The Peppol four-corner model
The four-corner model is the delivery architecture that makes the Peppol network work. It is the single most important picture to internalise when you start sending or receiving Peppol invoices. Below is an original SVG diagram, free to embed.
By Peppol Validator · Last updated
What the four corners actually are
A Peppol document touches four logical roles on its way from the seller to the buyer. Each role is one corner. Trading partners sit at corners 1 and 4. Their certified service providers, the Access Points, sit at corners 2 and 3. The corners are not physical locations: they are responsibilities.
Corner 1 is the seller. In practice this is the seller's ERP, accounting package, billing platform or invoicing app. It generates the invoice in whatever native format it uses internally and hands the document to its Access Point. Corner 1 only knows about its own Access Point: it never deals directly with the buyer's side of the network.
Corner 2 is the sender Access Point. It is a certified Peppol service provider. It converts the seller's native format into the agreed Peppol format (Peppol BIS Billing 3.0, which is UBL 2.1 with a CIUS on top of EN 16931), validates the document against the schematron rules, signs and encrypts the message, looks up the recipient endpoint via the SML and SMP, and transmits the document over the AS4 transport profile.
Corner 3 is the receiver Access Point. When the AS4 message arrives, corner 3 verifies the signature and the certificate, decrypts the payload, validates the document one more time, sends a Message Level Response back to corner 2 to acknowledge delivery, and forwards the invoice to the buyer in the format the buyer's system expects.
Corner 4 is the buyer. This is the buyer's ERP or accounting system. Because every field in the invoice is structured and mapped to a known place in the document, corner 4 can book the invoice automatically and reconcile it against a purchase order without manual data entry.
The discovery layer (SML and SMP)
The dashed callout in the diagram represents the Peppol discovery layer. It is a two-tier DNS-based lookup that lets any Access Point on the network find any other participant without prior bilateral configuration.
Each participant has a Peppol Participant Identifier composed of a four-digit scheme code and a value. The sender Access Point hashes the identifier and queries the Service Metadata Locator (SML), which returns the URL of the Service Metadata Publisher (SMP) that holds the recipient's metadata. The SMP returns the receiver Access Point's endpoint, the document types it accepts and the public certificate the sender must use. With those three things in hand, the sender can deliver the document directly over AS4.
In day-to-day use you do not see any of this. You give your Access Point a recipient identifier (often the buyer's VAT or company number) and the discovery, signing, transport and acknowledgement happens automatically.
Why four corners instead of two
The classic alternative is bilateral EDI: the seller and the buyer agree on a format, set up a dedicated connection (often AS2 or SFTP) and exchange documents directly. This works for two parties. It does not scale: every new trading partner is a new integration, a new format mapping and a new certificate to manage.
The four-corner model breaks the chain. Corner 1 only ever talks to corner 2. Corner 4 only ever talks to corner 3. The standard format and the discovery layer mean that any pair of Access Points can exchange documents without prior agreement. A buyer can switch ERP, an Access Point can change provider, or a seller can move to a different country, and the rest of the chain keeps working.
That is the whole point of Peppol: a single connection to one Access Point gives you reach to every other participant on the network, in any country where Peppol is live. Today that is more than 30 countries.
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Related references
- The Peppol network reference : long-form explanation of how Peppol works end to end.
- Peppol five-corner model diagram : how the four-corner architecture is extended for tax e-reporting.
- Peppol document flow diagram : the lifecycle of a single Peppol invoice in eight steps.
- Peppol Access Points : what corners 2 and 3 actually do, and how to choose one.
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Frequently asked questions
What is the Peppol four-corner model?
The four-corner model is the architecture Peppol uses to deliver electronic documents between trading partners. The seller (corner 1) hands a document to its certified Access Point (corner 2), which transmits it across the Peppol network to the buyer's Access Point (corner 3), which forwards it to the buyer's accounting system (corner 4). Senders and receivers never connect directly: they only ever talk to their own Access Point.
Why four corners and not two?
Because the Access Points decouple the trading partners from each other. With only two corners (sender and receiver) every business pair would need a bilateral integration, like classic EDI. The four-corner model means a single connection to your Access Point gives you reach to every other participant on Peppol, regardless of which software the other side uses.
What do the SML and SMP do?
The SML (Service Metadata Locator) is a DNS-based directory that, given a participant identifier, returns the URL of the SMP (Service Metadata Publisher) holding that participant's metadata. The SMP returns the recipient Access Point's endpoint URL, the document types it supports and its public certificate. The sender Access Point uses these together to know where and how to deliver a document.
Is this the same as the five-corner model?
No. The five-corner model adds a fifth corner: the tax authority. As an invoice flows from seller to buyer, a copy or summary is reported in real time to the national tax administration. Belgium will introduce a Peppol five-corner model on 1 January 2028 for tax e-reporting. See the dedicated five-corner diagram for the difference.
Can I use this diagram in my own article?
Yes. The diagram is an original SVG, free to embed with an attribution link back to peppolvalidator.com. The embed snippet on this page gives you the exact HTML to paste. The SVG file also carries a Dublin Core copyright block so the attribution travels with the image when downloaded.