Network Working Group P. Psenak, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track S. Hegde
Expires: April 10, 2021 Juniper Networks, Inc.
C. Filsfils
K. Talaulikar
Cisco Systems, Inc.
A. Gulko
Individual
October 7, 2020
IGP Flexible Algorithm
draft-ietf-lsr-flex-algo-12
Abstract
IGP protocols traditionally 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 proposes 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 April 10, 2021.
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Copyright Notice
Copyright (c) 2020 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Flexible Algorithm . . . . . . . . . . . . . . . . . . . . . 5
5. Flexible Algorithm Definition Advertisement . . . . . . . . . 6
5.1. ISIS Flexible Algorithm Definition Sub-TLV . . . . . . . 6
5.2. OSPF Flexible Algorithm Definition TLV . . . . . . . . . 7
5.3. Common Handling of Flexible Algorithm Definition TLV . . 9
6. Sub-TLVs of ISIS FAD Sub-TLV . . . . . . . . . . . . . . . . 10
6.1. ISIS Flexible Algorithm Exclude Admin Group Sub-TLV . . . 10
6.2. ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV . 11
6.3. ISIS Flexible Algorithm Include-All Admin Group Sub-TLV . 12
6.4. ISIS Flexible Algorithm Definition Flags Sub-TLV . . . . 12
6.5. ISIS Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 13
7. Sub-TLVs of OSPF FAD TLV . . . . . . . . . . . . . . . . . . 14
7.1. OSPF Flexible Algorithm Exclude Admin Group Sub-TLV . . . 14
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV . 14
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV . 15
7.4. OSPF Flexible Algorithm Definition Flags Sub-TLV . . . . 15
7.5. OSPF Flexible Algorithm Exclude SRLG Sub-TLV . . . . . . 16
8. ISIS Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . . 17
9. OSPF Flexible Algorithm Prefix Metric Sub-TLV . . . . . . . . 18
10. Advertisement of Node Participation in a Flex-Algorithm . . . 19
10.1. Advertisement of Node Participation for Segment Routing 19
10.2. Advertisement of Node Participation for Other
Applications . . . . . . . . . . . . . . . . . . . . . . 19
11. Advertisement of Link Attributes for Flex-Algorithm . . . . . 20
12. Calculation of Flexible Algorithm Paths . . . . . . . . . . . 20
12.1. Multi-area and Multi-domain Considerations . . . . . . . 22
13. Flex-Algorithm and Forwarding Plane . . . . . . . . . . . . . 23
13.1. Segment Routing MPLS Forwarding for Flex-Algorithm . . . 23
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13.2. SRv6 Forwarding for Flex-Algorithm . . . . . . . . . . . 24
13.3. Other Applications' Forwarding for Flex-Algorithm . . . 25
14. Operational considerations . . . . . . . . . . . . . . . . . 25
14.1. Inter-area Considerations . . . . . . . . . . . . . . . 25
14.2. Usage of SRLG Exclude Rule with Flex-Algorithm . . . . . 26
14.3. Max-metric consideration . . . . . . . . . . . . . . . . 26
15. Backward Compatibility . . . . . . . . . . . . . . . . . . . 27
16. Security Considerations . . . . . . . . . . . . . . . . . . . 27
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
17.1. IGP IANA Considerations . . . . . . . . . . . . . . . . 27
17.1.1. IGP Algorithm Types Registry . . . . . . . . . . . . 27
17.1.2. IGP Metric-Type Registry . . . . . . . . . . . . . . 27
17.2. Flexible Algorithm Definition Flags Registry . . . . . . 28
17.3. ISIS IANA Considerations . . . . . . . . . . . . . . . . 28
17.3.1. Sub TLVs for Type 242 . . . . . . . . . . . . . . . 28
17.3.2. Sub TLVs for for TLVs 135, 235, 236, and 237 . . . . 29
17.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-
TLV . . . . . . . . . . . . . . . . . . . . . . . . 29
17.4. OSPF IANA Considerations . . . . . . . . . . . . . . . . 30
17.4.1. OSPF Router Information (RI) TLVs Registry . . . . . 30
17.4.2. OSPFv2 Extended Prefix TLV Sub-TLVs . . . . . . . . 30
17.4.3. OSPFv3 Extended-LSA Sub-TLVs . . . . . . . . . . . . 30
17.4.4. OSPF Flexible Algorithm Definition TLV Sub-TLV
Registry . . . . . . . . . . . . . . . . . . . . . . 31
17.4.5. Link Attribute Applications Registry . . . . . . . . 32
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
19.1. Normative References . . . . . . . . . . . . . . . . . . 32
19.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
An IGP-computed path based on the shortest IGP metric must often be
replaced by a traffic-engineered path due to the traffic 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 is
reflecting the bandwidth on the link and the application traffic is
delay sensitive, the best IGP path may not reflect the best path from
such an application'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
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the engineered paths and mechanisms like one described in [RFC3906]
are used to replace the native IGP paths with such tunnel paths.
This document specifies a set of extensions to ISIS, OSPFv2, and
OSPFv3 that enable a router to advertise TLVs that identify (a)
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 SR Prefix-SIDs or SRv6 locators 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.
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
[BCP14] [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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.
Flexible 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 application configuration
state that expresses whether the node is participating in a
particular Flexible Algorithm.
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IGP Algorithm - value from the the "IGP Algorithm Types" registry
defined under "Interior Gateway Protocol (IGP) Parameters" IANA
registries. IGP Algorithms represents the triplet (Calculation Type,
Metric, Constraints), where the second and third elements of the
triple MAY be unspecified.
ABR - Area Border Router. In ISIS 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, we want to provide a mechanism that
allows a router to (a) identify a particular calculation-type, (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. We want the mapping between the Flex-Algorithm and its
meaning to be 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.
Flexible-Algorithm is a numeric identifier in the range 128-255 that
is associated via configuratin with the Flexible-Algorithm
Definition.
IANA "IGP Algorithm Types" registry defines the set of values for IGP
Algorithms. We propose to allocate the following values for Flex-
Algorithms from this registry:
128-255 - Flex-Algorithms
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5. Flexible Algorithm Definition Advertisement
To guarantee the 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.
5.1. ISIS Flexible Algorithm Definition Sub-TLV
The ISIS Flexible Algorithm Definition Sub-TLV (FAD Sub-TLV) is used
to advertise the definition of the Flex-Algorithm.
The ISIS FAD Sub-TLV is advertised as a Sub-TLV of the ISIS Router
Capability TLV-242 that is defined in [RFC7981].
ISIS FAD 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 | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 26
Length: variable, dependent on the included Sub-TLVs
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
Metric-Type: Type of metric to be used during the calculation.
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
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specified in [I-D.ietf-isis-te-app] and Section 11 of this
document.
2: Traffic Engineering Default Metric as defined in [RFC5305],
section 3.7, encoded as application specific link attribute as
specified in [I-D.ietf-isis-te-app] and Section 11 of this
document.
Calc-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, Constraints).
When used to specify the Calc-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 SHOULD appear
in this field.
Priority: Value between 0 and 255 inclusive that specifies the
priority of the advertisement.
Sub-TLVs - optional sub-TLVs.
The ISIS FAD Sub-TLV MAY be advertised in an LSP of any number, but a
router MUST NOT advertise more than one ISIS FAD Sub-TLV for a given
Flexible-Algorithm. A router receiving multiple ISIS FAD Sub-TLVs
for a given Flexible-Algorithm from the same originator SHOULD select
the first advertisement in the lowest numbered LSP.
The ISIS 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.
ISIS L1/L2 router MAY be configured to re-generate the winning FAD
from level 2, without any modification to it, to 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 case, the re-generated FAD
Sub-TLV will be advertised in the level 1 Router Capability TLV
originated by the L1/L2 router.
L1/L2 router MUST NOT re-generate any FAD Sub-TLV from level 1 to
level 2.
5.2. OSPF Flexible Algorithm Definition TLV
OSPF FAD TLV is advertised as a top-level TLV of the RI LSA that is
defined in [RFC7770].
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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:
Type: 16
Length: variable, dependent on the included Sub-TLVs
Flex-Algorithm:: Flex-Algorithm number. Value between 128 and 255
inclusive.
Metric-Type: Type of metric to be used during the calculation.
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 [I-D.ietf-ospf-te-link-attr-reuse] and Section 11
of this document.
2: Traffic Engineering metric as defined in [RFC3630], section
2.5.5, encoded as application specific link attribute as
specified in [I-D.ietf-ospf-te-link-attr-reuse] and Section 11
of this document.
Calc-Type: as described in Section 5.1
Priority: as described in Section 5.1
Sub-TLVs - optional sub-TLVs.
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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 by default unless
local configuration policy on the originating router indicates domain
wide flooding.
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 (ISIS). We will refer to it as FAD TLV in
this section, even though in case of ISIS 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 highest priority value.
2. If there are multiple advertisements of the FAD with the same
highest priority, select the one that is originated from the
router with the highest System-ID, in the case of ISIS, or Router
ID, in the case of OSPFv2 and OSPFv3. For ISIS, the System-ID is
described in [ISO10589]. For OSPFv2 and OSPFv3, standard Router
ID is described in [RFC2328] and [RFC5340] respectively.
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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 is
allowed to advertise FAD for such Flex-Algorithm. Receiving routers
MUST consider FAD advertisement regardless of the Flex-Algorithm
participation of the FAD 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.
If a node is configured to participate in a particular Flexible-
Algorithm, but 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 10
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 ISIS FAD Sub-TLV
6.1. ISIS 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 ISIS 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 12.
The ISIS Flexible Algorithm Exclude Admin Group Sub-TLV (FAEAG Sub-
TLV) is a Sub-TLV of the ISIS FAD Sub-TLV. It has the following
format:
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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 ISIS FAEAG Sub-TLV MUST NOT appear more then once in an ISIS FAD
Sub-TLV. If it appears more then once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
6.2. ISIS 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 ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV is used
to advertise include-any rule that is used during the Flex-Algorithm
path calculation as specified in Section 12.
The format of the ISIS Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the FAEAG Sub-TLV in
Section 6.1.
The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
2.
The ISIS Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more then once in an ISIS FAD Sub-TLV. If it appears more
then once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.
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6.3. ISIS Flexible Algorithm Include-All Admin Group Sub-TLV
The Flexible Algorithm definition can specify 'colors' that are used
by the operator to include link during the Flex-Algorithm path
computation.
The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV is used
to advertise include-all rule that is used during the Flex-Algorithm
path calculation as specified in Section 12.
The format of the ISIS Flexible Algorithm Include-All Admin Group
Sub-TLV is identical to the format of the FAEAG Sub-TLV in
Section 6.1.
The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV Type is
3.
The ISIS Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more then once in an ISIS FAD Sub-TLV. If it appears more
then once, the ISIS FAD Sub-TLV MUST be ignored by the receiver.
6.4. ISIS Flexible Algorithm Definition Flags Sub-TLV
The ISIS Flexible Algorithm Definition Flags Sub-TLV (FADF Sub-TLV)
is a Sub-TLV of the ISIS 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:
Type: 4
Length: variable, non-zero number of octets of the Flags field
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
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M-flag: when set, the Flex-Algorithm specific prefix metric
MUST be used, if advertised with the prefix. This flag is not
applicable to prefixes advertised as SRv6 locators.
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 ISIS FADF Sub-TLV MUST NOT appear more then once in an ISIS FAD
Sub-TLV. If it appears more then once, the ISIS FAD Sub-TLV MUST be
ignored by the receiver.
If the ISIS FADF Sub-TLV is not present inside the ISIS FAD Sub-TLV,
all the bits are assumed to be set to 0.
6.5. ISIS 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 ISIS 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 12.
The ISIS FAESRLG Sub-TLV is a Sub-TLV of the ISIS 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
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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 ISIS FAESRLG Sub-TLV MUST NOT appear more then once in an ISIS
FAD Sub-TLV. If it appears more then once, the ISIS FAD Sub-TLV MUST
be ignored by the receiver.
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. It's 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 then once in an OSPF FAD
TLV. If it appears more then once, the OSPF FAD TLV MUST be ignored
by the receiver.
7.2. OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV
The usage of this Sub-TLVs is described in Section 6.2.
The format of the OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
Section 7.1.
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The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
2.
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV MUST NOT
appear more then once in an OSPF FAD TLV. If it appears more then
once, the OSPF FAD TLV MUST be ignored by the receiver.
7.3. OSPF Flexible Algorithm Include-All Admin Group Sub-TLV
The usage of this Sub-TLVs is described in Section 6.3.
The format of the OSPF Flexible Algorithm Include-Any Admin Group
Sub-TLV is identical to the format of the OSPF FAEAG Sub-TLV in
Section 7.1.
The OSPF Flexible Algorithm Include-Any Admin Group Sub-TLV Type is
3.
The OSPF Flexible Algorithm Include-All Admin Group Sub-TLV MUST NOT
appear more then once in an OSPF FAD TLV. If it appears more then
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:
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0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M| | | ...
+-+-+-+-+-+-+-+-+...
M-flag: when set, the Flex-Algorithm specific prefix metric
MUST be used, if advertised with the prefix. This flag is not
applicable to prefixes advertised as SRv6 locators.
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 OSPF FADF Sub-TLV MUST NOT appear more then once in an OSPF FAD
TLV. If it appears more then 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.
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.
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Shared Risk Link Group Value: SRLG value as defined in [RFC4203].
The OSPF FAESRLG Sub-TLV MUST NOT appear more then once in an OSPF
FAD TLV. If it appears more then once, the OSPF FAD TLV MUST be
ignored by the receiver.
8. ISIS Flexible Algorithm Prefix Metric Sub-TLV
The ISIS Flexible Algorithm Prefix Metric (FAPM) Sub-TLV supports the
advertisement of a Flex-Algorithm specific prefix metric associated
with a given prefix advertisement.
The ISIS 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 ISIS FAPM Sub-TLV MAY appear multiple times in its parent TLV.
If it appears more then 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
then 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 12.
The ISIS FAPM Sub-TLV MUST NOT be advertised as a sub-TLV of the ISIS
SRv6 Locator TLV [I-D.ietf-lsr-isis-srv6-extensions]. The ISIS 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
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ISIS 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-TLVis a Sub-TLV of
the:
- OSPFv2 Extended Prefix TLV [RFC7684]
- Following OSPFv3 TLVs as defined in [RFC8362]:
Intra-Area Prefix TLV
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 3 for OSPFv2, 26 for OSPFv3
Length: 8 octets
Flex-Algorithm: Single octet value between 128 and 255 inclusive.
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 then once with the same Flex-Algorithm value, the
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first instance MUST be used and any subsequent instances MUST be
ignored.
The usage of the Flex-Algorithm prefix metric is described in
Section 12.
10. Advertisement of Node Participation in a Flex-Algorithm
When a router is configured to support a particular Flex-Algorithm,
we say it is participating in that Flex-Algorithm.
Paths computed for a specific Flex-Algorithm MAY be used by various
applications, each potentially using its own specific data plane for
forwarding traffic over such paths. To guarantee the presence of the
application specific forwarding state associated with a particular
Flex-Algorithm, a router MUST advertise its participation for a
particular Flex-Algorithm for each application specifically.
10.1. 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 the 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 (ISIS).
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.
10.2. Advertisement of Node Participation for Other Applications
This section describes considerations related to how other
applications can advertise their participation in a specific Flex-
Algorithm.
Application-specific Flex-Algorithm participation advertisements MAY
be topology specific or MAY be topology independent, depending on the
application itself.
Application-specific advertisement for Flex-Algorithm participation
MUST be defined for each application and is outside of the scope of
this document.
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11. 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.
Link attribute advertisements that are to be used during Flex-
Algorithm calculation MUST use the Application-Specific Link
Attribute (ASLA) advertisements defined in [I-D.ietf-isis-te-app] or
[I-D.ietf-ospf-te-link-attr-reuse]. In the case of IS-IS, this
includes use of the L-flag as defined in [I-D.ietf-isis-te-app]
Section 4.2 subject to the constraints discussed in Section 6 of the
[I-D.ietf-isis-te-app]. 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 [I-D.ietf-isis-te-app] or [I-D.ietf-ospf-te-link-attr-reuse]:
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. If the Administrative Group encoding
is used, then the first 32 bits of the corresponding FAD sub-TLVs are
mapped to the link attribute advertisements as specified in RFC 7308.
12. 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.
As described in Section 10, participation for any particular Flex-
Algorithm MUST be advertised on a per-application basis. Calculation
of the paths for any particular Flex-Algorithm MUST be application
specific.
The way applications handle nodes that do not participate in
Flexible-Algorithm is application specific. If the application only
wants to consider participating nodes during the Flex-Algorithm
calculation, then when computing paths for a given Flex-Algorithm,
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all nodes that do not advertise participation for that Flex-Algorithm
in their application-specific advertisements MUST be pruned from the
topology. Segment Routing, including both SR MPLS and SRv6, are
applications 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 AG or
EAG we uses term 'color'.
Rules, in the order as specified below, MUST be used to prune links
from the topology during the Flex-Algorithm computation.
For all links in the topology:
1. Check if any exclude 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 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 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
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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 case.
12.1. 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
sub-optimal 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 may be dropped by
the ABR/ASBR.
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.
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 between areas or domains.
Such metric will be equal to the metric to reach the prefix for a
given Flex-Algorithm in a 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.
If the FAD selected based on the rules defined in Section 5.3
includes the M-flag, the FAPM MUST be used during 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.
Flex-Algorithm prefix 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).
If the FAD selected based on the rules defined in Section 5.3 does
not includes the M-flag, it is NOT RECOMMENDED to use the Flex-
Algorithm for inter-area or inter-domain prefix reachability. The
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reason is that without the explicit Flex-Algorithm Prefix Metric
advertisement, 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-Algoritm
traffic for such prefix towards the ABR or ASBR may result in traffic
looping or black-holing.
The FAPM MUST NOT be advertised with ISIS 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 FAD is not applicable to prefixes advertised as SRv6
locators. The ISIS SRv6 Locator TLV includes the Algorithm and
Metric fields [I-D.ietf-lsr-isis-srv6-extensions]. When the ISIS
SRv6 Locator is advertised between areas or domains, the metric field
in the Locator TLV MUST be used irrespective of the M-flag in the FAD
advertisement.
13. Flex-Algorithm and Forwarding Plane
This section describes how Flex-Algorithm paths are used in
forwarding.
13.1. 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.
Loop Free Alternate (LFA) paths for a given Flex-Algorithm MUST be
computed using the same constraints as the calculation of the primary
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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.
13.2. SRv6 Forwarding for Flex-Algorithm
This section describes how Flex-Algorithm paths are used with SRv6
forwarding.
In SRv6 a node is provisioned with topology/algorithm specific
locators 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 IGPs
([I-D.ietf-lsr-isis-srv6-extensions]
[I-D.ietf-lsr-ospfv3-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 IGPs 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 palne.
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.
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
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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.
13.3. Other Applications' Forwarding for Flex-Algorithm
Any application that wants to use Flex-Algorithm specific forwarding
needs to install some form of Flex-Algorithm specific forwarding
entries.
Application-specific forwarding for Flex-Algorithm MUST be defined
for each application and is outside of the scope of this document.
14. Operational considerations
14.1. Inter-area Considerations
The scope of the FA computation is an area, so is the scope of the
FAD. In ISIS, the Router Capability TLV in which the FAD Sub-TLV is
advertised MUST have the S-bit clear, which prevents it to be flooded
outside of 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, ISIS 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.
Re-generation of FAD from a level 1 area to the level 2 area is not
supported in ISIS, so if the intent is to regenerate the FAD between
ISIS 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
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propagated to all routers in the OSPF routing domain by using the AS
flooding scope of the RI LSA.
14.2. 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.
14.3. Max-metric consideration
Both ISIS 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).
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-
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Algorithm ASLA delay advertisement for the link to the value of (2^24
- 1) in ISIS and (2^32 - 1) in OSPF.
15. Backward Compatibility
This extension brings no new backward compatibility issues.
16. 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).
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 ISIS, in
[RFC2328] and [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552]
for OSPFv3.
17. IANA Considerations
17.1. IGP IANA Considerations
17.1.1. 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).
17.1.2. IGP Metric-Type Registry
IANA is requested to set up a registry called "IGP Metric-Type
Registry" under a "Interior Gateway Protocol (IGP) Parameters" IANA
registries. The registration policy for this registry is "Standards
Action" ([RFC8126] and [RFC7120]).
Values in this registry come from the range 0-255.
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This document registers following values in the "IGP Metric-Type
Registry":
Type: 0
Description: IGP metric
Reference: This document (Section 5.1)
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)
17.2. Flexible Algorithm Definition Flags Registry
IANA is requested to set up a registry called "ISIS Flexible
Algorithm Definition Flags Registry" under a "Interior Gateway
Protocol (IGP) Parameters" IANA registries. The registration policy
for this registry is "Standards Action" ([RFC8126] and [RFC7120]).
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).
17.3. ISIS IANA Considerations
17.3.1. Sub TLVs for Type 242
This document makes the following registrations in the "sub-TLVs for
TLV 242" registry.
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Type: 26.
Description: Flexible Algorithm Definition.
Reference: This document (Section 5.1).
17.3.2. Sub TLVs for for TLVs 135, 235, 236, and 237
This document makes the following registrations in the "Sub-TLVs for
for TLVs 135, 235, 236, and 237" registry.
Type: 6
Description: Flexible Algorithm Prefix Metric.
Reference: This document (Section 8).
17.3.3. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
This document creates the following Sub-Sub-TLV Registry:
Registry: Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Registration Procedure: Expert review
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: 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
Reference: This document (Section 6.3).
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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).
17.4. OSPF IANA Considerations
17.4.1. OSPF Router Information (RI) TLVs Registry
This specification updates the OSPF Router Information (RI) TLVs
Registry.
Type: 16
Description: Flexible Algorithm Definition TLV.
Reference: This document (Section 5.2).
17.4.2. 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.
Reference: This document (Section 9).
17.4.3. OSPFv3 Extended-LSA Sub-TLVs
This document makes the following registrations in the "OSPFv3
Extended-LSA Sub-TLVs" registry.
Type: 26
Description: Flexible Algorithm Prefix Metric.
Reference: This document (Section 9).
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17.4.4. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
This document creates the following registry:
Registry: OSPF Flexible Algorithm Definition TLV sub-TLV
Registration Procedure: Expert review
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 and should be added to the "Open Shortest Path First (OSPF)
Parameters" registries group. New values can be allocated via IETF
Review or IESG Approval.
This document registers following Sub-TLVs in the "TLVs for Flexible
Algorithm Definition TLV" registry:
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).
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Types in the range 32768-33023 are for experimental use; these will
not be registered with IANA, and MUST NOT be mentioned by RFCs.
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.
17.4.5. 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 11).
18. 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 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 William Britto A J. for his suggestions.
Thanks to Acee Lindem for editorial comments.
19. References
19.1. Normative References
[BCP14] "Key words for use in RFCs to Indicate Requirement
Levels", <https://tools.ietf.org/html/bcp14>.
[I-D.ietf-isis-te-app]
Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
draft-ietf-isis-te-app-19 (work in progress), June 2020.
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[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extension to Support Segment Routing over
IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-10
(work in progress), September 2020.
[I-D.ietf-lsr-ospfv3-srv6-extensions]
Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
"OSPFv3 Extensions for SRv6", draft-ietf-lsr-
ospfv3-srv6-extensions-01 (work in progress), August 2020.
[I-D.ietf-ospf-te-link-attr-reuse]
Psenak, P., Ginsberg, L., Henderickx, W., Tantsura, J.,
and J. Drake, "OSPF Application-Specific Link Attributes",
draft-ietf-ospf-te-link-attr-reuse-16 (work in progress),
June 2020.
[ISO10589]
International Organization for Standardization,
"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, Nov 2002.
[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>.
[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>.
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[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>.
[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>.
19.2. Informative References
[I-D.gulkohegde-routing-planes-using-sr]
Hegde, S. and a. arkadiy.gulko@thomsonreuters.com,
"Separating Routing Planes using Segment Routing", draft-
gulkohegde-routing-planes-using-sr-00 (work in progress),
March 2017.
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[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
Francois, P., Voyer, D., Clad, F., and P. Camarillo,
"Topology Independent Fast Reroute using Segment Routing",
draft-ietf-rtgwg-segment-routing-ti-lfa-04 (work in
progress), August 2020.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[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>.
[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>.
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[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>.
Authors' Addresses
Peter Psenak (editor)
Cisco Systems
Apollo Business Center
Mlynske nivy 43
Bratislava, 82109
Slovakia
Email: ppsenak@cisco.com
Shraddha Hegde
Juniper Networks, Inc.
Embassy Business Park
Bangalore, KA, 560093
India
Email: shraddha@juniper.net
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
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Ketan Talaulikar
Cisco Systems, Inc.
S.No. 154/6, Phase I, Hinjawadi
PUNE, MAHARASHTRA 411 057
India
Email: ketant@cisco.com
Arkadiy Gulko
Individual
Email: arkadiy.gulko@gmail.com
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