Implement simple path measurements and path selection

[mlegner]

• Measure aliveness and latency of paths
  • In the Go implementation, this is done via a traceroute to the final hop, see here
• Choose path based on latency, path length (?)
• Ideally shared among multiple sockets/connections of the same application
• Potential optimization: Use RTT measurements of upper-layer protocol (i.e., QUIC).
• Question: Should this instead be handled by the SCION Daemon?
• Question: How to choose between IP and SCION?
  - Always choose SCION if it is available?
  - Compare performance?

Probably depends on #54

[jpcsmith]

Some thoughts on the implementation.

As an alternative to wrapping the socket, we could define and implement the trait PathAwareDatagramSocket on UdpSocket and move the methods that are related to path persistence to it:

trait PathAwareDatagramSocket {
  fn path();
  fn set_path();
  fn set_path_service(PathService);
  fn send_to();
  fn send();
  // Method to propagate paths from recvs to PathService
  fn on_received_path();
}

[mlegner]

General path selection mechanisms are implemented now. More strategies, e.g., based on measurements will be added in the next version.

[jpcsmith]

@mlegner The pan implementation in golang seems to use Ping instead of traceroute to select paths. Do we want to use the same approach?

The upside: we would be including the delay in the destination AS in the measurements.
The downside: Measurements are done per-destination and so cannot be shared between different destinations in the same AS.

[mlegner]

The pan implementation in golang seems to use Ping instead of traceroute to select paths. Do we want to use the same approach?

Yes, I became aware of that as well. The aliveness check (e.g., for scion showpaths) uses traceroute while the path selection in pan uses pings. In the current deployments, there is probably very little difference between the two in terms of measurement results as we usually have very small source and destination ASes.

I don't have a strong opinion but would probably go with the traceroute approach to start with.