O registro SRV é definido em RFC 2782 e, de acordo com o local, você pode escolher o que funciona para você:
Proto The symbolic name of the desired protocol, with an underscore (_) prepended to prevent collisions with DNS labels that occur in nature. _TCP and _UDP are at present the most useful values for this field, though any name defined by Assigned Numbers or locally may be used (as for Service). The Proto is case insensitive.