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 Tuning and Troubleshooting Network Cards.