1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt>
3
4 ============================
5 Linux Phonet protocol family
6 ============================
7
8 Introduction
9 ------------
10
11 Phonet is a packet protocol used by Nokia cellular modems for both IPC
12 and RPC. With the Linux Phonet socket family, Linux host processes can
13 receive and send messages from/to the modem, or any other external
14 device attached to the modem. The modem takes care of routing.
15
16 Phonet packets can be exchanged through various hardware connections
17 depending on the device, such as:
18
19 - USB with the CDC Phonet interface,
20 - infrared,
21 - Bluetooth,
22 - an RS232 serial port (with a dedicated "FBUS" line discipline),
23 - the SSI bus with some TI OMAP processors.
24
25
26 Packets format
27 --------------
28
29 Phonet packets have a common header as follows::
30
31 struct phonethdr {
32 uint8_t pn_media; /* Media type (link-layer identifier) */
33 uint8_t pn_rdev; /* Receiver device ID */
34 uint8_t pn_sdev; /* Sender device ID */
35 uint8_t pn_res; /* Resource ID or function */
36 uint16_t pn_length; /* Big-endian message byte length (minus 6) */
37 uint8_t pn_robj; /* Receiver object ID */
38 uint8_t pn_sobj; /* Sender object ID */
39 };
40
41 On Linux, the link-layer header includes the pn_media byte (see below).
42 The next 7 bytes are part of the network-layer header.
43
44 The device ID is split: the 6 higher-order bits constitute the device
45 address, while the 2 lower-order bits are used for multiplexing, as are
46 the 8-bit object identifiers. As such, Phonet can be considered as a
47 network layer with 6 bits of address space and 10 bits for transport
48 protocol (much like port numbers in IP world).
49
50 The modem always has address number zero. All other device have a their
51 own 6-bit address.
52
53
54 Link layer
55 ----------
56
57 Phonet links are always point-to-point links. The link layer header
58 consists of a single Phonet media type byte. It uniquely identifies the
59 link through which the packet is transmitted, from the modem's
60 perspective. Each Phonet network device shall prepend and set the media
61 type byte as appropriate. For convenience, a common phonet_header_ops
62 link-layer header operations structure is provided. It sets the
63 media type according to the network device hardware address.
64
65 Linux Phonet network interfaces support a dedicated link layer packets
66 type (ETH_P_PHONET) which is out of the Ethernet type range. They can
67 only send and receive Phonet packets.
68
69 The virtual TUN tunnel device driver can also be used for Phonet. This
70 requires IFF_TUN mode, _without_ the IFF_NO_PI flag. In this case,
71 there is no link-layer header, so there is no Phonet media type byte.
72
73 Note that Phonet interfaces are not allowed to re-order packets, so
74 only the (default) Linux FIFO qdisc should be used with them.
75
76
77 Network layer
78 -------------
79
80 The Phonet socket address family maps the Phonet packet header::
81
82 struct sockaddr_pn {
83 sa_family_t spn_family; /* AF_PHONET */
84 uint8_t spn_obj; /* Object ID */
85 uint8_t spn_dev; /* Device ID */
86 uint8_t spn_resource; /* Resource or function */
87 uint8_t spn_zero[...]; /* Padding */
88 };
89
90 The resource field is only used when sending and receiving;
91 It is ignored by bind() and getsockname().
92
93
94 Low-level datagram protocol
95 ---------------------------
96
97 Applications can send Phonet messages using the Phonet datagram socket
98 protocol from the PF_PHONET family. Each socket is bound to one of the
99 2^10 object IDs available, and can send and receive packets with any
100 other peer.
101
102 ::
103
104 struct sockaddr_pn addr = { .spn_family = AF_PHONET, };
105 ssize_t len;
106 socklen_t addrlen = sizeof(addr);
107 int fd;
108
109 fd = socket(PF_PHONET, SOCK_DGRAM, 0);
110 bind(fd, (struct sockaddr *)&addr, sizeof(addr));
111 /* ... */
112
113 sendto(fd, msg, msglen, 0, (struct sockaddr *)&addr, sizeof(addr));
114 len = recvfrom(fd, buf, sizeof(buf), 0,
115 (struct sockaddr *)&addr, &addrlen);
116
117 This protocol follows the SOCK_DGRAM connection-less semantics.
118 However, connect() and getpeername() are not supported, as they did
119 not seem useful with Phonet usages (could be added easily).
120
121
122 Resource subscription
123 ---------------------
124
125 A Phonet datagram socket can be subscribed to any number of 8-bits
126 Phonet resources, as follow::
127
128 uint32_t res = 0xXX;
129 ioctl(fd, SIOCPNADDRESOURCE, &res);
130
131 Subscription is similarly cancelled using the SIOCPNDELRESOURCE I/O
132 control request, or when the socket is closed.
133
134 Note that no more than one socket can be subscribed to any given
135 resource at a time. If not, ioctl() will return EBUSY.
136
137
138 Phonet Pipe protocol
139 --------------------
140
141 The Phonet Pipe protocol is a simple sequenced packets protocol
142 with end-to-end congestion control. It uses the passive listening
143 socket paradigm. The listening socket is bound to an unique free object
144 ID. Each listening socket can handle up to 255 simultaneous
145 connections, one per accept()'d socket.
146
147 ::
148
149 int lfd, cfd;
150
151 lfd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
152 listen (lfd, INT_MAX);
153
154 /* ... */
155 cfd = accept(lfd, NULL, NULL);
156 for (;;)
157 {
158 char buf[...];
159 ssize_t len = read(cfd, buf, sizeof(buf));
160
161 /* ... */
162
163 write(cfd, msg, msglen);
164 }
165
166 Connections are traditionally established between two endpoints by a
167 "third party" application. This means that both endpoints are passive.
168
169
170 As of Linux kernel version 2.6.39, it is also possible to connect
171 two endpoints directly, using connect() on the active side. This is
172 intended to support the newer Nokia Wireless Modem API, as found in
173 e.g. the Nokia Slim Modem in the ST-Ericsson U8500 platform::
174
175 struct sockaddr_spn spn;
176 int fd;
177
178 fd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
179 memset(&spn, 0, sizeof(spn));
180 spn.spn_family = AF_PHONET;
181 spn.spn_obj = ...;
182 spn.spn_dev = ...;
183 spn.spn_resource = 0xD9;
184 connect(fd, (struct sockaddr *)&spn, sizeof(spn));
185 /* normal I/O here ... */
186 close(fd);
187
188
189 .. Warning:
190
191 When polling a connected pipe socket for writability, there is an
192 intrinsic race condition whereby writability might be lost between the
193 polling and the writing system calls. In this case, the socket will
194 block until write becomes possible again, unless non-blocking mode
195 is enabled.
196
197
198 The pipe protocol provides two socket options at the SOL_PNPIPE level:
199
200 PNPIPE_ENCAP accepts one integer value (int) of:
201
202 PNPIPE_ENCAP_NONE:
203 The socket operates normally (default).
204
205 PNPIPE_ENCAP_IP:
206 The socket is used as a backend for a virtual IP
207 interface. This requires CAP_NET_ADMIN capability. GPRS data
208 support on Nokia modems can use this. Note that the socket cannot
209 be reliably poll()'d or read() from while in this mode.
210
211 PNPIPE_IFINDEX
212 is a read-only integer value. It contains the
213 interface index of the network interface created by PNPIPE_ENCAP,
214 or zero if encapsulation is off.
215
216 PNPIPE_HANDLE
217 is a read-only integer value. It contains the underlying
218 identifier ("pipe handle") of the pipe. This is only defined for
219 socket descriptors that are already connected or being connected.
220
221
222 Authors
223 -------
224
225 Linux Phonet was initially written by Sakari Ailus.
226
227 Other contributors include Mikä Liljeberg, Andras Domokos,
228 Carlos Chinea and Rémi Denis-Courmont.
229
230 Copyright |copy| 2008 Nokia Corporation.
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