Is it possible to detect from which node a transaction was issued?
When my node receive many spam transactions for example, can I be sure that my neighbor issue them?
The information about the IP address of the origin of each transaction is recorded by each IRI node in the local database. That is the IP that sent it to that node and does not mean it is the actual original IP address within the network.
If a cluster of nodes was formed, and the neighbor states also recorded and kept in sync, with some luck someone could approximate the original IP.
Honey pots could also be setup that offer tips that are marked. That is another way the origin could be narrowed down.
In practicality, the IP is indeterminate.