MS Browsing Overview Microsoft Network Browser System
When a browser receives an election datagram, the receiving browser compares the election version in the datagram with its own. If the receiving browser has a higher election version than any other browser, it is elected, regardless of any other election criteria. For example, a computer running any version of Windows NT is always elected over a computer running Windows 95. If the election versions are identical, the receiving browser compares the election criteria as follows: If the receiving browser has a higher election criterion than the sending browser, the receiving browser issues its own election datagram and enters the election in progress state. If the receiving browser does not have a higher election criterion than the sending browser, the receiving browser attempts to determine which computer is the new master browser. If there is still a tie, the browser that has been running longest is elected. If there is still a tie, the browser that has a lexically lower name is elected. For example, when all other criteria are equal, a server named A1 is elected over a server named B1. When a browser receives an election datagram indicating that it is elected, the browser enters the running election state. In the running election state, the browser sends an election request after a delay based on the browser’s current browser role: 200 ms delay for master browsers 400 ms delay for backup browsers 800 ms delay for all other browsers The browser broadcasts up to four election datagrams. If, after four election datagrams, no other browser with an election criterion that would win the election has responded, the browser becomes the master browser. If the browser receives an election datagram indicating that another system would be elected, the browser demotes itself to backup browser. To avoid unnecessary network traffic, a browser that has lost an election does not broadcast any unsent election datagrams. Role of Master BrowsersThe master browser maintains the browse list, which is the list of all servers in the master browser’s domain or workgroup, plus the list of all domains on the network. For a domain that spans more than one subnetwork or broadcast collision zone (Like a V-LAN), the master browser maintains the browse list for the portion of the domain on its subnetwork or broadcast collision zone. The rest of the domain is known based on domain announcements made by the domain master browser. Individual servers running Windows NT Server, Windows NT Workstation, Windows 95, Windows for Workgroups, or LAN Manager announce their presence by sending a directed datagram called a server announcement (HostAnnouncement) datagram to the domain or workgroup’s master browser. This announcement includes the server’s NetBIOS name of <01><02>MSBROWSE<02><01>, Role of Backup BrowsersBackup browsers call the master browser every 15 minutes to get the latest copy of the browse list, as well as a list of domains. Each backup browser caches these lists and returns the list of servers to any clients that send a remote NetServerEnum call to the backup browser. If the backup browser cannot find the master browser, it forces an election. Role of Domain Master BrowsersThe browser service running on a domain’s primary domain controller (PDC) has the special additional role of being the domain master browser. The PDC of a domain has a bias in browser elections to ensure that it becomes the master browser. On networks using TCP/IP where a domain spans more than one subnetwork, each subnetwork functions as an independent browsing entity, with its own master browser and backup browsers. Name-query datagrams should not cross routers. When routers or switches are misconfigured and propagate UDP port 137 and 138 broadcasts, large numbers of event 8003 appear in the event log. For more information, see the following Knowledge Base articles: Q135464 - 8003 Browsing Errors with UDP Forwarding and Q190930 - UDP Broadcast Forwarding by Cisco's IP Helper. To browse across the wide area network (WAN) to other subnetworks, at least one browser running Windows NT Server is required on the domain for each subnetwork. On the subnetwork where the PDC is located, this Windows NT Server–based computer is usually the PDC, which functions as the domain master browser. When a domain spans multiple subnetworks, the master browser for each subnetwork announces itself as the master browser to the domain master browser by sending a directed MasterAnnouncement datagram, using the computer’s NetBIOS The master browser on each subnetwork also sends a remote NetServerEnum call to the domain master browser to obtain the complete browse list for the domain. This complete browse list is thus available to browser clients on the subnetwork. Note: Microsoft networking workgroups cannot span multiple subnetworks. Any workgroup that spans subnetworks actually functions as two separate workgroups with identical names. To span multiple collision zones, a Windows NT domain must exist. Browser Failures with Microsoft NetworkingA failed server stops announcing itself. When the master browser does not receive a server announcement for three of the server’s current announcement periods, the master browser removes that server from the browse list. It might take up to an additional 15 minutes for the backup browsers to retrieve the updated browse list from the master browser, so it could take as long as 51 minutes from the time a server fails to the time when it is removed from all browse lists. Because a backup browser announces itself in the same way as a server, the procedure when a backup browser fails is the same as that for a server. If the name of this backup browser has been given to any clients, attempts made by those clients to contact this backup browser fail. The client then retries the NetServerEnum call on another backup browser on the client’s list of browsers. If all the backup browsers that a client knows have failed, the client attempts to get a new list of backup browsers from the master browser. If the client is unable to contact the master browser, it forces a browser election. The client then returns an error to the application, indicating that the master browser could not be found. When a master browser fails, the backup browsers detect the failure within 15 minutes. The first backup browser to detect the failure forces an election to select a new master browser. Between the time that the master browser fails and the time that a new master browser is elected, the domain could disappear from the list of domains in the browse list. If a client performs its first NetServerEnum call after the old master browser has failed but before a backup browser detects the failure, the client forces an election. If a master browser fails and there are no backup browsers, browsing in the workgroup or domain does not work correctly. When a domain master browser fails, other master browsers see only servers on their own local subnetwork. Eventually, all servers that are not on the local subnetwork are removed from the browse list. 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