Subject: [Gen-art] Gen-ART review of draft-ietf-mpls-gach-adv-06

From: Martin Thomson <[email-masked]>
Date: Fri, 15 Feb 2013 11:33:02 -0800

I am the assigned Gen-ART reviewer for this draft. For background on
Gen-ART, please see the FAQ at

<http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.

Please resolve these comments along with any other Last Call comments
you may receive.

Document: draft-ietf-mpls-gach-adv-06
Reviewer: Martin Thomson
Review Date: 2013-02-15
IETF LC End Date: 2013-02-27
IESG Telechat date: (if known)

Summary: The document is almost ready for publication as proposed
standard.  There is a major issue that should be easy to resolve.

Major issues:

Section 6.3 duplicates the description of HMAC provided in RFC 2104.
That is likely to cause a bug.

If you reference RFC 2104, then the only requirement is a clear
specification for what message is input to the HMAC.  Currently, this
is buried.  It is unclear if the input includes the G-ACh header
defined in RFC 5586 (it doesn't need to, but this needs to be made
explicit).

Filling the authentication part with 0x878FE1F3 seems unnecessary busy
work, but it's harmless as long as the hash function produces a
multiple of 32 bits of output.

Minor issues:

To avoid forward compatibility issues, reserved fields should come
with guidance that says: "Implementations of this protocol version
MUST set reserved fields to all zero bits when sending and ignore any
value when receiving messages."

In Section 4.4, how does the duration interact with the lifetime?
What happens when the duration is longer than lifetime such that the
TLV is expunged before the duration is up?

Section 5.2 states:
   [...] If one
   of the received TLV objects has the same Type as a previously
   received TLV then the data from the new object SHALL replace the data
   associated with that Type unless the X specification dictates a
   different behavior.

This leads to different retention characteristics depending on some
arbitrary application-specific requirements.  It also complicates
implementations.  Is there a strong motivation for the "unless the X
specification dictates a different behavior" part of this statement?

If this behaviour is desirable, a note regarding what happens to the
composed TLV when some of the values that contribute to it might
expire might be necessary.

Regarding the last paragraph of Section 6.3:
          This also means that the use of hash functions with larger
          output sizes will increase the size of the GAP message as
          transmitted on the wire.

If you want to prevent hash truncation, then use 'MUST'.  Personally,
I see no reason to do so.  It's a good way to get smaller messages,
with a corresponding reduction in the strength of the assurance
provided.

Nits:

Section 3 could use some subheadings to aid navigation (and referencing).

Section 3 describes the size of fields only through ASCII-art.  It's a
fairly simple thing to add a bit count to the description of each
field.  That includes the reserved fields, which have no descriptions.

I like the text in the editors note on page 8.  Why is it not the
actual text already?

Sections 4.2 and 4.3 could probably use a note that notes that
retention of these TLVs doesn't make any sense.  These could be sent
with zero lifetime, except that if these are sent along with the
Source Address TLV, that's not possible... unless you send multiple
application data blocks for the same application.  Is that possible?