| Internet Engineering Task Force | T. Stach, Ed. |
| Internet-Draft | A. Hutton |
| Intended status: Informational | Siemens Enterprise Communications |
| Expires: December 29, 2013 | J. Uberti |
| June 27, 2013 |
RTCWEB Considerations for NATs, Firewalls and HTTP proxies
draft-hutton-rtcweb-nat-firewall-considerations-01
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.
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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].
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.
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].
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.
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 RTCWEB media streams, the same statement would be true if the NAT/Firewall would allow UDP traffic for a restricted UDP port range only.
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
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.
For the purpose of relaying RTCWEB media streams or data channels a browser needs to be able to
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 ...
This memo includes no request to IANA.
TBD
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |