CS 775

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CS 775

• Microsoft Windows
• Browser Service
• Presented by
• Stan Bush
• (from material taken from MSCE study guides)

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• A computer must know what resources are available
  to it before it can access those resources. For
  Microsoft networks the service that collects,
  maintains and distributes that list of resources
  is called the browser service. This service
  should not be confused with an Internet browser.
  In MS Windows the network browser maintains a
  list of currently available resources on the
  network.

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In a MS Windows environment you see available
network resources via the Network Neighborhood
window as seen below
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Or, from the Windows Explorer window. Again,
do not confuse this function which is the follow
on to the old Win 3.1 file manager, to MS
Internet Explorer.
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• When a computer joins the network at boot time,
  it announces its presence and provides a list of
  resources that it will be making available to the
  network (this list can be modified at anytime by
  an authorized user).
• The purpose of the browser service is to provide
  a centralized location and manager for listing
  network resources. This prevents each
  workstation and server from having to maintain
  this list for themselves and greatly decreases
  network message traffic.
• Computers can perform the roles of a master
  browser, back-up browser, potential browser, or a
  non-browser.

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Master Browser(by default is the Primary Domain
Controller)

• Maintains the master list of all available
  network resources which is called the browse
  list.
• It listens for the announcements from computers
  and adds those computers and their list of
  available resources to the browse list.
• It then distributes that list to the back-up
  browsers.
• It also selects potential browsers for promotion
  to back-up browsers when necessary.
• Only one master browser can exist in a workgroup
  or domain, except in the case of TCP/IP
  internetworks. This is because TCP/IP does not
  route broadcasts and therefore the browsing
  process will not reveal shared resources across
  routers (certain routers can be configured to
  pass-on network broadcasts but this is not the
  preferred method as this can dramatically
  increase traffic.

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Back-up Browser

• Receives the list of available resources from the
  master browser
• Supplies the browse list to computers requesting
  it.
• A back-up browser periodically announces itself
  to the master browser and requests an updated
  browse list. If it cannot find the master
  browser it forces an election to elect a new
  master browser.
• One backup browser will be selected by the master
  browser for every 32 computers on the network up
  to a maximum of 3, providing of course that there
  are that many computers available that can assume
  the role of back-up browser.

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Potential Browser

• It, like all computers on the network announces
  itself and supplies a list of resources it is
  making available to the network and can use any
  available resources from other computers if it
  has the proper permissions.
• Is simply a computer on the network that is able,
  if needed, to be promoted to back-up or master
  browser.

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Non-Browser

• It, like all computers on the network announces
  itself and supplies a list of resources it is
  making available to the network and can use any
  available resources from other computers if it
  has the proper permissions.
• But, it is not eligible to be promoted to back-up
  or master browser.

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Preferred Master Browser

• A computer can be designated as a preferred
  master browser by an entry in the windows
  registry.
• When this computer joins the network it announces
  itself as the master browser, and if one already
  exists, forces an election that re-evaluates the
  roles of computers as browsers in the network.
• So, if you have a PDC that you do not want to
  assume the extra task of master browser. Set its
  registry to specify that it is to be a
  non-browser and designate another computer as the
  preferred master browser to off-load those
  duties.
• If the PDC is set to a non-browser and the
  preferred master browser is not on the network
  than another qualified computer will be elected
  as the master.

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How browsing works...

• A computer starts up and connects to the network,
  it announces itself to the master browser and if
  it has resources to share provides a list to the
  master browser.
• When a computer attempts to locate network
  resources for the first time the computer
  contacts the master browser and receives a list
  of back-up browsers.
• The computer then contacts one of the back-up
  browsers and requests the list of available
  resources.
• The back-up browser responds with a list of
  available resources.
• The requesting computer can then contact the
  owner of the resource that it wants to access and
  establish a session.

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Browser elections...

• An election occurs when one of the following
  occurs
• When a computer on the network attempts to
  contact the master browser and it cannot be
  found.
• When the current master browser forces an
  election because it is being shut down.
• When more that one master browser exists (i.e. a
  preferred master came on-line).

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Browser elections, cont.

• A client computer, back-up browser, or a
  preferred master browser can cause an election to
  occur by broadcasting an election packet over
  the network.
• All the browsers receive the packet and evaluate
  their precedence in relation to it.
• If a computer determines that it is of a higher
  precedence than the computer that transmitted the
  packet, it will broadcast one of its own, if it
  is of lower precedence it will do nothing.
• This continues until a computer is uncontested as
  the master browser. During this time the network
  is considered to be in a state of an election in
  progress.

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Browser elections, cont.

• Elections are preformed to determine who gets to
  be the master browser. The factors that
  determine the outcome of an election, in order of
  precedence are
• the designated role of the computer in network
  browsing
• the operating system of the computer
• the versions of the operating system

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Browser elections, cont.

• The order of selection by operating system is
• Windows NT server (Primary Domain Controller)
• Windows NT Server (Backup Domain Controller)
• Windows NT Server
• Windows NT Workstation
• Windows 98/95
• Windows for Workgroups
• The order of selection by version is
• 4.0
• 3.51
• 3.5
• 3.1

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Browsing Fundamentals

• When a computer initially joins a network it
  announces itself and continues to announce its
  continued presence once a minute for the first
  several minutes and then the interval increases
  until it reaches 12 minutes.
• If the master browser does not receive an
  announcement from a computer for more than 3
  periods it removes it and its resources from the
  browse list. This should only happen in the case
  of an abnormal shutdown or communication failure,
  as a normal shutdown is announced and the master
  browser immediately removes that computer from
  the list.
• So, as much as a 36-minute delay can occur from
  when a computer goes down before it is removed
  from the browse list.

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Browsing Fundamentals, cont.

• Back-up browsers announce themselves on the
  network like all other computers.
• They also request an updated browse list from the
  master browser every 15 minutes.
• This coupled with the delay from when a computer
  is removed from the list can cause a total of 51
  minutes to pass before a computer is removed from
  the browse list maintained at the backup browser,
  which is the list that is passed on to requesting
  computers.

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Browsing Fundamentals, cont.

• To allow browsing over internetworks or within a
  WAN using TCP/IP to be successful you would need
  to implement one of the following solutions
• Use the WINS service (Windows Internet Name
  Service) which dynamically maps NetBIOS names to
  IP addresses. (browsing is conducted using
  NetBIOS names which is a unique name assigned
  each network computer)
• Use LMHOSTS files static mapping of NetBIOS
  names to IP addresses via text file lookup.
• Configure UDP port 137 to forward browser related
  broadcasts over network routers.