214 lines
7 KiB
Text
214 lines
7 KiB
Text
Linux Phonet protocol family
|
|
============================
|
|
|
|
Introduction
|
|
------------
|
|
|
|
Phonet is a packet protocol used by Nokia cellular modems for both IPC
|
|
and RPC. With the Linux Phonet socket family, Linux host processes can
|
|
receive and send messages from/to the modem, or any other external
|
|
device attached to the modem. The modem takes care of routing.
|
|
|
|
Phonet packets can be exchanged through various hardware connections
|
|
depending on the device, such as:
|
|
- USB with the CDC Phonet interface,
|
|
- infrared,
|
|
- Bluetooth,
|
|
- an RS232 serial port (with a dedicated "FBUS" line discipline),
|
|
- the SSI bus with some TI OMAP processors.
|
|
|
|
|
|
Packets format
|
|
--------------
|
|
|
|
Phonet packets have a common header as follows:
|
|
|
|
struct phonethdr {
|
|
uint8_t pn_media; /* Media type (link-layer identifier) */
|
|
uint8_t pn_rdev; /* Receiver device ID */
|
|
uint8_t pn_sdev; /* Sender device ID */
|
|
uint8_t pn_res; /* Resource ID or function */
|
|
uint16_t pn_length; /* Big-endian message byte length (minus 6) */
|
|
uint8_t pn_robj; /* Receiver object ID */
|
|
uint8_t pn_sobj; /* Sender object ID */
|
|
};
|
|
|
|
On Linux, the link-layer header includes the pn_media byte (see below).
|
|
The next 7 bytes are part of the network-layer header.
|
|
|
|
The device ID is split: the 6 higher-order bits constitute the device
|
|
address, while the 2 lower-order bits are used for multiplexing, as are
|
|
the 8-bit object identifiers. As such, Phonet can be considered as a
|
|
network layer with 6 bits of address space and 10 bits for transport
|
|
protocol (much like port numbers in IP world).
|
|
|
|
The modem always has address number zero. All other device have a their
|
|
own 6-bit address.
|
|
|
|
|
|
Link layer
|
|
----------
|
|
|
|
Phonet links are always point-to-point links. The link layer header
|
|
consists of a single Phonet media type byte. It uniquely identifies the
|
|
link through which the packet is transmitted, from the modem's
|
|
perspective. Each Phonet network device shall prepend and set the media
|
|
type byte as appropriate. For convenience, a common phonet_header_ops
|
|
link-layer header operations structure is provided. It sets the
|
|
media type according to the network device hardware address.
|
|
|
|
Linux Phonet network interfaces support a dedicated link layer packets
|
|
type (ETH_P_PHONET) which is out of the Ethernet type range. They can
|
|
only send and receive Phonet packets.
|
|
|
|
The virtual TUN tunnel device driver can also be used for Phonet. This
|
|
requires IFF_TUN mode, _without_ the IFF_NO_PI flag. In this case,
|
|
there is no link-layer header, so there is no Phonet media type byte.
|
|
|
|
Note that Phonet interfaces are not allowed to re-order packets, so
|
|
only the (default) Linux FIFO qdisc should be used with them.
|
|
|
|
|
|
Network layer
|
|
-------------
|
|
|
|
The Phonet socket address family maps the Phonet packet header:
|
|
|
|
struct sockaddr_pn {
|
|
sa_family_t spn_family; /* AF_PHONET */
|
|
uint8_t spn_obj; /* Object ID */
|
|
uint8_t spn_dev; /* Device ID */
|
|
uint8_t spn_resource; /* Resource or function */
|
|
uint8_t spn_zero[...]; /* Padding */
|
|
};
|
|
|
|
The resource field is only used when sending and receiving;
|
|
It is ignored by bind() and getsockname().
|
|
|
|
|
|
Low-level datagram protocol
|
|
---------------------------
|
|
|
|
Applications can send Phonet messages using the Phonet datagram socket
|
|
protocol from the PF_PHONET family. Each socket is bound to one of the
|
|
2^10 object IDs available, and can send and receive packets with any
|
|
other peer.
|
|
|
|
struct sockaddr_pn addr = { .spn_family = AF_PHONET, };
|
|
ssize_t len;
|
|
socklen_t addrlen = sizeof(addr);
|
|
int fd;
|
|
|
|
fd = socket(PF_PHONET, SOCK_DGRAM, 0);
|
|
bind(fd, (struct sockaddr *)&addr, sizeof(addr));
|
|
/* ... */
|
|
|
|
sendto(fd, msg, msglen, 0, (struct sockaddr *)&addr, sizeof(addr));
|
|
len = recvfrom(fd, buf, sizeof(buf), 0,
|
|
(struct sockaddr *)&addr, &addrlen);
|
|
|
|
This protocol follows the SOCK_DGRAM connection-less semantics.
|
|
However, connect() and getpeername() are not supported, as they did
|
|
not seem useful with Phonet usages (could be added easily).
|
|
|
|
|
|
Resource subscription
|
|
---------------------
|
|
|
|
A Phonet datagram socket can be subscribed to any number of 8-bits
|
|
Phonet resources, as follow:
|
|
|
|
uint32_t res = 0xXX;
|
|
ioctl(fd, SIOCPNADDRESOURCE, &res);
|
|
|
|
Subscription is similarly cancelled using the SIOCPNDELRESOURCE I/O
|
|
control request, or when the socket is closed.
|
|
|
|
Note that no more than one socket can be subcribed to any given
|
|
resource at a time. If not, ioctl() will return EBUSY.
|
|
|
|
|
|
Phonet Pipe protocol
|
|
--------------------
|
|
|
|
The Phonet Pipe protocol is a simple sequenced packets protocol
|
|
with end-to-end congestion control. It uses the passive listening
|
|
socket paradigm. The listening socket is bound to an unique free object
|
|
ID. Each listening socket can handle up to 255 simultaneous
|
|
connections, one per accept()'d socket.
|
|
|
|
int lfd, cfd;
|
|
|
|
lfd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
|
|
listen (lfd, INT_MAX);
|
|
|
|
/* ... */
|
|
cfd = accept(lfd, NULL, NULL);
|
|
for (;;)
|
|
{
|
|
char buf[...];
|
|
ssize_t len = read(cfd, buf, sizeof(buf));
|
|
|
|
/* ... */
|
|
|
|
write(cfd, msg, msglen);
|
|
}
|
|
|
|
Connections are traditionally established between two endpoints by a
|
|
"third party" application. This means that both endpoints are passive.
|
|
|
|
|
|
As of Linux kernel version 2.6.39, it is also possible to connect
|
|
two endpoints directly, using connect() on the active side. This is
|
|
intended to support the newer Nokia Wireless Modem API, as found in
|
|
e.g. the Nokia Slim Modem in the ST-Ericsson U8500 platform:
|
|
|
|
struct sockaddr_spn spn;
|
|
int fd;
|
|
|
|
fd = socket(PF_PHONET, SOCK_SEQPACKET, PN_PROTO_PIPE);
|
|
memset(&spn, 0, sizeof(spn));
|
|
spn.spn_family = AF_PHONET;
|
|
spn.spn_obj = ...;
|
|
spn.spn_dev = ...;
|
|
spn.spn_resource = 0xD9;
|
|
connect(fd, (struct sockaddr *)&spn, sizeof(spn));
|
|
/* normal I/O here ... */
|
|
close(fd);
|
|
|
|
|
|
WARNING:
|
|
When polling a connected pipe socket for writability, there is an
|
|
intrinsic race condition whereby writability might be lost between the
|
|
polling and the writing system calls. In this case, the socket will
|
|
block until write becomes possible again, unless non-blocking mode
|
|
is enabled.
|
|
|
|
|
|
The pipe protocol provides two socket options at the SOL_PNPIPE level:
|
|
|
|
PNPIPE_ENCAP accepts one integer value (int) of:
|
|
|
|
PNPIPE_ENCAP_NONE: The socket operates normally (default).
|
|
|
|
PNPIPE_ENCAP_IP: The socket is used as a backend for a virtual IP
|
|
interface. This requires CAP_NET_ADMIN capability. GPRS data
|
|
support on Nokia modems can use this. Note that the socket cannot
|
|
be reliably poll()'d or read() from while in this mode.
|
|
|
|
PNPIPE_IFINDEX is a read-only integer value. It contains the
|
|
interface index of the network interface created by PNPIPE_ENCAP,
|
|
or zero if encapsulation is off.
|
|
|
|
PNPIPE_HANDLE is a read-only integer value. It contains the underlying
|
|
identifier ("pipe handle") of the pipe. This is only defined for
|
|
socket descriptors that are already connected or being connected.
|
|
|
|
|
|
Authors
|
|
-------
|
|
|
|
Linux Phonet was initially written by Sakari Ailus.
|
|
Other contributors include Mikä Liljeberg, Andras Domokos,
|
|
Carlos Chinea and Rémi Denis-Courmont.
|
|
Copyright (C) 2008 Nokia Corporation.
|