Documentation/networking/phonet.rst

Source file repositories/reference/linux-study-clean/Documentation/networking/phonet.rst

File Facts

System
Linux kernel
Corpus path
Documentation/networking/phonet.rst
Extension
.rst
Size
7326 bytes
Lines
231
Domain
Support Tooling And Documentation
Bucket
Documentation
Inferred role
Support Tooling And Documentation: documentation
Status
atlas-only

Why This File Exists

Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

Dependency Surface

Detected Declarations

Annotated Snippet

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.

Annotation

Implementation Notes