| ROLL | R. Jadhav, Ed. |
| Internet-Draft | Huawei |
| Intended status: Standards Track | April 20, 2020 |
| Expires: October 22, 2020 |
RPL Storing Root Ack
draft-jadhav-roll-storing-rootack-00
This document explains problems with DAO-ACK handling in RPL Storing MOP and provides updates to RFC6550 to solve those problems.
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The primary motivation for this draft is to enlist different issues with RPL operation and invoke a discussion within the working group. This draft by itself is not intended for RFC tracks but as a WG discussion track. This draft may in turn result in other work items taken up by the WG which may improvise on the issues mentioned herewith.
RPL [RFC6550] specifies a proactive distance-vector routing scheme designed for LLNs (Low Power and Lossy Networks). RPL enables the network to be formed as a DODAG and supports storing mode and non-storing mode of operations. Non-storing mode allows reduced memory resource usage on the nodes by allowing non-BR nodes to operate without managing a routing table and involves use of source routing by the Root to direct the traffic along a specific path. In storing mode of operation the routing happens on hop-by-hop basis and intermediate routers need to maintain routing tables.
DAO messaging helps to install downstream routing paths in the DODAG. DAOs are generated on hop-by-hop basis. DAO may contain multiple RPL Control Options. The Target Option identifies the address prefix for which the route has to be installed and the corresponding Transit Information Option identifies the parameters (such as lifetime, freshness-counter, etc) for the target. The DAO base object contains the 'K' flag indicating that a DAO-ACK is sought by the sender. The DAO, DAO-ACK progresses on hop-by-hop basis all the way till Root.
This draft highlights various issues with RPL DAO-ACK handling in Storing MOP. The draft provides requirements to solve the issues and provides an updates to RFC6550 based on these requirements.
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 RFC 2119.
MOP = Mode of Operation
NS-MOP = RPL Non-Storing Mode of Operation
S-MOP = RPL Storing Mode of Operation
This document uses terminology described in [RFC6550].
(Root)
|
|
|
(A)
/ \
/ \
/ \
(B) -(C)
| / |
| / |
| / |
(D)- (E)
\ ;
\ ;
\ ;
(F)
/ \
/ \
/ \
(G) (H)
Figure 1: Sample topology
Consider the following topology for the subsequent description:
Non-Storing MOP
| D ======== B ======== A ======== (Root)
| ---------------DAO------------>
| <-----------DAO-ACK------------
|
V
time
Figure 2: NS-MOP DAO/DAO-ACK handling
Storing MOP
| D ======== B ======== A ======== (Root)
| ---DAO--->
| <-DAO-ACK-
| ---DAO--->
| <-DAO-ACK-
| ---DAO--->
| <-DAO-ACK-
V
time
Figure 3: Storing MOP DAO/DAO-ACK handling
Nodes need to know whether the end to end path till the Root has been established before they can initiate application traffic. In case of NS-MOP, the DAO is addressed to the Root from the Target node and the Root sends DAO-ACK directly addressed back to the target node. Thus in case of NS-MOP, the node can make use of this DAO-ACK as an indication whether the necessary routes have been installed. However, in case of Storing MOP, the DAO/DAO-ACK signalling happens at every hop.
Consider Figure 1, when node D sends a DAO, the node B receives the DAO and instantly sends back DAO-ACK. Node B then subsequently generates the DAO with Target as Node D and sends it to node A. The DAO with Target as Node D may take time (since the DAO is scheduled with DAO_DELAY timer by every node) to finally reach the Root at which point the end to end path is established. There is no way for node D to know when the end to end path is established. This information is needed for node D to initiate its application traffic. Initiating application traffic prior to this might almost certainly lead to application packet retries causing congestion in the network.
It is possible that the intermediate 6LR goes down while attempting to generate DAO on behalf of the target node. In this case, the target node has no way of knowing to retry the DAO, in which case the route installation may not happen until the target node's DAO lifetime expires.
Consider Figure 1, assume that node A was generating DAO with Target node D and sending it to Root. Node A reboots before attempting to send DAO to Root. Node A has already sent DAO-ACK downstream to node B. In this case, the target node D is not aware that sending DAO has failed somewhere upstream. Note that as per RFC6550 upstream DAO is scheduled based on DAO_DELAY but DAO_ACK is sent instantaneously on DAO reception from downstream node.
Following are the requirements:
The draft defines a way for the RPL Root to send the DAO-ACK back directly addressed to the Target node. The Target node can receive the DAO-ACK directly thus getting an indication that the end to end path till the Root has been successfully established.
The Target node indicates that it wishes to receive DAO-ACK directly from Root by setting the newly defined 'K' flag in Transit Information Option.
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 = 0x06 | Option Length |E|I|K| Flags | Path Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Sequence | Path Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Updated Transit Information Option (New K flag added)
On receiving a DAO with Transit Information Option with 'K' flag set, the Root MUST respond with a DAO-ACK immediately to the address extracted from the corresponding Target Option.
The DAO-ACK MUST contain the Transit Information Option with parameters copied from the DAO's Transit Information Option based on which this DAO-ACK was generated. The PathSequence in the Transit Information Option helps the Target node to identify for which DAO it generated it has received the DAO-ACK. The DAOSequence in the base DAO object is ignored by the Target node.
IANA is requested to allocate bit 2 from the Transit Information Option Flags registry for the 'K' flag (Section 5.1).
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 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, DOI 10.17487/RFC6550, March 2012. |
| [Perlman83] | Perlman, R., "Fault-Tolerant Broadcast of Routing Information", North-Holland Computer Networks, Vol.7, December 1983. |