Root initiated routing state in RPL

3.1. RPL Applicability

RPL is optimized for situations where the power is scarce, the bandwidth is constrained and the transmissions are unreliable. This matches the use case of an IoT LLN where RPL is typically used today, but also situations of high relative mobility between the nodes in the network (aka swarming), e.g., within a variable set of vehicles with a similar global motion, or a platoon of drones.¶

To reach this goal, RPL is primarily designed to minimize the control plane activity, that is the relative amount of routing protocol exchanges vs. data traffic, and the amount of state that is maintained in each node. RPL does not need to converge, and provides connectivity to most nodes most of the time.¶

RPL may form multiple topologies called instances. Instances can be created to enforce various optimizations through objective functions, or to reach out through different Root Nodes. The concept of objective function allows to adapt the activity of the routing protocol to the use case, e.g., type, speed, and quality of the LLN links.¶

RPL instances operate as ships passing in the night, unbeknownst of one another. The RPL Root is responsible for selecting the RPL Instance that is used to forward a packet coming from the Backbone into the RPL domain and for setting the related RPL information in the packets. 6TiSCH leverages RPL for its distributed routing operations.¶

To reduce the routing exchanges, RPL leverages an anisotropic Distance Vector approach, which does not need a global knowledge of the topology, and only optimizes the routes to and from the RPL Root, allowing P2P paths to be stretched. Although RPL installs its routes proactively, it only maintains them lazily, in reaction to actual traffic, or as a slow background activity.¶

This is simple and efficient in situations where the traffic is mostly directed from or to a central node, such as the control traffic between routers and a controller of a Software Defined Networking (SDN) infrastructure or an Autonomic Control Plane (ACP).¶

But stretch in P2P routing is counter-productive to both reliability and latency as it introduces additional delay and chances of loss. As a result, [RPL] is not a good fit for the use cases listed in the RAW use cases document [USE-CASES], which demand high availability and reliability, and as a consequence require both short and diverse paths.¶

3.2. RPL Routing Modes

RPL first forms a default route in each node towards the Root, and those routes together coalesce as a Directed Acyclic Graph oriented upwards. RPL then constructs routes to destinations signaled as Targets in the reverse direction, down the same DODAG. To do so, a RPL Instance can be operated either in RPL Storing or Non-Storing Mode of Operation (MOP). The default route towards the Root is maintained aggressively and may change while a packet progresses without causing loops, so the packet will still reach the Root.¶

In Non-Storing Mode, each node advertises itself as a Target directly to the Root, indicating the parents that may be used to reach itself. Recursively, the Root builds and maintains an image of the whole DODAG in memory, and leverages that abstraction to compute source route paths for the packets to their destinations down the DODAG. When a node changes its point(s) of attachment to the DODAG, it takes a single unicast packet to the Root along the default route to update it, and the connectivity to the node is restored immediately; this mode is preferable for use cases where internet connectivity is dominant, or when the Root controls the network activity in the nodes, which is the case of this draft.¶

In Storing Mode, the routing information percolates upwards, and each node maintains the routes to the subDAG of its descendants down the DODAG. The maintenance is lazy, either reactive upon traffic or as a slow background process. Packets flow via the common parent and the routing stretch is reduced compared to Non-Storing MOP, for better P2P connectivity. However, a new route takes a longer time to propagate to the Root, since it takes time for the Distance-Vector protocol to operate hop-by-hop, and the connectivity from the internet to the node is restored more slowly upon node movement.¶

Either way, the RPL routes are injected by the Target nodes, in a distributed fashion. To complement RPL and eliminate routing stretch, this specification introduces a hybrid mode that combines Storing and Non-Storing operations to build and project routes onto the nodes where they should be installed. This specification uses the term Projected Route (P-Route) to refer to those routes.¶

A P-Route may be installed in either Storing and Non-Storing Mode, potentially resulting in hybrid situations where the Mode in which the P-Route operates is different from that of the RPL Main Instance. P-Routes can be used as stand-alone Segments meant to reduce the size of the source routing headers, thanks to loose source routing operations down the main RPL DODAG. P-Routes can also be combined with other P-Routes to form a more complex Track, consisting of local instances overlaid on an existing main RPL Instance.¶

3.4. On Tracks

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |1|D|   ID      |  Local RPLInstanceID in 0..63
       +-+-+-+-+-+-+-+-+

3.6. Complex Tracks

To increase the reliability of the P2P transmission, this specification enables building a collection of Legs between the same Ingress and Egress Nodes and combining them within the same TrackID, as shown in Figure 7. Legs may cross at the edges of loose hops or remain parallel.¶

The Segments that join the loose hops of a Leg are installed with the same TrackID as the Leg. But each individual Leg and Segment has its own P-RouteID which allows it to be managed separately. When Legs cross within - meaning that there is a node that belongs to a Segment from each Tracks, so it may reach the destination over either leg - the next loose hop (the current destination of the packet) indicates which Leg is being followed and a Segment that can reach that next loose hop is selected.¶

        CPF               CPF          CPF                 CPF

                       Southbound API

   _-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-
 _-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-._-

                      +----------+
                      | RPL Root |
                      +----------+
                        (      )
              (                                  )
        (              DODAG                              )
          (                                           )
  (                                                         )
                                                                  )
  <-    Leg 1            B,                            E ->
  <--- Segment 1 A,B ---> <------- Segment 2 C,D,E ------->

             FWD  --z  Relay --z   FWD  --z   FWD          Target 1
         z-- Node  z--  Node  z--  Node  z--  Node --z     /
      --z    (A)        (B) \      (C)        (D)  z--    /
Track                        \                       Track
Ingress                    Segment 5                 Egress - Tgt 2
  (I)                           \                     (E)
      --z                        \                 z--    \
       z-- FWD   --z  FWD  --z  Relay --z  FWD  --z        \
           Node   z-- Node  z-- Node   z-- Node            Target 3
           (F)        (G)       (H)        (J)

  <------ Segment 3 F,G,H ------> <---- Segment 4 J,E ---->
  <-      Leg 2                  H,                    E ->

  <--- Segment 1 A,B ---> <- S5-> <---- Segment 4 J,E ---->
  <-      Leg 3          B,      H,                    E ->
                                                                  )
   (
              (                                        )

Note that while this specification enables building both Segments inside a Leg (aka East-West), such as Segment 2 above which is within Leg 1, and Inter-Leg Segments (aka North-South), such as Segment 2 above which joins Leg 1 and Leg 2, it does not signal to the Ingress which Inter-Leg Segments are available, so the use of North-South Segments and associated PAREO functions is curently limited. The only possibility available at this time is to define overlapping Legs as illustrated in Figure 7, with Leg 3 that is congruent with Leg 1 until node B and congruent with Leg 2 from node H on, abstracting Segment 5 as an East-West Segment.¶

3.7. Scope and Expectations

4.1. Extending RFC 6550

This specification Extends RPL [RPL] to enable the Root to install East-West routes inside a Main DODAG that is operated as Non-Storing Mode. The Root issues a Projected DAO (P-DAO) message (see Section 4.1.1) to the Track Ingress; the P-DAO message contains a new Via Information Option (VIO) that installs a strict or a loose sequence of hops to form a Track Segment or a Track Leg, respectively.¶

The P-DAO Request (PDR) is a new message detailed in Section 5.1. As per [RPL] section 6, if a node receives this message and it does not understand this new Code, it then discards the message. When the Root initiates communication to a node that it has not communicated with before and which it has not ascertained to implement this specification (by means such as capabilities), then the Root SHOULD request a PDR-ACK.¶

A P-DAO Request (PDR) message enables a Track Ingress to request the Track from the Root. The resulting Track is also a DODAG for which the Track Ingress is the Root, the owner the address that serves as DODAGID and authoritative for the associated namespace from which the TrackID is selected. In the context of this specification, the installed route appears as a more specific route to the Track Targets, and the Track Ingress routes the packets towards the Targets via the Track using the longest match as usual.¶

To ensure that the PDR and P-DAO messages can flow at most times, it is RECOMMENDED that the nodes involved in a Track maintain multiple parents in the Main DODAG, advertise them all to the Root, and use them in turn to retry similar packets. It is also RECOMMENDED that the Root uses diverse source route paths to retry similar messages to the nodes in the Track.¶

 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|P|  Flags  |   Reserved    | DAOSequence   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                   DODAGID field set to the                    |
+               IPv6 Address of the Track Ingress               +
|              used to source encapsulated packets              |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Option(s)...
+-+-+-+-+-+-+-+-+

 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    |D|P| Reserved  |  DAOSequence  |    Status     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                   DODAGID field set to the                    |
+               IPv6 Address of the Track Ingress               +
|              used to source encapsulated packets              |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Option(s)...
+-+-+-+-+-+-+-+-+

 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 = 0x04 |Opt Length = 14|D| | | |A|       ...           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     +
                                |4 bits |

4.2. Extending RFC 6553

"The RPL Option for Carrying RPL Information in Data-Plane Datagrams" [RFC6553] describes the RPL Option for use among RPL routers to include the abstract RPL Packet Information (RPI) described in section 11.2. of [RPL] in data packets.¶

The RPL Option is commonly referred to as the RPI though the RPI is really the abstract information that is transported in the RPL Option. [RFC9008] updated the Option Type from 0x63 to 0x23.¶

This specification Amends the RPL Option to encode the 'P' flag 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
                                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                |  Option Type  |  Opt Data Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|R|F|P|0|0|0|0| RPLInstanceID |          SenderRank           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         (sub-TLVs)                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Option Type:

0x23 or 0x63, see [RFC9008]¶

Opt Data Len:

See [RFC6553]¶

'O', 'R' and 'F' flags:

See [RFC6553]. Those flags MUST be set to 0 by the sender and ignored by the receiver if the 'P' flag is set.¶

Projected-Route 'P':

1-bit flag as defined in Section 4.1.6.¶

RPLInstanceID:

See [RFC6553]. Indicates the TrackId if the 'P' flag is set, as discussed in Section 4.1.1.¶

SenderRank:

See [RFC6553]. This field MUST be set to 0 by the sender and ignored by the receiver if the 'P' flag is set.¶

4.3. Extending RFC 8138

The 6LoWPAN Routing Header [RFC8138] specification introduces a new IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) [RFC6282] dispatch type for use in 6LoWPAN route-over topologies, which initially covers the needs of RPL data packet compression.¶

Section 4 of [RFC8138] presents the generic formats of the 6LoWPAN Routing Header (6LoRH) with two forms, one Elective that can be ignored and skipped when the router does not understand it, and one Critical which causes the packet to be dropped when the router cannot process it. The 'E' Flag in the 6LoRH indicates its form. In order to skip the Elective 6LoRHs, their format imposes a fixed expression of the size, whereas the size of a Critical 6LoRH may be signaled in variable forms to enable additional optimizations.¶

When the [RFC8138] compression is used, the Root of the Main DODAG that sets up the Track also constructs the compressed routing header (SRH-6LoRH) on behalf of the Track Ingress, which saves the complexities of optimizing the SRH-6LoRH encoding in constrained code. The SRH-6LoRH is signaled in the NSM-VIO, in a fashion that it is ready to be placed as is in the packet encapsulation by the Track Ingress.¶

Section 6.3 of [RFC8138] presents the formats of the 6LoWPAN Routing Header of type 5 (RPI-6LoRH) that compresses the RPI for normal RPL operation. The format of the RPI-6LoRH is not suited for P-Routes since the O,R,F flags are not used and the Rank is unknown and ignored.¶

This specification extends [RFC8138] to introduce a new 6LoRH, the P-RPI-6LoRH that can be used in either Elective or Critical 6LoRH form, see Table 22 and Table 23 respectively. The new 6LoRH MUST be used as a Critical 6LoRH, unless an SRH-6LoRH is present and controls the routing decision, in which case it MAY be used in Elective form.¶

The P-RPI-6LoRH is designed to compress the RPI along RPL P-Routes. Its format is as follows:¶

     0                   1                   2
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|0|E| Length  |  6LoRH Type   | RPLInstanceID |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type:

IANA is requested to define the same value of the type for both Elective and Critical forms. A type of 8 is suggested.¶

Elective 'E':

See [RFC8138]. The 'E' flag is set to 1 to indicate an Elective 6LoRH, meaning that it can be ignored when forwarding.¶

RPLInstanceID :

In the context of this specification, the RPLInstanceID field signals the TrackID, see Section 3.4 and Section 6.3 .¶

Section 6.8 details how a Track Ingress leverages the P-RPI-6LoRH Header as part of the encapsulation of a packet to place it into a Track.¶

5.1. 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 where this node is Track Ingress, and signals whether an acknowledgment called PDR-ACK is requested or not. A positive PDR-ACK indicates that the Track was built and that the Root commits to maintaining the Track for the negotiated lifetime.¶

The main Root MAY indicate to the Track Ingress that the Track was terminated before its time and to do so, it MUST use an asynchronous PDR-ACK with a negative status. A status of "Transient Failure" (see Section 11.10) is an indication that the PDR may be retried after a reasonable time that depends on the deployment. Other negative status values indicate a permanent error; the attempt must be abandoned until a corrective action is taken at the application layer or through network management.¶

The source IPv6 address of the PDR signals the Track Ingress to-be of the requested Track, and the TrackID is indicated in the message itself. At least one RPL Target Option MUST be present in the message. If more than one RPL Target Option is present, the Root will provide a Track that reaches the first listed Target and a subset of the other Targets; the details of the subset selection are out of scope. The RTO signals the Track Egress (more in Section 6.2).¶

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)...
 +-+-+-+-+-+-+-+-+

TrackID:

8-bit field. In the context of this specification, the TrackID field signals the RPLInstanceID of the DODAG formed by the Track, see Section 3.4 and Section 6.3. To allocate a new Track, the Ingress Node must provide a value that is not in use at this time.¶

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 be 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). The value of 255 (0xFF) represents infinity.¶

A PDR with a fresher PDRSequence refreshes the lifetime, and a PDRLifetime of 0 indicates that the Track should be destroyed, e.g., when the application that requested the Track terminates.¶

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.¶

5.2. New PDR-ACK Control Message

The new PDR-ACK is sent as a response to a PDR message with the 'K' flag set. The RPL Control Code for the PDR-ACK is 0x0A, to be confirmed by IANA. Its format is as follows:¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    TrackID    |     Flags     | Track Lifetime|  PDRSequence  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PDR-ACK Status|                Reserved                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Option(s)...
+-+-+-+-+-+-+-+

TrackID:

Set to the TrackID indicated in the TrackID field of the PDR messages that this replies to.¶

Flags:

Reserved. The Flags field MUST be initialized to zero by the sender and MUST be ignored by the receiver.¶

Track Lifetime:

Indicates the remaining Lifetime for the Track, expressed in Lifetime Units; The value of 255 (0xFF) represents infinity. 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 15:¶

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

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 28 and Table 29.¶

Reserved:

The Reserved field MUST be initialized to zero by the sender and MUST be ignored by the receiver.¶

5.3. Via Information Options

A VIO signals the ordered list of IPv6 Via Addresses that constitutes the hops of either a Leg (using Non-Storing Mode) or a Segment (using Storing mode) of a Track. A Storing Mode P-DAO contains one Storing Mode VIO (SM-VIO) whereas a Non-Storing Mode P-DAO contains one Non-Storing Mode VIO (NSM-VIO).¶

The duration of the validity of a VIO is indicated in a Segment Lifetime field. A P-DAO message that contains a VIO with a Segment Lifetime of zero is referred as a No-Path P-DAO.¶

The VIO contains one or more SRH-6LoRH header(s), each formed of a SRH-6LoRH head and a collection of compressed Via Addresses, except in the case of a Non-Storing Mode No-Path P-DAO where the SRH-6LoRH header is not present.¶

In the case of a SM-VIO, or if [RFC8138] is not used in the data packets, then the Root MUST use only one SRH-6LoRH per Via Information Option, and the compression is the same for all the addresses, as shown in Figure 16, for simplicity.¶

In case of an NSM-VIO and if [RFC8138] is in use in the Main DODAG, the Root SHOULD optimize the size of the NSM-VIO if using different SRH-6LoRH Types would make the VIO globally shorter; this means that more than one SRH-6LoRH may be present.¶

The format of the Via Information Options 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     |   P-RouteID   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Segm. Sequence | Seg. Lifetime |        SRH-6LoRH head         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .           Via Address 1 (compressed by RFC 8138)              .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                              ....                             .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .           Via Address n (compressed by RFC 8138)              .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .              Additional SRH-6LoRH Header(s)                   .
    |                                                               |
    .                              ....                             .

Option Type:

0x0E for SM-VIO, 0x0F for NSM-VIO (to be confirmed by IANA) (see Table 26).¶

Option Length:

8-bit unsigned integer, representing the length in octets of the option, not including the Option Type and Length fields (see section 6.7.1. of [RPL]); the Option Length is variable, depending on the number of Via Addresses and the compression applied.¶

P-RouteID:

8-bit field that identifies a component of a Track or 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/Leg. In an SM-VIO, the P-RouteID indicates an actual Segment. In an NSM-VIO, it indicates a Leg, that is a serial subTrack that is added to the overall topology of the Track.¶

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 VIO deprecates any state for the Segment indicated by the P-RouteID within the indicated Track and sets up the new information.¶

A VIO with a Segment Sequence that is not as fresh as the current one is ignored.¶

A VIO for a given DODAGID with the same (TrackID, P-RouteID, 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.¶

SRH-6LoRH head:

The first 2 bytes of the (first) SRH-6LoRH as shown in Figure 6 of [RFC8138]. As an example, a 6LoRH Type of 4 means that the VIA Addresses are provided in full with no compression.¶

Via Address:

An IPv6 ULA or GUA of a node along the Segment. The VIO contains one or more IPv6 Via Addresses listed in the datapath order from Ingress to Egress. The list is expressed in a compressed form as signaled by the preceding SRH-6LoRH header.¶

In a Storing Mode P-DAO that updates or removes a section of an already existing Segment, the list in the SM-VIO may represent only the section of the Segment that is being updated; at the extreme, the SM-VIO updates only one node, in which case it contains only one IPv6 address. In all other cases, the list in the VIO MUST be complete.¶

In the case of an SM-VIO, the list indicates a sequential (strict) path through direct neighbors, the complete list starts at Ingress and ends at Egress, and the nodes listed in the VIO, including the Egress, MAY be considered as implicit Targets.¶

In the case of an NSM-VIO, the complete list can be loose and excludes the Ingress node, starting at the first loose hop and ending at a Track Egress; the Track Egress MUST be considered as an implicit Target, so it MUST NOT be signaled in a RPL Target Option.¶

5.4. Sibling Information Option

The Sibling Information Option (SIO) provides information about siblings that could be used by the Root to form P-Routes. One or more SIO(s) may be placed in the DAO messages that are sent to the Root in Non-Storing Mode.¶

To advertise a neighbor node, the router MUST have an active Address Registration from that sibling using [RFC8505], for an address (ULA or GUA) that serves as identifier for the node. If this router also registers an address to that sibling, and the link has similar properties in both directions, only the router with the lowest Interface ID in its registered address needs to report the SIO, with the B flag set, and the Root will assume symmetry.¶

The SIO carries a flag (B) that is set when similar performance can be expected in both directions, so the routing can consider that the information provided for one direction is valid for both. If the SIO is effectively received from both sides then the B flag MUST be ignored. The policy that describes the performance criteria, and how they are asserted is out of scope. In the absence of an external protocol to assert the link quality, the flag SHOULD NOT be set.¶

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 |S|B|Flags|Comp.|    Opaque     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Step in Rank       |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    .                                                               .
    .       Sibling DODAGID (if the D flag not set)               .
    .                                                               .
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    .                                                               .
    .                     Sibling Address                           .
    .                                                               .
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Option Type:

0x10 for SIO (to be confirmed by IANA) (see Table 26).¶

Option Length:

8-bit unsigned integer, representing the length in octets of the option, not including the Option Type and Length fields (see section 6.7.1. of [RPL]).¶

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.¶

S:

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 be 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.¶

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 present. The Compression reference is the Root of the Main DODAG.¶

Step in Rank:

16-bit unsigned integer. This is the Step in Rank [RPL] as computed by the Objective Function between this node and the sibling, that reflects the abstract Rank increment that would be computed by the OF if the sibling was the preferred parent.¶

Reserved:

The Reserved field MUST be 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 if and only if the D flag is not set.¶

Sibling Address:

2 to 16 bytes, an IPv6 Address of the sibling, with a scope that MUST be make it reachable from the Root, e.g., it cannot be a Link Local Address. The IPv6 address is encoded in the [RFC8138] compressed form indicated by the Compression Type field.¶

An SIO MAY be immediately followed by a DAG Metric Container. In that case the DAG Metric Container provides additional metrics for the hop from the Sibling to this node.¶

6.1. RPL Network Setup

To avoid the need of Path MTU Discovery, 6LoWPAN links are normally defined with a MTU of 1280 (see section 4 of [6LoWPAN]). Injecting packets in a Track typically involves an IP-in-IP encapsulation and additional IPv6 Extension Headers. This may cause fragmentation if the resulting packets exceeds the MTU that is defined for the RPL domain.¶

Though fragmentation is possible in a 6LoWPAN LLN, e.g., using [6LoWPAN], [RFC8930], and/or [RFC8931], it is RECOMMENDED to allow an MTU that is larger than 1280 in the main DODAG and which allows for the additional headers while exposing only 1280 to the 6LoWPAN Nodes.¶

6.2. Requesting a Track

This specification introduces the PDR message, used by an LLN node to request the formation of a new Track for which this node is the Ingress. Note that the namespace for the TrackID is owned by the Ingress node, and in the absence of a PDR, there must be some procedure for the Root to assign TrackIDs in that namespace while avoiding collisions (more in Section 6.3).¶

The PDR signals the desired TrackID and the duration for which the Track should be established. Upon a PDR, the Root MAY install the Track as requested, 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 DODAGID signaled in the P-DAO.¶

The Root designs the Track as it sees best, and updates / changes the Segments over time to serve the Track as needed. Note that there is no protocol element to notify to the requesting Track Ingress when changes happen deeper down the Track, so they are transparent to the Track Ingress. If the main Root cannot maintain an expected service level, then it needs to tear down the Track completely. 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 - vs. the trip time from the node to the Root, the requesting node SHOULD 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 requesting 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.¶

6.3. Identifying a Track

RPL defines the concept of an Instance to signal an individual routing topology, and multiple topologies can coexist in the same network. The RPLInstanceID is tagged in the RPI of every packet to signal which topology the packet actually follows.¶

This draft leverages the RPL Instance model as follows:¶

• The Root MAY use P-DAO messages to add better routes in the main (Global) RPL Instance in conformance with the routing objectives in that Instance.¶

  To achieve this, the Root MAY install a Segment 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 Section 3.3.1. The Root MAY also install a Track Leg across the Main DODAG to complement the routing topology.¶

  When adding a P-Route to the RPL Main DODAG, the Root MUST set the RPLInstanceID field of the P-DAO Base Object (see section 6.4.1. of [RPL]) to the RPLInstanceID of the Main DODAG, and MUST NOT use the DODAGID field. A P-Route provides a longer match to the Target Address than the default route via the Root, so it is preferred.¶

• The Root MAY also use P-DAO messages to install a Track as an independent routing topology (say, Traffic Engineered) to achieve particular routing characteristics from an Ingress to Egress Endpoints. To achieve this, the Root MUST set up a local RPL Instance (see section 5 of [RPL]), and the Local RPLInstanceID serves as the TrackID. The TrackID MUST be unique for the IPv6 ULA or GUA of the Track Ingress that serves as DODAGID for the Track.¶

  This way, a Track is uniquely identified by the tuple (DODAGID, TrackID) where the TrackID is always represented with the D flag set to 0 (see also section 5.1. of [RPL]), indicating when used in an RPI that the source address of the IPv6 packet signals the DODAGID.¶

  The P-DAO Base Object MUST indicate the tuple (DODAGID, TrackID) that identifies the Track as shown in Figure 8, and the P-RouteID that identifies the P-Route MUST be signaled in the VIO as shown in Figure 16.¶

  The Track Ingress is the Root of the DODAG ID formed by the local RPL Instance. It owns the namespace of its TrackIDs, so it can pick any unused value to request a new Track with a PDR. In a particular deployment where PDRs are not used, a portion of the namespace can be administratively delegated to the main Root, meaning that the main Root is authoritative for assigning the TrackIDs for the Tracks it creates.¶

  With this specification, the Root is aware of all the active Tracks, so it can also pick any unused value to form Tracks without a PDR. To avoid a collision of the Root and the Track Ingress picking the same value at the same time, it is RECOMMENDED that the Track Ingress starts allocating the ID value of the Local RPLInstanceID (see section 5.1. of [RPL]) used as TrackIDs with the value 0 incrementing, while the Root starts with 63 decrementing.¶

6.4. Installing a Track

A Serial Track can be installed by a single P-Route that signals the sequence of consecutive nodes, either in Storing Mode as a single-Segment Track, or in Non-Storing Mode as a single-Leg Track. A single-Leg Track can be installed as a loose Non-Storing Mode P-Route, in which case the next loose entry must recursively be reached over a Serial Track.¶

A Complex Track can be installed as a collection of P-Routes with the same DODAGID and Track ID. The Ingress of a Non-Storing Mode P-Route is the owner and Root of the DODAGID. The Ingress of a Storing Mode P-Route must be either the owner of the DODAGID, or a hop of a Leg of the same Track. In the latter case, the Targets of the P-Route must include the next hop of the Leg if there is one, to ensure forwarding continuity. 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.¶

A route along a Track for which the TrackID is not the RPLInstanceID of the Main DODAG MUST be installed with a higher precedence than the routes along the Main DODAG, meaning that:¶

• Longest match MUST be the prime comparison for routing.¶
• In case of equal length match, the route along the Track MUST be preferred vs. the one along the Main DODAG.¶
• There SHOULD NOT be 2 different Tracks leading to the same Target from same Ingress node, unless there's a policy for selecting which packets use which Track; such a policy is out of scope.¶
• A packet that was routed along a Track MUST NOT be routed along the main DODAG again; if the destination is not reachable as a neighbor by the node where the packet exits the Track then the packet MUST be dropped.¶

        ------+---------
              |          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                                          v

           ------+---------
                 |          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 Egress       X
        o  o  o  o             o    |
                                  Target
       o       o     LLN          o    o
     o          o             o     o

6.5. Tearing Down a P-Route

A P-DAO with a lifetime of 0 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. To tear down a Track, the Root must tear down all the Track Segments and Legs that compose it one by one.¶

Since the state about a Leg of a Track is located only on the Ingress Node, the Root cleans up the Leg by sending an NSM-VIO to the Ingress indicating the TrackID and the P-RouteID of the Leg being removed, a Segment Lifetime of 0 and a newer Segment Sequence. The SRH-6LoRH with the Via Addresses in the NSM-VIO are not needed; it SHOULD NOT be placed in the message and MUST be ignored by the receiver. Upon that NSM-VIO, the Ingress node removes all state for that Track if any, and replies positively anyway.¶

The Root cleans up a section of a Segment by sending an SM-VIO to the last node of the Segment, with the TrackID and the P-RouteID of the Segment being updated, a Segment Lifetime of zero (0) and a newer Segment Sequence. The Via Addresses in the SM-VIO indicates the section of the Segment being modified, from the first to the last node that is impacted. This can be the whole Segment if it is totally removed, or a sequence of one or more nodes that have been bypassed by a Segment update.¶

The No-Path P-DAO is forwarded normally along the reverse list, even if the intermediate node does not find a Segment state to clean up. This results in cleaning up the existing Segment state if any, as opposed to refreshing an existing one or installing a new one.¶

6.6. Maintaining a Track

Repathing a Track Segment or Leg may cause jitter and packet misordering. For critical flows that require timely and/or in-order delivery, it might be necessary to deploy the PAREO functions [RAW-ARCHI] over a highly redundant Track. This specification allows to use more than one Leg for a Track, and 1+N packet redundancy.¶

This section provides the steps to ensure that no packet is lost due to the operation itself. This is ensured by installing the new section from its last node to the first, so when an intermediate node installs a route along the new section, all the downstream nodes in the section have already installed their own. The disabled section is removed when the packets in-flight are forwarded along the new section as well.¶

6.7. Encapsulating and Forwarding Along a Track

When injecting a packet in a Track, the Ingress router must encapsulate the packet using IP-in-IP to add the Source Routing Header with the final destination set to the Track Egress.¶

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 RPL Main DODAG, e.g., by setting the "T" flag [RFC9035] in the RPL configuration option.¶

The Track Ingress that places a packet in a Track encapsulates it with an IP-in-IP header, a Routing Header, and an IPv6 Hop-by-Hop Option Header that contains the RPL Packet Information (RPI) as follows:¶

• In the uncompressed form, the source of the packet is the address that this router uses as DODAGID for the Track, the destination is the first Via Address in the NSM-VIO, and the RH is a Source Routing Header (SRH) [RFC6554] that contains the list of the remaining Via Addresses, ending with the Track Egress.¶

• The preferred alternative 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 NSM-VIO, also ending with 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 [RFC9035].¶

To signal the Track in the packet, this specification leverages the RPL Forwarding model as follows:¶

• In the data packets, the Track DODAGID and the TrackID MUST be respectively signaled as the IPv6 Source Address and the RPLInstanceID field of the RPI that MUST be placed in the outer chain of IPv6 Headers.¶

  The RPI carries a local RPLInstanceID called the TrackID, which, in association with the DODAGID, indicates the Track along which the packet is forwarded.¶

  The D flag in the RPLInstanceID MUST be set to 0 to indicate that the source address in the IPv6 header is set to the DODAGID (more in Section 6.3).¶

• This draft conforms to the principles of [RFC9008] with regards to packet forwarding and encapsulation along a Track, as follows:¶

  • With this draft, the Track is a RPL DODAG. From the perspective of that DODAG, the Track Ingress is the Root, the Track Egress is a RPL-Aware 6LR, and neighbors of the Track Egress that can be reached via the Track, but are external to it, are external destinations and treated as RPL-Unaware Leaves (RULs). The encapsulation rules in [RFC9008] apply.¶
  • 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.¶
  • So the Track Ingress must encapsulate the traffic that it did not originate, and add an RPI.¶

  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 as discussed in more details Section 3.5.2.3. 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.¶

The forwarding of a packet along a track will fail if the Track continuity is broken, e.g.:¶

• When forwarding along a Segment, if the next strict hop in the RIB for the destination of the packet is a direct Neighbor, the packet MUST be forwarded to that neighbor. Otherwise the packet MUST be dropped.¶

• When forwarding along a Track, if the next hop in the source route header is a direct Neighbor, the packet MUST be forwarded to that neighbor; else, the packet can be forwarded in the following case:¶

  1. the previous next hop has a common Neighbor that can relay to the next loose hop (e.g., learned through a SIO in a multicast DAO message, see Section 4.1.4), in which case the packet MUST be forwarded to that neighbor,¶
  2. else the previous next hop is Ingress of a Segment of the same Track for which the loose next hop is a target in which case the Segment MUST be used,¶
  3. else the previous next hop is Ingress of a (nested) Track to the loose next hop, in which case the (nested) Track MUST be used; another encapsulation takes place and the process may recurse.¶

  Otherwise the packet MUST be dropped to avoid loops; as an example, forwarding the packet along the main DODAG is disallowed since it may cause such a loop.¶

In case of a failure forwarding a packet along a Segment, e.g., the next hop is unreachable, the node that discovers the fault MUST send an ICMPv6 Error message [RFC4443] to the Root, with a new Code "Error in P-Route" (See Section 11.15). The Root can then repair by updating the broken Segment and/or Tracks, and in the case of a broken Segment, remove the leftover sections of the Segment using SM-VIOs with a lifetime of 0 indicating the section to one or more nodes being removed (See Section 6.6).¶

In case of a permanent 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 receiving this message, the encapsulating node SHOULD stop using the source route path for a reasonable period of time which depends on the deployment, and it SHOULD send an ICMP message with a Code "Error in P-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.¶

Either way, the ICMP message MUST be throttled in case of consecutive occurrences. It MUST be sourced at the ULA or a GUA that is used in this Track for the source node, so the Root can establish where the error happened.¶

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.¶

6.8. Compression of the RPL Artifacts

When using [RFC8138] in the Main DODAG operated in Non-Storing Mode in a 6LoWPAN LLN, a typical packet that circulates in the Main DODAG is formatted as shown in Figure 20, representing the case where an IPv6-in-IPv6 encapsulation is needed (see Table 19 of [RFC9008]):¶

+-+ ... -+- ... -+- ... -+-+- ... +-+-+-+ ... +-+-+ ... -+ ... +-...
|11110001|  SRH- | RPI-  | IP-in-IP | NH=1      |11110CPP| UDP | UDP
| Page 1 | 6LoRH | 6LoRH |  6LoRH   |LOWPAN_IPHC| UDP    | hdr |Payld
+-+ ... -+- ... -+- ... -+-+- ... +-+-+-+ ... +-+-+ ... -+ ... +-...
                                     <=        RFC 6282      =>
          <================ Inner packet ==================== = =

Since there is no page switch between the encapsulated packet and the encapsulation, the first octet of the compressed packet that acts as page selector is actually removed at encapsulation, so the inner packet used in the descriptions below starts with the SRH-6LoRH, and is exactly the packet represented in Figure 20, from the second octet onward.¶

When encapsulating that inner packet to place it in the Track, the first header that the Ingress appends at the head of the inner packet is an IP-in-IP 6LoRH Header; in that header, the encapsulator address, which maps to the IPv6 source address in the uncompressed form, contains a GUA or ULA IPv6 address of the Ingress node that serves as DODAG ID for the Track, expressed in the compressed form and using the DODAGID of the Main DODAG as compression reference. If the address is compressed to 2 bytes, the resulting value for the Length field shown in Figure 21 is 3, meaning that the SRH-6LoRH as a whole is 5-octets long.¶

 0                   1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-     ...     -+
|1|0|1| Length  | 6LoRH Type 6  |  Hop Limit    | Track DODAGID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-     ...     -+

At the head of the resulting sequence of bytes, the track Ingress then adds the RPI that carries the TrackID as RPLinstanceID as a P-RPI-6LoRH Header, as illustrated in Figure 12, using the TrackID as RPLInstanceID. Combined with the IP-in-IP 6LoRH Header, this allows to identify the Track without ambiguity.¶

The SRH-6LoRH is then added at the head of the resulting sequence of bytes as a verbatim copy of the content of the SR-VIO that signaled the selected Track Leg.¶

 0                   1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5  ..         ..        ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    -+-    -+ ... +-    -+
|1|0|0|  Size   |6LoRH Type 0..4| Hop1 | Hop2 |     | HopN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    -+-    -+ ... +-    -+
                                          Where N = Size + 1

The format of the resulting encapsulated packet in [RFC8138] compressed form is illustrated in Figure 23:¶

+-+ ... -+-+-+- ... -+-+-+- ... -+-+-+-+-+- ... +-+-+-+-+-+-+- ...
| Page 1 |  SRH-6LoRH  | P-RPI-6LoRH | IP-in-IP 6LoRH | Inner Packet
+-+ ... -+-+-+- ... -+-+-+- ... -+-+-+-+-+- ... +-+-+-+-+-+-+- ...

 Signals :  Loose Hops :    TrackID  :  Track DODAGID :

7.1. Storing Mode Main DODAG

This specification expects that the Main DODAG is operated in Non-Storing Mode. The reasons for that limitation are mostly related to LLN operations, power and spectrum conservation:¶

• In Non-Storing Mode, the Root already knowns the DODAG topology, so the additional topological information is reduced to the siblings.¶
• The downward routes are updated with unicast messages to the Root, which ensures that the Root can reach back to the LLN nodes after a repair faster than in the case of Storing Mode. Also the Root can control the use of the path diversity in the DODAG to reach the LLN nodes. For both reasons, Non-Storing Mode provides better capabilities for the Root to maintain the P-Routes.¶
• When the Main DODAG is operated in Non-Storing Mode, P-Routes enable loose Source Routing, which is only an advantage in that mode. Storing Mode does not use Source Routing Headers, and does not derive the same benefits from this capability.¶

On the other hand, since RPL is a Layer-3 routing protocol, its applicability extends beyond LLNs to a generic IP network. RPL requires less resources than alternative IGPs like OSPF, ISIS, EIGRP, BABEL or RIP at the expense of a route stretch vs. the shortest path routes to a destination that those protocols compute. P-Routes add the capability to install shortest and/or constrained routes to special destinations such as discussed in section A.9.4. of the ANIMA ACP [RFC8994].¶

In a powered and wired network, when enough memory to store the needed routes is available, the RPL Storing Mode proposes a better trade-off than the Non-Storing, as it reduces the route stretch and lowers the load on the Root. In that case, the control path between the Root and the LLN nodes is highly available compared to LLNs, and the nodes can be reached to maintain the P-Routes at most times.¶

This section specifies the additions that are needed to support Projected Routes when the Main DODAG is operated in Storing Mode. As long as the RPI can be processed adequately by the dataplane, the changes to this specification are limited to the DAO message. The Track structure, routes and forwarding operations remain the same. Since there is no capability negotiation, the expectation is that all the nodes in the network support this specification in the same fashion, or are configured the same way through management.¶

In Storing Mode, the Root misses the Child to Parent relationship that forms the Main DODAG, as well as the sibling information. To provide that knowledge the nodes in the network MUST send additional DAO messages that are unicast to the Root just like Non-Storing DAO messages are.¶

In the DAO message, the originating router advertises a set of neighbor nodes using Sibling Information Options (SIO)s, regardless of the relative position in the DODAG of the advertised node vs. this router.¶

The DAO message MUST be formed as follows:¶

• The originating router is identified by the source address of the DAO. That address MUST be the one that this router registers to neighbor routers so the Root can correlate the DAOs from those routers when they advertise this router as their neighbor. The DAO contains one or more sequences of one Transit Information Option and one or more Sibling Information Options. There is no RPL Target Option so the Root is not confused into adding a Storing Mode route to the Target.¶
• The TIO is formed as in Storing Mode, and the Parent Address is not present. The Path Sequence and Path Lifetime fields are aligned with the values used in the Address Registration of the node(s) advertised in the SIO, as explained in Section 9.1. of [RFC9010]. Having similar values in all nodes allows factorising the TIO for multiple SIOs as done with [RPL].¶
• The TIO is followed by one or more SIOs that provide an address (ULA or GUA) of the advertised neighbor node.¶

But the RPL routing information headers may not be supported on all type of routed network infrastructures, especially not in high-speed routers. When the RPI is not supported in the dataplane, there cannot be local RPL Instances and RPL can only operate as a single topology (the Main DODAG). The RPL Instance is that of the Main DODAG and the Ingress node that encapsulates is not the Root. The routes along the Tracks are alternate routes to those available along the Main DODAG. They MAY conflict with routes to children and MUST take precedence in the routing table. The Targets MUST be adjacent to the Track Egress to avoid loops that may form if the packet is reinjected in the Main DODAG.¶

7.2. A Track as a Full DODAG

This specification builds parallel or crossing Track Legs as opposed to a more complex DODAG with interconnections at any place desirable. The reason for that limitation is related to constrained node operations, and the capability to store large amount of topological information and compute complex paths:¶

• With this specification, the node in the LLN has no topological awareness, and does not need to maintain dynamic information about the link quality and availability.¶
• The Root has a complete topological information and statistical metrics that allow it or its PCE to perform a global optimization of all Tracks in its DODAG. Based on that information, the Root computes the Track Leg and produces the source route paths.¶
• The node merely selects one of the proposed paths and applies the associated pre-computed routing header in the encapsulation. This alleviates both the complexity of computing a path and the compressed form of the routing header.¶

The RAW Architecture [RAW-ARCHI] actually expects the PSE at the Track Ingress to react to changes in the forwarding conditions along the Track, and reroute packets to maintain the required degree of reliability. To achieve this, the PSE needs the full richness of a DODAG to form any path that could meet the Service Level Objective (SLO).¶

This section specifies the additions that are needed to turn the Track into a full DODAG and enable the main Root to provide the necessary topological information to the Track Ingress. The expectation is that the metrics that the PSE uses are of an order other than that of the PCE, because of the difference of time scale between routing and forwarding, more in [RAW-ARCHI]. It follows that the PSE will learn the metrics it needs from an alternate source, e.g., OAM frames.¶

To pass the topological information to the Ingress, the Root uses a P-DAO messages that contains sequences of Target and Transit Information options that collectively represent the Track, expressed in the same fashion as in classical Non-Storing Mode. The difference is that the Root is the source as opposed to the destination, and can report information on many Targets, possibly the full Track, with one P-DAO.¶

Note that the Path Sequence and Lifetime in the TIO are selected by the Root, and that the Target/Transit information tuples in the P-DAO are not those received by the Root in the DAO messages about the said Targets. The Track may follow sibling routes and does not need to be congruent with the Main DODAG.¶

11.1. RPL DODAG Configuration Option Flag

IANA is requested to assign a flag from the "DODAG Configuration Option Flags for MOP 0..6" [RFC9010] registry under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" as follows:¶

IANA is requested to add [THIS RFC] as a reference for MOP 7 in the RPL Mode of Operation registry.¶

11.2. Elective 6LoWPAN Routing Header Type

IANA is requested to update the "Elective 6LoWPAN Routing Header Type" registry that was created for [RFC8138] under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" and assign the following value:¶

11.3. Critical 6LoWPAN Routing Header Type

IANA is requested to update the "Critical 6LoWPAN Routing Header Type" registry that was created for [RFC8138] under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" and assign the following value:¶

11.4. Subregistry For The RPL Option Flags

IANA is requested to create a registry for the 8-bit "RPL Option Flags" field, as detailed in Figure 11, under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". The bits are indexed from 0 (leftmost) to 7. Each bit is tracked with the following qualities:¶

• Bit number (counting from bit 0 as the most significant bit)¶
• Indication When Set¶
• Reference¶

Registration procedure is "Standards Action" [RFC8126]. The initial allocation is as indicated in Table 24:¶

11.5. RPL Control Codes

IANA is requested to update the "RPL Control Codes" registry under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" as indicated in Table 25:¶

11.6. RPL Control Message Options

IANA is requested to update the "RPL Control Message Options" registry under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" as indicated in Table 26:¶

11.7. SubRegistry for the Projected DAO Request Flags

IANA is requested to create a registry for the 8-bit "Projected DAO Request (PDR)" field under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". The bits are indexed from 0 (leftmost) to 7. 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 27:¶

11.8. SubRegistry for the PDR-ACK Flags

IANA is requested to create a registry for the 8-bit "PDR-ACK Flags" field under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". The bits are indexed from 0 (leftmost) to 7. 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 assigned for the PDR-ACK Flags.¶

11.9. Subregistry for the PDR-ACK Acceptance Status Values

IANA is requested to create a registry for the 8-bit "PDR-ACK Acceptance Status Values" under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". Each value is tracked with the following qualities:¶

• Value¶
• Meaning¶
• Reference¶

the possible values are expressed as a 6-bit unsigned integer (0..63). the registration procedure is "Standards Action" [RFC8126].¶

The (suggected) initial allocation is as indicated in Table 28:¶

11.10. Subregistry for the PDR-ACK Rejection Status Values

IANA is requested to create a registry for the 6-bit "PDR-ACK Rejection Status Values" under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". Each value is tracked with the following qualities:¶

• Value¶
• Meaning¶
• Reference¶

the possible values are expressed as a 6-bit unsigned integer (0..63). the registration procedure is "Standards Action" [RFC8126].¶

The (suggected) initial allocation is as indicated in Table 28:¶

11.11. SubRegistry for the Via Information Options Flags

IANA is requested to create a registry for the 8-bit "Via Information Options (VIO) Flags" field under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". The bits are indexed from 0 (leftmost) to 7. 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 assigned for the PDR-ACK Flags.¶

11.12. SubRegistry for the Sibling Information Option Flags

IANA is requested to create a registry for the 5-bit "Sibling Information Option (SIO) Flags" field under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)". The bits are indexed from 0 (leftmost) to 4. 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 30:¶

11.13. Destination Advertisement Object Flag

IANA is requested to update the "Destination Advertisement Object (DAO) Flags" registry created in Section 20.11 of [RPL] under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" as indicated in Table 31:¶

11.14. Destination Advertisement Object Acknowledgment Flag

IANA is requested to update the "Destination Advertisement Object (DAO) Acknowledgment Flags" registry created in Section 20.12 of [RPL] under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" as indicated in Table 32:¶

11.15. New ICMPv6 Error Code

In some cases RPL will return an ICMPv6 error message when a message cannot be forwarded along a P-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 P-Route", with a suggested code value of 8, to be confirmed by IANA.¶

11.16. RPL Rejection Status values

IANA is requested to update the "RPL Rejection Statuss" registry under the heading "Routing Protocol for Low Power and Lossy Networks (RPL)" as indicated in Table 33:¶