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Re: Proposed data channel API

From: Randell Jesup <randell-ietf@jesup.org>
Date: Fri, 11 Nov 2011 13:36:55 -0500
Message-ID: <4EBD6B47.1070501@jesup.org>
To: public-webrtc@w3.org
On 11/11/2011 1:07 PM, Vincent Scheib wrote:
> May I ask for clarification on unreliable? See inline:
>
> On Mon, Oct 31, 2011 at 12:50 AM, Justin Uberti <juberti@google.com
>  > In reliable mode, in-order delivery of messaging is guaranteed, which
>  > implies head-of-line blocking. In unreliable mode, messages may
>  > arrive out
>  > of order. In either mode, notifications of message delivery are
>  > communicated to the application via a callback; in unreliable mode,
>  > failures (defined as an elapsing of 2 RTT without an acknowledgement) are
>  > communicated through the same callback.
>
> In unreliable mode I understand not all messages will be received.
> However, what will happen with out of order packets? This recent
> documentation from Ericsson Labs concerns me:
> https://labs.ericsson.com/apis/web-real-time-communication/documentation#Peer-to-Peer_data
> "This data channel is unreliable (as UDP is used for transport) and
> packets received out of order are discarded."

In unreliable mode, I would strongly disagree with that.  Out-of-order 
packets should be flagged to the application (if by no other means than 
by providing a sequence #, or by the application being responsible for 
adding sequence numbers to its own data packets), but the application 
should decide if they're important or processable.  I've had connections 
*at work* that could get >1% OOO packets on RTP, for *years*, and was 
running videophone calls from this network all the time.  (Some sort of 
weird router-and-bonded-T1 issue the provider never resolved.)

If the app wants to discard them, fine.  I would be *ok* (though mildly 
concerned) with an app asking the system to discard them for it.

> It seems preferable for applications to have control over what packets
> should be dropped. E.g. Experienced multiplayer game developers have
> raised concern over this.
> https://twitter.com/#!/azaferakis/status/135030240018841602 "They are
> making a severe mistake by automatically discarding out of order
> packets" "I agree, it should be up to the application to decide what to
> do with out of order packets" etc.

Yup.  My draft-jesup-rtcweb-data-01 says we'll do unreliable channels, 
but does NOT say anything about discarding OOO packets; in fact I 
believe it touches on the application getting the option for OOO 
delivery, as well as partial-reliability (bounded retries), both of 
which are properties of SCTP that we could expose.


-- 
Randell Jesup
randell-ietf@jesup.org
Received on Friday, 11 November 2011 18:38:16 GMT

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