Subject: AW: AW: Usage of IPv6 flow label

From: Metzler Jochen <[email-masked]>
Date: Thu, 30 Nov 2000 11:56:21 +0100

Hi Itojun,

I am not a kernel specialist, but I assume that from
a performance point of view it would be much more performant
to have several flows using only one socket instead of 
having a single socket for each flow? 
An other concern is the redundancy of information in 
ports and labels. 

Cheers,
Jochen

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Jochen Metzler               Siemens AG
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	-----Ursprüngliche Nachricht-----
	Von:	[email-masked] [SMTP:[email-masked]]
	Gesendet am:	Donnerstag, 30. November 2000 11:33
	An:	Metzler Jochen
	Cc:	'Elwyn Davies'; [email-masked]'
	Betreff:	Re: AW: Usage of IPv6 flow label

	>Hi,
	>I fully agree with Itojun's draft on generating the flow label within the network code. But
	>I see no reason, why it should not be (at least) readable from application layer and why
	>a single UDP or TCP session  should not handle more than one streams.

		I admit there could be people who wants to use:
		- multiple flows (= multiple flow label value) in a single TCP session
		- multiple TCP sessions with single flow label value

		however, I could not find a good (a simple and usable) UNIX API to make 
		the above option happen, so I simplified the story.
		At least, the latter case can easily be handled by "single TCP session
		per single flow label".

		I documented the issue in section 5 of my draft.

	itojun
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From: Jun-ichiro itojun Hagino <[email-masked]>
Date: Thu, 30 Nov 2000 21:19:17 +0900

>having a single socket for each flow? 

	I believe there's no performance issue here.  what kind of
	performace drawback do you have in mind?

	I can imagine a use of multiple flow label values for single TCP/UDP
	session, if:
	- a single TCP session includes traffic with different characteristics,
	  for example, MPEG key frames (full picture) and intermediate data
	  (differences between Nth full picture to (N+1)th full picture)
	- the application would like to enforce different RSVP characteristics
	  to key frames and intermediate data.
	- so, application would like to label packets with key frames with flow
	  label X, and intermediate data with flow label Y.  packets with flow
	  label X will have a higher priority than those with flow label Y.
	i'm not sure if the above example makes sense or not.
	even if the example is okay, there's absolutely no kernel performance
	issue here.  the reason why we use two flow label values (X and Y)
	is from the applicatoin needs, not from kernel performance issues.

>An other concern is the redundancy of information in 

	as i noted in previous messages, the purpose of flow label value is
	to provide quick lookup of L4 port pair and protocol type.  so
	I would say that the redundancy is expected.  the redundancy is
	what we really want - intermediate routers do not want to chase
	extension headers to find L4 header.

itojun