[SCION] Carbon aware SCION networking.

Frei Matthias matthias.frei at inf.ethz.ch
Wed Mar 24 10:48:49 CET 2021

Hi Chris,

before trying to answer your questions, I think it might be worth mentioning that energy and/or carbon optimised routing has increasingly become a focus of attention for research and development on SCION. There are multiple research projects looking into different aspects of this. And just in case this hasn't reached you yet, a press article written by researchers in our Network Security Group about this topic has recently been published, see this link:


I'll try to answer your first two questions, but I don't know how to answer part 3 and 4.

1. Exit nodes for SCION: first of, in our SCIONLab testbed network, we currently use the term "attachment point" to denote special infrastructer SCION ASes designated to act as providers for user's SCION ASes. I think this is rather distinct from what you are referring here and I will therefore avoid the term.

There is currently the concept of a SCION-IP-gateway (SIG). These gateways run in multiple SCION ASes and can be connected to form an IP-overlay network over SCION. For "legacy" IP end hosts and applications, these gateways act as fairly normal IP routers and the entire SCION network is hidden away into just a single IP hop. So:

origin (IP host)  --> local SIG --> multiple SCION hops  --> remote SIG --> destination (IP host)

In the "typical" setup, the SIGs only act as routers for IP destinations in their respective local (SCION-)AS.
However, with a suitable setup tied into the public BGP routing, this can also be used to create such "exit nodes" as you describe, to forward traffic to a destination "further out" in the IP Internet:

origin (IP host) --> local SIG --> multiple SCION hops --> remote SIG (exit node) --> multiple inter-domain, BGP routed, IP hops -->  destination (IP host)

This latter setup is the focus of some ongoing research projects.

The SCION book, section 10.3 describes this SIG and some possible deployment scenarios in much more detail:


The SIG is part of the open source SCION implementation, look for "gateway" in issues or the code: https://github.com/scionproto/scion

As far as I know, Anapaya also develops a proprietary, commercial version of the SIG.

2. Exposing path preferences for carbon routing

There is an existing system for including "metadata" in SCION's path beaconing. This is currently used for information such as latency, bandwidth, and geographical coordinates. Each AS along a path announces this information when the paths are being constructed. A technical description of this can be found here:


As far as I'm aware, there is an ongoing student project to implement and evaluate such a scheme also energy intensity.

With this mechanism, the energy intensity information needs to be fed into the configuration of the "control service" of each participating SCION AS. Currently, this is in the form of a json file, but this could be adapted quite freely if there is a need to e.g. dynamically consume this information from a separate service in some form. The current format of this configuration file is also described in the documentation page linked above.

An application "consuming" paths can then inspect the included information for any hop along each path and use it to evaluate any policy.

From: SCION <scion-bounces at lists.inf.ethz.ch> on behalf of Chris Adams <chris at thegreenwebfoundation.org>
Sent: Tuesday, 23 March 2021 5:49:19 PM
To: scion at lists.inf.ethz.ch
Subject: [SCION] Carbon aware SCION networking.

Hi SCION folks.

I’ve been looking at SCION as a potential technology to building a degree of carbon awareness into the internet, so that it’s possible to have path preference policies that optimise for carbon intensity of the routes chosen, but also to move compute jobs to areas, either geographically or temporally, where compute is particularly cheap and green.

I have a few questions. I hope that a kind soul on the group might help me answer.

Some background before the questions

For context, I work at the Green Web Foundation, and was introduced to SCION by Adrian earlier, in a call. I find the technology really exciting, and it’s really cool seeing problems solved at a protocol level that I see large companies building proprietary stacks to solve for themselves.

We’re looking for partners with experience using SCION, as part of a bid to the NGI Pointer funding call at the link below, to build a demonstrator of these ideas. We’re currently working with a small sustainable CDN provider, and a grid intensity data provider, come up with a user facing product that relies on features of SCION to make what I outlined in the my opening paragraph possible.

You can learn more about the funding bid we’re going for at the link below:

https://ngi-pointer-open-call-2.fundingbox.com <https://ngi-pointer-open-call-2.fundingbox.com/>

The questions

1. Exit nodes for SCION - it’s my understanding that there are libraries you can use, to allow connections from a device to go from regular internet ‘into’ the SCION internet via an attachment point, at which point all the clever routing can happen, and then on the destination server you want to reach.

origin -> attachment point -> multiple SCION   hops > destination

I understand there has been some work to make something like an 'exit node’, to allow connections like so:

origin -> attachment point -> multiple SCION   hops > ‘exit node' > destination

I can understand how you might do this, by encapsulating the original origin/destination packets, but I’m not sure where to look to see the code for this.

Would someone point me to where I should be looking, or if there is a specific issue tracker I ought to watch to learn more?

2. Exposing path preferences for carbon routing

I've shared a sample path preference policy based on assumed carbon intensity of SCION AS as a thought experiment - I’m guessing at some of this on the SCION side, but the intention of this policy object would be:

‘choose the greenest route possible, even if it results in more hops, as long as the total CO2 is lower along the longer route"

https://gist.github.com/mrchrisadams/12fa0d440b6873e8afef986aa78ec60a <https://gist.github.com/mrchrisadams/12fa0d440b6873e8afef986aa78ec60a> -

We’ve have a prometheus exporter that exports these figures, so that the numbers can be advertised for route planning purposes. Both of the providers we consume data from in the grid intensity exporter can provide readings at a time resolution that I think would match SCION (I think we can reliably get readings every 30 mins for most parts of the world)

https://github.com/thegreenwebfoundation/grid-intensity-exporter/ <https://github.com/thegreenwebfoundation/grid-intensity-exporter/>

However I think I need some help understanding how to go from having this information in an prometheus exporter, and having a path policy to them being advertised so they can be used in routing. How can I go from here to announcing it via the beacons, so numbers like this can be used in routing decisions?

3. Getting help and advice

We’re looking for someone who would be able to answer questions like this to join us on the bid, but I couldn’t tell who we should be contacting for this. We’re submitting this bid as a research project, and we couldn’t tell if it made sense to ask here or contact Anapaya directly, and once we knew that it wasn’t clear what an engagement might look like.

Are the companies that offer retainer-type services or a block of time we could work into the bid so when we come up against technical problems we have someone to ask for pointers?

Thanks in advance for my questions.

4. Prior work

Finally, if there’s prior work in this field on SCION and climate, or papers that are already published - would someone direct us to it? We’d like to be able to point to it, and find fellow travellers too.

I know about the this paper below, but I’m not aware of any others:

Modeling Data-Plane Power Consumption of Future Internet Architectures <https://www.scion-architecture.net/pdf/2016-energy.pdf>
Chen Chen, David Barrera and Adrian Perrig.
In Proceedings of the IEEE Conference on Collaboration and Internet Computing (CIC), November 2016.

Many thanks


Chris Adams


w: thegreenwebfoundation.org
e: chris at thegreenwebfoundation.org
t: @mrchrisadams

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