IS-IS Application-Specific Link Attributes
draft-ietf-isis-te-app-19

EDIT: Thanks for setting me straight on my DISCUSS point.

--

Since this document is in many parts a copy of draft-ietf-ospf-te-link-attr-reuse, I'm only reviewing this delta between them here:
https://www.ietf.org/rfcdiff?url1=draft-ietf-ospf-te-link-attr-reuse-14&url2=draft-ietf-isis-te-app-14

Section 2:
* "... expected to continue - so any discussion ..." -- change to "... expected to continue.  Therefore, any discussion ..."
* "... key points identified in the introduction - which are:" -- change hyphen to a comma

Section 3:
* "... advertisements include sub-TLVs for TLVs ..." -- Please define or expand "TLV" on first use.
* Please just name the registries, rather than giving multi-line URLs to them.

Section 3.1:
* As with the matching OSPF document, I don't see the benefit of citing current registry contents rather than just referencing the registry.

Section 4.3:
* Interestingly, the entries for IPv4 are not capitalized (e.g., "interface address"), but they are for IPv6 (e.g., "Interface Address").

Section 6.3.2:
* These two paragraphs read like they're in the wrong order.

Sections 7.1 and 7.2:
* These should refer back to Sections 4.2 and 4.3, respectively, where the new values are fully described.

(revised) 
** Section 4.1.  As I understand it, the SABM can be of 0 – 8 octets in length. The SABM Length field represents that length and has 7 bits to do that.  However, the maximum number of bits needed to represent 8 is only 4 bits.  What’s the thinking on those three extra bits?  Should they be marked as reserved?  I would have the same question for the UDABM mask.

** Section 6.2.  I didn’t follow what it means to send the sub-TLV in Section 4.2 with a zero length SABM Length and UDABM Length – that is two empty bitmasks?  Is that permitted?  What would it convey?

** Section 8.  Per “Tampering with the information defined in this document may have an effect on applications using it, including impacting Traffic Engineering.”, I have no disagreement with this statement.  However, I would recommend adding a brief statement what is the security impact of “impacting Traffic Engineering”.

** Section 8.  Per “This is similar in nature to the impacts associated  with (for example) [RFC5305]”, what specific text in RFC5305 was envisioned?  The SecCon section (Section 6) of RFC5305 contains only one sentence that points to RFC5304?

** Section 8.  Consider using the editorial framing of the first paragraph of Section 13 in draft-ietf-ospf-te-link-attr-reuse to introduce how the RFC5304 applies here.

** Editorial
-- Section 3.  Editorial.  Consider providing a reference for the registries instead of an inline URL.

-- Section 4.1.  The rendering of the sub-TLV diagram was split between Page 6 and 7 when this draft is read in TXT format.  IMO, it would be more readable if it was on one page.

-- Per Section 4.1.  Editorial.  Per “See the following section for a description …”, please explicitly name the section.

-- Section 4.2.  Typo. s/Identifer/Identifier/

I am balloting NoObj in the "I read the protocol action, and I trust the sponsoring AD so have no problem." sense, but I do have some concerns.

I found the document to be really hard to read, enough so that I largely gave up - this might just be that I'm overcommitted/grumpy this week, but the lack of punctuation in sentences like: "In cases where multiple applications wish to make use of these link  attributes the current advertisements do not support application specific values for a given attribute nor do they support indication  of which applications are using the advertised value for a given link." certainly don't help...

I was largely unable to follow Section 4.1.  Application Identifier Bit Mask, but as others / the responsible AD seem to get it, I'm just going to assume that my wife secretly swapped out my coffee for decaf this week, or that I'm just being dumb. 
As an example, I am unable to sensibly parse: "Standard Application Identifier Bits are defined/sent starting with Bit 0.  ... Bits that are not transmitted MUST be treated as if they are set to 0 on receipt.", especially the "Bits that are not transmitted" part.

Thank you for the work put into this document. I have only one nits in section 4.3: while I appreciate IPv6, there is no need to capitalize 'IPv6 Interface Address' as "IPv4 interface address" is not capitalized ;-)

Special thanks to Acee, as the document shepherd he managed to represent the conflicts within the WG.

I hope that this helps to improve the document,

Regards,

-éric

My co-AD has this covered.

What is the scope over which the user-defined application bits are
defined/allocated?

And, a general question, just to check my understanding: if I do need to
specify different values of a given attribute for different
applications, I do that by putting multiple copies of the new sub-TLV in
TLV 22/23/etc., with the flags set according to which application the
contained attributes apply to?  (Mostly I ask because I forget what the
rules are for having multiple instances of a given TLV/sub-TLV as
siblings in the same container.)

Section 3.1

Maybe mention (again, I know) that this is only the subset of sub-TLVs
that are used for RSVP-TE?

Section 4.2

   When the L-flag is set in the Application Identifier Bit Mask, all of
   the applications specified in the bit mask MUST use the legacy
   advertisements for the corresponding link found in TLVs 22, 23, 25,
   141, 222, and 223 or TLV 138 or TLV 139 as appropriate.  Link

nit(?): is this "found in sub-TLVs of TLVs 22, [...]"?

   attribute sub-sub-TLVs for the corresponding link attributes MUST NOT
   be advertised for the set of applications specified in the Standard/
   User Application Identifier Bit Masks and all such advertisements
   MUST be ignored on receipt.

Does this apply to just the (sub-)TLV with the L-flag set, or to other
instances of that (sub-)TLV as well?

   For a given application, the setting of the L-flag MUST be the same
   in all sub-TLVs for a given link.  In cases where this constraint is
   violated, the L-flag MUST be considered set for this application.

editorial: I suggest "the L-flag MUST be considered set for this
application for all sub-TLVs on the link in question".

   If link attributes are advertised associated with zero length
   Application Identifier Bit Masks for both standard applications and
   user defined applications, then any Standard Application and/or any

Do we need to talk about conflicts if there are multiple such sub-TLVs
for the link in question (that contain different values in the
sub-sub-TLV(s))?

   User Defined Application is permitted to use that set of link
   attributes so long as there is not another set of attributes
   advertised on that same link which is associated with a non-zero
   length Application Identifier Bit Mask with a matching Application
   Identifier Bit set.

nit: this phrasing of "matching Application Identifier Bit set" does not
seem as clear as it could be that the bit for the application in
question is what's checked (though I have a hard time believing that
someone would accidentally misinterpret the meaning).

   the link attribute sub-sub-TLV code points.  This document defines a
   sub-sub-TLV for each of the existing sub-TLVs listed in Section 3.1
   except as noted below.  The format of the sub-sub-TLVs matches the

Just to check that I'm matching things up properly: this leaves the only
attributes that do not have some form of exception noted as
administrative group, extended administrative group, and TE default
metric?

Section 4.2.1

   Maximum link bandwidth is an application independent attribute of the
   link.  When advertised using the Application Specific Link Attributes
   sub-TLV, multiple values for the same link MUST NOT be advertised.
   This can be accomplished most efficiently by having a single
   advertisement for a given link where the Application Identifier Bit
   Mask identifies all the applications which are making use of the
   value for that link.

If I want the same maximum link bandwidth to apply to all applications,
couldn't I just put it in a sub-TLV with both SABM and UDABM length of
zero?  (Is this somehow less efficient than setting all the bits for the
applications making use of the value?)  Section 4.2.3 seems to discuss
using the zero-length bit mask option in the context of TE metrics...
(I do note the note at the end of Section 5 about the "any application"
encoding leaving ambiguous whether an application is enabled, but I
don't see how this consideration differs between maximum link bandwidth
and extended TE metrics.)

Section 4.2.2

   Maximum Reservable Link Bandwidth and Unreserved Bandwidth are
   attributes specific to RSVP-TE.  When advertised using the
   Application Specific Link Attributes sub-TLV, bits other than the
   RSVP-TE (R-bit) MUST NOT be set in the Application Identifier Bit

Just to confirm: we find the risk of some future application that also
wants to do reservation-like things sufficiently low that we're okay
with preventing it from using these attributes?

Section 4.3

What are the semantics when I specify more than one link identifier
sub-TLV?  They are all supposed to identify the same link, and in some
case might be needed to disambiguate if there are (e.g.) multiple links
to the same neighbor?

Section 5

   In the case of SRTE, advertisement of application specific link
   attributes does not indicate enablement of SRTE on that link.  The

Is the SRTE specification sufficiently final that we're comfortable
enshrining in a (different) RFC this property of it?  I note that we
only list draft-ietf-spring-segment-routing-policy as an informative
reference, so it's entirely possible that this document will be
published as an RFC before that document is done.

Section 6.1

   the writing of this document.  Therefore, such applications have been
   deployed using the legacy advertisements.  The Standard Applications
   defined in this document may continue to use legacy advertisements
   for a given link so long as at least one of the following conditions
   is true:

nit(?): do we want to say something like "may safely continue" or "may
continue to use without ill effect"?

Section 6.3.2

   advertisements as defined in this document.  Attributes for
   applications other than RSVP-TE MUST be advertised using application
   specific advertisements which have the L-flag clear.  In cases where
   some link attributes are shared with RSVP-TE, this requires duplicate
   advertisements for those attributes.

Maybe repeat that this duplication is required because the L flag
applies per-application per-link, for all attributes?

Section 6.3.4

   2)Advertise all legacy link attributes using application specific
   advertisements with L-flag clear and R-bit set.

nit: I suggest clarifying that this involves duplicate advertisements
(legacy plus application-specific).  Or at least, I assume it does,
since step (3) is "remove legacy advertisements".

Section 7.3

   Note to IANA: For future codepoints, in cases where the document
   which defines the encoding is different from the document which
   assigns the codepoint, the encoding reference MUST be to the document
   which defines the encoding.

Why not list both as the reference?

   Note to designated experts: If a link attribute can be advertised
   both as a sub-TLV of TLVs 22, 23, 25, 141, 222, and 223 and as a sub-
   sub-TLV of the Application Specific Link Attributes sub-TLV defined
   in this document, then the same numerical code should be assigned to
   the link attribute whenever possible.

Are these notes intended to end up in the final RFC, attached to the
registry, both places, or neither place?

Section 7.4

   policy for this registry is "Standards Action" [RFC8126].  Bit
   definitions SHOULD be assigned in ascending bit order beginning with
   Bit 0 so as to minimize the number of octets that will need to be
   transmitted.  The following assignments are made by this document:

I worry a little bit that this will encourage codepoint squatting,
though in theory the user-defined bitmask should avoid the need for
squatting.

Section 7.5

   Note to IANA: For future codepoints, in cases where the document
   which defines the encoding is different from the document which
   assigns the codepoint, the encoding reference MUST be to the document
   which defines the encoding.

(As above, why not list both?)

I know very little about this, but just checking:
- I trust that a network that mixes routers that use application attributes, and not, will not lead to long-term routing loops in spite of them not having a common picture of the network?

- It is odd that a link that advertises a zero-length flags field means support for RSVP-TE is “ambiguous” (sec 5). What are the implications of this? When is it OK to use a zero-length flags field given this ambiguity? In a standard, can we not decide on a meaning to eliminate the uncertainty? I would appreciate some language here to answer at least the first two questions.

- as the TSVart review points out, the length field wastes 3 bits of 7 because the maximum length is 8. You could reserve them or even use them to encode these three legacy applications.

Nits:

Abstract: 
In “these link attributes for a given attribute” add a comma after both instances of attribute(s)

Sec 4 2)Application. Add a space

Sec 5. Missing a period at the end of “existence of link attribute advertisements”

I'm not sure whether this should really be a discuss ... I raised a similar concern on the OSPF document as part of a discuss, and presume consistency is helpful.

4.1.  Application Identifier Bit Mask

  UDABM Length: Indicates the length in octets (0-8) of the
       User Defined Application Identifier Bit Mask. The length SHOULD
       be the minimum required to send all bits which are set.

   User Defined Application Identifier Bits have no relationship to
   Standard Application Identifier Bits and are not managed by IANA or
   any other standards body.  It is recommended that bits are used
   starting with Bit 0 so as to minimize the number of octets required
   to advertise all UDAs.

In section 4.1, I think that the document should also add the sentence (taken from the previous paragraph) "Bits that are not transmitted MUST be treated as if they are set to 0 on receipt."  Note, that I also think that this behaviour is implicitly required from the description of UDABM Length.

Regards,
Rob