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draft-ietf-bier-te-arch-11.txt
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draft-ietf-bier-te-arch-11.txt
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Network Working Group T. Eckert, Ed.
Internet-Draft Futurewei
Intended status: Standards Track G. Cauchie
Expires: May 19, 2022 Bouygues Telecom
M. Menth
University of Tuebingen
November 15, 2021
Tree Engineering for Bit Index Explicit Replication (BIER-TE)
draft-ietf-bier-te-arch-11
Abstract
This memo describes per-packet stateless strict and loose path
steered replication and forwarding for Bit Index Explicit Replication
packets (RFC8279). It is called BIER Tree Engineering (BIER-TE) and
is intended to be used as the path steering mechanism for Traffic
Engineering with BIER.
BIER-TE introduces a new semantic for bit positions (BP) that
indicate adjacencies, as opposed to (non-TE) BIER in which BPs
indicate Bit-Forwarding Egress Routers (BFER). BIER-TE can leverage
BIER forwarding engines with little changes. Co-existence of BIER
and BIER-TE forwarding in the same domain is possible, for example by
using separate BIER sub-domains (SDs). 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Eckert, et al. Expires May 19, 2022 [Page 1]
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This Internet-Draft will expire on May 19, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . 5
2.2. BIER-TE Topology and adjacencies . . . . . . . . . . . . 8
2.3. Relationship to BIER . . . . . . . . . . . . . . . . . . 9
2.4. Accelerated/Hardware forwarding comparison . . . . . . . 11
3. Components . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 12
3.2. The BIER-TE Control Plane . . . . . . . . . . . . . . . . 12
3.2.1. The BIER-TE Controller . . . . . . . . . . . . . . . 13
3.2.1.1. BIER-TE Topology discovery and creation . . . . . 14
3.2.1.2. Engineered Trees via BitStrings . . . . . . . . . 14
3.2.1.3. Changes in the network topology . . . . . . . . . 15
3.2.1.4. Link/Node Failures and Recovery . . . . . . . . . 15
3.3. The BIER-TE Forwarding Plane . . . . . . . . . . . . . . 15
3.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 16
3.5. Traffic Engineering Considerations . . . . . . . . . . . 16
4. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 17
4.1. The Bit Index Forwarding Table (BIFT) . . . . . . . . . . 17
4.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 18
4.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 19
4.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 19
4.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2.4. Local Decap(sulation) . . . . . . . . . . . . . . . . 20
4.3. Encapsulation / Co-existence with BIER . . . . . . . . . 20
4.4. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . 21
4.5. Basic BIER-TE Forwarding Example . . . . . . . . . . . . 24
4.6. BFR Requirements for BIER-TE forwarding . . . . . . . . . 27
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5. BIER-TE Controller Operational Considerations . . . . . . . . 27
5.1. Bit position Assignments . . . . . . . . . . . . . . . . 27
5.1.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . 28
5.1.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . 28
5.1.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . 29
5.1.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . 29
5.1.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . 31
5.1.8. Forward Routed adjacencies . . . . . . . . . . . . . 33
5.1.8.1. Reducing bit positions . . . . . . . . . . . . . 33
5.1.8.2. Supporting nodes without BIER-TE . . . . . . . . 34
5.1.9. Reuse of bit positions (without DNC) . . . . . . . . 34
5.1.10. Summary of BP optimizations . . . . . . . . . . . . . 36
5.2. Avoiding duplicates and loops . . . . . . . . . . . . . . 37
5.2.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . 37
5.2.2. Duplicates . . . . . . . . . . . . . . . . . . . . . 37
5.3. Managing SI, sub-domains and BFR-ids . . . . . . . . . . 38
5.3.1. Why SI and sub-domains . . . . . . . . . . . . . . . 38
5.3.2. Assigning bits for the BIER-TE topology . . . . . . . 39
5.3.3. Assigning BFR-id with BIER-TE . . . . . . . . . . . . 40
5.3.4. Mapping from BFR to BitStrings with BIER-TE . . . . . 41
5.3.5. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . 42
5.3.6. Example bit allocations . . . . . . . . . . . . . . . 42
5.3.6.1. With BIER . . . . . . . . . . . . . . . . . . . . 42
5.3.6.2. With BIER-TE . . . . . . . . . . . . . . . . . . 43
5.3.7. Summary . . . . . . . . . . . . . . . . . . . . . . . 44
6. BIER-TE and Segment Routing . . . . . . . . . . . . . . . . . 45
7. Security Considerations . . . . . . . . . . . . . . . . . . . 46
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 47
10. Change log [RFC Editor: Please remove] . . . . . . . . . . . 47
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 57
11.1. Normative References . . . . . . . . . . . . . . . . . . 57
11.2. Informative References . . . . . . . . . . . . . . . . . 57
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 60
1. Overview
BIER-TE is based on architecture, terminology and packet formats with
(non-TE) BIER as described in [RFC8279] and [RFC8296]. This document
describes BIER-TE in the expectation that the reader is familiar with
these two documents.
BIER-TE introduces a new semantic for bit positions (BP) that
indicate 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
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Topology. Other BPs are empty in the BIFT. The BFR replicate and
forwards BIER packets to adjacent BPs that are set in the packet.
BPs are normally also 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
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fewer addressing bits than explicit BitStrings, but they introduce
the heuristic risk of false positives and cannot clear bits in the
BitString during forwarding to avoid loops. For these reasons, BIER-
TE uses explicit BitStrings like BIER. The explicit 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.
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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() to BFR2
BFR2: p1 -> forward_connected() to BFR1
p5 -> forward_connected() to BFR3
p8 -> forward_connected() to BFR4
BFR3: p3 -> forward_connected() to BFR2
p7 -> forward_connected() to BFR5
p13 -> local_decap
BFR4: p4 -> forward_connected() to BFR2
p10 -> forward_connected() to BFR5
p14 -> local_decap
BFR5: p6 -> forward_connected() to BFR3
p9 -> forward_connected() to BFR4
p12 -> forward_connected() to BFR6
BFR6: p11 -> forward_connected() to BFR5
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 bit positions (BP) used. All
BFRs can act as an ingress BFR (BFIR), BFR1, BFR3, BFR4 and BFR6 can
also be egress BFRs (BFERs). Forward_connected() is the name for
adjacencies that are representing subnet adjacencies of the network.
Local_decap() is the name of the adjacency to decapsulate BIER-TE
packets and pass their payload to higher layer processing.
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Assume a packet from BFR1 should be sent via BFR4 to BFR6. This
requires a BitString (p2,p8,p10,p12,p15). When this packet is
examined by BIER-TE on BFR1, the only bit position from the 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. p15 finally makes BFR6 receive and decapsulate the
packet.
To send in addition to BFR6 via BFR4 also a copy to BFR3, the
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 was (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. To emphasize non-L2, but routed/tunneled
forwarding of BIER-TE packets, these adjacencies are called
"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.
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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() to BFR3
p2 -> forward_routed() to BFR4
BFR3: p3 -> local_decap
p5 -> forward_routed() to BFR6
BFR4: p4 -> local_decap
p6 -> forward_routed() to BFR6
BFR6: p5 -> local_decap
p6 -> local_decap
p7 -> forward_routed() to BFR3
p8 -> 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 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).
2.2. BIER-TE Topology and adjacencies
The key new component in BIER-TE compared to (non-TE) BIER is the
BIER-TE topology as introduced through the two examples in
Section 2.1. It is used to control where replication can or should
happen and how to minimize the required number of BP for adjacencies.
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The BIER-TE Topology consists of the 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 as shown in the first example, this
may be called a "native" BIER-TE topology. A topology consisting
only of "forward_routed" adjacencies as shown in the second example
may be called an "overlay" BIER-TE topology. A BIER-TE topology with
both "forward_connected" and "forward_routed" adjacencies may be
called a "hybrid" BIER-TE topology.
2.3. Relationship to BIER
BIER-TE is designed so that is forwarding plane is a simple extension
to the (non-TE) BIER forwarding plane, hence allowing for it to be
added to BIER deployments where it can be beneficial.
BIER-TE is also intended as an option to expand the BIER architecture
into deployments where (non-TE) BIER may not be the best fit, such as
statically provisioned networks with needs for path 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 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 packet header
indicate a BFER and bits in the BIFT indicate the BIER
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control plane calculated next-hop toward that BFER. In BIER-
TE, bits in the BitString of a BIER packet header indicate an
adjacency in the BIER-TE topology, and only the BFRs that are
the upstream of this adjacency have this bit populated with
the adjacency in their BIFT.
2. In BIER, the implied reference option for the core part of
the BIER layer control plane are the BIER extensions for
distributed routing protocols, such as those standardized in
ISIS/OSPF extensions for BIER, [RFC8401] and [RFC8444]. The
reference option for the core part of the BIER-TE control
plane is the BIER-TE controller. Nevertheless, both BIER and
BIER-TE BIFT forwarding plane state could equally be
populated by any mechanism.
3. Assuming the reference options for the control plane, BIER-TE
replaces in-network autonomous path calculation by explicit
paths calculated by the BIER-TE controller.
3. The following elements/functions described in the BIER
architecture are not required by the BIER-TE architecture:
1. BIRTs are not required on BFRs for BIER-TE when using a BIER-
TE controller because the controller can directly populate
the BIFTs. In BIER, BIRTs are populated by the distributed
routing protocol support for BIER, allowing BFRs to populate
their BIFTs locally from their BIRTs. Other BIER-TE control
plane or management plane options may introduce requirements
for BIRTs for BIER-TE BFRs.
2. The BIER-TE layer forwarding plane does not require BFRs to
have a unique BP and therefore also no unique BFR-id. See
for example See Section 5.1.3.
3. Identification of BFRs by the BIER-TE control plane is
outside the scope of this specification. Whereas the BIER
control plane uses BFR-ids in its BFR to BFR signaling, a
BIER-TE controller may choose any form of identification
deemed appropriate.
4. BIER-TE forwarding does not use the BFR-id field of the BIER
packet header.
4. Co-existence of BIER and 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 encapsulation
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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 nights forwarding).
2. BIER-TE deployments will have to assign BFR-ids to BFRs and
insert them into the 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 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 (non-TE) BIER/BIFT forwarding can be modified to support
the 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's neighbor is skipped in BIER-TE
forwarding because it is not necessary and could not be done when
using BIER F-BM.
Whether to 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".
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<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
BIER-TE [BIER-TE Controller] <=> [BIER-TE Topology]
control ^ ^ ^
plane / | \ BIER-TE control protocol
| | | e.g. YANG/Netconf/RestConf
| | | PCEP/...
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|<----------------->|
BIER-TE forwarding plane
|<- BIER-TE domain->|
|<--------------------->|
Routing underlay
Figure 3: BIER-TE architecture
3.1. The Multicast Flow Overlay
The Multicast Flow Overlay has the same role as described for BIER in
[RFC8279], Section 4.3. See also Section 3.2.1.2.
3.2. The BIER-TE Control Plane
In the (non-TE) BIER architecture [RFC8279], the BIER control plane
is not explicitly separated from the BIER forwarding plane, but
instead their functions are summarized together in Section 4.2.
Example standardized options for the BIER control plane include ISIS/
OSPF extensions for BIER, [RFC8401] and [RFC8444].
For BIER-TE, the control plane includes at minimum the following
functionality.
1. During initial provisioning of the network and/or during
modifications of its topology and/or services: protocols and/or
procedures to establish BIER-TE BIFTs:
1. Determine the desired BIER-TE topology for a BIER-TE sub-
domains: the native and/or overlay adjacencies that are
assigned to BPs.
2. Determine the per-BFR BIFT from the BIER-TE topology.
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3. Optionally assign BFR-ids to BFIRs for later insertion into
BIER-TE headers on BFIRs. 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-ids
into BFIRs.
2. During operations of the network: Protocols and/or procedures to
support creation/change/removal of overlay flows on BFIRs:
1. Process the BIER-TE requirements for the multicast overlay
flow: BFIR and BFERs of the flow as well as policies for the
path selection of the flow.
2. Determine the BitStrings and optionally Entropy.
3. Install state on the BFIR to impose the 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
Notwithstanding other options, this architecture describes the BIER
control plane as shown in Figure 3 to consists of:
o A single centralized BIER-TE controller.
o Data-models and protocols to communicate between controller and
BFRs in step 1, such as YANG/Netconf/RestConf.
o Protocols to communicate between controller and BFIR in step 2,
such as BIER-TE extensions for [RFC5440].
The (non-TE) BIER control plane could equally be implemented without
any active dynamic components by an operator via CLI on the BFRs. In
that case, operator configured local policy on the BFIR would have to
determine how to set the appropriate BIER header fields. The BIER-TE
control plane could also be decentralized and/or distributed, but
this document does not consider any additional protocols and/or
procedures which would then be necessary to coordinate its entities
to achieve the above described functionality.
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3.2.1.1. BIER-TE Topology discovery and creation
Step 1.1 includes network topology discovery and BIER-TE 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 discovery may rely on protocols including
extending a Link-State-Protocol (LSP) based IGP into the BIER-TE
controller itself, [RFC7752] (BGP-LS) or [RFC8345] (Yang topology) as
well as BIER-TE 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 as easy as mapping
the network topology 1:1 to the BIER-TE topology by 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 BIER-TE topology is determined, the BIER-TE Controller then
pushes the BitPositions/adjacencies to the BIFT of the BFRs. On each
BFR only those SI:BitPositions are populated that are adjacencies to
other BFRs in the BIER-TE topology.
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. Likewise, the BitString from
the same BFIR to the same set of BFER can be 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.
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See also [I-D.ietf-bier-multicast-http-response] for an application
leveraging BIER-TE engineered trees.
3.2.1.3. Changes in the network topology
If the network topology changes (not failure based) so that
adjacencies that are assigned to bit positions are no longer needed,
the BIER-TE Controller can re-use those bit positions for new
adjacencies. First, these 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.
3.2.1.4. Link/Node Failures and Recovery
When link or nodes fail or recover in the topology, BIER-TE could
quickly respond with FRR procedures such as [I-D.eckert-bier-te-frr],
the details of which are out of scope for this document. 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.
3.3. The BIER-TE Forwarding 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 Plane receives a packet, it simply looks
up the 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
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copy, the BFR clears all BPs in the BitString of the packet for which
the BFR has one or more adjacencies in the BIFT, except when the
adjacency indicates "DoNotClear" (DNC, see Section 4.2.1). This is
done to inhibit that packets can loop. Because DNC raises the risk
of packets looping with inmakes it easier to
3.4. The Routing Underlay
For 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()
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 4.2.2 for the adjacency definition.
BIER relies on the routing underlay to calculate paths towards 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
through the BIER-TE Controller established adjacencies on the BFR for
the BPs of the BitString. There is thus no need for BFER specific
routing underlay extensions to forward BIER packets with BIER-TE
semantics.
Encapsulation parameters can be provisioned by the BIER-TE controller
into the forward_connected() or 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.
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 and BFR BIFT state. The mapping of user/IP traffic to
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specific BitStrings/BIER-TE flows is made based on policy. The
specific details of BIER-TE policies and how a controller uses 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. For example, when composing BIER-TE
BitStrings, a Controller 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 Controller 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() adjacencies are used (see Section 4.2.1 for the
definition of forward_routed() adjacencies). When 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
traffic and potentially 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
4.1. The Bit Index Forwarding Table (BIFT)
The Bit Index Forwarding Table (BIFT) exists in every BFR. For every
sub-domain in use, it is a table indexed by 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 sub-domains, each with a
separate BIFT.
In [RFC8279], Figure 2, indices into the BIFT are both SI:BitString
and BFR-id, where BitString is indicating a BP: BFR-id = SI * 2^BSL +
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BP. As shown in Figure 4, in BIER-TE, only SI:BP are used as indices
into a BIFT because they identify adjacencies and not BFR.
------------------------------------------------------------------
| Index: | Adjacencies: |
| SI:bit position | <empty> or one or more per entry |
==================================================================
| 0:1 | forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------
| 0:2 | forward_connected(interface,neighbor{,DNC}) |
| | 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.
Note that a BIFT index (SI:BP) may be populated in the BIFT of more
than one BFR. See Section 5.1.6 for an example of how a BIER-TE
controller could assign BPs to (logical) adjacencies shared across
multiple BFRs, Section 5.1.3 for an example of assigning the same BP
to different adjacencies, and Section 5.1.9 for 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.
4.2. Adjacency Types