[sfc] Genart last call review of draft-ietf-sfc-nsh-19

[Dan Romascanu <[e-mail]>]

Reviewer: Dan Romascanu
Review result: Almost Ready

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Document: draft-ietf-sfc-nsh-19
Reviewer: Dan Romascanu
Review Date: 2017-08-22
IETF LC End Date: 2017-08-25
IESG Telechat date: 2017-09-14

Summary:

This document describes the header (called Network Service Header - NSH) to be
inserted in packets and frames in order to create packets and frames that
realize service functions described by other SFC documents. It needs to be read
in conjunction with RFC 7665 and other documents as the SFC control plane I-D
in order to make sense of the required functionality. This part of the SFC
solution seems almost ready, but a few issues mentioned below need in my
opinion clarification before the document is approved.

Major issues:

1. Section 11.1 claims: 'An IEEE EtherType, 0x894F, has been allocated for
NSH'. I could not find this value in the tables displayed by the IEEE
Registration Authority (RAC) - for example at
http://standards-oui.ieee.org/ethertype/eth.txt. Neither does IANA at
https://www.iana.org/assignments/ieee-802-numbers/ieee-802-numbers.xhtml (which
would not be in any case the authoritative source). Can you please indicate the
source that this is indeed a confirmed IEEE EtherType registration.

2. Section 5 refers to ietf-rtgwg-dt-encap which is expired. If I understand
correctly the status of this work, there is a design team in place in the
Routing Area created to look at common issues for the different data plane
encapsulations being discussed in various WGs including SFC. This leaves the
issue unsettled at this stage and this may be a problem for a standards track
document. I suggest that first the reference to the expired document is
removed, second that the issue is marked as 'open' and subject for future work.

Minor issues:

1.  Two values 'Experiment 1' and 'Experiment 2' are defined in section 2.2 and
11.2.5 for the Next Protocol. This seems a little odd. Why 2? Why are they
defined at the end of the range? Maybe there is an explanation for those who
know the history but for an un-initiated reader or a future implementer this
seems odd.

2. In Section 2.2 I see:

> All other flag fields, marked U, are unassigned and available for
   future use, see Section 11.2.1.  Unassigned bits MUST be set to zero
   upon origination, and MUST be ignored and preserved unmodified by
   other NSH supporting elements.  Elements which do not understand the
   meaning of any of these bits MUST NOT modify their actions based on
   those unknown bits.

The way the last sentence is written it seems to open the path for elements
that claim to 'understand' the meaning of some Unassigned bit to modify its
behavior based upon it. This does not seem good, as at transmission all
Unassigned bits MUST be set to zero. I would suggest to make the statement
simpler and just say that at reception all elements MUST NOT modify their
actions based on these bits.

3. In Section 2.2 I see:

> 0xF - This value is reserved for experimentation and testing, as per
   [RFC3692].  Implementations not explicitly configured to be part of
   an experiment SHOULD silently discard packets with MD Type 0xF.

Why is this a SHOULD and not a MUST?

4. I believe that  [I-D.ietf-sfc-control-plane] needs to be a Normative
Reference. There are several places in the document (e.g. in Section 2.3) where
this document is referred to describe behavior or actions related to the fields
of the header.

5. The version number has only two bits assigned. Moreover, this document
reserves already two values without any explanation why (only 00 is mandatory
to use, as far as I understand). This needs to be explained (maybe 'historical'
reasons - but unclear for future readers and users) and may be a limitation as
the protocol develops.

Nits/editorial comments:

1. Please make sure that acronyms are expanded at first occurrence (e.g. ECMP)
and if necessary appropriate references are being provided.