Internet Engineering Task Force (IETF) R. Gellens
Request for Comments: 8917
Core Technology Consulting
Category: Standards Track October 2020
The LoST-Validation Straightforward-Naming Authority PoinTeR (S-NAPTR)
Application Service Tag
This document adds the 'LoST-Validation' service tag to the
Straightforward-Naming Authority PoinTeR (S-NAPTR) Application
Service Tag IANA registry. This tag can appear in a Naming Authority
Pointer (NAPTR) Domain Name System (DNS) record to assist clients of
the Location-to-Service Translation (LoST) Protocol in identifying
LoST servers designated for location validation. This tag and the
information about its use update RFC 5222
, which enables the explicit
discovery of a server that supports location validation.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8917
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) in effect on the date of
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Table of Contents 1.
Document Scope 2.
Requirements Language 3.
The LoST-Validation Application Service Tag 4.
Backwards Compatibility 5.
Security Considerations 6.
IANA Considerations 6.1.
S-NAPTR Registration 7.
Normative References 7.2.
1. Document Scope
This document adds 'LoST-Validation' to the S-NAPTR Application
Service Tag IANA registry and describes how this tag fits in the LoST
server discovery procedure described in [RFC5222
]. This tag is used
with Naming Authority Pointer (NAPTR) Domain Name System (DNS)
records so that clients of the Location-to-Service Translation (LoST)
] can identify servers designated for location
validation. This tag and the information on its use is an update to
] that enables the explicit discovery of a server that
supports location validation.
The LoST Protocol [RFC5222
] defines a mapping service with the
additional ability for a client to request that a civic address be
validated. The LoST protocol allows servers to ignore a request to
perform location validation. The National Emergency Number
Association (NENA) has defined an architecture for all-IP emergency
services (known as "i3" [NENA-i3]), which defines the mapping
(routing) and validation functions as two distinct functional
elements, defined as an Emergency Call Routing Function (ECRF) and a
Location Validation Function (LVF). NENA i3 requires that the
mapping (ECRF) and validation (LVF) functions be separable; an entity
responsible for a LoST server cluster can decide to provide mapping
and validation services using consolidated or separate server
clusters (i.e., using the same or separate boxes). The rationale is
that the mapping service is used in real time during emergency call
routing, while the validation service is used in advance, typically
when data is provisioned; therefore, the mapping service has much
higher availability and response-time requirements than the
validation service. An organization might choose to deploy these
services using different server clusters to make it easier to provide
higher levels of service for the mapping function while shielding it
from the potentially bursty load of validation. Another organization
might choose to use the same sets of servers for both services,
configured and deployed to offer the high service level demanded of
the mapping service.
In order to permit this separability, any entity querying a LoST
server needs to be able to resolve an Application Unique String (AUS)
into a URL for a LoST server designated for the required service
(mapping or validation). This separability needs to be maintained
throughout the LoST tree structure, from forest guide to leaf node
(LoST architecture is described in [RFC5582
]). Because LoST
referrals return an AUS rather than a URL, either a different service
tag or a DNS name convention (e.g., "ecrf.example.org" and
"lvf.example.org") is needed to differentiate between the services.
DNS name conventions are inflexible and fragile, making a different
service tag the preferred approach.
Because LoST servers may ignore a request to perform location
validation, a service tag explicitly for location validation also
reduces the likelihood (which has existed since [RFC5582
]) that a
client needing location validation will reach servers that are not
doing so (due to configuration and/or conditions).
2.1. Requirements Language
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in BCP
] when, and only when, they appear in all
capitals, as shown here.
3. The LoST-Validation Application Service Tag
This document adds 'LoST-Validation' to the "S-NAPTR Application
Service Tags" registry created by [RFC3958
]. The 'LoST-Validation'
tag serves as a counterpart to the 'LoST' tag added by [RFC5222
'LoST' tag identifies servers able to perform the core mapping
function, while 'LoST-Validation' identifies servers designated for
the validation function.
Because some servers might be configured to provide both mapping and
validation functions, a server identified using the 'LoST' service
tag might also perform the validation function (and resolving the two
tags might result in the same URL). Because the two functions might
be separate, clients seeking a LoST server for location validation
can first try a URI-Enabled NAPTR (U-NAPTR) resolution using the
'LoST-Validation' service tag and can fall back to the 'LoST' service
tag if this does not resolve to a usable LoST server.
] specifies that LoST servers are located by resolving
an AUS using U-NAPTR/DDDS (URI-Enabled NAPTR / Dynamic Delegation
Discovery Service) [RFC4848
] and defines the 'LoST' application
service tag. In order to permit separability of the mapping and
validation services performed using LoST, this document defines the
'LoST-Validation' service tag. This tag also reduces the likelihood
that a client needing location validation might reach servers that
are not performing validation (due to configuration and/or
conditions). NAPTR records for LoST servers available for location
validation contain the 'LoST-Validation' service tag. An entity
needing to perform location validation using LoST performs the
discovery procedure as described in [RFC5222
], except that the 'LoST-
Validation' service tag is used in preference to the 'LoST' service
tag. For both service tags, the HTTP and HTTPS URL schemes are used.
In the absence of any NAPTR records containing the 'LoST-Validation'
service tag, the 'LoST' service tag is used. Fallback to the 'LoST'
service tag may follow if the 'LoST-Validation' service tag fails to
result in a usable LoST server. The discovery procedure with the
'LoST-Validation' service tag might result in the same URL as the
'LoST' service tag, or it may result in a different URL. When the
URLs are different, they could lead to the same physical servers or
4. Backwards Compatibility
The primary use of LoST in general, and the location validation
functionality in particular, is within the emergency services area.
Within North America, the NENA i3 [NENA-i3] document specifies how
protocols including LoST are used. The i3 document is expected to
reference the 'LoST-Validation' service tag and specify its use in
both server NAPTR DNS records and client resolution of AUS.
LoST allows a server to refuse to perform location validation and
defines the 'locationValidationUnavailable' warning. LoST also
allows a server to refer to another server rather than answering
itself. So, in a deployment where a LoST tree has separate server
clusters for mapping and for validation, mapping servers receiving a
request for validation could either perform the validation as
requested or return the 'locationValidationUnavailable' warning and
potentially also include a <redirect> element to redirect to a
validation server. However, the <redirect> element contains an AUS,
so unless the AUSs for validation and mapping are different (e.g.,
'ecrf.example.org' and 'lvf.example.org'), we still need a different
service tag to allow for flexible deployment choices (i.e., not
requiring a DNS name convention).
LoST clients performing emergency services operations in North
America are expected to comply with the NENA i3 specification and
hence support the 'LoST-Validation' service tag when defined. A LoST
client implemented prior to the addition of the 'LoST-Validation' tag
would use the 'LoST' tag to resolve an AUS. Such a client might not
be performing location validation, but if it is, the LoST server it
contacts may perform the service. Even in a deployment where mapping
and validation are split, the data is identical; the split is a load
and deployment optimization strategy. Servers designated for mapping
might perform validation when requested (potentially depending on
load or other factors). If an older client attempts validation using
a designated mapping server that refuses the request, the client will
retry later, at which point the server might provide the function
(e.g., if its load or other conditions have changed). Even in the
case of a designated mapping server that refuses to perform
validation at any time, the server could return a redirect with a
different AUS (e.g., "lvf.example.com") that resolves to a designated
validation server. In the worst case, the client will be unable to
reach a server willing to perform validation and will follow up
(e.g., submit a discrepancy report as specified in NENA i3). The
resolution may be to update the client with the 'LoST-Validation'
service tag, update the AUS returned in a redirect and DNS to use a
different DNS host name, or permit the server to perform validation
when not under stress (or a combination). Note that, because LoST
does not require servers to perform validation, the situation
described can exist regardless of the addition of the 'LoST-
Validation' service tag. Use of the tag improves the likelihood that
a client is able to validate a location when needed.
5. Security Considerations
The security considerations described in [RFC3958
] apply here. No additional security aspects are foreseen by
the addition of an extra tag. Separation of services might be
desired, for example, to be able to allocate different levels of
resources (such as server capacity, attack mitigation, bandwidth,
etc.) to the mapping and validation services, in which case separate
tags are needed to allow LoST clients (which may include other LoST
servers) to identify the correct server cluster.
] descriptively discusses the use of DNS security [RFC4033
to mitigate the risk of DNS-based attacks. Because DNS security has
become more widely deployed since the publication of [RFC5222
be used when performing NAPTR resolution. Note that,
while there are valid reasons to proceed with a LoST mapping query
despite security failures while initiating or processing an emergency
call, these concerns generally do not apply to a LoST validation
query done in advance of an emergency call.
6. IANA Considerations
IANA has added 'LoST-Validation' to the "S-NAPTR Application Service
Tags" registry created by [RFC3958
]. This tag serves as a
counterpart to the 'LoST' tag added by [RFC5222
(Note that IANA and [RFC3958
] call this registry "S-NAPTR Application
Service Tags", while [RFC5222
] calls it "U-NAPTR application service
6.1. S-NAPTR Registration
This document registers an S-NAPTR application service tag:
Application Service Tag: LoST-Validation
Defining Publication: This document
7.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958
, DOI 10.17487/RFC3958
January 2005, <https://www.rfc-editor.org/info/rfc3958
] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC 4033
, DOI 10.17487/RFC4033
, March 2005,
] Daigle, L., "Domain-Based Application Service Location
Using URIs and the Dynamic Delegation Discovery Service
(DDDS)", RFC 4848
, DOI 10.17487/RFC4848
, April 2007,
] Hardie, T., Newton, A., Schulzrinne, H., and H.
Tschofenig, "LoST: A Location-to-Service Translation
Protocol", RFC 5222
, DOI 10.17487/RFC5222
, August 2008,
] 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/info/rfc8174
7.2. Informative References
[NENA-i3] National Emergency Number Association (NENA)
Interconnection and Security Committee, i3 Architecture
Working Group, "Detailed Functional and Interface
Standards for the NENA i3 Solution", 2016,
] Schulzrinne, H., "Location-to-URL Mapping Architecture and
Framework", RFC 5582
, DOI 10.17487/RFC5582
Many thanks to Ted Hardie, Ben Campbell, Dan Banks, Pete Resnick,
Shawn Emery, Robert Wilton, Roman Danyliw, and Benjamin Kaduk for
their helpful reviews and suggestions and to Barry Leiba for
shepherding the document.
Core Technology Consulting
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