> But what are the “awful effects” of sending FIN instead? Can someone explain?
The RFC explains.
The other side - the server - will be stuck on `send(sock_fd, more_bytes...)`. If it's the 90s and your FTP server is single-threaded, that means the server will appear completely stuck. This won't resolve for several minutes, or possibly forever if the server-side TCP stack is buggy or lacks timeouts.
The client's connection, even after the process is gone, will still be alive in the kernel. It will be in FIN-WAIT-2, waiting for the server to send its FIN. The remote sender will be stuck waiting for the zero-window state to pass, sending probes to figure out when the receive window opens (which will never happen). Things will be stuck in this state until one of the sides times out, which could be several minutes. Or if the implementations are buggy, it could be permanent.
From RFC 2525, 2.17
Failure to reset the connection can lead to permanently hung
connections, in which the remote endpoint takes no further action
to tear down the connection because it is waiting on the local TCP
to first take some action. This is particularly the case if the
local TCP also allows the advertised window to go to zero, and
fails to tear down the connection when the remote TCP engages in
"persist" probes (see example below).
The program in the boot sector is run similar to other software: the code is loaded into memory, and then the processor is jumped to the first instruction. This loading and jumping is done by the firmware, which is included in your hardware, separate from your disks.
Let's back up to the start. When you switch on a computer, the power rails on a bunch of the chips come up. As this happens, the chips enter a "reset" state with initial data that is baked into the circuitry. This is the power-on reset (PoR) circuitry. When the power is up and stable, the "reset" is over and the processor starts executing. The initial value of program counter / instruction pointer is called the reset vector, and this is where software execution begins. On an x86 PC, this is something like 0xFFFFFFF0. The memory controller on the system is configured for reads from this address to go NOT to the main RAM, but to a flash memory chip on the board that holds the firmware software. From there the firmware will find your bootable disks, load the bootloader, and pass control to it.
The RFC explains.
The other side - the server - will be stuck on `send(sock_fd, more_bytes...)`. If it's the 90s and your FTP server is single-threaded, that means the server will appear completely stuck. This won't resolve for several minutes, or possibly forever if the server-side TCP stack is buggy or lacks timeouts.
The client's connection, even after the process is gone, will still be alive in the kernel. It will be in FIN-WAIT-2, waiting for the server to send its FIN. The remote sender will be stuck waiting for the zero-window state to pass, sending probes to figure out when the receive window opens (which will never happen). Things will be stuck in this state until one of the sides times out, which could be several minutes. Or if the implementations are buggy, it could be permanent.
From RFC 2525, 2.17
https://datatracker.ietf.org/doc/html/rfc2525#page-50