Looking for a manga.. Round Tin Can....

It's about a guy suddenly getting a flat round tin can that can be opened up and close, this tin can receives a letter from someone out of nowhere. The guy got curious and he replied back and was able to communicate with the other person. The guy was talking to a boy in the past through the tin can and a few days passed the boy stopped sending him letters, the final letter was meant as a good bye to him since the boy from the past might die. The guy then realizes the one who he is talking too was his bestfriend's "past", his friend set it up so they could meet in the current timeline.