Re: [Qemu-devel] [Nbd] [PATCH 3/1] doc: Propose Structured Replies extension

[Wouter Verhelst]

Hi Eric,

After applying some of the other outstanding patches, this one doesn't apply
anymore. Can you rebase?

On Mon, Mar 28, 2016 at 09:56:36PM -0600, Eric Blake wrote:
> The existing transmission phase protocol is difficult to sniff,
> because correct interpretation of the server stream requires
> context from the client stream (or risks false positives if
> data payloads happen to contain the protocol magic numbers).  It
> also prohibits the ability to do short reads, or to return a
> read error without also sending length bytes of data.
> 
> Remedy this by adding a new global/client flag negotiation,
> which affects the response of the NBD_CMD_READ command, and sets
> forth rules for how future command responses must behave when
> they carry a data payload.
> 
> Signed-off-by: Eric Blake <address@hidden>
> ---
>  doc/proto.md | 123 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 123 insertions(+)
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 44579fc..f687e3e 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -209,6 +209,10 @@ same value for handle as was sent by the client for each 
> request that
>  the server is replying to, so that the client may correlate which
>  request is receiving a response.
> 
> +Note that it is impossible to tell by reading just the server traffic
> +whether a data field will be present.  To remedy this, the experimental
> +`Structured Reply` extension has been introduced; see below.
> +
>  ## Values
> 
>  This section describes the value and meaning of constants (other than
> @@ -231,6 +235,8 @@ the first magic number.
>  - bit 1, `NBD_FLAG_NO_ZEROES`; if set, and if the client replies with
>    `NBD_FLAG_C_NO_ZEROES` in the client flags field, the server MUST NOT
>    send the 124 bytes of zero at the end of the negotiation.
> +- bit 2, `NBD_FLAG_STRUCTURED_REPLY`; defined by the experimental
> +  `Structured Reply` extension; see below.
> 
>  The server MUST NOT set any other flags, and SHOULD NOT change behaviour
>  unless the client responds with a corresponding flag.  The server MUST
> @@ -265,6 +271,8 @@ receiving the global flags from the server.
>  - bit 1, `NBD_FLAG_C_NO_ZEROES`; MUST NOT be set if the server did not
>    set `NBD_FLAG_NO_ZEROES`. If set, the server MUST NOT send the 124
>    bytes of zeroes at the end of the negotiation.
> +- bit 2, `NBD_FLAG_C_STRUCTURED_REPLY`; defined by the experimental
> +  `Structured Reply` extension; see below.
> 
>  Clients SHOULD NOT set any other flags; the server MUST drop the
>  connection if the client sets an unknown flag, or a flag that does
> @@ -435,6 +443,10 @@ The following request types exist:
>      signalling no error), the server MUST immediately close the
>      connection; it MUST NOT send any further data to the client.
> 
> +    The experimental `Structured Reply` extension changes the set of
> +    valid replies, in part to allow recovery after a partial read; see
> +    below.
> +
>  * `NBD_CMD_WRITE` (1)
> 
>      A write request. Length and offset define the location and amount of
> @@ -609,6 +621,117 @@ option reply type.
>        message if they do not also send it as a reply to the
>        `NBD_OPT_SELECT` message.
> 
> +### `Structured Reply` extension
> +
> +Some major downsides of the default reply to `NBD_CMD_READ` is that it
> +is not possible to support partial reads (the command must succeed or
> +fail as a whole, and len bytes of data must be sent even on an error,
> +unless the connection is closed); nor is it possible to decode the
> +server traffic without also knowing what pending read requests were
> +sent by the client.
> +
> +To remedy this, a `Structured Reply` extension is envisioned. This
> +extension adds a global flag, a client flag, a new reply type during
> +the transmission phase, and alters the set of valid replies to an
> +existing command.
> +
> +* `NBD_FLAG_STRUCTURED_REPLY` (bit 2)
> +
> +  This is a global flag; if set, and if the client replies with
> +  `NBD_FLAG_C_STRUCTURED_REPLY` in the client flags field, the server
> +  MUST use structured replies to the `NBD_CMD_READ` command.
> +
> +* `NBD_FLAG_C_STRUCTURED_REPLY` (bit 2)
> +
> +  This is a client flag; MUST NOT be set if the server did not set
> +  `NBD_FLAG_STRUCTURED_REPLY`. If set, the server must use structured
> +  replies to the `NBD_CMD_READ` command.
> +
> +* Transmission phase
> +
> +  The transmission phase includes a third message type: the structured
> +  reply.  If `NBD_FLAG_C_STRUCTURED_REPLY` was negotiated, then the
> +  normal server reply will never contain a data payload, and all
> +  server replies that send data will be in the following form:
> +
> +  S: 32 bits, 0x668e33ef, magic (`NBD_STRUCTURED_REPLY_MAGIC`)  
> +  S: 32 bits, error  
> +  S: 64 bits, handle  
> +  S: 32 bits, length of payload (unsigned)  
> +  S: *length* bytes of payload data
> +
> +  If `NBD_FLAG_C_STRUCTURED_REPLY` was not negotiated, then the normal
> +  server reply with a data payload will be used for `NBD_CMD_READ`;
> +  but any other replies with a data payload will still use a
> +  structured reply (that is, only `NBD_CMD_READ` is allowed to send
> +  data in the non-structured form, and negotiating
> +  `NBD_FLAG_C_STRUCTURED_REPLY` only affects replies to
> +  `NBD_CMD_READ`).  This implies that any new commands that require
> +  data in the reply should be gated by their own new global and client
> +  flag.  A server MAY refuse to allow a client that negotiates a
> +  command that requires a structured reply, but does not also
> +  negotiate `NBD_FLAG_C_STRUCTURED_REPLY`.
> +
> +  In the generic form, the length field of a structured response MAY
> +  be zero if there is no data payload, and the error field may be set
> +  regardless of the length field (although where possible, the server
> +  SHOULD use a length of zero when setting the error field).  However,
> +  particular commands may document additional restrictions regarding
> +  what forms a valid response (for example, a structured response to
> +  `NBD_CMD_READ` MUST NOT set the error field, and MUST have a
> +  non-zero length of at least 9).
> +
> +* `NBD_CMD_READ`
> +
> +  If `NBD_FLAG_C_STRUCTURED_REPLY` was not negotiated, then a read
> +  request MUST always be answered by a single response (with magic
> +  0x67446698 `NBD_REPLY_MAGIC`); the response MUST include length
> +  bytes of data according to the client's request, although those
> +  bytes MAY be invalid if an error is returned, and the connection
> +  MUST if an error occurs after a header claiming no error.
> +
> +  Conversely, if the `NBD_FLAG_C_STRUCTURED_REPLY` was negotiated, the
> +  response MUST be a sequence of zero or more structured replies (with
> +  magic 0x668e33ef `NBD_STRUCTURED_REPLY_MAGIC`), followed by a
> +  concluding normal response (0x67446698 `NBD_REPLY_MAGIC`), where the
> +  final response MUST NOT have a data payload; all responses in the
> +  sequence use the same handle from the client.  The payload of each
> +  intermediate structured reply, called a read chunk, MUST be of the
> +  following form:
> +
> +  S: 64 bits, offset (unsigned)  
> +  S: (*length* - 8) bytes of data
> +
> +  Note that the amount of data returned in a read chunk is determined
> +  by the length field of the structured reply, and not the original
> +  length of the client's request.  If the server sends a single read
> +  chunk for a successful read, the server's length will be the
> +  client's length plus 8, because the server must account for the
> +  offset field in its reply.  Similarly, a successful client request
> +  for a read of 2^32-8 or more bytes MUST be split into at least two
> +  read chunks, so that the length field does not suffer from overflow.
> +
> +  The server MUST ensure that each read chunk lies within the original
> +  offset and length of the original client request, MUST NOT send read
> +  chunks that would cover the same offset more than once, and MUST
> +  send at least one byte of data in addition to the offset field of
> +  each read chunk.  The server MAY send read chunks out of order, and
> +  may interleave other responses between read replies.  The server
> +  MUST NOT set the error field of a read chunk; if an error occurs, it
> +  MAY immediately end the sequence of structured response messages,
> +  MUST send the error in the concluding normal response, and SHOULD
> +  keep the connection open.  The final non-structured response MUST
> +  set an error unless the sum of data sent by all read chunks totals
> +  the original client length request.
> +
> +  The client SHOULD immediately close the connection if it detects
> +  that the server has sent an offset more than once (whether or not
> +  the overlapping data claimed to have the same contents), or if
> +  receives the concluding normal reply without an error set but
> +  without all bytes covered by read chunk(s).  A future extension may
> +  add a command flag that would allow the server to skip read chunks
> +  for portions of the file that read as all zeroes.
> +
>  ## About this file
> 
>  This file tries to document the NBD protocol as it is currently
> -- 
> 2.5.5
> 
> 
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-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12