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diff --git a/Documentation/networking/phonet.txt b/Documentation/networking/phonet.txt new file mode 100644 index 000000000000..0e6e592f4f55 --- /dev/null +++ b/Documentation/networking/phonet.txt @@ -0,0 +1,175 @@ +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 consitute 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). + + +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 established between two endpoints by a "third party" +application. This means that both endpoints are passive; so connect() +is not possible. + +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 because possible again, unless non-blocking mode +becomes 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. + + +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. |