Re: [rtcweb] Opening unordered data channel issue

Victor Pascual Avila <> Fri, 23 August 2013 10:40 UTC

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Date: Fri, 23 Aug 2013 12:40:44 +0200
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From: Victor Pascual Avila <>
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Subject: Re: [rtcweb] Opening unordered data channel issue
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I agree -- +1 for b)

On Thu, Aug 22, 2013 at 7:12 PM, Justin Uberti <> wrote:
> I agree that b) is the cleanest choice. While this makes the handshake
> 2-way, it does not suffer from the performance issues that affected the
> previous 2- and 3-way handshake proposals, since data can be sent
> immediately without waiting for the ACK.
> On Thu, Aug 22, 2013 at 8:52 AM, Michael Tuexen
> <> wrote:
>> Dear all,
>> at the last IETF in Berlin we discussed how to handle the opening of
>> unordered data channels (no matter wether reliable or not). The problem
>> to be handled is that it is possible that user messages arrive at the
>> receiver before the DATA_CHANNEL_OPEN message is received. The result
>> of the discussion at the meeting was that the best solution is that
>> the sender sends user messages also ordered (on unordered channels)
>> until there is an indication from the peer that the DATA_CHANNEL_OPEN
>> has been processed. This makes sure that the DATA_CHANNEL_OPEN message
>> is not received before user messages and therefore the above problem
>> is solved.
>> The open issue is, what the indication from the peer is. There were
>> two options discussed at the meeting and it was decided that Randall
>> Stewart and myself bring this to the list with the pros and cons
>> of the alternatives. So here are the two options:
>> (a) Use any user message sent from the peer as an indication.
>> (b) Use a new ACK message.
>> Arguments:
>> * Using (b) introduces a 2-way handshake for unordered data channels.
>>   However, it also works for data channels being used unidirectional.
>> * Using (a) stays with the 1-way handshake, but in case of
>>   data channels being used unidirectional you will not see the benefits
>>   of unordered delivery.
>> We had some discussions offline and think that using (b) is the
>> cleaner solution. Please note that this 2-way handshake is not
>> similar to the one we had some time ago. The ACK does not contain
>> any additional information and it does not introduce any delay.
>> You can send user data right after opening the data channel.
>> For consistency we would even propose to use this 2 way handshake
>> for ordered channels. It doesn't have any drawback and would mean
>> that we use the same handshake for all data channels.
>> So the proposed procedure for opening an arbitrary data channel is:
>> 1) The opener sends a DATA_CHANNEL_OPEN message ordered and reliable.
>> 2) The opener can start sending user data immediately.
>>    It will always be sent ordered, no matter whether the data channel
>>    is ordered or not.
>> 3) The peer will send a DATA_CHANNEL_ACK message ordered and reliable
>>    in response to the DATA_CHANNEL_OPEN message.
>> 4) After reception of the DATA_CHANNEL_ACK message,
>>    the opener will sent user messages unordered,
>>    if the data channel is unordered and ordered, if the
>>    data channel is ordered.
>> Any opinions?
>> Best regards
>> Michael
>> _______________________________________________
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