[ndnSIM] some questions about 'limit'

[Alex Afanasyev]

Hi Chengcheng,

> On Nov 3, 2014, at 5:41 PM, Chengcheng Li <lengcangche at bupt.edu.cn> wrote:
> 
> Hi, everyone
> ​I'm so confused about the concept 'limit' in the ndnsim code.
> 1. On this page 'http://ndnsim.net/fw.html',  a equation is presented according to which limit is set. What are the meanings of Delay and Bandwidth? Is it about a single link or all the links?

This is something similar to TCP’s delay-bandwidth product (http://en.wikipedia.org/wiki/Bandwidth-delay_product <http://en.wikipedia.org/wiki/Bandwidth-delay_product>).  To “fill the pipe” with Interest-Data, you need to allow a certain number of outstanding Interests and this certain number would depend on path’s delay.

> 2. And I notice that the dimension(量纲) of limit in this equation is number of interest, not number per second. I think this doesn't comply with ns3::ndn::Limits::Rate <http://ndnsim.net/doxygen/classns3_1_1ndn_1_1_limits_1_1_rate.html> situation.

It still applies, but just in a different way. Limits::Rate implements a token bucket shaping algorithm (http://en.wikipedia.org/wiki/Token_bucket <http://en.wikipedia.org/wiki/Token_bucket>) and `limit` defines the size of the bucket = number of allowed burst (bucket is “refilled” with a certain rate).  Limits::Window implements a simple windowing mechanism, where `limit` is just a number of outstanding interests.

> 3. Following that, a code is presented: 'ndnHelper.EnableLimits (true, Seconds (0.2), 40, 1100);', does this mean that all the links' delay in topology are 0.2?

It doesn’t mean, but implies that in your simulation scenario paths over which Interest/Data are exchanges will be ~200ms.  This method makes a big assumption that this will be constant, which may not be applicable for some scenarios.

—
Alex


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