Re: [LARTC] Modems: Cable or DSL digital blunders that lartc may

[Ed Wildgoose <lists@wildgooses.com>]

>http://www.lammertbies.nl/comm/info/RS-232_flow_control.html
>ECN is software flow control.  There is also icmp for software FC?  The
>idea is too prevent 'buffer underruns' in the modem, any SNMP or other
>stats on this buffer would also provide SWFC.  
>  
>

I don't consider this to be "flow-control" in the sense of throttling 
sending.  This is really a syncronisation protocol.

>http://www.nwfusion.com/netresources/0913flow.html
>This is ethernets HWFC, that any directly connected modem could use.  The
>idea then is for the directly connected computer to use SwFC(ECN or
>?icmp?) to pass this FC onto other hosts using the modem.
>  
>

Hmm, I wasn't aware of a "pause" ability for ethernet.  But still this 
is a very low level layer 1 protocol.  In other words this will calm 
your 100mb net card talking to your 10mbit port on your cable modem.  
But it has no idea that the cable modem only has a 256kb link

You need something which works at IP level or above.  TCP (level higher) 
has some stuff, but (I repeat) it basically involves dropping traffic 
until the sender slows down.  There are protocols like ECN, but they are 
broadly unsupported.  ICMP stuff is frequently dropped by 
routers/firewalls making it problematic (look at how difficult it is 
just to do MTU discovery!)

What's your question though?  Read the LARTC howto and the ADSL QOS 
howto.  They are both excellent docs.  Also read up on some basic TCP 
notes.  There is nothing really clever that you can do - it's all in the 
docs you just have to work around the limitations of the protocols

Ed W
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