Tuning Network Memory Buffers (mbufs)
From the FreeBSD MBUF(9) man page:
An mbuf is a basic unit of memory management in the kernel IPC subsystem. Network packets and socket buffers are stored in mbufs. A network packet may span multiple mbufs arranged into a mbuf chain (linked list), which allows adding or trimming network headers with little overhead.
An mbuf consists of a variable-sized header and a small internal buffer for data.
For more details, read Network Buffer Allocation in the FreeBSD Operating System [May 2004].
If there are concerns about mbuf usage on a firewall, see :doc:`/hardware/tuning-and-troubleshooting-network-cards`.