NAME

Net::uTP - Micro Transport Protocol (μTP, BEP 29)

SYNOPSIS

use Net::uTP;

my $utp = Net::uTP->new(
    conn_id_send => 1234,
    conn_id_recv => 1235
);

# Initiate connection
my $syn_pkt = $utp->connect( );

# Handle incoming UDP data
my $reply = $utp->receive_packet( $raw_udp_payload );

# Send data (returns one or more μTP packets)
my $pkts = $utp->send_data( 'Hello World' );

# Periodically handle retransmissions
my $resends = $utp->tick( 0.1 );

DESCRIPTION

Net::uTP implements μTP, the Micro Transport Protocol (originally μTorrent transport protocol), a congestion-controlled UDP transport designed to prevent background traffic from "choking" other internet activity (like VoIP or gaming) on the same network.

While originally developed for BitTorrent, this module is general-purpose and can be used for any application requiring high-throughput, low-impact data transfer.

Key Mechanisms

• LEDBAT Congestion Control: Uses one-way delay measurements to detect network congestion before packet loss occurs. It targets a specific queuing delay (100ms) and yields to other traffic.
• Selective ACKs (SACK): Efficiently recovers from packet loss by acknowledging specific received packets out-of-order, reducing redundant retransmissions.
• Out-of-Order Reassembly: Automatically buffers and re-sequences packets received out of order, delivering a contiguous stream to the application.
• Fast Retransmit: Detects dropped packets via duplicate ACKs and resends them immediately.

METHODS

connect( )

Transitions to SYN_SENT state and returns the raw ST_SYN packet to be sent over UDP.

receive_packet( $data )

Processes an incoming raw μTP packet.

• Updates congestion windows and delay metrics.
• Emits data or connected events.
• Returns a ST_STATE (ACK) packet if a response is required.

send_data( $payload )

Fragments the payload into 1400-byte μTP data packets and returns them. Data is only sent if within the current congestion window.

tick( $delta )

Handles timers for retransmissions. If a packet has not been ACKed within the current RTO (Retransmission TimeOut), it is resent.

state( )

Returns the current connection state: CLOSED, SYN_SENT, CONNECTED, etc.

on( $event, $callback )

Registers a handler for:

• connected: Handshake successful.
• data: contiguous payload received.
• closed: Connection lost or reset.

AUTHOR

Sanko Robinson sanko@cpan.org

COPYRIGHT

Copyright (C) 2026 by Sanko Robinson.

This library is free software; you can redistribute it and/or modify it under the terms of the Artistic License 2.0.