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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 packets have a common header as follows:
+
+ 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 */
+ };
+
+On Linux, the link-layer header includes the pn_media byte (see below).
+The next 7 bytes are part of the network-layer header.
+
+The device ID is split: the 6 higher-order bits constitute the device
+address, while the 2 lower-order bits are used for multiplexing, as are
+the 8-bit 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. All other device have a their
+own 6-bit 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. Each Phonet network device shall prepend and set the media
+type byte as appropriate. For convenience, a common phonet_header_ops
+link-layer header operations structure is provided. It sets the
+media type according to the network device hardware address.
+
+Linux Phonet network interfaces support a dedicated link layer packets
+type (ETH_P_PHONET) which is out of the Ethernet type range. They can
+only send and receive Phonet packets.
+
+The virtual TUN tunnel device driver can also be used for Phonet. This
+requires IFF_TUN mode, _without_ the IFF_NO_PI flag. In this case,
+there is no link-layer header, so there is no Phonet media type byte.
+
+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).
+
+
+Resource subscription
+---------------------
+
+A Phonet datagram socket can be subscribed to any number of 8-bits
+Phonet resources, as follow:
+
+ uint32_t res = 0xXX;
+ ioctl(fd, SIOCPNADDRESOURCE, &res);
+
+Subscription is similarly cancelled using the SIOCPNDELRESOURCE I/O
+control request, or when the socket is closed.
+
+Note that no more than one socket can be subcribed to any given
+resource at a time. If not, ioctl() will return EBUSY.
+
+
+Phonet Pipe protocol
+--------------------
+
+The Phonet Pipe protocol is a simple sequenced packets protocol
+with end-to-end congestion control. It uses the passive listening
+socket paradigm. The listening socket is bound to an unique free object
+ID. Each listening socket can handle up to 255 simultaneous
+connections, one per accept()'d socket.
+
+ int lfd, cfd;
+
+ lfd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
+ listen (lfd, INT_MAX);
+
+ /* ... */
+ cfd = accept(lfd, NULL, NULL);
+ for (;;)
+ {
+ char buf[...];
+ ssize_t len = read(cfd, buf, sizeof(buf));
+
+ /* ... */
+
+ write(cfd, msg, msglen);
+ }
+
+Connections are traditionally established between two endpoints by a
+"third party" application. This means that both endpoints are passive.
+
+
+As of Linux kernel version 2.6.39, it is also possible to connect
+two endpoints directly, using connect() on the active side. This is
+intended to support the newer Nokia Wireless Modem API, as found in
+e.g. the Nokia Slim Modem in the ST-Ericsson U8500 platform:
+
+ struct sockaddr_spn spn;
+ int fd;
+
+ fd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
+ memset(&spn, 0, sizeof(spn));
+ spn.spn_family = AF_PHONET;
+ spn.spn_obj = ...;
+ spn.spn_dev = ...;
+ spn.spn_resource = 0xD9;
+ connect(fd, (struct sockaddr *)&spn, sizeof(spn));
+ /* normal I/O here ... */
+ close(fd);
+
+
+WARNING:
+When polling a connected pipe socket for writability, there is an
+intrinsic race condition whereby writability might be lost between the
+polling and the writing system calls. In this case, the socket will
+block until write becomes possible again, unless non-blocking mode
+is enabled.
+
+
+The pipe protocol provides two socket options at the SOL_PNPIPE level:
+
+ PNPIPE_ENCAP accepts one integer value (int) of:
+
+ PNPIPE_ENCAP_NONE: The socket operates normally (default).
+
+ PNPIPE_ENCAP_IP: The socket is used as a backend for a virtual IP
+ interface. This requires CAP_NET_ADMIN capability. GPRS data
+ support on Nokia modems can use this. Note that the socket cannot
+ be reliably poll()'d or read() from while in this mode.
+
+ PNPIPE_IFINDEX is a read-only integer value. It contains the
+ interface index of the network interface created by PNPIPE_ENCAP,
+ or zero if encapsulation is off.
+
+ PNPIPE_HANDLE is a read-only integer value. It contains the underlying
+ identifier ("pipe handle") of the pipe. This is only defined for
+ socket descriptors that are already connected or being connected.
+
+
+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.