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Versions: (draft-boucadair-intarea-nat-reveal-analysis) 00 01 02 03 04 05 06 07 08 09 10 RFC 6967

INTAREA WG                                                  M. Boucadair
Internet-Draft                                            France Telecom
Intended status: Informational                                  J. Touch
Expires: October 20, 2012                                        USC/ISI
                                                                P. Levis
                                                          France Telecom
                                                                R. Penno
                                                                   Cisco
                                                          April 18, 2012


Analysis of Solution Candidates to Reveal a Host Identifier (HOST_ID) in
                       Shared Address Deployments
               draft-ietf-intarea-nat-reveal-analysis-02

Abstract

   This document analyzes a set of solution candidates to mitigate some
   of the issues encountered when address sharing is used.  In
   particular, this document focuses on means to reveal a host
   identifier (HOST_ID) when a Carrier Grade NAT (CGN) or application
   proxies are involved in the path.  This host identifier must be
   unique to each host under the same shared IP address.

Requirements Language

   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 RFC 2119 [RFC2119].

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 20, 2012.

Copyright Notice



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   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
   publication of this document.  Please review these documents
   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.







































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Context  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Purpose and Scope  . . . . . . . . . . . . . . . . . . . .  4
   2.  Problem to Be Solved . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  IPv6 May Also Be Concerned . . . . . . . . . . . . . . . .  6
   3.  Solutions Analysis . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Synthesis  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Recommendation . . . . . . . . . . . . . . . . . . . . . .  9
   4.  HOST_ID and Privacy  . . . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Detailed Solutions Analysis . . . . . . . . . . . . . 14
     A.1.  Use the Identification Field of IP Header (IP-ID)  . . . . 14
       A.1.1.  Description  . . . . . . . . . . . . . . . . . . . . . 14
       A.1.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 14
     A.2.  Define an IP Option  . . . . . . . . . . . . . . . . . . . 14
       A.2.1.  Description  . . . . . . . . . . . . . . . . . . . . . 14
       A.2.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 15
     A.3.  Assign Port Sets . . . . . . . . . . . . . . . . . . . . . 15
       A.3.1.  Description  . . . . . . . . . . . . . . . . . . . . . 15
       A.3.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 15
     A.4.  Use ICMP . . . . . . . . . . . . . . . . . . . . . . . . . 16
       A.4.1.  Description  . . . . . . . . . . . . . . . . . . . . . 16
       A.4.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 16
     A.5.  Define a TCP Option  . . . . . . . . . . . . . . . . . . . 17
       A.5.1.  Description  . . . . . . . . . . . . . . . . . . . . . 17
       A.5.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 17
     A.6.  PROXY Protocol . . . . . . . . . . . . . . . . . . . . . . 18
       A.6.1.  Description  . . . . . . . . . . . . . . . . . . . . . 18
       A.6.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 19
     A.7.  Host Identity Protocol (HIP) . . . . . . . . . . . . . . . 19
       A.7.1.  Description  . . . . . . . . . . . . . . . . . . . . . 19
       A.7.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 19
     A.8.  Inject Application Headers . . . . . . . . . . . . . . . . 19
       A.8.1.  Description  . . . . . . . . . . . . . . . . . . . . . 19
       A.8.2.  Analysis . . . . . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20







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1.  Introduction

1.1.  Context

   As reported in [RFC6269], several issues are encountered when an IP
   address is shared among several subscribers.  Examples of such issues
   are listed below:

   o  Implicit identification (Section 13.2 of [RFC6269])
   o  SPAM (Section 13.3 of [RFC6269])
   o  Blacklisting a mis-behaving host (Section 13.1 of [RFC6269])
   o  Redirect users with infected machines to a dedicated portal
      (Section 5.1 of [RFC6269])
   The sole use of the IPv4 address is not sufficient to uniquely
   distinguish a host.  As a mitigation, it is tempting to investigate
   means which would help in disclosing an information to be used by the
   remote server as a means to uniquely disambiguate packets of hosts
   using the same IPv4 address.

   The risk of not mitigating these issues are: OPEX increase for IP
   connectivity service providers (costs induced by calls to a hotline),
   revenue loss for content providers (loss of users audience),
   customers unsatisfaction (low quality of experience, service
   segregation, etc.).

1.2.  Purpose and Scope

   The purpose of this document is to analyze a set of alternative
   channels to convey a host identifier and to assess to what extent
   they solve the problem described in Section 2.  Below are listed the
   candidates analyzed in the document:

   o  Use the Identification field of IP header (denoted as IP-ID,
      Appendix A.1).
   o  Define a new IP option (Appendix A.2).
   o  Assign port sets (Appendix A.3).
   o  Use ICMP (Appendix A.3).
   o  Define a new TCP Option (Appendix A.5).
   o  Enable Proxy Protocol ( (Appendix A.6)).
   o  Activate HIP (Appendix A.7).
   o  Inject application headers (Appendix A.8).

   A synthesis is provided in Section 3 while the detailed analysis is
   elaborated in Appendix A.

   Section 4 discusses privacy issues common to all HOST_ID solutions.
   It is out of scope of this document to elaborate on privacy issues
   specific to each solution.



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2.  Problem to Be Solved

   Observation:  Today, some servers use the source IPv4 address as an
             identifier to treat some incoming connections differently.
             Tomorrow, due to the introduction of CGNs (e.g., NAT44
             [RFC3022], NAT64 [RFC6146]), that address will be shared.
             In particular, when a server receives packets from the same
             source address, because this address is shared, the server
             does not know which host is the sending host [RFC6269].

   Objective:  The server should be able to sort out the packets by
             sending host.

   Requirement:  The server must have extra information than the source
             IP address to differentiate the sending host.  We call
             HOST_ID this information.

   For all solutions analyzed, we provide answers to the following
   questions:

   What is the HOST_ID?  It must be unique to each host under the same
        IP address.  It does not need to be globally unique.  Of course,
        the combination of the (public) IP source address and the
        identifier (i.e., HOST_ID) ends up being relatively unique.  As
        unique as today's 32-bit IPv4 addresses which, today, can change
        when a host re-connects.

   Where is the HOST_ID? (which protocol, which field):  If the HOST_ID
        is put at the IP level, all packets will have to bear the
        identifier.  If it is put at a higher connection-oriented level,
        the identifier is only needed once in the session establishment
        phase (for instance TCP three-way-handshake), then, all packets
        received in this session will be attributed to the HOST_ID
        designated during the session opening.

   Who puts the HOST_ID?  For almost all the analyzed solutions, the
        address sharing function injects the HOST_ID.  When there are
        several address sharing functions in the data path, we describe
        to what extent the proposed solution is efficient.  Another
        option to avoid potential performance degradation is to let the
        host inject its HOST_ID but the address sharing function will
        check its content (just like an IP anti-spoofing function).

   What are the security considerations?  Security considerations are
        common to all analyzed solutions (see Section 6).  Privacy-
        related aspects are discussed in Section 4.





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2.1.  IPv6 May Also Be Concerned

   Some of the issues mentioned in Section 2 are independent of IPv4 vs.
   IPv6.  Even in IPv6, address sharing can be used for a variety of
   reasons (e.g., to hide network topology, to defeat hosts from
   offering network services directly, etc.).

   A solution to reveal HOST_ID is also needed in IPv6 deployment.


3.  Solutions Analysis

3.1.  Requirements

   Whatever the channel used to convey the HOST_ID, the following
   requirements are to be met:

   Uniqueness of identifiers in HOST_ID:  It is RECOMMENDED that
      HOST_IDs be limited to providing local uniqueness rather than
      global uniqueness.

   Refresh rate of HOST_ID:  Address sharing function SHOULD NOT use
      permanent HOST_ID values.

   Manipulate HOST_IDs:  Address sharing function SHOULD be able to
      strip, re-write and add HOST_ID fields.

   Interference between HOST_IDs:  An address sharing function, able to
      inject HOST_IDs in several layers, SHOULD reveal subsets of the
      same information (e.g., full IP address, lower 16 bits of IP
      address, etc.).

3.2.  Synthesis

   The following Table 1 summarizes the approaches analyzed in this
   document.

   o  "Success ratio" indicates the ratio of successful communications
      when the option is used.  Provided figures are inspired from the
      results documented in [Options].
   o  "Deployable today" indicates if the solution can be generalized
      without any constraint on current architectures and practices.
   o  "Possible Perf Impact" indicates the level of expected performance
      degradation.  The rationale behind the indicated potential
      performance degradation is whether the injection requires some
      treatment at the IP level or not.





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   o  "OS TCP/IP Modif" indicates whether a modification of the OS
      TCP/IP stack is required at the server side.

















































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             +------+------+-------+-------+-------+------+-----+------+
             | IP   | TCP  | IP-ID | HTTP  | Proxy | Port | HIP | ICMP |
             |Option|Option|       | Header|       | Set  |     |      |
             |      |      |       | (XFF) |       |      |     |      |
   ----------+------+------+-------+-------+-------+------+-----+------+
   UDP       | Yes  | No   | Yes   | No    | No    | Yes  |     | Yes  |
   ----------+------+------+-------+-------+-------+------+-----+------+
   TCP       | Yes  | Yes  | Yes   | No    | Yes   | Yes  |     | Yes  |
   ----------+------+------+-------+-------+-------+------+-----+------+
   HTTP      | Yes  | Yes  | Yes   | Yes   | Yes   | Yes  |     | Yes  |
   ----------+------+------+-------+-------+-------+------+-----+------+
   Encrypted | Yes  | Yes  | Yes   | No    | Yes   | Yes  |     | Yes  |
   Traffic   |      |      |       |       |       |      |     |      |
   ----------+------+------+-------+-------+-------+------+-----+------+
   Success   | 30%  | 99%  | 100%  | 100%  | Low   | 100% |Low  | ~100%|
   Ratio     |      |      |       |       |       |      |     |  (6) |
   ----------+------+------+-------+-------+-------+------+-----+------+
   Possible  | High | Med  |  Low  |  Med  | High  | No   | N/A | High |
   Perf      |      |  to  |   to  |   to  |       |      |     |      |
   Impact    |      | High |  Med  |  High |       |      |     |      |
   ----------+------+------+-------+-------+-------+------+-----+------+
   OS TCP/IP | Yes  | Yes  | Yes   | No    | No    | No   |     | Yes  |
   Modif     |      |      |       |       |       |      |     |      |
   ----------+------+------+-------+-------+-------+------+-----+------+
   Deployable| Yes  | Yes  | Yes   | Yes   | No    | Yes  | No  | Yes  |
   Today     |      |      |       |       |       |      |     |      |
   ----------+------+------+-------+-------+-------+------+-----+------+
   Notes     |      |      |  (1)  |  (2)  |       | (1)  | (4) | (7)  |
             |      |      |       |       |       | (3)  | (5) |      |
   ----------+------+------+-------+-------+-------+------+-----+------+

    Notes:

    (1)  Requires mechanism to advertise NAT is participating in this
         scheme (e.g., DNS PTR record).
    (2)  This solution is widely deployed.
    (3)  When the port set is not advertised, the solution is less
         efficient for third-party services.
    (4)  Requires the client and the server to be HIP-compliant and HIP
         infrastructure to be deployed.
    (5)  If the client and the server are HIP-enabled, the address
         sharing function does not need to insert a host-hint.  If the
         client is not HIP-enabled, designing the device that performs
         address sharing to act as a UDP/TCP-HIP relay is not viable.
    (6)  Implementation-specific.
    (7)  The solution is inefficient in various scenarios as discussed
         in Section 3.




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             Figure 1: Table 1: Summary of analyzed solutions.

   According to the above table and the analysis elaborated in
   Appendix A:

   o  IP Option, IP-ID and Proxy Protocol proposals are broken;

   o  HIP is not largely deployed;

   o  The use of Port Set may contradict the port randomization
      [RFC6056] requirement identified in [RFC6269].  This solution can
      be used by a service provider for the delivery of its own service
      offerings relying on implicit identification.

   o  XFF is de facto standard deployed and supported in operational
      networks (e.g., HTTP Severs, Load-Balancers, etc.).

   o  From an application standpoint, the TCP Option is superior to XFF/
      Forwarded-For since it is not restricted to HTTP.  Nevertheless
      XFF/Forwarded-For is compatible with the presence of address
      sharing and load-balancers in the communication path.  To provide
      a similar functionality, the TCP Option may be extended to allow
      conveying a list of IP addresses and port numbers to not lose the
      source IP address in the presence of load-balancers.  Another
      alternative is to combine the usage of both the HOST_ID TCP Option
      and XFF/Forwarded-For.  Extending TCP is still possible as
      analyzed in [ExtendTCP].

3.3.  Recommendation

   Taking into account the analysis above and [RFC6269] context, the
   following recommendation is made to mitigate the problem formulated
   in Section 2:

      An address sharing function SHOULD support HOST_ID TCP Option
      (Appendix A.5).


4.  HOST_ID and Privacy

   IP address sharing is motivated by a number of different factors.
   For years, many network operators have conserved the use of public
   IPv4 addresses by making use of Customer Premises Equipment (CPE)
   that assigns a single public IPv4 address to all hosts within the
   customer's local area network and uses NAT [RFC3022] to translate
   between locally unique private IPv4 addresses and the CPE's public
   address.  With the exhaustion of IPv4 address space, address sharing
   between customers on a much larger scale is likely to become much



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   more prevalent.  While many individual users are unaware of and
   uninvolved in decisions about whether their unique IPv4 addresses get
   revealed when they send data via IP, some users realize privacy
   benefits associated with IP address sharing, and some may even take
   steps to ensure that NAT functionality sits between them and the
   public Internet.  IP address sharing makes the actions of all users
   behind the NAT function unattributable to any single host, creating
   room for abuse but also providing some identity protection for non-
   abusive users who wish to transmit data with reduced risk of being
   uniquely identified.

   The proposals considered in this document add a measure of uniqueness
   back to hosts that share a public IP address.  The extent of that
   uniqueness depends on which information is included in the HOST_ID.

   The volatility of the HOST_ID information is similar to the source IP
   address: a distinct HOST_ID may be used by the address sharing
   function when the host reboots or gets a new internal IP address.  As
   with persistent IP addresses, persistent HOST_IDs facilitate user
   tracking over time.

   As a general matter, the HOST_ID proposals do not seek to make hosts
   any more identifiable than they would be if they were using a public,
   non-shared IP address.  However, depending on the solution proposal,
   the addition of HOST_ID information may allow a device to be
   fingerprinted more easily than it otherwise would be.  Should
   multiple solutions be combined (e.g., TCP Option and XFF) that
   include different pieces of information in the HOST_ID,
   fingerprinting may become even easier.

   The trust placed in the information conveyed in the HOST_ID is likely
   to be the same as for current practices with source IP addresses.  In
   that sense, a HOST_ID can be spoofed as this is also the case for
   spoofing an IP address.  Furthermore, users of network-based
   anonymity services (like Tor) may be capable of stripping HOST_ID
   information before it reaches its destination.

   HOST_ID specification document(s) SHOULD explain the privacy impact
   of the solutions they specify, including the extent of HOST_ID
   uniqueness and persistence, assumptions made about the lifetime of
   the HOST_ID, whether and how the HOST_ID can be obfuscated or
   recycled, and the impact of the use of the HOST_ID on device or
   implementation fingerprinting.  [I-D.iab-privacy-considerations]
   provides further guidance.

   For more discussion about privacy, refer to [RFC6462].





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5.  IANA Considerations

   This document does not require any action from IANA.


6.  Security Considerations

   The same security concerns apply for the injection of an IP option,
   TCP Option and application-related content (e.g., XFF) by the address
   sharing device.  If the server trusts the content of the HOST_ID
   field, a third party user can be impacted by a misbehaving user to
   reveal a "faked" HOST_ID (e.g., original IP address).

   HOST_ID may be used to leak information about the internal structure
   of a network behind an address sharing function.  If this behavior is
   undesired for the network administrator, the address sharing function
   can be configured to strip any existing HOST_ID in received packets
   from internal hosts.

   HOST_ID specification documents SHOULD elaborate further on threats
   inherent to each individual solution to convey the HOST_ID (e.g., use
   of the IP-ID field to count hosts behind a NAT [Count]).


7.  Acknowledgments

   Many thanks to D. Wing and C. Jacquenet for their review, comments
   and inputs.

   Thanks also to P. McCann, T. Tsou, Z. Dong, B. Briscoe, T. Taylor, M.
   Blanchet, D. Wing and A. Yourtchenko for the discussions in Prague.

   Some of the issues related to defining a new TCP Option have been
   raised by L. Eggert.

   Privacy text is provided by A. Cooper.


8.  References

8.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

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




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   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              January 2011.

8.2.  Informative References

   [Count]    "A technique for counting NATted hosts",
              <http://www.cs.columbia.edu/~smb/papers/fnat.pdf>.

   [ExtendTCP]
              Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
              Handley, M. and H. Tokuda,, "Is it still possible to
              extend TCP?", November 2011,
              <http://nrg.cs.ucl.ac.uk/mjh/tmp/mboxes.pdf>.

   [I-D.abdo-hostid-tcpopt-implementation]
              Abdo, E., Boucadair, M., and J. Queiroz, "HOST_ID TCP
              Options: Implementation & Preliminary Test Results",
              draft-abdo-hostid-tcpopt-implementation-02 (work in
              progress), January 2012.

   [I-D.chen-intarea-v4-uid-header-option]
              Wu, Y., Ji, H., Chen, Q., and T. ZOU), "IPv4 Header Option
              For User Identification In CGN Scenario",
              draft-chen-intarea-v4-uid-header-option-00 (work in
              progress), March 2011.

   [I-D.iab-privacy-considerations]
              Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., and
              J. Morris, "Privacy Considerations for Internet
              Protocols", draft-iab-privacy-considerations-02 (work in
              progress), March 2012.

   [I-D.ietf-appsawg-http-forwarded]
              Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              draft-ietf-appsawg-http-forwarded-01 (work in progress),
              March 2012.

   [I-D.ietf-intarea-ipv4-id-update]
              Touch, J., "Updated Specification of the IPv4 ID Field",
              draft-ietf-intarea-ipv4-id-update-04 (work in progress),
              September 2011.

   [I-D.wing-nat-reveal-option]



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              Yourtchenko, A. and D. Wing, "Revealing hosts sharing an
              IP address using TCP option",
              draft-wing-nat-reveal-option-03 (work in progress),
              December 2011.

   [I-D.yourtchenko-nat-reveal-ping]
              Yourtchenko, A., "Revealing hosts sharing an IP address
              using ICMP Echo Request",
              draft-yourtchenko-nat-reveal-ping-00 (work in progress),
              March 2012.

   [Not_An_Option]
              R. Fonseca, G. Porter, R. Katz, S. Shenker, and I.
              Stoica,, "IP options are not an option", 2005, <http://
              www.eecs.berkeley.edu/Pubs/TechRpts/2005/
              EECS-2005-24.html>.

   [Options]  Alberto Medina, Mark Allman, Sally Floyd, "Measuring
              Interactions Between Transport Protocols and Middleboxes",
              2005, <http://conferences.sigcomm.org/imc/2004/papers/
              p336-medina.pdf>.

   [Proxy]    Tarreau, W., "The PROXY protocol", November 2010, <http://
              haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt>.

   [RFC2753]  Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
              for Policy-based Admission Control", RFC 2753,
              January 2000.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269,
              June 2011.

   [RFC6302]  Durand, A., Gashinsky, I., Lee, D., and S. Sheppard,
              "Logging Recommendations for Internet-Facing Servers",
              BCP 162, RFC 6302, June 2011.

   [RFC6462]  Cooper, A., "Report from the Internet Privacy Workshop",
              RFC 6462, January 2012.

   [Trusted_ISPs]



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              "Trusted XFF list", <http://meta.wikimedia.org/wiki/
              XFF_project#Trusted_XFF_list>.


Appendix A.  Detailed Solutions Analysis

A.1.  Use the Identification Field of IP Header (IP-ID)

A.1.1.  Description

   IP-ID (Identification field of IP header) can be used to insert an
   information which uniquely distinguishes a host among those sharing
   the same IPv4 address.  An address sharing function can re-write the
   IP-ID field to insert a value unique to the host (16 bits are
   sufficient to uniquely disambiguate hosts sharing the same IP
   address).  Note that this field is not altered by some NATs; hence
   some side effects such as counting hosts behind a NAT as reported in
   [Count].

   A variant of this approach relies upon the format of certain packets,
   such as TCP SYN, where the IP-ID can be modified to contain a 16 bit
   HOST_ID.  Address sharing devices performing this function would
   require to indicate they are performing this function out of band,
   possibly using a special DNS record.

A.1.2.  Analysis

   This usage is not compliant with what is recommended in
   [I-D.ietf-intarea-ipv4-id-update].

A.2.  Define an IP Option

A.2.1.  Description

   A solution alternative to convey the HOST_ID is to define an IP
   option [RFC0791].  HOST_ID IP option can be inserted by the address
   sharing function to uniquely distinguish a host among those sharing
   the same IP address.  An example of such option is documented in
   [I-D.chen-intarea-v4-uid-header-option].  This IP option allows to
   convey an IPv4 address, an IPv6 prefix, a GRE key, IPv6 Flow Label,
   etc.

   Another way for using IP option has been described in Section 4.6 of
   [RFC3022].







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A.2.2.  Analysis

   Unlike the solution presented in Appendix A.5, this proposal can
   apply for any transport protocol.  Nevertheless, it is widely known
   that routers (and other middleboxes) filter IP options.  IP packets
   with IP options can be dropped by some IP nodes.  Previous studies
   demonstrated that "IP Options are not an option" (Refer to
   [Not_An_Option], [Options]).

   As a conclusion, using an IP option to convey a host-hint is not
   viable.

A.3.  Assign Port Sets

A.3.1.  Description

   This solution does not require any action from the address sharing
   function to disclose a host identifier.  Instead of assuming all
   transport ports are associated with one single host, each host under
   the same external IP address is assigned a restricted port set.
   These port sets are then advertised to remote servers using off-line
   means.  This announcement is not required for the delivery of
   internal services (i.e., offered by the service provider deploying
   the address sharing function) relying on implicit identification.

   Port sets assigned to hosts may be static or dynamic.

   Port set announcements to remote servers do not require to reveal the
   identity of individual hosts but only to advertise the enforced
   policy to generate non-overlapping port sets (e.g., the transport
   space associated with an IP address is fragmented to contiguous
   blocks of 2048 port numbers).

A.3.2.  Analysis

   The solution does not require defining new fields nor options; it is
   policy-based.

   The solution may contradict the port randomization as identified in
   [RFC6269].  A mitigation would be to avoid assigning static port sets
   to individual hosts.

   The method is convenient for the delivery of services offered by the
   service provider offering also the IP connectivity service.







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A.4.  Use ICMP

A.4.1.  Description

   Another alternative is to convey the HOST_ID using a separate
   notification channel than the packets issued to invoke the service.

   An implementation example is defined in
   [I-D.yourtchenko-nat-reveal-ping].  This solution relies on a
   mechanism where the address sharing function encapsulates the
   necessary differentiating information into an ICMP Echo Request
   packet that it sends in parallel with the initial session creation
   (e.g., SYN).  The information included in the ICMP Request Data
   portion describes the five-tuples as seen on both of the sides of the
   address sharing function.

A.4.2.  Analysis

   o  This ICMP proposal is valid for both UDP and TCP.  Address sharing
      function may be configurable with the transport protocol which is
      allowed to trigger those ICMP messages.
   o  A hint should be provided to the ultimate server (or intermediate
      nodes) an ICMP Echo Request conveys a HOST_ID.  This may be
      implemented using magic numbers.
   o  Even if ICMP packets are blocked in the communication path, the
      user connection does not have to be impacted.
   o  Some implementations requiring to delay the establishment of a
      session until receiving the companion ICMP Echo Request, may lead
      to some user experience degradation.
   o  Because of the presence of load-balancers in the path, the
      ultimate server receiving the SYN packet may not be the one which
      may receive the ICMP message conveying the HOST_ID.
   o  Because of the presence of load-balancers in the path, the port
      number assigned by address sharing may be lost.  Therefore the
      mapping information conveyed in the ICMP may not be sufficient to
      associate a SYN packet with a received ICMP.
   o  The proposal is not compatible with the presence of cascaded NAT.
   o  The ICMP proposal will add a traffic overhead for both the server
      and the address sharing device.
   o  The ICMP proposal is similar to other mechanisms (e.g., syslog,
      netflow) for reporting dynamic mappings to a mediation platform
      (mainly for legal traceability purposes).  Performance degradation
      are likely to be experienced by address sharing functions because
      ICMP messages are to be sent in particular for each new
      instantiated mapping (and also even if the mapping exists).
   o  In some scenarios (e.g., Fixed-Mobile Convergence, Open WiFi,
      etc.), HOST_ID should be interpreted by intermediate devices which
      embed Policy Enforcement Points (PEP, [RFC2753]) responsible for



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      granting access to some services.  These PEPs need to inspect all
      received packets in order to find the companion (traffic) messages
      to be correlated with ICMP messages conveying HOST_IDs.  This
      induces more complexity to these intermediate devices.

A.5.  Define a TCP Option

A.5.1.  Description

   HOST_ID may be conveyed in a dedicated TCP Option.  An example is
   specified in [I-D.wing-nat-reveal-option] which defines a new TCP
   Option called USER_HINT.  This option encloses the TCP client's
   identifier (e.g., the lower 16 bits of their IPv4 address, their VLAN
   ID, VRF ID, subscriber ID).  The address sharing device inserts this
   TCP Option into the TCP SYN packet.

A.5.2.  Analysis

   Using a new TCP Option to convey the HOST_ID does not require any
   modification to the applications but it is applicable only for TCP-
   based applications.  Applications relying on other transport
   protocols are therefore left unsolved.

   [I-D.wing-nat-reveal-option] discusses the interference with other
   TCP Options.

   The risk related to handling a new TCP Option is low as measured in
   [Options].  [I-D.abdo-hostid-tcpopt-implementation] provides a
   detailed implementation and experimentation report of HOST_ID TCP
   Option.  [I-D.abdo-hostid-tcpopt-implementation] investigated in
   depth the impact of activation HOST_ID in host, address sharing
   function and the enforcement of policies at the server side.
   [I-D.abdo-hostid-tcpopt-implementation] reports a failure ratio of
   0,103% among top 100000 websites.

   Some downsides have been raised against defining a TCP Option to
   reveal a host identity:

   o  Conveying an IP address in a TCP Option may be seen as a violation
      of OSI layers but since IP addresses are already used for the
      checksum computation, this is not seen as a blocking point.
      Moreover, updated version of [I-D.wing-nat-reveal-option] does not
      allow anymore to convey an IP address (the HOST_ID is encoded in
      16bits).

   o  TCP Option space is limited, and might be consumed by the TCP
      client.  Earlier versions of [I-D.wing-nat-reveal-option] discuss
      two approaches to sending the HOST_ID: sending the HOST_ID in the



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      TCP SYN (which consumes more bytes in the TCP header of the TCP
      SYN) and sending the HOST_ID in a TCP ACK (which consumes only two
      bytes in the TCP SYN).  Content providers may find it more
      desirable to receive the HOST_ID in the TCP SYN, as that more
      closely preserves the HOST_ID received in the source IP address as
      per current practices.  It is more complicated to implement
      sending the HOST_ID in a TCP ACK, as it can introduce MTU issues
      if the ACK packet also contains TCP data, or a TCP segment is
      lost.  The latest specification of the HOST_ID TCP Option,
      documented at [I-D.wing-nat-reveal-option], allows only to enclose
      the HOST_ID in the TCP SYN packet.

   o  When there are several NATs in the path, the original HOST_ID may
      be lost.  In such case, the procedure may not be efficient.

   o  Interference with current usages such as X-Forwarded-For (see
      Appendix A.8) should be elaborated to specify the behavior of
      servers when both options are used; in particular specify which
      information to use: the content of the TCP Option or what is
      conveyed in the application headers.

   o  When load-balancers or proxies are in the path, this option does
      not allow to preserve the original source IP address and source
      port.  Preserving such information is required for logging
      purposes for instance (e.g., [RFC6302]).
      [I-D.abdo-hostid-tcpopt-implementation] defines a TCP Option which
      allows to reveal various combinations of source information (e.g.,
      source port, source port and source IP address, source IPv6
      prefix, etc.).

   More discussion about issues raised when extending TCP can be found
   at [ExtendTCP].

A.6.  PROXY Protocol

A.6.1.  Description

   The solution, referred to as Proxy Protocol [Proxy], does not require
   any application-specific knowledge.  The rationale behind this
   solution is to prepend each connection with a line reporting the
   characteristics of the other side's connection as shown in the
   example depicted in Figure 2:

                   PROXY TCP4 192.0.2.1 192.0.2.15 56324 443\r\n

                Figure 2: Example of PROXY conection report

   Upon receipt of a message conveying this line, the server removes the



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   line.  The line is parsed to retrieve the transported protocol.  The
   content of this line is recorded in logs and used to enforce
   policies.

A.6.2.  Analysis

   This solution can be deployed in a controlled environment but it can
   not be deployed to all access services available in the Internet.  If
   the remote server does not support the Proxy Protocol, the session
   will fail.  Other complications will raise due to the presence of
   firewalls for instance.

   As a consequence, this solution is broken and can not be recommended.

A.7.  Host Identity Protocol (HIP)

A.7.1.  Description

   [RFC5201] specifies an architecture which introduces a new namespace
   to convey an identity information.

A.7.2.  Analysis

   This solution requires both the client and the server to support HIP
   [RFC5201].  Additional architectural considerations are to be taken
   into account such as the key exchanges, etc.

   If the address sharing function is required to act as a UDP/TCP-HIP
   relay, this is not a viable option.

A.8.  Inject Application Headers

A.8.1.  Description

   Another option is to not require any change at the transport nor the
   IP levels but to convey at the application payload the required
   information which will be used to disambiguate hosts.  This format
   and the related semantics depend on its application (e.g., HTTP, SIP,
   SMTP, etc.).

   For HTTP, the X-Forwarded-For (XFF) or Forwarded-For
   ([I-D.ietf-appsawg-http-forwarded]) headers can be used to display
   the original IP address when an address sharing device is involved.
   Service Providers operating address sharing devices can enable the
   feature of injecting the XFF header which will enclose the original
   IPv4 address or the IPv6 prefix part (see the example shown in
   Figure 3).  The address sharing device has to strip all included XFF
   headers before injecting their own.  Servers may rely on the contents



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   of this field to enforce some policies such as blacklisting
   misbehaving users.  Note that XFF can also be logged by some servers
   (this is for instance supported by Apache).

                 Forwarded: for=192.0.2.1,for=[2001:db8::1]
                 Forwarded: proto=https;by=192.0.2.15

                    Figure 3: Example of Forwarded-For

A.8.2.  Analysis

   Not all applications impacted by the address sharing can support the
   ability to disclose the original IP address.  Only a subset of
   protocols (e.g., HTTP) can rely on this solution.

   For the HTTP case, to prevent users injecting invalid HOST_IDs, an
   initiative has been launched to maintain a list of trusted ISPs using
   XFF: See for example the list available at: [Trusted_ISPs] of trusted
   ISPs as maintained by Wikipedia.  If an address sharing device is on
   the trusted XFF ISPs list, users editing Wikipedia located behind the
   address sharing device will appear to be editing from their
   "original" IP address and not from the NATed IP address.  If an
   offending activity is detected, individual hosts can be blacklisted
   instead of all hosts sharing the same IP address.

   XFF header injection is a common practice of load balancers.  When a
   load balancer is in the path, the original content of any included
   XFF header should not be stripped.  Otherwise the information about
   the "origin" IP address will be lost.

   When several address sharing devices are crossed, XFF header can
   convey the list of IP addresses (e.g., Figure 3).  The origin HOST_ID
   can be exposed to the target server.

   XFF also introduces some implementation complexity if the HTTP packet
   is at or close to the MTU size.

   It has been reported that some "poor" implementation may encounter
   some parsing issues when injecting XFF header.

   For encrypted HTTP traffic, injecting XFF header may be broken.










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Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes,   35000
   France

   Email: mohamed.boucadair@orange.com


   Joe Touch
   USC/ISI


   Email: touch@isi.edu


   Pierre Levis
   France Telecom
   Caen,   14000
   France

   Email: pierre.levis@orange.com


   Reinaldo Penno
   Cisco
   USA

   Email: repenno@cisco.com





















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