Internet Engineering Task Force IJ. Wijnands, Ed.
Internet-Draft E. Rosen, Ed.
Intended status: Standards Track Cisco Systems, Inc.
Expires: March 26, 2015 A. Dolganow
Alcatel-Lucent
T. Przygienda
Ericsson
September 22, 2014
Multicast using Bit Index Explicit Replication
draft-wijnands-bier-architecture-00
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
any explicit tree-building protocol, 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 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. Elimination of the per-flow state and the explicit
tree-building protocols results in a considerable simplification.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents 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 26, 2015.
Wijnands, et al. Expires March 26, 2015 [Page 1]
Internet-Draft Multicast with BIER September 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The BFR Identifier and BFR-Prefix . . . . . . . . . . . . . . 5
3. Encoding BFR Identifiers in BitStrings . . . . . . . . . . . 6
4. Layering . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. The Routing Underlay . . . . . . . . . . . . . . . . . . 7
4.2. The BIER Layer . . . . . . . . . . . . . . . . . . . . . 8
4.3. The Multicast Flow Overlay . . . . . . . . . . . . . . . 9
5. Advertising BFR-ids and BFR-Prefixes . . . . . . . . . . . . 9
6. BIER Intra-Domain Forwarding Procedures . . . . . . . . . . . 10
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. BFR Neighbors . . . . . . . . . . . . . . . . . . . . . . 11
6.3. The Bit Index Routing Table . . . . . . . . . . . . . . . 12
6.4. The Bit Index Forwarding Table . . . . . . . . . . . . . 13
6.5. The BIER Forwarding Procedure . . . . . . . . . . . . . . 14
6.6. Examples of BIER Forwarding . . . . . . . . . . . . . . . 16
6.6.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . 16
6.6.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . 17
6.7. Equal Cost Multi-path Forwarding . . . . . . . . . . . . 19
6.8. Prevention of Loops and Duplicates . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
10. Contributor Addresses . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Wijnands, et al. Expires March 26, 2015 [Page 2]
Internet-Draft Multicast with BIER September 2014
1. Introduction
This document specifies a new architecture for the forwarding of
multicast data packets. It provides optimal forwarding of multicast
data packets through a "multicast domain". However, it does not
require any explicit tree-building protocol, and 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). A BIER domain is a connected set of BFRs, where each BFR has
a "BFR-id" that is unique within the domain. A BFR-id is a small
unstructured number. For instance, if a particular BIER domain
contains 1,374 BFRs, each one could be given a BFR-id in the range
1-1374.
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, a BFR
may be the BFIR for a given packet and may also be (one of) the
BFER(s), for that packet; it may also forward that packet to one or
more additional BFRs.
When a multicast packet arrives from outside the domain at a BFIR,
the BFIR determines the set of BFERs to which the packet must be
sent. It then 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 needs to receive a given
packet, the BFIR sets the bit corresponding to that BFER's BFR-id.
We will use 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 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 will have more BFERs
in a given 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
Wijnands, et al. Expires March 26, 2015 [Page 3]
Internet-Draft Multicast with BIER September 2014
SI together that determine the set of BFERs to which a given packet
will be delivered:
o by convention, the least significant (rightmost) bit in the
BitString is "bit 1", and the most significant (leftmost) bit is
"bit BitStringLength".
o if a BIER-encapsulated packet has an SI of n, and a BitString with
bit k set, then the packet must be delivered to the BFER whose
BFR-id 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", and the most significant (leftmost) bit is "bit 256".
Suppose that a given packet needs to be delivered to three BFERs,
where those BFERs have BFR-ids of 13, 126, and 235 respectively. The
BFIR would create a BIER encapsulation with the SI set to 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.
Note that it is generally advantageous to assign the BFR-ids so that
as many BFERs as possible can be represented in a single bit string.
Suppose a BFR, call it BFR-A, receives a packet whose BIER
encapsulation specifies an SI of 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 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,
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.
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, or to run a multicast tree-building
protocol.
Wijnands, et al. Expires March 26, 2015 [Page 4]
Internet-Draft Multicast with BIER September 2014
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 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) for
details.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. The BFR Identifier and BFR-Prefix
Each BFR MUST be assigned a "BFR-Prefix". A BFR's BFR-Prefix MUST be
an IP address (either IPv4 or IPv6) of the BFR, and MUST be routable
within the domain. It is RECOMMENDED that the BFR-prefix be a
loopback address of the BFR. Two BFRs in the same BIER domain MUST
NOT be assigned the same BFR-Prefix.
A "BFR Identifier" (BFR-id) is a number in the range [1,65535]. In
general, each BFR in a given BIER domain must be assigned a unique
number from this range (i.e., two BFRs in the same BIER domain MUST
NOT have the same BFR-id). However, if it is known that a given BFR
will never need to function as a BFER, then it is not necessary to
assign a BFR-id to that BFR.
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 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, 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.
Wijnands, et al. Expires March 26, 2015 [Page 5]
Internet-Draft Multicast with BIER September 2014
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
o The BitString has one bit position set. If the low-order bit is
bit 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
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.
Different BIER domains may use different values of BitStringLength.
Within a BIER domain, all BFRs MUST use the same BitStringLength
value; this value is known by provisioning. A BFR MUST support a
BitStringLength value of 256. Particular encapsulation types MAY
allow other BitStringLengths to be optionally supported. For
example, when using the encapsulation specified in
[MPLS_BIER_ENCAPS], a BFR may support any or all of the following
BitStringLengths: 64, 128, 256, 512, 1024, 2048, and 4096.
A BFR MUST support SI values in the range [0,15], and MAY support SI
values in the range [0,255].
It is possible to design procedures that allow BFRs within a given
BIER domain to use different BitStringLength values, and to enable
each BFR to discover the BitStringLength values used by all the other
BFRs in the domain. However, this document presupposes that they all
use the same value; procedures for the use of different values may be
specified in future documents.
When a BFIR determines that a multicast data packet needs to be
forwarded to a particular set of destination BFERs, it partitions
that set of BFERs into subsets, where each subset contains the target
BFERs whose BFR-ids 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, and contains
the BitString that represents all the BFR-ids in the corresponding
SI-subset.
Wijnands, et al. Expires March 26, 2015 [Page 6]
Internet-Draft Multicast with BIER September 2014
Suppose, for example, that a BFIR determines that a given packet
needs to be forwarded to three BFERs, whose BFR-ids 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 be
assigned "densely" (see Section 2) from the numbering space. This
will minimize the number of SIs that have to be used in the 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
for the "east coast" or for the "west coast". 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, 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. Then 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, 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), 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 expected that there will be several different forms of BIER
encapsulation. 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
will be given in companion documents. An encapsulation for use in
MPLS networks is described in [MPLS_BIER_ENCAPS]
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, 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),
Wijnands, et al. Expires March 26, 2015 [Page 7]
Internet-Draft Multicast with BIER September 2014
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" 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 Interior Gateway Protocol (IGP), e.g., OSPF, that is used
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 to B, one can use a different underlay. For example, if multi-
topology OSPF is being used, one OSPF topology could be used for
unicast traffic, and the other for multicast traffic. Alternatively,
one could deploy a routing underlay that creates a Steiner tree.
Then BIER could be used to forward multicast data packets along the
Steiner tree, while unicast packets follow the "ordinary" OSPF best
path. 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 to use.
By default, the routing underlay used by BIER is the unicast IGP.
Note that specification of the protocol 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 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 advertise each BFR's BFR-prefix and
BFR-id to all other BFRs in the same BIER domain.
o The imposition by a BFIR of a BIER header on a multicast data
packet.
o The procedures for forwarding BIER-encapsulated packets, and for
modifying the BIER header during transit.
Wijnands, et al. Expires March 26, 2015 [Page 8]
Internet-Draft Multicast with BIER September 2014
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 though
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 will be specified in a companion document.
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 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 a BFR-id (for a given
BIER domain). Each BFER must advertise this assignment to all the
other BFRs in the domain. Similarly, each BFR is assigned a
BFR-prefix (for a given BIER domain), and must advertise this
assignment to all the other BFRs in the domain. Finally, it is
useful for each BFR to advertise its value of BitStringLength (for a
given BIER domain)
If the BIER domain is also a link state routing IGP domain (i.e., an
OSPF or IS-IS domain), the advertisement of the BFR-id, BFR-prefix,
Wijnands, et al. Expires March 26, 2015 [Page 9]
Internet-Draft Multicast with BIER September 2014
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] for the extensions to OSPF.)
These advertisements enable each BFR to associate a given 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 the domain) BFR-id, two
different BFRs can advertise ownership of the same BFR-id only if
there has been a provisioning error. If BFR-A determines that BFR-B
and BFR-C have both advertised the same BFR-id, 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, 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 until
the provisioning error is fixed. However, that is preferable to
having them receive each other's traffic.
6. BIER Intra-Domain Forwarding Procedures
This section specifies the rules for forwarding a BIER-encapsulated
data packet within a BIER domain.
6.1. Overview
This section provides a brief overview of the BIER forwarding
procedures. Subsequent sub-sections specify the procedures in more
detail.
To forward a BIER-encapsulated packet:
1. Determine the packet's SI.
2. From the SI and the BitString, determine the set of destination
BFERs for the packet.
3. Using information provided by the routing underlay, determine the
next hop adjacency for each of the destination BFERs.
4. 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.
Wijnands, et al. Expires March 26, 2015 [Page 10]
Internet-Draft Multicast with BIER September 2014
5. 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 a BFR receives a BIER-encapsulated packet whose SI and BitString
identify 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 more than one bit set, 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 will be cleared in
all the copies.
When BIER on a BFER passes a packet to the multicast flow overlay, it
may need to provide contextual information obtained from the BIER
encapsulation. The information that needs to pass between the BIER
layer and the multicast flow layer is specific to the multicast flow
layer. Specification of the interaction between the BIER layer and
the multicast flow layer is outside the scope of this specification.
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 layer.
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 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 the BFR-ids of the BFERs to
which the packet is destined. Then using the information advertised
per Section 5, BFR-A can find the BFR-prefix of each destination
Wijnands, et al. Expires March 26, 2015 [Page 11]
Internet-Draft Multicast with BIER September 2014
BFER. Given the BFR-prefix of a particular destination BFER, say
BFER-D, BFR-A learns from the routing underlay 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-NH. BFR-A must then determine the BFR-prefix
of BFR-NH. (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 paths
from BFR-A to BFER-D, BFR-A may have multiple BFR-NBRs for BFER-D.
6.3. The Bit Index Routing Table
The Bit Index Routing Table (BIRT) is a table that maps from the
BFR-id of a BFER to the BFR-prefix of that BFER, and to the BFR-NBR
on the path to that BFER.
( 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.)
Wijnands, et al. Expires March 26, 2015 [Page 12]
Internet-Draft Multicast with BIER September 2014
--------------------------------------------
| 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.
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> 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 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 they have the same F-BM.
Wijnands, et al. Expires March 26, 2015 [Page 13]
Internet-Draft Multicast with BIER September 2014
-------------------------------------
| BFR-id | F-BM | BFR-NBR |
| (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) is programmed into the 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 for forwarding a BIER-encapsulated packet.
1. Determine the packet's SI.
2. Find the position of least significant (rightmost) bit in the
packet's BitString that is set. (Remember, bits are numbered
from 1, starting with the least significant bit.) Use that bit
position, together with the SI, as the 'index' into the BIFT.
3. Extract from the BIFT the F-BM and the BFR-NBR.
4. 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. 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.
5. Now update the original packet's BitString by AND'ing it with the
INVERSE of the F-BM (i.e., 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.
Note that 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.
Wijnands, et al. Expires March 26, 2015 [Page 14]
Internet-Draft Multicast with BIER September 2014
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 algorithms are outside the scope of this
document. However, 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
interface (if possible).
In some cases, the routing underlay may provide multiple equal cost
paths (through different BFR-NBRs) to a given BFER. This is known as
"Equal Cost Multiple Paths" (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 to the BFR-NBR SHOULD also be sent via that tunnel. This allows
any existing "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
Note 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. In this case,
Wijnands, et al. Expires March 26, 2015 [Page 15]
Internet-Draft Multicast with BIER September 2014
the "PacketSend" function sends the packet to the multicast flow
layer.
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 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 Forwarding Examples
6.6.1. Example 1
BFR-D, BFR-E and BFR-F are BFER's. BFR-A is the BFIR. Suppose that
BFIR-A has learned from the multicast flow layer 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 follows the procedures of Section 6.5:
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, and applies F-BM to the
copy: Copy->BitString &= 0111. The copy's Bitstring is now 0001
(0001 & 0111).
o The copy is now sent to BFR-B.
Wijnands, et al. Expires March 26, 2015 [Page 16]
Internet-Draft Multicast with BIER September 2014
o BFR-A then updates the packet's BitString by applying the inverse
of the F-BM: 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 as a 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 0000 and a BFR-NBR of BFR-D itself. This will cause a copy
of the packet to be delivered to the multicast flow layer 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 1, except that BFIR-A has learned
from the multicast flow layer 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., the BitString (assuming again a
BitStringLength of 4) is 0101. To forward the packet, BFIR-A follows
the procedures of Section 6.5:
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, and applies the F-BM to the
copy: Copy->BitString &= 0111. The copy's 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 &= ~F-BM. As a result, the
packet's BitString is now 0000 (0101 & 1000).
Wijnands, et al. Expires March 26, 2015 [Page 17]
Internet-Draft Multicast with BIER September 2014
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, and applies the F-BM to the
copy: Copy->BitString &= 0011. The copy's 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 &= 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, and applies the F-BM to the
copy: Copy->BitString &= 0100. The copy's 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 &= ~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 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.
At BFER-D, the BIFT entry (not pictured) for BFR-id 1 will specify an
F-BM of 0000 and a BFR-NBR of BFR-D itself. This will cause a copy
of the packet to be delivered to the multicast flow layer at BFR-D.
The packet's BitString will be set to 0000, and the packet will not
be forwarded any further.
Wijnands, et al. Expires March 26, 2015 [Page 18]
Internet-Draft Multicast with BIER September 2014
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 0000 and a BFR-NBR of BFR-E itself. This will cause a copy
of the packet to be delivered to the multicast flow layer 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 Forwarding
In many networks, the routing underlay will provide multiple 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 among those paths. This feature
is known as "equal cost multiple path forwarding", or "ECMP".
BIER supports ECMP, but the procedures of Section 6.5 must be
modified slightly. This is shown in Figure 6, which is based on the
topology of Figure 1.
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 | -------------------
-------------------
Figure 6: Example of ECMP
In this example, BFR-B has two equal cost paths to reach 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, 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
Wijnands, et al. Expires March 26, 2015 [Page 19]
Internet-Draft Multicast with BIER September 2014
split among two paths, and any "entropy" field in the packet's
encapsulation SHOULD be respected.
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.
6.8. 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 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 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 techniques beyond the
forwarding procedures specified in Section 6.5.
If, at some time, the routing underlay is not providing a loop free
path between BFIR-A and BFER-B, then 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" (RPF) check that is used in conventional IP multicast
forwarding.
7. IANA Considerations
This document contains no actions for IANA.
8. Security Considerations
When BIER is paired with a particular multicast flow layer, 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.
Wijnands, et al. Expires March 26, 2015 [Page 20]
Internet-Draft Multicast with BIER September 2014
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.
Every BFR must be provisioned to know which of its interfaces lead to
a BIER domain and which do not. 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.
9. Acknowledgements
The authors wish to thank Rajiv Asati, John Bettink, Nagendra Kumar,
Christian Martin, Neale Ranns, Greg Shepherd, and Ramji Vaithianathan
for their ideas and contributions to this work.
10. Contributor Addresses
Below is a list of other contributing authors in alphabetical order:
Wijnands, et al. Expires March 26, 2015 [Page 21]
Internet-Draft Multicast with BIER September 2014
Wim Henderickx
Alcatel-Lucent
Copernicuslaan 50
Antwerp 2018
Belgium
Email: wim.henderickx@alcatel-lucent.com
Martin Horneffer
Deutsche Telekom
Hammer Str. 216-226
Muenster 48153
DE
Email: Martin.Horneffer@telekom.de
Uwe Joorde
Deutsche Telekom
Hammer Str. 216-226
Muenster D-48153
DE
Email: Uwe.Joorde@telekom.de
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
US
Email: jeff.tantsura@ericsson.com
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[Boivie_Feldman]
Boivie, R. and N. Feldman, "Small Group Multicast",
(expired) draft-boivie-sgm-02.txt, February 2001.
Wijnands, et al. Expires March 26, 2015 [Page 22]
Internet-Draft Multicast with BIER September 2014
[MPLS_BIER_ENCAPS]
Wijnands, IJ., "BIER Encapsulation for MPLS Networks",
internet-draft draft-wijnands-mpls-bier-encaps-00.txt,
September 2014.
[OSPF_BIER_EXTENSIONS]
Kumar, N. and P. Psenak, "OSPF Extension for Bit Index
Explicit Replication", internet-draft draft-kumar-ospf-
bier-extension-00.txt, September 2014.
[RFC6513] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
VPNs", RFC 6513, February 2012.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, February 2012.
Authors' Addresses
IJsbrand Wijnands (editor)
Cisco Systems, Inc.
De Kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Eric C. Rosen (editor)
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, Massachusetts 01781
USA
Email: erosen@cisco.com
Andrew Dolganow
Alcatel-Lucent
600 March Rd.
Ottawa, Ontario K2K 2E6
Canada
Email: andrew.dolganow@alcatel-lucent.com
Wijnands, et al. Expires March 26, 2015 [Page 23]
Internet-Draft Multicast with BIER September 2014
Tony Przygienda
Ericsson
Email: antoni.przygienda@ericsson.com
Wijnands, et al. Expires March 26, 2015 [Page 24]