The driver just packs IP packets without anything appended or prepended into raw HDLC packets on the B channel and transfers them to a remote site. IP packets received from the remote site are queued into the local IP protocol stack.
The format of the resulting packet on the B channel is:
(HDLC
opening
flag)
(IP-packet)
(CRC)
(HDLC
closing
flag)
In the case where an IP packet for a remote site arrives in the driver and no connection has been established yet, the driver communicates with the isdnd(8) daemon to establish a connection.
The driver has support for interfacing to the bpf(4) subsystem for using tcpdump(8) with the irip interfaces.
The driver optionally (when compiled with the IRIP_VJ option) provides Van Jacobson header compression, under control of the link0 and link1 options to ifconfig(8):
The default values are on for link1 and off for link0.