLN typically
uses the Non-Storing Mode of Operation as represented in Figure 8.
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
Figure 8: 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).
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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.
A.2. Transversal Routes
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 9:
* 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.
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* 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.
------+---------
| 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 9: 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
path, or if the new route is engineered with a different objective,
and that it is even more critical in Non-Storing Mode than it is in
Storing Mode, because the routing stretch is wider. 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.
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------+---------
| 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 10: 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 11, 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 11: 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 12: 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 13: 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 14: Projected Transversal Route
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
06254 Mougins - Sophia Antipolis
France
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Phone: +33 497 23 26 34
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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