Key takeaways
- The Peppol network directory rests on two services: the SMP and the SML.
- The SMP publishes, for each recipient, the documents it accepts and its access point address.
- The SML is the only central service of the network: it tells you which SMP a recipient is in, via DNS.
- You never query these services yourself; your access point does it whenever it needs to locate a recipient.
The Peppol network directory: why it takes two services
With the B2B e-invoicing mandate, your invoices travel over the Peppol network instead of leaving by email. A practical question follows: how does a document know where to go? Nothing inside an invoice states the technical address of its recipient. That is the job of the Peppol network directory, which rests on two distinct services, the SMP and the SML.
The two complement each other. The SMP (Service Metadata Publisher) is the registry that publishes, for each business, what it can receive and where to send it. The SML (Service Metadata Locator) is the central service that, from a recipient's identifier, tells you which SMP holds that information. This article explains what each one does, how they chain together to route an invoice, and why you never have to touch them.
The SMP: the technical record of each recipient
The SMP, or Service Metadata Publisher, is a registry of technical metadata. For each registered participant, it publishes two essential pieces of information: the list of document types that participant accepts (invoice, credit note, and the matching profile, for example Peppol BIS Billing 3.0) and the address of its access point, meaning where a document is actually delivered to it.
The SMP is decentralised. There is no single SMP for all of Peppol: the network has several, run by different providers. When you connect, your business is registered in one of them, usually the one belonging to your access point provider. That registration is what makes you reachable: without it, no sender would know it can address an invoice to you, or how.
What the SMP publishes about each participant
The participant identifier
In Belgium, the enterprise number through the 0208 scheme.
The accepted documents
Invoice, credit note and the applicable profile, for example Peppol BIS Billing 3.0.
The access point address
The technical URL where the document must be delivered.
The SML: the central locator of the network
The SML, or Service Metadata Locator, is the only central component of the Peppol network. Where SMPs are many, there is a single SML for the whole network. Its role is narrower but essential: it does not hold recipients' addresses, it only tells you which SMP to find them in.
For that, the SML relies on DNS, the domain name system that already lets you find a website from its address. The principle is the same: from the recipient's identifier, the network builds a technical name using a digest of that identifier, then queries the SML, which returns the address of the relevant SMP. Every SMP operator is required to register its server with the SML: that registration is what makes the whole thing searchable.
central locator
a single SML for the whole Peppol network
where each address lives
one registry per participant, run by an operator
the SML's technology
the same logic as resolving website names
How an invoice finds the right recipient
The point of these two services becomes clear when you follow an invoice. In the four-corner model of Peppol, your access point (corner C2) must reach the recipient's (corner C3), which it does not know in advance. The resolution happens in a few steps, all invisible to you.
- 1
The recipient's identifier
The sender's access point starts from the recipient's Peppol identifier; in Belgium, the enterprise number through the 0208 scheme.
- 2
Querying the SML
From that identifier, it queries the SML, which returns the address of the SMP the recipient is registered in.
- 3
Consulting the SMP
It then queries that SMP, which states the documents the recipient accepts and the address of its access point.
- 4
Delivering the document
With that address, the sender's access point passes the invoice to the recipient's access point, which delivers it to them.
This mechanism explains why a single identifier is enough to reach any business on the network: the SML leads to the right SMP, and the SMP gives the final address. That is what sets Peppol apart from an ad hoc file exchange, where you would have to know each partner's technical address in advance.
SMP and SML: two complementary roles
The most common confusion treats the SMP and the SML as two versions of the same thing. They in fact occupy two distinct places in the chain. The table below places each role.
| SMP | SML | |
|---|---|---|
| Decentralised, run by several providers | ||
| Single central service for the whole network | ||
| Holds the recipient's access point address | ||
| Relies on DNS to locate the information | ||
| Queried first when sending |
Remember the order: you query the SML first to know where to look, then the SMP to get the address. The SML is the network's general index, the SMP a recipient's detailed record. This is also why your Peppol identifier must be exact: it is the key that opens the whole chain. On how to obtain that identifier, see the Peppol identifier.
Be reachable on Peppol, without managing the directory
YouInv connects you to the Peppol network through an access point: your business is registered and your invoices find their recipient automatically.
What the Peppol directory changes for your business
The good news is that none of this falls on you directly. You do not install an SMP, you do not query the SML: it is your Peppol access point that registers your business in an SMP when you connect, then queries the SML and the SMPs whenever it needs to locate a recipient (results may be cached). These services are the network's infrastructure, not a tool you operate.
Two points are worth keeping. First, being "on Peppol" means being registered in an SMP: that is what makes you reachable, and it is the first thing to check if a partner cannot address an invoice to you. Second, the reliability of this resolution rests on the accuracy of your identifier and your registration. Compliant software takes care of it, but it helps to understand what happens behind the apparent simplicity of a send.
Further reading
- The four-corner model of Peppol explained: the C1 to C4 roles the SMP and SML fit into.
- Peppol access point: what it is for: the component that queries the directory for you.
- The Peppol identifier: what it is for and how to get it: the key used by the SML and the SMP.
The reference source is authoritative: the Peppol infrastructure specifications published by OpenPeppol, which describe the SMP and the SML.
What is the Peppol network directory (SMP and SML)?
The Peppol network directory rests on two complementary services: the SMP (Service Metadata Publisher), which publishes for each recipient the documents it accepts and its access point address, and the SML (Service Metadata Locator), the central service that tells you which SMP a given recipient is registered in.
What is the SMP in Peppol?
The SMP, or Service Metadata Publisher, is a registry that publishes a participant's technical metadata: the document types it can receive and its access point address. Every Peppol participant is registered in an SMP, run by an operator. The network has several SMPs.
What is the SML in Peppol?
The SML, or Service Metadata Locator, is the only central component of the Peppol network. From a recipient's identifier, it tells you which SMP holds its metadata. It relies on DNS, the same technology that resolves website names.
What is the difference between the SMP and the SML?
The SML locates the right SMP from the recipient's identifier; the SMP then holds the real address of its access point and the list of documents it accepts. The SML is central and unique, the SMP is decentralised and run by multiple operators.
Do I have to manage the SMP and the SML myself?
No. Your Peppol access point registers your business in an SMP and queries the SML and the SMPs whenever it needs to locate a recipient. You enter your invoices; locating the recipient is handled entirely by the infrastructure.
