The
strip driver takes outbound network packets, encapsulates them using the Metricom “star mode” framing, and sends the packets out an RS-232 interface to a Metricom
Ricochet packet radio. Packets arriving from the packet radio via the serial link are decapsulated and then passed up to the local host's networking stack.
strip is an acronym for
STarmode
Radio
IP.
The
strip interfaces can be created by using the
ifconfig(8) create command. Each
strip interface is a pseudo-device driver for the Metricom
Ricochet packet radio, operating in peer-to-peer packet mode.
In many ways, the
strip driver is very much like the
sl(4) SLIP pseudo-device driver. A
strip device is attached to a tty line with
slattach(8). Once attached, the interface is configured via
ifconfig(8). The major difference between the
sl(4) SLIP pseudo-device driver and the
strip driver is that SLIP works only between two hosts over a dedicated point-to-point connection.
In contrast,
strip sends packets to a frequency-hopping packet radio, which can address packets to any peer Metricom
Ricochet packet radio, rather than just to a single host at the other end of a point-to-point line. Thus, one
strip pseudo-device is usually sufficient for any kernel.
In other respects, a
strip interface is rather like an Ethernet interface. Packets are individually addressed, and subsequent packets can be sent independently to different MAC addresses. However, the “star mode” framing and MAC addressing are not in any way compatible with Ethernet. Broadcast or multicast to more than one packet radio is not possible, due to the independent frequency-hopping operation of the packet radios. The interface flags
IFF_POINTOPOINT and
IFF_BROADCAST are not supported on the
strip interface.
In other words,
strip implements a multiple-access, non-broadcast device, accessed via an RS-232 serial line, using a proprietary packet framing scheme.
This version of the
strip driver maps IP addresses to Metricom
Ricochet packet radio addresses using statically configured entries in the normal routing table. These entries map IP addresses of peer packet radios to the MAC-level addresses. The exact syntax of this mapping and an example are discussed below. The Internet Assigned Numbers Authority (IANA) has allocated an ARP type code for use with STRIP. A future version of this driver will support
arp(4) to obtain the IP address of reachable peer packet radios dynamically.