Internet Engineering Task Force (IETF) IJ. Wijnands, Ed. Request for Comments: 8279 Cisco Systems, Inc. Category: Experimental E. Rosen, Ed. ISSN: 2070-1721 Juniper Networks, Inc. A. Dolganow Nokia T. Przygienda Juniper Networks, Inc. S. Aldrin Google, Inc. November 2017 Multicast Using Bit Index Explicit Replication (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 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 document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8279. Copyright Notice Copyright (c) 2017 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 (https://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 ....................................................4 2. The BFR Identifier and BFR-Prefix ...............................7 3. Encoding BFR Identifiers in BitStrings ..........................8 4. Layering .......................................................11 4.1. The Routing Underlay ......................................11 4.2. The BIER Layer ............................................12 4.3. The Multicast Flow Overlay ................................13 5. Advertising BFR-ids and BFR-Prefixes ...........................13 6. BIER Intra-Domain Forwarding Procedures ........................15 6.1. Overview ..................................................15 6.2. BFR Neighbors .............................................17 6.3. The Bit Index Routing Table ...............................18 6.4. The Bit Index Forwarding Table ............................19 6.5. The BIER Forwarding Procedure .............................20 6.6. Examples of BIER Forwarding ...............................23 6.6.1. Example 1 ..........................................23 6.6.2. Example 2 ..........................................24 6.7. Equal-Cost Multipath Forwarding ...........................26 6.7.1. Non-deterministic ECMP .............................27 6.7.2. Deterministic ECMP .................................28 6.8. Prevention of Loops and Duplicates ........................29 6.9. When Some Nodes Do Not Support BIER .......................30 6.10. Use of Different BitStringLengths within a Domain ........33 6.10.1. BitStringLength Compatibility Check ...............34 6.10.2. Handling BitStringLength Mismatches ...............36 6.10.3. Transitioning from One BitStringLength to Another ...........................................36 7. Operational Considerations .....................................37 7.1. Configuration .............................................37 8. IANA Considerations ............................................37 9. Security Considerations ........................................38 10. References ....................................................39 10.1. Normative References .....................................39 10.2. Informative References ...................................39 Acknowledgements ..................................................40 Contributors ......................................................41 Authors' Addresses ................................................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 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 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]. 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, 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 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 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 (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, and the most significant (leftmost) bit is bit 256. Suppose that a given packet has been assigned to sub-domain 0 and needs to be delivered to three BFERs, where those BFERs have BFR-ids in sub-domain 0 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. 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 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. 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 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 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, 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 6.5) 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 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" for each sub-domain to which it belongs. A BFR's 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 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 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 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 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=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. 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 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 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 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). 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 each legal smaller value of BitStringLength as (a) its Imposition BitStringLength, and (b) one of its Disposition 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] 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 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 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 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 intended either for the "east coast" or for the "west 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 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 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 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 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" 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 BFR-A to BFER-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. (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. 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 multiple sub-domains may be associated with the same routing 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 External BGP (EBGP) is used as the IGP, the underlay adjacencies, by default, are the BGP adjacencies. Specification of the 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 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 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]. 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.