The UDPTL, RTP, TPKT packetiser

A second level packetiser bundles the IFP packets for transmission. This functions in different ways, depending on the type of transport in use. For the unreliable transports (UDPTL and RTP) the packetiser adds forward error correction (FEC) information to the packets. This can greatly inprove the reliability of the T.38 protocol, at the expense of higher bandwidth. The amount of error correction information added to the packets is implementation dependant.

For UDPTL, two forms of FEC are defined. One simply repeats older T.38 packets as FEC information in the current UDPTL packet. The other generates parity packets over a series of T.38 packets, and includes that parity information in the current UDPTL packet. The type of FEC to be used is negotiated just before T.38 communication begins.

The RTP specifications (RFC3550, RFC3551, RFC2198, RFC2733) define common redundancy and FEC formats, to be used for any payload. Where T.38 packets are carried by RTP, the standard RTP FEC mechanisms are used.

TPKT (RFC1006, RFC2126) encapsulation works over a TCP transport. TCP provides full error correction, though retries may slow it considerably. TPKT encapsulation merely provides the ability to delineate the start and end of the IFP packets in the structureless TCP stream.