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Re: [core] Head of line message blocking?

Hannes Tschofenig <hannes.tschofenig@gmx.net> Mon, 20 November 2017 12:49 UTC

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To: Carsten Bormann <cabo@tzi.org>, Jim Schaad <ietf@augustcellars.com>
Cc: draft-ietf-core-coap-tcp-tls@ietf.org, core <core@ietf.org>
References: <040301d35f36$2ef6b830$8ce42890$@augustcellars.com> <97B706B3-B7AA-423B-A025-25369C6D8CA1@tzi.org>
From: Hannes Tschofenig <hannes.tschofenig@gmx.net>
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Date: Mon, 20 Nov 2017 13:48:26 +0100
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Subject: Re: [core] Head of line message blocking?
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I agree with the response Carsten provided.

Note: We do touch on the head-of-line blocking issue in TCP, which may
occur when large payloads (such as firmware images) are transported over
TCP and if you only use one TCP connection.

See https://tools.ietf.org/html/draft-ietf-core-coap-tcp-tls-10#section-6

Ciao
Hannes

On 11/17/2017 12:57 AM, Carsten Bormann wrote:
> 
>> On Nov 17, 2017, at 07:53, Jim Schaad <ietf@augustcellars.com> wrote:
>>
>> In UDP there is a vague requirement that a client should only have one message in transit to any given server at a time.  This means that it is up to the client, if it wants to break this rule, to ensure that a response to message A has been received before sending message B if there is going to be a dependency between them.  This is needed because UDP does not do ordered delivery among other things.
>>  
>> TCP however does have ordered delivery.  Thus the question is does the same rule still exist for a client when using a TCP channel.  
> 
> No.  Congestion control in CoAP-TCP is entirely handled by TCP.
> Fire away your requests at will.
> 
>> In theory, it would be possible to send message A and then send message B before getting the answer back and have an assumption that message A would complete before message B starts.  
> 
> There is no implied ordering of the responses as in HTTP/1.1 pipelining.
> The token provides the link between request and response, so that ordering is simply not needed.
> 
>> This implies that the server would need to have an enforcement of such a rule.  My server currently does parallel processing of requests and thus would not do this unless it was enforced by the transport.  Should such enforcement be done?  I see nothing about this in the document so I assume that the UDP rules would still apply.
> 
> Maybe the document should state explicitly why it is silent about this: It does not impose any rule on this.
> 
> Grüße, Carsten
> 
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