Internet Engineering Task Force T. Stach, Ed.
Internet-Draft A. Hutton
Intended status: Informational Siemens Enterprise Communications
Expires: December 29, 2013 J. Uberti
Google
June 27, 2013
RTCWEB Considerations for NATs, Firewalls and HTTP proxies
draft-hutton-rtcweb-nat-firewall-considerations-01
Abstract
This document describes mechanism to enable media stream
establishment for Real-Time Communication in WEB-browsers (RTCWEB) in
the presence of network address translators, firewalls and HTTP
proxies. HTTP proxy and firewall policies applied in many private
network domains introduce obstacles to the successful establishment
of media stream via RTCWEB. This document examines some of these
policies and develops requirements on the web browsers designed to
provide the best possible chance of media connectivity between RTCWEB
peers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 29, 2013.
Copyright Notice
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document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Considerations for NATs/Firewalls independent of HTTP proxies 3
2.1. NAT/Firewall open for outgoing UDP and TCP traffic . . . 3
2.2. NAT/Firewall open only for TCP traffic . . . . . . . . . 4
2.3. NAT/Firewall open only for TCP-based HTTP(s) traffic . . 4
3. Considerations for NATs/Firewalls in presence of HTTP proxies 5
3.1. HTTP proxy with NAT/Firewall open for
outgoing UDP and TCP traffic . . . . . . . . . . . . . . 5
3.2. HTTP proxy with NAT/Firewall open only for TCP traffic . 5
3.3. HTTP proxy assisted TURN server connection . . . . . . . 5
3.3.1. TURN server connection via TCP . . . . . . . . . . . 5
3.3.2. TURN server connection via UDP . . . . . . . . . . . 7
4. Other Approaches . . . . . . . . . . . . . . . . . . . . . . 7
4.1. TURN server connection via WebSocket . . . . . . . . . . 7
4.2. Port Control Protocol . . . . . . . . . . . . . . . . . . 7
4.3. HTTP Fallback for RTP Media Streams . . . . . . . . . . . 7
5. Requirements for RTCWEB-enabled browsers . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Many organizations, e.g. an enterprise, a public service agency or a
university, deploy Network Address Translators (NAT) and firewalls
(FW) at the border to the public internet. RTCWEB relies on ICE
[RFC5245] in order to establish a media path between two RTCWEB peers
in the presence of such NATs/FWs. As last resort in order to cater
for NAT/FWs with address and port dependent filtering characteristics
[RFC4787], the peers will introduce a TURN server [RFC5766] in the
public internet as a media relay. Some use cases and requirements
relating to RTCWEB NAT/FW traversal can be found in
[draft-ietf-rtcweb-use-cases-and-requirements].
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If an organization wants to support RTCWEB such a TURN server may be
located in the DMZ of the private network of that organization where
it is still under administrative control.
In certain environments with very restrictive FW policies a TURN
server in the public internet may not be sufficient to establish
connectivity towards the RTCWEB peer for RTP-based media [RFC3550].
Such policies can include blocking of all UDP based traffic and
allowing only HTTP(S) traffic to the TCP ports 80/443. In addition
access to the World Wide Web from inside an organization is often
only possible via a HTTP proxy.
This document examines impact of NAT/FW policies in Section 2.
Additional impacts due to the presence of a HTTP proxy are examined
in Section 3.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Considerations for NATs/Firewalls independent of HTTP proxies
This section covers aspects of how NAT/FW characteristic influence
the establishment of a media stream. Additional aspects introduced
by the presence of a HTTP proxy are covered in Section 3.
If the NATs serving caller and callee both show port and address
dependent filtering behavior the need for a TURN server arises in
order to establish connectivity for media streams. The TURN server
will relay the RTP packet to the RTCWEB peer using UDP. How the RTP
packets can be transported from the RTCWEB client within the private
network to the TURN server depends on what the firewall will let pass
through.
Other types of NATs do not require using the TURN relay.
Nevertheless, the FW rules and policies still affect how media
streams can be established.
2.1. NAT/Firewall open for outgoing UDP and TCP traffic
This scenario assumes that the NAT/FW is transparent for all outgoing
traffic independent of using UDP or TCP as transport protocol. This
case is used as starting point for introduction of more restrictive
firewall policies. It presents the least critical example with
respect to the establishment of the media streams.
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The TURN server can be reached directly from within the private
network via the NAT/FW and the ICE procedures will reveal that media
can be sent via the TURN server. The TURN client will send its media
to the allocated resources at the TURN server via UDP.
Dependent on the port range that is used for RTCWEB media streams,
the same statement would be true if the NAT/Firewall would allow UDP
traffic for a restricted UDP port range only.
2.2. NAT/Firewall open only for TCP traffic
This scenario assumes that the NAT/FW is transparent for outgoing
traffic only using TCP as transport protocol. This gives two options
for media stream establishment dependent on the NAT's filtering
characteristics. Either transport RTP over TCP or contacting the
TURN server via TCP.
In the first case the browser needs to use ICE-TCP [RFC6544] and
provide active, passive and/or simultaneous-open TCP candidates.
Assuming the peer also provides TCP candidates, a connectivity check
for a TCP connection between the two peers should be successful.
In the second case the browser needs to contact the TURN server via
TCP for allocation of an UDP-based relay address at the TURN server.
The ICE procedures will reveal that RTP media can be sent via the
TURN relay using the TCP connection between TURN client and TURN
server. The TURN server would then relay the RTP packets using UDP,
as well as other UDP-based protocols. ICE-TCP is not needed in this
context.
Note that the second case is not to be mixed up with TURN/TCP
[RFC6062], which deals with how to establish a TCP connection to the
peer. For this document we assume that the TURN server can reach the
peer always via UDP, possibly via a second TURN server.
2.3. NAT/Firewall open only for TCP-based HTTP(s) traffic
In this case the firewall blocks all outgoing traffic except for TCP
traffic to port 80 for HTTP or 443 for HTTPS. A TURN server
listening to its default ports (3478 for TCP/UDP, 5349 for TLS) would
not be reachable in this case.
However, the TURN server can still be reached when it is configured
to listen to the HTTP(S) ports as well. In addition the RTCWEB
clients need to be configured to contact the TURN server over the
HTTP(S) ports and/or needs to be able to tell the browser
accordingly.
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3. Considerations for NATs/Firewalls in presence of HTTP proxies
This section considers a scenario where all HTTP(S) traffic is routed
via an HTTP proxy. Note: If both RTCWEB clients are located behind
the same HTTP proxies, we, of course, assume that ICE would give us a
direct media connection within the private network. We consider this
case as out of the scope of this document.
3.1. HTTP proxy with NAT/Firewall open for outgoing UDP and TCP traffic
As in Section 2.1 we assume that the NAT/FW is transparent for all
outgoing traffic independent of using UDP or TCP as transport
protocol. The HTTP proxy has no impact on the transport of media
streams in this case. Consequently, the same considerations as in
Section 2.1 apply with respect to the traversal of the NAT/FW.
3.2. HTTP proxy with NAT/Firewall open only for TCP traffic
As in Section 2.2 we assume that the NAT/FW is transparent only for
outgoing TCP traffic. The HTTP proxy has no impact on the transport
of media streams in this case. Consequently, the same considerations
as in Section 2.2 apply with respect to the traversal of the NAT/FW.
3.3. HTTP proxy assisted TURN server connection
3.3.1. TURN server connection via TCP
Different from the previous scenarios, we assume that the NAT/FW
accepts outgoing traffic only via a TCP connection that is initiated
from the HTTP proxy. Consequently, a RTCWEB client would have to use
the HTTP CONNECT method [RFC2616] in order to get access to the TURN
server via the HTTP proxy. The HTTP CONNECT request needs to convey
the TURN Server URI or transport address. As a result the HTTP Proxy
will establish a TCP connection to the TURN server, i.e. the TURN
server only has to handle a standard TCP connection and an update to
the TURN protocol or the TURN software is not needed.
Afterwards, the RTCWEB client could upgrade the connection to use
TLS, forward STUN/TURN traffic via the HTTP proxy and use the TURN
server as media relay. Note that upgrading in this case is not to be
misunderstood as usage of the HTTP UPGRADE method as specified in
[RFC2817] as this would require the TURN server to support HTTP. We
rather envisage the following sequence:
o the browser opens a TCP connection to the HTTP proxy,
o the browser issues a HTTP CONNECT request to the HTTP proxy with
the TURN server address in the Request URI,
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o the HTTP proxy opens a TCP connection to the TURN server and
"bridges" the incoming and outgoing TCP connections together,
forming a virtual end-to-end TCP connection,
o the browser can do a TLS handshake over the virtual end-to-end TCP
connection with the TURN server.
If it is not possible to use HTTP CONNECT in this way it will not be
possible to establish connectivity between the RTCWEB peers and the
ICE connectivity checks will fail.
Strictly speaking the TLS upgrade is not necessary, but using TLS
would also prevent the HTTP proxy from sniffing into the data stream
and provides the same flow as HTTPS and might improve
interoperability with proxy servers. Some tests (done a while ago)
indicated that there are proxies performing Deep Packet inspection
(DPI) that expect to see at least a SSL handshake and, possibly,
valid SSL records. The application has the ability to control
whether SSL is used by the parameters it supplies to the TURN URI
(e.g. turns: vs. turn:), so the decision to do TURN/TCP to port 443
versus TURN/TLS to port 443 could be left up to the application or
possibly the browser configuration script.
In contrast to using UDP or TCP for transporting the STUN messages,
the browser would now need to first establish a HTTP over TCP
connection to the HTTP proxy, upgrade to using TLS and then switch to
using this TLS connection for transport of STUN messages. It is also
desirable that the browser detects the need to connect to the TURN
server through a HTTP proxy automatically in order to achieve
seamless deployment and interoperability. The browser should use the
same proxy selection procedure for TURN as currently done for HTTP.
The user or network administrator should not be required to change
browser or proxy script configuration.
Further considerations apply to the default connection timeout of the
HTTP proxy connection to the TURN server and the timeout of the TURN
server allocation. Whereas [RFC5766] specifies a 10 minutes default
lifetime of the TURN allocation, typical proxy connection lifetimes
are in the range of 60 seconds if no activity is detected. Thus, if
the RTCWEB client wants to pre-allocate TURN ressources it needs to
refresh TURN allocations more frequently in order to keep the TCP
connection to its TURN server alive.
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3.3.2. TURN server connection via UDP
If a local TURN server under administrative control of the
organization is deployed it is desirable to reach this TURN server
via UDP. The TURN server could be specified in the proxy
configuration script, giving the browser the possibility to learn how
to access it. Then, when gathering candidates, this TURN server
would always be used such that the RTCWEB client application could
get UDP traffic out to the internet.
4. Other Approaches
4.1. TURN server connection via WebSocket
The RTCWEB client could connect to a TURN server via WebSocket
[RFC6455] as described in [draft-chenxin-behave-turn-WebSocket].
This might have benefits in very restrictive environments where HTTPS
is not permitted through the proxy. However, such environments are
also likely to deploy DPI boxes which would eventually complain
against usage of WebSocket or block RTCWEB traffic based on other
heuristic means. It is also to be expected that an environment that
does not allow HTTPS will also forbid usage of WebSocket over TLS.
In addition, usage of TURN over WebSocket puts an additional burden
on existing TURN server implementation to support HTTP and WebSocket.
The resulting benefit seems rather small, thus TURN over WebSocket is
left for further study.
4.2. Port Control Protocol
As a further alternative, the Port Control Protocol (PCP) [RFC6887]
allows to configure how incoming IPv6 or IPv4 packets are translated
and forwarded by a NAT/FW. However, this document does not examine
benefits of PCP for the management of the local NAT/FW, but leaves
this for further study until PCP is deployed more widely.
4.3. HTTP Fallback for RTP Media Streams
As an alternative to using a TURN server it was proposed to send RTP
directly over HTTP [draft-miniero-rtcweb-http-fallback]. This
approach bears some similarities with TURN as it also uses a RTP
relay. However, it uses HTTP GET and POST requests to receive and
send RTP packets.
Despite a number of open issues, the proposal addreses some corner
cases. However, the expected benefit in form of an increased success
rate for establishment of a media stream seems rather small, thus
HTTP fallback is left for further study.
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5. Requirements for RTCWEB-enabled browsers
For the purpose of relaying RTCWEB media streams or data channels a
browser needs to be able to
o connect to a TURN server via UDP, TCP and TLS,
o connect to a TURN server via a HTTP proxy using the HTTP connect
method,
o connect to a TURN server via the HTTP(s) ports 80/443 instead of
the default STUN ports 3478/5349,
o upgrade the HTTP proxy-relayed connection to the TURN server to
use TLS,
o use the same proxy selection procedure for TURN as currently done
for HTTP,
o switch the usage of the HTTP proxy-relayed connection with the
TURN server from HTTP to STUN/TURN,
o use a preconfigured or standardized port range for UDP-based media
streams or data channels,
o learn from the proxy configuration script about the presence of a
local TURN server and use it for sending UDP traffic to the
internet,
o support ICE-TCP for TCP-based direct media connection to the
RTCWEB peer.
6. Acknowledgements
The authors want to thank Heinrich Haager for all his input during
many valuable discussions.
Furthermore, the authors want to thank for comments and suggestions
received from ...
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
TBD
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/
1.1", RFC 2817, May 2000.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[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, April 2010.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations", RFC
6062, November 2010.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
6455, December 2011.
[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, March 2012.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
2013.
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[draft-chenxin-behave-turn-WebSocket]
Xin. Chen , "Traversal Using Relays around NAT (TURN)
Extensions for WebSocket Allocations ", 2013, <http://
tools.ietf.org/html/draft-chenxin-behave-turn-WebSocket>.
[draft-ietf-rtcweb-use-cases-and-requirements]
C. Holmberg, S. Hakansson, G. Eriksson , "Web Real-Time
Communication Use-cases and Requirements ", 2012, <http://
tools.ietf.org/html/draft-ietf-rtcweb-use-cases-and-
requirements>.
[draft-miniero-rtcweb-http-fallback]
L. Miniero , "HTTP Fallback for RTP Media Streams ", 2012,
<http://tools.ietf.org/html/draft-miniero-rtcweb-http-
fallback>.
Authors' Addresses
Thomas Stach (editor)
Siemens Enterprise Communications
Dietrichgasse 27-29
Vienna 1030
AT
Email: thomas.stach@siemens-enterprise.com
Andrew Hutton
Siemens Enterprise Communications
Technology Drive
Nottingham NG9 1LA
UK
Email: andrew.hutton@siemens-enterprise.com
Justin Uberti
Google
747 6th Ave S
Kirkland, WA 98033
US
Email: justin@uberti.name
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