Subject: [ippm] Benjamin Kaduk's Discuss on draft-ietf-ippm-ioam-data-12: (with DISCUSS and COMMENT)

From: Benjamin Kaduk via Datatracker <[email-masked]>
Date: Tue, 23 Mar 2021 21:43:33 -0700

Benjamin Kaduk has entered the following ballot position for
draft-ietf-ippm-ioam-data-12: Discuss

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

I think we need some greater clarity on the relationship between IOAM
"layers" and IOAM-Namespaces.  For example, in Section 4 there is a
principle of "Layering" that seems to indicate that different layers
operate entirely independently, such as might occur when traffic from
one operator that uses IOAM is conveyed in a tunnel over a different
operator's network and both operators use IOAM independently.  But in
Section 5.3 we seem to see some discussion that IOAM-Namespaces can be
used to enforce a separation of layers ("IOAM-Namespaces provide
additional context [...] e.g. if an operator wishes to use different
node identifiers for different IOAM layers"), and that namespace
identifiers allow for determination of which IOAM-Option-Types need to
be processed "in case of a layered IOAM deployment".

I think there is also some internal inconsistency relating to the role
of IOAM transit nodes.  This may be localised in Section 5.2 where we
see both that a transit node is one that "read and/or write or process
[the] IOAM data" and that a transit node "updates one or more of the
IOAM-Data-Fields" (i.e., always writes), but I did not attempt an
exhaustive check for other instances.

I don't think the definition of the POSIX epoch is correct -- it seems
to be copied (roughly) from the definition of the PTP epoch (i.e., using
midnight TAI as the reference) but all the references I consulted
indicate that the POSIX epoch started at midnight UTC.

As foreshadowed in
https://mailarchive.ietf.org/arch/msg/last-call/Ak2NAIKQ7p4Rij9jfv123xeTXQY/
I think we need to have a discussion about the expectations and
provisions for cryptographic (e.g., integrity) protection of IOAM data.
>From my perspective, IOAM is a new (class of) protocols that is seeking
publication on the IETF stream as Proposed Standard.  While we do make
exceptions for modifications to protocols that were developed before we
realized how important integrated security mechanisms are, it's
generally the case that new protocols are held to the current IETF
expectations for what security properties are provided; the default
expectation is that a protocol is expected to provide secure operation
in the internet threat model of RFC 3552.  This draft currently only
provides a brief note in the security considerations that there exists
an individual draft (draft-brockners-ippm-ioam-data-integrity) that
might provide ways to protect the integrity of IOAM data fields.
Shouldn't the security mechanisms be getting developed side-by-side by
the protocol mechanisms, to ensure that they are properly integrated and
fit for purpose?  (This does not necessarily have to be in the same
document and could be part of a cluster of related documents, but I
don't think that an informative reference to a non-WG draft really
qualifies.)

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

Thanks to Shawn Emery for the secdir review.

I have grave misgivings about the proposed behavior that provides for a
separate domain boundary at the granularity of IOAM-Namespace.  It does
not quite rise to a Discuss because it is technically possible to
implement successfully if done perfectly, but it seems incredibly risky
and very hard to get right, and there does not seem to be sufficient
motivation presented for the benefits of this behavior to justify the
risk.  Specifically, by partitioning all IOAM behaviors by
IOAM-Namespace, we require a node to know whether or not it is to behave
as ingress/egress for a domain on a per-namespace basis.  In effect, it
allows for a proliferation of as many distinct and partially overlapping
IOAM Domains as there are namespace values, and whether a node is
ingress (or egress) for any given domain has to be looked up on the
basis of the specific IOAM-Namespace value.  I find this concerning
because the security properties of the system rely on properly policing
the domain boundary, so that IOAM markings from outside the domain are
rejected at ingress, and IOAM data is not leaked out of the domain by
policing at egress.  While we have ample evidence to suggest that it is
feasible to maintain a single tightly controlled domain that properly
polices ingress and egress (e.g., MPLS), I don't know of evidence to
suggest that it is feasible to do so with a spate of partially
overlapping domains where domains and domain membership are malleable
based the interaction of configuration and in-band protocol elements.
It seems incredibly likely that some domain boundary somewhere will be
inadvertently permeable, and compromise the security properties of the
system.

Section 1

(side note) There's perhaps something of a philosophical question of
whether a mechanism really qualifies as "in situ" if it involves
encapsulating the packet in a full-fledged tunnel (as at least the IPv6
encapsulation does, as is needed to add the additional EHs that hold
IOAM data fields).  That said, I don't really have any alternative
suggestions, and I am sure that IOAM is a well-established terminology,
so this is just a side node.

Section 4

   Scope: This document defines the data fields and associated data
   types for in-situ OAM.  The in-situ OAM data field can be
   encapsulated in a variety of protocols, including NSH, Segment
   Routing, Geneve, IPv6, or IPv4.  Specification details for these

I found drafts (split between targeting ippm and "the relevant working
groups") that would cover Geneve and IPv6, but not anything for IPv4.
Are we fully confident that the aim is actually achievable for IPv4?

   Deployment domain (or scope) of in-situ OAM deployment: IOAM is a
   network domain focused feature, with "network domain" being a set of
   network devices or entities within a single administration.  For
   example, a network domain can include an enterprise campus using
   physical connections between devices or an overlay network using
   virtual connections / tunnels for connectivity between said devices.

If these virtual connections/tunnels do not provide cryptographic
confidentiality and integrity protection, then the security
considerations for IOAM need to include the full physical deployment
scope including the underaly over which the overlay is constructed, not
just the "administrative domain" as defined here.

Furthermore, there seems to be a very questionable interaction between
labeling the IOAM deployment domain an administrative domain yet
allowing for multiple partially overlapping IOAM domains as
distinguished by namespace ID.  Does the administrative domain have to
encompass the union of all the different IOAM domains (as identified by
namespace ID)?

Section 5.2

   An "IOAM transit node" updates one or more of the IOAM-Data-Fields.

(per the discuss)
The previous discussion suggested that even a node that only reads
IOAM-DataFields is still considered a "transit node", and that updating
the field contents is not necessary in order to justify that moniker.
That said, the notion that a transit node is writing (e.g., to update
trace option-types) seems to be prevailing throughout later portions of
the document, so perhaps it is the text a few paragraphs earlier that is
in error.

   If both the Pre-allocated and the Incremental Trace Option-Types are
   present in the packet, each IOAM transit node based on configuration
   and available implementation of IOAM populates IOAM trace data in
   either Pre-allocated or Incremental Trace Option-Type but not both.

(per the discuss)
Likewise, is this "populates" mandatory?

   A transit node MUST ignore IOAM-Option-Types that it does not
   understand.  A transit node MUST NOT add new IOAM-Option-Types to a
   packet, MUST NOT remove IOAM-Option-Types from a packet, and MUST NOT
   change the IOAM-Data-Fields of an IOAM Edge-to-Edge Option-Type.

Almost all of this (not the Edge-to-Edge stuff) seems fairly
tautological, in that if a node did that stuff it would be an
encapsulating and/or decapsulating node, possibly in addition to being a
transit node.

   The role of an IOAM-encapsulating, IOAM-transit or IOAM-decapsulating
   node is always performed within a specific IOAM-Namespace.  This
   means that an IOAM node which is e.g. an IOAM-decapsulating node for
   IOAM-Namespace "A" but not for IOAM-Namespace "B" will only remove
   the IOAM-Option-Types for IOAM-Namespace "A" from the packet.  Note
   that this applies even for IOAM-Option-Types that the node does not
   understand, for example an IOAM-Option-Type other than the four
   described above, that is added in a future revision.  An IOAM
   decapsulating node situated at the edge of an IOAM domain MUST remove
   all IOAM-Option-Types and associated encapsulation headers for all
   IOAM-Namespaces from the packet.

I can only make sense of this paragraph as a whole if the last sentence
instead says "MUST remove from the packet [...] for all IOAM-Namespaces
for which the node is a decapsulating node.  The current text says to
remove literally all IOAM information for literally all IOAM-Namespaces,
which seems to contradict the separation of namespaces depicted earlier
in the paragraph.

Section 5.4

   A particular implementation of IOAM MAY choose to support only one of
   the two trace option types.  In the event that both options are
   utilized at the same time, the Incremental Trace-Option MUST be
   placed before the Pre-allocated Trace-Option.  Deployments which mix
   devices with either the Incremental Trace-Option or the Pre-allocated
   Trace-Option could result in both Option-Types being present in a
   packet.  Given that the operator knows which equipment is deployed in
   a particular IOAM, the operator will decide by means of configuration
   which type(s) of trace options will be used for a particular domain.

Up in Section 5.2 we said that "each IOAM transit node based on
configuration and available implementation" populates exactly one trace
option type.  I think I can read the two statements as being consistent
with each other, but it might be useful to harmonize the specific
language used to make it very clear that these refer to the same
behavior.

Section 5.4.1

Just to check my understanding: when I first read the discussion of this
being an "array" and there being a potential "Opaque State Snapshot"
component, I assumed that the length of this opaque snapshot would need
to be fixed per array element (i.e., as part of the namespace
definition).  Having read a bit more, it seems that my initial
assumption is incorrect, since there is a length field in the opaque
state snapshot framing, and so a node parsing the array of elements will
be able to skip over the approprate (variable) length for each element
in the array.  Please confirm that my updated understanding is correct.

   Namespace-ID:  16-bit identifier of an IOAM-Namespace.  The
      Namespace-ID value of 0x0000 is defined as the "Default-Namespace-
      ID" (see Section 5.3) and MUST be known to all the nodes
      implementing IOAM.  For any other Namespace-ID value that does not
      match any Namespace-ID the node is configured to operate on, the
      node MUST NOT change the contents of the IOAM-Data-Fields.

Though it may seem banal to do so, explicitly listing "change, add, or
remove" might help avoid future questions of how to interpret this text.
E.g., there is some similar text in RFC 2460 about "examined or
processed" that proved to be highly controversial and was changed in RFC
8200.

      A node receiving an IOAM Pre-allocated or Incremental Trace-Option
      relies on the NodeLen value, or it can ignore the NodeLen value
      and calculate the node length from the IOAM-Trace-Type bits (see
      below).

Allowing an implementation to pick one or the other option like this
introduces fragility to the system as a whole and requires strict
controls that encapsulating nodes always set the value even if their
implementation does not use it.  I would prefer if we had a single
well-specified required behavior for all nodes, that could be easily
tested for conformance.

      Bit 0  "Overflow" (O-bit) (most significant bit).  If there are
         not enough octets left to record node data, the network element
         MUST NOT add any fields and MUST set the overflow "O-bit" to
         "1" in the IOAM-Trace-Option header.  This is useful for
         transit nodes to ignore further processing of the option.

This makes things awkward for a lot of the earlier statements that
require transit nodes to populate trace data.  How is the document
internally consistent in this regard, if transit nodes are both required
to populate trace data and required to not add trace data?

      NodeLen - sizeof(opaque snapshot) in 4 octet units.  If
      RemainingLen in a pre-allocated trace option exceeds the length of
      the option, as specified in the preceding header, then the node
      MUST NOT add any fields.

What is the "preceding header"?  This seems like perhaps it originated
in a description of a concrete protocol encoding of this data item and
does not quite apply to the abstract description thereof.

      Bit 12-21  Undefined.  An IOAM encapsulating node MUST set the
               value of each of these bits to 0.  If an IOAM transit
               node receives a packet with one or more of these bits set
               to 1, it MUST either:

               1.  Add corresponding node data filled with the reserved
                   value 0xFFFFFFFF, after the node data fields for the
                   IOAM-Trace-Type bits defined above, such that the

Just to confirm: this means that there cannot be any future allocations
of bits to "wide format" items and thus that only bits 8-10 will ever
consume 2 units each of four-octets?  (This does not preclude allocating
two adjacent bits to indicate a single logical data item, of course,
with appropriate error handling for when only one of the two is set.)

      Bit 23   Reserved: MUST be set to zero upon transmission and
               ignored upon receipt.

I note that this description for the "reserved" bit has a different
specified behavior than the unallocated bits 12-21 (that require a node
to fill the data with ones if the bit is set).  Do we have a sense for
any ways in which this reserved bit's semantics could be used for future
extensibility, vs being locked into these (basically useless) semantics
forever?

Section 5.4.2

   Some IOAM-Data-Fields defined below, such as interface identifiers or
   IOAM-Namespace specific data, are defined in both "short format" as
   well as "wide format".  Their use is not exclusive.  A deployment
   could choose to leverage both.  [...]

While the text goes on to clarify that, based on per-domain
configuration, they can hold qualitatively different types of
information, I still must ask if there is any entity that is or might be
tasked with enforcing that there is consistency between the two fields
(mostly for when they are different representations of the same
information, but not necessarily exclusively so).

Section 5.4.2.1

   Hop_Lim:  1-octet unsigned integer.  It is set to the Hop Limit value
      in the packet at the node that records this data.  Hop Limit

"In the packet" at ingress to, or egress from, the node?

   node_id:  3-octet unsigned integer.  Node identifier field to
      uniquely identify a node within the IOAM-Namespace and associated
      IOAM-Domain.  The procedure to allocate, manage and map the
      node_ids is beyond the scope of this document.

Even if we attempt to leave allocation/management of node_ids out of
scope for this document, I think we still need to talk about what goes
wrong and how to recover in case of collision.

Section 5.4.2.2

I kind of expected some note that the interpretation of the interface
identifier fields here is (or might be?) going to be specified by the
node they apply to and can only be interpreted in that context.  Or are
these expected to be allocated by a central entity?

Section 5.4.2.12

   The "buffer occupancy" field is a 4-octet unsigned integer field.
   This field indicates the current status of the occupancy of the
   common buffer pool used by a set of queues.  The units of this field
   are implementation specific.  Hence, the units are interpreted within
   the context of an IOAM-Namespace and/or node-id if used.  The authors
   acknowledge that in some operational cases there is a need for the
   units to be consistent across a packet path through the network,
   hence it is RECOMMENDED for implementations to use standard units
   such as Bytes.

I guess I'm not sure that I understand exactly what this field should be
indicating, even if my implementation does adhere to the recommendation
to "use standard units such as Bytes" (which, by the way, could probably
stand to be a stronger recommendation for a single distinguished unit
chosen by the authors).  That is, if I suppose that the node in question
has some common pool of buffers that's shared across queues.  Maybe all
the buffers are the same size, maybe not.  But in order to measure
"occupancy", is it more important to know how many bytes are occupied,
how many buffers are in use, or what proportion of the total buffers are
in use?  Just knowing the number of bytes or buffers in use does not
convey much information if the total capacity is unknown, and having 40
MB of buffers in use would mean something very different for a CPE
router vs "big iron".  Can we give a bit more clarity into at least what
portions of the semantics need to be set at a per-namespace level, even
if we can't nail them down more tightly as part of the protocol spec?

Section 5.4.13

   Schema ID:  3-octet unsigned integer identifying the schema of Opaque
      data.

Just to check my understanding: the interpretation of this schema ID is
per IOAM-Namespace, which is mostly going to be something maintained by
the individual operators.  So in some sense each operator will have to
maintain and publish (internally) a registry of these Schema IDs and
avoid collisions.  Is this the sort of thing that is already a common
practice, or is there a risk of operational fragility being introduced?

Furthermore, some of the Namespace IDs are not operator-managed, e.g.,
0x0000.  Is the opaque state snapshot functionality just not going to be
used for those well-known namespaces?  If so, we should say so
explicitly.

Section 5.5

   o  Random: Unique identifier for the packet (e.g., 64-bits allow for
      the unique identification of 2^64 packets).

We should probably say that this is generated using a
cryptographic-strength PRNG (i.e., not rand()).  BCP 106 covers
randomness requirements for security.

Also, due to the birthday paradox, an actual 64-bit random identifier will
produce collisions well before 2^64 packets.  Since these identifiers
(AFAICT) need to be assigned in an uncoordinated fashion, the random
allocation scheme may well be the best scheme (or "least bad"), but if
that's the case I don't think we should make bold statements like "allow
for the unique identification of 2^64 packets".

   IOAM POT flags:  8-bit.  Following flags are defined:

It's slightly surprising to me that the flags are defined as having
global semantics across all POT values, as opposed to being interpreted
in the context of the POT type they are used with, but that's not
inherently problematic.

Section 5.5.1

Should we just say that the Namespace-ID, POT Type, and flags are as
defined in Section 5 rather than repeating the definitions wholesale
(or, as we currently do, having to modify the definition text slightly)?
In particular (but not exclusively), it's pretty distracting to have
Section 5 refer to "Bit 0" and "Bit 1-7" but Section 5.5.1 refer to the
"P bit" and "R (7 bits)"

   P bit:  1-bit.  "Profile-to-use" (P-bit) (most significant bit).
      Indicates which POT-profile is used to generate the Cumulative.
      Any node participating in POT will have a maximum of 2 profiles
      configured that drive the computation of cumulative.  The two
      profiles are numbered 0, 1.  This bit conveys whether profile 0 or
      profile 1 is used to compute the Cumulative.

Is it worth saying a few more words about how the P bit is used as a
"generation count" for performing incremental/in-place updates of what
profile to use, and that profile 0 can be repurposed for something new
once all uses of its previous interpretation have been removed from the
network?  ("No" is a fine answer, but it might be worth proactively
allaying the concern that you can only have two profiles, ever.)

Section 5.6

I suggest giving some guidance as to whether the initial sequence number
should (not) start at zero, for the fields indicated by bits 0 and 1.
I note that draft-gont-numeric-ids-sec-considerations has some
discussion supporting starting at non-zero values.  It seems that the
"increment by one for each packet" behavior is needed, though, in order
to be able to use this value to detect loss.

      Bit 0    (Most significant bit) When set indicates presence of a
               64-bit sequence number added to a specific "packet group"
               which is used to detect packet loss, packet reordering,
               or packet duplication within the group.  The "packet
               group" is deployment dependent and defined at the IOAM
               encapsulating node e.g. by n-tuple based classification
               of packets.

      Bit 1    When set indicates presence of a 32-bit sequence number
               added to a specific "packet group" which is used to
               detect packet loss, packet reordering, or packet
               duplication within that group.  The "packet group" is
               deployment dependent and defined at the IOAM
               encapsulating node e.g. by n-tuple based classification
               of packets.

When both of these bits are set, are the contained values agglomerated
into a hybrid 96-bit sequence number?  If so, in which order?

      Bit 2    When set indicates presence of timestamp seconds,
               [...]
               packet is encapsulated into the tunnel.  Each
               implementation has to document when the E2E timestamp
               that is going to be put in the packet is retrieved.  This

It seems a little awkward that an operator is going to have to base
their processing logic on knowledge of what *implementation* the
encapsulating node is, especially on a heterogeneous network.  But I
suppose it would be an RFC 6919 "MUST (BUT WE KNOW YOU WON'T)" if we
said that implementations had to allow configuring the different
possibilities, based on the IOAM-Namespace...

Section 6.3 (POSIX-based Timestamp Format)

      The POSIX-based timestamp format is affected by leap seconds; it
      represents the number of seconds since the epoch minus the number
      of leap seconds that have occurred since the epoch.  The value of
      a timestamp during or slightly after a leap second could be
      temporarily inaccurate.

(If I'm wrong about the Discuss point, this description seems
inconsistent with the POSIX epoch being midnight TAI.)

Section 8

I note that the "RFC Required" policy lets ISE and IRTF stream documents
allocate values, in theory without any IETF review at all.  It seems
unlikely that this is what is intended in all of the cases where "RFC
Required" is specified, such asa the 4-bit IOAM Trace-Flags registry
that has only three unallocated codepoints.

Section 8.4

   0: 16 Octet POT data

I'd suggest a slightly more descriptive name, as there may well be other
POT formats that want to use 16 octets for their data.

Section 8.7

I suggest removing the sentence "Upon a new allocation request, the
responsible AD will appoint a designated expert, who will review the
allocation request." as it is not really accurate (the IESG appoints
DEs) and is not helpful for the potential registrant.  It's arguably
also needlessly restrictive, preventing the IESG from appointing an
expert before there is a request to review.

Section 10

"Direct Export" sounds like a self-induced DoS attack by traffic
amplification, but that's probably more a matter to be discussed in that
document, not this one.  (Though, do we need to mention direct export
here at all?)

We should probably say that the opaque snapshot, namespace specific
data, etc., will have security considerations corresponding to their
defined data contents that should be described where those formats are
defined.

   From a confidentiality perspective, although IOAM options do not
   contain user data, they can be used for network reconnaissance,

Given that we provide multiple fields that essentially carry opaque or
operator-defined data, the blanket "do not contain" may be too strong of
a statement.  (What if someone decides to put a subscriber identifier in
the namespace-specific data?)  So maybe "are not expected to" is more
appropriate".

   allowing attackers to collect information about network paths,
   performance, queue states, buffer occupancy and other information.

One possible application of such reconiassance is to gauge the
effectiveness of an ongoing attack (e.g., if buffers and queues are
overflowing).  I don't know whether it's particularly useful to mention
that scenario here or not, though.

   IOAM can be used as a means for implementing Denial of Service (DoS)
   attacks, or for amplifying them.  For example, a malicious attacker
   can add an IOAM header to packets in order to consume the resources
   of network devices that take part in IOAM or entities that receive,
   collect or analyze the IOAM data.  [...]

Messing up the POT data seems worth calling out here as well (though the
particular behavior when proof of transit fails is not defined in this
document, of course).

   Notably, in most cases IOAM is expected to be deployed in specific
   network domains, thus confining the potential attack vectors to

Where does the "most cases" come from?  I thought the definitions
restricted IOAM to the IOAM domain.

                  Indeed, in order to limit the scope of threats
   mentioned above to within the current network domain the network
   operator is expected to enforce policies that prevent IOAM traffic
   from leaking outside of the IOAM domain, and prevent IOAM data from
   outside the domain to be processed and used within the domain.

It would be great if we could provide a bit more detail on the scope of
consequences if the operator fails to do so.

Section 12.1

The 2008 POSIX reference has since been superseded by the 2017 version.

NITS

Abstract

nits: NSH is not listed as "well known" at the RFC Editor's abbreviation list,
so should probably be written out in full.  Also, please put commas
before and after "e.g." (which will presumably help with the spurious
double-space as well).

Section 1

   cannot be considered passive.  In terms of the classification given
   in [RFC7799] IOAM could be portrayed as Hybrid Type 1.  IOAM

RFC 7799 writes it with a majuscule 'I', not the numeral '1'.

Section 3

Please use the updated RFC 8174 version of the BCP 14 boilerplate.

Section 4

   Scope: This document defines the data fields and associated data
   types for in-situ OAM.  The in-situ OAM data field can be
   encapsulated in a variety of protocols, including NSH, Segment
   Routing, Geneve, IPv6, or IPv4.  [...]

s/field/fields/, and s/or/and/

Section 5.3

   A subset or all of the IOAM-Option-Types and their corresponding
   IOAM-Data-Fields can be associated to an IOAM-Namespace.  IOAM-

The way this is written seems to imply that any given IOAM-Option-Type
is associated with at most one IOAM-Namespace, which I think is not the
intent.

   Namespaces add further context to IOAM-Option-Types and associated
   IOAM-Data-Fields.  Any IOAM-Namespace MUST interpret the IOAM-Option-
   Types and associated IOAM-Data-Fields per the definition in this
   document.  IOAM-Namespaces group nodes to support different

Presumably this ("MUST interpret") only applies to the option-type and
data fields defined in this document?

   deployment approaches of IOAM (see a few example use-cases below) as

IIUC the meaning here is "IOAM-Namespaces provide a way to group nodes"
and would be easier to read if formulated in that manner.

The RFC Editor will probably have a hard time in this section with which
things that end in 's' are possessives (and thus benefit from an
apostrophe) and which are not, though it may not be an efficient use of
time to try to tidy up before the document gets to them.

   o  whether IOAM-Option-Type(s) has to be removed from the packet,
      e.g. at a domain edge or domain boundary.

there's a singular/plural mismatch here (irrespective of the "(s)" --
"whether an [option-type] has to be removed" vs "whether [option-types]
have to be removed"

Section 5.4.2.3

   The "timestamp seconds" field is a 4-octet unsigned integer field.
   Absolute timestamp in seconds that specifies the time at which the

s/Absolute timestamp/It contains the absolute timestamp/

Section 5.4.2.4

   The "timestamp subseconds" field is a 4-octet unsigned integer field.
   Absolute timestamp in subseconds that specifies the time at which the

s/Absolute timestamp/It contains the absolute timestamp/

Section 5.4.2.9

Please use the exact same wording in the description of the Hop_Lim
field that was used in Section 5.4.2.1 (or just incorporate that
definition by reference).  (The node_id descriptions properly differ
only in the 3-octet vs 7-octet phrase.)

Section 5.4.3

   An entry in the "node data list" array can have different formats,
   following the needs of the deployment.  [...]

This phrasing seems needlessly confusing.  Within a single "node data
list" (i.e., a single packet), all the list entries have the same
format.  What we want to be describing is that the per-entry format can
vary across packets and across deployments.  So perhaps just "the format
used for the entries in a packet's "node data list" array can vary from
packet to packet and deployment to deployment".
(Also, there's a singular/plural mismatch between "an entry" and
"different formats".)

Section 5.5, 5.6

When we talk about how the different Data-Fields "MUST be 4-octet
aligned", having the figure show a variable-height entry might be
helpful; the current formulation looks like it's exactly a 32-bit field.

Section 5.5

   IOAM Proof of Transit Option-Type is to support path or service
   function chain [RFC7665] verification use cases.  Proof-of-transit

s/is to/is used to/

Sections 6.1, 6.2, 6.3

      Seconds: specifies the integer portion of the number of seconds
      since the epoch.

I suggest writing out "the PTP epoch", "the NTP epoch", and "the Unix
epoch" in the respective sections, to avoid giving the impression (via
the definite article) that there is a single distinguished epoch, when
there is not.  (Similarly for the Fractions.)