title: What are the chances of losing a UDP packet?
body: <p>Okay, so I am programming for my networking course and I have to implement a project in Java using UDP. We are implementing an HTTP server and client along with a 'gremlin' function that corrupts packets with a specified probability. The HTTP server has to break a large file up into multiple segments at the application layer to be sent to the client over UDP. The client must reassemble the received segments at the application layer. What I am wondering however is, if UDP is by definition unreliable, why am I having to simulate unreliability here?</p>
  <p>My first thought is that perhaps it's simply because my instructor is figuring in our case, both the client and the server will be run on the same machine and that the file will be transferred from one process to another 100% reliably even over UDP since it is between two processes on the same computer.</p>
  <p>This led me first to question whether or not UDP could ever actually lose a packet, corrupt a packet, or deliver a packet out of order if the server and client were guaranteed to be two processes on the same physical machine, guaranteed to be routed strictly over localhost only such that it won't ever go out over the network.</p>
  <p>I would also like to know, in general, for a given packet what is the rough probability that UDP will drop / corrupt / or deliver a packet out of order while being used to facilitate communication over the open internet between two hosts that are fairly geographically distant from one another (say something comparable to the route between the average broadband user in the US to one of Google's CDNs)? I'm mostly just trying to get a general idea of the conditions experienced when communicated over UDP, does it drop / corrupt / misorder something on the order of 25% of packets, or is it more like something on the order of 0.001% of packets?</p>
  <p>Much appreciation to anyone who can shed some light on any of these questions for me.</p>