You may have noticed the Data Port Pause Frames control on the LAN Configuration page of a PTP product. It looks like this:
In a network based on modern switching hardware, it is very rare for the network equipment to be configured with flow control using Pause frames. If your network doesn't use Pause frames then either Tunnel or Discard will do equally well on the PTP unit.
However, wireless links are sometimes used to connect older networks, and in this case it might be useful to review the behaviour of the two options here. The Tunnel option configures the PTP link to take Pause frames received at an Ethernet port at one end of the link and transmit them unchanged at the other end of the link. The Discard option configures the PTP link to simply drop any Pause frames received at an Ethernet port. In either case, the PTP products do not actually generate any additional Pause frames. Pause frames are used to prevent overflow of an input buffer and this never occurs in these PTP products where the hardware-based implementation can process any sequence of Ethernet frames at line speed. Also, the PTP products do not obey Pause frames received at an Ethernet port. This means that any use of flow control using the Tunnel option is strictly end-to-end between the connected networks, with the wireless link operating like a transparent pipe.