[sfc] Benjamin Kaduk's Discuss on draft-ietf-sfc-nsh-tlv-09: (with DISCUSS and COMMENT)

[Benjamin Kaduk via Datatracker <[e-mail]>]

Benjamin Kaduk has entered the following ballot position for
draft-ietf-sfc-nsh-tlv-09: Discuss

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DISCUSS:
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(1) RFC 8300 is pretty clear that "Metadata privacy and security
considerations are a matter for the documents that define metadata
format."  Some of the metadata context headers defined in this document
clearly have privacy considerations that need to be documented (e.g.,
policy ID and source/destination group serve to concretely identify
flows that are related in some way), though some may not have much that
needs documenting (e.g., the forwarding context metadata seems to just
be extracting out information that is already present in the packet
being wrapped).  Regardless, we need to have some discussion of the
privacy and security considerations of the new metadata context headers,
even if that is just "no new considerations" for some of them.

(2) I think we need to discuss the Flow ID context header further.  Is
it intended to just be a container to hold a flow identifier already
present in the contained packet (such as the IPv6 Flow Label or MPLS
Entropy Label that are called out), or can it also be used to apply a
new flow identifier at the SFC layer?

The named examples of a flow ID are both 20 bits long; if that is an
exhaustive listing, shouldn't we update the figure accordingly (to
include Length=3, four leading bits of padding, and a trailing byte of
padding)?  If that is not an exhaustive listing and longer flow
identifiers are expected, how do we know what length of flow identifier
is being conveyed?

(3) If we are to allow for specifying the "logical grouping of source and/or
destination objects" in §4.6 (emphasis on "and/or"), but the context
header always conveys both a source group and dest group field, do we
need to reserve a dedicated value for "no group information specified"?


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COMMENT:
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Section 1

   This document does not address metadata usage, updating/chaining of
   metadata, or other SFP functions.  Those topics are described in
   [RFC8300].

I'm not entirely sure what is meant by "chaining of metadata" (unless
it's just a synonym for "updating of metadata"?), and having reviewed
all 18 instances of "chain" and 88 instances of "metadata" in RFC 8300,
I am not sure which part you're intending to refer to by it.  Could you
please point to a more specific part of RFC 8300 (at least in this email
thread for now, not necessarily in a document update)?

Section 4.1

I suggest putting a context (forgive the pun) indicator in the figure
legends, e.g., "Figure 4: Forwarding Context - 2 (QinQ)".  The
information is already available elsewhere, but having it in the caption
helps focus the reader on the important/relevant part of the figure.

Section 4.3

It might be interesting to say something about when the SPI itself
suffices to identify the ingress node vs. when this metadata context
header is needed.

      Node ID: Represents an opaque value of the ingress network node
      ID.  The structure and semantics of this field are deployment
      specific.  For example, Node ID may be a 4 bytes IPv4 address Node
      ID, or a 16 bytes IPv6 address Node ID, or a 6 bytes MAC address,
      or 8 bytes MAC address (EUI-64), etc,.

There seems to be some dissonance between saying this field is "an
opaque value" and also saying that it might be an IP or MAC address.  In
the vein of Francesca's Discuss point, I don't think we can have
interoperability if the NSH implementation is expected to process this
as IP/MAC address information (as opposed to just an opaque identifier).

Section 8.2

URLs for [GROUPBASEDPOLICY] and [GROUPPOLICY] would be helpful.