20.4 Crossing the Street with the DMB

Browser roles are cumulative, as we keep saying, so the Domain Master Browser is also the Local Master Browser for its subnet and it must handle all of the duties of an LMB. One such duty is participation in local Browser Elections. Of course, since the DMB is the appointed workgroup president it is expected to win the election which it will do because the election is rigged. More on that when we go into detail regarding the election process.

The DMB listens on the workgroup <1B> name for (unicast) MasterAnnouncement messages from Local Master Browsers on remote subnets. It keeps track of these announcements and, periodically, contacts the LMBs and asks for a new copy of their local Browse List. The DMB merges the local Browse Lists collected from the various LMBs (including its own) into a master Browse List for the entire workgroup. The LMBs, in turn , will periodically query the DMB and add the remote entries collected in the workgroup master list to their own local Browse Lists. That's how local LANs get a complete copy of the combined workgroup Browse List.

The key to making this all work is the NBT Name Service, the N et B IOS N ame S erver (NBNS) in particular. The scattered LMBs use the NBNS (aka WINS server) to find the workgroup <1B> name, which is registered by the DMB. Without that, cross-subnet browsing would not work because the LMBs would be unable to announce themselves to the DMB, and would also be unable to request copies of the DMB's master list.

Note that B mode NBT nodes do not talk to the NBNS and, therefore, cannot find a remote Domain Master Browser. That's okay, though, because the scope of a B mode NBT LAN is limited to the local IP subnet anyway. Even if a B node could do cross-subnet browsing, it wouldn't (shouldn't) be able to connect to a server on a remote subnet.

In contrast, P nodes must transact all of their Browse Service business directly with the Domain Master Browser. The NBT Scope available to a P node is the set of names it can resolve via the NBNS. It doesn't do broadcasts, so the only Browser node that it can find is the DMB because the DMB is the only Browser node with a name that can be properly resolved via the NBNS. M and H nodes have the best of both worlds . They can send broadcasts and use the NBNS to resolve names.

Now that you have a basic idea of how this stuff works, think about what might have happened if Microsoft had correctly implemented group name handling in their WINS implementation and had also provided a working N et B IOS D atagram D istribution Server (NBDD). If they had done that, the broadcast datagrams used by the Browse Service the announcements and election requests and such would have reached the entire extent of the virtual NetBIOS LAN even if it spanned multiple subnets, even across WAN links and such. For better or worse , that would have changed the design and workings of the Browse Service entirely.