Extensions for Scalable DNS Service Discovery G. Akiwate
Internet-Draft T. Pauly
Intended status: Informational Apple Inc
Expires: 2 September 2025 1 March 2025
Use SVCB with DNS Service Discovery
draft-gakiwate-dnssd-use-svcb-00
Abstract
DNS Service Discovery (DNS-SD) relies on a sequence of steps to
enable the discovery and connection to local network services. The
use of Service Binding (SVCB) resource records during the service
discovery process enables service instances to advertise properties
such as Application-Layer Protocol Negotiation (ALPN) identifiers and
other endpoint configuration options. This document describes the
use of SVCB / HTTPS RRs in the DNS Service Discovery process as an
additional step to allow clients to connect to service instances
optimally.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://gakiwate.github.io/draft-gakiwate-dnssd-use-svcb/draft-
gakiwate-dnssd-use-svcb.html. Status information for this document
may be found at https://datatracker.ietf.org/doc/draft-gakiwate-
dnssd-use-svcb/.
Discussion of this document takes place on the Extensions for
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Use of SVCB with Service Instance Resolution . . . . . . . . 3
4.1. Example Use of SVCB RRs . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document describes the use of Service Binding (SVCB) resource
records in the DNS Service Discovery process. The use of SVCB
records alongside SRV records during service discovery enables richer
metadata exchange, enhancing the ability to identify and select
optimal connection parameters. Specifically, SVCB records allow
service instances to advertise properties such as Application-Layer
Protocol Negotiation (ALPN) identifiers and other endpoint
configuration options which streamlines client connections by
providing essential information for protocol negotiation and
connection setup during the discovery phase.
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2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Motivation
The DNS Service Discovery, with the SRV and TXT records, provides
some metadata about the service instance. The TXT records
specifically are designed to give additional information about the
service itself. The specific nature of the additional data in TXT
records, and how it is to be used, is service-dependent. However,
additional properties which are not service-specific, like supported
protocols, or privacy requirements which speed up connection
establishment and improve user privacy do not fit naturally in this
scheme.
This documents describes how with the use of SVCB / HTTPS resource
records [SVCB] in the DNS service discovery process we can support
these non service specific properties such as Application-Layer
Protocol Negotiation (ALPN) [ALPN] identifiers and other endpoint
configuration options such as Encrypted Client Hello (ECH) [ECH] so
that a client can select its preferred options to optimally initiate
the connection to the service instance.
4. Use of SVCB with Service Instance Resolution
Typically, the DNS-SD [DNSSD] process begins with a client
enumerating service instance names using a PTR record query. The
result of this PTR query is a list of zero or more PTR records each
pointing to a unique service instance. For example, a query for
_foo._tcp.example.com might return multiple PTR records, each
corresponding to a specific service instances, such as
service1._foo._tcp.example.com and service2._foo._tcp.example.com.
Once the client identifies the exact service instance it wants to
connect to, the client issues an SRV query for the service instance.
The SRV record for the service instance includes the hostname of the
service instance, the port number on which the service is listening,
and priority and weight fields for load balancing or failover. For
example, querying the SRV record for service1._foo._tcp.example.com
might return a hostname like host1.example.com and a port number,
such as 8080.
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After resolving the SRV record, the client issues A and AAAA queries
to map the hostname in the SRV record to an IP address. For example,
querying for host1.example.com might return an A record with the IPv4
address 192.0.2.3 or an AAAA record with the IPv6 address
2001:db8::1. Once the IP address is obtained, the client can
initiate a connection to the desired service instance using the port
number specified in the SRV record.
Once the client has resolved the SRV record to identify the hostname
and the port of the service instance, and along with or before the A
and AAAA queries are issued, the client can issue a query for an SVCB
or HTTPS record, depending on the relevant URI scheme the application
is using. The client is expected to use "Port Prefix Naming" [SVCB]
to encode the port learned from the SRV response, and the scheme from
the URI being accessed by the application. If there is no notion of
a URI scheme for the application, then SVCB or HTTPS queries SHOULD
NOT be made. If the URI scheme is "http" or "https", the client will
issue a query for an HTTPS record; and if the port learned from the
SRV response is the same as the default port (443), it will leave off
the port prefix.
4.1. Example Use of SVCB RRs
Consider an example where the client application starts with a
service name service1._foo._tcp.local and expects to use a URI scheme
of "https". The client first queries the SRV record for
service1._foo._tcp.example.com, which returns a hostname
host1.example.com and port number 8080.
service1._foo._tcp.example.com 3600 IN SRV (
0 0 8080 host1.example.com )
The client application then can synthesize the URI
"https://host1.example.com:8080".
The client then issues issue an HTTPS query for
_8080.host1.example.com, and A and AAAA queries for
host1.example.com.
The service instance operator can publish this HTTPS record:
_8080.host1.example.com 7200 IN HTTPS 1 . (
alpn=h2,h3 )
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All put together, the client in this case with the alpn values learns
that the service instance supports h3 (which the client may prefer
over h2) speeding up connection establishment with the service with
its preferred protocol. Other SvcParamKeys such as ipv4hint and
ipv6hint can also be present in the HTTPS record.
5. Security Considerations
TODO
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[DNSSD] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/rfc/rfc6763>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
and Parameter Specification via the DNS (SVCB and HTTPS
Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.
7.2. Informative References
[ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
[ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-23, 19 February 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-23>.
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Appendix A. Acknowledgments
Authors' Addresses
Gautam Akiwate
Apple Inc
Email: gakiwate@apple.com
Tommy Pauly
Apple Inc
Email: tpauly@apple.com
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