IS-IS and OSPF extensions for TVR (Time-Variant Routing)

Internet-Draft	Abbreviated Title	March 2024
Zhang, et al.	Expires 4 September 2024	[Page]

Abstract

TVR needs IGP to calculate different results depending on the time, without convergence after the detection of link or nodes. IGP nodes need to learn the predictable and scheduled changes in advance. This document defines the IGP extensions for predictable and scheduled changes of TVR.¶

Status of This Memo

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1. Introduction

This draft was submitted as [I-D.zw-tvr-igp-extensions].¶

[I-D.ietf-tvr-use-cases] and [I-D.ietf-tvr-requirements] describe a new challenge for non-terrestrial networks. In these networks, there are predictable and scheduled changes of link or nodes. The affected nodes which connected the changing link or node advertise the changes after finding the changes. After convergence all the nodes calculate a new routing table. But for the predictable and scheduled changes, the nodes can advertise the changes and do the calculation in advance to reduce the convergence time.¶

   -----------------------------------------------------
   |                 Satellite Network                 |
   |                                                   |
   |   ----------------            ----------------    |
   ----| Satellite R1 |------------| Satellite R2 |-----
       ----------------            ----------------
              |                            |
              |                            |
              |                            |
      -------------------         -------------------
  ----| Ground Station1 |---------| Ground Station2 |----
  |   -------------------         -------------------   |
  |                                                     |
  |                                                     |
  |                    Ground Network                   |
  -------------------------------------------------------

Figure 1

For example, in Figure 1, The groud station 1/2 connects the Satellite. Because of the satellite movement, the metric between satellite and ground station changes over time. Because the movement is periodic, the metric is changed periodically. Regardless the unexpected situation like bad weather, the metric variation is predicable and scheduled. The groud station 1/2 advertises the predicable and scheduled metric variation to the ground network. Then the nodes in the ground network can learn the scheduled metric and calculate the routing table in advance.¶

This document defines the a set of extensions to IS-IS, OSPFv2 and OSPFv3 for predictable and scheduled changes of TVR. These extensions can be advertised by the node self which has predictable and scheduled changes, or by the node which connected or adjacenct to the node which has predictable and scheduled changes.¶

This document helps the implementation of Intrinsic schedule defined in section 3.1.1 in [I-D.ietf-tvr-requirements]. And the schedule based on Flex Algorithm can solve the time overlap mentioned in section 3.2.7 in [I-D.ietf-tvr-requirements]. The schedule in different FA may be overlapped, but the constrain-based routing calculation will not be affected.¶

1.1. 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 [RFC2119].¶

4. Time Variant sub-TLVs of IS-IS and OSPF FAD Sub-TLV

A new type of Metric-Type is defined for time variant in IS-IS FAD sub-TLV which defined in section 5.1 [RFC9350] and OSPF FAD sub-TLV which defined in section 5.2 [RFC9350].¶

A new Time Variant sub-TLV is advertised as a sub-TLV of IS-IS FAD sub-TLV and OSPF FAD sub-TLV. The Time Variant sub-TLV has following formats:¶

    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     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Recurrence-type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Metric1                              |
   |                       Time-slot1-begin                        |
   |                       Time-slot1-end                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Metric2                              |
   |                       Time-slot2-begin                        |
   |                       Time-slot2-end                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   .                               .                               .
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Metric[m]                            |
   |                       Time-slot[m]-begin                      |
   |                       Time-slot[m]-end                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2

Type: 1-octet. TBD (To be assigned by IANA).
Length: 1-octet. The number of the set of metric and time-slot.
Recurrence-type: 4-octet. The schedule repetitiveness.
Metric: 4-octet. The metric value of specific time slot. The value of 0xffffffff indicates unreachable.
Time-slot-begin: 4-octet. The begin time of this time-slot.
Time-slot-end: 4-octet. The end time of this time-slot.

5. Handling of the time variant sub-TLV

The handling follows the definition in [RFC9350], except one or more timers are scheduled for different time-slot carried in time variant sub-tlv. When a scheduled time-slot is coming, the associated routing table needs to be calculated with the associated metric.¶

When the Recurrence-type in the time variant sub-TLV is set, the routing table needs to be calculated repeatedly. During the time which is not covered by the time-slot, the default metric is suggested to be used for routing calculation.¶

6. IANA Considerations

The document requests new allocations from the IANA registries as follows:¶

6.1. IGP Metric-type Registry

IANA is requested to allocate a new code points from the "IGP Metric-Type" registry.¶

Table 1: IGP Metric-Type
Type	Description	Reference
TBD1	Time Variant metric	This Document

6.2. IS-IS Registry

IANA is requested to allocate a new code points from the "IS-IS Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV" registry.¶

Table 2: IS-IS TVR sub-TLV
Type	Description	Reference
TBD2	Time Variant	This Document

6.3. OSPF Registry

IANA is requested to allocate a new code points from the "OSPF Flexible Algorithm Definition TLV Sub-TLVs" registry.¶

Table 3: OSPF TVR sub-TLV
Type	Description	Reference
TBD3	Time Variant	This Document

9. References

9.1. 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>.
[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>.

9.2. Informative References

[I-D.ietf-tvr-requirements]: King, D., Contreras, L. M., and B. Sipos, "TVR (Time-Variant Routing) Requirements", Work in Progress, Internet-Draft, draft-ietf-tvr-requirements-01, 23 October 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-tvr-requirements-01>.
[I-D.ietf-tvr-use-cases]: Birrane, E. J., Kuhn, N., Qu, Y., Taylor, R., and L. Zhang, "TVR (Time-Variant Routing) Use Cases", Work in Progress, Internet-Draft, draft-ietf-tvr-use-cases-09, 29 February 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-tvr-use-cases-09>.
[I-D.zw-tvr-igp-extensions]: Zhang, Z. and H. Wu, "IS-IS and OSPF extensions for TVR (Time-Variant Routing)", Work in Progress, Internet-Draft, draft-zw-tvr-igp-extensions-02, 17 October 2023, <https://datatracker.ietf.org/doc/html/draft-zw-tvr-igp-extensions-02>.