Internet DRAFT - draft-ietf-lsr-flex-algo
draft-ietf-lsr-flex-algo
Network Working Group P. Psenak, Ed.
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track S. Hegde
Expires: 20 April 2023 Juniper Networks, Inc.
C. Filsfils
Cisco Systems, Inc.
K. Talaulikar
Cisco Systems, Inc
A. Gulko
Edward Jones
17 October 2022
IGP Flexible Algorithm
draft-ietf-lsr-flex-algo-26
Abstract
IGP protocols historically compute best paths over the network based
on the IGP metric assigned to the links. Many network deployments
use RSVP-TE based or Segment Routing based Traffic Engineering to
steer traffic over a path that is computed using different metrics or
constraints than the shortest IGP path. This document specifies a
solution that allows IGPs themselves to compute constraint-based
paths over the network. This document also specifies a way of using
Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets
along the constraint-based paths.
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/.
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 20 April 2023.
Psenak, et al. Expires 20 April 2023 [Page 1]
Internet-Draft IGP Flexible Algorithm October 2022
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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 5
5. Flexible Algorithm Definition Advertisement . . . . . . . . . 6
5.1. IS-IS Flexible Algorithm Definition Sub-TLV . . . . . . . 6
5.2. OSPF Flexible Algorithm Definition TLV . . . . . . . . . 8
5.3. Common Handling of Flexible Algorithm Definition TLV . . 10
6. Sub-TLVs of IS-IS FAD Sub-TLV . . . . . . . . . . . . . . . . 11
6.1. IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV . . 11
6.2. IS-IS Flexible Algorithm Include-Any Admin Group
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. IS-IS Flexible Algorithm Include-All Admin Group
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.4. IS-IS Flexible Algorithm Definition Flags Sub-TLV . . . . 14
6.5. IS-IS Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 16
7. Sub-TLVs of OSPF FAD TLV . . . . . . . . . . . . . . . . . . 17
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . . 17
7.2. OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3. OSPF Flexible Algorithm Include-All Admin Group
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV . . . . 19
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 20
8. IS-IS Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . 21
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . . 22
10. OSPF Flexible Algorithm ASBR Reachability Advertisement . . . 23
10.1. OSPFv2 Extended Inter-Area ASBR LSA . . . . . . . . . . 23
10.1.1. OSPFv2 Extended Inter-Area ASBR TLV . . . . . . . . 25
10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV . . . . . . 26
11. Advertisement of Node Participation in a Flex-Algorithm . . . 28
Psenak, et al. Expires 20 April 2023 [Page 2]
Internet-Draft IGP Flexible Algorithm October 2022
11.1. Advertisement of Node Participation for Segment
Routing . . . . . . . . . . . . . . . . . . . . . . . . 28
11.2. Advertisement of Node Participation for Other
Data-planes . . . . . . . . . . . . . . . . . . . . . . 28
12. Advertisement of Link Attributes for Flex-Algorithm . . . . . 29
13. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 30
13.1. Multi-area and Multi-domain Considerations . . . . . . . 31
14. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . . 34
14.1. Segment Routing MPLS Forwarding for Flex-Algorithm . . . 34
14.2. SRv6 Forwarding for Flex-Algorithm . . . . . . . . . . . 35
14.3. Other Data-planes' Forwarding for Flex-Algorithm . . . . 36
15. Operational Considerations . . . . . . . . . . . . . . . . . 36
15.1. Inter-area Considerations . . . . . . . . . . . . . . . 36
15.2. Usage of SRLG Exclude Rule with Flex-Algorithm . . . . . 37
15.3. Max-metric consideration . . . . . . . . . . . . . . . . 37
15.4. FAD Definition and Changes . . . . . . . . . . . . . . . 38
15.5. Number of Flex-Algorithms . . . . . . . . . . . . . . . 38
16. Backward Compatibility . . . . . . . . . . . . . . . . . . . 38
17. Security Considerations . . . . . . . . . . . . . . . . . . . 38
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
18.1. IGP IANA Considerations . . . . . . . . . . . . . . . . 39
18.1.1. IGP Algorithm Types Registry . . . . . . . . . . . . 39
18.1.2. IGP Metric-Type Registry . . . . . . . . . . . . . . 39
18.2. Flexible Algorithm Definition Flags Registry . . . . . . 40
18.3. IS-IS IANA Considerations . . . . . . . . . . . . . . . 40
18.3.1. IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV . . . 40
18.3.2. IS-IS Sub-TLVs for TLVs Advertising Prefix
Reachability . . . . . . . . . . . . . . . . . . . . 41
18.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . 41
18.4. OSPF IANA Considerations . . . . . . . . . . . . . . . . 42
18.4.1. OSPF Router Information (RI) TLVs Registry . . . . . 42
18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs . . . . . . . . 42
18.4.3. OSPFv3 Extended-LSA Sub-TLVs . . . . . . . . . . . . 42
18.4.4. OSPF Flex-Algorithm Prefix Metric Bits . . . . . . . 43
18.4.5. OSPFv2 Opaque LSA Option Types . . . . . . . . . . . 43
18.4.6. OSPFv2 Extended Inter-Area ASBR TLVs . . . . . . . . 44
18.4.7. OSPFv2 Inter-Area ASBR Sub-TLVs . . . . . . . . . . 44
18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLV
Registry . . . . . . . . . . . . . . . . . . . . . . 44
18.4.9. Link Attribute Applications Registry . . . . . . . . 46
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 46
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
20.1. Normative References . . . . . . . . . . . . . . . . . . 46
20.2. Informative References . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
Psenak, et al. Expires 20 April 2023 [Page 3]
Internet-Draft IGP Flexible Algorithm October 2022
1. Introduction
An IGP-computed path based on the shortest IGP metric is often
replaced by a traffic-engineered path due to requirements which are
not reflected by the IGP metric. Some networks engineer the IGP
metric assignments in a way that the IGP metric reflects the link
bandwidth or delay. If, for example, the IGP metric reflects the
bandwidth on the link and user traffic is delay sensitive, the best
IGP path may not reflect the best path from such a user's
perspective.
To overcome this limitation, various sorts of traffic engineering
have been deployed, including RSVP-TE and SR-TE, in which case the TE
component is responsible for computing paths based on additional
metrics and/or constraints. Such paths need to be installed in the
forwarding tables in addition to, or as a replacement for, the
original paths computed by IGPs. Tunnels are often used to represent
the engineered paths and mechanisms like the one described in
[RFC3906] are used to replace the original IGP paths with such tunnel
paths.
This document specifies a set of extensions to IS-IS, OSPFv2, and
OSPFv3 that enable a router to advertise TLVs that (a) identify
calculation-type, (b) specify a metric-type, and (c) describe a set
of constraints on the topology, that are to be used to compute the
best paths along the constrained topology. A given combination of
calculation-type, metric-type, and constraints is known as a
"Flexible Algorithm Definition". A router that sends such a set of
TLVs also assigns a Flex-Algorithm value to the specified combination
of calculation-type, metric-type, and constraints.
This document also specifies a way for a router to use IGPs to
associate one or more "Segment Routing with the MPLS Data Plane (SR-
MPLS)" Prefix-SIDs [RFC8660], or "Segment Routing over IPv6 (SRv6)"
locators [RFC8986] with a particular Flex-Algorithm. Each such
Prefix-SID or SRv6 locator then represents a path that is computed
according to the identified Flex-Algorithm. In SRv6 it is the
locator, not the SID, that holds the binding to the algorithm.
2. 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.
Psenak, et al. Expires 20 April 2023 [Page 4]
Internet-Draft IGP Flexible Algorithm October 2022
3. Terminology
This section defines terms that are often used in this document.
Flexible Algorithm Definition (FAD) - the set consisting of (a)
calculation-type, (b) metric-type, and (c) a set of constraints.
Flex-Algorithm - a numeric identifier in the range 128-255 that is
associated via configuration with the Flexible Algorithm Definition.
Local Flexible Algorithm Definition - Flexible Algorithm Definition
defined locally on the node.
Remote Flexible Algorithm Definition - Flexible Algorithm Definition
received from other nodes via IGP flooding.
Flexible Algorithm Participation - per data-plane configuration state
that expresses whether the node is participating in a particular
Flexible Algorithm. Not all routers in a given network need to
participate in a given Flexible Algorithm. The Flexible Algorithm(s)
a given router participates in is determined by configuration.
IGP Algorithm - value from the "IGP Algorithm Types" registry defined
under "Interior Gateway Protocol (IGP) Parameters" IANA registry
grouping. IGP Algorithms represents the triplet (calculation-type,
metric-type, constraints), where the second and third elements of the
triple MAY be unspecified.
ABR - Area Border Router. In IS-IS terminology it is also known as
L1/L2 router.
ASBR - Autonomous System Border Router.
4. Flexible Algorithm
Many possible constraints may be used to compute a path over a
network. Some networks are deployed as multiple planes. A simple
form of constraint may be to use a particular plane. A more
sophisticated form of constraint can include some extended metric as
described in [RFC8570]. Constraints which restrict paths to links
with specific affinities or avoid links with specific affinities are
also possible. Combinations of these are also possible.
To provide maximum flexibility, a mechanism is provided that allows a
router to (a) identify a particular calculation-type and (b) metric-
type, (c) describe a particular set of constraints, and (d) assign a
numeric identifier, referred to as Flex-Algorithm, to the combination
of that calculation-type, metric-type, and those constraints. The
Psenak, et al. Expires 20 April 2023 [Page 5]
Internet-Draft IGP Flexible Algorithm October 2022
mapping between the Flex-Algorithm and its meaning is flexible and
defined by the user. As long as all routers in the domain have a
common understanding as to what a particular Flex-Algorithm
represents, the resulting routing computation is consistent and
traffic is not subject to any looping.
The set consisting of (a) calculation-type, (b) metric-type, and (c)
a set of constraints is referred to as a Flexible Algorithm
Definition.
Flex-Algorithm is a numeric identifier in the range 128-255 that is
associated via configuration with the Flexible Algorithm Definition.
The IANA "IGP Algorithm Types" registry defines the set of values for
IGP Algorithms. The following values area allocated by IANA from
this registry for Flex-Algorithms:
128-255 - Flex-Algorithms
5. Flexible Algorithm Definition Advertisement
To guarantee loop-free forwarding for paths computed for a particular
Flex-Algorithm, all routers that (a) are configured to participate in
a particular Flex-Algorithm, and (b) are in the same Flex-Algorithm
definition advertisement scope MUST agree on the definition of the
Flex-Algorithm. The following procedures ensure this condition is
fulfilled.
5.1. IS-IS Flexible Algorithm Definition Sub-TLV
The IS-IS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used
to advertise the definition of the Flex-Algorithm.
The IS-IS FAD Sub-TLV is advertised as a Sub-TLV of the IS-IS Router
Capability TLV-242 that is defined in [RFC7981].
IS-IS FAD Sub-TLV has the following format:
Psenak, et al. Expires 20 April 2023 [Page 6]
Internet-Draft IGP Flexible Algorithm October 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 |Flex-Algorithm | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 26
Length: variable number of octets, dependent on the included Sub-
TLVs
Flex-Algorithm: Flexible Algorithm number. Single octet value
between 128 and 255 inclusive.
Metric-Type: Type of metric from the "IGP Metric-Type Registry"
(Section 18.1.2) to be used during the calculation. The following
values are defined:
0: IGP Metric
1: Min Unidirectional Link Delay as defined in [RFC8570],
section 4.2, encoded as application specific link attribute as
specified in [RFC8919] and Section 12 of this document.
2: Traffic Engineering Default Metric as defined in [RFC5305],
section 3.7, encoded as application specific link attribute as
specified in [RFC8919] and Section 12 of this document.
Calc-Type: calculation-type, value from 0 to 127 inclusive from
the "IGP Algorithm Types" registry defined under "Interior Gateway
Protocol (IGP) Parameters" IANA registries. IGP algorithms in the
range of 0-127 have a defined triplet (calculation-type, metric-
type, constraints). When used to specify the calculation-type in
the FAD Sub-TLV, only the calculation-type defined for the
specified IGP Algorithm is used. The Metric/Constraints MUST NOT
be inherited. If the required calculation-type is Shortest Path
First, the value 0 MUST appear in this field.
Psenak, et al. Expires 20 April 2023 [Page 7]
Internet-Draft IGP Flexible Algorithm October 2022
Priority: Value between 0 and 255 inclusive that specifies the
priority of the advertisement. Numerically greater values are
preferred. Usage fo the priority is described in Section 5.3.
Sub-TLVs - optional sub-TLVs.
The IS-IS FAD Sub-TLV MAY be advertised in an LSP of any number. IS-
IS router MAY advertise more than one IS-IS FAD Sub-TLV for a given
Flexible Algorithm (see Section 6).
The IS-IS FAD Sub-TLV has an area scope. The Router Capability TLV
in which the FAD Sub-TLV is present MUST have the S-bit clear.
An IS-IS L1/L2 router MAY be configured to re-generate the winning
FAD from level 2, without any modification to it, to the level 1
area. The re-generation of the FAD Sub-TLV from level 2 to level 1
is determined by the L1/L2 router, not by the originator of the FAD
advertisement in the level 2. In such a case, the re-generated FAD
Sub-TLV will be advertised in the level 1 Router Capability TLV
originated by the L1/L2 router.
An L1/L2 router MUST NOT re-generate any FAD Sub-TLV from level 1 to
level 2.
5.2. OSPF Flexible Algorithm Definition TLV
The OSPF FAD TLV is advertised as a top-level TLV of the Router
Information (RI) LSA that is defined in [RFC7770].
The OSPF FAD TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Metric-Type | Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Psenak, et al. Expires 20 April 2023 [Page 8]
Internet-Draft IGP Flexible Algorithm October 2022
Type: 16
Length: variable number of octets, dependent on the included Sub-
TLVs
Flex-Algorithm: Flexible Algorithm number. Single octet value
between 128 and 255 inclusive.
Metric-Type: Type of metric from the "IGP Metric-Type Registry"
(Section 18.1.2) to be used during the calculation. The following
values are defined:
0: IGP Metric
1: Min Unidirectional Link Delay as defined in [RFC7471],
section 4.2, encoded as application specific link attribute as
specified in [RFC8920] and Section 12 of this document.
2: Traffic Engineering metric as defined in [RFC3630], section
2.5.5, encoded as application specific link attribute as
specified in [RFC8920] and Section 12 of this document.
Calc-Type: as described in Section 5.1
Priority: as described in Section 5.1
Sub-TLVs - optional sub-TLVs.
When multiple OSPF FAD TLVs, for the same Flexible Algorithm, are
received from a given router, the receiver MUST use the first
occurrence of the TLV in the Router Information LSA. If the OSPF FAD
TLV, for the same Flex-Algorithm, appears in multiple Router
Information LSAs that have different flooding scopes, the OSPF FAD
TLV in the Router Information LSA with the area-scoped flooding scope
MUST be used. If the OSPF FAD TLV, for the same algorithm, appears
in multiple Router Information LSAs that have the same flooding
scope, the OSPF FAD TLV in the Router Information (RI) LSA with the
numerically smallest Instance ID MUST be used and subsequent
instances of the OSPF FAD TLV MUST be ignored.
The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
OSPF FAD TLV advertisement, area-scoped flooding is REQUIRED. The
Autonomous System flooding scope SHOULD NOT be used unless local
configuration policy on the originating router indicates domain wide
flooding.
Psenak, et al. Expires 20 April 2023 [Page 9]
Internet-Draft IGP Flexible Algorithm October 2022
5.3. Common Handling of Flexible Algorithm Definition TLV
This section describes the protocol-independent handling of the FAD
TLV (OSPF) or FAD Sub-TLV (IS-IS). We will refer to it as FAD TLV in
this section, even though in the case of IS-IS it is a Sub-TLV.
The value of the Flex-Algorithm MUST be between 128 and 255
inclusive. If it is not, the FAD TLV MUST be ignored.
Only a subset of the routers participating in the particular Flex-
Algorithm need to advertise the definition of the Flex-Algorithm.
Every router, that is configured to participate in a particular Flex-
Algorithm, MUST select the Flex-Algorithm definition based on the
following ordered rules. This allows for the consistent Flex-
Algorithm definition selection in cases where different routers
advertise different definitions for a given Flex-Algorithm:
1. From the advertisements of the FAD in the area (including both
locally generated advertisements and received advertisements)
select the one(s) with the numerically greatest priority value.
2. If there are multiple advertisements of the FAD with the same
numerically greatest priority, select the one that is originated
from the router with the numerically greatest System-ID, in the
case of IS-IS, or Router ID, in the case of OSPFv2 and OSPFv3.
For IS-IS, the System-ID is described in [ISO10589]. For OSPFv2
and OSPFv3, standard Router ID is described in [RFC2328] and
[RFC5340] respectively.
The FAD selected according to these rules is also known as the
"winning FAD".
A router that is not configured to participate in a particular Flex-
Algorithm MUST ignore FAD Sub-TLVs advertisements for such Flex-
Algorithm.
A router that is not participating in a particular Flex-Algorithm MAY
advertise FAD for such Flex-Algorithm. Receiving routers MUST
consider a received FAD advertisement regardless of the Flex-
Algorithm participation of that FAD advertisement's originator.
Any change in the Flex-Algorithm definition may result in temporary
disruption of traffic that is forwarded based on such Flex-Algorithm
paths. The impact is similar to any other event that requires
network-wide convergence.
Psenak, et al. Expires 20 April 2023 [Page 10]
Internet-Draft IGP Flexible Algorithm October 2022
If a node is configured to participate in a particular Flexible
Algorithm, but there is no valid Flex-Algorithm definition available
for it, or the selected Flex-Algorithm definition includes
calculation-type, metric-type, constraint, flag, or Sub-TLV that is
not supported by the node, it MUST stop participating in such
Flexible Algorithm. That implies that it MUST NOT announce
participation for such Flexible Algorithm as specified in Section 11
and it MUST remove any forwarding state associated with it.
Flex-Algorithm definition is topology independent. It applies to all
topologies that a router participates in.
6. Sub-TLVs of IS-IS FAD Sub-TLV
One of the limitations of IS-IS [ISO10589] is that the length of a
TLV/sub-TLV is limited to a maximum of 255 octets. For the FAD sub-
TLV, there are a number of sub-sub-TLVs (defined below) which are
supported. For a given Flex-Algorithm, it is possible that the total
number of octets required to completely define a FAD exceeds the
maximum length supported by a single FAD sub-TLV. In such cases, the
FAD MAY be split into multiple such sub-TLVs and the content of the
multiple FAD sub-TLVs combined to provide a complete FAD for the
Flex-Algorithm. In such a case, the fixed portion of the FAD (see
Section 5.1) MUST be identical in all FAD sub-TLVs for a given Flex-
Algorithm from a given IS. In case the fixed portion of such FAD
Sub-TLVs differ, the values in the fixed portion in the FAD sub-TLV
in the first occurrence in the lowest numbered LSP from a given IS
MUST be used.
Any specification that introduces a new IS-IS FAD sub-sub-TLV MUST
specify whether the FAD sub-TLV may appear multiple times in the set
of FAD sub-TLVs for a given Flex-Algorithm from a given IS and how to
handle them if multiple are allowed.
6.1. IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV
The Flexible Algorithm definition can specify 'colors' that are used
by the operator to exclude links during the Flex-Algorithm path
computation.
The IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV is used to
advertise the exclude rule that is used during the Flex-Algorithm
path calculation as specified in Section 13.
The IS-IS Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-
TLV) is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following
format:
Psenak, et al. Expires 20 April 2023 [Page 11]
Internet-Draft IGP Flexible Algorithm October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The IS-IS FAEAG Sub-TLV MUST NOT appear more than once in a single
IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-
TLV MUST be ignored by the receiver.
The IS-IS FAEAG Sub-TLV MUST NOT appear more than once in the set of
FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it
appears more than once in such a set, the IS-IS FAEAG Sub-TLV in the
first occurrence in the lowest numbered LSP from a given IS MUST be
used and any other occurrences MUST be ignored.
6.2. IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV
The Flexible Algorithm definition can specify 'colors' that are used
by the operator to include links during the Flex-Algorithm path
computation.
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV is used
to advertise the include-any rule that is used during the Flex-
Algorithm path calculation as specified in Section 13.
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV is a
Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format:
Psenak, et al. Expires 20 April 2023 [Page 12]
Internet-Draft IGP Flexible Algorithm October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 2
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more than once in a single IS-IS FAD Sub-TLV. If it appears
more than once, the IS-IS FAD Sub-TLV MUST be ignored by the
receiver.
The IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more than once in the set of FAD sub-TLVs for a given Flex-
Algorithm from a given IS. If it appears more than once in such a
set, the IS-IS Flexible Algorithm Include-Any Admin Group Sub-TLV in
the first occurrence in the lowest numbered LSP from a given IS MUST
be used and any other occurrences MUST be ignored.
6.3. IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV
The Flexible Algorithm definition can specify 'colors' that are used
by the operator to include links during the Flex-Algorithm path
computation.
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV is used
to advertise the include-all rule that is used during the Flex-
Algorithm path calculation as specified in Section 13.
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV is is a
Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format:
Psenak, et al. Expires 20 April 2023 [Page 13]
Internet-Draft IGP Flexible Algorithm October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more than once in a single IS-IS FAD Sub-TLV. If it appears
more than once, the IS-IS FAD Sub-TLV MUST be ignored by the
receiver.
The IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more than once in the set of FAD sub-TLVs for a given Flex-
Algorithm from a given IS. If it appears more than once in such a
set, the IS-IS Flexible Algorithm Include-All Admin Group Sub-TLV in
the first occurrence in the lowest numbered LSP from a given IS MUST
be used and any other occurrences MUST be ignored.
6.4. IS-IS Flexible Algorithm Definition Flags Sub-TLV
The IS-IS Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
is a Sub-TLV of the IS-IS FAD Sub-TLV. It has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Psenak, et al. Expires 20 April 2023 [Page 14]
Internet-Draft IGP Flexible Algorithm October 2022
Type: 4
Length: variable, number of octets of the Flags field
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
M-flag: when set, the Flex-Algorithm specific prefix metric
MUST be used for inter-area and external prefix calculation.
This flag is not applicable to prefixes advertised as SRv6
locators.
A new IANA "IGP Flexible Algorithm Definition Flags Registry" is
defined for allocation of bits in the Flags field - see Section 18.2.
Bits are defined/sent starting with Bit 0 defined above. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order so as to minimize the number of bits that will
need to be transmitted.
Undefined bits MUST be transmitted as 0.
Bits that are not transmitted MUST be treated as if they are set to 0
on receipt.
The IS-IS FADF Sub-TLV MUST NOT appear more than once in a single IS-
IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-TLV
MUST be ignored by the receiver.
The IS-IS FADF Sub-TLV MUST NOT appear more than once in the set of
FAD sub-TLVs for a given Flex-Algorithm from a given IS. If it
appears more than once in such a set, the IS-IS FADF Sub-TLV in the
first occurrence in the lowest numbered LSP from a given IS MUST be
used and any other occurrences MUST be ignored.
If the IS-IS FADF Sub-TLV is not present inside the IS-IS FAD Sub-
TLV, all the bits are assumed to be set to 0.
If a node is configured to participate in a particular Flexible
Algorithm, but the selected Flex-Algorithm definition includes a bit
in the IS-IS FADF Sub-TLV that is not supported by the node, it MUST
stop participating in such Flexible Algorithm.
Psenak, et al. Expires 20 April 2023 [Page 15]
Internet-Draft IGP Flexible Algorithm October 2022
New flag bits may be defined in the future. Implementations MUST
check all advertised flag bits in the received IS-IS FADF Sub-TLV -
not just the subset currently defined.
M-flag MUST not be used when calculating prefix reachability for SRv6
Locator prefix.
6.5. IS-IS Flexible Algorithm Exclude SRLG Sub-TLV
The Flexible Algorithm definition can specify Shared Risk Link Groups
(SRLGs) that the operator wants to exclude during the Flex-Algorithm
path computation.
The IS-IS Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG) is used
to advertise the exclude rule that is used during the Flex-Algorithm
path calculation as specified in Section 13.
The IS-IS FAESRLG Sub-TLV is a Sub-TLV of the IS-IS FAD Sub-TLV. It
has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Value |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5
Length: variable, dependent on number of SRLG values. MUST be a
multiple of 4 octets.
Shared Risk Link Group Value: SRLG value as defined in [RFC5307].
The IS-IS FAESRLG Sub-TLV MUST NOT appear more than once in a single
IS-IS FAD Sub-TLV. If it appears more than once, the IS-IS FAD Sub-
TLV MUST be ignored by the receiver.
Psenak, et al. Expires 20 April 2023 [Page 16]
Internet-Draft IGP Flexible Algorithm October 2022
The IS-IS FAESRLG Sub-TLV MAY appear more than once in the set of FAD
sub-TLVs for a given Flex-Algorithm from a given IS. This may be
necessary in cases where the total number of SRLG values which are
specified cause the FAD sub-TLV to exceed the maximum length of a
single FAD sub-TLV. In such a case the receiver MUST use the union
of all values across all IS-IS FAESRLG Sub-TLVs from such set.
7. Sub-TLVs of OSPF FAD TLV
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV
The Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-TLV) is
a Sub-TLV of the OSPF FAD TLV. Its usage is described in
Section 6.1. It has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The OSPF FAEAG Sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV is a Sub-
TLV of the OSPF FAD TLV. The usage of this Sub-TLVs is described in
Section 6.2. It has the following format:
Psenak, et al. Expires 20 April 2023 [Page 17]
Internet-Draft IGP Flexible Algorithm October 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 2
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more than once in an OSPF FAD TLV. If it appears more than
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV
The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV is a Sub-
TLV of the OSPF FAD TLV. The usage of this Sub-TLVs is described in
Section 6.3. It has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Admin Group |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3
Length: variable, dependent on the size of the Extended Admin
Group. MUST be a multiple of 4 octets.
Extended Administrative Group: Extended Administrative Group as
defined in [RFC7308].
Psenak, et al. Expires 20 April 2023 [Page 18]
Internet-Draft IGP Flexible Algorithm October 2022
The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more than once in an OSPF FAD TLV. If it appears more than
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV
The OSPF Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
is a Sub-TLV of the OSPF FAD TLV. It has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 4
Length: variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
M-flag: when set, the Flex-Algorithm specific prefix and ASBR
metric MUST be used for inter-area and external prefix
calculation. This flag is not applicable to prefixes
advertised as SRv6 locators.
A new IANA "IGP Flexible Algorithm Definition Flags Registry" is
defined for allocation of bits in the Flags field - see Section 18.2.
Bits are defined/sent starting with Bit 0 defined above. Additional
bit definitions that may be defined in the future SHOULD be assigned
in ascending bit order so as to minimize the number of bits that will
need to be transmitted.
Undefined bits MUST be transmitted as 0.
Psenak, et al. Expires 20 April 2023 [Page 19]
Internet-Draft IGP Flexible Algorithm October 2022
Bits that are not transmitted MUST be treated as if they are set to 0
on receipt.
The OSPF FADF Sub-TLV MUST NOT appear more than once in an OSPF FAD
TLV. If it appears more than once, the OSPF FAD TLV MUST be ignored
by the receiver.
If the OSPF FADF Sub-TLV is not present inside the OSPF FAD TLV, all
the bits are assumed to be set to 0.
If a node is configured to participate in a particular Flexible
Algorithm, but the selected Flex-Algorithm definition includes a bit
in the OSPF FADF Sub-TLV that is not supported by the node, it MUST
stop participating in such Flexible Algorithm.
New flag bits may be defined in the future. Implementations MUST
check all advertised flag bits in the received OSPF FADF Sub-TLV -
not just the subset currently defined.
M-flag MUST not be used when calculating prefix reachability for SRv6
Locator prefix.
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV
The OSPF Flexible Algorithm Exclude SRLG Sub-TLV (FAESRLG Sub-TLV) is
a Sub-TLV of the OSPF FAD TLV. Its usage is described in
Section 6.5. It has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Shared Risk Link Group Value |
+- -+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 5
Length: variable, dependent on the number of SRLGs. MUST be a
multiple of 4 octets.
Shared Risk Link Group Value: SRLG value as defined in [RFC4203].
Psenak, et al. Expires 20 April 2023 [Page 20]
Internet-Draft IGP Flexible Algorithm October 2022
The OSPF FAESRLG Sub-TLV MUST NOT appear more than once in an OSPF
FAD TLV. If it appears more than once, the OSPF FAD TLV MUST be
ignored by the receiver.
8. IS-IS Flexible Algorithm Prefix Metric Sub-TLV
The IS-IS Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports
the advertisement of a Flex-Algorithm specific prefix metric
associated with a given prefix advertisement.
The IS-IS FAPM Sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237
and has the following format:
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 |Flex-Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 6
Length: 5 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Metric: 4 octets of metric information
The IS-IS FAPM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
If a prefix is advertised with a Flex-Algorithm prefix metric larger
than MAX_PATH_METRIC as defined in [RFC5305] this prefix MUST NOT be
considered during the Flexible Algorithm computation.
The usage of the Flex-Algorithm prefix metric is described in
Section 13.
Psenak, et al. Expires 20 April 2023 [Page 21]
Internet-Draft IGP Flexible Algorithm October 2022
The IS-IS FAPM Sub-TLV MUST NOT be advertised as a sub-TLV of the IS-
IS SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions]. The IS-IS
SRv6 Locator TLV includes the Algorithm and Metric fields which MUST
be used instead. If the FAPM Sub-TLV is present as a sub-TLV of the
IS-IS SRv6 Locator TLV in the received LSP, such FAPM Sub-TLV MUST be
ignored.
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV
The OSPF Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the
advertisement of a Flex-Algorithm specific prefix metric associated
with a given prefix advertisement.
The OSPF Flex-Algorithm Prefix Metric (FAPM) Sub-TLV is a Sub-TLV of
the:
- OSPFv2 Extended Prefix TLV [RFC7684]
- Following OSPFv3 TLVs as defined in [RFC8362]:
Inter-Area Prefix TLV
External Prefix TLV
OSPF FAPM Sub-TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3 for OSPFv2, 26 for OSPFv3
Length: 8 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Flags: One octet value
Psenak, et al. Expires 20 April 2023 [Page 22]
Internet-Draft IGP Flexible Algorithm October 2022
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|E| |
+-+-+-+-+-+-+-+-+
E bit : position 0: The type of external metric. If bit is
set, the metric specified is a Type 2 external metric. This
bit is applicable only to OSPF External and NSSA external
prefixes. This is semantically the same as the E bit in
section A.4.5 of [RFC2328] and section A.4.7 of [RFC5340] for
OSPFv2 and OSPFv3 respectively.
Bits 1 through 7: MUST be cleared by originator and ignored by
receiver.
Reserved: MUST be set to 0, ignored at reception.
Metric: 4 octets of metric information
The OSPF FAPM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
The usage of the Flex-Algorithm prefix metric is described in
Section 13.
10. OSPF Flexible Algorithm ASBR Reachability Advertisement
An OSPF ABR advertises the reachability of ASBRs in its attached
areas to enable routers within those areas to perform route
calculations for external prefixes advertised by the ASBRs. OSPF
extensions for advertisement of Flex-Algorithm specific reachability
and metric for ASBRs is similarly required for Flex-Algorithm
external prefix computations as described further in Section 13.1.
10.1. OSPFv2 Extended Inter-Area ASBR LSA
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA is an OSPF Opaque
LSA [RFC5250] that is used to advertise additional attributes related
to the reachability of the OSPFv2 ASBR that is external to the area
yet internal to the OSPF domain. Semantically, the OSPFv2 EIA-ASBR
LSA is equivalent to the fixed format Type 4 Summary LSA [RFC2328].
Unlike the Type 4 Summary LSA, the LSID of the EIA-ASBR LSA does not
carry the ASBR Router-ID - the ASBR Router-ID is carried in the body
of the LSA. The OSPFv2 EIA-ASBR LSA is advertised by an OSPFv2 ABR
and its flooding is defined to be area-scoped only.
Psenak, et al. Expires 20 April 2023 [Page 23]
Internet-Draft IGP Flexible Algorithm October 2022
An OSPFv2 ABR generates the EIA-ASBR LSA for an ASBR when it is
advertising the Type-4 Summary LSA for it and has the need for
advertising additional attributes for that ASBR beyond what is
conveyed in the fixed format Type-4 Summary LSA. An OSPFv2 ABR MUST
NOT advertise the EIA-ASBR LSA for an ASBR for which it is not
advertising the Type 4 Summary LSA. This ensures that the ABR does
not generate the EIA-ASBR LSA for an ASBR to which it does not have
reachability in the base OSPFv2 topology calculation. The OSPFv2 ABR
SHOULD NOT advertise the EIA-ASBR LSA for an ASBR when it does not
have additional attributes to advertise for that ASBR.
The OSPFv2 EIA-ASBR LSA has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Type | Opaque ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- TLVs -+
| ... |
LS age and Options fields are as defined in Section A.4.1. of
[RFC2328].
The LS Type MUST be 10, indicating that the Opaque LSA flooding scope
is area-local [RFC5250].
The Opaque Type used by the OSPFv2 EIA-ASBR LSA is 11. The Opaque
Type is used to differentiate the various types of OSPFv2 Opaque LSAs
and is described in Section 3 of [RFC5250].
The Opaque ID field is an arbitrary value used to maintain multiple
OSPFv2 EIA-ASBR LSAs. For OSPFv2 EIA-ASBR LSAs, the Opaque ID has no
semantic significance other than to differentiate OSPFv2 EIA-ASBR
LSAs originated by the same OSPFv2 ABR. If multiple OSPFv2 EIA-ASBR
LSAs specify the same ASBR, the attributes from the Opaque LSA with
the lowest Opaque ID SHOULD be used.
Psenak, et al. Expires 20 April 2023 [Page 24]
Internet-Draft IGP Flexible Algorithm October 2022
Advertising Router, LS sequence number, and LS checksum fields are as
defined in Section A.4.1. of [RFC2328].
The Length field is as defined in Section A.4.1. of [RFC5250]. It
represents the total length (in octets) of the Opaque LSA, including
the LSA header and all TLVs (including padding).
The format of the TLVs within the body of the OSPFv2 EIA-ASBR LSA is
the same as the format used by the Traffic Engineering Extensions to
OSPFv2 [RFC3630]. The variable TLV section consists of one or more
nested TLV tuples. Nested TLVs are also referred to as sub- TLVs.
The TLV Length field defines the length of the value portion in
octets (thus, a TLV with no value portion would have a length of 0).
The TLV is padded to 4-octet alignment; padding is not included in
the Length field (so a 3-octet value would have a length of 3, but
the total size of the TLV would be 8 octets). Nested TLVs are also
32-bit aligned. For example, a 1-octet value would have the Length
field set to 1, and 3 octets of padding would be added to the end of
the value portion of the TLV. The padding is composed of zeros.
10.1.1. OSPFv2 Extended Inter-Area ASBR TLV
The OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) TLV is a top-level TLV
of the OSPFv2 EIA-ASBR LSA and is used to advertise additional
attributes associated with the reachability of an ASBR.
The OSPFv2 EIA-ASBR TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASBR Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1
Length: variable number of octets
ASBR Router ID: four octets carrying the OSPF Router ID of the
ASBR whose information is being carried.
Psenak, et al. Expires 20 April 2023 [Page 25]
Internet-Draft IGP Flexible Algorithm October 2022
Sub-TLVs : variable
Only a single OSPFv2 EIA-ASBR TLV MUST be advertised in each OSPFv2
EIA-ASBR LSA and the receiver MUST ignore all instances of this TLV
other than the first one in an LSA.
OSPFv2 EIA-ASBR TLV MUST be present inside an OSPFv2 EIA-ASBR LSA and
MUST include at least a single sub-TLV, otherwise the OSPFv2 EIA-ASBR
LSA MUST be ignored by the receiver.
10.2. OSPF Flexible Algorithm ASBR Metric Sub-TLV
The OSPF Flexible Algorithm ASBR Metric (FAAM) Sub-TLV supports the
advertisement of a Flex-Algorithm specific metric associated with a
given ASBR reachability advertisement by an ABR.
The OSPF Flex-Algorithm ASBR Metric (FAAM) Sub-TLV is a Sub-TLV of
the:
- OSPFv2 Extended Inter-Area ASBR TLV as defined in Section 10.1.1
- OSPFv3 Inter-Area-Router TLV defined in [RFC8362]
OSPF FAAM Sub-TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flex-Algorithm | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 1 for OSPFv2, 33 for OSPFv3
Length: 8 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Reserved: Three octets. MUST be set to 0, ignored at reception.
Metric: 4 octets of metric information
Psenak, et al. Expires 20 April 2023 [Page 26]
Internet-Draft IGP Flexible Algorithm October 2022
The OSPF FAAM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more than once with the same Flex-Algorithm value, the
first instance MUST be used and any subsequent instances MUST be
ignored.
The advertisement of the ASBR reachability using the OSPF FAAM Sub-
TLV inside the OSPFv2 EIA-ASBR LSA follows Section 12.4.3 of
[RFC2328] and inside the OSPFv3 E-Inter-Area-Router LSA follows
Section 4.8.5 of [RFC5340]. The reachability of the ASBR is
evaluated in the context of the specific Flex-Algorithm.
The FAAM computed by the ABR will be equal to the metric to reach the
ASBR for a given Flex-Algorithm in a source area or the cumulative
metric via other ABR(s) when the ASBR is in a remote area. This is
similar in nature to how the metric is set when the ASBR reachability
metric is computed in the default algorithm for the metric in the
OSPFv2 Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA.
An OSPF ABR MUST NOT include the OSPF FAAM Sub-TLV with a specific
Flex-Algorithm in its reachability advertisement for an ASBR between
areas unless that ASBR is reachable for it in the context of that
specific Flex-Algorithm.
An OSPF ABR MUST include the OSPF FAAM Sub-TLVs as part of the ASBR
reachability advertisement between areas for any Flex-Algorithm for
which the winning FAD includes the M-flag and the ASBR is reachable
in the context of that specific Flex-Algorithm.
OSPF routers MUST use the OSPF FAAM Sub-TLV to calculate the
reachability of the ASBRs if the winning FAD for the specific Flex-
Algorithm includes the M-flag. OSPF routers MUST NOT use the OSPF
FAAM Sub-TLV to calculate the reachability of the ASBRs for the
specific Flex-Algorithm if the winning FAD for such Flex-Algorithm
does not include the M-flag. Instead, the OSPFv2 Type 4 Summary LSAs
or the OSPFv3 Inter-Area-Router-LSAs MUST be used instead as
specified in section 16.2 of [RFC2328] and section 4.8.5 of [RFC5340]
for OSPFv2 and OSPFv3 respectively.
The processing of a new or changed OSPF FAAM Sub-TLV triggers the
processing of External routes similar to what is described in section
16.5 of the [RFC2328] for OSPFv2 and section 4.8.5 of [RFC5340] for
OSPFv3 for the specific Flex-Algorithm. The External and NSSA
External route calculation should be limited to Flex-Algorithm(s) for
which the winning FAD(s) includes the M-flag.
Psenak, et al. Expires 20 April 2023 [Page 27]
Internet-Draft IGP Flexible Algorithm October 2022
Processing of the OSPF FAAM Sub-TLV does not require the existence of
the equivalent OSPFv2 Type 4 Summary LSA or the OSPFv3 Inter-Area-
Router-LSA that is advertised by the same ABR inside the area. The
presence of the base LSA is not mandatory for the usage of the
extended LSA with the OSPF FAAM Sub-TLV.
11. Advertisement of Node Participation in a Flex-Algorithm
When a router is configured to participate in a particular Flex-
Algorithm and is advertising such participation, it is participating
in that Flex-Algorithm.
Paths for various data-planes MAY be computed for a specific Flex-
Algorithm. Each data-plane uses its own specific forwarding over
such Flex-Algorithm paths. To guarantee the presence of the data-
plane specific forwarding, associated with a particular Flex-
Algorithm, a router MUST advertise its participation for a particular
Flex-Algorithm for each data-plane. Some data-planes may share a
common participation advertisement (e.g. SR-MPLS and SRv6).
Advertisement of the participation for any particular Flex-Algorithm
in any data-plane is subject to the condition specified in
Section 5.3.
11.1. Advertisement of Node Participation for Segment Routing
[RFC8667], [RFC8665], and [RFC8666] (IGP Segment Routing extensions)
describe how the SR-Algorithm is used to compute the IGP best path.
Routers advertise support for the SR-Algorithm as a node capability
as described in the above-mentioned IGP Segment Routing extensions.
To advertise participation for a particular Flex-Algorithm for
Segment Routing, including both SR-MPLS and SRv6, the Flex-Algorithm
value MUST be advertised in the SR-Algorithm TLV (OSPF) or sub-TLV
(IS-IS).
Segment Routing Flex-Algorithm participation advertisement is
topology independent. When a router advertises participation in an
SR-Algorithm, the participation applies to all topologies in which
the advertising node participates.
11.2. Advertisement of Node Participation for Other Data-planes
This section describes considerations related to how other data-
planes can advertise their participation in a specific Flex-
Algorithm.
Psenak, et al. Expires 20 April 2023 [Page 28]
Internet-Draft IGP Flexible Algorithm October 2022
Data-plane specific Flex-Algorithm participation advertisements MAY
be topology specific or MAY be topology independent, depending on the
data-plane itself.
Data-plane specific advertisement for Flex-Algorithm participation
MUST be defined for each data-plane and is outside the scope of this
document.
12. Advertisement of Link Attributes for Flex-Algorithm
Various link attributes may be used during the Flex-Algorithm path
calculation. For example, include or exclude rules based on link
affinities can be part of the Flex-Algorithm definition as defined in
Section 6 and Section 7.
Application-specific link attributes, as specified in [RFC8919] or
[RFC8920], that are to be used during Flex-Algorithm calculation MUST
use the Application-Specific Link Attribute (ASLA) advertisements
defined in [RFC8919] or [RFC8920], unless, in the case of IS-IS, the
L-Flag is set in the ASLA advertisement. When the L-Flag is set,
then legacy advertisements MUST be used, subject to the procedures
and constraints defined in [[RFC8919] Section 4.2 and Section 6.
The mandatory use of ASLA advertisements applies to link attributes
specifically mentioned in this document (Min Unidirectional Link
Delay, TE Default Metric, Administrative Group, Extended
Administrative Group and Shared Risk Link Group) and any other link
attributes that may be used in support of Flex-Algorithm in the
future.
A new Application Identifier Bit is defined to indicate that the ASLA
advertisement is associated with the Flex-Algorithm application.
This bit is set in the Standard Application Bit Mask (SABM) defined
in [RFC8919] or [RFC8920]:
Bit-3: Flexible Algorithm (X-bit)
ASLA Admin Group Advertisements to be used by the Flexible Algorithm
application MAY use either the Administrative Group or Extended
Administrative Group encodings.
A receiver supporting this specification MUST accept both ASLA
Administrative Group and Extended Administrative Group TLVs as
defined in [RFC8919] or [RFC8920]. In the case of IS-IS, if the
L-Flag is set in ASLA advertisement, as defined in [RFC8919]
Section 4.2, then the receiver MUST be able to accept both
Administrative Group TLV as defined in [RFC5305] and Extended
Administrative Group TLV as defined in [RFC7308].
Psenak, et al. Expires 20 April 2023 [Page 29]
Internet-Draft IGP Flexible Algorithm October 2022
13. Calculation of Flexible Algorithm Paths
A router MUST be configured to participate in a given Flex-Algorithm
K and MUST select the FAD based on the rules defined in Section 5.3
before it can compute any path for that Flex-Algorithm.
No specific two-way connectivity check is performed during the Flex-
Algorithm path computation. The result of the existing, Flex-
Algorithm agnostic, two-way connectivity check is used during the
Flex-Algorithm path computation.
As described in Section 11, participation for any particular Flex-
Algorithm MUST be advertised on a per data-plane basis. Calculation
of the paths for any particular Flex-Algorithm is data-plane
specific.
Multiple data-planes MAY use the same Flex-Algorithm value at the
same time, and as such, share the FAD for it. Traffic for each data-
plane will be forwarded based on the data-plane specific forwarding
entries.
Flex-Algorithm definition is data-plane independent and is used by
all Flex-Algorithm data-planes.
The way various data-planes handle nodes that do not participate in
Flexible Algorithm is data-plane specific. If the data-plane only
wants to consider participating nodes during the Flex-Algorithm
calculation, then when computing paths for a given Flex-Algorithm,
all nodes that do not advertise participation for that Flex-Algorithm
in their data-plane specific advertisements MUST be pruned from the
topology. Segment Routing, including both SR-MPLS and SRv6, are
data-planes that MUST use such pruning when computing Flex-Algorithm
paths.
When computing the path for a given Flex-Algorithm, the metric-type
that is part of the Flex-Algorithm definition (Section 5) MUST be
used.
When computing the path for a given Flex-Algorithm, the calculation-
type that is part of the Flex-Algorithm definition (Section 5) MUST
be used.
Various link include or exclude rules can be part of the Flex-
Algorithm definition. To refer to a particular bit within an Admin
Group or Extended Admin Group we use the term 'color'.
Rules, in the order as specified below, MUST be used to prune links
from the topology during the Flex-Algorithm computation.
Psenak, et al. Expires 20 April 2023 [Page 30]
Internet-Draft IGP Flexible Algorithm October 2022
For all links in the topology:
1. Check if any exclude AG rule is part of the Flex-Algorithm
definition. If such exclude rule exists, check if any color that
is part of the exclude rule is also set on the link. If such a
color is set, the link MUST be pruned from the computation.
2. Check if any exclude SRLG rule is part of the Flex-Algorithm
definition. If such exclude rule exists, check if the link is
part of any SRLG that is also part of the SRLG exclude rule. If
the link is part of such SRLG, the link MUST be pruned from the
computation.
3. Check if any include-any AG rule is part of the Flex-Algorithm
definition. If such include-any rule exists, check if any color
that is part of the include-any rule is also set on the link. If
no such color is set, the link MUST be pruned from the
computation.
4. Check if any include-all AG rule is part of the Flex-Algorithm
definition. If such include-all rule exists, check if all colors
that are part of the include-all rule are also set on the link.
If all such colors are not set on the link, the link MUST be
pruned from the computation.
5. If the Flex-Algorithm definition uses other than IGP metric
(Section 5), and such metric is not advertised for the particular
link in a topology for which the computation is done, such link
MUST be pruned from the computation. A metric of value 0 MUST NOT
be assumed in such a case.
13.1. Multi-area and Multi-domain Considerations
Any IGP Shortest Path Tree calculation is limited to a single area.
This applies to Flex-Algorithm calculations as well. Given that the
computing router does not have visibility of the topology of the next
areas or domain, the Flex-Algorithm specific path to an inter-area or
inter-domain prefix will be computed for the local area only. The
egress L1/L2 router (ABR in OSPF), or ASBR for inter-domain case,
will be selected based on the best path for the given Flex-Algorithm
in the local area and such egress ABR or ASBR router will be
responsible to compute the best Flex-Algorithm specific path over the
next area or domain. This may produce an end-to-end path, which is
suboptimal based on Flex-Algorithm constraints. In cases where the
ABR or ASBR has no reachability to a prefix for a given Flex-
Algorithm in the next area or domain, the traffic could be dropped by
the ABR/ASBR.
Psenak, et al. Expires 20 April 2023 [Page 31]
Internet-Draft IGP Flexible Algorithm October 2022
To allow the optimal end-to-end path for an inter-area or inter-
domain prefix for any Flex-Algorithm to be computed, the FAPM has
been defined in Section 8 and Section 9. For external route
calculation for prefixes originated by ASBRs in remote areas in OSPF,
the FAAM has been defined in Section 10.2 for the ABR to indicate its
ASBR reachability along with the metric for the specific Flex-
Algorithm.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, an ABR or ASBR MUST include the FAPM (Section 8,
Section 9) when advertising the prefix, that is reachable in a given
Flex-Algorithm, between areas or domains. Such metric will be equal
to the metric to reach the prefix for that Flex-Algorithm in its
source area or domain. This is similar in nature to how the metric
is set when prefixes are advertised between areas or domains for the
default algorithm. When a prefix is unreachable in its source area
or domain in a specific Flex-Algorithm, then an ABR or ASBR MUST NOT
include the FAPM for that Flex-Algorithm when advertising the prefix
between areas or domains.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, the FAPM MUST be used during the calculation of
prefix reachability for the inter-area and external prefixes. If the
FAPM for the Flex-Algorithm is not advertised with the inter-area or
external prefix reachability advertisement, the prefix MUST be
considered as unreachable for that Flex-Algorithm. Similarly, in the
case of OSPF, for ASBRs in remote areas, if the FAAM is not
advertised by the local ABR(s), the ASBR MUST be considered as
unreachable for that Flex-Algorithm and the external prefix
advertisements from such an ASBR are not considered for that Flex-
Algorithm.
Flex-Algorithm prefix metrics and the OSPF Flex-Algorithm ASBR
metrics MUST NOT be used during the Flex-Algorithm computation unless
the FAD selected based on the rules defined in Section 5.3 includes
the M-Flag, as described in (Section 6.4 or Section 7.4).
In the case of OSPF, when calculating external routes in a Flex-
Algorithm, if the winning FAD includes the M-Flag, and where the
advertising ASBR is in a remote area, the metric will be the sum of
the following:
* the FAPM for that Flex-Algorithm advertised with the external
route by the ASBR
* the metric to reach the ASBR for that Flex-Algorithm from the
local ABR i.e., the FAAM for that Flex-Algorithm advertised by the
ABR in the local area for that ASBR
Psenak, et al. Expires 20 April 2023 [Page 32]
Internet-Draft IGP Flexible Algorithm October 2022
* the Flex-Algorithm specific metric to reach the local ABR
This is similar in nature to how the metric is calculated for routes
learned from remote ASBRs in the default algorithm using the OSPFv2
Type 4 ASBR Summary LSA and the OSPFv3 Inter-Area-Router LSA.
If the FAD selected based on the rules defined in Section 5.3 does
not include the M-flag, then the IGP metrics associated with the
prefix reachability advertisements used by the base IS-IS and OSPF
protocol MUST be used for the Flex-Algorithm route computation.
Similarly, in the case of external route calculations in OSPF, the
ASBR reachability is determined based on the base OSPFv2 Type 4
Summary LSA and the OSFPv3 Inter-Area-Router LSA.
It is NOT RECOMMENDED to use the Flex-Algorithm for inter-area or
inter-domain prefix reachability without the M-flag set. The reason
is that without the explicit Flex-Algorithm Prefix Metric
advertisement (and the Flex-Algorithm ASBR metric advertisement in
the case of OSPF external route calculation), it is not possible to
conclude whether the ABR or ASBR has reachability to the inter-area
or inter-domain prefix for a given Flex-Algorithm in the next area or
domain. Sending the Flex-Algorithm traffic for such a prefix towards
the ABR or ASBR may result in traffic looping or persistent traffic
drop.
During the route computation, it is possible for the Flex-Algorithm
specific metric to exceed the maximum value that can be stored in an
unsigned 32-bit variable. In such scenarios, the value MUST be
considered to be of value 0xFFFFFFFF during the computation and
advertised as such.
The FAPM MUST NOT be advertised with IS-IS L1 or L2 intra-area,
OSPFv2 intra-area, or OSPFv3 intra-area routes. If the FAPM is
advertised for these route-types, it MUST be ignored during the
prefix reachability calculation.
The M-flag in the FAD is not applicable to prefixes advertised as
SRv6 locators. The IS-IS SRv6 Locator TLV
[I-D.ietf-lsr-isis-srv6-extensions] includes the Algorithm and Metric
fields. When the SRv6 Locator is advertised between areas or
domains, the metric field in the Locator TLV of IS-IS MUST be used
irrespective of the M-flag in the FAD advertisement.
OSPF external and NSSA external prefix advertisements MAY include a
non-zero forwarding address in the prefix advertisements in the base
protocol. In such a scenario, the Flex-Algorithm specific
reachability of the external prefix is determined by Flex-Algorithm
specific reachability of the forwarding address.
Psenak, et al. Expires 20 April 2023 [Page 33]
Internet-Draft IGP Flexible Algorithm October 2022
In OSPF, the procedures for translation of NSSA external prefix
advertisements into external prefix advertisements performed by an
NSSA ABR [RFC3101] remain unchanged for Flex-Algorithm. An NSSA
translator MUST include the OSPF FAPM Sub-TLVs for all Flex-
Algorithms that are in the original NSSA external prefix
advertisement from the NSSA ASBR in the translated external prefix
advertisement generated by it regardless of its participation in
those Flex-Algorithms or its having reachability to the NSSA ASBR in
those Flex-Algorithms.
An area could become partitioned from the perspective of the Flex-
Algorithm due to the constraints and/or metric being used for it,
while maintaining the continuity in the base algorithm. When that
happens, some destinations inside that area could become unreachable
in that Flex-Algorithm. These destinations will not be able to use
an inter-area path. This is the consequence of the fact that the
inter-area prefix reachability advertisement would not be available
for these intra-area destinations within the area. It is RECOMMENDED
to minimize the risk of such partitioning by providing enough
redundancy inside the area for each Flex-Algorithm being used.
14. Flex-Algorithm and Forwarding Plane
This section describes how Flex-Algorithm paths are used in
forwarding.
14.1. Segment Routing MPLS Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SR MPLS
forwarding.
Prefix SID advertisements include an SR-Algorithm value and, as such,
are associated with the specified SR-Algorithm. Prefix-SIDs are also
associated with a specific topology which is inherited from the
associated prefix reachability advertisement. When the algorithm
value advertised is a Flex-Algorithm value, the Prefix SID is
associated with paths calculated using that Flex-Algorithm in the
associated topology.
A Flex-Algorithm path MUST be installed in the MPLS forwarding plane
using the MPLS label that corresponds to the Prefix-SID that was
advertised for that Flex-algorithm. If the Prefix SID for a given
Flex-algorithm is not known, the Flex-Algorithm specific path cannot
be installed in the MPLS forwarding plane.
Traffic that is supposed to be routed via Flex-Algorithm specific
paths MUST be dropped when there are no such paths available.
Psenak, et al. Expires 20 April 2023 [Page 34]
Internet-Draft IGP Flexible Algorithm October 2022
Loop Free Alternate (LFA) paths ([RFC6571] or its variants) for a
given Flex-Algorithm MUST be computed using the same constraints as
the calculation of the primary paths for that Flex-Algorithm. LFA
paths MUST only use Prefix-SIDs advertised specifically for the given
algorithm. LFA paths MUST NOT use an Adjacency-SID that belongs to a
link that has been pruned from the Flex-Algorithm computation.
If LFA protection is being used to protect a given Flex-Algorithm
paths, all routers in the area participating in the given Flex-
Algorithm SHOULD advertise at least one Flex-Algorithm specific Node-
SID. These Node-SIDs are used to steer traffic over the LFA computed
backup path.
14.2. SRv6 Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SRv6
forwarding.
In SRv6 a node is provisioned with a (topology, algorithm) specific
locator for each of the topology/algorithm pairs supported by that
node. Each locator is an aggregate prefix for all SIDs provisioned
on that node which have the matching topology/algorithm.
The SRv6 locator advertisement in IS-IS
[I-D.ietf-lsr-isis-srv6-extensions] includes the MTID value that
associates the locator with a specific topology. SRv6 locator
advertisements also includes an Algorithm value that explicitly
associates the locator with a specific algorithm. When the algorithm
value advertised with a locator represents a Flex-Algorithm, the
paths to the locator prefix MUST be calculated using the specified
Flex-Algorithm in the associated topology.
Forwarding entries for the locator prefixes advertised in IS-IS MUST
be installed in the forwarding plane of the receiving SRv6 capable
routers when the associated topology/algorithm is participating in
them. Forwarding entries for locators associated with Flex-
Algorithms in which the node is not participating MUST NOT be
installed in the forwarding plane.
When the locator is associated with a Flex-Algorithm, LFA paths to
the locator prefix MUST be calculated using such Flex-Algorithm in
the associated topology, to guarantee that they follow the same
constraints as the calculation of the primary paths. LFA paths MUST
only use SRv6 SIDs advertised specifically for the given Flex-
Algorithm.
Psenak, et al. Expires 20 April 2023 [Page 35]
Internet-Draft IGP Flexible Algorithm October 2022
If LFA protection is being used to protect locators associated with a
given Flex-Algorithm, all routers in the area participating in the
given Flex-Algorithm SHOULD advertise at least one Flex-Algorithm
specific locator and END SID per node and one END.X SID for every
link that has not been pruned from such Flex-Algorithm computation.
These locators and SIDs are used to steer traffic over the LFA-
computed backup path.
14.3. Other Data-planes' Forwarding for Flex-Algorithm
Any data-plane that wants to use Flex-Algorithm specific forwarding
needs to install some form of Flex-Algorithm specific forwarding
entries.
Data-plane specific forwarding for Flex-Algorithm MUST be defined for
each data-plane and is outside the scope of this document.
15. Operational Considerations
15.1. Inter-area Considerations
The scope of the Flex-Algorithm computation is an area, so is the
scope of the FAD. In IS-IS, the Router Capability TLV in which the
FAD Sub-TLV is advertised MUST have the S-bit clear, which prevents
it from being flooded outside the level in which it was originated.
Even though in OSPF the FAD Sub-TLV can be flooded in an RI LSA that
has AS flooding scope, the FAD selection is performed for each
individual area in which it is being used.
There is no requirement for the FAD for a particular Flex-Algorithm
to be identical in all areas in the network. For example, traffic
for the same Flex-Algorithm may be optimized for minimal delay (e.g.,
using delay metric) in one area or level, while being optimized for
available bandwidth (e.g., using IGP metric) in another area or
level.
As described in Section 5.1, IS-IS allows the re-generation of the
winning FAD from level 2, without any modification to it, into a
level 1 area. This allows the operator to configure the FAD in one
or multiple routers in the level 2, without the need to repeat the
same task in each level 1 area, if the intent is to have the same FAD
for the particular Flex-Algorithm across all levels. This can
similarly be achieved in OSPF by using the AS flooding scope of the
RI LSA in which the FAD Sub-TLV for the particular Flex-Algoritm is
advertised.
Psenak, et al. Expires 20 April 2023 [Page 36]
Internet-Draft IGP Flexible Algorithm October 2022
Re-generation of the FAD from a level 1 area to the level 2 area is
not supported in IS-IS, so if the intent is to regenerate the FAD
between IS-IS levels, the FAD MUST be defined on router(s) that are
in level 2. In OSPF, the FAD definition can be done in any area and
be propagated to all routers in the OSPF routing domain by using the
AS flooding scope of the RI LSA.
15.2. Usage of SRLG Exclude Rule with Flex-Algorithm
There are two different ways in which SRLG information can be used
with Flex-Algorithm:
In a context of a single Flex-Algorithm, it can be used for
computation of backup paths, as described in
[I-D.ietf-rtgwg-segment-routing-ti-lfa]. This usage does not
require association of any specific SRLG constraint with the given
Flex-Algorithm definition.
In the context of multiple Flex-Algorithms, it can be used for
creating disjoint sets of paths by pruning the links belonging to
a specific SRLG from the topology on which a specific Flex-
Algorithm computes its paths. This usage:
Facilitates the usage of already deployed SRLG configurations
for setup of disjoint paths between two or more Flex-
Algorithms.
Requires explicit association of a given Flex-Algorithm with a
specific set of SRLG constraints as defined in Section 6.5 and
Section 7.5.
The two usages mentioned above are orthogonal.
15.3. Max-metric consideration
Both IS-IS and OSPF have a mechanism to set the IGP metric on a link
to a value that would make the link either non-reachable or to serve
as the link of last resort. Similar functionality would be needed
for the Min Unidirectional Link Delay and TE metric, as these can be
used to compute Flex-Algorithm paths.
The link can be made un-reachable for all Flex-Algorithms that use
Min Unidirectional Link Delay as metric, as described in Section 5.1,
by removing the Flex-Algorithm ASLA Min Unidirectional Link Delay
advertisement for the link. The link can be made the link of last
resort by setting the delay value in the Flex-Algorithm ASLA delay
advertisement for the link to the value of 16,777,215 (2^24 - 1).
Psenak, et al. Expires 20 April 2023 [Page 37]
Internet-Draft IGP Flexible Algorithm October 2022
The link can be made un-reachable for all Flex-Algorithms that use TE
metric, as described in Section 5.1, by removing the Flex-Algorithm
ASLA TE metric advertisement for the link. The link can be made the
link of last resort by setting the TE metric value in the Flex-
Algorithm ASLA delay advertisement for the link to the value of (2^24
- 1) in IS-IS and (2^32 - 1) in OSPF.
15.4. FAD Definition and Changes
When configuring a node to participate in a specific Flex-Algorithm,
the components of the FAD (calculation-type, metric-type,
constraints) should be considered carefully. The configuration of
participation in a particular Flex-Algorithm doesn't guarantee that
the node will actively participate in it, because it may not support
the calculation-type, metric type or some constraint advertised by
the winning FAD (see Section 5.3). Changes in the FAD configuration
should also be considered in light of the capabilities of the
participating routers in the scope of the FAD advertisement.
As Section 5.3 notes, a change in the Flex-Algorithm definition may
require network-wide SPF re-computation and network re-convergence.
This potential for disruption should be taken into consideration when
planning and making changes to the FAD.
15.5. Number of Flex-Algorithms
The maximum number of Flex-Algorithms is determined by the algorithm
range that is (128-255), as specified in Section 4. Although
possible, it is not expected that all of them will be used
simultaneously. Typically, only a limited subset of Flex-Algorithms
is expected to be deployed in the network.
16. Backward Compatibility
This extension brings no new backward compatibility issues. IS-IS,
OSPFv2 and OSPFv3 all have well-defined handling of unrecognized TLVs
and sub-TLVs that allows the introduction of new extensions, similar
to those defined here, without introducing any interoperability
issues.
17. Security Considerations
This draft adds two new ways to disrupt IGP networks:
An attacker can hijack a particular Flex-Algorithm by advertising
a FAD with a priority of 255 (or any priority higher than that of
the legitimate nodes).
Psenak, et al. Expires 20 April 2023 [Page 38]
Internet-Draft IGP Flexible Algorithm October 2022
An attacker could make it look like a router supports a particular
Flex-Algorithm when it actually doesn't, or vice versa.
Both of these attacks can be addressed by the existing security
extensions as described in [RFC5304] and [RFC5310] for IS-IS, in
[RFC2328] and [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552]
for OSPFv3.
If the node that is authenticated is taken over by an attacker, such
rogue node can advertise the FAD for any Flex-Algorithm. Doing so
may result in traffic for such Flex-Algorithm to be misrouted, or not
being delivered at all, for example, by using an unsupported metric-
type, calculation-type, or constraint. Such attack is not
preventable through authentication, and it is not different from
advertising any other incorrect information through IS-IS or OSPF.
18. IANA Considerations
18.1. IGP IANA Considerations
18.1.1. IGP Algorithm Types Registry
This document makes the following registrations in the "IGP Algorithm
Types" registry:
Type: 128-255.
Description: Flexible Algorithms.
Reference: This document (Section 4).
18.1.2. IGP Metric-Type Registry
IANA is requested to set up a registry called "IGP Metric-Type
Registry" under the "Interior Gateway Protocol (IGP) Parameters" IANA
grouping. The registration policy for this registry is "Standards
Action" ([RFC8126] and [RFC7120]).
Values in this registry come from the range 0-255.
This document registers following values in the "IGP Metric-Type
Registry":
Type: 0
Description: IGP metric
Reference: This document (Section 5.1)
Psenak, et al. Expires 20 April 2023 [Page 39]
Internet-Draft IGP Flexible Algorithm October 2022
Type: 1
Description: Min Unidirectional Link Delay as defined in
[RFC8570], section 4.2, and [RFC7471], section 4.2.
Reference: This document (Section 5.1)
Type: 2
Description: Traffic Engineering Default Metric as defined in
[RFC5305], section 3.7, and Traffic engineering metric as defined
in [RFC3630], section 2.5.5
Reference: This document (Section 5.1)
18.2. Flexible Algorithm Definition Flags Registry
IANA is requested to set up a registry called "IGP Flexible Algorithm
Definition Flags Registry" under the "Interior Gateway Protocol (IGP)
Parameters" IANA grouping. The registration policy for this registry
is "Standards Action" ([RFC8126] and [RFC7120]). New registrations
should be assigned in ascending bit order (see Section 6.4).
This document defines the following single bit in Flexible Algorithm
Definition Flags registry:
Bit # Name
----- ------------------------------
0 Prefix Metric Flag (M-flag)
Reference: This document (Section 6.4, Section 7.4).
18.3. IS-IS IANA Considerations
18.3.1. IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV
This document makes the following registrations in the "IS-IS Sub-
TLVs for IS-IS Router CAPABILITY TLV" registry.
Type: 26.
Description: Flexible Algorithm Definition (FAD)
Reference: This document (Section 5.1).
Psenak, et al. Expires 20 April 2023 [Page 40]
Internet-Draft IGP Flexible Algorithm October 2022
18.3.2. IS-IS Sub-TLVs for TLVs Advertising Prefix Reachability
This document makes the following registrations in the "IS-IS Sub-
TLVs for TLVs Advertising Prefix Reachability" registry.
Type: 6
Description: Flexible Algorithm Prefix Metric (FAPM).
Reference: This document (Section 8).
18.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
This document creates the following Sub-Sub-TLV Registry, under the
IS-IS TLV Codepoints grouping.
Registry: Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Registration Procedure: Expert review. (Note that the IS-IS TLV
Codepoints grouping includes Expert Review guidance that applies
to all registries thereunder.)
Reference: This document (Section 5.1)
This document defines the following Sub-Sub-TLVs in the "Sub-Sub-TLVs
for Flexible Algorithm Definition Sub-TLV" registry:
Type: 0
Description: Reserved
Reference: This document.
Type: 1
Description: Flexible Algorithm Exclude Admin Group
Reference: This document (Section 6.1).
Type: 2
Description: Flexible Algorithm Include-Any Admin Group
Reference: This document (Section 6.2).
Type: 3
Description: Flexible Algorithm Include-All Admin Group
Psenak, et al. Expires 20 April 2023 [Page 41]
Internet-Draft IGP Flexible Algorithm October 2022
Reference: This document (Section 6.3).
Type: 4
Description: Flexible Algorithm Definition Flags
Reference: This document (Section 6.4).
Type: 5
Description: Flexible Algorithm Exclude SRLG
Reference: This document (Section 6.5).
Type: 6-255
Description: Unassigned
Reference: This document.
18.4. OSPF IANA Considerations
18.4.1. OSPF Router Information (RI) TLVs Registry
This specification makes the following registration in the OSPF
Router Information (RI) TLVs Registry.
Type: 16
Description: Flexible Algorithm Definition (FAD) TLV.
Reference: This document (Section 5.2).
18.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs
This document makes the following registrations in the "OSPFv2
Extended Prefix TLV Sub-TLVs" registry.
Type: 3
Description: Flexible Algorithm Prefix Metric (FAPM).
Reference: This document (Section 9).
18.4.3. OSPFv3 Extended-LSA Sub-TLVs
This document makes the following registrations in the "OSPFv3
Extended-LSA Sub-TLVs" registry.
Psenak, et al. Expires 20 April 2023 [Page 42]
Internet-Draft IGP Flexible Algorithm October 2022
Type: 26
Description: Flexible Algorithm Prefix Metric (FAPM).
Reference: This document (Section 9).
Type: 33
Description: OSPF Flexible Algorithm ASBR Metric
Reference: This document (Section 10.2).
For both of these sub-TLVs the column L2BN in the registry is set to
"X" - meaning "sub-TLV is not a Router Link sub-TLV; it MUST NOT
appear in L2 Bundle Member sub-TLV".
18.4.4. OSPF Flex-Algorithm Prefix Metric Bits
This specification requests creation of the "OSPF Flex-Algorithm
Prefix Metric Bits" registry under the "Open Shortest Path First
(OSPF) Parameters" with the following initial values:
Bit Number: 0
Description: E bit - External Type
Reference: this document (Section 9).
The bits 1-7 are unassigned and the registration procedure to be
followed for this registry is IETF Review.
18.4.5. OSPFv2 Opaque LSA Option Types
This document makes the following registrations in the "Opaque Link-
State Advertisements (LSA) Option Types" registry under the "Open
Shortest Path First (OSPF) Opaque Link-State Advertisements (LSA)
Option Types" grouping.
Value: 11
Description: OSPFv2 Extended Inter-Area ASBR (EIA-ASBR) LSA
Reference: This document (Section 10.1).
Psenak, et al. Expires 20 April 2023 [Page 43]
Internet-Draft IGP Flexible Algorithm October 2022
18.4.6. OSPFv2 Extended Inter-Area ASBR TLVs
This specification requests creation of "OSPFv2 Extended Inter-Area
ASBR TLVs" registry under the OSPFv2 Parameters Registry with the
following initial values.
Value: 1
Description : Extended Inter-Area ASBR
Reference: this document
The values 2 to 32767 are unassigned, values 32768 to 33023 are
reserved for experimental use while the values 0 and 33024 to 65535
are reserved. The registration procedure to be followed for this
registry is IETF Review or IESG Approval.
18.4.7. OSPFv2 Inter-Area ASBR Sub-TLVs
This specification requests creation of "OSPFv2 Extended Inter-Area
ASBR Sub-TLVs" registry under the "Open Shortest Path First v2
(OSPFv2) Parameters" grouping, with the following initial values.
Value: 1
Description : OSPF Flexible Algorithm ASBR Metric
Reference: this document
The values 2 to 32767 are unassigned, values 32768 to 33023 are
reserved for experimental use while the values 0 and 33024 to 65535
are reserved. The registration procedure to be followed for this
registry is IETF Review or IESG Approval.
18.4.8. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
This document creates the following registry under the "Open Shortest
Path First (OSPF) Parameters" grouping:
Registry: OSPF Flexible Algorithm Definition TLV sub-TLVs
Registration Procedure: IETF Review or IESG Approval
Reference: This document (Section 5.2)
The "OSPF Flexible Algorithm Definition TLV sub-TLV" registry will
define sub-TLVs at any level of nesting for the Flexible Algorithm
TLV New values can be allocated via IETF Review or IESG Approval.
Psenak, et al. Expires 20 April 2023 [Page 44]
Internet-Draft IGP Flexible Algorithm October 2022
This document registers following Sub-TLVs in the "OSPF Flexible
Algorithm Definition TLV sub-TLV" registry:
Type: 0
Description: Reserved
Reference: This document (Section 7.1).
Type: 1
Description: Flexible Algorithm Exclude Admin Group
Reference: This document (Section 7.1).
Type: 2
Description: Flexible Algorithm Include-Any Admin Group
Reference: This document (Section 7.2).
Type: 3
Description: Flexible Algorithm Include-All Admin Group
Reference: This document (Section 7.3).
Type: 4
Description: Flexible Algorithm Definition Flags
Reference: This document (Section 7.4).
Type: 5
Description: Flexible Algorithm Exclude SRLG
Reference: This document (Section 7.5).
The values 6 to 32767 are unassigned, values 32768-33023 are for
experimental use; these will not be registered with IANA.
Types in the range 33024-65535 are not to be assigned at this time.
Before any assignments can be made in the 33024-65535 range, there
MUST be an IETF specification that specifies IANA Considerations that
covers the range being assigned.
Psenak, et al. Expires 20 April 2023 [Page 45]
Internet-Draft IGP Flexible Algorithm October 2022
18.4.9. Link Attribute Applications Registry
This document registers following bit in the Link Attribute
Applications Registry:
Bit-3
Description: Flexible Algorithm (X-bit)
Reference: This document (Section 12).
19. Acknowledgements
This draft, among other things, is also addressing the problem that
the [I-D.gulkohegde-routing-planes-using-sr] was trying to solve.
All authors of that draft agreed to join this draft.
Thanks to Eric Rosen, Tony Przygienda, William Britto A J, Gunter Van
De Velde, Dirk Goethals, Manju Sivaji and, Baalajee S for their
detailed review and excellent comments.
Thanks to Cengiz Halit for his review and feedback during initial
phase of the solution definition.
Thanks to Kenji Kumaki for his comments.
Thanks to Acee Lindem for editorial comments.
20. References
20.1. Normative References
[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extensions to Support Segment Routing over
IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
extensions-18.txt>.
[ISO10589] ISO, "Intermediate system to Intermediate system intra-
domain routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", ISO/
IEC 10589:2002, Second Edition, November 2002.
Psenak, et al. Expires 20 April 2023 [Page 46]
Internet-Draft IGP Flexible Algorithm October 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>.
[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.
[RFC5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
July 2008, <https://www.rfc-editor.org/info/rfc5250>.
[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>.
[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
Traffic Engineering (MPLS-TE)", RFC 7308,
DOI 10.17487/RFC7308, July 2014,
<https://www.rfc-editor.org/info/rfc7308>.
[RFC7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
2015, <https://www.rfc-editor.org/info/rfc7684>.
[RFC7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
February 2016, <https://www.rfc-editor.org/info/rfc7770>.
[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>.
[RFC8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
2018, <https://www.rfc-editor.org/info/rfc8362>.
Psenak, et al. Expires 20 April 2023 [Page 47]
Internet-Draft IGP Flexible Algorithm October 2022
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
[RFC8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
December 2019, <https://www.rfc-editor.org/info/rfc8666>.
[RFC8667] Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
Extensions for Segment Routing", RFC 8667,
DOI 10.17487/RFC8667, December 2019,
<https://www.rfc-editor.org/info/rfc8667>.
[RFC8919] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
RFC 8919, DOI 10.17487/RFC8919, October 2020,
<https://www.rfc-editor.org/info/rfc8919>.
[RFC8920] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 8920, DOI 10.17487/RFC8920, October 2020,
<https://www.rfc-editor.org/info/rfc8920>.
20.2. Informative References
[I-D.gulkohegde-routing-planes-using-sr]
Hegde, S. and A. Gulko, "Separating Routing Planes using
Segment Routing", Work in Progress, Internet-Draft, draft-
gulkohegde-routing-planes-using-sr-00, 13 March 2017,
<https://www.ietf.org/archive/id/draft-gulkohegde-routing-
planes-using-sr-00.txt>.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., 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-
08, 21 January 2022, <https://www.ietf.org/archive/id/
draft-ietf-rtgwg-segment-routing-ti-lfa-08.txt>.
Psenak, et al. Expires 20 April 2023 [Page 48]
Internet-Draft IGP Flexible Algorithm October 2022
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC3101] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
RFC 3101, DOI 10.17487/RFC3101, January 2003,
<https://www.rfc-editor.org/info/rfc3101>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
Protocol (IGP) Routes Over Traffic Engineering Tunnels",
RFC 3906, DOI 10.17487/RFC3906, October 2004,
<https://www.rfc-editor.org/info/rfc3906>.
[RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality
for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
<https://www.rfc-editor.org/info/rfc4552>.
[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>.
[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>.
[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>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<https://www.rfc-editor.org/info/rfc6571>.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, <https://www.rfc-editor.org/info/rfc7120>.
Psenak, et al. Expires 20 April 2023 [Page 49]
Internet-Draft IGP Flexible Algorithm October 2022
[RFC7471] Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
Previdi, "OSPF Traffic Engineering (TE) Metric
Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
<https://www.rfc-editor.org/info/rfc7471>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
Bratislava
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde
Juniper Networks, Inc.
Embassy Business Park
Bangalore, KA
560093
India
Email: shraddha@juniper.net
Psenak, et al. Expires 20 April 2023 [Page 50]
Internet-Draft IGP Flexible Algorithm October 2022
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Ketan Talaulikar
Cisco Systems, Inc
India
Email: ketant.ietf@gmail.com
Arkadiy Gulko
Edward Jones
Email: arkadiy.gulko@edwardjones.com
Psenak, et al. Expires 20 April 2023 [Page 51]
