Websocket documentation error ?

[dev31337]

Hi,

The websocket documentation here says that the flags element corresponds to the first byte as described in RFC 6455 section 5.2.

But when we look at the RFC the opcode is bits 4,5,6,7 while with the flags the opcode is bits 0,1,2,3.

The example makes it pretty clear by doing "msgtype = wm->flags & 0x0F;" to get the opcode.

This brings another question, if the flags element does not contain the FIN bit, that means we can not detect fragmented websocket messages,

Does that mean that Mongoose automatically buffers and de-fragments a fragmented websocket message and thus only issue one MG_EV_WS_MSG event when the final fragment is received ?

Regards.

[scaprile]

https://datatracker.ietf.org/doc/html/rfc6455#section-5.2 :

When encoded on the wire, the most significant bit is the leftmost in the ABNF

msg->flags is not the actual byte in the WS frame, but a copy of it. The order of the bytes in the frame is as seen left to right, and the order of the bits is MSb to LSb left to right. The docs explain msg->flags and that is nevertheless coincident with that.

We don't strip anything, you just don't need to use that, because:

mongoose/src/ws.c

Line 72 in cb1ae31

msg->flags = buf[0];

mongoose/src/ws.c

Line 179 in cb1ae31

uint8_t final = msg.flags & 128, op = msg.flags & 15;

mongoose/src/ws.c

Lines 211 to 230 in cb1ae31

	// Handle fragmented frames: strip header, keep in c->recv
	if (final == 0 || op == 0) {
	if (op) ofs++, len--, msg.header_len--; // First frame
	mg_iobuf_del(&c->recv, ofs, msg.header_len); // Strip header
	len -= msg.header_len;
	ofs += len;
	c->pfn_data = (void *) ofs;
	// MG_INFO(("FRAG %d [%.*s]", (int) ofs, (int) ofs, c->recv.buf));
	}
	// Remove non-fragmented frame
	if (final && op) mg_iobuf_del(&c->recv, ofs, len);
	// Last chunk of the fragmented frame
	if (final && !op) {
	m.flags = c->recv.buf[0];
	m.data = mg_str_n((char *) &c->recv.buf[1], (size_t) (ofs - 1));
	mg_call(c, MG_EV_WS_MSG, &m);
	mg_iobuf_del(&c->recv, 0, ofs);
	ofs = 0;
	c->pfn_data = NULL;
	}

Mongoose buffers the fragments

[dev31337]

Thank you for this very detailed answer ! I understand now...

This is really nice that Mongoose handles fragmented messages by itself, may I suggest that you tell it in the documentation because it is a great feature.