An Impression is a single occurence of an advertisement or creative displayed in a web browser.  Each time an advertisement loads onto a users screen, the ad server may count that loading as one impression. However, the ad server may be programmed to exclude from the count certain nonqualifying activity such as a reload, internal user actions, and other events that the merchant and ad serving company agreed to not count.

Technically speaking.... how does LinkTrust track ad impressions?
 

1. We first check to see if the img parameter is specified indicating we will be redirecting to that image.  If anything is specified for the img parameter we check if it is a valid url and if it is we use that as a redirect location.
2. We then attempt to parse out the campaign id.  If it is present we ensure it is a numeric value.  If it is not specified or is not numeric we will fail with a 3001 error.
3. We then attempt to parse out the affiliate id.   If it is present we ensure it is a numeric value.  If it is not specified or it is not numeric we will fail with a 3002 error.
   For the impression tracking if an error occurs such as those listed above we will redirect to the image url if it was specified and if not we just serve a blank 1x1 pixel.
   Next we try and load the campaign from the database.  If the campaign id we have is invalid meaning there is no campaign with that id then we fail with a 3001 error.  The same happens if the campaign is not for the current partner.  Note, when we do this check if a campaign has been soft deleted it will be treated as if it does not exist.
   Next we try and load the affiliate from the database.  If the affiliate id we have is invalid meaning nonexistent in the database or it isn’t for the current partner we fail with a 3002 error.
   If the affiliate is suspended we fail with a 3003 error
   If the affiliate is blocked from the campaign we fail with a 3003 error
   If the affiliate is unauthorized for the campaign and the campaign does not allow unauthorized affiliates we fail with a 3003 error.  If unauthorized affiliates are allowed we continue.
4. Next we look at the following other parameters: sub id, creative id (adid). 
   If the sub id is specified we use it.  If the sub id is over 100 characters we trim it to 100 characters.  If it contains any < or > characters we put in html escape codes for them.
   If the creative id is specified we use it if it is numeric, otherwise if it is not a number we ignore it.  If the creative id was specified we attempt to load the creative.  Assuming we find it we ensure it belongs to the current campaign.  If we cannot find the creative or it is not for the current campaign it is ignored.  If it is for the current campaign we check if it is a simple banner creative.  If so and the image url is valid we redirect to that image in the end.  If at any point the creative id was determined not to be good the impression is tracked with creative id 0.
5. We then look up the country the ip address belongs to.  If we cannot determine the country the impression will be tracked for a blank country.
6. We then check if this is a CPM campaign.  If so, we check if the duplicate ip filter is on.  If so we check if this is a duplicate ip address.  If it is we fail with an error code 1004.
   Assuming the duplicate filter is on but the ip is not a duplicate we add it to the duplicate list.  In impression tracking there is a potential for an ip to be allowed multiple times if they all come in close together.
7. We then check if the country for the ip address is authorized.  If not we fail with a 3004 error.  If we couldn’t determine the country from the ip address we always allow the traffic through.
8. At this point we release the user and either redirect them to the image url specified via the parameter or creative or we serve them a 1x1 pixel.
9. We then record that the impression occurred.  For CPM campaigns, if they are not authorized for the campaign, but were allowed because unauthorized affiliates are allowed, we record the impression but do not give them “credit for it”.  It won’t count towards their 1000 impressions in which every 1000 impressions result in a transaction.  For nonCPM campaigns, we track the impression regardless.