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From: ahi@duvan.UUCP (Anders Hillbo)
Newsgroups: net.lan
Subject: Problems with IP/UDP speed on Vax750/Unix4.2
Message-ID: <776@duvan.UUCP>
Date: Fri, 7-Feb-86 23:03:54 EST
Article-I.D.: duvan.776
Posted: Fri Feb  7 23:03:54 1986
Date-Received: Tue, 11-Feb-86 06:08:41 EST
Reply-To: ahi@duvan.UUCP (Anders Hillbo)
Organization: The Royal Inst. of Techn., Stockholm
Lines: 35

We want to connect a VAX 750, running UNIX BSD 4.2, with an Interlan
controller and a custom made display station (Intel iSBC 186/51 & 82586
based) to transfer image data. We decided to use Ethernet and an UDP/IP 
implementation on the displaystation to be able to run in user mode on the
Vax.

The problem is SPEED (on the 750). A typical image consists
of something between 64 kbyte and 1 Mbyte of data and the time to transfer
the picture is how long you wait for the picture to show on the screen.

The microprocessor based display station is just now capable of sending data
in up to 300 kbyte/s , while the VAX computer only send data in 33 kbyte/s
(UDP/IP, 1024 byte buffers, one user on the system). If raw IP is used
(super user alone on the system), the speed increases to 182 kbyte/s.

Also when sending TO the Vax the interface or something seems to drop packets
if 4 of them arrive immediatly after each other. (I thought Interlan
could handle back-to-back packets). We know that the packets leave the
sender correctly by spying on the ether from a another displaystation
which of course receives all packets with out any problem.

The goal is a transfer rate at about 100 kbyte/s.

The questions: 
	. Why cant the UDP-interface send packets in a faster speed?
	  Maybe if you could use bigger packets?
	. Is the limitation in the Vax750 or the interface?
	. Is this a well known problem? Anyone else with similar problems?
	. What are the real rules for trailer encapsulation. 4.2
	  only seems to do it when the package is 512 or 1024 bytes.


Anders Hillbo
CS Dept, Royal Inst of Tech.
Stockholm, Sweden