Root initiated routing state in RPL

3.1. Projected DAO

Section 6 of [RPL] introduces the RPL Control Message Options (CMO), including the RPL Target Option (RTO) and Transit Information Option (TIO), which can be placed in RPL messages such as the Destination Advertisement Object (DAO). This specification extends the DAO message with the Projected DAO (P-DAO); a P-DAO message signals a Projected Route to one or more Targets using the new CMOs presented therein. This specification enables to combine one or more Projected Routes into a DODAG called a Track, that is traversed to reach the Targets.¶

The Track is assimilated with the DODAG formed for a Local RPL Instance. The local RPLInstanceID of the Track is called the TrackID, more in Section 7.2. A P-DAO message for a Track signals the TrackID in the RPLInstanceID field. The Track Ingress is signaled in the DODAGID field of the Projected DAO Base Object; that field is elided in the case of the main RPL Instance. The Track Ingress is the Root of the Track, as shown in Figure 1. .¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    TrackID    |K|D|   Flags   |   Reserved    | DAOSequence   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+               IPv6 Address of the Track Ingress               +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Option(s)...
+-+-+-+-+-+-+-+-+

In RPL Non-Storing Mode, the TIO and RTO are combined in a DAO message to inform the DODAG Root of all the edges in the DODAG, which are formed by the directed parent-child relationships. Options may be factorized; multiple RTOs may be present to signal a collection of children that can be reached via the parent(s) indicated in the TIO(s) that follows the RTOs. This specification generalizes the case of a parent that can be used to reach a child with that of a whole Track through which both children and siblings of the Track Egress are reachable.¶

New CMOs called the Via Information Options (VIO) are introduced for use in P-DAO messages as a multihop alternative to the TIO. One VIO is the Stateful Via Information Option (SF-VIO); the SF-VIO installs Storing Mode Projected Route (SMPR) along a strict segment. The other is the Source-Routed SF-VIO (SR-VIO); the SR-VIO installs a Non-Storing Mode Projected Route (NMPR) at the Track Ingress, which uses that state to encapsulate a packet with a Routing Header (RH) to the Track Egress.¶

Like in a DAO message, the RTOs can be factorized in a P-DAO, but the Via Options cannot. A P-DAO contains one or more RTOs that indicate the destinations that can be reached via the Track, and exactly one Via Option that signals a sequence of nodes. In Non-Storing Mode, the Root sends the P-DAO to the Track Ingress where the source-routing state is stored. In Storing Mode, the P-DAO is sent to the Track Egress and forwarded along the Segment in the reverse direction, installing a Storing Mode state to the Track Egress at each hop. In both cases the Track Ingress is the owner of the Track, and it generates the P-DAO-ACK when the installation is successful.¶

3.2. Sibling Information Option

This specification adds another CMO called the Sibling Information Option (SIO) that is used by a RPL Aware Node (RAN) to advertise a selection of its candidate neighbors as siblings to the Root, more in Section 6.4. The sibling selection process is out of scope.¶

3.3. P-DAO Request

Two new RPL Control Messages are also introduced, to enable a RAN to request the establishment of a Track between self as the Track Ingress Node and a Track Egress. The RAN makes its request by sending a new P-DAO Request (PDR) Message to the Root. The Root confirms with a new PDR-ACK message back to the requester RAN, see Section 6.1 for more. A positive PDR-ACK indicates that the Track was built and that the Roots commits to maintain the Track for the negotiated lifetime. In the case of a complex Track, each Segment is maintained independently and asynchronously by the Root, with its own lifetime that may be shorter, the same, or longer than that of the Track. The Root may use an asynchronous PDR-ACK with an negative status to indicate that the Track was terminated before its time.¶

3.4. Extending the RPI

Sending a Packet within a RPL Local Instance requires the presence of the abstract RPL Packet Information (RPI) described in section 11.2. of [RPL] in the outer IPv6 Header chain (see [USEofRPLinfo]). The RPI carries a local RPLInstanceID which, in association with either the source or the destination address in the IPv6 Header, indicates the RPL Instance that the packet follows.¶

This specification extends [RPL] to create a new flag that signals that a packet is forwarded along a projected route.¶

Projected-Route 'P':

1-bit flag. It is set to 1 if this packet is sent over a projected route and set to 0 otherwise.¶

6.1. New P-DAO Request Control Message

The P-DAO Request (PDR) message is sent by a Node in the main DODAG to the Root. It is a request to establish or refresh a Track. Exactly one RTO MUST be present in a PDR. The RTO signals the Track Egress, more in Section 7.1.¶

The RPL Control Code for the PDR is 0x09, to be confirmed by IANA. The format of PDR Base Object is as follows:¶

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   TrackID     |K|R|   Flags   |  ReqLifetime  | PDRSequence   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Option(s)...
 +-+-+-+-+-+-+-+-+

TrackID:

8-bit field indicating the RPLInstanceID associated with the Track. It is set to zero upon the first request for a new Track and then to the TrackID once the Track was created, to either renew it of destroy it.¶

K:

The 'K' flag is set to indicate that the recipient is expected to send a PDR-ACK back.¶

R:

The 'R' flag is set to request a Complex Track for redundancy.¶

Flags:

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

ReqLifetime:

8-bit unsigned integer. The requested lifetime for the Track expressed in Lifetime Units (obtained from the DODAG Configuration option).¶

A PDR with a fresher PDRSequence refreshes the lifetime, and a PDRLifetime of 0 indicates that the track should be destroyed.¶

PDRSequence:

8-bit wrapping sequence number, obeying the operation in section 7.2 of [RPL]. The PDRSequence is used to correlate a PDR-ACK message with the PDR message that triggered it. It is incremented at each PDR message and echoed in the PDR-ACK by the Root.¶

6.2. 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:

The RPLInstanceID of the Track that was created. The value of 0x00 is used to when no Track was created.¶

Flags:

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

Track Lifetime:

Indicates that remaining Lifetime for the Track, expressed in Lifetime Units; the value of zero (0x00) indicates that the Track was destroyed or not created.¶

PDRSequence:

8-bit wrapping sequence number. It is incremented at each PDR message and echoed in the PDR-ACK.¶

PDR-ACK Status:

8-bit field indicating the completion. The PDR-ACK Status is substructured as indicated in Figure 6:¶

    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 7 and Table 8.¶

Reserved:

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

6.3. Via Information Options

An Via Option signals the ordered list of IPv6 Via Addresses that constitutes the hops of either a Serial Track or a Segment of a more Complex Track. An Via Option MUST contain at least one Via Address, and a Via Address MUST NOT be present more than once, otherwise the Via Option MUST be ignored. The format of the Via 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     |   SegmentID   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Segm. Sequence | Seg. Lifetime |      SRH-6LoRH header         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    .                                                               .
    .                     Via Address 1                             .
    .                                                               .
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                              ....                             .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    .                                                               .
    .                     Via Address n                             .
    .                                                               .
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Option Type:

0x0B for SF-VIO, 0x0C for SR-VIO (to be confirmed by IANA)¶

Option Length:

In bytes; variable, depending on the number of Via Addresses and the compression.¶

SegmentID:

8-bit field that identifies a Segment within a Track or the main DODAG as indicated by the TrackID field. The value of 0 is used to signal a Serial Track, i.e., made of a single segment.¶

Segment Sequence:

8-bit unsigned integer. The Segment Sequence obeys the operation in section 7.2 of [RPL] and the lollipop starts at 255.¶

When the Root of the DODAG needs to refresh or update a Segment in a Track, it increments the Segment Sequence individually for that Segment.¶

The Segment information indicated in the Via Option deprecates any state for the Segment indicated by the SegmentID within the indicated Track and sets up the new information.¶

An Via Option 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, SegmentID, Segment Sequence) indicates a retry; it MUST NOT change the Segment and MUST be propagated or answered as the first copy.¶

Segment Lifetime:

8-bit unsigned integer. The length of time in Lifetime Units (obtained from the Configuration option) that the Segment is usable.¶

The period starts when a new Segment Sequence is seen. The value of 255 (0xFF) represents infinity. The value of zero (0x00) indicates a loss of reachability.¶

A P-DAO message that contains a Via Information option with a Segment Lifetime of zero is referred as a No-Path P-DAO in this document.¶

SRH-6LoRH header:

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

Via Address:

An IPv6 addresse along the Segment.¶

In a SF-VIO, the list is a strict path between direct neighbors, from the segment ingress to egress, both included. In an SR-VIO, the list starts at the first hop and ends at a Track Egress. The list in an SR-VIO may be loose, provided that each listed node has a path to the next listed node, e.g., via a segment or another Track.¶

In the case of a SF-VIO, or if [RFC8138] is not used in the data packets, then the Root MUST use only one SRH-6LoRH per Via Option, and the compression is the same for all the addresses, as shown in Figure 7.¶

In case of an SR-VIO, and if [RFC8138] is in use in the main DODAG, then the Root SHOULD optimize the size of the SR-VIO; more than one SRH-6LoRH may be present, e.g., if the compression level changes inside the Segment and different SRH-6LoRH Types are required. The content of the SR-VIO starting at the first SRH-6LoRH header is thus verbatim the one that the Track Ingress places in the packet encapsulation to reach the Track Ingress.¶

6.4. Sibling Information Option

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

The format of the SIO is as follows:¶

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type        | Option Length |Comp.|B|D|Flags|    Opaque     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Step of Rank       |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    .                                                               .
    .         Sibling DODAGID (if 'D' flag not set)                 .
    .                                                               .
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    .                                                               .
    .                     Sibling Address                           .
    .                                                               .
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Option Type:

0x0D (to be confirmed by IANA)¶

Option Length:

In bytes, the size of the option.¶

Compression Type:

3-bit unsigned integer. This is the SRH-6LoRH Type as defined in figure 7 in section 5.1 of [RFC8138] that corresponds to the compression used for the Sibling Address and its DODAGID if resent. The Compression refernce is the Root of the main DODAG.¶

Reserved for Flags:

MUST be set to zero by the sender and MUST be ignored by the receiver.¶

B:

1-bit flag that is set to indicate that the connectivity to the sibling is bidirectional and roughly symmetrical. In that case, only one of the siblings may report the SIO for the hop. If 'B' is not set then the SIO only indicates connectivity from the sibling to this node, and does not provide information on the hop from this node to the sibling.¶

D:

1-bit flag that is set to indicate that sibling belongs to the same DODAG. When not set, the Sibling DODAGID is indicated.¶

Flags:

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

Opaque:

MAY be used to carry information that the node and the Root understand, e.g., a particular representation of the Link properties such as a proprietary Link Quality Information for packets received from the sibling. An industrial Alliance that uses RPL for a particular use / environment MAY redefine the use of this field to fit its needs.¶

Step of Rank:

16-bit unsigned integer. This is the Step of Rank [RPL] as computed by the Objective Function between this node and the sibling.¶

Reserved:

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

Sibling DODAGID:

2 to 16 bytes, the DODAGID of the sibling in a [RFC8138] compressed form as indicated by the Compression Type field. This field is present when the 'D' flag is not set.¶

Sibling Address:

2 to 16 bytes, the IPv6 Address of the sibling in a [RFC8138] compressed form as indicated by the Compression Type field.¶

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

7.1. Requesting a Track

A Node is free to ask the Root for a new Track at any time. This is done with a PDR message, that indicates in the Requested Lifetime field the duration for which the Track should be established. Upon a PDR, the Root MAY install the necessary Segments, in which case it answers with a PDR-ACK indicating the granted Track Lifetime. All the Segments MUST be of a same mode, either Storing or Non-Storing. All the Segments MUST be created with the same TrackID and the same DODAGID signaled in the P-DAO.¶

The Root is free to design the Track as it wishes, and to change the Segments overtime to serve the Track as needed, without notifying the resquesting Node. The Segment Lifetime in the P-DAO messages does not need to be aligned to the Requested Lifetime in the PDR, or between P-DAO messages for different Segments. The Root may use shorter lifetimes for the Segments and renew them faster than the Track is, or longer lifetimes in which case it will need to tear down the Segments if the Track is not renewed.¶

When the Track Lifetime that was returned in the PDR-ACK is close to elapse, the resquesting Node needs to resend a PDR using the TrackID in the PDR-ACK to extend the lifetime of the Track, else the Track will time out and the Root will tear down the whole structure.¶

If the Track fails and cannot be restored, the Root notifies the resquesting Node asynchronously with a PDR-ACK with a Track Lifetime of 0, indicating that the Track has failed, and a PDR-ACK Status indicating the reason of the fault.¶

7.2. Identifying a Track

RPL defines the concept of an Instance to signal an individual routing topology but does not have a concept of an administrative distance, which exists in certain proprietary implementations to sort out conflicts between multiple sources of routing information within one routing topology.¶

This draft leverages the RPL Instance model as follows:¶

• The Root MAY use P-DAO messages to add better routes in the main (Global) Instance in conformance with the routing objectives in that Instance. To achieve this, the Root MAY install an SMPR along a path down the main Non-Storing Mode DODAG. This enables a loose source routing and reduces the size of the Routing Header, see Appendix A.1.¶

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

  Once the Projected Route is installed, the intermediate nodes listed in the SF-VIO after first one (i.e. The ingress) can be elided from the RH in packets sent along the Segment signaled in the P-DAO. The resulting loose source routing header indicates (one of) the Target(s) as the next entry after the ingress.¶

• The Root MAY also use P-DAO messages to install a specific (say, Traffic Engineered) path as a Serial or as a Complex Track, to a particular endpoint that is the Track Egress. In that case, the Root MUST install a Local RPL Instance (see section 5 of [RPL]).¶

  In a that case, the TrackID MUST be unique for the Global Unique IPv6 Address (GUA) or Unique-Local Address (ULA) of the Track 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.¶

  The Track Egress Address and the TrackID MUST be signaled in the P-DAO message as shown in Figure 1.¶

7.3. Installing a Track

A Storing Mode P-DAO contains an SF-VIO that signals the strict sequence of consecutive nodes to form a segment between a segment ingress and a segment egress (both included). It installs a route of a higher precedence along the segment towards the Targets indicated in the Target Options. The segment is included in a DODAG indicated by the P-DAO Base Object, that may be the one formed by the main RPL Instance, or a Track associated with a local RPL Instance. A Track Egress is signaled as a Target in the P-DAO, and as the last entry is an SF-VIO of a last segment towards that Egress.¶

A Non-Storing Mode P-DAO signals a strict or loose sequence of nodes between the Track Ingress (excluded) and a Track Egress (included). It installs a source-routed path of a higher precedence within the Track indicated by the P-DAO Base Object, towards the Targets indicated in the Target Options. The source-routed path requires a Source-Routing header which implies an encapsulation to add the SRH to an existing packet.¶

The next entry in the sequence must be either a neighbor of the previous entry, or reachable as a Target via another Projected Route, either Storing or Non-Storing. If it is reachable over a Storing Mode Projected Route, the next entry in the loose sequence is the Target of a previous segment and the ingress of a next segment; the segments are associated with the same Track, which avoids the need of an encapsulation. Conversely, if it is reachable over a Non-Storing Mode Projected Route, the next loose source routed hop of the inner Track is a Target of a previous Track and the ingress of a next Track, which requires a de- and a re-encapsulation.¶

A Serial Track is installed by a single Projected Routes that signals the sequence of consecutive nodes, either in Storing or Non-Storing Mode. If can be a loose Non-Storing Mode Projected 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 Projected Routes with the same DODAGID and Track ID. The Ingress of a Non-Storing Mode Projected Route must be the owner of the DODAGID. The Ingress of a Storing Mode Projected Route must be either the owner of the DODAGID, or the egress of a preceding Storing Mode Projected Route in the same Track. In the latter case, the Targets of the Projected Route must be Targets of the preceding Projected Route to ensure that they are visible from the track Ingress.¶

7.4. Forwarding Along a Track

This draft leverages the RPL Forwarding model 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 ot the DODAGID, more in Section 7.4.¶

• This draft conforms the principles of [USEofRPLinfo] with regards to packet forwarding and encapsulation along a Track.¶

  • In that case, the Track is the DODAG, the Track Ingress is the Root, and the Track Egress is a RAL, and neighbors of the Track Egress that can be reached via the Track are RULs. The encapsulation rules in [USEofRPLinfo] 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 in any fashion.¶

  A packet that is being routed over the RPL Instance associated to a first Non-Storing Mode Track MAY be placed (encapsulated) in a second Track to cover one loose hop of the first Track. On the other hand, a Storing Mode Track must be strict and a packet that it placed in a Storing Mode Track MUST follow that Track till the Track Egress.¶

  When a Track Egress extracts a packet from a Track (decapsulates the packet), the Destination of the inner packet MUST be either this node or a direct neighbor, or a Target of another Segment of the same Track for which this node is ingress, otherwise the packet MUST be dropped.¶

All properties of a Track operations are inherited form the main RPL Instance that is used to install the Track. For instance, the use of compression per [RFC8138] is determined by whether it is used in the main instance, e.g., by setting the "T" flag [TURN-ON_RFC8138] in the RPL configuration option.¶

7.5. Non-Storing Mode Projected Route

As illustrated in Figure 9, a P-DAO that carries an SR-VIO enables the Root to install a source-routed path towards a Track Egress in any particular router.¶

           ------+---------
                 |          Internet
                 |
              +-----+
              |     | Border Router
              |     |  (RPL Root)
              +-----+                    |  P  ^ ACK
                 |        Track          | DAO |
           o    o   o  o  Ingress X      V     |   X
       o o   o  o   o  o     o   X   o             X Source
      o  o o  o o    o   o  o    X  o  o           X Routed
      o   o    °  o     o   o   o X     o          X Segment
     o  o   o  o   o  o    o  o     X Track        X
        o  o  o  o             o     Egress

       o       o               o    o
     o          o             o     o
                                   destination

                       LLN

A route indicated by an SR-VIO may be loose, meaning that the node that owns the next listed Via Address is not necessarily a neighbor. Without proper loop avoidance mechanisms, the interaction of loose source routing and other mechanisms may effectively cause loops.¶

When forwarding a packet to a destination for which the router determines that routing happens via the Track Egress, the router inserts the source routing header in the packet with the destination set to the Track Egress.¶

In order to signal the Segment, the router encapsulates the packet with an IP-in-IP header and a Routing Header as follows:¶

• In the uncompressed form the source of the packet is this router, the destination is the first Via Address in the SR-VIO, and the RH is a Source Routing Header (SRH) [RFC6554] that contains the list of the remaining Via Addresses terminating by the Track Egress.¶

• The preferred alternate in a network where 6LoWPAN Header Compression [RFC6282] is used is to leverage "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Paging Dispatch" [RFC8025] to compress the RPL artifacts as indicated in [RFC8138].¶

  In that case, the source routed header is the exact copy of the (chain of) SRH-6LoRH found in the SR-VIO, also terminating by the Track Egress. The RPI-6LoRH is appended next, followed by an IP-in-IP 6LoRH Header that indicates the Ingress Router in the Encapsulator Address field, see as a similar case Figure 20 of [TURN-ON_RFC8138].¶

In the case of a loose source-routed path, there MUST be either a neighbor that is adjacent to the loose next hop, on which case the packet is forwarded to that neighbor, or another Track to the loose next hop for which this node is Ingress; in the latter case, another encapsulation takes place and the process possibly recurses; otherwise the packet is dropped.¶

In case of a forwarding error along a Source Route path, the node that fails to forward SHOULD send an ICMP error with a code "Error in Source Routing Header" back to the source of the packet, as described in section 11.2.2.3. of [RPL]. Upon this message, the encapsulating node SHOULD stop using the source route path for a period of time and it SHOULD send an ICMP message with a Code "Error in Projected Route" to the Root. Failure to follow these steps may result in packet loss and wasted resources along the source route path that is broken.¶

7.6. Storing Mode Projected Route

As illustrated in Figure 10, a P-DAO that carries a SF-VIO enables the Root to install a stateful route towards a collection of Targets along a Segment between a Track Ingress and a Track Egress.¶

        ------+---------
              |          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

In order to install the relevant routing state along the Segment , the Root sends a unicast P-DAO message to the Track Egress router of the routing Segment that is being installed. The P-DAO message contains a SF-VIO with the direct sequence of Via Addresses. The SF-VIO follows one or more RTOs indicating the Targets to which the Track leads. The SF-VIO contains a Segment Lifetime for which the state is to be maintained.¶

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

The Egress node of the Segment is the only node in the path that does not install a route in response to the P-DAO; it is expected to be already able to route to the Target(s) on its own. If one of the Targets is not known, the node MUST answer to the Root with a negative DAO-ACK listing the Target(s) that could not be located (suggested status 10 to be confirmed by IANA).¶

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

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

Upon receiving a propagated DAO, all except the Egress Router MUST install a route towards the DAO Target(s) via their successor in the SF-VIO. The router MAY install additional routes towards the VIA Addresses that are the SF-VIO after the next one, if any, but in case of a conflict or a lack of resource, the route(s) to the Target(s) have precedence.¶

If a router cannot reach its predecessor in the SF-VIO, the router MUST answer to the Root with a negative DAO-ACK indicating the successor that is unreachable (suggested status 11 to be confirmed by IANA).¶

The process continues till the P-DAO is propagated to ingress router of the Segment, which answers with a DAO-ACK to the Root.¶

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

In case of a forwarding error along an SMPR, the node that fails to forward SHOULD send an ICMP error with a code "Error in Projected Route" to the Root. Failure to do so may result in packet loss and wasted resources along the Projected Route that is broken.¶

9.1. New Elective 6LoWPAN Routing Header Type

This document updates the IANA registry titled "Elective 6LoWPAN Routing Header Type" that was created for [RFC8138] and assigns the following value:¶

9.2. New Critical 6LoWPAN Routing Header Type

This document updates the IANA registry titled "Critical 6LoWPAN Routing Header Type" that was created for [RFC8138] and assigns the following value:¶

9.3. New Subregistry For The RPL Option Flags

IANA is required to create a subregistry for the 8-bit RPL Option Flags field, as detailed in Figure 2, under the "Routing Protocol for Low Power and Lossy Networks (RPL)" registry. 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 6:¶

9.4. New RPL Control Codes

This document extends the IANA Subregistry created by RFC 6550 for RPL Control Codes as indicated in Table 4:¶

9.5. New RPL Control Message Options

This document extends the IANA Subregistry created by RFC 6550 for RPL Control Message Options as indicated in Table 5:¶

9.6. SubRegistry for the Projected DAO Request Flags

IANA is required to create a registry for the 8-bit Projected DAO Request (PDR) Flags field. Each bit is tracked with the following qualities:¶

• Bit number (counting from bit 0 as the most significant bit)¶
• Capability description¶
• Reference¶

Registration procedure is "Standards Action" [RFC8126]. The initial allocation is as indicated in Table 6:¶

9.7. SubRegistry for the PDR-ACK Flags

IANA is required to create an subregistry for the 8-bit PDR-ACK Flags field. Each bit is tracked with the following qualities:¶

• Bit number (counting from bit 0 as the most significant bit)¶
• Capability description¶
• Reference¶

Registration procedure is "Standards Action" [RFC8126]. No bit is currently defined for the PDR-ACK Flags.¶

9.8. Subregistry for the PDR-ACK Acceptance Status Values

IANA is requested to create a Subregistry for the PDR-ACK Acceptance Status values.¶

• Possible values are 6-bit unsigned integers (0..63).¶
• Registration procedure is "Standards Action" [RFC8126].¶
• Initial allocation is as indicated in Table 7:¶

9.9. Subregistry for the PDR-ACK Rejection Status Values

IANA is requested to create a Subregistry for the PDR-ACK Rejection Status values.¶

• Possible values are 6-bit unsigned integers (0..63).¶
• Registration procedure is "Standards Action" [RFC8126].¶
• Initial allocation is as indicated in Table 8:¶

9.10. SubRegistry for the Via Information Options Flags

IANA is requested to create a Subregistry for the 5-bit Via Information Options (Via Option) Flags field. Each bit is tracked with the following qualities:¶

• Bit number (counting from bit 0 as the most significant bit)¶
• Capability description¶
• Reference¶

Registration procedure is "Standards Action" [RFC8126]. No bit is currently defined for the Via Information Options (Via Option) Flags.¶

9.11. SubRegistry for the Sibling Information Option Flags

IANA is required to create a registry for the 5-bit Sibling Information Option (SIO) Flags field. Each bit is tracked with the following qualities:¶

• Bit number (counting from bit 0 as the most significant bit)¶
• Capability description¶
• Reference¶

Registration procedure is "Standards Action" [RFC8126]. The initial allocation is as indicated in Table 9:¶

9.12. Error in Projected Route ICMPv6 Code

In some cases RPL will return an ICMPv6 error message when a message cannot be forwarded along a Projected Route. This ICMPv6 error message is "Error in Projected Route".¶

IANA has defined an ICMPv6 "Code" Fields Registry for ICMPv6 Message Types. ICMPv6 Message Type 1 describes "Destination Unreachable" codes. This specification requires that a new code is allocated from the ICMPv6 Code Fields Registry for ICMPv6 Message Type 1, for "Error in Projected Route", with a suggested code value of 8, to be confirmed by IANA.¶