I work at a company that has two separate instances of Lync 2010 running on child domains.  We're separated between a firewall and we're able to IM each other, but we can't initiate screen or filesharing when traversing the firewall.  If a user from the other child domain connects from inside of our network, we can do screen and filesharing with them. 

I'm wondering what ports need to be opened on our firewall to allow screen and filesharing between our clients.

Thanks

Hi,

That's a normal behavior, if not all needed ports are open.

https://technet.microsoft.com/en-us/library/gg398833.aspx

And here you will find the Lync worloadposter for all Lync applications with port described

http://www.microsoft.com/en-us/download/details.aspx?id=39968

You should look at the PSOM traffic on port 8057 and 8058