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

[Mike Mestnik <cheako911@yahoo.com>]

--- Ed Wildgoose <lists@wildgooses.com> wrote:
> 
> >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.
> 
Yes, SwFC is lame.  Howerver it would be a good band-aid, to have router
and modem syncronisation take place.

> >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
> 
The router(PC) has no idea of the buffer fullness, this is the underlying
problem!  The cable modem needs to triger a "pause" when it's buffer
reaches %80 full, regardless of any other rate limiting.  Linux kernel can
do this with some cards, under heavy CPU strain(htb).

CONFIG_NET_HW_FLOWCONTROL: net/Kconfig

> You need something which works at IP level or above.  TCP (level higher)
> 
No, the DEVICE is not > layer 3.  It's simply a bridge, with a stoplight
on one end.  The idea is to not make any cars crash, that has nothing todo
with the ball game a car might want to get too.

> has some stuff, but (I repeat) it basically involves dropping traffic 
> until the sender slows down.  There are protocols like ECN, but they are
> 
Yes, ECN will/should be used for routers attached to bridges.

> broadly unsupported.  ICMP stuff is frequently dropped by 
> routers/firewalls making it problematic (look at how difficult it is 
> just to do MTU discovery!)
> 
ECN makes this a non-issue.

> 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
> 
I would like to know is maby a ?$6000.00? Cisco cable mobem will not only
use HwFC but ECN as well?  Thought i'd like to find a $150.00 modem that
will just do HwFC, maby an internal one?

> Ed W
> 



		
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