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Re: RTCDataChannel characteristics and failures -API description -

From: Gunnar Hellstrom <gunnar.hellstrom@omnitor.se>
Date: Sat, 16 Aug 2014 20:10:30 +0200
Message-ID: <53EF9E96.4060403@omnitor.se>
To: Michael Tuexen <Michael.Tuexen@lurchi.franken.de>
CC: Martin Thomson <martin.thomson@gmail.com>, Adam Bergkvist <adam.bergkvist@ericsson.com>, Web-rtc <public-webrtc@w3.org>
We had some mail list discussions on failure handling in the 
RTCDataChannel, but nothing has been decided and entered into the spec.
http://dev.w3.org/2011/webrtc/editor/webrtc.html#rtcdatachannel

I suggest the following:

1. Section 5.2, third paragraph, ending with "(TODO: reference needed)"
  Replace "(TODO: reference needed)" with the same references as at the 
end of the first paragraph:
"[RTCWEB-DATA 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#bib-RTCWEB-DATA>] and 
[RTCWEB-DATA-PROTOCOL 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#bib-RTCWEB-DATA-PROTOCOL>]."

2. The reliable mode cannot ensure that data is delivered, it can just 
make efforts. It can fail after a limited number of retransmissions and 
it is important that implementers understand that.
Section 5.2, fourth paragraph, replace:
"A reliable channel ensures that the data is delivered at the other peer 
through retransmissions."
with
"A reliable channel makes efforts to deliver data at the other peer 
through limited retransmission. If these limited transmissions fail, the 
underlying transport will be abruptly aborted."

3. What happens when unreliable transmission fails need to be explained 
in section 5.2, fourth paragraph.
After the last sentence in paragraph four in 5.2, insert:
"If the limit for the unreliable transport is exhausted for a message to 
be transmitted, the transmission of the corresponding message is 
aborted. All kinds of channels may get their underlying transport 
abruptly aborted by failure in connectivity checks by the underlying 
transport."

4. The ordered characteristics is not mentioned in the introduction in 
section 5.2.
Since the other characteristics are mentioned, I suggest that also 
ordered/unordered is introduced in a new paragraph, after the fourth 
paragraph in 5.2.
"A||RTCDataChannel| 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#idl-def-RTCDataChannel>|can 
be configured to operate inordered or unordered mode. In (ordered) mode, 
transmitted messages are delivered in order. Both reliable and 
unreliable channels may be set to either of these ordered/unordered 
modes. Ordered mode is the default."

5. After point 6 in the second numbered list in section 5.2, I suggest 
to introduce a brief description of that the transport can be aborted in 
case of transmission errors.
Insert:
"An||RTCDataChannel| 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#idl-def-RTCDataChannel>|object'sunderlying 
data transport 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#dfn-underlying-data-transport>may 
be torn down in an abrupt manner by transmission errors.When that 
happens the user agent/must/||, queue a task that sets the 
object's|readyState 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#dom-datachannel-readystate>|attribute 
to|closing|. This will eventually render thedata transport 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#dfn-underlying-data-transport>closed 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#data-transport-closed>."

Please verify that this is what is supposed to happen!


6. In chapter 11, the error event in the first table needs an explanation.

Replace "TODO" in the third column for the error event the following:
"The||RTCDataChannel| 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#idl-def-RTCDataChannel>|object'sunderlying 
data transport 
<http://dev.w3.org/2011/webrtc/editor/webrtc.html#dfn-underlying-data-transport>has 
detected an error that caused the transport to be aborted".

7. Consider introducing an error event also for transmission failures in 
unreliable channels.
The change proposals above do not include any error indication when 
transmission of messages in unreliable channels fail because of 
exhausted retries.
The ambition is that WebRTC shall be as equivalent as possible to the 
Websocket API.  The Websocket API has however not specified any 
unreliable mode, so we are free to specify what happens on transmission 
errors on unreliable channels. A suitable result of a transmission error 
is to issue an error event but not abort the transport.  Note however 
that STCP Watchdog failure and failure of transmission in reliable 
channels will abort the transport also for the unreliable channels using 
that transport (=RTSP association).

Text proposal TBD.

Regards

Gunnar


------------------------------------------------------------------------
Gunnar Hellström
Omnitor
gunnar.hellstrom@omnitor.se
On 2014-03-03 15:57, Michael Tuexen wrote:
> On 04 Feb 2014, at 19:40, Gunnar Hellstrom <gunnar.hellstrom@omnitor.se> wrote:
>
>> On 2014-02-04 18:41, Martin Thomson wrote:
>>> On 4 February 2014 03:52, Adam Bergkvist <adam.bergkvist@ericsson.com> wrote:
>>>> Does the silence mean that people agree and are fine with these changes
>>>> going into the spec? If anyone have any comments or suggestions, please
>>>> comment.
>>> Please don't assume silence == acceptance.  I'd prefer silence ==
>>> ambivalence, or maybe silence == no thanks.
>>>
>>> The proposed text is correct in part and too specific elsewhere.  I
>>> agree with the note that reliable is instead equated with maximum
>>> retry {count + time}.  I don't agree with setting specific numbers on
>>> this, unless they are minimum requirements for support.
>> A follow-up question:
>>
>> If an application opens a data channel and requests the unreliable type with maximum 7 retries.
>> And it turns out that the reliable type by default makes maximum 5 retries.
>>
>> Will the channel establishment then accept to extend to 7 retries?
>> Or what will happen?
> Even if you configure the SCTP_PR_SCTP_RTX parameter larger than association.max.retrans
> the association will be aborted after association.max.retrans consecutive retransmissions.
>> It looks a bit strange to have an unreliable channel that is more reliable than the reliable one.
> It isn't. SCTP_PR_SCTP_RTX limits the number of retransmissions, it doesn't enforce them.
>> But without documentation about the default retries for the reliable channel this situation might very well happen because the application programmer does not know the figure.
> I think what is written in
> http://tools.ietf.org/html/draft-ietf-tsvwg-sctp-prpolicies-01
> is clear. If not, please let me know.
>
> Best regards
> Michael
>> If it is agreed that it is illogical to set the unreliable channel more reliable than the reliable one, an addition to the API could say:
>> "If the MaxRetries parameter is higher than the default maximum number of retries used for reliable channels, the channel will use the default number, but in other aspects perform as an unreliable channel (e.g. no channel disconnect after exceeded number of retries)."
>> That would meet your view that the proposed text had too much detail, but it covers a logical gap in the API specification.
>>
>> Gunnar
>>
>>
>>
Received on Saturday, 16 August 2014 18:11:07 UTC

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