20.3 Becoming a Backup Browser

A Potential Browser becomes a Backup Browser in one of three ways.

1. If the number of Providers on the local LAN exceeds a predefined limit, the Local Master Browser will select an available Potential Browser and promote it to Backup Browser.

   In theory, the LMB will appoint new Backup Browsers as needed to prevent itself from getting overloaded. Some documentation says that there should be one Backup Browser for every 32 Providers on the local LAN. Other documentation says other things. In any case, the LMB can promote a Potential Browser by sending a BecomeBackup Browser Frame .

   The BecomeBackup Browser Frame is another NBT broadcast datagram. In this case, it is sent to the group name workgroup <1E> , which means that all of the Potential Browsers will receive the message. The NetBIOS machine name of the node to be promoted is carried within the message.

2. A Potential Browser may decide on its own to become a Backup Browser. It simply announces its new status to the LMB by sending out a HostAnnouncement Browser Frame with a specific flag set. (The flags will be described when we go into detail about the NetServerEnum2 RAP call.)

   Note that the Backup Browser uses a HostAnnouncement to make itself known. That's the same thing a Provider does to announce services. In fact, the Backup Browser is announcing itself as a Provider: it provides access to the Browse List. This stuff gets mighty recursive at times.

   According to the Leach/Naik Browser Internet Draft, an LMB that loses a new election should demote itself to Backup Browser status instead of dropping all the way down to a Potential Browser. The theory is that it is the most likely to be promoted should something bad happen to the new LMB, so it should maintain a fairly up-to-date copy of the Browse List to ensure a smooth transition.

3. Browser roles are cumulative so, to be pedantic, the LMB is also a Backup Browser.

At the time that the Browse Service was created it may have been reasonable to be concerned about one computer bearing the brunt of all of the client requests , particularly on a LAN with a very large number of nodes. Today's computers are capable of handling the load and their own work without breaking a sweat. It would take an effort (a purposeful Denial of Service attack, for instance) to cause the LMB any real trouble.