The "Communication CPU to Application CPU Interface" (CAIF) is a packet
based connection-oriented MUX protocol developed by ST-Ericsson for use
with its modems. It is accessed from user space as sockets (PF_CAIF).

Say Y (or M) here if you build for a phone product (e.g. Android or
MeeGo ) that uses CAIF as transport, if unsure say N.

If you select to build it as module then CAIF_NETDEV also needs to be
built as modules. You will also need to say yes to any CAIF physical
devices that your platform requires.

See Documentation/networking/caif for a further explanation on how to
use and configure CAIF.

source code:

net/caif/