security considerations for UDP source port value

[bganne]

The PSP specification encourages to set the outer UDP header source to a flow identifier (flow hash).
I think it's important to clarify the security impacts:

• if a simple flow hash (not a strong crypto hash) is used as is hinted in the spec and implemented in the reference implementation, then an attacker can easily recover the original flow tuple.
• if using some other mechanism (eg. a table with unique id for each flow), then some information is still leaked, as packets can be split between flows.

Depending upon the usecase, this may or may not be an issue, but this should be called out.