Telechat Review of draft-ietf-6lo-deadline-time-03
review-ietf-6lo-deadline-time-03-rtgdir-telechat-frost-2019-01-07-00

Hello,

I have been selected as the Routing Directorate reviewer for this draft. The
Routing Directorate seeks to review all routing or routing-related drafts as
they pass through IETF last call and IESG review, and sometimes on special
request. The purpose of the review is to provide assistance to the Routing ADs.
For more information about the Routing Directorate, please see
​https://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir

Although these comments are primarily for the use of the Routing ADs, it would
be helpful if you could consider them along with any other IETF Last Call
comments that you receive, and strive to resolve them through discussion or by
updating the draft.

Document: draft-ietf-6lo-deadline-time-03
Reviewer: Dan Frost
Review Date: 2019-01-07
Intended Status: Standards Track

Summary:

I have some minor concerns about this document that I think should be resolved
before publication.

Comments:

This draft specifies a mechanism for including packet delivery deadline times,
in the form of an elective 6LoWPAN routing header, for use in low-power and
lossy networks with real-time requirements for end-to-end delay. Routers can
use packet deadline times to make informed scheduling decisions or discard
overdue packets. The timing metadata can also be useful for performance
monitoring and diagnostics.

The draft is, for the most part, clear, and the writing quality is good.

Major Issues:

No major issues found.

Minor Issues:

The main issue I see with the spec is the way timestamp formats are specified
with the TU (time units) field. The possible values for this field include
"seconds" and "microseconds". This is unusual, particularly in combination with
the EXP field, which leads to some time values having multiple representations.
And when representing absolute timestamps, we'd usually use well-known formats
like NTP or IEEE 1588. The draft probably needs to rework the timestamp
representation options along these lines, including specifying a single default
format for interoperability (we did this in RFC 6374, for example). An
important consideration here is the typical capabilities of the kinds of
devices expected to implement this spec; many devices only have good support
for one standard timestamp format. Industrial devices, a specfiic target of
this spec, usually expect IEEE 1588.

Making the Origination Time non-optional and specifying the Deadline Time as a
delta could also be considered.

Is the D flag (must drop if deadline exceeded) really necessary? Should the
semantics not just be to drop the overdue packet if there's congestion, and
forward it otherwise?

Nits:

Section 4: s/Whenever the packets crosses into a network/Whenever a packet
crosses into a network/

Cheers,
-d