Sign-on
The link comes up and is announced. Key exchange runs on the same message pair.
T-NET / Switch
A protocol engine you can configure, not a codebase you inherit.
ISO 8583-1:1987 with a data-driven packaging profile. The field map, the message-type matrix, the processing codes and the response codes are configuration — so a new endpoint is a profile rather than a release.
7.2 · Functional specification
The modules.
| Module | Specification |
|---|---|
| Protocol engine | ISO 8583-1:1987 with a data-driven packaging profile: field map, message-type matrix, processing codes and response codes are configuration, not code. Binary primary and secondary bitmaps, ASCII field bodies, and a two-byte binary length header on the TCP stream. |
| Message types | Authorisation and pre-authorisation (0100/0110), financial (0200/0210), advice (0220/0230), reversal (0400/0410), core-banking-side reversal (0420/0430) and network management (0800/0810) for sign-on, sign-off, echo and key exchange. |
| Processing codes | Purchase, purchase with cashback (cashback portion carried in the additional-amounts field), refund within the agreed window with the original reference, balance inquiry, card and token authentication, and settlement batch. Legacy codes are normalised at the edge of the switch. |
| Session management | Persistent TCP sessions with an active and a standby connection per endpoint, primary and DR ports, echo heartbeat every 60 seconds, three retries and automatic failover. |
| Pre-auth lifecycle | Authorised, incremented, completed or released states with expiry handling, matched through the original-data and replacement-amount fields. |
| Duplicate detection | Replay window keyed on message type, primary account number, processing code, amount, trace number, retrieval reference and terminal; a replay returns the stored response rather than a second authorisation. |
| Reversal engine | A financial-response timeout triggers an automatic reversal carrying the original data elements, retried three times and then queued with an alarm; a reversal with no original is answered with the agreed decline code. |
| HSM security services | PIN translation from the terminal's DUKPT base derivation key to the scheme working key and between working keys, PIN verification, MAC generation and verification on every inter-switch message, EMV cryptogram services and data-block encryption — delivered by a domain-separated HSM service whose acquiring command set is isolated from issuing and ATM commands. |
| Risk & fraud monitoring | Real-time rules per institution, merchant and terminal: velocity, high value, geography, rapid repeats, MCC restrictions and card-type restrictions; risk scoring with decline or alert actions. |
| AML screening hook | Screening call-outs on acquiring authorisation and settlement flows to the institution's AML system, with alert routing and case references. |
| Clearing, settlement & chargebacks | Clearing file generation and ingestion, settlement position calculation per merchant, institution and currency, settlement reports, and the retrieval, chargeback, representment and write-off case workflow. |
Sessions
A link that tells you it is alive.
Sign-on, then an echo every sixty seconds. A standby connection is held open beside the active one, across primary and DR ports. Three retries, then failover — without anyone being paged to notice first.
Every 60 seconds
A heartbeat on the active connection. Silence is a signal, not an absence of one.
Held, not dialled
The second connection is already open. Failover is a switch of traffic, not a reconnect.
A second address
Primary and DR ports per endpoint, so a site failure is a route change.
Reversals & duplicates
The two mechanisms that decide your Monday.
Reversal engine
A financial response that does not arrive within the timer is not an unknown. It is a reversal.
- Timeout on the financial response triggers an automatic 0400
- The reversal carries the original data elements, not a reconstruction
- Three retries, then queued for delivery with an alarm to operations
- A reversal received with no original is answered with the agreed decline code
Duplicate detection
A frozen screen and a second tap must not become a second authorisation.
- Replay window keyed on message type, PAN, processing code and amount
- Plus trace number, retrieval reference and terminal identification
- A match returns the stored response, unchanged
- The cardholder is authorised once, and the merchant is paid once
Pre-authorisation
Four states and an expiry.
Hotels, fuel and rental live here. Each state transition is matched through the original-data and replacement-amount fields, so a completion for a lower amount releases the difference rather than leaving it held.
Authorised
The hold is placed. The original data elements become the key for everything that follows.
Incremented
A further amount is added to the existing hold rather than raising a second, unrelated authorisation.
Completed
Captured at the final amount through the replacement-amount field; the unused portion of the hold is released.
Released or expired
An explicit release, or expiry handling when nothing completes the hold within the window.
Risk, fraud and AML
Rules that belong to an institution.
Rule sets are named in the merchant suite, assigned to merchants and terminals, and enforced here at authorisation time — scoped by institution so one acquirer's rule never fires on another's traffic.
Rules at authorisation
Velocity, high value, geography, rapid repeats, MCC restrictions and card-type restrictions, per institution, merchant and terminal.
Decline or alert
Risk scoring with a configured action — decline the transaction, or approve it and raise an alert for review.
Screening hook
Call-outs on acquiring authorisation and settlement flows to the institution's AML system, with alert routing and case references.
Next step
Send us your message spec. We will send back a profile.
Your scheme endpoint, your CBS interface and your terminal mix are enough for us to come back with an ISO 8583 packaging profile, a sizing note and a certification schedule.