1.Probe Mechanisms Think of the name database as a "perfectly" distributed object, with each participating station responsible for just the segment of the database containing it's own names.When query, use a single broadcast packet-a "probe"--which asks all stations at once to peruse their own database segment for the name.
Drawbacks:
1) annoyance of broadcast
2) lack of any centralized authority
3) what about probe get lost in transmission? need repetition and timeout periods. However, "Three times ought to be enough" is about as scientific as it gets.
2. Roll Call Mechanisms
broadcasts are't required--requestor could just probe other stations one at a time. Especially in nonbroadcast networks.
Drawbacks:
1.required that a list of stations be maintained, imposing a burden administrator would rather discard.
3.Centralized Database
use a "centralized" NBNS . The database is entrusted to the server. In practice, the name database is contained in both the central server and the individual stations, with the server calling the shots.
RFC1001：
Three types of end-nodes are part of this standard:
- Broadcast ("B") nodes
- Point-to-point ("P") nodes
- Mixed mode ("M") nodes
It is recommended that the administration of a NetBIOS scope avoid using both M and B nodes within the same scope.A NetBIOS scope should contain only B nodes or only P and nodes.
Name Format:
Each 4-bit, half-octet of the NetBIOS name is treated as an 8-bit,
right-adjusted, zero-filled binary number. This number is added to
value of the ASCII character 'A' (hexidecimal 41). The resulting 8-
bit number is stored in the appropriate byte.
The following diagram
The NetBIOS scope identifier is a valid domain name (without a
leading dot).
An ASCII dot (2E hexidecimal) and the scope identifier are appended
to the encoded form of the NetBIOS name, the result forming a valid
domain name.
Note that the first octet of a compressed name must contain one of
the following bit patterns. (An "x" indicates a bit whose value may
be either 0 or 1.):
00100000 - Netbios name, length must be 32 (decimal)
11xxxxxx - Label string pointer
10xxxxxx - Reserved
01xxxxxx - Reserved
B nodes and M nodes may be intermixed only on a local
broadcast area. B and M nodes should not be intermixed in
an internet environment.
Name Registration:
Each node maintains state information for each name it has
registered. This information includes:
- Whether the name is a group or unique name
- Whether the name is "in conflict"
- Whether the name is in the process of being deleted
B nodes perform name registration by broadcasting claim requests,
soliciting a defense from any node already holding the name.
P nodes perform name registration through the agency of the NBNS.
M nodes register names through an initial broadcast, like B nodes,
then, in the absence of an objection, by following the same
procedures as a P node. In other words, the broadcast action may
terminate the attempt, but is not sufficient to confirm the
registration.
Different end-node types perform name resolution using different
techniques, but using the same packet formats:
- B nodes solicit name information by broadcasting a request.
- P nodes ask the NBNS.
- M nodes broadcast a request. If that does not provide the
desired information, an inquiry is sent to the NBNS.
阅读(1656) | 评论(0) | 转发(0) |
