RTCWEB Considerations for NATs, Firewalls and HTTP proxies
draft-hutton-rtcweb-nat-firewall-considerations-02

Internet Engineering Task Force                            T. Stach, Ed.
Internet-Draft                                                 A. Hutton
Intended status: Informational         Siemens Enterprise Communications
Expires: March 24, 2014                                        J. Uberti
                                                                  Google
                                                      September 20, 2013

       RTCWEB Considerations for NATs, Firewalls and HTTP proxies
           draft-hutton-rtcweb-nat-firewall-considerations-02

Abstract

   This document describes mechanism to enable media stream
   establishment for Real-Time Communication in WEB-browsers (WebRTC) in
   the presence of network address translators, firewalls and HTTP
   proxies.  HTTP proxy and firewall deployed in many private network
   domains introduce obstacles to the successful establishment of media
   stream via WebRTC.  This document examines some of these deployment
   scenarios and develops requirements on the web browsers designed to
   provide the best possible chance of media connectivity between WebRTC
   peers.

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 March 24, 2014.

Copyright Notice

   Copyright (c) 2013 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

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   publication of this document.  Please review these 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   4
     2.1.  NAT/Firewall open for outgoing UDP and TCP traffic  . . .   4
     2.2.  NAT/Firewall open only for TCP traffic  . . . . . . . . .   4
     2.3.  NAT/Firewall open only for TCP on restricted ports  . . .   5
   3.  Considerations for NATs/Firewalls in presence of HTTP proxies   6
     3.1.  HTTP proxy with NAT/Firewall open for
           outgoing UDP and TCP traffic  . . . . . . . . . . . . . .   6
     3.2.  HTTP proxy with NAT/Firewall open only for TCP traffic  .   6
     3.3.  HTTP proxy assisted TURN server connection  . . . . . . .   6
       3.3.1.  TURN server connection via TCP  . . . . . . . . . . .   6
       3.3.2.  TURN server connection via UDP  . . . . . . . . . . .   8
   4.  Other Approaches  . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  TURN server connection via WebSocket  . . . . . . . . . .   8
     4.2.  Port Control Protocol . . . . . . . . . . . . . . . . . .   9
     4.3.  HTTP Fallback for RTP Media Streams . . . . . . . . . . .   9
   5.  Requirements for RTCWEB-enabled browsers  . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   WebRTC is a web-based technique for direct interactive rich
   communication using audio, video, and data between two peer browsers.

   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.  WebRTC relies on ICE
   [RFC5245] in order to establish a media path between two WebRTC peers
   in the presence of such NATs/FWs.

   When WebRTC is deployed by the corporate IT department one can assume
   that the corporate IT configures the corporate NATs, Firewalls, DPI

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   units, TURN servers accordingly.  If so desired by the organization
   WebRTC media streams can then be established to WebRTC peers outside
   of the organization subject to the applied policies.  In order to
   cater for NAT/FWs with address and port dependent mapping
   characteristics [RFC4787], the peers will introduce a TURN server
   [RFC5766] in the public internet as a media relay.  Such a TURN
   server could be deployed by the organization wanting to assert policy
   on WebRTC traffic.

   However, there are also environments that are not prepared for WebRTC
   and have NAT/FW deployed that prevent the media stream establishment
   although such blocking is not intentional.  These environments
   include e.g. internet cafes or hotels offering their customers access
   to the web and have opened the well-known HTTP(S) ports but nothing
   else.  In such an environment ICE will fail to establish
   connectivity.  Re-configuration of the NAT/FW is also often
   impracticable or not possible.

   In such an environment a WebRTC user may easily reach its WebRTC
   server possibly via an HTTP proxy and start establishing a WEBTRTC
   session, but will become frustrated when a media connection cannot be
   established.  A corresponding use case and its requirements relating
   to WebRTC NAT/FW traversal can be found in
   [draft-ietf-rtcweb-use-cases-and-requirements].

   The TURN server in the public internet is not sufficient to establish
   connectivity for RTP-based media [RFC3550] and the WebRTC data
   channel [draft-ietf-rtcweb-data-channel] towards external WebRTC
   peers since the FW policies include blocking of all UDP based traffic
   and allowing only traffic to the TCP ports 80/443 with the intent to
   support HTTP(S) [RFC2616].

   We explicitly don't address even more restricted environments, that
   deploy HTTP traffic validation.  This could e.g. be done by means of
   DPI validation or traffic pattern analysis to determine the contents
   of the packets that the traffic is, in fact, HTTP or HTTPS-looking or
   by an HTTP proxy that breaks into the TLS exchange and looks for HTTP
   in the traffic.  However we want to address the case when access to
   the World Wide Web from inside an organization is only possible via a
   transparent HTTP Proxy that just tunnels traffic after e.g. enforcing
   an acceptable use policy.

   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

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   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 mapping 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 WebRTC peer using UDP.  How the RTP
   packets can be transported from the WebRTC 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.

   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 WebRTC 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.  Theoretically, this
   gives two options for media stream establishment dependent on the
   NAT's mapping characteristics.  Either transporting RTP over TCP
   directly to the peer or contacting a TURN server via TCP that then
   relays RTP.

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   In the first case the browser does not use any TURN server to get
   through its NAT/FW.  However, 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 contacts 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 confused with TURN/TCP
   [RFC6062], which deals with how to establish a TCP connection from a
   TURN server 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, in case the WebRTC peer is located in a similar environment
   as described in this section.

   We don't see a need to support TURN/TCP since all WebRTC media is
   transported over UDP.  For the same reason we also prefer using TCP
   just as transport to the TURN server over using the ICE-TCP with an
   end-to-end TCP connection

2.3.  NAT/Firewall open only for TCP on restricted ports

   In this case the firewall blocks all outgoing traffic except for TCP
   traffic to specific ports, for example port 80 (HTTP) for HTTP or 443
   for HTTPS(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 e.g. the HTTP(S) ports.

      Open issue: Although
      [draft-ietf-rtcweb-use-cases-and-requirements] considers only a
      restriction to HTTP(S) similar consideration apply to other ports
      or port ranges.  A change to req F37 to "The browser must be able
      to send streams and data to a peer in the presence of FWs that
      only allows traffic via a HTTP Proxy." has been agreed and will be
      in the next update does this solve the issue.

   In addition the browser needs to be configured to contact the TURN
   server over the HTTP(S) ports and/or the WebRTC client has 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.  We assume that the HTTP proxy is tranparent and
   just tunnels traffic after e.g. enforcing an acceptable use policy
   with respect to domains that are allowed to be reached.  We don't
   consider cases where the HTTP proxy is used to deploy HTTP traffic
   validation.  This includes DPI validation that the traffic is, in
   fact, HTTP or HTTPS-looking or a HTTP proxy that breaks into the TLS
   exchange and looks for HTTP in the traffic.

   Note: If both WebRTC clients are located behind the same HTTP proxy,
   we, of course, assume that ICE would give us a direct media
   connection within the private network.  We don't consider this case
   in detail within 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 browser would have to use the
   HTTP CONNECT method [RFC2817] and request that the HTTP proxy
   establishes a tunnel connection on its behalf in order to get access
   to the TURN server.  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 and when
   successful the HTTP Proxy will answer the HTTP CONNECT request with a
   200OK response.  In case of a transparent proxy, the HTTP proxy will
   now switch into tunneling mode and will transparently relay the
   traffic to the TURN server.

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   We want to emphasize that by using the HTTP CONNECT method, the TURN
   server only has to handle a standard TCP connection.  An update to
   the TURN protocol or the TURN software is not needed.

      Open Issue: This assumes that the HTTP Proxy can handle
      establishing the TCP connection to the STUN ports 3478/5349.  The
      syntax of the Request-URI certainly allows this, but is this also
      supported in the major HTTP proxy implementations?  Section 4.3.6
      of [draft-ietf-httpbis-p2-semantics] describes tunneling to ports
      other than 80/443 and the corresponding security impacts.

   Afterwards, the browser 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, for example

         CONNECT turn_server.example.com:5349 HTTP/1.1 Host:
         turn_server.example.com:5349

   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 WebRTC 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 TLS handshake and, possibly,
   valid TLS records.  The application has the ability to control
   whether TLS is used by the parameters it supplies to the TURN URI
   (e.g. turns: vs. turn:), so the decision to access the TURN server
   via TCP versus TLS could be left up to the application or possibly
   the browser configuration script.

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   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 WebRTC 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.

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 WebRTC client application could
   get UDP traffic out to the internet.

4.  Other Approaches

4.1.  TURN server connection via WebSocket

   The WebRTC 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 WebRTC 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.

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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.

5.  Requirements for RTCWEB-enabled browsers

   For the purpose of relaying WebRTC media streams or data channels a
   browser needs to be able to

   o  connect to a TURN server via UDP, TCP and TLS,

   o  support the HTTP CONNECT method and request that a proxy establish
      a tunneled TCP connection to a TURN server on its behalf,

   o  connect to a TURN server through this HTTP proxy-tunnelled TCP
      connection,

   o  connect to the TURN server via application specified ports other
      than the default STUN ports including the HTTP(s) ports,

         Open issue: Do we want to allow the TURN server to relay data
         received via the HTTP(S) ports?  Is this a change to the TURN
         protocol, that needs work to be done in the BEHAVE WG?

   o  upgrade the HTTP proxy-tunneled connection to the TURN server to
      use TLS,

   o  use the same proxy selection procedure for TURN as currently done
      for HTTP (e.g. Web Proxy Autodiscovery Protocol (WPAD) and .pac-
      files for Proxy-Auto-Config),

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   o  switch the usage of the HTTP proxy-tunneled connection with the
      TURN server from HTTP to STUN/TURN,

         Note: Usually, the browser would send further HTTP requests
         over HTTP proxy-tunneled connection.  Here, the connection in
         just established using HTTP CONNECT, but afterwards used for
         STUN/TURN messages.

   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  as an option and if needed, support ICE-TCP for TCP-based direct
      media connection to the WebRTC 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 Bernard Aboba, Xavier Marjou, Dan Wing, ...

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   TBD

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.

   [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.

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   [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.

9.2.  Informative References

   [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.

   [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-httpbis-p2-semantics]
              R. Fielding, J. Reschke , "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content ", 2013, <http://
              tools.ietf.org/html/draft-ietf-
              httpbis-p2-semantics-23#section-4.3.6>.

   [draft-ietf-rtcweb-data-channel]

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              R. Jesup, S. Loreto, M. Tuexen , "RTCWeb Data Channels ",
              2013, <http://tools.ietf.org/html/draft-ietf-rtcweb-data-
              channel>.

   [draft-ietf-rtcweb-use-cases-and-requirements]
              C. Holmberg, S. Hakansson, G. Eriksson , "Web Real-Time
              Communication Use-cases and Requirements ", 2013, <http://
              tools.ietf.org/html/draft-ietf-WebRTC-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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