wdiff draft-ietf-bier-te-arch-09.txt draft-ietf-bier-te-arch-10.txt

Network Working Group T. Eckert, Ed.
Internet-Draft Futurewei
Intended status: Standards Track G. Cauchie
Expires: May 3, 2021 January 10, 2022 Bouygues Telecom
M. Menth
University of Tuebingen
Oct 30, 2020
July 9, 2021

Tree Engineering for Bit Index Explicit Replication (BIER-TE)
draft-ietf-bier-te-arch-09
draft-ietf-bier-te-arch-10

Abstract

This memo introduces describes per-packet stateless strict and loose path
steered replication and forwarding for Bit Index Explicit Replication
packets (RFC8279). This It is called BIER Tree Engineering (BIER-TE).
BIER-TE can (BIER-TE) and
is intended to be used as a the path steering mechanism in future for Traffic
Engineering solutions for BIER (BIER-TE).

BIER-TE leverages RFC8279 and extends it with BIER.

BIER-TE introduces a new semantic for bits bit positions (BP) that
indicate adjacencies, as opposed to BIER in the bitstring. which BPs indicate Bit-
Forwarding Egress Routers (BFER). BIER-TE can leverage BIER
forwarding engines with little or no changes.

In BIER, the BitPositions (BP) Co-existence of the packets bitstring indicate BIER
Forwarding Egress Routers (BFER), and hop-by-hop forwarding uses a
Routing Underlay such as an IGP.

In BIER-TE, BitPositions indicate adjacencies. The BIFT of each BFR
are only populated with BPs that are adjacent to the BFR in the BIER-
TE topology. The
BIER-TE topology can consist of layer 2 or remote
(routed) adjacencies. The BFR then replicates and forwards BIER
packets to those adjacencies. This results in the aforementioned
strict and loose path steering and replications.

BIER-TE can co-exist with BIER forwarding in the same domain, domain is possible, for example by
using separate BIER sub-domains. In sub-domains (SDs). Except for the absence of optional
routed adjacencies, BIER-TE does not require a BIER routing underlay,
and can then be operated therefore operate without requiring depending on an Interior Gateway
Routing protocol (IGP).

BIER-TE

As it operates without explicit in-network tree-state and carries
the multicast distribution tree in on the packet header. It same per-packet stateless forwarding
principles, BIER-TE can
therefore also be a good fit to support multicast path
steering in Segment Routing (SR) networks.

Name explanation

[RFC-editor: This section to be removed before publication.]

Explanation for name change from BIER-TE to mean "Traffic
Engineering" to BIER-TE "Tree Engineering" in WG last-call (to
benefit IETF/IESG reviewers):

This document started by calling itself BIER-TE, "Traffic
Engineering" as it is a mode of BIER specifically beneficial for
Traffic Engineering. It supports per-packet bitstring based policy
steering and replication. BIER-TE technology itself does not provide
a complete traffic engineering solution for BIER but would require
combination with other technologies for a full BIER based TE
solution, such as a PCE and queuing mechanisms to provide bandwidth
and latency reservations. It is also not the only option to build a
traffic engineering solution utilizing BIER, for example BIER trees
could be steered through IGP metric engineering, such as through
Flex-Topologies. The architecure for Traffic Engineering with either
modes of BIER (BIER-TE/BIER) is intended to be defined in a separate
document, most likely in TEAs WG.

Because the name of such an overall solution is intended to be BIER-
TE, the expansion of BIER-TE was therefore changed to name this BIER
mode "Tree Engineering", so the overall solution can be distinguished
better from its tree building/engineering method without having to
change the long time well-established abbreviation BIER-TE.

Status of This Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 3, 2021. January 10, 2022.

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Table of Contents

1. Introduction Overview . . . . . . . . . . . . . . . . . . . . . . . . 4 . . 3
1.1. Basic Examples Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. BIER-TE Topology and adjacencies
2.1. Basic Examples . . . . . . . . . . . . 8
1.3. Comparison with BIER . . . . . . . . . 5
2.2. BIER-TE Topology and adjacencies . . . . . . . . . . . . 9
1.4. Requirements Language 8
2.3. Relationship to BIER . . . . . . . . . . . . . . . . . . 9
2.
2.4. Accelerated/Hardware forwarding comparison . . . . . . . 11
3. Components . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1. 11
3.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 10
2.2. 12
3.2. The BIER-TE Controller Control Plane . . . . . . . . . . . . . . . . 12
3.2.1. The BIER-TE Controller . . 10
2.2.1. Assignment of BitPositions to adjacencies of the
network topology . . . . . . . . . . . . . 13
3.2.1.1. BIER-TE Topology discovery and creation . . . . . 11
2.2.2. Changes in the network topology 14
3.2.1.2. Engineered Trees via BitStrings . . . . . . . . . 14
3.2.1.3. Changes in the network topology . . 11
2.2.3. Set up per-multicast flow BIER-TE state . . . . . . . 11
2.2.4. 15
3.2.1.4. Link/Node Failures and Recovery . . . . . . . . . . . 12
2.3. 15
3.3. The BIER-TE Forwarding Layer Plane . . . . . . . . . . . . . . 12
2.4. 15
3.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 12
2.5. 16
3.5. Traffic Engineering Considerations . . . . . . . . . . . 13
3. 16
4. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 14
3.1. 17
4.1. The Bit Index Forwarding Table (BIFT) . . . . . . . . . . 14
3.2. 17
4.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 15
3.2.1. 18
4.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 15
3.2.2. Forward Routed 18
4.2.2. Forward_routed . . . . . . . . . . . . . . . . . . . 16
3.2.3. 19
4.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.4. 19
4.2.4. Local Decap . Decap(sulation) . . . . . . . . . . . . . . . . . . . . 16
3.3. 19
4.3. Encapsulation considerations . . . . . / Co-existence with BIER . . . . . . . . . 17
3.4. Basic 20
4.4. BIER-TE Forwarding Example Pseudocode . . . . . . . . . . . . 17
3.5. Forwarding comparison with BIER . . 21
4.5. Basic BIER-TE Forwarding Example . . . . . . . . . . . . 19
3.6. 24
4.6. BFR Requirements for BIER-TE forwarding . . . . . . . . . 26

5. BIER-TE Controller Operational Considerations . . . . . . . . 26
5.1. Bit position Assignments . . . . . 20
4. BIER-TE Controller BitPosition Assignments . . . . . . . . . 20
4.1. P2P Links . . 26
5.1.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . 21
4.2. 27
5.1.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.3. 27
5.1.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . . . 21
4.4. 27
5.1.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.5. 28
5.1.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . . . 22
4.6. 28
5.1.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.7. 29
5.1.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . . 24
4.8. Routed 30
5.1.8. Forward_routed adjacencies . . . . . . . . . . . . . . . . . . . 26
4.8.1. 32
5.1.8.1. Reducing BitPositions . . . bit positions . . . . . . . . . . . . . 26
4.8.2. 32
5.1.8.2. Supporting nodes without BIER-TE . . . . . . . . . . 27
4.9. 33
5.1.9. Reuse of BitPositions bit positions (without DNR) . . . DNC) . . . . . . . . 27
4.10. 33
5.1.10. Summary of BP optimizations . . . . . . . . . . . . . . . 28
5. 35
5.2. Avoiding duplicates and loops . . . . . . . . . . . . . . . . 29
5.1. 36
5.2.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2. 36
5.2.2. Duplicates . . . . . . . . . . . . . . . . . . . . . . . 30
6. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . . 30
7. 36
5.3. Managing SI, subdomains sub-domains and BFR-ids . . . . . . . . . . . . . 33
7.1. 37
5.3.1. Why SI and sub-domains . . . . . . . . . . . . . . . . . 34
7.2. Bit assignment comparison BIER and 37
5.3.2. Assigning bits for the BIER-TE topology . . . . . . . 35
7.3. Using 38
5.3.3. Assigning BFR-id with BIER-TE . . . . . . . . . . . . 39
5.3.4. Mapping from BFR to BitStrings with BIER-TE . . . . . 35
7.4. 40
5.3.5. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . . . 36
7.5. 41
5.3.6. Example bit allocations . . . . . . . . . . . . . . . . . 37
7.5.1. 41
5.3.6.1. With BIER . . . . . . . . . . . . . . . . . . . . . . 37
7.5.2. 41
5.3.6.2. With BIER-TE . . . . . . . . . . . . . . . . . . . . 38
7.6. 42
5.3.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 39
8. 43
6. BIER-TE and Segment Routing . . . . . . . . . . . . . . . . . 39
9. 44
7. Security Considerations . . . . . . . . . . . . . . . . . . . 40
10. 45
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
11. 46
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41
12. 46
10. Change log [RFC Editor: Please remove] . . . . . . . . . . . 41
13. 46
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
13.1. 56
11.1. Normative References . . . . . . . . . . . . . . . . . . 47
13.2. 56
11.2. Informative References . . . . . . . . . . . . . . . . . 47 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48 58

1. Introduction Overview

BIER-TE shares is based on architecture, terminology and packet formats with
BIER as described in [RFC8279] and [RFC8296]. This document
describes BIER-TE in the expectation that the reader is familiar with
these two documents.

In BIER-TE, BitPositions

BIER-TE introduces a new semantic for bit positions (BP) that
indicate adjacencies. The adjacencies, as opposed to BIER in which BPs indicate Bit-
Forwarding Egress Routers (BFER). With BIER-TE, the BIFT of each BFR
is only populated with BP that are adjacent to the BFR in the BIER-TE
Topology. Other BPs are left without adjacency. empty in the BIFT. The BFR replicate and
forwards BIER packets to adjacent BPs that are set in the packet.
BPs are normally also reset cleared upon forwarding to avoid duplicates and
loops. This is detailed further below.

BIER-TE can leverage BIER forwarding engines with little or no
changes. It can also co-exist with BIER forwarding in the same
domain, for example by using separate BIER sub-domains. Except for
the optional routed adjacencies, BIER-TE does not require a BIER
routing underlay, and can therefore operate without depending on an
Interior Gateway Routing protocol (IGP).

As it operates on the same per-packet stateless forwarding
principles, BIER-TE can also be a good fit to support multicast path
steering in Segment Routing (SR) networks.

This document is structured as follows:

o Section 2 introduces BIER-TE with two reference forwarding
examples, followed by an introduction of the new concepts of the
BIER-TE (overlay) topology and finally a summary of the
relationship between BIER and BIER-TE and a discussion of
accelerated hardware forwarding.

o Section 3 describes the components of the BIER-TE architecture,
Flow overlay, BIER-TE layer with the BIER-TE control plane
(including the BIER-TE controller) and BIER-TE forwarding plane,
and the routing underlay.

o Section 4 specifies the behavior of the BIER-TE forwarding plane
with the different type of adjacencies and possible variations of
BIER-TE forwarding pseudocode, and finally the mandatory and
optional requirements.

o Section 5 describes operational considerations for the BIER-TE
controller, foremost how the BIER-TE controller can optimize the
use of BP by using specific type of BIER-TE adjacencies for
different type of topological situations, but also how to assign
bits to avoid loops and duplicates (which in BIER-TE does not come
for free), and finally how SI, sub-domains and BFR-ids can be
managed by a BIER-TE controller, examples and summary.

o Section 6 concludes the technology specific sections of document
by further relating BIER-TE to Segment Routing (SR).

Note that related work, [I-D.ietf-roll-ccast] uses Bloom filters
[Bloom70] to represent leaves or edges of the intended delivery tree.
Bloom filters in general can support larger trees/topologies with
fewer addressing bits than explicit bitstrings, BitStrings, but they introduce
the heuristic risk of false positives and cannot reset clear bits in the
bitstring
BitString during forwarding to avoid loops. For these reasons, BIER-
TE uses explicit bitstrings BitStrings like BIER. The explicit bitstrings BitStrings of
BIER-TE can also be seen as a special type of Bloom filter, and this
is how related work [ICC] describes it.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119], [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Introduction

2.1. Basic Examples

BIER-TE forwarding is best introduced with simple examples.

BIER-TE Topology:

Diagram:

p5 p6
--- BFR3 ---
p3/ p13 \p7 p15
BFR1 ---- BFR2 BFR5 ----- BFR6
p1 p2 p4\ p14 /p10 p11 p12
--- BFR4 ---
p8 p9

(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):

BFR1: p1 -> local_decap
p2 -> forward_connected forward_connected() to BFR2

BFR2: p1 -> forward_connected forward_connected() to BFR1
p5 -> forward_connected forward_connected() to BFR3
p8 -> forward_connected forward_connected() to BFR4

BFR3: p3 -> forward_connected forward_connected() to BFR2
p7 -> forward_connected forward_connected() to BFR5
p13 -> local_decap

BFR4: p4 -> forward_connected forward_connected() to BFR2
p10 -> forward_connected forward_connected() to BFR5
p14 -> local_decap

BFR5: p6 -> forward_connected forward_connected() to BFR3
p9 -> forward_connected forward_connected() to BFR4
p12 -> forward_connected forward_connected() to BFR6

BFR6: p11 -> forward_connected forward_connected() to BFR5
p12
p15 -> local_decap

Figure 1: BIER-TE basic example

Consider the simple network in the above BIER-TE overview example
picture with 6 BFRs. p1...p14 are the BitPositions bit positions (BP) used. All
BFRs can act as an ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can
also be egress BFR (BFER). Forward_connected BFRs (BFERs). Forward_connected() is the name for
adjacencies that are representing subnet adjacencies of the network.
Local_decap
Local_decap() is the name of the adjacency to decapsulate BIER-TE
packets and pass their payload to higher layer processing.

Assume a packet from BFR1 should be sent via BFR4 to BFR6. This
requires a bitstring (p2,p8,p10,p12). BitString (p2,p8,p10,p12,p15). When this packet is
examined by BIER-TE on BFR1, the only BitPosition bit position from the bitstring BitString
that is also set in the BIFT is p2. This will cause BFR1 to send the
only copy of the packet to BFR2. Similarly, BFR2 will forward to
BFR4 because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6
because of p12. p12 also p15 finally makes BFR6 receive and decapsulate the
packet.

To send in addition to BFR6 via BFR4 also a copy to BFR3, the
bitstring
BitString needs to be (p2,p5,p8,p10,p12,p13). When this packet is
examined by BFR2, p5 causes one copy to be sent to BFR3 and p8 one
copy to BFR4. When BFR3 receives the packet, p13 will cause it to
receive and decapsulate the packet.

If instead the bitstring BitString was (p2,p6,p8,p10,p12,p13), (p2,p6,p8,p10,p12,p13,p15), the packet
would be copied by BFR5 towards BFR3 because of p6 instead of being
copied by BFR2 to BFR3 because of p5 in the prior case. This is
showing the ability of the shown BIER-TE Topology to make the traffic
pass across any possible path and be replicated where desired.

BIER-TE has various options to minimize BP assignments, many of which
are based on assumptions about the required multicast traffic paths
and bandwidth consumption in the network.

The following picture shows a modified example, in which Rtr2 and
Rtr5 are assumed not to support BIER-TE, so traffic has to be unicast
encapsulated across them. Unicast tunneling of BIER-TE packets can
leverage any feasible mechanism such as MPLS or IP, these
encapsulations are out of scope of this document. To emphasize non-
native non-L2, but routed/tunneled
forwarding of BIER-TE packets, these adjacencies are called
"forward_routed", but otherwise
"forward_routed". Otherwise there is no difference in their
processing over the aforementioned "forward_connected" adjacencies.

In addition, bits are saved in the following example by assuming that
BFR1 only needs to be BFIR but not BFER or transit BFR.

BIER-TE Topology:

Diagram:

p1 p3 p7
....> BFR3 <.... p5
........ ........>
BFR1 (Rtr2) (Rtr5) BFR6
........ ........>
....> BFR4 <.... p6
p2 p4 p8

(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):

BFR1: p1 -> forward_routed forward_routed() to BFR3
p2 -> forward_routed forward_routed() to BFR4

BFR3: p3 -> local_decap
p5 -> forward_routed forward_routed() to BFR6

BFR4: p4 -> local_decap
p6 -> forward_routed forward_routed() to BFR6

BFR6: p5 -> local_decap
p6 -> local_decap
p7 -> forward_routed forward_routed() to BFR3
p8 -> forward_routed forward_routed() to BFR4

Figure 2: BIER-TE basic overlay example

To send a BIER-TE packet from BFR1 via BFR3 to BFR6, the bitstring BitString is
(p1,p5). From BFR1 via BFR4 to BFR6 it is (p2,p6). A packet from
BFR1 to BFR3,BFR4 and from BFR3 to BFR6 uses (p1,p2,p3,p4,p5). A
packet from BFR1 to BFR3,BFR4 and from BFR4 to BFR uses
(p1,p2,p3,p4,p6). A packet from BFR1 to BFR4, and from BFR4 to BFR6
and from BFR6 to BFR3 uses (p2,p3,p4,p6,p7). A packet from BFR1 to
BFR3, and from BFR3 to BFR6 and from BFR6 to BFR4 uses
(p1,p3,p4,p5,p8).

1.2.

2.2. BIER-TE Topology and adjacencies

The key new component in BIER-TE compared to BIER is the BIER-TE
topology as introduced through the two examples in Section 1.1. 2.1. It
is used to control where replication can or should happen and how to
minimize the required number of BP for adjacencies.

The BIER-TE Topology consists of the BIFT BIFTs of all the BFR and can
also be expressed as a directed graph where the edges are the
adjacencies between the BFR labelled with the BP used for the
adjacency. Adjacencies are naturally unidirectional. BP can be
reused across multiple adjacencies as long as this does not lead to
undesired duplicates or loops as explained further down in the text.

If the BIER-TE topology represents (a subset of) the underlying
(layer 2) topology of the network, network as shown in the first example, this is
may be called a "native" BIER-TE topology. A topology consisting
only of "forward_routed" adjacencies as shown in the first
example. This can second example
may be freely mixed with called an "overlay" BIER-TE, in BIER-TE topology. A BIER-TE topology will
both "forward_connected" and "forward_routed" adjacencies are used.

1.3. Comparison with may be
called a "hybrid" BIER-TE topology.

2.3. Relationship to BIER

The key differences over

BIER-TE is designed so that is forwarding plane is a simple extension
to the BIER are:

o forwarding plane, hence allowing for it to be added to
BIER deployments where it can be beneficial.

BIER-TE replaces in-network autonomous is also intended as an option to expand the BIER architecture
into deployments where BIER may not be the best fit, such as
statically provisioned networks with needs for path calculation by
explicit paths calculated steering but
without desire for distributed routing protocols.

1. BIER-TE inherits the following aspects from BIER unchanged:

1. The fundamental purpose of per-packet signaled packet
replication and delivery via a BitString.

2. The overall architecture consisting of three layers, flow
overlay, BIER(-TE) layer and routing underlay.

3. The supportable encapsulations, [RFC8296] or other (future)
encapsulations.

4. The semantic of all [RFC8296] header elements used by the
BIER-TE Controller.

o In BIER-TE every BitPosition forwarding plane other than the semantic of the BP in
the BitString.

5. The BIER forwarding plane, with the exception of how bits
have to be cleared during replication.

2. BIER-TE has the following key changes with respect to BIER:

1. In BIER, bits in the BitString of a BIER-TE BIER packet
indicates one or more adjacencies - instead of header
indicate a BFER as and bits in BIER.

o BIER-TE the BIFT indicate the BIER
control plane calculated next-hop toward that BFER. In BIER-
TE, bits in each BFR has no routing table but only the BitString of a BIER packet header indicate an
adjacency in the BIER-TE
Forwarding Table (BIFT) indexed by SI:BitPosition topology, and only the BFRs that are
upstream of this adjacency have this bit populated with only those adjacencies the
adjacency in their BIFT.

2. In BIER, the implied reference option for the core part of
the BIER layer control plane is the BIER extension to which
distributed routing protocol, such as standardized in ISIS/
OSPF extensions for BIER, [RFC8401] and [RFC8444]. The
reference option for the core part of the BFR should replicate
packets to. BIER-TE headers use control
plane is the same format as BIER-TE controller. Nevertheless, both BIER headers. and
BIER-TE BIFT forwarding does not require/use the BFIR-ID. The BFIR-ID can
still plane state could equally be useful though for coordinated BFIR/BFER functions, such as
populated by any mechanism.

3. Assuming the context for upstream assigned labels reference options for MPLS payloads in MVPN
over BIER-TE.

If the control plane, BIER-TE domain is also running BIER, then
replaces in-network autonomous path calculation by explicit
paths calculated by the BFIR-ID BIER-TE controller.

3. The following element/functions described in the BIER
architecture are not required by the BIER-TE architecture:

1. BIRTs on BFR for BIER-TE are not required when using a BIER-
TE packets controller because the controller can be set to directly populate
the same BFIR-ID as used with BIER packets.

If BIFTs. In BIER, BIRTs are populated by the distributed
routing protocol support for BIER, allowing BFR to populate
their BIFTs locally from their BIRTs. Other BIER-TE domain is not running full BIER control
plane or management plane options may introduce requirements
for BIRTs for BIER-TE BFR.

2. The BIER-TE layer forwarding plane does not want to
reduce the need require BFR to allocate bits in BIER bitstrings
have a unique BP and therefore also no unique BFR-id. See
for BFIR-ID
values, then the allocation example See Section 5.1.3.

3. Identification of BFIR-ID values in BFR by the BIER-TE packets can
be done through other mechanisms control plane is outside
the scope of this document,
as long as this is appropriately agreed upon between all BFIR/BFER.

1.4. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described specification. Whereas the BIER control
plane uses BFR-ids in RFC 2119 [RFC2119].

2. Components

End its BFR to end BFR signaling, a BIER-TE operations consists
controller may choose any form of four mayor components: The
"Multicast Flow Overlay", identification deemed
appropriate.

4. BIER-TE forwarding does not use the "BIER-TE control plane" consisting BFR-id field of the "BIER-TE Controller" BIER
packet header.

4. Co-existence of BIER and its signaling channels BIER-TE in the same network requires the
following:

1. The BIER/BIER-TE packet header needs to allow addressing both
BIER and BIER-TE BIFT. Depending on the BFR, encapsulation
option, the same SD may or may not be reusable across BIER
and BIER-TE. See Section 4.3. In either case, a packet is
always only forwarded end-to-end via BIER or via BIER-TE
(ships in the
"Routing Underlay" nights forwarding).

2. BIER-TE deployments will have to assign BFR-ids to BFR and
insert them into the "BIER-TE BFR-id field of BIER packet headers as
BIER does, whenever the deployment uses (unchanged)
components developed for BIER that use BFR-id, such as
multicast flow overlays or BIER layer control plane elements.
See also Section 5.3.3.

2.4. Accelerated/Hardware forwarding layer". The Bier-TE
Controller comparison

Forwarding of BIER-TE is designed to easily build/program common
forwarding hardware with BIER. The pseudocode in Section 4.4 shows
how existing BIER/BIFT forwarding can be modified to support the new architectural component
REQUIRED BIER-TE forwarding functionality, by using BIER BIFT's
"Forwarding Bit Mask" (F-BM): Only the clearing of bits to avoid
duplicate packets to a BFR neighbor is skipped in BIER-TE compared forwarding
because it is not necessary and could not be done when using BIER
F-BM.

Whether to
BIER.

Picture 2: Components use BIER or BIER-TE forwarding is simply a choice of the
mode of the BIFT indicated by the packet (BIER or BIER-TE BIFT).
This is determined by the BFR configuration for the encapsulation,
see Section 4.3.

3. Components

BIER-TE can be thought of being constituted from the same three
layers as BIER: The "multicast flow overlay", the "BIER layer" and
the "routing underlay". The following picture also shows how the
"BIER layer" is constituted from the "BIER-TE forwarding plane" and
the "BIER-TE control plane" represent by the "BIER-TE Controller".

<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay

BIER-TE [BIER-TE Controller] <=> [BIER-TE Topology]
BIER-TE
control plane ^ ^ ^
plane / | \ BIER-TE control protocol
| | | e.g. Netconf/Restconf/Yang YANG/Netconf/RestConf
| | | PCEP/...
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr

|<----------------->|
BIER-TE forwarding layer plane

|<- BIER-TE domain->|

|<--------------------->|
Routing underlay

Figure 3: BIER-TE architecture

2.1.

3.1. The Multicast Flow Overlay

The Multicast Flow Overlay operates has the same role as described for BIER in BIER.
[RFC8279], Section 4.3. See [RFC8279].
Instead of interacting with also Section 3.2.1.2.

3.2. The BIER-TE Control Plane

In the BIER architecture [RFC8279], the BIER control plane is not
explicitly separated from the BIER forwarding layer (as plane, but instead
their functions are summarized together in BIER),
it interacts with Section 4.2. Example
standardized options for the BIER-TE Controller.

2.2. The BIER-TE Controller

The BIER-TE Controller is representing BIER control plane include ISIS/OSPF
extensions for BIER, [RFC8401] and [RFC8444].

For BIER-TE, the control plane of BIER-TE.
It communicates two sets of information with BFRs: includes at minimum the following
functionality.

1. During initial provisioning or modifications of the network topology,
the BIER-TE Controller discovers the network and/or during
modifications of its topology and creates and/or services: protocols and/or
procedures to establish BIER-TE BIFTs:

1. Determine the desired BIER-TE topology from it: determine which for a BIER-TE sub-
domains: the native and/or overlay adjacencies that are required/
desired and assign BitPositions
assigned to them. Then it signals BPs.

2. Determine the
resulting of BitPositions and their adjacencies to each BFR per-BFR BIFT from the BIER-TE topology.

3. Optionally assign BFR-id to set up
their BFIR for later insertion into
BIER-TE BIFTs. headers on BFIR. Alternatively, bfir-id in BIER
packet headers may be managed solely by the flow overlay
layer and/or be unused.

4. Install/update the BIFTs into the BFRs and optionally BFR-id
into BFIR.

2. During day-to-day operations of the network, the BIER-TE Controller
signals network: Protocols and/or procedures to BFIRs what multicast
support creation/change/removal of overlay flows are mapped to what BitStrings.

Communications between on BFIR:

1. Process the BIER-TE Controller and BFRs is ideally via
standardized protocols requirements for the multicast overlay
flow: BFIR and data-models such BFERs of the flow as Netconf/Restconf/Yang.
This is currently outside well as policies for the scope
path selection of this document. Vendor-
specific CLI the flow.

2. Determine the BitStrings and optionally Entropy.

3. Install state on the BFRs is also a possible stopgap option (as in
many other SDN solutions lacking definition of standardized data
model).

For simplicity, BFIR to imposition the procedures desired BIER
packet header(s) for packets of the overlay flow.

4. Install the necessary state on the BFERs to decapsulate the
BIER packet header and properly dispatch its payload.

3.2.1. The BIER-TE Controller are
described in

Notwithstanding other options, this document architecture describes the BIER
control plane as if it is a single, shown in Figure 3 to consists of:

o A single centralized
automated entity, BIER-TE controller.

o Data-models and protocols to communicate between controller and
BFR in step 1, such YANG/Netconf/RestConf.

o Protocols to communicate between controller and BFIR in step 2,
such as an SDN controller. It BIER-TE extensions for [RFC5440].

The BIER control plane could equally be implemented without any
active dynamic components by an operator setting up via CLI on the BFRs. Distribution of In
that case, operator configured local policy on the functions
of BFIR would have to
determine how to set the appropriate BIER header fields. The BIER-TE Controller is currently outside the scope of
control plane could also be decentralized and/or distributed, but
this
document.

2.2.1. Assignment of BitPositions document does not consider any additional protocols and/or
procedures which would then be necessary to adjacencies of coordinate its entities
to achieve the above described functionality.

3.2.1.1. BIER-TE Topology discovery and creation

Step 1.1 includes network topology

The discovery and BIER-TE Controller tracks topology
creation. The latter describes the process by which a Controller
determines which routers are to be configured as BFR and the
adjacencies between them.

In statically managed networks, such as in industrial environments,
both discovery and creation can be a manual/offline process.

In other networks, topology of discovery may rely on protocols including
extending a Link-State-Protocol (LSP) based IGP into the BIER-TE domain.
It determines what adjacencies require BitPositions so that
controller itself, [RFC7752] (BGP-LS) or [RFC8345] (Yang topology) as
well as BIER-TE
explicit paths specific methods, for example via
[I-D.ietf-bier-te-yang]. These options are non-exhaustive.

Dynamic creation of the BIER-TE topology can be built through them as desired easy as mapping
the network topology 1:1 to the BIER-TE topology by operator
policy.

The assigning a BP
for every network subnet adjacency. In larger networks, it likely
involves more complex policy and optimization decisions including how
to minimize the number of BP required and how to assign BP across
different BitStrings to minimize the number of duplicate packets
across links when delivering an overlay flow to BFER using different
SIs/BitStrings. These topics are discussed in Section 5.

When the topology is determined, the BIER-TE Controller then pushes
the BitPositions/adjacencies to the BIFT of the BFRs, populating only
those SI:BitPositions to the BIFT of each BFR to which that BFR
should be able to send packets to - adjacencies connecting to this
BFR.

2.2.2.

Communications between the BIER-TE Controller and BFRs (beside
topology discovery) is ideally via standardized protocols and data-
models such as Netconf/RestConf/Yang/PCEP. Vendor-specific CLI on
the BFRs is also an option (as in many other SDN solutions lacking
definition of standardized data model).

3.2.1.2. Engineered Trees via BitStrings

In BIER, the same set of BFER in a single sub-domain is always
encoded as the same BitString. In BIER-TE, the BitString used to
reach the same set of BFER in the same sub-domain can be different
for different overlay flows because the BitString encodes the paths
towards the BFER, so the BitStrings from different BFIR to the same
set of BFER will often be different, and the BitString from the same
BFIR to the same set of BFER can different for different overlay
flows for policy reasons such as shortest path trees, Steiner trees
(minimum cost trees), diverse path trees for redundancy and so on.

See also [I-D.ietf-bier-multicast-http-response] for a solution
describing this interaction.

3.2.1.3. Changes in the network topology

If the network topology changes (not failure based) so that
adjacencies that are assigned to BitPositions bit positions are no longer needed,
the BIER-TE Controller can re-use those BitPositions bit positions for new
adjacencies. First, these BitPositions bit positions need to be removed from any
BFIR flow state and BFR BIFT state, then they can be repopulated,
first into BIFT and then into the BFIR.

2.2.3. Set up per-multicast flow BIER-TE state

The BIER-TE Controller interacts with the multicast flow overlay to
determine what multicast flow needs to be sent by a BFIR to which set
of BFER. It calculates the desired distribution tree across the
BIER-TE domain based on algorithms outside the scope of this document
(e.g. CSFP, Steiner Tree, ...). It then pushes the calculated
BitString into the BFIR.

See [I-D.ietf-bier-multicast-http-response] for a solution describing
this interaction.

2.2.4.

3.2.1.4. Link/Node Failures and Recovery

When link or nodes fail or recover in the topology, BIER-TE can could
quickly respond with the optional out-of-scope FRR procedures described in [I-
D.eckert-bier-te-frr]. such as
[I-D.eckert-bier-te-frr]. It can also more slowly react by
recalculating the BitStrings of affected multicast flows. This
reaction is slower than the FRR procedure because the BIER-TE
Controller needs to receive link/node up/down indications,
recalculate the desired BitStrings and push them down into the BFIRs.
With FRR, this is all performed locally on a BFR receiving the
adjacency up/down notification.

2.3.

3.3. The BIER-TE Forwarding Layer Plane

The BIER-TE Forwarding Plane constitutes of the following components:

1. On BFIR imposition of BIER header for packets from overlay flows.
This is driven by a combination of state established by the BIER-
TE control plane and/or the multicast flow overlay as explained
in Section 3.1.

2. On BFR (including BFIR and BFER), forwarding/replication of BIER
packets according to their BitString as explained below and
optionally Entropy. Processing of other BIER header fields such
as DSCP is outside the scope of this document.

3. On BFER removal of BIER header and dispatching of the payload
according to state created by the BIER-TE control plane and/or
overlay layer.

When the BIER-TE Forwarding Layer Plane receives a packet, it simply looks
up the BitPositions bit positions that are set in the BitString of the packet in
the Bit Index Forwarding Table (BIFT) that was populated by the BIER-
TE Controller. For every BP that is set in the BitString, and that
has one or more adjacencies in the BIFT, a copy is made according to
the type of adjacencies for that BP in the BIFT. Before sending any
copy, the BFR resets clears all BP BPs in the BitString of the packet for which
the BFR has one or more adjacencies in the BIFT, except when the
adjacency indicates "DoNotReset" (DNR, "DoNotClear" (DNC, see Section 3.2.1). 4.2.1). This is
done to inhibit that packets can loop.

2.4.

3.4. The Routing Underlay

For forward_connected forward_connected() adjacencies, BIER-TE is sending BIER packets
to directly connected BIER-TE neighbors as L2 (unicasted) BIER
packets without requiring a routing underlay. For forward_routed forward_routed()
adjacencies, BIER-TE forwarding encapsulates a copy of the BIER
packet so that it can be delivered by the forwarding plane of the
routing underlay to the routable destination address indicated in the
adjacency. See Section 3.2.2 4.2.2 for the adjacency definition.

BIER relies on the routing underlay to calculate paths towards BFER BFERs
and derive next-hop BFR adjacencies for those paths. This commonly
relies on BIER specific extensions to the routing protocols of the
routing underlay but may also be established by a controller. In
BIER-TE, the next-hops of a packet are determined by the bitstring BitString
through the BIER-TE Controller established adjacencies on the BFR for
the BPs of the bitsring. BitString. There is thus no need for BFER specific
routing underlay extensions to forward BIER packets with BIER-TE
semantics.

BIER encapsulations may have BFER independent extensions in the
routing underlay, such as the label range for BIER packets in the
BIER over MPLS encapsulation ([RFC8296]). These BIER specific
functions of the routing underlay are equally useable by BIER-TE.
Alternatively, these encapsulation

Encapsulation parameters can be provisioned by the BIER-TE controller
into the forward_connected forward_connected() or forward_routed forward_routed() adjacencies directly
without relying on a routing underlay.

If the BFR intends to support FRR for BIER-TE, then the BIER-TE
forwarding plane needs to receive fast adjacency up/down
notifications: Link up/down or neighbor up/down, e.g. from BFD.
Providing these notifications is considered to be part of the routing
underlay in this document.

2.5.

3.5. Traffic Engineering Considerations

Traffic Engineering ([I-D.ietf-teas-rfc3272bis]) provides performance
optimization of operational IP networks while utilizing network
resources economically and reliably. The key elements needed to
effect TE are policy, path steering and resource management. These
elements require support at the control/controller level and within
the forwarding plane.

Policy decisions are made within the BIER-TE control plane, i.e.,
within BIER-TE Controllers. Controllers use policy when composing
BitStrings (BFR flow state) and BFR BIFT state. The mapping of user/
IP user/IP traffic to
specific BitStrings/BIER-TE flows is made based on policy. The specifics
specific details of BIER-TE policies and how a controller uses such them
are out of scope of this document.

Path steering is supported via the definition of a BitString.
BitStrings used in BIER-TE are composed based on policy and resource
management considerations. When For example, when composing BIER-TE
BitStrings, a Controller MUST must take into account the resources
available at each BFR and for each BP when it is providing congestion
loss free services such as Rate Controlled Service Disciplines
[RCSD94]. Resource availability could be provided for example via
routing protocol information, but may also be obtained via a BIER-TE
control protocol such as Netconf or any other protocol commonly used
by a PCE to understand the resources of the network it operates on.
The resource usage of the BIER-TE traffic admitted by the BIER-TE
controller can be solely tracked on the BIER-TE Controller based on
local accounting as long as no forward_routed forward_routed() adjacencies are used
(see Section 3.2.1 4.2.1 for the definition of forward_routed forward_routed()
adjacencies). When
forward_routed forward_routed() adjacencies are used, the paths
selected by the underlying routing protocol need to be tracked as
well.

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

4. BIER-TE Forwarding

3.1.

4.1. The Bit Index Forwarding Table (BIFT)

The Bit Index Forwarding Table (BIFT) exists in every BFR. For every
subdomain
sub-domain in use, it is a table indexed by SI:BitPosition SI:bit position and is
populated by the BIER-TE control plane. Each index can be empty or
contain a list of one or more adjacencies.

Like BIER, BIER-TE can support multiple subdomains like BIER. Each one sub-domains, each with a
separate BIFT BIFT.

In the BIER architecture, [RFC8279], Figure 2, indices into the BIFT are explained to be both BFR-id and SI:BitString (BitPosition). This is because there
and BFR-id, where BitString is indicating a 1:1 relationship between BP: BFR-id and SI:BitString - every bit in
every = SI is/can be assigned to a BFIR/BFER. In BIER-TE there are
more bits used * 2^BSL +
BP. As shown in each BitString than there are BFIR/BFER assigned to
the bitstring. This is because of the bits required to express the
engineered path through the topology. The BIER-TE forwarding
definitions do therefore not use the term BFR-id at all. Instead,
BFR-ids are Figure 4, in BIER-TE, only SI:BP are used as required by routing underlay, flow overlay
of BIER headers. Please refer to Section 7 for explanations how to
deal with SI, subdomains indices
into a BIFT because they identify adjacencies and BFR-id in BIER-TE. not BFR.

------------------------------------------------------------------
| Index: | Adjacencies: |
| SI:BitPosition SI:bit position | <empty> or one or more per entry |
==================================================================
| 0:1 | forward_connected(interface,neighbor{,DNR}) forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------
| 0:2 | forward_connected(interface,neighbor{,DNR}) forward_connected(interface,neighbor{,DNC}) |
| | forward_connected(interface,neighbor{,DNR}) forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------
| 0:3 | local_decap({VRF}) |
------------------------------------------------------------------
| 0:4 | forward_routed({VRF,}l3-neighbor) |
------------------------------------------------------------------
| 0:5 | <empty> |
------------------------------------------------------------------
| 0:6 | ECMP({adjacency1,...adjacencyN}, seed) |
------------------------------------------------------------------
...
| BitStringLength | ... |
------------------------------------------------------------------
Bit Index Forwarding Table

Figure 4: BIFT adjacencies

The BIFT is programmed into the data plane of BFRs by the BIER-TE
Controller and used to forward packets, according to the rules
specified in the BIER-TE Forwarding Procedures.

Adjacencies for the same BP when

Note that a BIFT index (SI:BP) may be populated in the BIFT of more
than one BFR by
the BFR. See Section 5.1.6 for an example of how a BIER-TE Controller does not have
controller could assign BPs to have (logical) adjacencies shared across
multiple BFRs, Section 5.1.3 for an example of assigning the same adjacencies.
This is up BP
to the BIER-TE Controller. BPs different adjacencies, and Section 5.1.9 for p2p links are one case
(see below).

{VRF}indicates guidelines regarding
re-use of BPs across different adjacencies.

{VRF} indicates the Virtual Routing and Forwarding context into which
the BIER payload is to be delivered. This is optional and depends on
the multicast flow overlay.

3.2.

4.2. Adjacency Types

3.2.1.

4.2.1. Forward Connected

A "forward_connected" adjacency is towards a directly connected BFR
neighbor using an interface address of that BFR on the connecting
interface. A forward_connected forward_connected() adjacency does not route packets
but only L2 forwards them to the neighbor.

Packets sent to an adjacency with "DoNotReset" (DNR) "DoNotClear" (DNC) set in the BIFT
will not
MUST NOT have the BitPosition bit position for that adjacency reset cleared when the
BFR creates a copy for it. The BitPosition bit position will still be reset cleared
for copies of the packet made towards other adjacencies. This can be
used for example in ring topologies as explained below.

3.2.2. Forward Routed

4.2.2. Forward_routed

A "forward_routed" adjacency is an adjacency towards a BFR that is
not a forward_connected adjacency: towards a loopback address of uses
a
BFR or towards an interface address that is non-directly connected.
Forward_routed packets are (tunneling) encapsulation which will cause the packet to be
forwarded via by the Routing Underlay.

If routing underlay toward the adjacent BFR. This can
leverage any feasible encapsulation, such as MPLS or tunneling over
IP/IPv6, as long as the Routing Underlay has multiple paths for a forward_routed
adjacency, it will perform ECMP independent of BIER-TE for packets
forwarded across packet can be identified as a forward_routed adjacency.
payload. This is independent of
BIER-TE ECMP identification can either rely on the BIER/BIER-TE co-
existence mechanisms described in Section 3.2.3.

If 4.3, or by explicit support
for a BIER-TE payload type in the Routing Underlay has FRR, it will perform tunneling encapsulation.

"forward_routed" adjacencies are necessary to pass BIER-TE traffic
across non BIER-TE capable routers or to minimize the number of
required BP by tunneling over (BIER-TE capable) routers on which
neither replication nor path-steering is desired, or simply to
leverage path redundancy and FRR of the routing underlay towards the
next BFR. They may also be useful to a multi-subnet adjacent BFR to
leverage the routing underlay ECMP independent of BIER-TE for packets forwarded across a forward_routed adjacency.

3.2.3. ECMP

The
(Section 4.2.3).

4.2.3. ECMP mechanisms in

BIER are ECMP is tied to the BIER BIFT processing semantic and are
therefore not directly useable usable with BIER-TE. The following
procedures describe ECMP for

A BIER-TE that we consider to be
lightweight but also well manageable. It leverages the existing
entropy parameter in the BIER header to keep packets of the flows on
the same path and it introduces a "seed" parameter to allow for
traffic flows to be polarized or randomized across multiple hops.

An "Equal Cost Multipath" (ECMP) adjacency has a list of two
or more non-ECMP adjacencies included in it. It copies the and a seed parameter. When a BIER-TE to
packet is copied onto such an ECMP adjacency, an implementation
specific so-called hash function will select one out of those the lists
adjacencies based on to which the packet is forwarded. This ECMP hash calculation. The BIER-TE ECMP
hash algorithm must
function MUST select the same adjacency from that list for all
packets with the same "entropy" value in the BIER-TE header if the
same number of adjacencies and same seed are given as parameters.
Further use of the entropy parameter. The seed parameter is explained below.

3.2.4. allows
to design hash functions that are easy to implement at high speed
without running into polarization issues across multiple consecutive
ECMP hops. See Section 5.1.7 for more explanations.

4.2.4. Local Decap Decap(sulation)

A "local_decap" adjacency passes a copy of the payload of the BIER-TE
packet to the packets NextProto protocol within the BFR (IPv4/IPv6,
Ethernet,...). Ethernet,...)
responsible for that payload according to the packet header fields.
A local_decap local_decap() adjacency turns the BFR into a BFER for matching
packets. Local_decap Local_decap() adjacencies require the BFER to support
routing or switching for NextProto to determine how to further
process the packet.

3.3.

4.3. Encapsulation considerations / Co-existence with BIER

Specifications for BIER-TE encapsulation are outside the scope of
this document. This section gives explanations and guidelines.

Because a BFR needs to interpret the BitString of a BIER-TE packet
differently from a BIER packet, it is necessary to distinguish BIER
from BIER-TE packets. This is subject to definitions in In the BIER encapsulation specifications.

MPLS encapsulation [RFC8296] for example assigns one label by which
BFRs recognizes [RFC8296], the BIFT-
id field of the packet indicates the BIFT of the packet. BIER packets for every (SI,subdomain) combination.
If it is desirable that every subdomain and
BIER-TE can forward only therefore be run simultaneously, when the BIFT-id address
space is shared across BIER or BIFT and BIER-TE packets, then BIFT. Partitioning the
BIFT-id address space is subject to BIER-TE/BIER control plane
procedures.

When [RFC8296] is used for BIER with MPLS, BIFT-id address ranges can
be dynamically allocated from MPLS label allocation could stay the same, and space only for the forwarding model (BIER/BIER-TE) would have set of
actually used SD:BSL BIFT. This allows to also allocate non-
overlapping label ranges for BIFT-ids that are to be defined per
subdomain. If used with BIER-
TE BIFTs.

With MPLS, it is desirable also possible to support reuse the same SD space for both
BIER-TE and BIER, so that the same SD has both a BIER BIFT and
according range of BIFT-ids and a disjoint BIER-TE BIFT and non-
overlapping range of BIFT-ids.

When a fixed mapping from BSL, SD, SI is used without specifically
distinguishing BIER and BIER-TE, such as proposed for non-MPLS
forwarding with [RFC8296] in the same subdomain, [I-D.ietf-bier-non-mpls-bift-encoding]
revision 04, section 5., then additional labels would need it is necessary to allocate disjoint
SDs to BIER and BIER-TE BIFT so that both can be assigned addressed by the
BIFT-ids. The encoding proposed in section 6. of the same document
does not statically encode BSL or SD into the BIFT-id, but allows for BIER-TE forwarding.
a mapping, and hence could provide for the same freedom as when MPLS
is being used (same or different SD for BIER/BIER-TE).

"forward_routed" requires an encapsulation permitting to unicast
BIER-TE packets to a specific interface address on a target BFR.
With MPLS encapsulation, this can simply be done via a label stack
with that addresses label as the top label - followed by the label
assigned to (SI,subdomain) - and if necessary (see above) BIER-TE. the (BSL,SD,SI) BitString. With non-MPLS encapsulation,
some form of IP encapsulation would be required (for example IP/GRE).

The encapsulation used for "forward_routed" adjacencies can equally
support existing advanced adjacency information such as "loose source
routes" via e.g. MPLS label stacks or appropriate header extensions
(e.g. for IPv6).

3.4.

4.4. BIER-TE Forwarding Pseudocode

The following pseudocode, Figure 5, for BIER-TE forwarding is based
on the BIER forwarding pseudocode of [RFC8279], section 6.5 with one
modification.

void ForwardBitMaskPacket_withTE (Packet)
{
SI=GetPacketSI(Packet);
Offset=SI*BitStringLength;
for (Index = GetFirstbit position(Packet->BitString); Index ;
Index = GetNextbit position(Packet->BitString, Index)) {
F-BM = BIFT[Index+Offset]->F-BM;
if (!F-BM) continue; [3]
BFR-NBR = BIFT[Index+Offset]->BFR-NBR;
PacketCopy = Copy(Packet);
PacketCopy->BitString &= F-BM; [2]
PacketSend(PacketCopy, BFR-NBR);
// The following must not be done for BIER-TE:
// Packet->BitString &= ~F-BM; [1]
}
}

Figure 5: BIER-TE Forwarding Pseudocode for required functions, based
on BIER Pseudocode

In step [2], the F-BM is used to clear bit(s) in PacketCopy. This
step exists in both BIER and BIER-TE, but the F-BMs need to be
populated differently for BIER-TE than for BIER for the desired
clearing.

In BIER, multiple bits of a BitString can have the same BFR-NBR.
When a received packets BitString has more than one of those bits
set, the BIER replication logic has to avoid that more than one
PacketCopy is sent to that BFR-NBR ([1]). Likewise, the PacketCopy
sent to a BFR-NBR must clear all bits in its BitString that are not
routed across BFR-NBR. This protects against BIER replication on any
possible further BFR to create duplicates ([2]).

To solve both [1] and [2] for BIER, the F-BM of each bit needs to
have all bits set that this BFR wants to route across BFR-NBR. [2]
clears all other bits in PacketCopy->BitString, and [1] clears those
bits from Packet->BitString after the first PacketCopy.

In BIER-TE, a BFR-NBR is an adjacency, forward_connected,
forward_routed or local_decap. There is no need for [2] to suppress
duplicates in the way BIER does because in general, different BP
would never have the same adjacency. If a BIER-TE controller
actually finds some optimization in which this would be desirable,
then the controller is also responsible to ensure that only one of
those bits is set in any Packet->BitString, unless the controller
explicitly wants for duplicates to be created.

For BIER-TE, F-BM is handled as follows:

1. The F-BM of all bits without an adjacency has all bits clear.
This will cause [3] to skip further processing of such a bit.

2. All bits with an adjacency (with DNC flag clear) have an F-BM
that has only those bits set for which this BFR does not have an
adjacency. This causes [2] to clear all bits from
PacketCopy->BitString for which this BFR does have an adjacency.

3. [1] is not performed for BIER-TE. All bit clearing required by
BIER-TE is performed by [2].

This Forwarding Pseudocode can support the REQUIRED BIER-TE
forwarding functions (see Section 4.6), forward_connected,
forward_routed() and local decap, but not the RECOMMENDED functions
DNC flag and multiple adjacencies per bit nor the OPTIONAL function,
ECMP adjacencies. The DNC flag cannot be supported when using only
[1] to mask bits.

The modified and expanded Forwarding Pseudocode in Figure 6 specifies
how to support all BIER-TE forwarding functions (required,
recommended and optional):

o This pseudocode eliminates per-bit F-BM, therefore reducing the
size of BIFT state by BitStringLength^2*SI and eliminating the
need for per-packet-copy masking operation except for adjacencies
with the DNC flag set:

* AdjacentBits[SI] are bits with a non-empty list of adjacencies.
This can be computed whenever the BIER-TE Controller updates
the adjacencies.

* Only the AdjacentBits need to be examined in the loop for
packet copies.

* The packets BitString is masked with those AdjacentBits before
the loop to avoid doing this repeatedly for every PacketCopy.

o The code loops over the adjacencies because there may be more than
one adjacency for a bit.

o When an adjacency has the DNC bit, the bit is set in the packet
copy (to save bits in rings for example).

o The ECMP adjacency is shown. Its parameters are a
ListOfAdjacencies from which one is picked.

o The forward_local, forward_routed, local_decap() adjacencies are
shown with their parameters.

void ForwardBitMaskPacket_withTE (Packet)
{
SI=GetPacketSI(Packet);
Offset=SI*BitStringLength;
AdjacentBits = Packet->BitString &= ~AdjacentBits[SI];
Packet->BitString &= AdjacentBits[SI];
for (Index = GetFirstbit position(AdjacentBits); Index ;
Index = GetNextbit position(AdjacentBits, Index)) {
foreach adjacency BIFT[Index+Offset] {
if(adjacency == ECMP(ListOfAdjacencies, seed) ) {
I = ECMP_hash(sizeof(ListOfAdjacencies),
Packet->Entropy, seed);
adjacency = ListOfAdjacencies[I];
}
PacketCopy = Copy(Packet);
switch(adjacency) {
case forward_connected(interface,neighbor,DNC):
if(DNC)
PacketCopy->BitString |= 1<<(Index-1);
SendToL2Unicast(PacketCopy,interface,neighbor);

case forward_routed({VRF},neighbor):
SendToL3(PacketCopy,{VRF,}l3-neighbor);

case local_decap({VRF},neighbor):
DecapBierHeader(PacketCopy);
PassTo(PacketCopy,{VRF,}Packet->NextProto);
}
}
}
}

Figure 6: Complete BIER-TE Forwarding Pseudocode for required,
recommended and optional functions

4.5. Basic BIER-TE Forwarding Example

[RFC Editor: remove this section.]

THIS SECTION TO BE REMOVED IN RFC BECAUSE IT WAS SUPERCEEDED BY
SECTION 1.1 EXAMPLE - UNLESS REVIEWERS CHIME IN AND EXPRESS DESIRE TO
KEEP THIS ADDITIONAL EXAMPLE SECTION. ALVARO RETANA DID NOT MIND
ANOTHER EXAMPLE.

Step by step example of basic BIER-TE forwarding. This example does
not use ECMP or forward_routed forward_routed() adjacencies nor does it try to
minimize the number of required BitPositions for the topology.

[BIER-TE Controller]
/ | \
v v v
. .
| p13 p1 | .
+- BFIR2 --+ | .
| . | p2 p6 | . LAN2
| . +-- BFR3 --+ . |
| . | | p7 p11 |
Src -+ . +-- BFER1 --+
| . | p3 p8 | . |
| . +-- BFR4 --+ . +-- Rcv1
| . | | . |
| . | .
| p14 p4 | .
+- BFIR1 --+ | .
| . +-- BFR5 --+ p10 p12 |
LAN1 . | p5 p9 +-- BFER2 --+
. | . +-- Rcv2
. . |
. . LAN3
. .
IP |..... BIER-TE network......| network.....| IP

Figure 5: 7: BIER-TE Forwarding Example

pXX indicate the BitPositions number assigned by the BIER-TE
Controller to adjacencies in the BIER-TE topology. For example, p9
is the adjacency towards BFR5 on the LAN connecting to BFER2.

BIFT BFIR2:
p13: local_decap() local_decap
p2: forward_connected(BFR3)

BIFT BFR3:
p1: forward_connected(BFIR2)
p7: forward_connected(BFER1)
p8: forward_connected(BFR4)

BIFT BFER1:
p11: local_decap() local_decap
p6: forward_connected(BFR3)
p8: forward_connected(BFR4)

Figure 6: 8: BIER-TE Forwarding Example Adjacencies

...and so on.

For example, we assume that some multicast traffic seen on LAN1 needs
to be sent via BIER-TE by BFIR2 towards Rcv1 and Rcv2. The BIER-TE
Controller determines it wants it to pass this traffic across the
following paths:

-> BFER1 ---------------> Rcv1
BFIR2 -> BFR3
-> BFR4 -> BFR5 -> BFER2 -> Rcv2

Figure 7: 9: BIER-TE Forwarding Example Paths

These paths equal to the following BitString: p2, p5, p7, p8, p10,
p11, p12.

This BitString is assigned by BFIR2 to the example multicast traffic
received from LAN1.

Then BFIR2 forwards this multicast traffic with BIER-TE based on that
BitString. The BIFT of BFIR2 has only p2 and p13 populated. Only p2
is in the BitString and this is an adjacency towards BFR3. BFIR2
therefore resets clears p2 in the BitString and sends a copy towards BFR2.

BFR3 sees a BitString of p5,p7,p8,p10,p11,p12. It is For those BPs, it has
only interested
in p1,p7,p8. adjacencies for p7,p8. It creates a copy of the packet to BFER1
(due to p7) and one to BFR4 (due to p8). It resets clears p7, p8 before
sending.

BFER1 sees a BitString of p5,p10,p11,p12. It is only interested in
p6,p7,p8,p11 and therefore considers For those BPs, it only has
an adjacency for p11. p11 is a "local_decap" adjacency installed by
the BIER-TE Controller because BFER1 should
pass packets to IP multicast. The local_decap adjacency instructs
BFER1 to create receive a copy, decapsulate it from copy of the BIER packet - dispose
of the BIER header and pass
it on to the NextProtocol, in this example payload to IP multicast. IP
multicast will then forward the packet out to LAN2 because it did
receive PIM or IGMP joins on LAN2 for the traffic.

Further processing of the packet in BFR4, BFR5 and BFER2 accordingly.

3.5. Forwarding comparison with BIER

Forwarding of

4.6. BFR Requirements for BIER-TE is designed to allow common forwarding hardware
with BIER. In fact, one of the main goals of this document is to
encourage the building of forwarding hardware that can not only
support BIER, but also BIER-TE - to allow experimentation with BIER-
TE and

BFR MUST support building of BIER-TE control plane code.

The pseudocode in Section 6 shows how existing BIER/BIFT forwarding
can be amended to support basic BIER-TE forwarding, by using BIER
BIFT's F-BM. Only configure the masking BIFT of bits due to avoid duplicates must
be skipped when forwarding is for BIER-TE.

Whether to use BIER or BIER-TE forwarding can simply be a configured
choice per subdomain and accordingly be set up by a BIER-TE
Controller. The BIER packet encapsulation [RFC8296] too can be
reused without changes except that the currently defined BIER-TE ECMP
adjacency does not leverage the entropy field sub-domains so that field would be
unused when BIER-TE forwarding is used.

3.6. Requirements

Basic BIER-TE forwarding MUST support to configure Subdomains to they
use
basic BIER-TE forwarding rules (instead instead of BIER). With basic BIER-TE
forwarding, every bit BIER forwarding rules. Every
BP in the BIFT MUST support to have zero or one adjacency. It
Forwarding MUST support the adjacency types forward_connected without DNR forward_connected() with
clear DNC flag,
forward_routed forward_routed() and local_decap. All other BIER-TE forwarding
features are optional. These basic BIER-TE requirements make As explained in
Section 4.4, these REQUIRED BIER-TE forwarding exactly functions can be
implement via the same Forwarding Pseudocode as BIER forwarding with the exception of
skipping the aforementioned F-BM
except for one modification (skipping one masking on egress. with F-BM).

BIER-TE forwarding SHOULD support the DNR forward_connected() adjacencies
with a set DNC flag, as this is highly useful to save bits in rings
(see Section 4.6). 5.1.6).

BIER-TE forwarding MAY SHOULD support more than one adjacency on a bit and
ECMP adjacencies. The importance of ECMP adjacencies is unclear when
traffic steering is used because it may be more desirable to
explicitly steer traffic across non-ECMP paths to make per-path
traffic calculation easier for BIER-TE Controllers. Having more than
one adjacency for a bit bit.
This allows further savings of to save bits in hub&spoke
scenarios, but unlike rings it is less "natural" to flood traffic
across multiple links unconditional. Both scenarios (see Section 5.1.5).

BIER-TE forwarding MAY support ECMP and multiple adjacencies are forwarding plane features that should be possible to
support later when needed as they do not impact the basic BIER-TE
replication loop. save bits in ECMP
scenarios, see Section 5.1.7 for an example. This is true a MAY
requirement, because there is no inter-copy
dependency through resetting the deployment importance of F-BM ECMP adjacencies
for BIER-TE is unclear as in BIER.

4. one can also leverage ECMP of the routing
underlay via forwarded_routed adjacencies and/or might prefer to have
more explicit control of the path chosen via explicit BP/adjacencies
for each ECMP path alternative.

5. BIER-TE Controller BitPosition Operational Considerations

5.1. Bit position Assignments

This section describes how the BIER-TE Controller can use the
different BIER-TE adjacency types to define the BitPositions bit positions of a
BIER-TE domain.

Because the size of the BitString is limiting limits the size of the BIER-TE
domain, many of the options described exist to support larger
topologies with fewer BitPositions bit positions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7,
4.8).

4.1.

5.1.1. P2P Links

Each P2p

On a P2P link in that connects two BFR, the same bit position can be
used on both BFR for the BIER-TE domain is assigned one unique
BitPosition with a forward_connected adjacency pointing to the
neighbor on the p2p link.

4.2. neighboring BFR. A P2P
link requires therefore only one bit position.

5.1.2. BFER

Every non-Leaf BFER is given a unique BitPosition bit position with a local_decap
adjacency.

4.3.

5.1.3. Leaf BFERs

BFR1(P) BFR2(P) BFR1(P) BFR2(P)
| \ / | | |
| X | | |
| / \ | | |
BFER1(PE) BFER2(PE) BFER1(PE)----BFER2(PE)

^ U-turn link

Leaf BFER / Non-Leaf BFER /
PE-router PE-router

Figure 8: 10: Leaf vs. non-Leaf BFER Example

Leaf BFERs are

A leaf BFERs is one where incoming BIER-TE packets never need to be
forwarded to another BFR but are only sent to the BFER to exit the
BIER-TE domain. For example, in networks where PEs Provider Edge (PE)
router are spokes connected to P Provider (P) routers, those PEs are
Leaf BFERs unless there is a U-turn between two PEs.

Consider how redundant disjoint traffic can reach BFER1/BFER2 in above picture:
Figure 10: When BFER1/BFER2 are Non-Leaf BFER as shown on the right
hand side, one traffic copy would be forwarded to BFER1 from BFR1,
but the other one could only reach BFER1 via BFER2, which makes BFER2
a non-Leaf BFER. Likewise BFER1 is a non-Leaf BFER when forwarding
traffic to BFER2. Note that the BFERs in the left hand picture are
only guaranteed to be leaf-BFER by fitting routing configuration that
prohibits transit traffic to pass through the BFERs, which is
commonly applied in these topologies.

All leaf-BFER leaf-BFERs in a BIER-TE domain can share a single BitPosition. bit position.
This is possible because the BitPosition bit position for the adjacency to reach
the BFER can be used to distinguish whether or not packets should
reach the BFER.

This optimization will not work if an upstream interface of the BFER
is using a BitPosition bit position optimized as described in the following two
sections (LAN, Hub and Spoke).

4.4.

5.1.4. LANs

In a LAN, the adjacency to each neighboring BFR on the LAN is given a unique BitPosition. bit
position. The adjacency of this BitPosition bit position is a
forward_connected
forward_connected() adjacency towards the BFR and this BitPosition bit position
is populated into the BIFT of all the other BFRs on that LAN.

BFR1
|p1
LAN1-+-+---+-----+
p3| p4| p2|
BFR3 BFR4 BFR7

Figure 9: 11: LAN Example

If Bandwidth on the LAN is not an issue and most BIER-TE traffic
should be copied to all neighbors on a LAN, then BitPositions bit positions can be
saved by assigning just a single BitPosition bit position to the LAN and
populating the BitPosition bit position of the BIFTs of each BFRs on the LAN with
a list of forward_connected forward_connected() adjacencies to all other neighbors on
the LAN.

This optimization does not work in the case of BFRs redundantly
connected to more than one LANs LAN with this optimization because these
BFRs would receive duplicates and forward those duplicates into the
opposite LANs. Adjacencies of such BFRs into their LANs LAN still need a
separate BitPosition.

4.5. bit position.

5.1.5. Hub and Spoke

In a setup with a hub and multiple spokes connected via separate p2p
links to the hub, all p2p links can share the same BitPosition. bit position. The
BitPosition
bit position on the hub's BIFT is set up with a list of
forward_connected
forward_connected() adjacencies, one for each Spoke.

This option is similar to the BitPosition bit position optimization in LANs:
Redundantly connected spokes need their own BitPositions. bit positions, unless
they are themselves Leaf-BFER.

This type of optimized BP could be used for example when all traffic
is "broadcast" traffic (very dense receiver set) such as live-TV or
situation-awareness (SA). This BP optimization can then be used to
explicitly steer different traffic flows across different ECMP paths
in Data-Center or broadband-aggregation networks with minimal use of
BPs.

4.6.

5.1.6. Rings

In L3 rings, instead of assigning a single BitPosition bit position for every p2p
link in the ring, it is possible to save BitPositions bit positions by setting the
"Do Not Reset" (DNR)
"DoNotClear" (DNC) flag on forward_connected forward_connected() adjacencies.

For the rings shown in the following picture, Figure 12, a single BitPosition bit position will suffice
to forward traffic entering the ring at BFRa or BFRb all the way up
to BFR1:

On BFRa, BFRb, BFR30,... BFR3, the BitPosition bit position is populated with a
forward_connected
forward_connected() adjacency pointing to the clockwise neighbor on
the ring and with DNR DNC set. On BFR2, the adjacency also points to the
clockwise neighbor BFR1, but without DNR DNC set.

Handling DNR DNC this way ensures that copies forwarded from any BFR in
the ring to a BFR outside the ring will not have the ring BitPosition bit
position set, therefore minimizing the chance to create loops.

v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc

Figure 10: 12: Ring Example

Note that this example only permits for packets intended to enter make it
all the way around the ring to enter it at BFRa and BFRb, and that
packets will always travel clockwise. If packets should be allowed
to enter the ring at any ring BFR, then one would have to use two
ring BitPositions. bit positions. One for clockwise, one for each direction: clockwise and
counterclockwise.

Both would be set up to stop rotating on the same link, e.g. L1.
When the ingress ring BFR creates the clockwise copy, it will reset clear
the counterclockwise BitPosition bit position because the DNR DNC bit only applies to
the bit for which the replication is done. Likewise for the
clockwise BitPosition bit position for the counterclockwise copy. In As a result,
the ring ingress BFR will send a copy in both directions, serving
BFRs on either side of the ring up to L1.

4.7.

5.1.7. Equal Cost MultiPath (ECMP)

The ECMP adjacency allows to use just one BP per link bundle between
two BFRs instead of one BP for each p2p member link of that link
bundle. In the following picture, Figure 13, one BP is used across L1,L2,L3.

--L1-----
BFR1 --L2----- BFR2
--L3-----

BIFT entry in BFR1:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({forward_connected(L1, BFR2), |
| | forward_connected(L2, BFR2), |
| | forward_connected(L3, BFR2)}, seed) |
------------------------------------------------------------------

BIFT entry in BFR2:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({forward_connected(L1, BFR1), |
| | forward_connected(L2, BFR1), |
| | forward_connected(L3, BFR1)}, seed) |
------------------------------------------------------------------

Figure 11: 13: ECMP Example

This document does not standardize any ECMP algorithm because it is
sufficient for implementations to document their freely chosen ECMP
algorithm. This allows the BIER-TE Controller to calculate ECMP
paths and seeds. The following picture Figure 14 shows an example ECMP algorithm:

forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)):
i = (packet(bier-header-entropy) XOR seed) % N
forward packet to adj(i)

Figure 12: 14: ECMP algorithm Example

In the following example, all traffic from BFR1 towards BFR10 is
intended to be ECMP load split equally across the topology. This
example is not meant as a likely setup, but to illustrate that ECMP
can be used to share BPs not only across link bundles, but also
across alternative paths across different transit BFR, and it
explains the use of the seed parameter.

BFR1 (BFIR)
/L11 \L12
/ \
BFR2 BFR3
/L21 \L22 /L31 \L32
/ \ / \
BFR4 BFR5 BFR6 BFR7
\ / \ /
\ / \ /
BFR8 BFR9
\ /
\ /
BFR10 (BFER)

BIFT entry in BFR1:
------------------------------------------------------------------
| 0:6 | ECMP({forward_connected(L11, BFR2), |
| | forward_connected(L12, BFR3)}, seed1) |
------------------------------------------------------------------

BIFT entry in BFR2:
------------------------------------------------------------------
| 0:7 | ECMP({forward_connected(L21, BFR4), |
| | forward_connected(L22, BFR5)}, seed1) |
------------------------------------------------------------------

BIFT entry in BFR3:
------------------------------------------------------------------
| 0:7 | ECMP({forward_connected(L31, BFR6), |
| | forward_connected(L32, BFR7)}, seed1) |
------------------------------------------------------------------

BIFT entry in BFR4, BFR5:
------------------------------------------------------------------
| 0:8 | forward_connected(Lxx, BFR8) |xx differs on BFR4/BFR5|
------------------------------------------------------------------

BIFT entry in BFR6, BFR7:
------------------------------------------------------------------
| 0:8 | forward_connected(Lxx, BFR9) |xx differs on BFR6/BFR7|
------------------------------------------------------------------

BIFT entry in BFR8, BFR9:
------------------------------------------------------------------
| 0:9 | forward_connected(Lxx, BFR10) |xx differs on BFR8/BFR9|
------------------------------------------------------------------

Figure 13: 15: Polarization Example

Note that for the following discussion of ECMP, only the BIFT ECMP
adjacencies on BFR1, BFR2, BFR3 are relevant. The re-use of BP
across BFR in this example is further explained in Section 4.9 5.1.9
below.

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

This issue in BFR2/BFR3 is called polarization. It results from the
re-use of the same hash function across multiple consecutive hops in
topologies like these. To resolve this issue, the ECMP adjacency on
BFR1 can be set up with a different seed2 than the ECMP adjacencies
on BFR2/BFR3. BFR2/BFR3 can use the same hash because packets will
not sequentially pass across both of them. Therefore, they can also
use the same BP 0:7.

Note that ECMP solutions outside of BIER often hide the seed by auto-
selecting it from local entropy such as unique local or next-hop
identifiers. The solutions chosen for BIER-TE to allow Allowing the BIER-TE Controller to explicitly set the
seed maximizes gives the ability of the
BIER-TE Controller to choose the seed, independent of such seed
source that the BIER-TE Controller may not be able for it to control well,
and even calculate optimized seeds for multi-hop cases.

4.8. Routed same/different path
selection across multiple consecutive ECMP hops.

5.1.8. Forward_routed adjacencies

4.8.1.

5.1.8.1. Reducing BitPositions

Routed bit positions

Forward_routed() adjacencies can reduce the number of BitPositions bit positions
required when the path steering requirement is not hop-by-hop
explicit path selection, but loose-hop selection. Routed Forward_routed()
adjacencies can also allow to operate BIER-TE across intermediate hop
routers that do not support BIER-TE.

...............
...BFR1--... ...--L1-- BFR2...
... .Routers. ...--L2--/
...BFR4--... ...------ BFR3...
............... |
LO
Network Area 1

Figure 14: Routed 16: Forward_routed Adjacencies Example

Assume the requirement in the above picture Figure 16 is to explicitly steer traffic
flows that have arrived at BFR1 or BFR4 via a shortest path in the
routing underlay "Network Area 1" to one of the following three next
segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via
BFR3.

To enable this, both BFR1 and BFR4 are set up with a forward_routed
adjacency BitPosition bit position towards an address of BFR2 on link L1, another
forward_routed BitPosition
forward_routed() bit position towards an address of BFR2 on link L2
and a third forward_routed Bitposition forward_routed() bit position towards a node address LO
of BFR3.

4.8.2.

5.1.8.2. Supporting nodes without BIER-TE

Routed

Forward_routed() adjacencies also enable incremental deployment of
BIER-TE. Only the nodes through which BIER-TE traffic needs to be
steered - with or without replication - need to support BIER-TE.
Where they are not directly connected to each other, forward_routed
adjacencies are used to pass over non BIER-TE enabled nodes.

4.9.

5.1.9. Reuse of BitPositions bit positions (without DNR)

BitPositions DNC)

bit positions can be re-used across multiple BFR to minimize the
number of BP needed. This happens when adjacencies on multiple BFR
use the DNR DNC flag as described above, but it can also be done for non-
DNR
DNC adjacencies. This section only discussses discusses this non-DNR non-DNC case.

Because BP are reset after cleared when passing a BFR with an adjacency for that
BP, reuse of BP across multiple BFR does not introduce any problems
with duplicates or loops that do not also exist when every adjacency
has a unique BP: Instead of setting one BP in a BitString that is
reused in N-adjacencies, one would get BP. Instead, the same or worse results if
each of these adjacencies had a unique challenge when reusing BP and all of them where set
in the BitString. Instead, based on is whether
it allows to still achieve the case, BPs can be reused
without limitation, or they introduce fewer path steering choices, or
they do not work. desired Tree Engineering goals.

BP cannot be reused across two BFR that would need to be passed
sequentially for some path: The first BFR will reset clear the BP, so those
paths cannot be built. BP can be set across BFR that would (A) only
occur across different paths or (B) across different branches of the
same tree.

An example of (A) was given in Figure 13, 15, where BP 0:7, BP 0:8 and BP
0:9 are each reused across multiple BFR BFRs because a single packet/path
would never be able to reach more than one BFR sharing the same BP.

Assume the example was changed: BFR1 has no ECMP adjacency for BP
0:6, but instead BP 0:5 with forward_connected forward_connected() to BFR2 and BP 0:6
with forward_connected forward_connected() to BFR3. Packets with both BP 0:5 and BP
0:6 would now be able to reach both BFR2 and BFR3 and the still
existing re-use of BP 0:7 between BFR2 and BFR3 is a case of (B)
where reuse of BP is perfect because it does not limit the set of
useful path choices:

If instead of reusing BP 0:7, BFR3 used a separate BP 0:10 for its
ECMP adjacency, no useful additional path steering options would be
enabled. If duplicates at BFR10 where undesirable, this would be
done by not setting BP 0:5 and BP 0:6 for the same packet. If the
duplicates where desirable (e.g.: resilient transmission), the
additional BP 0:10 would also not render additional value.

Figure 17: Reuse of BP

Reuse may also save BPs in larger topologies. Consider the topology
shown in Figure 17, but only the following explanations: 20. A BFIR/
sender BFIR/sender (e.g.: video headend) is attached
to area 1, and area 2...6 contain receivers/BFER. Assume each area
had a distribution ring, each with two BPs to indicate the direction
(as explained in before). These two BPs could be reused across the 5
areas. Packets would be replicated through other BPs for the Core to
the desired subset of areas, and once a packet copy reaches the ring
of the area, the two ring BPs come into play. This reuse is a case
of (B), but it limits the topology choices: Packets can only flow
around the same direction in the rings of all areas. This may or may
not be acceptable based on the desired path steering options: If
resilient transmission is the path engineering goal, then it is
likely a good optimization, if the bandwidth of each ring was to be
optimized separately, it would not be a good limitation.

4.10.

5.1.10. Summary of BP optimizations

This section reviewed a range of techniques by which a BIER-TE
Controller can create a BIER-TE topology in a way that minimizes the
number of necessary BPs.

Without any optimization, a BIER-TE Controller would attempt to map
the network subnet topology 1:1 into the BIER-TE topology and every
subnet adjacent neighbor requires a forward_connected forward_connected() BP and every
BFER requires a local_decap local_decap() BP.

The optimizations described are then as follows:

o P2p P2P links require only one BP (Section 4.1). 5.1.1).

o All leaf-BFER can share a single local_decap local_decap() BP (Section 4.3). 5.1.3).

o A LAN with N BFR needs at most N BP (one for each BFR). It only
needs one BP for all those BFR tha that are not redundanty redundantly connected
to multiple LANs (Section 4.4). 5.1.4).

o A hub with p2p connections to multiple non-leaf-BFER spokes can
share one BP to all spokes if traffic can be flooded to all
spokes, e.g.: because of no bandwidth concerns or dense receiver
sets (Section 4.5). 5.1.5).

o Rings of BFR can be built with just two BP (one for each
direction) except for BFR with multiple ring connections - similar
to LANs (Section 4.6). 5.1.6).

o ECMP adjacencies to N neighbors can replace N BP with 1 BP.
Multihop ECMP can avoid polarization through different seeds of
the ECMP algorithm (Section 4.7). 5.1.7).

o Routed Forward_routed() adjacencies allow to "tunnel" across non-BIER-TE
capable routers and across BIER-TE capable routers where no traffic-
steering
traffic-steering or replications are required (Section 4.8). 5.1.8).

o BP can generally be reused across nodes that do not need to be
consecutive in paths, but depending on scenario, this may limit
the feasible path steering options (Section 4.9). 5.1.9).

Note that the described list of optimizations is not exhaustive.
Especially when the set of required path steering choices is limited
and the set of possible subsets of BFER BFERs that should be able to
receive traffic is limited, further optimizations of BP are possible.
The hub & spoke optimization is a simple example of such traffic
pattern dependent optimizations.

5.2. Avoiding duplicates and loops

5.1.

5.2.1. Loops

Whenever BIER-TE creates a copy of a packet, the BitString of that
copy will have all BitPositions bit positions cleared that are associated with
adjacencies on the BFR. This inhibits looping of packets. The only
exception are adjacencies with DNR DNC set.

v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb ---------------------\
| . |
| ...... Wrong link wiring |
| . |
\- BFR1 - BFR2 BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc

Figure 18: Miswired Ring Example

With DNR DNC set, looping can happen. Consider in the ring picture Figure 18 that link L4
from BFR3 is (inadvertently) plugged into the L1 interface of BFRa. BFRa
(instead of BFR2). This creates a loop where the rings clockwise BitPosition bit
position is never reset cleared for copies of the packets traveling
clockwise around the ring.

To inhibit looping in the face of such physical misconfiguration,
only forward_connected forward_connected() adjacencies are permitted to have DNR DNC set,
and the link layer port unique unicast destination address of the
adjacency (e.g. MAC address) protects against closing the loop.
Link layers without port unique link layer addresses should not be
used with the DNR DNC flag set.

5.2.

5.2.2. Duplicates
BFIR1
/ \
/ p2 \ p3
BFR2 BFR3
\ p4 / p5
\ /
BFER4

Figure 19: Duplicates Example

Duplicates happen when the topology of the graph expressed by a BitString is not a
tree but redundantly connecting BFRs with each other. In Figure 19,
a BitString of p2,p3,p4,p5 would result in duplicate packets to
arrive on BFER4. The BIER-TE Controller must therefore ensure to
only create BitStrings that are
trees in the topology. trees.

When links are incorrectly physically re-connected before the BIER-TE
Controller updates BitStrings in BFIRs, duplicates can happen. Like
loops, these can be inhibited by link layer addressing in
forward_connected
forward_connected() adjacencies.

If interface or loopback addresses used in forward_routed forward_routed()
adjacencies are moved from one BFR to another, duplicates can equally
happen. Such re-addressing operations must be coordinated with the
BIER-TE Controller.

6. BIER-TE Forwarding Pseudocode

The following simplified pseudocode for BIER-TE forwarding is using
BIER forwarding pseudocode of [RFC8279], section 6.5 with the one
modification necessary to support basic BIER-TE forwarding. Like the
BIER pseudo forwarding code, for simplicity it does hide the details
of the adjacency processing inside PacketSend() which can be
forward_connected, forward_routed or local_decap.

void ForwardBitMaskPacket_withTE (Packet)
{
SI=GetPacketSI(Packet);
Offset=SI*BitStringLength;
for (Index = GetFirstBitPosition(Packet->BitString); Index ;
Index = GetNextBitPosition(Packet->BitString, Index)) {
F-BM = BIFT[Index+Offset]->F-BM;
if (!F-BM) continue;
BFR-NBR = BIFT[Index+Offset]->BFR-NBR;
PacketCopy = Copy(Packet);
PacketCopy->BitString &= F-BM; [2]
PacketSend(PacketCopy, BFR-NBR);
// The following must not be done for BIER-TE:
// Packet->BitString &= ~F-BM; [1]
}
}

Figure 15: Simplified BIER-TE Forwarding Pseudocode

The difference is that in BIER-TE, step [1] must not be performed,
but is replaced with [2] (when the forwarding plane algorithm is
implemented verbatim as shown above).

In BIER, the F-BM of a BP has all BP set that are meant to be
forwarded via the same neighbor. It is used to reset those BP in the
packet after the first copy to this neighbor has been made to inhibit
multiple copies to the same neighbor.

In BIER-TE, the F-BM of a particular BP with an adjacency is the list
of all BPs with an adjacency on this BFR except the particular BP
itself if it has an adjacency with the DNR bit set. The F-BM is used
to reset the F-BM BPs before creating copies.

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

The above simplified pseudocode is elaborated further as follows:

o This pseudocode eliminates per-bit F-BM, therefore reducing state
by BitStringLength^2*SI and eliminating the need for per-packet-
copy masking operation except for adjacencies with DNR flag set:

* AdjacentBits[SI] are bits with a non-empty list of adjacencies.
This can be computed whenever the BIER-TE Controller updates
the adjacencies.

* Only the AdjacentBits need to be examined in the loop for
packet copies.

* The packets BitString is masked with those AdjacentBits on
ingress to avoid packets looping.

o The code loops over the adjacencies because there may be more than
one adjacency for a bit.

o When an adjacency has the DNR bit, the bit is set in the packet
copy (to save bits in rings for example).

o The ECMP adjacency is shown. Its parameters are a
ListOfAdjacencies from which one is picked.

o The forward_local, forward_routed, local_decap adjacencies are
shown with their parameters.

void ForwardBitMaskPacket_withTE (Packet)
{
SI=GetPacketSI(Packet);
Offset=SI*BitStringLength;
AdjacentBitstring = Packet->BitString &= ~AdjacentBits[SI];
Packet->BitString &= AdjacentBits[SI];
for (Index = GetFirstBitPosition(AdjacentBits); Index ;
Index = GetNextBitPosition(AdjacentBits, Index)) {
foreach adjacency BIFT[Index+Offset] {
if(adjacency == ECMP(ListOfAdjacencies, seed) ) {
I = ECMP_hash(sizeof(ListOfAdjacencies),
Packet->Entropy, seed);
adjacency = ListOfAdjacencies[I];
}
PacketCopy = Copy(Packet);
switch(adjacency) {
case forward_connected(interface,neighbor,DNR):
if(DNR)
PacketCopy->BitString |= 2<<(Index-1);
SendToL2Unicast(PacketCopy,interface,neighbor);

case forward_routed({VRF},neighbor):
SendToL3(PacketCopy,{VRF,}l3-neighbor);

case local_decap({VRF},neighbor):
DecapBierHeader(PacketCopy);
PassTo(PacketCopy,{VRF,}Packet->NextProto);
}
}
}
}

Figure 16: BIER-TE Forwarding Pseudocode

5.3. Managing SI, subdomains sub-domains and BFR-ids

When the number of bits required to represent the necessary hops in
the topology and BFER exceeds the supported bitstring length, BitStringLength (BSL),
multiple SI SIs and/or subdomains sub-domains must be used. This section discusses
how.

BIER-TE forwarding does not require the concept of BFR-id, but
routing underlay, flow overlay and BIER headers may. This section
also discusses how BFR-ids can be assigned to BFIR/BFER for BIER-TE.

7.1.

5.3.1. Why SI and sub-domains

For BIER and BIER-TE forwarding, the most important result of using
multiple SI and/or subdomains sub-domains is the same: Packets that need to be
sent to BFER BFERs in different SI SIs or subdomains sub-domains require different BIER
packets: each one with a bitstring BitString for a different (SI,subdomain) (SI,sub-domain)
combination. Each such bitstring BitString uses one bitstring length BSL sized SI block in the
BIFT of the subdomain. sub-domain. We call this a BIFT:SI (block).

For BIER and BIER-TE forwarding itself themselves there is also no
difference whether different SI SIs and/or sub-domains are chosen, but
SI and
subdomain sub-domain have different purposes in the BIER architecture
shared by BIER-TE. This impacts how operators are managing them and
how especially flow overlays will likely use them.

By default, every possible BFIR/BFER in a BIER network would likely
be given a BFR-id in subdomain sub-domain 0 (unless there are > 64k BFIR/BFER).

If there are different flow services (or service instances) requiring
replication to different subsets of BFER, BFERs, then it will likely not be
possible to achieve the best replication efficiency for all of these
service instances via subdomain sub-domain 0. Ideal replication efficiency for
N BFER exists in a subdomain sub-domain if they are split over not more than
ceiling(N/bitstring-length)
ceiling(N/BitStringLength) SI.

If service instances justify additional BIER:SI state in the network,
additional subdomains sub-domains will be used: BFIR/BFER are assigned BFIR-id BFR-id in
those subdomains sub-domains and each service instance is configured to use the
most appropriate subdomain. sub-domain. This results in improved replication
efficiency for different services.

Even if creation of subdomains sub-domains and assignment of BFR-id to BFIR/BFER
in those subdomains sub-domains is automated, it is not expected that individual
service instances can deal with BFER in different subdomains. sub-domains. A
service instance may only support configuration of a single subdomain sub-
domain it should rely on.

To be able to easily reuse (and modify as little as possible)
existing BIER procedures including flow-overlay and routing underlay,
when BIER-TE forwarding is added, we therefore reuse SI and subdomain sub-
domain logically in the same way as they are used in BIER: All
necessary BFIR/BFER for a service use a single BIER-TE BIFT and are
split across as many SI SIs as necessary (see below). Section 5.3.2). Different
services may use different subdomains sub-domains that primarily exist to
provide more efficient replication (and for BIER-TE desirable path
steering) for different subsets of BFIR/BFER.

7.2. Bit assignment comparison BIER and

5.3.2. Assigning bits for the BIER-TE topology

In BIER, bitstrings BitStrings only need to carry bits for BFER, BFERs, which leads to
the model that BFR-ids map 1:1 to each bit in a bitstring. BitString.

In BIER-TE, bitstrings BitStrings need to carry bits to indicate not only the
receiving BFER but also the intermediate hops/links across which the
packet must be sent. The maximum number of BFER that can be
supported in a single bitstring BitString or BIFT:SI depends on the number of
bits necessary to represent the desired topology between them.

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

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

7.3. Using

5.3.3. Assigning BFR-id with BIER-TE

Because there is no 1:1 mapping between bits in the bitstring and
BFER,

BIER-TE cannot simply rely on forwarding does not use the BFR-id, not does it require for
the BFR-id field of the BIER 1:1 mapping between bits
in header to be set to a particular value.
However, other parts of a BIER-TE deployment may need a bitstring BFR-id,
specifically overlay signaling, and BFR-id.

In BIER, automatic schemes could assign all possible in that case BFR need to also
have BFR-ids
sequentially for BIER-TE SDs.

For example, for BIER overlay signaling, BFIR need to BFERs. This will not work have a BFR-id,
because this BFIR BFR-id is carried in BIER-TE. the BFR-id field of the BIER
header to indicate to the overlay signaling on the receiving BFER
which BFIR originated the packet.

In BIER-TE, BIER, BFR-id = BSL * SI + BP, such that the operator or BIER-TE Controller SI and BP of a BFER
can be calculated from the BFR-id and vice versa. This also means
that every BFR with a BFR-id has to determine a reserved BP in an SI, even if that
is not necessary for BIER forwarding, because the BFR may never be a
BFER but only a BFIR.

In BIER-TE, for a non-leaf BFER, there is usually a single BP for
that BFER with a local_decap() adjacency on the BFER. The BFR-id for each
such a BFER can therefore equally be determined as in each required subdomain. The BIER: BFR-id may or =
SI * BitStringLength + BP.

As explained in Section 5.1.3, leaf BFERs do not need such a unique
local_decap() adjacency, likewise, BFIR who are not also BFER may not
have a
relationship unique local_decap() adjacency either. For all those BFIR and
(leaf) BFER, the controller needs to determine unique BFR-ids that do
not collide with the BFR-ids derived from the non-leaf BFER
local_decap() BPs.

While this document defines no requirements how to allocate such BFR-
id, a bit in simple option is to derive it from the bitstring. Suggestions are detailed
below. Once determined, (SI,BP) of an adjacency
that is unique to the BFR-id BFR in question. For a BFIR this can then be configured he
first adjacency only populated on this BFIR, for a leaf-BFER, this
could be the
BFER and used by flow overlay, routing underlay and first BP with an adjacency towards that BFER.

5.3.4. Mapping from BFR to BitStrings with BIER-TE

In BIER, applications of the BIER header
almost flow overlay on a BFIR can calculate the same as
(SI,BP) of a BFER from the BFR-id in BIER.

The one exception are application/flow-overlays that automatically
calculate the bitstring(s) of the BFER and can therefore
easily determine the BitStrings for a BIER packets by converting BFR-id packet to
bits. a set of BFER
with known BFR-ids.

In BIER-TE, BIER-TE this operation can mapping needs to be done in equally supported for flow
overlays. This section outlines two ways: core options, based on how
"complex" the Tree Engineering is that the BIER-TE controller
performs for a particular application.

"Independent branches": For a given application or (set of) trees, flow overlay instance, the
branches from a BFIR to every BFER are calculated by the BIER-TE
controller to be independent of the branches to any other BFER. For example, shortest part
Shortest path trees have are the most common examples of trees with
independent branches.

"Interdependent branches": When a BFER is added or deleted from a
particular distribution tree, the BIER-TE controller has to
recalculate the branches to other BFER still in the
tree BFER, because they may need to
change. Steiner tree trees are examples of dependent interdependent branch trees.

If "independent branches" are sufficient, used, the BIER-TE Controller can
provide signal
to such applications for every BFR-id a SI:bitstring with the
BIER-TE bits BFIR flow overlay for every BFER an SI:BitString that
represents the branch towards to that BFER. The application flow overlay on the BIFR can
then independently of the controller calculate the SI:bitstring SI:BitString for
all desired BFER by OR'ing their bitstrings. BitStrings. This allows for flow
overlay applications to operate independently from the controller
whenever it needs to determine which subset of BFERs need to receive
a particular packet.

If "interdependent branches" are required, the application could call
a BIER-TE Controller API with would need
to inquire the list SI:BitString for a given set of required BFER-id and get
the required bitstring back. Whenever BFER whenever the set of BFER-id changes,
this is repeated.
changes.

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

7.4. Assigning BFR-ids for
SI:BitStrings.

Communications between BFIR flow overlay and BIER-TE

For a non-leaf BFER, there is usually a single bit k for that BFER
with a local_decap() adjacency on the controller
requires some way to identify BFER. The BFR-id for such a
BFER is therefore most easily the one it would have If BFR-ids are used in BIER: SI *
bitstring-length + k.

As explained earlier the
deployment, as outlined in Section 5.3.3, then those are the document, leaf BFERs do natural
BFR identifier. If BFR-ids are not need used, then any other unique
identifier, such a
separate bit because the fact alone that the BIER-TE packet is
forwarded to the leaf BFER indicates that as the BFER should decapsulate
it. Such a BFER will have one or more bits for BFR-prefix of the links leading
only to it. The BFR-id BFR as of [RFC8279] could therefore most easily
be the BFR-id
derived from the lowest bit for those links.

These two rules are only recommendations used.

5.3.5. Assigning BFR-ids for the operator or BIER-TE
Controller assigning the BFR-ids. Any allocation scheme can be used,
the BFR-ids just need to be unique across BFRs in each subdomain.

It is not currently determined if a single subdomain sub-domain could or should
be allowed to forward both BIER and BIER-TE packets. If this should
be supported, there are two options:

A. BIER and BIER-TE have different BFR-id in the same subdomain. sub-domain.
This allows higher replication efficiency for BIER because their BFR-
id can be assigned sequentially, while the bitstrings BitStrings for BIER-TE
will have also the additional bits for the topology. There is no
relationship between a BFR BIER BFR-id and BIER-TE BFR-id.

B. BIER and BIER-TE share the same BFR-id. The BFR-id BFR-ids are assigned
as explained above for BIER-TE and simply reused for BIER. The
replication efficiency for BIER will be as low as that for BIER-TE in
this approach. Depending on topology, only the same 20%..80% of bits
as possible for BIER-TE can be used for BIER.

7.5.

5.3.6. Example bit allocations

7.5.1.

5.3.6.1. With BIER

Consider a network setup with a bitstring length BSL of 256 for a network topology as
shown in the picture below. Figure 20. The network has 6 areas, each with ca. 170 BFR, BFRs,
connecting via a core with some larger 4 (core)
BFR. BFRs. To address all BFER BFERs with
BIER, 4 SI SIs are required. To send a BIER packet to all BFER in the
network, 4 copies need to be sent by the BFIR. On the BFIR it does
not make a difference how the BFR-id BFR-ids are allocated to BFER in the
network, but for efficiency further down in the network it does make
a difference.

area1 area2 area3
BFR1a BFR1b BFR2a BFR2b BFR3a BFR3b
| \ / \ / |
................................
. Core .
................................
| / \ / \ |
BFR4a BFR4b BFR5a BFR5b BFR6a BFR6b
area4 area5 area6

Figure 17: 20: Scaling BIER-TE bits by reuse

With random allocation of BFR-id to BFER, each receiving area would
(most likely) have to receive all 4 copies of the BIER packet because
there would be BFR-id for each of the 4 SI SIs in each of the areas.
Only further towards each BFER would this duplication subside - when
each of the 4 trees runs out of branches.

If BFR-id BFR-ids are allocated intelligently, then all the BFER in an area
would be given BFR-id with as few as possible different SI. SIs. Each
area would only have to forward one or two packets instead of 4.

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

This example shows that intelligent BFR-id allocation within at least
subdomain
sub-domain 0 can even be helpful or even necessary in BIER.

7.5.2.

5.3.6.2. With BIER-TE

In BIER-TE one needs to determine a subset of the physical topology
and attached BFER BFERs so that the "desired" representation of this
topology and the BFER fit into a single bitstring. BitString. This process
needs to be repeated until the whole topology is covered.

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

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

To set up BIER-TE efficiently for the above topology, the following
bit allocation methods method can be used. This method can easily be
expanded to other, similarly structured larger topologies.

Each area is allocated one or more SI SIs depending on the number of
future expected BFER BFERs and number of bits required for the topology in
the area. In this example, 6 SI, SIs, one per area.

In addition, we use 4 bits in each SI: bia, bib, bea, beb: bit
ingress a, bit ingress b, bit egress a, bit egress b. These bits
will be used to pass BIER packets from any BFIR via any combination
of ingress area a/b BFR and egress area a/b BFR into a specific
target area. These bits are then set up with the right
forward_routed
forward_routed() adjacencies on the BFIR and area edge BFR:

On all BFIR BFIRs in an area j, j|j=2...6, bia in each BIFT:SI is populated
with the same forward_routed(BFRja), and bib with
forward_routed(BFRjb). On all area edge BFR, bea in BIFT:SI=k
BIFT:SI=k|k=2...6 is populated with forward_routed(BFRka) and beb in
BIFT:SI=k with forward_routed(BFRkb).

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

7.6.

5.3.7. Summary

BIER-TE can can, like BIER BIER, support multiple SI SIs within a sub-domain to
allow re-using the concept of BFR-id and therefore minimize BIER-TE
specific functions in underlay routing, any possible BIER layer control plane used in
conjunction with BIER-TE, flow overlay methods and BIER headers.

The number of BFIR/BFER possible in a subdomain sub-domain is smaller than in
BIER because BIER-TE uses additional bits for topology.

Subdomains can

Sub-domains (SDs) in BIER-TE can be used like in BIER to create more
efficient replication to known subsets of BFER. BFERs.

Assigning bits for BFER BFERs intelligently into the right SI is more
important in BIER-TE than in BIER because of replication efficiency
and overall amount of bits required.

6. BIER-TE and Segment Routing

SR aims to enable lightweight path steering via loose source routing.
Compared to its more heavy-weight predecessor RSVP-TE, SR does for
example not require per-path signaling to each of these hops.

BIER-TE supports the same design philosophy for multicast. Like in
SR, it relies on source-routing - via the definition of a BitString.
Like SR, it only requires to consider the "hops" on which either
replication has to happen, or across which the traffic should be
steered (even without replication). Any other hops can be skipped
via the use of routed adjacencies.

BIER-TE BitPosition bit position (BP) can be understood as the BIER-TE equivalent
of "forwarding segments" in SR, but they have a different scope than
SR forwarding segments. Whereas forwarding segments in SR are global
or local, BPs in BIER-TE have a scope that is the group of BFR(s)
that have adjacencies for this BP in their BIFT. This can be called
"adjacency" scoped forwarding segments.

Adjacency scope could be global, but then every BFR would need an
adjacency for this BP, for example a forward_routed forward_routed() adjacency with
encapsulation to the global SR SID of the destination. Such a BP
would always result in ingress replication though. The first BFR
encountering this BP would directly replicate to it. Only by using
non-global adjacency scope for BPs can traffic be steered and
replicated on non-ingress BFR.

SR can naturally be combined with BIER-TE and help to optimize it.
For example, instead of defining BitPositions bit positions for non-replicating
hops, it is equally possible to use segment routing encapsulations
(eg: MPLS
(e.g. SR-MPLS label stacks) for the encapsulation of
"forward_routed" adjacencies.

Note that BIER itself can also be seen to be similar to SR. BIER BPs
act as global destination Node-SIDs and the BIER bitstring BitString is simply
a highly optimized mechanism to indicate multiple such SIDS SIDs and let
the network take care of effectively replicating the packet hop-by-
hop to each destination Node-SID. What BIER does not allow is to
indicate intermediate hops, or terms of SR the ability to indicate a
sequence of SID to reach the destination. This is what BIER-TE and
its adjacency scoped BP enables.

Both BIER and BIER-TE allow BFIR to "opportunistically" copy packets
to a set of desired BFER on a packet-by-packet basis. In BIER, this
is done by OR'ing the BP for the desired BFER. In BIER-TE this can
be done by OR'ing for each desired BFER a bitstring BitString using the
"independent branches" approach described in Section 7.3 5.3.3 and
therefore also indicating the engineered path towards each desired
BFER. This is the approach that
[I-D.ietf-bier-multicast-http-response] relies on.

7. Security Considerations

The

If [RFC8296] is used, BIER-TE shares its security considerations are considerations.

BIER-TE shares the same as for BIER security considerations of BIER, [RFC8279], with
the following differences: overriding or additional considerations.

In BIER, the standardized methods for the routing underlays as well
as to distribute BFR-ids and BFR-prefixes are not IGPs such as specified
in [RFC8401] for IS-IS and in [RFC8444] for OSPF. Attacking the
protocols for the BIER routing underlay or BIER layer control plane,
or impairment of any BFR in a domain may lead to successful attacks
against the results of the routing protocol, enabling DoS attacks
against paths or addressing (BFR-id, BFR-prefixes) used by BIER.

The reference model for the BIER-TE layer control plane is a BIER-TE
controller. When such a controller is used, impairment of individual
BFR in BIER-TE, nor are procedures a domain causes no impairment of the BIER-TE control plane on
other BFR. If a routing protocol is used to support forward_routed()
adjacencies, then this is still an attack vector as in BIER, but only
for their distribution, so these BIER-TE forward_routed() adjacencies, and no other adjacencies.

Whereas IGP routing protocols are most often not well secured through
cryptographic authentication and confidentiality, communications
between controllers and routers such as those to be considered for
the BIER-TE controller/control-plane can and are much more commonly
secured with those security properties, for example by using Secure
SHell (SSH), [RFC4253] for NetConf ([RFC6241]), or via Transport
Layer Security (TLS), such as [RFC8253] for PCEP, [RFC5440], or
[RFC7589] for NetConf.

For additional, BIER-TE independent security considerations for the
use of a central BIER-TE controller, the security section of the
protocols and security options in the previous paragraph apply. In
addition, the security considerations of [RFC4655] apply.

The most important attack vectors vector in BIER-TE is misconfiguration,
either on the BFR themselves or via the BIER-TE controller.
Forwarding entries with DNC could be set up to create persistent
loops, in which packets only expire because of TTL. To minimize the
impact of such attacks (or more likely unintentional misconfiguration
by operators and/or bad BIER-TE controller software), the BIER-TE
forwarding rules are defined to be as strict in clearing bits as they
are. The clearing of all bits with an adjacency on a BFR prohibits
that a looping packet creates additional packet amplification through
the misconfigured loop on the packets second or further times around
the loop, because all relevant adjacency bits would have been cleared
on the first round through the loop. In result, BIER-TE has the same
degree of looping packets as possible with unintentional or malicious
loops in the routing underlay with BIER or even with unicast traffic.

Deployments especially where BIER-TE would likely be beneficial may
include operational models where actual configuration changes from
the controller are only required during non-productive phases of the
networks life-cycle, such as in embedded networks or in manufacturing
networks during e.g. plant reworking/repairs. In these type of
deployments, configuration changes could be locked out when the
network is in production state and could only be (re-)enabled through
reverting the network/installation into non-productive state. Such
security designs would not only allow to provide additional layers of
protection against BIER-
TE.

10. configuration attacks, but would foremost protect
the active production process from such configuration attacks.

8. IANA Considerations

This document requests no action by IANA.

11.

9. Acknowledgements

The authors would like to thank Greg Shepherd, Ijsbrand Wijnands,
Neale Ranns, Dirk Trossen, Sandy Zheng, Lou Berger and Berger, Jeffrey Zhang Zhang,
Alvaro Retana and Wolfgang Braun for their reviews and suggestions.

12.

10. Change log [RFC Editor: Please remove]

draft-ietf-bier-te-arch:

10: AD review Alvaro Retana, summary:

Note: rfcdiff shows more changes than actually exist because text
moved around.

Summary:

1. restructuring: merged all controller sections under common
controller ops main section, moved unfitting stuff out to
other parts of doc. Split Intro section into Overview and
Intro. Shortened Abstract, moved text into Overview, added
sections overview.

2. enhanced/rewrote: 2.3 Comparison with -> Relationship to BIER-
TE

3. enhanced/rewrote: 3.2 BIER-TE controller -> BIER-TE control
plane, 3.2.1 BIER-TE controller, for consistency with rfc8279

4. additional subsections for Alvaros asks

5. added to: 3.3 BIER-TE forwarding plane (consistency with
rfc8279)

6. Enhanced description of 4.3/encap considerations to better
explain how BIER/BIER-TE can run together.

Notation: Markers (a),(b),... at end of points are references from
the review discussion with Alvaro to the changes made.

Details:.

Throughout text: changed term spelling to rfc8279 - bit positions,
sub-domain, ... (i).

Reset changed to clear, also DNR changed to DNC (Do Not Clear)
(q).

Abstract: Shortened. Removed name explanation note (Tree
Engineering), (a).

1. Introduction -> Overview: Moved important explanation
paragraph from abstract to Introduction. Fixed text, (a).

Added bullet point list explanation of structure of document (e).

Renamed to Overview because that is now more factually correct.

1.1. Fixed bug in example adding bit p15.(l).

2. (New - Introduction): Moved section 1.1 - 1.3 (examples,
comparison with BIER-TE) from Introduction into new "Overview"
section. Primarily so that "requirements language" section (at
end of Introduction) is not in middle of document after all the
Introduction.

2.1 Removed discussion of encap, moved to 4.2.2 (m).

2.2 enhanced paragraph suggesting native/overlay topology types,
also sugest type hybrid (n).

2.3 Overhauled comparison text BIER/BIER-TE, structured into
common, different, not-required-by-te, integration-bier-bier-te.
Changed title to "Relationship" to allow including last point.
(f).

2.4 moved Hardware forwarding comparison section into section 2 to
allow coalescing of sections into section 5 about the controller
operations (hardware forwarding was in the middle of it, wrong
place). Shortened/improved third paragraph by pointing to BIFT as
deciding element for selection between BIER/BIER-TE. Removed
notion of experimentation (this now targets standard) (g).

3. (Components): Aligned component name and descriptions better
with RFC8279. Now describe exactly same three layers. BIER layer
constituted from BIER-TE control plane and BIER-TE forwarding
plane. BIER-TE controller is now simply component of BIER-TE
control plane. (b).

3.1. shortened/improved paragraph explaining use of SI:BP instead
of also bfr-id as index into BIFT, rewrote paragraph talking about
reuse of BPs(o).

3.2. rewrote explanation of BIER-TE control plane in the style of
RFC8729 Section 4.2 (BIER layer) with numbered points. Note that
RFC8729 mixes control and forwarding plane bullet points (this doc
does not). Merged text from old sections 2.2.1 and 2.2.3 into
list. (b).

3.2.1. Expanded/improved explanation of BIER-TE Controller (b).

3.2.1.1. Added subsection for topology discovery and creation
(d).

3.2.1.2. Added subsection for engineered BitStrings as key novel
aspect not found in BIER. (X).

3.3. Added numbered list for components of BIER-TE forwarding
plane (completing the comparable text from RFC8729 Section 4.2).

3.4 Alvaro does not mind additional example, fixed bugs.

3.5 Removed notion about using IGP BIER extensions for BIER-TE,
such as BIFT address ranges. After -10 making use of BIFT
clearer, it now looks to authors as if use of IGP extensions would
not be beneficial, as long as we do need to use the BIER-TE
controller, e.g. unlike in BIER, a BFR could not learn from the
IGP information what traffic to send towards a particular BIFT-ID,
but instead that is the core of what the controller needs to
provide.

4.2.2 Improved text to explain requirement to identify BIER-TE in
the tunnel encap and compress description of use-cases (m).

4.2.3 enhanced ECMP text (p).

4.3. rewrote most of Encapsulation Considerations to better
explain to Alvaros question re sharing or not sharing SD via BIER/
BIER-TE. Added reference to I-D.ietf-bier-non-mpls-bift-encoding
as a very helpful example. (f).

4.3 Renamed title to "...Co-Existence with BIER" as this is what
it is about and to help finding it from abstract/intro ("co-
exist") (j).

4.4. Moved BIER-TE Forwarding Pseudocode here to coalesce text
logically. Changed text to better compare with BIER pseudo
forwarding code. Numerical list of how F-BM works for BIER-TE.
Removed efficiency comparison with BIER (too difficult to provide
sufficient justification, derails from focus of section) (j).

4.6. (Requirements) Restructured: Removed notion of "basic" BIER-
TE forwarding, simply referring to it now as "mandatory" BIER-TE
forwarding. Cleaned up text to have requirements for different
adjacencies in different paragraphs. (c).

5. Created new main section "BIER-TE Controller operational
considerations", coalesced old sections 4., 5., 7. into this new
main section. No text changes. (k).

5.1.9 Added new separate picture instead of referring to a picture
later in text, adjusted text (r).

5.3.2 Changed title to not include word "comparison" to avoid this
being accounted against Alvaros concern about scattering
comparison (IMHO text already has little comparison, so title was
misleading) (h).

co-authors internal review:

4.4 Added xref to Figure 5.

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

5.2.2 replace "topology" with graph (wrong word).

5.3.3 rewrote explanation of how to map BFR-id to SI:BP and assign
them, clarified BFR-id is option. Retitled to better explain
scope of section.

5.3.4 Removed considerations in 5.3.4 for sharing BFR-id across
BIER/BIER-TE (t), changed title to explain how BFIR/BIER-TE
controller interactions need some form of identifying BFR but this
does not have to be BFR-id.

7. Added new security considerations (u).

09: Incorporated fixes for feedback from Shepherd (Xuesong Geng).

Added references for Bloom Filders Filters and Rate Controlled Service
Disciplines.

1.1 Fixed numbering of example 1 topology explanation. Improved
language on second example (less abbreviating to avoid confusion
about meaning).

1.2 Improved explanation of BIER-TE topology, fixed terminology of
graphs (BIER-TE topology is a directed graph where the edges are
the adjacencies).

2.4 Fixed and amended routing underlay explanations: detailled detailed why
no need for BFER routing underlay routing protocol etensions, extensions, but
potential to re-use BIER routing underlay routing protocol
extensions for non-BFER related extensions.

3.1 Added explanation for VRF and its use in adjacencies.

08: Incorporated (with hopefully acceptable fixes) for Lou
suggested section 2.5, TE considerations.

Fixes are primarily to the point to a) emphasize that BIER-TE does
not depend on the routing underlay unless forward_routed forward_routed()
adjacencies are used, and b) that the allocation and tracking of
resources does not explicitly have to be tied to BPs, because they
are just steering labels. Instead, it would ideally come from
per-hop resource management that can be maintained only via local
accounting in the controller.

07: Further reworking text for Lou.

Renamed BIER-PE to BIER-TE standing for "Tree Engineering" after
votes from BIER WG.

Removed section 1.1 (introduced by version 06) because not
considered necessary in this doc by Lou (for framework doc).

Added [RFC editor pls. remove] Section to explain name change to
future reviewers.

06: Concern by Lou Berger re. BIER-TE as full traffic engineering
solution.

Changed title "Traffic Engineering" to "Path Engineering"

Added intro section of relationship BIER-PE to traffic
engineering.

Changed "traffic engineering" term in text" to "path engineering",
where appropriate

Other:

Shortened "BIER-TE Controller Host" to "BIER-TE Controller".
Fixed up all instances of controller to do this.

05: Review Jeffrey Zhang.

Part 2:

4.3 added note about leaf-BFER being also a propery of routing
setup.

4.7 Added missing details from example to avoid confusion with
routed adjacencies, also compressed explanatory text and better
justification why seed is explicitly configured by controller.

4.9 added section discussing generic reuse of BP methods.

4.10 added section summarizing BP optimizations of section 4.

6. Rewrote/compressed explanation of comparison BIER/BIER-TE
forwarding difference. Explained benefit of BIER-TE per-BP
forwarding being independent of forwarding for other BPs.

Part 1:

Explicitly ue forwarded_connected adjcency in ECMP adjcency
examples to avoid confusion.

4.3 Add picture as example for leav vs. non-leaf BFR in topology.
Improved description.

4.5 Exampe for traffic that can be broadcast -> for single BP in
hub&spoke.

4.8.1 Simplified example picture for routed adjacency, explanatory
text.

Review from Dirk Trossen:

Fixed up explanation of ICC paper vs. bloom filter.

04: spell check run.

Addded remaining fixes for Sandys (Zhang Zheng) review:

4.7 Enhance ECMP explanations:

example ECMP algorithm, highlight that doc does not standardize
ECMP algorithm.

Review from Dirk Trossen:

1. Added mentioning of prior work for traffic engineered paths
with bloom filters.

2. Changed title from layers to components and added "BIER-TE
control plane" to "BIER-TE Controller" to make it clearer, what it
does.

2.2.3. Added reference to I-D.ietf-bier-multicast-http-response
as an example solution.

2.3. clarified sentence about resetting BPs before sending copies
(also forgot to mention DNR here).

3.4. Added text saying this section will be removed unless IESG
review finds enough redeeming value in this example given how -03
introduced section 1.1 with basic examples.

7.2. Removed explicit numbers 20%/80% for number of topology bits
in BIER-TE, replaced with more vague (high/low) description,
because we do not have good reference material Added text saying
this section will be removed unless IESG review finds enough
redeeming value in this example given how -03 introduced section
1.1 with basic examples.

many typos fixed. Thanks a lot.

03: Last call textual changes by authors to improve readability:

removed Wolfgang Braun as co-authors (as requested).

Improved abstract to be more explanatory. Removed mentioning of
FRR (not concluded on so far).

Added new text into Introduction section because the text was too
difficult to jump into (too many forward pointers). This
primarily consists of examples and the early introduction of the
BIER-TE Topology concept enabled by these examples.

Amended comparison to SR.

Changed syntax from [VRF] to {VRF} to indicate its optional and to
make idnits happy.

Split references into normative / informative, added references.

02: Refresh after IETF104 discussion: changed intended status back
to standard. Reasoning:

Tighter review of standards document == ensures arch will be
better prepared for possible adoption by other WGs (e.g. DetNet)
or std. bodies.

Requirement against the degree of existing implementations is self
defined by the WG. BIER WG seems to think it is not necessary to
apply multiple interoperating implementations against an
architecture level document at this time to make it qualify to go
to standards track. Also, the levels of support introduced in -01
rev. should allow all BIER forwarding engines to also be able to
support the base level BIER-TE forwarding.

01: Added note comparing BIER and SR to also hopefully clarify
BIER-TE vs. BIER comparison re. SR.

- added requirements section mandating only most basic BIER-TE
forwarding features as MUST.

- reworked comparison with BIER forwarding section to only
summarize and point to pseudocode section.

- reworked pseudocode section to have one pseudocode that mirrors
the BIER forwarding pseudocode to make comparison easier and a
second pseudocode that shows the complete set of BIER-TE
forwarding options and simplification/optimization possible vs.
BIER forwarding. Removed MyBitsOfInterest (was pure
optimization).

- Added captions to pictures.

- Part of review feedback from Sandy (Zhang Zheng) integrated.

00: Changed target state to experimental (WG conclusion), updated
references, mod auth association.

- Source now on http://www.github.com/toerless/bier-te-arch

- Please open issues on the github for change/improvement requests
to the document - in addition to posting them on the list
(bier@ietf.). Thanks!.

draft-eckert-bier-te-arch:

06: Added overview of forwarding differences between BIER, BIER-
TE.

05: Author affiliation change only.

04: Added comparison to Live-Live and BFIR to FRR section
(Eckert).

04: Removed FRR content into the new FRR draft [I-D.eckert-bier-
te-frr] (Braun).

- Linked FRR information to new draft in Overview/Introduction

- Removed BTAFT/FRR from "Changes in the network topology"

- Linked new draft in "Link/Node Failures and Recovery"

- Removed FRR from "The BIER-TE Forwarding Layer"

- Moved FRR section to new draft

- Moved FRR parts of Pseudocode into new draft

- Left only non FRR parts
- removed FrrUpDown(..) and //FRR operations in
ForwardBierTePacket(..)

- New draft contains FrrUpDown(..) and ForwardBierTePacket(Packet)
from bier-arch-03

- Moved "BIER-TE and existing FRR to new draft

- Moved "BIER-TE and Segment Routing" section one level up

- Thus, removed "Further considerations" that only contained this
section

- Added Changes for version 04

03: Updated the FRR section. Added examples for FRR key concepts.
Added BIER-in-BIER tunneling as option for tunnels in backup
paths. BIFT structure is expanded and contains an additional
match field to support full node protection with BIER-TE FRR.

03: Updated FRR section. Explanation how BIER-in-BIER
encapsulation provides P2MP protection for node failures even
though the routing underlay does not provide P2MP.

02: Changed the definition of BIFT to be more inline with BIER.
In revs. up to -01, the idea was that a BIFT has only entries for
a single bitstring, BitString, and every SI and subdomain sub-domain would be a
separate BIFT. In BIER, each BIFT covers all SI. This is now
also how we define it in BIER-TE.

02: Added Section 7 5.3 to explain the use of SI, subdomains sub-domains and BFR-
id
BFR-id in BIER-TE and to give an example how to efficiently assign
bits for a large topology requiring multiple SI.

02: Added further detailed for rings - how to support input from
all ring nodes.

01: Fixed BFIR -> BFER for section 4.3.

01: Added explanation of SI, difference to BIER ECMP,
consideration for Segment Routing, unicast FRR, considerations for
encapsulation, explanations of BIER-TE Controller and CLI.

00: Initial version.

13.

11. References

13.1.

11.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.

[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.

13.2.

11.2. Informative References

[Bloom70] Bloom, B., "Space/time trade-offs in hash coding with
allowable errors", Comm. ACM 13(7):422-6, July 1970.

[I-D.eckert-bier-te-frr]
Eckert, T., Cauchie, G., Braun, W., and M. Menth,
"Protection Methods for BIER-TE", draft-eckert-bier-te-
frr-03 (work in progress), March 2018.

[I-D.ietf-bier-multicast-http-response]
Trossen, D., Rahman, A., Wang, C., and T. Eckert,
"Applicability of BIER Multicast Overlay for Adaptive
Streaming Services", draft-ietf-bier-multicast-http-
response-04
response-05 (work in progress), July January 2021.

[I-D.ietf-bier-non-mpls-bift-encoding]
Wijnands, I., Mishra, M., Xu, X., and H. Bidgoli, "An
Optional Encoding of the BIFT-id Field in the non-MPLS
BIER Encapsulation", draft-ietf-bier-non-mpls-bift-
encoding-03 (work in progress), November 2020.

[I-D.ietf-bier-te-yang]
Zhang, Z., Wang, C., Chen, R., Hu, F., Sivakumar, M., and
H. Chen, "A YANG data model for Traffic Engineering for
Bit Index Explicit Replication (BIER-TE)", draft-ietf-
bier-te-yang-02 (work in progress), August 2020.

[I-D.ietf-roll-ccast]
Bergmann, O., Bormann, C., Gerdes, S., and H. Chen,
"Constrained-Cast: Source-Routed Multicast for RPL",
draft-ietf-roll-ccast-01 (work in progress), October 2017.

[I-D.ietf-teas-rfc3272bis]
Farrel, A., "Overview and Principles of Internet Traffic
Engineering", draft-ietf-teas-rfc3272bis-01 draft-ietf-teas-rfc3272bis-11 (work in
progress), July 2020. April 2021.

[ICC] Reed, M., Al-Naday, M., Thomos, N., Trossen, D.,
Petropoulos, G., and S. Spirou, "Stateless multicast
switching in software defined networks", IEEE
International Conference on Communications (ICC), Kuala
Lumpur, Malaysia, 2016, May 2016,
<https://ieeexplore.ieee.org/document/7511036>.

[RCSD94] Zhang, H. and D. Domenico, "Rate-Controlled Service
Disciplines", Journal of High-Speed Networks, 1994, May
1994, <https://dl.acm.org/doi/10.5555/2692227.2692232>.

[RFC2119] Bradner,

[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.

[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.

[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.

[RFC7589] Badra, M., Luchuk, A., and J. Schoenwaelder, "Using the
NETCONF Protocol over Transport Layer Security (TLS) with
Mutual X.509 Authentication", RFC 7589,
DOI 10.17487/RFC7589, June 2015,
<https://www.rfc-editor.org/info/rfc7589>.

[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., "Key words Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.

[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for use the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/info/rfc8253>.

[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in RFCs to Indicate
Requirement Levels", BCP 14, MPLS and Non-
MPLS Networks", RFC 2119, 8296, DOI 10.17487/RFC2119, 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.

[RFC8345] Clemm, A., Medved, J., Varga, R., Bahadur, N.,
Ananthakrishnan, H., and X. Liu, "A YANG Data Model for
Network Topologies", RFC 8345, DOI 10.17487/RFC8345, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
2018, <https://www.rfc-editor.org/info/rfc8345>.

[RFC8401] Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
Zhang, "Bit Index Explicit Replication (BIER) Support via
IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,
<https://www.rfc-editor.org/info/rfc8401>.

[RFC8444] Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
Extensions for Bit Index Explicit Replication (BIER)",
RFC 8444, DOI 10.17487/RFC8444, November 2018,
<https://www.rfc-editor.org/info/rfc8444>.

Authors' Addresses

Toerless Eckert (editor)
Futurewei Technologies Inc.
2330 Central Expy
Santa Clara 95050
USA

Email: tte+ietf@cs.fau.de
Gregory Cauchie
Bouygues Telecom

Email: GCAUCHIE@bouyguestelecom.fr

Michael Menth
University of Tuebingen

Email: menth@uni-tuebingen.de