{Fraud?} dummynet

Robert Watson rwatson at FreeBSD.org
Fri Oct 27 09:59:00 BST 2006

On Wed, 25 Oct 2006, khoa phan wrote:

> I want to ask You:  when i configure my dummynet router with large delay 
> time, then can packet loss happen? ,and can you show me how to increase 
> network buffer size in dummynet router for overcoming it.

Each dummynet pipe is essentially a software queue with a set of delivery 
properties, including bandwidth, delay, queue size, and packet loss rate. 
Packet loss occurs for two reasons: (1) filling of the queue, and (2) as part 
of probabilistic loss configured on the pipe.  Since delay length affects how 
long a packet remains in the queue, it directly affects how fast data can move 
through the queue.  You may want to google "bandwidth-delay product" for more 
information on this topic.  You can use the "queue" argument when configuring 
a pipe to set the size in slots (packets) or in K.

Be warned that the whole issue of sizing queues/buffers in network stacks, 
routers, etc, is non-trivial, so use large pipe sizes with caution.  One 
particular issue is that FreeBSD places an upper bound on the size of 
in-flight packet data, and that if you start configuring many large pipes, you 
may need to increase that limit.  The main tuning parameter for this is 

Robert N M Watson
Computer Laboratory
University of Cambridge

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