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 Abstract This document explains problems with DAO-ACK handling in RPL Storing MOP and provides updates to RFC6550 to solve those problems. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 22, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Jadhav Expires October 22, 2020 [Page 1] Internet-Draft Storing Root Ack April 2020 Table of Contents 1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Requirements Language and Terminology . . . . . . . . . . 3 3. Problems with DAO-ACK in Storing MOP . . . . . . . . . . . . 3 3.1. End to End Path Establishment Indication . . . . . . . . 4 3.2. Target node unaware if it needs to retry the DAO . . . . 5 4. Requirements for DAO-ACK handling in Storing MOP . . . . . . 5 5. DAO-ACK from Root . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Transit Information Option update in DAO message . . . . 6 5.2. Root sends DAO-ACK addressed to Target . . . . . . . . . 6 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 9.1. Normative References . . . . . . . . . . . . . . . . . . 6 9.2. Informative References . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Motivation 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. 2. Introduction 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 Jadhav Expires October 22, 2020 [Page 2] Internet-Draft Storing Root Ack April 2020 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. 2.1. Requirements Language and Terminology 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 [RFC2119]. 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]. 3. Problems with DAO-ACK in Storing MOP Consider the following topology for the subsequent description: (Root) | | | (A) / \ / \ / \ (B) -(C) | / | | / | | / | (D)- (E) \ ; \ ; \ ; (F) / \ / \ / \ (G) (H) Figure 1: Sample topology Jadhav Expires October 22, 2020 [Page 3] Internet-Draft Storing Root Ack April 2020 3.1. End to End Path Establishment Indication 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. 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 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. Jadhav Expires October 22, 2020 [Page 4] Internet-Draft Storing Root Ack April 2020 3.2. Target node unaware if it needs to retry the DAO 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. 4. Requirements for DAO-ACK handling in Storing MOP Following are the requirements: Indicate end to end path establishment The Target node must know when to initiate the application traffic based on end to end path establishment. Handle multiple targets in DAOs A DAO message may contain multiple Target Options. The DAO-ACK mechanism must handle multiple targets in DAO. Handle DAOs with address prefix RPL DAO Target Option may contain an address prefix i.e., not the full address. Provide suitable way for target node to retry The Target node must have a way to know and retry the DAO in case the DAO transmission fails enroute. Backward compatible with current DAO-ACK The current per hop DAO-ACK must function as it is. Legacy nodes should be able to operate without any changes. 5. DAO-ACK from Root 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. Jadhav Expires October 22, 2020 [Page 5] Internet-Draft Storing Root Ack April 2020 5.1. Transit Information Option update in DAO message 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) 5.2. Root sends DAO-ACK addressed to Target 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. 6. Acknowledgements 7. IANA Considerations IANA is requested to allocate bit 2 from the Transit Information Option Flags registry for the 'K' flag (Section 5.1). 8. Security Considerations 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Jadhav Expires October 22, 2020 [Page 6] Internet-Draft Storing Root Ack April 2020 [RFC6550] Winter, T., Ed., Thubert, P., Ed., 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, . 9.2. Informative References [Perlman83] Perlman, R., "Fault-Tolerant Broadcast of Routing Information", North-Holland Computer Networks, Vol.7, December 1983. Author's Address Rahul Arvind Jadhav (editor) Huawei Whitefield, Bangalore, Karnataka 560037 India Email: rahul.ietf@gmail.com Jadhav Expires October 22, 2020 [Page 7]