Re: [hybi] Multiplexing extension spec draft 03

["Arman Djusupov" <[e-mail]>]

Hello Takeshi,

 

Currently the specification does not make it clear which side assigns a
channel ID when an AddChannel control message is sent. I think this could be
made flexible; for example if Objective Channel ID is set to 0 then this
means that the client prefers the server to select the channel ID, if it is
greater than 1 then the client requests a specific channel ID.

 

Giving the client the option to select the channel ID can facilitate the use
of application-specific channel IDs, where the predefined logical channel ID
is reserved for a specific role, e.g. 1 - Video channel, 2 - Voice channel,
3 - Control channel, 4 - Subtitles channel (optional). 

 

It is also worth mentioning in the spec that the protocol specified by the
Sec-WebSocket-Protocol header MAY define a default set of logical channels
that are considered as being implicitly established when the mux extension
is agreed. Some protocols, as mentioned above, may require a fixed set of
channels with predefined IDs. Negotiating the same set of channel IDs using
an AddChannel request<->response every time a new connection is established
would not be efficient. It should be permitted for the "protocol" to imply a
default set of channels that don't need to be negotiated (at least this
should not be against the specification).

 

As an example, when Sec-WebSocket-Protocol set to "smart-playback" would
mean then once the mux is negotiated the following logical channels are
considered already established: 1 - Video, 2 - Voice, 3 - Control,  4 -
Subtitles.

 

I think this would make the specification much more versatile.

 

With best regards,

Arman

 

 

From: Takeshi Yoshino [mailto:[e-mail]] 
Sent: Friday, March 23, 2012 8:22 AM
To: Arman Djusupov
Cc: [e-mail]
Subject: Re: [hybi] Multiplexing extension spec draft 03

 

The idea in my mind is that the DropChannel command works similar to TCP
FIN/RST when no mux is used. I.e., each logical channel still exchanges
close frames (with their channel ID in extension data portion) and uses
status code in them (e.g. CloseEvent's code attribute), and after handling
them, each side issues the DropChannel command as well as TCP shutdown.

 

So, there's no such problem with graceful shutdown for logical channels'
traffic. In terms of control traffic, we may receive some mux command for
logical channel X after sending DropChannel command for X, but for now we
can just ignore any of control commands safely:

- FlowControl X -> safe to ignore

- DropChannel X -> no problem. it's totally fine that both sides drop the
same channel at the same time.

- AddChannel X -> if we receive this, it just means the other peer is
misbehaving

 

That said, we can't distinguish this from invalid commands sent by broken
peers (e.g. logical channel Y never existed but received command for Y).
Maybe it's better to have endpoints respond to DropChannel as well as close
frame to sync active channel list.

 

Thanks
Takeshi



On Mon, Mar 19, 2012 at 23:37, Arman Djusupov <[e-mail]> wrote:

The procedure of closing the logical channel is not detailed enough. Closing
the logical channel should be performed in a way similar to closing the
websocket connection when no mux extension is used. We need a close control
frame to roundtrip in order to ensure the mutual agreement between the two
sides when a logical channel is closed. Currently the specification does not
require the side that receives a DropChannel control frame to reply to it
(unless I missed something), which does not ensure the graceful closure of
logical channels.

 

"When an endpoint received DropChannel, the endpoint MUST remove the logical
channel and the application instance that used the logical channel MUST
treat this as closure of underlying transport."
 
One side could be in the process of sending a message when it receives a
DropChannel frame, so it is important to ensure that the logical channel is
closed gracefully without dead channel frames left on the wire. The best way
to do it is to let the DropChannel frame roundtrip back to its sender. When
both sides have received the DropChannel frame they are in mutual agreement
to release the channel ID, being sure that no more frames with the same
channel ID are expected to arrive from the remote side.
 
With best regards,
Arman