wdiff rfc8279.alt-original rfc8279.txt

Internet Engineering Task Force (IETF) IJ. Wijnands, Ed.
Internet-Draft
Request for Comments: 8279 Cisco Systems, Inc.
Intended status:
Category: Experimental E. Rosen, Ed.
Expires: March 17, 2018
ISSN: 2070-1721 Juniper Networks, Inc.
A. Dolganow
Nokia
T. Przygienda
Juniper Networks, Inc.
S. Aldrin
Google, Inc.
September 13,
November 2017

Multicast using Using Bit Index Explicit Replication
draft-ietf-bier-architecture-08 (BIER)

Abstract

This document specifies a new architecture for the forwarding of
multicast data packets. It provides optimal forwarding of multicast
packets through a "multicast domain". However, it does not require a
protocol for explicitly building multicast distribution trees, nor
does it require intermediate nodes to maintain any per-flow state.
This architecture is known as "Bit Index Explicit Replication"
(BIER). When a multicast data packet enters the domain, the ingress
router determines the set of egress routers to which the packet needs
to be sent. The ingress router then encapsulates the packet in a
BIER header. The BIER header contains a bitstring bit string in which each bit
represents exactly one egress router in the domain; to forward the
packet to a given set of egress routers, the bits corresponding to
those routers are set in the BIER header. The procedures for
forwarding a packet based on its BIER header are specified in this
document. Elimination of the per-flow state and the explicit tree-
building protocols results in a considerable simplification.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.

Internet-Drafts are working documents
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a maximum candidate for any level of
Internet Standard; see Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on March 17, 2018.
https://www.rfc-editor.org/info/rfc8279.

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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................4
2. The BFR Identifier and BFR-Prefix . . . . . . . . . . . . . . 6 ...............................7
3. Encoding BFR Identifiers in BitStrings . . . . . . . . . . . 7 ..........................8
4. Layering . . . . . . . . . . . . . . . . . . . . . . . . . . 10 .......................................................11
4.1. The Routing Underlay . . . . . . . . . . . . . . . . . . 10 ......................................11
4.2. The BIER Layer . . . . . . . . . . . . . . . . . . . . . 11 ............................................12
4.3. The Multicast Flow Overlay . . . . . . . . . . . . . . . 12 ................................13
5. Advertising BFR-ids and BFR-Prefixes . . . . . . . . . . . . 12 ...........................13
6. BIER Intra-Domain Forwarding Procedures . . . . . . . . . . . 14 ........................15
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14 ..................................................15
6.2. BFR Neighbors . . . . . . . . . . . . . . . . . . . . . . 16 .............................................17
6.3. The Bit Index Routing Table . . . . . . . . . . . . . . . 16 ...............................18
6.4. The Bit Index Forwarding Table . . . . . . . . . . . . . 17 ............................19
6.5. The BIER Forwarding Procedure . . . . . . . . . . . . . . 18 .............................20
6.6. Examples of BIER Forwarding . . . . . . . . . . . . . . . 20 ...............................23
6.6.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . 21 ..........................................23
6.6.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . 22 ..........................................24
6.7. Equal Cost Multi-path Equal-Cost Multipath Forwarding . . . . . . . . . . . . 24 ...........................26
6.7.1. Non-deterministic ECMP . . . . . . . . . . . . . . . 24 .............................27
6.7.2. Deterministic ECMP . . . . . . . . . . . . . . . . . 25 .................................28
6.8. Prevention of Loops and Duplicates . . . . . . . . . . . 27 ........................29
6.9. When Some Nodes do not Do Not Support BIER . . . . . . . . . . . 28 .......................30
6.10. Use of Different BitStringLengths within a Domain . . . . 30 ........33
6.10.1. BitStringLength Compatibility Check . . . . . . . . 32 ...............34
6.10.2. Handling BitStringLength Mismatches . . . . . . . . 33 ...............36
6.10.3. Transitioning from One BitStringLength to
Another . 34 ...........................................36
7. Operational Considerations . . . . . . . . . . . . . . . . . 34 .....................................37
7.1. Configuration . . . . . . . . . . . . . . . . . . . . . . 35 .............................................37
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 ............................................37
9. Security Considerations . . . . . . . . . . . . . . . . . . . 35 ........................................38
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
11. Contributor Addresses . . . . . . . . . . . . . . . . . . . . 36
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
12.1. ....................................................39
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
12.2. .....................................39
10.2. Informative References . . . . . . . . . . . . . . . . . 38 ...................................39
Acknowledgements ..................................................40
Contributors ......................................................41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 ................................................43

1. Introduction

This document specifies a new architecture for the forwarding of
multicast data packets. The architecture provides optimal forwarding
of multicast data packets through a "multicast domain". However, it
does not require the use of a protocol for explicitly building
multicast distribution trees, and it does not require intermediate
nodes to maintain any per-flow state. This architecture is known as
"Bit Index Explicit Replication" (BIER).

A router that supports BIER is known as a "Bit-Forwarding Router"
(BFR). The BIER control plane control-plane protocols (see Section 4.2) run within
a "BIER domain", allowing the BFRs within that domain to exchange the
information needed for them to forward packets to each other
using BIER.

A multicast data packet enters a BIER domain at a "Bit-Forwarding
Ingress Router" (BFIR), and leaves the BIER domain at one or more
"Bit-Forwarding Egress Routers" (BFERs). A BFR that receives a
multicast data packet from another BFR in the same BIER domain, and
forwards the packet to another BFR in the same BIER domain, will be
known as a "transit BFR" for that packet. A single BFR may be a BFIR
for some multicast traffic while also being a BFER for some multicast
traffic and a transit BFR for some multicast traffic. In fact, for a
given packet, a BFR may be a BFIR and/or a transit BFR and/or (one
of) the BFER(s) for that packet.

A BIER domain may contain one or more sub-domains. Each BIER domain
MUST contain at least one sub-domain, the "default sub-domain" (also
denoted "sub-domain zero"). 0"). If a BIER domain contains more than one
sub-domain, each BFR in the domain MUST be provisioned to know the
set of sub-domains to which it belongs. Each sub-domain is
identified by a sub-domain-id in the range [0,255].

For each sub-domain to which a given BFR belongs, if the BFR is
capable of acting as a BFIR or a BFER, it MUST be provisioned with a
"BFR-id" that is unique within the sub-domain. A BFR-id is a small
unstructured positive integer. For instance, if a particular BIER
sub-domain contains 1,374 BFRs, each one could be given a BFR-id in
the range 1-1374. [1,1374].

If a given BFR belongs to more than one sub-domain, it may (though it
need not) have a different BFR-id for each sub-domain.

When a multicast packet arrives from outside the domain at a BFIR,
the BFIR determines the set of BFERs to which the packet will be
sent. The BFIR also determines the sub-domain in which the packet
will be sent. Determining the sub-domain in which a given packet
will be sent is known as "assigning the packet to a sub-domain".
Procedures for choosing the sub-domain to which a particular packet
is assigned are outside the scope of this document. However, once a
particular packet has been assigned to a particular sub-domain, it
remains assigned to that sub-domain until it leaves the BIER domain.
That is, the sub-domain to which a packet is assigned MUST NOT be
changed while the packet is in flight through the BIER domain.

Once the BFIR determines the sub-domain and the set of BFERs for a
given packet, the BFIR encapsulates the packet in a "BIER header".
The BIER header contains a bit string in which each bit represents a
single BFR-id. To indicate that a particular BFER is to receive a
given packet, the BFIR sets the bit corresponding to that BFER's
BFR-id in the sub-domain to which the packet has been assigned. We
will use the term "BitString" to refer to the bit string field in the
BIER header. We will use the term "payload" to refer to the packet
that has been encapsulated. Thus Thus, a "BIER-encapsulated" packet
consists of a "BIER header" followed by a "payload".

The number of BFERs to which a given packet can be forwarded is
limited only by the length of the BitString in the BIER header.
Different deployments can use different BitString lengths. We will
use the term "BitStringLength" to refer to the number of bits in the
BitString. It is possible that some deployment deployments will have more BFERs
in a given sub-domain than there are bits in the BitString. To
accommodate this case, the BIER encapsulation includes both the
BitString and a "Set Identifier" (SI). It is the BitString and the
SI together that determine the set of BFERs to which a given packet
will be delivered:

o by By convention, the least significant (rightmost) bit in the
BitString is "bit 1", and the most significant (leftmost) bit is
"bit BitStringLength".

o if If a BIER-encapsulated packet has an SI of n, n and a BitString with
bit k set, then the packet must be delivered to the BFER whose
BFR-id (in the sub-domain to which the packet has been assigned)
is n*BitStringLength+k.

For example, suppose the BIER encapsulation uses a BitStringLength of
256 bits. By convention, the least significant (rightmost) bit is
"bit 1",
bit 1, and the most significant (leftmost) bit is "bit 256". bit 256. Suppose
that a given packet has been assigned to sub-domain 0, 0 and needs to be
delivered to three BFERs, where those BFERs have BFR-ids in
sub-domain 0 of 13, 126, and 235 235, respectively. The BFIR would
create a BIER encapsulation with the SI set to zero, zero and with bits 13,
126, and 235 of the BitString set. (All other bits of the BitString
would be clear.) If the packet also needs to be sent to a BFER whose
BFR-id is 257, the BFIR would have to create a second copy of the
packet, and the BIER encapsulation would specify an SI of 1, and a
BitString with bit 1 set and all the other bits clear.

It is generally advantageous to assign the BFR-ids of a given
sub-domain so that as many BFERs as possible can be represented in a
single bit string.

Suppose a BFR, call BFR (call it BFR-A, "BFR-A") receives a packet whose BIER
encapsulation specifies an SI of 0, 0 and a BitString with bits 13, 26,
and 235 set. Suppose BFR-A has two BFR neighbors, BFR-B and BFR-C,
such that the best path to BFERs 13 and 26 is via BFR-B, but the best
path to BFER 235 is via BFR-C. Then BFR-A will then replicate the packet,
sending one copy to BFR-B and one copy to BFR-C. However, BFR-A will
clear bit 235 in the BitString of the packet copy it sends to BFR-B, BFR-B
and will clear bits 13 and 26 in the BitString of the packet copy it
sends to BFR-C. As a result, BFR-B will forward the packet only
towards BFERs 13 and 26, and BFR-C will forward the packet only
towards BFER 235. This ensures that each BFER receives only one copy
of the packet.

Detailed procedures for forwarding a BIER-encapsulated packet through
a BIER domain can be found in Section 6.

With this forwarding procedure, a multicast data packet can follow an
optimal path from its BFIR to each of its BFERs. Further, since the
set of BFERs for a given packet is explicitly encoded into the BIER
header, the packet is not sent to any BFER that does not need to
receive it. This allows for optimal forwarding of multicast traffic.
This optimal forwarding is achieved without any need for transit BFRs
to maintain per-flow state, state or to run a multicast tree-building
protocol.

The idea of encoding the set of egress nodes into the header of a
multicast packet is not new. For example, [Boivie_Feldman] proposes
to encode the set of egress nodes as a set of IP addresses, and
proposes mechanisms and procedures that are in some ways similar to
those described in the current document. However, since BIER encodes
each BFR-id as a single bit in a bit string, it can represent up to
128 BFERs in the same number of bits that it would take to carry the
IPv6 address of a single BFER. Thus Thus, BIER scales to a much larger
number of egress nodes per packet.

BIER does not require that each transit BFR look up the best path to
each BFER that is identified in the BIER header; the number of
lookups required in the forwarding path for a single packet can be
limited to the number of neighboring BFRs; this can be much smaller
than the number of BFERs. See Section 6 (especially Section 6.4) 6.5) for
details.

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 RFC 2119 [RFC2119].
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. The BFR Identifier and BFR-Prefix

Each BFR MUST be assigned a single "BFR-Prefix" "BFR-prefix" for each sub-domain
to which it belongs. A BFR's BFR-Prefix BFR-prefix MUST be an IP address
(either IPv4 or IPv6) of the BFR. It is RECOMMENDED that the
BFR-prefix be a loopback address of the BFR.

If a BFR belongs to more than one sub-domain, it may (though it need
not) have a different BFR-prefix in each sub-domain.

All BFR-Prefixes BFR-prefixes used within a given sub-domain MUST belong to the
same address family (either IPv4 or IPv6).

The BFR-prefix of a given BFR in a given sub-domain MUST be routable
in that sub-domain. Whether a particular BFR-Prefix BFR-prefix is routable in a
given sub-domain depends on the "routing underlay" associated with
that sub-domain. The notion of "routing underlay" is described in
Section 4.1.

A "BFR Identifier" (BFR-id) is a number in the range [1,65535].
Within a given sub-domain, every BFR that may need to function as a
BFIR or BFER MUST have a single BFR-id, which identifies it uniquely
within that sub-domain. A BFR that does not need to function as a
BFIR or BFER in a given sub-domain does not need to have a BFR-id in
that sub-domain.

The value 0 is not a legal BFR-id.

The procedure for assigning a particular BFR-id to a particular BFR
is outside the scope of this document. However, it is RECOMMENDED
that the BFR-ids for each sub-domain be assigned "densely" from the
numbering space, as this will result in a more efficient encoding
(see Section 3). That is, if there are 256 or fewer BFERs, it is
RECOMMENDED to assign all the BFR-ids from the range [1,256]. If
there are more than 256 BFERs, BFERs but less than 512, it is RECOMMENDED to
assign all the BFR-ids from the range [1,512], with as few "holes" as
possible in the earlier range. However, in some deployments, it may
be advantageous to depart from this recommendation; this is discussed
further in Section 3.

In some deployments, it may not be possible to support (in a given
sub-domain) the full range of 65535 65,535 BFR-ids. For example, if the
BFRs in a given sub-domain only support 16 SIs and if they only
support BitStringLengths of 256 or less, then only 16*256=4096 16*256=4,096
BFR-ids can be supported in that sub-domain.

3. Encoding BFR Identifiers in BitStrings

To encode a BFR-id in a BIER data packet, one must convert the BFR-id
to an SI and a BitString. This conversion depends upon the parameter
we are calling "BitStringLength". The conversion is done as follows.
If the BFR-id is N, then

o SI is the integer part of the quotient (N-1)/BitStringLength (N-1)/BitStringLength.

o The BitString has one bit position set. If the low-order bit is
bit 1, 1 and the high-order bit is bit BitStringLength, the bit
position that represents BFR-id N is
((N-1) modulo BitStringLength)+1.

If several different BFR-ids all resolve to the same SI, then all of
those BFR-ids can be represented in a single BitString. The
BitStrings for all of those BFR-ids are combined using a bitwise
logical OR operation.

Within a given BIER domain (or even within a given BIER sub-domain),
different values of BitStringLength may be used. Each BFR MUST be
provisioned to know the following:

o the The BitStringLength ("Imposition BitStringLength") and sub-domain
("Imposition sub-domain") to use when it imposes (as a BFIR) a
BIER encapsulation on a particular set of packets, and

o the The BitStringLengths ("Disposition BitStringLengths") that it will
process when (as a BFR or BFER) it receives packets from a
particular sub-domain.

It is not required that a BFIR use the same Imposition
BitStringLength or the same Imposition sub-domain for all packets on
which it imposes the BIER encapsulation. However, if a particular
BFIR is provisioned to use a particular Imposition BitStringLength
and a particular Imposition sub-domain when imposing the
encapsulation on a given set of packets, all other BFRs with BFR-ids
in that sub-domain SHOULD be provisioned to process received BIER
packets with that BitStringLength (i.e., all other BFRs with BFR-ids
in that sub-domain SHOULD be provisioned with that BitStringLength as
a Disposition BitStringLength for that sub-domain. sub-domain). Exceptions to
this rule MAY be made under certain conditions; this is discussed in
Section 6.10.

When a BIER encapsulation is specified, the specification MUST define
a default BitStringLength for the encapsulation. Every BFIR
supporting that encapsulation MUST be capable of being provisioned
with that default BitStringLength as its Imposition BitStringLength.
Every BFR and BFER supporting that encapsulation MUST be capable of
being provisioned with that default BitStringLength as a Disposition
BitStringLength.

The specification of a BIER encapsulation MAY also allow the use of
other BitStringLengths.

If a BFR is capable of being provisioned with a given value of
BitStringLength as an Imposition BitStringLength, it MUST also be
capable of being provisioned with that same value as one of its
Disposition BitStringLengths. It SHOULD be capable of being
provisioned with all each legal smaller values value of BitStringLength as both (a)
its Imposition BitStringLength, and (b) one of its Disposition BitStringLength.
BitStringLengths.

In order to support transition from one BitStringLength to another,
every BFR MUST be capable of being provisioned to simultaneously use
two different Disposition BitStringLengths.

A BFR MUST support SI values in the range [0,15], [0,15] and MAY support SI
values in the range [0,255]. ("Supporting the values in a given
range" means, in this context, that any value in the given range is
legal,
legal and will be properly interpreted.) Note that for a given
BitStringLength, the total number of BFR-ids that can be represented
is the product of the BitStringLength and the number of supported
SIs. For example, if a deployment uses (in a given sub-domain) a
BitStringLength of 64 and supports 256 SIs, that deployment can only
support 16384 BFR-ids in that sub-domain. Even a deployment that
supports 256 SIs will not be able to support 65535 65,535 BFR-ids unless it
uses a BitStringLength of at least 256.

When a BFIR determines that a multicast data packet, assigned to a
given sub-domain, needs to be forwarded to a particular set of
destination BFERs, the BFIR partitions that set of BFERs into
subsets, where each subset contains the target BFERs whose BFR-ids in
the given sub-domain all resolve to the same SI. Call these the
"SI-subsets" for the packet. Each SI-subset can be represented by a
single BitString. The BFIR creates a copy of the packet for each
SI-subset. The BIER encapsulation is then applied to each packet.
The encapsulation specifies a single SI for each packet, packet and contains
the BitString that represents all the BFR-ids in the corresponding
SI-subset. Of course, in order to properly interpret the BitString,
it must be possible to infer the sub-domain-id from the encapsulation
as well.

Suppose, for example, that a BFIR determines that a given packet
needs to be forwarded to three BFERs, whose BFR-ids (in the
appropriate sub-domain) are 27, 235, and 497. The BFIR will have to
forward two copies of the packet. One copy, associated with SI=0,
will have a BitString with bits 27 and 235 set. The other copy,
associated with SI=1, will have a BitString with bit 241 set.

In order to minimize the number of copies that must be made of a
given multicast packet, it is RECOMMENDED that the BFR-ids used in a
given sub-domain be assigned "densely" (see Section 2) from the
numbering space. This will minimize the number of SIs that have to
be used in that sub-domain. However, depending upon the details of a
particular deployment, other assignment methods may be more
advantageous. Suppose, for example, that in a certain deployment,
every multicast flow is either intended either for the "east coast" or for
the "west coast". coast", but not for both coasts. In such a deployment, it
would be advantageous to assign BFR-ids so that all the "west coast"
BFR-ids fall into the same SI-subset, SI-subset and so that all the "east coast"
BFR-ids fall into the same SI-subset.

When a BFR receives a BIER data packet, it will infer the SI from the
encapsulation. The set of BFERs to which the packet needs to be
forwarded can then be inferred from the SI and the BitString.

In some of the examples given later in this document, we will use a
BitStringLength of 4, 4 and will represent a BFR-id in the form
"SI:xyzw", where SI is the Set Identifier of the BFR-id (assuming a
BitStringLength of 4), 4) and xyzw is a string of 4 bits. A
BitStringLength of 4 is used only in the examples; we would not
expect actual deployments to have such a small BitStringLength.

It is possible that several different forms of BIER encapsulation
will be developed. If so, the particular encapsulation that is used
in a given deployment will depend on the type of network
infrastructure that is used to realize the BIER domain. Details of
the BIER encapsulation(s) will be given in companion documents. An
encapsulation for use in MPLS networks is described in
[MPLS_BIER_ENCAPS]; that document also describes a very similar
encapsulation that can be used in non-MPLS networks.

4. Layering

It is helpful to think of the BIER architecture as consisting of
three layers: the "routing underlay", the "BIER layer", and the
"multicast flow overlay".

4.1. The Routing Underlay

The "routing underlay" establishes "adjacencies" between pairs of
BFRs,
BFRs and determines one or more "best paths" from a given BFR to a
given set of BFRs. Each such path is a sequence of BFRs
<BFR(k), BFR(k+1), ..., BFR(k+n)> such that BFR(k+j) is "adjacent" to
BFR(k+j+1) (for 0<=j<n).

At a given BFR, say BFR-A, for every IP address that is the address
of a BFR in the BIER domain, the routing underlay will map that IP
address into a set of one or more "equal cost" "equal-cost" adjacencies. If a
BIER data packet has to be forwarded by BFR-A to a given BFER, say
BFER-B, the packet will follow the path from BFR-A to BFER-B that is
determined by the routing underlay.

It is expected that in a typical deployment, the routing underlay
will be the default topology that the Interior Gateway Protocol
(IGP), e.g., OSPF, uses for unicast routing. In that case, the
underlay adjacencies are just the OSPF adjacencies. A BIER data
packet traveling from BFR-A to BFER-B will follow the path that OSPF
has selected for unicast traffic from BFR-A to BFER-B.

If one wants to have multicast traffic from BFR-A to BFER-B travel a
path that is different from the path used by the unicast traffic from
A
BFR-A to B, BFER-B, one can use a different underlay. For example, if multi-
topology
multi-topology OSPF is being used, one OSPF topology could be used
for unicast traffic, traffic and the other for multicast traffic. (Each
topology would be considered to be a different underlay.)
Alternatively, one could deploy a routing underlay that creates a
multicast-specific tree of some sort. Then BIER could then be used to
forward multicast data packets along the multicast-specific tree,
while unicast packets follow the "ordinary" OSPF best path. (In a
case like this, many multicast flows could be traveling along a
single tree, and the BitString carried by a particular packet would
identify those nodes of the tree that need to receive that packet.)
It is even possible to have multiple routing underlays used by BIER,
as long as one can infer from a data packet's BIER encapsulation
which underlay is being used for that packet.

If multiple routing underlays are used in a single BIER domain, each
BIER sub-domain MUST be associated with a single routing underlay.
(Though underlay
(though multiple sub-domains may be associated with the same routing
underlay.)
underlay). A BFR that belongs to multiple sub-domains MUST be
provisioned to know which routing underlay is used by each
sub-domain. By default (i.e., in the absence of any provisioning to
the contrary), each sub-domain uses the default topology of the
unicast IGP as the routing underlay.

In scenarios where EBGP External BGP (EBGP) is used as the IGP, the
underlay adjacencies, by default, are the BGP adjacencies.

Specification of the protocol protocols and procedures of the routing underlay
is outside the scope of this document.

4.2. The BIER Layer

The BIER layer consists of the protocol protocols and procedures that are used
in order to transmit a multicast data packet across a BIER domain,
from its BFIR to its BFERs. This includes the following components:

o Protocols and procedures that a given BFR uses to advertise, to
all other BFRs in the same BIER domain:

* its BFR-prefix;

* its BFR-id in each sub-domain for which it has been provisioned
with a BFR-id;

* the set of Disposition BitStringLengths it has been provisioned
to use for each sub-domain;

* optionally, information about the routing underlay associated
with each sub-domain.

o The procedures used by a BFIR to impose a BIER header on a
multicast data packet.

o The procedures for forwarding BIER-encapsulated packets, packets and for
modifying the BIER header during transit.

o The procedures used by a BFER to decapsulate a BIER packet and
properly dispatch it.

4.3. The Multicast Flow Overlay

The "multicast flow overlay" consists of the set of protocols and
procedures that enable the following set of functions.

o When a BFIR receives a multicast data packet from outside the BIER
domain, the BFIR must determine the set of BFERs for that packet.
This information is provided by the multicast flow overlay.

o When a BFER receives a BIER-encapsulated packet from inside the
BIER domain, the BFER must determine how to further forward the
packet. This information is provided by the multicast flow
overlay.

For example, suppose the BFIR and BFERs are Provider Edge (PE)
routers providing Multicast Virtual Private Network (MVPN) service.
The multicast flow overlay consists of the protocols and procedures
described in [RFC6513] and [RFC6514]. The MVPN signaling described
in those RFCs enables an ingress PE to determine the set of egress
PEs for a given multicast flow (or set of flows); it also enables an
egress PE to determine the "Virtual Routing and Forwarding Tables"
(VRFs) to which multicast packets from the backbone network should be
sent. MVPN signaling also has several components that depend on the
type of "tunneling technology" used to carry multicast data through
the network. Since BIER is, in effect, a new type of "tunneling
technology", some extensions to the MVPN signaling are needed in
order to properly interface the multicast flow overlay with the BIER
layer. These are specified in [BIER-MVPN]. [BIER_MVPN].

MVPN is just one example of a multicast flow overlay. Protocols and
procedures for other overlays will be provided in companion
documents. It is also possible to implement the multicast flow
overlay by means of a "Software Defined "Software-Defined Network" (SDN) controller.
Specification of the protocols and procedures of the multicast flow
overlay is outside the scope of this document.

5. Advertising BFR-ids and BFR-Prefixes

As stated in Section 2, each BFER is assigned (by provisioning) a
BFR-id (for a given BIER sub-domain). Each BFER must advertise these
assignments to all the other BFRs in the domain. Similarly, each BFR
is assigned (by provisioning) a BFR-prefix (for a given BIER domain), domain)
and must advertise this assignment to all the other BFRs in the
domain. Finally, each BFR has been provisioned to use a certain set
of Disposition BitStringLengths for each sub-domain, sub-domain and must
advertise these to all other BFRs in the domain.

If the BIER domain is also a link state link-state routing IGP domain (i.e., an
OSPF or IS-IS domain), the advertisement of the BFR-prefix,
<sub-domain-id,BFR-id>
<sub-domain-id, BFR-id>, and BitStringLength can be done using the
advertisement capabilities of the IGP. For example, if a BIER domain
is also an OSPF domain, these advertisements can be done using the
OSPF "Opaque Link State Advertisement" (Opaque LSA) mechanism.
Details of the necessary extensions to OSPF and IS-IS will be
provided in companion documents. (See [OSPF_BIER_EXTENSIONS] and
[ISIS_BIER_EXTENSIONS].)

If, in a particular deployment, the BIER domain is not an OSPF or
ISIS
IS-IS domain, procedures suitable to the deployment must be used to
advertise this information. Details of the necessary procedures will
be provided in companion documents. For example, if BGP is the only
routing algorithm used in the BIER domain, the procedures of
[BGP_BIER_EXTENSIONS] may be used.

These advertisements enable each BFR to associate a given
<sub-domain-id, BFR-id> with a given BFR-prefix. As will be seen in
subsequent sections of this document, knowledge of this association
is an important part of the forwarding process.

Since each BFR needs to have a unique (in each sub-domain) BFR-id,
two different BFRs will not advertise ownership of the same
<sub-domain-id, BFR-id> unless there has been a provisioning error.

o If BFR-A determines that BFR-B and BFR-C have both advertised the
same BFR-id for the same sub-domain, BFR-A MUST log an error.
Suppose that the duplicate BFR-id is "N". When BFR-A is
functioning as a BFIR, it MUST NOT encode the BFR-id value N in
the BIER encapsulation of any packet that has been assigned to the
given sub-domain, even if it has determined that the packet needs
to be received by BFR-B and/or BFR-C.

This will mean that BFR-B and BFR-C cannot receive multicast
traffic at all in the given sub-domain until the provisioning
error is fixed. However, that is preferable to having them
receive each other's traffic.

o If Suppose that BFR-A has been provisioned with BFR-id N for a
particular
sub-domain, sub-domain but that it has not yet advertised its
ownership of BFR-id N for that sub-domain, but sub-domain. Suppose also that it
has received an advertisement from a different BFR (say BFR-B)
that is advertising ownership of BFR-id N for the same sub-domain, then sub-domain.
In such a case, BFR-A SHOULD log an error, error and MUST NOT advertise
its own ownership of BFR-id N for that sub-domain as long as the
advertisement from BFR-B is extant.

This procedure may prevent the accidental misconfiguration of a
new BFR from impacting an existing BFR.

If a BFR advertises that it has a BFR-id of 0 in a particular
sub-domain, other BFRs receiving the advertisement MUST interpret
that advertisement as meaning that the advertising BFR does not have
a BFR-id in that sub-domain.

6. BIER Intra-Domain Forwarding Procedures

This section specifies the rules for forwarding a BIER-encapsulated
data packet within a BIER domain. These rules are not intended to
specify an implementation strategy; to conform to this specification,
an implementation need only produce the same results that these rules
produce.

6.1. Overview

This section provides a brief overview of the BIER forwarding
procedures. Subsequent sub-sections subsections specify the procedures in more
detail.

To forward a BIER-encapsulated packet:

1. Determine the packet's sub-domain.

2. Determine the packet's BitStringLength and BitString.

3. Determine the packet's SI.

4. From the sub-domain, the SI SI, and the BitString, determine the set
of destination BFERs for the packet.

5. Using information provided by the routing underlay associated
with the packet's sub-domain, determine the next hop next-hop adjacency
for each of the destination BFERs.

6. It is possible that the packet's BitString will have one or more
bits that correspond to BFR-ids that are not in use. It is also
possible that the packet's BitString will have one or more bits
that correspond to BFERs that are unreachable, i.e., that have no
next hop
next-hop adjacency. In the following, we will consider the "next
hop
"next-hop adjacency" for all such bit positions to be the "null"
next hop.

7. Partition the set of destination BFERs such that all the BFERs in
a single partition have the same next hop. We will say that each
partition is associated with a next hop.

8. For each partition:

a. Make a copy of the packet.

b. Clear any bit in the packet's BitString that identifies a
BFER that is not in the partition.

c. Transmit the packet to the associated next hop. (If the
next hop is the null next hop, the packet is discarded.)

If a BFR receives a BIER-encapsulated packet whose <sub-domain, SI,
BitString> triple identifies that BFR itself, then the BFR is also a
BFER for that packet. As a BFER, it must pass the payload to the
multicast flow overlay. If the BitString has bits set for other
BFRs, the packet also needs to be forwarded further within the BIER
domain. If the BF(E)R also forwards one or more copies of the packet
within the BIER domain, the bit representing the BFR's own BFR-id
MUST be clear in all the copies.

When BIER on a BFER is to pass a packet to the multicast flow
overlay, it of course decapsulates the packet by removing the BIER
header. However, it may be necessary to provide the multicast flow
overlay with contextual information obtained from the BIER
encapsulation. The information that needs to pass between the BIER
layer and the multicast flow overlay is specific to the multicast
flow overlay. Specification of the interaction between the BIER
layer and the multicast flow overlay is outside the scope of this
specification.

If the BIER encapsulation contains a "Time to Live" (TTL) value, this
value is not, by default, inherited by the payload. If a particular
multicast flow overlay needs to know the TTL value, this needs to be
specified in whatever specification defines the interaction between
BIER and that multicast flow overlay.

If the BIER encapsulation contains a Traffic Class field, a
Type of Service field, a Differentiated Services field, or any field
of that sort, the value of that field is not, by default, passed to
the multicast flow overlay. If a particular multicast flow overlay
needs to know the values of such fields, this fact needs to be
specified in whatever specification defines the interaction between
BIER and that multicast flow overlay.

When BIER on a BFER passes a packet to the multicast flow overlay,
the overlay will determine how to further dispatch the packet. If
the packet needs to be forwarded into another BIER domain, then the
BFR will act as a BFER in one BIER domain and as a BFIR in another.

A BIER-encapsulated packet cannot pass directly from one BIER domain
to another; at the boundary between BIER domains, the packet must be
decapsulated and passed to the multicast flow overlay.

Note that when a BFR transmits multiple copies of a packet within a
BIER domain, only one copy will be destined to any given BFER.
Therefore
Therefore, it is not possible for any BIER-encapsulated packet to be
delivered more than once to any BFER.

6.2. BFR Neighbors

The "BFR Neighbors" (BFR-NBRs) of a given BFR, say BFR-A, are those
BFRs that, according to the routing underlay, are adjacencies of
BFR-A. Each BFR-NBR will have a BFR-prefix.

Suppose a BIER-encapsulated packet arrives at BFR-A. From the
packet's encapsulation, BFR-A learns (a) the sub-domain of the packet, packet
and (b) the BFR-ids (in that sub-domain) of the BFERs to which the
packet is destined. Then Then, using the information advertised per
Section 5, BFR-A can find the BFR-prefix of each destination BFER.
Given the BFR-prefix of a particular destination BFER, say BFER-D,
BFR-A learns from the routing underlay (associated with the packet's
sub-domain) an IP address of the BFR that is the next hop on the path
from BFR-A to BFER-D. Let's call this next hop BFR-B. "BFR-B". BFR-A must
then determine the BFR-prefix of BFR-B. (This determination can be
made from the information advertised per Section 5.) This BFR-prefix
is the BFR-NBR of BFR-A on the path from BFR-A to BFER-D.

Note that if the routing underlay provides multiple equal cost equal-cost paths
from BFR-A to BFER-D, BFR-A may have multiple BFR-NBRs for BFER-D.

Under certain circumstances, a BFR may have adjacencies (in a
particular routing underlay) that are not BFRs. Please see
Section 6.9 for a discussion of how to handle those circumstances.

6.3. The Bit Index Routing Table

The Bit "Bit Index Routing Table Table" (BIRT) is a table that maps from the
BFR-id (in a particular sub-domain) of a BFER to the BFR-prefix of
that BFER, and to the BFR-NBR on the path to that BFER. As an
example, consider the topology shown in Figure 1. In this diagram,
we represent the BFR-id of each BFR in the SI:xyzw form discussed in
Section 3.

( A ) ------------ ( B ) ------------ ( C ) ------------ ( D )
4 (0:1000) \ \ 1 (0:0001)
\ \
( E ) ( F )
3 (0:0100) 2 (0:0010)

Figure 1: BIER Topology 1
As an example, consider the topology shown in Figure 1. In this
diagram, we represent the BFR-id of each BFR in the SI:xyzw form
discussed in Section 3.

This topology will result in the BIRT of Figure 2 at BFR-B. The
first column shows the BFR-id as a number and also (in parentheses)
in the SI:BitString format that corresponds to a BitStringLength
of 4. (The actual minimum BitStringLength is 64, but we use 4 in the
examples.)

Note that a BIRT is specific to a particular BIER sub-domain.

--------------------------------------------
| BFR-id | BFR-Prefix | BFR-NBR |
| (SI:BitString) | of Dest BFER | |
============================================
| 4 (0:1000) | A | A |
--------------------------------------------
| 1 (0:0001) | D | C |
--------------------------------------------
| 3 (0:0100) | E | E |
--------------------------------------------
| 2 (0:0010) | F | C |
--------------------------------------------

Figure 2: Bit Index Routing Table at BFR-B

6.4. The Bit Index Forwarding Table

The "Bit Index Forwarding Table" (BIFT) is derived from the BIRT as
follows. (Note that a BIFT is specific to a particular sub-domain.)

Suppose that several rows in the BIRT have the same SI and the same
BFR-NBR. By taking the logical OR of the BitStrings of those rows,
we obtain a bit mask that corresponds to that combination of SI and
BFR-NBR. We will refer to this bit mask as the "Forwarding Bit Mask"
(F-BM) for that <SI,BFR-NBR> <SI, BFR-NBR> combination.

For example, in Figure 2, we see that two of the rows have the same
SI (0) and same BFR-NBR (C). The Bit Mask bit mask that corresponds to
<SI=0, BFR-NBR-C> is 0011 ("0001" OR'd with "0010").

The BIFT is used to map from the BFR-id of a BFER to the
corresponding F-BM and BFR-NBR. For example, Figure 3 shows the BIFT
that is derived from the BIRT of Figure 2. Note that BFR-ids 1 and 2
have the same SI and the same BFR-NBR, hence BFR-NBR; hence, they have the
same F-BM.

-------------------------------------
| BFR-id | F-BM | BFR-NBR |
| (SI:Bitstring) (SI:BitString) | | |
=====================================
| 1 (0:0001) | 0011 | C |
-------------------------------------
| 2 (0:0010) | 0011 | C |
-------------------------------------
| 3 (0:0100) | 0100 | E |
-------------------------------------
| 4 (0:1000) | 1000 | A |
-------------------------------------

Figure 3: Bit Index Forwarding Table

This Bit Index Forwarding Table (BIFT) BIFT is programmed into the data- data plane and used to forward
packets, applying the rules specified below in Section 6.5.

6.5. The BIER Forwarding Procedure

Below is the procedure that a BFR uses for forwarding a BIER-
encapsulated
BIER-encapsulated packet.

1. Determine the packet's SI, BitStringLength, and sub-domain.

2. If the BitString consists entirely of zeroes, discard the packet;
the forwarding process has been completed. Otherwise Otherwise, proceed to
step 3.

3. Find the position, call position (call it "k", "k") of the least significant (i.e.,
of the rightmost) bit that is set in the packet's BitString.
(Remember, bits are numbered from 1, starting with the least
significant bit.)

4. If bit k identifies the BFR itself, copy the packet, and send the
copy to the multicast flow overlay. Then clear bit k in the
original packet, and go to step 2. Otherwise, proceed to step 5.

5. Use the value k, together with the SI, sub-domain, and
BitStringLength, as the 'index' "index" into the BIFT.

6. Extract from the BIFT the F-BM and the BFR-NBR.

7. Copy the packet. Update the copy's BitString by AND'ing it with
the F-BM (i.e., PacketCopy->BitString &= F-BM). Then forward the
copy to the BFR-NBR. (If the BFR-NBR is null, the copy is just
discarded.) Note that when a packet is forwarded to a particular
BFR-NBR, its BitString identifies only those BFERs that are to be
reached via that BFR-NBR.

8. Now update the original packet's BitString by AND'ing it with the
INVERSE of the F-BM (i.e., Packet->Bitstring Packet->BitString &= ~F-BM). (This
clears the bits that identify the BFERs to which a copy of the
packet has just been forwarded.) Go to step 2.

This procedure causes the packet to be forwarded to a particular
BFR-NBR only once. The number of lookups in the BIFT is the same as
the number of BFR-NBRs to which the packet must be forwarded; it is
not necessary to do a separate lookup for each destination BFER.

When a packet is sent to a particular BFR-NBR, the BitString is not
the only part of the BIER header that needs to be modified. If there
is a TTL field in the BIER header, it will need to be decremented.
In addition, when either of the encapsulations of [MPLS_BIER_ENCAPS]
is used, the BIFT-id field is likely to require modification, based
on signaling from the BFR-NBR to which the packet is being sent. The
BIFT-id field of an incoming BIER packet implicitly identifies an SI,
a Set
Identifier, a Sub-Domain sub-domain, and a BitStringLength. If the packet is sent to a
particular BFR-NBR, the BIFT-id field must be changed to whatever
value that BFR-NBR has advertised for the same Set
Identifier, Sub-Domain, SI, sub-domain, and
BitStringLength. (If the encapsulation of Section 2.1 of
[MPLS_BIER_ENCAPS] is used, this is essentially an MPLS label swap
operation.)

Suppose it has been decided (by the above rules) to send a packet to
a particular BFR-NBR. If that BFR-NBR is connected via multiple
parallel interfaces, it may be desirable to apply some form of load
balancing. Load balancing Load-balancing algorithms are outside the scope of this
document. However, if the packet's encapsulation contains an
"entropy" entropy
field, the entropy field SHOULD be respected; two packets with the
same value of the entropy field SHOULD be sent on the same interface
(if possible).

In some cases, the routing underlay may provide multiple equal cost equal-cost
paths (through different BFR-NBRs) to a given BFER. This is known as
"Equal Cost Multiple Paths"
"Equal-Cost Multipath" (ECMP). The procedures described in this
section must be augmented in order to support load balancing over
ECMP. The necessary augmentations can be found in Section 6.7.

In the event that unicast traffic to the BFR-NBR is being sent via a
"bypass tunnel" of some sort, the BIER-encapsulated multicast traffic
send
sent to the BFR-NBR SHOULD also be sent via that tunnel. This allows
any existing "Fast Reroute" "fast reroute" schemes to be applied to multicast
traffic as well as to unicast traffic.

Some examples of these forwarding procedures can be found in
Section 6.6.

The rules given in this section can be represented by the following
pseudocode:

void ForwardBitMaskPacket (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;
BFR-NBR = BIFT[Index+Offset]->BFR-NBR;
PacketCopy = Copy(Packet);
PacketCopy->BitString &= F-BM;
PacketSend(PacketCopy, BFR-NBR);
Packet->BitString &= ~F-BM;
}
}

Figure 4: Pseudocode

This pseudocode assumes that that, at a given BFER, the BFR-NBR entry
corresponding to the BFER's own BFR-id will be the BFER's own
BFR-prefix. It also assumes that the corresponding F-BM has only
one bit set, the bit representing the BFER itself. In this case, the
"PacketSend" function sends the packet to the multicast flow overlay.

This pseudocode also assumes that the F-BM for the null next hop
contains a 1 in a given bit position if and only if that bit position
corresponds either to either an unused BFR-id or to an unreachable BFER. When
the BFR-NBR is null, the "PacketSend" function discards the packet.

6.6. Examples of BIER Forwarding

In this section, we give two examples of BIER forwarding, based on
the topology in Figure 1. In these examples, all packets have been
assigned to the default sub-domain, all packets have SI=0, and the
BitStringLength is 4. Figure 5 shows the BIFT entries for SI=0 only.
For compactness, we show the first column of the BIFT, the BFR-id,
only as an integer.

BFR-A BIFT BFR-B BIFT BFR-C BIFT
------------------- ------------------- -------------------
| Id | F-BM | NBR | | Id | F-BM | NBR | | Id | F-BM | NBR |
=================== =================== ===================
| 1 | 0111 | B | | 1 | 0011 | C | | 1 | 0001 | D |
------------------- ------------------- -------------------
| 2 | 0111 | B | | 2 | 0011 | C | | 2 | 0010 | F |
------------------- ------------------- -------------------
| 3 | 0111 | B | | 3 | 0100 | E | | 3 | 1100 | B |
------------------- ------------------- -------------------
| 4 | 1000 | A | | 4 | 1000 | A | | 4 | 1100 | B |
------------------- ------------------- -------------------

Figure 5: BIFTs for Used in the Forwarding Examples

6.6.1. Example 1

BFR-D, BFR-E BFR-E, and BFR-F are BFER's. BFERs. BFR-A is the BFIR. Suppose that
BFIR-A has learned from the multicast flow overlay that BFER-D is
interested in a given multicast flow. If BFIR-A receives a packet of
that flow from outside the BIER domain, BFIR-A applies the BIER
encapsulation to the packet. The encapsulation must be such that the
SI is zero. The encapsulation also includes a BitString, with just
bit 1 set and with all other bits clear (i.e., 0001). This indicates
that BFER-D is the only BFER that needs to receive the packet. Then
BFIR-A then follows the procedures of Section 6.5: 6.5, as follows:

o Since the packet's BitString is 0001, BFIR-A finds that the first
bit in the string is bit 1. Looking at entry 1 in its BIFT, BFR-A
determines that the bit mask F-BM is 0111 and the BFR-NBR is
BFR-B.

o BFR-A then makes a copy of the packet, packet and applies the F-BM to the
copy: Copy->BitString &= 0111. The copy's Bitstring BitString is now 0001
(0001 & 0111).

o The copy is now sent to BFR-B.

o BFR-A then updates the packet's BitString by applying the inverse
of the F-BM: Packet->Bitstring Packet->BitString &= ~F-BM. As a result, the
packet's BitString is now 0000 (0001 & 1000).

o As the packet's BitString is now zero, the forwarding procedure is
complete.

When BFR-B receives the multicast packet from BFR-A, it follows the
same procedure. The result is that a copy of the packet, with a
BitString of 0001, is sent to BFR-C. BFR-C applies the same
procedures, and
procedures and, as a result result, sends a copy of the packet, with a
BitString of 0001, to BFR-D.

At BFER-D, the BIFT entry (not pictured) for BFR-id 1 will specify an
F-BM of 0001 and a BFR-NBR of BFR-D itself. This will cause a copy
of the packet to be delivered to the multicast flow overlay at BFR-D.
The packet's BitString will be set to 0000, and the packet will not
be forwarded any further.

6.6.2. Example 2

This example is similar to Example example 1, except that BFIR-A has learned
from the multicast flow overlay that both BFER-D and BFER-E are
interested in a given multicast flow. If BFIR-A receives a packet of
that flow from outside the BIER domain, BFIR-A applies the BIER
encapsulation to the packet. The encapsulation must be such that the
SI is zero. The encapsulation also includes a BitString with
two bits set: bit 1 is set (as in example 1) to indicate that BFR-D
is a BFER for this packet, and bit 3 is set to indicate that BFR-E is
a BFER for this packet. I.e., That is, the BitString (assuming again a
BitStringLength of 4) is 0101. To forward the packet, BFIR-A follows
the procedures of Section 6.5: 6.5, as follows:

o Since the packet's BitString is 0101, BFIR-A finds that the first
bit in the string is bit 1. Looking at entry 1 in its BIFT, BFR-A
determines that the bit mask F-BM is 0111 and the BFR-NBR is
BFR-B.

o BFR-A then makes a copy of the packet, packet and applies the F-BM to the
copy: Copy->BitString &= 0111. The copy's Bitstring BitString is now 0101
(0101 & 0111).

o The copy is now sent to BFR-B.

o BFR-A then updates the packet's BitString by applying the inverse
of the F-BM: Packet->Bitstring Packet->BitString &= ~F-BM. As a result, the
packet's BitString is now 0000 (0101 & 1000).

o As the packet's BitString is now zero, the forwarding procedure is
complete.

When BFR-B receives the multicast packet from BFR-A, it follows the
procedure of Section 6.5, as follows:

o Since the packet's BitString is 0101, BFR-B finds that the first
bit in the string is bit 1. Looking at entry 1 in its BIFT, BFR-B
determines that the bit mask F-BM is 0011 and the BFR-NBR is
BFR-C.

o BFR-B then makes a copy of the packet, packet and applies the F-BM to the
copy: Copy->BitString &= 0011. The copy's Bitstring BitString is now 0001
(0101 & 0011).

o The copy is now sent to BFR-C.

o BFR-B then updates the packet's BitString by applying the inverse
of the F-BM: Packet->Bitstring Packet->BitString &= F-BM. ~F-BM. As a result, the
packet's BitString is now 0100 (0101 & 1100).

o Now BFR-B finds the next bit in the packet's (modified) BitString.
This is bit 3. Looking at entry 3 in its BIFT, BFR-B determines
that the F-BM is 0100 and the BFR-NBR is BFR-E.

o BFR-B then makes a copy of the packet, packet and applies the F-BM to the
copy: Copy->BitString &= 0100. The copy's Bitstring BitString is now 0100
(0100 & 0100).

o The copy is now sent to BFR-E.

o BFR-B then updates the packet's BitString by applying the inverse
of the F-BM: Packet->Bitstring Packet->BitString &= ~F-BM. As a result, the
packet's BitString is now 0000 (0100 & 1011).

o As the packet's BitString is now zero, the forwarding procedure is
complete.

Thus

Thus, BFR-B forwards two copies of the packet. One copy of the
packet, with BitString 0001, has now been sent from BFR-B to BFR-C.
Following the same procedures, BFR-C will forward the packet to
BFER-D.

The other copy of the packet has been sent from BFR-B to BFER-E, with
BitString 0100.

At BFER-E, the BIFT entry (not pictured) for BFR-id 3 will specify an
F-BM of 0100 and a BFR-NBR of BFR-E itself. This will cause a copy
of the packet to be delivered to the multicast flow overlay at BFR-E.
The packet's BitString will be set to 0000, and the packet will not
be forwarded any further.

6.7. Equal Cost Multi-path Equal-Cost Multipath Forwarding

In many networks, the routing underlay will provide multiple equal
cost
equal-cost paths from a given BFR to a given BFER. When forwarding
multicast packets through the network, it can be beneficial to take
advantage of this by load balancing load-balancing among those paths. This feature
is known as "equal cost multiple path forwarding", or "ECMP". "Equal-Cost Multipath (ECMP) forwarding".

BIER supports ECMP, ECMP forwarding, but the procedures of Section 6.5 must
be modified slightly. Two ECMP procedures are defined. In the first
(described in Section 6.7.1), the choice among equal-cost paths taken
by a given packet from a given BFR to a given BFER depends on
(a) routing, (b) the packet's entropy, and (b) (c) the other BFERs to
which that packet is destined. In the second (described in
Section 6.7.2), the choice depends only upon the packet's entropy.

There are tradeoffs trade-offs between the two forwarding procedures described
here. In the procedure of Section 6.7.1, the number of packet
replications is minimized. The procedure in Section 6.7.1 also uses
less memory in the BFR. In the procedure of Section 6.7.2, the path
traveled by a given packet from a given BFR to a given BFER is
independent of the other BFERs to which the packet is destined.
While the procedures of Section 6.7.2 may cause more replications,
they provide a more predictable behavior.

The two procedures described here operate on identical packet formats
and will interoperate correctly. However, if deterministic behavior
is desired, then all BFRs would need to use the procedure from
Section 6.7.2.

6.7.1. Non-deterministic ECMP

Figure 6 shows the operation of non-deterministic ECMP in BIER.

BFR-A BIFT BFR-B BIFT BFR-C BIFT
------------------- ------------------- -------------------
| Id | F-BM | NBR | | Id | F-BM | NBR | | Id | F-BM | NBR |
=================== =================== ===================
| 1 | 0111 | B | | 1 | 0011 | C | | 1 | 0001 | D |
------------------- ------------------- -------------------
| 2 | 0111 | B | | 2 | 0011 | C | | 2 | 0010 | F |
------------------- | | 0110 | E | -------------------
| 3 | 0111 | B | ------------------- | 3 | 1100 | B |
------------------- | 3 | 0110 | E | -------------------
| 4 | 1000 | A | ------------------| | 4 | 1100 | B |
------------------- | 4 | 1000 | A | -------------------
-------------------

( A ) ------------ ( B ) ------------ ( C ) ------------ ( D )
4 (0:1000) \ \ 1 (0:0001)
\ \
( E ) ------------ ( F )
3 (0:0100) 2 (0:0010)

Figure 6: Example of Non-deterministic ECMP

In this example, BFR-B has two equal cost equal-cost paths to reach BFER-F, BFER-F: one
via BFR-C and one via BFR-E. Since the BFR-id of BFER-F is 2, this
is reflected in entry 2 of BFR-B's BIFT. Entry 2 shows that BFR-B
has a choice of two BFR-NBRs for BFER-B, BFER-B and that a different F-BM is
associated with each choice. When BFR-B looks up entry 2 in the
BIFT, it can choose either BFR-NBR. However, when following the
procedures of Section 6.5, it MUST use the F-BM corresponding to the
BFR-NBR that it chooses.

How the choice is made is an implementation matter. However, the
usual rules for ECMP apply: packets of a given flow SHOULD NOT be
split among two paths, and any "entropy" entropy field in the packet's
encapsulation SHOULD be respected.

Note however

Note, however, that by the rules of Section 6.5, any packet destined
for both BFER-D and BFER-F will be sent via BFR-C.

6.7.2. Deterministic ECMP

With the procedures of Section 6.7.1, where ECMP paths exist, the
path a packet takes to reach any particular BFER depends not only on
routing and on the packet's entropy, entropy but also on the set of other
BFERs to which the packet is destined.

For example example, consider the following scenario in the network of
Figure 6.

o There is a sequence of packets being transmitted by BFR-A, some of
which are destined for both D and F, F and some of which are destined
only for F.

o All the packets in this sequence have the same entropy value, call value (call
it "Q". "Q").

o At BFR-B, when a packet with entropy value Q is forwarded via
entry 2 in the BIFT, the packet is sent to E.

Using the forwarding procedure of Section 6.7.1, packets of this
sequence that are destined for both D and F are forwarded according
to entry 1 in the BIFT, BIFT and thus will reach F via the path A-B-C-F.
However, packets of this sequence that are destined only for F are
forwarded according to entry 2 in the BIFT, BIFT and thus will reach F via
the path A-B-E-F.

That procedure minimizes the number of packets transmitted by BFR B. BFR-B.
However, consider the following scenario:

o Beginning at time t0, the multicast flow in question needs to be
received ONLY by BFER-F; BFER-F.

o Beginning at a later time, t1, the flow needs to be received by
both BFER-D and BFER-F.

o Beginning at a later time, t2, the flow no longer needs to be
received by D, but still needs to be received by F.

Then

Then, from t0 until t1, the flow will travel to F via the path
A-B-E-F. From t1 until t2, the flow will travel to F via the path
A-B-C-F. And from t2, the flow will again travel to F via the path
A-B-E-F.

The problem is that if D repeatedly joins and leaves the flow, the
flow's path from B to F will keep switching. This could cause F to
receive packets out of order. It also makes troubleshooting
difficult. For example, if there is some problem on the E-F link,
receivers at F will get good service when the flow is also going to D
(avoiding the E-F link), link) but bad service when the flow is not going
to D. Since it is hard to know which path is being used at any given
time, this may be hard to troubleshoot. Also, it is very difficult
to perform a traceroute that is known to follow the path taken by the
flow at any given time.

The source of this difficulty is that, in the procedures of
Section 6.7.1, the path taken by a particular flow to a particular
BFER depends upon whether there are lower numbered lower-numbered BFERs that are
also receiving the flow. Thus Thus, the choice among the ECMP paths is
fundamentally non-deterministic.

Deterministic forwarding can be achieved by using multiple BIFTs,
such that each row in a BIFT has only one path to each destination, destination
but the multiple ECMP paths to any particular destination are spread
across the multiple tables. When a BIER-encapsulated packet arrives
to be forwarded, the BFR uses a hash of the BIER Entropy entropy field to
determine which BIFT to use, and then the normal BIER forwarding
algorithm (as described in Sections 6.5 and 6.6) is used with the
selected BIFT.

As an example, suppose there are two paths to destination X (call
them X1 "X1" and X2), "X2") and four paths to destination Y (call them Y1, Y2,
Y3, "Y1",
"Y2", "Y3", and Y4). "Y4"). If there are, say, four BIFTs, one BIFT would
have paths X1 and Y1, one would have X1 and Y2, one would have X2 and
Y3, and one would have X2 and Y4. If traffic to X is split evenly
among these four BIFTs, the traffic will be split evenly between the
two paths to X; if traffic to Y is split evenly among these four
BIFTs, the traffic will be split evenly between the four paths to Y.

Note that if there are three paths to one destination and four paths
to another, 12 BIFTs would be required in order to get even splitting
of the load to each of those two destinations. Of course, each BIFT
uses some memory, and one might be willing to have less optimal
splitting in order to have fewer BIFTs. How that tradeoff trade-off is made
is an implementation or deployment decision.

6.8. Prevention of Loops and Duplicates

The BitString in a BIER-encapsulated packet specifies the set of
BFERs to which that packet is to be forwarded. When a
BIER-encapsulated packet is replicated, no two copies of the packet
will ever have a BFER in common. If one of the packet's BFERs
forwards the packet further, that BFER will first clear the bit that
identifies itself. As a result, duplicate delivery of packets is not
possible with BIER.

As long as the routing underlay provides a loop free loop-free path between
each pair of BFRs, BIER-encapsulated packets will not loop. Since
the BIER layer does not create any paths of its own, there is no need
for any BIER-specific loop prevention loop-prevention techniques beyond the
forwarding procedures specified in Section 6.5.

If, at some time, the routing underlay is not providing a loop free loop-free
path between BFIR-A and BFER-B, then BIER encapsulated BIER-encapsulated packets may
loop while traveling from BFIR-A to BFER-B. However, such loops will
never result in delivery of duplicate packets to BFER-B.

These properties of BIER eliminate the need for the "reverse path
forwarding" "Reverse Path
Forwarding" (RPF) check that is used in conventional IP multicast
forwarding.

6.9. When Some Nodes do not Do Not Support BIER

The procedures of section Section 6.2 presuppose that, within a given BIER
domain, all the nodes adjacent to a given BFR in a given routing
underlay are also BFRs. However, it is possible to use BIER even
when this is not the case, as long as the ingress and egress nodes
are BFRs. In this section, we describe procedures that can be used
if the routing underlay is an SPF-based IGP that computes a shortest
path
shortest-path tree from each node to all other nodes in the domain.

At a given BFR, say BFR B, "BFR-B", start with a copy of the IGP-computed
shortest path
shortest-path tree from BFR B BFR-B to each router in the domain. (This
tree is computed by the SPF algorithm of the IGP.) Let's call this
copy the "BIER-SPF tree rooted at BFR B." BFR B BFR-B". BFR-B then modifies this
BIER-SPF tree as follows.

1. BFR B BFR-B looks in turn at each of B's its child nodes on the BIER-SPF
tree.

2. If one of the child nodes does not support BIER, BFR B BFR-B removes
that node from the tree. The child nodes of the node that has
just been removed are then re-parented on the tree, so that BFR B BFR-B
now becomes their parent.

3. BFR B BFR-B then continues to look at each of its child nodes,
including any nodes that have been re-parented to B BFR-B as a
result of the previous step.

When all of the child nodes (the original child nodes plus any new
ones) have been examined, B's BFR-B's children on the BIER-SPF tree will
all be BFRs.

When the BIFT is constructed, B's BFR-B's child nodes on the BIER-SPF
tree are considered to be the BFR-NBRs. The F-BMs must be computed
appropriately, based on the BFR-NBRs.

BFR-B may now have BFR-NBRs that are not "directly connected" to B
BFR-B via
layer Layer 2. To send a packet to one of these BFR-NBRs, B BFR-B
will have to send the packet through a unicast tunnel. In an MPLS
network, this may be as simple as finding the IGP unicast next hop to
the child
node, node and pushing on (above the BIER encapsulation header)
an MPLS label that the IGP next hop has bound to an address of the
child node. (This assumes that the packet is using an MPLS-based
BIER encapsulation, such as the one specified in Section 2.1 of
[MPLS_BIER_ENCAPS].) Of course, the BIFT-id in the BIER
encapsulation header must be the BIFT-id advertised by the child node
for the packet's Set Index, Sub-Domain, SI, sub-domain, and BitStringLength.

If for some reason the unicast tunnel cannot be an MPLS tunnel, any
other kind of tunnel can be used, as long as the encapsulation for
that tunnel type has a way of indicating that the payload is a
BIER-encapsulated packet.

Note that if a BIER-encapsulated packet is not using an MPLS-based
BIER encapsulation, it will not be possible to send it through an
MPLS tunnel unless it is known that the tunnel only carries BIER
packets. The reason
packets; this is that because MPLS has no "next protocol type" field.
This is not a problem if an MPLS-based BIER encapsulation is used,
because in that case the BIER encapsulation begins with an MPLS label
that identifies the packet as a BIER-encapsulated packet.

Of course, the above is not meant as an implementation technique,
just as a functional description.

While the above description assumes that the routing underlay
provides an SPF tree, it may also be applicable to other types of
routing underlay. underlays.

The technique above can also be used to provide "node protection"
(i.e., to provide fast reroute around nodes that are believed to have
failed). If BFR B BFR-B has a failed BFR-NBR, B BFR-B can remove the failed
BFR-NBR from the BIER-SPF tree, tree and can then re-parent the child
BFR-NBRs of the failed BFR-NBR so that they appear to be B's BFR-B's own
child nodes on the tree (i.e., so that they appear to be B's BFR-B's
BFR-NBRs). Then the The usual BIER forwarding procedures then apply.
However, getting the packet from B BFR-B to the child nodes of the
failed BFR-NBR is a bit more complicated, as it may require using a
unicast bypass tunnel to get around the failed node.

A simpler variant of step 2 above would be the following:

If one of the child nodes does not support BIER, BFR B BFR-B removes
that node from the tree. All BFERs that are reached through that
child node are then re-parented on the tree, so that BFR B BFR-B now
becomes their parent.

This variant is simpler because the set of BFERs that are reached
through a particular child node of B BFR-B can be determined from the
F-BM in the BIFT. However, if this variant is used, the results are
less optimal, because packets will be unicast directly from B BFR-B to
the BFERs that are reachable through the non-BIER child node.

When using a unicast MPLS tunnel to get a packet to a BFR-NBR:

o the The TTL of the MPLS label entry representing the tunnel SHOULD be
set to a large value, rather than being copied from the TTL value
from the BIER encapsulation header, and

o when When the tunnel labels are popped off, the TTL from the tunnel
labels SHOULD NOT be copied to the BIER encapsulation header.

In other words, the TTL processing for the tunnel SHOULD be as
specified in [RFC3443] for "Pipe Model" and "Short Pipe Model" LSPs. Label
Switched Paths (LSPs). The same principle applies if the tunnels are
not MPLS tunnels; the BIER packet SHOULD NOT inherit the TTL from the
tunnel encapsulation. That way, the TTL of the BIER encapsulation
header constrains only the number of BFRs that the packet may
traverse, not the total number of hops.

If two BIER packets have the same value in the entropy field of their
respective BIER headers, headers and if both are transmitted through a given
tunnel, it is desirable for the tunnel encapsulation to preserve the
fact that the two packets have the same entropy.

The material in this section presupposes that if a given node router is either
a BFR or not, and that a BFR
BFR, then it supports BIER on all its interfaces. It
is however is, however,
possible that a router will have some line cards that support BIER
and some that do not. In such a case, one can think of the router as
a "partial-BFR", "partial BFR" that supports BIER only on some of its interfaces.
If it is desired to deploy such partial-BFRs, partial BFRs, one can use the
multi-topology features of the IGP to set up a BIER-specific
topology. This topology would exclude all the non-BIER-capable
interfaces that attach to BFRs. BIER would then have to be run in a
sub-domain that is bound to this topology. If unicast tunnels are
used to bypass non-BFRs, either (a) the tunnels have to be restricted
to this topology, topology or (b) the tunnel endpoints have to be BFRs that do
not have any non-BIER-capable interfaces.

6.10. Use of Different BitStringLengths within a Domain

The procedures of this section apply only when the same encapsulation
is used throughout the BIER domain. Consideration of the scenario
where both multiple encapsulations and multiple BitStringLengths are
used in a given BIER domain is outside the scope of this document.

It is possible for different BFRs within a BIER domain to be using
different Imposition and/or Disposition BitStringLengths. As stated
in Section 3:

"if a particular BFIR is provisioned to use a particular Imposition
BitStringLength and a particular Imposition sub-domain when imposing
the encapsulation on a given set of packets, all other BFRs with
BFR-ids in that sub-domain SHOULD be provisioned to process received
BIER packets with that BitStringLength (i.e., all other BFRs with
BFR-ids in that sub-domain SHOULD be provisioned with that
BitStringLength as a Disposition BitStringLength for that
sub-domain)."

Note that mis-provisioning can result in "black holes". If a BFIR
creates a BIER packet with a particular BitStringLength, BitStringLength and if that
packet needs to travel through a BFR that cannot process received
BIER packets with that BitStringLength, then it may be impossible to
forward the packet to all of the BFERs identified in its BIER header.
Section 6.10.1 defines a procedure, the "BitStringLength
Compatibility Check", that can be used to detect the possibility of
such black holes.

However, failure of the BitStringLength Compatibility Check does not
necessarily result in the creation of black holes; Section 6.10.2
specifies OPTIONAL procedures that allow BIER forwarding to proceed
without black holes, even if the BitStringLength Compatibility Check
fails.

If the procedures of Section 6.10.2 are not deployed, deployed but the
BitStringLength Compatibility Check fails at some BFIR, the BFIR has
two choices:

o Create BIER packets with the provisioned Imposition
BitStringLength, even though the packets may not be able to reach
all the BFERs identified in their BitStrings BitStrings.

o Use an Imposition BitStringLength that passes the Compatibility
Check (assuming that there is one), even if this is not the
provisioned Imposition BitStringLength.

Section 6.10.1 discusses the implications of making one or the other
of these choices.

There will be times when an operator wishes to change the
BitStringLengths used in a particular BIER domain. Section 6.10.3
specifies a simple procedure that can be used to transition a BIER
domain from one BitStringLength to another.

6.10.1. BitStringLength Compatibility Check

When a BFIR needs to encapsulate a packet, the BFIR first assigns the
packet to a sub-domain. Then the The BFIR then chooses an Imposition
BitStringLength L for the packet. The choice of Imposition
BitStringLength is determined by provisioning. However, the BFIR
should also perform the BitStringLength Compatibility Check defined
below.

The combination of Sub-Domain sub-domain S and Imposition BitStringLength L
passes the BitStringLength Compatibility Check if and only if the
following condition holds:

Every BFR that has advertised its membership in sub-domain S has
also advertised that it is using Disposition BitStringLength L
(and possibly other BitStringLengths as well) in that Sub-Domain. sub-domain.
(If the MPLS encapsulation (Section 2.1 of [MPLS_BIER_ENCAPS]) is
being used, this means that every BFR that is advertising a label
for Sub-Domain sub-domain S is advertising a label for the combination of
Sub-Domain
sub-domain S and Disposition BitStringLength L.)

If a BFIR has been provisioned to use a particular Imposition
BitStringLength and a particular sub-domain for some set of packets,
and if that combination of Imposition BitStringLength and sub-domain
does not pass the BitStringLength Compatibility Check, the BFIR
SHOULD log this fact as an error. It then has the following choice two
choices about what to do with the packets:

1. The BFIR MAY use the provisioned Imposition BitStringLength
anyway. If the procedure Paragraph of either option 2 or Paragraph option 3 of
Section 6.10.2 are is deployed, this will not cause black holes, holes and
may actually be the optimal result. It should be understood
though understood,
though, that the BFIR cannot determine by signaling whether those
procedures have been deployed.

2. If the BFIR is capable of using an Imposition BitStringlength BitStringLength
that does pass the BitStringLength Compatibility Check for the
particular sub-domain, the BFIR MAY use that Imposition
BitStringLength instead.

Which of these two choices to make is itself determined by
provisioning.

Note that discarding the packets is not one of the allowable choices.
Suppose, for example, that all the BFIRs are provisioned to use
Imposition BitStringLength L for a particular sub-domain S, S but one
BFR has not been provisioned to use Disposition BitStringLength L for
sub-domain S. This will cause the BitStringLength Compatibility
Check to fail. If the BFIR sends packets with BitStringLength L and
sub-domain S, the mis-provisioned BFR will not be able to forward
those packets, and thus the packets may only be able to reach a
subset of the BFERs to which they are destined. However, this is
still better than having the BFIRs drop the packets; if the BFIRs
discard the packets, the packets won't reach any of the BFERs to
which they are destined at all.

If the procedures of Section 6.10.2 have not been deployed, choice 2
above might seem like a better option. However, there might not be
any Imposition BitStringLength that a given BFIR can use that also
passes the BitStringLength Compatibility Check. If it is desired to
use choice 2 in a particular deployment, then there should be a
"Fallback Disposition BitStringLength", call BitStringLength" (call it F, "F") such that:

o Every BFR advertises that it uses BitStringLength F as a
Disposition BitStringLength for every sub-domain, and

o If a BFIR is provisioned to use Imposition BitStringLength X and
Imposition sub-domain S for a certain class of packets, packets but the
BitStringLength Compatibility check Check fails for the combination of
BitStringLength X and sub-domain S, then the BFIR will fall back
to using BitStringLength F as the Imposition BitStringLength
whenever the Imposition sub-domain is S.

It is RECOMMENDED that the value of F be the default BitStringLength
for the encapsulation being used.

6.10.2. Handling BitStringLength Mismatches

Suppose that a packet has been BIER-encapsulated with a
BitStringLength value of X, X and that the packet has arrived at BFR-A.
Now suppose that according to the routing underlay, underlay the next hop is
BFR-B, but BFR-B is not using X as one of its Disposition
BitStringLengths. What should BFR-A do with the packet? BFR-A has
three options. It MUST do one of the three, but the choice of which
procedure to follow is a local matter. The three options are:

1. BFR-A MAY discard the packet.

2. BFR-A MAY re-encapsulate the packet, using a BIER header whose
BitStringLength value is supported by BFR-B.

Note that if BFR-B only uses Disposition BitStringLength values
that are smaller than the BitStringLength value of the packet,
this may require creating additional copies of the packet.
Whether additional copies actually have to be created depends
upon the bits that are actually set in the original packet's
BitString.

3. BFR-A MAY treat BFR-B as if BFR-B did not support BIER at all,
and apply
in which case BFR-A applies the rules of Section 6.9.

Note that there is no signaling that enables a BFR to advertise which
of the three options it will use.

Option 2 can be useful if there is a region of the BIER domain where
the BFRs are capable of using a long BitStringLength, and BitStringLength as well as a
region where the BFRs are only capable of using a shorter
BitStringLength.

6.10.3. Transitioning from One BitStringLength to Another

Suppose one wants to migrate the BitStringLength used in a particular
BIER domain from one value (X) to another value (Y). The following
migration procedure can be used. This procedure allows the BFRs to
be reprovisioned one at a time, time and does not require a "flag day".

First, upgrade

1. Upgrade all the BFRs in the domain so that they use both value X
and value Y as their Disposition BitStringLengths. Once this is
done, reprovision

2. Reprovision the BFIRs so that they use BitStringLength value Y as
the Imposition BitStringLength. Once that is done, one

3. One may then optionally reprovision all the BFRs so that they no
longer use Disposition BitStringLength X.

7. Operational Considerations

BIER offers a radical simplification over current IPMulticast
operations; IP multicast
operations: no tree-building control plane, no per-flow forwarding
state, no Reverse Path Forwarding (RPF), no Rendezvous Point (RP) (RP),
etc. BIER packet forwarding/replication is along the unicast paths
to each bit position set in the packet, ensuring that the
encapsulated multicast packets follow the same path as unicast to
each set bit in the header. The BIER FIB can be derived from the
SPF-calculated unicast SPF
calculated unicast FIB, FIB or from any other forwarding path forwarding-path
calculation in or out of band. Each bit will follow this unicast
path from the entry point of the BIER domain to the edge device with
that assigned bit.

Due to these differences, operational expectation expectations from traditional
multicast solutions do not apply to a BIER domain. There is no
granular per-flow state at each node defining a tree. Monitoring
flows at the forwarding plane level, (S,G) entries, forwarding-plane level ((S,G) entries) is not provided
in a BIER node. BIER FIB packet counters may be maintained for BFR-
IDs
BFR-ids or next-hop neighbors. Any flow based flow-based metrics will require
deeper packet inspection which inspection; this topic is outside of the scope of this
document. In this way, BIER is again more like unicast.

It is this reduction in state that allows for one of the key
operational benefits of BIER, BIER: deterministic convergence. The BIER
FIB can converge immediately after the unicast FIB regardless of how
many multicast flows are transiting the links. Careful monitoring of
(S,G) utilization is not required within a BIER domain.

7.1. Configuration

A BIER domain requires that each edge node (BFER) be given a unique
bit position in the BIER mask (BFR-id). The BFR-id must be
configured on each BFER and associated with a unique IP address of
that BFER. Any existing manual or automated configuration tools must
provide access to BIER specific BIER-specific configuration. The association of
the BFR-id with a unique address of the BFER to which it is assigned
must also be advertised into the IGP of the BIER domain. This may be
implied from the BIER configuration or require IGP specific IGP-specific
configuration. This document does not dictate any specific
configuration methodology.

8. IANA Considerations

This document contains no actions for IANA. does not require any IANA actions.

9. Security Considerations

When BIER is paired with a particular multicast flow overlay, it
inherits the security considerations of that layer. Similarly, when
BIER is paired with a particular routing underlay, it inherits the
security considerations of that layer.

If the BIER encapsulation of a particular packet specifies an SI or a
BitString other than the one intended by the BFIR, the packet is
likely to be misdelivered. If the BIER encapsulation of a packet is
modified (through error or malfeasance) in a way other than that
specified in this document, the packet may be misdelivered. Some
modifications of the BIER encapsulation, e.g., setting every bit in
the BitString, may result in (intentional or unintentional) Denial of
Service
denial-of-service (DoS) attacks.

If a BFIR is compromised, it may impose a BIER encapsulation with all
the bits in the BitString set, which set; this would also result in a DoS
attack.

Every BFR MUST be provisioned to know which of its interfaces lead to
a BIER domain and which do not. BIER-encapsulated packets MUST NOT
be accepted from outside the BIER domain. (Reception of
BIER-encapsulated packets from outside the BIER domain would create
an attack vector for DoS attacks, as an attacker might set all the
bits in the BitString.)

If two interfaces lead to different BIER domains, the BFR MUST be
provisioned to know that those two interfaces lead to different BIER
domains. If the provisioning is not correct, BIER-encapsulated
packets from one BIER domain may "leak" into another; this is likely
to result in misdelivery of packets.

DoS attacks may also result from incorrect provisioning (through
error or malfeasance) of the BFRs.

If the procedures used for advertising BFR-ids and BFR-prefixes are
not secure, an attack on those procedures may result in incorrect
delivery of BIER-encapsulated packets.

12.

10. References

12.1.

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, DOI 10.17487/RFC3443, January 2003,
<https://www.rfc-editor.org/info/rfc3443>.

12.2.

[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>.

10.2. Informative References

[BGP_BIER_EXTENSIONS]
Xu, X., Ed., Chen, M., Patel, K., Wijnands, IJ., and T. A.
Przygienda, "BGP Extensions for BIER", internet-draft draft-
ietf-bier-idr-extensions-02.txt, January Work in Progress,
draft-ietf-bier-idr-extensions-03, August 2017.

[BIER-MVPN]

[BIER_MVPN]
Rosen, E., Ed., Sivakumar, M., Aldrin, S., Dolganow, A.,
and T. Przygienda, "Multicast VPN Using BIER", internet-draft
draft-ietf-bier-mvpn-07.txt, July Work in
Progress, draft-ietf-bier-mvpn-09, November 2017.

[Boivie_Feldman]
Boivie, R. and N. Feldman, "Small Group Multicast",
(expired) draft-boivie-sgm-02.txt, Work
in Progress, draft-boivie-sgm-02, February 2001.

[ISIS_BIER_EXTENSIONS]
Ginsberg, L., Ed., Przygienda, T., A., Aldrin, S., and J.
Zhang, "BIER Support via ISIS", internet-draft draft-ietf-bier-
isis-extensions-04.txt, March Work in Progress,
draft-ietf-bier-isis-extensions-06, October 2017.

[MPLS_BIER_ENCAPS]
Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication in MPLS and non-MPLS Networks
Networks", internet-draft Work in Progress, draft-ietf-bier-mpls-
encapsulation-07.txt, June
encapsulation-12, October 2017.

[OSPF_BIER_EXTENSIONS]
Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
Przygienda, T., Zhang, J., and S. Aldrin, "OSPF Extensions
for Bit Index Explicit Replication", internet-draft draft-
ietf-bier-ospf-bier-extensions-06.txt, June BIER", Work in Progress, draft-ietf-bier-ospf-bier-
extensions-09, October 2017.

[RFC6513] Rosen, E., Ed. Ed., and R. Aggarwal, Ed., "Multicast in MPLS/
BGP
MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513,
February 2012, <https://www.rfc-editor.org/info/rfc6513>.

[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<https://www.rfc-editor.org/info/rfc6514>.

10.

Acknowledgements

The authors wish to thank Rajiv Asati, Alia Atlas, John Bettink, Ross
Callon (who contributed much of the text on deterministic ECMP),
Nagendra Kumar, Christian Martin, Neale Ranns, Greg Shepherd, Albert Tian, Ramji
Vaithianathan, Xiaohu Xu Xu, and Jeffrey Zhang for their ideas and
contributions to this work.

The authors also wish to thank Sue Hares, Victor Kuarsingh, and Dan
Romascanu for their reviews of this document.

11. Contributor Addresses

Below is a list

Contributors

The following people contributed significantly to the content of other contributing authors in alphabetical order: this
document and should be considered co-authors:

Gregory Cauchie
Bouygues Telecom
Email: gcauchie@bouyguestelecom.fr

Mach (Guoyi)

Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com

Arkadiy Gulko
Thomson Reuters
195 Broadway
New York York, NY 10007
United States of America
Email: arkadiy.gulko@thomsonreuters.com

Wim Henderickx
Nokia
Copernicuslaan 50
Antwerp 2018
Belgium
Email: wim.henderickx@nokia.com

Martin Horneffer
Deutsche Telekom
Hammer Str. 216-226
Muenster 48153
Germany
Email: Martin.Horneffer@telekom.de

Uwe Joorde
Deutsche Telekom
Hammer Str. 216-226
Muenster D-48153
Germany

Email: Uwe.Joorde@telekom.de

Luay Jalil
Verizon
1201 E East Arapaho Rd.
Richardson, TX 75081
United States of America
Email: luay.jalil@verizon.com

Uwe Joorde
Deutsche Telekom
Hammer Str. 216-226
Muenster D-48153
Germany
Email: Uwe.Joorde@telekom.de
Greg Shepherd
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
United States of America
Email: shep@cisco.com

Jeff Tantsura
Email: jefftant.ietf@gmail.com

Authors' Addresses

IJsbrand Wijnands (editor)
Cisco Systems, Inc.
De Kleetlaan 6a
Diegem 1831
Belgium

Email: ice@cisco.com

Eric C. Rosen (editor)
Juniper Networks, Inc.
10 Technology Park Drive
Westford, Massachusetts 01886
United States of America

Email: erosen@juniper.net

Andrew Dolganow
Nokia
600 March Rd.
Ottawa, Ontario K2K 2E6
Canada
438B Alexandra Rd #08-07/10
Alexandra Technopark
Singapore 119968
Singapore

Email: andrew.dolganow@nokia.com

Tony Przygienda
Juniper Networks, Inc.
1194 N. Mathilda Ave.
Sunnyvale, California 94089
United States of America

Email: prz@juniper.net

Sam K K. Aldrin
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, California 94043
United States of America

Email: aldrin.ietf@gmail.com