|Title||Remote Service Discovery in the Service Location Protocol (SLP) via
|Author||W. Zhao, H. Schulzrinne, E. Guttman, C. Bisdikian, W.
Network Working Group W. Zhao
Request for Comments: 3832 H. Schulzrinne
Category: Experimental Columbia University
Remote Service Discovery in the
Service Location Protocol (SLP) via DNS SRV
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2004).
Remote service discovery refers to discovering desired services in
given remote (i.e., non-local) DNS domains. This document describes
remote service discovery in the Service Location Protocol (SLP) via
DNS SRV. It defines the DNS SRV Resource Records for SLP Directory
Agent services, discusses various issues in using SLP and DNS SRV
together for remote service discovery, and gives the steps for
discovering desired services in remote DNS domains.
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
2. The DNS SRV RRs for SLP DA services
According to RFC 2782 [RFC2782], 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 [RFC2782] for detailed
explanation of each field in DNS SRV RRs.
Next we show an example of using DNS SRV RRs to map a given DNS
domain to remotely accessible DAs in that domain. To discover
remotely accessible DAs in a remote domain (say, example.com), a UA
makes a DNS query [RFC1034,RFC1035] for QNAME=_slpda._tcp.example.com
(or QNAME=_slpda._udp.example.com), QCLASS=IN, and QTYPE=SRV. Then
the UA will receive a list of DNS SRV RRs in a DNS reply, which gives
all remotely accessible DAs in the domain example.com, such as:
;; Priority Weight Port Target
_slpda._tcp.example.com IN SRV 0 0 427 da1.example.com
_slpda._tcp.example.com IN SRV 0 0 427 da2.example.com
3. Remote Service Discovery in SLP via DNS SRV
SLP DAs can be discovered in two ways: (1) using the mechanisms
described in RFC 2608, and (2) using DNS SRV RRs as described in this
document. The second approach is useful for UAs to acquire service
information for remote DNS domains. For example, a mobile node
visiting a network (without the use of mobile IP) may want to obtain
information about services in its home network.
3.1. The DNS Domain of Interest for Remote Service Discovery
Using DNS SRV RRs to discover SLP DAs requires knowledge of the
domain where SLP DAs are registered. For remote service discovery,
it is assumed that the DNS domain of interest is known via a priori
knowledge. For example, a UA is configured with a domain name or the
user provides the domain name manually.
Note that there is no implied "search order" of DNS domains in
finding remote DAs. For instance, if a UA is looking for remote DAs
in the domain foo.bar.example.com, it SHOULD only look for
_slp._tcp.foo.bar.example.com (or _slp._udp.foo.bar.example.com), and
MUST NOT fall back to look for _slp._tcp.bar.example.com,
_slp._tcp.example.com, and so on.
3.2. SLP DAs for Remote Service Discovery
A UA discovers desired services in a given remote DNS domain by
unicasting requests to DAs in that domain. The UA uses remote DAs
according to these prioritized rules: (1) using DAs which it has been
configured with, and (2) using DAs which it has discovered via DNS
3.3. SLP Scopes for Remote Service Discovery
As SLP scopes are intended to be used only within one administrative
domain, they are likely incomprehensible to users outside of the
administrative domain. Thus, any remotely accessible service MUST be
registered in the "default" scope, but it MAY be registered in other
scopes at the same time. Similarly, all DAs advertised via DNS SRV
MUST serve the "default" scope, but they MAY serve other scopes at
the same time. As a result, users wishing to discover services at a
remote DNS domain SHOULD only search the "default" scope.
4. Steps for Remote Service Discovery
The steps for a User Agent U to discover desired services in a remote
DNS domain D are as follows.
(1) U makes a DNS query for QNAME=_slpda._tcp.D (or
QNAME=_slpda._udp.D), QCLASS=IN, and QTYPE=SRV. Then, U gets a
list of DNS SRV RRs (referred to as L) in a DNS reply, which
gives all remotely accessible DAs in D.
(2) U selects a DA X from L based on the priority and weight
information per RFC 2782.
(3) U queries X in the "default" scope to discover desired services
Note that the services discovered in the above steps may not
necessarily be remotely accessible.
5. Security Considerations
To support remote service discovery, a domain exposes its service
information to the Internet. Standard SLP authentication SHOULD be
used to protect valuable service information. First, there is a risk
that any SA could advertise any service on a DA accessible from the
Internet. Such a DA SHOULD reject all registrations and
deregistrations that cannot be authenticated. Secondly, to avoid
disclosing unintended service information to remote users, a DA
accessible from the Internet SHOULD at least authenticate service
queries that are not in the "default" scope. In addition, the
security considerations for DNS SRV [RFC2782] apply to this document.
Also, the DNS security extensions [RFC 2535] SHOULD be used to
provide origin authentication and integrity protection for DNS data.
6. Applicability Statements
This specification describes remote service discovery in SLP via DNS
SRV. It facilitates discovering services at a remote DNS domain if
the domain name is known via a priori knowledge. However, it does
not intend to solve the problem of Internet-wide service discovery.
Users should be aware of two constraints in using DNS SRV to discover
SLP DAs: (1) they SHOULD only use DNS SRV to discover DAs in the
"default" scope, and (2) an administrator may choose to register only
a subset of all DAs in the "default" scope via DNS SRV. Thus, to
discover local DAs, implementations MUST use the standard SLP
mechanisms per RFC 2608. In addition, implementations supporting
this specification MAY use DNS SRV to discover local DAs in the
As SLP scopes are not intended to be used outside the local
administrative domain, all remote service discovery in SLP SHOULD be
carried only in the "default" scope.
Note that the services discovered via DNS SRV and remote SLP DAs may
not necessarily be remotely accessible.
7. IANA Considerations
In the DNS SRV RRs for SLP DA services, the symbolic name for the
Service field is "slpda", supported protocols are "tcp" and "udp".
The following values have been registered with IANA:
Service Field Protocol Field Reference
------------- -------------- ---------
slpda tcp [RFC3832]
slpda udp [RFC3832]
The authors would like to thank Bernard Aboba, Kevin Arnold, Steven
Bellovin, Ted Hardie, James Kempf, Thomas Narten, Erik Nordmark, and
Jon Peterson for their valuable comments.
9. Normative References
[RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2 ", RFC 2608, June 1999.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2535] Eastlake 3rd, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
10. Authors' Addresses
Department of Computer Science
1214 Amsterdam Avenue, MC 0401
New York, NY 10027-7003
Department of Computer Science
1214 Amsterdam Avenue, MC 0401
New York, NY 10027-7003
Dr. Chatschik Bisdikian
IBM T. J. Watson Research Center
30 Saw Mill River Road, M/S 3S-B34
Hawthorne, NY 10532, USA
Phone: +1 914 784 7439
Fax: +1 914 784 6225
William F. Jerome
Thomas J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532, USA
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