TRAM P. Patil
Internet-Draft T. Reddy
Intended status: Standards Track D. Wing
Expires: January 21, 2016 Cisco
July 20, 2015
TURN Server Auto Discovery
draft-ietf-tram-turn-server-discovery-04
Abstract
Current Traversal Using Relays around NAT (TURN) server discovery
mechanisms are relatively static and limited to explicit
configuration. These are usually under the administrative control of
the application or TURN service provider, and not the enterprise,
ISP, or the network in which the client is located. Enterprises and
ISPs wishing to provide their own TURN servers need auto discovery
mechanisms that a TURN client could use with no or minimal
configuration. This document describes three such mechanisms for
TURN server discovery.
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
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This Internet-Draft will expire on January 21, 2016.
Copyright Notice
Copyright (c) 2015 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Discovery Procedure . . . . . . . . . . . . . . . . . . . . . 3
4. Discovery using Service Resolution . . . . . . . . . . . . . 4
4.1. Retrieving Domain Name . . . . . . . . . . . . . . . . . 4
4.1.1. DHCP . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.2. From own Identity . . . . . . . . . . . . . . . . . . 5
4.2. Resolution . . . . . . . . . . . . . . . . . . . . . . . 5
5. DNS Service Discovery . . . . . . . . . . . . . . . . . . . . 6
5.1. mDNS . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Discovery using Anycast . . . . . . . . . . . . . . . . . . . 8
7. Deployment Considerations . . . . . . . . . . . . . . . . . . 8
7.1. Mobility and Changing IP addresses . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8.1. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9.1. Service Resolution . . . . . . . . . . . . . . . . . . . 9
9.2. DNS Service Discovery . . . . . . . . . . . . . . . . . . 10
9.3. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 13
A.1. Change from draft-patil-tram-serv-disc-00 to -01 . . . . 13
A.2. Change from draft-ietf-tram-turn-server-discovery-01 to
02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
TURN [RFC5766] is a protocol that is often used to improve the
connectivity of Peer-to-Peer (P2P) applications (as defined in
section 2.7 of [RFC5128]). TURN allows a connection to be
established when one or both sides are incapable of a direct P2P
connection. It is an important building block for interactive, real-
time communication using audio, video, collaboration etc.
While TURN services are extensively used today, the means to auto
discover TURN servers do not exist. TURN clients are usually
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explicitly configured with a well known TURN server. To allow TURN
applications to operate seamlessly across different types of networks
and encourage the use of TURN without the need for manual
configuration, it is important that there exists an auto discovery
mechanism for TURN services. Web Real-Time Communication (WebRTC)
[I-D.ietf-rtcweb-overview] usages and related extensions, which are
mostly based on web applications, need this immediately.
This document describes three discovery mechanisms. The reason for
providing multiple mechanisms is to maximize the opportunity for
discovery, based on the network in which the TURN client finds
itself. The three discovery mechanisms are:
o A resolution mechanism based on straightforward Naming Authority
Pointer (S-NAPTR) resource records in the Domain Name System
(DNS). [RFC5928] describes details on retrieving a list of server
transport addresses from DNS that can be used to create a TURN
allocation.
o DNS Service Discovery
o A mechanism based on anycast address for TURN.
In general, if a client wishes to communicate using one of its
interfaces using a specific IP address family, it SHOULD query the
TURN server(s) that has been discovered for that specific interface
and address family. How to select an interface and IP address
family, is out of the scope of this document.
2. Terminology
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 [RFC2119].
3. Discovery Procedure
A TURN client that implements the auto discovery algorithm uses the
following mechanisms for discovery:
1. Local Configuration : Local or manual TURN configuration (i.e.,
TURN servers configured at the system level) should be tried
first, as it may be an explicit preferred choice of a user. An
implementation MAY give the user an opportunity (e.g., by means
of configuration file options or menu items) to specify a TURN
server for every address family.
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2. Service Resolution : The TURN client attempts to perform TURN
service resolution using the host's DNS domain.
3. DNS SD: DNS Service Discovery.
4. Anycast : Send TURN allocate request to the assigned TURN anycast
request for each combination of interface and address family.
Not all TURN servers may be discovered using NAPTR records or DNS SD;
Similarly, not all TURN servers may support anycast. For best
results, a client SHOULD implement all discovery mechanisms described
above.
The document does not prescribe a strict order that a client must
follow for discovery. An implementation may choose to perform steps
2,3 and 4 in parallel for discovery OR choose to follow any desired
order and stop the discovery procedure if a mechanism succeeds.
On hosts with more than one interface or address family (IPv4/v6),
the TURN server discovery procedure has to be performed for each
combination of interface and address family. A client MAY optionaly
choose to perform the discovery procedure only for a desired
interface/address combination if the client does not wish to discover
a TURN server for all combinations of interface and address family.
4. Discovery using Service Resolution
This mechanism is performed in two steps:
1. A DNS domain name is retrieved for each combination of interface
and address family.
2. Retrieved DNS domain names are then used for S-NAPTR lookups as
per [RFC5928]. Further DNS lookups may be necessary to determine
TURN server IP address(es).
4.1. Retrieving Domain Name
A client has to determine the domain in which it is located. The
following sections provide two possible mechanisms to learn the
domain name, but other means of retrieving domain names may be used,
which are outside the scope of this document e.g. local
configuration.
Implementations may allow the user to specify a default name that is
used if no specific name has been configured.
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4.1.1. DHCP
DHCP can be used to determine the domain name related to an
interface's point of network attachment. Network operators may
provide the domain name to be used for service discovery within an
access network using DHCP. Sections 3.2 and 3.3 of [RFC5986] define
DHCP IPv4 and IPv6 access network domain name options to identify a
domain name that is suitable for service discovery within the access
network. [RFC2132] defines the DHCP IPv4 domain name option; While
this option is less suitable, it may still be useful if the options
defined in [RFC5986] are not available.
For IPv6, the TURN server discovery procedure MUST try to retrieve
DHCP option 57 (OPTION_V6_ACCESS_DOMAIN). If no such option can be
retrieved, the procedure fails for this interface. For IPv4, the
TURN server discovery procedure MUST try to retrieve DHCP option 213
(OPTION_V4_ACCESS_DOMAIN). If no such option can be retrieved, the
procedure SHOULD try to retrieve option 15 (Domain Name). If neither
option can be retrieved the procedure fails for this interface. If a
result can be retrieved it will be used as an input for S-NAPTR
resolution.
4.1.2. From own Identity
For a TURN client with an understanding of the protocol mechanics of
calling applications, the client may wish to extract the domain name
from its own identity i.e canonical identifier used to reach the
user.
Example
SIP : 'sip:alice@example.com'
JID : 'alice@example.com'
email : 'alice@example.com'
'example.com' is retrieved from the above examples.
The means to extract the domain name may be different based on the
type of identifier and is outside the scope of this document.
4.2. Resolution
Once the TURN discovery procedure has retrieved domain names, the
resolution mechanism described in [RFC5928] is followed. An S-NAPTR
lookup with 'RELAY' application service and the desired protocol tag
is made to obtain information necessary to connect to the
authoritative TURN server within the given domain.
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In the example below, for domain 'example.net', the resolution
algorithm will result in IP address, port, and protocol tuples as
follows:
example.net.
IN NAPTR 100 10 "" RELAY:turn.udp "" example.net.
example.net.
IN NAPTR 100 10 S RELAY:turn.udp "" _turn._udp.example.net.
_turn._udp.example.net.
IN SRV 0 0 3478 a.example.net.
a.example.net.
IN A 192.0.2.1
+-------+----------+------------+------+
| Order | Protocol | IP address | Port |
+-------+----------+------------+------+
| 1 | UDP | 192.0.2.1 | 3478 |
+-------+----------+------------+------+
If no TURN-specific S-NAPTR records can be retrieved, the discovery
procedure fails for this domain name (and the corresponding interface
and IP protocol version). If more domain names are known, the
discovery procedure may perform the corresponding S-NAPTR lookups
immediately. However, before retrying a lookup that has failed, a
client MUST wait a time period that is appropriate for the
encountered error (NXDOMAIN, timeout, etc.).
5. DNS Service Discovery
DNS-based Service Discovery (DNS-SD) [RFC6763] and Multicast DNS
(mDNS) [RFC6762] provide generic solutions for discovering services
available in a local network. DNS-SD/ mDNS define a set of naming
rules for certain DNS record types that they use for advertising and
discovering services. PTR records are used to enumerate service
instances of a given service type. A service instance name is mapped
to a host name and a port number using a SRV record. If a service
instance has more information to advertise than the host name and
port number contained in its SRV record, the additional information
is carried in a TXT record.
Section 4.1 of [RFC6763] specifies that a service instance name in
DNS-SD has the following structure:
<Instance> . <Service> . <Domain>
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The <Domain> portion specifies the DNS sub-domain where the service
instance is registered. It may be "local.", indicating the mDNS
local domain, or it may be a conventional domain name such as
"example.com.". The <Service> portion of the TURN service instance
name MUST be "_turnserver._udp", "_turnserver._tcp".
The <Instance> portion is a DNS label, containing UTF-8-encoded text
[RFC5198], limited to 63 octets in length. It is meant to be a user-
friendly description of the service instance, suitable for a menu-
like user interface display. Thus it can contain any characters
including spaces, punctuation, and non-Latin characters as long as
they can be encoded in UTF-8.
For example, TURN server advertises the following DNS records :
_turnserver._udp.local. PTR example.com._turnserver._udp.local.
example.com._turnserver._udp.local. SRV 0 0 5030 example-turn-
server.local.
example-turn-server.local. A 192.168.1.2
In addition to the service instance name, IP address and the port
number, DNS-SD provides a way to publish other information pertinent
to the service being advertised. The additional data can be stored
as name/value attributes in a TXT record with the same name as the
SRV record for the service. Each name/value pair within the TXT
record is preceded by a single length byte, thereby limiting the
length of the pair to 255 bytes (See Section 6 of [RFC6763] and
Section 3.3.14 of [RFC1035] for details).
5.1. mDNS
A TURN client tries to discover the TURN servers being advertised in
the site by multicasting a PTR query "_turnserver._udp.local." or
"_turnserver._tcp.local" or the TURN server can send out gratuitous
multicast DNS answer packets whenever it starts up, wakes from sleep,
or detects a chance in network configuration. TURN clients receive
these gratuitous packet and cache the information contained in it.
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+------+ +-------------+
| TURN | | TURN Server |
|Client| | |
+------+ +-------------+
| |
| PTR query "_turnserver._udp.local." |
|--------------------------------------------->|
| PTR reply |
|<---------------------------------------------|
| SRV query |
|--------------------------------------------->|
| SRV reply |
|<---------------------------------------------|
| A/AAAA query reply |
|--------------------------------------------->|
| TURN Request |
|--------------------------------------------->|
| TURN Response |
|<---------------------------------------------|
Figure 1: TURN Server Discovery using mDNS
6. Discovery using Anycast
IP anycast can also be used for TURN service discovery. A packet
sent to an anycast address is delivered to the "topologically
nearest" network interface with the anycast address. Using the TURN
anycast address, the only two things that need to be deployed in the
network are the two things that actually use TURN.
When a client requires TURN services, it sends a TURN allocate
request to the assigned anycast address. The TURN anycast server
responds with a 300 (Try Alternate) error as described in [RFC5766];
The response contains the TURN unicast address in the ALTERNATE-
SERVER attribute. For subsequent communication with the TURN server,
the client uses the responding server's unicast address. This has to
be done because two packets addressed to an anycast address may reach
two different anycast servers. The client, thus, also needs to
ensure that the initial request fits in a single packet. An
implementation may choose to send out every new request to the
anycast address to learn the closest TURN server each time.
7. Deployment Considerations
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7.1. Mobility and Changing IP addresses
A change of IP address on an interface may invalidate the result of
the TURN server discovery procedure. For instance, if the IP address
assigned to a mobile host changes due to host mobility, it may be
required to re-run the TURN server discovery procedure without
relying on earlier gained information. New requests should be made
to the newly learned TURN servers learned after TURN discovery re-
run. However, if an earlier learned TURN server is still accessible
using the new IP address, procedures described for mobility using
TURN defined in [I-D.wing-tram-turn-mobility] can be used for ongoing
streams.
8. IANA Considerations
8.1. Anycast
IANA should allocate an IPv4 and an IPv6 well-known TURN anycast
address. 192.0.0.0/24 and 2001:0000::/48 are reserved for IETF
Protocol Assignments, as listed at
<http://www.iana.org/assignments/iana-ipv4-special-registry/> and
<http://www.iana.org/assignments/iana-ipv6-special-registry/>
9. Security Considerations
In general, it is recommended that a TURN client authenticate with
the TURN server to identify a rouge server. [RFC7350] can be
potentially used by a client to validate a previously unknown server.
9.1. Service Resolution
The primary attack against the methods described in this document is
one that would lead to impersonation of a TURN server. An attacker
could attempt to compromise the S-NAPTR resolution. Security
considerations described in [RFC5928] are applicable here as well.
In addition to considerations related to S-NAPTR, it is important to
recognize that the output of this is entirely dependent on its input.
An attacker who can control the domain name can also control the
final result. Because more than one method can be used to determine
the domain name, a host implementation needs to consider attacks
against each of the methods that are used.
If DHCP is used, the integrity of DHCP options is limited by the
security of the channel over which they are provided. Physical
security and separation of DHCP messages from other packets are
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commonplace methods that can reduce the possibility of attack within
an access network; alternatively, DHCP authentication [RFC3188] can
provide a degree of protection against modification. When using DHCP
discovery, clients are encouraged to use unicast DHCP INFORM queries
instead of broadcast queries which are more easily spoofed in
insecure networks.
9.2. DNS Service Discovery
Since DNS-SD is just a specification for how to name and use records
in the existing DNS system, it has no specific additional security
requirements over and above those that already apply to DNS queries
and DNS updates. For DNS queries, DNS Security Extensions (DNSSEC)
[RFC4033] should be used where the authenticity of information is
important. For DNS updates, secure updates [RFC2136][RFC3007] should
generally be used to control which clients have permission to update
DNS records.
For mDNS, in addition to what has been described above, a principal
security threat is a security threat inherent to IP multicast routing
and any application that runs on it. A rogue system can advertise
that it is a TURN server. Discovery of such rogue systems as TURN
servers, in itself, is not a security threat if there is a means for
the TURN client to authenticate and authorize the discovered TURN
servers.
9.3. Anycast
In a network without any TURN server that is aware of the TURN
anycast address, outgoing TURN requests could leak out onto the
external Internet, possibly revealing information.
Using an IANA-assigned well-known TURN anycast address enables border
gateways to block such outgoing packets. In the default-free zone,
routers should be configured to drop such packets. Such
configuration can occur naturally via BGP messages advertising that
no route exists to said address.
Sensitive clients that do not wish to leak information about their
presence can set an IP TTL on their TURN requests that limits how far
they can travel into the public Internet.
10. Acknowledgements
The authors would like to thank Simon Perrault, Paul Kyzivat, Troy
Shields, Eduardo Gueiros and Ted Hardie for their review and valuable
comments. Thanks to Adam Roach for his detailed review and
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suggesting DNS Service Discovery as an additional discovery
mechanism.
11. References
11.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<http://www.rfc-editor.org/info/rfc2132>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<http://www.rfc-editor.org/info/rfc2136>.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
<http://www.rfc-editor.org/info/rfc3007>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC3596, October 2003,
<http://www.rfc-editor.org/info/rfc3596>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
<http://www.rfc-editor.org/info/rfc5198>.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<http://www.rfc-editor.org/info/rfc5766>.
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[RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT
(TURN) Resolution Mechanism", RFC 5928,
DOI 10.17487/RFC5928, August 2010,
<http://www.rfc-editor.org/info/rfc5928>.
[RFC5986] Thomson, M. and J. Winterbottom, "Discovering the Local
Location Information Server (LIS)", RFC 5986,
DOI 10.17487/RFC5986, September 2010,
<http://www.rfc-editor.org/info/rfc5986>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
[RFC7216] Thomson, M. and R. Bellis, "Location Information Server
(LIS) Discovery Using IP Addresses and Reverse DNS",
RFC 7216, DOI 10.17487/RFC7216, April 2014,
<http://www.rfc-editor.org/info/rfc7216>.
[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
August 2014, <http://www.rfc-editor.org/info/rfc7350>.
11.2. Informative References
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-14
(work in progress), June 2015.
[I-D.kist-alto-3pdisc]
Kiesel, S., Krause, K., and M. Stiemerling, "Third-Party
ALTO Server Discovery (3pdisc)", draft-kist-alto-3pdisc-05
(work in progress), January 2014.
[I-D.wing-tram-turn-mobility]
Wing, D., Patil, P., Reddy, T., and P. Martinsen,
"Mobility with TURN", draft-wing-tram-turn-mobility-03
(work in progress), May 2015.
[RFC3188] Hakala, J., "Using National Bibliography Numbers as
Uniform Resource Names", RFC 3188, DOI 10.17487/RFC3188,
October 2001, <http://www.rfc-editor.org/info/rfc3188>.
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[RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March
2008, <http://www.rfc-editor.org/info/rfc5128>.
[RFC7286] Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
H. Song, "Application-Layer Traffic Optimization (ALTO)
Server Discovery", RFC 7286, DOI 10.17487/RFC7286,
November 2014, <http://www.rfc-editor.org/info/rfc7286>.
Appendix A. Change History
[Note to RFC Editor: Please remove this section prior to
publication.]
A.1. Change from draft-patil-tram-serv-disc-00 to -01
o Added IP address (Section 4.1.2) and Own identity (4.1.3) as new
means to obtain domain names
o New Section 4.2.1 SOA (inspired by draft-kist-alto-3pdisc)
o 300 (Try Alternate) response for Anycast
A.2. Change from draft-ietf-tram-turn-server-discovery-01 to 02
o Removed sections that describe reverse IP lookup
o Added DNS Service Discovery as an additional discovery mechanism
Authors' Addresses
Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: praspati@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
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Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
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