Internet-Draft M-LDP Signaling Through BIER Core October 2023
Bidgoli, et al. Expires 23 April 2024 [Page]
Network Working Group
Intended Status:
Standards Track
H. Bidgoli, Ed.
J. Kotalwar
I. Wijnands
Cisco System
M. Mishra
Cisco System
Z. Zhang
Juniper Networks
E. Leyton

M-LDP Signaling Through BIER Core


Consider an end-to-end Multipoint LDP (mLDP) network, where it is desirable to deploy BIER in a segment of this network. It might be desirable to deploy BIER with minimal disruption to the mLDP network or a redesign of the network.

This document describes a procedure needed for mLDP tunnels to be signaled over and stitched through a BIER core, allowing LDP routers to run traditional mLDP services through a BIER core.

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

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 23 April 2024.

Table of Contents

1. Introduction

Some operators that are using mLDP P2MP LSPs for their multicast transport would like to deploy BIER technology in some segments of their network. This draft explains a method to signal mLDP services through a BIER domain, with minimal disruption and operational impact to the mLDP domain.

2. Conventions used in this document

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 [RFC2119].

2.1. Definitions

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.


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 Bit Map into the packet. Each BFIR must have a valid BFR-id assigned. BFIR is a term used for data plane packet forwarding.


Bit-Forwarding Egress Router, A router that participates in Bit Index Forwarding as leaf. Each BFER must have a valid BFR-id assigned. BFER is a term used for data plane packet forwarding.


BIER Boundary router. The router between the LDP domain and BIER domain.


Ingress BIER Boundary Router. The ingress router from a signaling point of view. It maintains mLDP adjacency toward the LDP domain and determines if the Multipoint LDP FEC needs to be signaled across the BIER domain via Targeted LDP.


Egress BIER Boundary Router. The egress router in a BIER domain from signaling point of view. It terminates the targeted LDP signaling through the BIER domain. It also keeps track of all IBBRs that are part of this P2MP tree


Bit Index Forwarding Table.

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, all BFERs and BFIRs need to be assigned a BFR-id.


MPLS Incoming Label Map.

3. mLDP Signaling Through BIER domain

                    BBR                   BBR
     |---LDP Domain--|-----BIER domain-----|---LDP domain--|
S--( A )-----------( B ) ---- ( C ) ---- ( D )-----------( E )--h

                   EBBR                  IBBR
Sig  <----MLDP------|<----targeted LDP----|<---MLDP------

                  BFIR                  BFER
     ------------->|--------BIER-------->|------------->  Datapatah
Figure 1: BIER boundary router

As per figure 1, point-to-multipoint and multipoint-to-multipoint LSPs established via mLDP [RFC6388] can be signaled through a bier domain via Targeted LDP sessions. This procedure is explained in [RFC7060] (Using LDP Multipoint Extension on Targeted LDP Sessions).

This document provides details and defines some needed procedures.


3.1. Ingress BBR procedure

In Figure 1, the Ingress BBR (IBBR) is connected to the mLDP domain on downstream and a BIER domain on the upstream. To connect the LDP domains via BIER domain, IBBR needs to establish a targeted LDP session with EBBR closest to the root of the P2MP or MP2MP LSP. To do so IBBR will follow procedures in [RFC7060] in particular section 6 "Targeted mLDP with Multicast Tunneling".

The Target LDP session can be established manually via configuration or via automated mechanism.

3.1.1. Automatic tLDP session Creation

tLDP sessions can be signaled automatically from every IBBR to the appropriate EBBR. When mLDP FEC arrives at the IBBR from LDP domain, IBBR can automatically start a tLDP session to the EBBR closest to the root node. Both IBBR and EBBR should be in auto-discovery mode and react to the arriving tLDP signaling packets (i.e. targeted hellos, keep-alives etc...) to establish the session automatically.

The root node address in the mLDP FEC can be used to find the EBBR. To identify the EBBR, the same procedures as [RFC7060] section 2.1 can be used or the procedures as explained in the [draft-ietf-bier-pim-signaling] appendix A.

3.1.2. ECMP Method on IBBR

If the IBBR finds multiple equal cost EBBRs on the path to the root, it can use a vendor specific algorithm to choose between the EBBRs. These algorithms are beyond the scope of this draft. As an example the IBBR can use the lowest EBBR IP address to establish its mLDP signaling to.

3.2. Egress BBR procedure

The Egress BBR (EBBR) is connected to the upstream mLDP domain. The EBBR should accept the tLDP session generated form the IBBR. It should assign a unique "upstream assigned label" for each arriving FEC generated by the IBBRs.

The EBBR should follow the [RFC7060] procedures with the following modifications:

The EBBR will also generate a new label and FEC toward the root in the LDP domain. The EBBR Should stitch this generated label with the "upstream assigned label" to complete the P2MP or MP2MP LSP.

With the same token the EBBR should track all the arriving FECs and the IBBRs that are generating these FECs. The EBBR will use this information to build the bier header for each set of common FEC arriving from the IBBRs.

3.2.1. IBBR procedure for arriving upstream assigned label

Upon receiving the "upstream assigned label", the IBBR should create its own stitching instruction between the "upstream assigned label" and the downstream signaled label.

3.2.2. BIER Interface ID SUB-TLVs

As per [RFC6389] when LDP is used for upstream label assignment,the Interface ID TLV is used for signaling the Tunnel Identifier and it carries sub-TLVs. This document defines two new Interface ID sub-TLVs for BIER.

Below is the Interface ID BIER sub-TLV for IPv4 BIER prefix:

       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 (TBD1)                |               15              |
      |                      IBBR Prefix IPv4                         |
      |  sub-domain-id|      BFR-id                   |

Below is the Interface ID BIER sub-TLV for IPv6 BIER prefix:

       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 (TBD2)                |               23              |
      |                      IBBR Prefix IPv6                         |
      ~                         ........                              ~
      |  sub-domain-id|      BFR-id                   |

4. Datapath Forwarding

4.1. Datapath traffic flow

On the BFIR when the MPLS label for P2MP/MP2MP LSP arrives from upstream, a lookup in the ILM table is done and the label is swapped with the tLDP upstream assigned label. The BFIR will note all the BFERs that are interested in specific P2MP/MP2MP LSP (as per section 3.2). The BFIR will put the corresponding BIER header with the bit index set for all the IBBRs interested in this stream. The BFIR will set the BIERHeader.Proto = MPLS and will forward the BIER packet into the BIER domain.

In the BIER domain, normal BIER forwarding procedure will be done, as per [RFC8279]

The BFERs will receive the BIER packet and will look at the protocol field of BIER header, indicating MPLS protocol. The BFER will remove the BIER header and will do a lookup in the ILM table for the upstream assigned label and perform its corresponding action.

It should be noted that these procedures are also valid if BFIR is the ILER and/or BFER is the ELER as per [RFC7060]

5. Recursive FEC

The procedures above are also valid for mLDP recursive FEC. The root used to determine the EBBR is the outer root of the FEC. The entire recursive FEC needs to be preserved when it is forwarded via tLDP and the label request.

6. IANA Consideration

IANA maintains a registry of Interface ID Types for use in GMPLS in the registry "Generalized Multi-Protocol Label Switching (GMPLS) Signaling Parameters" and sub-registry "Interface_ID Types"

This document defines and requests two new Interface_ID Type for BIER from the Interface_ID Types space,

7. Security Considerations

While in the BIER domain the security considerations of [RFC8279] are relevant to this document.

The implementation should also take into account the security recommendations of [RFC6389].

8. Acknowledgments

Authors would like to acknowledge Jingrong Xie and Nabeel Cocker for his comments and help on this draft.

9. References

9.1. Normative References

"Z. Zhang, E. Rosen, W. Lin, Z. Li, I. Wijnands, "MVPN/EVPN Tunnel Aggregation with Common Labels"", .
"H, Bidgoli, F. Xu, J. Kotalwar, I. Wijnands, M. Mishra, Z. Zhang", .
"S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels"", .
"IJ. Wijnands, I. Minei, K. Kompella, B.Thomas "Label Distribution Protocol Extensions for Point-to-Multipoint and Multipoint-to-Multipoint LSP"".
"R Aggarwal, JL. Le Roux, "MPLS Upstream Label Assignment for LDP"", .
"M. Napierala, E. Rosen, I. Wijnands", .
"I. Wijnands, E. Rosen, A. ADolganow, T. Prygienda, S. Aldrin", .

9.2. Informative References

"Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang, "BIER Support via ISIS"", .
"Psenak, P., Kumar, N., Wijnands, IJ., Dolganow, A., Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions for Bit Index Explicit Replication"", .
"Rosen, E., Ed., Sivakumar, M., Wijnands, IJ., Aldrin, S.,Dolganow, A., and T. Przygienda, "Multicast VPN Using Index Explicit Replication (BIER)", .

Authors' Addresses

Hooman Bidgoli (editor)
Jayant Kotalwar
Montain View,
United States of America
IJsbrand Wijnands
Cisco System
Mankamana Mishra
Cisco System
United States of America
Zhaohui Zhang
Juniper Networks
United States of America
Eddie Leyton