This one spl. for anonymous (X-mas puzzle)

[Rajiv Malik]

Hi there

I remember u posted a nice puzzle long back this one
spl. for u :
-->
There are two persons. A pair of no is written on each's face. They can
see the no (pair of no) written on the other's face and they have to
tell the no written on their face. They start one by one. 
Lets say one person starts he can see the no written on his opponents
face , its (13,17). After 3 tries he tells the correct answer.
The nos exhibit the following properties:
1> All the nos are prime.
2> Any three no taken ( from the set of 4 nos, two no on each's face)
together form a triangle and the perimeter of the triangle is prime.

Now you have tell the no written on that persons face and why it took
him three tries.
The conversation between the two persons is as follows
Let p1 and p2 be the persons and 
p1 starts and p1 saw the no (13,17) written in p2.

p1> I don't know
p2> I don't know
p1> I don't know
p2> I don't know
p1> Tells the right no.

enjoy (Happy X-mas)
Rajiv

PS : I'm off till 28th So u have enough time.