QEMU supports the NBD protocol, and has an internal NBD client (see
block/nbd.c), an internal NBD server (see blockdev-nbd.c), and an
external NBD server tool (see qemu-nbd.c). The common code is placed
in nbd/*.

The NBD protocol is specified here:
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md

The following paragraphs describe some specific properties of NBD
protocol realization in QEMU.

= Metadata namespaces =

QEMU supports the "base:allocation" metadata context as defined in the
NBD protocol specification, and also defines an additional metadata
namespace "qemu".

== "qemu" namespace ==

The "qemu" namespace currently contains two available metadata context
types.  The first is related to exposing the contents of a dirty
bitmap alongside the associated disk contents.  That metadata context
is named with the following form:

    qemu:dirty-bitmap:<dirty-bitmap-export-name>

Each dirty-bitmap metadata context defines only one flag for extents
in reply for NBD_CMD_BLOCK_STATUS:

    bit 0: NBD_STATE_DIRTY, set when the extent is "dirty"

The second is related to exposing the source of various extents within
the image, with a single metadata context named:

    qemu:allocation-depth

In the allocation depth context, the entire 32-bit value represents a
depth of which layer in a thin-provisioned backing chain provided the
data (0 for unallocated, 1 for the active layer, 2 for the first
backing layer, and so forth).

For NBD_OPT_LIST_META_CONTEXT the following queries are supported
in addition to the specific "qemu:allocation-depth" and
"qemu:dirty-bitmap:<dirty-bitmap-export-name>":

* "qemu:" - returns list of all available metadata contexts in the
            namespace.
* "qemu:dirty-bitmap:" - returns list of all available dirty-bitmap
                         metadata contexts.

= Features by version =

The following list documents which qemu version first implemented
various features (both as a server exposing the feature, and as a
client taking advantage of the feature when present), to make it
easier to plan for cross-version interoperability.  Note that in
several cases, the initial release containing a feature may require
additional patches from the corresponding stable branch to fix bugs in
the operation of that feature.

* 2.6: NBD_OPT_STARTTLS with TLS X.509 Certificates
* 2.8: NBD_CMD_WRITE_ZEROES
* 2.10: NBD_OPT_GO, NBD_INFO_BLOCK
* 2.11: NBD_OPT_STRUCTURED_REPLY
* 2.12: NBD_CMD_BLOCK_STATUS for "base:allocation"
* 3.0: NBD_OPT_STARTTLS with TLS Pre-Shared Keys (PSK),
NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
* 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
NBD_CMD_FLAG_FAST_ZERO
* 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
* 7.1: NBD_FLAG_CAN_MULTI_CONN for shareable writable exports
* 8.2: NBD_OPT_EXTENDED_HEADERS, NBD_FLAG_BLOCK_STATUS_PAYLOAD