176 lines
5.8 KiB
Plaintext
176 lines
5.8 KiB
Plaintext
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 packets have a common header as follows:
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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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On Linux, the link-layer header includes the pn_media byte (see below).
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The next 7 bytes are part of the network-layer header.
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The device ID is split: the 6 higher-order bits consitute 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-bit 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. All other device have a their
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own 6-bit 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. Each Phonet network device shall prepend and set the media
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type byte as appropriate. For convenience, a common phonet_header_ops
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link-layer header operations structure is provided. It sets the
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media type according to the network device hardware address.
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Linux Phonet network interfaces support a dedicated link layer packets
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type (ETH_P_PHONET) which is out of the Ethernet type range. They can
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only send and receive Phonet packets.
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The virtual TUN tunnel device driver can also be used for Phonet. This
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requires IFF_TUN mode, _without_ the IFF_NO_PI flag. In this case,
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there is no link-layer header, so there is no Phonet media type byte.
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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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Phonet Pipe protocol
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--------------------
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The Phonet Pipe protocol is a simple sequenced packets protocol
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with end-to-end congestion control. It uses the passive listening
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socket paradigm. The listening socket is bound to an unique free object
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ID. Each listening socket can handle up to 255 simultaneous
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connections, one per accept()'d socket.
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int lfd, cfd;
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lfd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
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listen (lfd, INT_MAX);
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/* ... */
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cfd = accept(lfd, NULL, NULL);
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for (;;)
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{
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char buf[...];
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ssize_t len = read(cfd, buf, sizeof(buf));
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/* ... */
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write(cfd, msg, msglen);
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}
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Connections are established between two endpoints by a "third party"
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application. This means that both endpoints are passive; so connect()
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is not possible.
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WARNING:
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When polling a connected pipe socket for writability, there is an
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intrinsic race condition whereby writability might be lost between the
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polling and the writing system calls. In this case, the socket will
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block until write becomes possible again, unless non-blocking mode
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is enabled.
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The pipe protocol provides two socket options at the SOL_PNPIPE level:
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PNPIPE_ENCAP accepts one integer value (int) of:
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PNPIPE_ENCAP_NONE: The socket operates normally (default).
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PNPIPE_ENCAP_IP: The socket is used as a backend for a virtual IP
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interface. This requires CAP_NET_ADMIN capability. GPRS data
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support on Nokia modems can use this. Note that the socket cannot
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be reliably poll()'d or read() from while in this mode.
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PNPIPE_IFINDEX is a read-only integer value. It contains the
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interface index of the network interface created by PNPIPE_ENCAP,
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or zero if encapsulation is off.
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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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