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Server and Client Ports
The TFTP server uses a subnet-directed broadcast address as the destination
address. It also uses a well-known port as the port of datagrams sent to clients that
have requested the subnet broadcast option. The clients listen for and receive
datagrams on the well-known port. The keyword for the well-known port is
subntbcst_tftp, and its decimal value is 247.
The TFTP server sends subnet-directed broadcast datagrams to clients that request
the subnet broadcast option. The source ports from which the TFTP server sends
these datagrams do not have to be unique. They can be arbitrarily allocated.
Some routers filter or block subnet-directed broadcast datagrams. In support of
router filters, you can define restricted ports for the QTFTP profile. If you define
restricted ports for the QTFTP profile, the TFTP server uses only the defined
restricted ports as the source ports for the subnet-directed broadcast datagrams.
Network administrators define router filtering rules to allow subnet-directed
broadcast datagrams to pass through router filters based on the source port of
subnet-directed datagrams being one of the restricted ports defined for the QTFTP
profile.
TFTP Extensions
The following sections include information on the TFTP Transfer Size Option and
the Subnet Broadcast TFTP.
TFTP Transfer Size Option
The Transfer Size option allows the client to determine how much data is
transferred on a read request (RRQ). This is useful for requesting a subnet
broadcast of a file. The client finds the size of the buffer it needs in order to store
the file in memory. Drawing from this block size, the client determines the number
of blocks for the transfer. The number of blocks is helpful information for tracking
the blocks that have been received. You can also use it for the last block
acknowledgment (ACK), which must be sent to terminate a transfer normally.
Without the Transfer Size option, determining the size and the last block of the
transfer requires the client to wait for a block to be received that is smaller than the
block size of the transfer.
Note: For files transferred in
netascii
mode, this option might not be as useful if
you are converting the data during the transfer in a way that changes its
size. Also, the server might require additional processing time to determine
the transfer size due to conversion of the file to the appropriate CCSID.
TFTP Subnet Broadcast Option
With the increasing popularity of the Network Station, the possibility for
boot storms
also increases. These storms occur when large numbers of clients request their
boot code at the same time. When hundreds of stations are involved in booting, the
same data must be routed through each hop in the network between each Network
Station and the server.
The TFTP Subnet Broadcast option provides a solution to this problem. It allows the
server to broadcast the boot code to the Network Stations on a subnet basis. Using
Chapter 14. TFTP Server 385