At which point in message transmission from client to host (or vice versa) is the message actually sliced into packets?
From my current understanding, the application puts an entire file in the socket and hands it over to TCP entirely. TCP first buffers the file/message, and when the time is right (when is the time right?) cuts chunks of the buffer data (creates packets) and adds TCP headers to transform chunks into segments.
Why do we talk about packets in the application layer, if there are no packets in the application layer at all? Just whole files... This doesn't add right.
Can someone confirm my understanding?
A TCP-based application has a message to send. What the message is depends on the application — it could be just a small request, or a whole file. It passes the message to the transport layer (TCP), which chops up the message into segments and passes them one by one to the network layer (Internet Protocol). The network layer adds a header to each packet, and passes it to the link layer (Ethernet), which handles frames.
So, in principle, we have:
In practice, however, people are not that pedantic, and tend to mix the notions up. You'll often hear people speak about TCP packets (the correct term would be IP packets with a TCP payload), and they will even speak about the application sending packets (the correct formulation would be that the application passes messages to the transport layer). Most of the time the inexact terminology is not a problem, since context disambiguates things.