the user breaks it. While the control connection is up, a separate data connection is established
for each file transfer.
Figure 5-16. Rather than just creating a new connection, the client tells the server with an
application layer message what port number will be used for the new connection. The server
will not transfer the file (zzz, in this case) over any other data connection except the one to the
correct socket--the one with the client's IP address, TCP, and the port number declared to the
server (1031, in this case).
of the reasons that such an application is needed (when the more robust FTP is available) is that
TFTP takes little memory to load and takes little time to program. With the advent of extremely
low-cost memory and processing, such advantages seem trivial. Practically speaking, if you
intend to transfer files frequently from your PC, FTP is probably what you will use. However,
to transfer files into and out of IOS-based routers and switches, Cisco supports TFTP, not FTP.
layer. However, TFTP uses application layer recovery by embedding a small header between
the UDP header and the data. This header includes codes--for instance, read, write, and
acknowledgment--along with a numbering scheme that numbers 512-byte blocks of data.
These block numbers are used to acknowledge receipt and resend the data. TFTP sends one
block and waits on an acknowledgment before sending another block--essentially, the
equivalent of a window size of 1.
control commands