SACKs have been in TCP for 25-30 years now (Widely adopted as part of New Reno, although RFC 2018 proposed the TCP option and implementation back in 1996).
That said, the typical reason why TCP doesn't send packet N+1 is because its congestion window is full.
There is a related problem known as head-of-line blocking where the application won't receive packet N+1 from the kernel until packet N has been received, as a consequence of TCP delivering that in-order stream of bytes.
That said, the typical reason why TCP doesn't send packet N+1 is because its congestion window is full.
There is a related problem known as head-of-line blocking where the application won't receive packet N+1 from the kernel until packet N has been received, as a consequence of TCP delivering that in-order stream of bytes.