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TVR (Time-Variant Routing) Use Cases
draft-ietf-tvr-use-cases-07

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Authors Edward J. Birrane , Nicolas Kuhn , Yingzhen Qu , Rick Taylor , Li Zhang
Last updated 2024-02-29 (Latest revision 2024-02-26)
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draft-ietf-tvr-use-cases-07
Network Working Group                                         E. Birrane
Internet-Draft                                                   JHU/APL
Intended status: Informational                                   N. Kuhn
Expires: 29 August 2024                              Thales Alenia Space
                                                                   Y. Qu
                                                  Futurewei Technologies
                                                               R. Taylor
                                                          Ori Industries
                                                                L. zhang
                                                                  Huawei
                                                        26 February 2024

                  TVR (Time-Variant Routing) Use Cases
                      draft-ietf-tvr-use-cases-07

Abstract

   This document introduces use cases where Time-Variant Routing (TVR)
   computations (i.e. routing computations taking into considerations
   time-based or scheduled changes to a network) could improve routing
   protocol convergence and/or network performance.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-tvr-use-cases/.

   Discussion of this document takes place on the Time Variant Routing
   Working Group mailing list (mailto:tvr@ietf.org), which is archived
   at https://mailarchive.ietf.org/arch/browse/tvr/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/tvr/.

   Source for this draft and an issue tracker can be found at
   https://github.com/NasaDtn/tvr-bof-use-cases.

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 29 August 2024.

Copyright Notice

   Copyright (c) 2024 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Resource Preservation . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Routing Impacts . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Exemplar  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Operating Efficiency  . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Routing Impacts . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Exemplar : Cellular Network . . . . . . . . . . . . . . .  10
     4.4.  Another Exemplar : Tidal Network  . . . . . . . . . . . .  11
   5.  Dynamic Reachability  . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Routing Impacts . . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Exemplar : Mobile Satellites  . . . . . . . . . . . . . .  14
     5.4.  Another Exemplar : Predictable Moving Vessels . . . . . .  17
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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1.  Introduction

   There is a growing number of use cases where changes to the routing
   topology are an expected part of network operations.  In these use
   cases the pre-planned loss and restoration of an adjacency, or
   formation of an alternate adjacency, should be seen as a non-
   disruptive event.

   Expected changes to topologies can occur for a variety of reasons.
   In networks with mobile nodes, such as unmanned aerial vehicles and
   some orbiting spacecraft constellations, links are lost and re-
   established as a function of the mobility of the platforms.  In
   networks without reliable access to power, such as networks
   harvesting energy from wind and solar, link activity might be
   restricted to certain times of day.  Similarly, in networks
   prioritizing green computing and energy efficiency over data rate,
   network traffic might be planned around energy costs or expected user
   data volumes.

   This document defines three categories of use cases where a route
   computation might beneficially consider time information.  Each of
   these use cases includes the following information.

   1.  An overview of the use case describing how route computations
       might select different paths (or subpaths) as a function of time.

   2.  A set of assumptions made by the use case as to the nature of the
       network and data exchange.

   3.  Specific discussion on the routing impacts of the use case.

   4.  Exemplars of a network conformant to the use case.

   The use cases that are considered in this document are the following.

   1.  Resource Preservation (described in Section 3), where there is
       on-off link availability over time at the client level.  Time
       Variant Routing can utilize the predictability of the link
       availability to optimize network connectivity by taking into
       account end point resource preservation.

   2.  Operating Efficiency (described in Section 4), where there is a
       server cost or a path cost usage varying over time.  Time Variant
       Routing can exploit the predictability of the path cost to
       optimize the cost of the system exploitation.  The notion of cost
       of a path is extended by introducing the notion of its time-
       varying characteristic.

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   3.  Dynamic Reachability (described in Section 5), where there is on-
       off link availability variation between nodes taking part of the
       end-to-end path.  Time Variant Routing can exploit the
       predictability of the link availability to optimize in-network
       routing.

   The document may not represent the full set of cases where time-
   variant routing computations could beneficially impact network
   performance - new use cases are expected to be generated over time.
   Similarly, the concrete examples within each use case are meant to
   provide an existence proof of the use case, not to present any
   exhaustive enumeration of potential examples.  It is likely that
   there exist multiple example networks that could be claimed as
   instances of any given use case.

   The document focuses on deterministic scenarios.  Non-deterministic
   scenarios such as vehicle-to-vehicle communication is out of the
   scope of the document.

2.  Conventions and Definitions

   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.

3.  Resource Preservation

   Some nodes in a network might operate in resource-constrained
   environments or otherwise with limited internal resources.
   Constraints such as available power, thermal ranges, and on-board
   storage can all impact the instantaneous functionality of a node.  In
   particular, resource management on such a node can require that
   certain functionality be powered on (or off) to extend the ability of
   the node to participate in the network.

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   When power on a node is running low, non-critical functions on the
   node might be turned off in favor of extending node life.
   Alternatively, certain functions on a node may be turned off to allow
   the node to use available power to respond to an event, such as data
   collection.  When a node is in danger of violating a thermal
   constraint, normal processing might be paused in favor of a
   transition to a thermal safe mode until a regular operating condition
   is reestablished.  When local storage resources run low, a node might
   choose to expend power resources to fuse, delete, or transmit data
   off the node to free space for future data collection.  There might
   also be cases where a node experiences a planned offline state to
   save and accumulate power.

   In addition to power, thermal, and storage, other resource
   constraints may exist on a node such that the preservation of
   resources are necessary to preserve the existence (and proper
   function) of the node in the network.  Nodes operating in these
   conditions might benefit from TVR computations as the connectivity of
   the node changes over time as part of node preservation.

3.1.  Assumptions

   To manage on-board functionality as a function of available
   resources, a node must understand certain elements of how resources
   are used and replenished.  It is assumed that patterns of the
   environment, device construction, and operational configuration exist
   with enough regularity and stability to allow meaningful planning.
   The following assumptions are made with this use case.

   1.  Known resource expenditures.  It is assumed that there exists
       some determinable relationship between the resources available on
       a node and the resources needed to participate in a network.  A
       node would need to understand when it has met some condition for
       participating in, or dropping out of, a network.  This is
       somewhat similar to predicting the amount of battery life left on
       a laptop as a function of likely future usage.

   2.  Predictable resource accumulation.  It is assumed that the
       accumulation of resources on a node are predictable such that a
       node might expect (and be able to communicate) when it is likely
       to next rejoin a network.  This is similar to predicting the time
       at which a battery on a laptop will be fully charged.

   3.  Consistent cost functions.  It is assumed that resource
       management on a node is deterministic such that the management of
       a node as a function of resource expenditure and accumulation is
       consistent enough for link planning.

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3.2.  Routing Impacts

   Resource management in these scenarios might involve turning off
   elements of the node as part of on-board resource management.  These
   activities can affect data routing in a variety of ways.

   1.  Power Savings.  On-board radios may be turned off to allow other
       node processing.  This may happen on power-constrained devices to
       extend the battery life of the node or to allow a node to perform
       some other power-intensive task.

   2.  Thermal Savings.  On-board radios may be turned off if there are
       thermal considerations on the node, such as an increase in a
       node’s operating temperature.

   3.  Storage Savings.  On-board radios may be turned on with the
       purpose of transmitting data off the node to free local storage
       space to collect new data.

   Whenever a communications device on a node changes its powered state
   there is the possibility (if the node is within range of other nodes
   in a network) that the topology of the network is changed, which
   impacts route calculations through the network.  Additionally,
   whenever a node joins a network there may be a delay between the
   joining of the node to the network and any discovery that may take
   place relating to the status of the node’s functional neighborhood.
   During these times, forwarding to and from the node might be delayed
   pending some synchronization.

3.3.  Exemplar

   An illustrative example of a network necessitating resource
   preservation is an energy-harvesting wireless sensor network.  In
   such a network, nodes rely exclusively on environmental sources for
   power, such as solar panels.  On-board power levels may fluctuate
   based on various factors including sensor activity, processing
   demands, and the node's position and orientation relative to its
   energy source.

   Consider a contrived three node network where each node accumulates
   power through solar panels.  Power available for Radio Frequency (RF)
   transmission is shown below in Figure 1.  In this figure, each of the
   three nodes (Node 1, Node 2, and Node 3) have a different plot of
   available power over time.  This example assumes that a node will not
   power its radio until available power is over some threshold, which
   is shown by the horizontal line on each plot.

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            Node 1                   Node 2                   Node 3
 P |                      P |   -------            P |          --
 o |  ----       --       o |  /       \           o |         /  \
 w |~/~~~~\~~~~~/~~\~~    w |~/~~~~~~~~~\~~~~~~    w |~~~~~~~~/~~~~\~~~~
 e |/      \   /    \     e |/           \         e |       /      \
 r |        ---      -    r |             -----    r |-------        ---
   +---++----++----++-      +---++----++----++-      +---++----++----++-
       t1    t2    t3           t1    t2    t3           t1    t2    t3
            Time                     Time                     Time

                     Figure 1: Node Power Over Time

   The connectivity of this three node network changes over time in ways
   that may be predictable and are likely able to be communicated to
   other nodes in this small sensor network.  Examples of connectivity
   are shown in Figure 2.  This figure shows a sample of network
   connectivity at three times: t1, t2, and t3.

   *  At time t1 Node 1 and Node 2 have their radios powered and are
      expected to communicate.

   *  At time t2 it is expected that Node 1 has its radio off, but that
      Node 2 and Node 3 can communicate.

   *  Finally, at time t3 it is expected that Node 1 may be turning its
      radio off and that Node 2 and Node 3 are not powering their radios
      and there is no expectation of connectivity.

              +----------+        +----------+        +----------+
         t1   |  Node 1  |--------|  Node 2  |        |  Node 3  |
              +----------+        +----------+        +----------+

              +----------+        +----------+        +----------+
         t2   |  Node 1  |        |  Node 2  |--------|  Node 3  |
              +----------+        +----------+        +----------+

              +----------+        +----------+        +----------+
         t3   |  Node 1  |        |  Node 2  |        |  Node 3  |
              +----------+        +----------+        +----------+

                        Figure 2: Topology over Time

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4.  Operating Efficiency

   Some nodes in a network might alter their networking behavior to
   optimize metrics associated with the cost of a node's operation.
   While the resource preservation use case described in Section 3
   addresses node survival, this use case discusses non-survival
   efficiencies such as the financial cost to operate the node and the
   environmental impact (cost) of using that node.

   When a node operates using some pre-existing infrastructure there is
   (typically) some cost associated with the use of that infrastructure.
   Sample costs include items such as the following.

   1.  Nodes that use existing wireless communications such as a
       cellular infrastructure must pay to communicate to and through
       that infrastructure.

   2.  Nodes supplied with electricity from an energy provider pay for
       the power they use.

   3.  Nodes that cluster computation and activities might increase the
       temperature of the node and incur additional costs associated
       with cooling the node (or collection of nodes).

   4.  Beyond financial costs, assessing the environmental impact of
       operating a node may also be modeled as a cost associated with
       node operation, to include achieving carbon credits or other
       incentives for green computing.

   When the cost of using a node's resources changes over time, a node
   can benefit from predicting when data transmissions might optimize
   costs, environmental impacts, or other metrics associated with
   operation.

4.1.  Assumptions

   The ability to predict the impact of a node's resource utilization
   over time presumes that the node exists within a defined environment
   (or infrastructure).  Some necessary characteristics of these
   environments are listed as follows.

   1.  Cost Measureability.  The impacts of operating a node within its
       environment can be measured in a deterministic way.  For example,
       that the cost-per-bit of data over a cellular network or the
       cost-per-kilowatt of energy used are known.

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   2.  Cost Predictability.  Changes to the impacts of resource
       utilization are known in advance.  For example, if the cost of
       energy is less expensive in the evening than during the day,
       there exists some way of communicating this change to a node.

   3.  Cost Persistent.  Changes to the cost of operating in the
       environment persist for a sufficient amount of time such that
       behavior can be adjusted in response to changing costs.  If costs
       change rapidly or near continuously it is likely not possible to
       meaningfully react to their change.

   4.  Cost Magnitude.  The magnitude of cost changes are such that a
       node sees a minimum threshold cost reduction as a result of
       optimization.

4.2.  Routing Impacts

   Optimizing resource utilization can affect route computation in ways
   similar to those experienced with resource preservation.  The route
   computation may not change the available path but the topology as
   seen by an end point would be different.  Cost optimization can
   impact route calculation in a variety of ways, some of which are
   described as follows.

   1.  Link Filtering.  Data might be accumulated on a node waiting for
       a cost-effective time for data transmission.  Individual link
       costs might be annotated with cost information such that
       adjacencies with a too-high cost might not be used for
       forwarding.  This effectively filters which adjacencies are used
       (possibly as a function of the type of data being routed).

   2.  Burst Planning.  In cases where there is a cost savings
       associated with fewer longer transmissions (versus many smaller
       transmissions), nodes might refuse to forward data until a
       sufficient data volume exists to justify a transmission.

   3.  Environmental Measurement.  Nodes that measure the quality of
       individual links can compute the overall cost of using a link as
       a function of the signal strength of the link.  If link quality
       is insufficient due to environmental conditions (such as clouds
       on a free-space optical link or long distance RF transmission in
       a storm) the cost required to communicate over the link may be
       too much, even if access to infrastructure is otherwise in a less
       expensive time of day.

   In each of these cases, some consideration of the efficiency of
   transmission is prioritized over achieving a particular data rate.
   Waiting until data rate costs are lower takes advantage of platforms

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   using time-of-use rate plans – both for pay-as-you-go data and
   associated energy costs.  Accumulating data volumes and choosing more
   opportune times to transmit can also result in less energy
   consumption by radios and, thus, less operating cost for platforms.

4.3.  Exemplar : Cellular Network

   One example of a network where nodes might seek to optimize operating
   cost is a set of nodes operating over cellular connections that
   charge both On-Peak and Off-Peak data rates.  In this case,
   individual nodes may be allocated a fixed set of "On-Peak" minutes
   such that exceeding that amount of time results in expensive overage
   charges.  Generally, the concept of On-Peak and Off-Peak minutes
   exists to deter the use of a given network at times when the cellular
   network is likely to encounter heavy call volumes (such as during the
   workday).

   Just as pricing information can act as a deterrent (or incentive) for
   a human cellular user, this pricing information can be codified in
   ways that also allow machine-to-machine (M2M) connections to
   prioritize Off-Peak communications for certain types of data
   exchange.  Many M2M traffic exchanges involve schedulable activities,
   such as nightly bulk file transfers, pushing software updates,
   synchronizing datastores, and sending non-critical events and logs.
   These activities are usually already scheduled to minimize impact on
   businesses and customers, but can also be scheduled to minimize
   overall cost.

   Consider a contrived three node network, similar to the one pictured
   in Figure 1, except that in this case the resource that varies over
   time is the cost of the data exchange.  This case is illustrated
   below in Figure 3.  In this figure, a series of three plots are
   given, one for each of nodes Node 1, Node 2, and Node 3.  Each of
   these nodes exists in a different cellular service area which has
   different On-Peak and Off-Peak data rate times.  This is shown in
   each figure by times when the cost is low (Off-Peak) and when the
   cost is high (On-Peak).

 Node 1                     Node 2                   Node 3

C |       +---------     C |--+                    C |-------------+
o |       |              o |  |                    o |             |
s |       |              s |  |                    s |             |
t |-------+              t |  +----------------    t |             +-------
  |                        |                         |
  +---++----++----++--     +----++----++----++--     +----++----++-----++--
      t1    t2    t3            t1    t2    t3            t1    t2     t3
           Time                      Time                      Time

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                    Figure 3: Data Cost Over Time

   Given the presumption that peak times are known in advance, the cost
   of data exchange from Node 1 through Node 2 to Node 3 can be
   calculated.  Examples of these data exchanges are shown in Figure 4.
   From this figure, both times t1 and t3 result in a smaller cost of
   data exchange than choosing to communicate data at time t2.

          +-----------+          +-----------+          +-----------+
     t1   |  Node N1  |---LOW----|  Node N2  |---HIGH---|  Node N3  |
          +-----------+          +-----------+          +-----------+

          +-----------+          +-----------+          +-----------+
     t2   |  Node N1  |---HIGH---|  Node N2  |---HIGH---|  Node N3  |
          +-----------+          +-----------+          +-----------+

          +-----------+          +-----------+          +-----------+
     t3   |  Node N1  |---HIGH---|  Node N2  |----LOW---|  Node N3  |
          +-----------+          +-----------+          +-----------+

                   Figure 4: Data Exchange Cost over Time

   While not possible in every circumstance, a highly optimized plan
   could be to communicate from Node 1 to Node 2 at time t1 and then
   queue data at Node 2 until time t3 for delivery to Node 3.  This case
   is shown in Figure 5.

          +-----------+          +-----------+
     t1   |  Node N1  |---LOW----|  Node N2  |
          +-----------+          +-----------+
                                 +-----------+          +-----------+
     t3                          |  Node N2  |----LOW---|  Node N3  |
                                 +-----------+          +-----------+

                     Figure 5: Data Cost using Storage

4.4.  Another Exemplar : Tidal Network

   Another example related to operating efficiency is what is often
   referred to as a 'tidal network,' in which traffic volume undergoes
   significant fluctuations at different times.  Take, for instance, a
   campus network, where thousands of individuals go to classrooms and
   libraries during the daytime and retire to the dormitories at night.
   This results in a regular oscillation of network traffic across
   various locations within the campus.

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   In the context of a tidal network scenario, energy-saving methods may
   include the deactivation of some or all components of network nodes.
   These activities have the potential to alter network topology and
   impact data routing in a variety of ways.  Ports on network nodes can
   be selectively disabled or enabled based on traffic patterns, thereby
   reducing the energy consumption of nodes during periods of low
   network traffic.

   More information on Tidal Network can be found in [TIDAL].

5.  Dynamic Reachability

   When a node is placed on a mobile platform, the mobility of the
   platform (and thus the mobility of the node) may cause changes to the
   topology of the network over time.  Since the topology is realized by
   (pre)planned actions of the nodes, the impacts on the dynamics of the
   topology can be very important.  To the extent that the relative
   mobility between and among nodes in the network and the impacts of
   the environment on the signal propagation can be understood in
   advance, the associated loss and establishment of adjacencies can
   also be planned for.

   Mobility can cause the loss of an adjacent link in several ways, such
   as the following.

   1.  Node mobility can cause the distance between two nodes to become
       large enough that distance-related attenuation causes the mobile
       node to lose connectivity with one or more other nodes in the
       network.

   2.  Node mobility can also be used to maintain a required distance
       from other mobile nodes in the network.  While moving, external
       characteristics may cause the loss of links through occultation
       or other hazards of traversing a shared environment.

   3.  Node mobility can cause the distance between two nodes to vary
       quickly over the time making it complicated to establish and
       maintain connectivity.

   4.  Nodes that can change the orientation of their communication
       terminals will also establish and lose connectivity with other
       nodes as a function of that motion.

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   Mobile nodes (like any node) may have concerns related to resource
   preservation and cost efficiency, but they can also have concerns
   uniquely attributed to their mobility.  This on-off availability of
   the links may then induce dynamic pointing mechanisms at the node's
   level.  This use case focuses on understanding the routing
   implications of motion-related changes to a network topology.

5.1.  Assumptions

   Predicting the impact of node mobility on route computation requires
   some information relating to the nature of the mobility and the
   nature of the environment being moved through.  Some information
   presumed to exist for planning is listed as follows.

   1.  Path Predictability.  The path of a mobile node through its
       environment is known (or can be predicted) as a function of (at
       least) time. it is presumed that mobile nodes using time-variant
       algorithms would not exhibit purely random motion.

   2.  Environmental Knowledge.  When otherwise well-connected mobile
       nodes pass through certain elements of their environment (such as
       a storm, a tunnel, or the horizon) they may lose connectivity.
       The duration of this connectivity loss is assumed to be
       calculable as a function of node mobility and the environment
       itself.

5.2.  Routing Impacts

   Changing a network topology has a straightforward impact on the
   computation of paths (or subpaths) through that topology.  In
   particular, the following features can be implemented in a network
   with mobile nodes such that different paths might be computed over
   time.

   1.  Adjacent Link Expiration.  A node might be able to predict that
       an adjacency will expire as a function of that node's mobility,
       the other node's mobility, or some characteristic of the
       environment.  Determining that an adjacency has expired allows a
       route computation to plan for that loss, rather than default to
       an error recovery mechanism.

   2.  Adjacent Link Resumption.  Just as the loss of an adjacency can
       be predicted, it may be possible to predict when an adjacency
       will resume.

   3.  Data Rate Adjustments.  The achievable data rate over a given
       link is not constant over time, and may vary significantly as a
       function of both relative mobility between a transmitter and

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       receiver as well as the environment being transmitted through.
       Knowledge of both mobility and environmental state may allow for
       prediction of data rates which may impact path computation.

   4.  Adjacent Link Filtering.  Separate from the instantaneous
       presence or absence of an adjacency, a route computation might
       choose to not use an adjacency if that adjacency is likely to
       expire in the near future or if it is likely to experience a
       significant drop in predicted data rate.

5.3.  Exemplar : Mobile Satellites

   A relatively new type of mobile network that has emerged over the
   past several years is the Low Earth Orbit (LEO) networked
   constellation (LEO-NC).  There are a number of such constellations
   being built by both private industry and governments.  While this
   exemplar describes LEO satellites systems, the mobility events can be
   applied to satellite systems orbiting at different altitude
   (including Very LEO (V-LEO) or Medium Earth Orbit (MEO)).

   Many LEO-NCs have a similar operational concept of hundreds-to-
   thousands of inexpensive spacecraft that can communicate both with
   their orbital neighbors as well as down to any ground station that
   they happen to be passing over.  A ground station is a facility used
   to communicate with satellites in low Earth orbit.  The relationship
   between an individual spacecraft and an individual ground station
   becomes somewhat complex as each spacecraft may only be over a single
   ground station for a few minutes at a time.  Moreover, as a function
   of the constellation topology, there are scenarios where (1) the
   inter-satellite links need to be shut down for interference avoidance
   purposes or (2) the network topology changes, which modifies the
   neighbors of a given spacecraft.

   A LEO-NC represents a good example of planned mobility based on the
   predictability of spacecraft in orbit.  While other mobile vehicles
   might experience unpredictable significant changes to speed,
   spacecraft operate in a less impactful environment.  This determinism
   makes them an excellent candidate for time-variant route
   computations.  It is worth pointing out that unplanned inter-
   satellite links failures could still introduce randomness in the
   network topology.

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   Consider three spacecraft (N1, N2, and N3) following each other
   sequentially in the same orbit (this is sometimes called a string of
   pearls configuration).  Spacecraft N2 always maintains connectivity
   to its two neighbor spacecraft, N1 (which is behind in the orbit) and
   N3 (which is ahead in the orbit).  This configuration is illustrated
   in Figure 6.  While these spacecraft are all mobile, their relative
   mobility ensures that they are always in contact with each other
   (absent any true error condition).

          .--.                     .--.                     .--.
    ####-| N1 |-####  <--->  ####-| N2 |-####  <--->  ####-| N3 |-####
          \__/                     \__/                     \__/

                   Figure 6: Three Sequential Spacecraft

   Flying over a ground station imposes a non-relative motion between
   the ground and the spacecraft - namely that any given ground station
   will only be in view of the spacecraft for a short period of time.
   The times at which each spacecraft can see the ground station is
   shown in the plots in Figure 7.  In this figure, ground contact is
   shown when the plot is high, and a lack of ground contact is shown
   when the graph is low.  From this, we see that spacecraft N3 can see
   ground at time t1, N2 sees ground at time t2, and spacecraft N1 sees
   ground at time t3.

       Spacecraft N1            Spacecraft N2             Spacecraft N3
G |                      G |                       G |
r |              +--+    r |         +--+          r |   +--+
o |              |  |    o |         |  |          o |   |  |
u |              |  |    u |         |  |          u |   |  |
n |--------------+  +-   n |---------+  +-------   n |---+  +-------------
d |                      d |                       d |
  +---++----++----++--     +----++----++----++--     +----++----++----++--
      t1    t2    t3            t1    t2    t3            t1    t2    t3
           Time                      Time                      Time

            Figure 7: Spacecraft Ground Contacts Over Time

   Since the ground station in this example is stationary, each
   spacecraft will pass over it, resulting in a change to the network
   topology.  This topology change is shown in Figure 8.  At time t1,
   any message residing on N3 and destined for the ground could be
   forwarded directly to the ground station.  At time t2, that same
   message would need to, instead, be forwarded to N2 and then forwarded
   to ground.  By time t3, the same message would need to be forwarded
   from N2 to N1 and then down to ground.

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        +------+          +------+
    t1  |  N2  |----------|  N3  |
        +------+          +---+--+
                              |
                             /|\
                            \___/
                             / \
                           Ground
                           Station
    ------------------------------------------------------------------
        +------+          +------+          +------+
    t2  |  N1  |----------|  N2  |----------|  N3  |
        +------+          +---+--+          +------+
                              |
                             /|\
                            \___/
                             / \
                           Ground
                           Station
    ------------------------------------------------------------------
                          +------+          +------+          +------+
    t3                    |  N1  |----------|  N2  |----------|  N3  |
                          +---+--+          +------+          +------+
                              |
                             /|\
                            \___/
                             / \
                           Ground
                           Station
    ------------------------------------------------------------------

                 Figure 8: Constellation Topology Over Time

   This example focuses on the case where the spacecrafts fly over a
   ground station and introduce changes in the network topology.  There
   are also scenarios where the in-constellation network topology varies
   over time following a deterministic time-driven operation from the
   ground system.  More information on in-constellation network topology
   can be found in [LHAN-PROB] and [LWOOD-SCN].  For this example, and
   in particular for within constellation network topology changes, TVR
   approach is important to avoid the Interior Gateway Protocol (IGP)
   issues mentioned in [LHAN-PROB].

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5.4.  Another Exemplar : Predictable Moving Vessels

   Another relevant example for this use case involves the movement of
   vessels with predictable trajectories, such as ferries or planes.
   These end points often rely on a combination of satellite and
   terrestrial systems for Internet connectivity, capitalizing on their
   predictable journeys.

   This scenario also covers situations where nodes employ dynamic
   pointing solutions to track the mobility of other nodes.  In such
   cases, nodes dynamically adjust their antennas and application
   settings to determine the optimal timing for data transmission along
   the path.

6.  Acknowledgments

   Many thanks to Tony Li, Peter Ashwood-Smith, Abdussalam Baryun,
   Arashmid Akhavain, Dirk Trossen, Brian Sipos, Alexandre Petrescu,
   Haoyu Song, Hou Dongxu, Tianran Zhou, Jie Dong, Nkosinathi Nzima and
   Vinton Cerf for their useful comments that helped improve the
   document.

7.  Security Considerations

   While this document does not define a specific mechanism or solution,
   it serves to motivate the use of Time-Based Validation and Revocation
   (TVR).  Therefore, security considerations are anticipated to be
   addressed elsewhere, such as within a TVR schedule definition or
   through a protocol extension utilizing a TVR schedule.  However it's
   important to note that time synchronization is critical within a
   network employing a TVR schedule.  Any unauthorized changes to
   network clocks can disrupt network functionality, potentially leading
   to a Denial of Service (DoS) attack.

8.  IANA Considerations

   This document has no IANA actions.

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/rfc/rfc2119>.

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   [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/rfc/rfc8174>.

9.2.  Informative References

   [LHAN-PROB]
              Han, L., Li, R., Retana, A., Chen, M., Su, L., Jiang, T.,
              and N. Wang, "Problems and Requirements of Satellite
              Constellation for Internet", n.d.,
              <https://datatracker.ietf.org/doc/draft-lhan-problems-
              requirements-satellite-net/>.

   [LWOOD-SCN]
              Wood, L., "SATELLITE CONSTELLATION NETWORKS",
              Internetworking and Computing over Satellite Networks ,
              2003, <http://personal.ee.surrey.ac.uk/Personal/L.Wood/
              publications/zhang-book/zhang-book-wood-chapter-2.pdf>.

   [TIDAL]    Zang, L., Zhou, T., Dong, J., and N. Nzima, "Use Case of
              Tidal Network", n.d., <https://datatracker.ietf.org/doc/
              draft-zzd-tvr-use-case-tidal-network/>.

Authors' Addresses

   Ed Birrane
   JHU/APL
   Email: edward.birrane@jhuapl.edu

   Nicolas Kuhn
   Thales Alenia Space
   Email: nicolas.kuhn.ietf@gmail.com

   Yingzhen Qu
   Futurewei Technologies
   Email: yingzhen.ietf@gmail.com

   Rick Taylor
   Ori Industries
   Email: rick.taylor@ori.co

   Li zhang
   Huawei
   Email: zhangli344@huawei.com

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