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draft-ietf-bier-te-arch-04.xml
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draft-ietf-bier-te-arch-04.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-04" category="std">
<front>
<title abbrev="BIER-TE ARCH">Traffic 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="Gregory Cauchie" initials="G.C." surname="Cauchie">
<organization>Bouygues Telecom</organization>
<address>
<email>GCAUCHIE@bouyguestelecom.fr</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>
<date month="October" year="2019"/>
<abstract>
<t>
This memo introduces per-packet stateless strict and loose path
engineered replication and forwarding for Bit Index Explicit Replication
packets ([RFC8279]). This is called BIER-TE.
</t>
<t> BIER-TE leverages the BIER architecture (<xref target="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 (route) adjacencies.
The BFR then replicates and forwards BIER packets to those adjacencies.
This results in the aforementioned strict and loose path forwarding. </t>
<t>
BIER-TE can co-exist with BIER forwarding in the same domain, for example by using
separate 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 IGP routing protocol.
</t>
<t>BIER-TE operates without explicit in-network tree-building 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>
</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="ICC"/>, <xref target="I-D.ietf-roll-ccast"/>
uses bloom filters to represent leaves or edges of the intended delivery tree. Bloom filters
can support larger trees with fewer addressing bits, 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 does not use bloom filters, but explicit bitstrings like BIER.</t>
<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 p6 instead of BFR2 to BFR5
because of p6 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 BFR6
can use (p1,p2,p3,p4,p5) or (p1,p2,p3,p4,p6), or via
BFR6 (p2,p3,p4,p6,p7) or (p1.p3,p4,p5,p8).</t>
</section>
<section anchor="topology" title="BIER-TE Topology and adjacencies">
<t>The key new component in BIER-TE to control where replication
can or should happens and how to minimize the required BP for
segments is - as shown in these two examples - the BIER-TE topology.
</t>
<t>
The BIER-TE Topology effectively consists of the BIFT of all the BFR and
can also be expressed in a diagram as a graph where the edges are the adjacencies
between the BFR. 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 off-path by the BIER-TE controller host. </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 Host" and its signaling
channels to the BFR, the "Routing Underlay" and the "BIER-TE forwarding layer".
The Bier-TE Controller Host 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 Host] <=> [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 Host. </t>
</section>
<!-- flow-overlay -->
<section anchor="controller" title="The BIER-TE Controller Host">
<t>The BIER-TE controller host 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 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 controller signals to
BFIRs what multicast flows are mapped to what BitStrings.</t>
<t>Communications between the BIER-TE controller host to 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 host 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 host 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 host 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 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 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 host 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 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
host. 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>BIER-TE is sending BIER packets to directly connected
BIER-TE neighbors as L2 (unicasted) BIER packets without requiring a
routing underlay. BIER-TE forwarding uses the Routing underlay for
forward_routed adjacencies which copy BIER-TE packets to not-directly-connected
BFRs (see below for adjacency definitions).
</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>
<!-- 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 (traffic 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 host 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 controller does not have to have the same adjacencies. This is
up to the controller. BPs for p2p links are one case (see below).</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. </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 engineering traffic 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 tunneling (IP in IP, LISP, GRE) would
be required.</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 Host]
/ | \
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 host 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 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 host 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 cannot 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 host. 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
engineering is used because it may be more desirable to explicitly steer
traffic across non-ECMP paths to make per-path traffic calculation easier for
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 Host BitPosition Assignments">
<t>This section describes how the BIER-TE controller host 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 BFER is given a unique BitPosition with a local_decap adjacency.</t>
</section>
<!-- bfer -->
<section anchor="bfirs" title="Leaf BFERs">
<t>Leaf BFERs are BFERs where incoming BIER-TE packets never need to
be forwarded to another BFR but are only sent to the BFER
to exit the BIER-TE domain. For example, in networks where PEs
are spokes connected to P routers, those PEs are Leaf BFIRs unless
there is a U-turn between two PEs.</t>
<t>All leaf-BFER in a BIER-TE domain can share a single BitPosition.
This is possible because the BitPosition for the adjacency to reach the BFER
can be used to distinguish whether or not packets should reach the BFER.</t>
<t>This optimization will not work if an upstream interface of the BFER
is using a BitPosition optimized as described in the following two
sections (LAN, Hub and Spoke).</t>
</section>
<!-- bfirs -->
<section anchor="lans" title="LANs">
<t>In a LAN, the adjacency to each neighboring BFR on the LAN
is given a unique BitPosition. The adjacency of this BitPosition
is a forward_connected adjacency towards the BFR and this BitPosition
is populated into the BIFT of all the other BFRs on that LAN.</t>
<figure anchor="lan-picture" title="LAN Example">
<artwork align="left"><![CDATA[
BFR1
|p1
LAN1-+-+---+-----+
p3| p4| p2|
BFR3 BFR4 BFR7
]]></artwork></figure>
<t>If Bandwidth on the LAN is not an issue and most BIER-TE traffic
should be copied to all neighbors on a LAN, then BitPositions
can be saved by assigning just a single BitPosition to the LAN
and populating the BitPosition of the BIFTs of each BFRs on
the LAN with a list of forward_connected adjacencies to all other
neighbors on the LAN.</t>
<t>This optimization does not work in the face of BFRs redundantly
connected to more than one LANs with this optimization because
these BFRs would receive duplicates and forward those duplicates into
the opposite LANs. Adjacencies of such BFRs into their LANs still
need a separate BitPosition.</t>
</section>
<!-- lans -->
<section anchor="hubnspoke" title="Hub and Spoke">
<t>In a setup with a hub and multiple spokes connected via separate
p2p links to the hub, all p2p links can share the same BitPosition.
The BitPosition on the hubs BIFT is set up with a list of
forward_connected adjacencies, one for each Spoke.</t>
<t>This option is similar to the BitPosition optimization in
LANs: Redundantly connected spokes need their own BitPositions.</t>
</section>
<!-- hubnspoke -->
<section anchor="rings" title="Rings">
<t>In L3 rings, instead of assigning a single BitPosition for
every p2p link in the ring, it is possible to save BitPositions by
setting the "Do Not Reset" (DNR) flag on forward_connected adjacencies.</t>
<t>For the rings shown in the following picture, a single BitPosition
will suffice to forward traffic entering the ring at BFRa or BFRb
all the way up to BFR1:</t>
<t>On BFRa, BFRb, BFR30,... BFR3, the BitPosition is populated with
a forward_connected adjacency pointing to the clockwise neighbor
on the ring and with DNR set. On BFR2, the adjacency also points
to the clockwise neighbor BFR1, but without DNR set.</t>
<t>Handling DNR this way ensures that copies forwarded from any BFR in
the ring to a BFR outside the ring will not have the ring BitPosition set,
therefore minimizing the chance to create loops.</t>
<figure anchor="ring-picture" title="Ring Example">
<artwork align="left"><![CDATA[
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
]]></artwork></figure>
<t>Note that this example only permits for packets to enter the ring at
BFRa and BFRb, and that packets will always travel clockwise. If
packets should be allowed to enter the ring at any ring BFR, then one
would have to use two ring BitPositions. One for clockwise, one for
counterclockwise.</t>
<t>Both would be set up to stop rotating on the same link, e.g. L1. When the
ingress ring BFR creates the clockwise copy, it will reset the counterclockwise
BitPosition because the DNR bit only applies to the bit for which the
replication is done. Likewise for the clockwise
BitPosition for the counterclockwise copy. In result, the ring ingress
BFR will send a copy in both directions, serving BFRs on either side of the
ring up to L1.</t>
</section>
<!-- rings -->
<section anchor="ecmp" title="Equal Cost MultiPath (ECMP)">
<t>The ECMP adjacency allows to use just one BP per link
bundle between two BFRs instead of one BP for each p2p member
link of that link bundle. In the following picture, one BP
is used across L1,L2,L3 and BFR1/BFR2 have for the BP</t>
<figure anchor="ecmp-picture" title="ECMP Example">
<artwork align="left"><![CDATA[
--L1-----
BFR1 --L2----- BFR2
--L3-----
BIFT entry in BFR1:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({L1-to-BFR2,L2-to-BFR2,L3-to-BFR2}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({L1-to-BFR1,L2-to-BFR1,L3-to-BFR1}, seed) |
------------------------------------------------------------------
]]></artwork></figure>
<t>This document does not standardize any ECMP algorithm because it
is sufficient for implementations to document their freely chosen
ECMP algorithm. This allows the BIER-TE controller host to calculate ECMP
paths and seeds. The following picture shows an example ECMP algorithm:</t>
<figure anchor="ecmp-algo-picture" title="ECMP algorithm Example">
<artwork align="left"><![CDATA[
forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)):
i = (packet(bier-header-entropy) XOR seed) % N
forward packet to adj(i)
]]></artwork></figure>
<t>In the following example, all traffic from BFR1 towards BFR10 is
intended to be ECMP load split equally across the topology. This
example is not meant as a likely setup, but to illustrate that ECMP can
be used to share BPs not only across link bundles, and it explains
the use of the seed parameter.</t>