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draft-ietf-bier-te-arch.xml
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draft-ietf-bier-te-arch.xml
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]>
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<rfc ipr="trust200902" docName="draft-ietf-bier-te-arch-12" category="std">
<front>
<title abbrev="BIER-TE ARCH">Tree Engineering for Bit Index Explicit Replication (BIER-TE)</title>
<author role="editor" fullname="Toerless Eckert" initials="T.T.E." surname="Eckert">
<organization abbrev="Futurewei">Futurewei Technologies Inc.</organization>
<address>
<postal>
<street>2330 Central Expy</street>
<city>Santa Clara</city>
<code>95050</code>
<country>USA</country>
</postal>
<email>tte+ietf@cs.fau.de</email>
</address>
</author>
<author fullname="Michael Menth" initials="M.M." surname="Menth">
<organization>University of Tuebingen</organization>
<address>
<email>menth@uni-tuebingen.de</email>
</address>
</author>
<author fullname="Gregory Cauchie" initials="G.C." surname="Cauchie">
<organization>Bouygues Telecom</organization>
<address>
<email>GCAUCHIE@bouyguestelecom.fr</email>
</address>
</author>
<date month="January" year="2022"/>
<abstract>
<t> This memo describes per-packet stateless strict and loose path
steered replication and forwarding for "Bit Index Explicit Replication" (BIER, RFC8279) packets. It is called BIER Tree Engineering (BIER-TE) and is intended to be used as the path steering mechanism for Traffic Engineering
with BIER.</t>
<t>BIER-TE introduces a new semantic for "bit positions" (BP). They indicate adjacencies
of the network topology, as opposed to (non-TE) BIER in which BPs indicate
"Bit-Forwarding Egress Routers" (BFER). A BIER-TE packets BitString therefore indicates the
edges of the (loop-free) tree that the packet is forwarded across by BIER-TE.
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).</t>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="overview" title="Overview">
<t> BIER-TE is based on the (non-TE) BIER architecture, terminology and packet formats as described
in <xref target="RFC8279"/> and <xref target="RFC8296"/>.
This document describes BIER-TE in the expectation that the reader is familiar
with these two documents.</t>
<t>BIER-TE introduces a new semantic for "bit positions" (BP). They indicate adjacencies
of the network topology, as opposed to (non-TE) BIER in which BPs indicate
"Bit-Forwarding Egress Routers" (BFER). A BIER-TE packets BitString therefore indicates the
edges of the (loop-free) tree that the packet is forwarded across by BIER-TE.
With BIER-TE, the "Bit Index Forwarding Table" (BIFT) of each "Bit Forwarding Router" (BFR)
is only populated with BP that are adjacent to the BFR
in the BIER-TE 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.
</t>
<t>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).</t>
<t>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 (<xref target="RFC8402"/>).</t>
<t>This document is structured as follows:
<list style="symbols">
<t><xref target="introduction"/> introduces BIER-TE with two
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.</t>
<t><xref target="components"/> 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.</t>
<t><xref target="forwarding"/> 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.</t>
<t><xref target="controller-ops"/> 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 "Set Identifier" (SI), "sub-domain" (SD) and BFR-ids can be managed by a BIER-TE controller, examples and summary.</t>
<t><xref target="SR"/> concludes the technology specific sections of the document by further relating BIER-TE to SR.</t>
</list></t>
<t>Note that related work, <xref target="I-D.ietf-roll-ccast"/>
uses Bloom filters <xref target="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,
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 <xref target="ICC"/>
describes it.</t>
<!-- Removed for now by review with Lou Berger
<section anchor="te" title="BIER-TE and Traffic Engineering (BIER-TE)">
<t>BIER-TE is not a standalone, complete traffic engineering signaling solution such as RSVP with RSVP-TE
extensions (<xref target="RFC2205"/>, <xref target="RFC3209"/>). Instead it is a (non-TE) BIER derived architecture
and forwarding plane that allows to signal "source-routed" paths and replication points without
per-path, per-replication-point state on the transit nodes. This document introduces the name
"Tree Engineering" for BitStrings using this semantic. BIER-TE is therefore more similar to Segment Routing
(SR, (<xref target="RFC8402"/>)) than RSVP-TE. Note that SR does not provide stateless replication point
and receiver set signaling in its packet header. See <xref target="SR"/> for a more detailed discussion of
BIER-TE and SR.</t>
<t>BIER-TE can be used alone in use cases not requiring bandwidth or buffer resource reservations,
such as high resilient services through dual transmission with path diversity or optimization
of network capacity utilization through calculated paths/trees ("load balancing across non-ECMP paths").
Due to its stateless BIER approach, BIER-TE does not create per-flow/per-tree state on intermedia nodes.</t>
<t>BIER-TE can also be combined with bandwidth and buffer management functions to support
traffic engineering such as per-flow guaranteed bandwidth and guaranteed latency across BIER-TE
steered paths / trees. Combinations of BIER or BIER-TE with such per-tree/per-flow resource
guarantees are called BIER-TE. The following paragraphs summarize options and considerations.</t>
<t>In <xref target="components"/> below, the BIER-TE architecture specifies the BIER-TE Controller
as an entity calculating both the BIER-TE topology and desired paths/trees for overlay flows
based on the desired policies. A Path Computation Engine (PCE, see <xref target="RFC4655"/>)
that can calculate the BitString for BIER-TE is an instance of such a BIER-TE Controller.
If the PCE can also perform resource management such as per-flow bandwidth reservations and
optional latency guarantees, then it becomes a PCE for BIER-TE with traffic engineering.</t>
<t>To support bandwidth guarantees in the forwarding plane, the ingres BIER-TE node
(BFIR) may need to have a per-flow policer if ingressed traffic is not trusted to stay within
its admitted traffic envelope. This is a well understood policy function that can be deployed
without changes to BIER-TE.</t>
<t>If latency guarantees as required as for example by Guaranteed Services (<xref target="RFC2212"/>),
then additional per-hop latency control in the forwarding plane can be required. This can also
be added to BIER-TE deployments without changes to BIER-TE. Per-hop stateless solutions for this
such as in <xref target="I-D.qiang-detnet-large-scale-detnet"/> would allow to maintain
the per-hop stateless design goal of BIER-TE and expand it into BIER-TE. Per-hop stateful solutions like
per-flow, per-hop shaping may also be beneficial given how BIER-TE eliminates the need for
per-flow, per-hop multicast replication and steering state.</t>
<t>Mechanisms how to combine BIER-TE or BIER with other mechanisms to build BIER-TE are outside
the scope of this document. See <xref target="I-D.eckert-teas-bier-te-framework"/>.</t>
</section>
-->
<section anchor="boilerplate" title="Requirements Language">
<t>
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 <xref target="RFC2119"/>
<xref target="RFC8174"/> when, and only when, they appear in all capitals, as shown here.
</t>
</section>
</section>
<section anchor="introduction" title="Introduction">
<section anchor="examples" title="Basic Examples">
<t>BIER-TE forwarding is best introduced with simple examples. These examples
use formal terms defined later in the document (<xref target="adjacencies"/>),
including forward_connected(), forward_routed() and local_decap().
</t>
<figure anchor="basic-example" title="BIER-TE basic example">
<artwork align="left"><![CDATA[
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()
]]></artwork></figure>
<t>
Consider the simple network in the above BIER-TE overview example picture
with 6 BFRs. p1...p15 are the bit positions used. All BFRs can act as
an ingress BFR (BFIR), BFR1, BFR3, BFR4 and
BFR6 can also be 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.
</t>
<t>
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.
</t>
<t>To send a copy to BFR6 via BFR4 and also a copy to BFR3, the BitString needs
to be (p2,p5,p8,p10,p12,p13,p15). 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.
</t>
<t>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.
</t>
<t>BIER-TE has various options to minimize BP assignments,
many of which are based on out-of-band knowledge about the required multicast traffic
paths and bandwidth consumption in the network, such as from pre-deployment planning.</t>
<t><xref target="basic-overlay"/> 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.</t>
<t>In addition, bits are saved in the following example by assuming that BFR1 only
needs to be BFIR but not BFER or transit BFR.</t>
<figure anchor="basic-overlay" title="BIER-TE basic overlay example">
<artwork align="left"><![CDATA[
BIER-TE Topology:
Diagram:
p1 p3 p7
....> BFR3 <.... p5
........ ........>
BFR1 (Rtr2) (Rtr5) BFR6
........ ........> p9
....> 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: p7 -> forward_routed() to BFR3
p8 -> forward_routed() to BFR4
p9 -> local_decap()
]]></artwork></figure>
<t>To send a BIER-TE packet from BFR1 via BFR3 to be received by BFR6,
the BitString is (p1,p5,p9). From BFR1 via BFR4 to be received by BFR6,
the BitString is (p2,p6,p9). A packet from BFR1 to be received by BFR3,BFR4
and from BFR3 to be received by BFR6 uses (p1,p2,p3,p4,p5,p9). A packet
from BFR1 to be received by BFR3,BFR4 and from BFR4 to be received by BFR6
uses (p1,p2,p3,p4,p6,p9). A packet from BFR1 to be received by BFR4,
and from BFR4 to be received by BFR6 and from there to be received by BFR3 uses (p2,p3,p4,p6,p7,p9).
A packet from BFR1 to be received by BFR3, and from BFR3 to be received by BFR6
there to be received by BFR4 uses (p1,p3,p4,p5,p8,p9).</t>
</section>
<section anchor="topology" title="BIER-TE Topology and adjacencies">
<t>The key new component in BIER-TE compared to (non-TE) BIER is the BIER-TE topology
as introduced through the two examples in <xref target="examples"/>.
It is used to control where replication can or should happen and how to
minimize the required number of BP for adjacencies.
</t>
<t>
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 BFRs 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 in <xref target="avoiding"/>.
</t>
<t>If the BIER-TE topology represents (a subset of) the underlying (layer 2)
topology of the network as shown in the first example, this may be called a "native"
BIER-TE topology. A topology consisting only of "forward_routed" adjacencies as
shown in the second example may be called an "overlay" BIER-TE topology.
A BIER-TE topology with both forward_connected() and forward_routed() adjacencies
may be called a "hybrid" BIER-TE topology.</t>
</section>
<!-- topology -->
<section anchor="comparison" title="Relationship to BIER">
<t>BIER-TE is designed so that its 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.</t>
<t>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.</t>
<t><list style="numbers">
<t>BIER-TE inherits the following aspects from BIER unchanged:
<list style="numbers">
<t>The fundamental purpose of per-packet signaled replication and delivery via a BitString.</t>
<t>The overall architecture consisting of three layers, flow overlay, BIER(-TE) layer and routing underlay.</t>
<t>The supported encapsulations <xref target="RFC8296"/>.</t>
<t>The semantic of all <xref target="RFC8296"/> header elements used by the BIER-TE forwarding plane other than the semantic of the BP in the BitString.</t>
<t>The BIER forwarding plane, except for how bits have to be cleared during replication.</t>
</list></t>
<t>BIER-TE has the following key changes with respect to BIER:
<list style="numbers">
<t>In BIER, bits in the BitString of a BIER packet header indicate a BFER
and bits in the BIFT indicate the BIER control plane calculated next-hop
toward that BFER. In BIER-TE, a bit in the BitString of a BIER packet
header indicates an adjacency in the BIER-TE topology, and only the
BFR that is the upstream of that adjacency has its BP populated with
the adjacency in its BIFT.</t>
<t>In BIER, the implied reference option for the core part of the BIER layer
control plane are the BIER extensions for distributed routing protocols.
This includes ISIS/OSPF extensions for BIER, <xref target="RFC8401"/>
and <xref target="RFC8444"/>.</t>
<t>The reference option for the core part of the BIER-TE control plane is
the BIER-TE controller. Nevertheless, both the BIER and BIER-TE BIFTs forwarding
plane state could equally be populated by any mechanism.</t>
<t>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.</t>
</list></t>
<t>The following elements/functions described in the BIER architecture are not required by the BIER-TE architecture:
<list style="numbers">
<t>"Bit Index Routing Tables" (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.</t>
<t>The BIER-TE layer forwarding plane does not require BFRs to have a unique BP and therefore also no unique BFR-id. See <xref target="leaf-bfer"/>.</t>
<t>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.</t>
<t>BIER-TE forwarding does not require the BFIR-id field of the BIER packet header.</t>
</list></t>
<t>Co-existence of BIER and BIER-TE in the same network requires the following:
<list style="numbers">
<t>The BIER/BIER-TE packet header needs to allow addressing both BIER and BIER-TE BIFTs. Depending on the encapsulation option, the same SD may or may not be reusable across BIER and BIER-TE. See <xref target="encapsulation"/>.
In either case, a packet is always only forwarded end-to-end via BIER or via BIER-TE (ships in the nights forwarding).</t>
<t>BIER-TE deployments will have to assign BFR-ids to BFRs and insert them into the BFIR-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 <xref target="bfr-id"/>.</t>
</list></t>
</list></t>
</section>
<section anchor="fwd-comparison" title="Accelerated/Hardware forwarding comparison">
<t>BIER-TE forwarding rules, especially the BitString parsing are designed to be as close
as possible to those of BIER in the expectation that this eases the programming of BIER-TE forwarding
code and/or BIER-TE forwarding hardware on platforms supporting BIER.
The pseudocode in <xref target="pseudocode"/> shows how existing
(non-TE) BIER/BIFT forwarding can be modified to support the required BIER-TE forwarding
functionality (<xref target="requirements"/>), 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.</t>
<t>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 <xref target="encapsulation"/>.</t>
</section>
<!-- fwd-comparison -->
</section>
<!-- overview -->
<section anchor="components" title="Components">
<t>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".</t>
<figure anchor="architecture" title="BIER-TE architecture">
<artwork align="left"><![CDATA[
<------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
]]></artwork></figure>
<section anchor="flow-overlay" title="The Multicast Flow Overlay">
<t>The Multicast Flow Overlay has the same role as described for BIER
in <xref target="RFC8279"/>, Section 4.3. See also <xref target="engineered-bitstrings"/>.</t>
<t>When a BIER-TE controller is used, then the signaling for the Multicast Flow Overlay may
also be preferred to operate through a central point of control. For BGP based
overlay flow services such as "Multicast VPN Using BIER" (<xref target="RFC8556"/>) this
can be achieved by making the BIER-TE controller operate as a BGP Route
Reflector (<xref target="RFC4456"/>) and combining it with signaling through BGP
or a different protocol for the BIER-TE controller calculated BitStrings.
See <xref target="engineered-bitstrings"/> and <xref target="bitstring-mappings"/>.</t>
</section>
<!-- flow-overlay -->
<section anchor="control-plane" title="The BIER-TE Control Plane">
<t>In the (non-TE) BIER architecture <xref target="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, <xref target="RFC8401"/> and <xref target="RFC8444"/>.</t>
<t>For BIER-TE, the control plane includes at minimum the following functionality.</t>
<t><list style="hanging">
<t anchor="topology-control" hangText="1. BIER-TE topology control:">During initial provisioning of the network and/or during modifications of its topology and/or services, the protocols and/or procedures to establish BIER-TE BIFTs:
<list style="numbers">
<t anchor="topology-control-1">Determine the desired BIER-TE topology for a BIER-TE sub-domains: the native and/or overlay adjacencies that are assigned to BPs. Topology discovery is discussed in <xref target="topology-discovery"/> and the various aspects of the BIER-TE controllers determinations about the topology are discussed throughout <xref target="controller-ops"/></t>
<t>Determine the per-BFR BIFT from the BIER-TE topology. This is achieved by simply extracting the adjacencies of the BFR from the BIER-TE topology and populating the BFRs BIFT with them.</t>
<t>Optionally assign BFR-ids to BFIRs for later insertion into BIER headers on BFIRs as BFIR-id. Alternatively, BFIR-id in BIER packet headers may be managed solely by the flow overlay layer and/or be unused. This is discussed in <xref target="bfr-id"/>.</t>
<t>Install/update the BIFTs into the BFRs and optionally BFR-ids into BFIRs. This is discussed in <xref target="topology-discovery"/>.</t>
</list></t>
<t anchor="tree-control" hangText="2. BIER-TE tree control:">During operations of the network, protocols and/or procedures to support creation/change/removal of overlay flows on BFIRs:
<list style="numbers">
<t>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. This is discussed in <xref target="te-considerations"/>.</t>
<t>Determine the BitStrings and optionally Entropy. This is discussed in <xref target="engineered-bitstrings"/>, <xref target="te-considerations"/> and <xref target="bitstring-mappings"/>.</t>
<t>Install state on the BFIR to impose the desired BIER packet header(s) for packets of the overlay flow. Different aspects of this and the next point are discussed throughout <xref target="bier-te-controller"/> and in <xref target="encapsulation"/>, but the main responsibility of these two points is with the Multicast Flow Overlay (<xref target="flow-overlay"/>), which is architecturally inherited from BIER.</t>
<t>Install the necessary state on the BFERs to decapsulate the BIER packet header and properly dispatch its payload.</t>
</list></t>
</list></t>
<section anchor="bier-te-controller" title="The BIER-TE Controller">
<t>[RFC-Editor: the following text has three references to anchors topology-control, topology-control-1 and tree-control. Unfortunately, XMLv2 does not offer any tagging that reasonable references are generated (i had this problem already in RFCs last year. Please make sure there are useful-to-read cross-references in the RFC in these three places after you convert to XMLv3.]</t>
<t>This architecture describes the
BIER-TE control plane as shown in <xref target="architecture"/> to consist of:
<list style="symbols">
<t>A BIER-TE controller.</t>
<t>BFR data-models and protocols to communicate between controller and BFRs
in support of <xref target="topology-control">BIER-TE topology control</xref>,
such as YANG/NETCONF/RESTCONF (<xref target="RFC7950"/>/<xref target="RFC6241"/>/<xref target="RFC8040"/>).</t>
<t>BFR data-models and protocols to communicate between controller and BFIR in support of
<xref target="tree-control">BIER-TE tree control</xref>, such as BIER-TE extensions
for <xref target="RFC5440"/>.</t>
</list>
</t>
<t>The single, centralized BIER-TE controller is used in this document as reference option for the BIER-TE control plane but other options are equally feasible.
The BIER-TE control plane could equally be implemented without automated configuration/protocols,
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 (distributed/decentralized) entities to achieve the above described functionality.</t>
<section anchor="topology-discovery" title="BIER-TE Topology discovery and creation">
<t><xref target="topology-control-1">The first item of BIER-TE topology control</xref>
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 BFRs and the
adjacencies between them.</t>
<t>In statically managed networks, such as in industrial environments, both discovery and creation can be a manual/offline process.</t>
<t>In other networks, topology discovery may rely on protocols including extending a "Link-State-Protocol" based IGP into the BIER-TE controller itself, <xref target="RFC7752"/> (BGP-LS) or <xref target="RFC8345"/> (YANG topology) as well as BIER-TE specific methods, for example via <xref target="I-D.ietf-bier-te-yang"/>. These options are non-exhaustive.</t>
<t>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 BPs required and how to assign BPs 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 <xref target="controller-ops"/>.</t>
<t>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.</t>
<t>Communications between the BIER-TE Controller and BFRs for both BIER-TE topology
control and BIER-TE tree control 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 models).</t>
</section>
<section anchor="engineered-bitstrings" title="Engineered Trees via BitStrings">
<t>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.</t>
<t>See also <xref target="I-D.ietf-bier-multicast-http-response"/> for an application
leveraging BIER-TE engineered trees.</t>
</section>
<section anchor="changes-in-topo" title="Changes in the network topology">
<t>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.</t>
</section>
<!-- changes-in-topo -->
<section anchor="failures" title="Link/Node Failures and Recovery">
<t>When link or nodes fail or recover in the topology, BIER-TE could quickly
respond with FRR procedures such as <xref target="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.</t>
</section>
<!-- failures -->
</section>
<!-- control-plane -->
</section>
<!-- XXX -->
<section anchor="forwarding-plane" title="The BIER-TE Forwarding Plane">
<t>[RFC-editor Q: "is constituted from" / "consists of" / "composed from..." ???]</t>
<t>The BIER-TE Forwarding Plane is constituted from the following components:
<list style="numbers">
<t>On a BFIR, imposition of the 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 <xref target="flow-overlay"/>.</t>
<t>On BFRs (including BFIR and BFER), forwarding/replication of BIER packets according to their SD, SI, "BitStringLength" (BSL), BitString and optionally Entropy fields as explained in <xref target="forwarding"/>. Processing of other BIER header fields such as DSCP is outside the scope of this document.</t>
<t>On BFERs, removal of the BIER header and dispatching of the payload according to state created by the BIER-TE control plane and/or overlay layer.</t>
</list>
</t>
<t>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
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 clears all BPs in the BitString of the packet for which the
BFR has one or more adjacencies in the BIFT. Clearing these bits inhibits
packets from looping when the BitStrings erroneously includes a forwarding loop.
When a forward_connected() adjacency has the "DoNotClear" (DNC) flag
set, then this BP is re-set for the packet copied to that adjacency.
See <xref target="forward-connected"/>.</t>
</section>
<!-- forwarding-plane -->
<section anchor="routing-underlay" title="The Routing Underlay">
<t>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 <xref target="forward-routed"/> for the adjacency definition.</t>
<t>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 BFR specific routing underlay extensions to forward BIER packets with
BIER-TE semantics.</t>
<t>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.
</t>
<t>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.</t>
</section>
<!-- routing-underlay -->
<section anchor="te-considerations" title="Traffic Engineering Considerations">
<t>Traffic Engineering (<xref target="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.</t>
<t>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 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.</t>
<t>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 <xref target="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 <xref target="forward-connected"/> 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.</t>
<t>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.</t>
</section>
<!-- te-considerations -->
</section>
<!-- components -->
<section anchor="forwarding" title="BIER-TE Forwarding">
<section anchor="btft" title="The BIER-TE Bit Index Forwarding Table (BIFT)">
<t>The BIER-TE BIFT is the equivalent to the BIER BIFT for (non-TE) BIER. It
exists on every BFR running BIER-TE. For every BIER sub-domain (SD) in use for BIER-TE,
it is a table as shown shown in <xref target="adjacencies"/>. That example
BIFT assumes a BSL of 8 bit positions (BPs) in the packets BitString.
As in <xref target="RFC8279"/> this BSL is purely used for the example and not a BIER/BIER-TE
supported BSL (minimum BSL is 64).</t>
<t>A BIER-TE BIFT compares to a BIER BIFT as shown in <xref target="RFC8279"/> as
follows.</t>
<t>In both BIER and BIER-TE, BIFT rows/entries are indexed in their respective BIER pseudocode
(<xref target="RFC8279"/> Section 6.5) and BIER-TE pseudocode (<xref target="pseudocode"/>)
by the BIFT-index derived from the packets SI, BSL and the one bit position of the
packets BitString (BP) addressing the BIFT row: BIFT-index = SI * BSL + BP - 1.
BP within a BitString are numbered from 1 to BSL, hence the - 1 offset when converting
to a BIFT-index. This document also uses the notion SI:BP to indicate BIFT rows,
<xref target="RFC8279"/> uses the equivalent notion SI:BitString, where the BitString is
filled with only the BP for the BIFT row.</t>
<t>In BIER, each BIFT-index addresses one BFER by its BFR-id = BIFT-index + 1
and is populated on each BFR with the next-hop "BFR Neighbor" (BFR-NBR) towards that BFER.</t>
<t>In BIER-TE, each BIFT-index and therefore SI:BP indicates one or more adjacencies
between BFRs in the topology and is only populated with those adjacencies forwarding
entries on the BFR that is the upstream for these adjacencies. The BIFT entry are
empty on all other BFRs.</t>
<t>In BIER, each BIFT rows also requires a "Forwarding Bit Mask" (F-BM) entry
for BIER forwarding rules. In BIER-TE forwarding, F-BM is not required, but can be used
when implementing BIER-TE on forwarding hardware derived from BIER forwarding, that
must use F-BM. This is discussed in the first BIER-TE forwarding pseudocode in
<xref target="pseudocode"/>.</t>
<figure anchor="adjacencies" title="BIER-TE BIFT with different adjacencies">
<artwork align="left"><![CDATA[
------------------------------------------------------------------
| BIFT-index | | Adjacencies: |
| (SI:BP) |(FBM)| <empty> or one or more per entry |
==================================================================
| BIFT indices for Packets with SI=0 |
------------------------------------------------------------------
| 0 (0:1) | ... | forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------
| 1 (0:2) | ... | forward_connected(interface,neighbor{,DNC}) |
| | ... | forward_connected(interface,neighbor{,DNC}) |
------------------------------------------------------------------
| ... | ... | ... |
------------------------------------------------------------------
| 4 (0:5) | ... | local_decap({VRF}) |
------------------------------------------------------------------
| 5 (0:6) | ... | forward_routed({VRF,}l3-neighbor) |
------------------------------------------------------------------
| 6 (0:7) | ... | <empty> |
------------------------------------------------------------------
| 7 (0:8) | ... | ECMP((adjacency1,...adjacencyN){,seed}) |
-----------------------------------------------------------------
| BIFT indices for BitString/Packet with SI=1 |
------------------------------------------------------------------
| 9 (1:1) | | ... |
| ... |... | ... |
------------------------------------------------------------------
BIER-TE Bit Index Forwarding Table (BIFT)
]]></artwork></figure>
<t>The BIFT is configured for the BIER-TE data plane of a BFR by the BIER-TE
Controller through an appropriate protocol and data-model. The BIFT is then
used to forward packets, according to the rules
specified in the BIER-TE Forwarding Procedures.</t>
<t>Note that a BIFT index (SI:BP) may be populated in the BIFT of more
than one BFR to save BPs. See <xref target="rings"/> for an example of how a BIER-TE controller
could assign BPs to (logical) adjacencies shared across multiple BFRs,
<xref target="leaf-bfer"/> for an example of assigning the same BP to different
adjacencies, and <xref target="reuse"/> for general guidelines regarding re-use of BPs across different adjacencies.</t>
<t>{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.</t>
</section>
<!-- btft -->
<section anchor="atypes" title="Adjacency Types">
<section anchor="forward-connected" title="Forward Connected">
<t>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() adjacency does not route packets
but only L2 forwards them to the neighbor.</t>
<t>Packets sent to an adjacency with "DoNotClear" (DNC) set in the
BIFT MUST NOT have the bit position for that adjacency cleared when the
BFR creates a copy for it. The bit position will still be cleared for
copies of the packet made towards other adjacencies. This can be
used for example in ring topologies as explained in <xref target="rings"/>.</t>
<t>For protection against loops from misconfiguration (see <xref target="loops"/>),
DNC is only permissible for forward_connected() adjacencies. No need or benefit
of DNC for other type of adjacencies was identified and their risk was not analyzed.</t>
</section>
<!-- forward-connected -->
<section anchor="forward-routed" title="Forward Routed">
<t>A "forward_routed()" adjacency is an adjacency towards a BFR that
uses a (tunneling) encapsulation which will cause the packet to be
forwarded by the routing underlay toward the adjacent BFR. This can
leverage any feasible encapsulation, such as MPLS or tunneling over IP/IPv6,
as long as the BIER-TE packet can be identified as a payload. This identification
can either rely on the BIER/BIER-TE co-existence mechanisms described in
<xref target="encapsulation"/>, or by explicit support for a BIER-TE payload type
in the tunneling encapsulation.</t>
<t>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 ECMP (<xref target="forward-ecmp"/>).</t>
</section>
<!-- forward-routed -->
<section anchor="forward-ecmp" title="ECMP">
<t>(non-TE) BIER ECMP is tied to the BIER BIFT processing semantic and is therefore
not directly usable with BIER-TE.</t>
<t>A BIER-TE "Equal Cost Multipath" (ECMP()) adjacency as shown in <xref target="adjacencies"/>
for BIFT-index 7 has a list of two or more non-ECMP adjacencies as parameters and an optional
seed parameter. When a BIER-TE packet is copied
onto such an ECMP() adjacency, an implementation specific so-called hash function
will select one out of the list's adjacencies to which the packet is forwarded.
If the packet's encapsulation contains an entropy field, the entropy field SHOULD
be respected; two packets with the same value of the entropy field SHOULD be sent on
the same adjacency. The seed parameter allows to design
hash functions that are easy to implement at high speed without running into
polarization issues across multiple consecutive ECMP hops. See <xref target="ecmp"/>
for more explanations.</t>
</section>
<!-- forward-ecmp -->
<section anchor="forward-local" title="Local Decap(sulation)">
<t>A "local_decap()" adjacency passes a copy of the payload of
the BIER-TE packet to the protocol ("NextProto") within the BFR (IPv4/IPv6, Ethernet,...) responsible for
that payload according to the packet header fields.
A local_decap() adjacency turns the BFR into a BFER for matching
packets. Local_decap() adjacencies require the BFER to support
routing or switching for NextProto to determine how to further
process the packet.</t>
</section>
<!-- forward-local -->
</section>
<!-- atypes -->
<section anchor="encapsulation" title="Encapsulation / Co-existence with BIER">
<t>Specifications for BIER-TE encapsulation are outside the scope of this document.
This section gives explanations and guidelines.</t>
<t>Like <xref target="RFC8279"/>, handling of "Maximum Transmission Unit" (MTU)
limitations is outside the scope of this document and instead part of the
BIER-TE packet encapsulation and/or flow overlay. See for example <xref target="RFC8296"/>, Section 3.
It applies equally to BIER-TE as it does to BIER.</t>
<t>Because a BFR needs to interpret the BitString of a BIER-TE packet differently
from a (non-TE) BIER packet, it is necessary to distinguish BIER from BIER-TE packets.
In the BIER encapsulation <xref target="RFC8296"/>,
the BIFT-id field of the packet indicates the BIFT of the packet. BIER and BIER-TE can
therefore be run simultaneously, when the BIFT-id address space is shared across
BIER BIFT and BIER-TE BIFT. Partitioning the BIFT-id address space is subject
to BIER-TE/BIER control plane procedures.</t>
<t>When <xref target="RFC8296"/> is used for BIER with MPLS, BIFT-id address ranges
can be dynamically allocated from MPLS label space only for the set of actually
used SD:BSL BIFT. This allows to also allocate non-overlapping label ranges for BIFT-id
that are to be used with BIER-TE BIFTs.</t>
<t>With MPLS, it is also possible to reuse the
same SD space for both BIER-TE and BIER, so that the same SD has both a
BIER BIFT with a corresponding range of BIFT-ids and disjoint BIER-TE BIFTs with a non-overlapping range of BIFT-ids.</t>
<t>When a fixed mapping from BSL, SD and SI to BIFT-id is used which does
not explicitly partition the BIFT-id space between BIER and BIER-TE,
such as proposed for non-MPLS forwarding with <xref target="RFC8296"/> encapsulation
in <xref target="I-D.ietf-bier-non-mpls-bift-encoding"/>
revision 04, section 5, then it is necessary to allocate disjoint SDs to BIER
and BIER-TE BIFTs so that both can be 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 a mapping, and hence could provide for
the same freedom as when MPLS is being used (same or different SD for BIER/BIER-TE).</t>
<t>forward_routed() requires an encapsulation that permits to direct unicast encapsulated 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 the (BSL,SD,SI) BitString.
With non-MPLS encapsulation, some form of IP encapsulation would be required (for example IP/GRE).
</t>
<t>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).</t>
</section>
<!-- encapsulation -->
<section anchor="pseudocode" title="BIER-TE Forwarding Pseudocode">
<t>
The following pseudocode, <xref target="simple-pseudocode-picture"/>, for BIER-TE forwarding is based
on the (non-TE) BIER forwarding pseudocode of <xref target="RFC8279"/>, section 6.5 with one modification.</t>
<figure anchor="simple-pseudocode-picture" title="BIER-TE Forwarding Pseudocode for required functions, based on BIER Pseudocode">
<artwork align="left"><![CDATA[
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; [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]
}
}
]]></artwork></figure>
<t>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.</t>
<t>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]).</t>