Root initiated routing state in RPL
draft-ietf-roll-dao-projection-14
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| Last updated | 2020-10-02 | ||
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draft-ietf-roll-dao-projection-14
ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems
Updates: 6550 (if approved) R.A. Jadhav
Intended status: Standards Track Huawei Tech
Expires: 5 April 2021 M. Gillmore
Itron
2 October 2020
Root initiated routing state in RPL
draft-ietf-roll-dao-projection-14
Abstract
This document 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 Routing 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
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 5 April 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. New RPL Control Messages and Options . . . . . . . . . . . . 8
4.1. New P-DAO Request Control Message . . . . . . . . . . . . 8
4.2. New PDR-ACK Control Message . . . . . . . . . . . . . . . 9
4.3. Route Projection Options . . . . . . . . . . . . . . . . 10
4.4. Sibling Information Option . . . . . . . . . . . . . . . 12
5. Projected DAO . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Requesting a Track . . . . . . . . . . . . . . . . . . . 15
5.2. Identifying a Track . . . . . . . . . . . . . . . . . . . 16
5.3. Forwarding Along a Track . . . . . . . . . . . . . . . . 17
5.4. Non-Storing Mode Projected Route . . . . . . . . . . . . 18
5.5. Storing Mode Projected Route . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7.1. New RPL Control Codes . . . . . . . . . . . . . . . . . . 21
7.2. New RPL Control Message Options . . . . . . . . . . . . . 22
7.3. SubRegistry for the Projected DAO Request Flags . . . . . 22
7.4. SubRegistry for the PDR-ACK Flags . . . . . . . . . . . . 23
7.5. Subregistry for the PDR-ACK Acceptance Status Values . . 23
7.6. Subregistry for the PDR-ACK Rejection Status Values . . . 23
7.7. SubRegistry for the Route Projection Options Flags . . . 24
7.8. SubRegistry for the Sibling Information Option Flags . . 24
7.9. Error in Projected Route ICMPv6 Code . . . . . . . . . . 25
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
9. Normative References . . . . . . . . . . . . . . . . . . . . 25
10. Informative References . . . . . . . . . . . . . . . . . . . 26
Appendix A. Applications . . . . . . . . . . . . . . . . . . . . 27
A.1. Loose Source Routing . . . . . . . . . . . . . . . . . . 27
A.2. Transversal Routes . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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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. 6TiSCH uses RPL for its routing operations.
The "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, the PCE with a global visibility on the system
can compute direct Peer to Peer (P2P) routes that are optimized for
the needs expressed by an objective function. This document
specifies protocol extensions to RPL [RPL] that enable the Root of a
main DODAG to install centrally-computed routes inside the DODAG on
behalf of a PCE.
This specification expects that the 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.
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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 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 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 Track Egress.
The "Reliable and Available Wireless (RAW) Architecture/Framework"
[RAW-ARCHI] defines the Path Selection Engine (PSE) that adapts the
use of the path redundancy within a Track to defeat the diverse
causes of packet loss.
The PSE is a dataplane extension of the PCE; it controls the
forwarding operation of the packets within a Track, using Packet ARQ,
Replication, Elimination, and Overhearing (PAREO) functions over the
Track 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,
using long-term statistical metrics to perform global optimizations
at the scale of the whole network. Conversely, the PSE makes
forwarding decisions at the time scale of one or a small collection
of packets, based on a knowledge that is limited in scope to the
Track itself, so it can be refreshed at a 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 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 vertex (i.e., node) that has no outgoing edge (i.e., link)
LLN: Low-Power and Lossy Network
NMPR: Non-Storing Mode Projected Route
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
RH: Routing Header
RPI: RPL Packet Information
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.
SMPR: Storing Mode Projected Route
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 path segment that is
computed remotely, and installed and maintained 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 IPv6 Address to the Track Egress Node.
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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
Section 6 of [RPL] introduces the RPL Control Message Options (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). This specification extends the DAO
message with the Projected DAO (P-DAO); a P-DAO message signals one
or more Projected Route(s) using the new CMOs presented therein.
A Projected Route can be an additional route of higher precedence
within the main DODAG. In that case, it is installed with a P-DAO
using the parameters of the main DODAG, typically a global
RPLInstanceID and the DODAGID field elided as shown in Section 6.4.1.
of [RPL].
A Projected Route can also be a Segment within a Track. A stand-
alone Segment can be used as a Serial Track. Segments can also be
combined to form a Complex Track. The Root uses a local RPL Instance
rooted at the Track Egress to signal the Track. The local
RPLInstanceID of the Track is called the TrackID, more in
Section 5.2. A P-DAO message for a Track signals the IPv6 Address of
the Track Egress in the DODAGID field of the DAO Base Object, and the
TrackID in the RPLInstanceID field, as shown in Figure 1.
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
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In RPL Non-Storing Mode, the TIO and RTO are combined in a DAO
message to inform the DODAG Root of all the edges in the DODAG, which
are formed by the directed parent-child relationships. Options may
be factorized; multiple RTOs may be present to signal a collection of
children that can be reached via the parent(s) indicated in the
TIO(s) that follows the RTOs. This specification generalizes the
case of a parent that can be used to reach a child with that of a
whole Track through which both children and siblings of the Track
Egress are reachable.
New CMOs called the Route Projection Options (RPO) are introduced for
use in P-DAO messages as a multihop alternative to the TIO. One RPO
is the Via Information Option (VIO); the VIO installs a state at each
hop along a Storing Mode Projected Route (SMPR). The other is the
Source-Routed VIO (SRVIO); the SRVIO installs a source-routing state
at the Segment ingress, which uses that state to encapsulate a packet
with a Routing Header (RH) along a Non-Storing Mode Projected Route
(NMPR).
Like in a DAO message, the RTOs can be factorized in a P-DAO, but the
RPOs cannot. A P-DAO contains one or more RTOs that indicate the
destinations that can be reached via the Track, and exactly one RPO
that signals the sequence of nodes between the Track Ingress and the
Track Egress, both included. In Non-Storing Mode, the Root sends the
P-DAO to the 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 the reverse direction, installing a Storing Mode
state at each hop. In both cases the Track Ingress generates the P-
DAO-ACK when the installation is successful.
This specification adds another CMO called the Sibling Information
Option (SIO) that is used by a RPL Aware Node (RAN) to advertise a
selection of its candidate neighbors as siblings to the Root, more in
Section 4.4. The sibling selection process is out of scope.
Two new RPL Control Messages are also introduced, to enable a 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 4.1 for more. A positive PDR-ACK indicates that the Track
was built and that the Roots commits to maintain the Track for the
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 shorter, the same, or longer than that 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.
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4. New RPL Control Messages and Options
4.1. New P-DAO Request Control Message
The P-DAO Request (PDR) message is sent by a Node in the main DODAG
to the Root. It is a request to establish or refresh a Track.
Exactly one RTO MUST be present in a PDR. The RTO signals the Track
Egress, more in Section 5.1.
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 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 request a Complex Track for redundancy.
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 triggered it. It is
incremented at each PDR message and echoed in the PDR-ACK by the
Root.
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4.2. 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 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; the 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 4:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E|R| Value |
+-+-+-+-+-+-+-+-+
Figure 4: PDR-ACK status Format
E: 1-bit flag. Set to indicate a rejection. When not set, the
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, see Table 4 and Table 5.
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Reserved: The Reserved field MUST initialized to zero by the sender
and MUST be ignored by the receiver
4.3. Route Projection Options
An RPO signals the ordered list of IPv6 Via Addresses that
constitutes the hops of either a Serial Track or a Segment of a more
Complex Track. 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. The format of the 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 | Flags | SegmentID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Segm. Sequence | Seg. Lifetime | SRH-6LoRH header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 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 and the compression.
SegmentID: 8-bit field that identifies a Segment within a Track or
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the main DODAG as indicated by the TrackID field. The value of 0
is used to signal a 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 RPO deprecates any state
for the Segment indicated by the SegmentID within the indicated
Track and sets up the new information.
An RPO with a Segment Sequence that is not as fresh as the current
one is ignored.
An RPO for a given 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. The value
of 255 (0xFF) represents infinity. The value of zero (0x00)
indicates a loss of reachability.
A P-DAO message that contains a Via Information option with a
Segment Lifetime of zero for a Track Egress is referred as a No-
Path (for that Track Egress) in this document.
SRH-6LoRH header: The first 2 bytes of the (first) 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: An IPv6 addresse along the Segment.
In a VIO, the list is a strict path between direct neighbors,
whereas for an SRVIO, the list may be loose, provided that each
listed node has a path to the next listed node, e.g., via another
Track.
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In the case of a SMPR, or if [RFC8138] is not used in the data
packets, then the Root MUST use only one SRH-6LoRH per RPO, and
the compression is the same for all the addresses, as shown in
Figure 5.
In case of a NMPR, and if [RFC8138] is in use in the main DODAG,
then the Root SHOULD optimize the size of the SRVIO; more than one
SRH-6LoRH may be present, e.g., if the compression level changes
inside the Segment and different SRH-6LoRH Types are required.
4.4. Sibling Information Option
The Sibling Information Option (SIO) provides indication on siblings
that could be used by the Root to form Projected Routes. One or more
SIO(s) may be placed in the DAO messages that are sent to the Root in
Non-Storing Mode.
The format of the SIO 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 6: Sibling Information Option Format
Option Type: 0x0D (to be confirmed by IANA)
Option Length: In bytes, the size of the option.
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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 and
its DODAGID if resent. The Compression refernce is the Root of
the main DODAG.
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 industrial 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.
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5. Projected DAO
This draft adds a capability to RPL whereby the Root of a DODAG
projects a Track by sending one or more Projected-DAO (P-DAO)
messages to selected routers in the DODAG. The P-DAO signals a
collection of Targets in the RPL Target Option(s) (RTO). Those
Targets can be reached via a sequence of routers indicated in a Route
Projection Option (RPO). A P-DAO message MUST contain exactly one
RPO, which is either a VIO or an SRVIO, and MUST follow one or more
RTOs. There can be at most one such sequence of RTO(s) and an RPO.
A P-DAO MUST be sent from the address of the Root that serves as
DODAGID for the main DODAG. It MUST be sent to a GUA or a ULA of
either the ingress or the egress of the Segment, more below. If the
'K' Flag is present in the P-DAO, and unless the P-DAO does not reach
it, the ingress of the Segment is the node that acknowledges the
message, using a DAO-ACK that MUST be sent back to the address that
serves as DODAGID for the main DODAG.
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 5.4. A Non-Storing
Mode P-DAO carries an SRVIO with the loose list of Via Addresses
that forms a source-routed Segment to the Track Egress. The
recipient of the P-DAO is the ingress router of the source-routed
Segment. The ingress router MUST install a source-routed state to
the Track Egress and reply to the Root directly using a DAO-ACK
message if requested to.
* The Storing Mode is discussed in Section 5.5. A Storing Mode
P-DAO carries a VIO with the strict list of Via Addresses from the
ingress to the egress of the Segment in the data path order. The
routers listed in the Via Addresses, except the egress, MUST
install a routing state to the Target(s) 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 Segment, that it can also be neither but
it cannot be both.
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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 7.9). 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 RH if one is present, and
this hop of the RH 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 outer 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 5.4 and Section 5.5 respectively.
5.1. Requesting a Track
A Node is free to ask the Root for a new Track at any time. This is
done with a PDR message, that indicates in the Requested Lifetime
field the duration for which the Track should be established. Upon a
PDR, the Root MAY install the necessary Segments, in which case it
answers with a PDR-ACK indicating the granted Track Lifetime. 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 the same
Track Egress signaled in the P-DAO.
The Root is free to design the Track as it wishes, and to change the
Segments overtime to serve the Track as needed, without notifying the
resquesting Node. 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.
When the Track Lifetime that was 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 extend the lifetime of the Track, else the Track
will time out and the Root will tear down the whole structure.
If the Track fails and cannot be restored, 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.
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5.2. Identifying a Track
RPL defines the concept of an Instance to signal an individual
routing 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 leverages the RPL Instance model as follows:
* The Root 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 Root MAY install an SMPR
along a path down the main Non-Storing Mode DODAG. This enables a
loose source routing and reduces the size of the Routing Header,
see Appendix A.1.
When adding an SMPR to the main RPL Instance, the Root MUST set
the RPLInstanceID field of the P-DAO message (see section 6.4.1.
of [RPL]) to the RPLInstanceID of the main DODAG, and MUST NOT use
the DODAGID field. A Projected Route provides a longer match to
the Target Address than the default route via the Root, so it is
preferred.
Once the Projected Route is installed, the intermediate nodes
listed in the VIO after first one (i.e. The ingress) can be
elided from the RH in packets sent along the Segment signaled in
the P-DAO. The resulting loose source routing header indicates
(one of) the Target(s) as the next entry after the ingress.
* The Root MAY also use P-DAO messages to install a specific (say,
Traffic Engineered) path as a Serial or as a Complex Track, to a
particular endpoint that is the Track Egress. In that case, the
Root MUST install a Local RPL Instance (see section 5 of [RPL]).
In a that case, the TrackID MUST be unique for the Global Unique
IPv6 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 by the tuple (Track Egress Address, TrackID)
where the TrackID is always represented with the 'D' flag set.
The Track Egress Address and the TrackID MUST be signaled in the
P-DAO message as shown in Figure 1.
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5.3. Forwarding Along a Track
Sending a Packet within a RPL Local Instance requires the presence of
an RPL Packet Information (RPI) (see [USEofRPLinfo]) in the outer
IPv6 Header chain. The RPI carries a local RPLInstanceID which, in
association with the IPv6 final destination, indicates the RPL
Instance that the packet follows.
This draft leverages the RPL Forwarding model follows:
* The RPI carries a local RPLInstanceID called the TrackID, which,
in association with the IPv6 final destination, indicates the
Track along which the packet is forwarded. The 'D' flag in the
RPLInstanceID MUST be set to indicate that the final destination
address in the IPv6 header owns the local RPLInstanceID, more in
Section 5.3.
In the data packets, the Track Egress Address and the TrackID MUST
be respectively signaled as the IPv6 Address of the final
destination and the RPLInstanceID field of the RPI that MUST be
placed in the outer chain of IPv6 Headers.
In case of a NMPR, the outer chain of IPv6 Headers contains an
IPv6 RH as well. If it is not fully consumed, then the final
destination is the last entry in the RH; else it is the
Destination Address in the IPv6 Header. When using the [RFC8138]
compression, it is the last hop of the last SRH-6LoRH of the outer
header in either case.
* If the Track Ingress is the originator of the packet and the Track
Egress is the destination of the packet, there is no need 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
over the Track via the Track Egress but is located beyond it, then
an IP-in-IP encapsulation is needed.
A packet that is being routed over the RPL Instance associated to
a first Non-Storing Mode Track MAY be placed (encapsulated) in a
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
it placed in a Storing Mode Track MUST follow that Track till the
Track Egress.
When a Track Egress extracts a packet from a Track (decapsulates
the packet), the Destination of the inner packet MUST be either
this node or a direct neighbor, or a Target of another Segment of
the same Track for which this node is ingress, otherwise the
packet MUST be dropped.
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All properties of a Track operations are inherited form the main 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.
5.4. Non-Storing Mode Projected Route
As illustrated in Figure 7, a P-DAO that carries an SRVIO enables the
Root to install a source-routed path towards a Track Egress in any
particular router.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ | P ^ ACK
| Track | DAO |
o o o o Ingress X V | X
o o o o o o o X o X Source
o o o o o o o o X o o X Routed
o o ° o o o o X o X Segment
o o o o o o o o X Track X
o o o o o Egress
o o o o
o o o o
destination
LLN
Figure 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.
When forwarding a packet to a destination for which the router
determines that routing happens via the Track Egress, the router
inserts the source routing header in the packet with the destination
set to the Track Egress.
In order to signal the Segment, the router encapsulates the packet
with an IP-in-IP header and a Routing Header as follows:
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* In the uncompressed form the source of the packet is this router,
the destination is the first Via Address in the SRVIO, and the RH
is a Source Routing Header (SRH) [RFC6554] that contains the list
of the remaining Via Addresses terminating by the Track Egress.
* The preferred alternate in a network where 6LoWPAN Header
Compression [RFC6282] is used is to leverage "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 source routed header is the exact copy of the
(chain of) SRH-6LoRH found in the SRVIO, also terminating by the
Track Egress. The RPI-6LoRH is appended next, followed by an IP-
in-IP 6LoRH Header that indicates the Ingress Router in the
Encapsulator Address field, see as a similar case Figure 20 of
[TURN-ON_RFC8138].
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 another Track to the loose
next hop for which this node is Ingress; in the latter case, another
encapsulation takes place and the process possibly recurses;
otherwise the packet is dropped.
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.
5.5. Storing Mode Projected Route
As illustrated in Figure 8, a P-DAO that carries a VIO enables the
Root to install a stateful route towards a collection of Targets
along a Segment between a Track Ingress and a Track Egress.
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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 8: Projecting a route
In order to install the relevant routing state along the Segment ,
the Root sends a unicast P-DAO message to the Track Egress router of
the routing Segment that is being installed. The P-DAO message
contains a VIO with the direct sequence of Via Addresses. The VIO
follows one or more RTOs indicating the Targets to which the Track
leads. The VIO 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 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 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. If one of the
Targets is not known, 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.
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The address of the predecessor to be used as destination of the
propagated DAO message is found in the Via Address the precedes the
one that contain the address of the propagating node, which is used
as source of the message.
Upon receiving a propagated DAO, all except the Egress Router MUST
install a route towards the DAO Target(s) via their successor in the
VIO. The router MAY install additional routes towards the VIA
Addresses that are the VIO after the next one, if any, but in case of
a conflict or a lack of resource, the route(s) to the Target(s) have
precedence.
If a router cannot reach its predecessor in the VIO, 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).
The process continues till the P-DAO is propagated to ingress router
of the Segment, which answers with a DAO-ACK to the Root.
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 an SMPR, 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.
6. 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?
7. IANA Considerations
7.1. New RPL Control Codes
This document extends the IANA Subregistry created by RFC 6550 for
RPL Control Codes as indicated in Table 1:
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+======+=============================+===============+
| Code | Description | Reference |
+======+=============================+===============+
| 0x09 | Projected DAO Request (PDR) | This document |
+------+-----------------------------+---------------+
| 0x0A | PDR-ACK | This document |
+------+-----------------------------+---------------+
Table 1: New RPL Control Codes
7.2. 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
7.3. 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:
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+============+========================+===============+
| 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
7.4. 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.
7.5. 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
7.6. Subregistry for the PDR-ACK Rejection Status Values
IANA is requested to create a Subregistry for the PDR-ACK Rejection
Status values.
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* 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
7.7. 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.
7.8. 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:
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+============+===================================+===============+
| Bit number | Capability description | Reference |
+============+===================================+===============+
| 0 | Connectivity is bidirectional (B) | This document |
+------------+-----------------------------------+---------------+
Table 6: Initial SIO Flags
7.9. 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.
8. Acknowledgments
The authors wish to acknowledge JP Vasseur, Remy Liubing, James
Pylakutty and Patrick Wetterwald for their contributions to the ideas
developed here.
9. 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>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[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
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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>.
10. 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>.
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[TURN-ON_RFC8138]
Thubert, P. and L. Zhao, "A RPL DODAG Configuration Option
for the 6LoWPAN Routing Header", Work in Progress,
Internet-Draft, draft-ietf-roll-turnon-rfc8138-17, 30
September 2020, <https://tools.ietf.org/html/draft-ietf-
roll-turnon-rfc8138-17>.
[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-41,
21 September 2020, <https://tools.ietf.org/html/draft-
ietf-roll-useofrplinfo-41>.
[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 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.
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------+---------
| 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 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).
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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 a 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 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 an RH
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.
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------+---------
| 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 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.
------+---------
| 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 11: Projected Transversal Route
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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.
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
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