How do the shepherd and messenger know each other in Oedipus King?

In the play "Oedipus Rex" by Sophocles, the shepherd and the messenger do not know each other. The shepherd is an old man who once worked for King Laius, the father of Oedipus, and is responsible for raising Oedipus after he was abandoned as a baby. The messenger is a servant of King Oedipus who brings news of the death of Polybus, Oedipus's supposed father, and the subsequent revelation that Oedipus killed his real father, Laius, and married his mother, Jocasta.