Networking Working Group J. Ko
Internet-Draft J. Jeong
Intended status: Standards Track J. Park
Expires: August 16, 2014 J. Jun
N. Kim
Electronics and Telecommunications Research Institute
O. Gnawali
University of Houston
February 12, 2014
RPL Routing Pathology In a Network With a Mix of Nodes Operating in
Storing and Non-Storing Modes
draft-ko-roll-mix-network-pathology-04
Abstract
The RPL specification allows nodes running with storing or non-
storing modes to operate in the same network. We describe how such a
mix can result in network partitioning even when there are plenty of
physical links available in the network. The partitioning affects
both upwards (nodes to root) and downwards (root to leaf) traffic.
This routing pathology stems from a recommendation made in the RPL
specification forcing nodes with different modes of operation to join
the RPL network as leaf nodes only. We propose a solution that
modifies RPL by mandating that all the nodes parse and interpret
source routing headers and storing mode nodes to sometimes act like a
non-storing mode root by attaching source routing headers.
Status of This Memo
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This Internet-Draft will expire on August 16, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Storing and Non-storing modes . . . . . . . . . . . . . . . . 3
4. Routing Pathology . . . . . . . . . . . . . . . . . . . . . . 4
5. Fixing the Pathology . . . . . . . . . . . . . . . . . . . . 5
6. Considerations for Route Management . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
RPL [RFC6550] can operate in storing and non-storing modes. These
modes introduce two different ways to perform downward routing.
Downward routing is used when a node needs to send a packet to an
arbitrary node (e.g., non-DODAG root node) in the network: the packet
can go from a node "upward" towards the root and "downwards" to the
final destination.
The RPL specification allows operating a network with a mix of
storing and non-storing modes. RFC 6550 describes special rules to
operate such a network: a node that operates with a different Mode of
Operation (MOP) than the DODAG root will act as a leaf node in the
network. The consensus was that it is unknown if the network would
work properly because no one had designed such a network and was left
to be explored in the future.
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In this draft, we document a case in which we allow a mix of nodes
operating in storing and non-storing modes to form a single network
(e.g, despite having different MOPs) and show that RPL's two
downwards routing modes, as it is, can cause a routing pathology.
This pathology can partition the network, i.e., it can result in
scenarios where nodes cannot send packets to the root and the root
cannot send packets to the nodes even though these nodes have plenty
of multi-hop physical connectivity in the network.
We propose one approach of modifying RPL to prevent this routing
pathology. The methodology, introducing a new mode of operation
(MOP), has been implemented and tested on an LLN testbed and in
process of publication. It is possible there are more elegant
approaches to prevent the pathology described.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
The terminologies used in this document are consistent with the
terminologies described in [I-D.ietf-roll-terminology], [RFC6551],
and [RFC6550].
3. Storing and Non-storing modes
Before we describe the routing pathology that arises due to the
existence of a mix of nodes operating in storing and non-storing
modes, we review the storing and non-storing downwards routing modes
that RPL introduces.
In Storing mode, a node keeps a (not necessarily) complete list of
(destination, nexthop) for nodes in its subtree. When a node
receives a packet, it forwards the packet to the nexthop if the node
finds the destination in the list. If it does not find the
destination in the list, it forwards the packet to the preferred
parent.
In Non-storing mode, if a packet does not have routing path in the
header, it forwards the packet to the preferred parent. The root in
this mode collects and maintains topology information of the network.
If the packet makes it to the root, the root computes the path to the
destination based on this topology information. The root puts this
path in the header and sends it to the next hop. The nodes, upon
receiving a packet with a path in the header, forward the packet to
the next hop as indicated in the path in the header.
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4. Routing Pathology
We first examine the effect of this routing pathology for routing
collection traffic packets. Lets consider the following network
topology.
A -> B -> N -> S -> Root (Storing)
Note that RPL indicates that, storing mode nodes and non-storing mode
nodes use a different mode of operation (MOP) field. Furthermore, if
the MOP supported from a DODAG root is not supported at a RPL node,
the node can only participate in the RPL network as a leaf node. Say
that the Root of the topology is a storing mode node. In this case,
S (e.g., a storing mode node) can connect to the Root (a storing mode
root) properly as a RPL router node. On the other hand, using the
DIOs initiated at the Root, N (e.g., a non-storing mode node) will
notice that the Root's MOP and its MOP is different; therefore, will
only connect join the RPL network as a leaf node. As a result, A and
B, which physically have connections to the root will not be able to
join the RPL network. On a functional perspective, this means that
both upwards (e.g., collection traffic) and downwards traffic to or
from nodes A and B cannot reach the root or any other node in the RPL
network. Thus, there is needless network partitioning. While this
is an extreme case, other cases where using routes that non-storing
mode nodes provide can help optimize the collection and downwards
routes that RPL nodes form.
On a practical perspective, two RPL networks, each deployed for
different purposes and use different MOPs, can meet at the
geographically same area. Allowing these overlapping networks
cooperate as a single dense network will improve the routing
reliability with better connectivities. Since nodes of two networks
can interoperate with each other, a network can migrate into the
another network in case of root failures or power outages. It will
also improve the network system robustness.
On a system design perspective, as the size of LLN deployments
increase, using a homogeneous non-storing mode RPL network will
introduce a high level of communication overhead, and a homogeneous
network of strong mode nodes will require too much memory resources.
By allowing a single mixed RPL network with computationally powerful
nodes with route storing capabilities and low-cost nodes that do not
need to maintain a routing table, it will construct a more efficient
DODAG and resolve such system design issues.
Unfortunately, in such cases, the pathology that we discuss above can
arise and cause downwards packets to be dropped and even more,
restrict the formation of efficient collection routes.
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5. Fixing the Pathology
We describe one way to fix RPL to prevent the pathology described
above, while acknowledging that there might be more elegant
solutions. In this approach, we acknowledge the fact that non-
storing mode nodes are more likely to have strict resource
limitations compared to nodes implementing the storing mode.
Therefore, we make sure that the most of the required additional
capabilities occur at the storing mode nodes rather than the non-
storing mode nodes.
1. A new mode of operation (MOP) that allows a node to choose either
to implement the storing or non-storing features, or both. The
changes below are made compared to the original storing and non-
storing modes.
2. Require storing and non-storing nodes to implement source routing
header parsing capabilities.
3. RPL DODAG Root nodes supporting this MOP should have the
capability to store routes (similar to the non-storing mode
option) and also identify storing mode node children nodes.
4. Non-storing nodes send hop-by-hop DAO.
5. Storing nodes keep a table of all the DAO senders and a flag
indicating if each of those sender is operating in storing or
non-storing mode. This requires allocating one of the bits in
the DAO message for a node to indicate if it is operating in
storing or non-storing mode.
6. Change the forwarding mechanism in the storing mode node when it
receives a downward bound packet:
7.
1. If packet does not have source routing header and the next
hop is a storing-mode node, forward as in [RFC6550]. If the
next hop is a non-storing node, insert the source routing
header [RFC6554] into the packet and forward, i.e., act like
a non-storing root.
2. Using the flag indicating the storing status of nodes in its
sub-DODAG, a node constructing a source routing header MAY
choose to construct a source routing header only up to the
next storing mode node.
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3. If the incoming packet has a source routing header, a storing
mode node SHOULD obey the route specified in the source
routing header to comply with the strict source routing
requirements in [RFC6554].
If there is a mix of storing and non-storing nodes, we should also be
more aggressive about loop detection. More aggressive loop detection
will quickly remove the looping packets from the network. Even with
the implementation of this suggestion, nodes beyond storing / non-
storing nodes will still remain unreachable.
6. Considerations for Route Management
Given the lossy nature of the links and the variability in link
quality on a temporal scale, the DAO parent(s) that were originally
selected can change over time. In such cases, nodes take the
following steps.
1. Upon realizing a change in the DAO parent set, a RPL node SHOULD
send a DAO with an increased Path Sequence Number to its ``new"
DAO parent set.
2. Next, the node experiencing the change in its DAO parents, SHOULD
send a No-Path DAO message with an increased Path Sequence Number
to the set of DAO parents that have been deleted.
3. When nodes receive a DAO that contains a modified Transit
Information option for a specific Target option, they SHOULD
propagate this information to its DAO parents while respecting
the Path Control field in the Transit Information option.
4. When a node without the route storing capability fails in
forwarding a packet using the information provided by a source
routing header, the node initiates an ICMPv6 error message to the
node that constructed the source routing header.
This process alerts nodes keeping the routing state of the DAO
initiating nodes with the up-to-date DAO parent information. Nodes
with route storing capabilities will process the information and
nodes without this capability will pass the information up the DODAG.
When using the flag indicating the storing status, the propagation of
the updated DAO and ICMPv6 error message MAY stop at the first node
with route storing capability since this node takes the role of
managing the routing state for the target node.
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7. Acknowledgements
8. IANA Considerations
9. Security Considerations
Future work.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, March
2012.
10.2. Informative References
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-05 (work in
progress), March 2011.
Authors' Addresses
JeongGil Ko
Electronics and Telecommunications Research Institute
218 Gajeong-Ro
Yuseong-Gu, Daejeon 305-700
Korea
Phone: +82-42-860-5824
Email: jeonggil.ko@etri.re.kr
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Jongsoo Jeong
Electronics and Telecommunications Research Institute
218 Gajeong-Ro
Yuseong-Gu, Daejeon 305-700
Korea
Phone: +82-42-860-1806
Email: jsjeong@etri.re.kr
Jongjun Park
Electronics and Telecommunications Research Institute
218 Gajeong-Ro
Yuseong-Gu, Daejeon 305-700
Korea
Phone: +82-42-860-5413
Email: juny@etri.re.kr
Jong Arm Jun
Electronics and Telecommunications Research Institute
218 Gajeong-Ro
Yuseong-Gu, Daejeon 305-700
Korea
Phone: +82-42-860-4835
Email: jajun@etri.re.kr
Naesoo Kim
Electronics and Telecommunications Research Institute
218 Gajeong-Ro
Yuseong-Gu, Daejeon 305-700
Korea
Phone: +82-42-860-5214
Email: nskim@etri.re.kr
Omprakash Gnawali
University of Houston
PGH 577, University of Houston
Houston, TX 77204
USA
Phone: +1-713-743-3356
Email: gnawali@cs.uh.edu
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