Here's a strawman for the structured reply section. I haven't covered negotation.
Signed-off-by: Alex Bligh <a...@alex.org.uk> --- doc/proto.md | 114 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 111 insertions(+), 3 deletions(-) diff --git a/doc/proto.md b/doc/proto.md index aaae0a2..2ea81b9 100644 --- a/doc/proto.md +++ b/doc/proto.md @@ -195,15 +195,123 @@ C: 64 bits, offset (unsigned) C: 32 bits, length (unsigned) C: (*length* bytes of data if the request is of type `NBD_CMD_WRITE`) -The server replies with: +Replies take one of two forms. They may either be structured replies, +or unstructured replies. The server MUST NOT send structured replies +unless it has negotiated structured replies with the client using +`NBD_OPT_STUCTURED_REPLIES` (??). Subject to that, structured replies +may be sent in response to any command. + +Unstructured replies are problematic for error handling within +`NBD_CMD_READ`, therefore servers SHOULD support structured replies. + +#### Unstructured replies + +In an unstructured reply, the server replies with: S: 32 bits, 0x67446698, magic (`NBD_REPLY_MAGIC`) S: 32 bits, error S: 64 bits, handle S: (*length* bytes of data if the request is of type `NBD_CMD_READ`) -Replies need not be sent in the same order as requests (i.e., requests -may be handled by the server asynchronously). +#### Structured replies + +A structured reply consists of one or more chunks. The server +MUST send exactly one end chunk chunk (identified by +the chunk type `NBD_CHUNKTYPE_END`), and this MUST be the final +chunk within the reply. + +Each chunk consists of the following: + +S: 32 bits, 0x668e33ef, magic (`NBD_STRUCTURED_REPLY_MAGIC`) +S: 32 bits, flags (including type) +S: 64 bits, handle +S: 32 bits, payload length +S: (*length* bytes of payload data) + +The flags have the following meanings: + +* bits 0-7: `NBD_CHUNKTYPE`, an 8 bit unsigned integer +* bits 8-31: reserved (server MUST set these to zero) + +Possible values of `NBD_CHUNKTYPE` are as follows: + +* 0 = `NBD_CHUNKTYPE_END`: the final chunk +* 1 = `NBD_CHUNKTYPE_DATA`: data that has been read +* 2 = `NBD_CHUNKTYPE_ZERO`: data that should be considered zero + +The format of the payload data for each chunk type is as follows: + +##### `NBD_CHUNKTYPE_END` + +S: 32 bits, error code or zero for success +S: 64 bits, offset of error (if any) + +##### `NBD_CHUNKTYPE_DATA` + +S: 64 bits, offset of data +S: (*length-8* bytes of data as read) + +##### `NBD_CHUNKTYPE_ZERO` + +S: 64 bits, offset of data +S: 32 bits, number of zeroes to which this corresponds + + +Commands that return data (currently `NBD_CMD_READ`) therefore MUST +return zero or more chunks each of type `NBD_CHUNKTYPE_DATA` or +`NBD_CHUNKTYPE_ZERO` (collectively 'data chunks') followed +an `NBD_CHUNKTYPE_END`. + +The server MAY split the reply into any number of data +chunks (provided each consists of at least one byte) and +MAY send the data chunks in any order (though the +`NBD_CHUNKTYPE_END` must be the final chunk). This means the +client is responsible for reassembling the chunks in the correct +order. + +The server MUST NOT send chunks that overlap. The server +MUST NOT send chunks whose data exceeds the length +of data requested (for this purpose counting the data +within `NBD_CHUNKTYPE_ZERO` as the number of zero bytes +specified therein). The server MUST, in the case of a successesful +read send exactly the number of bytes requested (whether +represented by `NBD_CHUNKTYPE_DATA` or `NBD_CHUNKTYPE_ZERO`). +The server MUST NOT, in the case of an errored read, send +more than the number of byte requested. + +In order to avoid the burden of reassembly, the client +MAY send `NBD_CMD_FLAG_DF` (??), which instructs the server +not to fragment the reply. If this flag is set, the server +MUST send either zero or one data chunks and an `NBD_CHUNKTYPE_END` +only. Under such circumstances the server MAY error the command +with `ETOOBIG` if the length read exceeds [65,536 bytes | the +negotiated maximum fragment size]. + +If no errors are detected within an operation, the `NBD_CHUNKTYPE_END` +packet MUST contain an error value of zero and an error offset of +zero. + +If the server detects an error during an operation which it +is serving with a structured reply, it MUST complete +the transmission of the current data chunk if transmission +has started (by padding the current chunk with data +which MUST be zero), after which zero or more other +data chunks may be sent, followed by an `NBD_CHUNKTYPE_END` +chunk. The server MAY set the offset within `NBD_CHUNKTYPE_END` +to the offset of the error; if so, this MUST be within the +length requested. + +#### Ordering of replies + +The server MAY send replies in any order; the order of replies +need not correpsond to the order of requests, i.e., requests +may be handled by the server asynchronously). The server MAY +interleave the chunks relating to a single structured reply +with chunks relating to structured replies relating to +a different handle, or with unstructured replies relating +to a different handle. Note that there is a constraint on +the ordering of chunks within structured replies as set out +above. ## Values -- 1.9.1
