Network Working Group                                     R. Sparks, Ed.
Internet-Draft                                          Estacado Systems
Updates: 3261 (if approved)                                  S. Lawrence
Expires: October 2, 2006                                   Pingtel Corp.
                                                          A. Hawrylyshen
                                             Ditech Communications Corp.
                                                          March 31, 2006

      Addressing an Amplification Vulnerability in Forking Proxies

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on October 2, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document normatively updates RFC 3261, the Session Initiation
   Protocol (SIP), to address a security vulnerability identified in SIP
   proxy behavior.  This vulnerability enables an attack against SIP
   networks where a small number of legitimate, even authorized, SIP
   requests can stimulate massive amounts of proxy-to-proxy traffic.

Sparks, et al.           Expires October 2, 2006                [Page 1]

Internet-Draft                fork-loop-fix                   March 2006

   This document strengthens loop-detection requirements on SIP proxies
   when they fork requests (that is, forward a request to more than one

Table of Contents

   1.  Conventions and Definitions . . . . . . . . . . . . . . . . . . 3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Vulnerability: Leveraging Forking to Flood a Network  . . . . . 3
   4.  Normative changes to RFC 3261 . . . . . . . . . . . . . . . . . 5
   5.  Impact on overall network performance . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9

Sparks, et al.           Expires October 2, 2006                [Page 2]

Internet-Draft                fork-loop-fix                   March 2006

1.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

2.  Introduction

   Interoperability testing uncovered a vulnerability in the behavior of
   forking SIP proxies as defined in [RFC3261].  This vulnerability can
   be leveraged to cause a small number of valid SIP requests to
   generate an extremely large number of proxy-to-proxy messages.  A
   version of this attack demonstrates fewer than ten messages
   stimulating potentially 2^70 messages.

   This document specifies normative changes to the SIP protocol to
   address this vulnerability.  According to this update, when a SIP
   proxy forks a request to more than one destination, it is required to
   ensure it is not participating in a request loop.

3.  Vulnerability: Leveraging Forking to Flood a Network

   This section describes setting up an attack with a simplifying
   assumption, that two accounts on each of two different RFC 3261
   compliant proxy/registrar servers that do not perform loop-detection
   are available to an attacker.  This assumption is not necessary for
   the attack, but makes representing the scenario simpler.  The same
   attack can be realized with a single account on a single server.

   Consider two proxy/registrar services, P1 and P2, and four Addresses
   of Record, a@P1, b@P1, a@P2, and b@P2.  Using normal REGISTER
   requests, establish bindings to these AoRs as follows (non-essential
   details elided):

Sparks, et al.           Expires October 2, 2006                [Page 3]

Internet-Draft                fork-loop-fix                   March 2006

           REGISTER sip:P1 SIP/2.0
           To: <sip:a@P1>
           Contact: <sip:a@P2>, <sip:b@P2>

           REGISTER sip:P1 SIP/2.0
           To: <sip:b@P1>
           Contact: <sip:a@P2>, <sip:b@P2>

           REGISTER sip:P2 SIP/2.0
           To: <sip:a@P2>
           Contact: <sip:a@P1>, <sip:b@P1>

           REGISTER sip:P2 SIP/2.0
           To: <sip:b@P2>
           Contact: <sip:a@P1>, <sip:b@P1>

   With these bindings in place, introduce an INVITE to any of the four
   AoRs, say a@P1.  This request will fork to two requests handled by
   P2, which will fork to four requests handled by P1, which will fork
   to eight messages handled by P2, and so on:

                                      /       \
                                    /           \
                                  /               \
                                /                   \
                             a@P2                   b@P2
                             /  \                   /  \
                           /      \               /      \
                          /        \             /        \
                        a@P1       b@P1        a@P1       b@P1
                        /  \       /  \        /  \       /  \
                     a@P2  b@P2 a@P2  b@P2  a@P2  b@P2 a@P2  b@P2
                      /\    /\   /\    /\    /\    /\   /\    /\

   Figure 2

   Requests will continue to propagate down this tree until Max-Forwards
   reaches zero.  If the endpoint and two proxies involved follow RFC
   3261 recommendations, the tree will be 70 rows deep, representing
   2^70 requests.  The actual number of messages may be much larger if
   the time to process the entire tree worth of requests is longer than
   Timer C at either proxy.  In this case, a storm of 408s, and/or a
   storm of CANCELs will also be propagating through the tree along with

Sparks, et al.           Expires October 2, 2006                [Page 4]

Internet-Draft                fork-loop-fix                   March 2006

   the INVITEs.  Remember that there are only two proxies involved in
   this scenario - each having to hold the state for all the
   transactions it sees (at least 2^69 simultaneously active
   transactions near the end of the scenario).

   The attack can be simplified to one account at one server if the
   service can be convinced that contacts with varying attributes
   (parameters, schemes, embedded headers) are sufficiently distinct,
   and these parameters are not used as part of AOR comparisons when
   forwarding a new request.  Perhaps:

   REGISTER sip:P1 SIP/2.0
   To: <sip:a@P1>
   Contact: <sip:a@P1;unknown-param=whack>,<sip:a@P1;unknown-param=thud>

   This attack was realized in practice during one of the SIP
   Interoperability Test (SIPit) sessions.  The scenario was extended to
   include more than two proxies, and the participating proxies all
   limited Max-Forwards to be no larger than 20.  After a handful of
   messages to construct the attack, the participating proxies began
   bombarding each other.  Extrapolating from the several hours the
   experiment was allowed to run, the scenario would have completed in
   just under 10 days.  Had the proxies used the RFC 3261 recommended
   Max-Forwards value of 70, and assuming they performed linearly as the
   state they held increases, it would have taken 3 trillion years to
   complete the processing of the single INVITE that initiated the
   attack.  It is interesting to note that a few proxies rebooted during
   the scenario, and rejoined in the attack when they restarted (as long
   as they maintained registration state across reboots).  This points
   out that if this attack were launched on the Internet at large, it
   might require coordination among all the affected elements to stop

4.  Normative changes to RFC 3261

   The following requirements mitigate the risk of a proxy falling
   victim to the attack described in this document.

   When a SIP proxy forks a particular request to more than one
   destination, it MUST ensure that request is not looping through this
   proxy.  It is RECOMMENDED that proxies meet this requirement by
   performing the Loop-Detection steps defined as an optional step in
   Section 16.3 of RFC 3261.

   The requirement to use the loop-detection algorithm in RFC 3261 is
   set at should-strength since it is expected that other mechanisms

Sparks, et al.           Expires October 2, 2006                [Page 5]

Internet-Draft                fork-loop-fix                   March 2006

   that will allow a proxy to determine it is not looping will be
   standardized in the near future.  For example, a proxy forking to
   destinations established using the sip-outbound mechanism [I-D.ietf-
   sip-outbound] would know those branches will not loop.

   A SIP proxy forwarding a request to only one location MAY perform
   loop detection but is not required to.  When forwarding to only one
   location, the amplification risk being exploited is not present, and
   the Max-Forwards mechanism is sufficient to protect the network.  A
   proxy is not required to perform loop detection when forwarding a
   request to a single location even if it previously forked that
   request in its progression through the network.

5.  Impact on overall network performance

   These requirements and the recommendation to use the loop-detection
   mechanisms from RFC 3261 make the favorable trade of exponential
   message growth for work that is at worst case order n^2 as a message
   crosses n proxies.  Specifically, this work is order m*n where m is
   the number of proxies in the path that fork the request to more than
   one location.  In practice, m is expected to be small.

6.  IANA Considerations


7.  Security Considerations

   This document is entirely about addressing a vulnerability in SIP
   proxies as defined by RFC 3261 that can lead to an exponentially
   growing message exchange attack.

8.  Acknowledgements

   Thanks go to the implementors that subjected their code to this
   scenario and helped analyze the results at SIPit 17.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

Sparks, et al.           Expires October 2, 2006                [Page 6]

Internet-Draft                fork-loop-fix                   March 2006

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

9.2.  Informative References

              Jennings, C. and R. Mahy, "Managing Client Initiated
              Connections in the Session Initiation Protocol  (SIP)",
              draft-ietf-sip-outbound-03 (work in progress), March 2006.

Sparks, et al.           Expires October 2, 2006                [Page 7]

Internet-Draft                fork-loop-fix                   March 2006

Authors' Addresses

   Robert Sparks (editor)
   Estacado Systems
   17210 Campbell Road
   Suite 250
   Dallas, Texas  75254-4203


   Scott Lawrence
   Pingtel Corp.
   400 West Cummings Park
   Suite 2200
   Woburn, MA  01801

   Phone: +1 781 938 5306

   Alan Hawrylyshen
   Ditech Communications Corp.
   602 - 11 Ave SW
   Suite 310
   Calgary, Alberta  T2R 1J8

   Phone: +1 403 561 7313

Sparks, et al.           Expires October 2, 2006                [Page 8]

Internet-Draft                fork-loop-fix                   March 2006

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Sparks, et al.           Expires October 2, 2006                [Page 9]