CoRE K. Lynn
Internet-Draft Oracle+Dyn
Intended status: Standards Track P. van der Stok
Expires: September 12, 2019 Consultant
M. Koster
SmartThings
C. Amsuess
Energy Harvesting Solutions
March 11, 2019
CoRE Resource Directory: DNS-SD mapping
draft-ietf-core-rd-dns-sd-04
Abstract
Resource and service discovery are complementary. Resource discovery
provides fine-grained detail about the content of a web server, while
service discovery can provide a scalable method to locate servers in
large networks. This document defines a method for mapping between
CoRE Link Format attributes and DNS-Based Service Discovery records
to facilitate the use of either method to locate RESTful service
interfaces (APIs) in heterogeneous HTTP/CoAP environments.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction and Background . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. CoRE Resource Discovery . . . . . . . . . . . . . . . . . 4
1.3. CoRE Resource Directories . . . . . . . . . . . . . . . . 5
1.4. DNS-Based Service Discovery . . . . . . . . . . . . . . . 5
2. New Link-Format Attributes . . . . . . . . . . . . . . . . . 6
2.1. Export attribute "exp" . . . . . . . . . . . . . . . . . 7
2.2. Resource Instance attribute "ins=" . . . . . . . . . . . 7
2.3. Service Type attribute "st=" . . . . . . . . . . . . . . 7
3. Mapping CoRE Link Attributes to DNS-SD Record Fields . . . . 7
3.1. Mapping Resource Instance attribute "ins=" to <Instance> 7
3.2. Mapping Service Type attribute "st=" to <ServiceType> . . 8
3.3. <Domain> Mapping . . . . . . . . . . . . . . . . . . . . 8
3.4. TXT Record key=value strings . . . . . . . . . . . . . . 9
3.5. Exporting resource links into DNS-SD . . . . . . . . . . 9
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security considerations . . . . . . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction and Background
The Constrained RESTful Environments (CoRE) working group aims at
realizing the [REST] architecture in a suitable form for the most
constrained devices (e.g. 8-bit microcontrollers with limited RAM and
ROM) and networks (e.g. 6LoWPAN [RFC4944]). CoRE is aimed at
machine-to-machine (M2M) applications such as smart energy and
building automation. The main deliverable of CoRE is the Constrained
Application Protocol (CoAP) specification [RFC7252].
CoRE Link Format [RFC6690] is intended to support fine-grained
discovery of hosted resources, their attributes, and possibly other
related resources. Automated dynamic discovery of resources hosted
by a constrained server is critical in M2M applications, where human
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intervention is minimal and static configurations result in
brittleness.
DNS-Based Service Discovery (DNS-SD) [RFC6763] supports wide-area
search for instances of a given service type (i.e. servers that
support a particular application protocol stack). A service instance
consists of a server's name, IP address, and port number plus
additional meta-data about the server. This data may extend to
support multi-function devices, where multiple services are available
at the same endpoint. The result of the discovery process may
include a path to a resource representing the entry point to each
function's RESTful service interface and possibly a link to a formal
description of that interface (e.g. a JSON Hyper-Schema document
[I-D.handrews-json-schema-hyperschema]).
Resource and service discovery are complementary in the case of large
networks, where the latter can facilitate scaling. This document
defines a mapping between CoRE Link Format attributes and DNS-Based
Service Discovery records that permits discovery of CoAP services by
either method. It also addresses the CoRE charter goal to
interoperate with DNS-SD.
The primary use case for mapping between resource and service
discovery is to support heterogeneous HTTP/CoAP environments where,
for example, HTTP clients may discover and communicate with CoAP
servers that are behind a "cross proxy" [RFC8075].
1.1. Terminology
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
[RFC2119]. The term "byte" is used in its now conventional sense as
a synonym for "octet".
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC6690] and [RFC8288]. Readers
should also be familiar with the terms and concepts discussed in
[RFC7252].
This specification also incorporates the terminology of
[I-D.ietf-core-resource-directory].
In particular, the following terms are used frequently:
Endpoint: a web server associated with a specific IP address and
port; thus a physical device may host one or more endpoints.
Endpoints may also act as clients.
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Link: Web Linking [RFC8288] defines a Web Link (link) as a typed
connection between two resources, comprised of:
o a link context,
o a link relation type (see Section 2.1 of [RFC8288],
o a link target, and
o optionally, target attributes (see Section 2.2 of [RFC8288]).
A link can be viewed as a statement of the form "link context has a
link relation type resource at link target, which (optionally) has
target attributes", where link target and context are typically
Universal Resource Identifiers (URIs) [RFC3986]. For example,
"https://www.example.com/" has a "canonical" resource at
"https://example.com", which has a "type" of "text/html".
1.2. CoRE Resource Discovery
The main function of Resource Discovery is to return links to the
resources hosted by a server, complemented by attributes about those
resources and additional link relations. In CoRE this collection of
links and attributes is itself a resource (in contrast to HTTP, where
headers delivered with a specific resource describe its attributes).
Resource Discovery can be performed either unicast or multicast.
When a server's IP address is already known, either a priori or
resolved via the Domain Name System (DNS) [RFC1034][RFC1035], unicast
discovery is performed in order to locate the entry point to the
resource of interest. This is performed using a GET to "/.well-
known/core" on the server, which returns a payload in the CoRE Link
Format [RFC6690]. A client would then match the appropriate Resource
Type, Interface Description, and possible media type [RFC2045] for
its application. These attributes may also be included in the query
string in order to filter the number of links returned in a response.
Multicast Resource Discovery is useful when a client needs to locate
a resource within a limited scope, and that scope supports IP
multicast. A GET request to the appropriate multicast address is
made for "/.well-known/core". In order to limit the number and size
of responses, a query string is recommended with the known
attributes. Typically, a resource would be discovered based on its
Resource Type and/or Interface Description, along with possible
application-specific attributes.
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1.3. CoRE Resource Directories
In many M2M scenarios, direct discovery of resources is not practical
due to sleeping nodes, limited bandwidth, or networks where multicast
traffic is inefficient. These problems can be solved by deploying a
network element called a Resource Directory (RD), which hosts
descriptions of resources that originate on other endpoints and
allows indirect lookups to be performed for those resources.
The Resource Directory implements a set of REST interfaces for
endpoints to register and maintain collections of links, called
Resource Directory registrations. [I-D.ietf-core-resource-directory]
specifies the web interfaces that an RD supports for endpoints to
discover the RD and to register, maintain, lookup and remove resource
descriptions; for the RD to validate entries; and for clients to
lookup resources from the RD.
1.4. DNS-Based Service Discovery
DNS-Based Service Discovery (DNS-SD) defines a conventional method of
naming and configuring DNS PTR, SRV, and TXT resource records to
facilitate discovery of services (such as CoAP servers in a
subdomain) using the existing DNS infrastructure. This section gives
a brief overview of DNS-SD; for a detailed specification see
[RFC6763].
DNS-SD Service Names are limited to 255 bytes and are of the form:
Service Name = <Instance>.<ServiceType>.<Domain>
The Service Name identifies a SRV/TXT resource record (RR) pair. The
SRV RR specifies the hostname and port of an endpoint. The TXT RR
provides additional information in the form of key/value pairs. DNS-
Based Service Discovery is accomplished by sending a DNS request for
PTR records with the name <ServiceType>.<Domain>, which will return a
list of zero or more Service Names.
The <Domain> part of the Service Name is identical to the global (DNS
subdomain) part of the authority in URIs [RFC3986] that identify the
resources on an individual server or group of servers.
The <ServiceType> part is generally composed of two labels. The
first label of the pair is the application protocol name [RFC6335]
preceded by an underscore character. For example, an organization
such as the Open Connectivity Foundation [OCF] that specifies
Resource Types [RFC6335] might register application protocol names
beginning with "oic", which all servers that advertise OCF resources
would use as part of their ServiceType. The second label indicates
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the transport protocol binding and is typically "_udp" for CoAP
services.
The default <Instance> part of the Service Name SHOULD be set to a
default value at the factory and MAY be modified during the
commissioning process. It MUST uniquely identify an instance of
<ServiceType> within a <Domain>. Taken together, these three
elements comprise a unique name for an SRV/TXT record pair within the
DNS subdomain.
The granularity of a Service Name MAY be that of a host or group, or
it might represent a particular resource within a CoAP server. The
SRV record contains the host name (AAAA record name) and port of the
endpoint, while protocol is part of the Service Name. In the case
where a Service Name identifies a particular resource, the path part
of the URI must be carried in a corresponding TXT record.
A DNS TXT record is in practice limited to a few hundred bytes in
length, which is indicated in the resource record header in the DNS
response message (See section 6 of [RFC6763]). The data consist of
one or more strings comprising a key/value pair. By convention, the
first pair is txtver=<number> (to support different versions of a
service description). Each string is formatted as a single length
byte followed by 0-255 bytes of text. An example string is:
----------------------------------------
| 0x08 | t | x | t | v | e | r | = | 1 |
----------------------------------------
2. New Link-Format Attributes
When using the CoRE Link Format to describe resources being
discovered by or posted to a resource directory service, additional
information about those resources is often useful. This
specification defines the following new attributes for use in the
CoRE Link Format [RFC6690] to enable the data-driven mappings
described in Section 3:
link-extension = ( "exp" )
link-extension = ( "ins" "=" (ptoken | quoted-string) )
; The token or string is max 63 bytes
link-extension = ( "st" "=" (ptoken | quoted-string) )
; The token or string is max 15 bytes
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2.1. Export attribute "exp"
The Export "exp" attribute is used as a flag to indicate that a link
description MAY be exported from a resource directory to external
directories.
The CoRE Link Format is used for many purposes between CoAP
endpoints. Some are useful mainly locally; for example checking the
observability of a resource before accessing it, determining the size
of a resource, or traversing dynamic resource structures. However,
other links are very useful to be exported to other directories, for
example the entry point resource to a functional service. This
attribute MAY be used as a query parameter in the RD Lookup Function
Set defined in Section 7 of [I-D.ietf-core-resource-directory].
2.2. Resource Instance attribute "ins="
The Resource Instance "ins=" attribute is an identifier for this
resource, which makes it possible to distinguish it from other
similar resources in a Resource Directory. This attribute specifies
the value to be used for the <Instance> portion of an exported DNS-SD
Service Name (see Section 1.4), and SHOULD be unique across resources
with the same Resource Type "rt=" attribute in the domain in which it
is used.
A Resource Instance SHOULD be a descriptive human readable string
like "Ceiling Light, Room 3". This attribute MUST NOT be more than
63 bytes in length. The resource identifier attribute MUST NOT
appear more than once in a link description. This attribute MAY be
used as a query parameter in the RD Lookup Function Set defined in
Section 7 of [I-D.ietf-core-resource-directory].
2.3. Service Type attribute "st="
The Service Type instance "st=" attribute specifies the value to be
used for the <ServiceType> portion of an exported DNS-SD Service Name
(see Section 1.4). This attribute MUST NOT be more than 15 bytes in
length (see [RFC6335], Section 5.1) and MUST be present in the IANA
Service Name registry [st].
3. Mapping CoRE Link Attributes to DNS-SD Record Fields
3.1. Mapping Resource Instance attribute "ins=" to <Instance>
The Resource Instance "ins=" attribute maps directly to the
<Instance> part of a DNS-SD Service Name. It is stored directly in
the DNS as a single DNS label of canonical precomposed UTF-8
[RFC3629] "Net-Unicode" (Unicode Normalization Form C) [RFC5198]
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text. However, if the "ins=" attribute is chosen to match the DNS
host name of a service, it SHOULD use the syntax defined in
Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123].
The <Instance> part of the name of a service being offered on the
network SHOULD be configurable by the user setting up the service, so
that he or she may give it an informative name. However, the device
or service SHOULD NOT require the user to configure a name before it
can be used. A sensible choice of default name can allow the device
or service to be accessed in many cases without any manual
configuration at all (see Appendix D of [RFC6763]).
DNS labels are limited to 63 bytes in length and the entire Service
Name may not exceed 255 bytes.
3.2. Mapping Service Type attribute "st=" to <ServiceType>
The Service Type "st=" attribute maps directly to the <ServiceType>
part of a DNS-SD Service Name.
In practice, the ServiceType should unambiguously identify
interoperable devices. It is up to individual SDOs to specify how to
represent their registered Resource Type "rt=" values as registered
application protocol names according to [RFC6335]. The application
name is then used as the value of the resource "st=" attribute.
The resulting application protocol name MUST be composed of at least
a single Net-Unicode text string, without underscore '_' or period
'.' and limited to 15 bytes in length (see Section 5.1 of [RFC6335]).
This string is mapped to the DNS-SD <ServiceType> by prepending an
underscore and appending a period followed by the "_udp" label. For
example, rt="oic.d.light" might correspond to the registered
application protocol name st="oic-d-light" and would be mapped into
Service Type "_oic-d-light._udp".
The resulting string is used to form labels for DNS-SD records which
are stored directly in the DNS.
3.3. <Domain> Mapping
TBD: A method must be specified to determine which DNS zone the CoAP
service description should be exported to. See, for example,
Section 11 in [RFC6763] and Section 2 in
[I-D.sctl-service-registration].
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3.4. TXT Record key=value strings
DNS-SD key/value pairs may be derived from CoRE Link Format
information and exported as key=value strings in a DNS-SD TXT record
(See Section 6.3 of [RFC6763]).
The resource <URI> is exported as key/value pair "path=<URI>".
The Interface Description "if=" attribute is exported as key/value
pair "if=<Interface Description>".
The DNS TXT record can be further populated by importing any other
resource description attributes as they share the same key=value
format specified in Section 6 of [RFC6763].
3.5. Exporting resource links into DNS-SD
Assuming the ability to query a Resource Directory or multicast a GET
(?exp) over the local link, CoAP resource discovery may be used to
populate the DNS-SD database in an automated fashion. CoAP resource
descriptions (links) can be exported to DNS-SD for exposure to
service discovery by using the Resource Instance attribute as the
basis for a unique Service Name, composed with the Service Type
attribute as the <ServiceType>, and registered in the appropriate
<Domain>. The agent responsible for exporting records to the DNS
zone file SHOULD be authenticated to the DNS server. The following
example, using the example lookup location /rd-lookup, shows an agent
discovering a resource to be exported:
Req: GET /rd-lookup/res?exp
Res: 2.05 Content
<coap://[FDFD::1234]:5683/light/1>;
exp;st=oic-d-light;rt="oic.d.light";ins="Spot";
d="office";ep="node1"
The agent subsequently registers the following DNS-SD RRs, assuming a
derived DNS zone name "office.example.com":
_oic-d-light._udp.office.example.com IN PTR
Spot._oic-d-light._udp.office.example.com
Spot._oic-d-light._udp.office.example.com IN TXT
txtver=1;path=/light/1;rt=oic.d.light
Spot._oic-d-light._udp.office.example.com IN SRV 0 0 5683
node1.office.example.com.
node1.office.example.com. IN AAAA FDFD::1234
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4. IANA considerations
This specification defines new parameters for the registry "RD
Parameters" provided under "CoRE Parameters" (TBD).
+----------------+-------+---------------+-----+--------------------+
| Full name | Short | Validity | Use | Description |
+----------------+-------+---------------+-----+--------------------+
| ServiceType | st | | RLA | Name of the |
| | | | |Service Type, |
| | | | | max 63 bytes |
| Resource | ins | | RLA | Instance identifier|
| Instance | | | | of the resource |
| | | | | |
| Export | exp | | RLA | flag to indicate |
| | | | | exportation |
+----------------+-------+---------------+-----+--------------------+
5. Security considerations
TBD
6. References
6.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[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/info/rfc2119>.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
<https://www.rfc-editor.org/info/rfc5198>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
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6.2. Informative References
[I-D.handrews-json-schema-hyperschema]
Andrews, H. and A. Wright, "JSON Hyper-Schema: A
Vocabulary for Hypermedia Annotation of JSON", draft-
handrews-json-schema-hyperschema-01 (work in progress),
January 2018.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-19 (work in progress), January 2019.
[I-D.sctl-service-registration]
Cheshire, S. and T. Lemon, "Service Registration Protocol
for DNS-Based Service Discovery", draft-sctl-service-
registration-02 (work in progress), July 2018.
[OCF] Open Connectivity Foundation, "OCF Specification 2.0",
2018, <https://openconnectivity.org/developer/
specifications>.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine, 2000,
<http://www.ics.uci.edu/~fielding/pubs/dissertation/
fielding_dissertation.pdf>.
[rt] IANA, "Resource Type (rt=) Link Target Attribute
Values", 2012, <https://www.iana.org/assignments/core-
parameters/
core-parameters.xhtml#rt-link-target-att-value>.
[st] IANA, "Service Name and Transport Protocol Port Number
Registry", 2018, <https://www.iana.org/assignments/
service-names-port-numbers/
service-names-port-numbers.xml>.
Appendix A. Acknowledgments
This document was split out from [I-D.ietf-core-resource-directory].
Zach Shelby was a co-author of the original version of this draft.
The authors wish to thank Stuart Cheshire, Ted Lemon, and David
Thaler for their thorough reviews and clarifying suggestions.
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Authors' Addresses
Kerry Lynn
Oracle+Dyn
150 Dow Street, Tower Two
Manchester, NH 03101
USA
Phone: +1 978-460-4253
Email: kerlyn@ieee.org
Peter van der Stok
Consultant
Phone: +31 492474673 (Netherlands), +33 966015248 (France)
Email: consultancy@vanderstok.org
URI: www.vanderstok.org
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View, CA 94043
USA
Phone: +1 707-502-5136
Email: Michael.Koster@smartthings.com
Christian Amsuess
Energy Harvesting Solutions
Hollandstr. 12/4
1020
Austria
Phone: +43 664-9790639
Email: c.amsuess@energyharvesting.at
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