RTCWEB T. Reddy
Internet-Draft Muthu A M. Perumal
Intended status: Standards Track R. Ram Mohan
Expires: January 9, 2013 D. Wing
Cisco
July 8, 2012
STUN Extensions for Authenticated Firewall Traversal
draft-reddy-rtcweb-stun-auth-fw-traversal-00
Abstract
Some networks deploy firewalls configured to block UDP traffic. When
SIP user agents or WebRTC endpoints are deployed behind such
firewalls, media cannot be sent over UDP across the firewall, but
must be sent using TCP (which causes a different user experience) or
through a session border controller.
This draft describes an alternate model wherein extensions to ICE
connectivity checks can be examined by the firewall to permit
outgoing UDP flows across the firewall.
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 January 9, 2013.
Copyright Notice
Copyright (c) 2012 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Different Components and the Trust model . . . . . . . . . 5
5. Usage and Processing . . . . . . . . . . . . . . . . . . . . . 6
5.1. Generating FW-FLOWDATA Attribute . . . . . . . . . . . . 6
5.2. Sending FW-FLOWDATA Attribute in Binding Request . . . . . 6
5.3. Firewalls processing FW-FLOWDATA Attribute . . . . . . . . 7
6. STUN Attribute Format . . . . . . . . . . . . . . . . . . . . 9
7. Key Provisioning . . . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
To protect networks using real-time communications, firewalls or
session border controllers are typically deployed.
Firewalls include Application Layer Gateway functionality, which
intercepts and analyzes the session signaling traffic such as the
Session Initiation Protocol (SIP) traffic and creates dynamic mapping
to permit the media traffic. In particular, firewall extracts the
media transport addresses and ports from the session description and
creates dynamic mapping for media to flow through. This model has
the following problems:
1. It does not work if the session signaling is end-to-end encrypted
(say, using TLS).
2. It does not work if a non-standard session signaling is used that
the firewall does not understand.
3. It does not work if the session signaling and media traverse
different firewalls.
When an enterprise deploys WebRTC, the above problems are relevant
because:
1. The session signaling between the WebRTC application running in
the browser and the web server could be using TLS.
2. WebRTC does not enforce a particular session signaling protocol
to be used. So, the firewall may not be able to understand it.
3. This session signaling and the peer-to-peer media may traverse
different firewalls.
As a result the firewall may block ICE connectivity checks and media
traffic.
Session Border Controllers (SBCs) are active participants with call
signaling. Like firewalls, they also create dynamic mappings to
permit media traffic. This forces call signaling and media through
specific IP addresses, belonging to the SBC or an SBC-controlled
media relay device.
This draft has the WebRTC server generate a cryptographic token which
is passed to the WebRTC endpoint. The endpoint includes the token in
its ICE connectivity checks. The firewall intercepts the ICE
connectivity checks containing the token, validates it, and permits
the ICE connectivity checks and the subsequent media flow through the
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firewall.
2. Notational Conventions
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 [RFC2119].
This note uses terminology defined in [RFC5245].
3. Problem Statement
In the below topology, a WebRTC server is deployed in the enterprise
Data Center. Alice makes a WebRTC call to Bob. For the two endpoints
to successfully establish media sessions, firewalls FW1 and FW2 need
to permit the ICE connectivity checks and media traffic. In such
scenarios the mechanism described in this draft proposes a new
comprehension-optional FW-FLOWDATA STUN attribute to be included in
STUN Bind requests sent during ICE connectivity checks so that
firewalls will permit media traffic between internal peers. This
STUN attribute is created by the trusted WebRTC server and sent to
the endpoints to be propagated by the respective ICE agents during
ICE connectivity checks.
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====================
| WebRTC Server |
====================
| Data Center
|
|
==================
| WAN |-----+-+-------+---+----+-+-----+
================== |
| Branch office 2 |
Branch office 1 | |
| |
+-------+-------+ +----+-------+
| Firewall 1 | | Firewall 2 |
| | | |
+-------+-------+ +----+-------+
| |
| |
| |
---+-+-----+-----------+-+-----+-------- -----+-+-----+------
| |
+-+------+ +--------+
| Alice | | Bob |
+--------+ +--------+
Figure 1: WebRTC service in enterprise - internal call
4. Solution Overview
This section gives an overview of the solution and the different
components involved in the solution and the role of each component.
4.1. Different Components and the Trust model
Figure 1 above shows typical components involved in a WebRTC
scenario. As part of the call setup, the WebRTC endpoint would have
to gather its candidates from a STUN/TURN server, send the candidates
in the offer to the peer endpoint. On receiving the answer from the
peer endpoint it starts the ICE connectivity checks. As discussed in
the problem statement, firewalls would typically block these ICE
connectivity checks and media flowing there after. To allow this
traffic a firewall needs to authorize the flow.
o A new comprehensive optional STUN attribute called FW-FLOWDATA is
defined as part of this draft. This is used by WebRTC endpoints
requiring firewall traversal.
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o This STUN attribute FW-FLOWDATA is generated by the WebRTC server
in co-ordination with the WebRTC endpoint.
o Once the WebRTC session ends the firewall's dynamic mappings are
closed after timeout.
o DISCUSSION: Could we could have a FW-FLOWDATA attribute sent in a
STUN message to close the dynamic mappings in the firewalls?
5. Usage and Processing
An RTP endpoint which generates media can include the FW-FLOWDATA
attribute in its STUN Binding requests used in ICE connectivity
checks, to inform on-path firewalls to permit the flow.
5.1. Generating FW-FLOWDATA Attribute
The WebRTC server after processing the OFFER/ANSWER sends the FW-
FLOWDATA STUN attribute to both the peers to be included in the ICE
connectivity checks. The Authentication Tag field in the FW-FLOWDATA
attribute contains the digest of the FW-FLOWDATA attribute for data
origin authentication and integrity protection. The server first
selects the candidate address info based on OFFER/ANSWER exchange and
generates other fields of this attribute. The server then computes a
digest for the FW-FLOWDATA attribute using HMAC-SHA1. The key for
HMAC-SHA1 is provisioned using the technique in Section 7. The
result of which is truncated to 96 bits (retaining the left most
bits) to produce HMAC-SHA-1-96 and input into the Authentication Tag
field. The mechanism to send FW-FLOWDATA attribute from the WebRTC
server to the cient is outside the scope of this draft. But it is
assumed that signalling protocols used for WebRTC call setup will be
enhanced to deliver this new attribute to the WebRTC client. The
WebRTC server MUST provide a new FW-FLOWDATA to allow the media
session to continue before Lifetime expires.
5.2. Sending FW-FLOWDATA Attribute in Binding Request
Once a WebRTC endpoint receives the FW-FLOWDATA, it is responsible
for generating the STUN message and retransmitting the transactions
per the STUN specification. The FW-FLOWDATA attribute should be
placed before the FINGERPRINT attribute (if present) and after the
MESSAGE-INTEGRITY attribute. The STUN length field is adjusted to
point to the new end of the STUN message; that is, the STUN length
field always accurately indicates the length of the STUN message
(including the MESSAGE-INTEGRITY, FINGERPRINT, and FW-FLOWDATA
attributes). This does not interfere with 3rd party receivers of the
STUN message, as they will adjust the STUN length field to point to
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the end of the MESSAGE-INTEGRITY field. Receivers that do not
understand the FW-FLOWDATA will ignore it.
FW-FLOWDATA attribute received by the WebRTC client is passed to the
web browser's ICE agent (API to be added in in W3C WebRTC-API
specification [I.D.w3c-webrtc]). The ICE agent includes the FW-
FLOWDATA attribute with all ICE connectivity checks, so that on-path
firewalls can validate and permit the ICE connectivity checks and
forthcoming media.
For the FW-FLOWDATA attribute to be visible to the firewalls between
the client and the TURN server, the FW-FLOWDATA should be included in
the ALLOCATE request, channel bind or refresh messages going to the
TURN server. This is to avoid firewalls having to look for STUN
packets within STUN (TURN) packets.
5.3. Firewalls processing FW-FLOWDATA Attribute
Firewalls can reliably determine a UDP message is a STUN message
because all STUN messages sent as ICE connectivity checks include the
32-bit STUN magic cookie and the FINGERPRINT attribute. STUN
messages which are authenticated also include a MESSAGE-INTEGRITY
attribute which authenticates the fields prior to the MESSAGE-
INTEGRITY.
When the firewall receives a STUN binding request with FW-FLOWDATA
attribute it stores the Authentication Tag in the FW-FLOWDATA
attribute. The firewall then generates a digest for the FW-FLOWDATA
attribute using HMAC-SHA1. The result of which is truncated to 96
bits (retaining the left most bits) to produce HMAC-SHA-1-96. If the
value of the newly generated digest HMAC-SHA-1-96 is identical to the
stored one, the firewall can ensure that the FW-FLOWDATA attribute
has not been tampered with. Otherwise the packet is discarded.
To facilitate timestamp checking for replay attacks, each firewall
SHOULD store the following information for each host: (The timestamp
check is mostly brought from SEND [RFC3971])
o The receive time of the last received and accepted STUN message
with FW-FLOWDATA Attribute. This is called RDlast.
o The timestamp in the last received and accepted STUN message with
FW-FLOWDATA Attribute. This is called TSlast.
When a message is received from a new host (i.e., one that is not
stored in the cache), the received timestamp, TSnew, is checked, and
the packet is accepted if the timestamp is recent enough to the
reception time of the packet, RDnew:
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-Delta < (RDnew - TSnew) < +Delta
The recommended value for the allowed Delta is 180 seconds. If the
timestamp is NOT within the boundaries then discard the STUN message.
When a message is received from a known host (i.e., one that already
has an entry in the cache), the timestamp is checked against the
previously received STUN message with FW-FLOWDATA Attribute:
TSnew + fuzz > TSlast + (RDnew - RDlast) x (1 - drift) - fuzz
If this inequality does not hold, firewall SHOULD discard the
message. If, on the other hand, the inequality holds, then firewall
SHOULD process the message. Moreover, if the above inequality holds
and TSnew > TSlast, the receiver SHOULD update RDlast and TSlast.
Otherwise, the receiver MUST NOT update RDlast or TSlast. The
default value of fuzz is 1 second and drift 1 % (0.01).
The firewall also performs the following checks:
o Ensures that the source IP address and UDP port of the packet
matches with one of the local CAI entries or remote CAI entries in
the payload unless the firewall is configured to ignore this
check.
o Ensures the destination IP address and UDP port of the packet
matches with one of the remote CAI entries or local CAI entries in
the packet payload.
If all the above checks pass then the firewall creates the 5-tuple
dynamic mapping using the local candidate IP address, local candidate
port, remote candidate IP address, remote candidate port, transport
protocol. The session time of the dynamic mapping will be set to a
short lifetime (default value of 60 seconds).
o If the initial ICE connectivity check includes the ICE-CONTROLLING
attribute but does not include USE-CANDIDATE and a subsequent ICE
connectivity check includes both these attributes, the firewall can
determine that the ICE agent is the controlling agent using regular
nomination and this candidate pair is nominated for media flow. The
firewall then sets the session time of the dynamic mapping equal to
the Lifetime field in FW-FLOWDATA attribute.
o If the initial ICE connectivity check includes the ICE-CONTROLLED
attribute but does not include USE-CANDIDATE and a subsequent ICE
connectivity check includes both the attributes, the firewall can
determine that the ICE agent is the controlled agent using regular
nomination and this candidate pair is nominated for media flow. The
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firewall then sets the session time of the dynamic mapping equal to
the Lifetime field in FW-FLOWDATA attribute.
o If the initial ICE connectivity check includes the ICE-CONTROLLING
attribute and the USE-CANDIDATE attribute, firewall can determine
that the ICE agent is the controlling agent using aggressive using
nomination. It then waits for the media traffic to flow before
setting the session time of the dynamic mapping equal to Lifetime
field in FW-FLOWDATA attribute.
o If the initial ICE connectivity check includes the ICE-CONTROLLED
attribute and the USE-CANDIDATE attribute, the firewall can determine
that the ICE agent is the controlled agent using aggressive
nomination. Tt then waits for the media traffic to flow before
setting session time of the dynamic mapping equal to Lifetime field
in FW-FLOWDATA Attribute.
This technique would ensure that dynamic mappings created for pairs
in ICE checklist which are not nominated for sending media will be
removed after a short duration of time.
DISCUSSION: If WebRTC implementations of RTP support multiplexing of
multiple media sessions onto a single RTP session, FW-FLOWDATA
attribute can be enhanced to carry a flag indicating the same so that
firewall can immediately close the dynamic mapping created for other
pairs in the ICE checklist once media starts flowing on one the
candidate pairs. In case of multi-homing firewalls can track
multiple host IP addresses using authentication supplicant or, for
hosts lacking the supplicant, use address-based authentication
method.
6. STUN Attribute Format
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LCA Count | RCA Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Candidate Address Info |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authentication Tag |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: FW-FLOWDATA Attribute
Lifetime: 32-bit unsigned integer. The length of time in seconds
that the STUN attribute is valid for the purpose of firewall
creating dynamic mapping. The lifetime of the firewall dynamic
mapping is set to this value. After the lifetime expires the
mapping is deleted, unless the lifetime is extended using a
another FW-FLOWDATA attribute.
Nonce: 96-bit unsigned integer. Random value chosen by the WebRTC
Server that uniquely identifies the STUN attribute.
Timestamp: 64-bit unsigned integer field containing a timestamp.
The value indicates the number of seconds since January 1, 1970,
00:00 UTC, by using a fixed point format. In this format, the
integer number of seconds is contained in the first 48 bits of the
field, and the remaining 16 bits indicate the number of 1/64K
fractions of a second.
LCA Count: 8-bit unsigned integer. Number of local candidate
addresses.
RCA Count: 8-bit unsigned integer. Number of remote candidate
addresses.
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Reserved: 16-bit unsigned integer. An unused field. It MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Candidate Address Info:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Family | Protocol | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address (32 bits or 128 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It consists of an 8-bit address family, L4 protocol (for example
17 for UDP, 6 for TCP) and a 16-bit port, followed by a fixed-
length value representing the IP address. If the 16-bit port
value is 0 it indicates "all ports". The address family can take
on the following values: 0x01:IPv4 and 0x02:IPv6.
Authentication Tag: A 96-bit field that carries the Message
Authentication Code for the FW-FLOWDATA STUN attribute.
7. Key Provisioning
Static keys are preconfigured, either manually or through a network
management system. The simplest way to implement FW-FLOWDATA
validation is to use static keys. The provisioning of static keys
requires either manual operator intervention on the WebRTC Server and
each firewall in the enterprise or a network management system
performing the same task.
Alternatively using Dynamic Group Key Distribution, group keys are
dynamically distributed among the WebRTC server and enterprise
firewalls using GDOI [RFC6407]. In this way, each firewall requests
a group key from a key server as part of an encrypted and integrity-
protected key agreement protocol. Once the key server has
authenticated and authorized the firewalls, it distributes a group
key to the group member. The authentication in this model can be
based on public key mechanisms, thereby avoiding the need for static
key provisioning.
8. Security Considerations
Hosts using WebRTC calls will see lot of FW-FLOWDATA attributes.
They determine the key by trying a number of candidate keys and
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seeing if one of them is correct. The attack works when the keys
have low entropy, such as a word from the dictionary. This attack
can be mitigated by using strong keys with large entropy. In
situations where even stronger mitigation is required, the keys can
be dynamically changed using GDOI. The WebRTC server controls how
long a firewall session is kept open via the Lifetime value and
WebRTC server could use different Lifetime values depending on the
anticipated level of trust of the device (e.g. company provided
laptop might be trusted more than a Bring Your Own Device (BYOD));
the device with more trust need to obtain its authentication
attribute less often). Firewalls in addition to timestamp checking
can also maintain a cache of used Nonces, IP source addresses as an
effective countermeasure against replay attacks.
All the security considerations applicable to STUN [RFC5389] and ICE
[RFC5245] are applicable to this document as well.
9. IANA Considerations
Allocate new STUN attribute value for FW-FLOWDATA from the
[STUN-ATTR] registry.
10. Acknowledgements
The authors would like to thank Prashanth Patil and Ramesh Nethi for
review comments.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
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[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.
[RFC6407] Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", RFC 6407, October 2011.
11.2. Informative References
[I-D.ietf-rtcweb-use-cases-and-requirements]
Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use-cases and Requirements",
draft-ietf-rtcweb-use-cases-and-requirements-09 (work in
progress), June 2012.
[STUN-ATTR]
IANA, "IANA: STUN Attributes", December 2011, <http://
www.iana.org/assignments/stun-parameters/
stun-parameters.xml#stun-parameters-3>.
Authors' Addresses
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Muthu Arul Mozhi Perumal
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: mperumal@cisco.com
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Ram Mohan R
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: rmohanr@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
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