hegde-lsr-flex-algo-bw-con-02.txt

Internet DRAFT - draft-hegde-lsr-flex-algo-bw-con

draft-hegde-lsr-flex-algo-bw-con

Last Version:	draft-hegde-lsr-flex-algo-bw-con-02.txt	Tracker Entry
Date:	`17-Apr-2021`
Disposition:	.draft-ietf-lsr-flex-algo-bw-con
Previous Versions:	draft-hegde-lsr-flex-algo-bw-con-01.txt (diff) - 09-Mar-2021
	draft-hegde-lsr-flex-algo-bw-con-00.txt (diff) - 23-Feb-2021

SPRING S. Hegde
Internet-Draft W. Britto
Intended status: Standards Track R. Shetty
Expires: October 18, 2021 Juniper Networks Inc.
B. Decraene
Orange
P. Psenak
Cisco Systems
T. Li
Arista Networks
April 16, 2021

Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints
draft-hegde-lsr-flex-algo-bw-con-02

Abstract

Many networks configure the link metric relative to the link
capacity. High bandwidth traffic gets routed as per the link
capacity. Flexible algorithms provides mechanisms to create
constraint based paths in 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

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 October 18, 2021.

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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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Generic Metric Advertisement . . . . . . . . . . . . . . . . 4
2.1. ISIS Generic Metric sub-TLV . . . . . . . . . . . . . . . 5
2.2. OSPF Generic Metric sub-TLV . . . . . . . . . . . . . . . 6
3. FAD constraint sub-TLVs . . . . . . . . . . . . . . . . . . . 7
3.1. ISIS FAD constraint sub-TLVs . . . . . . . . . . . . . . 7
3.1.1. ISIS Exclude Minimum Bandwidth sub-TLV . . . . . . . 7
3.1.2. ISIS Exclude Maximum Delay sub-TLV . . . . . . . . . 8
3.2. OSPF FAD constraint sub-TLVs . . . . . . . . . . . . . . 9
3.2.1. OSPF Exclude Minimum Bandwidth sub-TLV . . . . . . . 9
3.2.2. OSPF Exclude Maximum Delay sub-TLV . . . . . . . . . 10
4. Bandwidth Metric Advertisement . . . . . . . . . . . . . . . 11
4.1. Automatic Metric Calculation . . . . . . . . . . . . . . 12
4.1.1. Automatic Metric Calculation Modes . . . . . . . . . 12
4.1.2. Automatic Metric Calculation Methods . . . . . . . . 13
4.1.3. ISIS FAD constraint sub-TLVs for automatic metric
calculation . . . . . . . . . . . . . . . . . . . . . 14
4.1.4. OSPF FAD constraint sub-TLVs for automatic metric
calculation . . . . . . . . . . . . . . . . . . . . . 18
5. Bandwidth metric considerations . . . . . . . . . . . . . . . 22
6. Calculation of Flex-Algorithm paths . . . . . . . . . . . . . 22
7. Backward Compatibility . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9.1. IGP Metric-Type Registry . . . . . . . . . . . . . . . . 23
9.2. ISIS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-
TLV . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.3. OSPF Sub-TLVs for Flexible Algorithm Definition Sub-TLV . 24
9.4. Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 . . . . . 24
9.5. Sub-sub-TLV Codepoints for Application-Specific Link
Attributes . . . . . . . . . . . . . . . . . . . . . . . 24

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9.6. OSPFv2 Extended Link TLV Sub-TLVs . . . . . . . . . . . . 25
9.7. Types for sub-TLVs of TE Link TLV (Value 2) . . . . . . . 25
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26

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 [I-D.ietf-lsr-flex-algo]
has already defined as a set of parameters consisting of calculation-
type, metric-type and a set of constraints for allowing operators to
have more control over network path computation. In this document,
we define further extensions to Flex-Algorithm that will allow
operators additional control over their traffic, especially with
respect to constraints about bandwidth.

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 become clear. Flex-Algorithm already
supports using the minimum link delay and the administratively
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.

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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 carry
various types of administratively assigned metrics. This document
proposes standard metric-types which require specific standard
document. This document also proposes user defined metric-types
where specifics are not defined, so that adminstrators are free to
assign semantics as they fit. This document also specifies a new
bandwidth based metric type to be used with Flex-Algorithm and other
applications in Section Section 4. Additional Flexible Algorithm
Definition (FAD) constraints are defined in Section Section 3 that
allow the network adminstrator to preclude the use of low bandwidth
links or high delay links. Section Section 4.1 defines mechanisms to
automatically calculate link metrics based on 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.

2. Generic Metric Advertisement

ISIS and OSPF advertise a metric for each link in their respective
link state advertisements. Multiple metric types are are already
supported. Administratively assigned metrics are described in the
original OSPF and ISIS 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.

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2.1. ISIS Generic Metric sub-TLV

The ISIS 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:

TLV-22 (Extended IS reachability) [RFC5305]

TLV-222 (MT-ISN) [RFC5120]

TLV-23 (IS Neighbor Attribute) [RFC5311]

TLV-223 (MT IS Neighbor Attribute) [RFC5311]

TLV-141 (inter-AS reachability information) [RFC5316]

sub-TLV 16 (Application-Specific Link Attributes) of TLV
22/222/23/223/141 [RFC8919]

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 : TBD (To be assigned by IANA)
Length: 4 octets
metric-type: A value from the IGP metric-type registry
Value : metric value range (1 - 16,777,215)

Figure 1: ISIS 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 same
metric type (and same application in case of ASLA) in one or more
received LSPDUs, the first one MUST be used and the subsequent ones
MUST be ignored.If the metric type indicates a standard metric type
for which there are other advertisement mechanisms (e.g., the IGP
metric, the Min Unidirectional Link Delay, or the Traffic Engineering

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Default Metric, as of this writing), the Generic Metric advertisement
MUST be ignored.

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:

sub-TLV of the OSPF Link TLV of OSPF extended Link LSA [RFC7684].

sub-TLV of TE Link TLV (2) of OSPF TE LSA [RFC3630].

sub-sub-TLV of Application-Specific Link Attributes sub-TLV [RFC
8920]

The Generic Metric sub-TLV is TLV type TBD (IANA), and is eight
octets in length.

Type : TBD (To be assigned by IANA)
Length: 8 octets
metric-type = A value from the IGP metric type registry

Value : metric value (1- 4,294,967,295)

Figure 2: OSPF Generic Metric sub-TLV

The Generic Metric sub-TLV MAY be advertised multiple times. For a
particular metric type, the Genreric Metric sub-TLV MUST be
advertised only once for a link when advertised in OSPF Link TLV of
Extended Link LSA and Link TLV of TE LSA. When Genreric 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 Genreric
Metric sub-TLVs advertised for a link for the same metric type and
for same application in one or more received LSPDUs, the first one
MUST be used and the subsequent ones MUST be ignored.If the metric

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type indicates a standard metric type for which there are other
advertisement mechanisms (e.g., the IGP metric, the Min
Unidirectional Link Delay, or the Traffic Engineering Default Metric,
as of this writing), the Generic Metric advertisement MUST be
ignored.

3. FAD constraint sub-TLVs

In networks that carry elephant flows, directing an elephant flow
down a low-bandwidth link would be catastrophic. 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 achived
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.

To implement this idea, this document defines a new 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 a Exclude Maximum Link Delay
constraint. Delay is an important consideration in High Frequency
Trading applications, networks with transparent L2 link recovery, or
in satellite networks, where link delay may fluctuate. Mechanisms
already exist to measure the link delay dynamically and advertised 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. ISIS FAD constraint sub-TLVs

3.1.1. ISIS Exclude Minimum Bandwidth sub-TLV

ISIS Flex-Algorithm Exclude Minimum Bandwidth sub-TLV (FAEMB) 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Min Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:

Type: TBA

Length: 4 octets.

Min Bandwidth: The link bandwidth is encoded in 32 bits in IEEE
floating point format. The units are bytes per second.

Figure 3: ISIS FAEMB sub-TLV

The FAEMB sub-TLV MUST appear at most once in the FAD sub-TLV. If it
appears more than once, the ISIS 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 [RFC 8919]. 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 by the sub-TLV 9 of the TLV
22/222/23/223/141 [RFC 5305] as defined in [RFC8919] 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 then the link MUST NOT be excluded from the topology based
on the Minimum Bandwidth constraint.

3.1.2. ISIS Exclude Maximum Delay sub-TLV

ISIS Flex-Algorithm Exclude Maximum Delay sub-TLV (FAEMD) is a sub-
TLV of the ISIS FAD sub-TLV. It has the following format.

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Type: TBD

Length: 3 octets

Max link delay: Maximum link delay in microseconds

Figure 4: ISIS FAEMD sub-TLV

The FAEMD sub-TLV MUST appear only once in the FAD sub-TLV. If it
appears more than once, the ISIS 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 [RFC 8919]. If 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 [RFC 8570] as defined in [RFC8919] 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.

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Type: TBD

Length: 4 octets.

Min Bandwidth: link bandwidth is encoded in 32 bits in IEEE
floating point format. The units are bytes per second.

Figure 5: OSPF FAEMB sub-TLV

The FAEMB sub-TLV MUST appear only 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 by the sub-sub-
TLV 23 of ASLA [RFC 8920] 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

OSPF Flex-Algorithm Exclude Maximum Delay sub-TLV (FAEMD) is a sub-
TLV of the OSPF FAD TLV. It has the following format.

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Type: TBD

Length: 3 octets

Max link delay: Maximum link delay in microseconds

Figure 6: OSPF FAEMD sub-TLV

The FAEMD sub-TLV MUST appear only once in the OSPF FAD TLV. If it
appears more than once, the OSPF FAD TLV MUST be ignored by the
receiver. The Min Unidirectional Link Delay as advertised by sub-
sub-TLV 12 of ASLA sub-TLV [RFC 8920], MUST be compared against the
Maximum delay advertised in FAEMD sub-TLV. If the Min Unidirectional
Link Delay is higher than the Maximum 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 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 have each node automatically
compute each link's metric based its bandwidth.

This document defines a new 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". ISIS and OSPF will advertise this new type of
metric in their link advertisements. Bandwidth metric is a link
attribute and for advertisement and processing of this attribute for
Flex-algorithm purposes, MUST follow the the section 12 of
[I-D.ietf-lsr-flex-algo]

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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 links
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 FAD TLV and these parameters are
applicable to applications such as Flex-algorithm that make use of
the FAD TLV.

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 advertisements 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] 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 face of parallel adjacencies. If there
are parallel adjacnecies 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.

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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.

A------B====C====F====D
| |
------E-------

Figure 7: Parallel interfaces

For exmple, 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 and the metric for each link will be the
same. Traffic from B to D will be forwarded B->E->D. Since the
bandwidth is higher on the B->C->F->D path, the metric for that path
should be lower, and that path should be selected. Interface Group
Mode is 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.

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

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
bandwdith 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

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computation to be done automatically. 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. ISIS 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 ISIS FAD sub-
TLV. It has the following format:

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where:

Type: TBD

Length: 9 octets.
Reference Bandwidth: Bandwidth encoded in 32 bits in IEEE floating point
format. The units are in bytes per second.
Granularity Bandwidth: Bandwidth encoded in 32 bits in IEEE floating point
format. The units are in bytes per second.

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.

Metric calculation: (Reference_bandwidth) /
(Total_link_bandwidth -
(Modulus of(Total_link_bandwidth,granularity_bw)))

Figure 8: ISIS FADRB sub-TLV

Granularity Bandwidth value ensures that the metric does not change
when there is a small change in the link bandwidth. The ISIS FADRB
Sub-TLV MUST NOT appear more than once in an ISIS FAD sub-TLV. If it
appears more than once, the ISIS FAD sub-TLV 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
advertised Bandwidth Metric, and MUST NOT use the automatically
derived metric for that link.

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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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric n-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth Threshold n-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Threshold Metric n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

Type: TBD

Length: 1 + n*7 octets. Here n is equal to number of Threshold Metrics specified.
n MUST be greater than or equal to 1.

Flags:

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|G| | | |
+-+-+-+-+-+-+-+-+

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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: Minimum Link Bandwidth is encoded in 32 bits in IEEE
floating point format. The units are bytes per second.
Bandwidth Threshold 2: Maximum Link Bandwidth is encoded in 32 bits in IEEE
floating point format. The units are bytes per second.
Threshold Metric 1 : metric value range (1 - 4,261,412,864)

Figure 9: ISIS FADBT sub-TLV

When G-flag is set, the cumulative bandwidth of the parallel links is
computed as described in section Section 4.1.1.2. If G-flag is not
set, the advertised Maximum Link Bandwidth is used.

When the computed link bandwidth is less than Bandwidth Threshold 1,
the MAX_METRIC value of 4,261,412,864 MUST be assigned as the
Bandwidth Metric on the link during Flex-Algorithm SPF calculation.

When the computed link bandwidth is greater than or equal to
Bandwidth Threshold 1 and less than Bandwidth Threshold 1, Threshold
Metric 1 MUST be assigned as the Bandwidth Metric on the link during
Flex-Algorithm SPF calculation.

Similarly, when the computed link bandwidth is greater than or equal
to Bandwidth Threshold 1 and less than Bandwidth Threshold 2,
Threshold Metric 2 MUST be assigned as the Bandwidth Metric on the
link during Flex-Algorithm SPF calculation.

In general, when the computed link bandwidth is greater than or equal
to Bandwidth Threshold X AND less than Bandwidth Threshold X+1,
Threshold Metric X MUST be assigned as the Bandwidth Metric on the
link during Flex-Algorithm SPF calculation.

Finally, when the computed link bandwidth is greater than or equal to
Bandwidth Threshold n, then Threshold Metric n MUST be assigned as
the Bandwidth Metric on the link during Flex-Algorithm SPF
calculation.

The ISIS FADBT Sub-TLV MUST NOT appear more than once in an ISIS FAD
sub-TLV. If it appears more than once, the ISIS FAD sub-TLV MUST
MUST stop participating in such flex-algorithm.

A FAD MUST NOT contain both FADBT sub-TLV and 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.

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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.

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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where:

Type: TBD

Length: 14 octets.
Reference Bandwidth: Bandwidth encoded in 32 bits in IEEE floating point
format. The units are in bytes per second.
Granularity Bandwidth: Bandwidth encoded in 32 bits in IEEE floating point
format. The units are in bytes per second.

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.

Metric calculation: (Reference_bandwidth) /
(Total_link_bandwidth -
(Modulus of(Total_link_bandwidth, Granularity_bw)))

Figure 10: OSPF FADRB sub-TLV

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. If a Generic Metric sub-TLV with Bandwidth metric type is
advertised for a link, the Flex-Algorithm calculation MUST use the

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advertised Bandwidth Metric on the link, and MUST NOT use the
automatically derived metric for that link.

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:

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: TBD

Length: 2 + n*8 octets. Here n is equal to number of Threshold Metrics specified.
n MUST be greater than or equal to 1.

Flags:

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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|G| | | |
+-+-+-+-+-+-+-+-+

G-flag: when set, interface group Mode MUST be used to derive total link bandwidth.

Figure 11: OSPF FADBT sub-TLV

When G-flag is set, the cumulative bandwidth of the parallel links is
computed as described in section Section 4.1.1.2. If G-flag is not
set, the advertised Maximum Link Bandwidth is used.

When the computed link bandwidth is less than Bandwidth Threshold 1 ,
the MAX_METRIC value of 4,294,967,296 MUST be assigned as the
Bandwidth Metric on the link during Flex-Algorithm SPF calculation.

The ISIS FADBT Sub-TLV MUST NOT appear more than once in an ISIS FAD
sub-TLV. If it appears more than once, the ISIS FAD sub-TLV MUST
stop participating in such flex-algorithm.

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A FAD MUST NOT contain both FADBT sub-TLV and 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.

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 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 Bandwidth Metric is
considered as not being advertised 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 Bandwidth Metric can not be calculated due to
lack of Flex-Algorithm specific ASLA advertisement of sub-sub-TLV
9 [RFC 8919], or in case of IS-IS, in presence of the L-Flag in
the Flex-Algorithm specific ASLA advertisement the lack of sub-TLV
9 in the TLV 22/222/23/223/141 [RFC 5305], the Bandwidth Metric is
considered as not being advertised for the link.

6. Calculation of Flex-Algorithm paths

Two new additional rules are added to the existing rules in the Flex-
rules specified in sec 13 of [I-D.ietf-lsr-flex-algo].

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 computation.

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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 computation.

7. Backward Compatibility

8. Security Considerations

TBD

9. IANA Considerations

9.1. IGP Metric-Type Registry

Type: Suggested 3 (TBA)

Description: Bandwidth metric

Reference: This document

Type: 128 to 255(TBA)

Description: User defned metric

Reference: This document

9.2. ISIS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV

Type: Suggested 6 (TBA)

Description: ISIS Exclude Minimum Bandwidth sub-TLV

Reference: This document Section 3.1.1

Type: Suggested 7 (TBA)

Description: ISIS Exclude Maximum Delay sub-TLV

Reference: This document Section 3.1.2

Type: Suggested 8 (TBA)

Description: ISIS Reference Bandwidth sub-TLV

Reference: This document Section 4.1.3.1

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Type: Suggested 9 (TBA)

Description: ISIS Threshold Metric sub-TLV

Reference: This document Section 4.1.3.2

9.3. OSPF Sub-TLVs for Flexible Algorithm Definition Sub-TLV

Type: Suggested 6 (TBA)

Description: OSPF Exclude Minimum Bandwidth sub-TLV

Reference: This document Section 3.2.1

Type: Suggested 7 (TBA)

Description: OSPF Exclude Maximum Delay sub-TLV

Reference: This document Section 3.2.2

Type: Suggested 8 (TBA)

Description: OSPF Reference Bandwidth sub-TLV

Reference: This document Section 4.1.4.1

Type: Suggested 9 (TBA)

Description: OSPF Threshold Metric sub-TLV

Reference: This document Section 4.1.4.2

9.4. Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223

Type: Suggested 45 (TBA)

Description: Generic metric

Reference: This document Section 2.1

9.5. Sub-sub-TLV Codepoints for Application-Specific Link Attributes

Type: Suggested 45 (TBA)

Description: Generic metric

Reference: This document Section 2.1

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9.6. OSPFv2 Extended Link TLV Sub-TLVs

Type: Suggested 45 (TBA)

Description: Generic metric

Reference: This document Section 2.2

9.7. Types for sub-TLVs of TE Link TLV (Value 2)

Type: Suggested 45 (TBA)

Description: Generic metric

Reference: This document Section 2.2

10. Acknowledgements

Many thanks to Chris Bowers, Krzysztof Szarcowitz, Julian Lucek, Ram
Santhanakrishnan for discussions and inputs.

11. Contributors

1. Salih K A

Juniper Networks

salih@juniper.net

12. References

12.1. Normative References

[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-13 (work in progress), October 2020.

[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>.

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[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>.

[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>.

12.2. 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", draft-bashandy-rtgwg-segment-routing-uloop-10
(work in progress), December 2020.

[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>.

[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>.

[RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
December 2008, <https://www.rfc-editor.org/info/rfc5316>.

[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>.

Authors' Addresses

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Shraddha Hegde
Juniper Networks Inc.
Exora Business Park
Bangalore, KA 560103
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

Email: bruno.decraene@orange.com

Peter Psenak
Cisco Systems

Email: ppsenak@cisco.com

Tony Li
Arista Networks

Email: tony.li@tony.li

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