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