Firewall Traversal for WebRTC

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Document Type Active Internet-Draft (individual)
Authors Pradeep Patel  , Cullen Jennings  , Suhas Nandakumar  , Jonathan Rosenberg  , Dan Wing 
Last updated 2015-07-20
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rtcweb                                                          P. Patel
Internet-Draft                                               C. Jennings
Intended status: Informational                             S. Nandakumar
Expires: January 21, 2016                                   J. Rosenberg
                                                                 D. Wing
                                                           July 20, 2015

                     Firewall Traversal for WebRTC


   Traversal of RTP through corporate firewalls has traditionally been
   complex, requiring the deployment of Session Border Controllers
   (SBCs) or wide open pinholes.  This draft proposes a simple technique
   that allows WebRTC based RTP traffic to traverse firewalls without
   complex firewall configuration and without deployment of SBCs or
   other middleboxes.

Status of This Memo

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   This Internet-Draft will expire on January 21, 2016.

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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Solution Requirements . . . . . . . . . . . . . . . . . . . .   4
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Firewall Processing . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Recognizing STUN packets  . . . . . . . . . . . . . . . .   6
     4.2.  Policy decision . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Creating the pinhole rules  . . . . . . . . . . . . . . .   7
     4.4.  Tracking media vs data  . . . . . . . . . . . . . . . . .   7
   5.  WebRTC Browsers . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Blocking Media Hiding in HTTP . . . . . . . . . . . . . . . .   8
   7.  Deployment Advice . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  WebRTC Servers  . . . . . . . . . . . . . . . . . . . . .   8
     7.2.  Firewall Admins . . . . . . . . . . . . . . . . . . . . .   9
   8.  Design Consideration  . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Why not just use TCP? . . . . . . . . . . . . . . . . . .   9
   9.  Security Concerns . . . . . . . . . . . . . . . . . . . . . .   9
   10. Alternate Approaches  . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Firewall Auth Tokens . . . . . . . . . . . . . . . . . .  10
     10.2.  Any Cast Whitelist . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Problem Statement

   WebRTC [I-D.ietf-rtcweb-overview] based voice and video
   communications systems are becoming far more common inside
   enterprises, which often need voice and video media to traverse the
   enterprise firewall.  This can happen when a device inside the
   firewall such as a web browser or phone is exchanging media with a
   conference bridge or gateway outside the firewall, or it can happen
   when a device inside the firewall is talking to a device in another
   enterprise or behind a different firewall.

   This problem is not unique to WebRTC media of course.  It is common
   practice for enterprise administrators to block outbound UDP through
   the corporate firewall.  This is done for several reasons:

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   1.  The lack of any kind of return messages means that there is no
       way to know that the recipient of the UDP traffic really wants
       it.  Infected computers within the enterprise could utilize UDP
       as the source of a DDoS attack.  If the firewall permitted such
       outbound traffic, the enterprise could in effect be a
       contributing source to such an attack.  By blocking UDP, the
       enterprise IT admin ensures that this cannot happen - at least
       not to external targets.

   2.  There have been prior attacks that have utilized UDP as a command
       and control channel for orchestrating DDoS attacks.  At the time,
       UDP had little usage within enterprises (most VoIP was internal
       to the enterprise when it existed at all).  Consequently, infosec
       departments have deemed it safer to block UDP outright in order
       to prevent such further incidents.

   3.  Many IT administrators enable various packet inspection
       operations on traffic flowing through the firewall.  High volume
       UDP traffic - such as voice or video - can be costly to inspect.
       As such, in cases where there is a need for traversal of such
       traffic, IT has preferred to deploy an SBC that, in essence,
       verifies that the traffic is VoIP and authorizes its egress.  The
       IT administrator then enables traffic to/from the SBC through the
       firewall.  In other words, VoIP authorization is delegated to an
       outsourced SBC.

   As more and more IP communications services move to the cloud, there
   is an increased need for VoIP traffic to traverse the enterprise
   firewall.  At the same time, the entire point of a cloud service is
   that it does not require the deployment of on premises
   infrastructure, making SBC-based solutions less desirable.  An
   alternative solution that has been historically used is to enable
   outbound UDP in the firewall to specific IP addresses, corresponding
   to the external service (TURN servers or conference servers) that the
   enterprise wishes to authorize.  With more applications running on
   virtual machines within cloud compute platforms like Amazon EC2, IP
   addresses are decreasingly usable as identifiers for a service.  VMs
   running TURN servers or conferencing servers may be established and
   torn down by the day, hour or even minute, with continuously changing
   IP addresses.  Given the multitenant nature of such providers, IT
   departments are unwilling to whitelist the IP addresses for the
   entire block used by such providers.

   Consequently, there is a growing need for solutions that allow VoIP
   traversal through the corporate firewall that alleviate the concerns
   above.  This issue is further exacerbated by the growing adoption of
   WebRTC by enterprise applications, which provide a ready source of
   RTP traffic which often needs to traverse the firewall.

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2.  Solution Requirements

   We believe the solution must meet the following requirements:

   REQ-1: The solution must enable traversal of real-time media without
   requiring deployment of additional media intermediaries on premise
   (e.g., no SBC required)

   REQ-2: The solution must not require the whitelisting of specific
   external IP addresses

   REQ-3: The solution must enable the enterprise to be sure that the
   receiving party of the traffic desires the traffic

   REQ-4: The solution must work with P2P calls between users in
   different enterprises without requiring a TURN server

   REQ-5: The solution must work with cloud services external to the
   enterprise which terminate media on servers, such as conference
   servers, voicemail servers, and so on.

   REQ-6: The solution must not require decryption of either signaling
   or media traffic at the firewall or at any other intermediary

   REQ-7: The solution must allow the IT department to easily make
   policy decisions about which applications are allowed, or not
   allowed, to traverse the firewall

   REQ-8: The solution must not require inspection of every single UDP
   packet that traverses the firewall

   REQ-9: The solution must provide a minimum level of proof that the
   traffic is WebRTC media or data and not something else

   REQ-10: The solution must work with WebRTC traffic.  Note that
   solving this for non-WebRTC is a non-requirement.

3.  Solution Overview

   Many of the reasons for blocking UDP at the corporate firewall have
   their origins in the lack of a three-way handshake for UDP traffic.
   TCP's three-way handshake ensures that the receiving party of the
   connection desires the traffic.  Similarly, HTTP traffic easily
   traverses the firewall since it provides application identification
   information in the URL.

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   Consequently, the solution proposed here relies on the ICE
   connectivity checks, which provide a similar handshake and ensure
   consent of the remote party.

   The firewall looks for an outbound ICE connectivity check and allows
   inbound ICE connectivity checks that are going to the same location
   that cared the outbound and that have the correct random ufrag value
   that was created by the client inside the firewall.  After a
   successful ICE connectivity check, the firewall allows other media to
   flow on the same 5 tuple that had the successful ICE connectivity
   check.  Timers are used to removed the various pinholes created.

   In addition, the initial outbound STUN packets can contain the STUN
   ORIGIN field which the firewall can use to make an authorization
   decision on the application.

   The end result is a system where:

   o  STUN packets are only allowed "in" if they know the crypto random
      username generated by a client inside the firewall

   o  Non STUN packets are only allowed "in" if they match a 5 tuple
      that a client inside the firewall sent a packet too

   o  Non STUN packets are only allowed "out" if the destination they
      are sending to did a stun consent handshake

4.  Firewall Processing

   The firewall processing is broken into three stages: recognizing STUN
   packets, making a policy decision as to whether each STUN packet
   should trigger a pinhole to be created, and managing the lifetime of
   any pinholes that are created.

   The term 3-tuple is used to refer to IP address, protocol (which is
   always UPD), and port that the firewall sees as the address of the
   client inside the firewall.

   The term 4-tuple is used to refer to 3-tuple plus the ice ufrag that
   was send in the STUN request message for the client inside the

   The term 5-tuple is used to refer to the 3-tuple plus the IP address
   and port of the device outside the firewall.

   When matching a ufrag, if it is a STUN request that came from outside
   the firewall, the two halves of the username on either side of the
   ":" need to be swapped before matching.

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4.1.  Recognizing STUN packets

   STUN messages all have a magic cookie value of 0x2112A442 in the 4th
   to 8th byte.  This can be used to quickly filter nearly all UDP
   packets that are not STUN packets.  Many firewalls are capable of
   doing this in hardware.  STUN supports an optional FINGERPRINT
   attribute that provides a 32 bit CRC over the message.

   Option A: Firewalls SHOULD look at outbound UDP packets and if they
   have the correct magic cookie they can classify them as STUN packets.

   Option B: The firewall looks for any outgoing STUN requests to the
   STUN port (3478).  When it finds one, it stores the 3 tuple of the
   source address port and protocol=UDP and for the next 30 seconds
   checks any packets from this 3 tuple to see if they are ICE
   connectivity checks.

   Open Issue: * decide between option A and B.  A requires looking at
   all UDP packets but will likely work better than B.  Most firewalls
   look at all TCP packets so probably bot a big deal.

   Firewalls that desire fewer false positives MAY also check that the
   FINGERPRINT attribute is correct.  Open Issue: MAY, MUST, MUST NOT -
   what do we want here.  If we put MAY or MUST, then browsers MUST
   include this.  If browsers are not required to provide this then I
   think we are more in the MUST not category.

4.2.  Policy decision

   Once the firewall has received a STUN packet from inside the
   firewall, it needs to decide if the packet is acceptable.  For most
   situations the firewall SHOULD accept all outbound STUN packets.
   This is similar to allowing all outbound TCP flows.  Some firewalls
   may choose to look at other factors including the outside UDP port
   and the ORIGIN attribute in the STUN packet.

   In general WebRTC media can be sent on a wide range of UDP ports but
   the two ports that are commonly used are the the RTP port (5004) and
   TURN port (3478).  Some firewalls MAY choose to only allow flows
   where the destination port on the outside of the firewall is one of

   The STUN ORIGIN attribute [I-D.ietf-tram-stun-origin] carries the
   origin of the web page that caused the various STUN requests.  So for
   example, if a browser was on a page such as and that page
   used the WebRTC calls to set up a connection, the STUN request's
   ORIGIN attribute would include  This allows the firewall
   to see the web application (in this case, that is

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   requesting the pinhole be opened.  The firewall MAY have a white list
   or black list for domains in STUN ORIGIN.

4.3.  Creating the pinhole rules

   Once a STUN packet is accepted, the firewall MUST create a temporary
   rule that causes the firewall to allow any inbound or outbound ICE
   messages on this 4-tuple.  This pinhole MUST to be valid for at least
   5 seconds from the time of creation.

   The firewall keeps track of the STUN transaction ID for all STUN
   requests messages that traverse the 4 tuple along with the 5 tuple
   they were sent on and direction (inbound or outbound).  If the
   firewall sees a STUN Success binding responses, with the same
   transaction ID, and on the same 5 tuple but in the opposite direction
   as the STUN request, then a valid ICE connectivity check has happened
   and the firewall MUST create a pinhole for this 5 tuple that allows
   any UDP traffic to flow across that 5 tuple.  This pinhole MUST to be
   valid for at least 30 seconds from the time of creation.

   The firewall continues watching ICE connectivity checks across this
   5-tuple as described in the previous paragraph and anytime the a
   valid ICE connectivity check happens, this effectively extends the
   lifetime of the pinhole by 30 seconds.  The procedures in
   [I-D.ietf-rtcweb-stun-consent-freshness] will ensure that an ICE
   connectivity check is done more often than every 30 seconds.

4.4.  Tracking media vs data

   WebRTC can send audio and video as well as carry a data channel.
   Confidential data could leave an enterprise by a video camera being
   pointed at a document, but IT departments are often more concerned
   about the data channel.  It is easy for the firewall to separately
   track the amount of RTP media and non-media data for each WebRTC
   flow.  If the first byte of the UDP message is 23, it is non-media
   data; if it is in the range 127 to 192 it is audio or video data.
   More information about this can be found in
   [I-D.ietf-avtcore-rfc5764-mux-fixes].  Network management systems on
   the firewall can track these two separately which can help identify
   unusual usage.

5.  WebRTC Browsers

   Open Issue: how much randomness for ICE ufrag

   o  ICE mandates at least 24 bits of randomness but we could require
      the browsers produce 64 bits of randomness

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   This specification would require browsers to include the FINGERPRINT
   and ORIGIN attributes in STUN for this to work correctly.

   Open Issue: Does adding the ORIGIN reduce user privacy?

   o  Consider the following case.  The user goes to and initiates a call with another Facebook
      user.  The domain will appear (unencrypted) in the
      STUN packets sent from the browser to Facebook's TURN server.
      Anyone along the network path could tell that the user is using
      Facebook's TURN server.  However, when the original TLS connection
      for the HTTP was made, the Server Name Indication (SNI) in the TLS
      of the HTTPS connection also revealed, largely for
      the same reasons - so that the firewall would be able to see which
      applications are using the network.

6.  Blocking Media Hiding in HTTP

   The IETF is designing systems to send interactive audio and video
   such that it looks like HTTPS and HTTP to the proxies and firewalls.
   The reasons for doing this is that sometimes the proxies and
   firewalls allow this to work when the mechanisms and channels
   designed for sending audio and video data have been explicitly
   disabled by the firewall administrators.  Many firewall
   administrators feel this circumvents the policy they are trying to
   enforce and desire a way to prevent this.  Any scheme for preventing
   this has some risk of impacting normal HTTP traffic, so there is a
   desire to provide guidance around ways to do that in this draft.

   Any HTTP or HTTPS connection that sends more than 10 requests per
   second for longer than 10 seconds should be paused for 1 second, and
   any HTTP/S requests from that client's IP address in the 1 second
   pause time should be buffered or simply dropped.  This strategy
   ensures there is no impact to clients other than the one exceeding
   the rate limit and minimizes the impact to other applications on the
   device while still reducing the incentive to try and run calls this

7.  Deployment Advice

7.1.  WebRTC Servers

   WebRTC media servers and TURN servers with public IP address(es) that
   can receive incoming packets from anywhere on the Internet are
   suggested to listen for UDP on ports 53 (DNS), 123 (NTP), and 5004
   for RTP media servers and 3478 for TURN servers.  UDP destined for
   port 53 or 123 if often allowed by firewalls that otherwise block

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7.2.  Firewall Admins

   Often the approach has been to lock down everything, so that all UDP
   is blocked.  This simply causes applications to do things like embed
   the data in normal looking HTTP or HTTPS requests.  Malware and
   viruses use similar approaches.  Just turning off all UDP results in
   a poor user experience some of the time, which results in users
   moving to applications and devices outside the firewall.  The IT
   department loses the visibility into what is going on and can no
   longer protect its users when their computers become compromised.
   Allowing things that users want to use to work and monitoring them to
   detect when things have gone wrong is very valuable.

8.  Design Consideration

8.1.  Why not just use TCP?


9.  Security Concerns

   Enterprises have a range of concerns around WebRTC traffic traversal
   of the firewall.  The major concerns that are raised include:

   1.  Unlike TCP, UDP does not have a connection where a device inside
       the firewall has confirmed that it wants to talk to the thing

   2.  Incoming UDP pinholes allow out of band packets to be spoofed
       into connecting as there is no equivalent of a TCP sequence
       number to check.

   3.  UDP has been used by malware command and control protocols so we
       block it.

   4.  We do not want enable ways for data to be exfiltrated outside the
       firewall with no monitoring.

   5.  An encrypted data channel in WebRTC can be used to bring malware
       into the company.

   6.  An encrypted media or data channel in WebRTC can be used as a
       command and control channel for malware inside the firewall.

   7.  An encrypted data channel in WebRTC can be used by an outside
       attacker to exfiltrate private files from inside the firewall.

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10.  Alternate Approaches

10.1.  Firewall Auth Tokens

   [I-D.reddy-rtcweb-stun-auth-fw-traversal] attempts to solve a similar
   problem by proposing a new comprehension-optional FW-FLOWDATA STUN
   attribute as part of ICE Connectivity checks enabling the firewall to
   permit outgoing UDP flows across the firewall.  FW-FLOWDATA STUN
   provides necessary information, such as lifetime, and candidate
   information, enabling a firewall to apply the required policy rules.
   However, [I-D.reddy-rtcweb-stun-auth-fw-traversal] requires
   establishing shared keys across the firewall(s) and the WebRTC server
   for successfully verifying the authenticity of the FW-FLOWDATA
   information.  In summary, we believe
   [I-D.reddy-rtcweb-stun-auth-fw-traversal] to have following short-

   1.  Requiring a tight coupling between the application server (WebRTC
       server) and firewall(s)

   2.  Requiring additional efforts for Firewall Admins within an
       enterprise to distribute and maintain the shared authentication
       keys needed to generate authentication tags for the FW-FLOWDATA

   3.  [I-D.reddy-rtcweb-stun-auth-fw-traversal] doesn't apply for
       distributing keys across firewalls in different administrative

10.2.  Any Cast Whitelist

   Deploying media or TURN servers on a single any-cast IP address also
   makes it easier for firewall administrators to whitelist the address.
   Concerns have been raised that two packets sent from the same host to
   a given any-cast address may get delivered to different servers.
   This is certainly possible in theory but in practice it does not seem
   be happen in limited experiments done so far.

11.  Acknowledgements

   Many thanks to Shaun Cooley and Alissa Cooper.

12.  References

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12.1.  Normative References

              Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              draft-ietf-avtcore-rfc5764-mux-fixes-02 (work in
              progress), March 2015.

              Johnston, A., Uberti, J., Yoakum, J., and K. Singh, "An
              Origin Attribute for the STUN Protocol", draft-ietf-tram-
              stun-origin-05 (work in progress), February 2015.

12.2.  Informative References

              Alvestrand, H., "Overview: Real Time Protocols for
              Browser-based Applications", draft-ietf-rtcweb-overview-14
              (work in progress), June 2015.

              Perumal, M., Wing, D., R, R., Reddy, T., and M. Thomson,
              "STUN Usage for Consent Freshness", draft-ietf-rtcweb-
              stun-consent-freshness-15 (work in progress), June 2015.

              Reddy, T., Perumal, M., and D. Wing, "STUN Extensions for
              Authenticated Firewall Traversal", draft-reddy-rtcweb-
              stun-auth-fw-traversal-00 (work in progress), July 2012.

Authors' Addresses

   Pradeep Patel


   Cullen Jennings


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   Suhas Nandakumar


   Jonathan Rosenberg


   Dan Wing


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