[Bier] [Shepherding AD review] Pre-IETF Last-Call review of for draft-ietf-bier-frr-04

["Gunter van de Velde (Nokia)" <[e-mail]>]

[Corrected email title]



# Gunter Van de Velde, RTG AD, comments for draft-ietf-bier-frr-04.txt



## Many thanks to Zheng (Sandy) Zhang for the shepherd writeup.



## This is a shepherding AD draft review before starting the process of the IETF LC.



## The shepherd write-up suggested a verification round with the rtgwg due to the experience of LFA in that WG.



  "The work on LFA-Based BIER-FRR is aligned with LFAs for IP FRR. Therefore,

  it should be reviewed by someone in rtgwg."



This seems a sensible request to comply towards. I performed a search in rtgwg and that yielded no review request:

https://mailarchive.ietf.org/arch/browse/rtgwg/?q=draft-ietf-bier-frr



## idnits revealed some outdated references (caused due the delay of the Shepherding AD review)



## The draft could use a section explaining the acronyms used. While these may be well known in the BIER technology area, it may not be the same for technology generalists. Please add a section or at least reference documents where terminology is defined and explained.



## the original text uses the word 'we' sporadically. i tried to clean that up with alternate suggested text removing the 'we' construct. It is to me unclear who exactly is 'we', is that the authors, is it the WG is that the IETF, is that a subgroup of authors?



## in the next part you will find suggestions for rewritten text to help the structure and technical precision of the content. I tried to keep the original intent. You will find the review rather lengthy, however i hope you find processing the suggestions and using copy/paste convenient



## In section "3.3.  Primary BIFT and Failure-Specific Backup BIFTs" it is unclear to me how the node protection failure-specific is illustrated. I see in the last column of figure 6 the textblob "Comment: protects: failure of some" but how is this correlated with node protection of B6? I am wondering if there is a column missing or if there is unclear description?



## section 5.1.3.  about implementation experience is better positioned in a document appendix. In few years the section is caught up by time and no longer relevant, while the remainder of the text keeps holding value.



## section 7. Security considerations is rather compact. The solution in the draft uses tunnels and LFA technology and these may have implicit relevant security considerations. I took liberty to add a set of those considerations at the end of this review message to look at and potentially validate against usage in a BIER enabled environment.



## the line numbering in this review is based upon the numbers from the idnits report

https://author-tools.ietf.org/api/idnits?url=https://www.ietf.org/archive/id/draft-ietf-bier-frr-04.txt



#DETAILED COMMENTS

#=================

##classified as [minor] and [major]



17           Abstract

18

19              BIER is a scalable multicast overlay that utilizes a routing

20              underlay, e.g., IP, to build up its Bit Index Forwarding Tables

21              (BIFTs).  This document proposes Fast Reroute for BIER (BIER-FRR).

22              It protects BIER traffic after detecting the failure of a link or

23              node in the core of a BIER domain until affected BIFT entries are

24              recomputed after reconvergence of the routing underlay.  BIER-FRR is

25              applied locally at the point of local repair (PLR) and does not

26              introduce any per-flow state.  The document specifies nomenclature

27              for BIER-FRR and gives examples for its integration in BIER

28              forwarding.  Furthermore, it presents operation modes for BIER-FRR.

29              Link and node protection may be chosen as protection level.

30              Moreover, the backup strategies tunnel-based BIER-FRR and LFA-based

31              BIER-FRR are defined and compared.



[minor]

My preference is to keep the abstract focussed around the content and objective of the draft itself. I woudl expect that readers of the draft have awareness of BIER already, Hence maybe the following alternate abstract:



"

This document describes a mechanism for Fast Reroute (FRR) in Bit Index Explicit Replication (BIER) networks. The proposed solution enhances the resiliency of BIER by providing a method to quickly reroute traffic in the event of a link or node failure, thereby minimizing packet loss and service disruption. The document details the procedures for detecting failures and selecting backup paths within the BIER domain, ensuring that multicast traffic continues to reach its intended destinations without requiring per-flow state or additional signaling. This FRR mechanism is designed to integrate seamlessly with existing BIER operations, offering a robust solution for improving network reliability.

"





131

132           BIER packets are usually forwarded without an outer IP header.  If a

133           link or node fails, the corresponding BFR neighbor (BFR-NBR) is no

134           longer reachable.  Fast reroute (FRR) mechanisms in the routing

135           underlay, e.g., IP-FRR, apply only to IP packets so that BIER traffic

136           would be dropped.  BIER traffic can be delivered again only after

137           reconvergence of the routing underlay and recalculation of the BIFT.

138           Thus, tunneling BIER packets can help to reach the BFR-NBR in case of

139           a link failure by leveraging FRR capabilities of the routing underlay

140           if such mechanisms are available.  However, this does not help in

141           case of a node failure.  Then, all destinations having the failed

142           node as BFR-NBR cannot be reached anymore.  As BIER carries multicast

143           traffic which has often realtime requirements, there is a particular

144           need to protect BIER traffic against too long outages after failures.

145

146           In this document we propose nomenclature for Fast Reroute in BIER

147           (BIER-FRR).  As soon as a BFR detects a BFR-NBR is unreachable, BIER-

148           FRR enables a BFR to quickly reroute affected BIER packets with the

149           help of backup forwarding entries.  To avoid redundant packets,

150           backup forwarding entries should be processed prior to normal

151           forwarding entries.  To achieve that goal, two possible

152           representations for backup forwarding entries are proposed.

153

154           The protection level can be either link protection or node

155           protection.  Link protection protects only the failure of a link.  It

156           is simple but may not work if a BFR fails.  Node protection is more

157           complex but also protects against the failure of BFRs.  The backup

158           strategy determines the selection of the backup forwarding entries.

159

160           Examples for backup strategies are tunnel-based BIER-FRR and LFA-

161           based BIER-FRR

162

163           *  Tunnel-based BIER-FRR leverages mechanisms of the routing underlay

164              for FRR purposes.  The routing underlay restores connectivity

165              faster than BIER as a reconverged routing underlay is prerequisite

166              for recalculation of the BIFT.  If the routing underlay leverages

167              FRR mechanisms, its forwarding ability is restored long before

168              reconvergence is completed.  To leverage fast restoration of the

169              routing underlay, BIER traffic affected by a failure is tunneled

170              over the routing underlay.

170

172           *  LFA-based BIER-FRR reroutes BIER traffic to alternative neighbors

173              in case of a failure.  It utilizes the principles of IP-FRR but

174              requires that LFAs are BFRs.  Normal BIER-LFAs can be reached

175              without tunneling, remote BIER-LFAs utilize a tunnel, and

176              topology-independent BIER-LFAs leverage explicit paths to reach

177              the backup BFR-NBR.  In contrast to tunnel-based FRR, LFA-based

178              BIER-FRR does not require fast reroute mechanisms in the routing

179              underlay.

180

181           BIER-FRR as presented in this document follows a primary/backup path

182           principle, also known as 1:1 protection.  It is opposite to 1+1

183           protection which denotes a live-live protection principle.  This has

184           been considered for BIER in [BrAl17].



[minor]

proposed rewrite:



"

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 [BrAl17].

"



192        2.1.  Definition of Forwarding Actions

193

194           A BFR-NBR is directly connected if it is a next hop on the network

195           layer, i.e., if it can be reached via the link layer technology.

196           Otherwise, the BFR-NBR is indirectly connected.

197

198           We define the following forwarding actions.

199

200           *  Plain: Sends the mere BIER packet to a BFR-NBR via a direct link

201              and without a tunnel header.  That means, the packet is not sent

202              over the routing underlay.

203

204           *  Tunnel: Encapsulates the BIER packet with a tunnel header towards

205              a BFR-NBR and sends it over the routing underlay.

206

207           *  Explicit: Forwards the packet over an explicit path to a BFR-NBR.

208              The path information must be given.  If segment routing is used

209              for this purpose, the segment IDs (SIDs) must be given.  Two

210              forwarding actions of type Explicit are equal only if they share

211              the same explicit path.

212

213           The forwarding actions in the BIFT as proposed in [RFC8279] are given

214           implicitly as they are derived from the connectedness of the BFR-NBR.

215           If the BFR-NBR is directly connected, the forwarding action is Plain.

216           If the BFR-NBR is not directly connected, the forwarding action is

217           Tunnel.



[minor]

proposed rewrite:



"

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 [RFC8279], 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.

"



219        2.2.  Definition of Backup Forwarding Entries

220

221           The BIFT as proposed in [RFC8279] contains a F-BM and a BFR-NBR for a

222           specific BFER.  They constitute a primary forwarding entry.  BIER-FRR

223           extends this regular BIFT by additional columns containing backup

224           forwarding entries.  A backup forwarding entry contains

225

226           *  a backup F-BM (BF-BM),

227

228           *  a backup BFR-NBR (BBFR-NBR),

229

230           *  a backup forwarding action (BFA), and

231

232           *  a backup entry active (BEA) flag.

233

234           Backup F-BM and backup BFR-NBR have the same structure as their

235           primary counterparts.  The backup forwarding action is a forwarding

236           action as defined in Section 2.1.  The BEA flag indicates whether the

237           backup forwarding entry is active.  When it is active, the backup

238           F-BM, backup BFR-NBR, and the backup forwarding action are used for

239           the forwarding of BIER packets instead of the primary forwarding

240           entry.  The structure of the extended BIFT is given in Figure 1.

241

242               +--------+------+---------+--------+----------+--------+----+

243               | BFR-id | F-BM | BFR-NBR | BF-BM  | BBFR-NBR |  BFA   | BEA|

244               +========+======+=========+========+==========+========+====+

245               |  ...   |  ... |   ...   |   ...  |   ...    |  ...   |    |

246               +--------+------+---------+--------+----------+--------+----+

247

248               Figure 1: Structure of an extended BIFT with backup forwarding

249                                          entries.

250

251           The primary action is not given in the BIFT as it is derived from the

252           BFR-NBR.  In contrast, the backup forwarding action is given in the

253           extended BIFT.  Moreover, an explicit path must be indicated in case

254           of forwarding action Explicit.  However, explicit paths are

255           implementation-specific and, therefore, this information is not

256           indicated in the table.  The values for the backup BFR-NBR and the

257           backup action depend on the desired protection level and the backup

258           strategy.  Examples for them are described in Section 5.1 and

259           Section 5.2.  The backup F-BM depends on the backup BFR-NBR.  Its

260           computation is explained in Section 2.4.



[minor]

proposed rewrite:



"

The BIFT as proposed in [RFC8279] 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 Section 2.1. 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 Figure 1.



                       +--------+------+---------+--------+----------+--------+----+

                       | BFR-id | F-BM | BFR-NBR | BF-BM  | BBFR-NBR |  BFA   | BEA|

                       +========+======+=========+========+==========+========+====+

                       |  ...   |  ... |   ...   |   ...  |   ...    |  ...   |    |

                       +--------+------+---------+--------+----------+--------+----+



                       Figure 1: Structure of an extended BIFT with backup forwarding

                                                  entries.



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 Sections 5.1 and 5.2. The Backup Forwarding Bit Mask (BF-BM) is determined based on the backup BFR-NBR, and its computation is described in Section 2.4.

"



279        2.4.  Computation of the Backup F-BM

280

281           The primary F-BM of a specific BFER indicates all BFERs that share

282           the same primary BFR-NBR.  The backup F-BM of a specific BFER

283           indicates

284

285           *  all BFERs that share the primary and backup BFR-NBR of the

286              specific BFER and

287

288           *  all BFERs that have the backup BFR-NBR of the specific BFER as

289              primary BFR-NBR.



[minor]

proposed rewrite:



"

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:



A* ll 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.

"



291        3.  Representations for BIER-FRR Forwarding Data

292

293           We show that backup entries need to be used first to reduce the

294           number of redundant packets in the single extended BIFT (presented in

295           Section 2.2).  This may be hard or cannot be achieved on some

296           hardware platforms.  Therefore, two alternate representations of

297           forwarding entries are proposed.  The first is a primary BIFT and

298           single backup BIFT (SBB).  The second is a primary BIFT and multiple

299           failure-specific backup BIFTs (FBB).



[minor]

proposed rewrite:



"

To minimize the occurrence of redundant packets, it is essential that backup entries are prioritized for use within the single extended BIFT, as described in Section 2.2. 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).

"



301        3.1.  Potential Emergence of Redundant Packets

302

303           The BIER forwarding procedure in failure-free scenarios avoids

304           redundant packets, i.e., it ensures that at most a single copy is

305           sent per link for every BIER packet.  However, this property might be

306           violated when BIER-FRR as presented in Section 2 is applied to

307           protect against a failure.

308

309           Figure 2 shows an example of a BIER network.  BFRs have the prefix

310           "B" and are numbered by their BFR-ids.  To simplify the example,

311           every BFR is a BFER and its bit position in the bitstring equals its

312           BFR-id.  The number on a link is its cost which is used by the

313           routing underlay for computing the shortest paths.

314

315                      1              1

316                B1 --------- B6 ------------ B5       BFR Bi is BFER

317                |            |               |       (i = 1,2,3,4,5,6,7;

318                |            |               |        i is BFR-id of Bi)

319              2 |            | 1             |

320                |      1     |               | 1     cost of link B1-B2 is 2

321                B2 --------- B7              |       cost of link B3-B4 is 4

322                |                            |       cost of other links is 1

323              1 |                            |

324                |                  4         |

325               B3 ------------------------- B4

326                              Figure 2: BIER network example.

327

328           The extended BIFT with backup forwarding entries for LFA-based BIER-

329           FRR with link protection built by BFR B1 is illustrated in Figure 3.

330

331              +------+----------+-------+----------+--------+----------+---+

332              |BFR-id|   F-BM   |BFR-NBR|  BF-BM   |BBFR-NBR|   BFA    |BEA|

333              +======+==========+=======+==========+========+==========+===+

334              |   2  | 0000110  |  B2   | 1111110  |   B6   |  Plain   |   |

335              +------+----------+-------+----------+--------+----------+---+

336              |   3  | 0000110  |  B2   | 1111110  |   B6   |  Plain   |   |

337              +------+----------+-------+----------+--------+----------+---+

338              |   4  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |   |

339              +------+----------+-------+----------+--------+----------+---+

340              |   5  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |   |

341              +------+----------+-------+----------+--------+----------+---+

342              |   6  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |   |

343              +------+----------+-------+----------+--------+----------+---+

344              |   7  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |   |

345              +------+----------+-------+----------+--------+----------+---+

346

347            Figure 3: B1's extended BIFT for LFA-based FRR with link protection.

348

349           We show how redundant packets can occur in case of a failure.  To

350           that end, we consider the extended BIFT for BFR 1 in Figure 3.  It

351           has backup forwarding entries for LFA-based FRR and link protection.

352           For a BIER packet with destinations B2 and B6 (i.e., bitstring

353           0100010), BFR B1 sends a single packet copy on link B1-B2 and on link

354           B1-B6 in the absence of a failure.

355

356           When the link B1-B6 fails, B1 as a PLR detects the failure.

357           Therefore, B1 sets the BEA flag for all destinations that have B6 as

358           BFR-NBR.  We consider again that B1 sends a BIER packet to B2 and B6.

359           At first, it sends a copy with bitstring 0000010 to B2 using the

360           corresponding primary forwarding entry in the extended BIFT in

361           Figure 3.

362

363           Then, B1 sends another copy of the packet with bitstring 0100000 for

364           B6 to B2 using the backup forwarding entry since the BEA flag is

365           activated.

366

367           This is a second (redundant) copy over the same link B1-B2.  It can

368           be prevented if the backup forwarding entry is used first.  When

369           using the backup forwarding entry, B1 sends only a single copy of the

370           packet with bitstring 0100010 to B2.  It will not send any copy of

371           the packet to B2 again since the bitstring in the packet will be all

372           cleaned by the BF-BM 1111110.  Thus, prioritized processing of BFERs

373           with unreachable BFR-NBRs helps to reduce redundant packet copies.



[minor]

proposed rewrite:



"

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 Section 2, is employed to provide protection against a failure.



Figure 2 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.



          1              1

    B1 --------- B6 ------------ B5       BFR Bi is BFER

    |            |               |       (i = 1,2,3,4,5,6,7;

    |            |               |        i is BFR-id of Bi)

  2 |            | 1             |

    |      1     |               | 1     cost of link B1-B2 is 2

    B2 --------- B7              |       cost of link B3-B4 is 4

    |                            |       cost of other links is 1

  1 |                            |

    |                  4         |

   B3 ------------------------- B4



    Figure 2: BIER network example.



The extended BIFT with backup forwarding entries for LFA-based BIER-FRR with link protection, as constructed by BFR B1, is illustrated in Figure 3.



   +------+----------+-------+----------+--------+----------+---+

   |BFR-id|   F-BM   |BFR-NBR|  BF-BM   |BBFR-NBR|   BFA    | BEA|

   +======+==========+=======+==========+========+==========+===+

   |   2  | 0000110  |  B2   | 1111110  |   B6   |  Plain   |    |

   +------+----------+-------+----------+--------+----------+---+

   |   3  | 0000110  |  B2   | 1111110  |   B6   |  Plain   |    |

   +------+----------+-------+----------+--------+----------+---+

   |   4  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |    |

   +------+----------+-------+----------+--------+----------+---+

   |   5  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |    |

   +------+----------+-------+----------+--------+----------+---+

   |   6  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |    |

   +------+----------+-------+----------+--------+----------+---+

   |   7  | 1111000  |  B6   | 1111110  |   B2   |  Plain   |    |

   +------+----------+-------+----------+--------+----------+---+



    Figure 3: B1's extended BIFT for LFA-based FRR with link protection



The emergence of redundant packets during a failure scenario is demonstrated as follows. Consider the extended BIFT for BFR B1 depicted in Figure 3. 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 Figure 3.



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.

"



375        3.2.  Primary BIFT and Single Backup BIFT

376

377           The extended BIFT may be separated into two BIFTs.  One is a primary

378           BIFT and the other is a single backup BIFT.  The primary BIFT is the

379           same as the regular BIFT.  The backup BIFT contains the backup

380           forwarding entries, including BF-BM, BBFR-NBR, BFA and BEA in the

381           extended BIFT.  When a BFR as a PLR detects that BFR-NBR N is

382           unreachable, it activates the BEA flag for all BFERs in the backup

383           BIFT that have BFR-NBR as primary BFR-NBR.  When a BFR forwards a

384           BIER packet, it processes the packet first using the backup BIFT and

385           then using the primary BIFT.  With this prioritization, the number of

386           redundant packet copies can be reduced.

387

388           B1's extended BIFT in Figure 3 is separated into the primary BIFT in

389           Figure 4 and the single backup BIFT in Figure 5.

390

391                               +--------+---------+---------+

392                               | BFR-id |   F-BM  | BFR-NBR |

393                               +========+=========+=========+

394                               |   2    | 0000110 |    B2   |

395                               +--------+---------+---------+

396                               |   3    | 0000110 |    B2   |

397                               +--------+---------+---------+

398                               |   4    | 1111000 |    B6   |

399                               +--------+---------+---------+

400                               |   5    | 1111000 |    B6   |

401                               +--------+---------+---------+

402                               |   6    | 1111000 |    B6   |

403                               +--------+---------+---------+

404                               |   7    | 1111000 |    B6   |

405                               +--------+---------+---------+

406

407                 Figure 4: B1's primary BIFT for the BIER network example.

408

409               +------+----------+--------+-----------+---+-----------------+

410               |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

411               |      |          |        |           |   |  failure of     |

412               +======+==========+========+===========+===+=================+

413               |   2  | 1111110  |   B6   |  Plain    |   |  Link B1->B2    |

414               +------+----------+--------+-----------+---+-----------------+

415               |   3  | 1111110  |   B6   |  Plain    |   |  Link B1->B2    |

416               +------+----------+--------+-----------+---+-----------------+

417               |   4  | 1111110  |   B2   |  Plain    |   |  Link B1->B6    |

418               +------+----------+--------+-----------+---+-----------------+

419               |   5  | 1111110  |   B2   |  Plain    |   |  Link B1->B6    |

420               +------+----------+--------+-----------+---+-----------------+

421               |   6  | 1111110  |   B2   |  Plain    |   |  Link B1->B6    |

422               +------+----------+--------+-----------+---+-----------------+

423               |   7  | 1111110  |   B2   |  Plain    |   |  Link B1->B6    |

424               +------+----------+--------+-----------+---+-----------------+

425

426                  Figure 5: B1's backup BIFT for the BIER network example.

427

428           Each forwarding entry in the backup BIFT contains BF-BM, BBFR-NBR,

429           BFA and BEA.  When a BFR-NBR fails, the BEA flag is activated for all

430           BFERs in the backup BIFT that have BFR-NBR as primary BFR-NBR.  For

431           example, BFERs B4, B5, B6 and B7 have BFR-NBR B6 as their primary

432           BFR-NBR.  When BFR-NBR B6 fails, the BEA flag for BFERs B4, B5, B6

433           and B7 is activated, i.e., the BEA in the last four entries in the

434           backup BIFT is set to one.



[minor]

proposed rewrite:



"

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 Figure 3 is divided into the primary BIFT shown in Figure 4 and the single backup BIFT shown in Figure 5.



   +--------+---------+---------+

   | BFR-id |   F-BM  | BFR-NBR |

   +========+=========+=========+

   |   2    | 0000110 |    B2   |

   +--------+---------+---------+

   |   3    | 0000110 |    B2   |

   +--------+---------+---------+

   |   4    | 1111000 |    B6   |

   +--------+---------+---------+

   |   5    | 1111000 |    B6   |

   +--------+---------+---------+

   |   6    | 1111000 |    B6   |

   +--------+---------+---------+

   |   7    | 1111000 |    B6   |

   +--------+---------+---------+



    Figure 4: B1's primary BIFT for the BIER network example.



   +------+----------+--------+-----------+---+-----------------+

   |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

   |      |          |        |           |   |  failure of     |

   +======+==========+========+===========+===+=================+

   |   2  | 1111110  |   B6   |  Plain    |   |  Link B1->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    |

   +------+----------+--------+-----------+---+-----------------+



    Figure 5: B1's backup BIFT for the BIER network example.



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.

"



436        3.3.  Primary BIFT and Failure-Specific Backup BIFTs

437

438           As an alternative, the information in the extended BIFT may be

439           represented in a primary BIFT and several, failure-specific backup

440           BIFTs.  A failure-specific backup BIFT is a backup BIFT for the

441           unreachability of BFR-NBR N.  A backup BIFT for the failure of N is

442           simply called a backup BIFT for N.  It has the same structure as the

443           regular BIFT but has an entry for a backup forwarding action.  Thus,

444           a BFR has a primary BIFT, which is the same as the regular BIFT, and

445           a backup BIFT for each of its BFR-NBRs.

446

447           The BFR uses the primary BIFT to forward BIER packets under failure-

448           free conditions.  When the BFR as a PLR detects that BFR-NBR N is

449           unreachable, it uses the backup BIFT for N to forward all BIER

450           packets.  After the routing underlay has re-converged on the new

451           network topology, the primary BIFT is re-computed.  Once the re-

452           computed primary BIFT is installed, it is used to forward all BIER

453           packets.

454

455           We illustrate the concept using the example from extended BIFT in

456           Figure 3.  Figure 4 shows the primary BIFT of B1 in this context.

457           BFR B1 in Figure 2 has two neighbors: B6 and B2.  B1 has two backup

458           BIFTs with link protection: one for B6 and another for B2.  B1 has

459           also two backup BIFTs with node protection.  Figure 6 is B1's backup

460           BIFT for B6 to react to the unreachability of B1 in a similar way as

461           with the extended BIFT in Figure 3.

462

463            +--------+---------+---------+-----------------+-----------------+

464            | BFR-id |  F-BM   | BFR-NBR |Forwarding Action|Comment: protects|

465            |        |         |         |                 |  failure of     |

466            +========+=========+=========+=================+=================+

467            |    2   | 1111110 |    B2   |   Plain         |                 |

468            +--------+---------+---------+-----------------+-----------------+

469            |    3   | 1111110 |    B2   |   Plain         |                 |

470            +--------+---------+---------+-----------------+-----------------+

471            |    4   | 1111110 |    B2   |   Plain         |  Link B1->B6    |

472            +--------+---------+---------+-----------------+-----------------+

473            |    5   | 1111110 |    B2   |   Plain         |  Link B1->B6    |

474            +--------+---------+---------+-----------------+-----------------+

475            |    6   | 1111110 |    B2   |   Plain         |  Link B1->B6    |

476            +--------+---------+---------+-----------------+-----------------+

477            |    7   | 1111110 |    B2   |   Plain         |  Link B1->B6    |

478            +--------+---------+---------+-----------------+-----------------+

479

480               Figure 6: B1's backup BIFT for B6 for LFA-based BIER FRR with

481                                      link protection.

482

483           Once B1 as a PLR detects that B6 is unreachable through the link to

484           B6, it uses the backup BIFT for B6 to forward all BIER packets.  As

485           this representation is equivalent to the concept of single primary

486           and single backup BIFT, redundant packets for the same forwarding

487           action are avoided.



[minor]

proposed rewrite:



"

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 Figure 3. Figure 4 depicts B1's primary BIFT in this context. BFR B1 in Figure 2 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. Figure 6 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 Figure 3.



   +--------+---------+---------+-----------------+-----------------+

   | BFR-id |  F-BM   | BFR-NBR |Forwarding Action|Comment: protects|

   |        |         |         |                 |  failure of     |

   +========+=========+=========+=================+=================+

   |    2   | 1111110 |    B2   |   Plain         |                 |

   +--------+---------+---------+-----------------+-----------------+

   |    3   | 1111110 |    B2   |   Plain         |                 |

   +--------+---------+---------+-----------------+-----------------+

   |    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    |

   +--------+---------+---------+-----------------+-----------------+



       Figure 6: B1's backup BIFT for B6 for LFA-based BIER FRR with

                              link protection.



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.

"



489        4.  Protection Levels

490

491           Link and node protection may be supported.  Link protection protects

492           against the failure of an adjacent link while node protection

493           protects against the failure of a neighboring node and the path

494           towards that node.  Depending on the supported service, link

495           protection or node protection may be relevant.  Both protection

496           levels can be combined with any backup strategy in Section 5.



[minor]

proposed rewrite:



"

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 Section 5.

"





498        4.1.  Link Protection

499

500           With link protection the backup path avoids the failed link (i.e.,

501           the failed primary path from the PLR to the primary BFR-NBR,

502           excluding the primary BFR-NBR), but the backup path may include the

503           primary BFR-NBR.  Therefore, the backup path is still operational if

504           the primary path fails.  The disadvantage of link protection is that

505           it fails if the primary BFR-NBR itself is not operational.  However,

506           link protection has also advantages.  It often leads to shorter

507           backup paths than node protection.  In case of tunnel-based BIER-FRR,

508           link protection causes at most one redundant packet while node

509           protection can cause more redundant packets.  In case of LFA-based

510           BIER-FRR, link protection can protect more BFERs with normal BIER-

511           LFAs than node protection.



[minor]

proposed rewrite:



"

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.

"



513        4.2.  Node Protection

514

515           With node protection, the backup path avoids the failed node and the

516           link to the node (i.e., the failed primary path from the PLR to the

517           primary BFR-NBR, including the primary BFR-NBR).  Therefore, the

518           backup path must not include the primary path or the primary BFR-NBR

519           so that the backup path is still operational if these elements fail.

520           If a BFER and its primary BFR-NBR are the same, only link protection

521           is possible for that BFER.  An advantage of node protection is the

522           improved protection quality compared to link protection.  However,

523           node protection has also disadvantages.  It often leads to longer

524           backup paths than link protection.  For tunnel-based BIER-FRR,

525           possibly more redundant packets are transmitted over a link than with

526           link protection.  For LFA-based BIER-FRR, possibly fewer BFERs can be

527           protected with normal BIER-LFAs so that more remote BIER-LFAs or

528           topology-independent BIER-LFAs are needed which are more complex.



[minor]

proposed rewrite:



"

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.

"



530        4.3.  Example

531

532           In Figure 2, B1's primary path towards BFER B5 is B1-B6-B5.  Node

533           protection for BFER B5 can be achieved only via the backup path

534           B1-B2-B3-B4-B5.  Link protection for BFER 5 is achieved via the

535           backup path B1-B2-B7-B6 and in addition via the backup path

536           B1-B2-B3-B4-B5-B6.  The backup entries depend on the protection level

537           and on the backup strategy.  Example BIFTs for link and node

538           protection are given in Section 5.



[minor]

proposed rewrite:



"

In the network depicted in Figure 2, the primary path from BFR B1 to BFER B5 is B1-B6-B5. Node protection for BFER B5 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 Section 5.

"



540        5.  Backup Strategies

541

542           The backup strategies determine the selection of the backup

543           forwarding entries.  They have an impact on the backup BFR-NBR and on

544           the backup action, and thereby on the backup path.  In the following,

545           tunnel-based BIER-FRR and LFA-based BIER-FRR are presented.

546

547        5.1.  Tunnel-Based BIER-FRR

548

549           The routing underlay may be able to forward packets towards their

550           destinations despite an existing failure.  This may be achieved,

551           e.g., due to FRR mechanisms in the routing underlay.  In that case,

552           the primary BFR-NBR is not reachable via the primary action (Plain),

553           but it may be reachable via a backup action (Tunnel).

554

555           Tunnel-based BIER-FRR encapsulates BIER packets affected by a failure

556           in the routing underlay to leverage its fast restoration capability.

557           The affected BIER packets can be delivered towards their destinations

558           as soon as the connectivity in the routing underlay is restored.  The

559           appropriate backup forwarding entries in a BIFT for BIER-FRR depend

560           on the desired protection level.



[minor]

proposed rewrite:



"

5. Backup Strategies



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.



5.1. Tunnel-Based BIER-FRR



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.

"



562        5.1.1.  Tunnel-Based BIER-FRR with Link Protection

563

564           With link protection, the backup BFR-NBRs equal the primary BFR-NBRs.

565           If a primary BFR-NBR is directly connected to the BFR as a PLR, the

566           corresponding backup forwarding action is Tunnel.  As a result, the

567           BIER packets affected by a failure are tunneled over the routing

568           underlay to their BFR-NBR instead of being sent directly as plain

569           BIER packets to the BFR-NBR.  If a primary BFR-NBR is not directly

570           connected to the BFR as a PLR (i.e., the implicit, primary action is

571           Tunnel), the corresponding backup action is also Tunnel.  The backup

572           F-BMs are the same as the primary F-BMs, which is in line with the

573           computation of the backup F-BMs in Section 2.4.

573

575               +------+----------+--------+-----------+---+-----------------+

576               |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

577               |      |          |        |           |   |  failure of     |

578               +======+==========+========+===========+===+=================+

579               |   2  | 0000110  |  B2    |  Tunnel   |   |  Link B1->B2    |

580               +------+----------+--------+-----------+---+-----------------+

581               |   3  | 0000110  |  B2    |  Tunnel   |   |  Link B1->B2    |

582               +------+----------+--------+-----------+---+-----------------+

583               |   4  | 1111000  |  B6    |  Tunnel   |   |  Link B1->B6    |

584               +------+----------+--------+-----------+---+-----------------+

585               |   5  | 1111000  |  B6    |  Tunnel   |   |  Link B1->B6    |

586               +------+----------+--------+-----------+---+-----------------+

587               |   6  | 1111000  |  B6    |  Tunnel   |   |  Link B1->B6    |

588               +------+----------+--------+-----------+---+-----------------+

589               |   7  | 1111000  |  B6    |  Tunnel   |   |  Link B1->B6    |

590               +------+----------+--------+-----------+---+-----------------+

591

592               Figure 7: B1's backup BIFT for tunnel-based BIER-FRR with link

593                                        protection.

594

595           Figure 7 shows B1's backup BIFT for tunnel-based BIER-FRR with link

596           protection for the BIER network example of Figure 2.  The backup BFR-

597           NBRs and backup F-BMs in this backup BIFT are the same as the primary

598           BFR-NBRs and primary F-BMs in the primary BIFT in Figure 4, but the

599           backup actions in this backup BIFT are Tunnel while the primary

600           actions in the primary BIFT are Plain (which are not shown, but

601           implied).

602

603           When B1 as a PLR detects failure of its link to B6, a BIER packet

604           with bitstring 0100000 for B6 is tunneled by B1 towards B6 via the

605           routing underlay.  The exact path of the backup tunnel depends on the

606           routing underlay.  It may be B1-B2-B7-B6 or B1-B2-B3-B4-B5-B6.

607

608           If a BIER packet is destined to {B2, B5, B7}, first an encapsulated

609           packet copy is forwarded via link B1-B2 to backup BFR-NBR B6 with

610           backup action Tunnel to deliver packet copies to BFER B5 and B7.

611           Then, a non-encapsulated packet copy is forwarded via link B1-B2 to

612           BFR-NBR B2 with primary action Plain to deliver a packet copy to BFER

613           B2.  Thus, with tunnel-based BIER-FRR, a single redundant packet copy

614           can occur in case of a failure because an encapsulated packet copy

615           and a non-encapsulated packet copy are forwarded over the same link.

616           This happens although BIER packets affected by failures are forwarded

617           before BIER packets not affected by failures.

618

619           A BIER packet with bitstring 1000000 for B7 is forwarded on the

620           backup path B1-B2-B7-B6-B7 as it is first delivered to the backup

621           BFR-NBR B6.  Thus, the backup path can be unnecessarily long.  This

622           phenomenon is known from facility backup method in [RFC4090] which

623           takes similar paths as tunnel-based BIER-FRR.



[minor]

proposed rewrite:



"

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 Section 2.4.



   +------+----------+--------+-----------+---+-----------------+

   |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

   |      |          |        |           |   |  failure of     |

   +======+==========+========+===========+===+=================+

   |   2  | 0000110  |  B2    |  Tunnel   |   |  Link B1->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    |

   +------+----------+--------+-----------+---+-----------------+



   Figure 7: B1's backup BIFT for tunnel-based BIER-FRR with link protection.



Figure 7 illustrates B1's backup BIFT for tunnel-based BIER-FRR with link protection in the BIER network example depicted in Figure 2. 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 Figure 4. 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 [RFC4090], which employs paths analogous to those in tunnel-based BIER-FRR.

"



625        5.1.2.  Tunnel-Based BIER-FRR with Node Protection

626

627           To determine the backup forwarding entries with node protection, a

628           case analysis for the BFER to protect is needed.  If the BFER is the

629           same as its primary BFR-NBR, node protection is not possible for that

630           BFER.  Therefore, link protection is applied, i.e., the backup BFR-

631           NBR is set to the primary BFR-NBR.  If that level of protection is

632           not sufficient, egress protection in [I-D.chen-bier-egress-protect]

633           may be applied.  Otherwise (i.e., the BFER is different from its

634           primary BFR-NBR), the backup BFR-NBR is set to the primary BFR-NBR's

635           primary BFR-NBR for that BFER, i.e., the backup BFR-NBR is a next

636           next hop BFR.  In all cases, the backup action is Tunnel.  In the

637           first case, the backup F-BM is set to all zeroes plus the bit enabled

638           for the BFER to protect.  In the second case, the backup F-BM is

639           computed in the way described in Section 2.4.

640

641                +------+----------+--------+----------+---+-----------------+

642                |BFR-id|  BF-BM   |BBFR-NBR|   BFA    |BEA|Comment: protects|

643                |      |          |        |          |   |  failure of     |

644                +======+==========+========+==========+===+=================+

645                |   2  | 0000010  |   B2   |  Tunnel  |   |  Link B1->B2    |

646                +------+----------+--------+----------+---+-----------------+

647                |   3  | 0000100  |   B3   |  Tunnel  |   |  BFR-NBR B2     |

648                +------+----------+--------+----------+---+-----------------+

649                |   4  | 0011000  |   B5   |  Tunnel  |   |  BFR-NBR B6     |

650                +------+----------+--------+----------+---+-----------------+

651                |   5  | 0011000  |   B5   |  Tunnel  |   |  BFR-NBR B6     |

652                +------+----------+--------+----------+---+-----------------+

653                |   6  | 0100000  |   B6   |  Tunnel  |   |  Link B1->B6    |

654                +------+----------+--------+----------+---+-----------------+

655                |   7  | 1000000  |   B7   |  Tunnel  |   |  BFR-NBR B6     |

656                +------+----------+--------+----------+---+-----------------+

657

658               Figure 8: B1's backup BIFT for tunnel-based BIER-FRR with node

659                                        protection.

660

661           Figure 8 shows B1's backup BIFT for tunnel-based BIER-FRR with node

662           protection for the BIER network example in Figure 2.  BFERs B2 and B6

663           are direct neighbors of B1.  To protect them, only link protection is

664           applied as B1's primary BFR-NBR for them are those nodes themselves.

665           According to the description above, only the bit for B2 is set in the

666           backup F-BM of B2.  The same holds for B6.  For BFER B5, the backup

667           BFR-NBR is B5 as it is B1's next next hop BFR towards B5.  Similarly,

668           for BFER B7, the backup BFR-NBR is B7.  When B1 as a PLR detects the

669           failure of its BFR-NBR B6, a BIER packet with bitstring 1010010 for

670           {B2, B5, B7} is processed as follows.  An encapsulated copy of the

671           packet is sent via tunnel B1-B2-B3-B4-B5, another encapsulated copy

672           is sent via tunnel B1-B2-B7, and a non-encapsulated copy is sent via

673           link B1-B2.  In this example, two redundant packets are sent on link

674           B1-B2.  Thus, with node protection, more redundant packets copies may

675           be sent than with link protection.

676

677           Caveat: If the routing underlay does not provide node protection,

678           tunnel-based BIER-FRR cannot provide node protection, either.  This

679           is shown by the following example.  The underlay in the networking

680           example of Figure 2 offers only link protection.  B6 fails and B1

681           must forward a packet to B5.  According to the backup BIFT in

682           Figure 8 the packet is tunneled towards B5 and the tunnel path

683           B1-B2-B7-B6-B5 may be taken for this purpose by the underlay due to

684           FRR with link protection.  However, B6 is also unreachable at B7 so

685           that the packet is returned to B2 and the packet loops between B2 and

686           B7.



[minor]

proposed rewrite:



"

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 [I-D.chen-bier-egress-protect] 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 Section 2.4



   +------+----------+--------+----------+---+-----------------+

   |BFR-id|  BF-BM   |BBFR-NBR|   BFA    |BEA|Comment: protects|

   |      |          |        |          |   |  failure of     |

   +======+==========+========+==========+===+=================+

   |   2  | 0000010  |   B2   |  Tunnel  |   |  Link B1->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     |

   +------+----------+--------+----------+---+-----------------+



   Figure 8: B1's backup BIFT for tunnel-based BIER-FRR with node protection.



Figure 8 illustrates B1's backup BIFT for tunnel-based BIER-FRR with node protection in the BIER network example provided in Figure 2. 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 Figure 2, 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 Figure 8, 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.

"



688        5.1.3.  Implementation Experience



690           Tunnel-based BIER-FRR has been implemented in P4 for the software-

691           switch bmv2 [MeLi20b] and the hardware switching ASIC Tofino

692           [MeLi21].  Performance results have been provided.



[major]

This section is better placed in appendix and not be in the body of the document.





694        5.2.  LFA-based BIER-FRR

694

696           LFA-based BIER-FRR leverages alternate BFRs to deliver BIER packets

697           to BFERs for which the primary BFR-NBR is unreachable.  It does not

698           rely on any fast restoration/protection mechanisms in the underlay.

699           First, some prerequisites for LFA-based BIER-FRR are clarified, BIER-

700           LFAs are defined, and then link and node protection for LFA-based

701           BIER-FRR are discussed using a single backup BIFT.

702

703        5.2.1.  Relation of BIER-LFAs to IP-LFAs and Prerequisites

704

705           A loop-free alternate (LFA) for a specific destination is an

706           alternate node to which a packet is sent if the primary next hop for

707           this destination is not reachable.  This alternate node should be

708           able to forward the packet without creating a forwarding loop.  LFAs

709           have been defined for IP networks in [RFC5286], [RFC7490] and

710           [I-D.ietf-rtgwg-segment-routing-ti-lfa].  We denote such LFAs as IP-

711           LFAs.  BIER-LFAs are very similar to IP-LFAs, but a BIER-LFA node

712           must be a BFR.  If only a subset of the nodes in the routing underlay

713           are BFRs, some IP-LFAs in the routing underlay may not be usable as

714           BIER-LFAs.  To compute BIER-LFAs, network topology and link cost

715           information from the routing underlay are needed.  This is a

716           difference to tunnel-based BIER-FRR where knowledge about the primary

717           BIFTs of a PLR and its BFR-NBRs is sufficient.

718

719           LFA-based BIER-FRR may reuse IP-LFAs in the following sense as BIER-

720           LFAs.  If an IP-LFA node for the destination of a specific BFER is a

721           BFR, it may be reused as backup BFR-NBR for that BFER together with

722           the backup action that is applied for that IP-LFA on the IP layer.  A

723           normal IP-LFA corresponds to backup action plain, a remote IP-LFA to

724           Tunnel, and a TI-IP-LFA to Explicit.

725

726        5.2.2.  Definition of BIER-LFAs

727

728           As for IP-LFAs, there are several, different types of BIER-LFAs:

729

730           *  A BFR is a normal BIER-LFA for a specific BFER if it is directly

731              connected to the PLR and

732

733              1.  the BFER can be reached from it through the BIER domain

734              2.  both the path from the PLR to it and the path from it to the

735                  BFER are disjoint with the primary path from the PLR to the

736                  primary BFR-NBR.  These paths

737

738                  -  may contain the primary BFR-NBR for link protection, and

739

740                  -  must not contain the primary BFR-NBR for node protection.

741

742           *  A BFR is a remote BIER-LFA for a specific BFER if it is not

743              directly connected to the PLR, if it can be reached via a tunnel

744              from the PLR, and if it also satisfies the aforementioned

745              conditions 1 and 2.

746

747           *  A BFR is a TI-BIER-LFA for a specific BFER if it is not directly

748              connected to the PLR, if it cannot be reached via a tunnel from

749              the PLR, if it is reachable from the PLR via an explicit path

750              (i.e., with the help of a SR header), and if it also satisfies the

751              aforementioned conditions 1 and 2.

752

753           For some BFERs, one or more normal BIER-LFAs are available at a

754           specific PLR.  For other BFERs, only remote and TI-LFAs are

755           available.  And there may be some BFERs, for which only TI-LFAs are

756           available.

757

758           The backup actions to reroute BIER packets depending on the BIER-LFA

759           types are:

760

761           *  For normal BIER-LFA: Plain

762

763           *  For remote BIER-LFA: Tunnel

764

765           *  For TI-BIER-LFA: Explicit

766

767        5.2.3.  Protection Coverage of BIER-LFA Types

768

769           The protection coverage is the set of BFERs that can be protected

770           with a desired protection level by a certain BIER-LFA type.  The

771           BIER-LFA types have the following properties:

772

773           *  Normal BIER-LFAs

774

775              -  The protection coverage is the least because some or many BFERs

776                 cannot be protected with the desired protection level or even

777                 not at all.

778

779              -  Redundant packet copies are avoided.

780

781              -  No encapsulation overhead.

782

783           *  Remote BIER-LFAs

784

785              -  They increase the protection coverage of normal BIER-LFAs.

786

787              -  Redundant packet copies may occur on a link similar to tunnel-

788                 based BIER-FRR.

789

790              -  Same encapsulation overhead as with tunnel-based BIER-FRR.

791

792           *  TI-BIER-LFAs

793

794              -  They complement the protection coverage of normal and remote

795                 BIER-LFAs to 100%.

796

797              -  Redundant packets may occur on a link similar to tunnel-based

798                 BIER-FRR.

799

800              -  Same or similar encapsulation overhead as with tunnel-based

801                 BIER-FRR depending on the FRR mechanism in the routing

802                 underlay.



[minor]

proposed rewrite:



"

5.2. LFA-based BIER-FRR



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.



5.2.1. Relation of BIER-LFAs to IP-LFAs and Prerequisites



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 [RFC5286], [RFC7490], and [I-D.ietf-rtgwg-segment-routing-ti-lfa], 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.



5.2.2. Definition of BIER-LFAs



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:



  1. the BFER can be reached from it through the BIER domain.

  2. 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



5.2.3. Protection Coverage of BIER-LFA Types



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.

"



804        5.2.4.  Sets of Supported BIER-LFAs

805

806           Normal BIER-LFAs are simplest, as they require neither tunneling nor

807           explicit paths.  Remote BIER-LFAs are more powerful, but entail more

808           header overhead and require more functionality from the PLR.  TI-

809           BIER-LFAs are most complex as they require the use of explicit paths.

810           When LFA-based BIER-FRR is utilized, the set of supported BIER-LFAs

811           must be indicated.  The following options are available:

812

813           *  Option 1: only normal BIER-LFAs are supported

814

815           *  Option 2: normal and remote BIER-LFAs are supported

816

817           *  Option 3: all BIER-LFA types are supported



[minor]

proposed rewrite:



"

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.

"



819        5.2.5.  Link Protection

820

821           With link protection, normal BIER-LFAs are preferred over remote LFAs

822           and remote BIER-LFAs are preferred over TI-BIER-LFAs.  Depending on

823           the set of supported BIER-LFAs, a BFER may not be protectable.

824

825           Figure 5 illustrates B1's backup BIFT for LFA-based BIER-FRR with

826           link protection in the networking example of Figure 2.

827

828           If the link B1-B6 fails, B1 cannot reach the BFERs B4, B5, B6, and B7

829           over their primary BFR-NBR.  Therefore, B1 sends their traffic via

830           the backup BFR-NBR B2 together with the traffic for B2 and B3 as B2

831           is their primary BFR-NBR.  As a consequence, the backup F-BM is

832           1111110 in that case.  Likewise, if the link B1-B2 fails, B1 sends

833           all traffic to B6, and the backup F-BM is 1111110 also in that case.

834

835           B1 requires only normal BIER-LFAs to protect all BFERs.  This can be

836           substantially different for other BFRs.  Figure 9 and Figure 10 show

837           the backup BIFTs for B7 and B5 respectively.  BFR B7 requires one

838           normal BIER-LFA, three remote BIER-LFAs, and two TI-BIER-LFAs to

839           protect all BFERs.  And BFR B5 requires even one normal BIER-LFA, one

840           remote BIER-LFA, and four TI-BIER-LFAs as backup BFR-NBRs.  Thus,

841           depending on the set of supported BIER-LFAs, a BFER cannot be

842           protected by BIER-FRR.

843

844           We now assume B7 has a BIER packet with destinations {B1, B4, B5,

845           B6}.  When link B7-B6 fails, the packet copy for B1 is sent to B2

846           using forwarding action Plain, the packet copy to B4 is tunneled via

847           B2 to B3, and the packet copies towards B5 and B6 are sent via

848           explicit paths towards B4 and B1 respectively.  As these packet

849           copies have different headers, they all need to be sent.  Hence, we

850           observe three redundant copies.

851

852               +------+----------+--------+-----------+---+-----------------+

853               |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

854               |      |          |        |           |   |  failure of     |

855               +======+==========+========+===========+===+=================+

856               |   1  | 0000111  |   B2   |  Plain    |   |  Link B7->B6    |

857               +------+----------+--------+-----------+---+-----------------+

858               |   2  | 0000110  |   B1   |  Tunnel   |   |  Link B1->B2    |

859               +------+----------+--------+-----------+---+-----------------+

860               |   3  | 0000110  |   B1   |  Tunnel   |   |  Link B1->B2    |

861               +------+----------+--------+-----------+---+-----------------+

862               |   4  | 0001000  |   B3   |  Tunnel   |   |  Link B1->B6    |

863               +------+----------+--------+-----------+---+-----------------+

864               |   5  | 0010000  |   B4   |  Explicit |   |  Link B1->B6    |

865               +------+----------+--------+-----------+---+-----------------+

866               |   6  | 0100000  |   B1   |  Explicit |   |  Link B1->B6    |

867               +------+----------+--------+-----------+---+-----------------+

868

869                      Figure 9: B7's backup BIFT with link protection.

870

871               +------+----------+--------+-----------+---+-----------------+

872               |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

873               |      |          |        |           |   |  failure of     |

874               +======+==========+========+===========+===+=================+

875               |   1  | 1100011  |   B3   |  Explicit |   |  Link B5->B6    |

876               +------+----------+--------+-----------+---+-----------------+

877               |   2  | 1100011  |   B3   |  Explicit |   |  Link B5->B6    |

878               +------+----------+--------+-----------+---+-----------------+

879               |   3  | 0000100  |   B4   |  Plain    |   |  Link B5->B6    |

880               +------+----------+--------+-----------+---+-----------------+

881               |   4  | 0001000  |   B3   |  Tunnel   |   |  Link B5->B4    |

882               +------+----------+--------+-----------+---+-----------------+

883               |   6  | 1100011  |   B3   |  Explicit |   |  Link B5->B6    |

884               +------+----------+--------+-----------+---+-----------------+

885               |   7  | 1100011  |   B3   |  Explicit |   |  Link B5->B6    |

886               +------+----------+--------+-----------+---+-----------------+

887

888                     Figure 10: B5's backup BIFT with link protection.



[minor]

proposed rewrite:



"

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.



Figure 5 illustrates B1's backup BIFT for LFA-based BIER-FRR with link protection, using the network example provided in Figure 2.



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. Figures 9 and 10 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.



   +------+----------+--------+-----------+---+-----------------+

   |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

   |      |          |        |           |   |  failure of     |

   +======+==========+========+===========+===+=================+

   |   1  | 0000111  |   B2   |  Plain    |   |  Link B7->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    |

   +------+----------+--------+-----------+---+-----------------+



   Figure 9: B7's backup BIFT with link protection.



   +------+----------+--------+-----------+---+-----------------+

   |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

   |      |          |        |           |   |  failure of     |

   +======+==========+========+===========+===+=================+

   |   1  | 1100011  |   B3   |  Explicit |   |  Link B5->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    |

   +------+----------+--------+-----------+---+-----------------+



   Figure 10: B5's backup BIFT with link protection.

"



890        5.2.6.  Node Protection

891

892           To determine the backup forwarding entries with node protection, a

893           case analysis for the BFER to protect is needed again.  If the BFER

894           is the same as its primary BFR-NBR, node protection is not possible

895           for that BFER.  In this case, link protection is applied.  Otherwise,

896           the BFER must be protected by a node-protecting BIER-LFA.  Thereby,

897           normal BIER-LFAs are preferred over remote BIER-LFAs and remote BIER-

898           LFAs are preferred over TI-BIER-LFAs.  Depending on the set of

899           allowed BIER-LFAs, a BFER may not be protectable.

900

901           Figure 11 illustrates B1's backup BIFT for the LFA-based BIER-FRR

902           with node protection in the networking example of Figure 2.

903

904               +------+----------+--------+-----------+---+-----------------+

905               |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

906               |      |          |        |           |   |  failure of     |

907               +======+==========+========+===========+===+=================+

908               |   2  | 1111010  |   B6   |  Plain    |   |  BFR-NBR B2     |

909               +------+----------+--------+-----------+---+-----------------+

910               |   3  | 0000100  |   B4   |  Tunnel   |   |  BFR-NBR B2     |

911               +------+----------+--------+-----------+---+-----------------+

912               |   4  | 0001000  |   B3   |  Tunnel   |   |  BFR-NBR B6     |

913               +------+----------+--------+-----------+---+-----------------+

914               |   5  | 0010000  |   B4   |  Explicit |   |  BFR-NBR B6     |

915               +------+----------+--------+-----------+---+-----------------+

916               |   6  | 1100100  |   B2   |  Plain    |   |  BFR-NBR B6     |

917               +------+----------+--------+-----------+---+-----------------+

918               |   7  | 1100100  |   B2   |  Plain    |   |  BFR-NBR B6     |

919               +------+----------+--------+-----------+---+-----------------+

920

921                     Figure 11: B1's backup BIFT with node protection.

922

923           As the primary BFR-NBR of B1 for BFER B6 is B6 itself, only link

924           protection can be applied.  Therefore, B2 is used as normal, link-

925           protection BIER-LFA to protect B6.  Likewise, the primary BFR-NBR of

926           B1 for BFER B2 is B2, and therefore, B2 is protected with B6 as

927           normal, link-protecting BIER-LFA.  BFER B7 is protected against the

928           failure of node B6 with B2 as normal, node-protecting BIER-LFA as B2

929           has a shortest path towards B7 that does not traverse B6.  The backup

930           F-BMs for BFER 6 and BFER 7 are {B2, B6, B7} because if B6 is

931           unreachable, the traffic for these BFERs is sent via link B1-B2 with

932           forwarding action Plain.

933

934           BFER B4 is not reachable through a normal LFA when BFR B6 fails.

935           However, B3 is a remote, node-protecting BIER-LFA for BFER B4 because

936           B3 has a shortest path towards B4, and B3 is reachable through a

937           shortest path from B1, and the resulting backup path from B1 to B4

938           does not traverse B6.  Likewise, B4 is a remote LFA for BFER B3 if

939           BFR B2 fails.

940

941           BFER B5 is neither reachable through a normal BIER-LFA nor through a

942           remote BIER-LFA when BFR B6 fails.  However, B4 is a node-protecting

943           TI-LFA for BFER B5 because B4 has a shortest path towards B5 that

944           does not traverse B6.  Moreover, B4 is reachable through the explicit

945           path B1-B2-B3-B4.



[minor]

proposed rewrite:



"

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.



Figure 11 illustrates B1's backup BIFT for LFA-based BIER-FRR with node protection, using the network example provided in Figure 2.



   +------+----------+--------+-----------+---+-----------------+

   |BFR-id|  BF-BM   |BBFR-NBR|   BFA     |BEA|Comment: protects|

   |      |          |        |           |   |  failure of     |

   +======+==========+========+===========+===+=================+

   |   2  | 1111010  |   B6   |  Plain    |   |  BFR-NBR B2     |

   +------+----------+--------+-----------+---+-----------------+

   |   3  | 0000100  |   B4   |  Tunnel   |   |  BFR-NBR B2     |

   +------+----------+--------+-----------+---+-----------------+

   |   4  | 0001000  |   B3   |  Tunnel   |   |  BFR-NBR B6     |

   +------+----------+--------+-----------+---+-----------------+

   |   5  | 0010000  |   B4   |  Explicit |   |  BFR-NBR B6     |

   +------+----------+--------+-----------+---+-----------------+

   |   6  | 1100100  |   B2   |  Plain    |   |  BFR-NBR B6     |

   +------+----------+--------+-----------+---+-----------------+

   |   7  | 1100100  |   B2   |  Plain    |   |  BFR-NBR B6     |

   +------+----------+--------+-----------+---+-----------------+



   Figure 11: B1's backup BIFT with node protection.



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.

"



947        5.2.7.  Optimization Potential to Reduce Redundant BIER Packets in

948                Failure Cases

949

950           Redundant packets occur with LFA-based BIER-FRR if BIER packets are

951           sent over a specific link in different forms.  These forms are

952           *  plain BIER packets (plain primary transmission or reroute to

953              normal BIER-LFA)

954

955           *  BIER packets encapsulated to a specific BFR-NBR (tunneled primary

956              transmission or reroute to remote BIER-LFA)

957

958           *  BIER packets with an encoded explicit path (reroute to TI-LFA)

959

960           When different remote LFAs are addressed, even multiple redundant

961           packets can be caused through remote LFAs.  The same can happen with

962           TI-LFAs.  Some redundant packets can be avoided if remote LFAs or TI-

963           LFAs are chosen such that they can protect several BFERs and thereby

964           avoid the need for another remote LFA or TI-LFA.  However, this may

965           lead to longer backup paths.  This is a new, potential optimization

966           objective for the choice of remote or TI-BIER-LFAs which does not

967           exist for IP-FRR.  Its relevance may depend on the use case.

968

969           We illustrate this optimization potential.  We consider LFA-based

970           BIER-FRR with link protection for B7.  Its backup BIFT is given in

971           Figure 9.  As observed in Section 5.2.5, B7 needs to send four copies

972           to forward a packet to {B1, B4, B5, B6}.  If we choose the more

973           complex TI-BIER-LFA B4 to protect BFER B4 instead of the remote BIER-

974           LFA B3, then only two redundant copies need to be sent.



[minor]

proposed rewrite:



"

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 Figure 9. As noted in Section 5.2.5, 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.

"



976        6.  Comparison

977

978           This section first discusses the difference of IP-LFAs for IP-FRR and

979           BIER-LFAs for BIER-FRR.  Then it discusses advantages and

980           disadvantages of tunnel-based and LFA-based BIER-FRR.

981

982        6.1.  Comparison of LFA-Based Protection for IP-FRR and BIER-FRR

983

984           LFAs have been first proposed for IP networks.  They are simple in

985           the sense that they do not require any tunneling overhead.  However,

986           some destinations cannot be protected against some link failures and

987           even more destinations cannot be protected against some node

988           failures.  Therefore, remote LFAs (R-LFAs) have been defined to

989           improve that coverage by tunneling the affected traffic to another

990           node from where the traffic can reach the destination via normal

991           forwarding.  Nevertheless, there may be still some destinations that

992           cannot be protected against link or node failures.  Therefore,

993           topology-independent LFAs (TI-LFAs) have been defined where affected

994           traffic is tunneled via an explicit path (preferably using segment

995           routing headers) to another node from where the traffic can reach its

996           destination via normal IP forwarding.  With TI-LFAs, all destinations

997           can be protected against any failures as long as connectivity exists.

998

999           LFA-based BIER-FRR adopts the idea of LFAs.  It differs from IP-FRR

1000        as the LFA target node, i.e., the node to which the traffic is

1001        deviated, must be a BFR.  If an IP-LFA target is a BFR, it can be

1002        utilized as a BIER-LFA; otherwise it cannot serve as BIER-LFA.  Thus,

1003        if only some nodes of the underlay are BFRs, the BIER-LFAs will be

1004        substantially different from IP-LFAs.  Moreover, this makes it more

1005        difficult to find normal LFAs for which tunneling is not needed.

1006        That means, LFA-based BIER-FRR is likely to require more remote LFAs

1007        and TI-LFAs than IP-FRR under such conditions.



[minor]

proposed rewrite:



"

6. Comparison



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.



6.1. Comparison of LFA-Based Protection for IP-FRR and 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.

"



1009     6.2.  Advantages and Disadvantages of Tunnel-Based BIER-FRR

1010

1011     6.2.1.  Advantages

1012

1013        *  Computation of backup forwarding entries is very simple.  Only

1014           primary BIFTs of a PLR and its BFR-NBRs are needed to compute the

1015           backup forwarding entries.  Routing information from the routing

1016           underlay is not needed.

1017

1018        *  The forwarding action Explicit is not needed.  However, depending

1019           on the underlay, explicit forwarding may be used to achieve FRR in

1020           the underlay.

1021

1022     6.2.2.  Disadvantages

1023

1024        *  It requires a FRR mechanism on the underlay.

1025

1026        *  It is limited to the protection level of the underlay.  E.g., if

1027           the underlay supports only link protection, tunnel-based BIER-FRR

1028           cannot provide node protection.

1029

1030        *  Redundant packet copies may occur in tunnel-based BIER-FRR.

1031

1032        *  In case of node protection, backup paths may be extended more than

1033           needed.

1034

1035        *  Requires a tunneling header for any rerouting, which creates

1036           header overhead.

1037

1038     6.3.  Advantages and Disadvantages of LFA-Based BIER-FRR

1039

1040     6.3.1.  Advantages

1041

1042        *  Does not rely on any fast protection of the underlay.

1043

1044        *  Can provide better protection on the BIER layer than on the IP

1045           layer; this is possible if LFA-based BIER-FRR utilizes BIER-LFAs

1046           with better protection level than LFA-based IP-FRR.  E.g., the

1047           underlay may provide only FRR with link protection while BIER-FRR

1048           may provide node protection for BIER traffic.

1049

1050        *  Avoids header overhead for normal BIER-LFAs.

1051

1052     6.3.2.  Disadvantages

1053

1054        *  Computation of backup forwarding entries requires routing

1055           information from the underlay.

1056

1057        *  Computation of backup forwarding entries more complex if some

1058           nodes of the underlay are not BFRs.

1059

1060        *  Need for forwarding action Tunnel to protect some BFERs, which

1061           adds header overhead.

1062

1063        *  Need for forwarding action Explicit to achieve full protection

1064           coverage for some topologies; otherwise only partial protection

1065           coverage.  This requires support for explicit paths, e.g., segment

1066           routing.

1067

1068        *  More remote and TI-LFAs needed than for IP-FRR if some nodes in

1069           the routing underlay are not BFRs.

1070

1071        *  Redundant packet copies may occur in LFA-based BIER-FRR (but it's

1072           less than with tunnel-based BIER-FRR).



[minor]

proposed rewrite:



"

6.2. Advantages and Disadvantages of Tunnel-Based BIER-FRR



6.2.1. Advantages



* 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.



6.2.2. Disadvantages



* 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.



6.3. Advantages and Disadvantages of LFA-Based BIER-FRR



6.3.1. Advantages



* 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.



6.3.2. Disadvantages



* 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.

"



1074     7.  Security Considerations

1075

1076        This document does not introduce any new security issues beyond those

1077        discussed in BIER architecture [RFC8279].



[major]

The security section looks very compact. Is this really correct?

This work suggests to use tunnels and various flavours of LFA for example.

Do these not have securty considerations that impact BIER traffic?



for example have a look at some key LFA security considerations and reflect relevance when applied to BIER:



1. Spoofing and False Route Advertisements

* LFA/R-LFA/TI-LFA Dependencies on Routing Information:

  - LFAs depend on accurate routing information to determine alternate paths. If an attacker can inject false routing information (e.g., by spoofing link-state advertisements), it could cause the network to select suboptimal or malicious paths for LFAs.

  - R-LFA and TI-LFA also depend on accurate routing information, particularly for determining the tunneling paths or explicit paths. False route advertisements could mislead the network into using insecure or compromised paths.



2. Man-in-the-Middle (MitM) Attacks

* Use of Alternate Paths:

  - By rerouting traffic through alternate paths, especially those that traverse multiple hops (as in R-LFA and TI-LFA), the risk of MitM attacks increases if any of the intermediate routers on the alternate path are compromised.

  - TI-LFA, which uses explicit paths, might expose traffic to routers that were not originally in the primary path, potentially allowing for interception or alteration of the traffic.



3. Confidentiality and Integrity

* Traffic Encapsulation:

  - R-LFA and TI-LFA involve encapsulating traffic, which may expose it to vulnerabilities if the encapsulation mechanisms are not secure. For instance, if IPsec or another secure encapsulation method is not used, an attacker might be able to intercept or alter the traffic in transit.

* Protection of Explicit Paths:

  - TI-LFA relies on explicit paths that are typically defined using segment routing. If these paths are not properly protected, an attacker could manipulate the segment list to reroute traffic through malicious nodes.



4. Increased Attack Surface

* Extended Topology:

  - By introducing LFAs, R-LFAs, and TI-LFAs, the network increases its reliance on additional routers and links, thereby expanding the potential attack surface. Compromise of any router in these alternate paths could expose traffic to unauthorized access or disruption.



5. Complexity and Configuration Errors

* Misconfigurations:

  - The complexity of configuring LFAs, R-LFAs, and TI-LFAs increases the risk of misconfigurations, which could inadvertently create security vulnerabilities, such as routing loops or the selection of insecure paths.

* Overhead and Resource Exhaustion:

  - The additional overhead introduced by encapsulation in R-LFA and the use of segment routing in TI-LFA can be exploited by attackers to exhaust network resources, such as CPU and memory, on routers.



6. Trust Model

* Trust in Routing Infrastructure:

  - The security of LFA, R-LFA, and TI-LFA mechanisms relies heavily on the trustworthiness of the underlying routing infrastructure. If the control plane is compromised (e.g., through route poisoning), the alternate paths selected by these mechanisms could lead to traffic being routed through untrusted or compromised parts of the network.



7. Securing the Routing Protocols

* Use of Secure Routing Protocols:

  - To mitigate the risks associated with false route advertisements and MitM attacks, it is recommended to use secure routing protocols (e.g., OSPFv3 with IPsec, or ISIS HMAC-SHA256 ) that provide authentication and integrity protection for routing updates.



8. Monitoring and Detection

* Anomaly Detection:

  - Implementing monitoring and anomaly detection mechanisms can help identify when traffic is being rerouted in unexpected ways, which could indicate a security issue with LFA, R-LFA, or TI-LFA mechanisms.









Brgds,

Gunter Van de Velde

Shepherding AD