Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints
draft-ietf-lsr-flex-algo-bw-con-18
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9843.
|
|
|---|---|---|---|
| Authors | Shraddha Hegde , William Britto , Rajesh Shetty , Bruno Decraene , Peter Psenak , Tony Li | ||
| Last updated | 2025-01-31 (Latest revision 2025-01-19) | ||
| Replaces | draft-hegde-lsr-flex-algo-bw-con | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Acee Lindem | ||
| Shepherd write-up | Show Last changed 2024-09-23 | ||
| IESG | IESG state | Became RFC 9843 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs 2 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Gunter Van de Velde | ||
| Send notices to | acee.ietf@gmail.com | ||
| IANA | IANA review state | IANA OK - Actions Needed | |
| IANA expert review state | Expert Reviews OK |
draft-ietf-lsr-flex-algo-bw-con-18
LSR S. Hegde
Internet-Draft W. Britto
Intended status: Standards Track R. Shetty
Expires: 23 July 2025 Juniper Networks Inc.
B. Decraene
Orange
P. Psenak
Cisco Systems
T. Li
Juniper Networks Inc.
19 January 2025
Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints
draft-ietf-lsr-flex-algo-bw-con-18
Abstract
Many networks configure the link metric relative to the link
capacity. High bandwidth traffic gets routed as per the link
capacity. Flexible algorithms provide mechanisms to create
constraint based paths in an IGP. This draft documents a generic
metric type and set of bandwidth related constraints to be used in
Flexible Algorithms.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14, [RFC2119], [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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."
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This Internet-Draft will expire on 23 July 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Generic Metric Advertisement . . . . . . . . . . . . . . . . 5
2.1. IS-IS Generic Metric Sub-TLV . . . . . . . . . . . . . . 5
2.2. OSPF Generic Metric Sub-TLV . . . . . . . . . . . . . . . 7
2.3. Generic Metric applicability to Flexible Algorithms
Multi-domain/Multi-area networks . . . . . . . . . . . . 9
3. FAD constraint Sub-TLVs . . . . . . . . . . . . . . . . . . . 9
3.1. IS-IS FAD constraint Sub-TLVs . . . . . . . . . . . . . . 10
3.1.1. IS-IS Exclude Minimum Bandwidth sub-TLV . . . . . . . 10
3.1.2. IS-IS Exclude Maximum Delay Sub-TLV . . . . . . . . . 11
3.2. OSPF FAD constraint Sub-TLVs . . . . . . . . . . . . . . 12
3.2.1. OSPF Exclude Minimum Bandwidth Sub-TLV . . . . . . . 12
3.2.2. OSPF Exclude Maximum Delay Sub-TLV . . . . . . . . . 13
4. Bandwidth Metric Advertisement . . . . . . . . . . . . . . . 14
4.1. Automatic Metric Calculation . . . . . . . . . . . . . . 15
4.1.1. Automatic Metric Calculation Modes . . . . . . . . . 15
4.1.2. Automatic Metric Calculation Methods . . . . . . . . 16
4.1.3. IS-IS FAD constraint Sub-TLVs for automatic metric
calculation . . . . . . . . . . . . . . . . . . . . . 17
4.1.4. OSPF FAD constraint Sub-TLVs for automatic metric
calculation . . . . . . . . . . . . . . . . . . . . . 22
5. Bandwidth metric considerations . . . . . . . . . . . . . . . 28
6. Calculation of Flex-Algorithm paths . . . . . . . . . . . . . 29
7. Backward Compatibility . . . . . . . . . . . . . . . . . . . 29
8. Security Considerations . . . . . . . . . . . . . . . . . . . 29
9. Operational Considerations . . . . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
10.1. IGP Metric-Type Registry . . . . . . . . . . . . . . . . 30
10.2. IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . 30
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10.3. OSPF Sub-TLVs for Flexible Algorithm Definition
Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . 31
10.4. IS-IS Sub-TLVs for TLVs Advertising Neighbor
Information . . . . . . . . . . . . . . . . . . . . . . 31
10.5. Sub-sub-TLV Codepoints for Application-Specific Link
Attributes . . . . . . . . . . . . . . . . . . . . . . . 32
10.6. OSPFv2 Extended Link TLV Sub-TLVs . . . . . . . . . . . 32
10.7. Types for Sub-TLVs of TE Link TLV (Value 2) . . . . . . 32
10.8. OSPFv3 Extended-LSA Sub-TLVs . . . . . . . . . . . . . . 32
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32
13. APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . 33
13.1. Updated list of rules for pruning links in Flex-algorithm
topology . . . . . . . . . . . . . . . . . . . . . . . . 33
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . 34
14.2. Informative References . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
High bandwidth traffic such as residential Internet traffic and
machine-to-machine elephant flows benefit from using high capacity
links. Accordingly, many network operators define a link's metric
relative to its capacity to help direct traffic to higher bandwidth
links, but this is no guarantee that lower bandwidth links will be
avoided, especially in failure scenarios. To ensure that elephant
flows are only placed on high capacity links, it would be useful to
explicitly exclude the high bandwidth traffic from utilizing links
below a certain capacity. Flex-Algorithm [RFC9350] provides a
mechanism to create constrained paths by defining a set of parameters
consisting of calculation-type, metric-type, and a set of
constraints. In this document, we define further extensions to Flex-
Algorithm Definition (FAD) that will allow operators additional
control over their traffic flows, especially with respect to
bandwidth constraints.
Historically, IGPs have done path computation by minimizing the sum
of the link metrics along the path from source to destination. While
the metric has been administratively defined, implementations have
defaulted to a metric that is inversely proportional to link
bandwidth. This has driven traffic to higher bandwidth links and has
required manual metric manipulation to achieve the desired loading of
the network.
Over time, with the addition of different traffic types, the need for
alternate types of metrics has evolved. Flex-Algorithm already
supports using the minimum link delay and the administratively
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assigned traffic-engineering metrics in path computation. However,
it is clear that additional metrics may be of interest in different
situations. A network operator may seek to minimize their
operational costs and thus may want a metric that reflects the actual
fiscal costs of using a link. Other traffic may require low jitter,
leading to an entirely different set of metrics. With Flex-
Algorithm, all of these different metrics, and more, could be used
concurrently on the same network.
In some circumstances, path computation constraints, such as
administrative groups, can be used to ensure that traffic avoids
particular portions of the network. These strict constraints are
appropriate when there is an absolute requirement to avoid parts of
the topology, even in failure conditions. If, however, the
requirement is less strict, then using a high metric in a portion of
the topology may be more appropriate.
This document defines a family of generic metrics that can advertise
various types of administratively assigned metrics. This document
proposes standard metric-types which have specific semantics and
require to be standardized. This document also proposes user defined
metric-types where specifics are not defined, so that administrators
are free to assign semantics as they see fit.
In Section 4, this document specifies a new bandwidth based metric
type to be used with Flex-Algorithm and other applications.
Section 3 defines additional Flexible Algorithm Definition (FAD)
[RFC9350] constraints that allow the network administrator to
preclude the use of low bandwidth links or high delay links.
Section 4.1 defines mechanisms to automatically calculate link
metrics based on the parameters defined in the FAD and the advertised
Maximum Link Bandwidth of each link. This is advantageous because
administrators can change their criteria for metric assignment
centrally, without individual modification of each link metric
throughout the network. The procedures described in this document
are intended to assign a metric to a link based on the total link
capacity and they are not intended to update the metric based on
actual traffic flow. Thus, the procedures described in this document
are not a replacement to the capability of a PCE which has a dynamic
view of the network and provides real-time bandwidth management or a
distributed bandwidth management protocol.
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2. Generic Metric Advertisement
IS-IS and OSPF advertise a metric for each link in their respective
link state advertisements. Multiple metric types are already
supported. Administratively assigned metrics are described in the
original OSPF and IS-IS specifications. The Traffic Engineering
Default Metric is defined in [RFC5305] and [RFC3630] and the Min
Unidirectional delay metric is defined in [RFC8570] and [RFC7471].
Other metrics, such as jitter, reliability, and fiscal cost may be
helpful, depending on the traffic class. Rather than attempt to
enumerate all possible metrics of interest, this document specifies a
generic mechanism for advertising metrics.
Each generic metric advertisement is on a per-link and per-metric
type basis. The metric advertisement consists of a metric type field
and a value for the metric. The metric type field is assigned by the
"IGP metric type" IANA registry. Metric types 0-127 are standard
metric types as assigned by IANA. This document further specifies a
user-defined metric type space of metric types 128-255. These are
user defined and can be assigned by an operator for local use.
Implementations MUST support sending and receiving generic metric
sub-TLV in ASLA encodings as well as in the TLV 22/extended link LSA/
TE-LSAs. The usage of a generic metric by an individual application
is subject to the same rules that apply to other link attributes as
defined in [RFC3630], [RFC5305], [RFC9479], [RFC9492] and [RFC9350].
2.1. IS-IS Generic Metric Sub-TLV
The IS-IS Generic Metric sub-TLV specifies the link metric for a
given metric type. Typically, this metric is assigned by a network
administrator. The Generic Metric sub-TLV is advertised in the TLVs/
sub-TLVs below:
a. TLV-22 (Extended IS reachability) [RFC5305]
b. TLV-222 (MT-ISN) [RFC5120]
c. TLV-23 (IS Neighbor Attribute) [RFC5311]
d. TLV-223 (MT IS Neighbor Attribute) [RFC5311]
e. TLV-141 (inter-AS reachability information) [RFC9346]
f. sub-TLV 16 (Application-Specific Link Attributes (ASLA)) of TLV
22/222/23/223/141 [RFC9479]
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g. TLV 25 (L2 Bundle Member Attributes) [RFC8668] Marked as
"y(s)" (shareable among bundle members)
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 | metric-type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (1 octet):
An 8-bit field assigned by IANA (To Be Determined as TBD1).
This value uniquely identifies the Generic Metric TLV.
Length (1 octet):
An 8-bit field indicating the total length, in octets,
of the subsequent fields. For this TLV, the Length is set to 4.
Metric-Type (1 octet):
An 8-bit field specifying the type of metric.
The value is taken from the "IGP metric-type"
registry maintained by IANA.
Value (3 octets):
A 24-bit unsigned integer representing the metric value.
The valid range is from 0 to 16,777,215 (0XFFFFFF).
Figure 1: IS-IS Generic Metric Sub-TLV
The Generic Metric sub-TLV MAY be advertised multiple times. For a
particular metric type, the Generic Metric sub-TLV MUST be advertised
only once for a link when advertised in TLV 22, 222, 23, 223 and 141.
When Generic metric sub-TLV is advertised in ASLA, each metric type
MUST be advertised only once per-application for a link. If there
are multiple Generic Metric sub-TLVs advertised for a link for the
same metric type (and same application in case of ASLA) in one or
more received LSPDUs, advertisement in the lowest numbered fragment
MUST be used and the subsequent instances MUST be ignored.
For a link, if the metric type corresponds to a metric type for which
legacy advertisement mechanisms exist (e.g., the IGP metric, the
Minimum Unidirectional Link Delay, or the Traffic Engineering Default
Metric), the legacy metric types MUST be utilized from the existing
TLV or sub-TLVs. If a Generic Metric advertises a legacy metric, it
MUST be ignored. .
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A metric value of 0xFFFFFF is considered as maximum link metric and a
link having this metric value MUST be used during Flex-algorithm
calculations as a last resort link as described in sec 15.3 of
[RFC9350]. A link can be made unusable by Flex-algorithm by leaving
out Generic metric advertisement of the particular metric-type that
the Flex-algorithm uses as described in [RFC9350].
During the router maintenance activity, the Generic Metric for all
the links on the node MAY be set to a maximum value of 16,777,215
(0XFFFFFF), as it is the maximum usable link metric for the Flex-
algorithm calculations.
2.2. OSPF Generic Metric Sub-TLV
The OSPF Generic Metric sub-TLV specifies the link metric for a given
metric type. Typically, this metric is assigned by a network
administrator. The Generic Metric sub-TLV is advertised in the TLVs
below:
a. sub-TLV of TE Link TLV (2) of OSPF TE LSA [RFC3630].
b. sub-TLV of TE Link TLV (2) of OSPFv2 Inter-AS-TE-v2 LSA
[RFC5392].
c. sub-TLV of TE Link TLV (2) of OSPFv3 Intra-Area-TE-LSA
[RFC5329].
d. sub-TLV of TE Link TLV (2) of OSPFv3 Inter-AS-TE-v3 LSA
[RFC5392].
e. sub-TLV of Application-Specific Link Attributes (ASLA) sub-TLV
[RFC9492] of the OSPFv2 Extended Link TLV [RFC7684].
f. sub-TLV of Application-Specific Link Attributes (ASLA) sub-TLV
[RFC9492] of the OSPFv3 Router-Link TLV [RFC8362].
g. sub-TLV of the OSPFv2 L2 Bundle Member Attributes sub-TLV
[RFC9356].
h. sub-sub-TLV of the OSPFv3 L2 Bundle Member Attributes sub-TLV
[RFC9356].
The Generic Metric sub-TLV is TLV type TBD21/TBD22/TB23 (IANA), and
is eight octets in length.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| metric-type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 octets):
A 16-bit field assigned by IANA (To Be Determined as TBD21/TBD22/TBD23).
This value uniquely identifies the Generic Metric TLV.
Length (2 octets):
A 16-bit field indicating the total length, in octets,
of the subsequent fields. For this TLV, the Length is set to 8.
Metric-Type (1 octet):
An 8-bit field specifying the type of metric.
The value is taken from the "IGP metric-type"
registry maintained by IANA.
Value (4 octets):
A 32-bit unsigned integer representing the metric value.
The valid range is from 0 to 4,294,967,295(0XFFFFFFFF).
Figure 2: OSPF Generic Metric Sub-TLV
The Generic Metric sub-TLV MAY be advertised multiple times. For a
particular metric type, the Generic Metric sub-TLV MUST be advertised
only once for a link when advertised as (a) through (d) above. When
Generic Metric sub-TLV is advertised as sub-sub-TLV of ASLA, it MUST
be advertised only once per-application for a link. If there are
multiple Generic Metric sub-TLVs advertised for a link for the same
metric type (and same application in case of ASLA) in one or more
received LSAs, advertisement in the lowest numbered LSA MUST be used
and the subsequent instances MUST be ignored.
For a link, if the metric type corresponds to a metric type for which
legacy advertisement mechanisms exist (e.g., the IGP metric, the
Minimum Unidirectional Link Delay, or the Traffic Engineering Default
Metric), the legacy metric types MUST be utilized from the existing
TLV or sub-TLVs. If a Generic Metric advertises a legacy metric, it
MUST be ignored.
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A metric value of 0xFFFFFFFF is considered as maximum link metric and
a link having this metric value MUST be used during Flex-algorithm
calculations as a last resort link as described in sec 15.3 of
[RFC9350].
A link can be made unusable by Flex-algorithm by leaving out Generic
metric advertisement of the particular metric-type that the Flex-
algorithm uses as described in [RFC9350].
During the router maintenance activity, the Generic Metric for all
the links on the node MAY be set to a maximum value of 4,294,967,295
((0XFFFFFFFF), as it is the maximum usable link metric for the Flex-
algorithm calculations.
2.3. Generic Metric applicability to Flexible Algorithms Multi-domain/
Multi-area networks
Generic Metric can be used by Flex-Algorithms by specifying the
metric type in the Flexible Algorithm Definitions. When Flex-
Algorithms is used in a multi-area network, [RFC9350] defines the
Flexible Algorithm Prefix Metric (FAPM) sub-TLV that carries the
Flexible-Algorithm-specific metric. Metrics carried in FAPM will be
equal to the metric to reach the prefix for that Flex-Algorithm in
its source area or domain (source area from the ABR perspective).
When Flex-Algorithm uses Generic metric, the same procedures as
described in section 13 of [RFC9350] are used to send and process
FAPM sub-TLV.
3. FAD constraint Sub-TLVs
In networks that carry elephant flows, directing an elephant flow
down a low-bandwidth link might congest the link and cause other
critical application traffic flowing on the link to drop. Thus, in
the context of Flex-Algorithm, it would be useful to be able to
constrain the topology to only those links capable of supporting a
minimum amount of bandwidth.
If the capacity of a link is constant, this can already be achieved
through the use of administrative groups. However, when a layer-3
link is actually a collection of layer-2 links (LAG/layer-2 Bundle),
the link bandwidth will vary based on the set of active constituent
links. This could be automated by having an implementation vary the
advertised administrative groups based on bandwidth, but this seems
unnecessarily complex and expressing this requirement as a direct
constraint on the topology seems simpler. This is also advantageous
if the minimum required bandwidth changes, as this constraint would
provide a single centralized, coordinated point of control.
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To satisfy this requirement, this document defines an Exclude Minimum
Bandwidth constraint. When this constraint is advertised in a FAD, a
link will be pruned from the Flex-Algorithm topology if the link's
advertised Maximum Link Bandwidth is below the advertised Minimum
Bandwidth value.
Similarly, this document defines an Exclude Maximum Link Delay
constraint.Applications, such as High-Frequency Trading are sensitive
to link delays and may perform poorly in networks prone to delay
variability, such as those with transparent Layer 2 link recovery
mechanisms or satellite links.". Mechanisms already exist to measure
the link delay dynamically and advertise it in the IGP. Networks
that employ dynamic link-delay measurement, may want to exclude links
that have a delay over a given threshold.
3.1. IS-IS FAD constraint Sub-TLVs
3.1.1. IS-IS Exclude Minimum Bandwidth sub-TLV
IS-IS Flex-Algorithm Exclude Minimum Bandwidth sub-TLV (FAEMB) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (1 octet):
An 8-bit field assigned by IANA (To Be Determined as TBD3).
This value uniquely identifies the FAEMB sub-TLV.
Length (1 octet):
An 8-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, the Length is set to 4.
Min Bandwidth (4 octets):
A 32-bit field specifying the link bandwidth encoded in IEEE
floating point format (32 bits). The units are bytes-per-second.
Figure 3: IS-IS FAEMB Sub-TLV
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The FAEMB sub-TLV MUST appear at most once in the FAD sub-TLV. If it
appears more than once, the IS-IS FAD sub-TLV MUST be ignored by the
receiver.
The Minimum bandwidth advertised in FAEMB sub-TLV MUST be compared
with Maximum Link Bandwidth advertised in sub-sub-TLV 9 of ASLA sub-
TLV [RFC9479]. If L-Flag is set in the ASLA sub-TLV, the Minimum
bandwidth advertised in FAEMB sub-TLV MUST be compared with Maximum
Link Bandwidth as advertised in the sub-TLV 9 of the TLV
22/222/23/223/141 [RFC5305] as defined in [RFC9479] Section 4.2.
If the Maximum Link Bandwidth is lower than the Minimum link
bandwidth advertised in FAEMB sub-TLV, the link MUST be excluded from
the Flex-Algorithm topology. If a link does not have the Maximum
Link Bandwidth advertised but the FAD contains this sub-TLV, then
that link MUST NOT be excluded from the topology based on the Minimum
Bandwidth constraint.
3.1.2. IS-IS Exclude Maximum Delay Sub-TLV
IS-IS Flex-Algorithm Exclude Maximum Delay sub-TLV (FAEMD) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Link Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (1 octet):
An 8-bit field assigned by IANA (To Be Determined as TBD4).
This value uniquely identifies the FAEMD sub-TLV.
Length (1 octet):
An 8-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, the Length is set to 3.
Max link delay (3 octets):
A 24-bit field specifying the Maximum link delay in microseconds.
Figure 4: IS-IS FAEMD Sub-TLV
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The FAEMD sub-TLV MUST appear only once in the FAD sub-TLV. If it
appears more than once, the IS-IS FAD sub-TLV MUST be ignored by the
receiver.
The Maximum link delay advertised in FAEMD sub-TLV MUST be compared
with Min Unidirectional Link Delay advertised in sub-sub-TLV 34 of
ASLA sub-TLV [RFC9479]. If the L-Flag is set in the ASLA sub-TLV,
the Maximum link delay advertised in FAEMD sub-TLV MUST be compared
with Min Unidirectional Link Delay as advertised by the sub-TLV 34 of
the TLV 22/222/23/223/141 [RFC8570] as defined in [RFC9479]
Section 4.2.
If the Min Unidirectional Link Delay value is higher than the Maximum
link delay advertised in FAEMD sub-TLV, the link MUST be excluded
from the Flex-Algorithm topology. If a link does not have the Min
Unidirectional Link Delay advertised but the FAD contains this sub-
TLV, then that link MUST NOT be excluded from the topology based on
the Maximum Delay constraint.
3.2. OSPF FAD constraint Sub-TLVs
3.2.1. OSPF Exclude Minimum Bandwidth Sub-TLV
OSPF Flex-Algorithm Exclude Minimum Bandwidth sub-TLV (FAEMB) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (2 octets):
A 16-bit field assigned by IANA (To Be Determined as TBD5).
This value uniquely identifies the OSPF FAEMB sub-TLV.
Length (2 octets):
A 16-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, the Length is set to 4.
Min Bandwidth (4 octets):
A 32-bit field specifying the link bandwidth encoded in IEEE
floating point format (32 bits). The units are bytes-per-second.
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Figure 5: OSPF FAEMB Sub-TLV
The FAEMB sub-TLV MUST only appear once in the FAD sub-TLV. If it
appears more than once, the OSPF FAD TLV MUST be ignored by the
receiver.
The Maximum Link Bandwidth as advertised in the Extended Link TLV in
the Extended Link Opaque LSA in OSPFv2 [RFC7684] or as a sub-TLV of
the Router-Link TLV of the E-Router-LSA Router-Link TLV in OSPFv3
[RFC8362] MUST be compared against the Minimum bandwidth advertised
in FAEMB sub-TLV. If the link bandwidth is lower than the Minimum
bandwidth advertised in FAEMB sub-TLV, the link MUST be excluded from
the Flex-Algorithm topology.
If a link does not have the Maximum Link Bandwidth advertised but the
FAD contains this sub-TLV, then that link MUST be included in the
topology and proceed to apply further pruning rules for the link.
3.2.2. OSPF Exclude Maximum Delay Sub-TLV
The OSPF Flex-Algorithm Exclude Maximum Delay sub-TLV (FAEMD) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESERVED | Max link Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (2 octets):
A 16-bit field assigned by IANA (To Be Determined as TBD6).
This value uniquely identifies the OSPF FAEMD sub-TLV.
Length (2 octets):
A 16-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, the Length is set to 4.
Max link delay (4 octets):
A 24-bit field specifying the Maximum link delay in microseconds.
Figure 6: OSPF FAEMD Sub-TLV
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The FAEMD sub-TLV MUST only appear once in the OSPF FAD TLV. If it
appears more than once, the OSPF FAD TLV MUST be ignored by the
receiver.
The Min Delay value advertised via the Min/Max Unidirectional Link
Delay of ASLA sub-TLV [RFC9492], MUST be compared against the Maximum
delay advertised in the FAEMD sub-TLV. If the Min Unidirectional
Link Delay is higher than the Maximum delay advertised in the FAEMD
sub-TLV, the link MUST be excluded from the Flex-Algorithm topology.
If the Min/Max Unidirectional Link Delay is not advertised for a link
but the FAD contains this sub-TLV,then then that link MUST NOT be
excluded from the topology based on the Maximum Delay constraint.
4. Bandwidth Metric Advertisement
Historically, IGP implementations have made default metric
assignments based on link bandwidth. This has proven to be useful,
but has suffered from having different defaults across
implementations and from the rapid growth of link bandwidths. With
Flex-Algorithm, the network administrator can define a function that
will produce a metric for each link and have each node automatically
compute each link's metric based its bandwidth.
This document defines a standard metric type for this purpose called
the "Bandwidth Metric". The Bandwidth Metric MAY be advertised in
the Generic Metric sub-TLV with the metric type set to "Bandwidth
Metric". IS-IS and OSPF will advertise this type of metric in their
link advertisements. Bandwidth metric is a link attribute and for
the advertisement and processing of this attribute for Flex-
algorithm, MUST follow the section 12 of [RFC9350]
Flex-Algorithm uses this metric type by specifying the bandwidth
metric as the metric type in a FAD TLV. A FAD TLV may also specify
an automatic computation of the bandwidth metric based on a link's
advertised bandwidth. An explicit advertisement of a link's
bandwidth metric using the Generic Metric sub-TLV overrides this
automatic computation. The automatic bandwidth metric calculation
sub-TLVs are advertised in the FAD TLV and these parameters are
applicable to applications such as Flex-algorithm that make use of
the FAD TLV.
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4.1. Automatic Metric Calculation
Networks which are designed to be highly regular and follow uniform
metric assignment may want to simplify their operations by
automatically calculating the bandwidth metric. When a FAD
advertises the metric type as Bandwidth Metric and the link does not
have the Bandwidth Metric advertised, automatic metric derivation can
be used with additional FAD constraint advertisement as described in
this section.
If a link's bandwidth changes, then the delay in learning about the
change may create the possibility of micro-loops in the topology.
This is no different from the IGP's susceptibility to micro-loops
during a metric change. The micro-loop avoidance procedures
described in [I-D.bashandy-rtgwg-segment-routing-uloop] or any other
mechanism as described in the framework [RFC5715] can be used to
avoid micro-loops when the automatic metric calculation is deployed.
Computing the metric between adjacent systems based on bandwidth
becomes more complex in the case of parallel adjacencies. If there
are parallel adjacencies between systems, then the bandwidth between
the systems is the sum of the bandwidth of the parallel links. This
is somewhat more complex to deal with, so there is an optional mode
for computing the aggregate bandwidth.
4.1.1. Automatic Metric Calculation Modes
4.1.1.1. Simple Mode
In simple mode, the Maximum Link Bandwidth of a single layer-3 link
is used to derive the metric. This mode is suitable for deployments
that do not use parallel layer-3 links. In this case, the
computation of the metric is straightforward. If a layer-3 link is
composed of a layer-2 bundle, then the link bandwidth is the sum of
the bandwidths of the working components and may vary with layer-2
link failures.
4.1.1.2. Interface Group Mode
The simple mode of metric calculation may not work well when there
are multiple parallel layer-3 interfaces between two nodes. Ideally,
the metric between two systems should be the same given the same
bandwidth, whether the bandwidth is provided by parallel layer-2
links or parallel layer-3 links. To address this, in Interface Group
Mode, nodes MUST compute the aggregate bandwidth of all parallel
adjacencies, MUST derive the metric based on the aggregate bandwidth,
and MUST apply the resulting metric to each of the parallel
adjacencies. Note that a single elephant flow is normally pinned to
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a single layer-3 interface. If the single layer-3 link bandwidth is
not sufficient for any single elephant flow, the mechanisms to solve
this issue are outside the scope of this document.
A------B====C====F====D
| |
------E-------
Figure 7: Parallel interfaces
For example, in the above diagram, there are two parallel links
between B->C, C->F, F->D. Let us assume the link bandwidth is
uniform 10Gbps on all links. When bandwidth is used to derive the
metric for the links, the metric for each link will be the same.
Traffic from B to D will be forwarded B->E->D as the metric will be
lower. Since the bandwidth is higher on the B->C->F->D path, the
metric for that path should be lower than the B->E->D path to attract
the traffic on B->C->F->D path. Interface Group Mode should be
preferred in cases where there are parallel layer-3 links.
In the interface group mode, every node MUST identify the set of
parallel links between a pair of nodes based on IGP link
advertisements and MUST consider cumulative bandwidth of the parallel
links while arriving at the metric of each link.
The parallel layer-3 links between two nodes may not have the same
bandwidth. In such cases the method described in interface group
mode will result in same metric being used for all the parallel links
which may cause undesired load-balancing on the links. In such
cases, a device may locally apply load-balancing factor relative to
the link bandwidth on the ECMP nexthops. The load-balancing
mechanisms are outside the scope of this document.
4.1.2. Automatic Metric Calculation Methods
In automatic metric calculation for simple and interface group mode,
Maximum Link Bandwidth of the links is used to derive the metric.
There are two types of automatic metric derivation methods.
1. Reference bandwidth method
2. Bandwidth thresholds method
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4.1.2.1. Reference Bandwidth method
In many networks, the metric is inversely proportional to the link
bandwidth. The administrator or implementation selects a reference
bandwidth and the metric is derived by dividing the reference
bandwidth by the advertised Maximum Link Bandwidth. Advertising the
reference bandwidth in the FAD constraints allows the metric
computation to be done on every node for each link. The metric is
computed using reference bandwidth and the advertised link bandwidth.
Centralized control of this reference bandwidth simplifies management
in the case that the reference bandwidth changes. In order to ensure
that small bandwidth changes do not change the link metric, it is
useful to define the granularity of the bandwidth that is of
interest. The link bandwidth will be truncated to this granularity
before deriving the metric.
For example,
reference bandwidth = 1000G
Granularity = 20G
The derived metric is 10 for link bandwidth in the range 100G to
119G
4.1.2.2. Bandwidth Thresholds method
The reference bandwidth approach described above provides a uniform
metric value for a range of link bandwidths. In certain cases there
may be a need to define non-proportional metric values for the
varying ranges of link bandwidth. For example, bandwidths from 10G
to 30G are assigned metric value 100, bandwidth from 30G to 70G get a
metric value of 50, and bandwidths greater than 70G have a metric of
10. In order to support this, a staircase mapping based on bandwidth
thresholds is supported in the FAD. This advertisement contains a
set of threshold values and associated metrics.
4.1.3. IS-IS FAD constraint Sub-TLVs for automatic metric calculation
4.1.3.1. Reference Bandwidth Sub-TLV
This section provides FAD constraint advertisement details for the
reference bandwidth method of metric calculation as described in
Section 4.1.2.1. The Flexible Algorithm Definition Reference
Bandwidth sub-TLV (FADRB sub-TLV) is a sub-TLV of the IS-IS 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 | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Granularity Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (1 octet):
An 8-bit field assigned by IANA (To Be Determined as TBD7).
This value uniquely identifies the IS-IS FADRB sub-TLV.
Length (1 octet):
An 8-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, the Length is set to 9.
Flags (1 octet):
An 8-bit field containing flags.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|G| |
+-+-+-+-+-+-+-+-+
G-flag: When set, Interface Group Mode MUST be used to
derive total link bandwidth.
Reference Bandwidth (4 octets):
A 32-bit field with Bandwidth encoded in IEEE floating point
format. The units are bytes-per-second.
Granularity Bandwidth (4 octets):
A 32-bit field with Bandwidth encoded in IEEE floating point
format.The units are bytes-per-second.
When granularity_bw is less than or equal to Total_link_bandwidth
Metric calculation: (Reference_bandwidth) /
(Total_link_bandwidth -
(Modulus of(Total_link_bandwidth,
granularity_bw)))
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When granularity_bw is greater than Total_link_bandwidth
Metric calculation: Reference_bandwidth /
Total_link_bandwidth
The division used here is integer division.
Figure 8: IS-IS FADRB Sub-TLV
The Granularity Bandwidth value ensures that the metric does not
change when there is a small change in the link bandwidth. The IS-IS
FADRB sub-TLV MUST NOT appear more than once in an IS-IS FAD sub-TLV.
If it appears more than once, the IS-IS FAD sub-TLV MUST be ignored
by the receiver. The value advertised in the Reference Bandwidth
field MUST be non-zero. If a zero value is advertised in the
Reference Bandwidth Field in the IS-IS FADRB sub-TLV, the sub-TLV
MUST be ignored.
If a Generic Metric sub-TLV with Bandwidth metric type is advertised
for a link, the Flex-Algorithm calculation MUST use the advertised
Bandwidth Metric, and MUST NOT use the automatically derived metric
for that link. In case of Interface Group Mode, if all the parallel
links have been advertised with the Bandwidth Metric, The individual
link Bandwidth Metric MUST be used. If only some links among the
parallel links have the Bandwidth Metric advertisement, the Bandwidth
Metric for such links MUST be ignored and automatic Metric
calculation MUST be used to derive link metric.
If the calculated metric evaluates to zero, a metric of 1 MUST be
used.
If the calculated metric evaluates to a number greater than 0xFFFFFF,
it is set to 0xFFFFFF.
4.1.3.2. Bandwidth Thresholds Sub-TLV
This section provides FAD constraint advertisement details for the
Bandwidth Thresholds method of metric calculation as described in
Section 4.1.2.2. The Flexible Algorithm Definition Bandwidth
Threshold sub-TLV (FADBT 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 1 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric N-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (1 octet):
An 8-bit field assigned by IANA (To Be Determined as TBD8).
This value uniquely identifies the IS-IS FADBT sub-TLV.
Length (1 octet):
An 8-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV,
the Length is calculated as (1+n*7). Here n is equal
to number of Threshold Metrics specified.
n MUST be greater than or equal to 1
Flags (1 octet):
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|G| | | |
+-+-+-+-+-+-+-+-+
G-flag: when set, interface group Mode MUST be used to derive
total link bandwidth.
Staircase bandwidth threshold and associated metric values.
Bandwidth Threshold 1 (4 octets):
Minimum Link Bandwidth is encoded in in IEEE floating
point format (32 bits).The units are bytes-per-second.
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Threshold Metric 1 (3 octets):
Metric value range (1 - 4,261,412,864)
Bandwidth Threshold n (4 octets):
Maximum Link Bandwidth is encoded in IEEE floating
point format (32 bits).The units are bytes-per-second.
Threshold Metric n (3 octest) :
Metric value range (1 - 4,261,412,864)
Figure 9: IS-IS FADBT Sub-TLV
When G-flag is set, the cumulative bandwidth of the parallel links is
computed as described in Section 4.1.1.2. If G-flag is not set, the
advertised Maximum Link Bandwidth is used.
Assignment of Bandwidth Metric Based on Computed Link Bandwidth:
The Bandwidth Metric for a link during the Flex-Algorithm Shortest
Path First (SPF) calculation MUST be assigned according to the
following rules:
1. When the computed link bandwidth is less than Bandwidth
Threshold 1:
- The Bandwidth Metric MUST be set to the maximum metric value of
4,261,412,864.
2. When the computed link bandwidth is greater than or equal to
Bandwidth Threshold 1 and less than Bandwidth Threshold 2:
- The Bandwidth Metric MUST be set to Threshold Metric 1.
3. When the computed link bandwidth is greater than or equal to
Bandwidth Threshold 2 and less than Bandwidth Threshold 3:
- The Bandwidth Metric MUST be set to Threshold Metric 2.
4. In general, for all integer values of X such that 1 ≤ X < N:
- When the computed link bandwidth is greater than or equal to
Bandwidth Threshold X and less than Bandwidth Threshold X+1:
- The Bandwidth Metric MUST be set to Threshold Metric X.
5. When the computed link bandwidth is greater than or equal to
Bandwidth Threshold N:
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- The Bandwidth Metric MUST be set to Threshold Metric N.
Notes:
The term Bandwidth Threshold X refers to a predefined threshold
value corresponding to the index X.
The term Threshold Metric X refers to the metric value associated
with Bandwidth Threshold X.
N represents the total number of bandwidth thresholds defined in
the system.
Implementations MUST ensure that these rules are consistently applied
to maintain interoperability and optimal path computation within the
network.
The IS-IS FADBT sub-TLV MUST NOT appear more than once in an IS-IS
FAD sub-TLV. If it appears more than once, the IS-IS FAD sub-TLV
MUST be ignored by the receiver.
A FAD MUST NOT contain both the FADBT sub-TLV and the FADRB sub-TLV.
If both these sub-TLVs are advertised in the same FAD for a Flexible
Algorithm, the FAD MUST be ignored by the receiver.
If a Generic Metric sub-TLV with Bandwidth metric type is advertised
for a link, the Flex-Algorithm calculation MUST use the Bandwidth
Metric advertised on the link, and MUST NOT use the automatically
derived metric for that link.
In case of Interface Group Mode, if all the parallel links have been
advertised with the Bandwidth Metric, The individual link Bandwidth
Metric MUST be used. If only some links among the parallel links
have the Bandwidth Metric advertisement, the Bandwidth Metric for
such links MUST be ignored and automatic Metric calculation MUST be
used to derive link metric.
4.1.4. OSPF FAD constraint Sub-TLVs for automatic metric calculation
4.1.4.1. Reference Bandwidth Sub-TLV
The Flexible Algorithm Definition Reference Bandwidth sub-TLV (FADRB
sub-TLV) is a sub-TLV of the OSPF FAD 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Granularity Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (2 octets):
A 16-bit field assigned by IANA (To Be Determined as TBD9).
This value uniquely identifies the OSPF FADRB sub-TLV.
Length (2 octets):
A 16-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, Length is set to 14.
Flags (1 octet):
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|G| | | |
+-+-+-+-+-+-+-+-+
G-flag: When set, Interface Group Mode MUST be used
to derive total link bandwidth.
Reference Bandwidth (4 octets):
Bandwidth encoded in 32 bits in IEEE floating point format.
The units are in bytes per second.
Granularity Bandwidth (4 octets):
Bandwidth encoded in 32 bits in IEEE floating point format.
The units are in bytes per second.
When granularity_bw is less than or equal to Total_link_bandwidth
Metric calculation: (Reference_bandwidth) /
(Total_link_bandwidth -
(Modulus of(Total_link_bandwidth,
granularity_bw)))
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When granularity_bw is greater than Total_link_bandwidth
Metric calculation: Reference_bandwidth/
Total_link_bandwidth
The division used here is integer division.
Figure 10: OSPF FADRB Sub-TLV
The Granularity Bandwidth value is used to ensure that the metric
does not change when there is a small change in the link bandwidth.
The OSPF FADRB 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.The value advertised in the Reference Bandwidth field
MUST be non-zero. If a zero value is advertised in the Reference
Bandwidth Field in the OSPF FADRB sub-TLV, the sub-TLV MUST be
ignored. If a Generic Metric sub-TLV with Bandwidth metric type is
advertised for a link, the Flex-Algorithm calculation MUST use the
advertised Bandwidth Metric on the link, and MUST NOT use the
automatically derived metric for that link. In the case of Interface
Group Mode, the following procedures apply:
When all parallel links have advertised the Bandwidth Metric: The
individual link Bandwidth Metric MUST be used for each link.
When only a subset of the parallel links have advertised the
Bandwidth Metric: The Bandwidth Metric advertisements for those
links MUST be ignored. In this scenario, automatic metric
calculation MUST be used to derive the link metrics for all
parallel links.
If the calculated metric evaluates to zero, a metric of 1 MUST be
used.
If the calculated metric evaluates to a number greater than
0xFFFFFFFF, it is set to 0xFFFFFFFF.
4.1.4.2. Bandwidth Threshold Sub-TLV
The Flexible Algorithm Definition Bandwidth Thresholds sub-TLV (FADBT
sub-TLV) is a sub-TLV of the OSPF FAD 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric N-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type (2 octets):
A 16-bit field assigned by IANA (To Be Determined as TBD10).
This value uniquely identifies the OSPF FADBT sub-TLV.
Length (2 octets):
A 16-bit field indicating the total length, in octets,
of the subsequent fields. For this sub-TLV, Length is set
2 + N*8 octets. Here N is equal to number of
Threshold Metrics specified. N MUST be greater than or equal to 1.
Flags (1 octet):
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|G| |
+-+-+-+-+-+-+-+-+
G-flag: when set, interface group Mode MUST be used to
derive total link bandwidth.
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Staircase bandwidth threshold and associated metric values.
Bandwidth Threshold 1 (4 octets):
Minimum Link Bandwidth is encoded
in IEEE floating point format (32 bits).
The units are bytes per second.
Threshold Metric 1 (4 octets):
Metric value range (1 - 4,294,967,296)
Bandwidth Threshold N (4 octets):
Maximum Link Bandwidth is encoded
in IEEE floating point format (32 bits).
The units are bytes per second.
Threshold Metric N (4 octets):
Metric value range (1 - 4,294,967,296)
Figure 11: OSPF FADBT Sub-TLV
When G-flag is set, the cumulative bandwidth of the parallel links is
computed as described in Section 4.1.1.2. If G-flag is not set, the
advertised Maximum Link Bandwidth is used.
Assignment of Bandwidth Metric Based on Computed Link Bandwidth:
The Bandwidth Metric for a link during the Flex-Algorithm Shortest
Path First (SPF) calculation MUST be assigned according to the
following rules:
1. When the computed link bandwidth is less than Bandwidth
Threshold 1:
- The Bandwidth Metric MUST be set to the maximum metric value of
4,294,967,296.
2. When the computed link bandwidth is greater than or equal to
Bandwidth Threshold 1 and less than Bandwidth Threshold 2:
- The Bandwidth Metric MUST be set to Threshold Metric 1.
3. When the computed link bandwidth is greater than or equal to
Bandwidth Threshold 2 and less than Bandwidth Threshold 3:
- The Bandwidth Metric MUST be set to Threshold Metric 2.
4. In general, for all integer values of X where 1 ≤X < N:
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- When the computed link bandwidth is greater than or equal to
Bandwidth Threshold X and less than Bandwidth Threshold X+1:
- The Bandwidth Metric MUST be set to Threshold Metric X.
5. When the computed link bandwidth is greater than or equal to
Bandwidth Threshold N:
- The Bandwidth Metric MUST be set to Threshold Metric N.
Notes:
Bandwidth Threshold X refers to the predefined bandwidth threshold
corresponding to index X.
Threshold Metric X is the metric value associated with Bandwidth
Threshold X.
N represents the total number of bandwidth thresholds defined in
the system.
Implementations MUST consistently apply these rules to ensure
accurate path computations and maintain interoperability within the
network.
The OSPF FADBT sub-TLV MUST NOT appear more than once in an OSPF FAD
sub-TLV. If it appears more than once, the OSPF FAD MUST be ignored
by the receiver.
A FAD MUST NOT contain both the FADBT sub-TLV and the FADRB sub-TLV.
If both these sub-TLVs are advertised in the same FAD for a Flexible
Algorithm, the FAD MUST be ignored by the receiver.
If a Generic Metric sub-TLV with Bandwidth metric type is advertised
for a link, the Flex-Algorithm calculation MUST use the Bandwidth
Metric advertised on the link, and MUST NOT use the automatically
derived metric for that link.
Metric Assignment in Interface Group Mode:
When a link does not have a configured Bandwidth Metric, the
automatically derived metric MUST be used for that link.
In the context of Interface Group Mode, the following rules apply to
parallel links:
If all parallel links have advertised the Bandwidth Metric:
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- The individual link Bandwidth Metrics MUST be used for each
link during path computation.
If only some of the parallel links have advertised the Bandwidth
Metric:
- The Bandwidth Metric advertisements for those links MUST be
ignored.
- Automatic metric calculation MUST be used to derive the link
metrics for all parallel links.
This approach ensures consistent metric calculation and avoids
discrepancies caused by partial Bandwidth Metric advertisements among
parallel links.
5. Bandwidth metric considerations
This section specifies the rules of deriving the Bandwidth Metric if
and only if the winning FAD for the Flex-Algorithm specifies the
metric-type as "Bandwidth Metric".
1. If the Generic Metric sub-TLV with Bandwidth metric type is
advertised for the link as described in Section 4, it MUST be used
during the Flex-Algorithm calculation.
2. If the Generic Metric sub-TLV with Bandwidth metric type is
not advertised for the link and the winning FAD for the Flex-
Algorithm does not specify the automatic bandwidth metric
calculation (as defined in Section 4.1 ), the the link is treated
as if the Bandwidth Metric is not available for the link.
3. If the Generic Metric sub-TLV with Bandwidth metric type is
not advertised for the link and the winning FAD for the Flex-
Algorithm specifies the automatic bandwidth metric calculation (as
defined in Section 4.1), the Bandwidth Metric metric MUST be
automatically calculated as per the procedures defined in
Section 4.1. If the Link Bandwidth is not advertised for a link,
the link MUST be pruned for the Flex-Algorithm calculations.
4.In ISIS the Link Bandwidth for Flex-Algorithm purposes is
advertised as a sub-sub-TLV 9 of the Flex-algorithm specific ASLA
sub-TLV. It is also possible to advertise the link bandwidth or
Flex-Algorithm, in sub-TLV 9 of TLV 22/222/23/223/141 [RFC5305],
together with the L-Flag set in the Flex-Algorithm specific ASLA
advertisement. In the absence of both of these advertisements,
the bandwidth of the link is not available for Flex-Algorithm
purposes.
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6. Calculation of Flex-Algorithm paths
Two new additional rules are added to the existing rules in the Flex-
Algorithm calculations specified in sec 13 of [RFC9350].
6. Check if any exclude FAEMB rule is part of the Flex-Algorithm
definition. If such exclude rule exists and the link has Maximum
Link Bandwidth advertised, check if the link bandwidth satisfies
the FAEMB rule. If the link does not satisfy the FAEMB rule, the
link MUST be pruned from the Flex-Algorithm computation.
7. Check if any exclude FAEMD rule is part of the Flex-Algorithm
definition. If such exclude rule exists and the link has Min
Unidirectional link delay advertised, check if the link delay
satisfies the FAEMD rule. If the link does not satisfy the FAEMD
rule, the link MUST be pruned from the Flex-Algorithm computation.
7. Backward Compatibility
This extension brings no new backward-compatibility issues. This
document defines new FAD constraints in Section 3 Section 4.1.3 and
Section 4.1.4. As described in [RFC9350], any node that does not
understand sub-TLVs in a FAD TLV, stops participation in the
corresponding Flex-Algorithm. The new extensions can be deployed
among the nodes that are upgraded to understand the new extensions
without affecting the nodes that are not upgraded. This document
also defines a new metric advertisement as described in Section 2.
As per Sec 13 of [RFC9350], the links that do not advertise the
metric-type specified by the selected FAD, the link is pruned from
Flex-Algorithm calculations. The new metric-types and the Flex-
Algorithms using new metric-types can be deployed in the network
without affecting existing deployment.
8. Security Considerations
This document inherits security considerations from [RFC9350].
9. Operational Considerations
Operational consideration defined in [RFC9350] generally apply to the
extensions defined in this document as well. This document defines
metric-type range for user defined metrics. When user defined
metrics are used in an inter-area or inter-level network, all the
domains should assign same meaning to the particular metric-type.
Before the router goes into maintenance activity, the traffic needs
to be diverted away from the router. This is achieved by setting the
overload bit or setting link metrics on the router to a high value.
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In case of Generic Metric, the link metrics can be set to Maximum
usable metric for OSPF and ISIS. The traffic will be diverted away
from the router to a shorter available path. If there are no
alternate paths available, traffic will stay on the router as the
links are not removed from Flex-algorithm calculation when they are
set to maximum metric as per [RFC9350]
10. IANA Considerations
10.1. IGP Metric-Type Registry
IGP Metric-type Registry is updated to include another column
specifying whether the pariticular metric-type is allowed in the
generic-metric sub-TLV or not.
Type Description Reference Allowed in
generic-metric
----------------------------------------------------------------
0 IGP Metric [RFC9350] No
Section 5.1
1 Min Unidirectional [RFC9350] No
Link Delay as defined Section 5.1
in [RFC8570,
Section 4.2],and
[RFC7471, Section 4.2]
2 Traffic Engineering Default [RFC9350] No
Metric as defined in Section 5.1
[RFC5305,Section 3.7],
and [RFC3630, Section 2.5.5]
3(TBA) Bandwidth Metric this document yes
128-255(TBA) User defined metric this document yes
Figure 12: IANA IGP Metric-Type Registry
10.2. IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Type: 6(TBD3)
Description: IS-IS Exclude Minimum Bandwidth Sub-TLV
Reference: This document Section 3.1.1
Type: 7 (TBD4)
Description: IS-IS Exclude Maximum Delay Sub-TLV
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Reference: This document Section 3.1.2
Type: 8 (TBD7)
Description: IS-IS Reference Bandwidth Sub-TLV
Reference: This document Section 4.1.3.1
Type: 9(TBD8)
Description: IS-IS Threshold Metric Sub-TLV
Reference: This document Section 4.1.3.2
10.3. OSPF Sub-TLVs for Flexible Algorithm Definition Sub-TLV
Type:6 (TBD5)
Description: OSPF Exclude Minimum Bandwidth Sub-TLV
Reference: This document Section 3.2.1
Type: 7(TBD6)
Description: OSPF Exclude Maximum Delay Sub-TLV
Reference: This document Section 3.2.2
Type: 8(TBD9)
Description: OSPF Reference Bandwidth Sub-TLV
Reference: This document Section 4.1.4.1
Type: 9 (TBD10)
Description: OSPF Threshold Metric Sub-TLV
Reference: This document Section 4.1.4.2
10.4. IS-IS Sub-TLVs for TLVs Advertising Neighbor Information
Type:17 (TBD1)
Description: Generic metric
Reference: This document Section 2.1
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TLV 22,23,25, 141, 222 and 223 set to Y
10.5. Sub-sub-TLV Codepoints for Application-Specific Link Attributes
Type: 17 (TBD1)
Description: Generic metric
Reference: This document Section 2.1
10.6. OSPFv2 Extended Link TLV Sub-TLVs
Type: 25(TBD21)
Description: Generic metric
Reference: This document Section 2.2
L2BM set to Y
10.7. Types for Sub-TLVs of TE Link TLV (Value 2)
Type: 36 (TBD22)
Description: Generic metric
Reference: This document Section 2.2
10.8. OSPFv3 Extended-LSA Sub-TLVs
Type: 34 (TBD23)
Description: Generic metric
Reference: This document Section 2.2
L2BM set to Y
11. Acknowledgements
Many thanks to Chris Bowers, Krzysztof Szarcowitz, Julian Lucek, Ram
Santhanakrishnan, Ketan Talaulikar and Acee Lindem for discussions
and inputs.
12. Contributors
1. Salih K A
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Juniper Networks
salih@juniper.net
13. APPENDIX
13.1. Updated list of rules for pruning links in Flex-algorithm
topology
This section lists the entire set of rules to prune links from Flex-
Algorithm topology during Flexible-algorithm calculation. It
includes the original rules defined in Section 13 of [RFC9350] as
well as new additions proposed in this document.
For all links in the topology:
1. Check if any exclude Administrative Group 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 a lso 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 Administrative Group 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 Administrative Group 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 something other than the
IGP metric (Section 5 of [RFC9350]), 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.
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6. Check if any exclude FAEMB rule is part of the Flex-Algorithm
definition. If such exclude rule exists and the link has Maximum
Link Bandwidth advertised, check if the link bandwidth satisfies
the FAEMB rule. If the link does not satisfy the FAEMB rule, the
link MUST be pruned from the Flex-Algorithm computation.
7. Check if any exclude FAEMD rule is part of the Flex-Algorithm
definition. If such exclude rule exists and the link has Min
Unidirectional link delay advertised, check if the link delay
satisfies the FAEMD rule. If the link does not satisfy the FAEMD
rule, the link MUST be pruned from the Flex-Algorithm computation.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[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>.
[RFC5329] Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
"Traffic Engineering Extensions to OSPF Version 3",
RFC 5329, DOI 10.17487/RFC5329, September 2008,
<https://www.rfc-editor.org/info/rfc5329>.
[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
January 2009, <https://www.rfc-editor.org/info/rfc5392>.
[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>.
[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>.
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[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>.
[RFC8668] Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
M., and E. Aries, "Advertising Layer 2 Bundle Member Link
Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
December 2019, <https://www.rfc-editor.org/info/rfc8668>.
[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
DOI 10.17487/RFC9350, February 2023,
<https://www.rfc-editor.org/info/rfc9350>.
[RFC9356] Talaulikar, K., Ed. and P. Psenak, "Advertising Layer 2
Bundle Member Link Attributes in OSPF", RFC 9356,
DOI 10.17487/RFC9356, January 2023,
<https://www.rfc-editor.org/info/rfc9356>.
[RFC9479] Ginsberg, L., Psenak, P., Previdi, S., Henderickx, W., and
J. Drake, "IS-IS Application-Specific Link Attributes",
RFC 9479, DOI 10.17487/RFC9479, October 2023,
<https://www.rfc-editor.org/info/rfc9479>.
[RFC9492] Psenak, P., Ed., Ginsberg, L., Henderickx, W., Tantsura,
J., and J. Drake, "OSPF Application-Specific Link
Attributes", RFC 9492, DOI 10.17487/RFC9492, October 2023,
<https://www.rfc-editor.org/info/rfc9492>.
14.2. Informative References
[I-D.bashandy-rtgwg-segment-routing-uloop]
Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
Francois, P., and P. Psenak, "Loop avoidance using Segment
Routing", Work in Progress, Internet-Draft, draft-
bashandy-rtgwg-segment-routing-uloop-17, 29 June 2024,
<https://datatracker.ietf.org/doc/html/draft-bashandy-
rtgwg-segment-routing-uloop-17>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
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[RFC5311] McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
Shand, "Simplified Extension of Link State PDU (LSP) Space
for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
<https://www.rfc-editor.org/info/rfc5311>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <https://www.rfc-editor.org/info/rfc5715>.
[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>.
[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>.
[RFC9346] Chen, M., Ginsberg, L., Previdi, S., and D. Xiaodong, "IS-
IS Extensions in Support of Inter-Autonomous System (AS)
MPLS and GMPLS Traffic Engineering", RFC 9346,
DOI 10.17487/RFC9346, February 2023,
<https://www.rfc-editor.org/info/rfc9346>.
Authors' Addresses
Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore 560103
KA
India
Email: shraddha@juniper.net
William Britto A J
Juniper Networks Inc.
Email: bwilliam@juniper.net
Rajesh Shetty
Juniper Networks Inc.
Email: mrajesh@juniper.net
Bruno Decraene
Orange
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Email: bruno.decraene@orange.com
Peter Psenak
Cisco Systems
Email: ppsenak@cisco.com
Tony Li
Juniper Networks Inc.
Email: tony.li@tony.li
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