  The growing energy consumption of Information and Communication Technology
(ICT) has raised concerns about its environmental impact. However, the carbon
efficiency of data transmission over the Internet has so far received little
attention. This carbon efficiency can be enhanced effectively by sending
traffic over carbon-efficient inter-domain paths. However, challenges in
estimating and disseminating carbon intensity of inter-domain paths have
prevented carbon-aware path selection from becoming a reality.
  In this paper, we take advantage of path-aware network architectures to
overcome these challenges. In particular, we design CIRo, a system for
forecasting the carbon intensity of inter-domain paths and disseminating them
across the Internet. We implement a proof of concept for CIRo on the codebase
of the SCION path-aware Internet architecture and test it on the SCIONLab
global research testbed. Further, we demonstrate the potential of CIRo for
reducing the carbon footprint of endpoints and end domains through large-scale
simulations. We show that CIRo can reduce the carbon intensity of
communications by at least 47% for half of the domain pairs and the carbon
footprint of Internet usage by at least 50% for 87% of end domains.
