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subnet-directed broadcast, Subnet Broadcast data packets are unicast between
routers until they reach the subnet on which the Network Stations reside. At this
point, the router at the destination subnet broadcasts the data packets to the
Network Stations on the subnet. Disinterested hosts on the subnet throw the data
packets away. The packets are usually thrown away by the host’s IP layer after it
determines that no applications are interested in receiving data on the port to which
the broadcast was directed. See Figure 235 on page 387 for an illustration of a
subnet-directed broadcast. This solution can drastically reduce the network traffic as
well as the time that it takes many Network Stations to boot when booting
simultaneously.
The TFTP Subnet Broadcast option enables clients to join a broadcasting filegroup.
It also allows clients to receive all subsequent blocks for a file until the client
becomes the master client. A client becomes the master client when it receives an
Option Acknowledge (OACK) packet from the TFTP server that indicates that it is
the master client. A client must keep track of blocks that it receives. After a client
becomes the master client, it can request the blocks that it has not received. The
master client requests blocks by sending ACK packets that include the block
number of the block
prior to
the block that the master client requires. For example,
if the client wants block 5, it sends an ACK with a block number of 4.
When a client receives an OACK packet that indicates that it is the master client,
the client must send an ACK that requests the first block it requires. From then on,
the client must request blocks in ascending but not necessarily consecutive order. A
master client continues to send ACK packets to the server to indicate the next block
that it requires. When the master client receives all of the blocks it requires, it sends
an ACK with the number of the last block on the file being transferred. Once the
server receives an ACK with the last block number of the file being transferred, the
transfer to the client sending the ACK is considered complete. A client can terminate
its transfer at any time by sending an ACK for the last block or by sending an Error
(ERR) packet. A client can terminate this transfer regardless of whether it is the
master client or not.
Note: This TFTP Subnet Broadcast option is designed to improve simultaneous
transfer of large files to multiple clients on a common subnet. This option
does not help with files that require only a few blocks to transfer or single
client transfers.
386 OS/400 TCP/IP Configuration and Reference V4R4