An SRV or Service Record is a category of data in the DNS specifying information on available services. When looking up for a service, you must first look up the SRV Record for the service to see which server actually handles it. Then it looks up the Address Record for the server to connect to its IP Address.
The SRV Record has a priority field similar to an MX Record's priority value. Clients always use the SRV Record with the lowest priority value first and only fall back to other SRV Records if the connection with this Record's host fails. If a service has multiple SRV records with the same priority value, clients use the weight field to determine which host to use. The weight value is relevant only in relation to other weight values for the service, and only among SRV Records with the same priority value.
Newer Internet Protocols such as SIP (Session Initiation Protocol) and XMPP (Extensible Messaging and Presence Protocol) often require SRV support from clients.