One document matched: draft-ietf-tram-turn-server-discovery-00.xml
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<rfc category="std" docName="draft-ietf-tram-turn-server-discovery-00"
ipr="trust200902">
<front>
<title abbrev="TURN server auto disc">TURN Server Auto Discovery</title>
<author fullname="Prashanth Patil" initials="P." surname="Patil">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street/>
<street/>
<city>Bangalore</city>
<country>India</country>
</postal>
<email>praspati@cisco.com</email>
</address>
</author>
<author fullname="Tirumaleswar Reddy" initials="T." surname="Reddy">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>Cessna Business Park, Varthur Hobli</street>
<street>Sarjapur Marathalli Outer Ring Road</street>
<city>Bangalore</city>
<region>Karnataka</region>
<code>560103</code>
<country>India</country>
</postal>
<email>tireddy@cisco.com</email>
</address>
</author>
<author fullname="Dan Wing" initials="D." surname="Wing">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>California</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>dwing@cisco.com</email>
</address>
</author>
<date/>
<workgroup>TRAM</workgroup>
<abstract>
<t>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 or the ISP, 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 two
such mechanisms for TURN server discovery.</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>TURN <xref target="RFC5766"/> is a protocol that is often used to
improve the connectivity of P2P applications. By providing a relay
service, TURN ensures that a connection can be established even when one
or both sides is 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 explicitly configured with a well known TURN server.
To allow TURN applications 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. WebRTC usages and related extensions, which
are mostly based on web applications, need this immediately.</t>
<t>This document describes two discovery mechanisms. The reason for
providing two mechanisms is to maximize the opportunity for discovery,
based on the network in the which the TURN client sees itself.</t>
<t><list style="symbols">
<t>A resolution mechanism based on straightforward Naming Authority
Pointer (S-NAPTR) resource records in the Domain Name System (DNS).
<xref target="RFC5928"/> describes details on retrieving a list of
server transport addresses from DNS that can be used to create a
TURN allocation.</t>
<t>A mechanism based on anycast address for TURN.</t>
</list>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.</t>
</section>
<section anchor="term" title="Terminology">
<t>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 <xref
target="RFC2119"/>.</t>
</section>
<section title="Discovery Procedure">
<t>A TURN client that implements the auto discovery algorithm MUST
proceed with discovery in the following order:</t>
<t><list style="numbers">
<t>Local Configuration : Local or manual configuration 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.</t>
<t>Service Resolution : The TURN client attempts to perform TURN
service resolution using the DNS domain name that the host belongs
to OR the hosts' global IP address. The TURN client will attempt to
do this for each combination of interface and address family. The
retrieved DNS domain names OR IP addresses are then used for NAPTR
lookups.</t>
<t>Anycast : Send TURN allocate request to the assigned TURN anycast
request for each combination of interface and address family.</t>
</list></t>
<t>While it is expected that Step-3 be performed if Step-2 fails, an
implementation may choose to perform steps 2 and 3 in parallel.</t>
</section>
<section title="Discovery using Service Resolution">
<t>This mechanism is performed in two steps:</t>
<t>1. A DNS domain name is retrieved for each combination of interface
and address family.</t>
<t>2. Retrieved DNS domain names are then used for S-NAPTR lookups as
per <xref target="RFC5928"/>. Further DNS lookups may be necessary to
determine TURN server IP address(es).</t>
<t>On hosts with more than one interface or address family (IPv4/v6),
the TURN server discovery procedure has to be run for each combination
of interface and address family.</t>
<section title="Retrieving Domain Name">
<t>The domain, in which the client is located, can be determined using
one of the techniques provided below. An implementation can choose to
use any or all techniques.</t>
<t>Implementations may allow the user to specify a default name that
is used if no specific name has been configured. Other means of
retrieving domain names may be used, which are outside the scope of
this document e.g. local configuration.</t>
<section title="DHCP">
<t>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. <xref target="RFC5986"/> defines DHCP
IPv4 and IPv6 access network domain name options to identify a
domain name that is suitable for service discovery within the access
network. <xref target="RFC2132"/> defines the DHCP IPv4 domain name
option. While this option is less suitable, it still may be useful
if the option defined in <xref target="RFC5986"/> is not
available.</t>
<t>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.</t>
</section>
<section title="IP Address">
<t>Typically, but not necessarily, the DNS domain name is the domain
name in which the client is located, i.e., a PTR lookup on the
client's IP address (according to <xref target="RFC1035"/>, Section
3.5 for IPv4 or <xref target="RFC3596"/>, Section 2.5 for IPv6)
would yield a similar name. However, due to the widespread use of
Network Address Translation (NAT), the client MAY need to determine
its public IP address using mechanisms described in <xref
target="RFC7216"/>.</t>
</section>
<section title="From own Identity">
<t>A TURN client could also wish to extract the domain name from its
own identity i.e canonical identifier used to reach the user.</t>
<t><figure>
<artwork><![CDATA[Example
SIP : 'sip:alice@example.com'
JID : 'alice@example.com'
email : 'alice@example.com'
'example.com' is retrieved from the above examples.]]></artwork>
</figure></t>
<t>The means to extract the domain name may be different based on
the type of identifier and is outside the scope of this
document.</t>
</section>
</section>
<section title="Resolution">
<t>Once the TURN discovery procedure has retrieved domain names, the
resolution mechanism described in <xref target="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.</t>
<t>In the example below, for domain 'example.net', the resolution
algorithm will result in IP address, port, and protocol tuples as
follows:</t>
<t><figure>
<artwork><![CDATA[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 |
+-------+----------+------------+------+
]]></artwork>
</figure>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.).</t>
<section title="SOA">
<t>If no TURN-specific S-NAPTR records can be retrieved using the
previous step, additional steps described in this section have to be
followed. First, an SOA record for the "reverse zone" i.e., the zone
in the in-addr.arpa. or ip6.arpa. domain that contains the IP
address(s) in question, has to be retrieved. IP addresses can be
determined, if not done already, as described in Section 4.1.2.</t>
<t><figure>
<artwork><![CDATA[ A sample SOA record could be:
100.51.198.in-addr.arpa
IN SOA dns1.isp.example.net. hostmaster.isp.example.net. (
1 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL]]></artwork>
</figure>If this lookup fails, the discovery procedure is aborted
without a result.</t>
<t>Once the SOA record is available, the discovery procedure
extracts the MNAME field, i.e., the responsible master name server
from the SOA record. An example MNAME could be:
dns1.isp.example.net. Then, an S-NAPTR lookup as specified in the
previous step Section 4.2 is performed on this MNAME to discover the
TURN service. 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).</t>
</section>
</section>
</section>
<section title="Discovery using Anycast">
<t>IP anycast is an elegant solution 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.</t>
<t>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 <xref
target="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.</t>
</section>
<section title="Deployment Considerations">
<section title="Mobility and Changing IP addresses">
<t>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 <xref target="I-D.wing-mmusic-ice-mobility"/> can be used
for ongoing streams.</t>
</section>
</section>
<section anchor="iana" title="IANA Considerations">
<section title="Anycast">
<t>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</t>
<t><http://www.iana.org/assignments/iana-ipv4-special-registry/>
and</t>
<t><http://www.iana.org/assignments/iana-ipv6-special-registry/></t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>In general, it is recommended that a TURN client authenticate with
the TURN server to identify a rouge server. <xref
target="I-D.petithuguenin-tram-turn-dtls"> </xref> can be potentially
used by a client to validate a previously unknown server.</t>
<section title="Service Resolution">
<t>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 <xref target="RFC5928"/> are applicable
here as well.</t>
<t>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.</t>
<t>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
commonplace methods that can reduce the possibility of attack within
an access network; alternatively, DHCP authentication <xref
target="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.</t>
</section>
<section title="Anycast">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
</section>
<section anchor="ack" title="Acknowledgements">
<t>Discovery using Service Resolution described in Section 4 of this
document was derived from similar techniques described in ALTO Server
Discovery <xref target="I-D.ietf-alto-server-discovery"/> and <xref
target="I-D.kist-alto-3pdisc"/>.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.5766"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.5986"?>
<?rfc include="reference.RFC.2132"
?>
<?rfc include="reference.RFC.1035"?>
<?rfc include="reference.RFC.5928"
?>
<?rfc include="reference.RFC.3596"?>
<?rfc include="reference.I-D.wing-mmusic-ice-mobility"?>
<?rfc include="reference.I-D.petithuguenin-tram-turn-dtls"
?>
<?rfc include="reference.RFC.7216"
?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.ietf-alto-server-discovery'
?>
<?rfc include="reference.RFC.3188"?>
<?rfc include="reference.I-D.kist-alto-3pdisc"?>
<!---->
</references>
<section title="Change History">
<t>[Note to RFC Editor: Please remove this section prior to
publication.]</t>
<section title="Change from draft-patil-tram-serv-disc-00 to -01">
<t><list style="symbols">
<t>Added IP address (Section 4.1.2) and Own identity (4.1.3) as
new means to obtain domain names</t>
<t>New Section 4.2.1 SOA (inspired by draft-kist-alto-3pdisc)</t>
<t>300 (Try Alternate) response for Anycast</t>
</list></t>
</section>
</section>
</back>
</rfc>
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