[ale] PPP Perfromance

[Dow Hurst]

The local LAN hardware that supports your LAN has to be able to handle 
jumbo frames if you want to have larger packet sizes than 1500 bytes.  
That is why Linux clusters using Gigabit switches have had to pay more 
for the switches that would support jumbo frames if they want to 
maximize the efficiency of the bandwidth.  The 1500 byte max is also a 
limiting factor on PPPoE since the LAN behind the DSL router should have 
an MTU of less than 1500.  How much less is debatable but the Roaring 
Penguin software recommended 1412 if I remember correctly.  You want the 
LAN ethernet frames to be efficiently packaged inside the PPP frame 
which is packaged inside the ATM frame.  No wonder everyone wants a 
straight ethernet instead of PPPoE for their broadband!!  Fragmenting 
the local LAN packets just lowers the efficiency of your PPPoE 
connection and the overhead of the router's rebuilding the fragmented 
packets.

To switch to another topic of LAN efficiency at a different layer:  For 
TCP and UDP packet size over a local LAN I've played with NFS packet 
sizes to see what is most efficient. Here in our VPN at KSU I found that 
udp and tcp required different values.  So, packet size at the ethernet 
layer and at the higher layer has a big effect on efficiency.  Within a 
local LAN on normal hardware 1500 MTU is best since you get the largest 
ethernet packet per packet transmission while a UDP/NFS packet size of 
64K instead of the default 8K might increase efficiency.  It is good to 
spend an afternoon testing such things to tune the network traffic.  I 
use SGI IRIX so my NFS packet sizes for UDP and TCP might be tuned 
differently than a Solaris or Linux based LAN.  I would imagine that as 
Gigabit switches drop in price and that the models with jumbo packet 
capability also drop that eventually we might be able to take advantage 
on home LANs for passing video and such.  I don't know alot about jumbo 
packets so I can't say anything other than just this little bit.  Jeff 
Layton might have a comment?
Dow


Doug McNash wrote:

>
> The max number of data bytes one can put in a 802.3 Ethernet frame is 
> 1500 per IEEE standard
>
>
> On Fri, 31 Oct 2003 10:53:49 -0500
>  Geoffrey <esoteric at 3times25.net> wrote:
>
>> Doug McNash wrote:
>> Getting back to the mtu issue, I'm now curious as to whether that 
>> might be part of the problem why I couldn't get to my mother-in-laws 
>> benefit web site.  I've played with mtu a bit and found a couple 
>> things out. Currently all my ethernet networks are set at mtu 1500.  
>> I can easily reduce this by simply:
>>
>> ifconfig eth0 mtu 1000
>>
>> But if I attempt to increase it, I get an error.  Me thinks this 
>> would because there's a max set somewhere, although after running 
>> around in /proc for a while, I couldn't find anything.  Anyone else 
>> have any ideas?
>>
>>
>> -- 
>> Until later, Geoffrey    esoteric at 3times25.net
>>
>> Building secure systems inspite of Microsoft
>>
>> _______________________________________________
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>> Ale at ale.org
>> http://www.ale.org/mailman/listinfo/ale
>
>
> -- 
> Doug McNash <dmcnash at yahoo.com>
> _______________________________________________
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> Ale at ale.org
> http://www.ale.org/mailman/listinfo/ale
>

-- 
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Dow Hurst                  Office: 770-499-3428            *
Systems Support Specialist    Fax: 770-423-6744            *
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