in the european eDelivery standard PEPPOL AS4, which protocol is used to retrieve the information about the location of participant capabilities?

in the european eDelivery standard PEPPOL AS4, which protocol is used to retrieve the information about the location of participant capabilities?

• isdn
• DNS
• https
• http 

EXPLANATION

This section includes specifications and resources to enable access to the PEPPOL eDelivery Network and to implement the following components: Access Points, gateways which ensure a standard and solid transport mechanism to exchange different types of documents across different communities or countries; Service Metadata Publishers (SMPs), which contain the exact location and the capabilities of participants; Service Metadata Locator (SML),
used to add/update/delete the IDs of the participants, and which offers a DNS-based mechanism to discover the SMP where the specific participant data is described.