BIER Fast ReRoute
draft-ietf-bier-frr-04
| Document | Type | Active Internet-Draft (bier WG) | |
|---|---|---|---|
| Authors | Huaimo Chen , Mike McBride , Steffen Lindner , Michael Menth , Aijun Wang , Gyan Mishra | ||
| Last updated | 2024-02-01 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | In WG Last Call | |
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draft-ietf-bier-frr-04
Network Working Group H. Chen, Ed.
Internet-Draft M. McBride
Intended status: Informational Futurewei
Expires: 4 August 2024 S. Lindner
M. Menth
University of Tuebingen
A. Wang
China Telecom
G. Mishra
Verizon Inc.
1 February 2024
BIER Fast ReRoute
draft-ietf-bier-frr-04
Abstract
BIER is a scalable multicast overlay that utilizes a routing
underlay, e.g., IP, to build up its Bit Index Forwarding Tables
(BIFTs). This document proposes Fast Reroute for BIER (BIER-FRR).
It protects BIER traffic after detecting the failure of a link or
node in the core of a BIER domain until affected BIFT entries are
recomputed after reconvergence of the routing underlay. BIER-FRR is
applied locally at the point of local repair (PLR) and does not
introduce any per-flow state. The document specifies nomenclature
for BIER-FRR and gives examples for its integration in BIER
forwarding. Furthermore, it presents operation modes for BIER-FRR.
Link and node protection may be chosen as protection level.
Moreover, the backup strategies tunnel-based BIER-FRR and LFA-based
BIER-FRR are defined and compared.
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
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This Internet-Draft will expire on 4 August 2024.
Copyright Notice
Copyright (c) 2024 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition of BIER-FRR . . . . . . . . . . . . . . . . . . . 4
2.1. Definition of Forwarding Actions . . . . . . . . . . . . 5
2.2. Definition of Backup Forwarding Entries . . . . . . . . . 5
2.3. Activating and Deactivating Backup Forwarding Entries . . 6
2.4. Computation of the Backup F-BM . . . . . . . . . . . . . 7
3. Representations for BIER-FRR Forwarding Data . . . . . . . . 7
3.1. Potential Emergence of Redundant Packets . . . . . . . . 7
3.2. Primary BIFT and Single Backup BIFT . . . . . . . . . . . 9
3.3. Primary BIFT and Failure-Specific Backup BIFTs . . . . . 10
4. Protection Levels . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Link Protection . . . . . . . . . . . . . . . . . . . . . 12
4.2. Node Protection . . . . . . . . . . . . . . . . . . . . . 12
4.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Backup Strategies . . . . . . . . . . . . . . . . . . . . . . 12
5.1. Tunnel-Based BIER-FRR . . . . . . . . . . . . . . . . . . 13
5.1.1. Tunnel-Based BIER-FRR with Link Protection . . . . . 13
5.1.2. Tunnel-Based BIER-FRR with Node Protection . . . . . 14
5.1.3. Implementation Experience . . . . . . . . . . . . . . 16
5.2. LFA-based BIER-FRR . . . . . . . . . . . . . . . . . . . 16
5.2.1. Relation of BIER-LFAs to IP-LFAs and Prerequisites . 16
5.2.2. Definition of BIER-LFAs . . . . . . . . . . . . . . . 16
5.2.3. Protection Coverage of BIER-LFA Types . . . . . . . . 17
5.2.4. Sets of Supported BIER-LFAs . . . . . . . . . . . . . 18
5.2.5. Link Protection . . . . . . . . . . . . . . . . . . . 18
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5.2.6. Node Protection . . . . . . . . . . . . . . . . . . . 20
5.2.7. Optimization Potential to Reduce Redundant BIER Packets
in Failure Cases . . . . . . . . . . . . . . . . . . 21
6. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1. Comparison of LFA-Based Protection for IP-FRR and
BIER-FRR . . . . . . . . . . . . . . . . . . . . . . . . 22
6.2. Advantages and Disadvantages of Tunnel-Based BIER-FRR . . 23
6.2.1. Advantages . . . . . . . . . . . . . . . . . . . . . 23
6.2.2. Disadvantages . . . . . . . . . . . . . . . . . . . . 23
6.3. Advantages and Disadvantages of LFA-Based BIER-FRR . . . 23
6.3.1. Advantages . . . . . . . . . . . . . . . . . . . . . 23
6.3.2. Disadvantages . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Specific Backup Strategy Examples . . . . . . . . . 26
A.1. LFA-based BIER-FRR using Single BIFT . . . . . . . . . . 26
A.2. LFA-based BIER-FRR using Multiple Backup BIFTs . . . . . 27
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 29
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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.
BIER packets are usually forwarded without an outer IP header. If a
link or node fails, the corresponding BFR neighbor (BFR-NBR) is no
longer reachable. Fast reroute (FRR) mechanisms in the routing
underlay, e.g., IP-FRR, apply only to IP packets so that BIER traffic
would be dropped. BIER traffic can be delivered again only after
reconvergence of the routing underlay and recalculation of the BIFT.
Thus, tunneling BIER packets can help to reach the BFR-NBR in case of
a link failure by leveraging FRR capabilities of the routing underlay
if such mechanisms are available. However, this does not help in
case of a node failure. Then, all destinations having the failed
node as BFR-NBR cannot be reached anymore. As BIER carries multicast
traffic which has often realtime requirements, there is a particular
need to protect BIER traffic against too long outages after failures.
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In this document we propose nomenclature for Fast Reroute in BIER
(BIER-FRR). As soon as a BFR detects a BFR-NBR is unreachable, BIER-
FRR enables a BFR to quickly reroute affected BIER packets with the
help of backup forwarding entries. To avoid redundant packets,
backup forwarding entries should be processed prior to normal
forwarding entries. To achieve that goal, two possible
representations for backup forwarding entries are proposed.
The protection level can be either link protection or node
protection. Link protection protects only the failure of a link. It
is simple but may not work if a BFR fails. Node protection is more
complex but also protects against the failure of BFRs. The backup
strategy determines the selection of the backup forwarding entries.
Examples for backup strategies are tunnel-based BIER-FRR and LFA-
based BIER-FRR
* Tunnel-based BIER-FRR leverages mechanisms of the routing underlay
for FRR purposes. The routing underlay restores connectivity
faster than BIER as a reconverged routing underlay is prerequisite
for recalculation of the BIFT. If the routing underlay leverages
FRR mechanisms, its forwarding ability is restored long before
reconvergence is completed. To leverage fast restoration of the
routing underlay, BIER traffic affected by a failure is tunneled
over the routing underlay.
* LFA-based BIER-FRR reroutes BIER traffic to alternative neighbors
in case of a failure. It utilizes the principles of IP-FRR but
requires that LFAs are BFRs. Normal BIER-LFAs can be reached
without tunneling, remote BIER-LFAs utilize a tunnel, and
topology-independent BIER-LFAs leverage explicit paths to reach
the backup BFR-NBR. In contrast to tunnel-based FRR, LFA-based
BIER-FRR does not require fast reroute mechanisms in the routing
underlay.
BIER-FRR as presented in this document follows a primary/backup path
principle, also known as 1:1 protection. It is opposite to 1+1
protection which denotes a live-live protection principle. This has
been considered for BIER in [BrAl17].
2. 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.
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2.1. Definition of Forwarding Actions
A BFR-NBR is directly connected if it is a next hop on the network
layer, i.e., if it can be reached via the link layer technology.
Otherwise, the BFR-NBR is indirectly connected.
We define the following forwarding actions.
* Plain: Sends the mere BIER packet to a BFR-NBR via a direct link
and without a tunnel header. That means, the packet is not sent
over the routing underlay.
* Tunnel: Encapsulates the BIER packet with a tunnel header towards
a BFR-NBR and sends it over the routing underlay.
* Explicit: Forwards the packet over an explicit path to a BFR-NBR.
The path information must be given. If segment routing is used
for this purpose, the segment IDs (SIDs) must be given. Two
forwarding actions of type Explicit are equal only if they share
the same explicit path.
The forwarding actions in the BIFT as proposed in [RFC8279] are given
implicitly as they are derived from the connectedness 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.
2.2. Definition of Backup Forwarding Entries
The BIFT as proposed in [RFC8279] contains a F-BM and a BFR-NBR for a
specific BFER. They constitute a primary forwarding entry. BIER-FRR
extends this regular BIFT by additional columns containing backup
forwarding entries. A backup forwarding entry contains
* a backup F-BM (BF-BM),
* a backup BFR-NBR (BBFR-NBR),
* a backup forwarding action (BFA), and
* a backup entry active (BEA) flag.
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Backup F-BM and backup BFR-NBR have the same structure as their
primary counterparts. The backup forwarding action is a forwarding
action as defined in Section 2.1. The BEA flag indicates whether the
backup forwarding entry is active. When it is active, the backup
F-BM, backup BFR-NBR, and the backup forwarding action are used for
the forwarding of BIER packets instead of the primary forwarding
entry. The structure of the extended BIFT is given 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 given in the BIFT as it is derived from the
BFR-NBR. In contrast, the backup forwarding action is given in the
extended BIFT. Moreover, an explicit path must be indicated in case
of forwarding action Explicit. However, explicit paths are
implementation-specific and, therefore, this information is not
indicated in the table. The values for the backup BFR-NBR and the
backup action depend on the desired protection level and the backup
strategy. Examples for them are described in Section 5.1 and
Section 5.2. The backup F-BM depends on the backup BFR-NBR. Its
computation is explained in Section 2.4.
2.3. 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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2.4. Computation of the Backup F-BM
The primary F-BM of a specific BFER indicates all BFERs that share
the same primary BFR-NBR. The backup F-BM of a specific BFER
indicates
* all BFERs that share the primary and backup BFR-NBR of the
specific BFER and
* all BFERs that have the backup BFR-NBR of the specific BFER as
primary BFR-NBR.
3. Representations for BIER-FRR Forwarding Data
We show that backup entries need to be used first to reduce the
number of redundant packets in the single extended BIFT (presented in
Section 2.2). This may be hard or cannot be achieved on some
hardware platforms. Therefore, two alternate representations of
forwarding entries are proposed. The first is a primary BIFT and
single backup BIFT (SBB). The second is a primary BIFT and multiple
failure-specific backup BIFTs (FBB).
3.1. Potential Emergence of Redundant Packets
The BIER forwarding procedure in failure-free scenarios avoids
redundant packets, i.e., it ensures that at most a single copy is
sent per link for every BIER packet. However, this property might be
violated when BIER-FRR as presented in Section 2 is applied to
protect against a failure.
Figure 2 shows an example of a BIER network. BFRs have the prefix
"B" and are numbered by their BFR-ids. To simplify the example,
every BFR is a BFER and its bit position in the bitstring equals its
BFR-id. The number on a link is its cost which is used by the
routing underlay for computing 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
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Figure 2: BIER network example.
The extended BIFT with backup forwarding entries for LFA-based BIER-
FRR with link protection built 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.
We show how redundant packets can occur in case of a failure. To
that end, we consider the extended BIFT for BFR 1 in Figure 3. It
has backup forwarding entries for LFA-based FRR and link protection.
For a BIER packet with destinations B2 and B6 (i.e., bitstring
0100010), BFR B1 sends a single packet copy on link B1-B2 and on link
B1-B6 in the absence of a failure.
When the link B1-B6 fails, B1 as a PLR detects the failure.
Therefore, B1 sets the BEA flag for all destinations that have B6 as
BFR-NBR. We consider again that B1 sends a BIER packet to B2 and B6.
At first, it sends a copy with bitstring 0000010 to B2 using the
corresponding primary forwarding entry in the extended BIFT in
Figure 3.
Then, B1 sends another copy of the packet with bitstring 0100000 for
B6 to B2 using the backup forwarding entry since the BEA flag is
activated.
This is a second (redundant) copy over the same link B1-B2. It can
be prevented if the backup forwarding entry is used first. When
using the backup forwarding entry, B1 sends only a single copy of the
packet with bitstring 0100010 to B2. It will not send any copy of
the packet to B2 again since the bitstring in the packet will be all
cleaned by the BF-BM 1111110. Thus, prioritized processing of BFERs
with unreachable BFR-NBRs helps to reduce redundant packet copies.
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3.2. Primary BIFT and Single Backup BIFT
The extended BIFT may be separated into two BIFTs. One is a primary
BIFT and the other is a single backup BIFT. The primary BIFT is the
same as the regular BIFT. The backup BIFT contains the backup
forwarding entries, including BF-BM, BBFR-NBR, BFA and BEA in the
extended BIFT. When a BFR as a PLR detects that BFR-NBR N is
unreachable, it activates the BEA flag for all BFERs in the backup
BIFT that have BFR-NBR as primary BFR-NBR. When a BFR forwards a
BIER packet, it processes the packet first using the backup BIFT and
then using the primary BIFT. With this prioritization, the number of
redundant packet copies can be reduced.
B1's extended BIFT in Figure 3 is separated into the primary BIFT in
Figure 4 and the single backup BIFT 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.
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+------+----------+--------+-----------+---+-----------------+
|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 contains 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 BFR-NBR as primary BFR-NBR. For
example, BFERs B4, B5, B6 and B7 have BFR-NBR B6 as their primary
BFR-NBR. When BFR-NBR B6 fails, the BEA flag for BFERs B4, B5, B6
and B7 is activated, i.e., the BEA in the last four entries in the
backup BIFT is set to one.
3.3. Primary BIFT and Failure-Specific Backup BIFTs
As an alternative, the information in the extended BIFT may be
represented in a primary BIFT and several, failure-specific backup
BIFTs. A failure-specific backup BIFT is a backup BIFT for the
unreachability of BFR-NBR N. A backup BIFT for the failure of N is
simply called a backup BIFT for N. It has the same structure as the
regular BIFT but has an entry for a backup forwarding action. Thus,
a BFR has a primary BIFT, which is the same as the regular BIFT, and
a backup BIFT for each of its BFR-NBRs.
The BFR uses the primary BIFT to forward BIER packets under failure-
free conditions. When the BFR as a PLR detects that BFR-NBR N is
unreachable, it uses the backup BIFT for N to forward all BIER
packets. After the routing underlay has re-converged on the new
network topology, the primary BIFT is re-computed. Once the re-
computed primary BIFT is installed, it is used to forward all BIER
packets.
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We illustrate the concept using the example from extended BIFT in
Figure 3. Figure 4 shows the primary BIFT of B1 in this context.
BFR B1 in Figure 2 has two neighbors: B6 and B2. B1 has two backup
BIFTs with link protection: one for B6 and another for B2. B1 has
also two backup BIFTs with node protection. Figure 6 is B1's backup
BIFT for B6 to react to the unreachability of B1 in a similar way as
with the extended BIFT 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 a PLR detects that B6 is unreachable through the link to
B6, it uses the backup BIFT for B6 to forward all BIER packets. As
this representation is equivalent to the concept of single primary
and single backup BIFT, redundant packets for the same forwarding
action are avoided.
4. Protection Levels
Link and node protection may be supported. Link protection protects
against the failure of an adjacent link while node protection
protects against the failure of a neighboring node and the path
towards that node. Depending on the supported service, link
protection or node protection may be relevant. Both protection
levels can be combined with any backup strategy in Section 5.
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4.1. Link Protection
With link protection the backup path avoids the failed link (i.e.,
the failed primary path from the PLR to the primary BFR-NBR,
excluding the primary BFR-NBR), but the backup path may include the
primary BFR-NBR. Therefore, the backup path is still operational if
the primary path fails. The disadvantage of link protection is that
it fails if the primary BFR-NBR itself is not operational. However,
link protection has also advantages. It often leads to shorter
backup paths than node protection. In case of tunnel-based BIER-FRR,
link protection causes at most one redundant packet while node
protection can cause more redundant packets. In case of LFA-based
BIER-FRR, link protection can protect more BFERs with normal BIER-
LFAs than node protection.
4.2. Node Protection
With node protection, the backup path avoids the failed node and the
link to the node (i.e., the failed primary path from the PLR to the
primary BFR-NBR, including the primary BFR-NBR). Therefore, the
backup path must not include the primary path or the primary BFR-NBR
so that the backup path is still operational if these elements fail.
If a BFER and its primary BFR-NBR are the same, only link protection
is possible for that BFER. An advantage of node protection is the
improved protection quality compared to link protection. However,
node protection has also disadvantages. It often leads to longer
backup paths than link protection. For tunnel-based BIER-FRR,
possibly more redundant packets are transmitted over a link than with
link protection. For LFA-based BIER-FRR, possibly fewer BFERs can be
protected with normal BIER-LFAs so that more remote BIER-LFAs or
topology-independent BIER-LFAs are needed which are more complex.
4.3. Example
In Figure 2, B1's primary path towards BFER B5 is B1-B6-B5. Node
protection for BFER B5 can be achieved only via the backup path
B1-B2-B3-B4-B5. Link protection for BFER 5 is achieved via the
backup path B1-B2-B7-B6 and in addition via the backup path
B1-B2-B3-B4-B5-B6. The backup entries depend on the protection level
and on the backup strategy. Example BIFTs for link and node
protection are given in Section 5.
5. Backup Strategies
The backup strategies determine the selection of the backup
forwarding entries. They have an impact on the backup BFR-NBR and on
the backup action, and thereby on the backup path. In the following,
tunnel-based BIER-FRR and LFA-based BIER-FRR are presented.
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5.1. Tunnel-Based BIER-FRR
The routing underlay may be able to forward packets towards their
destinations despite an existing failure. This may be achieved,
e.g., due to FRR mechanisms in the routing underlay. In that case,
the primary BFR-NBR is not reachable via the primary action (Plain),
but it may be reachable via a backup action (Tunnel).
Tunnel-based BIER-FRR encapsulates BIER packets affected by a failure
in the routing underlay to leverage its fast restoration capability.
The affected BIER packets can be delivered towards their destinations
as soon as the connectivity in the routing underlay is restored. The
appropriate backup forwarding entries in a BIFT for BIER-FRR depend
on the desired protection level.
5.1.1. 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
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 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.
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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.
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.
5.1.2. 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 2.4.
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+------+----------+--------+----------+---+-----------------+
|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
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.
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5.1.3. Implementation Experience
Tunnel-based BIER-FRR has been implemented in P4 for the software-
switch bmv2 [MeLi20b] and the hardware switching ASIC Tofino
[MeLi21]. Performance results have been provided.
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. 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.
5.2.1. 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]. We denote such LFAs 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.
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.
5.2.2. 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
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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
5.2.3. 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.
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* 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.
5.2.4. 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
* Option 2: normal and remote BIER-LFAs are supported
* Option 3: all BIER-LFA types are supported
5.2.5. 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
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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.
We now 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, we
observe 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.
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+------+----------+--------+-----------+---+-----------------+
|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.
5.2.6. 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.
Figure 11 illustrates B1's backup BIFT for the LFA-based BIER-FRR
with node protection in the networking example of Figure 2.
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+------+----------+--------+-----------+---+-----------------+
|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.
5.2.7. 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
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* 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.
We illustrate this optimization potential. We consider LFA-based
BIER-FRR with link protection for B7. Its backup BIFT is given in
Figure 9. As observed in Section 5.2.5, B7 needs to send four copies
to forward a packet to {B1, B4, B5, B6}. If we choose the more
complex TI-BIER-LFA B4 to protect BFER B4 instead of the remote BIER-
LFA B3, then only two redundant copies need to be sent.
6. 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.
6.1. 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
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.
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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.
6.2. Advantages and Disadvantages of Tunnel-Based BIER-FRR
6.2.1. 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.
6.2.2. 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.
6.3. Advantages and Disadvantages of LFA-Based BIER-FRR
6.3.1. Advantages
* Does not rely on any fast protection of the underlay.
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* 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.
6.3.2. 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).
7. Security Considerations
This document does not introduce any new security issues beyond those
discussed in BIER architecture [RFC8279].
8. IANA Considerations
No requirements for IANA.
9. References
9.1. Normative References
[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>.
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[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>.
9.2. 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-06, 26
December 2023, <https://datatracker.ietf.org/doc/html/
draft-chen-bier-egress-protect-06>.
[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-
13, 16 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-13>.
[MeLi20b] Merling, D., Lindner, S., and M. Menth, "P4-Based
Implementation of BIER and BIER-FRR for Scalable and
Resilient Multicast", November 2020.
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[MeLi21] Merling, D., Lindner, S., and M. Menth, "Hardware-based
Evaluation of Scalable and Resilient Multicast with BIER
in P4", March 2020.
[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 2.4, 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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