## 5 Transport mechanisms

### 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

### 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.

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