Internet Engineering Task Force (IETF) T. Eckert, Ed.
Request for Comments: 9262 Futurewei
Category: Standards Track M. Menth
ISSN: 2070-1721 University of Tuebingen
G. Cauchie
KOEVOO
October 2022
Tree Engineering for Bit Index Explicit Replication (BIER-TE)
Abstract
This memo describes per-packet stateless strict and loose path
steered replication and forwarding for "Bit Index Explicit
Replication" (BIER) packets (RFC 8279); it is called "Tree
Engineering for Bit Index Explicit Replication" (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" (BPs). These
BPs indicate adjacencies of the network topology, as opposed to (non-
TE) BIER in which BPs indicate "Bit-Forwarding Egress Routers"
(BFERs). A BIER-TE "packets BitString" therefore indicates the edges
of the (loop-free) tree across which the packets are forwarded 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
"subdomains" (SDs). Except for the optional routed adjacencies,
BIER-TE does not require a BIER routing underlay and can therefore
operate without depending on a routing protocol such as the "Interior
Gateway Protocol" (IGP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9262.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
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Table of Contents
1. Overview
2. Introduction
2.1. Requirements Language
2.2. Basic Examples
2.3. BIER-TE Topology and Adjacencies
2.4. Relationship to BIER
2.5. Accelerated Hardware Forwarding Comparison
3. Components
3.1. The Multicast Flow Overlay
3.2. The BIER-TE Control Plane
3.2.1. The BIER-TE Controller
3.2.1.1. BIER-TE Topology Discovery and Creation
3.2.1.2. Engineered Trees via BitStrings
3.2.1.3. Changes in the Network Topology
3.2.1.4. Link/Node Failures and Recovery
3.3. The BIER-TE Forwarding Plane
3.4. The Routing Underlay
3.5. Traffic Engineering Considerations
4. BIER-TE Forwarding
4.1. The BIER-TE Bit Index Forwarding Table (BIFT)
4.2. Adjacency Types
4.2.1. Forward Connected
4.2.2. Forward Routed
4.2.3. ECMP
4.2.4. Local Decap(sulation)
4.3. Encapsulation / Co-existence with BIER
4.4. BIER-TE Forwarding Pseudocode
4.5. BFR Requirements for BIER-TE Forwarding
5. BIER-TE Controller Operational Considerations
5.1. Bit Position Assignments
5.1.1. P2P Links
5.1.2. BFERs
5.1.3. Leaf BFERs
5.1.4. LANs
5.1.5. Hub and Spoke
5.1.6. Rings
5.1.7. Equal-Cost Multipath (ECMP)
5.1.8. Forward Routed Adjacencies
5.1.8.1. Reducing Bit Positions
5.1.8.2. Supporting Nodes without BIER-TE
5.1.9. Reuse of Bit Positions (without DNC)
5.1.10. Summary of BP Optimizations
5.2. Avoiding Duplicates and Loops
5.2.1. Loops
5.2.2. Duplicates
5.3. Managing SIs, Subdomains, and BFR-ids
5.3.1. Why SIs and Subdomains?
5.3.2. Assigning Bits for the BIER-TE Topology
5.3.3. Assigning BFR-ids with BIER-TE
5.3.4. Mapping from BFRs to BitStrings with BIER-TE
5.3.5. Assigning BFR-ids for BIER-TE
5.3.6. Example Bit Allocations
5.3.6.1. With BIER
5.3.6.2. With BIER-TE
5.3.7. Summary
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. BIER-TE and Segment Routing (SR)
Acknowledgements
Authors' Addresses
1. Overview
"Tree Engineering for Bit Index Explicit Replication" (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, with the expectation that the reader is familiar
with these two documents.
BIER-TE introduces a new semantic for "bit positions" (BPs). These
BPs indicate adjacencies of the network topology, as opposed to (non-
TE) BIER in which BPs indicate "Bit-Forwarding Egress Routers"
(BFERs). A BIER-TE "packets BitString" therefore indicates the edges
of the (loop-free) tree across which the packets are forwarded by
BIER-TE. With BIER-TE, the "Bit Index Forwarding Table" (BIFT) of
each "Bit-Forwarding Router" (BFR) is only populated with BPs that
are adjacent to the BFR in the BIER-TE topology. Other BPs are empty
in the BIFT. The BFR replicates and forwards BIER packets to
adjacent BPs that are set in the packets. 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 subdomains. Except for
the optional routed adjacencies, BIER-TE does not require a BIER
routing underlay and can therefore operate without depending on a
routing protocol such as the "Interior Gateway Protocol" (IGP).
This document is structured as follows:
* Section 2 introduces BIER-TE with two forwarding examples,
followed by an introduction to 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:
the multicast flow overlay, the BIER-TE layer with the BIER-TE
control plane (including the BIER-TE controller), the BIER-TE
forwarding plane, and the routing underlay.
* Section 4 specifies the behavior of the BIER-TE forwarding plane
with the different types 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, primarily how the BIER-TE controller can optimize the
use of BPs by using specific types of BIER-TE adjacencies for
different types of topological situations. It also describes how
to assign bits to avoid loops and duplicates (which, in BIER-TE,
does not come "for free"). Finally, it discusses how "Set
Identifiers" (SIs), "subdomains" (SDs), and BFR-ids can be managed
by a BIER-TE controller; examples and a summary are provided.
* Appendix A concludes this document; details regarding the
relationship between BIER-TE and "Segment Routing" (SR) are
discussed.
Note that related work [CONSTRAINED-CAST] 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
BitStrings during forwarding to avoid loops. For these reasons,
BIER-TE, like BIER, uses explicit BitStrings. Explicit BitStrings as
used by BIER-TE can also be seen as a special type of Bloom filter,
and this is how other related work [ICC] describes it.
2. Introduction
2.1. 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.
2.2. Basic Examples
BIER-TE forwarding is best introduced with simple examples. These
examples use formal terms defined later in this document (Figure 4 in
Section 4.1), including forward_connected(), forward_routed(), and
local_decap().
Consider the simple network in the BIER-TE overview example shown in
Figure 1, with six BFRs. p1...p15 are the bit positions used. All
BFRs can act as a "Bit-Forwarding Ingress Router" (BFIR); BFR1, BFR3,
BFR4, and BFR6 can also be BFERs. "Forward_connected()" is the name
used for adjacencies that represent subnet adjacencies of the
network. "Local_decap()" is the name used for the adjacency that
decapsulates BIER-TE packets and passes their payload to higher-layer
processing.
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 (BIFTs):
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
Assume that 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
demonstrates the ability of the BIER-TE topology, as shown in
Figure 1, to make the traffic pass across any possible path and be
replicated where desired.
BIER-TE has various options for minimizing BP assignments, many of
which are based on out-of-band knowledge about the required multicast
traffic paths and bandwidth consumption in the network, e.g., from
predeployment 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 explicitly distinguish routed/tunneled forwarding of
BIER-TE packets from Layer 2 forwarding (forward_connected()), these
adjacencies are called "forward_routed()" adjacencies. 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 a BFIR -- not a BFER or a transit BFR.
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 (BIFTs):
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). A packet from BFR1 via BFR4 to be
received by BFR6 uses the BitString (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, then from BFR4 to be
received by BFR6, and finally from BFR6 to be received by BFR3, uses
(p2,p3,p4,p6,p7,p9). A packet from BFR1 to be received by BFR3, then
from BFR3 to be received by BFR6, and finally from BFR6 to be
received by BFR4, uses (p1,p3,p4,p5,p8,p9).
2.3. 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 2.2. It is used to control where replication can or should
happen and how to minimize the required number of BPs for
adjacencies.
The BIER-TE topology consists of the BIFTs of all the BFRs and can
also be expressed as a directed graph where the edges are the
adjacencies between the BFRs labeled with the BP used for the
adjacency. Adjacencies are naturally unidirectional. A 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 an "underlay" 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.
2.4. 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 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 that need path steering but do not
want distributed routing protocols.
1. BIER-TE inherits the following aspects from BIER unchanged:
1.a The fundamental purpose of per-packet signaled replication
and delivery via a BitString.
1.b The overall architecture, which consists of three layers:
the flow overlay, the BIER(-TE) layer, and the routing
underlay.
1.c The supported encapsulations [RFC8296].
1.d The semantics of all BIER header elements [RFC8296] used by
the BIER-TE forwarding plane, other than the semantic of the
BP in the BitString.
1.e 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:
2.a In BIER, bits in the BitString of a BIER packet header
indicate a BFER, and bits in the BIFT indicate the BIER
control plane's calculated next hop towards that BFER. In
BIER-TE, a bit in the BitString of a BIER packet header
indicates an adjacency in the BIER-TE topology, and only the
BFR that is the upstream of that adjacency has its BP
populated with the adjacency in its BIFT.
2.b In BIER, the implied reference options for the core part of
the BIER layer control plane are the BIER extensions for
distributed routing protocols. These include IS-IS and OSPF
extensions for BIER, as specified in [RFC8401] and
[RFC8444], respectively.
2.c 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.
2.d Assuming the reference options for the control plane, BIER-
TE replaces in-network autonomous path calculations with
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:
3.a "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.
3.b The BIER-TE layer forwarding plane does not require BFRs to
have a unique BP; see Section 5.1.3. Therefore, BFRs may
not have a unique BFR-id; see Section 5.3.3.
3.c 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.
3.d 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:
4.a The BIER/BIER-TE packet header needs to allow the addressing
of 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 forwarded only end to end via BIER or via
BIER-TE ("ships in the night" forwarding).
4.b BIER-TE deployments will have to assign BFR-ids to BFRs and
insert them into the BFIR-id field of BIER packet headers,
as does BIER, whenever the deployment uses (unchanged)
components developed for BIER that use BFR-ids, such as
multicast flow overlays or BIER layer control plane
elements. See also Section 5.3.3.
2.5. Accelerated Hardware Forwarding Comparison
BIER-TE forwarding rules, especially BitString parsing, are designed
to be as close as possible to those of BIER, with 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 the BIER BIFT's "Forwarding Bit
Mask" (F-BM): only the clearing of bits to avoid sending 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 a 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 as being composed of the same three layers
as BIER: the "multicast flow overlay", the "BIER layer", and the
"routing underlay". Figure 3 also shows how the BIER layer is
composed of the "BIER-TE forwarding plane" and the "BIER-TE control
plane" as represented by the "BIER-TE controller".
<------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 that described for
BIER in [RFC8279], Section 4.3. See also Section 3.2.1.2.
When a BIER-TE controller is used, it might also be preferable that
multicast flow overlay signaling be performed through a central point
of control. For BGP-based overlay flow services such as "Multicast
VPN Using Bit Index Explicit Replication (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's calculated
BitStrings. See Sections 3.2.1.2 and 5.3.4.
3.2. The BIER-TE Control Plane
In the (non-TE) BIER architecture [RFC8279], the BIER layer is
summarized in Section 4.2 of [RFC8279]. This summary includes both
the functions of the BIER-layer control plane and forwarding plane,
without using those terms. Example standardized options for the BIER
control plane include IS-IS and OSPF extensions for BIER, as
specified in [RFC8401] and [RFC8444], respectively.
For BIER-TE, the control plane includes, at a minimum, the following
functionality.
1. BIER-TE topology control: During initial provisioning of the
network and/or during modifications of its topology and/or
services, the protocols and/or procedures to establish BIER-TE
BIFTs:
1.a Determine the desired BIER-TE topology for BIER-TE
subdomains: the adjacencies that are assigned to BPs.
Topology discovery is discussed in Section 3.2.1.1, and the
various aspects of the BIER-TE controller's determinations
regarding the topology are discussed throughout Section 5.
1.b 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 BFR's BIFT with
them.
1.c Optionally assign BFR-ids to BFIRs for later insertion into
BIER headers on BFIRs as BFIR-ids. Alternatively, BFIR-ids
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.
1.d 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 network operations, protocols and/or
procedures to support creation/change/removal of overlay flows on
BFIRs:
2.a Process the BIER-TE requirements for the multicast overlay
flow: BFIRs and BFERs of the flow as well as policies for
the path selection of the flow. This is discussed in
Section 3.5.
2.b Determine the BitStrings and, optionally, entropy.
BitStrings are discussed in Sections 3.2.1.2, 3.5, and
5.3.4. Entropy is discussed in Section 4.2.3.
2.c Install state on the BFIR to impose the desired BIER packet
header(s) for packets of the overlay flow. Different
aspects of this point, as well as the next point, are
discussed throughout Section 3.2.1 and in Section 4.3. The
main component responsible for these two points is the
multicast flow overlay (Section 3.1), which is
architecturally inherited from BIER.
2.d Install the necessary state on the BFERs to decapsulate the
BIER packet header and properly dispatch its payload.
3.2.1. The BIER-TE Controller
This architecture describes the BIER-TE control plane, as shown in
Figure 3, as consisting of:
* A BIER-TE controller.
* BFR data models and protocols to communicate between the
controller and BFRs in support of BIER-TE topology control (see
the list under "BIER-TE topology control"), such as YANG/NETCONF/
RESTCONF [RFC7950] [RFC6241] [RFC8040].
* BFR data models and protocols to communicate between the
controller and BFIRs in support of BIER-TE tree control (see
Section 3.2, point 2.), such as BIER-TE extensions for [RFC5440].
The single, centralized BIER-TE controller is used in this document
as the 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 a 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 that 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 listed for BIER-TE topology control (Section 3.2,
point 1.a.) 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, e.g., industrial environments, both
discovery and creation can be a manual/offline process.
In other networks, topology discovery may rely on such protocols as
those that include extending an IGP based on a link-state protocol
into the BIER-TE controller itself, e.g., BGP-LS [RFC7752] or YANG
topology [RFC8345], as well as methods specific to BIER-TE -- for
example, via [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 BFERs using
different SIs:BitStrings. These topics are discussed in Section 5.
When the BIER-TE topology has been determined, the BIER-TE controller
pushes the BPs/adjacencies to the BIFT of the BFRs. On each BFR,
only those SIs:BPs that are adjacencies to other BFRs in the BIER-TE
topology are populated.
Communications between the BIER-TE controller and BFRs for both BIER-
TE topology control and BIER-TE tree control are ideally via
standardized protocols and data models such as NETCONF/RESTCONF/YANG/
PCEP. A vendor-specific CLI on the BFRs is also an option (as in
many other "Software-Defined Network" (SDN) solutions lacking
definitions of standardized data models).
3.2.1.2. Engineered Trees via BitStrings
In BIER, the same set of BFERs in a single subdomain is always
encoded as the same BitString. In BIER-TE, the BitString used to
reach the same set of BFERs in the same subdomain can be different
for different overlay flows because the BitString encodes the paths
towards the BFERs, so the BitStrings from different BFIRs to the same
set of BFERs will often be different. Likewise, the BitString from
the same BFIR to the same set of BFERs can be different for different
overlay flows if different policies should be applied to those
overlay flows, such as shortest path trees, Steiner trees (minimum
cost trees), diverse path trees for redundancy, and so on.
See also [BIER-MCAST-OVERLAY] for an application leveraging BIER-TE
engineered trees.
3.2.1.3. Changes in the Network Topology
If the network topology changes (not failure based) so that
adjacencies that are assigned to bit positions are no longer needed,
the BIER-TE controller can reuse 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 the BIFT and then into the BFIR.
3.2.1.4. Link/Node Failures and Recovery
When links or nodes fail or recover in the topology, BIER-TE could
quickly respond with "Fast Reroute" (FRR) procedures such as those
described in [BIER-TE-PROTECTION], the details of which are out of
scope for this document. It can also more slowly react by
recalculating the BitStrings of affected multicast flows. This
reaction is slower than the FRR procedure because the BIER-TE
controller needs to receive link/node up/down indications,
recalculate the desired BitStrings, and push them down into the
BFIRs. With FRR, this is all performed locally on a BFR receiving
the adjacency up/down notification.
3.3. The BIER-TE Forwarding Plane
The BIER-TE forwarding plane consists of the following components:
1. On a BFIR, imposition of the BIER header for packets from overlay
flows. This is driven by state established by the BIER-TE
control plane, the multicast flow overlay as explained in
Section 3.1, or a combination of both.
2. On BFRs (including BFIRs and BFERs), 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 the
"Differentiated Services Code Point" (DSCP) field, 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 has one or more adjacencies in the
BIFT, a copy is made according to the types of adjacencies for that
BP in the BIFT. Before sending any copies, 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 prevents packets from
looping when a BitString erroneously includes a forwarding loop.
When a forward_connected() adjacency has the "DoNotClear" (DNC) flag
set, this BP is reset for the packet copied to that adjacency. See
Section 4.2.1.
3.4. The Routing Underlay
For forward_connected() adjacencies, BIER-TE sends BIER packets to
directly connected BIER-TE neighbors as L2 (unicast) 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 details on forward_routed()
adjacencies.
BIER relies on the routing underlay to calculate paths towards BFERs
and derive next-hop BFR adjacencies for those paths. These two steps
commonly rely 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 for 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
"Bidirectional Forwarding Detection" (BFD). Providing these
notifications is considered to be part of the routing underlay in
this document.
3.5. Traffic Engineering Considerations
Traffic Engineering [TE-OVERVIEW] provides performance optimization
of operational IP networks while utilizing network resources
economically and reliably. The key elements needed to effect Traffic
Engineering are policy, path steering, and resource management.
These elements require support at the control/controller level and
within the forwarding plane.
Policy decisions are made within the BIER-TE control plane, i.e.,
within BIER-TE controllers. Controllers use policy when composing
BitStrings and BFR BIFT state. The mapping of user/IP traffic to
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 for 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 on which it operates. 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 4.2.2 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 for the forwarding plane beyond
the BIER-TE-defined steering of packets; this includes allocation of
buffers to guarantee the worst-case requirements for 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 equivalent to the (non-TE) BIER BIFT. It exists
on every BFR running BIER-TE. For every BIER "subdomain" (SD) in use
for BIER-TE, the BIFT is constructed per the example shown in
Figure 4. The BIFT in the figure assumes a BSL of 8 "bit positions"
(BPs) in the packets BitString. As in [RFC8279], this BSL is purely
used as an example and is not a BSL supported by BIER/BIER-TE
(minimum BSL is 64).
A BIER-TE BIFT is compared 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 packet's
SI, BSL, and the one bit position of the packets BitString (BP)
addressing the BIFT row: BIFT-index = SI * BSL + BP - 1. BPs 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 BPs 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,
in the case of reuse of SI:BP, more than one adjacency between BFRs
in the topology. The SI:BP is populated with the adjacency on the
upstream BFR of the adjacency. The BIFT entries 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, an F-BM is
not required but can be used when implementing BIER-TE on forwarding
hardware, derived from BIER forwarding, that must use an F-BM. This
is discussed in the first variation of BIER-TE forwarding pseudocode
shown in Section 4.4.
-------------------------------------------------------------------
| BIFT-index | | Adjacencies: |
| (SI:BP) |(F-BM)| <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) | | ... |
| ... | ... | ... |
-------------------------------------------------------------------
Figure 4: BIER-TE Bit Index Forwarding Table (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 procedures
for the BIER-TE forwarding plane as specified in Section 3.3.
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 the reuse 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
4.2.1. Forward Connected
A "forward_connected()" adjacency is an adjacency towards a directly
connected BFR-NBR using an interface address of that BFR on the
connecting interface. A forward_connected() adjacency does not route
packets; 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 a 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 caused by misconfiguration (see
Section 5.2.1), DNC is only permissible for forward_connected()
adjacencies. No need or benefit of DNC for other types of
adjacencies was identified, and associated risks were not analyzed.
4.2.2. Forward Routed
A "forward_routed()" adjacency is an adjacency towards a BFR that
uses a (tunneling) encapsulation that will cause a packet to be
forwarded by the routing underlay towards the adjacent BFR indicated
via the l3-neighbor parameter of the forward_routed() adjacency.
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 rely on either the
BIER/BIER-TE co-existence mechanisms described in Section 4.3 or
explicit support for a BIER-TE payload type in the tunneling
encapsulation.
Forward_routed() adjacencies are necessary to pass BIER-TE traffic
across routers that are not BIER-TE capable or to minimize the number
of required BPs by tunneling over (BIER-TE-capable) routers on which
neither replication nor path steering is desired, or simply to
leverage the routing underlay's path redundancy and FRR towards the
next BFR. They may also be useful to a multi-subnet adjacent BFR for
leveraging the routing underlay ECMP independently 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
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
permits the design of 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 details.
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 (IP/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 packets.
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.
The handling of "Maximum Transmission Unit" (MTU) limitations is
outside the scope of this document and is not discussed in [RFC8279]
either. Instead, this process is part of the BIER-TE packet
encapsulation and/or flow overlay; for example, see [RFC8296],
Section 3. It applies equally to BIER-TE and 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 packets from BIER-TE packets. In 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 BIFTs and BIER-TE
BIFTs. 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 BIFTs. This also permits the allocation of non-
overlapping label ranges for BIFT-ids 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.
Assume that a fixed mapping from BSL, SD, and SI to a BIFT-id is
used, which does not explicitly partition the BIFT-id space between
BIER and BIER-TE -- for example, as proposed for non-MPLS forwarding
with BIER encapsulation [RFC8296] in [NON-MPLS-BIER-ENCODING],
Section 5. In this case, 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 [NON-MPLS-BIER-ENCODING] does
not statically encode the BSL or SD into the BIFT-id, but the
encoding permits a mapping and hence could provide the same freedom
as when MPLS is being used (the same SD, or different SDs for BIER/
BIER-TE).
Forward_routed() requires an encapsulation that permits directing
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 address's 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, for example, MPLS label stacks or appropriate header
extensions (e.g., for IPv6).
4.4. BIER-TE Forwarding Pseudocode
The pseudocode for BIER-TE forwarding, as shown in Figure 5, is based
on the (non-TE) BIER forwarding pseudocode provided in [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]
}
}
Figure 5: BIER-TE Forwarding Pseudocode for Required Functions,
Based on BIER Pseudocode
In step [2], the F-BM is used to clear one or more bits 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, BIER's replication logic has to prevent more than one PacketCopy
from being 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 a BFR-NBR. This prevents BIER's replication logic from
creating duplicates on any possible further BFRs ([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 a 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 same way that BIER does,
because in general, different BPs 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 for ensuring that only one of those bits is set in any
Packet->BitString, unless the controller explicitly wants duplicates
to be created.
The following points describe how the 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 the 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].
This forwarding pseudocode can support the required BIER-TE
forwarding functions (see Section 4.5) -- forward_connected(),
forward_routed(), and local_decap() -- but cannot support the
recommended functions (DNC flag and multiple adjacencies per bit) or
the optional function (i.e., 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):
1. This pseudocode eliminates per-bit F-BMs, therefore reducing the
size of BIFT state by SI*BSL^2 and eliminating the need for per-
packet-copy BitString masking operations, except for adjacencies
with the DNC flag set:
1.a 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 (adds/removes) adjacencies in
the BIFT.
1.b 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.
1.c All bit positions for 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.
2. BIFT entries may contain more than one adjacency in support of
specific configurations, such as a hub and multiple spokes
(Section 5.1.5). The code therefore includes a loop over these
adjacencies.
3. The ECMP() adjacency is also shown in the figure. Its parameters
are a seed and "ListOfAdjacencies", from which one is picked.
4. The forward_connected(), forward_routed(), and local_decap()
adjacencies are shown with their parameters.
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 the 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
BFRs that support BIER-TE and BIER MUST support a configuration that
enables BIER-TE instead of (non-TE) BIER forwarding rules for all
BIFTs of one or more BIER subdomains. Every BP in a BIER-TE BIFT
MUST support having 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
Section 4.4, these required BIER-TE forwarding functions can be
implemented via the same forwarding pseudocode as that used for BIER
forwarding, except for one modification (skipping one masking with an
F-BM).
BIER-TE forwarding SHOULD support forward_connected() adjacencies
with the DNC flag set, as this is very useful for saving bits in
rings (see Section 5.1.6).
BIER-TE forwarding SHOULD support more than one adjacency on a bit.
This allows bits to be saved 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 the routing underlay ECMP via forward_routed()
adjacencies and/or might prefer to have more explicit control of the
path chosen via explicit BPs/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 here exist to support larger
topologies with fewer bit positions.
5.1.1. P2P Links
On a "point-to-point" (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 therefore requires only one bit
position.
5.1.2. BFERs
Every non-leaf BFER is given a unique bit position with a
local_decap() adjacency.
5.1.3. Leaf BFERs
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)
routers 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 as
shown in Figure 7: when BFER1/BFER2 are non-leaf BFERs 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 on the left-hand
side of the figure are only guaranteed to be leaf BFERs by correctly
applying a routing configuration that prohibits transit traffic from
passing through the BFERs, which is commonly applied in these
topologies.
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
In most situations, leaf BFERs 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 also need to 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 BFERs
addressed by the BitString.
The possible exceptions to this leaf BFER bit position optimization
scenario 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 -- for
example, as described in Sections 5.1.4 and 5.1.5. With either of
these two optimizations, the sender of the packet could only control
explicitly 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 a 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 packets need 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 packets were delivered unnecessarily by the BIER-TE
layer. If the payload has no such membership indication and the BFIR
wants to have explicit control regarding which BFERs are to receive
and decapsulate a packet, then these two optimizations cannot be used
together with shared bit position optimization for a 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 BFR 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. These BFRs
would receive duplicates and forward those duplicates into the other
LANs. Such BFRs require separate bit positions for each LAN they
connect to.
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 BFERs.
This type of optimized BP could be used, for example, when all
traffic is "broadcast" traffic (very dense receiver sets), such as
live TV or many-to-many telemetry, including situational awareness.
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 ring 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, as follows.
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 BFRs in
the ring to a BFR outside the ring will not have the ring bit
position set, therefore minimizing the risk of creating loops.
v v
| |
L1 | L2 | L3
/-------- BFRa ---- BFRb --------------------\
| |
\- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
| | L4 | |
p33| p15|
BFRd BFRc
Figure 9: Ring Example
Note that this example only permits packets intended to make it all
the way around the ring to enter it at BFRa and BFRb. Note also that
packets will always travel clockwise. If packets should be allowed
to enter the ring at any of the ring's BFRs, 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 ring's BFIR 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's
BFIR 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)
An ECMP() adjacency allows the use of just one BP to deliver packets
to one of N adjacencies instead of one BP for each adjacency. In the
common example case shown in Figure 10, a link bundle of three links
L1,L2,L3 connects BFR1 and BFR2, and only one BP is used instead of
three BPs to deliver packets from BFR1 to BFR2.
--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 on 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 example shown in Figure 12, 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; rather, it illustrates
that ECMP can be used to share BPs not only across link bundles but
also across alternative paths across different transit BFRs, and it
explains the use of the seed parameter.
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
Note that for the following discussion of ECMP, only the BIFT ECMP()
adjacencies on BFR1, BFR2, and BFR3 are relevant. The reuse of BPs
across BFRs in this example is further explained in Section 5.1.9
below.
With the ECMP setup shown 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 two adjacencies
given as parameters to the ECMP: link L11-to-BFR2. BFR2 again
performs 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. Therefore, L22 and BFR5 see no traffic
(likewise for L31 and BFR6).
This issue in BFR2/BFR3 is called "polarization". It results from
the reuse 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 (i.e., 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 BIER-TE controller the ability to control the
selection of the same path or different paths 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 rather is loose-hop selection.
Forward_routed() adjacencies can also permit BIER-TE operation across
intermediate-hop routers that do not support BIER-TE.
Assume that 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 next three
segments: (1) BFR2 via link L1, (2) BFR2 via link L2, or (3) via BFR3
and then not caring whether the packet is forwarded via L3 or L4.
...............
...BFR1--... ...--L1-- BFR2...
... .Routers. ...--L2--/
...BFR4--... ...--L3-- BFR3...
... ...--L4--/ |
............... |
LO
Network Area 1
Figure 13: Forward Routed Adjacencies Example
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 nodes that are not BIER-TE enabled.
5.1.9. Reuse of Bit Positions (without DNC)
BPs can be reused across multiple BFRs to minimize the number of BPs
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 a given BP is cleared when passing a BFR with an adjacency
for that BP, reusing BPs 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 BPs
is whether the desired Tree Engineering goals can still be achieved.
A 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. A BP can be set across BFRs that would only
occur across (A) different paths or (B) 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 that the example was changed: BFR1 has no ECMP() adjacency for
BP 0:6 but instead has 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 0:6 would now be able to reach both BFR2 and BFR3, and the still-
existing reuse of BP 0:7 between BFR2 and BFR3 is a case of (B) where
reusing a BP is perfect because it does not limit the set of useful
path choices, as in the following example.
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 were undesirable, this would be done
by not setting BP 0:5 and BP 0:6 for the same packet. If the
duplicates were desirable (e.g., resilient transmission), the
additional BP 0:10 would also not render additional value.
Reuse may also save BPs in larger topologies. Consider the topology
shown in Figure 14.
area1
BFR1a BFR1b
/ \
....................................
. Core .
....................................
| / \ / \ |
BFR2a BFR2b BFR3a BFR3b BFR6a BFR6b
/-------\ /---------\ /--------\
| area2 | | area3 | ... | area6 |
| ring | | ring | | ring |
\-------/ \---------/ \--------/
more BFRs more BFRs more BFRs
Figure 14: Reuse of BPs
A BFIR/sender (e.g., video headend) is attached to area 1, and the
five areas 2...6 contain receivers/BFERs. Assume that each area has
a distribution ring, each with two BPs to indicate the direction (as
explained before). These two BPs could be reused across the five
areas. Packets would be replicated through other BPs from 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; however, if the bandwidth of each ring
were to be optimized separately, it would not be a good limitation.
5.1.10. Summary of BP Optimizations
In this section, we 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.
Without any optimization, a BIER-TE controller would attempt to map
the network subnet topology 1:1 into the BIER-TE topology, every
adjacent neighbor in the subnet would require a forward_connected()
BP, and every BFER would require a local_decap() BP.
The optimizations described in this document are then as follows:
1. P2P links require only one BP (Section 5.1.1).
2. All leaf BFERs can share a single local_decap() BP
(Section 5.1.3).
3. A LAN with N BFRs needs at most N BPs (one for each BFR). It
only needs one BP for all those BFRs that are not redundantly
connected to multiple LANs (Section 5.1.4).
4. A hub with P2P connections to multiple non-leaf BFER spokes can
share one BP with all of the spokes if traffic can be flooded to
all of those spokes, e.g., because of no bandwidth concerns or
dense receiver sets (Section 5.1.5).
5. Rings of BFRs can be built with just two BPs (one for each
direction), except for BFRs with multiple ring connections --
similar to LANs (Section 5.1.6).
6. ECMP() adjacencies to N neighbors can replace N BPs with one BP.
Multihop ECMP can avoid polarization through different seeds of
the ECMP algorithm (Section 5.1.7).
7. Forward_routed() adjacencies permit "tunneling" across routers
that are either BIER-TE capable or not BIER-TE capable where no
traffic steering or replications are required (Section 5.1.8).
8. A 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 the scenario, this may limit
the feasible path steering options (Section 5.1.9).
Note that this list of optimizations is not exhaustive. Further
optimizations of BPs are possible, especially when both the set of
required path steering choices and the possible subsets of BFERs that
should be able to receive traffic are limited. The hub-and-spoke
optimization is a simple example of such traffic-pattern-dependent
optimizations.
5.2. Avoiding Duplicates and Loops
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 prevents packets from looping. The
only exceptions are adjacencies with DNC set.
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 ring's clockwise
bit position is never cleared for copies of the packets traveling
clockwise around the ring.
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
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., "Media Access Control" (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
Duplicates happen when the graph expressed by a BitString is not a
tree but is redundantly connecting BFRs with each other. In
Figure 16, a BitString of p2,p3,p4,p5 would result in duplicate
packets arriving on BFER4. The BIER-TE controller must therefore
ensure that only BitStrings that are trees are created.
BFIR1
/ \
/ p2 \ p3
BFR2 BFR3
\ p4 / p5
\ /
BFER4
Figure 16: Duplicates Example
When links are incorrectly physically reconnected 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 are equally
likely to happen. Such readdressing operations must be coordinated
with the BIER-TE controller.
5.3. Managing SIs, Subdomains, and BFR-ids
When the number of bits required to represent the necessary hops in
the topology and BFERs exceeds the supported "BitStringLength" (BSL),
multiple SIs and/or subdomains must be used. This section discusses
how this is done.
BIER-TE forwarding does not require the concept of BFR-ids, but
routing underlay, flow overlay, and BIER headers may. This section
also discusses how BFR-ids can be assigned to BFIRs/BFERs for BIER-
TE.
5.3.1. Why SIs and Subdomains?
For (non-TE) BIER and BIER-TE forwarding, the most important result
of using multiple SIs and/or subdomains is the same: multicast flow
overlay packets that need to be sent to BFERs in different SIs or
subdomains require multiple BIER packets, each one with a BitString
for a different (SI,subdomain) combination. Each such BitString uses
one BSL-sized SI block in the BIFT of the subdomain. We call this a
BIFT:SI (block).
SIs and subdomains have different purposes in the BIER architecture
and also the BIER-TE architecture. This impacts how operators manage
them and especially how flow overlays will likely use them.
By default, every possible BFIR/BFER in a BIER network would likely
be given a BFR-id in subdomain 0 (unless there are > 64k BFIRs/
BFERs).
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 subdomain 0. Ideal replication efficiency for
N BFERs exists in a subdomain if they are split over no more than
ceiling(N/BitStringLength) SIs.
If service instances justify additional BIER:SI state in the network,
additional subdomains will be used: BFIRs/BFERs are assigned BFR-ids
in those subdomains, and each service instance is configured to use
the most appropriate subdomain. This results in improved replication
efficiency for different services.
Even if creation of subdomains and assignment of BFR-ids to BFIRs/
BFERs in those subdomains is automated, it is not expected that
individual service instances can deal with BFERs in different
subdomains. A service instance may only support configuration of a
single subdomain 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 SIs
and subdomains logically in the same way as they are used in BIER:
all necessary BFIRs/BFERs 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 subdomains that primarily exist
to provide more efficient replication (and, for BIER-TE, desirable
path steering) for different subsets of BFIRs/BFERs.
5.3.2. Assigning Bits for the BIER-TE Topology
In BIER, BitStrings only need to carry bits for BFERs; this leads to
the model where 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 BFERs 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 means that it depends on the physical topology and
the operator's desire to
1. permit explicit path steering across every single hop (which
requires more bits), or
2. reduce 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, for
path steering reasons, different trees do not need to take
different paths.
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 will allow
the expression of desirable path steering alternatives.
5.3.3. Assigning BFR-ids with BIER-TE
BIER-TE forwarding does not use BFR-ids, nor does it require that the
BFIR-id field of the BIER header 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; 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 (i.e., will only be 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
that used for (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-ids, 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 that is only populated on this BFIR; for a
leaf BFER, this could be the first BP with an adjacency towards that
BFER.
5.3.4. Mapping from BFRs 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 perform 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 to or deleted from a
particular distribution tree, the BIER-TE controller has to
recalculate the branches to other BFERs, 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 BFIR can
then, independently of the controller, calculate the SI:BitString for
all desired BFERs by ORing their BitStrings. This allows flow
overlay applications to operate independently of the controller
whenever they need to determine which subset of BFERs needs to
receive a particular packet.
If "interdependent branches" are required, an application would need
to query the SI:BitString for a given set of BFERs whenever the set
changes.
Note that in either case (unlike the scenario for BIER), the bits may
need to change upon link/node failure/recovery, network expansion, or
network resource consumption by other traffic as part of achieving
Traffic Engineering goals (e.g., reoptimization of lower-priority
traffic flows). Interactions between such BFIR applications and the
BIER-TE controller do therefore need to support dynamic updates to
the SIs:BitStrings.
Communications between the BFIR flow overlay and the BIER-TE
controller require some way to identify the BFERs. If BFR-ids are
used in the deployment, as outlined in Section 5.3.3, then those are
the "natural" BFR-ids. If BFR-ids are not used, then any other
unique identifier, such as a BFR's BFR-prefix [RFC8279], could be
used.
5.3.5. Assigning BFR-ids for BIER-TE
It is not currently determined if a single subdomain 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-ids in the same subdomain.
This allows higher replication efficiency for BIER because the
BIER BFR-ids can be assigned sequentially, while the BitStrings
for BIER-TE will also have to assign the additional bits for the
topology adjacencies. 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 six areas, each with 170 BFERs,
connecting via a core with four (core) BFRs. To address all BFERs
with BIER, four SIs are required. To send a BIER packet to all BFERs
in the network, four copies need to be sent by the BFIR. On the
BFIR, it does not matter how the BFR-ids are allocated to BFERs in
the network, but it does matter for efficiency further down in the
network.
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
With random allocation of BFR-ids to BFERs, each receiving area would
(most likely) have to receive all four copies of the BIER packet
because there would be BFR-ids for each of the four SIs in each of
the areas. Only further towards each BFER would this duplication
subside -- when each of the four trees runs out of branches.
If BFR-ids are allocated intelligently, then all the BFERs in an area
would be given BFR-ids with as few different SIs as possible. Each
area would only have to forward one or two packets instead of four.
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-ids
in one SI might end up with many SIs over a longer period of growth.
Allocating SIs to areas that initially have sufficiently many spare
bits for growth can help alleviate this issue. Alternatively, BFERs
can be renumbered after network expansion. In this example, one may
consider using six SIs and assigning one to each area.
This example shows that intelligent BFR-id allocation within at least
subdomain 0 can 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 BFERs fit into a single BitString. This process
needs to be repeated until the whole topology is covered.
Once bits/SIs are assigned to the topology and BFERs, BFR-ids are
just a derived set of identifiers from the operator / BIER-TE
controller as explained above.
Whenever different subtopologies have overlap, bits need to be
repeated across the BitStrings, increasing the overall amount of bits
required across all BitStrings/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 SIs and therefore reduces the
number of BFERs 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 shown in Figure 17,
the following bit allocation method can be used. This method can
easily be expanded to other, similarly structured larger topologies.
Each area is allocated one or more SIs, depending on the number of
future expected BFERs and the number of bits required for the
topology in the area. In this example, six SIs are used, one per
area.
In addition, we use four bits in each SI:
bia: (b)it (i)ngress (a)
bib: (b)it (i)ngress (b)
bea: (b)it (e)gress (a)
beb: (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 BFRs and egress area a/b BFRs into a
specific target area. These bits are then set up with the right
forward_routed() adjacencies on the BFIRs and area edge BFRs as
follows.
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 BFRs, 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 six copies, with SI=1...SI=6. In
each packet, the BitString includes bits for one area and the BFERs
in that area, 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
BFERs that are in the same area as the BFIR, these four bits are not
used.
5.3.7. Summary
BIER-TE can, like BIER, support multiple SIs within a subdomain.
This allows application of the mapping BFR-id = SI * BSL + BP. This
also permits the reuse of the BIER architecture concept of BFR-ids
and, therefore, minimization of 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 BFIRs/BFERs possible in a subdomain is smaller than in
BIER because BIER-TE uses additional bits for the topology.
Subdomains in BIER-TE can be used as they are 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 the overall amount of bits required.
6. Security Considerations
If "Encapsulation for Bit Index Explicit Replication (BIER) in MPLS
and Non-MPLS Networks" [RFC8296] is used, its security considerations
also apply to BIER-TE.
The security considerations of "Multicast Using Bit Index Explicit
Replication (BIER)" [RFC8279] also apply to BIER-TE, 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 not
only includes the need to filter received unicast "tunneled" BIER
packets to prohibit the injection of such "tunneled" BIER packets
from outside the BIER domain but also the need to prohibit
forward_routed() adjacencies from leaking 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 unicast "tunneled"
BIER packets even if the interface cannot 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 the impairment of
any BFRs in a domain, may lead to successful attacks against the
information that BIER-TE learns from the routing protocol (routes,
next hops, BFR-ids, ...), enabling DoS attacks against paths or the
addressing (BFR-ids, 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, the 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.
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 "A Path Computation Element (PCE)-Based
Architecture" [RFC4655] apply.
The most important attack vector in BIER-TE is misconfiguration,
either on the BFRs 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 a looping packet from creating
additional packet amplification through the misconfigured loop on the
packet's second time or subsequent times around the loop, because all
relevant adjacency bits would have been cleared on the first round
through the loop. As a result, looping packets can occur in BIER-TE
to the same degree as is 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, e.g., in embedded networks or in manufacturing
networks during such activities as plant reworking or repairs. In
these types 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 to non-
production state. Such security designs would not only allow a
deployment to provide additional layers of protection against
configuration attacks but would, first and foremost, protect the
active production process from such configuration attacks.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. 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>.
8.2. Informative References
[BIER-MCAST-OVERLAY]
Trossen, D., Rahman, A., Wang, C., and T. Eckert,
"Applicability of BIER Multicast Overlay for Adaptive
Streaming Services", Work in Progress, Internet-Draft,
draft-ietf-bier-multicast-http-response-06, 10 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-
multicast-http-response-06>.
[BIER-TE-PROTECTION]
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://datatracker.ietf.org/doc/html/draft-eckert-bier-
te-frr-03>.
[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-05, 1 May 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-bier-te-
yang-05>.
[Bloom70] Bloom, B. H., "Space/time trade-offs in hash coding with
allowable errors", Comm. ACM 13(7):422-6,
DOI 10.1145/362686.362692, July 1970,
<https://dl.acm.org/doi/10.1145/362686.362692>.
[CONSTRAINED-CAST]
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://datatracker.ietf.org/doc/html/
draft-ietf-roll-ccast-01>.
[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, DOI 10.1109/ICC.2016.7511036, May 2016,
<https://ieeexplore.ieee.org/document/7511036>.
[NON-MPLS-BIER-ENCODING]
Wijnands, IJ., 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://datatracker.ietf.org/doc/html/draft-ietf-bier-
non-mpls-bift-encoding-04>.
[RCSD94] Zhang, H. and D. Ferrari, "Rate-Controlled Service
Disciplines", Journal of High Speed Networks, Volume 3,
Issue 4, pp. 389-412, DOI 10.3233/JHS-1994-3405, October
1994, <https://content.iospress.com/articles/journal-of-
high-speed-networks/jhs3-4-05>.
[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>.
[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>.
[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>.
[TE-OVERVIEW]
Farrel, A., Ed., "Overview and Principles of Internet
Traffic Engineering", Work in Progress, Internet-Draft,
draft-ietf-teas-rfc3272bis-21, 11 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-teas-
rfc3272bis-21>.
Appendix A. BIER-TE and Segment Routing (SR)
SR [RFC8402] aims to enable lightweight path steering via loose
source routing. For example, compared to its more heavyweight
predecessor, RSVP-TE, SR does not require per-path signaling to each
of these hops.
BIER-TE supports the same design philosophy for multicast. Like SR,
BIER-TE
* relies on source routing (via a BitString), and
* only requires consideration of the "hops" either (1) on which
replication has to happen or (2) across which the traffic should
be steered (even without replication).
Any other hops can be skipped via the use of routed adjacencies.
BIER-TE "bit positions" (BPs) can be understood as the BIER-TE
equivalent of "forwarding segments" in SR, but they have a different
scope than do forwarding segments in SR. Whereas forwarding segments
in SR are global or local, BPs in BIER-TE have a scope that is
comprised of one or more BFRs that have adjacencies for the BPs in
their BIFTs. These segments can be called "adjacency-scoped"
forwarding segments.
Adjacency scope could be global, but then every BFR would need an
adjacency for a given BP -- for example, a forward_routed() adjacency
with encapsulation to the global SR "Segment Identifier" (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 traffic on it. Only by using non-global adjacency
scope for BPs can traffic be steered and replicated on a non-BFIR.
SR can naturally be combined with BIER-TE and can help optimize it.
For example, instead of defining bit positions for non-replicating
hops, it is equally possible to use SR encapsulations (e.g., SR-MPLS
label stacks) for the encapsulation of "forward_routed()"
adjacencies.
Note that (non-TE) BIER itself can also be seen as being 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. BIER does not
allow the indication of intermediate hops or, in terms of SR, the
ability to indicate a sequence of SIDs to reach the destination. On
the other hand, BIER-TE and its adjacency-scoped BPs provide these
capabilities.
Acknowledgements
The authors would like to thank Greg Shepherd, IJsbrand Wijnands,
Neale Ranns, Dirk Trossen, Sandy Zheng, Lou Berger, Jeffrey Zhang,
Carsten Bormann, and Wolfgang Braun for their reviews and
suggestions.
Special thanks to Xuesong Geng for shepherding this document.
Special thanks also for IESG review/suggestions by Alvaro Retana
(responsible AD/RTG), Benjamin Kaduk (SEC), Tommy Pauly (TSV),
Zaheduzzaman Sarker (TSV), Éric Vyncke (INT), Martin Vigoureux (RTG),
Robert Wilton (OPS), Erik Kline (INT), Lars Eggert (GEN), Roman
Danyliw (SEC), Ines Robles (RTGDIR), Robert Sparks (Gen-ART),
Yingzhen Qu (RTGDIR), and Martin Duke (TSV).
Authors' Addresses
Toerless Eckert (editor)
Futurewei Technologies Inc.
2330 Central Expy
Santa Clara, CA 95050
United States of America
Email: tte@cs.fau.de
Michael Menth
University of Tuebingen
Germany
Email: menth@uni-tuebingen.de