Someone

.---------------+. 
| A = g^a mod p |_\
|                 |  
|_________________|
      /
     / 
    /
 Alice          (g, p, A)   Bob            |
 A = g^a mod p -----------> B = g^b mod p  |
                            K = A^b mod p  |
                                           |
                     B                     |
 Alice         <----------- Bob            |
 K = B^a mod p                             |
                                           V