Forefront TMG URL mapping for internal website pages

Hi, 
How can do the following on Forefront TMG :
I have the following Internal URLS
http://Internal_domain/directory1/SubDirectory2/page1.aspx
http://Internal_domain/directory2/SubDirectory3/page2.aspx
I want the end user to access these two internal pages from outside via the following URLs  by hiding internal path and using short name
 ex.
http://external_domain/page1  => http://Internal_domain/directory1/SubDirectory2/page1.aspx
http://external_domain/page2  => http://Internal_domain/directory1/SubDirectory3/page2.aspx

thanks