TIP is getting deprecated - see the note at http://msdn2.microsoft.com/en-us/library/ms679484(VS.85).aspx
If the native OleTx protocol used by MSDTC and System.Transactions is not friendly enough for your firewall, you should consider the WS-AtomicTransactions protocol provided through Windows Communication Foundation in .Net Framework 3.0 and higher: http://msdn2.microsoft.com/en-us/library/ms730266.aspx