Internet Engineering Task Force L. Ginsberg, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track A. Przygienda
Expires: August 27, 2018 Juniper Networks
S. Aldrin
J. Zhang
Juniper Networks, Inc.
February 23, 2018

BIER support via ISIS


This document defines ISIS extensions to support multicast forwarding using the Bit Index Explicit Replication (BIER) architecture.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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

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This Internet-Draft will expire on August 27, 2018.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

Bit Index Explicit Replication (BIER) [RFC8279] defines an architecture where all intended multicast receivers are encoded as bitmask in the Multicast packet header within different encapsulations such as [RFC8296]. A router that receives such a packet will forward the packet based on the Bit Position in the packet header towards the receiver(s), following a precomputed tree for each of the bits in the packet. Each receiver is represented by a unique bit in the bitmask.

This document presents necessary extensions to the currently deployed ISIS for IP [RFC1195] protocol to support distribution of information necessary for operation of BIER domains and sub-domains. This document defines a new TLV to be advertised by every router participating in BIER signaling.

This document defines support for MPLS encapsulation as specified in [RFC8296]. Support for other encapsulation types is outside the scope of this document. The use of multiple encapsulation types is outside the scope of this document.

2. Terminology

Some of the terminology specified in [RFC8279] is replicated here and extended by necessary definitions:

Bit Index Explicit Replication (The overall architecture of forwarding multicast using a Bit Position).
BIER Overlay Signaling. (The method for the BFIR to learn about BFER's).
Bit Forwarding Router (A router that participates in Bit Index Multipoint Forwarding). A BFR is identified by a unique BFR-prefix in a BIER domain.
Bit Forwarding Ingress Router (The ingress border router that inserts the BM into the packet). Each BFIR must have a valid BFR-id assigned.
Bit Forwarding Egress Router. A router that participates in Bit Index Forwarding as leaf. Each BFER must be a BFR. Each BFER must have a valid BFR-id assigned.
Bit Forwarding Tree used to reach all BFERs in a domain.
BIER sub-domain:
A further distinction within a BIER domain identified by its unique sub-domain identifier. A BIER sub-domain can support multiple BitString Lengths.
An optional, unique identifier for a BFR within a BIER sub-domain.
Invalid BFR-id:
Unassigned BFR-id. The special value 0 is reserved for this purpose.
BIER Algorithm. Used to calculate underlay next hops.
IGP Algorithm. May be used to modify, enhance or replace the calculation of underlay paths as defined by the BAR value
Shortest Path First routing calculation based on IGP link metric

3. IANA Considerations

This document adds the following new sub-TLV to the registry of Sub-TLVs for TLVs 135, 235, 236, and 237.

Value: 32 (suggested - to be assigned by IANA)

Name: BIER Info

This document also introduces a new registry for sub-sub-TLVs for the BIER Info sub-TLV added above. The registration policy is Expert Review as defined in [RFC8126]. This registry is part of the "IS-IS TLV Codepoints" registry. The name of the registry is "sub-sub-TLVs for BIER Info sub-TLV". The defined values are:

  Type    Name
  ----    ----
  1       BIER MPLS Encapsulation

IANA is requested to set up a registry called "BIER Algorithm Registry" under category "Bit Index Explicit Replication". The registration policies [RFC8126] for this registry are:

The initial values in the BIER Algorithm Registry are:

4. Concepts

4.1. BIER Domains and Sub-Domains

An ISIS signalled BIER domain is aligned with the scope of distribution of BFR-prefixes that identify the BFRs within ISIS. ISIS acts in such a case as the supporting BIER underlay.

Within such a domain, the extensions defined in this document advertise BIER information for one or more BIER sub-domains. Each sub-domain is uniquely identified by a subdomain-id (SD). Each subdomain is associated with a single ISIS topology (MT) [RFC5120], which may be any of the topologies supported by ISIS. Local configuration controls which <MT,SD> pairs are supported by a router. The mapping of sub-domains to topologies MUST be consistent within the IS-IS flooding domain used to advertise BIER information.

Each BIER sub-domain has as its unique attributes the encapsulation used and the type of tree it is using to forward BIER frames (currently always SPF). Additionally, per supported bitstring length in the sub-domain, each router will advertise the necessary label ranges to support it.

4.2. Advertising BIER Information

BIER information advertisements are associated with a new sub-TLV in the extended reachability TLVs. BIER information is always associated with a host prefix which MUST be a node address for the advertising node. If this is not the case the advertisement MUST be ignored. Therefore the following restrictions apply:

5. Procedures

5.1. Multi Topology and Sub-Domain

A given sub-domain is supported within one and only one topology. All routers in the flooding scope of the BIER sub-TLVs MUST advertise the same sub-domain within the same multi-topology. A router receiving an <MT,SD> advertisement which does not match the locally configured pair MUST report a misconfiguration of the received <MT,SD> pair. All received BIER advertisements associated with the conflicting <MT,SD> pair MUST be ignored. Note that in the presence of such a misconfiguration this will lead to partitioning of the sub-domian.


The following combination of advertisements are valid: <0,0> <0,1> <2,2>.

The following combination of advertisements are invalid: <0,0> <0,1> <2,0>. Advertisements associated with <0,0> and <2,0> must be ignored.

5.2. BFR-id Advertisements

If a BFER/BFIR is configured with a BFR-id then it advertises this value in its BIER advertisements. If no BFR-id is configured then the value "Invalid BFR-id" is advertised. A valid BFR-id MUST be unique within the flooding scope of the BIER advertisements. All BFERs/BFIRs MUST detect advertisement of duplicate valid BFR-IDs for a given <MT, SD>. When such duplication is detected all of the routers advertising duplicates MUST be treated as if they did not advertise a valid BFR-id. This implies they cannot act as BFER or BFIR in that <MT,SD>.

5.3. Logging Misconfiguration

Whenever an advertisement is received which violates any of the constraints defined in this document the receiving router MUST support logging this occurrence. Logging SHOULD be dampened to avoid excessive output.

5.4. Flooding Reduction

It is expected that changes in BIER domain information which is advertised by IS-IS occur infrequently. If this expectation is not met for an extended period of time (more than a few seconds of burstiness) changes will increase the number of Link State PDU (LSP) updates and negatively impact performance in the network. Implementations SHOULD protect against this possibility e.g., by dampening updates if they occur over an extended period of time.

6. Packet Formats

All ISIS BIER information is carried within the TLVs 235, 237 [RFC5120] or TLVs 135 [RFC5305], or TLV 236.

6.1. BIER Info sub-TLV

This sub-TLV carries the information for the BIER sub-domains that the router participates in as BFR. This sub-TLV MAY appear multiple times in a given prefix-reachability TLV - once for each sub-domain supported in the associated topology.

The sub-TLV advertises a single <MT,SD> combination followed by optional sub-sub-TLVs as described in the following sections.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |    Type       |   Length      |
      |   BAR         |    IPA        | subdomain-id  |
      |     BFR-id                    |
      |  sub-sub-TLVs (variable)                                      |

[RFC8279] and additional methods may be defined in the future).

as indicated in IANA section.
BIER Algorithm. Specifies a BIER specific algorithm used to calculate underlay paths to reach BFERs. Values are allocated from the BIER Algorithm Registry. 1 octet
IGP algorithm. Specifies an IGP Algorithm to either modify, enhance or replace the calculation of underlay paths to reach BFERs as defined by the BAR value. Values are from the IGP Algorithm registry. 1 octet
Unique value identifying the BIER sub-domain. 1 octet
A 2 octet field encoding the BFR-id, as documented in [RFC8279]. If no BFR-id has been assigned the value of this field is set to "Invalid BFR-id", which is defined as illegal in [RFC8279] .

The use of non-zero values in either the BAR field or the IPA field is outside the scope of this document. If an implementation does not support the use of non-zero values in these fields, but receives a BIER Info sub-TLV containing non-zero values in these fields, it SHOULD treat the advertising router as incapable of supporting BIER (one way of handling incapable routers is documented in section 6.9 of

6.2. BIER MPLS Encapsulation sub-sub-TLV

This sub-sub-TLV carries the information for the BIER MPLS encapsulation including the label range for a specific bitstring length for a certain <MT,SD>. It is advertised within the BIER Info sub-TLV (Section 6.1) . This sub-sub-TLV MAY appear multiple times within a single BIER info sub-TLV.

If the same Bitstring length is repeated in multiple sub-sub-TLVs inside the same BIER Info Sub-TLV, the BIER Info sub-TLV MUST be ignored.

Label ranges within all BIER MPLS Encapsulation sub-sub-TLVs across all BIER Info sub-TLVs advertised by the same BFR MUST NOT overlap. If overlap is detected, the advertising router MUST be treated as if it did not advertise any BIER sub-TLVs.

Label values MUST NOT match any of the reserved values defined in [RFC3032]

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |    Type       |   Length      |
      |   Max SI      |BS Len |                    Label              |

value of 1 indicating MPLS encapsulation.
Local BitString Length (BS Len):
Encoded bitstring length as per [RFC8296]. 4 bits.
Max SI
Maximum Set Identifier (section 1 of [RFC8279]) used in the encapsulation for this BIER sub-domain for this bitstring length, 1 octet. Each SI maps to a single label in the label range. The first label is for SI=0, the second label is for SI=1, etc.
First label of the range, 20 bits. The labels are as defined in [RFC8296].

7. Security Considerations

Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310]. Advertisement of the additional information defined in this document introduces no new security concerns.

BIER specific security considerations are discussed in [RFC8279].

8. Acknowledgements

The RFC is aligned with the [I-D.draft-ietf-bier-ospf-bier-extensions-15] draft as far as the protocol mechanisms overlap.

Many thanks for comments from (in no particular order) Hannes Gredler, Ijsbrand Wijnands, Peter Psenak and Chris Bowers.

9. References

9.1. Normative References

[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, DOI 10.17487/RFC1195, December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001.
[RFC5120] Przygienda, T., Shen, N. and N. Sheth, "M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs)", RFC 5120, DOI 10.17487/RFC5120, February 2008.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, DOI 10.17487/RFC5308, October 2008.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R. and M. Fanto, "IS-IS Generic Cryptographic Authentication", RFC 5310, DOI 10.17487/RFC5310, February 2009.
[RFC7794] Ginsberg, L., Decraene, B., Previdi, S., Xu, X. and U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794, March 2016.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

9.2. Informative References

[I-D.draft-ietf-bier-ospf-bier-extensions-15] Psenak et al., P., "OSPF Extension for Bit Index Explicit Replication", internet-draft draft-ietf-bier-ospf-bier-extensions-15, February 2018.
[RFC8126] Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017.
[RFC8279] Wijnands, IJ., Rosen, E., Dolganow, A., Przygienda, T. and S. Aldrin, "Multicast Using Bit Index Explicit Replication (BIER)", RFC 8279, DOI 10.17487/RFC8279, November 2017.
[RFC8296] Wijnands, IJ., Rosen, E., Dolganow, A., Tantsura, J., Aldrin, S. and I. Meilik, "Encapsulation for Bit Index Explicit Replication (BIER) in MPLS and Non-MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January 2018.

Authors' Addresses

Les Ginsberg (editor) Cisco Systems 510 McCarthy Blvd. Milpitas, CA 95035 USA EMail:
Tony Przygienda Juniper Networks EMail:
Sam Aldrin Google 1600 Amphitheatre Parkway Mountain View, CA USA EMail:
Jeffrey (Zhaohui) Zhang Juniper Networks, Inc. 10 Technology Park Drive Westford, MA 01886 USA EMail: