IS-IS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering
draft-ietf-lsr-isis-rfc5316bis-07
| Document | Type | Active Internet-Draft (lsr WG) | |
|---|---|---|---|
| Authors | Mach Chen , Les Ginsberg , Stefano Previdi , Xiaodong Duan | ||
| Last updated | 2022-11-10 (Latest revision 2022-09-28) | ||
| Replaces | draft-chen-lsr-isis-rfc5316bis | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
OPSDIR Last Call review
(of
-03)
Has Nits
|
||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Christian Hopps | ||
| Shepherd write-up | Show Last changed 2021-08-15 | ||
| IESG | IESG state | RFC Ed Queue | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | John Scudder | ||
| Send notices to | chopps@chopps.org, teas@ietf.org | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA action state | RFC-Ed-Ack | ||
| IANA expert review state | Expert Reviews OK | ||
| RFC Editor | RFC Editor state | RFC-EDITOR | |
| Details |
draft-ietf-lsr-isis-rfc5316bis-07
Internet Engineering Task Force M. Chen
Internet-Draft Huawei
Obsoletes: 5316 (if approved) L. Ginsberg
Intended status: Standards Track Cisco Systems
Expires: 1 April 2023 S. Previdi
Huawei Technologies
D. Xiaodong
China Mobile
28 September 2022
IS-IS Extensions in Support of Inter-Autonomous System (AS) MPLS and
GMPLS Traffic Engineering
draft-ietf-lsr-isis-rfc5316bis-07
Abstract
This document describes extensions to the Intermediate System to
Intermediate System (IS-IS) protocol to support Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering
(TE) for multiple Autonomous Systems (ASs). It defines IS-IS
extensions for the flooding of TE information about inter-AS links,
which can be used to perform inter-AS TE path computation.
No support for flooding information from within one AS to another AS
is proposed or defined in this document.
This document builds on RFC 5316 by adding support for IPv6-only
operation.
This document obsoletes RFC 5316.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [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.
Chen, et al. Expires 1 April 2023 [Page 1]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
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/.
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 1 April 2023.
Copyright Notice
Copyright (c) 2022 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
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
2.1. A Note on Non-Objectives . . . . . . . . . . . . . . . . 4
2.2. Per-Domain Path Determination . . . . . . . . . . . . . . 5
2.3. Backward Recursive Path Computation . . . . . . . . . . . 6
3. Extensions to ISIS-TE . . . . . . . . . . . . . . . . . . . . 7
3.1. Choosing the TE Router ID Value . . . . . . . . . . . . . 8
3.2. Inter-AS Reachability TLV . . . . . . . . . . . . . . . . 9
3.3. TE Router ID . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Sub-TLVs for Inter-AS Reachability TLV . . . . . . . . . 11
3.4.1. Remote AS Number Sub-TLV . . . . . . . . . . . . . . 11
3.4.2. IPv4 Remote ASBR ID Sub-TLV . . . . . . . . . . . . . 12
3.4.3. IPv6 Remote ASBR ID Sub-TLV . . . . . . . . . . . . . 12
3.4.4. IPv6 Local ASBR ID sub-TLV . . . . . . . . . . . . . 13
3.5. Sub-TLVs for IS-IS Router Capability TLV . . . . . . . . 14
3.5.1. IPv4 TE Router ID sub-TLV . . . . . . . . . . . . . . 14
3.5.2. IPv6 TE Router ID sub-TLV . . . . . . . . . . . . . . 14
4. Procedure for Inter-AS TE Links . . . . . . . . . . . . . . . 15
4.1. Origin of Proxied TE Information . . . . . . . . . . . . 16
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
Chen, et al. Expires 1 April 2023 [Page 2]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6.1. Inter-AS Reachability TLV . . . . . . . . . . . . . . . . 17
6.2. Sub-TLVs for the Inter-AS Reachability TLV . . . . . . . 18
6.3. Sub-TLVs for the IS-IS Router Capability TLV . . . . . . 18
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Changes to RFC 5316 . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
[RFC5305] defines extensions to the IS-IS protocol [RFC1195] to
support intra-area Traffic Engineering (TE). The extensions provide
a way of encoding the TE information for TE-enabled links within the
network (TE links) and flooding this information within an area. The
extended IS reachability TLV and traffic engineering router ID TLV,
which are defined in [RFC5305], are used to carry such TE
information. The extended IS reachability TLV has several nested
sub-TLVs that describe the TE attributes for a TE link.
[RFC6119] and [RFC5307] define similar extensions to IS-IS in support
of IPv6 and Generalized Multiprotocol Label Switching (GMPLS) TE
respectively.
Requirements for establishing Multiprotocol Label Switching (MPLS) TE
Label Switched Paths (LSPs) that cross multiple Autonomous Systems
(ASes) are described in [RFC4216]. As described in [RFC4216], a
method SHOULD provide the ability to compute a path spanning multiple
ASes. So a path computation entity that may be the head-end Label
Switching Router (LSR), an AS Border Router (ASBR), or a Path
Computation Element (PCE) [RFC4655] needs to know the TE information
not only of the links within an AS, but also of the links that
connect to other ASes.
In this document, a new TLV, which is referred to as the inter-AS
reachability TLV, is defined to advertise inter-AS TE information,
and three new sub-TLVs are defined for inclusion in the inter-AS
reachability TLV to carry the information about the remote AS number
and remote ASBR ID. The sub-TLVs defined in [RFC5305][RFC6119] and
other documents for inclusion in the extended IS reachability TLV for
describing the TE properties of a TE link are applicable to be
included in the Inter-AS Reachability TLV for describing the TE
properties of an inter-AS TE link as well. Also, two more new sub-
TLVs are defined for inclusion in the IS-IS router capability TLV to
carry the TE Router ID when the TE Router ID is needed to reach all
routers within an entire IS-IS routing domain. The extensions are
Chen, et al. Expires 1 April 2023 [Page 3]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
equally applicable to IPv4 and IPv6 as identical extensions to
[RFC5305] and [RFC6119]. Detailed definitions and procedures are
discussed in the following sections.
This document does not propose or define any mechanisms to advertise
any other extra-AS TE information within IS-IS. See Section 2.1 for
a full list of non-objectives for this work.
2. Problem Statement
As described in [RFC4216], in the case of establishing an inter-AS TE
LSP that traverses multiple ASes, the Path message [RFC3209] may
include the following elements in the Explicit Route Object (ERO) in
order to describe the path of the LSP:
* a set of AS numbers as loose hops; and/or
* a set of LSRs including ASBRs as loose hops.
Two methods for determining inter-AS paths have been described
elsewhere. The per-domain method [RFC5152] determines the path one
domain at a time. The backward recursive method [RFC5441] uses
cooperation between PCEs to determine an optimum inter-domain path.
The sections that follow examine how inter-AS TE link information
could be useful in both cases.
2.1. A Note on Non-Objectives
It is important to note that this document does not make any change
to the confidentiality and scaling assumptions surrounding the use of
ASes in the Internet. In particular, this document is conformant to
the requirements set out in [RFC4216].
The following features are explicitly excluded:
* There is no attempt to distribute TE information from within one
AS to another AS.
* There is no mechanism proposed to distribute any form of TE
reachability information for destinations outside the AS.
* There is no proposed change to the PCE architecture or usage.
* TE aggregation is not supported or recommended.
* There is no exchange of private information between ASes.
* No IS-IS adjacencies are formed on the inter-AS link.
Chen, et al. Expires 1 April 2023 [Page 4]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
2.2. Per-Domain Path Determination
In the per-domain method of determining an inter-AS path for an MPLS-
TE LSP, when an LSR that is an entry-point to an AS receives a Path
message from an upstream AS with an ERO containing a next hop that is
an AS number, it needs to find which LSRs (ASBRs) within the local AS
are connected to the downstream AS. That way, it can compute a TE
LSP segment across the local AS to one of those LSRs and forward the
Path message to that LSR and hence into the next AS. See Figure 1
for an example.
R1------R3----R5-----R7------R9-----R11
| | \ | / |
| | \ | ---- |
| | \ | / |
R2------R4----R6 --R8------R10----R12
: :
<-- AS1 -->:<---- AS2 --->:<--- AS3 --->
Figure 1: Inter-AS Reference Model
The figure shows three ASes (AS1, AS2, and AS3) and twelve LSRs (R1
through R12). R3 and R4 are ASBRs in AS1. R5, R6, R7, and R8 are
ASBRs in AS2. R9 and R10 are ASBRs in AS3.
If an inter-AS TE LSP is planned to be established from R1 to R12,
the AS sequence will be: AS1, AS2, AS3.
Suppose that the Path message enters AS2 from R3. The next hop in
the ERO shows AS3, and R5 must determine a path segment across AS2 to
reach AS3. It has a choice of three exit points from AS2 (R6, R7,
and R8), and it needs to know which of these provide TE connectivity
to AS3, and whether the TE connectivity (for example, available
bandwidth) is adequate for the requested LSP.
Alternatively, if the next hop in the ERO is an entry ASBR for AS3
(say R9), R5 needs to know which of its exit ASBRs has a TE link that
connects to R9. Since there may be multiple ASBRs that are connected
to R9 (both R7 and R8 in this example), R5 also needs to know the TE
properties of the inter-AS TE links so that it can select the correct
exit ASBR.
Once the Path message reaches the exit ASBR, any choice of inter-AS
TE link can be made by the ASBR if not already made by the entry ASBR
that computed the segment.
More details can be found in Section 4 of [RFC5152], which clearly
points out why advertising of inter-AS links is desired.
Chen, et al. Expires 1 April 2023 [Page 5]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
To enable R5 to make the correct choice of exit ASBR, the following
information is needed:
* List of all inter-AS TE links for the local AS.
* TE properties of each inter-AS TE link.
* AS number of the neighboring AS connected to by each inter-AS TE
link.
* Identity (TE Router ID) of the neighboring ASBR connected to by
each inter-AS TE link.
In GMPLS networks, further information may also be required to select
the correct TE links as defined in [RFC5307].
The example above shows how this information is needed at the entry-
point ASBRs for each AS (or the PCEs that provide computation
services for the ASBRs). However, this information is also needed
throughout the local AS if path computation functionality is fully
distributed among LSRs in the local AS, for example to support LSPs
that have start points (ingress nodes) within the AS.
2.3. Backward Recursive Path Computation
Another scenario using PCE techniques has the same problem.
[RFC5441] defines a PCE-based TE LSP computation method (called
Backward Recursive Path Computation) to compute optimal inter-domain
constrained MPLS-TE or GMPLS LSPs. In this path computation method,
a specific set of traversed domains (ASes) are assumed to be selected
before computation starts. Each downstream PCE in domain(i) returns
to its upstream neighbor PCE in domain(i-1) a multipoint-to-point
tree of potential paths. Each tree consists of the set of paths from
all boundary nodes located in domain(i) to the destination where each
path satisfies the set of required constraints for the TE LSP
(bandwidth, affinities, etc.).
So a PCE needs to select boundary nodes (that is, ASBRs) that provide
connectivity from the upstream AS. In order for the tree of paths
provided by one PCE to its neighbor to be correlated, the identities
of the ASBRs for each path need to be referenced. Thus, the PCE must
know the identities of the ASBRs in the remote AS that are reached by
any inter-AS TE link, and, in order to provide only suitable paths in
the tree, the PCE must know the TE properties of the inter-AS TE
links. See the following figure as an example.
Chen, et al. Expires 1 April 2023 [Page 6]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
PCE1<------>PCE2<-------->PCE3
/ : :
/ : :
R1------R3----R5-----R7------R9-----R11
| | \ | / |
| | \ | ---- |
| | \ | / |
R2------R4----R6 --R8------R10----R12
: :
<-- AS1 -->:<---- AS2 --->:<--- AS3 --->
Figure 2: BRPC for Inter-AS Reference Model
The figure shows three ASes (AS1, AS2, and AS3), three PCEs (PCE1,
PCE2, and PCE3), and twelve LSRs (R1 through R12). R3 and R4 are
ASBRs in AS1. R5, R6, R7, and R8 are ASBRs in AS2. R9 and R10 are
ASBRs in AS3. PCE1, PCE2, and PCE3 cooperate to perform inter-AS
path computation and are responsible for path segment computation
within their own domain(s).
If an inter-AS TE LSP is planned to be established from R1 to R12,
the traversed domains are assumed to be selected: AS1->AS2->AS3, and
the PCE chain is: PCE1->PCE2->PCE3. First, the path computation
request originated from the PCC (R1) is relayed by PCE1 and PCE2
along the PCE chain to PCE3. Then, PCE3 begins to compute the path
segments from the entry boundary nodes that provide connection from
AS2 to the destination (R12). But, to provide suitable path
segments, PCE3 must determine which entry boundary nodes provide
connectivity to its upstream neighbor AS (identified by its AS
number), and must know the TE properties of the inter-AS TE links.
In the same way, PCE2 also needs to determine the entry boundary
nodes according to its upstream neighbor AS and the inter-AS TE link
capabilities.
Thus, to support Backward Recursive Path Computation, the same
information listed in Section 2.2 is required. The AS number of the
neighboring AS connected to by each inter-AS TE link is particularly
important.
3. Extensions to ISIS-TE
Note that this document does not define mechanisms for distribution
of TE information from one AS to another, does not distribute any
form of TE reachability information for destinations outside the AS,
does not change the PCE architecture or usage, does not suggest or
recommend any form of TE aggregation, and does not feed private
information between ASes. See Section 2.1.
Chen, et al. Expires 1 April 2023 [Page 7]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
In this document, for the advertisement of inter-AS TE links, a new
TLV, which is referred to as the inter-AS reachability TLV, is
defined. Three new sub-TLVs are also defined for inclusion in the
inter-AS reachability TLV to carry the information about the
neighboring AS number and the remote ASBR ID of an inter-AS link.
The sub-TLVs defined in [RFC5305], [RFC6119], and other documents for
inclusion in the extended IS reachability TLV are applicable to be
included in the inter-AS reachability TLV for inter-AS TE links
advertisement.
This document also defines two new sub-TLVs for inclusion in the IS-
IS router capability TLV to carry the TE Router ID when the TE Router
ID is needed to reach all routers within an entire IS-IS routing
domain.
While some of the TE information of an inter-AS TE link may be
available within the AS from other protocols, in order to avoid any
dependency on where such protocols are processed, this mechanism
carries all the information needed for the required TE operations.
3.1. Choosing the TE Router ID Value
Subsequent sections specify advertisement of a TE Router ID value for
IPv4 and/or IPv6. This section defines how this value is chosen.
A TE Router ID MUST be an address which is unique within the IS-IS
domain and stable i.e., it can always be referenced in a path that
will be reachable from multiple hops away, regardless of the state of
the node's interfaces.
When advertising an IPv4 address as a TE Router ID, if the Traffic
Engineering Router ID TLV [RFC5305] is being advertised, then the
address SHOULD be identical to the address in the Traffic Engineering
Router ID TLV. The TE Router ID MAY be identical to an IP Interface
Address [RFC1195] advertised by the originating IS so long as the
address meets the requirements specified above.
When advertising an IPv6 address as a TE Router ID, if the IPv6 TE
Router ID TLV [RFC6119] is being advertised, then the address SHOULD
be identical to the address in the IPv6 TE Router ID TLV. The TE
Router ID MAY be identical to a non-link-local IPv6 Interface Address
advertised by the originating IS in a Link State PDU using the IPv6
Intf. Addr TLV [RFC5308] so long as the address meets the
requirements specified above.
Chen, et al. Expires 1 April 2023 [Page 8]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
3.2. Inter-AS Reachability TLV
The inter-AS reachability TLV has type 141 (see Section 6.1) and
contains a data structure consisting of:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| default metric | (3 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | (1 octet)
+-+-+-+-+-+-+-+-+
|sub-TLVs length| (1 octet)
+-+-+-+-+-+-+-+-+-+-+-+-
| sub-TLVs ... (0-246 octets)
+-+-+-+-+-+-+-+-+-+-+-+-
Flags consists of the following:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S|D| Rsvd |
+-+-+-+-+-+-+-+-+
where:
S bit: If the S bit is set(1), the Inter-AS Reachability TLV
MUST be flooded across the entire routing domain. If the S bit is
not set(0), the TLV MUST NOT be leaked between levels. This bit MUST
NOT be altered during the TLV leaking.
D bit: When the Inter-AS Reachability TLV is leaked from
Level 2 (L2) to Level 1 (L1), the D bit MUST be set. Otherwise, this
bit MUST be clear. Inter-AS Reachability TLVs with the D bit set
MUST NOT be leaked from Level 1 to Level 2. This is to prevent TLV
looping.
Reserved(Rsvd) bits MUST be zero when originated and ignored
when received.
Compared to the extended reachability TLV which is defined in
[RFC5305], the inter-AS reachability TLV replaces the "7 octets of
System ID and Pseudonode Number" field with a "4 octets of Router ID"
field and introduces an extra "control information" field, which
consists of a flooding-scope bit (S bit), an up/down bit (D bit), and
6 reserved bits.
Chen, et al. Expires 1 April 2023 [Page 9]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
The Router ID field of the inter-AS reachability TLV is 4 octets in
length and has a value as defined in Section 3.1. If the originating
node does not support IPv4, then the reserved value 0.0.0.0 MUST be
used in the Router ID field and the IPv6 Router ID sub-TLV MUST be
present in the inter-AS reachability TLV. The Router ID could be
used to indicate the source of the inter-AS reachability TLV.
The flooding procedures for inter-AS reachability TLV are identical
to the flooding procedures for the GENINFO TLV, which are defined in
Section 4 of [RFC6823]. These procedures have been previously
discussed in [RFC7981]. The flooding-scope bit (S bit) SHOULD be set
to 0 if the flooding scope is to be limited to within the single IGP
area to which the ASBR belongs. It MAY be set to 1 if the
information is intended to reach all routers (including area border
routers, ASBRs, and PCEs) in the entire IS-IS routing domain. The
choice between the use of 0 or 1 is an AS-wide policy choice, and
configuration control SHOULD be provided in ASBR implementations that
support the advertisement of inter-AS TE links.
The sub-TLVs defined in [RFC5305], [RFC6119], and other documents for
describing the TE properties of a TE link are also applicable to the
inter-AS reachability TLV for describing the TE properties of an
Inter-AS TE link. Apart from these sub-TLVs, four new sub-TLVs are
defined for inclusion in the inter-AS reachability TLV defined in
this document:
Sub-TLV type Length Name
------------ ------ ---------------------------
24 4 remote AS number
25 4 IPv4 remote ASBR identifier
26 16 IPv6 remote ASBR identifier
TBD1 16 IPv6 local ASBR identifier
Detailed definitions of the four new sub-TLVs are described in
Sections 3.3.1, 3.3.2, 3.3.3, and 3.3.4.
3.3. TE Router ID
The Traffic Engineering router ID TLV and IPv6 TE Router ID TLV,
which are defined in [RFC5305] and [RFC6119] respectively, only have
area flooding-scope. When performing inter-AS TE, the TE Router ID
MAY be needed to reach all routers within an entire IS-IS routing
domain and it MUST have the same flooding scope as the Inter-AS
Reachability TLV does.
Chen, et al. Expires 1 April 2023 [Page 10]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
[RFC7981] defines a generic advertisement mechanism for IS-IS which
allows a router to advertise its capabilities within an IS-IS area or
an entire IS-IS routing domain. [RFC7981] also points out that the
TE Router ID is a candidate to be carried in the IS-IS router
capability TLV when performing inter-area TE.
This document uses such mechanism for TE Router ID advertisement when
the TE Router ID is needed to reach all routers within an entire IS-
IS Routing domain. Two new sub-TLVs are defined for inclusion in the
IS-IS Router Capability TLV to carry the TE Router IDs.
Sub-TLV type Length Name
------------ ------ -----------------
11 4 IPv4 TE Router ID
12 16 IPv6 TE Router ID
Detailed definitions of the new sub-TLVs are described in
Section 3.4.1 and 3.4.2.
3.4. Sub-TLVs for Inter-AS Reachability TLV
3.4.1. Remote AS Number Sub-TLV
A new sub-TLV, the remote AS number sub-TLV, is defined for inclusion
in the inter-AS reachability TLV when advertising inter-AS links.
The remote AS number sub-TLV specifies the AS number of the
neighboring AS to which the advertised link connects.
The remote AS number sub-TLV is TLV type 24 (see Section 6.2) and is
4 octets in length. The format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote AS Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The remote AS number field has 4 octets. When only 2 octets are used
for the AS number, the left (high-order) 2 octets MUST be set to 0.
The remote AS number sub-TLV MUST be included when a router
advertises an inter-AS TE link.
Chen, et al. Expires 1 April 2023 [Page 11]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
3.4.2. IPv4 Remote ASBR ID Sub-TLV
A new sub-TLV, which is referred to as the IPv4 remote ASBR ID sub-
TLV, is defined for inclusion in the inter-AS reachability TLV when
advertising inter-AS links. The IPv4 remote ASBR ID sub-TLV
specifies the IPv4 identifier of the remote ASBR to which the
advertised inter-AS link connects. The value advertised is selected
as defined in Section 3.1.
The IPv4 remote ASBR ID sub-TLV is TLV type 25 (see Section 6.2) and
is 4 octets in length. The format of the IPv4 remote ASBR ID sub-TLV
is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The IPv4 remote ASBR ID sub-TLV MUST be included if the neighboring
ASBR has an IPv4 address. The value advertised is selected as
defined in Section 3.1. If the neighboring ASBR does not have an
IPv4 address, the IPv6 remote ASBR ID sub-TLV MUST be included
instead. An IPv4 remote ASBR ID sub-TLV and IPv6 remote ASBR ID sub-
TLV MAY both be present in an extended IS reachability TLV.
3.4.3. IPv6 Remote ASBR ID Sub-TLV
A new sub-TLV, which is referred to as the IPv6 remote ASBR ID sub-
TLV, is defined for inclusion in the inter-AS reachability TLV when
advertising inter-AS links. The IPv6 remote ASBR ID sub-TLV
specifies the IPv6 identifier of the remote ASBR to which the
advertised inter-AS link connects. The value advertised is selected
as defined in Section 3.1.
The IPv6 remote ASBR ID sub-TLV is TLV type 26 (see Section 6.2) and
is 16 octets in length. The format of the IPv6 remote ASBR ID sub-
TLV is as follows:
Chen, et al. Expires 1 April 2023 [Page 12]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The IPv6 remote ASBR ID sub-TLV MUST be included if the neighboring
ASBR has an IPv6 address. If the neighboring ASBR does not have an
IPv6 address, the IPv4 remote ASBR ID sub-TLV MUST be included
instead. An IPv4 remote ASBR ID sub-TLV and IPv6 remote ASBR ID sub-
TLV MAY both be present in an extended IS reachability TLV.
3.4.4. IPv6 Local ASBR ID sub-TLV
The IPv6 Local ASBR ID sub-TLV is TLV type TBD1 (see Section 6.3) and
is 16 octets in length. The format of the IPv6 Local ASBR ID sub-TLV
is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local ASBR ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local ASBR ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value advertised is selected as defined in Section 3.1.
If the originating node does not support IPv4, the IPv6 Local ASBR ID
sub-TLV MUST be present in the inter-AS reachability TLV. Inter-AS
reachability TLVs which have a Router ID of 0.0.0.0 and do not have
the IPv6 Local ASBR ID sub-TLV present MUST be ignored.
Chen, et al. Expires 1 April 2023 [Page 13]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
3.5. Sub-TLVs for IS-IS Router Capability TLV
3.5.1. IPv4 TE Router ID sub-TLV
The IPv4 TE Router ID sub-TLV is TLV type 11 (see Section 6.3) and is
4 octets in length. The format of the IPv4 TE Router ID sub-TLV is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TE Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value advertised is selected as defined in Section 3.1.
When the TE Router ID is needed to reach all routers within an entire
IS-IS routing domain, the IS-IS Router capability TLV MUST be
included in its LSP. If an ASBR supports Traffic Engineering for
IPv4 and if the ASBR has an IPv4 TE Router ID, the IPv4 TE Router ID
sub-TLV MUST be included. If the ASBR does not have an IPv4 TE
Router ID, the IPv6 TE Router sub-TLV MUST be included instead. An
IPv4 TE Router ID sub-TLV and IPv6 TE Router ID sub-TLV MAY both be
present in an IS-IS router capability TLV.
3.5.2. IPv6 TE Router ID sub-TLV
The IPv6 TE Router ID sub-TLV is TLV type 12 (see Section 6.3) and is
16 octets in length. The format of the IPv6 TE Router ID sub-TLV is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TE Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TE Router ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TE Router ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TE Router ID (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value advertised is selected as defined in Section 3.1.
Chen, et al. Expires 1 April 2023 [Page 14]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
When the TE Router ID is needed to reach all routers within an entire
IS-IS routing domain, the IS-IS router capability TLV MUST be
included in its LSP. If an ASBR supports Traffic Engineering for
IPv6 and if the ASBR has an IPv6 TE Router ID, the IPv6 TE Router ID
sub-TLV MUST be included. If the ASBR does not have an IPv6 TE
Router ID, the IPv4 TE Router sub-TLV MUST be included instead. An
IPv4 TE Router ID sub-TLV and IPv6 TE Router ID sub-TLV MAY both be
present in an IS-IS router capability TLV.
4. Procedure for Inter-AS TE Links
When TE is enabled on an inter-AS link and the link is up, the ASBR
SHOULD advertise this link using the normal procedures for [RFC5305].
When either the link is down or TE is disabled on the link, the ASBR
SHOULD withdraw the advertisement. When there are changes to the TE
parameters for the link (for example, when the available bandwidth
changes), the ASBR SHOULD re-advertise the link but MUST take
precautions against excessive re-advertisements.
Hellos MUST NOT be exchanged over the inter-AS link, and
consequently, an IS-IS adjacency MUST NOT be formed.
The information advertised comes from the ASBR's knowledge of the TE
capabilities of the link, the ASBR's knowledge of the current status
and usage of the link, and configuration at the ASBR of the remote AS
number and remote ASBR TE Router ID.
Legacy routers receiving an advertisement for an inter-AS TE link are
able to ignore it because they do not know the new TLV and sub-TLVs
that are defined in Section 3 of this document. They will continue
to flood the LSP, but will not attempt to use the information
received.
In the current operation of ISIS-TE, the LSRs at each end of a TE
link emit LSPs describing the link. The databases in the LSRs then
have two entries (one locally generated, the other from the peer)
that describe the different 'directions' of the link. This enables
Constrained Shortest Path First (CSPF) to do a two-way check on the
link when performing path computation and eliminate it from
consideration unless both directions of the link satisfy the required
constraints.
In the case we are considering here (i.e., of a TE link to another
AS), there is, by definition, no IGP peering and hence no
bidirectional TE link information. In order for the CSPF route
computation entity to include the link as a candidate path, we have
to find a way to get LSPs describing its (bidirectional) TE
properties into the TE database.
Chen, et al. Expires 1 April 2023 [Page 15]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
This is achieved by the ASBR advertising, internally to its AS,
information about both directions of the TE link to the next AS. The
ASBR will normally generate an LSP describing its own side of a link;
here we have it 'proxy' for the ASBR at the edge of the other AS and
generate an additional LSP that describes that device's 'view' of the
link.
Only some essential TE information for the link needs to be
advertised; i.e., the Interface Address, the remote AS number, and
the remote ASBR ID of an inter-AS TE link.
Routers or PCEs that are capable of processing advertisements of
inter-AS TE links SHOULD NOT use such links to compute paths that
exit an AS to a remote ASBR and then immediately re-enter the AS
through another TE link. Such paths would constitute extremely rare
occurrences and SHOULD NOT be allowed except as the result of
specific policy configurations at the router or PCE computing the
path.
4.1. Origin of Proxied TE Information
Section 4 describes how an ASBR advertises TE link information as a
proxy for its neighbor ASBR, but does not describe where this
information comes from.
Although the source of the information described in Section 4 is
outside the scope of this document, it is possible that it will be a
configuration requirement at the ASBR, as are other local properties
of the TE link. Further, where BGP is used to exchange IP routing
information between the ASBRs, a certain amount of additional local
configuration about the link and the remote ASBR is likely to be
available.
We note further that it is possible, and may be operationally
advantageous, to obtain some of the required configuration
information from BGP. Whether and how to utilize these possibilities
is an implementation matter.
5. Security Considerations
The protocol extensions defined in this document are relatively minor
and can be secured within the AS in which they are used by the
existing IS-IS security mechanisms (e.g., using the cleartext
passwords or Hashed Message Authentication Codes, which are defined
in [RFC1195], [RFC5304], and [RFC5310] separately).
Chen, et al. Expires 1 April 2023 [Page 16]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
There is no exchange of information between ASes, and no change to
the IS-IS security relationship between the ASes. In particular,
since no IS-IS adjacency is formed on the inter-AS links, there is no
requirement for IS-IS security between the ASes.
Some of the information included in these new advertisements (e.g.,
the remote AS number and the remote ASBR ID) is obtained manually
from a neighboring administration as part of a commercial
relationship. The source and content of this information should be
carefully checked before it is entered as configuration information
at the ASBR responsible for advertising the inter-AS TE links.
It is worth noting that in the scenario we are considering, a Border
Gateway Protocol (BGP) peering may exist between the two ASBRs and
that this could be used to detect inconsistencies in configuration
(e.g., the administration that originally supplied the information
may provide incorrect information, or some manual mis-configurations
or mistakes may be made by the operators). For example, if a
different remote AS number is received in a BGP OPEN [RFC4271] from
that locally configured to ISIS-TE, as we describe here, then local
policy SHOULD be applied to determine whether to alert the operator
to a potential mis-configuration or to suppress the IS-IS
advertisement of the inter-AS TE link. Advertisement of incorrect
information could result in an inter-AS TE LSP that traverses an
unintended AS. Note further that if BGP is used to exchange TE
information as described in Section 4.1, the inter-AS BGP session
SHOULD be secured using mechanisms such as described in [RFC5925] to
provide authentication and integrity checks.
For a discussion of general security considerations for IS-IS, see
[RFC5304].
6. IANA Considerations
IANA is requested to make the following allocations from registries
under its control.
6.1. Inter-AS Reachability TLV
This document defines the following new IS-IS TLV type, described in
Section 3.1, which has been registered in the IS-IS TLV codepoint
registry:
Type Description IIH LSP SNP Purge Reference
---- ---------------------- --- --- --- ----- ---------
141 inter-AS reachability n y n n [This.I-D]
information
Chen, et al. Expires 1 April 2023 [Page 17]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
6.2. Sub-TLVs for the Inter-AS Reachability TLV
This document defines the following new sub-TLV types (described in
Sections 3.3.1, 3.3.2, 3.3.3, and, 3.3.4) of top-level TLV 141 (see
Section 6.1 above). Three of these sub-TLVs have been registered in
the IS-IS Sub-TLVs for TLVs Advertising Neighbor Information registry
by [RFC5316]. One additional sub-TLV (IPv6 local ASBR identifier) is
introduced by this document and needs to be added to the same
registry.
Type Description 22 23 25 141 222 223 Reference
---- ----------------------------- --- --- --- --- --- --- ---------
24 remote AS number n n n y n n [This.I-D]
25 IPv4 remote ASBR identifier n n n y n n [This.I-D]
26 IPv6 remote ASBR identifier n n n y n n [This.I-D]
TBD1 IPv6 local ASBR identifier n n n y n n [This.I-D]
As described above in Section 3.1, the sub-TLVs which are defined in
[RFC5305], [RFC6119] and other documents for describing the TE
properties of a TE link are applicable to describe an inter-AS TE
link and MAY be included in the inter-AS reachability TLV when
adverting inter-AS TE links.
6.3. Sub-TLVs for the IS-IS Router Capability TLV
This document defines the following new sub-TLV types, described in
Sections 3.4.1 and 3.4.2, of top-level TLV 242 (which is defined in
[RFC7981]) that have been registered in the IS-IS Sub-TLVs for IS-IS
Router CAPABILITY TLV registry:
Type Description Reference
---- ------------------------------ ---------
11 IPv4 TE Router ID [This.I-D]
12 IPv6 TE Router ID [This.I-D]
7. Acknowledgements
For the original version of [RFC5316] the authors thanked Adrian
Farrel, Jean-Louis Le Roux, Christian Hopps, and Hannes Gredler for
their review and comments on this document.
8. References
8.1. Normative References
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.
Chen, et al. Expires 1 April 2023 [Page 18]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6119] Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
February 2011, <https://www.rfc-editor.org/info/rfc6119>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[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>.
8.2. Informative References
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC4216] Zhang, R., Ed. and J.-P. Vasseur, Ed., "MPLS Inter-
Autonomous System (AS) Traffic Engineering (TE)
Requirements", RFC 4216, DOI 10.17487/RFC4216, November
2005, <https://www.rfc-editor.org/info/rfc4216>.
Chen, et al. Expires 1 April 2023 [Page 19]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC5152] Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
Per-Domain Path Computation Method for Establishing Inter-
Domain Traffic Engineering (TE) Label Switched Paths
(LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
<https://www.rfc-editor.org/info/rfc5152>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310, February
2009, <https://www.rfc-editor.org/info/rfc5310>.
[RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
December 2008, <https://www.rfc-editor.org/info/rfc5316>.
[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441,
DOI 10.17487/RFC5441, April 2009,
<https://www.rfc-editor.org/info/rfc5441>.
[RFC6823] Ginsberg, L., Previdi, S., and M. Shand, "Advertising
Generic Information in IS-IS", RFC 6823,
DOI 10.17487/RFC6823, December 2012,
<https://www.rfc-editor.org/info/rfc6823>.
Appendix A. Changes to RFC 5316
The following is a summary of the substantive changes this document
makes to RFC 5316. Some editorial changes were also made.
Chen, et al. Expires 1 April 2023 [Page 20]
Internet-Draft ISIS Extensions for Inter-AS TE September 2022
RFC 5316 only allowed a 32 bit Router ID in the fixed header of TLV
141. This is problematic in an IPv6-only deployment where an IPv4
address may not be available. This document specifies:
1. The Router ID should be identical to the value advertised in the
Traffic Engineering Router ID TLV (134) if available.
2. If no Traffic Engineering Router ID is assigned the Router ID
should be identical to an IP Interface Address [RFC1195] advertised
by the originating IS.
3. If the originating node does not support IPv4, then the reserved
value 0.0.0.0 must be used in the Router ID field and the new IPv6
Local ASBR identifier sub-TLV must be present in the TLV.
Authors' Addresses
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
Les Ginsberg
Cisco Systems
Email: ginsberg@cisco.com
Stefano Previdi
Huawei Technologies
Italy
Email: stefano@previdi.net
Xiaodong Duan
China Mobile
Email: duanxiaodong@chinamobile.com
Chen, et al. Expires 1 April 2023 [Page 21]