Track M · Messages and integrity

You will be able to

• Frame a message with a 2-byte binary or 4-byte ASCII length, or STX / ETX / LRC
• Explain TPDU, timeouts, retries and reversal on timeout
• Use STAN and RRN, and echo tests

The lesson

1. Why it matters
2. The concept
3. Step by step: a request that times out
4. Common mistakes
5. What to test

In the module

• The lesson, with diagrams, common mistakes and what to test
• A worked example you can change: Framing a message
• Practice: 3 kinds of generated exercise, each with a step-by-step solution
• A quiz of 8 questions

Key terms

Duplicate detection

Recognising a repeated request so it is not processed twice, typically by STAN, terminal, date and amount.

Framing

How a receiver finds where a message starts and ends on a stream: length headers, STX and ETX, or delimiters.

Keep-alive

Messages or TCP options that keep an idle link open and detect a dead peer. On ISO 8583 links, echo tests serve this purpose.

Length header

Bytes in front of each message on a TCP link giving its length, often 2 bytes binary big-endian, sometimes 4 ASCII digits. Both sides must agree whether it counts itself.

Message matching

Pairing a response with its request using the MTI, STAN, terminal and date, especially on links that carry many transactions at once.

STX / ETX

Start of text (02) and end of text (03): bytes that frame a message on serial and some dial links, usually followed by an LRC.

Timeout

The time a sender waits for a response before acting. A timeout on a financial request must lead to a reversal, not a silent retry.

TPDU

A 5-byte header used on some terminal networks with a type and source and destination addresses, used by network access points to route.

Tools for this module: LRC / CRCISO 8583 Message

Read the opening of the lesson →