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Remi Denis-Courmont953f5512008-09-22 20:09:46 -07001Linux 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
Rémi Denis-Courmontac2dc8c2008-09-30 02:52:01 -070024Phonet packets have a common header as follows:
Remi Denis-Courmont953f5512008-09-22 20:09:46 -070025
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
Rémi Denis-Courmontac2dc8c2008-09-30 02:52:01 -070036On 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 consitute 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
Remi Denis-Courmont953f5512008-09-22 20:09:46 -070042network layer with 6 bits of address space and 10 bits for transport
43protocol (much like port numbers in IP world).
44
Rémi Denis-Courmontac2dc8c2008-09-30 02:52:01 -070045The modem always has address number zero. All other device have a their
46own 6-bit address.
Remi Denis-Courmont953f5512008-09-22 20:09:46 -070047
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
Rémi Denis-Courmontac2dc8c2008-09-30 02:52:01 -070055perspective. 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.
Remi Denis-Courmont953f5512008-09-22 20:09:46 -070059
Rémi Denis-Courmontac2dc8c2008-09-30 02:52:01 -070060Linux 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.
Remi Denis-Courmont953f5512008-09-22 20:09:46 -070067
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
Rémi Denis-Courmont95430c02008-10-05 11:16:36 -0700115Phonet Pipe protocol
116--------------------
117
118The Phonet Pipe protocol is a simple sequenced packets protocol
119with end-to-end congestion control. It uses the passive listening
120socket paradigm. The listening socket is bound to an unique free object
121ID. Each listening socket can handle up to 255 simultaneous
122connections, 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
141Connections are established between two endpoints by a "third party"
142application. This means that both endpoints are passive; so connect()
143is not possible.
144
145WARNING:
146When polling a connected pipe socket for writability, there is an
147intrinsic race condition whereby writability might be lost between the
148polling and the writing system calls. In this case, the socket will
Randy Macleod3497b2f2008-10-14 13:49:38 -0700149block until write becomes possible again, unless non-blocking mode
150is enabled.
Rémi Denis-Courmont95430c02008-10-05 11:16:36 -0700151
152
153The 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
Remi Denis-Courmont953f5512008-09-22 20:09:46 -0700169Authors
170-------
171
172Linux Phonet was initially written by Sakari Ailus.
173Other contributors include Mikä Liljeberg, Andras Domokos,
174Carlos Chinea and Rémi Denis-Courmont.
175Copyright (C) 2008 Nokia Corporation.