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# Transport mechanisms

## 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 useful for flow control.

- `unblock` \- Unblock the transport. _Optional_

For more detail, see the `m:megaco_transport` behaviour definition.

## Examples

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