“Bump In The Stack” (BITS) and “Bump In The Wire” (BITW)

Three different architectures or implementation models are defined for IPSec. The best is integrated architecture, where IPSec is built into the IP layer of devices directly. The other two are “Bump In The Stack” (BITS) and “Bump In The Wire” (BITW), which both are ways of layering IPSec underneath regular IP, using software and hardware solutions respectively.

Bump in the Stack: http://www.tcpipguide.com/free/t_IPSecArchitecturesandImplementationMethods-2.htm

Bump in the Wire:http://www.tcpipguide.com/free/t_IPSecArchitecturesandImplementationMethods-3.htm

用在其它地方基本也是这个含义，只是稍微变通一下就好了。