BIER support via ISIS

Bit Index Explicit Replication (BIER) defines an architecture where all intended multicast receivers are encoded as bitmask in the Multicast packet header within different encapsulations such as . 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 protocol to support distribution of information necessary for operation of BIER domains. This document defines a new TLV to be distributed by every router participating in such BIER domains.

Some of the terminology specified in 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 Mask (A bit stream of a certain fixed length. Each Bit represents a receiver). Packet Bit Mask (A Bit Mask included in the Multicast Packet). Bit Position (A single Bit from the Bit Mask that represents a receiver). Bit Forwarding Router (A router that participates in Bit Index Multipoint Forwarding). Bit Forwarding Ingress Router (The ingress border router that inserts the BM into the packet). Bit Forwarding Egress Router. A router that participates in Bit Index Forwarding as leaf. Each BFER must be a BFR. Bit Forwarding Tree used to reach all BFERs in a domain. Bit Index Forwarding Table (A Bit index forwarding table). Bit Mask Set. Set containing bit positions of all BFER participating in a set. Bit Mask Position, a given bit in a BMS.

This document requests IANA to assign sub-TLV type values from the ISIS router capability TLV registry.

This draft introduces a sub-TLV in the router capabilites TLV to distribute the information. Any of the router's loopback addresses that it originates are considered BFR prefixes as required by . The question whether a particular loopback address is routable in a specific topology can be resolved by .

ISIS can carry BIER information not only for a single BIER domain but for multiple, distinct domains. This allows to run many disjoint BIER layers within the same Multi-Topology easily instead of always forcing different multicast overlays to share the exactly same set of BFRs and resources. Moreover, multi topology can be used for the purpose of restricting links that certain set of BIER domains can use or change metrics of such links. A BIER set is therefore always uniquely identified by the tuple of topology T, domain D it belongs to and its number S, denoted as <T,D,S>. The domain itself has as its unique attributes the encapsulation, bitmask length and the type of tree it is using to forward BIER frames (currently always SPF).

A given domain D in a multi-topology T (denoted as <T,D> from now on) is normally not advertised to preserve the scaling of the protocol (i.e. ISIS carries no TLVs containing any of the elements related to <T,D>) and is enabled by a first BIER sub-TLV () containing <T,D> being advertised into the area. The trigger itself is outside the scope of this draft but can be .e.g. a VPN desiring to initiate a domain as MP2MP or P2MP tree or a BMP being administratively assigned to a BFER and advertised via BIER TLV into the area or any other means within Multicast BIER Overlay(s) using BIER domains.

All routers in the flooding scope of the BIER TLVs SHOULD advertise the same bit mask length for a given <T,D>. A router discovering bitmask lengths advertised that are shorter than its own MUST report a misconfiguration of a specific <T,D>. Each router MUST compute BFTs for <T,D> using only routers having the same mask length as its own advertised Bit Mask Length in BIER sub-TLV for <T,D>.

The same router MAY advertise for different <T,D> combinations two different mask lengths. This allows to cleanly delineate domains crossing the same router but using different mask lengths in the encoding, even within the same topology.

Since encapsulation is an attribute of a domain <T,D> just like bitmask length, all rules that apply to Bitmask Length per apply to it well.

Each router MAY advertise within the sub-TLV of an according <T,D> (denoted further as TLV<T,D>) a valid starting label value and a non-zero range length. It MUST advertise a valid label value and a non-zero range length IF it has computed itself as being on the BFT rooted at any of the BFRs with valid BFR-ids (except itself) participating in <T,D>. A router CAN withdraw its TLV<T,D> if it does not want to participate in the domain due to resource constraints, label space optimization, administrative configuration or any other reasons. In case a router advertises a label range size of 0 for <T,D> it MUST be excluded from the BIER BFTs for <T,D>.

A router MUST advertise a for <T,D> label range size that guarantees to cover the maximum BFR-id injected into <T,D> (which implies a certain set id as described in ). Any router that violates this condition MUST be excluded from BIER BFTs for <T,D>.

Each BFER MAY advertise with its TLV<T,D> the according BFR-id that it has administratively chosen. If two BFRs advertise in their TLV<T,D> the same value for BFR-id, all routers MUST report it as misconfiguration and disregard those routers for all BIER calculations and procedures to align with . Such routers with colliding assignments MAY still act as BFIRs but will be never able to receive traffic.

All ISIS BIER information is carried within the router capability TLV with S bit clear.

This sub-TLV carries the information for the BIER domains that the router participates in as BFR. It can repeat multiple times. If the same <T,D> is advertised more than once, the first one in the first sub-TLV in the fragment with the lowest ID MUST be used.

TBD1. 2 octets. Multi-Topology, 1 octet. Unique identifier for a BIER domain, 2 octets. Number of labest in the range used on encapsulation for this BIER domain, 1 octet. First label of the range used on encapsulation for this BIER domain, 20 bits. The label is used by e.g. to forward traffic to sets of BFERs. Bitmask length for this BFR per . The BIER encapsulation type, 1 octet. Allowed values are: MPLS per . Indicates administratively set value if set, otherwise the BFR-id value MUST be considered as not assigned in this TLV. Reserved for future use. MUST be 0. Reserved for future use. MUST be 0. MUST be 0 on send, ignored on receive.

The extension does not introduce any known new protocol vulnerabilities.

The draft is aligned with the draft as far as the protocol mechanisms overlap.