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+Linux Phonet protocol family
+============================
+
+Introduction
+------------
+
+Phonet is a packet protocol used by Nokia cellular modems for both IPC
+and RPC. With the Linux Phonet socket family, Linux host processes can
+receive and send messages from/to the modem, or any other external
+device attached to the modem. The modem takes care of routing.
+
+Phonet packets can be exchanged through various hardware connections
+depending on the device, such as:
+ - USB with the CDC Phonet interface,
+ - infrared,
+ - Bluetooth,
+ - an RS232 serial port (with a dedicated "FBUS" line discipline),
+ - the SSI bus with some TI OMAP processors.
+
+
+Packets format
+--------------
+
+Phonet packet have a common header as follow:
+
+ struct phonethdr {
+ uint8_t pn_media; /* Media type (link-layer identifier) */
+ uint8_t pn_rdev; /* Receiver device ID */
+ uint8_t pn_sdev; /* Sender device ID */
+ uint8_t pn_res; /* Resource ID or function */
+ uint16_t pn_length; /* Big-endian message byte length (minus 6) */
+ uint8_t pn_robj; /* Receiver object ID */
+ uint8_t pn_sobj; /* Sender object ID */
+ };
+
+The device ID is split: the 6 higher order bits consitutes the device
+address, while the 2 lower order bits are used for multiplexing, as are
+the 8-bits object identifiers. As such, Phonet can be considered as a
+network layer with 6 bits of address space and 10 bits for transport
+protocol (much like port numbers in IP world).
+
+The modem always has address number zero. Each other device has a its
+own 6-bits address.
+
+
+Link layer
+----------
+
+Phonet links are always point-to-point links. The link layer header
+consists of a single Phonet media type byte. It uniquely identifies the
+link through which the packet is transmitted, from the modem's
+perspective.
+
+Linux Phonet network interfaces use a dedicated link layer type
+(ETH_P_PHONET) which is out of the Ethernet type range. They can only
+send and receive Phonet packets.
+
+Note that Phonet interfaces are not allowed to re-order packets, so
+only the (default) Linux FIFO qdisc should be used with them.
+
+
+Network layer
+-------------
+
+The Phonet socket address family maps the Phonet packet header:
+
+ struct sockaddr_pn {
+ sa_family_t spn_family; /* AF_PHONET */
+ uint8_t spn_obj; /* Object ID */
+ uint8_t spn_dev; /* Device ID */
+ uint8_t spn_resource; /* Resource or function */
+ uint8_t spn_zero[...]; /* Padding */
+ };
+
+The resource field is only used when sending and receiving;
+It is ignored by bind() and getsockname().
+
+
+Low-level datagram protocol
+---------------------------
+
+Applications can send Phonet messages using the Phonet datagram socket
+protocol from the PF_PHONET family. Each socket is bound to one of the
+2^10 object IDs available, and can send and receive packets with any
+other peer.
+
+ struct sockaddr_pn addr = { .spn_family = AF_PHONET, };
+ ssize_t len;
+ socklen_t addrlen = sizeof(addr);
+ int fd;
+
+ fd = socket(PF_PHONET, SOCK_DGRAM, 0);
+ bind(fd, (struct sockaddr *)&addr, sizeof(addr));
+ /* ... */
+
+ sendto(fd, msg, msglen, 0, (struct sockaddr *)&addr, sizeof(addr));
+ len = recvfrom(fd, buf, sizeof(buf), 0,
+ (struct sockaddr *)&addr, &addrlen);
+
+This protocol follows the SOCK_DGRAM connection-less semantics.
+However, connect() and getpeername() are not supported, as they did
+not seem useful with Phonet usages (could be added easily).
+
+
+Authors
+-------
+
+Linux Phonet was initially written by Sakari Ailus.
+Other contributors include Mikä Liljeberg, Andras Domokos,
+Carlos Chinea and Rémi Denis-Courmont.
+Copyright (C) 2008 Nokia Corporation.