DAP4 Chunking

Overview

Chunking Scheme

       chunked-body     =  *chunked-section last-chunk

       chunked-section  = chunk | chunk-extension
      
       chunk-extension  = chunk-size "x"  1*(chunk-ext-name [ "=" chunk-ext-val ] ";")

       chunk            = chunk-size "d" chunk-data 

       chunk-size       = 7HEXDIG
       last-chunk       = 7("0") "d"

       chunk-ext-name   = token
                          ; sequence of 7-bit ASCII printable chars

       chunk-ext-val    = token | quoted-string
                          ; quoted-string is DQUOTE token DQUOTE

       chunk-data       = chunk-size(OCTET)
                          ; exactly chunk-size bytes

Graphical representation of chunking scheme

size - Stored in the first seven bytes are the size of the chunk (in bytes). The size does not include the first 8 bytes which are the chunk size and the chunk type (x or d). The size is a 16 bit integer encoded in 7 bytes as 7 ASCII characters that represent the size as a 7 hexadecimal (base 16) digits. If the size is 0, this signifies the end of the transmission, no more chunks follow.

type - The eighth byte is the type of chunk that follows. The type can be one of

• x - extension, one or more name=value; pairs
• d - data, actual data

data - The data part of the chunk, meaning either extensions or data, not both

Extensions are a name/value pair and can represent information needed by the underlying communication layer. For example: status=error; would mean that an error has occurred. This chunk would not contain the error/exception information itself. Following chunks would hold that information and would be of type d (for data).

It is possible that the chunk may not come across in one read call to the underlying socket layer. The first 8 bytes represent information about the chunk, the first seven bytes being the size of the chunk and the eighth byte being the type of data the chunk contains. Read should be called until the entire chunk has been received.

Read/receive should be called until the last chunk in the transmission is received. The last chunk is represented by chunk size of 0.

Client/server exit

The exit handshake used to take place with an exit token. Now, instead, it is done with an exit extension. The chunk will look like this:

0000014xstatus=PPT_EXIT_NOW;

followed by the last chunk signifying the end of the transmission.

0000000d

Chunking State Diagram

Error response

Built in to the BES currently is the error message response to a BES client request. Specifically, if the BES client issues a command and the BES server has a problem with it (either with the command itself, the processing of the response to the command, or sending of the response) then the BES server will send an extension chunk to the BES client with the name/value pair "status=error;", and, following that chunk, data chunks will contain the error information followed by the last chunk. The extension chunk with the error status could come following a series of data chunks containing the partial response.