Toward Loosely Coupled Orchestration for the LEO Satellite Edge
Vaibhav Bhosale, Ketan Bhardwaj, Ada Gavrilovska
Georgia Institute of Technology
Abstract
Low Earth Orbit (LEO) satellites are envisioned to be capa-
ble of providing Internet services for billions of users who
currently lack reliable Internet connectivity. This calls for a
new LEO edge capable of providing edge computing benefits
from space. This paper proposes an orchestration approach
for the LEO edge that incorporates path models, temporal
compensation and affinity chains as the primary scheduling
constructs, and presents preliminary results that illustrate op-
portunities for achieving improved service availability and
improved performance for a stateful (caching) edge function.
1 Introduction
By 2025 as many as 1,100 satellites could be launching each
year. Just one project, SpaceX’s ambitious Starlink, aims to
fly 12,000 LEO satellites by 2027 [
20
,
21
]. As per predictions
from early 2019, Figure 1 shows the total number of planned
launches yearly. In addition to this meteoric rise in launches,
the Internet based on Low Earth Orbit (LEO) satellites is en-
visioned as being capable of providing Internet services to
billions of users worldwide [
34
]. What this points to is that,
within the next couple of years, the LEO satellites-based back-
haul will support a significant fraction of Internet services.
The more interesting aspect of these satellites is the avail-
ability of computational resources onboard the satellite [
25
],
opening up an opportunity for a new form of edge – the LEO
satellite edge.
Traditionally, LEO satellites are used in a “bent pipe” ar-
chitecture where every satellite simply acts as a communi-
cation link and pushes off the processing to the terrestrial
datacenters [
35
]. This has already been shown to help provide
Internet to remote areas and settings where deploying fiber
is challenging or not possible (such as for airplanes, ships,
etc.) [
5
]. Treating LEO satellites as a "new edge" brings in-
tuitive advantages such as reduced latency to serve requests
and reduced down link utilization since all requests need not
go to the data-center. In our experiments, we observe that a
Figure 1:
Satellites currently in orbit, along with plans announced
for future years. Source: Union of Concerned Scientists; TR projec-
tions based on announced plans
simple LRU web cache (with size 4.5 GB) for the Wikipedia
web service can achieve a cache hit rate upwards of 50%, thus
providing half the latency to end users using a LEO satellite
edge compared to one that uses LEO satellites with the bent
pipe architecture. Similarly, an image analytics function on a
LEO edge is shown to alleviate pressure on downlink capac-
ity by
20
x [
26
,
27
]. Deploying such applications on the LEO
edge would require orchestration support.
However, most of the currently commissioned satellites in
space serve a single mission. There satellites typically capture
images for weather forecasting, disaster management or mon-
itoring regions on land for security. Some of these scenarios,
such as collecting remote sensing data over Greenland (Ullo-
riaq) [
15
,
28
], are so specialized that the satellite is idle for
most of the duration of its orbit. This has led to development
of an orchestration stack that only needs to deploy specialized
and well-planned functions on satellites.
With recent developments to commercialize LEO satel-
lites [
11
,
13
] and the availability of a larger number of geo-
graphically distributed ground stations as a service [
2
], we
argue that fixed-function deployment will be insufficient to
harness the full potential of the LEO edge. This is particularly
the case due to the costs and timescales associated with the
satellite life cycles. We foresee the need for future capabilities
that enable dynamic configuration and deployment of LEO
edge functions [
22
]. Furthermore, the mobility and temporal
visibility of a satellite raise the need for those functions to be
selectively active in a target service area. Finally, to maximize
the benefit from the LEO edge, which could play a role in
helping LEO satellites achieve economic feasibility, it will be
critical to enable capabilities for its on-demand, multi-tenant
and dynamic operation. In short, the LEO edge will require
orchestration capabilities, much like what is required (and
being developed) for the terrestrial edge [32, 37].
A good starting point in the design space for LEO edge or-
chestration are current terrestrial orchestration systems. How-
ever, the current design of the orchestration systems is hinged
on a tight coupling between the orchestrator and the infras-
tructure it manages, making it unsuitable for the LEO edge
due to its inherent characteristics. First, the time-of-sight of a
particular LEO satellite is severely restricted due to the con-
tinuous motion of the satellite with respect to the Earth. With
a satellite visible only for a period of
7 30
minutes depend-
ing on its altitude [
36
], it is imperative that applications be
rescheduled onto the next incoming satellite to prevent the
risk of reduced availability of the edge functions. Second, the
lack of accuracy in predicting the position of LEO satellites is
well known and continues to be an active area of research in
aerospace engineering [
24
,
38
]. This presents difficulties in
determining the position of, and thus in predicting the trajec-
tory of, the LEO satellites and further compounds the problem
of a limited time-of-sight.
Addressing the above-mentioned problems is already non-
trivial in terms of selecting the right LEO edge nodes and
determining a schedule for application orchestration. A so-
lution focused on these two aspects alone, would only be
sufficient for stateless applications like geographical image
analytics. For stateful applications, like a web cache, another
key aspect that needs to be handled is graceful state transfer.
If the state is rebuilt every
7 10
minutes, there would not be
much benefit that can be extracted from the edge functions on
LEO satellites. Without access to state, the idea of localiza-
tion, a key reason for edge benefits, cannot be realized in the
LEO edge. This highlights a clear need to improve the state
handling in orchestration to deliver tangible benefits from
LEO satellite edge.
In response, we present
Krios
1
an orchestration system
for the LEO edge.
Krios
provides fundamental constructs and
directives which can be incorporated on top of any current
orchestration framework, with a few additional modules to
address the requirements of the LEO edge.
Krios
achieves its
goals through the key idea of loose coupling between edge
function orchestration and the underlying infrastructure.
Loose coupling in
Krios
is achieved through three new
mechanisms as part of its scheduling constructs. First, incor-
porating satellite path projection models in orchestration en-
ables
Krios
to predict the trajectory, and thus future positions,
of LEO satellites with respect to regions in which they are
visible (availability) with known bounds on the error, depend-
1
derived from the Greek god of constellations, Crius
ing on models used. This allows
Krios
to select appropriate
satellite nodes and (re)schedule the edge functions across the
LEO edge without impacting their availability in the target
region. Second, temporal compensation enables ahead-of-
time hand-off, i.e. deployment and preemptive killing of edge
functions on LEO satellites, masking the initialization times.
Third, cluster affinity chains that span individual clusters al-
low
Krios
to choose the satellite among many which may
be visible ahead of time, i.e. before those nodes join a given
ground station cluster. This reduces the time complexity of
choosing the satellite and makes deployment more predictable.
These mechanisms are also leveraged to orchestrate the edge
functions’ state transfers across the LEO edge.
In this paper, we use Kubernetes [
8
] to demonstrate the
benefits of
Krios
. It is, after all, the starting point for the ter-
restrial edge, as it leverages all the technology developments
in the datacenter space and has gained acceptance in industry
as the go-to orchestrator for web services in the cloud and
edge functions at the edge [7].
Summarizing, the contributions of this paper are: (i) quan-
tifying the limitations of terrestrial orchestration solutions
when used in the LEO edge due to the tight coupling to in-
frastructure inherent in their design, and (ii) design of an
end-to-end platform –
Krios
– that embodies the new system
support needed to loosen this coupling and to make LEO edge
orchestration feasible. Finally, we demonstrate the promise
of the approach through our preliminary experimental evalua-
tion.
2 Background
Case for Satellite Internet:
LEO satellites have been in exis-
tence since the late 1990s when they were designed to support
voice, data, facsimile, and paging. IRIDIUM was one of the
major players, having launched 95 satellites between 1997
and 2002. But LEO satellites could not compete with the
GSM services and hence, could not gain much traction. Since
then, a number of factors have changed, making a case for
the success of Internet services based on LEO satellites. The
biggest of those is the decreasing cost of space access, aided
by the commercialization of launch vehicles [
10
,
29
]. While
the lifecycle costs per satellite for Iridium were about US $ 5.7
billion [
3
], the estimated cost for a
4425
LEO constellation is
about US $10 billion [18].
In addition to this, while the first use case of LEO satellites
targeted the existing sets of mobile and Internet users, the
focus has now shifted to the next billion users. This provides
the benefit of manufacturing at scale and results in further
reduction of operational and manufacturing costs. Further,
there is a significant design difference in how the system was
supposed to work earlier in comparison to today. Earlier, a
user would need a specialized handheld device to communi-
cate with a satellite. These devices with significantly powerful
antennas (to directly connect to the satellites) led to higher
Orbit
Krios
federa-
tion master
Krios
ground
station master
Krios
satellite
State handler
Temporal compensation
Path projection models
Cluster affinity chain
WirelessWired
Figure 2: High level design of Krios components - geographically(left) and the network connections (right).
power consumption [
34
] than the competing GSM devices.
In the newer design, devices connect first to ground stations
using standard communication interfaces and the ground sta-
tions provide the connectivity to the satellites [
19
] and has
been argued to be power efficient [
34
]. These factors make a
strong case for the success of LEO satellites as a medium for
providing Internet services.
Review of current satellite systems:
Ground stations are a
key component in the current satellite systems. End users’
communication requests are passed to the satellites via the
ground stations. As per the bent-pipe architecture, on receiv-
ing a request the satellite bounces it off to another specific
ground station server. These ground servers are either con-
nected to a datacenter that serves the request or pass it onto
its next hop to the next visible satellite, creating an eventual
zigzag pattern from the request flow [
30
,
31
]. There are mul-
tiple ground stations depending on the geographical spread
of users, and they are strategically positioned so that there
is always at least one visible satellite that would be used to
serve the requests.
Another important aspect of the system is support for inter-
satellite communication links. The presence of these links can
significantly reduce the time required to reach the datacenter
due to the direct inter-satellite communication, compared to
the zigzag pattern incorporating the ground servers. Among
the current LEO providers, OneWeb’s system will not have
inter-satellite links, Starlink has plans to incorporate these
links at a later time, though the satellites being launched cur-
rently do not have them, and Telesat will have these links
when they start service in 2022 [25, 30, 31].
Potential role of LEO edge in current and emerging use
cases:
A LEO edge could play a role in satellite-supported
application scenarios like remote sensing [
39
], weather fore-
casting, space analysis [
23
], etc. Most of the current scenarios
continuously send the captured media content to ground sys-
tems. A LEO edge would be able to perform the initial pro-
cessing and only transmit necessary metadata to the ground
systems, utilizing low bandwidth and lesser resources on the
ground systems [26, 27].
The proposed satellites have a high capacity capable of
providing aggregate downlink capacity ranging from 17 to
23 Gbps [
19
]. Thus, a LEO edge will play a crucial role in
adding coverage for sparsely populated areas and regions
where setting up terrestrial systems is difficult. Further, it will
be able to supplement communication during disasters when
the terrestrial systems might have been impacted. A LEO
edge can also be used in providing communication services
in airplanes [4, 12] and ships.
An important aspect to consider is that the LEO edge would
likely never be standalone; the terrestrial counterpart will con-
tinue to exist. Hence, to aid with economic feasibility, it is
important to ensure that the developers’ experiences remain
similar irrespective of whether an application is being de-
ployed in space or terrestrially.
3 Design Challenges and Feasibility
The goal of the design of
Krios
is to address the high dy-
namism inherent in the LEO satellite environment.
Krios
achieves this by federating ground stations as
Krios
-ground
station masters responsible for tracking the mobility of LEO
satellites and for triggering the necessary orchestration ac-
tions needed for application hand-off on
Krios
-satellites. This
is done in a way that leverages opportunities afforded by the
periodicity of the LEO satellites’ motion, while also explicitly
compensating for the inherent inaccuracy in the predicted
satellite positions. Furthermore,
Krios
allows for current or-
chestration architectures such as Kubernetes to be seamlessly
leveraged, while minimizing overheads related to the need
for more frequent orchestration. Finally, unlike [
27
],
Krios
can work well for a constellation without inter-satellite links
provided there are a sufficient number of ground stations avail-
able. Next, we briefly touch on the key design elements:
Achieving LEO edge orchestration with existing stacks:
Using current orchestration stacks for a LEO edge will lead to
considerable downtime. The reasons for this are tied to the un-
derlying assumptions about the liveness of the infrastructure,
the job lifetimes and the connectivity between infrastructure
nodes. While generally failures in infrastructure nodes are
dealt with in a reactive manner,
Krios
takes a preemptive ap-
proach, i.e. an edge application may need to be preempted and
rescheduled even when the node it is running on is healthy.
For instance, the number of times an application is initialized
on a new LEO edge node is considerably higher than that on
a terrestrial edge node since every time a satellite goes out
of sight from a ground station, the application needs to be
scheduled on a different node. An application that needs to
be available 24 hours at a given location through a LEO edge
in orbit at an altitude of 200 km would need to be initialized
more than 200 times even without any true node or network
failure. To put this in perspective, even 5 seconds of initializa-
tion time, commonly associated with Kubernetes pod startup
time [1, 9], results in at least 16 minutes unavailability.
Incorporating mobility of the LEO edge:
A major differ-
ence between the LEO edge relative to current terrestrial
systems is that the satellite nodes are continuously in motion,
leading to dynamic network connections with the ground
stations. As stated earlier, an intuitive starting point for multi-
tenant and dynamic operation over mobile satellites would be
to use current terrestrial systems and to model a satellite going
out of sight as a node failure, with the orchestrator handling
the subsequent application hand-off to another node (in this
case to a visible satellite).
In current terrestrial systems, individual node failures are
fairly sporadic and random, whereas in a LEO edge scenario,
due to the constant rotation, the failures are periodic and high
frequency. For example, at the height of 500 km, every satel-
lite will experience an out of sight failure every 10 minutes. In
addition to this, it would be very difficult to identify true node
failures in the system, making troubleshooting actual issues
much harder. Similarly to [
27
], we observe that using path
models and incorporating information generated via them
into the scheduling constructs helps to solve this problem.
However, adding them to the satellites would incur additional
resource usage in the already resource constrained environ-
ment. This points to a more suitable design decision to run
and cleanly incorporate information generated by path models
into the scheduling constructs of
Krios
-ground station mas-
ters. The key insight here is that to leverage existing path
models, one does not need to modify the orchestrator itself.
Krios
provides for use of pluggable path models to predict the
location of satellites and, in that manner, their connectivity
to a particular ground station. This enables
Krios
to drive
scheduling decisions “just in time”.
Leveraging periodicity in LEO edge mobility:
With the
mobility of the nodes incorporated in orchestration, the num-
ber of application handoffs will increase. In a LEO constel-
lation, there are multiple satellites visible at any particular
time depending on their different orbital radius, rotation in
different rotational planes and also different visibility. Thus,
it is important to add support for disambiguating the decision
to hand-off an application to a particular satellite as per some
pre-defined policy. For example, a policy could be as simple
as hand-off to a satellite that has the longest visibility, which
would reduce the need for recurring hand-offs and improve
the overall network utilization.
By using a central master node,
Krios
is aware of the dif-
ferent satellites in the constellation and their physical spec-
ifications. Using this information, it builds an affinity chain
as the list of satellites depending on the edge function or on
a system-defined policy. Satellites in the same affinity chain
would ensure that there would not be any specification mis-
match for an application and also ensure that the new satellite
would be in sight for a larger amount of time. Use of affinity
chains allows
Krios
to make potential scheduling decisions
ahead of time and to reduce the runtime scheduling overheads.
Compensating for error in mobility prediction of LEO
satellites:
Intuitively, the use of path models to drive orches-
tration seems straightforward. However, the reality is that it is
difficult to accurately predict a satellite’s position and thus its
connectivity at a particular time. The fundamental reason is
that the path models (or even real measurements through high
power antennas) generally have an error component associ-
ated with their prediction of a position of a particular LEO
satellite.
The satellite positioning error implies that a satellite may
become out-of-reach of a ground station sooner than what is
predicted by path models. If using current terrestrial systems,
the orchestrator would consider this a potential node failure,
and wait some predefined time before triggering a reschedul-
ing operation. In Kubernetes, for example, this defaults to
300 seconds [
14
], which, as shown in §4, can lead to up to
85% unavailability of a LEO edge. To avoid this,
Krios
incor-
porates a preemptive hand-off of the application before the
satellite actually moves out of sight. With this,
Krios
achieves
a “just-ahead-of-time” (re)scheduling response by incorporat-
ing the error associated with the path model it is running and
the initialization time for an application.
Krios
is designed to contend with the worst possible sce-
narios and uses the combined error bounds in the predicted
availability time to trigger early preemptive edge applications
hand-off, referred to as temporal compensation. Empirically,
this preemption time for rescheduling in
Krios 4t
can be
written as:
4t = 4t
error
+ t
initialize
+ t
RT T
where
4t
error
is the time error due to inaccuracy of path
model,
t
initialize
the time to initialize the application on the new
node, and
t
RT T
the round trip time for network communication
between satellite and ground station.
We posit that there are many assumptions that need to be
reconsidered for specific LEO edge scenarios, and this discus-
sion presents initial steps toward highlighting and addressing
some of them.
4 Preliminary Evaluation
We run two sets of experiments. The first set of experiments
uses Kubernetes to demonstrate the benefits of using path
models and temporal compensation. We use the stateless Ng-
inx service for this experiment. The second set of experiments
shows the benefit of preemptive state transfer in improving
the utility of even simple edge functions. For this, we run the
Least Recently Used (LRU) eviction policy for an in-memory
web cache and calculate the hit ratio. We use the Simplified
General Perturbations 4 (SGP4) [
33
] model to evaluate the
orbital position of individual satellites relative to the Earth-
centered coordinate axis, also accounting for Earth’s rotation.
Figure 3:
Downtime for the four scenarios - default Kubernetes,
pessimistic Kubernetes, Kubernetes with just path awareness and
Krios handoff.
Experimental Setup:
We run these tests on five virtual ma-
chines, using one of them as the master (
Krios
-ground station
master) and the rest as the worker nodes (
Krios
-satellites).
The VMs are hosted on local x86 servers interconnected by 1
Gbps Ethernet. To model the satellites going out of sight, we
periodically reboot the worker machines.
Results:
In our preliminary experiments, we seek answers to
the following questions:
Q. How much downtime is expected with and without
Krios
?
Figure 3 shows the service availability around the time a
satellite goes out of sight. We compare four scenarios here:
default Kubernetes, modeling the loss of connection as node
failure; a slightly modified Kubernetes pessimistic to failure,
i.e., with lower rescheduling timeout upon a node failure; a
path-aware Kubernetes which initiates application handoff
right before the loss of a connection; and a
Krios
application
handoff incorporating path-awareness and temporal compen-
sation before the loss of connection. The graph show the
service downtime observed due to satellites moving out-of-
sight of a ground station.
With the default configuration of Kubernetes, we see a
downtime of about 350-400 seconds during every handoff.
For a satellite at height 200 km which is visible for 7 minutes,
this implies an overall 85% downtime. To be fair and conser-
vative for
Krios
, we configured Kubernetes to be pessimistic
and aggressively reactive to failure. Specifically, we change
(i) node-monitor-period to 2 seconds (default 5 seconds [
6
]),
(ii) tolerations.tolerationSeconds to 2 seconds (default 300
seconds [
14
]), and (iii) node-monitor-grace-period to 16 sec-
onds (default 40 seconds [
6
]). These changes lead to more
frequent monitoring of the nodes and a faster response to
a failure. The downside to these changes is higher network
utilization which in itself is not practical for LEO satellites
as downlink and uplink are both expensive, and a reactive
Kubernetes highly prone to false positives due to monitoring
failures or oversight.
Figure 4:
Cache Hits for different cache sizes showcasing benefits
of state transfer from the outgoing satellite to the incoming satellite.
Even with an aggressive eviction on top of default Kuber-
netes, we observe a downtime of about 70-80 seconds. For
a satellite at height 200 km with a 7 minute visiblity, this
implies an overall 16-18% downtime. For the path aware Ku-
bernetes which initiates the application handoff right before
the satellite goes out sight, we still see about 40 seconds down-
time, or an overall 10% downtime for a 200 km altitude orbit.
For the Krios scenario, there is no observed downtime.
These results demonstrate the value of incorporating path
projection models and temporal compensation in an orchestra-
tor such as Kubernetes (even when configured in aggressive,
pessimistic mode) for determining the right (re)scheduling
policy for applications even with a fast network connection.
Q. How much benefit do stateful functions gain with Krios?
Figure 4 shows the cache hit ratio for varying cache sizes
comparing scenarios with and without state transfer from the
outgoing to the incoming satellite, when running a web cache
for a Wikipedia service. We use the Wikipedia daily page
counts dataset [
17
] assuming each page to have size
3kb
[
16
].
Utilising
Krios
state transfer helps improve the cache hit
ratio by about 5-12%. A cache hit eliminates the entire back-
haul time from the satellite to the destination datacenter via
ground station, and the associated bandwidth utilization. Use
of state transfer amplifies these benefits.
5 Summary
We propose
Krios
as an orchestration service which enables
utilizing the LEO edge to its full potential.
Krios
achieves this
by incorporating path projection models, temporal compen-
sation, and developing affinity chains of plausible candidates
for (re)scheduling operations. More generally, we conclude
that considering LEO satellites as edge, akin to the terrestrial
edge, does have benefits. However, designing systems and
services for a LEO edge requires more than just migrating the
ones developed for the terrestrial edge. There remain many
interesting tradeoffs to be explored and new mechanisms to
be developed toward a solution for efficient use of a LEO
edge. This paper is a step in that direction.
6 Discussion
In this paper, we focus on a few specific design aspects for
LEO edge orchestration. However, there are assumptions in
different areas that need to be revisited. These include the reli-
ability of the network, the availability of power resources, the
complexity of application design, the impact of soft hardware
failures on orchestrators, and data compliance.
The network assumptions made in terrestrial systems do not
hold in the LEO edge scenario. The large RTT and increased
link error rate impact the reliability of the network. Further,
LEO satellite networks are considerably more dynamic with
regard to connectivity and latency. Thus, this requires new de-
velopments in improving the reliability of the network and in
routing mechanisms. Power availability is a crucial assump-
tion made in terrestrial systems. A LEO edge orchestrator
needs to be aware of the power availability on different nodes
while making scheduling decisions.
The stateful function we described is a rather simple one.
The state to be maintained is fairly simple. More complex
functions like machine learning inference would have a far
more complicated state that needs to be considered and de-
signed accordingly. There will be a need to reconsider how to
structure these functions in order to benefit from a LEO edge.
Hardware faults such as flipping of RAM bits are higher in
satellites due to the severe radiation effects in space. Thus, this
requires new development in both the hardware and software
stack to be able to handle these failures, in addition to the
the discussion of failures presented in this paper. Handling
these different types of failure scenarios may expose different
tradeoffs that a solution must co-optimize for.
Data compliance has been one of the pressing issues in
recent times. While there are no borders in space, the appli-
cations deployed would be serving people on the ground and
hence there would be policies that might be applicable. This
calls for policy changes to govern the compliance in space and
also solutions to incorporate clean multi-tenancy to remain
compliant.
This work highly depends on the latest events occurring
in the industry concerning LEO satellites. As long as the
industry players continue on that path, the relevance of this
research could increae.
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