tree ebd7f75cbb41c7b709bd20fa3a64edc07bb8b057
parent 65546237d0768d3ecd17c8de4ffd4447bf39ad4b
author QUICHE team <quiche-dev@google.com> 1782843923 -0700
committer Copybara-Service <copybara-worker@google.com> 1782843963 -0700

QBONE Bonnet: use `writev` to eliminate copies when adding eth headers.

In TAP mode, Bonnet needs to take L3 packets decapped from the QUIC connection and deliver them as L2 packets to the kernel.  It does this by prepending an ethernet header to the packet write.

For bonnet, this header is a fixed dummy header with the MAC of the device as src/dest and proto IPv6 and so can be generated once.  Additionally, instead of copying the payload into a new buffer, we can use `writev` to prepend the fixed header when present.

Most bonnet clients do not use TAP mode, but Borneo is using TAP mode to make passing the device into their TIVM easier.

Removal of copies from the read path occurred in cl/934614328.

PiperOrigin-RevId: 940562139