Host Discovery

Host discovery is the process through which a domain owner optionally specifies which web host to interrogate during capability discovery. The host discovery process involves the creation and subsequent query of SRV DNS records.

The use of an SRV record was chosen for the following reasons:

• Domain owners may choose to use a third-party paymail service provider. Delegating authority to this provider is a one-time activity (the creation of a DNS SRV record).
• TXT records were considered, however should the paymail service be reconfigured (for example, the root URI change from .../api/v1/... to .../api/v2/...), the domain owner would have to coordinate with the service provider to ensure the TXT record (containing a full endpoint URI) was updated. With SRV records (plus the Capability Discovery protocol), the Host Discovery phase is set-and-forget.
• As an optional step, if the canonical Capability Discovery host is the same as the domain found in the paymail alias, the DNS record can be omitted entirely.
• DNS records (including SRV) feature a TTL, which clients can use for caching responses. All common DNS clients implement this caching out-of-the-box, meaning implementers do not have to roll this themselves.

Setup

A domain owner may create an SRV record with the following parameters:

Parameter	Value
Service	_bsvalias
Proto	_tcp
Name	<domain>.<tld>.
TTL	3600 (see notes)
Class	IN
Priority	10
Weight	10
Port	443
Target	<endpoint-discovery-host>

The TTL parameter should be set very low for test configurations (a few seconds), whereas for production deployments this should be set higher, to allow caching to work. A value of 3600 is suggested for production deployments.

Although the DNS system allows for multiple records with a variety of priorities and weights, which allows for some level of traffic management, resilience, and load-balancing via DNS records, it is recommended by this specification that these considerations be handled by more modern infrastructure and only a single SRV record be created.

See https://en.wikipedia.org/wiki/SRV_record for more information on SRV DNS records.

Client Queries

Given a paymail alias <alias>@<domain>.<tld>, a paymail client would perform a DNS lookup for an SRV record matching _bsvalias._tcp.<domain>.<tld>. The combination of Target and Port fields are then used for Capability Discovery. Should no record be returned, a paymail client should assume a host of <domain>.<tld> and a port of 443.