1Linux Phonet protocol family 2============================ 3 4Introduction 5------------ 6 7Phonet is a packet protocol used by Nokia cellular modems for both IPC 8and RPC. With the Linux Phonet socket family, Linux host processes can 9receive and send messages from/to the modem, or any other external 10device attached to the modem. The modem takes care of routing. 11 12Phonet packets can be exchanged through various hardware connections 13depending 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 21Packets format 22-------------- 23 24Phonet 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 36On Linux, the link-layer header includes the pn_media byte (see below). 37The next 7 bytes are part of the network-layer header. 38 39The device ID is split: the 6 higher-order bits constitute the device 40address, while the 2 lower-order bits are used for multiplexing, as are 41the 8-bit object identifiers. As such, Phonet can be considered as a 42network layer with 6 bits of address space and 10 bits for transport 43protocol (much like port numbers in IP world). 44 45The modem always has address number zero. All other device have a their 46own 6-bit address. 47 48 49Link layer 50---------- 51 52Phonet links are always point-to-point links. The link layer header 53consists of a single Phonet media type byte. It uniquely identifies the 54link through which the packet is transmitted, from the modem's 55perspective. Each Phonet network device shall prepend and set the media 56type byte as appropriate. For convenience, a common phonet_header_ops 57link-layer header operations structure is provided. It sets the 58media type according to the network device hardware address. 59 60Linux Phonet network interfaces support a dedicated link layer packets 61type (ETH_P_PHONET) which is out of the Ethernet type range. They can 62only send and receive Phonet packets. 63 64The virtual TUN tunnel device driver can also be used for Phonet. This 65requires IFF_TUN mode, _without_ the IFF_NO_PI flag. In this case, 66there is no link-layer header, so there is no Phonet media type byte. 67 68Note that Phonet interfaces are not allowed to re-order packets, so 69only the (default) Linux FIFO qdisc should be used with them. 70 71 72Network layer 73------------- 74 75The 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 85The resource field is only used when sending and receiving; 86It is ignored by bind() and getsockname(). 87 88 89Low-level datagram protocol 90--------------------------- 91 92Applications can send Phonet messages using the Phonet datagram socket 93protocol from the PF_PHONET family. Each socket is bound to one of the 942^10 object IDs available, and can send and receive packets with any 95other 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 110This protocol follows the SOCK_DGRAM connection-less semantics. 111However, connect() and getpeername() are not supported, as they did 112not seem useful with Phonet usages (could be added easily). 113 114 115Resource subscription 116--------------------- 117 118A Phonet datagram socket can be subscribed to any number of 8-bits 119Phonet resources, as follow: 120 121 uint32_t res = 0xXX; 122 ioctl(fd, SIOCPNADDRESOURCE, &res); 123 124Subscription is similarly cancelled using the SIOCPNDELRESOURCE I/O 125control request, or when the socket is closed. 126 127Note that no more than one socket can be subcribed to any given 128resource at a time. If not, ioctl() will return EBUSY. 129 130 131Phonet Pipe protocol 132-------------------- 133 134The Phonet Pipe protocol is a simple sequenced packets protocol 135with end-to-end congestion control. It uses the passive listening 136socket paradigm. The listening socket is bound to an unique free object 137ID. Each listening socket can handle up to 255 simultaneous 138connections, 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 157Connections are traditionally established between two endpoints by a 158"third party" application. This means that both endpoints are passive. 159 160 161As of Linux kernel version 2.6.39, it is also possible to connect 162two endpoints directly, using connect() on the active side. This is 163intended to support the newer Nokia Wireless Modem API, as found in 164e.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 180WARNING: 181When polling a connected pipe socket for writability, there is an 182intrinsic race condition whereby writability might be lost between the 183polling and the writing system calls. In this case, the socket will 184block until write becomes possible again, unless non-blocking mode 185is enabled. 186 187 188The 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 208Authors 209------- 210 211Linux Phonet was initially written by Sakari Ailus. 212Other contributors include Mikä Liljeberg, Andras Domokos, 213Carlos Chinea and Rémi Denis-Courmont. 214Copyright (C) 2008 Nokia Corporation. 215