[6tisch] Thomas' review of draft-richardson-6tisch-enrollment-enhanced-beacon-00

[Thomas Watteyne <[e-mail]>]

 co-chair hat off.

The draft is easy to understand and to follow.

The main recommendation I have is that the rationale/context is VERY poorly
introduced. The abstract and intro discuss the fact that a node cannot send
many EBs, but the actual contents of the draft is something COMPLETELY
different, which is pretty much orthogonal to the statement inf the
abstract/intro. This spec simply defines an IE that goes into EBs and which
contains a number of priorities. AFAICT, that is information that is used
for a pledge to make an informed decision about (1) which LLN to join and
(2) which proxy to pick.

I imagine the use case is a super dense network in which a pledge hears may
different LLNs. Without this spec, some other LLN selection mechanisms can
be imagined (PANID being the obvious one). This spec prevents the pledge
from trying every LLN until it find the best one.

None of that discussion is anywhere in the draft. My recommendatoins are:

   - give that IE a name
   - rename the draft the name you just chose
   - rewrite the intro entirely, drop all the talk about the slow rate of
   EBs, and justify what it is you're trying to define.
   - in particular, describe in plain English how a node picks the proxy,
   pan and rank priorities, and what rule a pledge has when selecting a JP
   based on that info. Rewriting the intro will already be a good step forward
   in proving the relevance of the work, but I'm anticipating a lot of
   pushback if the logictics behind picking these numbers aren't clearly
   stated.

Bellow is the draft contents with my annotations, starting with "TW>"

Thomas

===

6lo Working Group                                             D. Dujovne
Internet-Draft                                Universidad Diego Portales
Intended status: Informational                             M. Richardson
Expires: August 12, 2018                        Sandelman Software Works
                                                       February 08, 2018

TW> looks like Diego is editor (from the authors addresses section)
TW> I'm surprised his name at the top doesn't contain the usual ", Ed."
TW> annotation.
TW> also, wouldn't this mean a renaming of the doc to
TW> draft-dujovne-6tisch-enrollment-enhanced-beacon-00?

  IEEE802.15.4 Informational Element encapsulation of 6tisch Join and
                         Enrollment Information
         draft-richardson-6tisch-enrollment-enhanced-beacon-00

Abstract

   In TSCH mode of IEEE802.15.4,
TW> In *the* TSCH
   as described by [RFC8180],
   opportunities for broadcasts are limited to specific times and
   specific channels.
TW> I don't fully understand this sentence. What makes opportunities
TW> limited? The number of broadcast timeslots?
   Nodes in a TSCH network typically frequently send
   Enhanced Beacon (EB) frames to announce the presence of the network.
   This document provides a mechanism by which small details critical
   for new nodes (pledges) and long sleeping nodes may be carried within
   the Enhanced Beacon.
TW> I would simplify this abstract to basically a single sentence
TW> "This document defines the X and Y that are carried in Enhanced
TW> Beacon (EB) frames to allow Z."

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 http://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 August 12, 2018.

Copyright Notice

   Copyright (c) 2018 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
   (http://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



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Layer-2 Synchronization . . . . . . . . . . . . . . . . .   3
     1.3.  Layer-3 synchronization IPv6 Router solicitations and
           advertisements  . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Definition . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Protocol Example  . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Change history . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   [RFC7554] describes the use of the time-slotted channel hopping
   (TSCH) mode of [ieee802154].  As further details in [RFC8180], an
   Enhanced Beacon is transmitted during a slot designated a broadcast
   slot.

   EDNOTE: Explain why broadcasts are rare, and why we need them.  What
   the Enhanced Beacon is, and what Information Elements are, and how
   the IETF has a subtype for that area.  Explain what kind of things
   could be placed in Information Elements, how big they could be, and
   how they could be compressed.
TW> also talk about encryption of IEs.

1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant STuPiD
   implementations.
TW> STuPiD implementations? I need to know more about that!







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1.2.  Layer-2 Synchronization

   As explained in section 6 of [RFC8180], the Enhanced Beacon has a
   number of purposes:
TW> "has a number of purposes" is too vague IMO. I suggest to rephrase
TW> to something like:
TW> the Enhanced Beacon allows a pledge to detect the presence of the
TW> network, and learn enough about the configurations of the network
TW> to start joining the network, if it wishes to do so.
   synchronization of ASN and Join Metric, timeslot
   template identifier, the channel hopping sequence identifier, TSCH
   SlotFrame and Link IE.

   The Enhanced Beacon (EB) is used by nodes already part of a TSCH
   network to annouce its existance.
TW> typo
   Receiving an EB allows a Joining
   Node (pledge)
TW> the term JN disappeared, just use pledge throughout
   to learn about the network and synchronize to it.  The
   EB may also be used as a means for a node already part of the network
   to re-synchronize [RFC7554].

   There are a limited number of timeslots designated as a broadcast
   slot by each router.  These slots are rare, and with 10ms slots, with
   a slot-frame length of 100,
TW> I would use the example slotframe length from RFC8180, which is 101
   there may be only 1 slot/s for the
   beacon.

1.3.  Layer-3 synchronization IPv6 Router solicitations and
      advertisements

   At layer 3, [RFC2461] defines a mechanism by which nodes learn about
   routers by listening for multicasted
TW> multicasted -> multicast ?
   Router Advertisements (RA).  If
   no RA is heard within a set time, then a Router Solicitation (RS) may
   be multicast, to which an RA will be received, usually unicast.

   Although [RFC6775] reduces the amount of multicast
TW> multicast*s*
   necessary to do
   address resolution via Neighbor Solicitation messages, it still
   requires multicast of either RAs or RS.  This is an expensive
   operation for two reasons: there are few multicast timeslots for
   unsolicited RAs; if a pledge node does not hear an RA, and decides to
   send a RS (consuming a broadcast aloha slot with unencrypted
   traffic), many unicast RS may be sent in response.

   This is a particularly acute issue for the join process for the
   following reasons:

   1.  use of a multicast slot by even a non-malicious unauthenticated
       node for a Router Solicitation may overwhelm that time slot.

   2.  it may require many seconds of on-time before a new pledge hears
       a Router Soliciation
TW> typo
       that it can use.

   3.  a new pledge may listen to many Enhanced Beacons before it can
       pick an appropriate network and/or closest Join Assistant
TW> JA -> JP
       attach to.  If it must listen for a
TW> an
       RS as well as find the
       Enhanced Beacon, then the process may take a very long time.




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2.  Protocol Definition

   [RFC8137] creates a registry for new IETF IE subtypes.  This document
   allocates a new subtype TBD-XXX.

   This document documents
TW> document documents reads strange -> defines?
   a new IE subtype structure is
TW> remove is"
   as follows.  As
   explained in [RFC8137] the length of the Sub-Type Content can be
   calculated from the container, so no length information is necessary.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   TBD-XXX     |R|pan priority |proxy prio.  | rank prio.      |
   +-+-+-+-+-+-+-+-+-+-------------+-------------+-----------------+
   |                                                               |
   +                                                               +
   |                           network ID                          |
   +                                                               +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

TW> please put the description of the fields in the same order as the
TW> fields themselves.

   proxy priority  the proxy prority value contains a number from 0 to
      0x7f.  Lower numbers are considered to be a higher preference.  A
      priority of 0x7f indicates that the announcer should never be
      considered as a viable enrollment proxy.  Lower value indicates
      willing to act as a Join Proxy as described in
      [I-D.ietf-6tisch-minimal-security].  Only unenrolled
TW> unenrolled -> un-enrolled (please look for the same everywhere)
      pledges look
      at this value.
TW> can you give us a hint at how this proxy priority would be managed?
TW> what's the rationale behind its use?
TW> for sure, this should be described much earlier on in this spec.

   pan priority  the pan priority is a value set by the 6LBR to indicate
      the relative priority of this LLN compared to those with different
      PANIDs.  This value may be used as part of the enrollment
      priority, but typically is used by devices which have already
      enrolled, and need to determine which PAN to pick.
TW> what is your definition of enrollment? AFAICT, a pledge is not enrolled
TW> picks an LLN, and joins that. I don't see a case where a node is
TW> enrolled but not joined to an LLN.
      Unenrolled
      pledges MAY consider this value when selecting a PAN to join.
      Enrolled devices MAY consider this value when looking for an
      elegible
TW> eligible
      parent device.

   rank priority  the rank priority is set by the 6LR which sent the
      beacon and is an indication of how willing this 6LR is to serve as
      an RPL parent within a particular network ID.  This is a local
      value to be determined in other work.  It might be calculated from
      RPL rank, and it may include some modifications based upon current
      number of children, or number of neighbor cache entries available.
      This value MUST be ignored by pledges, it is for enrolled devices
      only.
TW> I fail to understand the difference between proxy priority and
TW> rank priority. I understand the fact that each deals with a different
TW> part of the stack (proxying vs routing), but in the end, a pledge
TW> receives an EB which says "pick me", or "don't pick me". What happens
TW> if a pledge receives an EB with a low proxy priority but no
TW> rank priorioty? Again, the intro needs to put more context and
TW> justification of why this is an important problem to solve.




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   R  the Router Advertisement flag is set if the sending node will act
      as a Router for host-only nodes that need addressing via unicast
      Router Solicitation messages.
TW> please specify what a sender means when not setting that flag.

   network ID  this is an opaque 16-byte identifier that uniquely
      identifies this network, potentially among many networks that are
      operating in the same frequencies in overlapping physical space.

   In a 6tisch
TW> 6TiSCH (casing)
   network, where RPL is used as the mesh routing protocol,
   the network ID can be constructed from a SHA256 hash of the prefix
   (/64) of the network.  That is just a suggestion for a default value.
   In some LLNs where multiple PANIDs may lead to the same management
   device (the JRC), then a common value that is the same across all
   PANs MUST be configured.

2.1.  Protocol Example

   Here will be three examples of processing.
TW> please put a clear "TODO" statement. I took me a second to parse what
TW> you meant. If you're saying 3 examples, justify why 3, and ideally
TW> which ones. If 3 is just a magic value, just put "TODO".

3.  Security Considerations

   All of the contents of this Information Element are sent in the
   clear.  The containing Enhanced Beacon is not encrypted.

   The Enhanced Beagon
TW> beacon
   is authenticated at the layer-2 level using
   802.15.4 mechanisms using the network-wide keying material.  Nodes
   which are enrolled will have the network-wide keying material and can
   validate the beacon.

   Pledges which have not yet enrolled are unable to authenticate the
   beacons.
TW> you are describing the "mechanics" of which, easy to follow.
TW> but I was expected some form of conclusion about security, e.g.
TW> "this is no problem because X", or "this makes the system
TW> vulnerable to Y, but Z".

4.  Privacy Considerations

   The use of a network ID may reveal information about the network.
   The use of a SHA256 hash of the DODAGID, rather than using the
   DODAGID directly provides some cover
TW> "for"? looks like a word is missing.
   the addresses used within the
   network.  The DODAGID is usually the IPv6 address of the root of the
   RPL mesh.

   An interloper with a radio sniffer would be able to use the network
   ID to map out the extend of the mesh network.

5.  IANA Considerations

   Allocate a new number TBD-XXX from Registry IETF IE Sub-type ID.
   This entry should be called 6tisch-Join-Info.




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6.  Acknowledgements
TW> Acknowledgment

   Thomas Watteyne provided extensive editorial comments on the
   document.
TW> did I? it's a -00.

7.  References

7.1.  Normative References

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-13 (work
              in progress), November 2017.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-04 (work in progress), October
              2017.

   [ieee802154]
              IEEE Standard, ., "802.15.4-2015 - IEEE Standard for Low-
              Rate Wireless Personal Area Networks (WPANs)", 2015,
              <http://standards.ieee.org/findstds/
              standard/802.15.4-2015.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              DOI 10.17487/RFC2461, December 1998, <https://www.rfc-
              editor.org/info/rfc2461>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015, <https://www.rfc-
              editor.org/info/rfc7554>.




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   [RFC8137]  Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
              2017, <https://www.rfc-editor.org/info/rfc8137>.

7.2.  Informative References

   [I-D.ietf-6tisch-dtsecurity-secure-join]
              Richardson, M., "6tisch Secure Join protocol", draft-ietf-
              6tisch-dtsecurity-secure-join-01 (work in progress),
              February 2017.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.

Appendix A.  Change history
TW> please add a marker (e.g. "[TEMPORARY]") to indicate this section
TW> is not going to be there when the document is finished.

   The extension was originally for the use of Pledges only during the
   enrollment/join process.  Additional information was desired for
   nodes which have already enrolled in order to aid in the joining
   (selecting of a parent) of an RPL DAG.  The term "join" was realized
   to be ambiguous, meaning different things to different groups, and so
   the activity where the pledge finds a "Join Proxy" has been named
   "enrollment"
TW> the information is this paragraph should be at the very top of the
TW> spec...

   This is an evolution of an earlier proposal which provided for
   storing an entire IPv6 Router Adverisement in an Informational
   Element.  It was deemed too general a solution, possibly subject to
   mis-use.  This proposal restricts the use to just the key pieces of
   information required.

Authors' Addresses

   Diego Dujovne (editor)
   Universidad Diego Portales
   Escuela de Informatica y Telecomunicaciones, Av. Ejercito 441
   Santiago, Region Metropolitana
   Chile

   Phone: [phone]
   Email: [e-mail]


   Michael Richardson
   Sandelman Software Works

   Email: [e-mail]



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