BIER Fast ReRoute

With BIER , a Bit-Forwarding Router (BFR) forwards BIER packets based on a bitstring in the BIER header using the information in the Bit Index Forwarding Table (BIFT). Its entries are locally derived from a routing underlay or set by a controller. In case of a persistent link or node failure, BIER traffic may not be delivered until the BIFT has been updated based on the reconverged routing underlay or by the controller. Typically, BIER packets are forwarded without an outer IP header. Consequently, if a link or node failure occurs, the corresponding BFR Neighbor (BFR-NBR) becomes unreachable. Fast Reroute (FRR) mechanisms in the routing underlay, such as IP-FRR, apply exclusively to IP packets, leading to potential loss of BIER traffic. BIER traffic can only be restored after the routing underlay has reconverged and the BIFT has been recalculated. Tunneling BIER packets can serve as a solution to reach the BFR-NBR in the case of a link failure by leveraging the FRR capabilities of the routing underlay, provided such mechanisms are available. However, this approach does not address node failures, as all destinations that rely on the failed node as their BFR-NBR become unreachable. Given that BIER often carries multicast traffic with real-time requirements, there is a particular need to protect BIER traffic against prolonged outages following failures. This document introduces a nomenclature for Fast Reroute in BIER (BIER-FRR). Upon detecting that a BFR-NBR is unreachable, BIER-FRR enables a BFR to quickly reroute affected BIER packets using backup forwarding entries. To avoid the generation of redundant packets, backup forwarding entries should be processed before normal forwarding entries. To achieve this, two potential representations for backup forwarding entries are proposed. The protection level offered by BIER-FRR can be either link protection or node protection. Link protection is limited to safeguarding against link failures and is simpler to implement but may not be effective if a BFR itself fails. Node protection, while more complex, also guards against the failure of BFRs. The choice of backup strategy determines the selection of backup forwarding entries. Examples of backup strategies include tunnel-based BIER-FRR and Loop-Free Alternate (LFA)-based BIER-FRR: Tunnel-based BIER-FRR: This approach leverages the mechanisms of the routing underlay for FRR purposes. The routing underlay typically restores connectivity faster than BIER, as the reconvergence of the routing underlay is a prerequisite for the recalculation of the BIFT. When the routing underlay utilizes FRR mechanisms, its forwarding capabilities are restored well before reconvergence is completed. To benefit from the rapid restoration of the routing underlay, BIER traffic affected by a failure is tunneled over the routing underlay. LFA-based BIER-FRR: This approach reroutes BIER traffic to alternative neighbors in the event of a failure. It applies the principles of IP-FRR, requiring that LFAs are also BFRs. Normal BIER-LFAs can be reached without tunneling, remote BIER-LFAs employ a tunnel, and topology-independent BIER-LFAs use explicit paths to reach the backup BFR-NBR. Unlike tunnel-based FRR, LFA-based BIER-FRR does not depend on fast reroute mechanisms in the routing underlay. The BIER-FRR mechanism described in this document adheres to a primary/backup path model, also known as 1:1 protection, which contrasts with the 1+1 protection model, where traffic is duplicated across both primary and backup paths, as explored for BIER in .

This document uses the following definitions: BIER: Bit Indexed Explicit Routing BIER FRR: Bit Indexed Explicit Routing Fast ReRoute BFR: Bit-Forwarding Router BFR-NBR: Bit-Forwarding Neighbor BFIR: Bit-Forwarding Ingress Router BFER: Bit-Forwarding Egress Router BIFT: Bit Index Forwarding Table F-BM: Forwarding Bit Mask PLR: Point of Local Repair LFA: Loop Free Alternate BF-BM: Backup F-BM BBFR-NBR: Backup BFR-NBR BFA: Backup Forwarding Action BEA: Backup Entry Active

In this section, forwarding actions and backup forwarding entries are defined. Then, the BIER forwarding process with BIER-FRR and the computation of the backup F-BM are explained.

A BFR-NBR is considered directly connected if it is a next hop at the network layer, meaning it can be reached via link layer technology. Conversely, if the BFR-NBR cannot be reached directly through the link layer, it is regarded as indirectly connected. The following forwarding actions are defined: Plain: The BIER packet is sent directly to a BFR-NBR via a direct link without encapsulation in a tunnel header. This indicates that the packet is not routed through the underlying network. Tunnel: The BIER packet is encapsulated with a tunnel header and forwarded to a BFR-NBR over the routing underlay. Explicit: The packet is forwarded along an explicit path to a BFR-NBR. The specific path information must be provided. If segment routing is employed for this purpose, the segment IDs (SIDs) must be specified. Two forwarding actions of type Explicit are considered equivalent only if they utilize the same explicit path. In the BIFT as outlined in , the forwarding actions are implicitly determined by the connectivity status of the BFR-NBR. If the BFR-NBR is directly connected, the forwarding action is Plain. If the BFR-NBR is not directly connected, the forwarding action is Tunnel.

The BIFT as proposed in includes a Forwarding Bit Mask (F-BM) and a BFR-NBR for a specific BFER. These elements constitute a primary forwarding entry. The BIER-FRR (Fast Reroute) mechanism extends the conventional BIFT by introducing additional columns that contain backup forwarding entries. A backup forwarding entry comprises the following components: Backup Forwarding Bit Mask (BF-BM) Backup BFR Neighbor (BBFR-NBR) Backup Forwarding Action (BFA) Backup Entry Active (BEA) Flag The BF-BM and BBFR-NBR share the same structure as their primary counterparts. The BFA is defined as a forwarding action according to . The BEA flag indicates whether the backup forwarding entry is currently active. When active, the BF-BM, BBFR-NBR, and BFA are utilized for forwarding BIER packets in place of the primary forwarding entry. The structure of the extended BIFT is illustrated in .

The primary action is not explicitly stated in the BIFT, as it is derived from the BFR-NBR. In contrast, the backup forwarding action is explicitly defined in the extended BIFT. Additionally, in the case of an Explicit forwarding action, the explicit path must be indicated. However, since explicit paths are implementation-specific, this information is not detailed in the table. The values for the backup BFR-NBR and the backup action depend on the desired level of protection and the chosen backup strategy. Examples of these are provided in and . The Backup Forwarding Bit Mask (BF-BM) is determined based on the backup BFR-NBR, and its computation is described in .

When a primary BFR-NBR is not reachable over the implicit primary action, a failure is observed. Then, the BEA flag of the corresponding backup forwarding entry is set. If the primary BFR-NBR is directly connected, the information about the failed interface is sufficient to detect its unreachability. If the primary BFR-NBR is indirectly connected, a BFD session between the BFR as PLR and the BFR-NBR may be used to monitor its reachability. If the primary BFR-NBR is reachable again, the BEA flag is deactivated. This may be caused by the disappearance of the failure or by a change of the primary BFR-NBR due to a reconfiguration of the BIFT.

The primary F-BM of a specific BFER identifies all BFERs that share the same primary Bit-Forwarding Router Neighbor (BFR-NBR). The backup F-BM for a specific BFER is computed to indicate: All BFERs that share both the primary and backup BFR-NBRs of the specific BFER, and All BFERs for which the backup BFR-NBR of the specific BFER serves as the primary BFR-NBR.

To minimize the occurrence of redundant packets, it is essential that backup entries are prioritized for use within the single extended BIFT. However, implementing this priority may be challenging or infeasible on certain hardware platforms. Consequently, two alternative representations of forwarding entries are proposed. The first representation involves a primary BIFT and a Single Backup BIFT (SBB). The second representation comprises a primary BIFT along with multiple Failure-Specific Backup BIFTs (FBB).

The BIER forwarding procedure in failure-free scenarios is designed to avoid the generation of redundant packets, ensuring that at most a single copy is transmitted per link for each BIER packet. However, this property may be compromised when BIER-FRR, as described in is employed to provide protection against a failure. presents an example of a BIER network. In this example, BFRs are identified by the prefix "B" followed by their BFR-ids. For simplicity, each BFR also serves as a BFER, and its bit position in the bitstring corresponds to its BFR-id. The number assigned to each link represents its cost, which the routing underlay uses to compute the shortest paths.

The extended BIFT with backup forwarding entries for LFA-based BIER-FRR with link protection, as constructed by BFR B1, is illustrated in .

The emergence of redundant packets during a failure scenario is demonstrated as follows. Consider the extended BIFT for BFR B1 depicted in . This BIFT includes backup forwarding entries for LFA-based FRR with link protection. In a failure-free scenario, when forwarding a BIER packet destined for B2 and B6 (bitstring 0100010), BFR B1 sends a single copy of the packet on the link B1-B2 and another on the link B1-B6. In the event of a failure on link B1-B6, BFR B1, acting as the PLR, detects the failure. Consequently, B1 sets the BEA flag for all destinations that have B6 as their BFR-NBR. If B1 is to send a BIER packet to B2 and B6 under these conditions, it first sends a copy with bitstring 0000010 to B2 using the corresponding primary forwarding entry in the extended BIFT shown in . Subsequently, B1 sends another copy of the packet with bitstring 0100000 to B2 for B6, using the backup forwarding entry, since the BEA flag is activated. This results in a second (redundant) copy being sent over the same link B1-B2. This redundancy can be avoided if the backup forwarding entry is prioritized. By using the backup forwarding entry first, B1 would send only a single copy of the packet with bitstring 0100010 to B2, and no additional copy would be sent to B2, as the bitstring in the packet would be cleared by the BF-BM 1111110. Therefore, prioritizing the processing of BFERs with unreachable BFR-NBRs helps to reduce the generation of redundant packet copies.

The extended BIFT can be divided into two distinct BIFTs: one serving as the primary BIFT, and the other as a single backup BIFT. The primary BIFT functions in the same manner as the regular BIFT. The backup BIFT, however, contains the backup forwarding entries, including the BBF-BM, BBFR-NBR, BFA, and BEA flag from the extended BIFT. When a BFR, acting as the PLR, detects that a BFR-NBR has become unreachable, it activates the BEA flag for all BFERs in the backup BIFT that have the affected BFR-NBR as their primary BFR-NBR. When forwarding a BIER packet, the BFR processes the packet using the backup BIFT first, followed by the primary BIFT. This prioritization helps to reduce the number of redundant packet copies. B1's extended BIFT from is divided into the primary BIFT shown in and the single backup BIFT shown in .

B2 | +------+----------+--------+-----------+---+-----------------+ | 3 | 1111110 | B6 | Plain | | Link B1->B2 | +------+----------+--------+-----------+---+-----------------+ | 4 | 1111110 | B2 | Plain | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 5 | 1111110 | B2 | Plain | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 6 | 1111110 | B2 | Plain | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 7 | 1111110 | B2 | Plain | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+]]> Each forwarding entry in the backup BIFT includes the BF-BM, BBFR-NBR, BFA, and BEA. When a BFR-NBR fails, the BEA flag is activated for all BFERs in the backup BIFT that have the affected BFR-NBR as their primary BFR-NBR. For instance, BFERs B4, B5, B6, and B7 have BFR-NBR B6 as their primary BFR-NBR. If BFR-NBR B6 fails, the BEA flag for BFERs B4, B5, B6, and B7 is activated, setting the BEA in the last four entries in the backup BIFT to one.

As an alternative to the single extended BIFT, the information can be represented using a primary BIFT along with several failure-specific backup BIFTs. A failure-specific backup BIFT is associated with the unreachability of a particular BFR-NBR. A backup BIFT for the failure of BFR-NBR N is simply referred to as a backup BIFT for N. This backup BIFT mirrors the structure of the regular BIFT but includes entries for backup forwarding actions. Therefore, a BFR maintains a primary BIFT, identical to the regular BIFT, and a separate backup BIFT for each of its BFR-NBRs. Under normal, failure-free conditions, the BFR utilizes the primary BIFT to forward BIER packets. Upon detecting that BFR-NBR N has become unreachable, the BFR, acting as the PLR, switches to the backup BIFT for N to forward all BIER packets. Once the routing underlay has re-converged to reflect the updated network topology, the primary BIFT is re-computed. The newly computed primary BIFT is then reinstated for forwarding all BIER packets. This concept can be illustrated using the example of the extended BIFT in . depicts B1's primary BIFT in this context. BFR B1 in has two neighbors: B6 and B2. Consequently, B1 maintains two backup BIFTs with link protection: one for B6 and another for B2. Additionally, B1 also maintains two backup BIFTs with node protection. represents B1's backup BIFT for B6, which is utilized in response to the unreachability of B6, functioning similarly to the extended BIFT shown in .

B6 | +--------+---------+---------+-----------------+-----------------+ | 5 | 1111110 | B2 | Plain | Link B1->B6 | +--------+---------+---------+-----------------+-----------------+ | 6 | 1111110 | B2 | Plain | Link B1->B6 | +--------+---------+---------+-----------------+-----------------+ | 7 | 1111110 | B2 | Plain | Link B1->B6 | +--------+---------+---------+-----------------+-----------------+]]> Once B1, as the PLR, detects that B6 has become unreachable via the link to B6, it switches to the backup BIFT for B6 to forward all BIER packets. Since this representation aligns with the concept of a single primary and single backup BIFT, the occurrence of redundant packets for the same forwarding action is avoided.

Both link protection and node protection may be supported. Link protection is designed to safeguard against the failure of an adjacent link, whereas node protection addresses the failure of a neighboring node and the associated path leading to that node. The relevance of link or node protection depends on the specific service being supported. Additionally, both protection levels can be combined with any of the backup strategies outlined in .

In link protection, the backup path is designed to circumvent the failed link (i.e., the failed primary path from the PLR to the primary BFR-NBR), while still potentially including the primary BFR-NBR itself. Consequently, the backup path remains operational even if the primary path fails. The primary limitation of link protection is its inability to provide protection if the primary BFR-NBR itself becomes inoperative. However, link protection also offers certain advantages. It typically results in shorter backup paths compared to node protection. In the case of tunnel-based BIER-FRR, link protection generates at most one redundant packet, whereas node protection may result in multiple redundant packets. Additionally, for LFA-based BIER-FRR, link protection is more effective in safeguarding a greater number of BFERs using normal BIER-LFAs than node protection.

In node protection, the backup path is designed to avoid both the failed node and the link to that node (i.e., the failed primary path from the PLR to the primary BFR-NBR, including the primary BFR-NBR). Consequently, the backup path must exclude both the primary path and the primary BFR-NBR to remain operational in the event of their failure. If a BFER and its primary BFR-NBR are the same, only link protection is feasible for that BFER. The primary advantage of node protection is its enhanced protection quality compared to link protection. However, node protection also has certain drawbacks. It typically results in longer backup paths than link protection. In the context of tunnel-based BIER-FRR, node protection may lead to the transmission of a greater number of redundant packets over a link than with link protection. Furthermore, for LFA-based BIER-FRR, fewer BFERs may be protected using normal BIER-LFAs, necessitating the use of more remote or topology-independent BIER-LFAs, which are inherently more complex.

In the network depicted in , the primary path from BFR B1 to BFER B5 is B1-B6-B5. Protecting BFER B5 from a BFR-NBR B6 node failure can only be provided through the backup path B1-B2-B3-B4-B5. Link protection for BFER B5 is achieved via the backup path B1-B2-B7-B6, and additionally through the backup path B1-B2-B3-B4-B5-B6. The specific backup entries are determined by the selected protection level and backup strategy. Example BIFTs illustrating link and node protection are provided in .

Backup strategies determine the selection of backup forwarding entries, influencing both the choice of the backup BFR-NBR and the backup action, and consequently, the backup path. The following sections present tunnel-based BIER-FRR and LFA-based BIER-FRR as potential strategies.

The routing underlay may possess the capability to forward packets to their destinations even in the presence of a failure, potentially due to FRR mechanisms within the routing underlay. In such scenarios, while the primary BFR-NBR may no longer be reachable via the primary action (Plain), it could still be accessible through a backup action (Tunnel). Tunnel-based BIER-FRR encapsulates BIER packets impacted by a failure within the routing underlay, thereby leveraging the routing underlay's fast restoration capabilities. As soon as connectivity in the routing underlay is reestablished, the affected BIER packets can be forwarded to their intended destinations. The appropriate backup forwarding entries in a BIFT for BIER-FRR are determined by the desired protection level.

In the context of link protection, the backup BFR-NBRs are identical to the primary BFR-NBRs. If a primary BFR-NBR is directly connected to the BFR acting as the Point of Local Repair (PLR), the corresponding backup forwarding action is Tunnel. Consequently, BIER packets affected by a failure are tunneled through the routing underlay to their BFR-NBR, rather than being directly sent as plain BIER packets. If the primary BFR-NBR is not directly connected to the BFR as a PLR (i.e., the implicit primary action is Tunnel), the corresponding backup action is also Tunnel. The backup F-BMs are identical to the primary F-BMs, consistent with the computation of backup F-BMs described in .

B2 | +------+----------+--------+-----------+---+-----------------+ | 3 | 0000110 | B2 | Tunnel | | Link B1->B2 | +------+----------+--------+-----------+---+-----------------+ | 4 | 1111000 | B6 | Tunnel | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 5 | 1111000 | B6 | Tunnel | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 6 | 1111000 | B6 | Tunnel | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 7 | 1111000 | B6 | Tunnel | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+]]> illustrates B1's backup BIFT for tunnel-based BIER-FRR with link protection in the BIER network example depicted in . The backup BFR-NBRs and backup F-BMs in this backup BIFT correspond to the primary BFR-NBRs and primary F-BMs in the primary BIFT shown in . However, the backup actions in this backup BIFT are Tunnel, while the primary actions in the primary BIFT are Plain (which are not explicitly shown but implied). When B1, acting as the PLR, detects a failure of its link to B6, a BIER packet with the bitstring 0100000 destined for B6 is tunneled by B1 through the routing underlay towards B6. The specific path of the backup tunnel depends on the routing underlay and could be B1-B2-B7-B6 or B1-B2-B3-B4-B5-B6. If a BIER packet is destined for {B2, B5, B7}, an encapsulated packet copy is first forwarded via link B1-B2 to backup BFR-NBR B6 using the backup action Tunnel to deliver packet copies to BFERs B5 and B7. Subsequently, a non-encapsulated packet copy is forwarded via link B1-B2 to BFR-NBR B2 using the primary action Plain to deliver a packet copy to BFER B2. Therefore, with tunnel-based BIER-FRR, a single redundant packet copy may occur in the event of a failure because an encapsulated and a non-encapsulated packet copy are forwarded over the same link. This redundancy occurs even though BIER packets affected by failures are forwarded before those unaffected by failures. A BIER packet with the bitstring 1000000 destined for B7 is forwarded along the backup path B1-B2-B7-B6-B7, as it is first delivered to the backup BFR-NBR B6. Consequently, the backup path may be unnecessarily long. This phenomenon is similar to the facility backup method described in which employs paths analogous to those in tunnel-based BIER-FRR..

To determine the backup forwarding entries for node protection, a case-by-case analysis of the BFER to be protected is required. If the BFER is the same as its primary BFR-NBR, node protection is not feasible for that BFER. In such cases, link protection is applied, meaning the backup BFR-NBR is set to the primary BFR-NBR. If this level of protection is deemed insufficient, egress protection as described in may be applied.If the BFER is different from its primary BFR-NBR, the backup BFR-NBR is set to the primary BFR-NBR's primary BFR-NBR for that BFER, making the backup BFR-NBR a next-next-hop BFR. In all scenarios, the backup action is Tunnel. In the first case, the backup F-BM is set to all zeros with the bit for the BFER to be protected enabled. In the second case, the backup F-BM is computed as described in .

B2 | +------+----------+--------+----------+---+-----------------+ | 3 | 0000100 | B3 | Tunnel | | BFR-NBR B2 | +------+----------+--------+----------+---+-----------------+ | 4 | 0011000 | B5 | Tunnel | | BFR-NBR B6 | +------+----------+--------+----------+---+-----------------+ | 5 | 0011000 | B5 | Tunnel | | BFR-NBR B6 | +------+----------+--------+----------+---+-----------------+ | 6 | 0100000 | B6 | Tunnel | | Link B1->B6 | +------+----------+--------+----------+---+-----------------+ | 7 | 1000000 | B7 | Tunnel | | BFR-NBR B6 | +------+----------+--------+----------+---+-----------------+]]> illustrates B1's backup BIFT for tunnel-based BIER-FRR with node protection in the BIER network example provided in . BFERs B2 and B6 are direct neighbors of B1. To protect them, only link protection is applied, as B1's primary BFR-NBR for these nodes is the nodes themselves. As described above, only the bit for B2 is set in the backup F-BM of B2, and similarly for B6. For BFER B5, the backup BFR-NBR is B5, as it is B1's next-next-hop BFR towards B5. Similarly, for BFER B7, the backup BFR-NBR is B7. When B1, acting as the PLR, detects the failure of its BFR-NBR B6, a BIER packet with bitstring 1010010 destined for {B2, B5, B7} is processed as follows: an encapsulated copy of the packet is sent via tunnel B1-B2-B3-B4-B5, another encapsulated copy is sent via tunnel B1-B2-B7, and a non-encapsulated copy is sent via link B1-B2. In this example, two redundant packets are sent over link B1-B2. Therefore, node protection may result in more redundant packet copies than link protection.. Caveat: If the routing underlay does not support node protection, tunnel-based BIER-FRR will similarly be unable to provide node protection. This limitation is illustrated in the following example. In the network depicted in , the underlay offers only link protection. If BFR-NBR B6 fails and B1 must forward a packet to B5, according to the backup BIFT in the packet is tunneled towards B5. The underlay may route the packet along the path B1-B2-B7-B6-B5 due to FRR with link protection. However, since B6 is also unreachable from B7, the packet is returned to B2, resulting in a loop between B2 and B7.

LFA-based BIER-FRR leverages alternate BFRs to deliver BIER packets to BFERs for which the primary BFR-NBR is unreachable. This approach does not rely on any fast restoration or protection mechanisms in the underlying routing infrastructure. First, the prerequisites for LFA-based BIER-FRR are clarified, followed by the definition of BIER-LFAs. Subsequently, link and node protection for LFA-based BIER-FRR are discussed using a single backup BIFT.

A LFA for a specific destination is an alternate node to which a packet is sent if the primary next hop for that destination is unreachable. This alternate node should be capable of forwarding the packet without creating a forwarding loop. LFAs have been defined for IP networks in , and , and such LFAs are referred to as IP-LFAs. BIER-LFAs are similar to IP-LFAs, but a BIER-LFA node must be a BFR. If only a subset of the nodes in the routing underlay are BFRs, some IP-LFAs in the routing underlay may not be usable as BIER-LFAs. To compute BIER-LFAs, network topology and link cost information from the routing underlay are required. This differs from tunnel-based BIER-FRR, where knowledge of the primary BIFTs of a PLR and its BFR-NBRs is sufficient. LFA-based BIER-FRR may reuse IP-LFAs as BIER-LFAs under the following conditions: if an IP-LFA node for the destination of a specific BFER is a BFR, it may be reused as the backup BFR-NBR for that BFER, along with the backup action applied for that IP-LFA at the IP layer. A normal IP-LFA corresponds to the backup action Plain, a remote IP-LFA to Tunnel, and a TI-IP-LFA to Explicit.

As with IP-LFAs, there are several types of BIER-LFAs: A BFR is considered a normal BIER-LFA for a specific BFER if it is directly connected to the PLR and: the BFER can be reached from it through the BIER domain. both the path from the PLR to the BFR and the path from the BFR to the BFER are disjoint from the primary path from the PLR to the primary BFR-NBR. These paths: may include the primary BFR-NBR for link protection. must not include the primary BFR-NBR for node protection. A BFR is considered a remote BIER-LFA for a specific BFER if it is not directly connected to the PLR, can be reached via a tunnel from the PLR, and satisfies the aforementioned conditions 1 and 2. A BFR is considered a TI-BIER-LFA for a specific BFER if it is not directly connected to the PLR, cannot be reached via a tunnel from the PLR, but is reachable from the PLR via an explicit path (e.g., with the assistance of a Segment Routing (SR) header), and satisfies the aforementioned conditions 1 and 2. For some BFERs, one or more normal BIER-LFAs may be available at a specific PLR. For other BFERs, only remote or TI-BIER-LFAs may be available. There may also be BFERs for which only TI-BIER-LFAs are available. The backup actions for rerouting BIER packets depending on the type of BIER-LFA are: For normal BIER-LFA: Plain For remote BIER-LFA: Tunnel For TI-BIER-LFA: Explicit

Protection coverage refers to the set of BFERs that can be protected with a desired level of protection by a particular type of BIER-LFA. The BIER-LFA types exhibit the following characteristics: Normal BIER-LFAs The protection coverage is the least, as some or many BFERs may not be protected at the desired level or at all. Redundant packet copies are avoided. There is no encapsulation overhead. Remote BIER-LFAs They enhance the protection coverage of normal BIER-LFAs. Redundant packet copies may occur on a link, similar to tunnel-based BIER-FRR. The encapsulation overhead is similar to that of tunnel-based BIER-FRR. TI-BIER-LFAs They complement the protection coverage of normal and remote BIER-LFAs to achieve 100% coverage. Redundant packets may occur on a link, similar to tunnel-based BIER-FRR. The encapsulation overhead is similar or equivalent to that of tunnel-based BIER-FRR, depending on the FRR mechanism employed in the routing underlay.

Normal BIER-LFAs are the simplest option, as they do not require tunneling or explicit paths. Remote BIER-LFAs offer greater capabilities but introduce additional header overhead and require more functionality from the PLR. TI-BIER-LFAs are the most complex, necessitating the use of explicit paths. When implementing LFA-based BIER-FRR, it is essential to specify the set of supported BIER-LFAs. The available options are as follows: Option 1: Only normal BIER-LFAs are supported. Option 2: Both normal and remote BIER-LFAs are supported. Option 3: All types of BIER-LFAs are supported.

In link protection, normal BIER-LFAs are prioritized over remote LFAs, and remote BIER-LFAs are preferred over TI-BIER-LFAs. Depending on the set of supported BIER-LFAs, it may not be possible to protect all BFERs. illustrates B1's backup BIFT for LFA-based BIER-FRR with link protection, using the network example provided in . If the link between B1 and B6 fails, B1 cannot reach the BFERs B4, B5, B6, and B7 via their primary BFR-NBR. Consequently, B1 forwards their traffic via the backup BFR-NBR B2, along with the traffic for B2 and B3, as B2 is their primary BFR-NBR. In this scenario, the backup F-BM is set to 1111110. Similarly, if the link between B1 and B2 fails, B1 routes all traffic to B6, with the backup F-BM also set to 1111110. B1 requires only normal BIER-LFAs to protect all BFERs. However, this situation can vary significantly for other BFRs. and present the backup BIFTs for B7 and B5, respectively. BFR B7 requires one normal BIER-LFA, three remote BIER-LFAs, and two TI-BIER-LFAs to protect all BFERs. BFR B5 requires one normal BIER-LFA, one remote BIER-LFA, and four TI-BIER-LFAs as backup BFR-NBRs. Thus, depending on the set of supported BIER-LFAs, it may not be possible to protect all BFERs using BIER-FRR. Consider a scenario where B7 holds a BIER packet with destinations {B1, B4, B5, B6}. If the link between B7 and B6 fails, the packet copy for B1 is sent to B2 using the forwarding action Plain, the packet copy for B4 is tunneled via B2 to B3, and the packet copies for B5 and B6 are sent via explicit paths to B4 and B1, respectively. Since these packet copies have different headers, all of them must be transmitted, resulting in three redundant copies.

B6 | +------+----------+--------+-----------+---+-----------------+ | 2 | 0000110 | B1 | Tunnel | | Link B1->B2 | +------+----------+--------+-----------+---+-----------------+ | 3 | 0000110 | B1 | Tunnel | | Link B1->B2 | +------+----------+--------+-----------+---+-----------------+ | 4 | 0001000 | B3 | Tunnel | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 5 | 0010000 | B4 | Explicit | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+ | 6 | 0100000 | B1 | Explicit | | Link B1->B6 | +------+----------+--------+-----------+---+-----------------+]]>

B6 | +------+----------+--------+-----------+---+-----------------+ | 2 | 1100011 | B3 | Explicit | | Link B5->B6 | +------+----------+--------+-----------+---+-----------------+ | 3 | 0000100 | B4 | Plain | | Link B5->B6 | +------+----------+--------+-----------+---+-----------------+ | 4 | 0001000 | B3 | Tunnel | | Link B5->B4 | +------+----------+--------+-----------+---+-----------------+ | 6 | 1100011 | B3 | Explicit | | Link B5->B6 | +------+----------+--------+-----------+---+-----------------+ | 7 | 1100011 | B3 | Explicit | | Link B5->B6 | +------+----------+--------+-----------+---+-----------------+]]>

To determine the backup forwarding entries for node protection, it is necessary to conduct a case-by-case analysis of the BFER to be protected. If the BFER is the same as its primary BFR-NBR, node protection is not feasible for that BFER, and link protection must be applied instead. In all other cases, the BFER should be protected by a node-protecting BIER-LFA. In this context, normal BIER-LFAs are prioritized over remote BIER-LFAs, and remote BIER-LFAs are preferred over TI-BIER-LFAs. Depending on the set of supported BIER-LFAs, it may not be possible to protect certain BFERs. illustrates B1's backup BIFT for LFA-based BIER-FRR with node protection, using the network example provided in .

As B6 serves as the primary BFR-NBR for BFER B6, only link protection can be applied. Consequently, B2 is utilized as a normal, link-protecting BIER-LFA to safeguard B6. Similarly, as B2 is the primary BFR-NBR for BFER B2, B2 is protected with B6 as its normal, link-protecting BIER-LFA. BFER B7 is protected against the failure of node B6 by using B2 as its normal, node-protecting BIER-LFA, as B2 has a shortest path to B7 that does not traverse B6. The backup F-BMs for BFERs B6 and B7 are set to {B2, B6, B7}, as traffic for these BFERs is routed via link B1-B2 with the forwarding action Plain when B6 is unreachable. BFER B4 cannot be reached via a normal LFA when BFR B6 fails. However, B3 serves as a remote, node-protecting BIER-LFA for BFER B4, as B3 has a shortest path to B4, is reachable from B1 via a shortest path, and the resulting backup path from B1 to B4 does not traverse B6. Similarly, B4 serves as a remote LFA for BFER B3 if BFR B2 fails. BFER B5 is neither reachable through a normal BIER-LFA nor through a remote BIER-LFA when BFR B6 fails. However, B4 acts as a node-protecting TI-LFA for BFER B5, as B4 has a shortest path to B5 that does not traverse B6. Additionally, B4 is reachable through the explicit path B1-B2-B3-B4.

Redundant packets can occur with LFA-based BIER-FRR when BIER packets are transmitted over a specific link in different forms, including: Plain BIER packets (either primary transmission or reroute to a normal BIER-LFA). BIER packets encapsulated for transmission to a specific BFR-NBR (either tunneled primary transmission or reroute to a remote BIER-LFA). BIER packets routed with an encoded explicit path (reroute to a TI-LFA). When different remote LFAs are utilized, multiple redundant packets may be generated through remote LFAs. A similar situation can arise with TI-LFAs. However, some redundant packets can be mitigated if remote LFAs or TI-LFAs are selected such that they can protect multiple BFERs, thereby reducing the need for additional remote LFAs or TI-LFAs. This approach, while potentially leading to longer backup paths, introduces a new optimization objective for the selection of remote or TI-BIER-LFAs, which does not exist in IP-FRR. The relevance of this optimization may vary depending on the specific use case. To illustrate this optimization potential, consider LFA-based BIER-FRR with link protection for B7, as described in its backup BIFT in . As noted in , B7 needs to transmit four copies to forward a packet to {B1, B4, B5, B6}. If the more complex TI-BIER-LFA B4 is chosen to protect BFER B4 instead of the remote BIER-LFA B3, only two redundant copies need to be transmitted.

This section first addresses the differences between IP-LFAs for IP-FRR and BIER-LFAs for BIER-FRR. It then examines the advantages and disadvantages of tunnel-based and LFA-based BIER-FRR.

LFAs were initially proposed for IP networks. They are straightforward in that they do not require any tunneling overhead. However, certain destinations cannot be protected against specific link failures, and even more destinations may be unprotected against certain node failures. To improve coverage, remote LFAs (R-LFAs) were introduced, which tunnel affected traffic to another node from which the traffic can reach the destination through normal forwarding. Despite this, there may still be destinations that remain unprotected against link or node failures. To address this, topology-independent LFAs (TI-LFAs) were developed, wherein affected traffic is tunneled via an explicit path (preferably using segment routing headers) to another node from which the traffic can reach its destination through standard IP forwarding. With TI-LFAs, all destinations can be protected against any failures as long as connectivity exists. LFA-based BIER-FRR adopts the principles of LFAs but differs from IP-FRR in that the LFA target node, i.e., the node to which traffic is diverted, must be a BFR. If an IP-LFA target is a BFR, it can be utilized as a BIER-LFA; otherwise, it cannot serve as a BIER-LFA. Consequently, if only a subset of nodes in the underlay are BFRs, the BIER-LFAs will differ substantially from IP-LFAs. Furthermore, this makes it more challenging to identify normal LFAs that do not require tunneling. As a result, LFA-based BIER-FRR is likely to require more remote LFAs and TI-LFAs than IP-FRR under such conditions.

The computation of backup forwarding entries is straightforward, requiring only the primary BIFTs of a PLR and its BFR-NBRs. No routing information from the routing underlay is necessary. The forwarding action "Explicit" is not required. However, depending on the underlay, explicit forwarding may still be utilized to achieve FRR in the underlay.

It relies on the presence of a FRR mechanism in the underlay. It is constrained by the protection level provided by the underlay. For instance, if the underlay supports only link protection, tunnel-based BIER-FRR cannot offer node protection. Redundant packet copies may occur in tunnel-based BIER-FRR. In the case of node protection, backup paths may be unnecessarily extended. A tunneling header is required for any rerouting, resulting in additional header overhead.

It does not depend on any fast protection mechanisms in the underlay. It can provide superior protection at the BIER layer compared to the IP layer, particularly if LFA-based BIER-FRR utilizes BIER-LFAs with a higher protection level than those used in LFA-based IP-FRR. For example, the underlay may only offer FRR with link protection, while BIER-FRR can provide node protection for BIER traffic. It avoids header overhead for normal BIER-LFAs.

The computation of backup forwarding entries requires routing information from the underlay. The computation of backup forwarding entries is more complex when some nodes in the underlay are not BFRs. The "Tunnel" forwarding action is required to protect certain BFERs, which adds header overhead. The "Explicit" forwarding action is necessary to achieve full protection coverage in some topologies; without it, only partial protection coverage is possible. This requires support for explicit paths, such as segment routing. More remote and TI-LFAs are needed compared to IP-FRR if some nodes in the routing underlay are not BFRs. Redundant packet copies may occur in LFA-based BIER-FRR, though this is less frequent than with tunnel-based BIER-FRR.

This specification does not introduce additional security concerns beyond those already discussed in the BIER architecture along with the IP FRR and LFA specifications.

No requirements for IANA.

Performance Comparison of Resilience Mechanisms for Stateless Multicast Using BIER

This appendix demonstrates the computations of some specific backup strategy options in details.

In the LFA-based BIER-FRR using single BIFT, every BFR has a single BIFT for a level of protection. Its structure is the same as the one in . The following presents the details in BFR B1 in for building the BIFT for BIER-FRR link protection. At first, BFR B1 obtains a BIER-LFA as BBFR-NBR for each BFER. B6 is normal BIER-LFA as BBFR-NBR for BFER B2 and B3. B2 is normal BIER-LFA as BBFR-NBR for BFER B4, B5, B6 and B7. illustrates B1's intermediate BIFT for link protection filled with values for BBFR-NBRs and BFAs.

From the intermediate BIFT, BFERs B2 and B3 have the same BFR-NBR B2 and BBFR-NBR B6, BFERs B4 to B7 have the same BFR-NBR B6 as the BBFR-NBR B6 for BFER B2. According to , the BF-BM for BFER B2 has the bits for B2 and B3 as well as the bits for B4 to B7, which is 1111110. The BF-BM for BFER B3 is also 1111110. Similarly, the BF-BM for each of BFERs B3 to B7 is computed, which is 1111110. With the BF-BMs, BFR B1 has the BIFT for link protection, which is illustrated in .

For the LFA-based BIER-FRR using multiple backup BIFTs, in addition to a primary BIFT, a BFR has a backup BIFT for each of its BFR-NBRs with a level of protection. The backup BIFT for BFR-NBR N with link protection (or simply called the backup BIFT for link to N) assumes that the link to N failed. The BFR uses it to protect against the failure of its link to N. The backup BIFT for N with node protection (or simply called the backup BIFT for N) assumes that node N failed. The BFR uses it to protect against the failure of N. Once the BFR as a PLR detects the failure of its link to N, it uses the backup BIFT for link to N to forward all BIER packets. When the BFR as a PLR detects the failure of its BFR-NBR N, it uses the backup BIFT for N to forward all BIER packets. Even though a BFR has multiple backup BIFTs, the LFA-based BIER-FRR using multiple backup BIFTs is scalable. Both the size of a backup BIFT and the number of backup BIFTs on the BFR are small. Assume a BIER network has 1000 BFRs and 100 BFERs, and each BFR has 10 BFR-NBRs on average. The size of a backup BIFT is 100 forwarding entries. The number of backup BIFTs on the BFR is 20 on average. The total size of all backup BIFTs is 100*20 = 2000 forwarding entries. The following presents the details in BFR B1 in for building the backup BIFT for link to B2 to support BIER-FRR link protection. To support link protection, BFR B1 in has two backup BIFTs: one for link to B2 and the other for link to B6. The backup BIFT for link to B2 is illustrated in .

B2 | +--------+---------+---------+-----------------+-----------------+ | 3 | 1111110 | B6 | Plain | Link B1->B2 | +--------+---------+---------+-----------------+-----------------+ | 4 | 1111110 | B6 | Plain | | +--------+---------+---------+-----------------+-----------------+ | 5 | 1111110 | B6 | Plain | | +--------+---------+---------+-----------------+-----------------+ | 6 | 1111110 | B6 | Plain | | +--------+---------+---------+-----------------+-----------------+ | 7 | 1111110 | B6 | Plain | | +--------+---------+---------+-----------------+-----------------+ ]]> BFR B1 builds the backup BIFT for link to B2 in two steps. In the first step, it builds the backup BIRT for link to B2 through copying its regular BIRT to the backup BIRT and then changing BFR-NBR B2 in the backup BIRT to a backup BFR-NBR to protect against the failure of the link to B2. The backup BIRT for link to B2 built by B1 is illustrated in .

B2 | +------+-------------+---------+-----------------+-----------------+ | 3 | B3 | B6 | Plain | Link B1->B2 | +------+-------------+---------+-----------------+-----------------+ | 4 | B4 | B6 | Plain | | +------+-------------+---------+-----------------+-----------------+ | 5 | B5 | B6 | Plain | | +------+-------------+---------+-----------------+-----------------+ | 6 | B6 | B6 | Plain | | +------+-------------+---------+-----------------+-----------------+ | 7 | B7 | B6 | Plain | | +------+-------------+---------+-----------------+-----------------+]]> The BFR-NBR in each of the first two routing entries with BFR-NBR B2 originally from the BIRT is changed to its corresponding backup BFR-NBR. The BFR-NBR B2 in the first entry is changed to backup BFR-NBR BIER-LFA B6. The BFR-NBR B2 in the second entry is changed to backup BFR-NBR BIER-LFA B6. In the second step, BFR B1 derives the backup BIFT for link to B2 from the backup BIRT for link to B2 in the same way as it derives its regular BIFT from its BIRT defined in . The result of the backup BIFT for link to B2 is the one shown in . When BFR B1 as a PLR detects the failure of its link to B2, it forwards all the BIER packets using the FRR-BIFT for link to B2. There is no redundant packet. For example, for a BIER packet with destinations B2 and B6 (i.e., bitstring 0100010), BFR B1 sends a single packet copy on the link to B6 using the backup BIFT for link to B2 after detecting the failure of its link to B2. It will not send any copy of the packet to B6 again since the bitstring in the packet will be all cleaned by the F-BM 1111110 after sending the packet to B6 via its link to B6. Similarly, for a BIER packet with destinations B2, B5 and B7 (i.e., bitstring 1010010), BFR B1 sends a single packet copy on its link to B6 using the backup BIFT for link to B2 after detecting the failure of its link to B2.

Acknowledgments The authors would like to thank Daniel Merling, Jeffrey Zhang, Tony Przygienda and Shaofu Peng for their comments to this work. A special thank you to Gunter van de Velde for his extensive editing to help bring this document to publication.

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