The S flag indicates whether the source address in the IP header is derived from
the MRI or is the signalling source address. In the case where the originator is
an end system, these addresses are the same anyway (at least for a single-homed
node), so it is not clear which S value should be used.

In practice, the S flag is used for two purposes:
- to decide if errors can be sent (safely) to the IP source address
- to decide how the IP source address can be examined to detect the presence of
a GIMPS-unaware NAT

In that sense, it is almost certainly more helpful to know that the IP source
address is the signalling source address (in other words, for S to be set),
since if this is not known, there is nothing useful that the message receiver
can do. In other words, S SHOULD be set in messages from end systems.