Design
Websocket data is divided into frames, consisting of a header containing a length followed by a sequence of bytes called the payload. A message can be broken up into multiple frames, while a control frame is always delivered in a single frame. All but the first frame of a message is called a continuation frame, while the last frame of a message is called a final frame.
The diagram below shows a complete frame followed by an incomplete frame:
| frame | partial...Reading
When network I/O is performed, ideal results are achieved when the amount of work achieved per operation is maximized. This is because of the significant fixed cost for each I/O. The fraction of resources lost to overhead can be reduced by using the largest practically sized buffer possible in each read operation.
The ws_proto::frame_stream decoder uses a persistent, fixed-size internal
buffer to hold incoming bytes from the peer and decode the frames inside.
Because the buffer does not resize, this can produce zero or more complete
frames and up to two partial frames. At the beginning of a session, the first
input buffer will usually contain zero or more complete frames, and up to one
partial frame:
| frame | frame | partial...A partial frame is created when the last frame in the buffer cannot fit completely. In this case, the remainder of the frame payload will appear in the next buffer of data received from the peer:
...partial | frame | frame |Depending on the size of the read buffer and the length of incoming frames, the decoded contents may not contain any complete frames:
...partial | partial...It is also possible that the current contents of the read buffer do not contain enough frame data to produce a complete message:
...partial...The maximum payload size for a control frame is 127 bytes. A partial control frame can appear at the beginning of the read buffer, the end of the read buffer, or both ends. The library buffers partial control frame data and always delivers complete control frames to the caller as a single contiguous span of bytes.