Microsoft Network Browser System
The browser is a network resource enumeration tool first created for LAN Manager 1.0. Windows for Workgroups enhanced the browser to make it truly client/server, and finally it was enhanced for domain browsing on a Windows NT® operating system–based network. The Microsoft networking browser maintains a list called the browse list. The browse list is an enumeration of all the available servers, workgroups, and domains (for Windows NT and LAN Manager networks). For example, when a user attempts to connect to any network resource, using Network Neighborhood, the list of servers that appears is the browse list. The browse list is provided by a browser in the local computer’s workgroup or domain.
The Microsoft networking browser system consists of a domain master browser, master browsers, backup browsers, and client computers. The domain master browser maintains the complete domain browse list. The master browsers periodically query the domain master browser to obtain a complete list of domain resources. The master browser maintains the browse list and periodically sends copies to the backup browsers. When a browser client needs information, it obtains the current browse list by sending a NetServerEnum2 call to either the master browser or a backup browser.
NetServerEnum2 returns information about the server type (SV_TYPE_SERVER) entries in a domain or workgroup. It allows client computers to view a specific set of servers in the workgroup or domain. NetServerEnum2 provides a server type mask that allows users to query for various types of servers, such as workstations, servers, time servers, domain controllers, and so on. Printer browsing and Windows Terminal Server clients rely on this type of browsing to enumerate their respective resources
This centralized browser architecture for Windows networking resources reduces the number of broadcast datagrams on the network. A datagram is a network packet that is sent to a mailslot on a specified computer (a directed datagram) or to a mailslot on any number of computers (a broadcast datagram). The centralized browser architecture also reduces the demands on the client computer’s CPU and memory.
Note: For Microsoft networking using the Microsoft TCP/IP protocol, broadcast name resolution is a direct implementation of Request for Comments (RFCs) 1001 and 1002 (NetBIOS Service Protocols). This implementation of NetBIOS over TCP/IP is wholly compliant with the RFCs, and does not involve any method of what has been referred to as NetBEUI encapsulation. For more information about NetBIOS over TCP/IP (NetBT), see the discussion of name resolution for Windows-based networking in Windows NT Server Networking Guide.
How Servers Manage Browsing for Microsoft Networking
In a Windows NT domain, every Windows NT Server–based computer is a browser. The primary domain controller (PDC), if there is one, is the domain master browser, and the other computers running Windows NT Server, Windows NT Workstation, or the Windows® 95 operating system are backup browsers. If there is more than one Windows NT Server–based computer in the domain, no computer running Windows NT Workstation or Windows 95 should ever be a master browser in the domain. In a workgroup containing computers running Windows 95 and Windows NT Workstation, there is always one master browser. If there are at least two Windows 95- or Windows NT Workstation–based computers in the workgroup, there is also one backup browser. For every 32 computers running Windows 95 and Windows NT Workstation in the workgroup, there is another backup browser. The following section describes how computers become browsers and summarizes the roles played by master and backup browsers.
Multiprotocol Considerations
The browser maintains all of its server information on a per-transport basis. A transport is the physical network card bound to a protocol. When a client application issues a NetServerEnum call, the code running on the workstation enumerates all the serviced networks and forwards the NetServerEnum call to a browser server for that particular transport. When a browser server receives the request, it returns only those servers on the same network from which it received the transaction originally. Therefore, browsing occurs on each transport independently, regardless of other available transports.
Master Browser Elections
Each computer running Windows 95, Windows for Workgroups, Windows NT Workstation, or Windows NT Server has configuration settings that determine whether that computer can become a browser. For servers running Windows NT, the value for MaintainServerList for the browser service determines its possible role as a browser. By default, servers are configured to have the potential to become browsers. Unless the server is specifically configured to never be a browser, the Microsoft networking browser service starts automatically when the computer starts, and the server announces itself on the networking using the special NetBIOS name <1e>.
A master browser election always occurs in the following circumstances:
When a computer cannot find its master browser at system startup
When a computer determines that a master browser has disappeared
When a computer running Windows NT Server starts (that is, a preferred master browser is started)
When any server needs to force a master browser election, it notifies the other browsers on the system by broadcasting an election datagram that contains the sending browser’s election version and election criteria. The election datagram and subsequent communications are sent to the NetBIOS name <1e>.
The election version is a constant value that identifies the version of the browser election protocol. If the election versions are identical for both computers, the election criteria are compared. The election criteria consist of a 4-byte hexadecimal value. Specific groups of bytes are masked and their values set according to the following table:
Election Criterion
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Value
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Operating system type
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0xFF000000
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Windows NT Server
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0x20000000
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Windows NT Workstation
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0x10000000
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Windows 95
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0x01000000
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Windows for Workgroups
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0x01000000
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Election version
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0x00FFFF00
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Per-version criterion
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0x000000FF
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Primary domain controller
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0x00000080
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WINS client
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0x00000020
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Preferred master browser
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0x00000008
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Running master browser
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0x00000004
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MaintainServerList or BrowseMaster = yes
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0x00000002
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Running backup browser
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0x00000001
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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 Browsers
The 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>, <1d>, or <1e>, as appropriate for the type of server. When the master browser receives a server announcement from a computer, it adds that computer to the browse list. The master browser then returns lists of backup browsers to computers running Windows NT Server, Windows NT Workstation, Windows 95, and Windows for Workgroups. When a computer starts that is configured to automatically become a browser if required, the current master browser must tell that computer whether to become a backup browser. If its browse list is empty when a computer first becomes a master browser, it can force all servers to register with it by broadcasting an announce request (AnnouncementRequest) datagram. All computers receiving this datagram must respond by sending a server announcement at a random time within the next 30 seconds. The randomized delay ensures that the network and the master browser itself are not overwhelmed with responses. When a master browser receives a server announcement from another computer that claims to be the master browser, the receiving master browser demotes itself and forces an election as described in the previous section. This ensures that there is always only one master browser in each domain or workgroup.
Role of Backup Browsers
Backup 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 Browsers
The 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 <1d>. The domain master browser then sends a remote NetServerEnum call to each master browser to collect each subnetwork’s list of servers. The domain master browser merges the server list from each subnetwork master browser with its own server list to form the browse list for the domain. This process is repeated every 15 minutes to ensure that the domain master browser has a complete browse list of all the servers in the domain.
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 Networking
A 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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