[6tisch] answers Ralph comments for minimal
Xavi Vilajosana Guillen <xvilajosana@uoc.edu> Fri, 20 January 2017 12:17 UTC
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From: Xavi Vilajosana Guillen <xvilajosana@uoc.edu>
Date: Fri, 20 Jan 2017 13:17:04 +0100
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Subject: [6tisch] answers Ralph comments for minimal
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Dear Ralph, all, here answers of your comments after reviewing the minimal draft and publishing v18. 1 ) MOST IMPORTANT COMMENT Most importantly, this document needs to explain its intended use. From the Abstract: "This document describes a minimal mode of operation for a 6TiSCH Network." Exactly what is a "minimal mode of operation"? Is the document intended to describe: a starting point for design and deployment of a "6TiSCH network"? 2) a baseline set of required protocols and modes of operation, which will be extended by future specifications (as suggested in section 1)? 3) initial behavior that will guarantee out of the box interoperability among devices from, say, different vendors? 4) something else? As an example, if I were reading the document for point 1, I can see the value of pointing out, as in this rev of the document, various operational choices. However, if I were reading the document for point 3, I don't see how interoperability can be guaranteed given the variety of specific choices a vendor might make in the production of a device. RESOLUTION: Introduction and abstract have been updated following Brian and your comments. We have tried to clarify what is the inteded use of the document as well as indicate what a minimal mode of operation is. E.g., Abstract: This document describes a minimal mode of operation for a 6TiSCH Network. A minimal mode of operation is a baseline set of protocols, recommended configurations and modes of operation sufficient to enable a 6TiSCH functional network. 6TiSCH provides IPv6 connectivity over a Time Synchronized Channel Hopping (TSCH) mesh composed of IEEE Std 802.15.4 TSCH links. This minimal mode uses a collection of protocols with the respective configurations, including the 6LoWPAN framework, enabling interoperable IPv6 connectivity over IEEE Std 802.15.4 TSCH. This minimal configuration provides the necessary bandwidth for network and security bootstrap and defines the proper link between the IETF protocols that interface to the IEEE Std 802.15.4 TSCH. This minimal mode of operation should be implemented by all 6TiSCH compliant devices. Introduction A 6TiSCH Network provides IPv6 connectivity [RFC2460] over a Time Synchronized Channel Hopping (TSCH) mesh [RFC7554] composed of IEEE Std 802.15.4 TSCH links [IEEE802154-2015]. IPv6 connectivity is obtained by the use of the 6LoWPAN framework ([RFC4944], [RFC6282], [RFC8025],[I-D.ietf-6lo-routing-dispatch] and [RFC6775]), RPL [RFC6550], and its Objective Function 0 (OF0) [RFC6552]. This specification defines operational parameters and procedures for a minimal mode of operation to build a 6TiSCH Network. Any 6TiSCH complaint device SHOULD implement this mode of operation. This operational parameters configuration provides the necessary bandwidth for nodes to bootstrap the network. The bootstrap process includes initial network configuration and security bootstrap. In this specification, the 802.15.4 TSCH mode, the 6LoWPAN framework, RPL [RFC6550], and its Objective Function 0 (OF0) [RFC6552], are used unmodified. Parameters and particular operations of TSCH are specified to guarantee interoperability between nodes in a 6TiSCH Network. RPL is specified to provide the framework for time synchronization in an 802.15.4 TSCH network. The specifics for interoperable interaction between RPL and TSCH are described. In a 6TiSCH network, nodes follow a communication schedule as per 802.15.4 TSCH. In it, nodes learn the schedule of the network when joining. When following this specification, the learned schedule is the same for all nodes and does not change over time. Future specifications may define mechanisms for dynamically managing the communication schedule. Dynamic scheduling solutions are out of scope of this document. IPv6 addressing and compression are achieved by the 6LoWPAN framework. The framework includes [RFC4944], [RFC6282], [RFC8025], the 6LoWPAN Routing Header dispatch [I-D.ietf-6lo-routing-dispatch] for addressing and header compression, and [RFC6775] for duplicate address detection (DAD) and address resolution. More advanced work is expected in the future to complement the Minimal Configuration with dynamic operations that can adapt the schedule to the needs of the traffic at run time. 2) MAJOR ISSUE 1 If this document is concerned with "IPv6 connectivity", why is there no mention of address selection, prefix advertisement, use of ND, etc.? Similarly, why is there no advice about 6LoWPAN compression modes? DISCUSSION This document’s main goal is to explain how to use the IETF LLN upper stack on top of IEEE802.15.4 TSCH We treat the IETF LLN upper stack as a functional set of standards, and (now) list them: The framework includes <xref target="RFC4944"/>, <xref target="RFC6282"/>, <xref target="RFC8025"/>, the 6LoWPAN Routing Header dispatch <xref target="I-D.ietf-6lo-routing-dispatch"/> for addressing and header compression, and <xref target="RFC6775"/> for duplicate address detection (DAD) and address resolution. What we are explicitly trying to avoid discussion such as “here is how you use 6LoWPAN and RPL together”. Not that such discussions aren’t interesting (sidenote: should the IETF/a WG/the IoT directorate produce such docs?), they are, but the goal of this document is much more modest. We absolutely understand the point of your comments, but need guidance. For example, about “there no advice about 6LoWPAN compression modes”, what could we put? Are you looking for text like “SAC=1 SAM=1”? But mandating something like thing would go against RFC6282’s MUSTs? RESOLUTION: We clarified the use of the 6lowpan framework in the abstract and introduction, but we have not specified any particular compression mode (for example). We understand that the compression mode used in 6lowpan does not affect the underlying TSCH configuration and vice-versa. In opposition, for example, the Join Metric when RPL is used impacts the mapping of the L2 overlay and the L3 overlay and hence we define this mapping. 3) MAJOR ISSUE 2 What is the relationship between the recommended configuration in this document and protocol semantics that provide the same configuration parameters? For example, there are operational parameters provided in EBs that are also specified in this document. How does a node choose which parameters to use? RESOLUTION: this document defines a set of parameters so the network can be bootstrap. The aim of the defined parameters is to provide the simplest configuration so a network can be form. Despite nodes learn some of the configurations, we need to guide vendors to a configuration that supports the network bootstrap and adapts the underlying MAC layer to support higher layer protocols such as RPL or security bootstrap mechanisms. 4) MAJOR ISSUE 3 A document such as this one that specifies the use of protocols specified elsewhere should use pointers to the other specifications wherever possible and should avoid repeating a description of those other protocols. The danger is getting the specifications out of sync or describing them in a misleading or incorrect way. In the case of this document, in my opinion there is text that repeats the description of aspects of the IEEE 802.15.4 and RPL specifications that should be elided. For example, section 4.2 and 4.3 describe details of cell transmission that appear to be redundant relative to the IEEE 802.15.4 specification. RESOLUTION: This point was raised a couple of revisions ago, which has triggered an editorial overhaul to (1) avoid copy-pasting potentially copyrighted text from other standards, in particular IEEE802.15.4 and (2) avoid getting out-of-sync exactly as you describe In our opinion, this exercise has been done. To answer your specific comments: About Section 4.2: IEEE802.15.4 only states this each cell can be assigned bits/flags This spec specifies the bits to set (TX, RX, Shared, Timekeeping) About Section 4.3: IEEE802.15.4 doesn’t specify a number of retries We say it should be 3 To be crystal clear (the questions has been raised multiple times), we indicate that these are retransmissions 5) SPECIFIC ISSUE 1 >From the Introduction: "a node learns the schedule of the network when joining, the schedule is static and the same for all nodes". This statement of operational behavior seems important. How does a node learn the schedule? Is it up to the network administrator to configure all the nodes so that the schedule is "static and the same for all nodes"? What happens if different nodes advertise different schedules (or other operational parameters) in EBs? Is that situation considered an administrative error? DISCUSSION We agree this is important A node doesn’t contain a hardcoded schedule; it learns every thing is needs by reading the contents of an EB If only this specification is implemented, nodes advertise the same schedule. Future specifications which can be used alongside this specification MAY cause different nodes to generate EBs with a different contents. This situation is therefore not considered and administrative error. RESOLUTION OLD When following this specification, a node learns the schedule of the network when joining, the schedule is static and the same for all nodes. NEW When following this specification, the learnt schedule is the same for all nodes and does not change over time. Future specifications may define mechanisms for dynamically managing the communication schedule. Dynamic scheduling solutions are out of scope of this document. 6) SPECIFIC ISSUE 2 In a related issue, I found the text in section 4.1 about future use of unscheduled cells confusing, after the assertion that "There is only a single scheduled cell in the slotframe“ All remaining cells in the slotframe are unscheduled. Dynamic scheduling solutions MAY be defined in the future which schedule those cells. One example is the 6top Protocol (6P) [I-D.ietf-6tisch-6top-protocol]. Dynamic scheduling solutions are out of scope of this document. Details about the usage of the non- scheduled cells are out of scope of this document. In particular, this specification does not make any restriction on the Link Option bitmap associated with those dynamically scheduled cells (i.e. they can be "Hard" or "Soft" cells, see [I-D.ietf-6tisch-terminology]). RESOLUTION: We have changed the text: OLD See above NEW All remaining cells in the slotframe are unscheduled. Dynamic scheduling solutions MAY be defined in the future which schedule those cells. One example is the 6top Protocol (6P) <xref target="I-D.ietf-6tisch-6top-protocol"/>. Dynamic scheduling solutions are out of scope of this document. 7) SPECIFIC ISSUE 3 Similarly, in section 4.2, if the schedule is static, why is it necessary to state that "The scheduled cell... cannot be moved or relocated by any dynamic scheduling mechanism"? The scheduled cell is a "Hard cell" [I-D.ietf-6tisch-terminology], i.e. it cannot be moved or relocated by any dynamic scheduling mechanism. DISCUSSION Agreed, this is a “historical leftover” when this text actively discussed hard and soft cells. RESOLUTION Remove the paragraph 8) SPECIFIC ISSUE 4 Section 4.4 seems to me to be a rewording of the IEEE 802.15.4 specification. Is there some specific reason that the text is included here? DISCUSSION Agreed, this is a repeat of the IEEE802.15.4 standard. RESOLUTION OLD Entire 4.4 section NEW Keep section 4.4, but containing only the following sentence: Per <xref target="tab_recommended_tsch_settings"/>, the RECOMMENDED timeslot template is the default one (macTimeslotTemplateId=0) defined in <xref target="IEEE802154-2015"/>. 9) SPECIFIC ISSUE 6 In section 5.2, what does the word "supported" mean? Which mode of operation should be implemented according to this specification? DISCUSSION Agreed, this terminology is not the right one. “implemented” is RESOLUTION OLD When RPL is used, nodes MUST support the non-storing (<xref target="RFC6550"/> Section 9.7) mode of operation. The storing (<xref target="RFC6550"/> Section 9.8) mode of operation SHOULD be supported by nodes with enough capabilities. Nodes not supporting RPL MUST join as leaf nodes. NEW When RPL is used, nodes MUST implement the non-storing (<xref target="RFC6550"/> Section 9.7) mode of operation. The storing (<xref target="RFC6550"/> Section 9.8) mode of operation SHOULD be implemented by nodes with enough capabilities. Nodes not implementing RPL MUST join as leaf nodes. 10) SPECIFIC ISSUE 7 I don't understand the relation between sections 6.2, "Initial Time Source Neighbor Selection", 6.4, "Time Source Neighbor Selection" and section 6.5, "Hysteresis". In particular, section 6.4 seems to give only a requirement that a node must have a selected time source neighbor, but gives no advice on how the node makes the selection. DISCUSSION This is indeed confusing, as “Initial Time Source Neighbor Selection” and “Time Source Neighbor Selection” related to a very similar topic The node’s time source neighbor must be part of its RPL parent set, i.e. RPL gives you a time source neighbor selection process “for free” RESOLUTION Section "Initial Time SOurce Neighbor Selection" has been merged with "Time Source Neighbor Selection. Hysteresis is kept in a separate section as deals with time source parent stability. 11) EDITORIAL ISSUE 1 In the Introduction, why is RPL called out specifically as a "natural choice" without any justification? Why not just give the explicit explanation that RPL is specified to provide the framework for IEEE802.15.4 time sync? DISCUSSION Good point, thanks RESOLUTION OLD RPL is a natural choice for routing on top of IEEE802.15.4 TSCH, and the specifics for interoperable interaction between RPL and TSCH are described. NEW RPL is specified to provide the framework for time synchronization in IEEE802.15.4 TSCH network. The specifics for interoperable interaction between RPL and TSCH are described. 12) EDITORIAL ISSUE 2 Check for consistency of parameter names and protocol fields; e.g., I assume macTsRxAckDelay and tsRxAckDelay refer to the same information element. RESOLUTION Using the following: macTsCCAOffset macTsCCA macTsTxOffset macTsRxOffset macTsRxAckDelay macTsTxAckDelay macTsRxWait macTsAckWait macTsRxTx macTsMaxAck macTsMaxTx macTsTimeslotLength 13) EDITORIAL ISSUE 3 In section 4.1, this instance of "MAY" is not an RFC 2119 usage: "Dynamic scheduling solutions MAY be defined in the future [...]" DISCUSSION Agreed, thanks RESOLUTION OLD Dynamic scheduling solutions MAY be defined in the future which schedule those cells. NEW Dynamic scheduling solutions may be defined in the future which schedule those cells. 14) EDITORIAL ISSUE 4 In section 4.5.1, the first and third sentences seem to be redundant. DISCUSSION Agreed. A similar change was also suggested by Brian. RESOLUTION OLD The IEEE802.15.4 header of BEACON, DATA and ACKNOWLEDGMENT frames SHOULD include the Source Address field and the Destination Address field. The Frame Version field SHOULD be set to 0b10 (Frame Version 2). The IEEE802.15.4 header SHOULD include Source Address field and the Destination Address field. The Sequence Number field MAY be elided. NEW The Frame Version field SHOULD be set to 0b10 (Frame Version 2). The Sequence Number field MAY be elided. -- Dr. Xavier Vilajosana Wireless Networks Lab *Internet Interdisciplinary Institute (IN3)Professor* (+34) 646 633 681 xvilajosana@uoc.edu <usuari@uoc.edu> http://xvilajosana.org http://wine.rdi.uoc.edu Parc Mediterrani de la Tecnologia Av Carl Friedrich Gauss 5, B3 Building 08860 Castelldefels (Barcelona). Catalonia. Spain [image: Universitat Oberta de Catalunya]
- [6tisch] answers Ralph comments for minimal Xavi Vilajosana Guillen