wdiff draft-ietf-roll-dao-projection-12.txt draft-ietf-roll-dao-projection-13.txt

ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 6550 (if approved) R.A. Jadhav
Intended status: Standards Track Huawei Tech
Expires: 25 March 2 April 2021 M. Gillmore
Itron
21
29 September 2020

Root initiated routing state in RPL
draft-ietf-roll-dao-projection-12
draft-ietf-roll-dao-projection-13

Abstract

This document enables updates RFC 6550 to enable 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
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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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This Internet-Draft will expire on 25 March 2 April 2021.

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Please review these documents carefully, as they describe your rights
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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 Track . . . . . . . . . . . . . . . . . . . . . 7 8
5. New RPL Control Messages and Options . . . . . . . . . . . . 8 9
5.1. New P-DAO Request Control Message . . . . . . . . . . . . 8 9
5.2. New PDR-ACK Control Message . . . . . . . . . . . . . . . 9 10
5.3. Route Projection Options . . . . . . . . . . . . . . . . 10 12
5.4. Sibling Information Option . . . . . . . . . . . . . . . 13 14
6. Projected DAO . . . . . . . . . . . . . . . . . . . . . . . . 14 16
6.1. Requesting a Track . . . . . . . . . . . . . . . . . . . 16 17
6.2. Routing over a Track . . . . . . . . . . . . . . . . . . 16 18
6.3. Non-Storing Mode Projected Route . . . . . . . . . . . . 17 18
6.4. Storing-Mode Storing Mode Projected Route . . . . . . . . . . . . . . 18 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 22
8.1. New RPL Control Codes . . . . . . . . . . . . . . . . . . 21 22
8.2. New RPL Control Message Options . . . . . . . . . . . . . 21 22
8.3. SubRegistry for the Projected DAO Request Flags . . . . . 22 23
8.4. SubRegistry for the PDR-ACK Flags . . . . . . . . . . . . 22 23
8.5. Subregistry for the PDR-ACK Acceptance Status Values . . 22 24
8.6. Subregistry for the PDR-ACK Rejection Status Values . . . 23 24
8.7. SubRegistry for the Route Projection Options Flags . . . 23 24
8.8. SubRegistry for the Sibling Information Option Flags . . 24 25
8.9. Error in Projected Route ICMPv6 Code . . . . . . . . . . 24 25
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 25
10. Normative References . . . . . . . . . . . . . . . . . . . . 24 26
11. Informative References . . . . . . . . . . . . . . . . . . . 25 26
Appendix A. Applications . . . . . . . . . . . . . . . . . . . . 26 28
A.1. Loose Source Routing . . . . . . . . . . . . . . . . . . 27 28
A.2. Transversal Routes . . . . . . . . . . . . . . . . . . . 28 29
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 30 31
B.1. Using Storing Mode P-DAO in Non-Storing Mode MOP . . . . 30 31
B.2. Projecting a storing-mode Storing Mode transversal route . . . . . . . 31 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 34

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] 6TiSCH uses RPL for its routing operations.

The 6TiSCH Architecture "6TiSCH Architecture" [6TiSCH-ARCHI] 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 the PCE with a global visibility on the system
can compute P2P direct Peer to Peer (P2P) routes that are more optimized for
the current needs as expressed by the an objective function. This draft proposes document
specifies protocol extensions to RPL [RPL] that enable the Root of a
main DODAG 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 on
behalf of a base RPL Instance. PCE.

This specification expects that the base main 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. Projected Routes can be used to
reduce the size of the source routing headers with loose source
routing operations down the main RPL DODAG. Projected Routes can
also be used to build transversal routes for route optimization and
Traffic Engineering purposes, between nodes of the DODAG.

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 potentially 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. Track Egress.

The "Reliable and Available Wireless (RAW) Architecture/Framework"
[RAW-ARCHI] enables a dynamic defines the Path Selection Engine (PSE) that adapts the
use of the path selection redundancy within the a Track to
increase defeat the transmission diversity and combat diverse
causes of packet loss.

To that effect, RAW defines the Path Selection Engine (PSE) as

The PSE is a
complement dataplane extension of the PCE operating in the dataplane. The PSE PCE; it controls the use
forwarding operation of the packets within a Track, using Packet ARQ,
Replication, Elimination, and Overhearing (PAREO) functions over the
Track segments.

While segments, to provide a dynamic balance between the reliability
and availability requirements of the flows and the need to conserve
energy and spectrum.

The time scale at which the PCE (re)computes the Track can be long, for an operation based on
using long-term statistical metrics to perform global optimizations
at the scale of the whole network, network. Conversely, the PSE makes
forwarding decision decisions at the time scale of one or a small collection
of packets, using based on a knowledge that is changing rapidly but limited in scope of to the
Track itself. This way, the PSE itself, so it can provide be refreshed at a dynamic balance between the reliability and availability
requirements of the flows and the need to conserve energy and
spectrum. fast pace.

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, 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.

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 vertex (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 segment that is
computed,
computed remotely, and 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 TrackID
is associated with a Target address that is IPv6 Address to the Track destination. Egress Node.

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 6 of [RPL] specifies introduces the RPL Control Message Options (CMO)
to (CMO),
including the RPL Target Option (RTO) and Transit Information Option
(TIO), which can be placed in RPL messages such as the Destination
Advertisement Object (DAO) message. (DAO). This specification extends the DAO
message with the Projected DAO (P-DAO); a P-DAO message signals a
Projected Route using new CMOs presented therein.

A Projected Route can be an additional route of higher precedence
within the main DODAG, in which case it is installed with the
RPLInstanceID and DODAGID of the main DODAG.

A Projected Route can also be a Segment within a Track. A stand-
alone Segment can be used as a Serial (end-to-end) Track. Segments
can also be combined to form a Complex Track. The Root uses a local
RPL Target Option (RTO) Instance rooted at the Track Egress to establish and maintain the Transit
Information Option (TIO) are such options.

In Non-Storing Mode,
Track. The local RPLInstanceID of the TIO option Track is used called the TrackID,
more in Section 4.

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|D| Flags | Reserved | DAOSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address of the Track Egress +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+

Figure 1: Projected DAO Format for a Track

A P-DAO message to
inform signals the root IPv6 Address of the parent-child relationships within Track Egress in the DODAG,
DODAGID field of the DAO Base Object, and the Root has a full knowledge of TrackID in the
RPLInstanceID field, as shown in Figure 1.

In RPL Non-Storing Mode, the DODAG structure. The TIO
applies and RTO are combined in a DAO
message to inform the RTOs that preceed it immediately DODAG Root of all the edges in the message. DODAG, which
are formed by the directed parent-child relationships. Options may
be factorized; multiple TIOs RTOs may be present to indicate
multiple routes to signal a collection of
children that can be reached via the one or more contiguous addresses parent(s) indicated in the RTOs
TIO(s) that immediately precede the TIOs in follows the RPL message. RTOs.

This specification introduces two new CMOs referred generalizes the case of a parent that can be used
to as reach a child with that of a whole Track through which both
children and siblings may be reached.

New CMOs called the Route Projection Options (RPO) are introduced for
use in P-DAO messages as a multihop alternative to install Projected Routes. the TIO. One RPO
is the Via Information Option (VIO) and the other is (VIO); the Source-Routed VIO
(SRVIO). The VIO installs a route on state at each
hop along a Projected
Route (in a fashion analogous to RPL Storing Mode) whereas Mode Projected Route. The other is the Source-
Routed VIO (SRVIO); the SRVIO installs a source-routing state at the ingress node,
Segment ingress, which uses that state to encapsulate a packet with an IPv6 Routing a
Source-Routing Header in along a
fashion similar to RPL Non-Storing Mode. Mode Projected Route.

Like in a DAO message, the TIO, RTOs can be factorized in a P-DAO, but the RPOs MUST
CMOs cannot. A P-DAO contains one or more RTOs that indicate the
destinations that can be preceded by exactly reached via the Track, and either one RTO to which
they apply, SRVIO
or one VIO signal the sequence of hops between the Track Ingress and SRVIOs MAY be factorized, though VIOs MUST NOT be.
Factorized contiguous SRVIOs indicate alternate paths
the (penultimate) node before the Track Egress. In Non-Storing Mode,
the Root sends the P-DAO to the Target,
more Track Ingress where the source-
routing state is stored. In Storing Mode, the P-DAO is sent to the
Track Egress and forwarded along the Segment in Section 5.3. the reverse
direction, installing a Storing Mode state at each hop.

This specification also introduces a new adds another CMO to enable called the Sibling Information
Option (SIO) that is used by a RAN RPL Aware Node (RAN) to advertise a
selection of its candidate neighbors as siblings to the Root, using a new Sibling Information Option (SIO) as specified more in
Section 5.4.

4. Identifying The sibling selection process is out of scope.

Two new RPL Control Messages are also introduced, to enable a Path

It must RAN to
request the establishment of a Track between self as the Track
Ingress Node and a Track Egress. 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. A positive PDR-ACK indicates that the Track
was built and that the Roots commits to maintain the Track for a
negotiated lifetime.

In the case of a complex Track, each Segment is maintained
independently and asynchronously by the Root, with its own lifetime
that may be noted shorter, the same, or longer than that RPL has of the Track. The
Root may use an asynchronous PDR-ACK with an negative status to
indicate that the Track was terminated before its time.

4. Identifying a Track

RPL defines the concept of an Instance to represent
different signal an individual
routing topologies topology 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 RPL Instance model as follows:

* If the The PCE needs to influence a particular Instance MAY use P-DAO messages to add better routes in the main
(Global) Instance in conformance with the routing objectives in
that Instance. To achieve this, the PCE MAY install a Storing
Mode Projected Route along a path down the main (Non-Storing Mode)
DODAG. This enables a loose source routing and reduces the size
of the Source Routing Header, see Appendix A.1.

When adding a Storing Mode Projected Route to the main RPL
Instance, it may do the Root MUST set the RPLInstanceID field of the DAO
message (see section 6.4.1. of [RPL]) to the RPLInstanceID of the
main DODAG, and set the DODAGID field to the Segment Egress. The
Projected Route provides a longer match to the Egress than the
default route via the Root, so as long as it does not create a loop. A is preferred. Once the
Projected Route is always preferred over installed, the intermediate nodes listed in the
VIO between the first (excluded) and the last (included) can be
elided in a route Source-Route Header that is learned
via RPL. signals that Segment.

* The PCE may Root MAY also use P-DAOs P-DAO messages to install a specific (say,
Traffic Engineered) and possibly complex path, that we refer to path as a Serial of a Complex Track, towards to a
particular Target. endpoint that is the Track Egress. In that case it case, the
Root 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 the Global Unique IPv6 address. The Address
(GUA) or Unique-Local Address (ULA) of the Track Egress that
serves as DODAGID for the Track. This way, a Track is uniquely
identified within the RPL domain by the tuple (Target
address, (Track Egress Address, TrackID) where the
TrackID is always represented with the 'D' flag set. The Track where a packet is placed is
Egress Address and the TrackID are signaled by a in the P-DAO message
as shown in Figure 1.

* In the data packets, the Track Egress Address and the TrackID are
respectively signaled in IPv6 Address of the final destination and
the RPLInstanceID field of the 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

If the outer chain of IPv6 header Headers contains a Source-Routing
Header that is not fully consumed, then the Target that final destination is used to identify
the Track, more last entry in Section 6.2.

* The PCE may also install a projected Route as a complement to the
main DODAG, e.g., using Source-Routing Header; else it is the Storing-Mode Mode along a Source-
Routed path
Destination Address in order to enable loose source routing and reduce the
Routing IPv6 Header. In that case, When using the global RPLInstanceID [RFC8138]
compression, it is the last hop of the
main DODAG is signaled in place last SRH-6LoRH of the TrackId on outer
header in either case.

The 'D' flag in the P-DAO, and RPLInstanceID MUST be set to indicate that the RPI
final destination address in the packet indicates IPv6 header owns the global local
RPLInstanceID, more in
Appendix A.1. Section 6.2.

* A packet that is being routed over the RPL Instance associated to
a first Non-Storing Mode Track MUST NOT MAY be placed over (encapsulated) in a different RPL Instance again.
Conversely,
second Track to cover one loose hop of the first Track. On the
other hand, a Storing Mode Track must be strict and a packet that is
it placed on a Global Instance MAY be
injected in a Local Instance based on a network policy and Storing Mode Track MUST follow that Track till the
Local Instance configuration.

A Projected Route is
Track Egress.

When a serial path that may represent Track Egress extracts a packet from a Track (decapsulates
the end-to-end
route packet), the Destination of the inner packet MUST be either
this node or only a Segment in a complex Track, in which case multiple
Projected Routes are installed with direct neighbor, otherwise the same tuple (Target address,
TrackID) and packet MUST be
dropped. That Destination may be the next Hop in a different Segment ID each. Non-Storing
Mode Track.

All properties of a Track operations are inherited form the main
instance RPL
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 to the
Target
Track Egress indicated in the Target Option TIO as a full path of a collection of
Segments in a Track. Exactly one Target Option TIO 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: 2: 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

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 triggered 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:

Figure 2: 3: 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.

The PDR-ACK Status is substructured as indicated in Figure 3: 4:

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E|R| Value |
+-+-+-+-+-+-+-+-+

Figure 3: 4: 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 Serial Track in full
or a Segment of a more complex Complex Track. In Non-Storing Mode, multiple
RPO may be placed after a same Target Option TIO to reflect different Segments
originated at this node. The Track is identified by a TrackID that
is a Local RPLInstanceID to the Target Track Egress 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 |C| | Flags | Reserved SegmentID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TrackID | SegmentID
|Segm. Sequence | Seg. Lifetime | SRH-6LoRH header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: 5: 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 and the Via Addresses are expressed in 128 bits. compression.

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
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 TrackID field. A Value of 0 is
used to signal a serial path, Serial Track, 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 Track Egress
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 Track Egress is referred as a No-Path (for that Target) Track
Egress) in this document.

Via Address:

SRH-6LoRH header: The collection first 2 bytes of the SRH-6LoRH as shown in
Figure 6 of [RFC8138]. A 6LoRH Type of 4 means that the VIA
Addresses are provided in full with no compression.

Via Address: A Luistof Via Addresses along one Segment, indicated in
the order of the path from the ingress to the egress nodes. If

In a VIO, the "C" flag list is set, a strict path between direct neighbors,
whereas for an SRVIO, the fields Via Address 1 .. Via
Address n in Figure 4 are replaced by one or more of list may be loose, provided that each
listed node has a path to the headers
illustrated in Fig. 6 of [RFC8138]. next listed node, e.g., via another
Track.

In the case of a VIO, or if [RFC8138] is turned off, then the Root
MUST use only one SRH-
6LoRH, SRH-6LoRH per RPO, and the compression is the
same for all addresses. the addresses, as shown in Figure 5.

If [RFC8138] is turned on, then the Root SHOULD optimize the size
of the SRVIO; in that case, more than one SRH-6LoRH are may be needed
if the compression of the addresses change changes 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.

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: 6: 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
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 Track by sending one or more extended DAO message called
Projected-DAO (P-DAO) messages to an arbitrary router chosen routers 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 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 Segment to the Target. Track Egress. The recipient of the P-DAO is
the ingress router of the source-routed path. Segment. Upon a Non-Storing Non-
Storing Mode P-DAO, the ingress router installs a source-routed
state to the
Target Track Egress 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 single
VIO, within which are signaled 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
Track Egress 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)
[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 TrackID and
Target
Track Egress 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 placed in a Track
MUST be that of the Target Track Egress of Track. The outer header of the
packet MUST contain an RPI that indicates the TrackId 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 for an
encapsulation. Else, i.e., if the Track Ingress is forwarding a
packet into the Track, or if the the final destination is reached via
is not the Target, Track Egress, 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, 7, a P-DAO that carries an SRVIO enables the
Root to install a source-routed path towards a Target Track Egress 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 Track Egress or can
be reached via the Target. Track Egress.

LLN

Figure 6: 7: 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, Track Egress, the router
inserts the source routing header in the packet with the destination
set to reach the Target. Track Egress. In order to add a source-routing header,
the router encapsulates the packet with an IP-in-IP header and a Non-Storing 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.
Track Egress.

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
SRVIO and then the Target Track Egress 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 Storing Mode Projected Route

As illustrated in Figure 7, 8, 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 Ingress and an egress router; Egress router this enables the routers to
forward along that Segment any packet for which the next loose hop is
the said Target, Egress node, for Instance instance a loose source routed packet for which
the next loose hop is the Target, Egress node, or a packet for which the
router has a routing state to the final destination via the Target. Egress
node.

------+---------
| 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: 8: 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 last 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
first Via Address in the first VIO.

The egress 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, address in the VIO,
and uses as next hop the address found in the previous Via Address
field in the following VIO. The router MAY install additional routes towards
the addresses that are located in VIOs VIA Addresses that are the VIO after the next one, if any, but in
case of a conflict or a lack of resource, a
route the route(s) to a Target installed by the Root has
Target(s) have 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.

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

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].

* 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.

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>.

[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>.

[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

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. 9.
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.

Figure 8: 9: 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.

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: 10:

* 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.

* 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.

Figure 9: 10: 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.

Figure 10: 11: 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, 12, if the source is node 41
and the destination is node 52, then the common parent is node 22.

------+---------
| 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: 12: 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 Storing Mode transversal route

In this example, say that a PCE determines that a path must be
installed between node S I and node D via routers A, B and C, E, in order
to serve the needs of a particular application.

The Root sends a P-DAO to node E, with a Target option an RTO indicating the
destination D D, a TIO optionally indicating the Track Egress in the
Parent Address field, and a sequence of Via Information option, options
indicating the hops, one for S, which is the ingress router of the
Segment, one for A A, and then one for B, which are an respectively the
intermediate routers, and one for C, which is the egress
router. penultimate routers.

Figure 12: 13: 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.

Figure 13: 14: P-DAO-ACK to Root

As a result, a transversal route is installed that does not need to
follow the DODAG structure.

Figure 14: 15: 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
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