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From: Jan Engelhardt <jengelh@linux01.gwdg.de>
To: vandrove@vc.cvut.cz
Cc: Pierre Ossman <drzeus-list@drzeus.cx>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: ncpfs and TCP vs UDP
Date: Sat, 27 Jan 2007 15:51:42 +0100 (MET)	[thread overview]
Message-ID: <Pine.LNX.4.61.0701271550210.22295@yvahk01.tjqt.qr> (raw)
In-Reply-To: <1169846450.45ba70b2698c2@imap.vc.cvut.cz>


On Jan 26 2007 22:20, vandrove@vc.cvut.cz wrote:
>Quoting Pierre Ossman <drzeus-list@drzeus.cx>:
>
>TCP is definitely preferred.  There are couple of reasons why you should
>prefer TCP:
>
>(1) There is server configuration option to disable NCP/UDP.  You cannot
>disable NCP/TCP that easily.
>
>(2) TCP (NCP over TCP) retransmits only missing data, and it can ask for
>retransmit much sooner as it knows what link latency is.  NCP/UDP can only ask
>for complete packet retransmission, and it has no good idea what's link
>latency because there is no ACK from server when it receives request - you can
>only resend after usual link latency + time for process request, so you'll
>wait longer for retransmit, and on retransmit you need to send again complete
>request (which can be 64KB of data if you use 64KB buffer size...)
>
>(3) To avoid problems with retransmits ncpfs uses default buffer size 60KB for
>TCP (SOCK_STREAM), while 1KB for UDP/IPX (it must be multiple of sector size,
>so using 1.4KB is not an option).  So if you read 1 page, you get 1
>request/reply when using TCP, but 4 requests/replies in UDP/IPX.  And as all
>this is fully synchronous, and for today's link latency is dominating factor,
>it will take 4 times longer...
[and (4)]

Well, probably the same reason as NFS over UDP is discouraged. See nfs(5)
section WARNINGS (in short: IP fragment ID can wrap quite fast on Gigabit)



	-`J'
-- 

  reply	other threads:[~2007-01-27 14:53 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-26 15:54 Pierre Ossman
2007-01-26 21:20 ` vandrove
2007-01-27 14:51   ` Jan Engelhardt [this message]
2007-01-27 15:03     ` Pierre Ossman
2007-01-27 16:06       ` Jan Engelhardt

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