Layer 6 header) is then passed to the application layer (Layer 7)
software.
can examine the true end-user data. This header signifies
agreement to operating parameters by the applications on Host A
and Host B. The headers are used to signal the values for all
parameters; therefore, the header typically is sent and received at
application initialization time only. For example, for file transfer,
the size of the file to be transferred and the file formats used would
be communicated (application parameters).
functions. For example, the transport layer (Layer 4) can send data, but if another computer
does not acknowledge that the data was received, the sender will not know when to perform
error recovery. Likewise, the sending computer encodes a destination network layer address
(Layer 3) in the network layer header. If the intervening routers do not cooperate by performing
their network layer tasks, the packet will not be delivered to the true destination.
cases, a trailer. Headers and trailers are additional data bits, created by the sending computer's
software or hardware, that are placed before or after the data given to Layer N by Layer N+1.
The information needed for this layer to communicate with the same layer process on the other
computer is encoded in the header and trailer. The receiving computer's Layer N software or
hardware interprets the headers and trailers created by the sending computer's Layer N, learning
how Layer N's processing is being handled, in this case.
layer on Host A communicates with the application layer on Host B. Likewise, the transport,
session, and presentation layers on Host A and Host B also communicate. The bottom three
layers of the OSI model have to do with delivery of the data; Router 1 is involved in that process.
Host A's network, physical, and data link layers communicate with Router 1; likewise, Router
1 communicates with Host B's physical, data link, and network layers. Figure 3-3 provides a
visual representation of the same-layer interaction concepts.