Section 1.5. Browsing

1.5. Browsing

Browsing is the process of finding the other computers and shared resources in the Windows network. Note that this is unrelated to web browsing on the Internet, apart from the general idea of "discovering what's there." On the other hand, browsing the Windows network is like the Web in one way: what's out there can change without warning. Also be aware that browsing is not the same thing as searching Active Directory (AD) for hosts or resources. Although the NetBIOS browse service and AD are each a type of directory service, the implementation details are completely different. The comments in this section apply to browsing NetBIOS networks, not AD.

Before browsing existed, users had to know the name of the computer they wanted to connect to on the network and then manually enter a UNC such as \\rain\documents to an application or file manager to access resources. Browsing is much more convenient, making it possible to examine the contents of a network by using the point-and-click My Network Places GUI interface on a Windows client.^[*]

^[*] This was originally called Network Neighborhood in Windows 95/98/NT. Microsoft has changed the name to My Network Places in the more recent Windows Me/2000/XP.

You will encounter two types of browsing in an SMB network:

• Browsing a list of computers and shared resources

• Browsing the shared resource of a specific computer

Let's look at the first type. On each LAN (or subnet) with a workgroup or domain, one computer has the responsibility of maintaining a list of the computers that are currently accessible through the network. This computer is called the local master browser, and the list it maintains is called the browse list. Computers on a subnet use the browse list to cut down on the amount of network traffic generated while browsing. Instead of each computer dynamically polling to determine a list of the currently available computers, the computer can simply query the local master browser to obtain a complete, up-to-date list.

To browse the resources on a computer, a user must connect to the specific computer; this information cannot be obtained from the browse list. Browsing the list of resources on a computer can be done by double-clicking the computer's icon when it is presented in My Network Places. As you saw at the opening of the chapter, the computer responds with a list of shared resources that can be accessed after the user is successfully authenticated.

Each server in a Windows workgroup is required to announce its presence to the local master browser after it has registered a NetBIOS name, and (theoretically) announce that it is leaving the workgroup when it is shut down. It is the local master browser's responsibility to record what the servers have announced.

The My Network Places application can behave oddly, until you select a particular computer to browse. You might see a list of computers that is not quite up-to-date, including hosts that are not longer on the network or new ones that have not been been noticed yet. Put succinctly, once you've selected a server and connected to it, you can be a lot more confident that the shares and printers really exist on the network.

Unlike the roles you've seen earlier, almost any Windows system can act as a local master browser. The local subnet can also have one or more backup browsers that will take over in the event that the local master browser fails or becomes inaccessible. The local master browser creates one backup browser for each group of 32 Windows NT based hosts on the subnet,^[dagger] or each group of 16 Windows 95/98/ME hosts on the subnet (or a fraction of such a group). To ensure fluid operation, the local backup browsers synchronize their browse list frequently with the local master browser. There is currently no upper limit on the number of backup browsers that can be allocated by the local master browser.

^[dagger] Windows 2000 and later operating systems are all based on Windows NT technology.

1.5.1. Browsing Elections

Browsing is a critical aspect of any Windows workgroup. However, this can go wrong on any network. For example, let's say that a computer running Windows on the desk of a small company's office manager is the local master browserthat is, until she switches it off to plug in a fax machine. At this point, the Windows XP Workstation in the spare parts department might agree to take over the job. However, that computer is currently running a large, poorly written program that has brought its processor to its knees. The moral: browsing has to be very tolerant of servers coming and going. Because nearly every Windows system can serve as a browser, there has to be a way of deciding at any time who will take on the job. This decision-making process is called an election.

An election algorithm is built into all Windows operating systems so that they can agree who is going to be a local master browser and who will be local backup browsers. An election can be forced at any time. For example, let's assume that the office manager has finished using the fax machine and reboots her desktop PC. As the server comes online, it announces its presence, and an election takes place to see whether the PC in the spare parts department should still be the master browser.

When an election is performed, each computer broadcasts information about itself via datagrams. This information includes the following:

• The version of the election protocol used

• The operating system on the computer

• The amount of time the client has been on the network

• The name of the client

These values determine which operating system has seniority and will fulfill the role of the local master browser. (Chapter 8 describes the election process in more detail.) The architecture developed to achieve this is inelegant, and has no built-in security to prevent rogue machines from taking over. Thus it is possible for any computer running a browser service to register itself as participating in the browsing election and (after winning) being able to change the browse list. Nevertheless, browsing is a key feature in many Windows networks, and backward-compatibility requirements will ensure that it is in use for years to come.