BIER Fast ReRoute
draft-ietf-bier-frr-05
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Authors | Huaimo Chen , Mike McBride , Steffen Lindner , Michael Menth , Aijun Wang , Gyan Mishra | ||
| Last updated | 2025-02-18 (Latest revision 2024-02-01) | ||
| Replaces | draft-merling-bier-frr, draft-chen-bier-frr | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Zheng Zhang | ||
| Shepherd write-up | Show Last changed 2024-05-22 | ||
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| Responsible AD | Gunter Van de Velde | ||
| Send notices to | zhang.zheng@zte.com.cn |
draft-ietf-bier-frr-05
Network Working Group H. Chen, Ed.
Internet-Draft M. McBride
Intended status: Informational Futurewei
Expires: 22 August 2025 S. Lindner
M. Menth
University of Tuebingen
A. Wang
China Telecom
G. Mishra
Verizon Inc.
18 February 2025
BIER Fast ReRoute
draft-ietf-bier-frr-05
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.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] [RFC8174]
when, and only when, they appear in all capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 August 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Definition of BIER-FRR . . . . . . . . . . . . . . . . . . . 6
3.1. Definition of Forwarding Actions . . . . . . . . . . . . 6
3.2. Definition of Backup Forwarding Entries . . . . . . . . . 6
3.3. Activating and Deactivating Backup Forwarding Entries . . 7
3.4. Computation of the Backup F-BM . . . . . . . . . . . . . 8
4. Representations for BIER-FRR Forwarding Data . . . . . . . . 8
4.1. Potential Emergence of Redundant Packets . . . . . . . . 8
4.2. Primary BIFT and Single Backup BIFT . . . . . . . . . . . 10
4.3. Primary BIFT and Failure-Specific Backup BIFTs . . . . . 12
5. Protection Levels . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Link Protection . . . . . . . . . . . . . . . . . . . . . 13
5.2. Node Protection . . . . . . . . . . . . . . . . . . . . . 13
5.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Backup Strategies . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Tunnel-Based BIER-FRR . . . . . . . . . . . . . . . . . . 14
6.1.1. Tunnel-Based BIER-FRR with Link Protection . . . . . 14
6.1.2. Tunnel-Based BIER-FRR with Node Protection . . . . . 16
6.2. LFA-based BIER-FRR . . . . . . . . . . . . . . . . . . . 17
6.2.1. Relation of BIER-LFAs to IP-LFAs and Prerequisites . 17
6.2.2. Definition of BIER-LFAs . . . . . . . . . . . . . . . 18
6.2.3. Protection Coverage of BIER-LFA Types . . . . . . . . 19
6.2.4. Sets of Supported BIER-LFAs . . . . . . . . . . . . . 19
6.2.5. Link Protection . . . . . . . . . . . . . . . . . . . 20
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6.2.6. Node Protection . . . . . . . . . . . . . . . . . . . 21
6.2.7. Optimization Potential to Reduce Redundant BIER Packets
in Failure Cases . . . . . . . . . . . . . . . . . . 23
7. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Comparison of LFA-Based Protection for IP-FRR and
BIER-FRR . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2. Advantages and Disadvantages of Tunnel-Based BIER-FRR . . 24
7.2.1. Advantages . . . . . . . . . . . . . . . . . . . . . 24
7.2.2. Disadvantages . . . . . . . . . . . . . . . . . . . . 24
7.3. Advantages and Disadvantages of LFA-Based BIER-FRR . . . 24
7.3.1. Advantages . . . . . . . . . . . . . . . . . . . . . 25
7.3.2. Disadvantages . . . . . . . . . . . . . . . . . . . . 25
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Specific Backup Strategy Examples . . . . . . . . . 27
A.1. LFA-based BIER-FRR using Single BIFT . . . . . . . . . . 27
A.2. LFA-based BIER-FRR using Multiple Backup BIFTs . . . . . 28
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 30
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
With BIER [RFC8279], a Bit-Forwarding Router (BFR) forwards BIER
packets based on a bitstring in the BIER header using the information
in the Bit Index Forwarding Table (BIFT). Its entries are locally
derived from a routing underlay or set by a controller. In case of a
persistent link or node failure, BIER traffic may not be delivered
until the BIFT has been updated based on the reconverged routing
underlay or by the controller.
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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
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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].
2. Terminology
This document uses the following definitions:
BIER: Bit Indexed Explicit Routing
BIER FRR: Bit Indexed Explicit Routing Fast ReRoute
BFR: Bit-Forwarding Router
BFR-NBR: Bit-Forwarding Neighbor
BFIR: Bit-Forwarding Ingress Router
BFER: Bit-Forwarding Egress Router
BIFT: Bit Index Forwarding Table
F-BM: Forwarding Bit Mask
PLR: Point of Local Repair
LFA: Loop Free Alternate
BF-BM: Backup F-BM
BBFR-NBR: Backup BFR-NBR
BFA: Backup Forwarding Action
BEA: Backup Entry Active
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3. Definition of BIER-FRR
In this section, forwarding actions and backup forwarding entries are
defined. Then, the BIER forwarding process with BIER-FRR and the
computation of the backup F-BM are explained.
3.1. Definition of Forwarding Actions
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.
3.2. Definition of Backup Forwarding Entries
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)
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* 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 3.1. 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 Section 6.1 and Section 6.2. The Backup
Forwarding Bit Mask (BF-BM) is determined based on the backup BFR-
NBR, and its computation is described in Section 3.4.
3.3. Activating and Deactivating Backup Forwarding Entries
When a primary BFR-NBR is not reachable over the implicit primary
action, a failure is observed. Then, the BEA flag of the
corresponding backup forwarding entry is set.
If the primary BFR-NBR is directly connected, the information about
the failed interface is sufficient to detect its unreachability. If
the primary BFR-NBR is indirectly connected, a BFD session between
the BFR as PLR and the BFR-NBR may be used to monitor its
reachability.
If the primary BFR-NBR is reachable again, the BEA flag is
deactivated. This may be caused by the disappearance of the failure
or by a change of the primary BFR-NBR due to a reconfiguration of the
BIFT.
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3.4. Computation of the Backup F-BM
The primary F-BM of a specific BFER identifies all BFERs that share
the same primary Bit-Forwarding Router Neighbor (BFR-NBR). The
backup F-BM for a specific BFER is computed to indicate:
* All BFERs that share both the primary and backup BFR-NBRs of the
specific BFER, and
* All BFERs for which the backup BFR-NBR of the specific BFER serves
as the primary BFR-NBR.
4. Representations for BIER-FRR Forwarding Data
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 3.2). 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).
4.1. Potential Emergence of Redundant Packets
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 3 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.
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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.
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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.
4.2. Primary BIFT and Single Backup BIFT
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.
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+--------+---------+---------+
| 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.
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4.3. Primary BIFT and Failure-Specific Backup BIFTs
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.
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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.
5. Protection Levels
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 6.
5.1. Link Protection
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.
5.2. Node Protection
In node protection, the backup path is designed to avoid both the
failed node and the link to that node (i.e., the failed primary path
from the PLR to the primary BFR-NBR, including the primary BFR-NBR).
Consequently, the backup path must exclude both the primary path and
the primary BFR-NBR to remain operational in the event of their
failure. If a BFER and its primary BFR-NBR are the same, only link
protection is feasible for that BFER. The primary advantage of node
protection is its enhanced protection quality compared to link
protection. However, node protection also has certain drawbacks. It
typically results in longer backup paths than link protection. In
the context of tunnel-based BIER-FRR, node protection may lead to the
transmission of a greater number of redundant packets over a link
than with link protection. Furthermore, for LFA-based BIER-FRR,
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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.
5.3. Example
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 6.
6. 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.
6.1. 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.
6.1.1. Tunnel-Based BIER-FRR with Link Protection
With link protection, the backup BFR-NBRs equal the primary BFR-NBRs.
If a primary BFR-NBR is directly connected to the BFR as a PLR, the
corresponding backup forwarding action is Tunnel. As a result, the
BIER packets affected by a failure are tunneled over the routing
underlay to their BFR-NBR instead of being sent directly as plain
BIER packets to the BFR-NBR. If a primary BFR-NBR is not directly
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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 the same as the primary F-BMs, which is in line with the
computation of the backup F-BMs in Section 3.4.
+------+----------+--------+-----------+---+-----------------+
|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 shows B1's backup BIFT for tunnel-based BIER-FRR with link
protection for the BIER network example of Figure 2. The backup BFR-
NBRs and backup F-BMs in this backup BIFT are the same as the primary
BFR-NBRs and primary F-BMs in the primary BIFT in Figure 4, but the
backup actions in this backup BIFT are Tunnel while the primary
actions in the primary BIFT are Plain (which are not shown, but
implied).
When B1 as a PLR detects failure of its link to B6, a BIER packet
with bitstring 0100000 for B6 is tunneled by B1 towards B6 via the
routing underlay. The exact path of the backup tunnel depends on the
routing underlay. It may be B1-B2-B7-B6 or B1-B2-B3-B4-B5-B6.
If a BIER packet is destined to {B2, B5, B7}, first an encapsulated
packet copy is forwarded via link B1-B2 to backup BFR-NBR B6 with
backup action Tunnel to deliver packet copies to BFER B5 and B7.
Then, a non-encapsulated packet copy is forwarded via link B1-B2 to
BFR-NBR B2 with primary action Plain to deliver a packet copy to BFER
B2. Thus, with tunnel-based BIER-FRR, a single redundant packet copy
can occur in case of a failure because an encapsulated packet copy
and a non-encapsulated packet copy are forwarded over the same link.
This happens although BIER packets affected by failures are forwarded
before BIER packets not affected by failures.
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A BIER packet with bitstring 1000000 for B7 is forwarded on the
backup path B1-B2-B7-B6-B7 as it is first delivered to the backup
BFR-NBR B6. Thus, the backup path can be unnecessarily long. This
phenomenon is known from facility backup method in [RFC4090] which
takes similar paths as tunnel-based BIER-FRR.
6.1.2. Tunnel-Based BIER-FRR with Node Protection
To determine the backup forwarding entries with node protection, a
case analysis for the BFER to protect is needed. If the BFER is the
same as its primary BFR-NBR, node protection is not possible for that
BFER. Therefore, link protection is applied, i.e., the backup BFR-
NBR is set to the primary BFR-NBR. If that level of protection is
not sufficient, egress protection in [I-D.chen-bier-egress-protect]
may be applied. Otherwise (i.e., 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, i.e., the backup BFR-NBR is a next
next hop BFR. In all cases, the backup action is Tunnel. In the
first case, the backup F-BM is set to all zeroes plus the bit enabled
for the BFER to protect. In the second case, the backup F-BM is
computed in the way described in Section 3.4.
+------+----------+--------+----------+---+-----------------+
|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 shows B1's backup BIFT for tunnel-based BIER-FRR with node
protection for the BIER network example 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 them are those nodes themselves.
According to the description above, only the bit for B2 is set in the
backup F-BM of B2. The same holds for B6. For BFER B5, the backup
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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 as a PLR detects the
failure of its BFR-NBR B6, a BIER packet with bitstring 1010010 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 on link
B1-B2. Thus, with node protection, more redundant packets copies may
be sent than with link protection.
Caveat: If the routing underlay does not provide node protection,
tunnel-based BIER-FRR cannot provide node protection, either. This
is shown by the following example. The underlay in the networking
example of Figure 2 offers only link protection. B6 fails and B1
must forward a packet to B5. According to the backup BIFT in
Figure 8 the packet is tunneled towards B5 and the tunnel path
B1-B2-B7-B6-B5 may be taken for this purpose by the underlay due to
FRR with link protection. However, B6 is also unreachable at B7 so
that the packet is returned to B2 and the packet loops between B2 and
B7.
6.2. 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. It does not
rely on any fast restoration/protection mechanisms in the underlay.
First, some prerequisites for LFA-based BIER-FRR are clarified, BIER-
LFAs are defined, and then link and node protection for LFA-based
BIER-FRR are discussed using a single backup BIFT.
6.2.1. Relation of BIER-LFAs to IP-LFAs and Prerequisites
A loop-free alternate (LFA) for a specific destination is an
alternate node to which a packet is sent if the primary next hop for
this destination is not reachable. This alternate node should be
able to forward 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]. Such LFAs are denoted as
IP-LFAs. BIER-LFAs are very 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 needed. This is a
difference to tunnel-based BIER-FRR where knowledge about the primary
BIFTs of a PLR and its BFR-NBRs is sufficient.
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LFA-based BIER-FRR may reuse IP-LFAs in the following sense as BIER-
LFAs. If an IP-LFA node for the destination of a specific BFER is a
BFR, it may be reused as backup BFR-NBR for that BFER together with
the backup action that is applied for that IP-LFA on the IP layer. A
normal IP-LFA corresponds to backup action plain, a remote IP-LFA to
Tunnel, and a TI-IP-LFA to Explicit.
6.2.2. Definition of BIER-LFAs
As for IP-LFAs, there are several, different types of BIER-LFAs:
* A BFR is 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 it and the path from it to the
BFER are disjoint with the primary path from the PLR to the
primary BFR-NBR. These paths
- may contain the primary BFR-NBR for link protection, and
- must not contain the primary BFR-NBR for node protection.
* A BFR is a remote BIER-LFA for a specific BFER if it is not
directly connected to the PLR, if it can be reached via a tunnel
from the PLR, and if it also satisfies the aforementioned
conditions 1 and 2.
* A BFR is a TI-BIER-LFA for a specific BFER if it is not directly
connected to the PLR, if it cannot be reached via a tunnel from
the PLR, if it is reachable from the PLR via an explicit path
(i.e., with the help of a SR header), and if it also satisfies the
aforementioned conditions 1 and 2.
For some BFERs, one or more normal BIER-LFAs are available at a
specific PLR. For other BFERs, only remote and TI-LFAs are
available. And there may be some BFERs, for which only TI-LFAs are
available.
The backup actions to reroute BIER packets depending on the BIER-LFA
types are:
* For normal BIER-LFA: Plain
* For remote BIER-LFA: Tunnel
* For TI-BIER-LFA: Explicit
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6.2.3. Protection Coverage of BIER-LFA Types
The protection coverage is the set of BFERs that can be protected
with a desired protection level by a certain BIER-LFA type. The
BIER-LFA types have the following properties:
* Normal BIER-LFAs
- The protection coverage is the least because some or many BFERs
cannot be protected with the desired protection level or even
not at all.
- Redundant packet copies are avoided.
- No encapsulation overhead.
* Remote BIER-LFAs
- They increase the protection coverage of normal BIER-LFAs.
- Redundant packet copies may occur on a link similar to tunnel-
based BIER-FRR.
- Same encapsulation overhead as with tunnel-based BIER-FRR.
* TI-BIER-LFAs
- They complement the protection coverage of normal and remote
BIER-LFAs to 100%.
- Redundant packets may occur on a link similar to tunnel-based
BIER-FRR.
- Same or similar encapsulation overhead as with tunnel-based
BIER-FRR depending on the FRR mechanism in the routing
underlay.
6.2.4. Sets of Supported BIER-LFAs
Normal BIER-LFAs are simplest, as they require neither tunneling nor
explicit paths. Remote BIER-LFAs are more powerful, but entail more
header overhead and require more functionality from the PLR. TI-
BIER-LFAs are most complex as they require the use of explicit paths.
When LFA-based BIER-FRR is utilized, the set of supported BIER-LFAs
must be indicated. The following options are available:
* Option 1: only normal BIER-LFAs are supported
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* Option 2: normal and remote BIER-LFAs are supported
* Option 3: all BIER-LFA types are supported
6.2.5. Link Protection
With link protection, normal BIER-LFAs are preferred over remote LFAs
and remote BIER-LFAs are preferred over TI-BIER-LFAs. Depending on
the set of supported BIER-LFAs, a BFER may not be protectable.
Figure 5 illustrates B1's backup BIFT for LFA-based BIER-FRR with
link protection in the networking example of Figure 2.
If the link B1-B6 fails, B1 cannot reach the BFERs B4, B5, B6, and B7
over their primary BFR-NBR. Therefore, B1 sends their traffic via
the backup BFR-NBR B2 together with the traffic for B2 and B3 as B2
is their primary BFR-NBR. As a consequence, the backup F-BM is
1111110 in that case. Likewise, if the link B1-B2 fails, B1 sends
all traffic to B6, and the backup F-BM is 1111110 also in that case.
B1 requires only normal BIER-LFAs to protect all BFERs. This can be
substantially different for other BFRs. Figure 9 and Figure 10 show
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. And BFR B5 requires even 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, a BFER cannot be
protected by BIER-FRR.
Assume B7 has a BIER packet with destinations {B1, B4, B5, B6}. When
link B7-B6 fails, the packet copy for B1 is sent to B2 using
forwarding action Plain, the packet copy to B4 is tunneled via B2 to
B3, and the packet copies towards B5 and B6 are sent via explicit
paths towards B4 and B1 respectively. As these packet copies have
different headers, they all need to be sent. Hence, there are three
redundant copies.
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+------+----------+--------+-----------+---+-----------------+
|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.
6.2.6. Node Protection
To determine the backup forwarding entries with node protection, a
case analysis for the BFER to protect is needed again. If the BFER
is the same as its primary BFR-NBR, node protection is not possible
for that BFER. In this case, link protection is applied. Otherwise,
the BFER must be protected by a node-protecting BIER-LFA. Thereby,
normal BIER-LFAs are preferred over remote BIER-LFAs and remote BIER-
LFAs are preferred over TI-BIER-LFAs. Depending on the set of
allowed BIER-LFAs, a BFER may not be protectable.
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Figure 11 illustrates B1's backup BIFT for the LFA-based BIER-FRR
with node protection in the networking example of 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 the primary BFR-NBR of B1 for BFER B6 is B6 itself, only link
protection can be applied. Therefore, B2 is used as normal, link-
protection BIER-LFA to protect B6. Likewise, the primary BFR-NBR of
B1 for BFER B2 is B2, and therefore, B2 is protected with B6 as
normal, link-protecting BIER-LFA. BFER B7 is protected against the
failure of node B6 with B2 as normal, node-protecting BIER-LFA as B2
has a shortest path towards B7 that does not traverse B6. The backup
F-BMs for BFER 6 and BFER 7 are {B2, B6, B7} because if B6 is
unreachable, the traffic for these BFERs is sent via link B1-B2 with
forwarding action Plain.
BFER B4 is not reachable through a normal LFA when BFR B6 fails.
However, B3 is a remote, node-protecting BIER-LFA for BFER B4 because
B3 has a shortest path towards B4, and B3 is reachable through a
shortest path from B1, and the resulting backup path from B1 to B4
does not traverse B6. Likewise, B4 is 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 is a node-protecting
TI-LFA for BFER B5 because B4 has a shortest path towards B5 that
does not traverse B6. Moreover, B4 is reachable through the explicit
path B1-B2-B3-B4.
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6.2.7. Optimization Potential to Reduce Redundant BIER Packets in
Failure Cases
Redundant packets occur with LFA-based BIER-FRR if BIER packets are
sent over a specific link in different forms. These forms are
* plain BIER packets (plain primary transmission or reroute to
normal BIER-LFA)
* BIER packets encapsulated to a specific BFR-NBR (tunneled primary
transmission or reroute to remote BIER-LFA)
* BIER packets with an encoded explicit path (reroute to TI-LFA)
When different remote LFAs are addressed, even multiple redundant
packets can be caused through remote LFAs. The same can happen with
TI-LFAs. Some redundant packets can be avoided if remote LFAs or TI-
LFAs are chosen such that they can protect several BFERs and thereby
avoid the need for another remote LFA or TI-LFA. However, this may
lead to longer backup paths. This is a new, potential optimization
objective for the choice of remote or TI-BIER-LFAs which does not
exist for IP-FRR. Its relevance may depend on the use case.
This optimization potential can be illustrated by LFA-based BIER-FRR
with link protection for B7. Its backup BIFT is given in Figure 9.
As observed in Section 6.2.5, B7 needs to send 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, then
only two redundant copies need to be sent.
7. Comparison
This section first discusses the difference of IP-LFAs for IP-FRR and
BIER-LFAs for BIER-FRR. Then it discusses advantages and
disadvantages of tunnel-based and LFA-based BIER-FRR.
7.1. Comparison of LFA-Based Protection for IP-FRR and BIER-FRR
LFAs have been first proposed for IP networks. They are simple in
the sense that they do not require any tunneling overhead. However,
some destinations cannot be protected against some link failures and
even more destinations cannot be protected against some node
failures. Therefore, remote LFAs (R-LFAs) have been defined to
improve that coverage by tunneling the affected traffic to another
node from where the traffic can reach the destination via normal
forwarding. Nevertheless, there may be still some destinations that
cannot be protected against link or node failures. Therefore,
topology-independent LFAs (TI-LFAs) have been defined where affected
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traffic is tunneled via an explicit path (preferably using segment
routing headers) to another node from where the traffic can reach its
destination via normal IP forwarding. With TI-LFAs, all destinations
can be protected against any failures as long as connectivity exists.
LFA-based BIER-FRR adopts the idea of LFAs. It differs from IP-FRR
as the LFA target node, i.e., the node to which the traffic is
deviated, 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 BIER-LFA. Thus,
if only some nodes of the underlay are BFRs, the BIER-LFAs will be
substantially different from IP-LFAs. Moreover, this makes it more
difficult to find normal LFAs for which tunneling is not needed.
That means, LFA-based BIER-FRR is likely to require more remote LFAs
and TI-LFAs than IP-FRR under such conditions.
7.2. Advantages and Disadvantages of Tunnel-Based BIER-FRR
7.2.1. Advantages
* Computation of backup forwarding entries is very simple. Only
primary BIFTs of a PLR and its BFR-NBRs are needed to compute the
backup forwarding entries. Routing information from the routing
underlay is not needed.
* The forwarding action Explicit is not needed. However, depending
on the underlay, explicit forwarding may be used to achieve FRR in
the underlay.
7.2.2. Disadvantages
* It requires a FRR mechanism on the underlay.
* It is limited to the protection level of the underlay. E.g., if
the underlay supports only link protection, tunnel-based BIER-FRR
cannot provide node protection.
* Redundant packet copies may occur in tunnel-based BIER-FRR.
* In case of node protection, backup paths may be extended more than
needed.
* Requires a tunneling header for any rerouting, which creates
header overhead.
7.3. Advantages and Disadvantages of LFA-Based BIER-FRR
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7.3.1. Advantages
* Does not rely on any fast protection of the underlay.
* Can provide better protection on the BIER layer than on the IP
layer; this is possible if LFA-based BIER-FRR utilizes BIER-LFAs
with better protection level than LFA-based IP-FRR. E.g., the
underlay may provide only FRR with link protection while BIER-FRR
may provide node protection for BIER traffic.
* Avoids header overhead for normal BIER-LFAs.
7.3.2. Disadvantages
* Computation of backup forwarding entries requires routing
information from the underlay.
* Computation of backup forwarding entries more complex if some
nodes of the underlay are not BFRs.
* Need for forwarding action Tunnel to protect some BFERs, which
adds header overhead.
* Need for forwarding action Explicit to achieve full protection
coverage for some topologies; otherwise only partial protection
coverage. This requires support for explicit paths, e.g., segment
routing.
* More remote and TI-LFAs needed than for IP-FRR if some nodes in
the routing underlay are not BFRs.
* Redundant packet copies may occur in LFA-based BIER-FRR (but it's
less than with tunnel-based BIER-FRR).
8. Security Considerations
This document does not introduce any new security issues beyond those
discussed in BIER architecture [RFC8279].
9. IANA Considerations
No requirements for IANA.
10. References
10.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015,
<https://www.rfc-editor.org/info/rfc7490>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
10.2. Informative References
[BrAl17] Braun, W., Albert, M., Eckert, T., and M. Menth,
"Performance Comparison of Resilience Mechanisms for
Stateless Multicast Using BIER", May 2017.
[I-D.chen-bier-egress-protect]
Chen, H., McBride, M., Wang, A., Mishra, G. S., Liu, Y.,
Menth, M., Khasanov, B., Geng, X., Fan, Y., Liu, L., and
X. Liu, "BIER Egress Protection", Work in Progress,
Internet-Draft, draft-chen-bier-egress-protect-07, 28
March 2024, <https://datatracker.ietf.org/doc/html/draft-
chen-bier-egress-protect-07>.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
20, 2 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-20>.
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[RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
DOI 10.17487/RFC4090, May 2005,
<https://www.rfc-editor.org/info/rfc4090>.
Appendix A. Specific Backup Strategy Examples
This appendix demonstrates the computations of some specific backup
strategy options in details.
A.1. LFA-based BIER-FRR using Single BIFT
In the LFA-based BIER-FRR using single BIFT, every BFR has a single
BIFT for a level of protection. Its structure is the same as the one
in Figure 1.
The following presents the details in BFR B1 in Figure 2 for building
the BIFT for BIER-FRR link protection.
At first, BFR B1 obtains a BIER-LFA as BBFR-NBR for each BFER. B6 is
normal BIER-LFA as BBFR-NBR for BFER B2 and B3. B2 is normal BIER-
LFA as BBFR-NBR for BFER B4, B5, B6 and B7. Figure 12 illustrates
B1's intermediate BIFT for link protection filled with values for
BBFR-NBRs and BFAs.
+------+---------+-------+----------+--------+----------+---+
|BFR-id| F-BM |BFR-NBR| BF-BM |BBFR-NBR| BFA |BEA|
+======+=========+=======+==========+========+==========+===+
| 2 | 0000110 | B2 | | B6 | Plain | |
+------+---------+-------+----------+--------+----------+---+
| 3 | 0000110 | B2 | | B6 | Plain | |
+------+---------+-------+----------+--------+----------+---+
| 4 | 1111000 | B6 | | B2 | Plain | |
+------+---------+-------+----------+--------+----------+---+
| 5 | 1111000 | B6 | | B2 | Plain | |
+------+---------+-------+----------+--------+----------+---+
| 6 | 1111000 | B6 | | B2 | Plain | |
+------+---------+-------+----------+--------+----------+---+
| 7 | 1111000 | B6 | | B2 | Plain | |
+------+---------+-------+----------+--------+----------+---+
Figure 12: B1's intermediate BIFT for link protection.
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From the intermediate BIFT, BFERs B2 and B3 have the same BFR-NBR B2
and BBFR-NBR B6, BFERs B4 to B7 have the same BFR-NBR B6 as the BBFR-
NBR B6 for BFER B2. According to Section 3.4, the BF-BM for BFER B2
has the bits for B2 and B3 as well as the bits for B4 to B7, which is
1111110. The BF-BM for BFER B3 is also 1111110. Similarly, the BF-
BM for each of BFERs B3 to B7 is computed, which is 1111110.
With the BF-BMs, BFR B1 has the BIFT for link protection, which is
illustrated in Figure 13.
+------+---------+-------+----------+--------+----------+---+
|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 13: B1's BIFT for BIER-FRR link protection.
A.2. LFA-based BIER-FRR using Multiple Backup BIFTs
For the LFA-based BIER-FRR using multiple backup BIFTs, in addition
to a primary BIFT, a BFR has a backup BIFT for each of its BFR-NBRs
with a level of protection. The backup BIFT for BFR-NBR N with link
protection (or simply called the backup BIFT for link to N) assumes
that the link to N failed. The BFR uses it to protect against the
failure of its link to N. The backup BIFT for N with node protection
(or simply called the backup BIFT for N) assumes that node N failed.
The BFR uses it to protect against the failure of N. Once the BFR as
a PLR detects the failure of its link to N, it uses the backup BIFT
for link to N to forward all BIER packets. When the BFR as a PLR
detects the failure of its BFR-NBR N, it uses the backup BIFT for N
to forward all BIER packets.
Even though a BFR has multiple backup BIFTs, the LFA-based BIER-FRR
using multiple backup BIFTs is scalable. Both the size of a backup
BIFT and the number of backup BIFTs on the BFR are small. Assume a
BIER network has 1000 BFRs and 100 BFERs, and each BFR has 10 BFR-
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NBRs on average. The size of a backup BIFT is 100 forwarding
entries. The number of backup BIFTs on the BFR is 20 on average.
The total size of all backup BIFTs is 100*20 = 2000 forwarding
entries.
The following presents the details in BFR B1 in Figure 2 for building
the backup BIFT for link to B2 to support BIER-FRR link protection.
To support link protection, BFR B1 in Figure 2 has two backup BIFTs:
one for link to B2 and the other for link to B6. The backup BIFT for
link to B2 is illustrated in Figure 14.
+--------+---------+---------+-----------------+-----------------+
| BFR-id | F-BM | BFR-NBR |Forwarding Action|Comment: protects|
| | | | | failure of |
+========+=========+=========+=================+=================+
| 2 | 1111110 | B6 | Plain | Link B1->B2 |
+--------+---------+---------+-----------------+-----------------+
| 3 | 1111110 | B6 | Plain | Link B1->B2 |
+--------+---------+---------+-----------------+-----------------+
| 4 | 1111110 | B6 | Plain | |
+--------+---------+---------+-----------------+-----------------+
| 5 | 1111110 | B6 | Plain | |
+--------+---------+---------+-----------------+-----------------+
| 6 | 1111110 | B6 | Plain | |
+--------+---------+---------+-----------------+-----------------+
| 7 | 1111110 | B6 | Plain | |
+--------+---------+---------+-----------------+-----------------+
Figure 14: B1's backup BIFT for link to B2.
BFR B1 builds the backup BIFT for link to B2 in two steps. In the
first step, it builds the backup BIRT for link to B2 through copying
its regular BIRT to the backup BIRT and then changing BFR-NBR B2 in
the backup BIRT to a backup BFR-NBR to protect against the failure of
the link to B2. The backup BIRT for link to B2 built by B1 is
illustrated in Figure 15.
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+------+-------------+---------+-----------------+-----------------+
|BFR-id|BFER's Prefix| BFR-NBR |Forwarding Action|Comment: protects|
| | | | | failure of |
+======+=============+=========+=================+=================+
| 2 | B2 | B6 | Plain | Link B1->B2 |
+------+-------------+---------+-----------------+-----------------+
| 3 | B3 | B6 | Plain | Link B1->B2 |
+------+-------------+---------+-----------------+-----------------+
| 4 | B4 | B6 | Plain | |
+------+-------------+---------+-----------------+-----------------+
| 5 | B5 | B6 | Plain | |
+------+-------------+---------+-----------------+-----------------+
| 6 | B6 | B6 | Plain | |
+------+-------------+---------+-----------------+-----------------+
| 7 | B7 | B6 | Plain | |
+------+-------------+---------+-----------------+-----------------+
Figure 15: B1's backup BIRT for link to B2.
The BFR-NBR in each of the first two routing entries with BFR-NBR B2
originally from the BIRT is changed to its corresponding backup BFR-
NBR. The BFR-NBR B2 in the first entry is changed to backup BFR-NBR
BIER-LFA B6. The BFR-NBR B2 in the second entry is changed to backup
BFR-NBR BIER-LFA B6.
In the second step, BFR B1 derives the backup BIFT for link to B2
from the backup BIRT for link to B2 in the same way as it derives its
regular BIFT from its BIRT defined in [RFC8279]. The result of the
backup BIFT for link to B2 is the one shown in Figure 14.
When BFR B1 as a PLR detects the failure of its link to B2, it
forwards all the BIER packets using the FRR-BIFT for link to B2.
There is no redundant packet. For example, for a BIER packet with
destinations B2 and B6 (i.e., bitstring 0100010), BFR B1 sends a
single packet copy on the link to B6 using the backup BIFT for link
to B2 after detecting the failure of its link to B2. It will not
send any copy of the packet to B6 again since the bitstring in the
packet will be all cleaned by the F-BM 1111110 after sending the
packet to B6 via its link to B6. Similarly, for a BIER packet with
destinations B2, B5 and B7 (i.e., bitstring 1010010), BFR B1 sends a
single packet copy on its link to B6 using the backup BIFT for link
to B2 after detecting the failure of its link to B2.
Acknowledgments
The authors would like to thank Daniel Merling, Jeffrey Zhang, Tony
Przygienda and Shaofu Peng for their comments to this work.
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Contributors
Yisong Liu
China Mobile
Email: liuyisong@chinamobile.com
Yanhe Fan
Casa Systems
United States of America
Email: yfan@casa-systems.com
Lei Liu
Fujitsu
United States of America
Email: liulei.kddi@gmail.com
Xufeng Liu
Alef Edge
United States of America
Email: xufeng.liu.ietf@gmail.com
Xuesong Geng
China
Email: gengxuesong@huawei.com
Authors' Addresses
Huaimo Chen (editor)
Futurewei
Boston, MA,
United States of America
Email: hchen.ietf@gmail.com
Mike McBride
Futurewei
Email: michael.mcbride@futurewei.com
Steffen Lindner
University of Tuebingen
Email: steffen.lindner@uni-tuebingen.de
Michael Menth
University of Tuebingen
Email: menth@uni-tuebingen.de
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Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing
102209
China
Email: wangaj3@chinatelecom.cn
Gyan S. Mishra
Verizon Inc.
13101 Columbia Pike
Silver Spring, MD 20904
United States of America
Phone: 301 502-1347
Email: gyan.s.mishra@verizon.com
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