wdiff draft-ietf-roll-dao-projection-03.txt draft-ietf-roll-dao-projection-04.txt

ROLL P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track R. Jadhav, Ed. Jadhav
Expires: September 20, December 21, 2018 Huawei Tech
J. Pylakutty
Cisco
March
June 19, 2018

Root initiated routing state in RPL
draft-ietf-roll-dao-projection-03
draft-ietf-roll-dao-projection-04

Abstract

This document proposes a protocol extension to RPL that enables to
install a limited amount of centrally-computed routes in a RPL graph,
enabling loose source routing down a non-storing mode DODAG, or
transversal routes inside the DODAG. As opposed to the classical
route injection in RPL that are injected by the end devices, this
draft enables the root of the DODAG to projects the routes that are
needed on the nodes where they should be installed.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 20, December 21, 2018.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. New RPL Control Message Options
2.1. BCP 14 . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Via Information Option
2.2. References . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Subset of a 6LoWPAN Glossary . . . . . . . . . . . . . . 4
2.4. New Terms . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Extending RFC 6550 . . . . . . . . . . . . . . . . . . . . . 5
4. New RPL Control Message Options . . . . . . . . . . . . . . . 5
5. Projected DAO . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. 7
5.1. Non-storing Mode Projected DAO . Route . . . . . . . . . . . . 6
4.2. 8
5.2. Storing-Mode Projected DAO . Route . . . . . . . . . . . . . . 8
5. 9
6. Applications . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. 11
6.1. Loose Source Routing in Non-storing Mode . . . . . . . . 10
5.2. 11
6.2. Transversal Routes in storing and non-storing modes . . . 11
6. 13
7. RPL Instances . . . . . . . . . . . . . . . . . . . . . . . . 13
7. 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. 15
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
10. 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 15
10.2. 16
11.2. Informative References . . . . . . . . . . . . . . . . . 15 17
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 16 18
A.1. Using storing mode P-DAO in non-storing mode MOP . . . . 16 18
A.2. Projecting a storing-mode transversal route . . . . . . . 17 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 20

1. Introduction

The "Routing Protocol for Low Power and Lossy Networks" [RFC6550]
(LLN)(RPL) is a generic Distance Vector protocol that is well suited
for application in a variety of low energy Internet of Things (IoT)
networks. RPL forms Destination Oriented Directed Acyclic Graphs
(DODAGs) in which the root often acts as the Border Router to connect
the RPL domain to the Internet. The root is responsible to select
the RPL Instance that is used to forward a packet coming from the
Internet into the RPL domain and set the related RPL information in
the packets.

The 6TiSCH architecture [I-D.ietf-6tisch-architecture] leverages RPL
for its routing operation and considers the Deterministic Networking
Architecture [I-D.ietf-detnet-architecture] as one possible model
whereby the device resources and capabilities are exposed to an
external controller which installs routing states into the network
based on some objective functions that reside in that external
entity.

Based on heuristics of usage, path length, and knowledge of device
capacity and available resources such as battery levels and
reservable buffers, a Path Computation Element ([PCE]) with a global
visibility on the system could install additional P2P routes that are
more optimized for the current needs as expressed by the objective
function.

This draft enables a RPL root, with optionally the assistance of a
PCE, root to install and maintain additional storing and non-storing mode projected
routes (P-routes) within the RPL domain, its DODAG, along a selected set of nodes and
that may or may not include self, for a
selected duration, thus providing chosen duration. This
potentially enables routes that are more suitable optimized than those
obtained with the distributed operation of RPL. Those routes 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. P-routes may
be installed in either storing Storing and non-storing modes Non-Storing Modes Instances of the
classical RPL instances, operation, resulting in potentially hybrid situations
where the mode of the
projected routes some P-routes is different from that of the other
routes in the
instance. RPL Instance.

Projected routes must be used with the parsimony to limit the amount
of state that is installed in each device to fit within its
resources, and to limit the amount of rerouted traffic to fit within
the capabilities of the transmission links. The algorithm used to
compute the paths and the protocol used to learn the topology of the
network and the resources that are available in devices and in the
network are out of scope for this document. Possibly with the
assistance of a Path Computation Element ([PCE]) that could have a
better visibility on the larger system, the root computes which
segment could be optimized and uses this draft to install the
corresponding projected routes.

2. Terminology

2.1. BCP 14

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The Terminology used BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.

2.2. References

In this document is consistent with document, readers will encounter terms and
incorporates concepts that described are
discussed in the following documents:

o "Routing Protocol for Low Power and Lossy Networks" [RFC6550], and

o "Terminology in Low power And Lossy
Networks"[RFC7102] Networks" [RFC7102].

2.3. Subset of a 6LoWPAN Glossary

This document often uses the following acronyms:

6BBR: 6LoWPAN Backbone Router

6LBR: 6LoWPAN Border Router

6LN: 6LoWPAN Node

6LR: 6LoWPAN Router

6CIO: Capability Indication Option

EARO: (Extended) Address Registration Option -- (E)ARO

EDAR: (Extended) Duplicate Address Request -- (E)DAR

EDAC: (Extended) Duplicate Address Confirmation -- (E)DAC

DAD: Duplicate Address Detection

DODAG: Destination-Oriented Directed Acyclic Graph

LLN: Low-Power and [RFC6550].

3. Lossy Network

NA: Neighbor Advertisement

NCE: Neighbor Cache Entry

ND: Neighbor Discovery

NDP: Neighbor Discovery Protocol

NS: Neighbor Solicitation

RPL: IPv6 Routing Protocol for LLNs (pronounced ripple) [RFC6550]

RA: Router Advertisement
RS: Router Solicitation

2.4. New Terms

Projected Route: A route that is installed remotely by a RPL Control Message Options root.

3. Extending RFC 6550

Section 6.7 of RPL [RFC6550] specifies Control Message Options (CMO)
to be placed in RPL messages such as the Destination Advertisement
Object (DAO) message. The RPL Target Option and the Transit
Information Option (TIO) are such options; the former indicates a
node to be reached and the latter specifies a parent that can be used
to reach that node. Options may be factorized; one or more
contiguous TIOs apply to the one or more contiguous Target options
that immediately precede the TIOs in the RPL message.

This specification introduces a 2 new Control Message Option, Options referred
to as Route Projection Options (RPO). One RPO is the Via Information
option (VIO). (VIO) and the other is the Source-Routed VIO (SRVIO). The VIO
installs a route on each hop along a projected route (in a fashion
analogous to RPL Storing Mode) whereas the SRVIO installs a source-
routing state at the ingress node, which uses it to insert a routing
header in a fashion similar to Non-Storing Mode.

Like the TIO, the VIO RPOs MUST be preceded by one or more RPL Target options
Options to which it applies. Unlike the TIO,
the VIO are not they apply, and they can be factorized: multiple
contiguous Via options RPOs indicate
an ordered sequence of routers alternate paths to reach the target(s), presented in
the order of the packet stream, source to destination, and in which a
routing state must be installed.

The Via Information option MUST contain at least one Via Address.

3.1. Via Information Option target(s).

4. New RPL Control Message Options

The Via Information option MAY be present in DAO messages, and its 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 = 0x0A | Option Length | Path Sequence | Path Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Via Information option format

Option Type: 0x0A for VIO, 0x0B for SRVIO (to be confirmed by IANA)

Option Length: In bytes; variable, depending on the number of Via
Addresses.

Path Sequence: 8-bit unsigned integer. When a RPL Target option is
issued by the root of the DODAG (i.e. in a DAO message), that
root sets the Path Sequence and increments the Path Sequence
each time it issues a RPL Target option with updated
information. The indicated sequence deprecates any state for a
given Target that was learned from a previous sequence and adds
to any state that was learned for that sequence.

Path Lifetime: 8-bit unsigned integer. The length of time in
Lifetime Units (obtained from the Configuration option) that
the prefix is valid for route determination. The period starts
when a new Path Sequence is seen. A value of all one bits
(0xFF) represents infinity. A value of all zero bits (0x00)
indicates a loss of reachability. A DAO message that contains
a Via Information option with a Path Lifetime of 0x00 for a
Target is referred as a No-Path (for that Target) in this
document.

Via Address: 16 bytes. IPv6 Address of the next hop towards the
destination(s) indicated in the target option that immediately
precede the VIO. RPO. Via Addresses are indicated in the order of
the data path from the ingress to the egress nodes. TBD: See
how the /64 prefix can be elided if it is the same as that of
(all of) the target(s). In that case, the Next-Hop Address
could be expressed as the 8-bytes suffix only, otherwise it is expressed as 16 bytes, only.

An RPO MUST contain at least in
storing mode.

4. one Via Address, and a Via Address MUST
NOT be present more than once, otherwise the RPO MUST be ignored.

5. Projected DAO

This draft adds a capability to RPL whereby the root of a DODAG
projects a route
through by sending an extended DAO message called a
Projected-DAO (P-DAO) to an arbitrary router down in the DODAG, indicating a next hop
one or a sequence more sequence(s) of routers inside the DODAG via which a certain destination the
target(s) indicated in the Target Information option may Option(s) (TIO) can be
reached.

A P-DAO is sent from a global address of the root to a global address
of the recipient, and MUST be confirmed by a DAO-ACK, which is sent
back to a global address of the root.

A P-DAO message MUST contain at least a Target Information option one TIO and at least one VIA Information option RPO
following it. There can be at most one such sequence of TIOs and
then RPOs.

Like a classical DAO message, a P-DAO is processed only if it is
"new" per section 9.2.2. "Generation of DAO Messages" of the RPL
specification [RFC6550]; this is determined using the Path Sequence
information from the VIO RPO as opposed to a TIO. Also, a Path Lifetime
of 0 in a VIO an RPO indicates that a route is to be removed.

There are two kinds of P-DAO, operation for the storing mode projected routes, the
Storing Mode and the non-storing
mode ones. Non-Storing Mode.

The non-storing mode P-DAO Non-Storing Mode is discussed in section Section 4.1 has 5.1. It uses
an SRVIO that carries a
single VIO with one or more Via Addresses in it, the list of Via Addresses indicating the to be used as a
source-routed path to the target to be
installed in target. The recipient of the P-DAO is
the ingress router that receives of the message, which source-routed path. Upon a Non-Storing
Mode P-DAO, the ingress router installs a source-routed state to
the target and replies to the root directly with a DAO-ACK
message.

The storing mode P-DAO Storing Mode is discussed in section Section 4.2 has at
least two Via Information options 5.2. It uses a
VIO with one Via Address each, for per consecutive hop, from the ingress and to
the egress of the path, and more if there are including the list of all intermediate routers.
routers in the data path order. The Via Addresses indicate the
routers in which the routing state to the target have to be
installed via the next Via Address in the sequence of 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.

The root is expected to use these mechanisms optimally and with
required parsimony to limit the state installed in the devices to fit
within their resources, but how the root figures the amount of
resources that is available in each device is out of scope for this
document.

In particular, the draft expects that the root has enough information
about the capability for each node to store a number of routes, which
can be discovered for instance using a Network Management System
(NMS) and/or the RPL routing extensions specified in "Routing for
Path Calculation in LLNs" [RFC6551].

A route that is installed by a P-DAO is not necessarily installed
along the DODAG, though how the root and the optional PCE obtain the
additional topological information to compute other routes is out of
scope for this document

4.1.

5.1. Non-storing Mode Projected DAO Route

As illustrated in Figure 2, the non-storing mode a P-DAO that carries an SRVIO enables the
root to install a source-routed path towards a target in any
particular router; with this path information the router can add a
source routed header reflecting the path P-route to any packet for which
the current destination either is the said target or can be reached
via the target, for instance a loose source routed packet for which the
next loose hop is the target, or a packet for which the router has a
routing state to the final destination via the target.

LLN

Figure 2: Projecting a Non-Storing route Route

A router route indicated by an SRVIO may be loose, meaning that receives the node
that owns the next listed Via Address is not necessarily a non-storing P-DAO 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 source P-route
does not have a connected route (a direct adjacency) to the next
soure routed
path towards each hop and fails to locate it as a neighbor or a neighbor
of the consecutive targets via a source neighbor, then it MUST ensure that it has another projected
route path
indicated in to the following VIO. next loose hop under the control of the same route
computation system, otherwise the P-DAO is rejected.

When forwarding a packet to a destination for which the router
determines that routing happens via the target, the router inserts
the source routing header in the packet to reach the target. In the
case of a loose source-routed path, there MUST be either a neighbor
that is adjacent to the loose next hop, on which case the packet s
forwarded to that neighbor, or a source-routed path to the loose next
hop; in the latter case, another encapsulation takes place and the
process possibly recurses; otherwise the packet is dropped.

In order to do so, add a source-routing header, the router encapsulates the
packet with an IP in
IP IP-in-IP header and a non-storing mode source routing
header (SRH) [RFC6554].

In the uncompressed form the source of the packet would be self, the
destination would be the first Via Address in the VIO, SRVIO, and the SRH
would contain the list of the remaining Via Addresses and then the
target.

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 the "6LoWPAN Routing
Header" [RFC8138] specification. In that case, the router indicates
self as encapsulator in an IP-in-IP 6LoRH Header, and places the list
of Via Addresses in the order of the VIO and then the target in the
SRH 6LoRH Header.

4.2.

5.2. Storing-Mode Projected DAO Route

As illustrated in Figure 3, the storing mode P-DAO enables Storing Mode projected iq used by the
root to install a routing state towards a target in the routers along
a segment between an ingress and an egress router; this enables the
routers to forward along that segment any packet for which the next
loose hop is the said target, for instance a loose source routed
packet for which the next loose hop is the target, or a packet for
which the router has a routing state to the final destination via the
target.

------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ | ^ |
| | DAO | ACK |
o o o o | | |
o o o o o o o o o | ^ | Projected .
o o o o o o o o o o | | DAO | Route .
o o o o o o o o o | ^ | .
o o o o o o o o v | DAO v .
o o LLN o o o |
o o o o o Loose Source Route Path |
o o o o From Root To Destination v

Figure 3: Projecting a route

Based on available topological, usage and capabilities node
information, the root or an associated PCE computes which segment
should be optimized and which relevant state should be installed in
which nodes. The algorithm is out of scope but it is envisaged that
the root could compute the ratio between the optimal path (existing
path not traversing the root, and the current path), the application
service level agreement (SLA) for specific flows that could benefit
from shorter paths, the energy wasted in the network, local
congestion on various links that would benefit from having flows
routed along alternate paths.

In order to install the relevant routing state along the segment
between an ingress and an egress routers, the root sends a unicast
P-DAO message to the egress router of the routing segment that must
be installed. The P-DAO message contains the ordered list of hops
along the segment as a direct sequence of Via Information options
that are preceded by one or more RPL Target options to which they
relate. Each Via Information option contains a Path Lifetime for
which the state is to be maintained.

The root sends the P-DAO directly to the egress node of the segment,
which segment.
In that P-DAO, the destination IP address matches the Via Address in
the last VIO. This is how the egress recognizes its role. In a
similar fashion, the ingress node recognizes its role as it matches
Via Address in the first VIO.

The egress node of the segment is the only node in the path that does
not install a route in response to the P-DAO; it is expected to be
already able to route to the target(s) on its own. It may either be
the target, or may have some existing information to reach the
target(s), such as a connected route or an already installed
projected route. If one of the targets cannot be located, the node
MUST answer to the root with a negative DAO-ACK listing the target(s)
that could not be located (suggested status 10 to be confirmed by
IANA).

If the egress node can reach all the targets, then it forwards the
P-DAO with unchanged content to its loose predecessor in the segment
as indicated in the list of Via Information options, and recursively
the message is propagated unchanged along the sequence of routers
indicated in the P-DAO, but in the reverse order, from egress to
ingress.

The address of the predecessor to be used as destination of the
propagated DAO message is found in the Via Information option the
precedes the one that contain the address of the propagating node,
which is used as source of the packet.

Upon receiving a propagated DAO, an intermediate router as well as
the ingress router install a route towards the DAO target(s) via its
successor in the P-DAO; the router locates the VIO that contains its
address, and uses as next hop the address found in the Via Address
field in the following VIO. The router MAY install additional routes
towards the addresses that are located in VIOs that are after the
next one, if any, but in case of a conflict or a lack of resource, a
route to a target installed by the root has precedence.

The process recurses till the P-DAO is propagated to ingress router
of the segment, which answers with a DAO-ACK to the root.

Also, the path indicated in a P-DAO may be loose, in which case the
reachability to the next hop has to be asserted. Each router along
the path indicated in a P-DAO is expected to be able to reach its
successor, either with a connected route (direct neighbor), or by
routing, for instance following a route installed previously by a DAO
or a P-DAO message. If that route is not connected then a recursive
lookup may take place at packet forwarding time to find the next hop
to reach the target(s). If it does not and cannot reach the next
router in the P-DAO, the router MUST answer to the root with a
negative DAO-ACK indicating the successor that is unreachable
(suggested status 11 to be confirmed by IANA).

A Path Lifetime of 0 in a Via Information option is used to clean up
the state. The P-DAO is forwarded as described above, but the DAO is
interpreted as a No-Path DAO and results in cleaning up existing
state as opposed to refreshing an existing one or installing a new
one.

6. Applications

5.1.

6.1. Loose Source Routing in Non-storing Mode

A RPL implementation operating in a very constrained LLN typically
uses the Non-Storing Mode of Operation as represented in Figure 4.
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 4: 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.

5.2.

6.2. Transversal Routes in storing and non-storing modes

RPL is optimized for Point-to-Multipoint (P2MP), root to leaves and
Multipoint-to-Point (MP2P) leaves to root operations, whereby routes
are always installed along the RPL DODAG. Transversal Peer to Peer
(P2P) routes in a RPL network will generally suffer from some stretch
since routing between 2 peers always happens via a common parent, as
illustrated in Figure 5:

o 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.

o 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.

Figure 5: 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.
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 6: 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.

7. RPL Instances

It must be noted that RPL has a concept of instance 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. This draft conforms the instance
model as follows:

o If the PCE needs to influence a particular instance to add better
routes in conformance with the routing objectives in that
instance, it may do so. When the PCE modifies an existing
instance then the added routes must not create a loop in that
instance. This is achieved by always preferring a route obtained
from the PCE over a route that is learned via RPL.

o If the PCE installs a more specific (say, Traffic Engineered)
route between a particular pair of nodes then it SHOULD use a
Local Instance from the ingress node of that path. A packet
associated with that instance will be routed along that path and
MUST NOT be placed over a Global Instance again. A packet that is
placed on a Global Instance may be injected in the Local Instance
based on node policy and the Local Instance paramenters.

In all cases, the path is indicated by a new Via Information option,
and the flow is similar to the flow used to obtain loose source
routing.

8. Security Considerations

This draft uses messages that are already present in RPL [RFC6550]
with optional secured versions. The same secured versions may be
used with this draft, and whatever security is deployed for a given
network also applies to the flows in this draft.

9. IANA Considerations

This document extends the IANA registry created by RFC 6550 for RPL
Control Codes as follows:

+------+-------------+---------------+

+------+-------------------+---------------+
| Code | Description | Reference |
+------+-------------+---------------+
+------+-------------------+---------------+
| 0x0A | Via | This document |
+------+-------------+---------------+
| | | |
| 0x0B | Source-Routed Via | This document |
+------+-------------------+---------------+

RPL Control Codes

This document is updating the registry created by RFC 6550 for the
RPL 3-bit Mode of Operation (MOP) as follows:

DIO Mode of operation

10. Acknowledgments

The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for
their contributions to the ideas developed here.

10.

11. References

10.1.

11.1. 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>.

[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.

[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<https://www.rfc-editor.org/info/rfc6551>.

[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.

[RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
RFC 8025, DOI 10.17487/RFC8025, November 2016,
<https://www.rfc-editor.org/info/rfc8025>.

[RFC8138] Thubert, P., Ed., Bormann, C., Toutain, L., and R. Cragie,
"IPv6 over Low-Power Wireless Personal Area Network
(6LoWPAN) Routing Header", RFC 8138, DOI 10.17487/RFC8138,
April 2017, <https://www.rfc-editor.org/info/rfc8138>.

10.2.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2. Informative References

[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-13 draft-ietf-6tisch-architecture-14 (work
in progress), November 2017. April 2018.

[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-04
detnet-architecture-05 (work in progress), October 2017. May 2018.

[PCE] IETF, "Path Computation Element",
<https://datatracker.ietf.org/doc/charter-ietf-pce/>.

[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>.

[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>.

Appendix A. Examples

A.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 7, 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 7: 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.

A.2. Projecting a storing-mode transversal route

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

The root sends a P-DAO with a target option indicating the
destination D and a sequence Via Information option, one for S, which
is the ingress router of the segment, one for A and then for B, which
are an intermediate routers, and one for C, which is the egress
router.

Figure 8: Projected 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 9: Projected DAO-ACK to root

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

Figure 10: Projected Transversal Route

Authors' Addresses
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE

Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com

Rahul Arvind Jadhav (editor)
Huawei Tech
Kundalahalli Village, Whitefield,
Bangalore, Karnataka 560037
India

Phone: +91-080-49160700
Email: rahul.ietf@gmail.com

James Pylakutty
Cisco Systems
Cessna Business Park
Kadubeesanahalli
Marathalli ORR
Bangalore, Karnataka 560087
INDIA

Phone: +91 80 4426 4140
Email: mundenma@cisco.com