Network administrators who do not appreciate this sort of hole in their firewall and are worried about abuse, are left with only one option – they have to block outgoing UDP traffic, or limit it to essential individual cases. UDP is not required for normal internet communication anyway – the web, e-mail and suchlike all use TCP. Streaming protocols may, however, encounter problems, as they often use UDP because of the reduced overhead.

via How Skype & Co. get round firewalls – The H Security: News and Features.