[6lo] Benjamin Kaduk's Discuss on draft-ietf-6lo-plc-06: (with DISCUSS and COMMENT)

Benjamin Kaduk via Datatracker <noreply@ietf.org> Wed, 11 August 2021 17:13 UTC

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Subject: [6lo] Benjamin Kaduk's Discuss on draft-ietf-6lo-plc-06: (with DISCUSS and COMMENT)
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Benjamin Kaduk has entered the following ballot position for
draft-ietf-6lo-plc-06: Discuss

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----------------------------------------------------------------------
DISCUSS:
----------------------------------------------------------------------

Further details in the COMMENT, but can we briefly discuss the apparent
requirement for the PANID/NID to have a couple bits set to zero (the
ones that would be U/L and Individual/Group in the resulting IID)?  It
seems like (but is not entirely clear to me) this is a new requirement
on the layer-2 behavior that is being imposed by the IPv6 adaptation
layer, and in particular that this is setting up a scenario where
certain existing layer-2 deployments would be unable to utilize the IPv6
adaptation layer, which would be a very surprising behavior for an IETF
Proposed Standard.  What alternatives were explored and rejected before
settling on this approach that introduces new limitations on the
underlying PLC deployments?

I mention in a few places in the COMMENT scenarios where we pull in part
of the functionality from RFC 6282 and RFC 4944, e.g., the IP header
compression scheme and the fragmentation format.  It seems to me that
the intent is that our payload always use the RFC 4944 "dispatch" scheme
and that we only use a subset of (and only sometimes?) the particular
functionality that RFC 4944/6282 can dispatch to.  But the current text
doesn't mention the dispatch behavior at all, so it's hard for me to be
certain that my understanding is correct.  It seems that some more
explicit treatment in the document of how what we are specifying
interacts with/uses the RFC 4944 dispatch layer would be important in
order for someone to be able to implement from this document.

I support Roman and Éric's Discusses.


----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

Section 4.1

   Since the derived Interface ID is not global, the "Universal/Local"
   (U/L) bit (7th bit) and the Individual/Group bit (8th bit) MUST both
   be set to zero.  In order to avoid any ambiguity in the derived
   Interface ID, these two bits MUST NOT be used to generate the PANID
   (for IEEE 1901.2 and ITU-T G.9903) or NID (for IEEE 1901.1).  In
   other words, the PANID or NID MUST always be chosen so that these
   bits are zeros.

Is this a new requirement on the PANID/NID not already imposed by the
underlying specifications?  If so, it seems that it presents a
limitation on the ability of already deployed PLC networks to adopt this
IPv6 adaptation layer.

   For privacy reasons, the IID derived from the MAC address SHOULD only
   be used for link-local address configuration.  A PLC host SHOULD use
   the IID derived from the link-layer short address to configure the
   IPv6 address used for communication with the public network;
   otherwise, the host's MAC address is exposed.  As per [RFC8065], when
   short addresses are used on PLC links, a shared secret key or version
   number from the Authoritative Border Router Option [RFC6775] can be
   used to improve the entropy of the hash input, thus the generated IID
   can be spread out to the full range of the IID address space while
   stateless address compression is still allowed.

The phrasing "derived from" is a little ambiguous to me, since it can
encompass procedures ranging from the "flip the U/L bit and append PLC
IID to the network's prefix" procedure to RFC 7217-style stable but
opaque IIDs that incorporate the MAC address into the pseudorandom
function's inputs.  Given the follow-up text about "host's MAC address
is exposed", it feels like this is implying more of the former
procedure.  Wouldn't the latter type of procedure be preferred, though
(as implied by the "hash input" in the last sentence)?  In particular,
the last sentence seems to imply that there is *always* a hash input,
which is at odds with the "former" interpretation that I present for
"derived from".  I'm not confident that I understand the intent of this
paragraph.

Section 4.3.1

   In order to avoid the possibility of duplicated IPv6 addresses, the
   value of the NID MUST be chosen so that the 7th and 8th bits of the
   first byte of the NID are both zero.

As above, it's not clear that the NID is something that this adaptation
layer can assert control over.

Section 4.3.2

   In order to avoid the possibility of duplicated IPv6 addresses, the
   value of the PAN ID MUST be chosen so that the 7th and 8th bits of
   the first byte of the PAN ID are both zero.

(likewise)

Section 4.5

   The compression of IPv6 datagrams within PLC MAC frames refers to
   [RFC6282], which updates [RFC4944].  Header compression as defined in
   [RFC6282] which specifies the compression format for IPv6 datagrams
   on top of IEEE 802.15.4, is the basis for IPv6 header compression in
   PLC.  For situations when PLC MAC MTU cannot support the 1280-octet
   IPv6 packet, headers MUST be compressed according to [RFC6282]
   encoding formats.

RFC 6282 refers to both a "Dispatch" value and the LOWPAN_IPHC header
compression encoding.  I strongly suggest clarifying whether both, or
just LOWPAN_IPHC, is used.

   For IEEE 1901.2 and G.9903, the IP header compression follows the
   instruction in [RFC6282].  However, additional adaptation MUST be
   considered for IEEE 1901.1 since it has a short address of 12 bits
   instead of 16 bits.  The only modification is the semantics of the
   "Source Address Mode" when set as "10" in the section 3.1 of
   [RFC6282], which is illustrated as following.

Is there anything useful to say about how carrying 12 vs 16 bits affects
byte alignment of the overal compressed message?  A quick survey of RFC
6282 finds many items that retain byte alignment, and I didn't actually
find anything that left the encoded bit stream in a non-aligned state.

   SAM: Source Address Mode:

I see that RFC 6282 also has procedures for Destination Address Mode
(DAM), including a scenario that involves conveying a 16-bit address
component.  Do we need to treat that DAM analogously to how we treat the
SAM here?  (This might also handle the byte alignment question from my
previous remark...)

Section 4.6

   In IEEE 1901.1 and IEEE 1901.2, the MAC layer supports payloads as
   big as 2031 octets and 1576 octets respectively.  However when the
   channel condition is noisy, it is possible to configure smaller MTU
   at the MAC layer.  If the configured MTU is smaller than 1280
   octects, the fragmentation and reassembly defined in [RFC4944] MUST
   be used.

Does this imply that implementing the IPv6 adaptation layer
fragmentation+reassembly logic is mandatory for implementations of IPv6
over IEEE 1901.1 and 1901.2, since the implementation might be
configured in a way that requires that support?  Please be clear about
what is required of implementations and in what circumstances.

Also, as above, please be clear about the interaction with the RFC 4944
dispatch layer.

Section 5

   node; PAN Devices are typically PLC meters and sensors.  The PANC
   also serves as the Routing Registrar for proxy registration and DAD
   procedures, making use of the updated registration procedures in
   [RFC8505].  IPv6 over PLC networks are built as tree, mesh or star

If the PANC always serves as the Routing Registrar (and thus the RFC
8505 procedures are always used), why do we allow for both RFC 6775 and
8505 DAD procedures up in §4.4?

Section 8

We should probably incorporate by reference the security considerations
of the documents whose technologies we are adopting.

One might hope that it goes without saying, but it's nonetheless
probably worth noting that the PANC, being in a position to observe all
traffic, is necessarily a trusted entity.

   Due to the high accessibility of power grid, PLC might be susceptible
   to eavesdropping within its communication coverage, e.g., one
   apartment tenant may have the chance to monitor the other smart
   meters in the same apartment building.  Thus link layer security
   mechanisms are designed in the PLC technologies mentioned in this
   document.

Key management for these security mechanisms will of course be quite
important.  IoT devices are notoriously vulnerable to physical attacks
and key extraction, so there may be something useful to say about the
importance of key management and what is exposed if the key material
available to a single device is compromised.

It's quite hard to make an evaluation of the actual security properties
provided by the link-layer mechanisms without access to the actual
specification documents for those technologies.  I'd actually seriously
consider adding another clause that "and additional end-to-end security
services can be used for sensitive traffic and as additional
protection against compromised PLC nodes" (or something in that general
vein).

Additionally, it's often the case that the link-layer security
mechanisms involve group-shared symmetric keys, so that a compromise of
even a single device puts the entire network, or a large chunk of the
network, at risk.  If this is the case for the PLC link layers, it seems
imperative to mention that risk in this document.

   Malicious PLC devices could paralyze the whole network via DOS
   attacks, e.g., keep joining and leaving the network frequently, or
   multicast routing messages containing fake metrics.  A device may

Is there potential for interfering with/corrupting legitimate traffic as
a DoS vector, as well?
   illegal users.  Mutual authentication of network and new device can
   be conducted during the onboarding process of the new device.
   Methods include protocols such as [RFC7925] (exchanging pre-installed
   certificates over DTLS) , [I-D.ietf-6tisch-minimal-security] (which
   uses pre-shared keys), and
   [I-D.ietf-6tisch-dtsecurity-zerotouch-join] (which uses IDevID and
   MASA service).  It is also possible to use EAP methods such as
   [I-D.ietf-emu-eap-noob] via transports like PANA [RFC5191].  No
   specific mechanism is specified by this document as an appropriate
   mechanism will depend upon deployment circumstances.

Would SZTP (RFC 8572) be applicable for these scenarios?
(Also, I would recognize "BRSKI" more than "IDevID and [a] MASA
service", though I don't know if I am the right population to be
sampling for readibility data.)

   scanning.  Schemes such as limited lease period in DHCPv6 [RFC3315],
   Cryptographically Generated Addresses (CGAs) [RFC3972], privacy
   extensions [RFC4941], Hash-Based Addresses (HBAs) [RFC5535], or
   semantically opaque addresses [RFC7217] SHOULD be considered to
   enhance the IID privacy.

"SHOULD be considered" is a fairly weak guidance; I would think that
"SHOULD be used" would be more consistent with the IETF consensus
position, while still leaving ample space for other behaviors.

Section 10.2

I would consider classifying RFC 4291 as normative.

NITS

   meters for electricity.  The inherent advantage of existing
   electricity infrastructure facilitates the expansion of PLC
   deployments, and moreover, a wide variety of accessible devices
   raises the potential demand of IPv6 for future applications.  This

"Advantage" typically implies a comparison with some other thing or
things as measured on a particular axis or axes.  While one might
presume that this refers to the advantages of using existing wires over
new wires in terms of cost and ease of deployment, it's probably worth
stating it more clearly.

Section 1

   century.  With the advantage of existing power grid, Power Line
   Communication (PLC) is a good candidate for supporting various
   service scenarios such as in houses and offices, in trains and

As above, what is "the advantage of existing power grid"?

Section 2

   PANC: PAN Coordinator, a coordinator which also acts as the primary
         controller of a PAN.

PAN is not marked as "well-known" at
https://www.rfc-editor.org/materials/abbrev.expansion.txt (in fact, is
not even defined there), and thus should get its own expansion.

Section 4.4

   information in the replied Neighbor Advertisements from the 6LR.  If
   DHCPv6 is used to assign addresses or the IPv6 address is derived
   from unique long or short link layer address, Duplicate Address
   Detection (DAD) MUST NOT be utilized.  Otherwise, the DAD MUST be
   performed at the 6LBR (as per [RFC6775]) or proxied by the routing
   registrar (as per [RFC8505]).  The registration status is feedbacked
   via the DAC or EDAC message from the 6LBR and the Neighbor
   Advertisement (NA) from the 6LR.

A few words on how the 6LR+6LBR must know whether 6775 or 8505 is in use
on the network, and thus there is no ambiguity about which entity is
performing DAD, might be helpful.

Section 4.5

   10:   12 bits.  The first 116 bits of the address are elided.The
         value of the first 64 bits is the link-local prefix padded with

spaces after the sentence break.

Section 5

   [RFC8505].  IPv6 over PLC networks are built as tree, mesh or star
   according to the use cases.  Generally, each PLC network has one

I think "as a tree, mesh or star topology"

   the size of PLC networks.  A simple use case is the smart home
   scenario where the ON/OFF state of air conditioning is controlled by
   the state of home lights (ON/OFF) and doors (OPEN/CLOSE).  AODV-RPL

Almost all the other examples in the document refer to PLC meters or
sensors (mostly meters), so the "smart home" scenario sticks out as
being rather different when only mentioned in passing like this.  I
don't question the conclusion, but the overall writing style of the
document might be improved if we introduced this scenario earlier on so
that it was a more continual theme.

   enables direct PAN device to PAN device communication, without being
   obliged to transmit frames through the PANC, which is a requirement
   often cited for AMI infrastructure.

The only earlier mention of AODV-RPL was in §3.4; we might consider
repeating the reference here in case the reader missed it the previous
time.

Section 6

   self-managed.  The software or firmware is flushed into the devices

s/flushed/flashed/?

   before deployment by the vendor or operator.  And during the
   deployment process, the devices are bootstrapped, and no extra
   configuration is needed to get the device connected to each other.

s/device/devices/

   gateway.  The recently-formed iotops WG in IETF is aming to design
   more features for the management of IOT networks.

s/aming/aiming/
Also, a reference to the WG's datatracker page might be worthwhile.

Section 8

   Malicious PLC devices could paralyze the whole network via DOS
   attacks, e.g., keep joining and leaving the network frequently, or
   multicast routing messages containing fake metrics.  A device may

I think s/multicast/sending/multicast/

   also join a wrong or even malicious network, exposing its data to
   illegal users.  Mutual authentication of network and new device can

Maybe "inadvertently join"?

   IP addresses may be used to track devices on the Internet; such
   devices can in turn be linked to individuals and their activities.

I think s/can in turn/can often in turn/.  There are some IoT devices
that are basically uncorrelated to individual humans.

   Cryptographically Generated Addresses (CGAs) [RFC3972], privacy
   extensions [RFC4941], Hash-Based Addresses (HBAs) [RFC5535], or

RFC 4941 has been obsoleted by RFC 8981.