diff --git a/doc/proto.md b/doc/proto.md
index dfdd06d..fe7ae53 100644
--- a/doc/proto.md
+++ b/doc/proto.md
@@ -768,8 +768,6 @@ The field has the following format:
   to that command to the client. In the absense of this flag, clients
   SHOULD NOT multiplex their commands over more than one connection to
   the export.
-- bit 9, `NBD_FLAG_SEND_BLOCK_STATUS`; defined by the experimental
-  `BLOCK_STATUS` extension; see below.
 
 Clients SHOULD ignore unknown flags.
 
@@ -871,6 +869,46 @@ of the newstyle negotiation.
 
     Defined by the experimental `INFO` [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
 
+- `NBD_OPT_ALLOC_CONTEXT` (10)
+
+    Return a list of `NBD_REP_ALLOC_CONTEXT` replies, one per context,
+    followed by an `NBD_REP_ACK`. If a server replies to such a request
+    with no error message, clients MAY send NBD_CMD_BLOCK_STATUS
+    commands during the transmission phase.
+
+    If the query string is syntactically invalid, the server SHOULD send
+    `NBD_REP_ERR_INVALID`. If the query string is syntactically valid
+    but finds no allocation contexts, the server MUST send a single
+    reply of type `NBD_REP_ACK`.
+
+    This option MUST NOT be requested unless structured replies have
+    been negotiated first. If a client attempts to do so, a server
+    SHOULD send `NBD_REP_ERR_INVALID`.
+
+    Data:
+    - 32 bits, type
+    - String, query to select a subset of the available allocation
+      contexts. If this is not specified (i.e., length is 4 and no
+      command is sent), then the server MUST send all the allocation
+      contexts it knows about. If specified, this query string MUST
+      start with a name that uniquely identifies a server
+      implementation; e.g., the reference implementation that
+      accompanies this document would support query strings starting
+      with 'nbd-server:'
+
+    The type may be one of:
+    - `NBD_ALLOC_LIST_CONTEXT` (1): the list of allocation contexts
+      selected by the query string is returned to the client without
+      changing any state (i.e., this does not add allocation contexts
+      for further usage).
+    - `NBD_ALLOC_ADD_CONTEXT` (2): the list of allocation contexts
+      selected by the query string is added to the list of existing
+      allocation contexts.
+    - `NBD_ALLOC_DEL_CONTEXT` (3): the list of allocation contexts
+      selected by the query string is removed from the list of used
+      allocation contexts. Servers SHOULD NOT reuse existing allocation
+      context IDs.
+
 #### Option reply types
 
 These values are used in the "reply type" field, sent by the server
@@ -901,6 +939,18 @@ during option haggling in the fixed newstyle negotiation.
 
     Defined by the experimental `INFO` [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
 
+- `NBD_REP_ALLOC_CONTEXT` (4)
+
+    A description of an allocation context. Data:
+
+    - 32 bits, NBD allocation context ID. If the request was NOT of type
+      `NBD_ALLOC_LIST_CONTEXT`, this field MUST NOT be zero.
+    - String, name of the allocation context. This is not required to be
+      a human-readable string, but it MUST be valid UTF-8 data.
+
+    Allocation context ID 0 is implied, and always exists. It cannot be
+    removed.
+
 There are a number of error reply types, all of which are denoted by
 having bit 31 set. All error replies MAY have some data set, in which
 case that data is an error message string suitable for display to the user.
@@ -938,15 +988,47 @@ case that data is an error message string suitable for display to the user.
 
     Defined by the experimental `INFO` [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
 
-* `NBD_REP_ERR_SHUTDOWN` (2^32 + 7)
+* `NBD_REP_ERR_SHUTDOWN` (2^31 + 7)
 
     The server is unwilling to continue negotiation as it is in the
     process of being shut down.
 
-* `NBD_REP_ERR_BLOCK_SIZE_REQD` (2^32 + 8)
+* `NBD_REP_ERR_BLOCK_SIZE_REQD` (2^31 + 8)
 
     Defined by the experimental `INFO` [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-info/doc/proto.md).
 
+##### Allocation contexts
+
+Allocation context 0 is the basic "exists at all" allocation context. If
+an extent is not allocated at allocation context 0, it MUST NOT be
+listed as allocated at another allocation context. This supports sparse
+file semantics on the server side. If a server has only one allocation
+context (the default), then writing to an extent which is allocated in
+that allocation context 0 MUST NOT fail with ENOSPC.
+
+For all other cases, this specification requires no specific semantics
+of allocation contexts. Implementations could support allocation
+contexts with semantics like the following:
+
+- Incremental snapshots; if a block is allocated in one allocation
+  context, that implies that it is also allocated in the next level up.
+- Various bits of data about the backend of the storage; e.g., if the
+  storage is written on a RAID array, an allocation context could
+  return information about the redundancy level of a given extent
+- If the backend implements a write-through cache of some sort, or
+  synchronises with other servers, an allocation context could state
+  that an extent is "allocated" once it has reached permanent storage
+  and/or is synchronized with other servers.
+
+The only requirement of an allocation context is that it MUST be
+representable with the flags as defined for `NBD_CMD_BLOCK_STATUS`.
+
+Likewise, the syntax of query strings is not specified by this document.
+
+Server implementations SHOULD document their syntax for query strings
+and semantics for resulting allocation contexts in a document like this
+one.
+
 ### Transmission phase
 
 #### Flag fields
@@ -983,6 +1065,9 @@ valid may depend on negotiation during the handshake phase.
    content chunk in reply.  MUST NOT be set unless the transmission
    flags include `NBD_FLAG_SEND_DF`.  Use of this flag MAY trigger an
    `EOVERFLOW` error chunk, if the request length is too large.
+- bit 3, `NBD_CMD_FLAG_REQ_ONE`; valid during `NBD_CMD_BLOCK_STATUS`. If
+  set, the client is interested in only one extent per allocation
+  context.
 
 ##### Structured reply flags
 
@@ -1371,38 +1456,48 @@ adds a new `NBD_CMD_BLOCK_STATUS` command which returns a list of
 ranges with their respective states.  This extension is not available
 unless the client also negotiates the `STRUCTURED_REPLY` extension.
 
-* `NBD_FLAG_SEND_BLOCK_STATUS`
-
-    The server SHOULD set this transmission flag to 1 if structured
-    replies have been negotiated, and the `NBD_CMD_BLOCK_STATUS`
-    request is supported.
-
 * `NBD_REPLY_TYPE_BLOCK_STATUS`
 
-    *length* MUST be a positive integer multiple of 8.  This reply
+    *length* MUST be 4 + (a positive integer multiple of 8).  This reply
     represents a series of consecutive block descriptors where the sum
     of the lengths of the descriptors MUST not be greater than the
-    length of the original request.  This chunk type MUST appear at most
-    once in a structured reply. Valid as a reply to
+    length of the original request. This chunk type MUST appear at most
+    once per allocation ID in a structured reply. Valid as a reply to
     `NBD_CMD_BLOCK_STATUS`.
 
-    The payload is structured as a list of one or more descriptors,
-    each with this layout:
+    Servers MUST return an `NBD_REPLY_TYPE_BLOCK_STATUS` chunk for every
+    allocation context ID, except if the semantics of particular
+    allocation contexts mean that the information for one allocation
+    context is implied by the information for another.
+
+    The payload starts with:
+
+        * 32 bits, allocation context ID
+
+    and is followed by a list of one or more descriptors, each with this
+    layout:
 
         * 32 bits, length (unsigned, MUST NOT be zero)
         * 32 bits, status flags
 
-    The definition of the status flags is determined based on the
-    flags present in the original request.
+    If the client used the `NBD_CMD_FLAG_REQ_ONE` flag in the request,
+    then every reply chunk MUST NOT contain more than one descriptor.
+
+    Even if the client did not use the `NBD_CMD_FLAG_REQ_ONE` flag in
+    its request, the server MAY return less descriptors in the reply
+    than would be required to fully specify the whole range of requested
+    information to the client, if the number of descriptors would be
+    over 16 otherwise and looking up the information would be too
+    resource-intensive for the server.
 
 * `NBD_CMD_BLOCK_STATUS`
 
-    A block status query request. Length and offset define the range
-    of interest. Clients SHOULD NOT use this request unless the server
-    set `NBD_CMD_SEND_BLOCK_STATUS` in the transmission flags, which
-    in turn requires the client to first negotiate structured replies.
-    For a successful return, the server MUST use a structured reply,
-    containing at most one chunk of type `NBD_REPLY_TYPE_BLOCK_STATUS`.
+    A block status query request. Length and offset define the range of
+    interest. Clients SHOULD NOT use this request unless allocation
+    contexts have been negotiated, which in turn requires the client to
+    first negotiate structured replies. For a successful return, the
+    server MUST use a structured reply, containing at least one chunk of
+    type `NBD_REPLY_TYPE_BLOCK_STATUS`.
 
     The list of block status descriptors within the
     `NBD_REPLY_TYPE_BLOCK_STATUS` chunk represent consecutive portions
@@ -1427,18 +1522,12 @@ unless the client also negotiates the `STRUCTURED_REPLY` extension.
     requested length, and *status* of 0 rather than reporting the
     error.
 
-    The type of information requested by the client is determined by
-    the request flags, as follows:
-
-    1. Block provisioning status
+    Upon receiving an `NBD_CMD_BLOCK_STATUS` command, the server MUST
+    return the status of the device, where the status field of each
+    descriptor is determined by the following bits (all combinations of
+    these bits are possible):
 
-    Upon receiving an `NBD_CMD_BLOCK_STATUS` command with the flag
-    `NBD_FLAG_STATUS_DIRTY` clear, the server MUST return the
-    provisioning status of the device, where the status field of each
-    descriptor is determined by the following bits (all four
-    combinations of these two bits are possible):
-
-      - `NBD_STATE_HOLE` (bit 0); if set, the block represents a hole
+      - `NBD_STATE_HOLE` (bit 0): if set, the block represents a hole
         (and future writes to that area may cause fragmentation or
         encounter an `ENOSPC` error); if clear, the block is allocated
         or the server could not otherwise determine its status.  Note
@@ -1446,43 +1535,34 @@ unless the client also negotiates the `STRUCTURED_REPLY` extension.
         that the server MAY report a hole even where trim has not been
         requested, and also that a server MAY report allocation even
         where a trim has been requested.
-      - `NBD_STATE_ZERO` (bit 1), if set, the block contents read as
+      - `NBD_STATE_ZERO` (bit 1): if set, the block contents read as
         all zeroes; if clear, the block contents are not known.  Note
         that the use of `NBD_CMD_WRITE_ZEROES` is related to this
         status, but that the server MAY report zeroes even where write
         zeroes has not been requested, and also that a server MAY
         report unknown content even where write zeroes has been
         requested.
-
-    The client SHOULD NOT read from an area that has both
-    `NBD_STATE_HOLE` set and `NBD_STATE_ZERO` clear.
-
-    2. Block dirtiness status
-
-    This command is meant to operate in tandem with other (non-NBD)
-    channels to the server.  Generally, a "dirty" block is a block
-    that has been written to by someone, but the exact meaning of "has
-    been written" is left to the implementation.  For example, a
-    virtual machine monitor could provide a (non-NBD) command to start
-    tracking blocks written by the virtual machine.  A backup client
-    can then connect to an NBD server provided by the virtual machine
-    monitor and use `NBD_CMD_BLOCK_STATUS` with the
-    `NBD_FLAG_STATUS_DIRTY` bit set in order to read only the dirty
-    blocks that the virtual machine has changed.
-
-    An implementation that doesn't track the "dirtiness" state of
-    blocks MUST either fail this command with `EINVAL`, or mark all
-    blocks as dirty in the descriptor that it returns.  Upon receiving
-    an `NBD_CMD_BLOCK_STATUS` command with the flag
-    `NBD_FLAG_STATUS_DIRTY` set, the server MUST return the dirtiness
-    status of the device, where the status field of each descriptor is
-    determined by the following bit:
-
-      - `NBD_STATE_CLEAN` (bit 2); if set, the block represents a
+      - `NBD_STATE_CLEAN` (bit 2): if set, the block represents a
         portion of the file that is still clean because it has not
         been written; if clear, the block represents a portion of the
         file that is dirty, or where the server could not otherwise
-        determine its status.
+        determine its status. The server MUST NOT set this bit for
+        allocation context 0, where it has no meaning.
+      - `NBD_STATE_ACTIVE` (bit 3): if set, the block represents a
+	portion of the file that is "active" in the given allocation
+	context. The server MUST NOT set this bit for allocation context
+	0, where it has no meaning.
+
+    The exact semantics of what it means for a block to be "clean" or
+    "active" at a given allocation context is not defined by this
+    specification, except that the default in both cases should be to
+    clear the bit. That is, when the allocation context does not have
+    knowledge of the relevant status for the given extent, or when the
+    allocation context does not assign any meaning to it, the bits
+    should be cleared.
+
+    A client SHOULD NOT read from an area that has both `NBD_STATE_HOLE`
+    set and `NBD_STATE_ZERO` clear.
 
 A client MAY close the connection if it detects that the server has
 sent an invalid chunks (such as lengths in the
@@ -1492,9 +1572,9 @@ request including one or more sectors beyond the size of the device.
 
 The extension adds the following new command flag:
 
-- `NBD_CMD_FLAG_STATUS_DIRTY`; valid during `NBD_CMD_BLOCK_STATUS`.
-  SHOULD be set to 1 if the client wants to request dirtiness status
-  rather than provisioning status.
+- `NBD_CMD_FLAG_REQ_ONE`; valid during `NBD_CMD_BLOCK_STATUS`.
+  SHOULD be set to 1 if the client wants to request information for only
+  one extent per allocation context.
 
 ## About this file

