Internet DRAFT - draft-ietf-bier-te-arch
draft-ietf-bier-te-arch
Network Working Group T.T.E. Eckert, Ed.
Internet-Draft Futurewei
Intended status: Standards Track M.M. Menth
Expires: 27 October 2022 University of Tuebingen
G.C. Cauchie
KOEVOO
April 2022
Tree Engineering for Bit Index Explicit Replication (BIER-TE)
draft-ietf-bier-te-arch-13
Abstract
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.
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).
Status of This Memo
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This Internet-Draft will expire on 3 October 2022.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . 5
2.2. BIER-TE Topology and adjacencies . . . . . . . . . . . . 8
2.3. Relationship to BIER . . . . . . . . . . . . . . . . . . 9
2.4. Accelerated/Hardware forwarding comparison . . . . . . . 11
3. Components . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. The Multicast Flow Overlay . . . . . . . . . . . . . . . 12
3.2. The BIER-TE Control Plane . . . . . . . . . . . . . . . . 12
3.2.1. The BIER-TE Controller . . . . . . . . . . . . . . . 14
3.2.1.1. BIER-TE Topology discovery and creation . . . . . 14
3.2.1.2. Engineered Trees via BitStrings . . . . . . . . . 15
3.2.1.3. Changes in the network topology . . . . . . . . . 16
3.2.1.4. Link/Node Failures and Recovery . . . . . . . . . 16
3.3. The BIER-TE Forwarding Plane . . . . . . . . . . . . . . 16
3.4. The Routing Underlay . . . . . . . . . . . . . . . . . . 17
3.5. Traffic Engineering Considerations . . . . . . . . . . . 17
4. BIER-TE Forwarding . . . . . . . . . . . . . . . . . . . . . 18
4.1. The BIER-TE Bit Index Forwarding Table (BIFT) . . . . . . 18
4.2. Adjacency Types . . . . . . . . . . . . . . . . . . . . . 20
4.2.1. Forward Connected . . . . . . . . . . . . . . . . . . 21
4.2.2. Forward Routed . . . . . . . . . . . . . . . . . . . 21
4.2.3. ECMP . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2.4. Local Decap(sulation) . . . . . . . . . . . . . . . . 22
4.3. Encapsulation / Co-existence with BIER . . . . . . . . . 22
4.4. BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . 23
4.5. BFR Requirements for BIER-TE forwarding . . . . . . . . . 26
5. BIER-TE Controller Operational Considerations . . . . . . . . 27
5.1. Bit Position Assignments . . . . . . . . . . . . . . . . 27
5.1.1. P2P Links . . . . . . . . . . . . . . . . . . . . . . 27
5.1.2. BFER . . . . . . . . . . . . . . . . . . . . . . . . 27
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5.1.3. Leaf BFERs . . . . . . . . . . . . . . . . . . . . . 27
5.1.4. LANs . . . . . . . . . . . . . . . . . . . . . . . . 29
5.1.5. Hub and Spoke . . . . . . . . . . . . . . . . . . . . 30
5.1.6. Rings . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1.7. Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . 31
5.1.8. Forward Routed adjacencies . . . . . . . . . . . . . 34
5.1.8.1. Reducing bit positions . . . . . . . . . . . . . 34
5.1.8.2. Supporting nodes without BIER-TE . . . . . . . . 35
5.1.9. Reuse of bit positions (without DNC) . . . . . . . . 35
5.1.10. Summary of BP optimizations . . . . . . . . . . . . . 36
5.2. Avoiding duplicates and loops . . . . . . . . . . . . . . 37
5.2.1. Loops . . . . . . . . . . . . . . . . . . . . . . . . 38
5.2.2. Duplicates . . . . . . . . . . . . . . . . . . . . . 38
5.3. Managing SI, sub-domains and BFR-ids . . . . . . . . . . 39
5.3.1. Why SI and sub-domains . . . . . . . . . . . . . . . 39
5.3.2. Assigning bits for the BIER-TE topology . . . . . . . 40
5.3.3. Assigning BFR-id with BIER-TE . . . . . . . . . . . . 41
5.3.4. Mapping from BFR to BitStrings with BIER-TE . . . . . 42
5.3.5. Assigning BFR-ids for BIER-TE . . . . . . . . . . . . 43
5.3.6. Example bit allocations . . . . . . . . . . . . . . . 43
5.3.6.1. With BIER . . . . . . . . . . . . . . . . . . . . 43
5.3.6.2. With BIER-TE . . . . . . . . . . . . . . . . . . 44
5.3.7. Summary . . . . . . . . . . . . . . . . . . . . . . . 45
6. Security Considerations . . . . . . . . . . . . . . . . . . . 46
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 47
9. Change log [RFC Editor: Please remove] . . . . . . . . . . . 48
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 61
10.1. Normative References . . . . . . . . . . . . . . . . . . 61
10.2. Informative References . . . . . . . . . . . . . . . . . 61
Appendix A. BIER-TE and Segment Routing (SR) . . . . . . . . . . 64
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 65
1. Overview
BIER-TE is based on the (non-TE) BIER architecture, terminology and
packet formats as described in [RFC8279] and [RFC8296]. This
document describes BIER-TE in the expectation that the reader is
familiar with these two documents.
BIER-TE introduces a new semantic for "bit positions" (BP). 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.
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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.
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).
This document is structured as follows:
* Section 2 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.
* Section 3 describes the components of the BIER-TE architecture,
Flow overlay, BIER-TE layer with the BIER-TE control plane
(including the BIER-TE controller) and BIER-TE forwarding plane,
and the routing underlay.
* Section 4 specifies the behavior of the BIER-TE forwarding plane
with the different type of adjacencies and possible variations of
BIER-TE forwarding pseudocode, and finally the mandatory and
optional requirements.
* Section 5 describes operational considerations for the BIER-TE
controller, foremost how the BIER-TE controller can optimize the
use of BP by using specific type of BIER-TE adjacencies for
different type of topological situations, but also how to assign
bits to avoid loops and duplicates (which in BIER-TE does not come
for free), and finally how "Set Identifier" (SI), "sub-domain"
(SD) and BFR-ids can be managed by a BIER-TE controller, examples
and summary.
* Appendix A concludes the technology specific sections of the
document by further relating BIER-TE to Segment Routing (SR).
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Note that related work, [I-D.ietf-roll-ccast] uses Bloom filters
[Bloom70] to represent leaves or edges of the intended delivery tree.
Bloom filters in general can support larger trees/topologies with
fewer addressing bits than explicit BitStrings, but they introduce
the heuristic risk of false positives and cannot clear bits in the
BitString during forwarding to avoid loops. For these reasons, BIER-
TE uses explicit BitStrings like BIER. The explicit BitStrings of
BIER-TE can also be seen as a special type of Bloom filter, and this
is how related work [ICC] describes it.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Introduction
2.1. Basic Examples
BIER-TE forwarding is best introduced with simple examples. These
examples use formal terms defined later in the document (Figure 4),
including forward_connected(), forward_routed() and local_decap().
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BIER-TE Topology:
Diagram:
p5 p6
--- BFR3 ---
p3/ p13 \p7 p15
BFR1 ---- BFR2 BFR5 ----- BFR6
p1 p2 p4\ p14 /p10 p11 p12
--- BFR4 ---
p8 p9
(simplified) BIER-TE Bit Index Forwarding Tables (BIFT):
BFR1: p1 -> local_decap()
p2 -> forward_connected() to BFR2
BFR2: p1 -> forward_connected() to BFR1
p5 -> forward_connected() to BFR3
p8 -> forward_connected() to BFR4
BFR3: p3 -> forward_connected() to BFR2
p7 -> forward_connected() to BFR5
p13 -> local_decap()
BFR4: p4 -> forward_connected() to BFR2
p10 -> forward_connected() to BFR5
p14 -> local_decap()
BFR5: p6 -> forward_connected() to BFR3
p9 -> forward_connected() to BFR4
p12 -> forward_connected() to BFR6
BFR6: p11 -> forward_connected() to BFR5
p15 -> local_decap()
Figure 1: BIER-TE basic example
Consider the simple network in the above BIER-TE overview example
picture with 6 BFRs. p1...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.
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Assume a packet from BFR1 should be sent via BFR4 to BFR6. This
requires a BitString (p2,p8,p10,p12,p15). When this packet is
examined by BIER-TE on BFR1, the only bit position from the BitString
that is also set in the BIFT is p2. This will cause BFR1 to send the
only copy of the packet to BFR2. Similarly, BFR2 will forward to
BFR4 because of p8, BFR4 to BFR5 because of p10 and BFR5 to BFR6
because of p12. p15 finally makes BFR6 receive and decapsulate the
packet.
To send 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.
If instead the BitString was (p2,p6,p8,p10,p12,p13,p15), the packet
would be copied by BFR5 towards BFR3 because of p6 instead of being
copied by BFR2 to BFR3 because of p5 in the prior case. This is
showing the ability of the shown BIER-TE Topology to make the traffic
pass across any possible path and be replicated where desired.
BIER-TE has various options to minimize BP assignments, many of which
are based on out-of-band knowledge about the required multicast
traffic paths and bandwidth consumption in the network, such as from
pre-deployment planning.
Figure 2 shows a modified example, in which Rtr2 and Rtr5 are assumed
not to support BIER-TE, so traffic has to be unicast encapsulated
across them. To emphasize non-L2, but routed/tunneled forwarding of
BIER-TE packets, these adjacencies are called "forward_routed".
Otherwise, there is no difference in their processing over the
aforementioned forward_connected() adjacencies.
In addition, bits are saved in the following example by assuming that
BFR1 only needs to be BFIR but not BFER or transit BFR.
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BIER-TE Topology:
Diagram:
p1 p3 p7
....> BFR3 <.... p5
........ ........>
BFR1 (Rtr2) (Rtr5) BFR6
........ ........> 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()
Figure 2: BIER-TE basic overlay example
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).
2.2. BIER-TE Topology and adjacencies
The key new component in BIER-TE compared to (non-TE) BIER is the
BIER-TE topology as introduced through the two examples in
Section 2.1. It is used to control where replication can or should
happen and how to minimize the required number of BP for adjacencies.
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The BIER-TE Topology consists of the BIFTs of all the BFR and can
also be expressed as a directed graph where the edges are the
adjacencies between the 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 Section 5.2.
If the BIER-TE topology represents (a subset of) the underlying
(layer 2) topology of the network as shown in the first example, this
may be called a "native" BIER-TE topology. A topology consisting
only of "forward_routed" adjacencies as shown in the second example
may be called an "overlay" BIER-TE topology. A BIER-TE topology with
both forward_connected() and forward_routed() adjacencies may be
called a "hybrid" BIER-TE topology.
2.3. Relationship to BIER
BIER-TE is designed so that 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.
BIER-TE is also intended as an option to expand the BIER architecture
into deployments where (non-TE) BIER may not be the best fit, such as
statically provisioned networks with needs for path steering but
without desire for distributed routing protocols.
1. BIER-TE inherits the following aspects from BIER unchanged:
1. The fundamental purpose of per-packet signaled replication
and delivery via a BitString.
2. The overall architecture consisting of three layers, flow
overlay, BIER(-TE) layer and routing underlay.
3. The supported encapsulations [RFC8296].
4. The semantic of all [RFC8296] header elements used by the
BIER-TE forwarding plane other than the semantic of the BP in
the BitString.
5. The BIER forwarding plane, except for how bits have to be
cleared during replication.
2. BIER-TE has the following key changes with respect to BIER:
1. In BIER, bits in the BitString of a BIER packet header
indicate a BFER and bits in the BIFT indicate the BIER
control plane calculated next-hop toward that BFER. In BIER-
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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.
2. In BIER, the implied reference options 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, [RFC8401] and [RFC8444].
3. 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.
4. Assuming the reference options for the control plane, BIER-TE
replaces in-network autonomous path calculation by explicit
paths calculated by the BIER-TE controller.
3. The following elements/functions described in the BIER
architecture are not required by the BIER-TE architecture:
1. "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.
2. The BIER-TE layer forwarding plane does not require BFRs to
have a unique BP and therefore also no unique BFR-id. See
Section 5.1.3.
3. Identification of BFRs by the BIER-TE control plane is
outside the scope of this specification. Whereas the BIER
control plane uses BFR-ids in its BFR to BFR signaling, a
BIER-TE controller may choose any form of identification
deemed appropriate.
4. BIER-TE forwarding does not require the BFIR-id field of the
BIER packet header.
4. Co-existence of BIER and BIER-TE in the same network requires the
following:
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1. 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 Section 4.3. In either case, a packet is
always only forwarded end-to-end via BIER or via BIER-TE
(ships in the nights forwarding).
2. BIER-TE deployments will have to assign BFR-ids to BFRs and
insert them into the 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 Section 5.3.3.
2.4. Accelerated/Hardware forwarding comparison
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 Section 4.4 shows how existing (non-TE) BIER/BIFT
forwarding can be modified to support the required BIER-TE forwarding
functionality (Section 4.5), by using BIER BIFT's "Forwarding Bit
Mask" (F-BM): Only the clearing of bits to avoid duplicate packets to
a BFR's neighbor is skipped in BIER-TE forwarding because it is not
necessary and could not be done when using BIER F-BM.
Whether to use BIER or BIER-TE forwarding is simply a choice of the
mode of the BIFT indicated by the packet (BIER or BIER-TE BIFT).
This is determined by the BFR configuration for the encapsulation,
see Section 4.3.
3. Components
BIER-TE can be thought of being constituted from the same three
layers as BIER: The "multicast flow overlay", the "BIER layer" and
the "routing underlay". The following picture also shows how the
"BIER layer" is constituted from the "BIER-TE forwarding plane" and
the "BIER-TE control plane" represent by the "BIER-TE Controller".
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<------BGP/PIM----->
|<-IGMP/PIM-> multicast flow <-PIM/IGMP->|
overlay
BIER-TE [BIER-TE Controller] <=> [BIER-TE Topology]
control ^ ^ ^
plane / | \ BIER-TE control protocol
| | | e.g. YANG/NETCONF/RESTCONF
| | | PCEP/...
v v v
Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr
|<----------------->|
BIER-TE forwarding plane
|<- BIER-TE domain->|
|<--------------------->|
Routing underlay
Figure 3: BIER-TE architecture
3.1. The Multicast Flow Overlay
The Multicast Flow Overlay has the same role as described for BIER in
[RFC8279], Section 4.3. See also Section 3.2.1.2.
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" ([RFC8556]) this can be achieved by
making the BIER-TE controller operate as a BGP Route Reflector
([RFC4456]) and combining it with signaling through BGP or a
different protocol for the BIER-TE controller calculated BitStrings.
See Section 3.2.1.2 and Section 5.3.4.
3.2. The BIER-TE Control Plane
In the (non-TE) BIER architecture [RFC8279], the BIER control plane
is not explicitly separated from the BIER forwarding plane, but
instead their functions are summarized together in Section 4.2.
Example standardized options for the BIER control plane include ISIS/
OSPF extensions for BIER, [RFC8401] and [RFC8444].
For BIER-TE, the control plane includes at minimum the following
functionality.
1. BIER-TE topology control: During initial provisioning of the
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network and/or during modifications of its topology and/or
services, the protocols and/or procedures to establish BIER-TE
BIFTs:
1. Determine the desired BIER-TE topology for a BIER-TE sub-
domains: the native and/or overlay adjacencies that are
assigned to BPs. Topology discovery is discussed in
Section 3.2.1.1 and the various aspects of the BIER-TE
controllers determinations about the topology are discussed
throughout Section 5
2. 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.
3. 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 Section 5.3.3.
4. Install/update the BIFTs into the BFRs and optionally BFR-ids
into BFIRs. This is discussed in Section 3.2.1.1.
2. BIER-TE tree control: During operations of the network,
protocols and/or procedures to support creation/change/removal of
overlay flows on BFIRs:
1. Process the BIER-TE requirements for the multicast overlay
flow: BFIR and BFERs of the flow as well as policies for the
path selection of the flow. This is discussed in Section 3.5.
2. Determine the BitStrings and optionally Entropy. This is
discussed in Section 3.2.1.2, Section 3.5 and Section 5.3.4.
3. 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
Section 3.2.1 and in Section 4.3, but the main responsibility
of these two points is with the Multicast Flow Overlay
(Section 3.1), which is architecturally inherited from BIER.
4. Install the necessary state on the BFERs to decapsulate the
BIER packet header and properly dispatch its payload.
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3.2.1. The BIER-TE Controller
[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.]
This architecture describes the BIER-TE control plane as shown in
Figure 3 to consist of:
* A BIER-TE controller.
* BFR data-models and protocols to communicate between controller
and BFRs in support of BIER-TE topology control (Section 3.2),
such as YANG/NETCONF/RESTCONF ([RFC7950]/[RFC6241]/[RFC8040]).
* BFR data-models and protocols to communicate between controller
and BFIR in support of BIER-TE tree control (Section 3.2), such as
BIER-TE extensions for [RFC5440].
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.
3.2.1.1. BIER-TE Topology discovery and creation
The first item of BIER-TE topology control (Section 3.2, Paragraph 3,
Item 2.2.1) includes network topology discovery and BIER-TE topology
creation. The latter describes the process by which a Controller
determines which routers are to be configured as BFRs and the
adjacencies between them.
In statically managed networks, such as in industrial environments,
both discovery and creation can be a manual/offline process.
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In other networks, topology discovery may rely on protocols including
extending a "Link-State-Protocol" based IGP into the BIER-TE
controller itself, [RFC7752] (BGP-LS) or [RFC8345] (YANG topology) as
well as BIER-TE specific methods, for example via
[I-D.ietf-bier-te-yang]. These options are non-exhaustive.
Dynamic creation of the BIER-TE topology can be as easy as mapping
the network topology 1:1 to the BIER-TE topology by assigning a BP
for every network subnet adjacency. In larger networks, it likely
involves more complex policy and optimization decisions including how
to minimize the number of 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 Section 5.
When the BIER-TE topology is determined, the BIER-TE Controller then
pushes the BitPositions/adjacencies to the BIFT of the BFRs. On each
BFR only those SI:BitPositions are populated that are adjacencies to
other BFRs in the BIER-TE topology.
Communications between the BIER-TE Controller and BFRs 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).
3.2.1.2. Engineered Trees via BitStrings
In BIER, the same set of BFER in a single sub-domain is always
encoded as the same BitString. In BIER-TE, the BitString used to
reach the same set of BFER in the same sub-domain can be different
for different overlay flows because the BitString encodes the paths
towards the BFER, so the BitStrings from different BFIR to the same
set of BFER will often be different. Likewise, the BitString from
the same BFIR to the same set of BFER can be different for different
overlay flows for policy reasons such as shortest path trees, Steiner
trees (minimum cost trees), diverse path trees for redundancy and so
on.
See also [I-D.ietf-bier-multicast-http-response] for an application
leveraging BIER-TE engineered trees.
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3.2.1.3. Changes in the network topology
If the network topology changes (not failure based) so that
adjacencies that are assigned to bit positions are no longer needed,
the BIER-TE Controller can re-use those bit positions for new
adjacencies. First, these bit positions need to be removed from any
BFIR flow state and BFR BIFT state, then they can be repopulated,
first into BIFT and then into the BFIR.
3.2.1.4. Link/Node Failures and Recovery
When link or nodes fail or recover in the topology, BIER-TE could
quickly respond with FRR procedures such as [I-D.eckert-bier-te-frr],
the details of which are out of scope for this document. It can also
more slowly react by recalculating the BitStrings of affected
multicast flows. This reaction is slower than the FRR procedure
because the BIER-TE Controller needs to receive link/node up/down
indications, recalculate the desired BitStrings and push them down
into the BFIRs. With FRR, this is all performed locally on a BFR
receiving the adjacency up/down notification.
3.3. The BIER-TE Forwarding Plane
[RFC-editor Q: "is constituted from" / "consists of" / "composed
from..." ???]
The BIER-TE Forwarding Plane is constituted from the following
components:
1. 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 Section 3.1.
2. 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
Section 4. Processing of other BIER header fields such as DSCP
is outside the scope of this document.
3. 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.
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
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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 Section 4.2.1.
3.4. The Routing Underlay
For forward_connected() adjacencies, BIER-TE is sending BIER packets
to directly connected BIER-TE neighbors as L2 (unicasted) BIER
packets without requiring a routing underlay. For forward_routed()
adjacencies, BIER-TE forwarding encapsulates a copy of the BIER
packet so that it can be delivered by the forwarding plane of the
routing underlay to the routable destination address indicated in the
adjacency. See Section 4.2.2 for the adjacency definition.
BIER relies on the routing underlay to calculate paths towards BFERs
and derive next-hop BFR adjacencies for those paths. This commonly
relies on BIER specific extensions to the routing protocols of the
routing underlay but may also be established by a controller. In
BIER-TE, the next-hops of a packet are determined by the BitString
through the BIER-TE Controller established adjacencies on the BFR for
the BPs of the BitString. There is thus no need for BFR specific
routing underlay extensions to forward BIER packets with BIER-TE
semantics.
Encapsulation parameters can be provisioned by the BIER-TE controller
into the forward_connected() or forward_routed() adjacencies directly
without relying on a routing underlay.
If the BFR intends to support FRR for BIER-TE, then the BIER-TE
forwarding plane needs to receive fast adjacency up/down
notifications: Link up/down or neighbor up/down, e.g. from BFD.
Providing these notifications is considered to be part of the routing
underlay in this document.
3.5. Traffic Engineering Considerations
Traffic Engineering ([I-D.ietf-teas-rfc3272bis]) provides performance
optimization of operational IP networks while utilizing network
resources economically and reliably. The key elements needed to
effect TE are policy, path steering and resource management. These
elements require support at the control/controller level and within
the forwarding plane.
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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.
Path steering is supported via the definition of a BitString.
BitStrings used in BIER-TE are composed based on policy and resource
management considerations. For example, when composing BIER-TE
BitStrings, a Controller must take into account the resources
available at each BFR and for each BP when it is providing
congestion-loss-free services such as Rate Controlled Service
Disciplines [RCSD94]. Resource availability could be provided for
example via routing protocol information, but may also be obtained
via a BIER-TE control protocol such as NETCONF or any other protocol
commonly used by a Controller to understand the resources of the
network it operates on. The resource usage of the BIER-TE traffic
admitted by the BIER-TE controller can be solely tracked on the BIER-
TE Controller based on local accounting as long as no
forward_routed() adjacencies are used (see Section 4.2.1 for the
definition of forward_routed() adjacencies). When forward_routed()
adjacencies are used, the paths selected by the underlying routing
protocol need to be tracked as well.
Resource management has implications on the forwarding plane beyond
the BIER-TE defined steering of packets. This includes allocation of
buffers to guarantee the worst case requirements of admitted RCSD
traffic and potentially policing and/or rate-shaping mechanisms,
typically done via various forms of queuing. This level of resource
control, while optional, is important in networks that wish to
support congestion management policies to control or regulate the
offered traffic to deliver different levels of service and alleviate
congestion problems, or those networks that wish to control latencies
experienced by specific traffic flows.
4. BIER-TE Forwarding
4.1. The BIER-TE Bit Index Forwarding Table (BIFT)
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
Figure 4. That example BIFT assumes a BSL of 8 bit positions (BPs)
in the packets BitString. As in [RFC8279] this BSL is purely used
for the example and not a BIER/BIER-TE supported BSL (minimum BSL is
64).
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A BIER-TE BIFT compares to a BIER BIFT as shown in [RFC8279] as
follows.
In both BIER and BIER-TE, BIFT rows/entries are indexed in their
respective BIER pseudocode ([RFC8279] Section 6.5) and BIER-TE
pseudocode (Section 4.4) 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, [RFC8279] uses the equivalent notion
SI:BitString, where the BitString is filled with only the BP for the
BIFT row.
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.
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.
In BIER, each BIFT row 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 Section 4.4.
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------------------------------------------------------------------
| 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)
Figure 4: BIER-TE BIFT with different adjacencies
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.
Note that a BIFT index (SI:BP) may be populated in the BIFT of more
than one BFR to save BPs. See Section 5.1.6 for an example of how a
BIER-TE controller could assign BPs to (logical) adjacencies shared
across multiple BFRs, Section 5.1.3 for an example of assigning the
same BP to different adjacencies, and Section 5.1.9 for general
guidelines regarding re-use of BPs across different adjacencies.
{VRF} indicates the Virtual Routing and Forwarding context into which
the BIER payload is to be delivered. This is optional and depends on
the multicast flow overlay.
4.2. Adjacency Types
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4.2.1. Forward Connected
A "forward_connected()" adjacency is towards a directly connected BFR
neighbor using an interface address of that BFR on the connecting
interface. A forward_connected() adjacency does not route packets
but only L2 forwards them to the neighbor.
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 Section 5.1.6.
For protection against loops from misconfiguration (see
Section 5.2.1), 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.
4.2.2. Forward Routed
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 Section 4.3, or by explicit support
for a BIER-TE payload type in the tunneling encapsulation.
forward_routed() adjacencies are necessary to pass BIER-TE traffic
across non BIER-TE capable routers or to minimize the number of
required BP by tunneling over (BIER-TE capable) routers on which
neither replication nor path-steering is desired, or simply to
leverage path redundancy and FRR of the routing underlay towards the
next BFR. They may also be useful to a multi-subnet adjacent BFR to
leverage the routing underlay ECMP independent of BIER-TE ECMP
(Section 4.2.3).
4.2.3. ECMP
(non-TE) BIER ECMP is tied to the BIER BIFT processing semantic and
is therefore not directly usable with BIER-TE.
A BIER-TE "Equal Cost Multipath" (ECMP()) adjacency as shown in
Figure 4 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
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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 Section 5.1.7 for more explanations.
4.2.4. Local Decap(sulation)
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.
4.3. Encapsulation / Co-existence with BIER
Specifications for BIER-TE encapsulation are outside the scope of
this document. This section gives explanations and guidelines.
Like [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 [RFC8296], Section 3. It applies equally to BIER-TE as it
does to BIER.
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
[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.
When [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.
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.
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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 [RFC8296]
encapsulation in [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).
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).
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).
4.4. BIER-TE Forwarding Pseudocode
The following pseudocode, Figure 5, for BIER-TE forwarding is based
on the (non-TE) BIER forwarding pseudocode of [RFC8279], section 6.5
with one modification.
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]
}
}
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Figure 5: BIER-TE Forwarding Pseudocode for required functions,
based on BIER Pseudocode
In step [2], the F-BM is used to clear bit(s) in PacketCopy. This
step exists in both BIER and BIER-TE, but the F-BMs need to be
populated differently for BIER-TE than for BIER for the desired
clearing.
In BIER, multiple bits of a BitString can have the same BFR-NBR.
When a received packets BitString has more than one of those bits
set, the BIER replication logic has to avoid that more than one
PacketCopy is sent to that BFR-NBR ([1]). Likewise, the PacketCopy
sent to a BFR-NBR must clear all bits in its BitString that are not
routed across BFR-NBR. This protects against BIER replication on any
possible further BFR to create duplicates ([2]).
To solve both [1] and [2] for BIER, the F-BM of each bit index needs
to have all bits set that this BFR wants to route across BFR-NBR. [2]
clears all other bits in PacketCopy->BitString, and [1] clears those
bits from Packet->BitString after the first PacketCopy.
In BIER-TE, a BFR-NBR in this pseudocode is an adjacency,
forward_connected(), forward_routed() or local_decap(). There is no
need for [2] to suppress duplicates in the way BIER does because in
general, different BP would never have the same adjacency. If a
BIER-TE controller actually finds some optimization in which this
would be desirable, then the controller is also responsible to ensure
that only one of those bits is set in any Packet->BitString, unless
the controller explicitly wants for duplicates to be created.
The following points describe how the forwarding bit mask (F-BM) for
each BP is configured in the BIFT and how this impacts the BitString
of the packet being processed with that BIFT:
1. The F-BMs of all BIFT BPs without an adjacency have all their
bits clear. This will cause [3] to skip further processing of
such a BP.
2. All BIFT BPs with an adjacency (with DNC flag clear) have an F-BM
that has only those BPs set for which this BFR does not have an
adjacency. This causes [2] to clear all bits from
PacketCopy->BitString for which this BFR does have an adjacency.
3. [1] is not performed for BIER-TE. All bit clearing required by
BIER-TE is performed by [2].
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This Forwarding Pseudocode can support the required BIER-TE
forwarding functions (see Section 4.5), forward_connected(),
forward_routed() and local_decap(), but not the recommended functions
DNC flag and multiple adjacencies per bit nor the optional function,
ECMP() adjacencies. The DNC flag cannot be supported when using only
[1] to mask bits.
The modified and expanded Forwarding Pseudocode in Figure 6 specifies
how to support all BIER-TE forwarding functions (required,
recommended and optional):
* This pseudocode eliminates per-bit F-BM, therefore reducing the
size of BIFT state by BSL^2*SI and eliminating the need for per-
packet-copy BitString masking operations except for adjacencies
with the DNC flag set:
- AdjacentBits[SI] are bit positions with a non-empty list of
adjacencies in this BFR BIFT. This can be computed whenever
the BIER-TE Controller updates (add/removes) adjacencies in the
BIFT.
- The BFR needs to create packet copies for these adjacent bits
when they are set in the packets BitString. This set of bits
is calculated in PktAdjacentBits.
- All bit positions to which the BFR creates copies have to be
cleared in packet copies to avoid loops. This is done by
masking the BitString of the packet with ~AdjacentBits[SI].
When an adjacency has DNC set, this bit position is set again
only for the packet copy towards that bit position.
* BIFT entries may contain more than one adjacency in support of
specific configurations such as Section 5.1.5. The code therefore
includes a loop over these adjacencies.
* The ECMP() adjacency is shown. Its parameters are a seed and a
ListOfAdjacencies from which one is picked.
* The forward_connected(), forward_routed(), local_decap()
adjacencies are shown with their parameters.
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void ForwardBitMaskPacket_withTE (Packet)
{
SI = GetPacketSI(Packet);
Offset = SI * BitStringLength;
// Determine adjacent bits in the Packets BitString
PktAdjacentBits = Packet->BitString & AdjacentBits[SI];
// Clear adjacent bits in Packet header to avoid loops
Packet->BitString &= ~AdjacentBits[SI];
// Loop over PktAdjacentBits to create packet copies
for (Index = GetFirstBitPosition(PktAdjacentBits); Index ;
Index = GetNextBitPosition(PktAdjacentBits, Index)) {
for adjacency in BIFT[Index+Offset]->Adjacencies {
if(adjacency.type == ECMP(ListOfAdjacencies,seed) ) {
I = ECMP_hash(sizeof(ListOfAdjacencies),
Packet->Entropy,seed);
adjacency = ListOfAdjacencies[I];
}
PacketCopy = Copy(Packet);
switch(adjacency.type) {
case forward_connected(interface,neighbor,DNC):
if(DNC)
PacketCopy->BitString |= 1<<(Index-1);
SendToL2Unicast(PacketCopy,interface,neighbor);
case forward_routed({VRF,}l3-neighbor):
SendToL3(PacketCopy,{VRF,}l3-neighbor);
case local_decap({VRF},neighbor):
DecapBierHeader(PacketCopy);
PassTo(PacketCopy,{VRF,}Packet->NextProto);
}
}
}
}
Figure 6: Complete BIER-TE Forwarding Pseudocode for required,
recommended and optional functions
4.5. BFR Requirements for BIER-TE forwarding
BFR that support BIER-TE and BIER MUST support configuration that
enables BIER-TE instead of (non-TE) BIER forwarding rules for all
BIFT of one or more BIER sub-domains. Every BP in a BIER-TE BIFT
MUST support to have zero or one adjacency. BIER-TE forwarding MUST
support the adjacency types forward_connected() with the DNC flag not
set, forward_routed() and local_decap(). As explained in
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Section 4.4, these required BIER-TE forwarding functions can be
implemented via the same Forwarding Pseudocode as BIER forwarding
except for one modification (skipping one masking with F-BM).
BIER-TE forwarding SHOULD support forward_connected() adjacencies
with a set DNC flag, as this is highly useful to save bits in rings
(see Section 5.1.6).
BIER-TE forwarding SHOULD support more than one adjacency on a bit.
This allows to save bits in hub and spoke scenarios (see
Section 5.1.5).
BIER-TE forwarding MAY support ECMP() adjacencies to save bits in
ECMP scenarios, see Section 5.1.7 for an example. This is an
optional requirement, because for ECMP deployments using BIER-TE one
can also leverage ECMP of the routing underlay via forwarded_routed
adjacencies and/or might prefer to have more explicit control of the
path chosen via explicit BP/adjacencies for each ECMP path
alternative.
5. BIER-TE Controller Operational Considerations
5.1. Bit Position Assignments
This section describes how the BIER-TE Controller can use the
different BIER-TE adjacency types to define the bit positions of a
BIER-TE domain.
Because the size of the BitString limits the size of the BIER-TE
domain, many of the options described exist to support larger
topologies with fewer bit positions.
5.1.1. P2P Links
On a P2P link that connects two BFRs, the same bit position can be
used on both BFRs for the adjacency to the neighboring BFR. A P2P
link requires therefore only one bit position.
5.1.2. BFER
Every non-Leaf BFER is given a unique bit position with a
local_decap() adjacency.
5.1.3. Leaf BFERs
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BFR1(P) BFR2(P) BFR1(P) BFR2(P)
| \ / | | |
| X | | |
| / \ | | |
BFER1(PE) BFER2(PE) BFER1(PE)----BFER2(PE)
^ U-turn link
Leaf BFER / Non-Leaf BFER /
PE-router PE-router
Figure 7: Leaf vs. non-Leaf BFER Example
A leaf BFER is one where incoming BIER-TE packets never need to be
forwarded to another BFR but are only sent to the BFER to exit the
BIER-TE domain. For example, in networks where Provider Edge (PE)
router are spokes connected to Provider (P) routers, those PEs are
Leaf BFERs unless there is a U-turn between two PEs.
Consider how redundant disjoint traffic can reach BFER1/BFER2 in
Figure 7: When BFER1/BFER2 are Non-Leaf BFER as shown on the right-
hand side, one traffic copy would be forwarded to BFER1 from BFR1,
but the other one could only reach BFER1 via BFER2, which makes BFER2
a non-Leaf BFER. Likewise, BFER1 is a non-Leaf BFER when forwarding
traffic to BFER2. Note that the BFERs in the left-hand picture are
only guaranteed to be leaf-BFER by fitting routing configuration that
prohibits transit traffic to pass through the BFERs, which is
commonly applied in these topologies.
In most situations, leaf-BFER that are to be addressed via the same
BitString can share a single bit position for their local_decap()
adjacency in that BitString and therefore save bit positions. On a
non-leaf BFER, a received BIER-TE packet may only need to transit the
BFER or it may need to also be decapsulated. Whether or not to
decapsulate the packet therefore needs to be indicated by a unique
bit position populated only on the BIFT of this BFER with a
local_decap() adjacency. On a leaf-BFER, packets never need to pass
through; any packet received is therefore usually intended to be
decapsulated. This can be expressed by a single, shared bit position
that is populated with a local_decap() adjacency on all leaf-BFER
addressed by the BitString.
The possible exception from this leaf-BFER bit position optimization
can be cases where the bit position on the prior BIER-TE BFR (which
created the packet copy for the leaf-BFER in question) is populated
with multiple adjacencies as an optimization, such as in
Section 5.1.4 or Section 5.1.5. With either of these two
optimizations, the sender of the packet could only control explicitly
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whether the packet was to be decapsulated on the leaf-BFER in
question, if the leaf-BFER has a unique bit position for its
local_decap() adjacency.
However, if the bit position is shared across leaf-BFER, and packets
are therefore decapsulated potentially unnecessarily, this may still
be appropriate if the decapsulated payload of the BIER-TE packet
indicates whether or not the packet needs to be further processed/
received. This is typically true for example if the payload is IP
multicast because IP multicast on a BFER would know the membership
state of the IP multicast payload and be able to discard it if the
packet was delivered unnecessarily by the BIER-TE layer. If the
payload has no such membership indication, and the BFIR wants to have
explicit control about which BFER are to receive and decapsulate a
packet, then these two optimizations can not be used together with
shared bit positions optimization for leaf-BFER.
5.1.4. LANs
In a LAN, the adjacency to each neighboring BFR is given a unique bit
position. The adjacency of this bit position is a
forward_connected() adjacency towards the BFR and this bit position
is populated into the BIFT of all the other BFRs on that LAN.
BFR1
|p1
LAN1-+-+---+-----+
p3| p4| p2|
BFR3 BFR4 BFR7
Figure 8: LAN Example
If Bandwidth on the LAN is not an issue and most BIER-TE traffic
should be copied to all neighbors on a LAN, then bit positions can be
saved by assigning just a single bit position to the LAN and
populating the bit position of the BIFTs of each BFRs on the LAN with
a list of forward_connected() adjacencies to all other neighbors on
the LAN.
This optimization does not work in the case of BFRs redundantly
connected to more than one LAN with this optimization because these
BFRs would receive duplicates and forward those duplicates into the
opposite LANs. Adjacencies of such BFRs into their LAN still need a
separate bit position.
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5.1.5. Hub and Spoke
In a setup with a hub and multiple spokes connected via separate p2p
links to the hub, all p2p adjacencies from the hub to the spokes
links can share the same bit position. The bit position on the hub's
BIFT is set up with a list of forward_connected() adjacencies, one
for each Spoke.
This option is similar to the bit position optimization in LANs:
Redundantly connected spokes need their own bit positions, unless
they are themselves Leaf-BFER.
This type of optimized BP could be used for example when all traffic
is "broadcast" traffic (very dense receiver set) such as live-TV or
many-to-many telemetry including situation-awareness (SA). This BP
optimization can then be used to explicitly steer different traffic
flows across different ECMP paths in Data-Center or broadband-
aggregation networks with minimal use of BPs.
5.1.6. Rings
In L3 rings, instead of assigning a single bit position for every p2p
link in the ring, it is possible to save bit positions by setting the
"DoNotClear" (DNC) flag on forward_connected() adjacencies.
For the rings shown in Figure 9, a single bit position will suffice
to forward traffic entering the ring at BFRa or BFRb all the way up
to BFR1:
On BFRa, BFRb, BFR30,... BFR3, the bit position is populated with a
forward_connected() adjacency pointing to the clockwise neighbor on
the ring and with DNC set. On BFR2, the adjacency also points to the
clockwise neighbor BFR1, but without DNC set.
Handling DNC this way ensures that copies forwarded from any BFR in
the ring to a BFR outside the ring will not have the ring bit
position set, therefore minimizing the chance to create loops.
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
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Figure 9: Ring Example
Note that this example only permits for packets intended to make it
all the way around the ring to enter it at BFRa and BFRb, and that
packets will always travel clockwise. If packets should be allowed
to enter the ring at any ring BFR, then one would have to use two
ring bit positions. One for each direction: clockwise and
counterclockwise.
Both would be set up to stop rotating on the same link, e.g. L1.
When the ingress ring BFR creates the clockwise copy, it will clear
the counterclockwise bit position because the DNC bit only applies to
the bit for which the replication is done. Likewise for the
clockwise bit position for the counterclockwise copy. As a result,
the ring ingress BFR will send a copy in both directions, serving
BFRs on either side of the ring up to L1.
5.1.7. Equal Cost MultiPath (ECMP)
[RFC-Editor: A reviewer (Lars Eggert) noted that the infinite "to
use" in the following sentence is not correct. The same was also
noted for several other similar instances. The following URL seems
to indicate though that this is a per-case decision, which seems
undefined: https://writingcenter.gmu.edu/guides/choosing-between-
infinitive-and-gerund-to-do-or-doing. What exactly should be done
about this ?].
An ECMP() adjacency allows to use just one BP to deliver packets to
one of N adjacencies instead of one BP for each adjacency. In the
common example case Figure 10, a link-bundle of three links L1,L2,L3
connects BFR1 and BFR2, and only one BP is used instead of three BP
to deliver packets from BFR1 to BFR2.
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--L1-----
BFR1 --L2----- BFR2
--L3-----
BIFT entry in BFR1:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({forward_connected(L1, BFR2), |
| | forward_connected(L2, BFR2), |
| | forward_connected(L3, BFR2)}, seed) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| Index | Adjacencies |
==================================================================
| 0:6 | ECMP({forward_connected(L1, BFR1), |
| | forward_connected(L2, BFR1), |
| | forward_connected(L3, BFR1)}, seed) |
------------------------------------------------------------------
Figure 10: ECMP Example
This document does not standardize any ECMP algorithm because it is
sufficient for implementations to document their freely chosen ECMP
algorithm. Figure 11 shows an example ECMP algorithm, and would
double as its documentation: A BIER-TE controller could determine
which adjacency is chosen based on the seed and adjacencies
parameters and the packet entropy.
forward(packet, ECMP(adj(0), adj(1),... adj(N-1), seed)):
i = (packet(bier-header-entropy) XOR seed) % N
forward packet to adj(i)
Figure 11: ECMP algorithm Example
In the following example, all traffic from BFR1 towards BFR10 is
intended to be ECMP load split equally across the topology. This
example is not meant as a likely setup, but to illustrate that ECMP
can be used to share BPs not only across link bundles, but also
across alternative paths across different transit BFR, and it
explains the use of the seed parameter.
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BFR1 (BFIR)
/L11 \L12
/ \
BFR2 BFR3
/L21 \L22 /L31 \L32
/ \ / \
BFR4 BFR5 BFR6 BFR7
\ / \ /
\ / \ /
BFR8 BFR9
\ /
\ /
BFR10 (BFER)
BIFT entry in BFR1:
------------------------------------------------------------------
| 0:6 | ECMP({forward_connected(L11, BFR2), |
| | forward_connected(L12, BFR3)}, seed1) |
------------------------------------------------------------------
BIFT entry in BFR2:
------------------------------------------------------------------
| 0:7 | ECMP({forward_connected(L21, BFR4), |
| | forward_connected(L22, BFR5)}, seed1) |
------------------------------------------------------------------
BIFT entry in BFR3:
------------------------------------------------------------------
| 0:7 | ECMP({forward_connected(L31, BFR6), |
| | forward_connected(L32, BFR7)}, seed1) |
------------------------------------------------------------------
BIFT entry in BFR4, BFR5:
------------------------------------------------------------------
| 0:8 | forward_connected(Lxx, BFR8) |xx differs on BFR4/BFR5|
------------------------------------------------------------------
BIFT entry in BFR6, BFR7:
------------------------------------------------------------------
| 0:8 | forward_connected(Lxx, BFR9) |xx differs on BFR6/BFR7|
------------------------------------------------------------------
BIFT entry in BFR8, BFR9:
------------------------------------------------------------------
| 0:9 | forward_connected(Lxx, BFR10) |xx differs on BFR8/BFR9|
------------------------------------------------------------------
Figure 12: Polarization Example
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Note that for the following discussion of ECMP, only the BIFT ECMP
adjacencies on BFR1, BFR2, BFR3 are relevant. The re-use of BP
across BFR in this example is further explained in Section 5.1.9
below.
With the setup of ECMP in the topology above, traffic would not be
equally load-split. Instead, links L22 and L31 would see no traffic
at all: BFR2 will only see traffic from BFR1 for which the ECMP hash
in BFR1 selected the first adjacency in the list of 2 adjacencies
given as parameters to the ECMP. It is link L11-to-BFR2. BFR2
performs again ECMP with two adjacencies on that subset of traffic
using the same seed1, and will therefore again select the first of
its two adjacencies: L21-to-BFR4. And therefore L22 and BFR5 sees no
traffic. Likewise for L31 and BFR6.
This issue in BFR2/BFR3 is called polarization. It results from the
re-use of the same hash function across multiple consecutive hops in
topologies like these. To resolve this issue, the ECMP() adjacency
on BFR1 can be set up with a different seed2 than the ECMP()
adjacencies on BFR2/BFR3. BFR2/BFR3 can use the same hash because
packets will not sequentially pass across both of them. Therefore,
they can also use the same BP 0:7.
Note that ECMP solutions outside of BIER often hide the seed by auto-
selecting it from local entropy such as unique local or next-hop
identifiers. Allowing the BIER-TE Controller to explicitly set the
seed gives the ability for it to control same/different path
selection across multiple consecutive ECMP hops.
5.1.8. Forward Routed adjacencies
5.1.8.1. Reducing bit positions
Forward_routed() adjacencies can reduce the number of bit positions
required when the path steering requirement is not hop-by-hop
explicit path selection, but loose-hop selection. Forward_routed()
adjacencies can also allow to operate BIER-TE across intermediate hop
routers that do not support BIER-TE.
...............
...BFR1--... ...--L1-- BFR2...
... .Routers. ...--L2--/
...BFR4--... ...--L3-- BFR3...
... ...--L4--/ |
............... |
LO
Network Area 1
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Figure 13: Forward Routed Adjacencies Example
Assume the requirement in Figure 13 is to explicitly steer traffic
flows that have arrived at BFR1 or BFR4 via a path in the routing
underlay "Network Area 1" to one of the following three next
segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via BFR3
and then nor caring whether the packet is forwarded via L3 or L4.
To enable this, both BFR1 and BFR4 are set up with a forward_routed
adjacency bit position towards an address of BFR2 on link L1, another
forward_routed() bit position towards an address of BFR2 on link L2
and a third forward_routed() bit position towards a node address LO
of BFR3.
5.1.8.2. Supporting nodes without BIER-TE
Forward_routed() adjacencies also enable incremental deployment of
BIER-TE. Only the nodes through which BIER-TE traffic needs to be
steered - with or without replication - need to support BIER-TE.
Where they are not directly connected to each other, forward_routed
adjacencies are used to pass over non BIER-TE enabled nodes.
5.1.9. Reuse of bit positions (without DNC)
Bit positions can be re-used across multiple BFRs to minimize the
number of BP needed. This happens when adjacencies on multiple BFRs
use the DNC flag as described above, but it can also be done for non-
DNC adjacencies. This section only discusses this non-DNC case.
Because BP are cleared when passing a BFR with an adjacency for that
BP, reuse of BP across multiple BFRs does not introduce any problems
with duplicates or loops that do not also exist when every adjacency
has a unique BP. Instead, the challenge when reusing BP is whether
it allows to still achieve the desired Tree Engineering goals.
BP cannot be reused across two BFRs that would need to be passed
sequentially for some path: The first BFR will clear the BP, so those
paths cannot be built. BP can be set across BFR that would (A) only
occur across different paths or (B) across different branches of the
same tree.
An example of (A) was given in Figure 12, where BP 0:7, BP 0:8 and BP
0:9 are each reused across multiple BFRs because a single packet/path
would never be able to reach more than one BFR sharing the same BP.
Assume the example was changed: BFR1 has no ECMP() adjacency for BP
0:6, but instead BP 0:5 with forward_connected() to BFR2 and BP 0:6
with forward_connected() to BFR3. Packets with both BP 0:5 and BP
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0:6 would now be able to reach both BFR2 and BFR3 and the still
existing re-use of BP 0:7 between BFR2 and BFR3 is a case of (B)
where reuse of BP is perfect because it does not limit the set of
useful path choices:
If instead of reusing BP 0:7, BFR3 used a separate BP 0:10 for its
ECMP() adjacency, no useful additional path steering options would be
enabled. If duplicates at BFR10 where undesirable, this would be
done by not setting BP 0:5 and BP 0:6 for the same packet. If the
duplicates where desirable (e.g.: resilient transmission), the
additional BP 0:10 would also not render additional value.
area1
BFR1a BFR1b
/ \
....................................
. Core .
....................................
| / \ / \ |
BFR2a BFR2b BFR3a BFR3b BFR6a BFR6b
/-------\ /---------\ /--------\
| area2 | | area3 | ... | area6 |
| ring | | ring | | ring |
\-------/ \---------/ \--------/
more BFR more BFR more BFR
Figure 14: Reuse of BP
Reuse may also save BPs in larger topologies. Consider the topology
shown in Figure 14. A BFIR/sender (e.g.: video headend) is attached
to area 1, and area 2...6 contain receivers/BFER. Assume each area
had a distribution ring, each with two BPs to indicate the direction
(as explained before). These two BPs could be reused across the 5
areas. Packets would be replicated through other BPs for the Core to
the desired subset of areas, and once a packet copy reaches the ring
of the area, the two ring BPs come into play. This reuse is a case
of (B), but it limits the topology choices: Packets can only flow
around the same direction in the rings of all areas. This may or may
not be acceptable based on the desired path steering options: If
resilient transmission is the path engineering goal, then it is
likely a good optimization, if the bandwidth of each ring was to be
optimized separately, it would not be a good limitation.
5.1.10. Summary of BP optimizations
This section reviewed a range of techniques by which a BIER-TE
Controller can create a BIER-TE topology in a way that minimizes the
number of necessary BPs.
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Without any optimization, a BIER-TE Controller would attempt to map
the network subnet topology 1:1 into the BIER-TE topology and every
subnet adjacent neighbor requires a forward_connected() BP and every
BFER requires a local_decap() BP.
The optimizations described are then as follows:
* P2P links require only one BP (Section 5.1.1).
* All leaf-BFER can share a single local_decap() BP (Section 5.1.3).
* A LAN with N BFR needs at most N BP (one for each BFR). It only
needs one BP for all those BFR that are not redundantly connected
to multiple LANs (Section 5.1.4).
* A hub with p2p connections to multiple non-leaf-BFER spokes can
share one BP to all spokes if traffic can be flooded to all
spokes, e.g.: because of no bandwidth concerns or dense receiver
sets (Section 5.1.5).
* Rings of BFR can be built with just two BP (one for each
direction) except for BFR with multiple ring connections - similar
to LANs (Section 5.1.6).
* ECMP() adjacencies to N neighbors can replace N BP with 1 BP.
Multihop ECMP can avoid polarization through different seeds of
the ECMP algorithm (Section 5.1.7).
* Forward_routed() adjacencies allow to "tunnel" across non-BIER-TE
capable routers and across BIER-TE capable routers where no
traffic-steering or replications are required (Section 5.1.8).
* BP can generally be reused across a set of nodes where it can be
guaranteed that no path will ever need to traverse more than one
node of the set. Depending on scenario, this may limit the
feasible path steering options (Section 5.1.9).
Note that the described list of optimizations is not exhaustive.
Especially when the set of required path steering choices is limited
and the set of possible subsets of BFERs that should be able to
receive traffic is limited, further optimizations of BP are possible.
The hub and spoke optimization is a simple example of such traffic
pattern dependent optimizations.
5.2. Avoiding duplicates and loops
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5.2.1. Loops
Whenever BIER-TE creates a copy of a packet, the BitString of that
copy will have all bit positions cleared that are associated with
adjacencies on the BFR. This inhibits looping of packets. The only
exception are adjacencies with DNC set.
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb ---------------------\
| . |
| ...... Wrong link wiring |
| . |
\- BFR1 - BFR2 BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
Figure 15: Miswired Ring Example
With DNC set, looping can happen. Consider in Figure 15 that link L4
from BFR3 is (inadvertently) plugged into the L1 interface of BFRa
(instead of BFR2). This creates a loop where the rings clockwise bit
position is never cleared for copies of the packets traveling
clockwise around the ring.
To inhibit looping in the face of such physical misconfiguration,
only forward_connected() adjacencies are permitted to have DNC set,
and the link layer port unique unicast destination address of the
adjacency (e.g. MAC address) protects against closing the loop.
Link layers without port unique link layer addresses should not be
used with the DNC flag set.
5.2.2. Duplicates
BFIR1
/ \
/ p2 \ p3
BFR2 BFR3
\ p4 / p5
\ /
BFER4
Figure 16: Duplicates Example
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Duplicates happen when the graph expressed by a BitString is not a
tree but redundantly connecting BFRs with each other. In Figure 16,
a BitString of p2,p3,p4,p5 would result in duplicate packets to
arrive on BFER4. The BIER-TE Controller must therefore ensure to
only create BitStrings that are trees.
When links are incorrectly physically re-connected before the BIER-TE
Controller updates BitStrings in BFIRs, duplicates can happen. Like
loops, these can be inhibited by link layer addressing in
forward_connected() adjacencies.
If interface or loopback addresses used in forward_routed()
adjacencies are moved from one BFR to another, duplicates can equally
happen. Such re-addressing operations must be coordinated with the
BIER-TE Controller.
5.3. Managing SI, sub-domains and BFR-ids
When the number of bits required to represent the necessary hops in
the topology and BFER exceeds the supported BitStringLength (BSL),
multiple SIs and/or sub-domains must be used. This section discusses
how.
BIER-TE forwarding does not require the concept of BFR-id, but
routing underlay, flow overlay and BIER headers may. This section
also discusses how BFR-ids can be assigned to BFIR/BFER for BIER-TE.
5.3.1. Why SI and sub-domains
For (non-TE) BIER and BIER-TE forwarding, the most important result
of using multiple SI and/or sub-domains is the same: Packets that
need to be sent to BFERs in different SIs or sub-domains require
different BIER packets: each one with a BitString for a different
(SI,sub-domain) combination. Each such BitString uses one BSL sized
SI block in the BIFT of the sub-domain. We call this a BIFT:SI
(block).
For BIER and BIER-TE forwarding themselves there is also no
difference whether different SIs and/or sub-domains are chosen, but
SI and sub-domain have different purposes in the BIER architecture
shared by BIER-TE. This impacts how operators are managing them and
how especially flow overlays will likely use them.
By default, every possible BFIR/BFER in a BIER network would likely
be given a BFR-id in sub-domain 0 (unless there are > 64k BFIR/BFER).
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If there are different flow services (or service instances) requiring
replication to different subsets of BFERs, then it will likely not be
possible to achieve the best replication efficiency for all of these
service instances via sub-domain 0. Ideal replication efficiency for
N BFER exists in a sub-domain if they are split over not more than
ceiling(N/BitStringLength) SI.
If service instances justify additional BIER:SI state in the network,
additional sub-domains will be used: BFIR/BFER are assigned BFR-id in
those sub-domains and each service instance is configured to use the
most appropriate sub-domain. This results in improved replication
efficiency for different services.
Even if creation of sub-domains and assignment of BFR-id to BFIR/BFER
in those sub-domains is automated, it is not expected that individual
service instances can deal with BFER in different sub-domains. A
service instance may only support configuration of a single sub-
domain it should rely on.
To be able to easily reuse (and modify as little as possible)
existing BIER procedures including flow-overlay and routing underlay,
when BIER-TE forwarding is added, we therefore reuse SI and sub-
domain logically in the same way as they are used in BIER: All
necessary BFIR/BFER for a service use a single BIER-TE BIFT and are
split across as many SIs as necessary (see Section 5.3.2). Different
services may use different sub-domains that primarily exist to
provide more efficient replication (and for BIER-TE desirable path
steering) for different subsets of BFIR/BFER.
5.3.2. Assigning bits for the BIER-TE topology
In BIER, BitStrings only need to carry bits for BFERs, which leads to
the model that BFR-ids map 1:1 to each bit in a BitString.
In BIER-TE, BitStrings need to carry bits to indicate not only the
receiving BFER but also the intermediate hops/links across which the
packet must be sent. The maximum number of BFER that can be
supported in a single BitString or BIFT:SI depends on the number of
bits necessary to represent the desired topology between them.
"Desired" topology because it depends on the physical topology, and
on the desire of the operator to allow for explicit path steering
across every single hop (which requires more bits), or reducing the
number of required bits by exploiting optimizations such as unicast
(forward_routed()), ECMP() or flood (DNC) over "uninteresting" sub-
parts of the topology - e.g. parts where different trees do not need
to take different paths due to path steering reasons.
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The total number of bits to describe the topology vs. the number of
BFERs in a BIFT:SI can range widely based on the size of the topology
and the amount of alternative paths in it. In a BIER-TE topology
crafted by a BIER-TE expert, the higher the percentage of non-BFER
bits, the higher the likelihood, that those topology bits are not
just BIER-TE overhead without additional benefit, but instead that
they will allow to express desirable path steering alternatives.
5.3.3. Assigning BFR-id with BIER-TE
BIER-TE forwarding does not use the BFR-id, nor does it require for
the BFIR-id field of the BIER header to be set to a particular value.
However, other parts of a BIER-TE deployment may need a BFR-id,
specifically multicast flow overlay signaling and multicast flow
overlay packet disposition, and in that case BFRs need to also have
BFR-ids for BIER-TE SDs.
For example, for BIER overlay signaling, BFIRs need to have a BFR-id,
because this BFIR BFR-id is carried in the BFIR-id field of the BIER
header to indicate to the overlay signaling on the receiving BFER
which BFIR originated the packet.
In BIER, BFR-id = SI * BSL + BP, such that the SI and BP of a BFER
can be calculated from the BFR-id and vice versa. This also means
that every BFR with a BFR-id has a reserved BP in an SI, even if that
is not necessary for BIER forwarding, because the BFR may never be a
BFER but only a BFIR.
In BIER-TE, for a non-leaf BFER, there is usually a single BP for
that BFER with a local_decap() adjacency on the BFER. The BFR-id for
such a BFER can therefore be determined using the same procedure as
in (non-TE) BIER: BFR-id = SI * BSL + BP.
As explained in Section 5.1.3, leaf BFERs do not need such a unique
local_decap() adjacency. Likewise, BFIRs that are not also BFERs may
not have a unique local_decap() adjacency either. For all those
BFIRs and (leaf) BFERs, the controller needs to determine unique BFR-
ids that do not collide with the BFR-ids derived from the non-leaf
BFER local_decap() BPs.
While this document defines no requirements on how to allocate such
BFR-id, a simple option is to derive it from the (SI,BP) of an
adjacency that is unique to the BFR in question. For a BFIR this can
be the first adjacency only populated on this BFIR, for a leaf-BFER,
this could be the first BP with an adjacency towards that BFER.
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5.3.4. Mapping from BFR to BitStrings with BIER-TE
In BIER, applications of the flow overlay on a BFIR can calculate the
(SI,BP) of a BFER from the BFR-id of the BFER and can therefore
easily determine the BitStrings for a BIER packet to a set of BFERs
with known BFR-ids.
In BIER-TE this mapping needs to be equally supported for flow
overlays. This section outlines two core options, based on what type
of Tree Engineering the BIER-TE controller needs to performs for a
particular application.
"Independent branches": For a given flow overlay instance, the
branches from a BFIR to every BFER are calculated by the BIER-TE
controller to be independent of the branches to any other BFER.
Shortest path trees are the most common examples of trees with
independent branches.
"Interdependent branches": When a BFER is added or deleted from a
particular distribution tree, the BIER-TE controller has to
recalculate the branches to other BFER, because they may need to
change. Steiner trees are examples of interdependent branch trees.
If "independent branches" are used, the BIER-TE Controller can signal
to the BFIR flow overlay for every BFER an SI:BitString that
represents the branch to that BFER. The flow overlay on the BIFR can
then independently of the controller calculate the SI:BitString for
all desired BFERs by OR'ing their BitStrings. This allows for flow
overlay applications to operate independently of the controller
whenever it needs to determine which subset of BFERs need to receive
a particular packet.
If "interdependent branches" are required, the application would need
to inquire the SI:BitString for a given set of BFER whenever the set
changes.
Note that in either case (unlike in BIER), the bits may need to
change upon link/node failure/recovery, network expansion and network
resource consumption by other traffic as part of traffic engineering
goals (e.g.: re-optimization of lower priority traffic flows).
Interactions between such BFIR applications and the BIER-TE
Controller do therefore need to support dynamic updates to the
SI:BitStrings.
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Communications between the BFIR flow overlay and the BIER-TE
controller requires some way to identify the BFER. If BFR-ids are
used in the deployment, as outlined in Section 5.3.3, then those are
the natural BFR identifier. If BFR-ids are not used, then any other
unique identifier, such as the BFR-prefix of the BFR ([RFC8279])
could be used.
5.3.5. Assigning BFR-ids for BIER-TE
It is not currently determined if a single sub-domain could or should
be allowed to forward both (non-TE) BIER and BIER-TE packets. If
this should be supported, there are two options:
A. BIER and BIER-TE have different BFR-id in the same sub-domain.
This allows higher replication efficiency for BIER because their BFR-
id can be assigned sequentially, while the BitStrings for BIER-TE
will have also the additional bits for the topology. There is no
relationship between a BFR BIER BFR-id and its BIER-TE BFR-id.
B. BIER and BIER-TE share the same BFR-id. The BFR-ids are assigned
as explained above for BIER-TE and simply reused for BIER. The
replication efficiency for BIER will be as low as that for BIER-TE in
this approach.
5.3.6. Example bit allocations
5.3.6.1. With BIER
Consider a network setup with a BSL of 256 for a network topology as
shown in Figure 17. The network has 6 areas, each with 170 BFERs,
connecting via a core with 4 (core) BFRs. To address all BFERs with
BIER, 4 SIs are required. To send a BIER packet to all BFER in the
network, 4 copies need to be sent by the BFIR. On the BFIR it does
not make a difference how the BFR-ids are allocated to BFER in the
network, but for efficiency further down in the network it does make
a difference.
area1 area2 area3
BFR1a BFR1b BFR2a BFR2b BFR3a BFR3b
| \ / \ / |
................................
. Core .
................................
| / \ / \ |
BFR4a BFR4b BFR5a BFR5b BFR6a BFR6b
area4 area5 area6
Figure 17: Scaling BIER-TE bits by reuse
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With random allocation of BFR-id to BFER, each receiving area would
(most likely) have to receive all 4 copies of the BIER packet because
there would be BFR-id for each of the 4 SIs in each of the areas.
Only further towards each BFER would this duplication subside - when
each of the 4 trees runs out of branches.
If BFR-ids are allocated intelligently, then all the BFER in an area
would be given BFR-id with as few as possible different SIs. Each
area would only have to forward one or two packets instead of 4.
Given how networks can grow over time, replication efficiency in an
area will then also go down over time when BFR-ids are only allocated
sequentially, network wide. An area that initially only has BFR-id
in one SI might end up with many SIs over a longer period of growth.
Allocating SIs to areas with initially sufficiently many spare bits
for growths can help to alleviate this issue. Or renumber BFERs
after network expansion. In this example one may consider to use 6
SIs and assign one to each area.
This example shows that intelligent BFR-id allocation within at least
sub-domain 0 can even be helpful or even necessary in BIER.
5.3.6.2. With BIER-TE
In BIER-TE one needs to determine a subset of the physical topology
and attached BFERs so that the "desired" representation of this
topology and the BFER fit into a single BitString. This process
needs to be repeated until the whole topology is covered.
Once bits/SIs are assigned to topology and BFERs, BFR-id is just a
derived set of identifiers from the operator/BIER-TE Controller as
explained above.
Every time that different sub-topologies have overlap, bits need to
be repeated across the BitStrings, increasing the overall amount of
bits required across all BitString/SIs. In the worst case, one
assigns random subsets of BFERs to different SIs. This will result
in an outcome much worse than in (non-TE) BIER: It maximizes the
amount of unnecessary topology overlap across SI and therefore
reduces the number of BFER that can be reached across each individual
SI. Intelligent BFER to SI assignment and selecting specific
"desired" subtopologies can minimize this problem.
To set up BIER-TE efficiently for the topology of Figure 17, the
following bit allocation method can be used. This method can easily
be expanded to other, similarly structured larger topologies.
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Each area is allocated one or more SIs depending on the number of
future expected BFERs and number of bits required for the topology in
the area. In this example, 6 SIs, one per area.
In addition, we use 4 bits in each SI: bia, bib, bea, beb: (b)it
(i)ngress (a), (b)it (i)ngress (b), (b)it (e)gress (a), (b)it
(e)gress (b). These bits will be used to pass BIER packets from any
BFIR via any combination of ingress area a/b BFR and egress area a/b
BFR into a specific target area. These bits are then set up with the
right forward_routed() adjacencies on the BFIR and area edge BFR:
On all BFIRs in an area j|j=1...6, bia in each BIFT:SI is populated
with the same forward_routed(BFRja), and bib with
forward_routed(BFRjb). On all area edge BFR, bea in
BIFT:SI=k|k=1...6 is populated with forward_routed(BFRka) and beb in
BIFT:SI=k with forward_routed(BFRkb).
For BIER-TE forwarding of a packet to a subset of BFERs across all
areas, a BFIR would create at most 6 copies, with SI=1...SI=6, In
each packet, the bits indicate bits for topology and BFER in that
topology plus the four bits to indicate whether to pass this packet
via the ingress area a or b border BFR and the egress area a or b
border BFR, therefore allowing path steering for those two "unicast"
legs: 1) BFIR to ingress area edge and 2) core to egress area edge.
Replication only happens inside the egress areas. For BFER in the
same area as in the BFIR, these four bits are not used.
5.3.7. Summary
BIER-TE can, like BIER, support multiple SIs within a sub-domain.
This allows to apply the mapping BFR-id = SI * BSL + BP. This allows
to re-use the BIER architecture concept of BFR-id and therefore
minimize BIER-TE specific functions in possible BIER layer control
plane mechanisms with BIER-TE, including flow overlay methods and
BIER header fields.
The number of BFIR/BFER possible in a sub-domain is smaller than in
BIER because BIER-TE uses additional bits for topology.
Sub-domains (SDs) in BIER-TE can be used like in BIER to create more
efficient replication to known subsets of BFERs.
Assigning bits for BFERs intelligently into the right SI is more
important in BIER-TE than in BIER because of replication efficiency
and overall amount of bits required.
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6. Security Considerations
If [RFC8296] is used, BIER-TE shares its security considerations.
BIER-TE shares the security considerations of BIER, [RFC8279], with
the following overriding or additional considerations.
BIER-TE forwarding explicitly supports unicast "tunneling" of BIER
packets via forward_routed() adjacencies. The BIER domain security
model is based on a subset of interfaces on a BFR that connect to
other BFRs of the same BIER domain. For BIER-TE, this security model
equally applies to such unicast "tunneled" BIER packets. This does
not only include the need to filter received unicast "tunneled" BIER
packets to prohibit injection of such "tunneled" BIER packets from
outside the BIER domain, but also prohibiting forward_routed()
adjacencies to leak BIER packets from the BIER domain. It SHOULD be
possible to configure interfaces to be part of a BIER domain solely
for sending and receiving of unicast "tunneled" BIER packets even if
the interface can not send/receive BIER encapsulated packets.
In BIER, the standardized methods for the routing underlays are IGPs
with extensions to distribute BFR-ids and BFR-prefixes. [RFC8401]
specifies the extensions for IS-IS and [RFC8444] specifies the
extensions for OSPF. Attacking the protocols for the BIER routing
underlay or (non-TE) BIER layer control plane, or impairment of any
BFR in a domain may lead to successful attacks against the results of
the routing protocol, enabling DoS attacks against paths or the
addressing (BFR-id, BFR-prefixes) used by BIER.
The reference model for the BIER-TE layer control plane is a BIER-TE
controller. When such a controller is used, impairment of an
individual BFR in a domain causes no impairment of the BIER-TE
control plane on other BFRs. If a routing protocol is used to
support forward_routed() adjacencies, then this is still an attack
vector as in BIER, but only for BIER-TE forward_routed() adjacencies,
and not other adjacencies.
Whereas IGP routing protocols are most often not well secured through
cryptographic authentication and confidentiality, communications
between controllers and routers such as those to be considered for
the BIER-TE controller/control-plane can be and are much more
commonly secured with those security properties, for example by using
Secure SHell (SSH), [RFC4253] for NETCONF ([RFC6242]), or via
Transport Layer Security (TLS), such as [RFC8253] for PCEP,
[RFC5440], or [RFC7589] for NETCONF. BIER-TE controllers SHOULD use
security equal to or better than these mechanisms.
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When any of these security mechanisms/protocols are used for
communications between a BIER-TE controller and BFRs, their security
considerations apply to BIER-TE. In addition, the security
considerations of PCE, [RFC4655] apply.
The most important attack vector in BIER-TE is misconfiguration,
either on the BFR themselves or via the BIER-TE controller.
Forwarding entries with DNC could be set up to create persistent
loops, in which packets only expire because of TTL. To minimize the
impact of such attacks (or more likely unintentional misconfiguration
by operators and/or bad BIER-TE controller software), the BIER-TE
forwarding rules are defined to be as strict in clearing bits as
possible. The clearing of all bits with an adjacency on a BFR
prohibits that a looping packet creates additional packet
amplification through the misconfigured loop on the packet's second
or further times around the loop, because all relevant adjacency bits
would have been cleared on the first round through the loop. In
result, BIER-TE has the same degree of looping packets as possible
with unintentional or malicious loops in the routing underlay with
BIER or even with unicast traffic.
Deployments where BIER-TE would likely be beneficial may include
operational models where actual configuration changes from the
controller are only required during non-production phases of the
network's life-cycle, such as in embedded networks or in
manufacturing networks during e.g. plant reworking/repairs. In these
type of deployments, configuration changes could be locked out when
the network is in production state and could only be (re-)enabled
through reverting the network/installation into non-production state.
Such security designs would not only allow to provide additional
layers of protection against configuration attacks, but would
foremost protect the active production process from such
configuration attacks.
7. IANA Considerations
This document requests no action by IANA.
8. Acknowledgements
The authors would like to thank Greg Shepherd, Ijsbrand Wijnands,
Neale Ranns, Dirk Trossen, Sandy Zheng, Lou Berger, Jeffrey Zhang,
Carsten Borman and Wolfgang Braun for their reviews and suggestions.
Special thanks to Xuesong Geng for shepherding the document and for
IESG review/suggestions by Alvaro Retana (responsible AD/RTG),
Benjamin Kaduk (SEC), Tommy Pauly (TSV), Zaheduzzaman Sarker (TSV),
Eric Vyncke (INT), Martin Vigoureux (RTG), Robert Wilton (OPS), Eric
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Kline (INT), Lars Eggert (GEN), Roman Danyliv (SEC), Ines Robles
(RTGDIR), Robert Sparks (Gen-ART), Yingzhen Qu (RTGdir), Martin Duke
(TSV).
9. Change log [RFC Editor: Please remove]
draft-ietf-bier-te-arch:
13:
Changed Gregs author association/email.
Fixed Nits in -12 from Ben Kaduk.
Fixed Alvaro's concerns: (1) Removed references to SR in Abstract/
Overview (2) removed section 4.5.
12:
AD review Alvaro Retana.
Various textual/editorial nits including adding () to all
instances of forwarding adjacency name instances.
3.1 Added new paragraph outlining possible use of BGP as RR in
BIER-TE controller as core of multicast flow overlay component of
BIER-TE.
3.2 added xref's to relevant sections to the listed control plane
points.
4.1 rewrote paragraphs of 4.1 leading up to Figure 4. to eliminate
any confusion in how the BIFT work and how it compares to the
notions in rfc8279, as well as better linking it to the
Pseudocode.
Moved SR section into appendix.
TSV review Martin Duke.
Text/editorial nits.
4.4 improved text describing handling of F-BM.
RTGdir review Yingzhen Qu.
Various text/editorial nits.
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Added notion that BitStrings represent loop free tree for packet
to abstract and intro.
Various text nit and editorial improvements.
Fixed some BFR-id field -> BFIR-id field mistakes.
Capitalized NETCONF/RESTCONF/YANG, added RFC references.
Improved Figure 16 with explicitly two links into BFR3 and
explanatory text.
Gen-ART review Robert Sparks.
Various textual nits, editorial improvements.
3.2 Introduced terms "BIER-TE topology control" and "BIER-TE tree
control" for the two functional components of the control plane.
3.2.1 - 3.2 change introduces the open RFC-editor issue of
appropriate xrfs (to be resolved by RFC-editor).
3.3 Rewrote last paragraph to better describe loop prevention
through clearing of bits in BitString.
4.1 Fixed up text/formula describing mapping between bfr-id, SI:BP
and SI,BSL and BP. Fix offset bug.
5.3.6.2 Improved description paragraph explaining overlap of
topology for different SI.
5.3.7 Improved first summary paragraph.
7. Rephrased applicability statement of control plane protocol
security considerations to BIER-TE security.
RTGDIR review Ines Robles.
Fixed up adjacencies in Example 2 and explanation text to be
explicit about which BFR not only passes, but also receives the
packet.
7. (security considerations). Added paragraph about
forward_routed() and prohibiting BIER packet leaking in/out of
domain.
IESG review Roman Danyliv (SEC).
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Several textual/sentence nits/editorials.
IESG review Lars Eggert (GEN).
Various good editorial word fixed.
Pointer to non-false-positive bloom filter work that looks like it
happened after our IETF discussions documented in this doc, so
will not add it to doc, but here is URL for folks interested:
https://ieeexplore.ieee.org/document/8486415.
Did not change "native" to a different word for inclusivity
because of my worry there is no estavblished single replacement
word, making reading/searching/understanding more difficult.
IESG review Martin Vigeureux (RTG).
Added back reference to RFC8402. Textual fixes.
IESG review Eric Kline (INT).
2.1 Fixed typo in BFR* explanations.
4.3 Added explanatio about MTU handling.
IESG review Eric Vyncke (INT).
Fixed up initial text to introduce various abbreviations.
2.4 refined wording to "with the _intent_ to easily build common
forwarding planes...".
4.2.3 refined text about entropy in ECMP - now taken text from
rfc8279.
IESG review Zaheduzzaman Sarker (TSV).
5.1.7 Refined text explaining documentation of ECMP algorithm.
5.3.6.2. fixed range of areas/SI over which to build the example
large network BPs - removed explanation of the large network shown
to be only used for sources in area 1 (IPTV), because it was a
stale explanation.
IESG review Ben Kaduk (round 2):
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4.4 Advanced pseudocode still had one wrong "~". Root cause seems
to have been day 0 problem in pseudocode written for -01, "~" was
inserted in the wrong one of two code lines. Also enhanced
textual description and comments in pseudocode, changed variable
name AdjacentBits to PktAdjacentBits to avoid confusion with
AdjacentBits[SI].
5.1.3 Rewrote last two paragraphs explaining the sharing of bit
positions for lead-BFER hopefully better. Also detailled how it
interacts with other optimizations and the type of payload BIER-TE
packets may carry.
4.4 (from Carsten Borman) changed spacing in pseudocode to be
consistent. Fixed {VRF}, clarified pseudocode object syntax,
typos.
11: IESG review Ben Kaduk, summary:
One discuss for bug in pseudocode. turned out to be one cahrcter
typo.
Added (non-TE) prefix in places where BIER by itsels had to be
better disambiguated.
enhanced text for hub-and-spoke to indicate we're only talking
about hub to spoke traffic.
long list ot language fixes/improvement (nits). Thanks a lot!.
add suggestion to SHOULD use known confidentiality protocols
between controller and BFR.
10: AD review Alvaro Retana, summary:
Note: rfcdiff shows more changes than actually exist because text
moved around.
Summary:
1. restructuring: merged all controller sections under common
controller ops main section, moved unfitting stuff out to
other parts of doc. Split Intro section into Overview and
Intro. Shortened Abstract, moved text into Overview, added
sections overview.
2. enhanced/rewrote: 2.3 Comparison with -> Relationship to BIER-
TE
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3. enhanced/rewrote: 3.2 BIER-TE controller -> BIER-TE control
plane, 3.2.1 BIER-TE controller, for consistency with rfc8279
4. additional subsections for Alvaros asks
5. added to: 3.3 BIER-TE forwarding plane (consistency with
rfc8279)
6. Enhanced description of 4.3/encap considerations to better
explain how BIER/BIER-TE can run together.
Notation: Markers (a),(b),... at end of points are references from
the review discussion with Alvaro to the changes made.
Details:.
Throughout text: changed term spelling to rfc8279 - bit positions,
sub-domain, ... (i).
Reset changed to clear, also DNR changed to DNC (Do Not Clear)
(q).
Abstract: Shortened. Removed name explanation note (Tree
Engineering), (a).
1. Introduction -> Overview: Moved important explanation
paragraph from abstract to Introduction. Fixed text, (a).
Added bullet point list explanation of structure of document (e).
Renamed to Overview because that is now more factually correct.
1.1. Fixed bug in example adding bit p15.(l).
2. (New - Introduction): Moved section 1.1 - 1.3 (examples,
comparison with BIER-TE) from Introduction into new "Overview"
section. Primarily so that "requirements language" section (at
end of Introduction) is not in middle of document after all the
Introduction.
2.1 Removed discussion of encap, moved to 4.2.2 (m).
2.2 enhanced paragraph suggesting native/overlay topology types,
also sugest type hybrid (n).
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2.3 Overhauled comparison text BIER/BIER-TE, structured into
common, different, not-required-by-te, integration-bier-bier-te.
Changed title to "Relationship" to allow including last point.
(f).
2.4 moved Hardware forwarding comparison section into section 2 to
allow coalescing of sections into section 5 about the controller
operations (hardware forwarding was in the middle of it, wrong
place). Shortened/improved third paragraph by pointing to BIFT as
deciding element for selection between BIER/BIER-TE. Removed
notion of experimentation (this now targets standard) (g).
3. (Components): Aligned component name and descriptions better
with RFC8279. Now describe exactly same three layers. BIER layer
constituted from BIER-TE control plane and BIER-TE forwarding
plane. BIER-TE controller is now simply component of BIER-TE
control plane. (b).
3.1. shortened/improved paragraph explaining use of SI:BP instead
of also bfr-id as index into BIFT, rewrote paragraph talking about
reuse of BPs(o).
3.2. rewrote explanation of BIER-TE control plane in the style of
RFC8729 Section 4.2 (BIER layer) with numbered points. Note that
RFC8729 mixes control and forwarding plane bullet points (this doc
does not). Merged text from old sections 2.2.1 and 2.2.3 into
list. (b).
3.2.1. Expanded/improved explanation of BIER-TE Controller (b).
3.2.1.1. Added subsection for topology discovery and creation
(d).
3.2.1.2. Added subsection for engineered BitStrings as key novel
aspect not found in BIER. (X).
3.3. Added numbered list for components of BIER-TE forwarding
plane (completing the comparable text from RFC8729 Section 4.2).
3.4 Alvaro does not mind additional example, fixed bugs.
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3.5 Removed notion about using IGP BIER extensions for BIER-TE,
such as BIFT address ranges. After -10 making use of BIFT
clearer, it now looks to authors as if use of IGP extensions would
not be beneficial, as long as we do need to use the BIER-TE
controller, e.g. unlike in BIER, a BFR could not learn from the
IGP information what traffic to send towards a particular BIFT-ID,
but instead that is the core of what the controller needs to
provide.
4.2.2 Improved text to explain requirement to identify BIER-TE in
the tunnel encap and compress description of use-cases (m).
4.2.3 enhanced ECMP text (p).
4.3. rewrote most of Encapsulation Considerations to better
explain to Alvaros question re sharing or not sharing SD via BIER/
BIER-TE. Added reference to I-D.ietf-bier-non-mpls-bift-encoding
as a very helpful example. (f).
4.3 Renamed title to "...Co-Existence with BIER" as this is what
it is about and to help finding it from abstract/intro ("co-
exist") (j).
4.4. Moved BIER-TE Forwarding Pseudocode here to coalesce text
logically. Changed text to better compare with BIER pseudo
forwarding code. Numerical list of how F-BM works for BIER-TE.
Removed efficiency comparison with BIER (too difficult to provide
sufficient justification, derails from focus of section) (j).
4.6. (Requirements) Restructured: Removed notion of "basic" BIER-
TE forwarding, simply referring to it now as "mandatory" BIER-TE
forwarding. Cleaned up text to have requirements for different
adjacencies in different paragraphs. (c).
5. Created new main section "BIER-TE Controller operational
considerations", coalesced old sections 4., 5., 7. into this new
main section. No text changes. (k).
5.1.9 Added new separate picture instead of referring to a picture
later in text, adjusted text (r).
5.3.2 Changed title to not include word "comparison" to avoid this
being accounted against Alvaros concern about scattering
comparison (IMHO text already has little comparison, so title was
misleading) (h).
co-authors internal review:
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4.4 Added xref to Figure 5.
5.2.1 Duplicated ring picture, added visuals for described
miswiring (s).
5.2.2 replace "topology" with graph (wrong word).
5.3.3 rewrote explanation of how to map BFR-id to SI:BP and assign
them, clarified BFR-id is option. Retitled to better explain
scope of section.
5.3.4 Removed considerations in 5.3.4 for sharing BFR-id across
BIER/BIER-TE (t), changed title to explain how BFIR/BIER-TE
controller interactions need some form of identifying BFR but this
does not have to be BFR-id.
7. Added new security considerations (u).
09: Incorporated fixes for feedback from Shepherd (Xuesong Geng).
Added references for Bloom Filters and Rate Controlled Service
Disciplines.
1.1 Fixed numbering of example 1 topology explanation. Improved
language on second example (less abbreviating to avoid confusion
about meaning).
1.2 Improved explanation of BIER-TE topology, fixed terminology of
graphs (BIER-TE topology is a directed graph where the edges are
the adjacencies).
2.4 Fixed and amended routing underlay explanations: detailed why
no need for BFER routing underlay routing protocol extensions, but
potential to re-use BIER routing underlay routing protocol
extensions for non-BFER related extensions.
3.1 Added explanation for VRF and its use in adjacencies.
08: Incorporated (with hopefully acceptable fixes) for Lou
suggested section 2.5, TE considerations.
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Fixes are primarily to the point to a) emphasize that BIER-TE does
not depend on the routing underlay unless forward_routed()
adjacencies are used, and b) that the allocation and tracking of
resources does not explicitly have to be tied to BPs, because they
are just steering labels. Instead, it would ideally come from
per-hop resource management that can be maintained only via local
accounting in the controller.
07: Further reworking text for Lou.
Renamed BIER-PE to BIER-TE standing for "Tree Engineering" after
votes from BIER WG.
Removed section 1.1 (introduced by version 06) because not
considered necessary in this doc by Lou (for framework doc).
Added [RFC editor pls. remove] Section to explain name change to
future reviewers.
06: Concern by Lou Berger re. BIER-TE as full traffic engineering
solution.
Changed title "Traffic Engineering" to "Path Engineering"
Added intro section of relationship BIER-PE to traffic
engineering.
Changed "traffic engineering" term in text" to "path engineering",
where appropriate
Other:
Shortened "BIER-TE Controller Host" to "BIER-TE Controller".
Fixed up all instances of controller to do this.
05: Review Jeffrey Zhang.
Part 2:
4.3 added note about leaf-BFER being also a propery of routing
setup.
4.7 Added missing details from example to avoid confusion with
routed adjacencies, also compressed explanatory text and better
justification why seed is explicitly configured by controller.
4.9 added section discussing generic reuse of BP methods.
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4.10 added section summarizing BP optimizations of section 4.
6. Rewrote/compressed explanation of comparison BIER/BIER-TE
forwarding difference. Explained benefit of BIER-TE per-BP
forwarding being independent of forwarding for other BPs.
Part 1:
Explicitly ue forwarded_connected adjcency in ECMP adjcency
examples to avoid confusion.
4.3 Add picture as example for leav vs. non-leaf BFR in topology.
Improved description.
4.5 Exampe for traffic that can be broadcast -> for single BP in
hub&spoke.
4.8.1 Simplified example picture for routed adjacency, explanatory
text.
Review from Dirk Trossen:
Fixed up explanation of ICC paper vs. bloom filter.
04: spell check run.
Addded remaining fixes for Sandys (Zhang Zheng) review:
4.7 Enhance ECMP explanations:
example ECMP algorithm, highlight that doc does not standardize
ECMP algorithm.
Review from Dirk Trossen:
1. Added mentioning of prior work for traffic engineered paths
with bloom filters.
2. Changed title from layers to components and added "BIER-TE
control plane" to "BIER-TE Controller" to make it clearer, what it
does.
2.2.3. Added reference to I-D.ietf-bier-multicast-http-response
as an example solution.
2.3. clarified sentence about resetting BPs before sending copies
(also forgot to mention DNR here).
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3.4. Added text saying this section will be removed unless IESG
review finds enough redeeming value in this example given how -03
introduced section 1.1 with basic examples.
7.2. Removed explicit numbers 20%/80% for number of topology bits
in BIER-TE, replaced with more vague (high/low) description,
because we do not have good reference material Added text saying
this section will be removed unless IESG review finds enough
redeeming value in this example given how -03 introduced section
1.1 with basic examples.
many typos fixed. Thanks a lot.
03: Last call textual changes by authors to improve readability:
removed Wolfgang Braun as co-authors (as requested).
Improved abstract to be more explanatory. Removed mentioning of
FRR (not concluded on so far).
Added new text into Introduction section because the text was too
difficult to jump into (too many forward pointers). This
primarily consists of examples and the early introduction of the
BIER-TE Topology concept enabled by these examples.
Amended comparison to SR.
Changed syntax from [VRF] to {VRF} to indicate its optional and to
make idnits happy.
Split references into normative / informative, added references.
02: Refresh after IETF104 discussion: changed intended status back
to standard. Reasoning:
Tighter review of standards document == ensures arch will be
better prepared for possible adoption by other WGs (e.g. DetNet)
or std. bodies.
Requirement against the degree of existing implementations is self
defined by the WG. BIER WG seems to think it is not necessary to
apply multiple interoperating implementations against an
architecture level document at this time to make it qualify to go
to standards track. Also, the levels of support introduced in -01
rev. should allow all BIER forwarding engines to also be able to
support the base level BIER-TE forwarding.
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01: Added note comparing BIER and SR to also hopefully clarify
BIER-TE vs. BIER comparison re. SR.
- added requirements section mandating only most basic BIER-TE
forwarding features as MUST.
- reworked comparison with BIER forwarding section to only
summarize and point to pseudocode section.
- reworked pseudocode section to have one pseudocode that mirrors
the BIER forwarding pseudocode to make comparison easier and a
second pseudocode that shows the complete set of BIER-TE
forwarding options and simplification/optimization possible vs.
BIER forwarding. Removed MyBitsOfInterest (was pure
optimization).
- Added captions to pictures.
- Part of review feedback from Sandy (Zhang Zheng) integrated.
00: Changed target state to experimental (WG conclusion), updated
references, mod auth association.
- Source now on https://www.github.com/toerless/bier-te-arch
- Please open issues on the github for change/improvement requests
to the document - in addition to posting them on the list
(bier@ietf.). Thanks!.
draft-eckert-bier-te-arch:
06: Added overview of forwarding differences between BIER, BIER-
TE.
05: Author affiliation change only.
04: Added comparison to Live-Live and BFIR to FRR section
(Eckert).
04: Removed FRR content into the new FRR draft [I-D.eckert-bier-
te-frr] (Braun).
- Linked FRR information to new draft in Overview/Introduction
- Removed BTAFT/FRR from "Changes in the network topology"
- Linked new draft in "Link/Node Failures and Recovery"
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- Removed FRR from "The BIER-TE Forwarding Layer"
- Moved FRR section to new draft
- Moved FRR parts of Pseudocode into new draft
- Left only non FRR parts
- removed FrrUpDown(..) and //FRR operations in
ForwardBierTePacket(..)
- New draft contains FrrUpDown(..) and ForwardBierTePacket(Packet)
from bier-arch-03
- Moved "BIER-TE and existing FRR to new draft
- Moved "BIER-TE and Segment Routing" section one level up
- Thus, removed "Further considerations" that only contained this
section
- Added Changes for version 04
03: Updated the FRR section. Added examples for FRR key concepts.
Added BIER-in-BIER tunneling as option for tunnels in backup
paths. BIFT structure is expanded and contains an additional
match field to support full node protection with BIER-TE FRR.
03: Updated FRR section. Explanation how BIER-in-BIER
encapsulation provides P2MP protection for node failures even
though the routing underlay does not provide P2MP.
02: Changed the definition of BIFT to be more inline with BIER.
In revs. up to -01, the idea was that a BIFT has only entries for
a single BitString, and every SI and sub-domain would be a
separate BIFT. In BIER, each BIFT covers all SI. This is now
also how we define it in BIER-TE.
02: Added Section 5.3 to explain the use of SI, sub-domains and
BFR-id in BIER-TE and to give an example how to efficiently assign
bits for a large topology requiring multiple SI.
02: Added further detailed for rings - how to support input from
all ring nodes.
01: Fixed BFIR -> BFER for section 4.3.
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01: Added explanation of SI, difference to BIER ECMP,
consideration for Segment Routing, unicast FRR, considerations for
encapsulation, explanations of BIER-TE Controller and CLI.
00: Initial version.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.
10.2. Informative References
[Bloom70] Bloom, B. H., "Space/time trade-offs in hash coding with
allowable errors", Comm. ACM 13(7):422-6, July 1970,
<https://dl.acm.org/doi/10.1145/362686.362692>.
[I-D.eckert-bier-te-frr]
Eckert, T., Cauchie, G., Braun, W., and M. Menth,
"Protection Methods for BIER-TE", Work in Progress,
Internet-Draft, draft-eckert-bier-te-frr-03, 5 March 2018,
<https://www.ietf.org/archive/id/draft-eckert-bier-te-frr-
03.txt>.
[I-D.ietf-bier-multicast-http-response]
Trossen, D., Rahman, A., Wang, C., and T. Eckert,
"Applicability of BIER Multicast Overlay for Adaptive
Streaming Services", Work in Progress, Internet-Draft,
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draft-ietf-bier-multicast-http-response-06, 10 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-bier-
multicast-http-response-06.txt>.
[I-D.ietf-bier-non-mpls-bift-encoding]
Wijnands, I., Mishra, M., Xu, X., and H. Bidgoli, "An
Optional Encoding of the BIFT-id Field in the non-MPLS
BIER Encapsulation", Work in Progress, Internet-Draft,
draft-ietf-bier-non-mpls-bift-encoding-04, 30 May 2021,
<https://www.ietf.org/archive/id/draft-ietf-bier-non-mpls-
bift-encoding-04.txt>.
[I-D.ietf-bier-te-yang]
Zhang, Z., Wang, C., Chen, R., Hu, F., Sivakumar, M., and
H. Chen, "A YANG data model for Tree Engineering for Bit
Index Explicit Replication (BIER-TE)", Work in Progress,
Internet-Draft, draft-ietf-bier-te-yang-04, 7 November
2021, <https://www.ietf.org/archive/id/draft-ietf-bier-te-
yang-04.txt>.
[I-D.ietf-roll-ccast]
Bergmann, O., Bormann, C., Gerdes, S., and H. Chen,
"Constrained-Cast: Source-Routed Multicast for RPL", Work
in Progress, Internet-Draft, draft-ietf-roll-ccast-01, 30
October 2017, <https://www.ietf.org/archive/id/draft-ietf-
roll-ccast-01.txt>.
[I-D.ietf-teas-rfc3272bis]
Farrel, A., "Overview and Principles of Internet Traffic
Engineering", Work in Progress, Internet-Draft, draft-
ietf-teas-rfc3272bis-16, 24 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-teas-
rfc3272bis-16.txt>.
[ICC] Reed, M. J., Al-Naday, M., Thomos, N., Trossen, D.,
Petropoulos, G., and S. Spirou, "Stateless multicast
switching in software defined networks", IEEE
International Conference on Communications (ICC), Kuala
Lumpur, Malaysia, 2016, May 2016,
<https://ieeexplore.ieee.org/document/7511036>.
[RCSD94] Zhang, H. and D. Domenico, "Rate-Controlled Service
Disciplines", Journal of High-Speed Networks, 1994, May
1994, <https://dl.acm.org/doi/10.5555/2692227.2692232>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
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[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.
[RFC7589] Badra, M., Luchuk, A., and J. Schoenwaelder, "Using the
NETCONF Protocol over Transport Layer Security (TLS) with
Mutual X.509 Authentication", RFC 7589,
DOI 10.17487/RFC7589, June 2015,
<https://www.rfc-editor.org/info/rfc7589>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC7988] Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
Replication Tunnels in Multicast VPN", RFC 7988,
DOI 10.17487/RFC7988, October 2016,
<https://www.rfc-editor.org/info/rfc7988>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
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[RFC8253] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)",
RFC 8253, DOI 10.17487/RFC8253, October 2017,
<https://www.rfc-editor.org/info/rfc8253>.
[RFC8345] Clemm, A., Medved, J., Varga, R., Bahadur, N.,
Ananthakrishnan, H., and X. Liu, "A YANG Data Model for
Network Topologies", RFC 8345, DOI 10.17487/RFC8345, March
2018, <https://www.rfc-editor.org/info/rfc8345>.
[RFC8401] Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
Zhang, "Bit Index Explicit Replication (BIER) Support via
IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,
<https://www.rfc-editor.org/info/rfc8401>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8444] Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
Extensions for Bit Index Explicit Replication (BIER)",
RFC 8444, DOI 10.17487/RFC8444, November 2018,
<https://www.rfc-editor.org/info/rfc8444>.
[RFC8556] Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S.,
and A. Dolganow, "Multicast VPN Using Bit Index Explicit
Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April
2019, <https://www.rfc-editor.org/info/rfc8556>.
Appendix A. BIER-TE and Segment Routing (SR)
SR ([RFC8402]) aims to enable lightweight path steering via loose
source routing. Compared to its more heavy-weight predecessor RSVP-
TE, SR does for example not require per-path signaling to each of
these hops.
BIER-TE supports the same design philosophy for multicast. Like in
SR, it relies on source-routing - via the definition of a BitString.
Like SR, it only requires to consider the "hops" on which either
replication has to happen, or across which the traffic should be
steered (even without replication). Any other hops can be skipped
via the use of routed adjacencies.
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BIER-TE bit position (BP) can be understood as the BIER-TE equivalent
of "forwarding segments" in SR, but they have a different scope than
SR forwarding segments. Whereas forwarding segments in SR are global
or local, BPs in BIER-TE have a scope that is the group of BFR(s)
that have adjacencies for this BP in their BIFT. This can be called
"adjacency" scoped forwarding segments.
Adjacency scope could be global, but then every BFR would need an
adjacency for this BP, for example a forward_routed() adjacency with
encapsulation to the global SR SID of the destination. Such a BP
would always result in ingress replication though (as in [RFC7988]).
The first BFR encountering this BP would directly replicate to it.
Only by using non-global adjacency scope for BPs can traffic be
steered and replicated on non-ingress BFR.
SR can naturally be combined with BIER-TE and help to optimize it.
For example, instead of defining bit positions for non-replicating
hops, it is equally possible to use segment routing encapsulations
(e.g. SR-MPLS label stacks) for the encapsulation of
"forward_routed" adjacencies.
Note that (non-TE) BIER itself can also be seen to be similar to SR.
BIER BPs act as global destination Node-SIDs and the BIER BitString
is simply a highly optimized mechanism to indicate multiple such SIDs
and let the network take care of effectively replicating the packet
hop-by-hop to each destination Node-SID. What BIER does not allow is
to indicate intermediate hops, or in terms of SR the ability to
indicate a sequence of SID to reach the destination. This is what
BIER-TE and its adjacency scoped BP enables.
Authors' Addresses
Toerless Eckert (editor)
Futurewei Technologies Inc.
2330 Central Expy
Santa Clara, 95050
United States of America
Email: tte+ietf@cs.fau.de
Michael Menth
University of Tuebingen
Email: menth@uni-tuebingen.de
Gregory Cauchie
KOEVOO
Email: gregory@koevoo.tech
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