> On 16 May 2018, at 15:49, Erik Kline <e...@google.com> wrote:
> 
> In the latter case especially, what becomes clear is that the UE needs
> to be able to receive an unsolicited packet.  ICMP is a canonical
> example of receiving and processing an unsolicited packet.  But it
> could also be something like a UDP socket listening on a well known
> port that receives a 1-byte datagram, which causes the UE to enqueue
> (for rate-limiting purposes) a captive API query.

I think UDP on a well-known port could be a viable alternative to ICMP. The 
interface should have deliver at least the same level of trust as the ICMP 
unreachable can provide. In the unreachable ICMP it is possible to relate the 
packet to the packet that caused the notification. I am not claiming the UDP 
interface (or other interface) should use this exact same approach for 
reliability, but if we use it as just a single byte message, we would actually 
make it easier for malicious users to trigger calls to the capport-api (even 
though it’s rate limited). Maybe some token as payload, or a better idea that 
has lesser dependencies on moving parts in the capport deployment.

Gr.,
Vincent van Dam
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