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Versions: 00 01 02 03 04 05 06 draft-ietf-rtcweb-return

Network Working Group                                        B. Schwartz
Internet-Draft                                                 J. Uberti
Intended status: Standards Track                                  Google
Expires: October 9, 2015                                   April 7, 2015


 Recursively Encapsulated TURN (RETURN) for Connectivity and Privacy in
                                 WebRTC
                    draft-schwartz-rtcweb-return-06

Abstract

   In the context of WebRTC, the concept of a local TURN proxy has been
   suggested, but not reviewed in detail.  WebRTC applications are
   already using TURN to enhance connectivity and privacy.  This
   document explains how local TURN proxies and WebRTC applications can
   work together.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 9, 2015.

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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Visual Overview of RETURN . . . . . . . . . . . . . . . . . .   4
   3.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Connectivity  . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Independent Path Control  . . . . . . . . . . . . . . . .   9
   4.  Concepts  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Proxy . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Virtual interface . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Proxy configuration leakiness . . . . . . . . . . . . . .  10
     4.4.  Sealed proxy rank . . . . . . . . . . . . . . . . . . . .  10
   5.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  ICE candidates produced in the presence of a proxy  . . .  11
     5.2.  Leaky proxy configuration . . . . . . . . . . . . . . . .  11
     5.3.  Sealed proxy configuration  . . . . . . . . . . . . . . .  11
     5.4.  Proxy rank  . . . . . . . . . . . . . . . . . . . . . . .  11
     5.5.  Multiple physical interfaces  . . . . . . . . . . . . . .  12
     5.6.  IPv4 and IPv6 . . . . . . . . . . . . . . . . . . . . . .  12
     5.7.  Unspecified leakiness . . . . . . . . . . . . . . . . . .  12
     5.8.  Interaction with SOCKS5-UDP . . . . . . . . . . . . . . .  12
     5.9.  Encapsulation overhead, fragmentation, and Path MTU . . .  13
     5.10. Interaction with alternate TURN server fallback . . . . .  13
     5.11. Reusing the same TURN server  . . . . . . . . . . . . . .  13
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Firewalled enterprise network with a basic application  .  14
     6.2.  Conflicting proxies configured by Auto-Discovery and
           local           policy  . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   TURN [RFC5766] is a protocol for communication between a client and a
   TURN server, in order to route UDP traffic to and from one or more
   peers.  As noted in [RFC5766], the TURN relay server "typically sits
   in the public Internet".  In a WebRTC context, if a TURN server is to
   be used, it is typically provided by the application, either to
   provide connectivity between users whose NATs would otherwise prevent




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   it, or to obscure the identity of the participants by concealing
   their IP addresses from one another.

   In many enterprises, direct UDP transmissions are not permitted
   between clients on the internal networks and external IP addresses,
   so media must flow over TCP.  To enable WebRTC services in such a
   situation, clients must use TURN-TCP, or TURN-TLS.  These
   configurations are not ideal: they send all traffic over TCP, which
   leads to higher latency than would otherwise be necessary, and they
   force the application provider to operate a TURN server because
   WebRTC endpoints behind NAT cannot typically act as TCP servers.
   These configurations may result in especially bad behaviors when
   operating through TCP or HTTP proxies that were not designed to carry
   real-time media streams.

   To avoid forcing WebRTC media streams through a TCP stage, enterprise
   network operators may operate a TURN server for their network, which
   can be discovered by clients using TURN Auto-Discovery
   [I-D.ietf-tram-turn-server-discovery], or through a proprietary
   mechanism.  This TURN server may be placed inside the network, with a
   firewall configuration allowing it to communicate with the public
   internet, or it may be operated by the a third party outside the
   network, with a firewall configuration that allows hosts inside the
   network. to communicate with it.  Use of the specified TURN server
   may be the only way for clients on the network to achieve a high
   quality WebRTC experience.  This scenario is required to be supported
   by the WebRTC requirements document
   [I-D.ietf-rtcweb-use-cases-and-requirements] Section 3.3.5.1.

   When the application intends to use a TURN server for identity
   cloaking, and the enterprise network administrator intends to use a
   TURN server for connectivity, there is a conflict.  In current WebRTC
   implementations, TURN can only be used on a single-hop basis in each
   candidate, but using only the enterprise's TURN server reveals
   information about the user (e.g. organizational affiliation), and
   using only the application's TURN server may be blocked by the
   network administrator, or may require using TURN-TCP or TURN-TLS,
   resulting in a significant sacrifice in latency.

   To resolve this conflict, we introduce Recursively Encapsulated TURN,
   a procedure that allows a WebRTC endpoint to route traffic through
   multiple TURN servers, and get improved connectivity and privacy in
   return.








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2.  Visual Overview of RETURN

      ____________    inside network     ||           outside network
     /            \                      ||  NAT/FW
     |       host O              ________||________
     |            |             /        ||        \
     |       srflx|.............|..................O      ___________
     |            |             |        ||        |     /           \
     |       relay|- - - - - - -|- - - - - - - - - |- - -|- - - - - -O
     |            |             |        ||        |     |           |
     |            |             |        ||        |     |           |
     |            |             |        ||        |     |           |
     |            |             |        ||        |     |           |
     |            |             |        ||        |     \___________/
     |            |             |        ||        |
     |            |             |        ||        |    Application TURN
     |            |             |        ||        |    server in cloud
     \____________/             \________||________/
                                         ||
         Browser                         ||
                                         ||
     KEY     O    Candidate
           .....  Non encapsulated
           - - -  TURN encapsulated
             ||   Network edge

          Figure 1: Basic WebRTC ICE Candidates with TURN Server

   Figure 1 shows a browser located inside a home or enterprise network
   which connects to the Internet through a Network Address Translator
   and Firewall (NAT/FW).  A TURN server in the Internet cloud is also
   shown, which is provided by the WebRTC application via the JavaScript
   IceServers object.

   A WebRTC application can use a TURN server to provide NAT traversal,
   but also to provide privacy, routing optimizations, logging, or
   possibly other functionality.  The application can accomplish this by
   forcing all traffic to flow through the TURN server using the
   JavaScript RTCIceTransportPolicy object [I-D.ietf-rtcweb-jsep].
   Since this TURN server is injected by the application, we will refer
   to it as an Application TURN server.










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       ____________    inside network     ||           outside network
      /            \                      ||  NAT/FW
      |       host O              ________||________
      |            |             /        ||        \
      |       srflx|.............|..................O
      |            |             |        ||        |
      |            |             |        ||        |
      |            |             |   _____||_____   |
      |            |             |  /     ||     \  |
      |    (border)|- - - - - - -| -|- - - - - - O  |
      |            |             |  |     ||     |  |
      |            |             |  |     ||     |  |
      |            |             |  |     ||     |  |
      |            |             |  |     ||     |  |
      |            |             |  \_____||_____/  |
      \____________/             \________||________/
                                          ||
          Browser           Border TURN   ||
                           server in DMZ  ||
      KEY     O    Candidate
            .....  Non encapsulated
            - - -  TURN encapsulated
              ||   Network edge

           Figure 2: WebRTC ICE Candidates with DMZ TURN Server

   Figure 2 shows a TURN server co-resident with the NAT/FW, i.e. in the
   DMZ of the FW.  This TURN server might be used by an enterprise, ISP,
   or home network to enable WebRTC media flows that would otherwise be
   blocked by the firewall, or to improve quality of service on flows
   that pass through this TURN server.  This TURN server is not part of
   a particular application, and is managed as part of the border
   control system, so we call it a Border TURN Server.

   Figure 2 shows the port allocated on this TURN server as "(border)",
   not any particular candidate type, to distinguish it from the other
   ports, which have been represented as ICE candidates in accordance
   with the WebRTC specifications.  This case is different, because
   unlike an Application TURN server, there is not yet any specification
   for how WebRTC should interact with a Border TURN server.  Under what
   conditions should WebRTC allocate a port on a Border TURN server?
   How should WebRTC represent that port as an ICE candidate?  This
   draft serves to answer these two questions.








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                      inside network     ||           outside network
      ____________                       ||  NAT/FW
     /            \              ________||________
     |            |             /        ||        \
     |            |             |        ||        |
     |       host O.............X        ||        |
     |            |             |        ||        |
     |       srflx|.............X..................O      ___________
     |            |             |        ||        |     /           \
     |       relay|- - - - - - -X- - - - - - - - - |- - -|- - - - - -O
     |            |             |   _____||_____   |     |           |
     |            |             |  /     ||     \  |     |           |
     |    (border)|- - - - - - -|- |- - - - - - O  |     |           |
     |            |             |  |     ||     |  |     \___________/
     |            |             |  |     ||     |  |
     |            |             |  |     ||     |  |    Application TURN
     |            |             |  |     ||     |  |         server
     |            |             |  \_____||_____/  |
     \____________/             \________||________/
                                         ||
         Browser             Border TURN ||
                               server    ||
     KEY     O    Candidate
           .....  Non encapsulated
           - - -  TURN encapsulated
             ||   Network edge
             X    Firewall block

     Figure 3: WebRTC ICE Candidates with Application and Border TURN
                                  Servers

   In Figure 3, there is both an Application TURN server and a Border
   TURN server.  The Firewall is blocking UDP traffic except for UDP
   traffic to/from the Border TURN server, so only the "(border)" port
   allocation will work.  However, there is no specified way for WebRTC
   to use this port as a candidate.  Moreover, this port on its own
   would not be sufficient to satisfy the user's needs.  Both TURN
   servers provide important functionality, so we need a way for WebRTC
   to select a candidate that uses both TURN servers.

   The solution proposed in this draft is for the browser to implement
   RETURN, which provides a candidate that traverses both TURN servers,
   as shown in Figure 4.








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       ____________    inside network     ||           outside network
      /            \                      ||  NAT/FW
      |       host O              ________||________
      |            |             /        ||        \
      |       srflx|.............|..................O      ___________
      |            |             |        ||        |     /           \
      |       relay|- - - - - - -|- - - - - - - - - |- - -|- - - - - -O
      |            |             |   _____||_____   |     |           |
      |            |             |  /     ||     \  |     |           |
      |      relay2|-------------|--|------------| -|- - -|- - - - - -O
      |            |             |  |     ||     |  |     \___________/
      |      srflx2|- - - - - - -|- |- - - - - - O  |
      |            |             |  |     ||     |  |   Application TURN
      |      host2 |- - - - - - -|- |- - - - - - O  |         server
      |            |             |  \_____||_____/  |
      \____________/             \________||________/
                                          ||
          Browser       Border TURN Proxy ||
                              server      ||
      KEY     O    Candidate
            .....  Non encapsulated
            - - -  TURN encapsulated
            -----  Double TURN encapsulated
              ||   Network edge

   Figure 4: WebRTC ICE Candidates with Application TURN and Border TURN
                               Proxy Servers

   The Browser in Figure 4 implements RETURN, so it allocates a port on
   the Border TURN server, now referred to as a Border TURN Proxy by
   analogy to an HTTP CONNECT or SOCKS Proxy (see Figure 5), and then
   runs STUN and TURN over this allocation, resulting in three
   candidates: relay2, srflx2, and host2.  The relay2 candidate causes
   traffic to flow through both TURN servers by encapsulating TURN
   within TURN - hence the name Recursively Encapsulated TURN (RETURN).

   The host2 and srflx2 candidates are probably identical, so one will
   be dropped by ICE.  If the NAT/FW blocks UDP and the application uses
   only relay candidates, then the relay2 candidate will be selected.
   Otherwise, the other candidates will be used, in accordance with the
   usual ICE procedure.

   Only the browser needs to implement the RETURN behavior - both the
   Border TURN Proxy and Application TURN servers' TURN protocol usage
   is unchanged.

      Note that this arrangement preserves the end-to-end security and
      privacy features of WebRTC media flows.  The ability to steer the



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      media flows through multiple TURN servers while still allowing
      end-to-end encryption and authentication is a key benefit of
      RETURN.

       ____________    inside network     ||           outside network
      /            \                      ||  NAT/FW
      |            |              ________||________
      |            |             /   _____||_____   \
      |            |             |  /     ||     \  |
      |            |             |  |     ||     |  |
      |            |             |  |     ||     |  |
      | Web Traffic|=============|==|============H  |
      |            |             |  |     ||     |  |
      |            |       HTTP/ |  |     ||     |  |
      |            |       HTTPS |  \_____||_____/  |
      |            |       Proxy |        ||        |
      |            |             |   _____||_____   |
      |            |             |  /     ||     \  |
      |            |             |  |     ||     |  |
      |            |             |  |     ||     |  |
      |WebRTC Media|- - - - - - -|- |- - - - - - O  |
      |            |             |  |     ||     |  |
      |            |             |  |     ||     |  |
      |            |       TURN  |  \_____||_____/  |
      \____________/       Proxy \________||________/
                                          ||
          Browser                         ||
                                          ||
      KEY     O    TURN Proxy Candidate
              H    HTTP/HTTPS Proxy Address
            - - -  TURN encapsulated
            =====  HTTP/HTTPS web traffic
              ||   Network edge

       Figure 5: Similarity between HTTP/HTTPS Proxy and TURN Proxy

3.  Goals

   These goals are requirements on this document (not on implementations
   of the specification).

3.1.  Connectivity

   As noted in [I-D.ietf-rtcweb-use-cases-and-requirements]
   Section 3.3.5.1 and requirement F20, a WebRTC browser endpoint MUST
   be able to direct UDP connections through a designated TURN server
   configured by enterprise policy (a "proxy").




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   It MUST be possible to configure a WebRTC endpoint that supports
   proxies to achieve connectivity no worse than if the endpoint were
   operating at the proxy's address.

   For efficiency, network administrators SHOULD be able to prevent
   browsers from attempting to send traffic through routes that are
   already known to be blocked.

3.2.  Independent Path Control

   Both network administrators and application developers may wish to
   direct all their UDP flows through a particular TURN server.  There
   are many goals that might motivate such a choice, including

   o  improving quality of service by tunneling packets through a
      network that is faster than the public internet,

   o  monitoring the usage of UDP services,

   o  troubleshooting and debugging problematic services,

   o  logging connection metadata for legal or auditing reasons,

   o  recording the entire contents of all connections, or

   o  providing partial IP address anonymization (as described in
      [I-D.ietf-rtcweb-security] Section 4.2.4).

4.  Concepts

   To achieve our goals, we introduce the following new concepts:

4.1.  Proxy

   In this document a "proxy" is any TURN server that was provided by
   any mechanism other than through the standard WebRTC-application ICE
   candidate provisioning API [I-D.ietf-rtcweb-jsep].  We call it a
   "proxy" by analogy with SOCKS proxies and similar network services,
   because it performs a similar function and can be configured in a
   similar fashion.

   If a proxy is to be used, it will be the destination of traffic
   generated by the client.  (There is no analogue to the transparent/
   intercepting HTTP proxy configuration, which modifies traffic at the
   network layer.)  Mechanisms to configure a proxy include Auto-
   Discovery [I-D.ietf-tram-turn-server-discovery] and local policy
   ([I-D.ietf-rtcweb-jsep], "ICE candidate policy").




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   In an application context, a proxy may be "active" (producing
   candidates) or "inactive" (not in use, having no effect on the
   context).

4.2.  Virtual interface

   A typical WebRTC browser endpoint may have multiple network
   interfaces available, such as wired ethernet, wireless ethernet, and
   WAN.  In this document, a "virtual interface" is a procedure for
   generating ICE candidates that are not simply generated by a
   particular physical interface.  A virtual interface can produce
   "host", "server-reflexive", and "relay" candidates, but may be
   restricted to only some type of candidate (e.g.  UDP-only).

4.3.  Proxy configuration leakiness

   "Leakiness" is an attribute of a proxy configuration.  This document
   defines two values for the "leakiness" of a proxy configuration:
   "leaky" and "sealed".  Proxy configuration, including leakiness, may
   be set by local policy ([I-D.ietf-rtcweb-jsep], "ICE candidate
   policy") or other mechanisms.

   A leaky configuration adds a proxy and also allows the browser to use
   routes that transit directly via the endpoint's physical interfaces
   (not through the proxy).  In a leaky configuration, setting a proxy
   augments the available set of ICE candidates.  Multiple leaky-
   configuration proxies may therefore be active simultaneously.

   A sealed proxy configuration requires the browser to route all WebRTC
   traffic through the proxy, eliminating all ICE candidates that do not
   go through the proxy.  Only one sealed proxy may be active at a time.

   Leaky proxy configurations allow more efficient routes to be
   selected.  For example, two peers on the same LAN can connect
   directly (peer to peer) if a leaky proxy is enabled, but must
   "hairpin" through the TURN proxy if the configuration is sealed.
   However, sealed proxy configurations can be faster to connect,
   especially if many of the peer-to-peer routes that ICE will try first
   are blocked by the network's firewall policies.

4.4.  Sealed proxy rank

   In some configurations, an endpoint may be subject to multiple sealed
   proxy settings at the same time.  In that case, one of those settings
   will have highest rank, and it will be the active proxy.  In a given
   application context (e.g. a webpage), there is at most one active
   sealed proxy.  This document does not specify a representation for
   rank.



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5.  Requirements

5.1.  ICE candidates produced in the presence of a proxy

   When a proxy is configured, by Auto-Discovery or a proprietary means,
   the browser MUST NOT report a "relay" candidate representing the
   proxy.  Instead, the browser MUST connect to the proxy and then, if
   the connection is successful, treat the TURN tunnel as a UDP-only
   virtual interface.

   For a virtual interface representing a TURN proxy, this means that
   the browser MUST report the public-facing IP address and port
   acquired through TURN as a "host" candidate, the browser MUST perform
   STUN through the TURN proxy (if STUN is configured), and it MUST
   perform TURN by recursive encapsulation through the TURN proxy,
   resulting in TURN candidates whose "raddr" and "rport" attributes
   match the acquired public-facing IP address and port on the proxy.

   Because the virtual interface has some additional overhead due to
   indirection, it SHOULD have lower priority than the physical
   interfaces if physical interfaces are also active.  Specifically,
   even host candidates generated by a virtual interface SHOULD have
   priority 0 when physical interfaces are active (similar to [RFC5245]
   Section 4.1.2.2, "the local preference for host candidates from a VPN
   interface SHOULD have a priority of 0").

5.2.  Leaky proxy configuration

   If the active proxy for an application is leaky, the browser should
   undertake the standard ICE candidate discovery mechanism [RFC5245] on
   the available physical and virtual interfaces.

5.3.  Sealed proxy configuration

   If the active proxy for an application is sealed, the browser MUST
   NOT gather or produce any candidates on physical interfaces.  The
   WebRTC implementation MUST direct its traffic from those interfaces
   only to the proxy, and perform ICE candidate discovery only on the
   single virtual interface representing the active proxy.

5.4.  Proxy rank

   Any browser mechanism for specifying a proxy SHOULD allow the caller
   to indicate a higher rank than the proxy provided by Auto-Discovery
   [I-D.ietf-tram-turn-server-discovery].






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5.5.  Multiple physical interfaces

   Some operating systems allow the browser to use multiple interfaces
   to contact a single remote IP address.  To avoid producing an
   excessive number of candidates, WebRTC endpoints MUST NOT use
   multiple physical interfaces to connect to a single proxy
   simultaneously.  (If this were violated, it could produce a number of
   virtual interfaces equal to the product of the number of physical
   interfaces and the number of active proxies.)

   For strategies to choose the best interface for communication with a
   proxy, see [I-D.reddy-mmusic-ice-best-interface-pcp].  Similar
   considerations apply when connecting to an application-specified TURN
   server in the presence of physical and virtual interfaces.

5.6.  IPv4 and IPv6

   A proxy MAY have both an IPv4 and an IPv6 address (e.g. if the proxy
   is specified by DNS and has both A and AAAA records).  The client MAY
   try both of these addresses, but MUST select one, preferring IPv6,
   before allocating any remote addresses.  This corresponds to the the
   Happy Eyeballs [RFC6555] procedure for dual-stack clients.

   A proxy MAY provide both IPv4 and IPv6 remote addresses to clients
   [RFC6156].  A client SHOULD request both address families.  If both
   requests are granted, the client SHOULD treat the two addresses as
   host candidates on a dual-stack virtual interface.

5.7.  Unspecified leakiness

   If a proxy configuration mechanism does not specify leakiness,
   browsers SHOULD treat the proxy as leaky.  This is similar to current
   WebRTC implementations' behavior in the presence of SOCKS and HTTP
   proxies: the candidate allocation code continues to generate UDP
   candidates that do not transit through the proxy.

5.8.  Interaction with SOCKS5-UDP

   The SOCKS5 proxy standard [RFC1928] permits compliant SOCKS proxies
   to support UDP traffic.  However, most implementations of SOCKS5
   today do not support UDP.  Accordingly, WebRTC browsers MUST by
   default (i.e. unless deliberately configured otherwise) treat SOCKS5
   proxies as leaky and having lower rank than any configured TURN
   proxies.







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5.9.  Encapsulation overhead, fragmentation, and Path MTU

   Encapsulating a link in TURN adds overhead on the path between the
   client and the TURN server, because each packet must be wrapped in a
   TURN message.  This overhead is sometimes doubled in RETURN proxying.
   To avoid excessive overhead, client implementations SHOULD use
   ChannelBind and ChannelData messages to connect and send data through
   proxies and application TURN servers when possible.  Clients MAY
   buffer messages to be sent until the ChannelBind command completes
   (requiring one round trip to the proxy), or they MAY use
   CreatePermission and Send messages for the first few packets to
   reduce startup latency at the cost of higher overhead.

   Adding overhead to packets on a link decreases the effective Maximum
   Transmissible Unit on that link.  Accordingly, clients that support
   proxying MUST NOT rely on the effective MTU complying with the
   Internet Protocol's minimum MTU requirement.

   ChannelData messages have constant overheard, enabling consistent
   effective PMTU, but Send messages do not necessarily have constant
   overhead.  TURN messages may be fragmented and reassembled if they
   are not marked with the Don't Fragment (DF) IP bit or the DONT-
   FRAGMENT TURN attribute.  Client implementors should keep this in
   mind, especially if they choose to implement PMTU discovery through
   the proxy.

5.10.  Interaction with alternate TURN server fallback

   As per [RFC5766], a TURN server MAY respond to an Allocate request
   with an error code of 300 and an ALTERNATE-SERVER indication.  When
   connecting to proxies or application TURN servers, clients SHOULD
   attempt to connect to the specified alternate server in accordance
   with [RFC5766].  The client MUST route a connection to the alternate
   server through the proxy if and only if the original connection
   attempt was routed through the proxy.

5.11.  Reusing the same TURN server

   It is possible that the same TURN server may appear more than once in
   the network path.  For example, if both endpoints configure the same
   sealed proxy, then each peer will only provide candidates on this
   proxy.  This is not a problem, and will work as expected.

   It is also possible that the same TURN server could be used by both
   the enterprise and the application.  It might appear attractive to
   connect to this server only once, rathering connecting to it through
   itself, in order to avoid imposing unnecessary server load.  However,




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   a RETURN client MUST connect to the server twice, even when this
   appears redundant, to ensure correct session attribution.

   For example, consider a TURN service operator that issues different
   authentication credentials to different customers, and then allows
   each customer to observe the source and destination IP addresses used
   with their credentials.  Suppose the application and enterprise both
   have accounts on this service: the application uses it to prevent the
   enterprise from learning its peers' IP addresses, and the enterprise
   uses it to prevent the application from learning its employees' IP
   addresses.  If the client only connects to the service once, then
   either the enterprise or the application will learn IP address
   information (via the TURN provider's metadata reporting) that was
   meant to be kept secret.

   As a result of this requirement, it is possible for the same TURN
   server to appear up to four times in a RETURN network path: once as
   each peer's application's TURN server, and once as each peer's sealed
   proxy.

6.  Examples

6.1.  Firewalled enterprise network with a basic application

   In this example, an enterprise network is configured with a firewall
   that blocks all UDP traffic, and a TURN server is advertised for
   Auto-Discovery in accordance with
   [I-D.ietf-tram-turn-server-discovery].  The proxy leakiness of the
   TURN server is unspecified, so the browser treats it as leaky.

   The application specifies a STUN and TURN server on the public net.
   In accordance with the ICE candidate gathering algorithm RFC 5245
   [RFC5245], it receives a set of candidates like:

   1.  A host candidate acquired from one interface.

       *  e.g. candidate:1610808681 1 udp 2122194687 [internal ip addr
          for interface 0] 63555 typ host generation 0

   2.  A host candidate acquired from a different interface.

       *  e.g. candidate:1610808681 1 udp 2122194687 [internal ip addr
          for interface 1] 54253 typ host generation 0

   3.  The proxy, as a host candidate.

       *  e.g. candidate:3458234523 1 udp 24584191 [public ip addr for
          the proxy] 54606 typ host generation 0



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   4.  The virtual interface also generates a STUN candidate, but it is
       eliminated because it is redundant with the host candidate, as
       noted in [RFC5245] Sec 4.1.2..

   5.  The application-provided TURN server as seen through the virtual
       interface.  (Traffic through this candidate is recursively
       encapsulated.)

       *  e.g. candidate:702786350 1 udp 24583935 [public ip addr of the
          application TURN server] 52631 typ relay raddr [public ip addr
          for the proxy] rport 54606 generation 0

   There are no STUN or TURN candidates on the physical interfaces,
   because the application-specified STUN and TURN servers are not
   reachable through the firewall.

   If the remote peer is within the same network, it may be possible to
   establish a direct connection using both peers' host candidates.  If
   the network prevents this kind of direct connection, the path will
   instead take a "hairpin" route through the enterprise's proxy, using
   one peer's physical "host" candidate and the other's virtual "host"
   candidate, or (if that is also disallowed by the network
   configuration) a "double hairpin" using both endpoints' virtual
   "host" candidates.

6.2.  Conflicting proxies configured by Auto-Discovery and local policy

   Consider an enterprise network with TURN and HTTP proxies advertised
   for Auto-Discovery with unspecified leakiness (thus defaulting to
   leaky).  The browser endpoint configures an additional TURN proxy by
   a proprietary local mechanism.

   If the locally configured proxy is leaky, then the browser MUST
   produce candidates representing any physical interfaces (including
   SSLTCP routes through the HTTP proxy), plus candidates for both UDP-
   only virtual interfaces created by the two TURN servers.

   There MUST NOT be any candidate that uses both proxies.  Multiple
   configured proxies are not chained recursively.

   If the locally configured proxy is "sealed", then the browser MUST
   produce only candidates from the virtual interface associated with
   that proxy.

   If both proxies are configured for "sealed" use, then the browser
   MUST produce only candidates from the virtual interface associated
   with the proxy with higher rank.




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7.  Security Considerations

   A RETURN proxy can capture, block, and otherwise interfere with all
   of its clients' WebRTC network activity.  Therefore, browsers and
   other WebRTC endpoints MUST NOT use RETURN proxies that are provided
   by untrusted sources.  For example, endpoints MUST NOT implement a
   configuration based on unauthenticated network multicast (e.g. mDNS)
   unless the endpoint will only be used on networks where all other
   users are fully trusted to intercept all WebRTC traffic.  In
   contrast, endpoints MAY implement mechanisms to configure RETURN
   proxies by system-wide policy, which can only be modified by trusted
   system administrators.

   This document describes web browser behaviors that, if implemented
   correctly, allow users to achieve greater identity-confidentiality
   during WebRTC calls under certain configurations.

   If a site administrator offers the site's users a TURN proxy,
   websites running in the users' browsers will be able to initiate a
   UDP-based WebRTC connection to any UDP transport address via the
   proxy.  Websites' connections will quickly terminate if the remote
   endpoint does not reply with a positive indication of ICE consent,
   but no such restriction applies to other applications that access the
   TURN server.  Administrators should take care to provide TURN access
   credentials only to the users who are authorized to have global UDP
   network access.

   TURN proxies and application TURN servers can provide some privacy
   protection by obscuring the identity of one peer from the other.
   However, unencrypted TURN provides no additional privacy from an
   observer who can monitor the link between the TURN client and server,
   and even encrypted TURN ([I-D.ietf-tram-stun-dtls] Section 4.6) does
   not provide significant privacy from an observer who sniff traffic on
   both legs of the TURN connection, due to packet timing correlations.

8.  IANA Considerations

   This document requires no actions from IANA.

9.  Acknowledgements

   Thanks to Harald Alvestrand, Philipp Hancke, Tirumaleswar Reddy, Alan
   Johnston, John Yoakum, and Cullen Jennings for suggestions to improve
   the content and presentation.  Special thanks to Alan Johnston for
   contributing the visual overview in Section 2.






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10.  References

10.1.  Normative References

   [I-D.ietf-rtcweb-jsep]
              Uberti, J. and C. Jennings, "Javascript Session
              Establishment Protocol", draft-ietf-rtcweb-jsep-06 (work
              in progress), February 2014.

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 5766, March
              1996.

   [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.

   [RFC6156]  Camarillo, G., Novo, O., and S. Perreault, "Traversal
              Using Relays around NAT (TURN) Extension for IPv6", RFC
              6156, April 2011.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

10.2.  Informative References

   [I-D.ietf-rtcweb-security]
              Rescorla, E., "Security Considerations for WebRTC", ietf-
              rtcweb-security-07 (work in progress), July 2014.

   [I-D.ietf-rtcweb-use-cases-and-requirements]
              Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use-cases and Requirements", ietf-
              rtcweb-use-cases-and-requirements-14 (work in progress),
              February 2014.

   [I-D.ietf-tram-stun-dtls]
              Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities for NAT (STUN)", ietf-rtcweb-use-cases-and-
              requirements-14 (work in progress), June 2014.





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   [I-D.ietf-tram-turn-server-discovery]
              Patil, P., Reddy, T., and D. Wing, "TURN Server Auto
              Discovery", draft-ietf-tram-turn-server-discovery-00 (work
              in progress), July 2014.

   [I-D.reddy-mmusic-ice-best-interface-pcp]
              Reddy, T., Wing, D., VerSteeg, B., Penno, R., and V.
              Singh, "Improving ICE Interface Selection Using Port
              Control Protocol (PCP) Flow Extension", draft-ietf-tram-
              turn-server-discovery-00 (work in progress), October 2013.

Authors' Addresses

   Benjamin M. Schwartz
   Google, Inc.
   111 8th Ave
   New York, NY  10011
   USA

   Email: bemasc@webrtc.org


   Justin Uberti
   Google, Inc.
   747 6th Street South
   Kirkland, WA  98033
   USA

   Email: justin@uberti.name






















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