about address A1 ad A2 problem in LDP of mpls

I cannot understand the meaning of the words:"    2.  LSR1 determines
whether it will play the active or passive role
         in session establishment by comparing addresses A1 and A2 as

Andersson, et al.                                              [Page 14]

Internet Draft         draft-ietf-mpls-ldp-06.txt           October 1999

         unsigned integers.  If A1 > A2, LSR1 plays the active role;
         otherwise it is passive.  "
I want to know what kind of address of A1&A2, IP address or TCP port
No.? why the role is determined by the size of A1 &A2?

thanks.
jinglin