Root initiated routing state in RPL
Cisco Systems
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Cisco Systems
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Routing
ROLL
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.
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 () 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 .The Terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks'
and .
Section 6.7 of 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:
0x0A (to be confirmed by IANA)In bytes; variable, depending on the number
of Via Addresses.8-bit unsigned integer. When a RPL
Target option is issued by the root of the DODAG
(i.e. in a DAO message), that root sets the Path Sequence and
increments the Path Sequence each time it issues a RPL Target
option with updated information. The indicated sequence
deprecates any state for a given Target that was learned from a
previous sequence and adds to any state that was learned for
that sequence.8-bit unsigned integer. The length
of time in Lifetime Units (obtained from the Configuration
option) that the prefix is valid for route determination. The
period starts when a new Path Sequence is seen. A value of all
one bits (0xFF) represents infinity. A value of all zero bits
(0x00) indicates a loss of reachability. A DAO message that
contains a Via Information option with a Path Lifetime of
0x00 for a Target is referred as a No-Path (for that Target) in
this document.16 bytes. IPv6 Address of the
next hop towards the destination(s) indicated in the target option
that immediately precede the VIO. TBD: See how the /64 prefix can be elided if
it is the same as that of (all of) the target(s). In that case,
the Next-Hop Address could be expressed as the 8-bytes suffix only,
otherwise it is expressed as 16 bytes, at least in storing mode.
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 non-storing mode P-DAO discussed in section
has a single VIO with one or more Via Addresses in it, the list of Via
Addresses indicating the source-routed path to the target to be installed
in the router that receives the message, which replies to the root directly
with a DAO-ACK message.
The storing mode P-DAO discussed in section has at
least two Via Information options with one Via Address each, for the ingress
and the egress of the path, and more if there are intermediate routers.
The Via Addresses indicate the routers in which the routing state to the
target have to be installed via the next Via Address in the sequence of VIO.
In normal operations, the P-DAO is propagated along the chain of Via Routers
from the egress router of the path till the ingress one, which confirms the
installation to the root with a DAO-ACK message.
Note that the root may be the ingress and it may be the egress of the
path, that it can also be neither but it cannot be both.
The root is expected to use these mechanisms optimally and
with required parsimony to limit the state installed in the devices
to fit within their resources, but how the
root figures the amount of resources that is available in each device
is out of scope for this document.
In particular, the draft expects that the root has enough information about
the capability
for each node to store a number of routes, which can be discovered for
instance using a Network Management System (NMS) and/or the RPL routing
extensions specified in
Routing for Path Calculation in LLNs.
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 , 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.
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)
. 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 , 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.
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 whereby 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.
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 :
in non-storing mode, all packets
routed within the DODAG flow all the way up to the root of the DODAG. If
the destination is in the same DODAG, the root must encapsulate the packet
to place a Routing Header that has the strict source route information down
the DODAG to the destination. This will be the case even if the destination
is relatively close to the source and the root is relatively far off.
In storing mode, unless the destination is a child of the source,
the packets will follow the default route up the DODAG as well.
If the destination is in the same DODAG, they will eventually reach a
common parent that has a route to the destination; at worse, the common
parent may also be the root. From that common parent, the packet will
follow a path down the DODAG that is optimized for the Objective Function
that was used to build the DODAG.
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.
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. This draft
conforms the instance model as follows:
if the PCE needs to influence a particular instance to add better routes
in conformance with the routing objectives in that instance, it may do so.
When the PCE modifies an existing instance then the added routes
must not create a loop in that instance. This is achieved by always
preferring a route obtained from the PCE over a route that is learned via
RPL.
If the PCE installs a more specific (Traffic Engineering) route between
a particular pair of nodes then it should use a Local Instance from the
ingress node of that path. Only packets associated with that instance will
be routed along that path.
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
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 updates the IANA registry for the Mode of Operation
(MOP)
4: Non-Storing with Projected routes [this]
This document updates IANA registry for the RPL Control Message
Options
0x0A: Via descriptor [this]
The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for their
contributions to the ideas developed here.Path Computation ElementIETF
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 , if the source is node 41
and the destination is node 52, then the common parent is node 22.
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.
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.
As a result, a transversal route is installed that does not need to follow
the DODAG structure.