IPv6 Operations Working Group (v6ops)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Informational                               N. Hilliard
Expires: April 17, 2016                                             INEX
                                                              G. Doering
                                                             SpaceNet AG
                                                                  W. Liu
                                                     Huawei Technologies
                                                               W. Kumari
                                                        October 15, 2015

    Operational Implications of IPv6 Packets with Extension Headers


   This document summarizes the security and operational implications of
   IPv6 extension headers, and attempts to analyze reasons why packets
   with IPv6 extension headers may be dropped in the public Internet.

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
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   This Internet-Draft will expire on April 17, 2016.

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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Previous Work on IPv6 Extension Headers . . . . . . . . . . .   3
   3.  Security Implications . . . . . . . . . . . . . . . . . . . .   3
   4.  Operational Implications  . . . . . . . . . . . . . . . . . .   5
     4.1.  Requirement to process required layer-3/layer-4
           information . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Route-Processor Protection  . . . . . . . . . . . . . . .   7
     4.3.  Inability to Perform Fine-grained Filtering . . . . . . .   7
   5.  A Possible Attack Vector  . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   IPv6 Extension Headers (EHs) allow for the extension of the IPv6
   protocol, and provide support for core functionality such as IPv6
   fragmentation.  However, common implementation limitations suggest
   that EHs present a challenge for IPv6 packet routing equipment, and
   evidence exists to suggest that IPv6 packets with EHs may be
   intentionally dropped on the public Internet in some network

   Discussions about the security and operational implications of IPv6
   extension headers are a regular feature in IETF working groups and
   other places.  Often in these discussions, important security and
   operational issues are overlooked.

   This document tries to raise awareness about the security and
   operational implications of IPv6 Extension Headers, and presents
   reasons why some networks drop packets containing IPv6 Extension

   Section 2 of this document summarizes the work that has been done in
   the area of IPv6 extension headers.  Section 3 discusses the security

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   implications of IPv6 Extension Headers, while Section 4 discusses
   their operational implications.

2.  Previous Work on IPv6 Extension Headers

   Some of the implications of IPv6 Extension Headers have been
   discussed in IETF circles.  For example, [I-D.taylor-v6ops-fragdrop]
   discusses a rationale for which operators drop IPv6 fragments.
   [I-D.wkumari-long-headers] discusses possible issues arising from
   "long" IPv6 header chains.  [RFC7045] clarifies how intermediate
   nodes should deal with IPv6 extension headers.  [RFC7112] discusses
   the issues arising in a specific case where the IPv6 header chain is
   fragmented into two or more fragments (and formally forbids such
   case).  [I-D.kampanakis-6man-ipv6-eh-parsing] describes how
   inconsistencies in the way IPv6 packets with extension headers are
   parsed by different implementations may result in evasion of security
   controls, and presents guidelines for parsing IPv6 extension headers
   with the goal of providing a common and consistent parsing
   methodology for IPv6 implementations.  [RFC6980] analyzes the
   security implications of employing IPv6 fragmentation with Neighbor
   Discovery for IPv6, and formally recommends against such usage.
   Finally, [RFC7123] discusses how some popular RA-Guard
   implementations are subject to evasion by means of IPv6 extension

   Some preliminary measurements regarding the extent to which packet
   containing IPv6 EHs are dropped in the public Internet have been
   presented in [PMTUD-Blackholes], [Gont-IEPG88], [Gont-Chown-IEPG89],
   and [Linkova-Gont-IEPG90].  [I-D.ietf-v6ops-ipv6-ehs-in-real-world]
   presents more comprehensive results and documents the methodology for
   obtaining the presented results.

3.  Security Implications

   The security implications of IPv6 Extension Headers generally fall
   into one or more of these categories:

   o  Evasion of security controls

   o  DoS due to processing requirements

   o  DoS due to implementation errors

   o  Extension Header-specific issues

   Unlike IPv4 packets where the upper-layer protocols can be trivially
   found by means of the "IHL" ("Internet Header Length") IPv4 header
   field, the structure of IPv6 packets is more flexible and complex.

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   Locating upper-layer protocol information requires that all IPv6
   extension headers be examined.  This has presented implementation
   difficulties, and packet filtering mechanisms on several security
   devices can be trivially evaded by inserting IPv6 Extension Headers
   between the main IPv6 header and the upper layer protocol.  [RFC7113]
   describes this issue for the RA-Guard case, but the same techniques
   can be employed to circumvent other IPv6 firewall and packet
   filtering mechanisms.  Additionally, implementation inconsistencies
   in packet forwarding engines may result in evasion of security
   controls [I-D.kampanakis-6man-ipv6-eh-parsing] [Atlasis2014]

   As noted in Section 4, packets that use IPv6 Extension Headers may
   have a negative performance impact on the handling devices.  Unless
   appropriate mitigations are put in place (e.g., packet dropping and/
   or rate-limiting), an attacker could simply send a large amount of
   IPv6 traffic employing IPv6 Extension Headers with the purpose of
   performing a Denial of Service (DoS) attack.

      NOTE: In the most trivial case, a packet that includes a Hop-by-
      Hop Options header will typically go through the slow forwarding
      path, and be processed by the router's CPU.  An implementation-
      dependent case might be that in which a router that has been
      configured to enforce an ACL based on upper-layer information
      (e.g., upper layer protocol or TCP Destination Port), needs to
      process the entire IPv6 header chain (in order to find the
      required information) and this causes the packet to be processed
      in the slow path [Cisco-EH-Cons].  We note that, for obvious
      reasons, the aforementioned performance issues may also affect
      other devices such as firewalls, Network Intrusion Detection
      Systems (NIDS), etc.  [Zack-FW-Benchmark].  The extent to which
      these devices are affected will typically be implementation-

   IPv6 implementations, like all other software, tend to mature with
   time and wide-scale deployment.  While the IPv6 protocol itself has
   existed for almost 20 years, serious bugs related to IPv6 Extension
   Header processing continue to be discovered.  Because there is
   currently little operational reliance on IPv6 Extension headers, the
   corresponding code paths are rarely exercised, and there is the
   potential that bugs still remain to be discovered in some

   IPv6 Fragment Headers are employed to allow fragmentation of IPv6
   packets.  While many of the security implications of the
   fragmentation / reassembly mechanism are known from the IPv4 world,
   several related issues have crept into IPv6 implementations.  These
   range from denial of service attacks to information leakage, for

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   example [I-D.ietf-6man-predictable-fragment-id], [Bonica-NANOG58] and

4.  Operational Implications

4.1.  Requirement to process required layer-3/layer-4 information

   Intermediate systems and middleboxes often need to process the entire
   IPv6 extension header chain to find the layer-4 header.  When such
   devices are unable to obtain the required information, they may
   simply drop the corresponding packets.  The following subsections
   discuss some of reasons for which such layer-4 information may be
   needed by an intermediate systems or middlebox, and why packets
   containing IPv6 extension headers may represent a challenge in such

4.1.1.  Enforcing infrastructure ACLs

   Generally speaking, infrastructure ACLs drop unwanted packets
   destined to parts of a provider's infrastructure, because they are
   not operationally needed and can be used for attacks of different
   sorts against the router's control plane.  Some traffic needs to be
   differentiated depending on layer-3 or layer-4 criteria to achieve a
   useful balance of protection and functionality, for example:

   o  Permit some amount of ICMP echo (ping) traffic towards the
      router's addresses for troubleshooting.

   o  Permit BGP sessions on the shared network of an exchange point
      (potentially differentiating between the amount of packets/seconds
      permitted for established sessions and connection establishment),
      but do not permit other traffic from the same peer IP addresses.

4.1.2.  DDoS Management and Customer Requests for Filtering

   The case of customer DDoS protection and edge-to-core customer
   protection filters is similar in nature to the infrastructure ACL
   protection.  Similar to iACL protection, layer-4 ACLs generally need
   to be applied as close to the edge of the network as possible, even
   though the intent is to protect the customer edge rather than the
   provider core.  Application of layer-4 DDoS protection to a network
   edge is often automated using Flowspec [RFC5575].

   For example, a web site which normally only handled traffic on TCP
   ports 80 and 443 could be subject to a volumetric DDoS attack using
   NTP and DNS packets with randomised source IP address, thereby
   rendering useless traditional [RFC5635] source-based real-time black
   hole mechanisms.  In this situation, DDoS protection ACLs could be

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   configured to block all UDP traffic at the network edge without
   impairing the web server functionality in any way.  Thus, being able
   to block arbitrary protocols at the network edge can avoid DDoS-
   related problems both in the provider network and on the customer
   edge link.

4.1.3.  ECMP and Hash-based Load-Sharing

   In the case of ECMP (equal cost multi path) load sharing, the router
   on the sending side of the link needs to make a decision regarding
   which of the links to use for a given packet.  Since round-robin
   usage of the links is usually avoided in order to prevent packet
   reordering, forwarding engines need to use a mechanism which will
   consistently forward the same data streams down the same forwarding
   paths.  Most forwarding engines achieve this by calculating a simple
   hash using an n-tuple gleaned from a combination of layer-2 through
   to layer-4 packet header information.  This n-tuple will typically
   use the src/dst MAC address, src/dst IP address, and if possible
   further layer-4 src/dst port information.  As layer-4 port
   information increases the entropy of the hash, it is highly desirable
   to use it where possible.

4.1.4.  Packet Forwarding Engine Constraints

   Most modern routers use dedicated hardware (e.g.  ASICs or NPUs) to
   determine how to forward packets across their internal fabrics.  One
   of the common methods of handling next-hop lookup is to send a small
   portion of the ingress packet to a lookup engine with specialised
   hardware (e.g.  Tertiary CAM or RLDRAM) to determine the packet's
   next-hop.  Technical constraints mean that there is a trade-off
   between the amount of data sent to the lookup engine and the overall
   performance of the lookup engine.  If more data is sent, the lookup
   engine can inspect further into the packet, but the overall
   performance of the system will be reduced.  If less data is sent, the
   overall performance of the router will be increased but the packet
   lookup engine may not be able to inspect far enough into a packet to
   determine how it should be handled.

      Note: For example, current high-end routers at the time of
      authorship of this document can use up to 192 bytes of header
      (Cisco ASR9000 Typhoon) or 384 bytes of header (Juniper MX Trio)

   If a hardware forwarding engine on a modern router cannot make a
   forwarding decision about a packet because critical information is
   not sent to the look-up engine, then the router will normally drop
   the packet.  Historically, some packet forwarding engines punted
   packets of this form to the control plane for more in-depth analysis,
   but this is unfeasible on most current router architectures as a

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   result of the vast difference between the hardware forwarding
   capacity of the router and the size of the management link which
   connects the control plane to the forwarding plane.

   If an IPv6 header chain is sufficiently long that its header exceeds
   the packet look-up capacity of the router, then it may be dropped due
   to hardware inability to determine how it should be handled.

4.2.  Route-Processor Protection

   Most modern routers have a fast hardware-assisted forwarding plane
   and a loosely coupled control plane, connected together with a link
   that has much less capacity than the forwarding plane could handle.
   Traffic differentiation cannot be done by the control plane side,
   because this would overload the internal link connecting the
   forwarding plane to the control plane.

   The Hop-by-Hop Options header is particularly challenging since, in
   most (if not all) implementations, it causes the corresponding packet
   to be punted to a software path.  As a result, operators usually drop
   IPv6 packets containing this extension header.

4.3.  Inability to Perform Fine-grained Filtering

   Some routers lack of fine-grained filtering of IPv6 extension
   headers.  For example, an operator may want to drop packets
   containing Routing Header Type 0 (RHT0) but may only be able to
   filter on the extension header type (Routing Header).  As a result,
   the operator may end up enforcing a more coarse filtering policy
   (e.g. "drop all packets containing a Routing Header" vs. "only drop
   packets that contain a Routing Header Type 0").

5.  A Possible Attack Vector

   The widespread drop of IPv6 packets employing IPv6 Extension Headers
   can, in some scenarios, be exploited for malicious purposes: if
   packets employing IPv6 EHs are known to be dropped on the path from
   system A to system B, an attacker could cause packets sent from A to
   B to be dropped by sending a forged ICMPv6 Packet Too Big (PTB)
   [RFC4443] error message to A (advertising an MTU smaller than 1280),
   such that subsequent packets from A to B include a fragment header
   (i.e., they result in atomic fragments [RFC6946]).

   Possible scenarios where this attack vector could be exploited
   include (but are not limited to):

   o  Communication between any two systems through to public network
      (e.g., client from/to server or server from/to server), where

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      packets with IPv6 extension headers are dropped by some
      intermediate router

   o  Communication between two BGP peers employing IPv6 transport,
      where these BGP peers implement ACLs to drop IPv6 fragments (to
      avoid control-plane attacks)

   The aforementioned attack vector is exacerbated by the following

   o  The attacker does not need to forge the IPv6 Source Address of his
      attack packets.  Hence, deployment of simple BCP38 filters will
      not help as a counter-measure.

   o  Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6
      payload need to be forged.  While one could envision filtering
      devices enforcing BCP38-style filters on the ICMPv6 payload, the
      use of extension headers (by the attacker) could make this
      difficult, if not impossible.

   o  Many implementations fail to perform validation checks on the
      received ICMPv6 error messages, as recommended in Section 5.2 of
      [RFC4443] and documented in [RFC5927].  It should be noted that in
      some cases, such as when an ICMPv6 error message has (supposedly)
      been elicited by a connection-less transport protocol (or some
      other connection-less protocol being encapsulated in IPv6), it may
      be virtually impossible to perform validation checks on the
      received ICMPv6 error messages.  And, because of IPv6 extension
      headers, the ICMPv6 payload might not even contain any useful
      information on which to perform validation checks.

   o  Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
      error messages, the Destination Cache [RFC4861] is usually updated
      to reflect that any subsequent packets to such destination should
      include a Fragment Header.  This means that a single ICMPv6
      "Packet Too Big" error message might affect multiple communication
      instances (e.g.  TCP connections) with such destination.

   o  A router or other middlebox cannot simply drop all incoming ICMPv6
      Packet Too Big error messages, as this would create a PMTUD

   Possible mitigations for this issue include:

   o  Dropping incoming ICMPv6 Packet Too Big error messages that
      advertise a Next-Hop MTU smaller than 1280 bytes.

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   o  Artificially reducing the MTU to 1280 bytes and dropping incoming
      ICMPv6 PTB error messages.

   Both of these mitigations come at the expense of possibly preventing
   communication through SIIT [RFC6145] that relies on IPv6 atomic
   fragments (see [I-D.ietf-6man-deprecate-atomfrag-generation]), and
   also implies that the filtering device has the ability to filter ICMP
   PTB messages based on the contents of the MTU field.

   [I-D.ietf-6man-deprecate-atomfrag-generation] has recently proposed
   to deprecate the generation of IPv6 atomic fragments, and update SIIT
   [RFC6145] such that it does not rely on ICMPv6 atomic fragments.
   Thus, any of the above mitigations would eliminate the attack vector
   without any interoperability implications.

6.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an

7.  Security Considerations

   The security implications of IPv6 extension headers are discussed in
   Section 3.  A specific attack vector that could leverage the
   widespread dropping of packets with IPv6 EHs (along with possible
   countermeasures) is discussed in Section 5.  This document does not
   introduce any new security issues.

8.  Acknowledgements

   The authors would like to thank (in alphabetical order) Mikael
   Abrahamsson, and Eric Vyncke, for providing valuable comments on
   earlier versions of this document.  Additionally, the authors would
   like to thank participants of the v6ops working group for their
   valuable input on the topics that led to the publication of this

   Fernando Gont would like to thank Fernando Gont would like to thank
   Jan Zorz / Go6 Lab <http://go6lab.si/>, and Jared Mauch / NTT
   America, for providing access to systems and networks that were
   employed to perform experiments and measurements involving packets
   with IPv6 Extension Headers.  Additionally, he would like to thank
   SixXS <https://www.sixxs.net> for providing IPv6 connectivity.

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9.  References

9.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", RFC 4443,
              DOI 10.17487/RFC4443, March 2006,

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013,

9.2.  Informative References

              Atlasis, A., "Attacking IPv6 Implementation Using
              Fragmentation",  BlackHat Europe 2012. Amsterdam,
              Netherlands. March 14-16, 2012,

              Atlasis, A., "A Novel Way of Abusing IPv6 Extension
              Headers to Evade IPv6 Security Devices", May 2014,

              Atlasis, A., Rey, E., and R. Schaefer, "Evasion of High-
              End IDPS Devices at the IPv6 Era",  BlackHat Europe 2014,
              2014, <https://www.ernw.de/download/eu-14-Atlasis-Rey-

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              Bonica, R., "IPv6 Extension Headers in the Real World
              v2.0",  NANOG 58. New Orleans, Louisiana, USA. June 3-5,
              2013, <https://www.nanog.org/sites/default/files/

              Cisco, , "IPv6 Extension Headers Review and
              Considerations", October 2006,

              Gont, F. and T. Chown, "A Small Update on the Use of IPv6
              Extension Headers", IEPG 89. London, UK. March 2, 2014,

              Gont, F., "Fragmentation and Extension header Support in
              the IPv6 Internet",  IEPG 88. Vancouver, BC, Canada.
              November 13, 2013, <http://www.iepg.org/2013-11-ietf88/

              Gont, F., LIU, S., and T. Anderson, "Deprecating the
              Generation of IPv6 Atomic Fragments", draft-ietf-6man-
              deprecate-atomfrag-generation-03 (work in progress), July

              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", draft-ietf-6man-predictable-
              fragment-id-10 (work in progress), October 2015.

              Gont, F., Linkova, J., Chown, T., and S. LIU,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", draft-ietf-v6ops-
              ipv6-ehs-in-real-world-01 (work in progress), October

              Kampanakis, P., "Implementation Guidelines for parsing
              IPv6 Extension Headers", draft-kampanakis-6man-ipv6-eh-
              parsing-01 (work in progress), August 2014.

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              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
              progress), December 2013.

              Kumari, W., Jaeggli, J., Bonica, R., and J. Linkova,
              "Operational Issues Associated With Long IPv6 Header
              Chains", draft-wkumari-long-headers-03 (work in progress),
              June 2015.

              Linkova, J. and F. Gont, "IPv6 Extension Headers in the
              Real World v2.0",  IEPG 90. Toronto, ON, Canada. July 20,
              2014, <http://www.iepg.org/2014-07-20-ietf90/

              De Boer, M. and J. Bosma, "Discovering Path MTU black
              holes on the Internet using RIPE Atlas", July 2012,

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,

   [RFC5635]  Kumari, W. and D. McPherson, "Remote Triggered Black Hole
              Filtering with Unicast Reverse Path Forwarding (uRPF)",
              RFC 5635, DOI 10.17487/RFC5635, August 2009,

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927,
              DOI 10.17487/RFC5927, July 2010,

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980,
              DOI 10.17487/RFC6980, August 2013,

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,

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   [RFC7112]  Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", RFC 7112,
              DOI 10.17487/RFC7112, January 2014,

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,

   [RFC7123]  Gont, F. and W. Liu, "Security Implications of IPv6 on
              IPv4 Networks", RFC 7123, DOI 10.17487/RFC7123, February
              2014, <http://www.rfc-editor.org/info/rfc7123>.

              RIPE, , "RIPE Atlas", <https://atlas.ripe.net/>.

              Zack, E., "Firewall Security Assessment and Benchmarking
              IPv6 Firewall Load Tests",  IPv6 Hackers Meeting #1,
              Berlin, Germany. June 30, 2013,

Authors' Addresses

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Nick Hilliard
   4027 Kingswood Road
   Dublin  24

   Email: nick@inex.ie

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   Gert Doering
   SpaceNet AG
   Joseph-Dollinger-Bogen 14
   Muenchen  D-80807

   Email: gert@space.net

   Will (Shucheng) Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: liushucheng@huawei.com

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: warren@kumari.net

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