Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints

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Table of Contents

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. A Flex-Algorithm [RFC9350] is defined as a set of parameters consisting of calculation-type, metric-type, and a set of constraints to allow operators to have more control over the network path computation. In this document, we define further extensions to Flex-Algorithm 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 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.¶

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].¶

    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).

       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).

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 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.¶

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. 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 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.¶

    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.

    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.

    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.

    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.

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.¶

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

    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)))

    When granularity_bw is greater than Total_link_bandwidth
            Metric calculation: Reference_bandwidth /
                                       Total_link_bandwidth

    The division used here is integer division.

    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 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.

        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)

    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)))

    When granularity_bw is greater than Total_link_bandwidth

        Metric calculation: Reference_bandwidth/
                    Total_link_bandwidth

    The division used here is integer division.

    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.

        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)

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.¶

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. 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

     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

11. Acknowledgements

Many thanks to Chris Bowers, Krzysztof Szarcowitz, Julian Lucek, Ram Santhanakrishnan, Ketan Talaulikar and Acee Lindem for discussions and inputs.¶

13. APPENDIX

Authors' Addresses