Implement SCMP

[mlegner]

• [x] How is ICMP handled typically?
• [x] How does the Go code handle SCMP? -> snet.packet, slayers.scmp_msg
• [x] How should our interface look like to send SCMP packets? Raw SCION socket?
• [x] Potentially refactor UDP socket to build on this raw socket?

[jpcsmith]

The UDP man pages states that received errors are returned on subsequent calls to the socket. Additionally, errors can be set to be collected and requested using a separate API.

Error handling All fatal errors will be passed to the user as an error return even when the socket is not connected. This includes asynchronous errors received from the network. You may get an error for an earlier packet that was sent on the same socket. This behavior differs from many other BSD socket implementations which don't pass any errors unless the socket is connected. Linux's behavior is mandated by RFC 1122. [...] When the IP_RECVERR option is enabled, all errors are stored in the socket error queue, and can be received by recvmsg(2) with the MSG_ERRQUEUE flag set.

[jpcsmith]

The SCMP message handling can be seen in netsec-ethz/scion-apps/blob/master/pkg/pan/raw.go, which used by their UDP socket.

Their current approach is to notify the PathStats monitor that the path in the message is down when the socket receives either ExternalInterfaceDown or InternalConnectivityDown. All other SCMP messages are propagated as an error.