Chapter2 TCP, FTP, TFTP, Telnet, NTP, and NetBIOS over IP Concepts
NetBIOS Overview
The Network Basic Input/Output System (NetBIOS)
is a session layer communications service used by client and server applications
in IBM token ring and PC LAN networks.
NetBIOS provides applications with a programming interface for sharing
services and information across a variety of lower-layer network protocols,
including IP. Figure
2-7 shows the position of NetBIOS and IP in a simple network architecture.
Figure 2-7. NetBIOS over IP
There are three categories of NetBIOS services: the name service, the
session service, and the datagram service.
The NetBIOS name service allows an application
to:
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Verify that its own NetBIOS name is unique. The application issues an add
name query to NetBIOS. NetBIOS broadcasts the add name query, containing
the name. NetBIOS applications that receive the query return an add name
response or a name-in-conflict response. If no response to the query is
received after (typically) six broadcasts, the name is considered to be
unique.
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Delete a NetBIOS name that the application no longer requires.
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Use a server's NetBIOS name to determine the server's network address.
The application issues a name query request to NetBIOS, containing the
target server's NetBIOS name. NetBIOS broadcasts the name query request.
The server that recognizes the name returns a name query response containing
its network address.
The NetBIOS session service allows an application
to conduct a reliable, sequenced exchange of messages with another application.
The messages can be up to 131,071 bytes long.
The NetBIOS datagram service allows an application
to exchange datagrams with a specific application or to broadcast datagrams
to a group and receive datagrams from the group. Datagrams allow applications
to communicate without establishing a session. When a NetBIOS application
wants to send information that does not require acknowledgment from the
destination application, the application can transmit a NetBIOS datagram.
NetBIOS in an IP Environment
The NetBIOS name service and datagram service rely on the capability of
the underlying network to broadcast name query requests to all NetBIOS
applications. In a NetBIOS over IP environment, it is the responsibility
of the IP router to ensure that the broadcast queries reach all appropriate
network segments. To do this, the router:
1. Analyzes each NetBIOS packet received on any NetBIOS interface to
determine whether the packet is a broadcast packet
2. Rebroadcasts each broadcast packet out all appropriate interfaces,
except the one on which it was received (readdressing the packet if required)
If alternate paths exist between different network segments, broadcasting
loops can occur. To prevent such loops, the router:
1. Stamps the data portion of the IP packet with the IP address of the
router from which the packet was rebroadcast
2. Parses the IP addresses included in the data portion of the IP packet
to determine if the packet has already been rebroadcast by that router
In Figure
2-8, for example, client C on the network connected to router
B wishes to communicate with server S, which is located on the network
connected to router A.
The following steps occur:
1. The client issues a name query request to NetBIOS on the host, specifying
the server application by its NetBIOS name. The IP service on the host
broadcasts the name query request.
2. Router B receives the name query request, determines that it is a
broadcast message, and rebroadcasts it out each of its NetBIOS interfaces
(except for the one on which it arrived).
3. Router A receives the broadcast equest and rebroadcasts to its local
network.
4. The server on router A receives the IP broadcast request and recognizes
its own name.
Figure 2-8. Broadcasting a Name Query
Request
The server responds to the name query request by issuing a positive
name query response, containing the IP address of the server, to NetBIOS
on the host. The following steps occur (Figure
2-9):
1. NetBIOS sends the response to router A as a unicast message.
2. Router A and router B forward the unicast response to the awaiting
client.
Now that the client has obtained the server's IP address from the name
query response, client and server can communicate by exchanging IP messages.
Figure 2-9. Returning a Unicast Name
Query Response
Forwarding Name Queries over an Unnumbered Interface
NetBIOS cannot be configured directly on an unnumbered interface. Because
of this restriction, name query requests cannot be broadcast over an unnumbered
interface.
To forward name query requests over unnumbered interfaces, the network
administrator configures a static NetBIOS name entry to the proper NetBIOS
name server. In this way, name query requests will traverse the unnumbered
interfaces as unicast IP packets.
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NetBIOS
in an IP Environment
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Forwarding
Name Queries over an Unnumbered Interface
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Configuring IP Utilities (117358-A Rev. A) - 29 SEP 1997
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