RPL Routing Pathology In a Network With a Mix of Nodes Operating in Storing and Non-Storing Modes
draft-ko-roll-mix-network-pathology-00

Abstract

Nodes can run RPL in storing or non-storing modes for downward routing. When a downward-bound packet traverses a node running in storing to a node running in non-storing mode, there is a routing pathology that makes the packet bounce between the two nodes. Due to this pathology, the packet never reaches the destination if it lies beyond these nodes in the multi-hop path. The solution is to mandate that all the nodes parse and interpret source routing headers and storing nodes to sometimes act like non-storing mode root.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Storing and Non-storing modes
• 4. Routing Pathology
• 5. Fixing the Pathology
• 6. Acknowledgements
• 7. IANA Considerations
• 8. Security Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

RPL [RFC6550] can use storing and non-storing mode operation to compute paths for downward routing. Downward routing is used when a node needs to send a packet to an arbitrary node in the network: the packet might go from a node upward towards the root and downwards to the destination.

The only discussion about what would happen if a single network had a mix of nodes running in storing and non-storing modes that the specification of RPL introduces is that node that operate with a different Mode of Operation (MOP) than the DODAG root will act as a leaf node in the network. The consensus it is unknown if the network would work properly because no one had required or built such a network and left to be explored in the future. In this draft, we document a case in which we allow a mix of nodes running in storing and non-storing modes to form a single network (e.g, despite having different MOPs) and introduce that RPL's two downwards routing modes, as it is, can cause a routing pathology that results in packets bouncing between the two nodes on the path and never reaching the destination.

We also propose one approach to modifying RPL to prevent this routing pathology. The methodology, introducing a new mode of operation, 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 arise due to the existence of a mix of nodes running in storing and non-storing nodes, we review storing and non-storing modes of RPL.

In Storing mode, a node keeps a (not necessarily) complete list of (nodid, 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.

4. Routing Pathology

Lets imagine a network of five nodes as shown below:

A -> B -> N -> S -> Root

In this network, N is operating in non-storing and S is operating in storing mode. N wants to send a packet to A. N sends this packet to S because S is the preferred parent. S is operating in storing mode so it looks up node A in its forwarding table and finds that the next hop to reach A is using node N. With the assumption that node N will also know how to reach node A it will forward the packet back to node N. N is operating in non-storing mode so without a source routing header, it will forward the packet back to S. Thus the packet bounces between N and S.

RPL indicates that, since storing mode nodes and non-storing mode nodes use a different mode of operation (MOP) field, 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. Notice that when following RPL as it is, there is no routing pathology since nodes with different MOPs will not be forwarding downwards packets interchangeably. In such cases a path of storing mode nodes can forward packets to a non-storing leaf node and vice versa. However, we can envision a network where a part of the network consists of computationally powerful nodes with route storing capabilities and the other part of the network with low-resource nodes that use a non-storing mode and operate together in a single RPL network. Furthermore, on a practical perspective, it is meaningful to use nodes that can contribute in constructing a more efficient DODAG that optimizes the data collection process rather than ignoring a node just because it supports a different MOP. Unfortunately, in such cases, the pathology that we discuss above can arise and cause downwards packets to be dropped before reaching the destination.

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.

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 capability.
3. Non-storing nodes send hop-by-hop DAO.
4. 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.
5. Change the forwarding mechanism in the storing mode node when it receives a downward bound packet:
6. 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.
   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. Acknowledgements

7. IANA Considerations

8. Security Considerations

Future work.

9. References

9.1. Normative References

9.2. Informative References

Authors' Addresses