A Message Length Indicator Encoder/Decoder

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Message Length Indicators (MLI) are commonly used in communications over raw TCP/IP sockets. This method of denoting message length is especially popular with users of ISO 8583 messages, a common communication protocol for financial transactions.

This package provides an easy-to-use Encoder and Decoder for Message Length Indicators.


import (

func main() {
	msg := []byte("This is a message")
	// Encoding Example
	mli, err := simplemli.Encode(simplemli.2I, len(msg))
	if err != nil {
		// Do something
	// Append the MLI to the message
	msg := append(mli, msg)
	// Write to TCP Connection
	_, err = conn.Write(msg)
	// Reading MLI from TCP Connection
	b := make([]byte, simplemli.Size2I)
	_, err = conn.Read(&b) // only read the MLI from buffer
	if err != nil {
		// Do something
	// Decoding Example
	length, err := simplemli.Decode(simplemli.2I, &b)
	if err != nil {
		// Do something
	// Reading Message from TCP Connection
	msg := make([]byte, length)
	_, err = conn.Read(&msg)

Message Length Types

There are many common ways to encode message lengths and this library attempts to provide the most common MLI types.

Valid Options are listed in the table below.

Name Description
2I 2-byte network byte order with MLI included
2E 2-byte network byte order with MLI excluded
4I 4-byte network byte order with MLI included
4E 4-byte network byte order with MLI excluded
2EE 2-byte network byte order with MLI excluded, additional 2-byte header is included with message
2BCD2 2-byte Header with a 2-byte binary-coded decimal with MLI excluded
A4E 4-byte ASCII string with MLI excluded

Inclusive vs. Exclusive MLI

An inclusive MLI is an MLI type where the length of the Message Length Indicator itself is included in the MLI value.

For example, if a message is 1500 bytes and 2I encoded, the resulting MLI will have a value of 1502. The reverse is true for Exclusive MLI types, a 1500-byte message with a 2E encoded MLI will have a value of 1500.

When calling the Decoder, the MLI inclusive/exclusive nature is already taken care of. If you pass an MLI with a value of 1502 and decode it with 2I encoding. The resulting integer will be 1500.


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