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 constitute 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 Resource subscription
 116 ---------------------
 117
 118 A Phonet datagram socket can be subscribed to any number of 8-bits
 119 Phonet resources, as follow:
 120
 121   uint32_t res = 0xXX;
 122   ioctl(fd, SIOCPNADDRESOURCE, &res);
 123
 124 Subscription is similarly cancelled using the SIOCPNDELRESOURCE I/O
 125 control request, or when the socket is closed.
 126
 127 Note that no more than one socket can be subcribed to any given
 128 resource at a time. If not, ioctl() will return EBUSY.
 129
 130
 131 Phonet Pipe protocol
 132 --------------------
 133
 134 The Phonet Pipe protocol is a simple sequenced packets protocol
 135 with end-to-end congestion control. It uses the passive listening
 136 socket paradigm. The listening socket is bound to an unique free object
 137 ID. Each listening socket can handle up to 255 simultaneous
 138 connections, one per accept()'d socket.
 139
 140   int lfd, cfd;
 141
 142   lfd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
 143   listen (lfd, INT_MAX);
 144
 145   /* ... */
 146   cfd = accept(lfd, NULL, NULL);
 147   for (;;)
 148   {
 149     char buf[...];
 150     ssize_t len = read(cfd, buf, sizeof(buf));
 151
 152     /* ... */
 153
 154     write(cfd, msg, msglen);
 155   }
 156
 157 Connections are traditionally established between two endpoints by a
 158 "third party" application. This means that both endpoints are passive.
 159
 160
 161 As of Linux kernel version 2.6.39, it is also possible to connect
 162 two endpoints directly, using connect() on the active side. This is
 163 intended to support the newer Nokia Wireless Modem API, as found in
 164 e.g. the Nokia Slim Modem in the ST-Ericsson U8500 platform:
 165
 166   struct sockaddr_spn spn;
 167   int fd;
 168
 169   fd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
 170   memset(&spn, 0, sizeof(spn));
 171   spn.spn_family = AF_PHONET;
 172   spn.spn_obj = ...;
 173   spn.spn_dev = ...;
 174   spn.spn_resource = 0xD9;
 175   connect(fd, (struct sockaddr *)&spn, sizeof(spn));
 176   /* normal I/O here ... */
 177   close(fd);
 178
 179
 180 WARNING:
 181 When polling a connected pipe socket for writability, there is an
 182 intrinsic race condition whereby writability might be lost between the
 183 polling and the writing system calls. In this case, the socket will
 184 block until write becomes possible again, unless non-blocking mode
 185 is enabled.
 186
 187
 188 The pipe protocol provides two socket options at the SOL_PNPIPE level:
 189
 190   PNPIPE_ENCAP accepts one integer value (int) of:
 191
 192     PNPIPE_ENCAP_NONE: The socket operates normally (default).
 193
 194     PNPIPE_ENCAP_IP: The socket is used as a backend for a virtual IP
 195       interface. This requires CAP_NET_ADMIN capability. GPRS data
 196       support on Nokia modems can use this. Note that the socket cannot
 197       be reliably poll()'d or read() from while in this mode.
 198
 199   PNPIPE_IFINDEX is a read-only integer value. It contains the
 200     interface index of the network interface created by PNPIPE_ENCAP,
 201     or zero if encapsulation is off.
 202
 203   PNPIPE_HANDLE is a read-only integer value. It contains the underlying
 204     identifier ("pipe handle") of the pipe. This is only defined for
 205     socket descriptors that are already connected or being connected.
 206
 207
 208 Authors
 209 -------
 210
 211 Linux Phonet was initially written by Sakari Ailus.
 212 Other contributors include Mikä Liljeberg, Andras Domokos,
 213 Carlos Chinea and Rémi Denis-Courmont.
 214 Copyright (C) 2008 Nokia Corporation.