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AD Sites and Services Transports: IP, RPC and SMTP. What's the difference?

I am running AD on several Windows 2000 and 2003 servers.  The domain is in native mode and the GCs / PDC emulators are all Windows 2003 server.
I have 4 sites in a hub/spoke topology connected via T1s and VPN:
New York (NY)  -  HUB - 2 Domain Controllers:  NYDC1 and NYDC2
Wash D.C. (DC) - 2 Domain Controllers:  DCDC1 and DCDC2
Las Angeles (LA) - 1 Domain Controller:  LADC1
Sydney, AU  (SY) - 1 Domain Controller:  SYDC1

We have one domain and 4 different subnets for each corresponding location.  In AD Sites and Services, I have IP and SMTP listed as Inte-Site Transports and I have turned off the "Bridge all site links" to clean up the replication partners.  All domain controllers replicate with NYDC1 and NYDC2 as they are in the Hub site.

Now, when I go into the NTDS Settings for NYDC1, I see all other Domain Controllers appear as replication partners which is normal.  If I click on the replication connection for NYDC2 and go to properties, I see that it is configured to use RPC for the replication transport which is again normal since both DCs are in the same site.

My question is about the replication transport options.  If I click it the Transport pulldown on the properties for a connection I have three options:  IP, RPC and SMTP

I do not recall being taught about 3 transport options... I recall SMTP being used for remote sites (with latency), and RPC being for DCs within one site (fast connection) but I do not remember the IP option.  What should I be using for the remote sites to replicate with the Hub?  I have them set for IP now, but I suspect that IP and RPC are synonymous (but why have them both listed!)

Any ideas?
1 Solution
There is a very extensive explanation of Active Directory replication in the Technet Library:

In that document, no other replication transports other than synchonous RPC-over-IP and asynchronous SMTP-over-IP are mentioned.

Automatically created replication connectors default to RPC for intrasite servers, and IP for intersite servers, but both transports use synchronous RPC-over-IP.
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