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draft-ietf-bier-te-arch-09.xml
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<?xml version="1.0" encoding="US-ASCII"?>
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<rfc ipr="trust200902" docName="draft-ietf-bier-te-arch-09" 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>[email protected]</email>
</address>
</author>
<author fullname="Gregory Cauchie" initials="G.C." surname="Cauchie">
<organization>Bouygues Telecom</organization>
<address>
<email>[email protected]</email>
</address>
</author>
<author fullname="Michael Menth" initials="M.M." surname="Menth">
<organization>University of Tuebingen</organization>
<address>
<email>[email protected]</email>
</address>
</author>
<date month="Oct" year="2020"/>
<abstract>
<t>
This memo introduces per-packet stateless strict and loose path
steered replication and forwarding for Bit Index Explicit Replication
packets (RFC8279). This is called BIER Tree Engineering (BIER-TE).
BIER-TE can be used as a path steering mechanism in future Traffic
Engineering solutions for BIER (BIER-TE).
</t>
<t> BIER-TE leverages RFC8279
and extends it with a new semantic for bits in the bitstring. BIER-TE
can leverage BIER forwarding engines with little or no changes.</t>
<t>In BIER, the BitPositions (BP) of the packets bitstring indicate BIER Forwarding Egress
Routers (BFER), and hop-by-hop forwarding uses a Routing Underlay such as an IGP.</t>
<t>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. </t>
<t>
BIER-TE can co-exist with BIER forwarding in the same domain, for example by using
separate BIER sub-domains. In the absence of routed adjacencies, BIER-TE does not
require a BIER routing underlay, and can then be operated without requiring an Interior Gateway Routing protocol (IGP).
</t>
<t>BIER-TE operates without explicit in-network tree-state and carries the multicast distribution tree in the packet header. It can therefore be a good fit to support multicast path steering in Segment Routing (SR) networks.</t>
</abstract>
<note title="Name explanation">
<t>[RFC-editor: This section to be removed before publication.]</t>
<t>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):</t>
<t>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.</t>
<t>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.</t>
</note>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t> BIER-TE shares architecture, terminology and packet formats with BIER 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>In BIER-TE, BitPositions (BP) indicate adjacencies.
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. The BFR replicate
and forwards BIER packets to adjacent BPs that are set in the packet.
BPs are normally also reset upon forwarding to avoid duplicates and loops.
This is detailed further below.
</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 reset 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 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 detailled 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 desired paths/trees 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.</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="examples" title="Basic Examples">
<t>BIER-TE forwarding is best introduced with simple examples.</t>
<figure anchor="basic-example" title="BIER-TE basic example">
<artwork align="left"><![CDATA[
BIER-TE Topology:
Diagram:
p5 p6
--- BFR3 ---
p3/ p13 \p7
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
p12 -> local_decap
]]></artwork></figure>
<t>
Consider the simple network in the above BIER-TE overview example picture
with 6 BFRs. p1...p14 are the BitPositions (BP) used. All BFRs can act as
ingress BFR (BFIR), BFR1, BFR3, BFR4 and
BFR6 can also be egress BFR (BFER). 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). When this packet is examined by BIER-TE
on BFR1, the only BitPosition 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. p12 also makes BFR6 receive
and decapsulate the packet.
</t>
<t>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.
</t>
<t>If instead the bitstring was (p2,p6,p8,p10,p12,p13), 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 assumptions about the required multicast traffic
paths and bandwidth consumption in the network.</t>
<t>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 forwarding of BIER-TE packets,
these adjacencies are called "forward_routed", but 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
........ ........>
....> 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
]]></artwork></figure>
<t>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).</t>
</section>
<section anchor="topology" title="BIER-TE Topology and adjacencies">
<t>The key new component in BIER-TE compared to 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 BIFT 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.
</t>
<t>If the BIER-TE topology represents the underlying (layer 2) topology of the
network, this is called "native" BIER-TE as shown in the first example. This
can be freely mixed with "overlay" BIER-TE, in "forward_routed" adjacencies
are used.</t>
</section>
<!-- topology -->
<section anchor="overview" title="Comparison with BIER">
<t> The key differences over BIER are: </t>
<t><list style="symbols">
<t> BIER-TE replaces in-network autonomous path calculation by explicit
paths calculated by the BIER-TE Controller. </t>
<t> In BIER-TE every BitPosition of the BitString of a BIER-TE packet
indicates one or more adjacencies - instead of a BFER as
in BIER.</t>
<t> BIER-TE in each BFR has no routing table but only a BIER-TE Forwarding
Table (BIFT) indexed by SI:BitPosition and populated with only those
adjacencies to which the BFR should replicate packets to. </t>
</list></t>
<t>BIER-TE headers use the same format as BIER headers.</t>
<t>BIER-TE forwarding does not require/use the BFIR-ID. The BFIR-ID can
still be useful though for coordinated BFIR/BFER functions, such as
the context for upstream assigned labels for MPLS payloads in MVPN
over BIER-TE. </t>
<t>If the BIER-TE domain is also running BIER, then the BFIR-ID in
BIER-TE packets can be set to the same BFIR-ID as used with BIER
packets.</t>
<t>If the BIER-TE domain is not running full BIER or does not
want to reduce the need to allocate bits in BIER bitstrings for
BFIR-ID values, then the allocation of BFIR-ID values in BIER-TE packets can
be done through other mechanisms outside the scope of this document,
as long as this is appropriately agreed upon between all BFIR/BFER.</t>
</section>
<!-- comparison -->
<section anchor="boilerplate" title="Requirements Language">
<t>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 in <xref target="RFC2119">RFC 2119</xref>.</t>
</section>
<!-- requirements -->
</section>
<!-- intro -->
<section anchor="components" title="Components">
<t>End to end BIER-TE operations consists of four mayor
components: The "Multicast Flow Overlay", the "BIER-TE control plane"
consisting of the "BIER-TE Controller" and its signaling
channels to the BFR, the "Routing Underlay" and the "BIER-TE forwarding layer".
The Bier-TE Controller is the new architectural component in
BIER-TE compared to BIER.</t>
<figure anchor="architecture" title="BIER-TE architecture">
<artwork align="left"><![CDATA[
Picture 2: Components of BIER-TE
<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
[BIER-TE Controller] <=> [BIER-TE Topology]
BIER-TE control plane
^ ^ ^
/ | \ BIER-TE control protocol
| | | e.g. Netconf/Restconf/Yang
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|<----------------->|
BIER-TE forwarding layer
|<- BIER-TE domain->|
|<--------------------->|
Routing underlay
]]></artwork></figure>
<section anchor="flow-overlay" title="The Multicast Flow Overlay">
<t>The Multicast Flow Overlay operates as in BIER. See
<xref target="RFC8279"/>. Instead of
interacting with the BIER forwarding layer (as in BIER),
it interacts with the BIER-TE Controller. </t>
</section>
<!-- flow-overlay -->
<section anchor="controller" title="The BIER-TE Controller">
<t>The BIER-TE Controller is representing the control plane of
BIER-TE. It communicates two sets of information with BFRs:</t>
<t>During initial provisioning or modifications of the network topology, the BIER-TE Controller discovers
the network topology and creates the BIER-TE topology from it: determine which
adjacencies are required/desired and assign BitPositions to them. Then it signals the resulting
of BitPositions and their adjacencies to each BFR to set up their BIER-TE BIFTs.</t>
<t>During day-to-day operations of the network, the BIER-TE Controller signals to
BFIRs what multicast flows are mapped to what BitStrings.</t>
<t>Communications between the BIER-TE Controller and BFRs is ideally
via standardized protocols and data-models such as Netconf/Restconf/Yang.
This is currently outside the scope of this document. Vendor-specific CLI
on the BFRs is also a possible stopgap option (as in many other SDN solutions lacking
definition of standardized data model).</t>
<t>For simplicity, the procedures of the BIER-TE Controller are described in
this document as if it is a single, centralized automated entity, such as an SDN
controller. It could equally be an operator setting up CLI on the BFRs. Distribution
of the functions of the BIER-TE Controller is currently outside the scope of this
document.</t>
<section anchor="assignment" title="Assignment of BitPositions to adjacencies of the network topology">
<t>The BIER-TE Controller tracks the BFR topology of the
BIER-TE domain. It determines what adjacencies require
BitPositions so that BIER-TE explicit paths can be built
through them as desired by operator policy.</t>
<t>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.</t>
</section>
<!-- assignment -->
<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 BitPositions are no longer needed, the BIER-TE Controller can
re-use those BitPositions for new adjacencies. First, these BitPositions
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="setup" title="Set up per-multicast flow BIER-TE state">
<t>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.</t>
<t>See <xref target="I-D.ietf-bier-multicast-http-response"/> for a solution
describing this interaction.</t>
</section>
<!-- setup -->
<section anchor="failures" title="Link/Node Failures and Recovery">
<t>When link or nodes fail or recover in the topology, BIER-TE can quickly
respond with the optional FRR procedures described in
[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.</t>
</section>
<!-- failures -->
</section>
<!-- controller -->
<section anchor="forwarding-layer" title="The BIER-TE Forwarding Layer">
<t>When the BIER-TE Forwarding Layer receives a packet, it simply looks
up the BitPositions 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 all BP 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, see <xref target="forward-connected"/>). This is done to inhibit that packets can loop.</t>
</section>
<!-- forwarding-layer -->
<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 BFER 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 bitsring.
There is thus no need for BFER specific routing underlay extensions to forward BIER packets with
BIER-TE semantics.</t>
<t>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 (<xref target="RFC8296"/>).
These BIER specific functions of the routing underlay are equally useable by BIER-TE.
Alternatively, these 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 (BFR
flow state) and BFR BIFT state. The mapping of user/IP traffic to specific
BitStrings/BIER-TE flows is made based on policy. The specifics details of
BIER-TE policies and how a controller uses such 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. 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 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 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 trafic
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.</t>
</section>
<!-- te-considerations -->
</section>
<!-- components -->
<section anchor="forwarding" title="BIER-TE Forwarding">
<section anchor="btft" title="The Bit Index Forwarding Table (BIFT)">
<t>The Bit Index Forwarding Table (BIFT) exists in every BFR. For every
subdomain in use, it is a table indexed by SI:BitPosition and is populated by the
BIER-TE control plane. Each index can be empty or contain a list of one or more
adjacencies.</t>
<t>BIER-TE can support multiple subdomains like BIER. Each one with a separate BIFT</t>
<t>In the BIER architecture, indices into the BIFT are explained to be both
BFR-id and SI:BitString (BitPosition). This is because there is a 1:1 relationship
between 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 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 only used as required
by routing underlay, flow overlay of BIER headers. Please refer to <xref target="mgmt-stuff"/>
for explanations how to deal with SI, subdomains and BFR-id in BIER-TE.</t>
<figure anchor="adjacencies" title="BIFT adjacencies">
<artwork align="left"><![CDATA[
------------------------------------------------------------------
| Index: | Adjacencies: |
| SI:BitPosition | <empty> or one or more per entry |
==================================================================
| 0:1 | forward_connected(interface,neighbor{,DNR}) |
------------------------------------------------------------------
| 0:2 | forward_connected(interface,neighbor{,DNR}) |
| | forward_connected(interface,neighbor{,DNR}) |
------------------------------------------------------------------
| 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
]]></artwork></figure>
<t>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.</t>
<t>Adjacencies for the same BP when populated in more than one BFR
by the BIER-TE Controller does not have to have the same adjacencies. This is
up to the BIER-TE Controller. BPs for p2p links are one case (see below).</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 "DoNotReset" (DNR) set in the
BIFT will not have the BitPosition for that adjacency reset when the
BFR creates a copy for it. The BitPosition will still be reset for
copies of the packet made towards other adjacencies. This can be
used for example in ring topologies as explained below.</t>
</section>
<!-- forward-connected -->
<section anchor="forward-routed" title="Forward Routed">
<t>A "forward_routed" adjacency is an adjacency towards a BFR that
is not a forward_connected adjacency: towards a loopback address
of a BFR or towards an interface address that is non-directly
connected. Forward_routed packets are forwarded via the Routing
Underlay.</t>
<t>If the Routing Underlay has multiple
paths for a forward_routed adjacency, it will perform ECMP independent
of BIER-TE for packets forwarded across a forward_routed adjacency.
This is independent of BIER-TE ECMP described in <xref target="forward-ecmp"/>.</t>
<t>If the Routing Underlay has FRR, it will perform FRR independent
of BIER-TE for packets forwarded across a forward_routed adjacency.</t>
</section>
<!-- forward-routed -->
<section anchor="forward-ecmp" title="ECMP">
<t>The ECMP mechanisms in BIER are tied to the BIER BIFT and are therefore
not directly useable with BIER-TE. The following procedures describe ECMP
for 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.</t>
<t>An "Equal Cost Multipath" (ECMP) adjacency has a list of two or
more adjacencies included in it. It copies the BIER-TE to
one of those adjacencies based on the ECMP hash calculation.
The BIER-TE ECMP hash algorithm 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
seed parameter is explained below.</t>
</section>
<!-- forward-ecmp -->
<section anchor="forward-local" title="Local Decap">
<t>A "local_decap" adjacency passes a copy of the payload of
the BIER-TE packet to the packets NextProto within the BFR (IPv4/IPv6, Ethernet,...).
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 considerations">
<t>Specifications for BIER-TE encapsulation are outside the scope of this document.
This section gives explanations and guidelines.</t>
<t>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 BIER encapsulation specifications.</t>
<t>MPLS encapsulation <xref target="RFC8296"/> for
example assigns one label by which BFRs recognizes BIER packets for every
(SI,subdomain) combination. If it is desirable that every subdomain can
forward only BIER or BIER-TE packets, then the label allocation could stay the
same, and only the forwarding model (BIER/BIER-TE) would have to be defined
per subdomain. If it is desirable to support both BIER and BIER-TE forwarding
in the same subdomain, then additional labels would need to be assigned for
BIER-TE forwarding.</t>
<t>"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.
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="basic" title="Basic BIER-TE Forwarding Example">
<t>[RFC Editor: remove this section.]</t>
<t>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.</t>
<t>Step by step example of basic BIER-TE forwarding. This does not
use ECMP or forward_routed adjacencies nor does it try to minimize
the number of required BitPositions for the topology.</t>
<figure anchor="forwarding-example" title="BIER-TE Forwarding Example">
<artwork align="left"><![CDATA[
[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......| IP
]]></artwork></figure>
<t>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.</t>
<figure anchor="example-adjacencies" title="BIER-TE Forwarding Example Adjacencies">
<artwork align="left"><![CDATA[
BIFT BFIR2:
p13: 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()
p6: forward_connected(BFR3)
p8: forward_connected(BFR4)
]]></artwork></figure>
<t>...and so on.</t>
<t>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:</t>
<figure anchor="example-paths" title="BIER-TE Forwarding Example Paths">
<artwork align="left"><![CDATA[
-> BFER1 ---------------> Rcv1
BFIR2 -> BFR3
-> BFR4 -> BFR5 -> BFER2 -> Rcv2
]]></artwork></figure>
<t>These paths equal to the following BitString:
p2, p5, p7, p8, p10, p11, p12.</t>
<t>This BitString is assigned by BFIR2 to the example multicast traffic received from LAN1.</t>
<t>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 p2 in the BitString
and sends a copy towards BFR2.</t>
<t>BFR3 sees a BitString of p5,p7,p8,p10,p11,p12.
It is only interested in p1,p7,p8. It creates a copy of the
packet to BFER1 (due to p7) and one to BFR4 (due to p8). It
resets p7, p8 before sending.</t>
<t>BFER1 sees a BitString of p5,p10,p11,p12.
It is only interested in p6,p7,p8,p11 and therefore considers
only 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 a copy, decapsulate it from the BIER header
and pass it on to the NextProtocol, in this example 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. </t>
<t>Further processing of the packet in BFR4, BFR5 and BFER2
accordingly.</t>
</section>
<!-- basic -->
<section anchor="fwd-comparison" title="Forwarding comparison with BIER">
<t>Forwarding of 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 support building of BIER-TE
control plane code.</t>
<t>The pseudocode in <xref target="pseudocode"/> shows how existing
BIER/BIFT forwarding can be amended to support basic BIER-TE forwarding,
by using BIER BIFT's F-BM. Only the masking of bits due to avoid duplicates
must be skipped when forwarding is for BIER-TE.</t>
<t>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 <xref target="RFC8296"/> too can be reused without
changes except that the currently defined BIER-TE ECMP adjacency does not leverage the
entropy field so that field would be unused when BIER-TE forwarding is used.</t>
</section>
<section anchor="requirements" title="Requirements">
<t>Basic BIER-TE forwarding MUST support to configure Subdomains to use basic
BIER-TE forwarding rules (instead of BIER). With basic BIER-TE forwarding,
every bit MUST support to have zero or one adjacency. It MUST support the
adjacency types forward_connected without DNR flag, forward_routed and local_decap. All
other BIER-TE forwarding features are optional. These basic BIER-TE requirements
make BIER-TE forwarding exactly the same as BIER forwarding with the exception
of skipping the aforementioned F-BM masking on egress.</t>
<t>BIER-TE forwarding SHOULD support the DNR flag, as this is highly useful to
save bits in rings (see <xref target="rings"/>).</t>
<t>BIER-TE forwarding MAY 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 allows further savings of
bits in hub&spoke scenarios, but unlike rings it is less "natural" to flood
traffic across multiple links unconditional. Both 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. This
is true because there is no inter-copy dependency through resetting of F-BM as
in BIER.</t>
</section>
</section>
<!-- forwarding -->
<section anchor="bitpositions" title="BIER-TE Controller BitPosition Assignments">
<t>This section describes how the BIER-TE Controller can use the
different BIER-TE adjacency types to define the BitPositions of a BIER-TE domain.</t>
<t>Because the size of the BitString is limiting the size of the
BIER-TE domain, many of the options described exist to support larger
topologies with fewer BitPositions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8).</t>
<section anchor="p2p-links" title="P2P Links">
<t>Each P2p link in the BIER-TE domain is assigned one unique BitPosition
with a forward_connected adjacency pointing to the neighbor on the
p2p link.</t>
</section>
<!-- p2p-links -->
<section anchor="bfer" title="BFER">
<t>Every non-Leaf BFER is given a unique BitPosition with a local_decap adjacency.</t>
</section>
<!-- bfer -->
<section anchor="leaf-bfers" title="Leaf BFERs">
<figure anchor="leaf-bfer-picture" title="Leaf vs. non-Leaf BFER Example">
<artwork align="left"><![CDATA[
BFR1(P) BFR2(P) BFR1(P) BFR2(P)
| \ / | | |