Re: [Bier] AD Review of draft-ietf-bier-te-arch-09

Toerless Eckert <> Sat, 10 July 2021 01:34 UTC

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Date: Sat, 10 Jul 2021 03:34:21 +0200
From: Toerless Eckert <>
To: Alvaro Retana <>
Cc:, "Gengxuesong (Geng Xuesong)" <>, BIER WG Chairs <>, BIER WG <>
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Subject: Re: [Bier] AD Review of draft-ietf-bier-te-arch-09
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On Fri, May 14, 2021 at 01:14:43PM -0700, Alvaro Retana wrote:
> Dear authors:
> Thank you for the interesting work!

Thanks for the thorough review and suggestions.

The -10 version should resolve all the concerns you raised. 
Replies inline.

Doc & diff:

High level:

Your concern about the Controller description also had me to revisit how rfc8279
described the BIER layer, so i enhanced the text of section 3 to use the same structure
as rfc8729 section 4.2, except that the TE draft splits it up into
now section 3.2 for the control plane (of which the TE controller is now a part),
and section 3.3 for the forwarding plane. Both together constitute the BIER-TE layer.

Your concerns about comparison BIER/BIER-TE and co-existance was resolved
not only be th coalescing you suggested, but the resulting section 2.3 is
renamed from "comparison" to "relationship" with section for unchanged/changed/
not-neeed/co-existance, and the encap consideration section 4.3 beter details
how co-existance depends on the BIER packet header and BIFT assignment mechanism

Detailled answers to your 2 mail concerns from here on.  The (a) ... (i) markers
in the following reply are repeated in the changelog, so that other reviewers
can track back all changes made to your specific concerns/suggestions here.

--- Second Mail from Alvaro:

> > "replaced" is certainly how the doc is written, so as to make it read
> > as simple as possible. The intent of course is to "amend" BIER by sharing
> > as much as possible with it such that a joint BIER/BIER-TE implementation
> > is but slightly more work than just a BIER implementation.
> Simplicity is good, but being clear is better.  I would like to see a
> list of the things in the BIER layer that are not replaced.  I
> remember text about the differences -- think of something similar for
> the things that don't change.

Done, see below

--- First mail from Alvaro:

> I have a couple of high-level comments/concerns.
> (1) What is this document specifying?
>   To start, I believe it is ok for an architecture document to be in the
>   Standards Track.

Ack. RFC8279 is also standards track. No change in draft required.

>   As far as I can see, this document mainly specifies alternate semantics for
>   the BitString.

Yes, (a)

>   Beyond that, the BIER-TE architecture (§2) maps well onto
>   the layering described in §4/rfc8297, where the BIER layer (§4.2/rfc8297) is
>   "replaced" with the BIER-TE Control Plane and the BIER-TE forwarding layer.
>   Is this a fair high-level representation of what is defined in this
>   document?

Yes, (b) significantly enhanced text in 3. and its subsections to align
terminology and explanations bette with rfc8279. Primarily by explicitly
now describing a BIER-TE control plane of which the controller is but the
mayor component.

>   In general, the BIER-TE forwarding is well specified.  I would prefer it if
>   the definition of the adjacencies included normative language.  The
>   requirements section (§3.6) introduces some confusion with the use of "basic
>   BIER-TE forwarding" (vs "BIER-TE forwarding").  Suggestion: the
>   specification should include required ("basic") and recommended/optional
>   behaviors.  See specific comments in-line.

I could no find good places in the description of the adjacencies (4.2.*)
where i could make text better by adding MUST/SHOULD. If there are specific
suggestions, please mak them. In general, find the more compact summarizing of
the requirements in 4.5 better.

(c) I did clean up section 4.5 to remove "basic". Instead i now refer to
tha subset of BIER as the "mandatory" subset of BIER forwarding in the other places where
i used "basic", hence avoiding introduction of new name/term basic.

Also cleanup up text by separating out other requirements
into separate paragraphs.

>   OTOH, the functions of the BIER-TE control plane are described (not
>   specified) in what I consider a set of operational considerations (things
>   the controller could consider -- including sections 4, 5, and 7).  Having an
>   extensive set of operational considerations is a good thing, specially given
>   how much BIER-TE relies on the controller.  The BIER-TE control protocol is
>   central to the operation/implementation of BIER-TE, but left out of scope
>   (see my comments in §2.2).
>   The BIER-TE topology is a "key new component in BIER-TE".  The document
>   doesn't specify, explain, or leave out of scope how "BIER-TE Controller
>   discovers the network topology and creates the BIER-TE topology from it"
>   (§2.2).  This omission is a significant hole in the architecture.

(d) Added section with text we already discussed giving examples
to topology discovery options.

>   It would be ideal if the Introduction included a high-level overview of the
>   document.

(e) Added bullet point iss.

> (2) Can BIER and BIER-TE really coexist in the same network?
>   The Abstract mentions that they can:
>      BIER-TE can co-exist with BIER forwarding in the same domain, for
>      example by using separate BIER sub-domains.
>   The result of separate sub-domains is more akin to ships-in-the-night than
>   having them be "mixed": in the same sub-domain using a single BIFT
>   (populated by different sources).   Is this correct?

Yes, there is no intention in this architecture to state "one bit in a
BIFT can have a BIER-TE semantic, another can have a BIER semantic". Purely
from HW-forwarding this would not be an issue, and i thouht long time about
it, but operationally i never found a simple example case where this would
be more beneficial than what you call ships-in-the-night per-SD.

Nevertheless, in many encap options you can actually have a BIER-TE BIFT
and ships-in-the-night a BIER BIFT, because rfc8296 only says "BIFT" in
the header, and the decision whether a BIFT is BIER or BIER-TE can
be taken elsewhere (config, controller,...). Whether reuse of the same
SD numbers is beneficial or confusing is an operational question.

(f) This is now better explained in section 4.3

>   §3.3 speculates about potential "definitions in BIER encapsulation
>   specifications" to "distinguish BIER from BIER-TE packets"

Right. We had a draft proposing modifying the BIER header to have e.g.:
a bit indicating BIER-TE instead of BIER. That was met with little
support, but text leftover.

The section 4.3 (f) has removed this statement.

>    -- and offers a
>   workaround if the MPLS encapsulation is used.

The reworked text of 4.3 (f) makes he situation hopefully very clear.

>   Even here, a "mixed" environment would seem to at least require independent
>   BIFTs, and not be possible with non-MPLS encapsulations.

Always require separate BIFT, doesn't mater whether you share SD
address space or partition it.

>   My conclusion is that using the encapsulation from rfc8296 it is not
>   possible to have a "mixed" BIER/BIER-TE network -- unless using MPLS with
>   extra labels and separate BIFTs.   This is just a guess --  the coexistence
>   topic deserves better coverage so that no one has to guess.

Right. The new text in 4.3 (f) and 2.3, point 4 (f) is addressing these points.

> (3) Organization
>   The document jumps right into examples and a short discussion of the BIER-TE
>   topology -- including a quick comparison with BIER (§1.2).  There are 3
>   other sections that are also called "comparison with BIER" (§1.3, §3.5, and
>   §7.2). It may make the document clearer if the "baseline" comparison with
>   BIER was set from the start (you can dig deeper later of course).


Actual comparisons merged into section 2 introduction.
1.3 is now 2.3 - Relationship to BIER
3.5 is now 2.4 - Accelerated/Hardware forwarding comparison

5.3.2 removed "comparison" from title (was misleading anyhow)

>   §3 describes BIER-TE forwarding, but sample pseudocode is in §6.  Please
>   move that to §3.


Cleaned up section 3 (now section 4) by removing section 3.4 (old example,
previously marked for RFC editor removal).

Section 6 (Pseudocode) is now section 4.4.

>   As I mentioned above, several of the sections (4, 5, and 7) include
>   considerations for the BIER-TE Controller.  It would be great if these
>   sections were consolidated under a single heading: Operational
>   Considerations for the BIER-TE Controller (for example).

Merged into new section 5 "BIER-TE Controller operational considerations"

> [Line numbers from idnits.]
> 13	Abstract
> [] In general, I think this Abstract is longer than needed -- in fact,
> it is longer than the initial part of the Introduction.  Consider
> making it shorter.

(a) Done

> 15	   This memo introduces per-packet stateless strict and loose path
> 16	   steered replication and forwarding for Bit Index Explicit Replication
> 17	   packets (RFC8279).  This is called BIER Tree Engineering (BIER-TE).
> 18	   BIER-TE can be used as a path steering mechanism in future Traffic
> 19	   Engineering solutions for BIER (BIER-TE).
> [major] "BIER-TE" has two different meanings?  I'm assuming the last
> mention is just a leftover.

(a) Removed.

> 25	   In BIER, the BitPositions (BP) of the packets bitstring indicate BIER
> 26	   Forwarding Egress Routers (BFER), and hop-by-hop forwarding uses a
> 27	   Routing Underlay such as an IGP.
> [major] The terminology used here doesn't correspond to what is used
> in rfc8279.  Please be consistent and don't make up new terminology
> unless it is to present something new.
> "BitPositions (BP)" doesn't appear in rfc8297.  Instead, "bit position" is used.
> s/bitstring/BitString/g
> s/BIER Forwarding Egress Routers (BFER)/Bit-Forwarding Egress Routers (BFERs)

(i) Ack. Thanks. Also s/subdomain/sub-domain/.

> 29	   In BIER-TE, BitPositions indicate adjacencies.  The BIFT of each BFR
> 30	   are only populated with BPs that are adjacent to the BFR in the BIER-
> 31	   TE topology.  The BIER-TE topology can consist of layer 2 or remote
> 32	   (routed) adjacencies.  The BFR then replicates and forwards BIER
> 33	   packets to those adjacencies.  This results in the aforementioned
> 34	   strict and loose path steering and replications.
> [minor] Expand all acronyms in the Abstract *and* on first mention later on

Did my best. Pretty sure i will need to do another run.

> [nit] s/The BIFT of each BFR are only populated/The BIFT of each BFR
> is only populated

removed by shortened abstract.

> 36	   BIER-TE can co-exist with BIER forwarding in the same domain, for
> 37	   example by using separate BIER sub-domains.  In the absence of routed
> 38	   adjacencies, BIER-TE does not require a BIER routing underlay, and
> 39	   can then be operated without requiring an Interior Gateway Routing
> 40	   protocol (IGP).
> [] This paragraph, for example, provides information that doesn't seem
> to be easily located in the document body.  I can't find another
> mention of "co-exist" or easily determine where running separate
> sub-domains (for BIER and BIER-TE) is covered.  [The spelling is
> different, but I finally found some discussion about "subdomains" in
> §3.3.]

Changed title of 4.3 to "Encapsulation / Co-existance with BIER-TE", also
used term Co-exist.. in Abstract, Overview and Intro 2.3, point 4.  (j)

> ...
> 47	Name explanation
> 49	   [RFC-editor: This section to be removed before publication.]
> [] As I mentioned above, and is explained below, "BIER-TE" now has two
> different meanings. :-(

Well, i think you did follow the WG last-call discussion. 

> IMO, this section will only result in
> distracting from the contents of the document.  Because it will be
> deleted before publication anyway, I strongly suggest that you remote
> it.  It anything, the Shepherd may want to include it in the write-up.

Removed whole section (a). 
> 51	   Explanation for name change from BIER-TE to mean "Traffic
> 52	   Engineering" to BIER-TE "Tree Engineering" in WG last-call (to
> 53	   benefit IETF/IESG reviewers):
> [nit] Up to this point, the IESG hasn't reviewed this document.  The
> comments did come from an AD, but it wasn't during IESG Evaluation.
> This fact doesn't mean that the comments are more or less valid, I'm
> just clarifying that the IESG hasn't looked at this document, so
> mentioning "IESG reviewers" may, again, distract form the
> specification.
> ...
> 172	1.  Introduction
> 174	   BIER-TE shares architecture, terminology and packet formats with BIER
> 175	   as described in [RFC8279] and [RFC8296].  This document describes
> 176	   BIER-TE in the expectation that the reader is familiar with these two
> 177	   documents.
> [minor] "BIER-TE shares architecture..."  Not the complete
> architecture since the BitString indicates something different.  Maybe
> write something like s/architecture/most architectural concepts

"is based on" (k)

> ...
> 186	   Note that related work, [I-D.ietf-roll-ccast] uses Bloom filters
> 187	   [Bloom70] to represent leaves or edges of the intended delivery tree.
> 189	   Bloom filters in general can support larger trees/topologies with
> 190	   fewer addressing bits than explicit bitstrings, but they introduce
> 191	   the heuristic risk of false positives and cannot reset bits in the
> 192	   bitstring during forwarding to avoid loops.  For these reasons, BIER-
> 193	   TE uses explicit bitstrings like BIER.  The explicit bitstrings of
> 194	   BIER-TE can also be seen as a special type of Bloom filter, and this
> 195	   is how related work [ICC] describes it.
> [minor] I don't see any value in including these last 2 paragraphs:
> you're basically telling the reader that someone else didn't chose the
> same approach.

No, that is not the important message of these paragraphs. The important
message is that bloom filters have been proposed as a way to better scale
bitstring forwarding (fewer bits in packet to represent mor bits in topology),
but that this BIER-TE proposal can not use such (compressing) heuristic solution
because the false positives would likely cause loops.

> 197	1.1.  Basic Examples
> ...
> 239	   Consider the simple network in the above BIER-TE overview example
> 240	   picture with 6 BFRs. p1...p14 are the BitPositions (BP) used.  All
> 241	   BFRs can act as ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can
> 242	   also be egress BFR (BFER).  Forward_connected is the name for
> 243	   adjacencies that are representing subnet adjacencies of the network.
> 244	   Local_decap is the name of the adjacency to decapsulate BIER-TE
> 245	   packets and pass their payload to higher layer processing.
> [nit] s/act as ingress/act as an ingress
> [nit] s/egress BFR/egress BFRs


> 247	   Assume a packet from BFR1 should be sent via BFR4 to BFR6.  This
> 248	   requires a bitstring (p2,p8,p10,p12).  When this packet is examined
> 249	   by BIER-TE on BFR1, the only BitPosition from the bitstring that is
> 250	   also set in the BIFT is p2.  This will cause BFR1 to send the only
> 251	   copy of the packet to BFR2.  Similarly, BFR2 will forward to BFR4
> 252	   because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6 because
> 253	   of p12. p12 also makes BFR6 receive and decapsulate the packet.
> [minor] §1 says that "BPs are normally also reset upon forwarding to
> avoid duplicates and loops."  Doesn't that mean that BFR6 won't
> receive p12 set?

Yikes. great catch. I think i had started with an example that was using DNR
for BFR6 and later tried to simplify it (to not introduce DNR this early). Fixed by
adding p15 for BFR6 and appropriate description in text. (l)
> ...
> 271	   The following picture shows a modified example, in which Rtr2 and
> 272	   Rtr5 are assumed not to support BIER-TE, so traffic has to be unicast
> 273	   encapsulated across them.  Unicast tunneling of BIER-TE packets can
> 274	   leverage any feasible mechanism such as MPLS or IP, these
> 275	   encapsulations are out of scope of this document.  To emphasize non-
> 276	   native forwarding of BIER-TE packets, these adjacencies are called
> 277	   "forward_routed", but otherwise there is no difference in their
> 278	   processing over the aforementioned "forward_connected" adjacencies.
> [major] "leverage any feasible mechanism such as MPLS or IP, these
> encapsulations are out of scope of this document."
> I can see why the encapsulation is outside the scope of this document,
> but there are basic considerations (for example, that the
> encapsulation is able to indicate that the payload is a BIER-TE packet
> --- as explained in rfc8279/§6.9 for BIER) that should be mentioned
> (or referenced) here.

Removed the sentence in question as this is too early in the document to go into
explanations. Rewrote text of definition section 4.2.2 (forward_routed)
to resolve the concern, but also to make the text better and shorter. Relevant
sentence there is now:

 This can leverage any feasible encapsulation, such as MPLS or tunneling over IP/IPv6, as long as the BIER-TE packet can be identified as a payload. This identification can either rely on the BIER/BIER-TE co-existance mechanisms described in <xref target="encapsulation"/>, or by explicit support for a BIER-TE payload type in the tunneling encapsulation.</t>


> 322	1.2.  BIER-TE Topology and adjacencies
> ...
> 329	   The BIER-TE Topology consists of the BIFT of all the BFR and can also
> 330	   be expressed as a directed graph where the edges are the adjacencies
> 331	   between the BFR labelled with the BP used for the adjacency.
> 332	   Adjacencies are naturally unidirectional.  BP can be reused across
> 333	   multiple adjacencies as long as this does not lead to undesired
> 334	   duplicates or loops as explained further down in the text.
> [nit] s/BIFT of all the BFR/BIFTs of all the BFRs
> [nit] s/between the BFR/between the BFRs


> 336	   If the BIER-TE topology represents the underlying (layer 2) topology
> 337	   of the network, this is called "native" BIER-TE as shown in the first
> 338	   example.  This can be freely mixed with "overlay" BIER-TE, in
> 339	   "forward_routed" adjacencies are used.
> [nit] s/This/This type of topology (?)

<t>If the BIER-TE topology represents (a subset of) the underlying (layer 2)
topology of the network as shown in the first example, this may be called a "native"
BIER-TE topology. A topology consisting only of "forward_routed" adjacencies as
shown in the second example may be called an "overlay" BIER-TE topology.
A BIER-TE topology will both "forward_connected" and "forward_routed" adjacencies
may be called a "hybrid" BIER-TE topology.</t>

(using "may", as i don't really want to standardize such name callin but it is helpfull).


> 341	1.3.  Comparison with BIER
> 343	   The key differences over BIER are:
> [minor] s/over/with respect to

Ack (different place not in restructured section 2.3).

> ...
> 351	   o  BIER-TE in each BFR has no routing table but only a BIER-TE
> 352	      Forwarding Table (BIFT) indexed by SI:BitPosition and populated
> 353	      with only those adjacencies to which the BFR should replicate
> 354	      packets to.
> [minor] "BIER-TE Forwarding Table (BIFT)"  Is a BIFT a "BIER-TE
> Forwarding Table" or a "Bit Index Forwarding Table" (rfc8279)?  Please
> don't overload the meaning.


> ...
> 358	   BIER-TE forwarding does not require/use the BFIR-ID.  The BFIR-ID can
> 359	   still be useful though for coordinated BFIR/BFER functions, such as
> 360	   the context for upstream assigned labels for MPLS payloads in MVPN
> 361	   over BIER-TE.
> [minor] s/BFIR-ID/BFIR-id/g
> That is the syntax from rfc8296.

Ack. Actually only BFR-id. Never used as BFIR-id.
And rfc8279 has a bunch of "bfr-id", but more "BFR-id" *sigh*

> [major] "BIER-TE forwarding does not require/use the BFIR-ID."  If the
> rfc8296 encapsulation is used, the BFIR-id is a required field.
> I understand that the BFIR-IDs are not used in the same way, but they
> are still required.  The last paragraph (below) talks about
> assignment, which contradicts the not-required characterization here.
> I think the issue is with using "required"...


This now explained in detail in section 2.3, points 3.2, 3.3 and 3.4.

[ Arguably, IMHO, BIER layer forwarding plane does not require BFR-id either,
  but rfc8279 isn't really clear about that, so i think point 3.4 is still
  correct as it refers to BIER architecture aka rfc8279. ]

So No: BFR-id is not required by BIER-TE unless the controller likes
them and/or some non-controller BIER-TE layer control plane is defined
that requires them.

BFR-ID is really only required when elements of BIER are used, this 
is detailled in section 2.3, point 4.2.

This is all a lot of fine-grained disection of functionality, hope i wrote it
down correctly. Maybe not very interesting for typical BIER/BIER-TE SP
neworks, but maybe more for BIER-TE only in e.g.: embedded/IoT BIER-TE networks.

> [minor] "The BFIR-ID can still be useful..."  The utility is just an
> example, right?   Because the paragraph started by stating that
> BFIR-IDs are not required/used, the second sentence sounds out of
> place and potentially confusing: there's no further mention in this
> document, no reference...

replaced by above (f) changes.

> ...
> 366	   If the BIER-TE domain is not running full BIER or does not want to
> 367	   reduce the need to allocate bits in BIER bitstrings for BFIR-ID
> 368	   values, then the allocation of BFIR-ID values in BIER-TE packets can
> 369	   be done through other mechanisms outside the scope of this document,
> 370	   as long as this is appropriately agreed upon between all BFIR/BFER.
> [] "reduce the need to allocate bits in BIER bitstrings for BFIR-ID
> values"  What does this phrase mean?

I removed his paragraph as it was too much discussion about operations of
BIER-TE controller for this section. Section 5.3 should capture all the
relevant aspects.

The paragraph had a factual bug.

> [] Related to the other comments (above) about the BFIR-id, this
> paragraph indicates that they are needed.

FYI: When a BIER-TE deployment does not re-use parts of BIER layer control plane,
then bfr-id does not need to be derived from SD,BSL,SI. When BIER-TE deployment
does not use a flow overlay (such as MVPN/BGP) that uses bfr-id to know
the BFIR on the BFER, then bfr-id in packets is not needed a all.

> [minor] "allocation of BFIR-ID values... can be done through other
> mechanisms outside the scope of this document"   Ok -- but §7.4 deals
> specifically with the assignment of BFR-ids.  I guess that pointing at
> §7.4 as an example/set of considerations is ok.

See above.

> 372	1.4.  Requirements Language
> 374	   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
> 375	   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
> 376	   document are to be interpreted as described in RFC 2119 [RFC2119].
> [major] Use the rfc8174 template.


> 378	2.  Components
> 380	   End to end BIER-TE operations consists of four mayor components: The
> 381	   "Multicast Flow Overlay", the "BIER-TE control plane" consisting of
> 382	   the "BIER-TE Controller" and its signaling channels to the BFR, the
> 383	   "Routing Underlay" and the "BIER-TE forwarding layer".  The Bier-TE
> 384	   Controller is the new architectural component in BIER-TE compared to
> 385	   BIER.
> [nit] s/Bier/BIER


> 387	      Picture 2: Components of BIER-TE
> [nit] The figure number/legend is also below the figure.  No need for this text.


> ...
> 417	2.2.  The BIER-TE Controller
> 419	   The BIER-TE Controller is representing the control plane of BIER-TE.
> 420	   It communicates two sets of information with BFRs:
> [nit] s/is representing/represents
> [nit] s/with/to

not present in -10 rewrite of secion.

> 422	   During initial provisioning or modifications of the network topology,
> 423	   the BIER-TE Controller discovers the network topology and creates the
> 424	   BIER-TE topology from it: determine which adjacencies are required/
> 425	   desired and assign BitPositions to them.  Then it signals the
> 426	   resulting of BitPositions and their adjacencies to each BFR to set up
> 427	   their BIER-TE BIFTs.
> [major] "BIER-TE Controller discovers the network topology and creates
> the BIER-TE topology from it"   How?  There are two steps here:
> discovery of the network topology and creating the BIER-TE topology.


Check, this hopefully answers both points appropriately.

> [nit] s/resulting of BitPositions/resulting BitPositions

Not in rewritten text.

> 432	   Communications between the BIER-TE Controller and BFRs is ideally via
> 433	   standardized protocols and data-models such as Netconf/Restconf/Yang.
> 434	   This is currently outside the scope of this document.  Vendor-
> 435	   specific CLI on the BFRs is also a possible stopgap option (as in
> 436	   many other SDN solutions lacking definition of standardized data
> 437	   model).
> [major] Even though there is explicit mention of data models and the
> use of specific protocols, this document doesn't dictate a specific
> configuration methodology -- just like BIER (rfc8279/§7.1).   This is
> ok.
> PCE is not mentioned as a possibility.  I point this out simply
> because there are 2 PCE-related active individual drafts
> (draft-chen-pce-bier, draft-chen-pce-bier-te-path), but no active YANG
> work (I know the drafts are expired).
> My suggestion is to avoid mentioning specifics and provide a statement
> similar to rfc8279 so that the choice of control protocol is left
> open.

Hmm.. for those readers who are new to the subject i do like to give examples,
in the same way as you did explicitly ask for examples about topology discovery
protocols/mechanisms (at least thats how i did read you). Even if just
for consistency in the degree of detail across discovery and creation.

So for now i have added PCEP to the list. Lets see what further reviewers say.

> [minor] s/currently outside the scope/outside the scope/g

Ok, i removed this "outside scope" disclaimer sentence. Get's too repetitive
for an architecture doc.

> [minor] s/a possible stopgap option/an option
> [nit] s/of standardized/of a standardized


> ...
> 446	2.2.1.  Assignment of BitPositions to adjacencies of the network
> 447	        topology
> 449	   The BIER-TE Controller tracks the BFR topology of the BIER-TE domain.
> 450	   It determines what adjacencies require BitPositions so that BIER-TE
> 451	   explicit paths can be built through them as desired by operator
> 452	   policy.
> [major] "BIER-TE Controller tracks the BFR topology"  How does it do that? does not have this sentence anymore, because i tried to uplevel,
and avoid repeating too much of a detailled explanation of how a PCE works,
which effectively is what a BIER-TE controller is. Aka: via the topology
discovery mechanisms and/or maybe TBD OAM mechanisms to discover path failures.

The fact that the BIER-TE controller is a PCE alas was in the initial
section we cut out (relationship to traffic engineering), but i do think
instead of adding more and more to the BIER-TE controller section here,
it would rather make sense to revive draft-eckert-teas-bier-te-framework
because that would also have the discussion about those aspects take place
 in the appropriate WG.

> ...
> 480	2.2.4.  Link/Node Failures and Recovery
> 482	   When link or nodes fail or recover in the topology, BIER-TE can
> 483	   quickly respond with the optional FRR procedures described in [I-
> 484	   D.eckert-bier-te-frr].  It can also more slowly react by
> 485	   recalculating the BitStrings of affected multicast flows.  This
> 486	   reaction is slower than the FRR procedure because the BIER-TE
> 487	   Controller needs to receive link/node up/down indications,
> 488	   recalculate the desired BitStrings and push them down into the BFIRs.
> 489	   With FRR, this is all performed locally on a BFR receiving the
> 490	   adjacency up/down notification.
> [minor] There's no reference for I-D.eckert-bier-te-frr.


> ...
> 505	2.4.  The Routing Underlay
> ...
> 515	   BIER relies on the routing underlay to calculate paths towards BFER
> 516	   and derive next-hop BFR adjacencies for those paths.  This commonly
> 517	   relies on BIER specific extensions to the routing protocols of the
> 518	   routing underlay but may also be established by a controller.  In
> 519	   BIER-TE, the next-hops of a packet are determined by the bitstring
> 520	   through the BIER-TE Controller established adjacencies on the BFR for
> 521	   the BPs of the bitsring.  There is thus no need for BFER specific
> 522	   routing underlay extensions to forward BIER packets with BIER-TE
> 523	   semantics.
> [nit] s/towards BFER/towards BFERs


> [major] Just to make sure I'm understanding.  In BIER-TE there is no
> need to learn the BP's because they are all locally significant, and,
> more importantly, defined by the controller.  Is that it?    [It makes
> sense to me, just asking because I see several BIER-TE drafts related
> to IGP extensions in the datatracker.]

Something like draft-chen-bier-te-isis-00.txt ?

I have not looked into detail on any of these drafts. The reuse of the
BAR / IPA fields for example may not make sense for BIER-TE, lets not
delve into those details here.

So, i wouldn't say it doesn't make sense to distribute BP of BIER-TE via
an IGP. Section 3.2 (BIER-TE Control Plane) now calls a single centralized
controller a component of the reference mode for the BIER-TE control plane.
If you have such a single controller that does everything there is likely
no benefit in distributing BIER-TE BP via IGP.

But think  about a decentralized system. Maybe there is one centralized
controller that is creating the topology, pushing the BP. Then the IGP
distributes them. Then every BFIR has a lightweight built-in controller
only responsible to calculate the bitstrings for trees originating on this
BFIR. And that conroller would get the info about available BP from the
IGP. Think original CSPF model in RSVP-TE. Obviously more resilient
han having the centralized controller send the info to all BFIR out-of-band.
By leveraing the IGP one could ensure only the actually BFR installed BP
would be signalled for example.

Did i mention a different home like draft-eckert-teas-bier-te-framework
as the appropriate place to elaborate on any such options ? ;-)

> ...
> 539	2.5.  Traffic Engineering Considerations
> ...
> 548	   Policy decisions are made within the BIER-TE control plane, i.e.,
> 549	   within BIER-TE Controllers.  Controllers use policy when composing
> 550	   BitStrings (BFR flow state) and BFR BIFT state.  The mapping of user/
> 551	   IP traffic to specific BitStrings/BIER-TE flows is made based on
> 552	   policy.  The specifics details of BIER-TE policies and how a
> 553	   controller uses such are out of scope of this document.
> [] "composing BitStrings (BFR flow state)"  It feels like you really
> want to say something more here, but the mention of "BFR flow state"
> is confusing without other context.  Please elaborate or take the
> phrase out.


> [nit] s/specifics details/specific details


> [nit] s/uses such/uses them


> 555	   Path steering is supported via the definition of a BitString.
> 556	   BitStrings used in BIER-TE are composed based on policy and resource
> 557	   management considerations.  When composing BIER-TE BitStrings, a
> 558	   Controller MUST take into account the resources available at each BFR
> 559	   and for each BP when it is providing congestion loss free services
> 560	   such as Rate Controlled Service Disciplines [RCSD94].  Resource
> 561	   availability could be provided for example via routing protocol
> 562	   information, but may also be obtained via a BIER-TE control protocol
> 563	   such as Netconf or any other protocol commonly used by a PCE to
> 564	   understand the resources of the network it operates on.  The resource
> 565	   usage of the BIER-TE traffic admitted by the BIER-TE controller can
> 566	   be solely tracked on the BIER-TE Controller based on local accounting
> 567	   as long as no forward_routed adjacencies are used (see Section 3.2.1
> 568	   for the definition of forward_routed adjacencies).  When
> 569	   forward_routed adjacencies are used, the paths selected by the
> 570	   underlying routing protocol need to be tracked as well.
> [major] "Controller MUST take into account..."  If the specific
> details of how a controller uses a policy are out of scope, how can
> then also be normatively required?   s/MUST/must


> [minor] "congestion loss free services"  This is the only time when a
> specific type of service is called out.  Is it necessary to do so?  Is
> this the only type of traffic that would be traffic engineered?
> Should others be specifically mentioned elsewhere?   I don't think so,
> just asking...

Prepended sentence with "For example, "

> [minor] "used by a PCE"  This is also the only place where a PCE is
> specifically mentioned.  In other places a generic "BIER-TE
> controller" is used -- please be consistent.  More importantly, §2.2
> doesn't even come close to mentioning the possibility of using PCEP as
> the BIER-TE control protocol.   There's nothing wrong with not
> mentioning PCE/PCEP there, it just feels sloppy that many options are
> mentioned but not a single one required in a standards track document.

The earlier text now also includes PCEP.

The initial section 2.2 is enlisting a bunch of of example protocols sufficient
to do network topology discovery. 

Only this section talks about traffic engineering, and for that you
also need network topology resource discovery. aka: not only what
links there are but speeds and qos, and usage of them. NetConf and PCEP
come to mind, but i rally wanted to keep this section as short as possible
(did i mention draft-eckert-teas-bier-te-framework ? ;-).

Also: this is an architecture document. Remember that rfc8279 does
also not mandate any specific control plane protocol option (or even
enlist them).

IMHO: In reality, both rfc8279 and this document have normative section(s)
about the forwarding plane and informational parts about the dependencies
to build them.

Note that in RFC8994 we had the same discussion and
resolved it by labelling mayor sections with Normative vs. Informative.
This would equally be possible with rc8279bis and this very draft.

> 572	   Resource management has implications on the forwarding plane beyond
> 573	   the BIER-TE defined steering of packets.  This includes allocation of
> 574	   buffers to guarantee the worst case requirements of admitted RCSD
> 575	   trafic and potential policing and/or rate-shaping mechanisms,
> 576	   typically done via various forms of queuing.  This level of resource
> 577	   control, while optional, is important in networks that wish to
> 578	   support congestion management policies to control or regulate the
> 579	   offered traffic to deliver different levels of service and alleviate
> 580	   congestion problems, or those networks that wish to control latencies
> 581	   experienced by specific traffic flows.
> [nit] s/trafic/traffic


> ...
> 585	3.1.  The Bit Index Forwarding Table (BIFT)
> ...
> 592	   BIER-TE can support multiple subdomains like BIER.  Each one with a
> 593	   separate BIFT
> [minor] s/subdomains/sub-domains/g  That is how rfc8279 uses the term.

global fixed.

> [] Suggestion>
>    Like BIER, BIER-TE can support multiple sub-domains, each with a separate
>    BIFT.


 Btw: i think there is also some fundamental problem with the term BIFT
across our documents. I think in general we call a BIFT the information about
all BP in a sub-domain, which can be one or more SI, aka: bitstrings. But
then we use terms like BIFT-id in the rfc8296 header, which is not id)entifying
a BIFT, but a single BIFT:SI, so that would have better been called BIFTSI-id
or the like... And i think ther are other places where you have to infer
whether BIFT really means BIFT or BIFT:SI. And of course we have
BIFT enumeration proposal where the bift-id is not derived from SI... Oh well.

> 595	   In the BIER architecture, indices into the BIFT are explained to be
> 596	   both BFR-id and SI:BitString (BitPosition).  This is because there is
> 597	   a 1:1 relationship between BFR-id and SI:BitString - every bit in
> 598	   every SI is/can be assigned to a BFIR/BFER.  In BIER-TE there are
> 599	   more bits used in each BitString than there are BFIR/BFER assigned to
> 600	   the bitstring.  This is because of the bits required to express the
> 601	   engineered path through the topology.  The BIER-TE forwarding
> 602	   definitions do therefore not use the term BFR-id at all.  Instead,
> 603	   BFR-ids are only used as required by routing underlay, flow overlay
> 604	   of BIER headers.  Please refer to Section 7 for explanations how to
> 605	   deal with SI, subdomains and BFR-id in BIER-TE.
> [minor] "SI:BitString (BitPosition)"  The BitString is the collection
> of all BitPositions -- it is not clear to me what you're trying to
> indicate here (which is not in rfc8279).

Ok, radically shortened and hopefully improved paragraph:

<t>In <xref target="RFC8279"/>, Figure 2, indices into the BIFT are both SI:BitString and
BFR-id, where BitString is indicating a BP: BFR-id = SI * 2^BSL + BP.
As shown in <xref target="adjacencies"/>, in BIER-TE, only SI:BP are used as indices into a BIFT because they identify adjacencies
and not BFR.</t>

> [nit] "in each BitString...assigned to the bitstring"  Is there a
> difference between BitString and bitstring?  Maybe you meant bit
> string in the second case.

BitString is referring to the term used in Figure 2 of rfc8279.

> [minor] "In BIER-TE there are more bits used in each BitString than
> there are BFIR/BFER assigned to the bitstring."  BIER-TE doesn't
> assign bits to the BFERs.  I'm not sure what you're trying to say
> here.
> [nit] s/by routing underlay/by the routing underlay

Yep, all deleted. already explained now in section 2.

> [?] "flow overlay of BIER headers"  ??   I'm assuming you mean the
> Multicast Flow Overlay (still don't understand the BIER headers part).
>   Please be consistent in the naming: s/flow overlay/Multicast Flow
> Overlay/g

Yep, all deleted. already explained now in section 2.

> 607	     ------------------------------------------------------------------
> 608	     | Index:          |  Adjacencies:                                |
> 609	     | SI:BitPosition  |  <empty> or one or more per entry            |
> 610	     ==================================================================
> 611	     | 0:1             |  forward_connected(interface,neighbor{,DNR}) |
> 612	     ------------------------------------------------------------------
> 613	     | 0:2             |  forward_connected(interface,neighbor{,DNR}) |
> 614	     |                 |  forward_connected(interface,neighbor{,DNR}) |
> 615	     ------------------------------------------------------------------
> 616	     | 0:3             |  local_decap({VRF})                          |
> 617	     ------------------------------------------------------------------
> 618	     | 0:4             |  forward_routed({VRF,}l3-neighbor)           |
> 619	     ------------------------------------------------------------------
> 620	     | 0:5             |  <empty>                                     |
> 621	     ------------------------------------------------------------------
> 622	     | 0:6             |  ECMP({adjacency1,...adjacencyN}, seed)      |
> 623	     ------------------------------------------------------------------
> 624	     ...
> 625	     | BitStringLength |  ...                                         |
> 626	     ------------------------------------------------------------------
> 627	                      Bit Index Forwarding Table
> 629	                        Figure 4: BIFT adjacencies
> [] This table isn't referred to or explicitly explained anywhere.  The
> following sections seem to cover some of the entries, but no pointer
> back to it.

Added forward pointer to the picture in the paragraph (o).

> ...
> 635	   Adjacencies for the same BP when populated in more than one BFR by
> 636	   the BIER-TE Controller does not have to have the same adjacencies.
> 637	   This is up to the BIER-TE Controller.  BPs for p2p links are one case
> 638	   (see below).
> [nit] "Adjacencies ...does not have to have the same adjacencies."
> Maybe there's a better way to avoid redundancy when explaining.

You mean a less redundant way to redundantly write redundant text ? Thats hard.

Replaced paragraph with:

<t>Note that a BIFT index (SI:BP) may be populated in the BIFT of more
than one BFR. See <xref target="rings"/> for an example of how a BIER-TE controller
could assign BPs to (logical) adjacencies shared across multiple BFRs,
<xref target="leaf-bfer"> for an example of assigning the same BP to different
adjacencies, and <xref target="reuse"/> for guidelines regarding re-use of BPs across different 


> 640	   {VRF}indicates the Virtual Routing and Forwarding context into which
> 641	   the BIER payload is to be delivered.  This is optional and depends on
> 642	   the multicast flow overlay.
> [nit] s/{VRF}indicates/{VRF} indicates


> ...
> 646	3.2.1.  Forward Connected
> 648	   A "forward_connected" adjacency is towards a directly connected BFR
> 649	   neighbor using an interface address of that BFR on the connecting
> 650	   interface.  A forward_connected adjacency does not route packets but
> 651	   only L2 forwards them to the neighbor.
> [] Does this imply that the non-MPLS encapsulation from rfc8296 is used?

Nobody says rfc8296 must be used, but lets assume as it does not make
a difference IMHO.

But answer is no: forward_connected just means you do not need a routing underlay
to figure out how to forward. Any address is fine. Ethernet, IPv6 link-local
come to mind, but also the non-routable MPLS label (forgot right term for them).
subnet-local signaling to resolve those addresses to L2 encap addresses is fine.

> 653	   Packets sent to an adjacency with "DoNotReset" (DNR) set in the BIFT
> 654	   will not have the BitPosition for that adjacency reset when the BFR
> 655	   creates a copy for it.  The BitPosition will still be reset for
> 656	   copies of the packet made towards other adjacencies.  This can be
> 657	   used for example in ring topologies as explained below.
> [] I would prefer to see some normative language in this part of the
> specification: "DNR...MUST NOT reset the BPs.."   ??


> ...
> 674	3.2.3.  ECMP
> 676	   The ECMP mechanisms in BIER are tied to the BIER BIFT and are
> 677	   therefore not directly useable with BIER-TE.  The following
> 678	   procedures describe ECMP for BIER-TE that we consider to be
> 679	   lightweight but also well manageable.  It leverages the existing
> 680	   entropy parameter in the BIER header to keep packets of the flows on
> 681	   the same path and it introduces a "seed" parameter to allow for
> 682	   traffic flows to be polarized or randomized across multiple hops.
> [minor] "The following procedures describe ECMP..."  Which procedures?
>  The paragraph below just has one instruction ("must select the same
> adjacency..."), but I wouldn't call that a procedure (much less
> procedures).  Is it possible to at least illustrate?

Ok, shortened and hopefully improved text of his section as follows.
No picture instead reference to the section that excessively elaborates
on ECMP with picures.

<t>BIER ECMP is tied to the BIER BIFT processing semantic and are therefore
not directly useable with BIER-TE.</t>

<t>A BIER-TE "Equal Cost Multipath" (ECMP) adjacency has a list of two or
more non-ECMP adjacencies and a seed parameter. When a BIER-TE packet is copied
onto such an ECMP adjacency, an implementation specific so-called hash function
will select one out of the lists adjacencies to which the packet is forwarded.
This ECMP hash function MUST select the same adjacency from that list for all
packets with the same entropy parameter.  The seed parameter allows to design
hash functions that are easy to implement at high speed without running into
polarization issues across multiple consecutive ECMP hops. See <xref target="ecmp"/>
for more explanations.</t>

> [style nit] "we consider"  Don't write in first person.  s/.../is considered


> 684	   An "Equal Cost Multipath" (ECMP) adjacency has a list of two or more
> 685	   adjacencies included in it.  It copies the BIER-TE to one of those
> 686	   adjacencies based on the ECMP hash calculation.  The BIER-TE ECMP
> 687	   hash algorithm must select the same adjacency from that list for all
> 688	   packets with the same "entropy" value in the BIER-TE header if the
> 689	   same number of adjacencies and same seed are given as parameters.
> 690	   Further use of the seed parameter is explained below.
> [minor] s/copies the BIER-TE/copies the (BIER-TE) traffic

fixed in above shown text.
> [minor] "Further use of the seed parameter is explained below."  Add a
> reference to §4.7.

fixed in above shown text.

> 692	3.2.4.  Local Decap
> [minor] s/Local Decap/Local Decapsulation

used Decap(sulation)

> 694	   A "local_decap" adjacency passes a copy of the payload of the BIER-TE
> 695	   packet to the packets NextProto within the BFR (IPv4/IPv6,
> 696	   Ethernet,...).  A local_decap adjacency turns the BFR into a BFER for
> 697	   matching packets.  Local_decap adjacencies require the BFER to
> 698	   support routing or switching for NextProto to determine how to
> 699	   further process the packet.
> [major] "NextProto"  The name of this field in rfc8296 is simply
> "Proto", or you might want to call it "Next Protocol" with a pointer
> to the header (so it is not confused with the next protocol at other
> layers).

changed to:

A "local_decap" adjacency passes a copy of the payload of
the BIER-TE packet to the protocol within the BFR (IPv4/IPv6, Ethernet,...) responsible for
that payload according to the packet header fields

I hope this is no too generic, but i am not in he mood of explaining all
the options possible here. Could be self-identifying payload, could be
ue of the "Proto" field in rfc8296...

> 701	3.3.  Encapsulation considerations
> 703	   Specifications for BIER-TE encapsulation are outside the scope of
> 704	   this document.  This section gives explanations and guidelines.
> [major] I've been assuming all along that the rfc8296 encapsulation is
> used.  In fact, the previous section points at a field there.  That
> doesn't seem to be "out of scope".  What am I missing?

Independent of me just having fixed that mention to be more generic,
there is no reason why different headers could not have the same field.

I have never read rfc8279 to imply that there can be only one BIER encapsulation.
It only refers to information that needs to be carried by a BIER header.

That is also how we wrote the BIER-TE architecture. Even if there would ever 
and always be only one encapsulation does not mean its specification would be in scope
of this spec, right ?

Especially given how i would hope BIER-TE might get into other areas like ROLL
or other IOT as well, i think it would be career limiting to think there should
be only one encap, especially given how rfc8296 is a great superset of MPLS
and IP, which more constrained environments may consider to be "too great" (large).

> ...
> 720	   "forward_routed" requires an encapsulation permitting to unicast
> 721	   BIER-TE packets to a specific interface address on a target BFR.
> 722	   With MPLS encapsulation, this can simply be done via a label stack
> 723	   with that addresses label as the top label - followed by the label
> 724	   assigned to (SI,subdomain) - and if necessary (see above) BIER-TE.
> 725	   With non-MPLS encapsulation, some form of IP encapsulation would be
> 726	   required (for example IP/GRE).
> [minor] "and if necessary (see above) BIER-TE"  I guess you mean a
> "BIER-TE label", right?

hmm... can't figure out. Looks like sentence leftover. Replaces sentence in before:

followed by the label assigned to the (BSL,SD,SI) BitString.
> ...
> 733	3.4.  Basic BIER-TE Forwarding Example
> 735	   [RFC Editor: remove this section.]
> [] I don't mind the extra example.

Let's see, for now its gone. If additional review bring up questions where
he example would help, i will revive. But document is long enough that
an even more complex example doesn't seem to be helpful for reviewers to get
through the doc.

> 741	   Step by step example of basic BIER-TE forwarding.  This does not use
> 742	   ECMP or forward_routed adjacencies nor does it try to minimize the
> 743	   number of required BitPositions for the topology.
> [nit] s/This does not/This example does not
> ...
> 775	      BIFT BFIR2:
> 776	        p13: local_decap()
> 777	         p2: forward_connected(BFR3)
> [] Shouldn't BFIR2 also know about p14?  I'm assuming that LAN1 is
> running IGMP/MLD and that maybe BFIR2 is the DR.

In this example, LAN1 has only Src, e.g.: IPTV server side and
the TE controller has figured out that that BFIR1 would never need
to send to BFIR2 or vice versa. That same would IMHO even be true
if hose two where BFER.

> [nit] s/local_decap()/local_decap/g   To match how the rest of the
> document uses local_decap.


> ...
> 815	   BFR3 sees a BitString of p5,p7,p8,p10,p11,p12.  It is only interested
> 816	   in p1,p7,p8.  It creates a copy of the packet to BFER1 (due to p7)
> 817	   and one to BFR4 (due to p8).  It resets p7, p8 before sending.
> [] There's no p1 in the BitString.  I guess you mean that it is
> "interested" in p1 because that is in the BFIT.  The terminology is a
> little confusing because the BitString is presented first.  Perhaps
> reword as something like "BFR3 only has p1, p7 and p8 in it's BIFT, so
> it will only..."

fixed to: For those BP it has only adjacencies for p7,p8

> 819	   BFER1 sees a BitString of p5,p10,p11,p12.  It is only interested in
> 820	   p6,p7,p8,p11 and therefore considers only p11. p11 is a "local_decap"
> 821	   adjacency installed by the BIER-TE Controller because BFER1 should
> 822	   pass packets to IP multicast.  The local_decap adjacency instructs
> 823	   BFER1 to create a copy, decapsulate it from the BIER header and pass
> 824	   it on to the NextProtocol, in this example IP multicast.  IP
> 825	   multicast will then forward the packet out to LAN2 because it did
> 826	   receive PIM or IGMP joins on LAN2 for the traffic.
> [minor] s/p6,p7,p8,p11/p6,p8,p11

Ack. I actually shortened the text to say "for those BP (of the received
Bitstring) it has only an adjacency for p11.

> [] The second and third sentences are redundant.

Yepp. shortened paragraph.

> ...
> 830	3.5.  Forwarding comparison with BIER
> 832	   Forwarding of BIER-TE is designed to allow common forwarding hardware
> 833	   with BIER.  In fact, one of the main goals of this document is to
> 834	   encourage the building of forwarding hardware that can not only
> 835	   support BIER, but also BIER-TE - to allow experimentation with BIER-
> 836	   TE and support building of BIER-TE control plane code.
> [major] "main goals of this document...allow experimentation with
> BIER-TE and support building of BIER-TE control plane code."
> control plane!?!?   I know this document was
> tagged as Experimental before -- maybe this text is just a leftover.
> ???

Yes. removed sentence (g).

> 838	   The pseudocode in Section 6 shows how existing BIER/BIFT forwarding
> 839	   can be amended to support basic BIER-TE forwarding, by using BIER
> 840	   BIFT's F-BM.  Only the masking of bits due to avoid duplicates must
> 841	   be skipped when forwarding is for BIER-TE.
> [major] What is "basic BIER-TE forwarding"?  I'm guessing it is a
> sub-set of what is discussed in this document, but which sub-set?  Is
> the reader to assume that "BIER-TE forwarding" (without "basic") is
> different?

Replaced "basic" with "mandatory" - as "basic" was removed in the Requirements
section (4.6) which now only talks about MUST (mandatory) forwarding
features and SHOULD / MAY (optional) features. (g)

> Note that §6 mixes the terms when introducing the pseudocode:  "The
> following simplified pseudocode for BIER-TE support
> basic BIER-TE forwarding."

Not sure what was wrong with the old text except the word "basic" (removed).
Maybe read again and tell me ?

> [minor] Please expand F-BM on first mention.


> [] "Only the masking of bits due to avoid duplicates must be skipped
> when forwarding is for BIER-TE."  I'm having a hard time parsing this
> sentence.

Only the resetting of bits to avoid duplicate
packets to a BFR neighbor is skipped in BIER-TE forwarding because it is not necessary
and could not be done when using BIER F-BM.
> 843	   Whether to use BIER or BIER-TE forwarding can simply be a configured
> 844	   choice per subdomain and accordingly be set up by a BIER-TE
> 845	   Controller.  The BIER packet encapsulation [RFC8296] too can be
> 846	   reused without changes except that the currently defined BIER-TE ECMP
> 847	   adjacency does not leverage the entropy field so that field would be
> 848	   unused when BIER-TE forwarding is used.
> [major] "BIER-TE ECMP adjacency does not leverage the entropy field"
> §3.2.3 says the opposite.

Sentence was removed as part of prior changes.

> 850	3.6.  Requirements
> [] I made the comments in this section as I was reading and (mostly)
> before realizing that it is here where you try to make the distinction
> between "basic BIER-TE forwarding" and "BIER-TE forwarding".  Please
> see some comments at the end related to the distinction.

Yes. Fixed to just MUST (called mandatory requirements in other sections)
and SHOULD/MAY (called optional in other sections) requirements. (c).

> 852	   Basic BIER-TE forwarding MUST support to configure Subdomains to use
> 853	   basic BIER-TE forwarding rules (instead of BIER).  With basic BIER-TE
> 854	   forwarding, every bit MUST support to have zero or one adjacency.  It
> 855	   MUST support the adjacency types forward_connected without DNR flag,
> 856	   forward_routed and local_decap.  All other BIER-TE forwarding
> 857	   features are optional.  These basic BIER-TE requirements make BIER-TE
> 858	   forwarding exactly the same as BIER forwarding with the exception of
> 859	   skipping the aforementioned F-BM masking on egress.
> [minor] s/support to configure/support configuring

All changes in this section under bullet (c) in changelog.

Eliminated by prior simplification/clarification of the text.
> [nit] "Basic BIER-TE forwarding MUST support to configure Subdomains
> to use basic BIER-TE forwarding rules (instead of BIER)."  There's a
> circular reference in basic BIER-TE supporting something to use basic
> BIER-TE...

Fixed to:
BFR MUST support to configure the BIFT of sub-domains so that they use
BIER-TE forwarding rules instead of BIER forwarding rules.

> [major] "...every bit MUST support to have zero or one adjacency."  It
> sounds like you're saying that each bit is required to represent at
> most one adjacency, or nothing.  This requirement forbids reusing the
> bits, or associating them with more than one adjacency:
> This is the same thing that has been described before, for example:
>    §1.2: "BP can be reused across multiple adjacencies..."
>    §1.3: "every BitPosition...indicates one or more adjacencies"
>    §3.2.3: "An "Equal Cost Multipath" (ECMP) adjacency has a list of two or
>    more adjacencies included in it."
> Presumably the statement in this section is still true for a local
> BFR, but putting it in the same paragraph as the initial requirement
> related to a whole sub-domain creates confusion, at best.

Title of section changed to: BFR Requirements for BIER-TE forwarding
(s as to correctly scope the section to what we're talking about:
individual BFR requirements.

These requirements do not impact the ability to do §1.2. They are
not sufficient to support more than one adjacency or ECMP though, that
is added through the later optional requirements.  Reason of course
being to have he MUST requirments be as close to BIER as possible.

> [major] "All other BIER-TE forwarding features are optional."  Are
> optional for basic BIER-TE forwarding?   If so, then what
> distinguishes the two modes?

Yepp, eliminated this bad wording by eliminating "basic". multiple
adjacency on a single BP and ECMP are SHOULD/MAY.

> [minor] Mixing of terms: "These basic BIER-TE requirements make
> BIER-TE forwarding..."

eliminated through new text.

> [minor] "aforementioned F-BM masking on egress"   Maybe this is what I
> didn't understand in §3.5, but I didn't see any mention of egress.


> 861	   BIER-TE forwarding SHOULD support the DNR flag, as this is highly
> 862	   useful to save bits in rings (see Section 4.6).
> [major] "SHOULD support the DNR flag"  When is it ok for BIER-TE
> forwarding to not support the DNR flag?  IOW, why is this a
> recommendation and not a requirement?

Note: changed name of DNR to DNC because of your comment further below.

Refined text that forward_routed, forward_conneced with a clear DNC is 
REQUIRED, and that a forward_routed, forward_connected with a set DNC flag

The text explains that this is beenficial for saving bits in rings.
The reason why is is not MUST is by implication of explanation in the
text that the MUST requirements are those that can be implemented with
one minimal change via the same Forwarding Pseudocode as used for BIER,
whereas DNC, multiple adjacencies and ECMP require more changes/enhancements,
as are shown in the second Forwarding Pseudocode.

> 864	   BIER-TE forwarding MAY support more than one adjacency on a bit and
> 865	   ECMP adjacencies.  The importance of ECMP adjacencies is unclear when
> 866	   traffic steering is used because it may be more desirable to
> 867	   explicitly steer traffic across non-ECMP paths to make per-path
> 868	   traffic calculation easier for BIER-TE Controllers.  Having more than
> 869	   one adjacency for a bit allows further savings of bits in hub&spoke
> 870	   scenarios, but unlike rings it is less "natural" to flood traffic
> 871	   across multiple links unconditional.  Both ECMP and multiple
> 872	   adjacencies are forwarding plane features that should be possible to
> 873	   support later when needed as they do not impact the basic BIER-TE
> 874	   replication loop.  This is true because there is no inter-copy
> 875	   dependency through resetting of F-BM as in BIER.
> [major] "BIER-TE forwarding MAY support more than one adjacency..."
> This text makes this support optional for both basic BIER-TE
> forwarding and BIER-TE forwarding.

See above, "basic" was eliminated, there are now only REQUIRED, RECOMMENDED
and OPTIONAL forwarding features.

Note too that i upgraded multiple adjacencies from MAY to SHOULD because
their bit savings in hub&spoke are very much comparable to DNC savings
in rings, and both hub&spoke and rins are equally found in network

> [major] "The importance of ECMP adjacencies is unclear..."  If
> unclear, why is it specified?  What are the operational considerations
> that should be taken into account when deciding to use ECMP
> adjacencies (if supported)?

Added reference to section that describes ECMP savings though an elaborate example.
Added to justification why ECMP is only MAY also the notion that
one can alernatively leverage ECMP of the routing underlay via

> ... 
> [] "further savings of bits in hub&spoke scenarios, but unlike rings"
> This sounds like a good start for operational considerations related
> to how to save bits.

This sentence replaced by one that also points to the right conroller
section for hub&spoke case as justification.

> [] "it is less "natural" to flood traffic across multiple links"
> Hmmm. I thought it was "natural" for multicast to forward traffic
> across multiple links.  Note that his is a specification --
> characterizing a behavior should be specific.

Right. Removed, see also previous comment.

> [nit] s/links unconditional/links unconditionally


> [] "features that should be possible to support later when needed"
> Again, great material for operational considerations.  When are these
> features needed?

When features are needed is answered through the above fixes, but i also
removed the explanation of implementations being able to add support
for these features into implementation "later", as this was too
much of assumptions against incremental forwarding plane implementation.

> [minor] "basic BIER-TE replication loop"   Using "loop" is not the
> best idea when talking about forwarding.  What is the "replication
> loop" anyway??

The replication loop is the For loop in the BIER Forwading Pseudocode,
but reworderd to avoid stumbing into such type of understanding problems.

> [] "...there is no inter-copy dependency through resetting of F-BM as
> in BIER."  You lost me again.  BTW, I couldn't find a mention of
> resetting (anything!) in rfc8279/rfc8296.

*argh*!!! (q)

I actually did not notice the difference in the documents choice of words until
you hereby dipped me into it: 8279 uses "clear" to set a bit to 0, and i used reset.
Set/Reset sounded best to me, and reset is a great word for a non-native,
because most inflections of reset ar also reset ;-)

Then again, clear is probably clearer (pun intended), because reset might be read as returning
a bit not to 0, but to whatever its original setting was after a change to
it. I now changed all instances of reset to clear. And of course also DoNotReset/DNR
to DoNotClear/DNC.

> [major] After reading this section several times, I think that using
> the "basic" terminology introduces significant confusion, especially
> because there's no way to distinguish whether a node only supports
> "basic" or not.  Please define the requirements as required for the
> "basic" flavor, and recommended/optional for the complete solution.

Yes, all fixed. Now i just wonder why RFC8174 does not include MANDATORY. The places
where i sed basic, i now have to say REQUIRED, but IMHO MANDATORY would sound better.
Oh well. Fun question for an rfc editor in i guess ;-)

> 877	4.  BIER-TE Controller BitPosition Assignments
> ...
> 883	   Because the size of the BitString is limiting the size of the BIER-TE
> 884	   domain, many of the options described exist to support larger
> 885	   topologies with fewer BitPositions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7,
> 886	   4.8).
> [nit] s/is limiting the size/limits the size


> 888	4.1.  P2P Links
> 890	   Each P2p link in the BIER-TE domain is assigned one unique
> 891	   BitPosition with a forward_connected adjacency pointing to the
> 892	   neighbor on the p2p link.
> [nit] s/P2p/P2P


> [minor] You mean the same BP for both directions, right?  It might be
> good to clarify.

Prephrased sentence.

> ...
> 899	4.3.  Leaf BFERs
> ...
> 912	   Leaf BFERs are BFERs where incoming BIER-TE packets never need to be
> 913	   forwarded to another BFR but are only sent to the BFER to exit the
> 914	   BIER-TE domain.  For example, in networks where PEs are spokes
> 915	   connected to P routers, those PEs are Leaf BFERs unless there is a
> 916	   U-turn between two PEs.  Consider how redundant disjoint traffic can
> 917	   reach BFER1/BFER2 in above picture: When BFER1/BFER2 are Non-Leaf
> 918	   BFER as shown on the right hand side, one traffic copy would be
> 919	   forwarded to BFER1 from BFR1, but the other one could only reach
> 920	   BFER1 via BFER2, which makes BFER2 a non-Leaf BFER.  Likewise BFER1
> 921	   is a non-Leaf BFER when forwarding traffic to BFER2.
> [minor] Please expand P/PE on first use.


> [nit] s/Leaf BFERs are BFERs/A leaf BFER is one


> [nit] s/in above picture/in Figure 8


> [minor] s/U-turn between two PEs/U-turn
> 923	   Note that the BFERs in the left hand picture are only guaranteed to
> 924	   be leaf-BFER by fitting routing configuration that prohibits transit
> 925	   traffic to pass through the BFERs, which is commonly applied in these
> 926	   topologies.
> [minor] This paragraph continues discussion about the left hand side
> of the picture -- this description started above and introduced the
> term "U-turn".  Consider grouping the common descriptions together --
> and avoid duplication.


> 928	   All leaf-BFER in a BIER-TE domain can share a single BitPosition.
> 929	   This is possible because the BitPosition for the adjacency to reach
> 930	   the BFER can be used to distinguish whether or not packets should
> 931	   reach the BFER.
> [nit] s/leaf-BFER/leaf-BFERs


> ...
> 937	4.4.  LANs
> 939	   In a LAN, the adjacency to each neighboring BFR on the LAN is given a
> 940	   unique BitPosition.  The adjacency of this BitPosition is a
> 941	   forward_connected adjacency towards the BFR and this BitPosition is
> 942	   populated into the BIFT of all the other BFRs on that LAN.
> [nit] s/In a LAN, the adjacency to each neighboring BFR on the LAN/
>         In a LAN, the adjacency to each neighboring BFR


> ...
> 952	   If Bandwidth on the LAN is not an issue and most BIER-TE traffic
> 953	   should be copied to all neighbors on a LAN, then BitPositions can be
> 954	   saved by assigning just a single BitPosition to the LAN and
> 955	   populating the BitPosition of the BIFTs of each BFRs on the LAN with
> 956	   a list of forward_connected adjacencies to all other neighbors on the
> 957	   LAN.
> [] "If Bandwidth on the LAN is not an issue..."  I don't understand
> how bw comes into play if the traffic needs to be forwarded to all
> neighbors anyway.  It seems that using a single BP may lead to L2
> multicast, while different BPs might now.  Just thinking out loud...

I did not include an L2 multicast adjacency into this document because
that one definitely requires an extension over rfc8296, so it could
equally be defined though such an encap extension RFC. But for BIER,
there would likely be more work. 

> [minor] "most BIER-TE traffic should be copied to all neighbors"  If
> the LAN shares a BP, how is traffic that doesn't need to be copied to
> all differentiated?  It seems that a separate BP per BFR would still
> be needed.  Am I missing something?

No, you understand perfectly: If most traffic should be flooded, the
assumption/hope is that you would not have to assign the same number
of BP to the non-L2 "flooded" traffic as you would do in the general purpose
case (BIER-TE topology designed to support any type of traffic with
individual BP for every underlay adjacency).

> 959	   This optimization does not work in the case of BFRs redundantly
> 960	   connected to more than one LANs with this optimization because these
> 961	   BFRs would receive duplicates and forward those duplicates into the
> 962	   opposite LANs.  Adjacencies of such BFRs into their LANs still need a
> 963	   separate BitPosition.
> [nit] s/one LANs/one LAN


> [nit] s/their LANs/their LAN


> 965	4.5.  Hub and Spoke
> ...
> 972	   This option is similar to the BitPosition optimization in LANs:
> 973	   Redundantly connected spokes need their own BitPositions.
> [minor] Why?  In this case the spokes are leaf-BFRs.

Added "unless they are themselves Leaf-BFER". Think about hub&spoke flooding 
to a bunch of branch edge-routers. There may be more hops beyond such a spoke.

> ...
> 982	4.6.  Rings
> ...
> 988	   For the rings shown in the following picture, a single BitPosition
> 989	   will suffice to forward traffic entering the ring at BFRa or BFRb all
> 990	   the way up to BFR1:
> [minor] s/the following picture/Figure 10


> ...
> 1013	   Note that this example only permits for packets to enter the ring at
> 1014	   BFRa and BFRb, and that packets will always travel clockwise.  If
> 1015	   packets should be allowed to enter the ring at any ring BFR, then one
> 1016	   would have to use two ring BitPositions.  One for clockwise, one for
> 1017	   counterclockwise.
> [minor] "only permits for packets to enter the ring at BFRa and BFRb"
> As long as the direction is maintained (clockwise), then the packets
> should be able to enter through any BFR.  Am I missing something here?

Fixed to: only permits for packets intended to make it all the way around the ring to enter it at BFRa and BFRb
> [nit] s/One for clockwise, one for counterclockwise./One for each
> direction: clockwise and counterclockwise.


> 1019	   Both would be set up to stop rotating on the same link, e.g.  L1.
> 1020	   When the ingress ring BFR creates the clockwise copy, it will reset
> 1021	   the counterclockwise BitPosition because the DNR bit only applies to
> 1022	   the bit for which the replication is done.  Likewise for the
> 1023	   clockwise BitPosition for the counterclockwise copy.  In result, the
> 1024	   ring ingress BFR will send a copy in both directions, serving BFRs on
> 1025	   either side of the ring up to L1.
> [nit] s/In result/As a result


> 1027	4.7.  Equal Cost MultiPath (ECMP)
> 1029	   The ECMP adjacency allows to use just one BP per link bundle between
> 1030	   two BFRs instead of one BP for each p2p member link of that link
> 1031	   bundle.  In the following picture, one BP is used across L1,L2,L3.
> [minor] s/the following picture/Figure 11


> ...
> 1057	   This document does not standardize any ECMP algorithm because it is
> 1058	   sufficient for implementations to document their freely chosen ECMP
> 1059	   algorithm.  This allows the BIER-TE Controller to calculate ECMP
> 1060	   paths and seeds.  The following picture shows an example ECMP
> 1061	   algorithm:
> [minor] s/The following picture/Figure 12


> ...
> 1069	   In the following example, all traffic from BFR1 towards BFR10 is
> 1070	   intended to be ECMP load split equally across the topology.  This
> 1071	   example is not meant as a likely setup, but to illustrate that ECMP
> 1072	   can be used to share BPs not only across link bundles, and it
> 1073	   explains the use of the seed parameter.
> [minor] "ECMP can be used to share BPs not only across link bundles"
> Remove "not only".   ???

Changed to: not only across link bundles, but also across alternative paths across different transit BFR

> ...
> 1128	   With the setup of ECMP in above topology, traffic would not be
> 1129	   equally load-split.  Instead, links L22 and L31 would see no traffic
> 1130	   at all: BFR2 will only see traffic from BFR1 for which the ECMP hash
> 1131	   in BFR1 selected the first adjacency in the list of 2 adjacencies
> 1132	   given as parameters to the ECMP.  It is link L11-to-BFR2.  BFR2
> 1133	   performs again ECMP with two adjacencies on that subset of traffic
> 1134	   using the same seed1, and will therefore again select the first of
> 1135	   its two adjacencies: L21-to-BFR4.  And therefore L22 and BFR5 sees no
> 1136	   traffic.  Likewise for L31 and BFR6.
> [nit] s/in above topology/in the topology above


> ...
> 1146	   Note that ECMP solutions outside of BIER often hide the seed by auto-
> 1147	   selecting it from local entropy such as unique local or next-hop
> 1148	   identifiers.  The solutions chosen for BIER-TE to allow the BIER-TE
> 1149	   Controller to explicitly set the seed maximizes the ability of the
> 1150	   BIER-TE Controller to choose the seed, independent of such seed
> 1151	   source that the BIER-TE Controller may not be able to control well,
> 1152	   and even calculate optimized seeds for multi-hop cases.
> [] "independent of such seed source that the BIER-TE Controller may
> not be able to control well"  Not sure what is meant here -- the
> sentence seems to read well with out this text.

Changed to: Allowing the BIER-TE Controller to explicitly set the seed gives
the ability for it to control same/different path selection across multiple
consecutive ECMP hops.
> 1154	4.8.  Routed adjacencies
> [minor] Do you mean Forward Routed adjacencies?  Later on you mention
> it, but it is not clear at first read because "routed" is not one of
> the defined types in §3.2.

Fixed all "Routed" to "Forward_routed" across text to avoid lazy abbreviating.

> 1156	4.8.1.  Reducing BitPositions
> ...
> 1174	   Assume the requirement in the above picture is to explicitly steer
> 1175	   traffic flows that have arrived at BFR1 or BFR4 via a shortest path
> 1176	   in the routing underlay "Network Area 1" to one of the following
> 1177	   three next segments: (1) BFR2 via link L1, (2) BFR2 via link L2, (3)
> 1178	   via BFR3.
> [minor] s/the above picture/Figure 14


> [nit] s/L2, (3)/L2, or (3)


> ...
> 1193	4.9.  Reuse of BitPositions (without DNR)
> ...
> 1200	   Because BP are reset after passing a BFR with an adjacency for that
> 1201	   BP, reuse of BP across multiple BFR does not introduce any problems
> 1202	   with duplicates or loops that do not also exist when every adjacency
> 1203	   has a unique BP: Instead of setting one BP in a BitString that is
> 1204	   reused in N-adjacencies, one would get the same or worse results if
> 1205	   each of these adjacencies had a unique BP and all of them where set
> 1206	   in the BitString.  Instead, based on the case, BPs can be reused
> 1207	   without limitation, or they introduce fewer path steering choices, or
> 1208	   they do not work.
> [?] "same or worse"   Worse?

The more BP i have the more bits i need to clear to avoid a loop.

Lets say i have 5 BFR each with a different adjacency. In one case i
figure out how i can reuse the same BP across those 5 adjacencies. Each
of those BFR would reset the BP when a packet passes by. If instead
each adjacency has a separate BP, there are 5 BP that all need to be
cleared to avoid a loop.

Ok, too academic.
Good BIER-TE controller developers will be happy
i figure this out themselves, haven seen comparable analysis in TEAS
documents either ;-))

Deleted starting from ":".

> [?] "BPs can be reused without limitation, or they introduce fewer
> path steering choices, or they do not work."   What?

Replaced text after ":" with:

"Instead, the challenge when reusing BP is whether it
allows to still achieve the desired Tree Engineering goals."

This sets the stage for the following paragraph explainin exclusions.

> ...
> 1216	   An example of (A) was given in Figure 13, where BP 0:7, BP 0:8 and BP
> 1217	   0:9 are each reused across multiple BFR because a single packet/path
> 1218	   would never be able to reach more than one BFR sharing the same BP.
> [nit] s/multiple BFR/multiple BFRs


> ...
> 1235	   Reuse may also save BPs in larger topologies.  Consider the topology
> 1236	   shown in Figure 17, but only the following explanations: A BFIR/
> 1237	   sender (e.g.: video headend) is attached to area 1, and area 2...6
> 1238	   contain receivers/BFER.  Assume each area had a distribution ring,
> 1239	   each with two BPs to indicate the direction (as explained in before).
> 1240	   These two BPs could be reused across the 5 areas.  Packets would be
> 1241	   replicated through other BPs to the desired subset of areas, and once
> 1242	   a packet copy reaches the ring of the area, the two ring BPs come
> 1243	   into play.  This reuse is a case of (B), but it limits the topology
> 1244	   choices: Packets can only flow around the same direction in the rings
> 1245	   of all areas.  This may or may not be acceptable based on the desired
> 1246	   path steering options: If resilient transmission is the path
> 1247	   engineering goal, then it is likely a good optimization, if the
> 1248	   bandwidth of each ring was to be optimized separately, it would not
> 1249	   be a good limitation.
> [] Figure 17 is all the way in §7.5.1.  Consider duplicating it here
> to help in the reading/continuity.

(r) Copied picture, enhanced it to show described details, also makes it not
just a copy anymore. Adjusted text to fit.

> [minor] "but only the following explanations"   I haven't read §7.5.1
> yet, but assume that there is an alternate description of the figure
> there.  Without that knowledge (a guess at this point) this text feels
> out of place.  Another reason to consider duplicating the figure
> here...

uhmmm.. lazyness in prior update where i added this text in response to review. Sorry.
Any linkage to 7.5.1 is gone now.

> [nit] s/as explained in before/as explained before


> 1251	4.10.  Summary of BP optimizations
> [] Maybe move the the start.

I prefer this summary to be at the end, re-emphasizing that controllers have a 
wide range of optimization options. This is all informational/educational, so
one rather re-summarize to uplevel what was explained in before instead of
brining it in h beginning where it might confuse the audience es it hasn't been

> ...
> 1268	   o  A LAN with N BFR needs at most N BP (one for each BFR).  It only
> 1269	      needs one BP for all those BFR tha are not redundanty connected to
> 1270	      multiple LANs (Section 4.4).
> [nit] s/tha are not redundanty/that are not redundantly


> ...
> 1302	5.1.  Loops
> ...
> 1309	   With DNR set, looping can happen.  Consider in the ring picture that
> 1310	   link L4 from BFR3 is plugged into the L1 interface of BFRa.  This
> 1311	   creates a loop where the rings clockwise BitPosition is never reset
> 1312	   for copies of the packets traveling clockwise around the ring.
> [minor] "the ring picture"  Which one?  Refer to a Figure and consider
> duplicating it closer to where it is refered to.

(s) Duplicated picture, added visuals for described miswiring.

> [minor] "link L4 from BFR3 is plugged into the L1 interface of BFRa"
> Assuming you're talking about Figure 10...  L4 seems to be the link
> between BFR3 and BFR2, and L1 in BFRa is connected to BFR1 -- I don't
> understand which changes you mean.  Again, consider putting a figure
> closer to this description.

Done, see visuals in new picture.

> ...
> 1321	5.2.  Duplicates
> 1323	   Duplicates happen when the topology of the BitString is not a tree
> 1324	   but redundantly connecting BFRs with each other.  The BIER-TE
> 1325	   Controller must therefore ensure to only create BitStrings that are
> 1326	   trees in the topology.
> [] Can you provide an example?

Added example picture and text for it:

                /    \
               / p2   \ p3
              BFR2   BFR3
               \ p4   / p5
                \    /

Bitstring p2,p3,p4,p5, duplicates on BFER4

> 1338	6.  BIER-TE Forwarding Pseudocode
> [] Placing this section here feels completely out of place because
> BIER-TE forwarding is otherwise described in §3.  Please move this
> there.


> ...
> 1366	   The difference is that in BIER-TE, step [1] must not be performed,
> 1367	   but is replaced with [2] (when the forwarding plane algorithm is
> 1368	   implemented verbatim as shown above).
> [minor] "step [1] replaced with [2]"   Step 2 is already present
> in the original pseudocode, so it is not really a replacement...Step 1
> is simply not performed.

Yes, all paragraph explaining the first pseudocode was rewritten in response
to your feedback.
> 1370	   In BIER, the F-BM of a BP has all BP set that are meant to be
> 1371	   forwarded via the same neighbor.  It is used to reset those BP in the
> 1372	   packet after the first copy to this neighbor has been made to inhibit
> 1373	   multiple copies to the same neighbor.
> [nit] s/all BP/all BPs


> [nit] s/those BP/those BPs

> 1375	   In BIER-TE, the F-BM of a particular BP with an adjacency is the list
> 1376	   of all BPs with an adjacency on this BFR except the particular BP
> 1377	   itself if it has an adjacency with the DNR bit set.  The F-BM is used
> 1378	   to reset the F-BM BPs before creating copies.
> [minor] "with the DNR bit set"   Theis pseudocode reflects "basic"
> BIER-TE, right?  I thought the DNR flag is not required/supported in
> the "basic" version.

Yes, this was a bug in explanation of the pseudocode, which is another reason
why i rewrote the paragraphs here. If we want to keep the BIER F-BPM processing
step [2] in BIER-TE, then we can not support DNC flag set, because that
would require a different F-BM for the copy to the bit itself (keep the bit
itself set as part of DNC), but clear the bit on all other copies.

> 1380	   In BIER, the order of BPs impacts the result of forwarding because of
> 1381	   [1].  In BIER-TE, forwarding is not impacted by the order of BPs.  It
> 1382	   is therefore possible to further optimize forwarding than in BIER.
> 1383	   For example, BIER-TE forwarding can be parallelized such that a
> 1384	   parallel instance (such as an egres linecard) can process any subset
> 1385	   of BPs without any considerations for the other BPs - and without any
> 1386	   prior, cross-BP shared processing.
> [nit] s/because of [1]/because of step [1]

Actually i removed the paragraph proactively because it was a) derailing),
and if i was b) challenged to provide an example, it wold be longer than make
sense in this normative section.

> 1388	   The above simplified pseudocode is elaborated further as follows:
> [] By "elaborated further" do you mean extended, enhanced, or
> something like that?  I first thought you meant you were explaining it
> (elaborating on its meaning), but Figure 16 seems like an extension.
> ??

Yes, bad wording. Replaced with a new sentence:

<t>The modified and expanded Forwarding Pseudocode in <xref target="pseudocode-picture"/> specifies how to
support all BIER-TE forwarding functions (required, recommended and optional):

> 1390	   o  This pseudocode eliminates per-bit F-BM, therefore reducing state
> 1391	      by BitStringLength^2*SI and eliminating the need for per-packet-
> 1392	      copy masking operation except for adjacencies with DNR flag set:
> [nit] s/This pseudocode/The updated pseudocode in Figure 16

Solved throuh previous parapgraph fix introducing xref to picture.

> [nit] s/with DNR flag/with the DNR flag


> ...
> 1448	                 Figure 16: BIER-TE Forwarding Pseudocode
> [] I assume the intent is for this pseudocode to be a representation
> of what is specified elsewhere -- is that correct?   Even then, it
> would be very nice if the functions/operations were explained.

Well, this Pseudocode is, like in rfc8279 for BIER the likely most formal
specification of the forwarding plane behavior, but it is a duplication
of what was written more informally in 3.3 and 4 (adjacencies an
resetting of bits).

I am not sure what explanations you feel are missing. The bullet list
preceeding the pseudocode do as far as i hope (AFAIH ;-) provide the
explanation of the pieces of the pseudocode that IMHO are not self

If you think explanations are missing, maybe be more specific ?!

> [major] Is this pseducode expected to "replace" the one in Figure 15?
> Does it represent "full" BIER-TE forwarding?  Neither is clear from
> the text.

I hope the fixed up txt o the pseudocode section makes it clear that
the first pseudocode is for required functions utilizing BIER pseudocode,
and the second one is for complete BIER-TE functions. Also fixed
up titles of pictures accordingly.

> 1450	7.  Managing SI, subdomains and BFR-ids
> 1452	   When the number of bits required to represent the necessary hops in
> 1453	   the topology and BFER exceeds the supported bitstring length,
> 1454	   multiple SI and/or subdomains must be used.  This section discusses
> 1455	   how.
> [minor] s/bitstring length/BitStringLength/s    From rfc8279.


> [nit] s/multiple SI/multiple SIs/g


> ...
> 1461	7.1.  Why SI and sub-domains
> 1463	   For BIER and BIER-TE forwarding, the most important result of using
> 1464	   multiple SI and/or subdomains is the same: Packets that need to be
> 1465	   sent to BFER in different SI or subdomains require different BIER
> 1466	   packets: each one with a bitstring for a different (SI,subdomain)
> 1467	   combination.  Each such bitstring uses one bitstring length sized SI
> 1468	   block in the BIFT of the subdomain.  We call this a BIFT:SI (block).
> [nit] s/sent to BFER/sent to BFERs


> [nit] s/different SI/different SIs/g


> 1470	   For BIER and BIER-TE forwarding itself there is also no difference
> 1471	   whether different SI and/or sub-domains are chosen, but SI and
> 1472	   subdomain have different purposes in the BIER architecture shared by
> 1473	   BIER-TE.  This impacts how operators are managing them and how
> 1474	   especially flow overlays will likely use them.
> [nit] s/itself/themselves,


> ...
> 1479	   If there are different flow services (or service instances) requiring
> 1480	   replication to different subsets of BFER, then it will likely not be
> 1481	   possible to achieve the best replication efficiency for all of these
> 1482	   service instances via subdomain 0.  Ideal replication efficiency for
> 1483	   N BFER exists in a subdomain if they are split over not more than
> 1484	   ceiling(N/bitstring-length) SI.
> [minor] s/bitstring-length/BitStringLength/g


> ...
> 1498	   To be able to easily reuse (and modify as little as possible)
> 1499	   existing BIER procedures including flow-overlay and routing underlay,
> 1500	   when BIER-TE forwarding is added, we therefore reuse SI and subdomain
> 1501	   logically in the same way as they are used in BIER: All necessary
> 1502	   BFIR/BFER for a service use a single BIER-TE BIFT and are split
> 1503	   across as many SI as necessary (see below).  Different services may
> 1504	   use different subdomains that primarily exist to provide more
> 1505	   efficient replication (and for BIER-TE desirable path steering) for
> 1506	   different subsets of BFIR/BFER.
> [minor] "see below"  Where?

nex section, replaced below with xref.

> 1508	7.2.  Bit assignment comparison BIER and BIER-TE
> [nit] s/comparison BIER/comparison between BIER

Title was changed to "Assigning bits for the BIER-TE topology"
(because of your prior concern of spreading "comparisons" across different
sections throughtout the doc. The "comparison" in the title here was
actually misleading.

> ...
> 1519	   "Desired" topology because it depends on the physical topology, and
> 1520	   on the desire of the operator to allow for explicit path steeering
> 1521	   across every single hop (which requires more bits), or reducing the
> 1522	   number of required bits by exploiting optimizations such as unicast
> 1523	   (forward_route), ECMP or flood (DNR) over "uninteresting" sub-parts
> 1524	   of the topology - e.g. parts where different trees do not need to
> 1525	   take different paths due to path steering reasons.
> [nit] s/steeering/steering


> 1527	   The total number of bits to describe the topology vs. the BFER in a
> 1528	   BIFT:SI can range widely based on the size of the topology and the
> 1529	   amount of alternative paths in it.  The higher the percentage, the
> 1530	   higher the likelihood, that those topology bits are not just BIER-TE
> 1531	   overhead without additional benefit, but instead that they will allow
> 1532	   to express desirable path steering alternatives.
> [minor] s/vs. the BFER/vs. the number of BFERs


> [minor] "The higher the percentage" of what?

Fixed to: "The higher the percentage for non-BFER bits..."

> 1534	7.3.  Using BFR-id with BIER-TE
> ...
> 1569	   If "interdependent branches" are required, the application could call
> 1570	   a BIER-TE Controller API with the list of required BFER-id and get
> 1571	   the required bitstring back.  Whenever the set of BFER-id changes,
> 1572	   this is repeated.
> [minor] s/BFER-id/BFR0id   To be consistent with the rest of the text.

following fixes to 7.3/7.4 under (t).


> [] "call a BIER-TE Controller API"   Where is that defined?
> Presumably (from the previous paragraph) "the BIER-TE Controller can
> provide to such applications for every BFR-id a SI:bitstring with the
> BIER-TE bits..." using the same API, right?   Isn't this out of scope?

In rfc8279, there is equal text explaining expectations against 
the routing underlay. Just because specification of HOW exactly
controller and/or routing undelay do things doesn't mean that the
architecture can not describe that they must do something ?!

However i reworded the sentence to the word API does not occur anymore.

> [minor] "SI:bitstring with the BIER-TE bits"  I assume that by
> "BIER-TE bits" you mean the BPs that are set (or something to that
> effect), right?  Please don't introduce new terminology unless it is
> necessary -- generically using "BIER-TE bits" may be confusing.

fixed by removing BIER-TE.

> 1574	   Note that in either case (unlike in BIER), the bits in BIER-TE may
> 1575	   need to change upon link/node failure/recovery, network expansion and
> 1576	   network resource consumption by other traffic as part of traffic
> 1577	   engineering goals (e.g.: re-optimization of lower priority traffic
> 1578	   flows).  Interactions between such BFIR applications and the BIER-TE
> 1579	   Controller do therefore need to support dynamic updates to the
> 1580	   bitstrings.
> [minor] As above, "bits in BIER-TE"...


> 1582	7.4.  Assigning BFR-ids for BIER-TE
> [] Isn't assignment also covered in the previous section?

Indeed. Somehow this ended up being a strange split of explanations across 7.3/7.4
In  -10 this text from 7.4 was merged into old 7.3, now 5.3.3 - new title "Assigning and using BFR-id with BIER-TE".

> [major] §1.3 says that "allocation of BFIR-ID values...[is]...outside
> the scope of this document".

The new improved text of 5.3 reiterates the condition under which BFR-ids are
required and that its optional.

I split out the scond part of 7.4 which is about the independent/interdependent
branches and this is now 5.3.4 because all these procedures between a BFIR
and the BIER-TE controller need some way to identify the BFER, but that does
no have to be the BFR-id. Therefore i did not want this text to be in the
BFR-id section 5.3.3 because BFR-id is optional in BIER-TE. If there are no
BFR-id's set up, the BFIR to BIER-TE controller signaling could identify
BFER for example by their BFR-prefix (loopback address). Which would be
simpler, eliminating one additional address space (BFR-id), when not needed.

There is one additional paragraph now explaining that in 5.3.4.

> 1584	   For a non-leaf BFER, there is usually a single bit k for that BFER
> 1585	   with a local_decap() adjacency on the BFER.  The BFR-id for such a
> 1586	   BFER is therefore most easily the one it would have in BIER: SI *
> 1587	   bitstring-length + k.
> [minor] There's some redundancy (too many mentions of BFER) in the
> first sentence.

I rewrote that into a single sentence and its earlier up in 5.3.3.

> Suggestion>
>    For a non-leaf BFER, there is usually a single bit k set with a local_decap
>    adjacency on it.

> ...
> 1600	   It is not currently determined if a single subdomain could or should
> 1601	   be allowed to forward both BIER and BIER-TE packets.  If this should
> 1602	   be supported, there are two options:
> [major] If it is "not currently determined", then why even include it?
>  Isn't this also out of scope?

Yes, the considerations here are beyond what is written elsewhere
in th document and reading them now, they sound very unattractive
and speculative. Removed.

> ...
> 1618	7.5.1.  With BIER
> 1620	   Consider a network setup with a bitstring length of 256 for a network
> 1621	   topology as shown in the picture below.  The network has 6 areas,
> 1622	   each with ca. 170 BFR, connecting via a core with some larger (core)
> 1623	   BFR.  To address all BFER with BIER, 4 SI are required.  To send a
> 1624	   BIER packet to all BFER in the network, 4 copies need to be sent by
> 1625	   the BFIR.  On the BFIR it does not make a difference how the BFR-id
> 1626	   are allocated to BFER in the network, but for efficiency further down
> 1627	   in the network it does make a difference.
> [minor] s/the picture below/Figure 17


> [nit] s/170 BFR/170 BFRs


> [nit] s/some larger (core) BFR/some larger (core) BFRs


> [nit] s/all BFER/all BFERs


> [nit] s/4 SI/4 SIs


> [minor] "4 SI are required"  That is true if each area has 170 BFRs
> (not "about 170") and there are 4 core BFRs (not just "some").  IOW,
> the example is ok, but somewhat sloppy.

Ok. removed "ca." and replaced "some" with 4. I felt that a less
strict example would be strong (proctive hackler defense: "but how about 169 or 171 ?"),
but if your wisdom is to be strict on the numbers, i am happy to take that lead.

> [nit] s/BFR-id are allocated to BFER/BFR-ids are allocated to BFERs

s/BFR-id are/BFR-ids are/g

> ...
> 1641	   With random allocation of BFR-id to BFER, each receiving area would
> 1642	   (most likely) have to receive all 4 copies of the BIER packet because
> 1643	   there would be BFR-id for each of the 4 SI in each of the areas.
> 1644	   Only further towards each BFER would this duplication subside - when
> 1645	   each of the 4 trees runs out of branches.
> [nit] s/BFR-id for each of the 4 SI/BFR-ids for each of the 4 SIs


> 1647	   If BFR-id are allocated intelligently, then all the BFER in an area
> 1648	   would be given BFR-id with as few as possible different SI.  Each
> 1649	   area would only have to forward one or two packets instead of 4.
> [nit] s/BFR-id/BFR-ids


> 1651	   Given how networks can grow over time, replication efficiency in an
> 1652	   area will also easily go down over time when BFR-id are network wide
> 1653	   allocated sequentially over time.  An area that initially only has
> 1654	   BFR-id in one SI might end up with many SI over a longer period of
> 1655	   growth.  Allocating SIs to areas with initially sufficiently many
> 1656	   spare bits for growths can help to alleviate this issue.  Or renumber
> 1657	   BFR-id after network expansion.  In this example one may consider to
> 1658	   use 6 SI and assign one to each area.
> [nit] s/BFR-id/BFR-ids


> [nit] s/many SI/many SIs


> [minor] s/renumber BFR-id/renumber BFERs

Good catch. fixed.

> [nit] s/6 SI/6 SIs


> ...
> 1663	7.5.2.  With BIER-TE
> 1665	   In BIER-TE one needs to determine a subset of the physical topology
> 1666	   and attached BFER so that the "desired" representation of this
> 1667	   topology and the BFER fit into a single bitstring.  This process
> 1668	   needs to be repeated until the whole topology is covered.
> [nit] s/BFER/BFERs


> 1670	   Once bits/SIs are assigned to topology and BFER, BFR-id is just a
> 1671	   derived set of identifiers from the operator/BIER-TE Controller as
> 1672	   explained above.
> [nit] s/BFER/BFERs


> 1674	   Every time that different sub-topologies have overlap, bits need to
> 1675	   be repeated across the bitstrings, increasing the overall amount of
> 1676	   bits required across all bitstring/SIs.  In the worst case, random
> 1677	   subsets of BFER are assigned to different SI.  This is much worse
> 1678	   than in BIER because it not only reduces replication efficiency with
> 1679	   the same number of overall bits, but even further - because more bits
> 1680	   are required due to duplication of bits for topology across multiple
> 1681	   SI.  Intelligent BFER to SI assignment and selecting specific
> 1682	   "desired" subtopologies can minimize this problem.
> [nit] s/subsets of BFER are assigned to different SI/subsets of BFERs


> are assigned to different SIs


> [nit] s/multiple SI/multiple SIs


> 1684	   To set up BIER-TE efficiently for above topology, the following bit
> 1685	   allocation methods can be used.  This method can easily be expanded
> 1686	   to other, similarly structured larger topologies.
> [nit] s/for above/for the above


> [nit] s/methods/method


> 1688	   Each area is allocated one or more SI depending on the number of
> 1689	   future expected BFER and number of bits required for the topology in
> 1690	   the area.  In this example, 6 SI, one per area.
> [nit] s/SI/SIs


> [nit] s/BFER/BFERs


> ...
> 1699	   On all BFIR in an area j, bia in each BIFT:SI is populated with the
> 1700	   same forward_routed(BFRja), and bib with forward_routed(BFRjb).  On
> 1701	   all area edge BFR, bea in BIFT:SI=k is populated with
> 1702	   forward_routed(BFRka) and beb in BIFT:SI=k with
> 1703	   forward_routed(BFRkb).
> [nit] s/BFIR/BFIRs


> [minor] The meaning of what you mean with this nomenclature may not be
> clear to everyone.  You might want to define it -- or change how it is
> explained.

yes, where is ltex when you need it. I guess even XMLv3, but i don't think
i want to start with sub/superscripting now in XML.

I added: j=2...6 and k=2...6, hopefullt that helps to explain te
not well enough explained terms BFRja, BFRjb, BFRja, BFRkb. If
not i can try to do another even more elaborate expansion...

> 1705	   For BIER-TE forwarding of a packet to some subset of BFER across all
> 1706	   areas, a BFIR would create at most 6 copies, with SI=1...SI=6, In
> 1707	   each packet, the bits indicate bits for topology and BFER in that
> 1708	   topology plus the four bits to indicate whether to pass this packet
> 1709	   via the ingress area a or b border BFR and the egress area a or b
> 1710	   border BFR, therefore allowing path steering for those two "unicast"
> 1711	   legs: 1) BFIR to ingress are edge and 2) core to egress area edge.
> 1712	   Replication only happens inside the egress areas.  For BFER in the
> 1713	   same area as in the BFIR, these four bits are not used.
> [nit] s/some subset of BFER/a subset of BFERs


> 1715	7.6.  Summary
> 1717	   BIER-TE can like BIER support multiple SI within a sub-domain to
> 1718	   allow re-using the concept of BFR-id and therefore minimize BIER-TE
> 1719	   specific functions in underlay routing, flow overlay methods and BIER
> 1720	   headers.
> [] Maybe I missed it, but I don't remember seeing a discussion on
> "specific functions in underlay routing, flow overlay methods".

Th flow overlay was with respect to what is now 5.3.4, e.g. how flow
overlay for example knows about BP of BFR relying on BFR-id.

routing underlay is a different point:

When i wrote this text originally, i was under the impression that BIER extensions
to IGPs for example where part of the routing underlay in BIER because
the IGP routing protocol is in the routing underlay and how could you
split a single protocol across two layers.

When i now re-read 8279 for this review feedback, i cam to realize that
rfc8279 would likely be correctly interpreted by saying that BIER extensions for
the IGPs are actually part of the BIER layer control plane and only the
non-bier stuff in IGPs is the routing underlay.

Yada yada, changed sentence to:

....functions in any possible BIER layer control plane used in conjunction with BIER-TE, flow overlay methods and BIER headers.

> [nit] s/BIER-TE can like BIER/BIER-TE can, like BIER,


> [nit] s/multiple SI/multiple SIs


> ...
> 1725	   Subdomains can in BIER-TE be used like in BIER to create more
> 1726	   efficient replication to known subsets of BFER.
> [nit] s/BFER/BFERs

s/subsets of BFER/subsets of BFERs/g

> 1728	   Assigning bits for BFER intelligently into the right SI is more
> 1729	   important in BIER-TE than in BIER because of replication efficiency
> 1730	   and overall amount of bits required.
> [nit] s/BFER/BFERs

s/bits for BFER/bits for BFERs/g

> 1732	8.  BIER-TE and Segment Routing
> [] What is the purpose of this section?  It seems to somehow compare
> BIER/BIER-TE with SR -- but, why?  In the context of this document,
> why is mentioning SR needed?  At times the text seems to even try to
> position BIER-TE as some type of SR alternative.  Even then it talks
> about how they can "naturally be combined"...
> I don't understand the purpose and think it would be better to remove it.

To me, BIER and BIER-TE are really the multicast extension for SR
because both are source-routed, per-hop, per-tree stateless:
BIER for the most common SR deployment of "get rid of LDP", eg. without
explicit path steering, and BIER-TE when you do want explicit path steering.

The core purpose of the section is of course "Upsell BIER/BIER-TE into
SR networks". Yes, BIER/BIER-TE fits perfectly into your SR network.

Of course, with BIER being its own working group and SPRING likely
claiming exclusive ownership of anything in IETF branded in conjunction
with "SR", i choose less direct explanations through terms like
"same design philosophy", which makes it somewhat difficult to read.

The other part is pointing to the fact that
SR is great for routed adjcencies to save bits when you only want steering
without a replication.

If you think the purpose is fine but the text sucks, i'd like
to attempt to improve on next rev. If you think he purpose suck,
and we should not can not upsell BIER/BIER-TE to SR networks, then
how abot moving it into an appendix first and then let further IETF
review decide final outcome ?

> ...
> 1784	9.  Security Considerations
> 1786	   The security considerations are the same as for BIER with the
> 1787	   following differences:
> 1789	   BFR-ids and BFR-prefixes are not used in BIER-TE, nor are procedures
> 1790	   for their distribution, so these are not attack vectors against BIER-
> 1791	   TE.
> [minor] BFR-ids are used -- not for BIER-TE-specific actions but
> because of the encapsulation; they are still there.

Right. removed. Might not be used in BIER-TE but small fish to fry in te
security section.

> [major] Add references to the relevant documents where BIER security
> is discussed.


> [major] For the most part the BIER-TE architecture seems close enough
> to the BIER architecture to have the same security properties.
> However, I think that short security considerations tend to attract
> more scrutiny.  A couple of suggestions:
> (1) The big change is that "BIER-TE replaces in-network autonomous
> path calculation by explicit paths calculated by the BIER-TE
> Controller."  There are all kinds of security vulnerabilities that
> could come from incorrect (because of an error or malicious action)
> path calculation and the subsequent programming.


While writing this document and therefore being also the defense
lawyer for the BIER-TE controller, i would say that incorrect
path calculation and forwarding plane programmin from route
processors is equal or more likely a problem.

I am also always wondering if good willed but erroneous operator or SDN developer
actions should be in the security considerations. I guess we sometimes
if not often do this, but it should really be more in a "resilience
considerations" section.

> Even if the specific
> BIER-TE control protocol is not specified, some type of generic
> security considerations should be provided.  Consider rfc7428.

Is rfc7428 a typo ? "Transmission of IPv6 Packets over ITU-T G.9959 Networks" ??

> (2) The concept of BIER-TE topology is also introduced.  An
> invalid/wrong network topology (because of an error or malicious
> action) can result in an invalid/wrong BIER-TE topology which, in
> turn, can result in all kinds of bad paths through the network.  As
> above, it would be ideal to include general security considerations
> related to topology discovery.  There is some text that could be
> reused in rfc7752, but it is probably not a good general reference.

So, i described the hopefully biggest/most important fish to fry which
is that of persistent loops (through attacks or misconfigs) and
the fact how/that BIER-TE has the same protections against that
as BIER and unicast because of its strict bit clearing
rules. That should be entertaining for the unicast security reviewers ;-)

> (3) In general it would be a good idea to mention why the changes in
> BIER-TE wrt BIER (at least the major ones) don't affect the security.
> For example, the use of the BitString is different, but the same
> vulnerabilities exist because the packets may still be misdirected if
> the packet is not processed appropriately (there's some text about
> this in rfc8279).

Check out the paragraphs i wrote comparing BIER/BIER-TE control plane
security and impact of attacks. I did of course (see above, defnse lawyer
for BIER-TE controller ;-) manage to explain how BIER-TE control plane
would on average be better secured than that of BIER (fault
of still pretty weak IGP security given IETF's failure to do
better there).

I also added another hopefully interesting paragraph about security
models in industrial/embedded where you could prohibi misconfigs
a lot easier than our typical Service Provider networks.

The securiy considerations should not be long and interesting enough for round 1 IETF/IESG ;-)

> ...
> 2089	13.2.  Informative References
> ...
> 2121	   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
> 2122	              Requirement Levels", BCP 14, RFC 2119,
> 2123	              DOI 10.17487/RFC2119, March 1997,
> 2124	              <>.
> [major] This reference (and the one to rfc8174) should be Normative.
> [End of Review -09]

Thank you so much, excellent review. Hope my solutions are also good,
i certainly think the review helped a lot to improve the oc.