RFC 3832 (rfc3832) - Page 2 of 8

Remote Service Discovery in the Service Location Protocol (SLP) via DNS SRV

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RFC 3832          Remote Discovery in SLP via DNS SRV          July 2004


   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
   [RFC 2119].

2.  The DNS SRV RRs for SLP DA services

   According to RFC 2782 [RFC 2782], the DNS SRV RRs for SLP DA services
   are defined as follows:

   _slpda._Proto.Name TTL Class SRV Priority Weight Port Target

   where "slpda" is the symbolic name for SLP DA services, the Proto
   field is either "tcp" or "udp", and the Target field is the domain
   name of an SLP DA.  Please refer to [RFC 2782] for detailed
   explanation of each field in DNS SRV RRs.





Zhao, et al.                  Experimental
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