# Tor Hidden Services: A Systematic Literature Review
## 5 Results and Discussion
### What Are the Main Research Areas and the Main Findings Regarding HS?
#### 5.1.3 Discovery and Measurement  
This section includes articles that retrieved as many.onion addresses as possible, and tried measuring the size of the Tor hidden network from different perspectives.  
The first document is from 2009 [81], and [PERSON] et al. were the first researchers to mention that the dark network is widely interconnected. They manually located 39 hidden services, including directory sites, and from those 39, they found 20,499 more HS. Later, this concept was taken up again in 2017 by [PERSON] et al. [36], but they showed that the THS that offer specific services contain few or no references to other HS, whereas those that advertise and link to a large number of other pages are much more likely to be known and accessible from public websites.  
[PERSON] and [PERSON] [34] attempted to discover HS by operating 40 ORs over six months, each with a bandwidth of approximately 50 kB/s and left active continuously for 25 h, with the intention that their nodes or one of them would be eligible to obtain the HSDir indicator and be able to recover the maximum number of.onion addresses possible. In the study period, they were able to observe approximately 80,000 HS. This is the first article that exposed the short life of the hidden services, many of which were observed to exist at most a few weeks before their closure. In all, only 15% of the HS they found persisted through their six-month observation period. [PERSON] et al. [76] also took up this technique again, but with a cost evaluation, they quantified the relationship between resources consumed and hidden services collected.  
Taking into account [PERSON] and [PERSON]'s work [34] that exposed the short lifetimes of the services in Tor, [PERSON] et al. [82] indicated that the dark web is much smaller than many people think, since most onion services do tend to be ephemeral, so they do not all coexist at once.  
Other studies [35, 45] were based on the discovery of hidden services through search engines with specific keywords that allowed them to extract.onion addresses--for example,from sites on the dark web such as wikis, or using common Surface Web search engines such as Google. Thus, [PERSON] et al. in [45] emphasised that during their collection process, they only reached 25% to 35% of the total number of hidden services that Tor says are published daily. They also pointed out that a very high percentage of these nodes are without outbound links. Furthermore, [PERSON] et al. [35] obtained 173,667.onion addresses, but only 4,857 of these were online.  
Finally, as we saw in the previous section, some articles focused on finding the most popular services on the Tor network [25, 32, 34, 44], but most of them did so by conducting actions to discover HS and locating the most referenced.onion addresses, and the researchers who had access to HsDir [32] estimated the popularity of hidden services by looking at the request rates of descriptors.  
To a lesser extent, the research analysed covered issues related to hidden services' performances and the analysis of specific parts of the protocol that they proposed improvements to. These studies are detailed below.