Author Topic: SBS2003 with SBS Suite download slow down  (Read 3551 times)

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zpolaris

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SBS2003 with SBS Suite download slow down
« on: September 29, 2006, 12:24:24 PM »
I'm trying SBS Suite, before purchasing, and i experience slow download speed in file retrieved from web (tried driver for EpsonNet JobTrackerWM  50MB). ISA2004 seem correctly configured. The file download speed in other PC is about 250KB/s, on SBS2003 with Avast is about 30KB/s.

I try a direct file transfer from a PC (in external network) to SBS2003 and it perform at full speed.

Can anyone help me?

Thank's in advance.
Cla.


Offline Vlk

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Re: SBS2003 with SBS Suite download slow down
« Reply #1 on: September 29, 2006, 04:40:40 PM »
This might be a "mirage" caused by the way the avast ISA provider works. The thing is, the provider caches the data stream on the server so that it can scan it safely.

Let me explain this in a bit more detail. Because of the stream nature of the HTTP protocol, it is not easy to implement a realtime HTTP virus scanner that would work correctly and still not influence the data tranfer characteristics. The fact (that cannot be changed) is that virus scanning is not a STREAM algorithm; many times, it needs the object in its entirety to reliably decide whether it's infected or not (think e.g. of packed contents). Therefore, what we do is cache the file on the server, "trickling" only a certain percentage of it to the consumer (the browser) - therefore the user sees reduced download speed, while in fact the speed is only reduced from ISA to the user, not from the Internet to ISA. When the last part of the file is eventually recived, avast (running on the ISA server) scans it, and if it's considered safe, blasts it to the browser (at once). I.e. typically, what the user sees is reduced download speed, then the download meter reaches certain threshold (by default, 25%) - this is the point when the file is already downloaded to ISA - and then, the meter suddenly jumps to 100% (because transfer speed from ISA to the user is typically very high - it's on LAN).

The "trickle ratio" can even be customized in the avast4.ini file. By default, it has a value of 4 (i.e. for each 4 bytes received on the ISA server, 1 byte is sent to the client - hence the threshold discussed above is 25%). Setting the value higher will make the download speed optically lower, but will improve the chance that no viral contents will be leaked. On the other hand, setting it lower number will cause download meters to move faster but will increase the risk of leaking infected contents...

The relevant key in avast4.ini is like this:

[MS Proxy/ISA]
TrickleRatio=<number>


You will need to restart the ISA service for the changes to become effective.


Hope this helps,
Vlk
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zpolaris

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Re: SBS2003 with SBS Suite download slow down
« Reply #2 on: September 29, 2006, 05:26:13 PM »
Wow... this is so called EXPLAIN the problem.....

Next i try it....

Thank again.