A method, medium, and communication system are provided for preventing fragmentation when tunneling between a first and second communication network. The communication system includes nodes and routers that transmit packets over the first and second communication networks. The nodes generate requests to identify the maximum transmission sizes of the first and second communication networks. In turn, the nodes format packets based on the minimum of the maximum transmission sizes of the first and second communication networks. The routers transmit the packets formatted by the nodes over the first and second communication networks without fragmenting the packets.