Glad to hear that Darkcoin has built-in protection for this attack vector, it's certainly a valid concern.
It doesn't have that, it was just something I was suggesting in DRK thread earlier and asking what people's thoughts about it are. But if the idea is feasible and provides additional security, the implementation probably should be pretty straightforward.
XC does also, in that every single wallet is a node. Thus, there's no central point that can be targeted for a ddos.
It's the same thing. XC node that participates in anonymization requires an open incoming TCP port, and the attacker can ddos those nodes just as well as drk masternodes.
In addition, transactions are multisig so they require several parties to sign before they are processed. If a bad node alters a transaction to steal funds, or refuses to sign, the transaction won't be processed, so you can't lose funds. Also, since there are redundant connections between nodes that provide multiple pathways for funds, if a node fails to sign a transaction there are others ready to take its place. Pretty cool stuff, really.
The goal when ddos'ing nodes is to first set up many nodes yourself, then ddos the other nodes, so you can then have a control of big enough percentage of the nodes so you can spy the transactions with good enough success.
You're right that it's quite possible to DDOS any coin's nodes like this.
However it'll take up massively more resources to DDOS XC nodes with any noticeable severity, since every single node is a "masternode". The number of nodes an attacker would need to control is proportionally huge.