Re: error model/losses

[Tarik Alj <aljtarik@ozias.inrs-telecom.uquebec.ca>]

Chris,

	links and LANs are not exaxctly defined the same, LANs use virtual links 
to represent connectivity within the LAN. And a virtual node so that it is seen 
as a single node from IP level.
	Since everybody on a LAN basically shares the same link, you would need 
to insert the errors at the src and dst interfaces. The filter is there to 
filter packets coming from src, otherwise errors would happen on packets coming 
from all the nodes on the LAN.
			
> X-Authentication-Warning: tuppes.cse.ucsc.edu: chris owned process doing -bs
> Date: Tue, 8 Feb 2000 10:22:57 -0800 (PST)
> From: Chris Parsa <chris@cse.ucsc.edu>
> To: ns-users@mash.cs.berkeley.edu
> cc: Chris Parsa <chris@cse.ucsc.edu>
> Subject: error model/losses
> MIME-Version: 1.0
> 
> Hi,
> 
> I've been looking at the script snoop.tcl to see an example of adding
> losses to a LAN.   I am really confused why the function add-errors
> uses a Filter and commands such as:
> 
> proc add-error {lan src dst errmodel} {
> 	$lan instvar lanIface_
> 	set nif $lanIface_([$dst id])
> 	set filter [new Filter/Field]
> 	$nif add-receive-filter $filter
> 	$filter filter-target $errmodel
> 	$errmodel target [$filter target]
> 	$filter set offset_ [PktHdr_offset PacketHeader/Mac macSA_]
> 	$filter set match_ [$src node-addr] 
> }
> 
> Can't we simply do something like this:
> $ns lossmodel $errormodel $node0 $node1
> 
> Is there something different about inserting link errors on Lans compared
> to point-to-point links?
> 
> Thanks for any clarification!!
> 
> Chris

Tarik