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

Alvaro Retana <aretana.ietf@gmail.com> Fri, 14 May 2021 20:14 UTC

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Subject: [Bier] AD Review of draft-ietf-bier-te-arch-09
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Dear authors:

Thank you for the interesting work!

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.

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

  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.

  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.

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

(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?

  §3.3 speculates about potential "definitions in BIER encapsulation
  specifications" to "distinguish BIER from BIER-TE packets" -- and offers a
  workaround if the MPLS encapsulation is used.  Even here, a "mixed"
  environment would seem to at least require independent BIFTs, and not be
  possible with non-MPLS encapsulations.

  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.

(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).

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

  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).



[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.

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.

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/BIER Forwarding Egress Routers (BFER)/Bit-Forwarding Egress Routers (BFERs)

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.

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

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

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. :-(   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.

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

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

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.

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?

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.

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 (?)

341	1.3.  Comparison with BIER

343	   The key differences over BIER are:

[minor] s/over/with respect to

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.

[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"...

[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...

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?

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

[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.

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

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.

[nit] s/resulting of BitPositions/resulting BitPositions

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

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

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

[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?

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.]

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...

[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.

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.

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

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).

[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.

[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

[nit] s/by routing underlay/by the routing underlay

[?] "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

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.

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.

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?

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?

[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

[minor] "Further use of the seed parameter is explained below."  Add a
reference to §4.7.

692	3.2.4.  Local Decap

[minor] s/Local Decap/Local Decapsulation

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

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?

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?

733	3.4.  Basic BIER-TE Forwarding Example

735	   [RFC Editor: remove this section.]


[] I don't mind the extra example.

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.

[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..."

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

[] The second and third sentences are redundant.

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."

Experimentation...build control plane!?!?   I know this document was
tagged as Experimental before -- maybe this text is just a leftover.

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

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

[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

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.

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.

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

[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

[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.

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

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

[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?

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.

[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)?

[] "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.

[] "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.

[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?

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

[] "...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.

[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.

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.

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...

[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?

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.

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?

[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".   ???

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.

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.

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?

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

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.

[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

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

1251	4.10.  Summary of BP optimizations

[] Maybe move the the start.

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.

[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.

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?

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

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]...is 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.

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.

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]

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.

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

[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.

[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.

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?

1508	7.2.  Bit assignment comparison BIER and BIER-TE

[nit] s/comparison BIER/comparison between BIER

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?

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.

[] "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?

[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.

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?

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

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.

   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?

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.

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

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

[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

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".

[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

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

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.

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.

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

(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.

(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).

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	              <https://www.rfc-editor.org/info/rfc2119>.

[major] This reference (and the one to rfc8174) should be Normative.

[End of Review -09]