| ROLL | P. Thubert, Ed. |
| Internet-Draft | J. Pylakutty |
| Intended status: Standards Track | Cisco |
| Expires: March 23, 2018 | September 19, 2017 |
Root initiated routing state in RPL
draft-ietf-roll-dao-projection-02
This document proposes a protocol extension to RPL that enables to install a limited amount of centrally-computed routes in a RPL graph, enabling loose source routing down a non-storing mode DODAG, or transversal routes inside the DODAG. As opposed to the classical route injection in RPL that are injected by the end devices, this draft enables the root of the DODAG to projects the routes that are needed on the nodes where they should be installed.
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This Internet-Draft will expire on March 23, 2018.
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The "Routing Protocol for Low Power and Lossy Networks" (LLN)(RPL) is a generic Distance Vector protocol that is well suited for application in a variety of low energy Internet of Things (IoT) networks. RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) in which the root often acts as the Border Router to connect the RPL domain to the Internet. The root is responsible to select the RPL Instance that is used to forward a packet coming from the Internet into the RPL domain and set the related RPL information in the packets.
The 6TiSCH architecture leverages RPL for its routing operation and considers the Deterministic Networking Architecture as one possible model whereby the device resources and capabilities are exposed to an external controller which installs routing states into the network based on some objective functions that reside in that external entity.
Based on heuristics of usage, path length, and knowledge of device capacity and available resources such as battery levels and reservable buffers, a Path Computation Element ([PCE]) with a global visibility on the system could install additional P2P routes that are more optimized for the current needs as expressed by the objective function.
This draft enables a RPL root, with optionally the assistance of a PCE, to install and maintain additional storing and non-storing mode routes within the RPL domain, along a selected set of nodes and for a selected duration, thus providing routes more suitable than those obtained with the distributed operation of RPL. Those routes may be installed in either storing and non-storing modes RPL instances, resulting in potentially hybrid situations where the mode of the projected routes is different from that of the other routes in the instance.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The Terminology used in this document is consistent with and incorporates that described in "Terminology in Low power And Lossy Networks"[RFC7102] and [RFC6550].
Section 6.7 of RPL [RFC6550] specifies Control Message Options (CMO) to be placed in RPL messages such as the Destination Advertisement Object (DAO) message. The RPL Target Option and the Transit Information Option (TIO) are such options; the former indicates a node to be reached and the latter specifies a parent that can be used to reach that node. Options may be factorized; one or more contiguous TIOs apply to the one or more contiguous Target options that immediately precede the TIOs in the RPL message.
This specification introduces a new Control Message Option, the Via Information option (VIO). Like the TIO, the VIO MUST be preceded by one or more RPL Target options to which it applies. Unlike the TIO, the VIO are not factorized: multiple contiguous Via options indicate an ordered sequence of routers to reach the target(s), presented in the order of the packet stream, source to destination, and in which a routing state must be installed.
The Via Information option MUST contain at least one Via Address.
The Via Information option MAY be present in DAO messages, and 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x0A | Option Length | Path Sequence | Path Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Via Information option format
This draft adds a capability to RPL whereby the root projects a route through an extended DAO message called a Projected-DAO (P-DAO) to an arbitrary router down the DODAG, indicating a next hop or a sequence of routers via which a certain destination indicated in the Target Information option may be reached.
A P-DAO message MUST contain at least a Target Information option and at least one VIA Information option following it.
Like a classical DAO message, a P-DAO is processed only if it is "new" per section 9.2.2. "Generation of DAO Messages" of the RPL specification; this is determined using the Path Sequence information from the VIO as opposed to a TIO. Also, a Path Lifetime of 0 in a VIO indicates that a route is to be removed.
There are two kinds of P-DAO, the storing mode and the non-storing mode ones.
The root is expected to use these mechanisms optimally and with required parsimony to limit the state installed in the devices to fit within their resources, but how the root figures the amount of resources that is available in each device is out of scope for this document.
In particular, the draft expects that the root has enough information about the capability for each node to store a number of routes, which can be discovered for instance using a Network Management System (NMS) and/or the RPL routing extensions specified in "Routing for Path Calculation in LLNs".
A route that is installed by a P-DAO is not necessarily installed along the DODAG, though how the root and the optional PCE obtain the additional topological information to compute other routes is out of scope for this document
As illustrated in Figure 2, the non-storing mode P-DAO enables the root to install a source-routed path towards a target in any particular router; with this path information the router can add a source routed header reflecting the path to any packet for which the current destination either is the said target or can be reached via the target, for instance a loose source routed packet for which the next loose hop is the target, or a packet for which the router has a routing state to the final destination via the target.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ | P ^ |
| | DAO | ACK | Loose
o o o o router V | | Source
o o o o o o o o o | P-DAO . Route
o o o o o o o o o o | Source . Path
o o o o o o o o o | Route . From
o o o o o o o o | Path . Root
o o o o o target V . To
o o o o | Desti-
o o o o | nation
destination V
LLN
Figure 2: Projecting a Non-Storing route
A router that receives a non-storing P-DAO installs a source routed path towards each of the consecutive targets via a source route path indicated in the following VIO.
When forwarding a packet to a destination for which the router determines that routing happens via the target, the router inserts the source routing header in the packet to reach the target.
In order to do so, the router encapsulates the packet with an IP in IP header and a non-storing mode source routing header (SRH) [RFC6554].
In the uncompressed form the source of the packet would be self, the destination would be the first Via Address in the VIO, and the SRH would contain the list of the remaining Via Addresses and then the target.
In practice, the router will normally use the "IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Paging Dispatch" to compress the RPL artifacts as indicated in the "6LoWPAN Routing Header" specification. In that case, the router indicates self as encapsulator in an IP-in-IP 6LoRH Header, and places the list of Via Addresses in the order of the VIO and then the target in the SRH 6LoRH Header.
As illustrated in Figure 3, the storing mode P-DAO enables the root to install a routing state towards a target in the routers along a segment between an ingress and an egress router; this enables the routers to forward along that segment any packet for which the next loose hop is the said target, for instance a loose source routed packet for which the next loose hop is the target, or a packet for which the router has a routing state to the final destination via the target.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ | ^ |
| | DAO | ACK |
o o o o | | |
o o o o o o o o o | ^ | Projected .
o o o o o o o o o o | | DAO | Route .
o o o o o o o o o | ^ | .
o o o o o o o o v | DAO v .
o o LLN o o o |
o o o o o Loose Source Route Path |
o o o o From Root To Destination v
Figure 3: Projecting a route
Based on available topological, usage and capabilities node information, the root or an associated PCE computes which segment should be optimized and which relevant state should be installed in which nodes. The algorithm is out of scope but it is envisaged that the root could compute the ratio between the optimal path (existing path not traversing the root, and the current path), the application service level agreement (SLA) for specific flows that could benefit from shorter paths, the energy wasted in the network, local congestion on various links that would benefit from having flows routed along alternate paths.
In order to install the relevant routing state along the segment between an ingress and an egress routers, the root sends a unicast P-DAO message to the egress router of the routing segment that must be installed. The P-DAO message contains the ordered list of hops along the segment as a direct sequence of Via Information options that are preceded by one or more RPL Target options to which they relate. Each Via Information option contains a Path Lifetime for which the state is to be maintained.
The root sends the P-DAO directly to the egress node of the segment, which In that P-DAO, the destination IP address matches the Via Address in the last VIO. This is how the egress recognizes its role. In a similar fashion, the ingress node recognizes its role as it matches Via Address in the first VIO.
The egress node of the segment is the only node in the path that does not install a route in response to the P-DAO; it is expected to be already able to route to the target(s) on its own. It may either be the target, or may have some existing information to reach the target(s), such as a connected route or an already installed projected route. If one of the targets cannot be located, the node MUST answer to the root with a negative DAO-ACK listing the target(s) that could not be located (suggested status 10 to be confirmed by IANA).
If the egress node can reach all the targets, then it forwards the P-DAO with unchanged content to its loose predecessor in the segment as indicated in the list of Via Information options, and recursively the message is propagated unchanged along the sequence of routers indicated in the P-DAO, but in the reverse order, from egress to ingress.
The address of the predecessor to be used as destination of the propagated DAO message is found in the Via Information option the precedes the one that contain the address of the propagating node, which is used as source of the packet.
Upon receiving a propagated DAO, an intermediate router as well as the ingress router install a route towards the DAO target(s) via its successor in the P-DAO; the router locates the VIO that contains its address, and uses as next hop the address found in the Via Address field in the following VIO. The router MAY install additional routes towards the addresses that are located in VIOs that are after the next one, if any, but in case of a conflict or a lack of resource, a route to a target installed by the root has precedence.
The process recurses till the P-DAO is propagated to ingress router of the segment, which answers with a DAO-ACK to the root.
Also, the path indicated in a P-DAO may be loose, in which case the reachability to the next hop has to be asserted. Each router along the path indicated in a P-DAO is expected to be able to reach its successor, either with a connected route (direct neighbor), or by routing, for instance following a route installed previously by a DAO or a P-DAO message. If that route is not connected then a recursive lookup may take place at packet forwarding time to find the next hop to reach the target(s). If it does not and cannot reach the next router in the P-DAO, the router MUST answer to the root with a negative DAO-ACK indicating the successor that is unreachable (suggested status 11 to be confirmed by IANA).
A Path Lifetime of 0 in a Via Information option is used to clean up the state. The P-DAO is forwarded as described above, but the DAO is interpreted as a No-Path DAO and results in cleaning up existing state as opposed to refreshing an existing one or installing a new one.
A RPL implementation operating in a very constrained LLN typically uses the Non-Storing Mode of Operation as represented in Figure 4. In that mode, a RPL node indicates a parent-child relationship to the root, using a Destination Advertisement Object (DAO) that is unicast from the node directly to the root, and the root typically builds a source routed path to a destination down the DODAG by recursively concatenating this information.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+ ^ | |
| | DAO | ACK |
o o o o | | | Strict
o o o o o o o o o | | | Source
o o o o o o o o o o | | | Route
o o o o o o o o o | | |
o o o o o o o o | v v
o o o o
LLN
Figure 4: RPL non-storing mode of operation
Based on the parent-children relationships expressed in the non-storing DAO messages,the root possesses topological information about the whole network, though this information is limited to the structure of the DODAG for which it is the destination. A packet that is generated within the domain will always reach the root, which can then apply a source routing information to reach the destination if the destination is also in the DODAG. Similarly, a packet coming from the outside of the domain for a destination that is expected to be in a RPL domain reaches the root.
It results that the root, or then some associated centralized computation engine such as a PCE, can determine the amount of packets that reach a destination in the RPL domain, and thus the amount of energy and bandwidth that is wasted for transmission, between itself and the destination, as well as the risk of fragmentation, any potential delays because of a paths longer than necessary (shorter paths exist that would not traverse the root).
As a network gets deep, the size of the source routing header that the root must add to all the downward packets becomes an issue for nodes that are many hops away. In some use cases, a RPL network forms long lines and a limited amount of well-targeted routing state would allow to make the source routing operation loose as opposed to strict, and save packet size. Limiting the packet size is directly beneficial to the energy budget, but, mostly, it reduces the chances of frame loss and/or packet fragmentation, which is highly detrimental to the LLN operation. Because the capability to store a routing state in every node is limited, the decision of which route is installed where can only be optimized with a global knowledge of the system, a knowledge that the root or an associated PCE may possess by means that are outside of the scope of this specification.
This specification enables to store source-routed or storing mode state in intermediate routers, which enables to limit the excursion of the source route headers in deep networks. Once a P-DAO exchange has taken place for a given target, if the root operates in non storing mode, then it may elide the sequence of routers that is installed in the network from its source route headers to destination that are reachable via that target, and the source route headers effectively become loose.
RPL is optimized for Point-to-Multipoint (P2MP), root to leaves and Multipoint-to-Point (MP2P) leaves to root operations, whereby routes are always installed along the RPL DODAG. Transversal Peer to Peer (P2P) routes in a RPL network will generally suffer from some stretch since routing between 2 peers always happens via a common parent, as illustrated in Figure 5:
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
X
^ v o o
^ o o v o o o o o
^ o o o v o o o o o
^ o o v o o o o o
S o o o D o o o
o o o o
LLN
Figure 5: Routing Stretch between S and D via common parent X
It results that it is often beneficial to enable transversal P2P routes, either if the RPL route presents a stretch from shortest path, or if the new route is engineered with a different objective. For that reason, earlier work at the IETF introduced the "Reactive Discovery of Point-to-Point Routes in Low Power and Lossy Networks", which specifies a distributed method for establishing optimized P2P routes. This draft proposes an alternate based on a centralized route computation.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
|
o o o o
o o o o o o o o o
o o o o o o o o o o
o o o o o o o o o
S>>A>>>B>>C>>>D o o o
o o o o
LLN
Figure 6: Projected Transversal Route
This specification enables to store source-routed or storing mode state in intermediate routers, which enables to limit the stretch of a P2P route and maintain the characteristics within a given SLA. An example of service using this mechanism oculd be a control loop that would be installed in a network that uses classical RPL for asynchronous data collection. In that case, the P2P path may be installed in a different RPL Instance, with a different objective function.
It must be noted that RPL has a concept of instance but does not have a concept of an administrative distance, which exists in certain proprietary implementations to sort out conflicts between multiple sources of routing information. This draft conforms the instance model as follows:
In all cases, the path is indicated by a new Via Information option, and the flow is similar to the flow used to obtain loose source routing.
This draft uses messages that are already present in RPL [RFC6550] with optional secured versions. The same secured versions may be used with this draft, and whatever security is deployed for a given network also applies to the flows in this draft.
This document extends the IANA registry created by RFC 6550 for RPL Control Codes as follows:
| Code | Description | Reference |
|---|---|---|
| 0x0A | Via | This document |
This document is updating the registry created by RFC 6550 for the RPL 3-bit Mode of Operation (MOP) as follows:
| MOP value | Description | Reference |
|---|---|---|
| 5 | Non-Storing mode of operation with Projected routes | This document |
| 6 | Storing mode of operation with Projected routes | This document |
The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for their contributions to the ideas developed here.
| [I-D.ietf-6tisch-architecture] | Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-12, August 2017. |
| [I-D.ietf-detnet-architecture] | Finn, N., Thubert, P., Varga, B. and J. Farkas, "Deterministic Networking Architecture", Internet-Draft draft-ietf-detnet-architecture-03, August 2017. |
| [PCE] | IETF, "Path Computation Element" |
| [RFC6997] | Goyal, M., Baccelli, E., Philipp, M., Brandt, A. and J. Martocci, "Reactive Discovery of Point-to-Point Routes in Low-Power and Lossy Networks", RFC 6997, DOI 10.17487/RFC6997, August 2013. |
| [RFC7102] | Vasseur, JP., "Terms Used in Routing for Low-Power and Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January 2014. |
In non-storing mode, the DAG root maintains the knowledge of the whole DODAG topology, so when both the source and the destination of a packet are in the DODAG, the root can determine the common parent that would have been used in storing mode, and thus the list of nodes in the path between the common parent and the destination. For instance in the diagram shown in Figure 7, if the source is node 41 and the destination is node 52, then the common parent is node 22.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| \ \____
/ \ \
o 11 o 12 o 13
/ | / \
o 22 o 23 o 24 o 25
/ \ | \ \
o 31 o 32 o o o 35
/ / | \ | \
o 41 o 42 o o o 45 o 46
| | | | \ |
o 51 o 52 o 53 o o 55 o 56
LLN
Figure 7: Example DODAG forming a logical tree topology
With this draft, the root can install a storing mode routing states along a segment that is either from itself to the destination, or from one or more common parents for a particular source/destination pair towards that destination (in this particular example, this would be the segment made of nodes 22, 32, 42).
In the example below, say that there is a lot of traffic to nodes 55 and 56 and the root decides to reduce the size of routing headers to those destinations. The root can first send a DAO to node 45 indicating target 55 and a Via segment (35, 45), as well as another DAO to node 46 indicating target 56 and a Via segment (35, 46). This will save one entry in the routing header on both sides. The root may then send a DAO to node 35 indicating targets 55 and 56 a Via segment (13, 24, 35) to fully optimize that path.
Alternatively, the root may send a DAO to node 45 indicating target 55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46 indicating target 56 and a Via segment (13, 24, 35, 46), indicating the same DAO Sequence.
In this example, say that a PCE determines that a path must be installed between node S and node D via routers A, B and C, in order to serve the needs of a particular application.
The root sends a P-DAO with a target option indicating the destination D and a sequence Via Information option, one for S, which is the ingress router of the segment, one for A and then for B, which are an intermediate routers, and one for C, which is the egress router.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| Projected DAO message to C
o | o o
o o o | o o o o o
o o o | o o o o o o
o o V o o o o o o
S A B C D o o o
o o o o
LLN
Figure 8: Projected DAO from root
Upon reception of the P-DAO, C validates that it can reach D, e.g. using IPv6 Neighbor Discovery, and if so, propagates the P-DAO unchanged to B.
B checks that it can reach C and of so, installs a route towards D via C. Then it propagates the P-DAO to A.
The process recurses till the P-DAO reaches S, the ingress of the segment, which installs a route to D via A and sends a DAO-ACK to the root.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
^ Projected DAO-ACK from S
/ o o o
/ o o o o o o o
| o o o o o o o o o
| o o o o o o o o
S A B C D o o o
o o o o
LLN
Figure 9: Projected DAO-ACK to root
As a result, a transversal route is installed that does not need to follow the DODAG structure.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
|
o o o o
o o o o o o o o o
o o o o o o o o o o
o o o o o o o o o
S>>A>>>B>>C>>>D o o o
o o o o
LLN
Figure 10: Projected Transversal Route