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have been recognized by the United Nations and by most governments as have been recognized by the United Nations and by most governments as
one of the big challenges of our time. As a result, curbing those one of the big challenges of our time. As a result, curbing those
emissions is becoming increasingly important for society and for many emissions is becoming increasingly important for society and for many
industries. The networking industry is no exception. industries. The networking industry is no exception.
The science behind greenhouse gas emissions and their relationship The science behind greenhouse gas emissions and their relationship
with climate change is complex. However, there is overwhelming with climate change is complex. However, there is overwhelming
scientific consensus pointing toward a clear correlation between scientific consensus pointing toward a clear correlation between
climate change and a rising amount of greenhouse gases in the climate change and a rising amount of greenhouse gases in the
atmosphere. When we say 'greenhouse gases' or GHG, we are referring atmosphere. When we say 'greenhouse gases' or GHG, we are referring
to gases in the Earth’s atmosphere that trap heat and contribute to to gases in the Earth's atmosphere that trap heat and contribute to
the greenhouse effect. They include carbon dioxide (CO2), methane the greenhouse effect. They include carbon dioxide (CO2), methane
(CH4), nitrous oxide (N2O), and Fluorinated gases (as covered under (CH4), nitrous oxide (N2O), and fluorinated gases (as covered under
the Kyoto Protocol and Paris Agreement). In terms of emissions from the Kyoto Protocol and Paris Agreement). In terms of emissions from
human activity, the dominant greenhouse gas is CO2; consequently, it human activity, the dominant greenhouse gas is CO2; consequently, it
often becomes shorthand for “all GHGs”. However, other gases are also often becomes shorthand for "all GHGs". However, other gases are
converted into “CO2-equivalents”, or CO2e. One greenhouse gas of also converted into "CO2-equivalents", or CO2e. One greenhouse gas
particular concern, but by no means the only one, is carbon dioxide of particular concern, but by no means the only one, is carbon
(CO2). Carbon dioxide is emitted in the process of burning fuels to dioxide (CO2). Carbon dioxide is emitted in the process of burning
generate energy that is used, for example, to power electrical fuels to generate energy that is used, for example, to power
devices such as networking equipment. Notable here is the use of electrical devices such as networking equipment. Notable here is the
fossil fuels (such as oil, which releases CO2 that had long been use of fossil fuels (such as oil, which releases CO2 that had long
removed from the earth's atmosphere), as opposed to the use of been removed from the earth's atmosphere), as opposed to the use of
renewable or sustainable fuels that do not "add" to the amount of CO2 renewable or sustainable fuels that do not "add" to the amount of CO2
in the atmosphere. There are additional gases associated with in the atmosphere. There are additional gases associated with
electricity generation, in particular methane (CH4) and nitrous oxide electricity generation, in particular methane (CH4) and nitrous oxide
(N2O). Although they exist in smaller quantities, they have an even (N2O). Although they exist in smaller quantities, they have an even
higher Global Warming Potential (GWP). higher Global Warming Potential (GWP).
Greenhouse gas emissions are in turn correlated with the need to Greenhouse gas emissions are in turn correlated with the need to
power technology, including networks. Reducing those emissions can power technology, including networks. Reducing those emissions can
be achieved by reducing the amount of fossil fuels needed to generate be achieved by reducing the amount of fossil fuels needed to generate
the energy that is needed to power those networks. This can be the energy that is needed to power those networks. This can be
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achieved through steady year-on-year efficiency gains, although these achieved through steady year-on-year efficiency gains, although these
are still partly offset by simultaneous growth in data volume. The are still partly offset by simultaneous growth in data volume. The
same report highlights an important corporate goal: continuing on same report highlights an important corporate goal: continuing on
this trajectory and further reducing overall greenhouse gas this trajectory and further reducing overall greenhouse gas
emissions. emissions.
1.2. Approaching the Problem 1.2. Approaching the Problem
One way in which gains in network sustainability can be achieved One way in which gains in network sustainability can be achieved
involves reducing the amount of energy needed to provide involves reducing the amount of energy needed to provide
communication services and improving the efficiency with with communication services and improving the efficiency with which
networks utilize power during their use phase. However, for a networks utilize power during their use phase. However, for a
holistic approach, other aspects need to be considered as well. holistic approach, other aspects need to be considered as well.
The environmental footprint is not determined by energy consumption The environmental footprint is not determined by energy consumption
alone. The sustainability of power sources needs to be considered as alone. The sustainability of power sources needs to be considered as
well. A deployment that includes devices that are less energy well. A deployment that includes devices that are less energy
efficient but powered by a sustainable energy source can arguably be efficient but powered by a sustainable energy source can arguably be
considered greener than a deployment that includes highly efficient considered greener than a deployment that includes highly efficient
devices that are powered by diesel generators. In fact, in the same devices that are powered by diesel generators. In fact, in the same
Telefónica report mentioned earlier, extensive reliance on renewable Telefónica report mentioned earlier, extensive reliance on renewable
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efficiency ratings of traversed equipment.) It also includes aspects efficiency ratings of traversed equipment.) It also includes aspects
such as considering the incremental carbon footprint in routing such as considering the incremental carbon footprint in routing
decisions. Optimizing cost has long been an area of focus in decisions. Optimizing cost has long been an area of focus in
networking; many of the existing mechanisms can be leveraged for networking; many of the existing mechanisms can be leveraged for
greener networking simply by introducing the carbon footprint as a greener networking simply by introducing the carbon footprint as a
cost factor. Low-hanging fruit includes adding carbon-related cost factor. Low-hanging fruit includes adding carbon-related
parameters as a cost parameter in control planes, whether distributed parameters as a cost parameter in control planes, whether distributed
(e.g., IGP) or conceptually centralized via SDN controllers. (e.g., IGP) or conceptually centralized via SDN controllers.
Likewise, there are opportunities to correctly place functionality in Likewise, there are opportunities to correctly place functionality in
the network for optimal effectiveness. An example is placement of the network for optimal effectiveness. An example is placement of
virtualized network functions in carbon-optimized ways. for exmaple, virtualized network functions in carbon-optimized ways. For example,
virtualized network functions can be cohosted on fewer servers to virtualized network functions can be cohosted on fewer servers to
achieve higher server utilization, which is more effective from an achieve higher server utilization, which is more effective from an
energy and carbon perspective than larger numbers of servers with energy and carbon perspective than larger numbers of servers with
lower utilization. Likewise, they can be placed in close proximity lower utilization. Likewise, they can be placed in close proximity
to each other in order to avoid unnecessary overhead in long-distance to each other in order to avoid unnecessary overhead in long-distance
control traffic. control traffic.
Other opportunities concern adding carbon awareness to dynamic path Other opportunities concern adding carbon awareness to dynamic path
selection schemes. This is sometimes referred to as "energy-aware selection schemes. This is sometimes referred to as "energy-aware
networking" (or "pollution-aware networking" [Hossain2019] or networking" (or "pollution-aware networking" [Hossain2019] or
"carbon-aware networking", when parameters beyond simply energy "carbon-aware networking", when parameters beyond simply energy
consumption are taken into account). Again, considerable energy consumption are taken into account). Again, considerable energy
savings can potentially be realized by taking resources offline savings can potentially be realized by taking resources offline
(e.g., putting them into power-saving or hibernation mode) when they (e.g., putting them into power-saving or hibernation mode) when they
are not needed under current network demand and load conditions. are not needed under current network demand and load conditions.
Therefore, weaning resources from traffic is an important Therefore, weaning resources from traffic is an important
consideration for energy-efficient traffic steering. This approach consideration for energy-efficient traffic steering. This approach
contrasts and indeed conflicts with existing schemes that typically contrasts and indeed conflicts with existing schemes that typically
aim to to create redundancy and load-balance traffic across a network aim to create redundancy and load-balance traffic across a network to
to achieve even resource utilization across larger numbers of network achieve even resource utilization across larger numbers of network
resources as a means to increase network resilience, optimize service resources as a means to increase network resilience, optimize service
levels, and ensure fairness. Thus, a big challenge is how resource- levels, and ensure fairness. Thus, a big challenge is how resource-
weaning schemes to realize energy savings can be accommodated without weaning schemes to realize energy savings can be accommodated without
cannibalizing other important goals, counteracting other established cannibalizing other important goals, counteracting other established
mechanisms, or destabilizing the network. mechanisms, or destabilizing the network.
An opportunity may lie in making a distinction between "energy modes" An opportunity may lie in making a distinction between "energy modes"
of different domains. For instance, in a highly trafficked core, the of different domains. For instance, in a highly trafficked core, the
energy challenge is to transmit the traffic efficiently. The amount energy challenge is to transmit the traffic efficiently. The amount
of traffic is relatively fluid (due to multiplexing of multiple of traffic is relatively fluid (due to multiplexing of multiple
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* Is compression more energy efficient once factoring in the * Is compression more energy efficient once factoring in the
computation cost of compression vs. transmitting uncompressed computation cost of compression vs. transmitting uncompressed
data? Which compression scheme is more energy efficient? data? Which compression scheme is more energy efficient?
* Is energy saving of computing at an efficient hyperscale DC * Is energy saving of computing at an efficient hyperscale DC
compensated by the networking cost to reach that DC? compensated by the networking cost to reach that DC?
* Is the overhead of gathering and transmitting fine-grained energy * Is the overhead of gathering and transmitting fine-grained energy
telemetry data offset by the total energy gain resulting from the telemetry data offset by the total energy gain resulting from the
better decisions that this data enables? better decisions that this data enables?
* Is the energy cost needed to transmit data to a Low Earth Orbit * Is the energy cost needed to transmit data to a Low Earth Orbit
(LEO) satellite constellation offset by the fact that the (LEO) satellite constellation offset by the fact that the
constallation and any networking within it are powered by solar constellation and any networking within it are powered by solar
energy? energy?
* Is the energy cost of sending rockets to place routers in LEO * Is the energy cost of sending rockets to place routers in LEO
amortized over time? amortized over time?
Determining where the sweet spots are and optimizing networks along Determining where the sweet spots are and optimizing networks along
those lines will be a key towards making networks greener. We expect those lines will be a key towards making networks greener. We expect
to see significant advances across these areas and believe that to see significant advances across these areas and believe that
researchers, developers, and operators of networking technology have researchers, developers, and operators of networking technology have
an important role to play in this. an important role to play in this.
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