ePub specifications clarification needed

Question

just wanted some help on ePub specifications.. Is it mandatory for the toc.ncx to have the src(ie. the xhtml) .I have observed the same content src is also available in the .opf file.

Answer 1

Yes, that is mandatory and that's a design problem :

Overlap between NCX and OPF metadata

Because the NCX is borrowed from another standard, there is some overlap between the information encoded in the NCX and that in the OPF. This is rarely a problem when you generate EPUBs programmatically, where the same code can output to two different files. Take care to put the same information in both places, as different EPUB readers might use the values from one or the other.

Source

That's not the case anymore in the future version of the norm (ePub 3), but please note that no device support it currently.