# 5 Transport mechanisms

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5.1 Callback interface

The callback interface of the transport module contains several functions. Some of which are mandatory while others are only optional:

- send_message - Send a message. Mandatory

- block - Block the transport. Optional

  This function is usefull for flow control.

- unblock - Unblock the transport. Optional

For more detail, see the megaco_transport behaviour definition.

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5.2 Examples

The Megaco/H.248 application contains implementations for the two protocols specified by the Megaco/H.248 standard; UDP, see megaco_udp, and TCP/TPKT, see megaco_tcp.