Documentation/networking/phonet.txt

   1 Linux Phonet protocol family
   2 ============================
   3
   4 Introduction
   5 ------------
   6
   7 Phonet is a packet protocol used by Nokia cellular modems for both IPC
   8 and RPC. With the Linux Phonet socket family, Linux host processes can
   9 receive and send messages from/to the modem, or any other external
  10 device attached to the modem. The modem takes care of routing.
  11
  12 Phonet packets can be exchanged through various hardware connections
  13 depending on the device, such as:
  14   - USB with the CDC Phonet interface,
  15   - infrared,
  16   - Bluetooth,
  17   - an RS232 serial port (with a dedicated "FBUS" line discipline),
  18   - the SSI bus with some TI OMAP processors.
  19
  20
  21 Packets format
  22 --------------
  23
  24 Phonet packets have a common header as follows:
  25
  26   struct phonethdr {
  27     uint8_t  pn_media;  /* Media type (link-layer identifier) */
  28     uint8_t  pn_rdev;   /* Receiver device ID */
  29     uint8_t  pn_sdev;   /* Sender device ID */
  30     uint8_t  pn_res;    /* Resource ID or function */
  31     uint16_t pn_length; /* Big-endian message byte length (minus 6) */
  32     uint8_t  pn_robj;   /* Receiver object ID */
  33     uint8_t  pn_sobj;   /* Sender object ID */
  34   };
  35
  36 On Linux, the link-layer header includes the pn_media byte (see below).
  37 The next 7 bytes are part of the network-layer header.
  38
  39 The device ID is split: the 6 higher-order bits consitute the device
  40 address, while the 2 lower-order bits are used for multiplexing, as are
  41 the 8-bit object identifiers. As such, Phonet can be considered as a
  42 network layer with 6 bits of address space and 10 bits for transport
  43 protocol (much like port numbers in IP world).
  44
  45 The modem always has address number zero. All other device have a their
  46 own 6-bit address.
  47
  48
  49 Link layer
  50 ----------
  51
  52 Phonet links are always point-to-point links. The link layer header
  53 consists of a single Phonet media type byte. It uniquely identifies the
  54 link through which the packet is transmitted, from the modem's
  55 perspective. Each Phonet network device shall prepend and set the media
  56 type byte as appropriate. For convenience, a common phonet_header_ops
  57 link-layer header operations structure is provided. It sets the
  58 media type according to the network device hardware address.
  59
  60 Linux Phonet network interfaces support a dedicated link layer packets
  61 type (ETH_P_PHONET) which is out of the Ethernet type range. They can
  62 only send and receive Phonet packets.
  63
  64 The virtual TUN tunnel device driver can also be used for Phonet. This
  65 requires IFF_TUN mode, _without_ the IFF_NO_PI flag. In this case,
  66 there is no link-layer header, so there is no Phonet media type byte.
  67
  68 Note that Phonet interfaces are not allowed to re-order packets, so
  69 only the (default) Linux FIFO qdisc should be used with them.
  70
  71
  72 Network layer
  73 -------------
  74
  75 The Phonet socket address family maps the Phonet packet header:
  76
  77   struct sockaddr_pn {
  78     sa_family_t spn_family;    /* AF_PHONET */
  79     uint8_t     spn_obj;       /* Object ID */
  80     uint8_t     spn_dev;       /* Device ID */
  81     uint8_t     spn_resource;  /* Resource or function */
  82     uint8_t     spn_zero[...]; /* Padding */
  83   };
  84
  85 The resource field is only used when sending and receiving;
  86 It is ignored by bind() and getsockname().
  87
  88
  89 Low-level datagram protocol
  90 ---------------------------
  91
  92 Applications can send Phonet messages using the Phonet datagram socket
  93 protocol from the PF_PHONET family. Each socket is bound to one of the
  94 2^10 object IDs available, and can send and receive packets with any
  95 other peer.
  96
  97   struct sockaddr_pn addr = { .spn_family = AF_PHONET, };
  98   ssize_t len;
  99   socklen_t addrlen = sizeof(addr);
 100   int fd;
 101
 102   fd = socket(PF_PHONET, SOCK_DGRAM, 0);
 103   bind(fd, (struct sockaddr *)&addr, sizeof(addr));
 104   /* ... */
 105
 106   sendto(fd, msg, msglen, 0, (struct sockaddr *)&addr, sizeof(addr));
 107   len = recvfrom(fd, buf, sizeof(buf), 0,
 108                  (struct sockaddr *)&addr, &addrlen);
 109
 110 This protocol follows the SOCK_DGRAM connection-less semantics.
 111 However, connect() and getpeername() are not supported, as they did
 112 not seem useful with Phonet usages (could be added easily).
 113
 114
 115 Phonet Pipe protocol
 116 --------------------
 117
 118 The Phonet Pipe protocol is a simple sequenced packets protocol
 119 with end-to-end congestion control. It uses the passive listening
 120 socket paradigm. The listening socket is bound to an unique free object
 121 ID. Each listening socket can handle up to 255 simultaneous
 122 connections, one per accept()'d socket.
 123
 124   int lfd, cfd;
 125
 126   lfd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
 127   listen (lfd, INT_MAX);
 128
 129   /* ... */
 130   cfd = accept(lfd, NULL, NULL);
 131   for (;;)
 132   {
 133     char buf[...];
 134     ssize_t len = read(cfd, buf, sizeof(buf));
 135
 136     /* ... */
 137
 138     write(cfd, msg, msglen);
 139   }
 140
 141 Connections are established between two endpoints by a "third party"
 142 application. This means that both endpoints are passive; so connect()
 143 is not possible.
 144
 145 WARNING:
 146 When polling a connected pipe socket for writability, there is an
 147 intrinsic race condition whereby writability might be lost between the
 148 polling and the writing system calls. In this case, the socket will
 149 block until write because possible again, unless non-blocking mode
 150 becomes enabled.
 151
 152
 153 The pipe protocol provides two socket options at the SOL_PNPIPE level:
 154
 155   PNPIPE_ENCAP accepts one integer value (int) of:
 156
 157     PNPIPE_ENCAP_NONE: The socket operates normally (default).
 158
 159     PNPIPE_ENCAP_IP: The socket is used as a backend for a virtual IP
 160       interface. This requires CAP_NET_ADMIN capability. GPRS data
 161       support on Nokia modems can use this. Note that the socket cannot
 162       be reliably poll()'d or read() from while in this mode.
 163
 164   PNPIPE_IFINDEX is a read-only integer value. It contains the
 165     interface index of the network interface created by PNPIPE_ENCAP,
 166     or zero if encapsulation is off.
 167
 168
 169 Authors
 170 -------
 171
 172 Linux Phonet was initially written by Sakari Ailus.
 173 Other contributors include Mikä Liljeberg, Andras Domokos,
 174 Carlos Chinea and Rémi Denis-Courmont.
 175 Copyright (C) 2008 Nokia Corporation.