Should I order references on surname or paper title when using Harvard style? I use the harvard referencing style.
One of our assignment instructions is: 


  The reference list should be in alphabetical order


Lets use the following references as an example 


  Kopecky, J. and Simperl, E., 2008. Semantic Web Service Offer
  Discovery for E-commerce.
  
  Sanvido, F., Sanchez, D., Mendoza, A. and Lopez, A., 2011. Dynamic
  Negotiation Layer for Secure Semantic Service Oriented Architectures.


Following the instruction I was given to order the references, is it generally implied that I should order the references by the first authors surname: Kopecky then Sanvido.
Or is it implied that I should order by the paper titles: "Dynamic Negotiation Layer..." then "Semantic Web Service..."
From Harvard website: 


  The entries in the list should be in alphabetical order by the
  author's last name [sic].


As pointed out by aeismail in the comment, the second criterion is the date:


  Works by the same author should be ordered chronologically, from oldest to most recent


Of course, you might have many articles with the same author and same year, in that case, they say that:


  If your reference list includes multiple works by the same author in
  the same year, identify them in your parenthetical citations and in
  your reference list by a lowercase letter after the year, assigning
  each letter in alphabetical order by the title of the work