path: root/usb-redirection-protocol.txt

USB Network Redirection protocol description version 0.2 (unreleased/wip)

Revisions
---------

Version 0.1
-Initial version (released as initial RFC without a version number)

Version 0.2
-Remove usb_redir_report_descriptor packet, as it is not possible to get
 the cached descriptors from the OS on all platforms and we can do without
-Replace vm-host with usb-guest
-Replace the synchroneous / asynchroneous commands nomenclature with
 control / data packets
-Move the packet id to the main packet header shared by all packets
-Add note: "All integers in the protocol are send over the pipe in least
 significant byte first order."
-Add note: "All structs are packed"
-s/data_size/length/
-Add a usb_redir_cancel_data_packet packet
-Add usb_redir_reset and usb_redir_reset_status packets

Version 0.3
-Add a usb_redir_ep_info packet
-Report usb_redir_stall as iso status error to indicate a stream stop
-Add a usb_redir_device_info packet


USB redirerection protocol (draft)
----------------------------------

The protocol described in this document is meant for tunneling usb transfers
to a single usb device. Note: not an entire hub, only a single device.

The most significant use case for this is taking a usb device attached to
some machine "a" which acts as a client / viewer to a virtual machine "v"
hosted on another machine "b", and make the usb device show up inside the
virtual machine as if it were attached directly to the virtual machine "v".

The described protocol assumes a reliable ordered bidirectional transport is
available, for example a tcp socket. All integers in the protocol are send
over the pipe in least significant byte first order. All structs send over
the pipe are packed (no padding).

Definitions:
usb-device: The usb-device whose usb transfers are being tunneled.
usb-guest: The entity connecting to the usb-device and using it as if
    connected directly to it. For example a virtual machine running a guest
    os which accesses a usb-device over the network as if it is part of the
    virtual machine.
usb-host: The entity making the usb-device available for use by a usb-guest.
    For example a deamon on a machine which "exports" the usb-device over the
    network which then "appears" inside a virtual machine on another machine.


Basic packet structure / communication
--------------------------------------

Each packet exchanged between the usb-guest and the usb-host starts with a
usb_redir_header, followed by an optional packet type specific header
follow by optional additional data.

The usb_redir_header each packet starts with looks as follows:

struct usb_redir_header {
    uint32_t type;
    uint32_t length;
    uint32_t id;
}

type:    This identifies the type of packet, from the type enum
length:  Length of the optional type specific packet header + the optional
         additional data. Can be 0.
id:      A unique id, generated by the usb-guest when sending a packet,
         the usb-host will use the same id in its response packet, allowing
         the usb-guest to match responses to its original requests.

There are 2 types of packets:

1) control packets
2) data packets

Control packets are handled synchroneously inside the usb-host, it will hand
the request over to the host os and then *wait* for a response. The usb-host
will thus stop processing further packets. Where as for data packets the
usb-host hands them over to the host os with the request to let the usb-host
process know when there is a respone from the usb-device.

Note that control packets should only be send to the usb-host when no data
packets are pending on the device / interface / endpoint affected by the
control packet. Any pending data packets will get dropped, and any active
iso streams / allocated bulk streams will get stopped / free-ed.


Packet type list
----------------

control packets:
usb_redir_hello
usb_redir_device_info
usb_redir_ep_info
usb_redir_reset
usb_redir_reset_status
usb_redir_set_configuration
usb_redir_get_configuration
usb_redir_configuration_status
usb_redir_set_alt_setting
usb_redir_get_alt_setting
usb_redir_alt_setting_status
usb_redir_start_iso_stream
usb_redir_stop_iso_stream
usb_redir_iso_stream_status
usb_redir_alloc_bulk_streams
usb_redir_free_bulk_streams
usb_redir_bulk_streams_status
usb_redir_cancel_data_packet

data packets:
usb_redir_control_packet
usb_redir_bulk_packet
usb_redir_iso_packet


usb_redir_hello
---------------

usb_redir_header.type:    usb_redir_hello
usb_redir_header.length:  <see description>
usb_redir_header.id:      0 (always as this is an unsolicited packet)

struct usb_redir_hello_header {
    char     version[64];
    uint32_t capabilities[0];
}

No packet type specific additional data.

A packet of this type is send by both sides as soon as a connection is
establised. This packet contains:
version:       A free form 0 terminated version string, useful for logging
               should not be parsed! Suggested format: "qemu 0.13",
               "usb-redir-daemon 0.1", etc.
capabilities:  A variable length array for announcing capabilities.

The value of the length field depends on the size of the capabilities array.
If we cross the 32 capabilities count, it will go from 1 uint32_t to 2,
etc. the value is 64 + capabilities-array-size * sizeof(uint32_t).

Currently the following capabilities are defined:
usb_redir_cap_bulk_streams: USB 3 bulk streams are supported

usb_redir_device_info
---------------------

usb_redir_header.type:    usb_redir_device_info
usb_redir_header.length:  sizeof(usb_redir_device_info_header)
usb_redir_header.id:      0 (always as this is an unsolicited packet)

enum {
    usb_redir_speed_low,
    usb_redir_speed_full,
    usb_redir_speed_high,
    usb_redir_speed_super,
    usb_redir_speed_unknown = 255
}

struct usb_redir_device_info_header {
    uint8_t speed;
}

No packet type specific additional data.

This packet gets send by the usb-host to let the usb-guest know the speed
of the device. It gets send directly after the hello message (so also as
soon as a connection gets established).

usb_redir_ep_info
-----------------

usb_redir_header.type:    usb_redir_ep_info
usb_redir_header.length:  sizeof(usb_redir_ep_info_header)
usb_redir_header.id:      0 (always as this is an unsolicited packet)

enum {
    /* Note these 4 match the usb spec! */
    usb_redir_type_control,
    usb_redir_type_iso,
    usb_redir_type_bulk,
    usb_redir_type_interrupt,
    usb_redir_type_invalid = 255
}

struct usb_redir_ep_info_header {
    uint8_t type[32];
    uint8_t interval[32];
    uint8_t interface[32];
}

No packet type specific additional data.

This packet gets send by the usb-host to let the usb-guest know the endpoint
type, interval and interface it belongs to for all possible endpoints,
first 0-15 out, then 0-15 in. This gets send after a (successful) initial
connection, reset, set_config and set_alt_setting.

usb_redir_reset
---------------

usb_redir_header.type:    usb_redir_reset
usb_redir_header.length:  0

No packet type specific header.

No packet type specific additional data.

This packet can be send by the usb-guest to cause a reset of the usb
device.

usb_redir_reset_status
----------------------

usb_redir_header.type:    usb_redir_reset_status
usb_redir_header.length:  sizeof(usb_redir_reset_status_header)

struct usb_redir_reser_status_header {
    uint8_t status;
}

No packet type specific additional data.

This packet is send by the usb-host in response to a usb_redir_reset packet.
Note that an error status means that the usb-host was unable to re-connect
to the device after the reset!


usb_redir_set_configuration
---------------------------

usb_redir_header.type:    usb_redir_set_configuration
usb_redir_header.length:  sizeof(usb_redir_set_configuration_header)

struct usb_redir_set_configuration_header {
    uint8_t configuration;
}

No packet type specific additional data.

This packet can be send by the usb-guest to set (change) the active
configuration of the usb-device.

usb_redir_get_configuration
---------------------------

usb_redir_header.type:    usb_redir_get_configuration
usb_redir_header.length:  0

No packet type specific header.

No packet type specific additional data.

This packet can be send by the usb-guest to get (query) the active
configuration of the usb-device.

usb_redir_configuration_status
------------------------------

usb_redir_header.type:    usb_redir_configuration_status
usb_redir_header.length:  sizeof(usb_redir_configuration_status_header)

struct usb_redir_configuration_status_header {
    uint8_t status;
    uint8_t configuration;
}

No packet type specific additional data.

This is send by the usb-host in response to a usb_redir_set_configuration /
usb_redir_get_configuration packet. It reports a status code and on success
the resulting / active configuration.


usb_redir_set_alt_setting
-------------------------

usb_redir_header.type:    usb_redir_set_alt_setting
usb_redir_header.length:  sizeof(usb_redir_set_alt_setting_header)

struct usb_redir_set_alt_setting_header {
    uint8_t interface;
    uint8_t alt;
}

No packet type specific additional data.

This packet can be send by the usb-guest to set (change) the alt_setting of
interface <interface> to <alt>.

usb_redir_get_alt_setting
---------------------------

usb_redir_header.type:    usb_redir_get_alt_setting
usb_redir_header.length:  sizeof(usb_redir_get_alt_setting_header)

struct usb_redir_get_alt_setting_header {
    uint8_t interface;
}

No packet type specific additional data.

This packet can be send by the usb-guest to get (query) the active
alt_setting of an interface of the usb-device.

usb_redir_alt_setting_status
----------------------------

usb_redir_header.type:    usb_redir_alt_setting_status
usb_redir_header.length:  sizeof(usb_redir_alt_setting_status_header)

struct usb_redir_alt_setting_status_header {
    uint8_t status;
    uint8_t interface;
    uint8_t alt;
}

No packet type specific additional data.

This is send by the usb-host in response to a usb_redir_set_alt_setting /
usb_redir_get_alt_setting packet. It reports a status code, the affected
interface and on success the resulting / active alt_setting for that interface.


usb_redir_start_iso_stream
--------------------------

usb_redir_header.type:    usb_redir_start_iso_stream
usb_redir_header.length:  sizeof(usb_redir_start_iso_stream_header)

struct usb_redir_start_iso_stream_header {
    uint8_t endpoint;
    uint8_t pkts_per_urb;
    uint8_t no_urbs;
}

No packet type specific additional data.

This packet can be send by the usb-guest to start a iso stream on the
designated endpoint of the usb-device.

This function allocates no_urbs urbs with pkts_per_urb iso packets/frames
per urb. For iso input endpoints these urbs will get submitted to the
device *immediately*, for iso output endpoints the usb-host will wait till
it has received (pkts_per_urb * no_urbs / 2) packets to fill its buffers,
before submitting the first urb.

usb_redir_stop_iso_stream
-------------------------

usb_redir_header.type:    usb_redir_stop_iso_stream
usb_redir_header.length:  sizeof(struct usb_redir_start_iso_stream_header)

struct usb_redir_stop_iso_stream_header {
    uint8_t endpoint;
};

No packet type specific additional data.

This packet can be send by the usb-guest to stop an iso stream on the
designated endpoint. This will cancel all pending urbs, flush the usb-host's
buffers and free all relevant resources. Note that the usb-guest can still
receive isoc data packets from an isoc in endpoint after sending this, as
some data packets may already be inside the transport pipe.

usb_redir_iso_stream_status
---------------------------

usb_redir_header.type:    usb_redir_iso_stream_status
usb_redir_header.length:  sizeof(usb_redir_iso_stream_status_header)

struct usb_redir_iso_stream_status_header {
    uint8_t status;
    uint8_t endpoint;
}

No packet type specific additional data.

This packet is send by the usb-host in response to a
usb_redir_start_iso_stream or usb_redir_stop_iso_stream packet. Note that
for the starting of output iso streams a success status only indicates that
all the buffers were successfully allocated, the actual stream is not
started until enough packets are buffered.

Note that this can also be send unsolicited by a usb-host in case of an
error with an iso output stream, see usb_redir_iso_packet.

To allow the usb-guest to detect if the stream was adversely stopped, the
usb-host will always report usb_redir_stall as status if the stream was
stopped for any reason other then an usb_redir_stop_iso_stream.


usb_redir_alloc_bulk_streams
----------------------------

usb_redir_header.type:    usb_redir_alloc_bulk_streams
usb_redir_header.length:  sizeof(usb_redir_alloc_bulk_streams_header)

struct usb_redir_alloc_bulk_streams_header {
    uint8_t endpoint;
    uint8_t no_streams;
}

No packet type specific additional data.

This packet can be send by the usb-guest to the usb-host to request
that the usb-host allocates IDs so the usb-guest can use up to no_streams
stream IDs.

usb_redir_free_bulk_streams
----------------------------

usb_redir_header.type:    usb_redir_free_bulk_streams
usb_redir_header.length:  sizeof(usb_redir_free_bulk_streams_header)

struct usb_redir_free_bulk_streams_header {
    uint8_t endpoint;
}

No packet type specific additional data.

This packet can be send by the usb-guest to the usb-host to free any
bulk streams previouisly allocated on the endpoint.

usb_redir_bulk_streams_status
-----------------------------

usb_redir_header.type:    usb_redir_bulk_streams_status
usb_redir_header.length:  sizeof(usb_redir_bulk_streams_status_header)

struct usb_redir_bulk_streams_status_header {
    uint8_t status;
    uint8_t endpoint;
    uint8_t no_streams;
}

No packet type specific additional data.

This packet is send by the usb-host in response to a
usb_redir_alloc_bulk_streams or usb_redir_free_bulk_streams packet. Note
that on a success status in response to a usb_redir_alloc_bulk_streams
no_streams may be less then requested due to host controller / device
limitations. On a success status in response to a usb_redir_alloc_bulk_streams
the usb-guest may use stream ids 1 through no_streams.


usb_redir_cancel_data_packet
----------------------------

usb_redir_header.type:    usb_redir_cancel_data_packet
usb_redir_header.id       <id of packet to cancel>
usb_redir_header.length:  0

No packet type specific header.

No packet type specific additional data.

This packet can be send by the usb-guest to cancel an earlier send data
packet, the id should be set to the id used when sending the packet the
guest no wishes to cancel.

Note that the usb-guest will always receive back a data packet of the same type
and with the same id, the usb-giuest can check if the packet completed
normally (before the cancel packet was processed by the usb-host), or was
cancelled by looking at the return data packet's status field.


usb_redir_control_packet
------------------------

usb_redir_header.type:    usb_redir_control_packet
usb_redir_header.length:  sizeof(usb_redir_control_packet_header) [+ length]

struct usb_redir_control_packet_header {
    uint8_t endpoint;
    uint8_t request;
    uint8_t requesttype;
    uint8_t status;
    uint16_t value;
    uint16_t index;
    uint16_t length;
}

The additional data contains the control msg data to be send / received.

Packets of this type can be send by the usb-guest to the usb-host to
initiate a control transfer on the usb-device. endpoint, request, requesttype,
value and index have their standard meaning for usb control messages.
The status field is only used in the usb-host's response.

length is the amount of data the usb-guest is sending / expects to read
(in the USB_DIR_IN case). Note that the length should only be added
to usb_redir_header.length in one direction (and the actual packet
length should match).

When the control msg has been processed by the usb-device the usb-host sends
a usb_redir_control_packet back to the usb-guest, with all fields unchanged
except for the status field and length which get updated to match the
actual results.


usb_redir_bulk_packet
---------------------

usb_redir_header.type:    usb_redir_bulk_packet
usb_redir_header.length:  sizeof(usb_redir_bulk_packet_header) [+ length]

struct usb_redir_bulk_packet_header {
    uint8_t endpoint;
    uint8_t status;
    uint16_t length;
    uint32_t stream_id;
}

The additional data contains the bulk msg data to be send / received.

Packets of this type can be send by the usb-guest to the usb-host to
initiate a bulk transfer on the usb-device. endpoint and stream_id have
their standard meaning for usb bulk messages. The status field is only used
in the usb-host's response. length is the amount of data the usb-guest is
sending / expects to read (depending on the direction of the endpoint).

When the bulk msg has been processed by the usb-device the usb-host sends
a usb_redir_bulk_packet back to the usb-guest, with the status field and
length updated to match the actual results.

Note just as usb_redir_control_packet this packet only has additional data
in one direction depending on the direction of the endpoint.


usb_redir_iso_packet
--------------------

usb_redir_header.type:    usb_redir_iso_packet
usb_redir_header.length:  sizeof(usb_redir_iso_packet_header) + length

struct usb_redir_iso_packet_header {
    uint8_t endpoint;
    uint8_t status;
    uint16_t length;
}

The additional data contains the iso msg data to be send / received.

Packets of this type should be send continuesly (at the endpoint interval
speed) as soon as an iso stream is started using usb_redir_start_iso_stream
the direction in which they gets send depends on the endpoints direction.

The status field only has meaning for packets send from the usb-host to
the usb-guest (for iso input endpoints). Due to buffering it is not possibly
to timely notify the usb-guest of transfer errors for iso output packets. The
usb-host will try to clear any error conditions itself. If it fails to do
so it will send a usb_redir_iso_stream_status to the usb-guest indicating
there is a problem with the iso stream.

Since usb_redir_iso_packet's are send continuously by the usb-host once
a stream is started on an iso input endpoint, the usb-host cannot set the
usb_redir_header.id to the id of the corresponding received packet. So for
usb_redir_iso_packet's the usb-host simply starts with an id of 0 and
increments this every packet. Note that when the usb-host has recovered from
a stall the id will restart at 0!