[Nfd-dev] Why the tags are 64 bit? What is packet overhead?

[Alex Afanasyev]

Hi Aman,

The tags are just in-memory annotation of the packet and have no direct impact on the size of the actual packet.  To encode any value of the tag, you will need to provision it as part of, for example, NDNLP encoding process.   Therefore, if you just added a custom tag on the packet and didn't change any encoding, you will not see any changes in packet size, nor see anything when dissecting the packet.

The code has a few examples for tag encoding, look in face.cpp and detail/face-impl.hpp.

--
Alex

> On Nov 17, 2018, at 7:26 PM, Aman Maldar <amanmaldar at gmail.com <mailto:amanmaldar at gmail.com>> wrote:
> 
> Hello All,
>  I have 2 questions.
> First,
> I have added hopCount tag to the packet. But my questions is why these tags have to be 64 bit log? I tried to use 16/32 bit long tags. I got compilation issues.
> I see the following tags are also defined as 64 bit long. ref - https://github.com/named-data/ndn-cxx/blob/master/src/lp/tags.hpp <https://github.com/named-data/ndn-cxx/blob/master/src/lp/tags.hpp>
> 
> typedef SimpleTag<uint64_t, 10> IncomingFaceIdTag;
> typedef SimpleTag<uint64_t, 11> NextHopFaceIdTag;
> 
> Second,
> Also, is there a way I can calculate the interest packet overhead after adding a new tag?
> If I do following before and after adding the tags, would I get correct result? 
> auto intSize = interest.wireEncode().size();
> 
> Ndn-dissect didn't work very well, I didn't get what I was looking for. Some fields were missing.  
> 
> -- 
> Regards,
> Mr. Maldar Aman M.
> University of Massachusetts, Lowell
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