Is the sending of SMS more like TCP, where handshaking occurs to guarantee arrival, or is it more like UDP, where the message is sent "in the dark" with the hope that it arrives, or something else in between?
P.s. I'm not sure if this is the best stackexchange site to have posted this question on. Can anyone suggest the most appropriate stack exchange site for a question on this topic?
Short answer: it's rather like TCP. Messages can fail to deliver but in that case delivery will be retried for a configured period or number of tries. For more details, I recommend the technical details on the Wikipedia page on the subject or asking a more precise question.