1. Introduction

   This document describes remote service discovery in the Service
   Location Protocol (SLP) [RFC2608] via DNS SRV [RFC2782].  We consider
   remote service discovery as discovering desired services in given
   remote DNS domains, and local service discovery as discovering
   desired services within the local administrative domain.

   SLP provides a scalable framework for local service discovery and
   selection.  In SLP, User Agents (UAs) discover desired services in
   the local administrative domain by querying all Service Agents (SAs)
   via multicast or querying a Directory Agent (DA) via unicast.  To

   query a DA using unicast, a UA needs to first learn about the DA via
   DHCP, static configuration or multicast (listening for DAAdvert
   multicast or issuing DA discovery SrvRqst multicast).

   DNS SRV provides good support for remote service discovery.  However,
   if multiple servers are discovered via DNS SRV for a service, only
   priority and weight can be used to make a selection.  If additional
   service properties (such as cost, speed and service quality) need to
   be considered in the selection process, DNS SRV becomes insufficient.

   We propose that using SLP and DNS SRV together can provide better
   support for remote service discovery.  First, a UA uses DNS SRV to
   find SLP DAs at a remote DNS domain.  Then, the UA uses SLP to query
   one of those DAs to discover desired services.  In this way, we can
   avoid the limitations in using SLP and DNS SRV separately.  On one
   hand, without DNS SRV, an SLP UA needs to depend on static
   configuration to learn about remote DAs because DHCP and multicast DA
   discovery are not generally applicable beyond the local
   administrative domain.  On the other hand, without SLP, DNS SRV has
   limited support for service selection.

   In this document, we first define the DNS SRV Resource Records (RRs)
   for SLP DA services, which are used to map a given DNS domain to
   remotely accessible (i.e., accessible from the Internet) DAs in that
   domain.  Then, we discuss various issues in using SLP and DNS SRV
   together for remote service discovery.  Finally, we give the steps
   for discovering services in remote DNS domains.

1.1. Notation Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].