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Versions: (draft-boschi-ipfix-anon) 00 01 02 03 04 05 06 RFC 6235

IPFIX Working Group                                            E. Boschi
Internet-Draft                                               B. Trammell
Intended status: Experimental                             Hitachi Europe
Expires: August 19, 2010                               February 15, 2010


                     IP Flow Anonymisation Support
                      draft-ietf-ipfix-anon-02.txt

Abstract

   This document describes anonymisation techniques for IP flow data and
   the export of anonymised data using the IPFIX protocol.  It
   categorizes common anonymisation schemes and defines the parameters
   needed to describe them.  It provides guidelines for the
   implementation of anonymised data export and storage over IPFIX, and
   describes an information model and Options-based method for
   anonymisation technique metadata export within the IPFIX protocol or
   storage in IPFIX Files.

Status of this Memo

   This Internet-Draft is submitted to IETF 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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   Drafts.

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   This Internet-Draft will expire on August 19, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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
   to this document.  Code Components extracted from this document must
   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 BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  IPFIX Protocol Overview  . . . . . . . . . . . . . . . . .  4
     1.2.  IPFIX Documents Overview . . . . . . . . . . . . . . . . .  5
     1.3.  Anonymisation within the IPFIX Architecture  . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Categorisation of Anonymisation Techniques . . . . . . . . . .  7
   4.  Anonymisation of IP Flow Data  . . . . . . . . . . . . . . . .  8
     4.1.  IP Address Anonymisation . . . . . . . . . . . . . . . . . 10
       4.1.1.  Truncation . . . . . . . . . . . . . . . . . . . . . . 10
       4.1.2.  Reverse Truncation . . . . . . . . . . . . . . . . . . 11
       4.1.3.  Permutation  . . . . . . . . . . . . . . . . . . . . . 11
       4.1.4.  Prefix-preserving Pseudonymisation . . . . . . . . . . 11
     4.2.  Hardware Address Anonymisation . . . . . . . . . . . . . . 12
       4.2.1.  Reverse Truncation . . . . . . . . . . . . . . . . . . 12
       4.2.2.  Permutation  . . . . . . . . . . . . . . . . . . . . . 13
       4.2.3.  Structured Pseudonymisation  . . . . . . . . . . . . . 13
     4.3.  Timestamp Anonymisation  . . . . . . . . . . . . . . . . . 13
       4.3.1.  Precision Degradation  . . . . . . . . . . . . . . . . 13
       4.3.2.  Enumeration  . . . . . . . . . . . . . . . . . . . . . 14
       4.3.3.  Random Shifts  . . . . . . . . . . . . . . . . . . . . 14
     4.4.  Counter Anonymisation  . . . . . . . . . . . . . . . . . . 14
       4.4.1.  Precision Degradation  . . . . . . . . . . . . . . . . 15
       4.4.2.  Binning  . . . . . . . . . . . . . . . . . . . . . . . 15
       4.4.3.  Random Noise Addition  . . . . . . . . . . . . . . . . 15
     4.5.  Anonymisation of Other Flow Fields . . . . . . . . . . . . 16
       4.5.1.  Binning  . . . . . . . . . . . . . . . . . . . . . . . 16
       4.5.2.  Permutation  . . . . . . . . . . . . . . . . . . . . . 16
   5.  Parameters for the Description of Anonymisation Techniques . . 16
     5.1.  Stability  . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.2.  Truncation Length  . . . . . . . . . . . . . . . . . . . . 17
     5.3.  Bin Map  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.4.  Permutation  . . . . . . . . . . . . . . . . . . . . . . . 18
     5.5.  Shift Amount . . . . . . . . . . . . . . . . . . . . . . . 18
   6.  Anonymisation Export Support in IPFIX  . . . . . . . . . . . . 18
     6.1.  Anonymisation Options Template . . . . . . . . . . . . . . 18



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     6.2.  Recommended Information Elements for Anonymisation
           Metadata . . . . . . . . . . . . . . . . . . . . . . . . . 20
       6.2.1.  informationElementIndex  . . . . . . . . . . . . . . . 20
       6.2.2.  anonymisationFlags . . . . . . . . . . . . . . . . . . 20
       6.2.3.  anonymisationTechnique . . . . . . . . . . . . . . . . 22
   7.  Applying Anonymisation Techniques to IPFIX Export and
       Storage  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.1.  Arrangement of Processes in IPFIX Anonymisation  . . . . . 24
     7.2.  IPFIX-Specific Anonymisation Guidelines  . . . . . . . . . 27
       7.2.1.  Appropriate Use of Information Elements for
               Anonymised Data  . . . . . . . . . . . . . . . . . . . 27
       7.2.2.  Export of Perimeter-Based Anonymisation Policies . . . 28
       7.2.3.  Anonymisation of Header Data . . . . . . . . . . . . . 28
       7.2.4.  Anonymisation of Options Data  . . . . . . . . . . . . 29
       7.2.5.  Special-Use Address Space Considerations . . . . . . . 30
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 35
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     12.2. Informative References . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36




























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1.  Introduction

   The standardisation of an IP flow information export protocol
   [RFC5101] and associated representations removes a technical barrier
   to the sharing of IP flow data across organizational boundaries and
   with network operations, security, and research communities for a
   wide variety of purposes.  However, with wider dissemination comes
   greater risks to the privacy of the users of networks under
   measurement, and to the security of those networks.  While it is not
   a complete solution to the issues posed by distribution of IP flow
   information, anonymisation (i.e., the deletion or transformation of
   information that is considered sensitive and could be used to reveal
   the identity of subjects involved in a communication) is an important
   tool for the protection of privacy within network measurement
   infrastructures.

   This document presents a mechanism for representing anonymised data
   within IPFIX and guidelines for using it.  It begins with a
   categorization of anonymisation techniques.  It then describes
   applicability of each technique to commonly anonymisable fields of IP
   flow data, organized by information element data type and semantics
   as in [RFC5102]; enumerates the parameters required by each of the
   applicable anonymisation techniques; and provides guidelines for the
   use of each of these techniques in accordance with best practices in
   data protection.  Finally, it specifies a mechanism for exporting
   anonymised data and binding anonymisation metadata to templates using
   IPFIX Options.

1.1.  IPFIX Protocol Overview

   In the IPFIX protocol, { type, length, value } tuples are expressed
   in templates containing { type, length } pairs, specifying which {
   value } fields are present in data records conforming to the
   Template, giving great flexibility as to what data is transmitted.
   Since Templates are sent very infrequently compared with Data
   Records, this results in significant bandwidth savings.  Various
   different data formats may be transmitted simply by sending new
   Templates specifying the { type, length } pairs for the new data
   format.  See [RFC5101] for more information.

   The IPFIX information model [RFC5102] defines a large number of
   standard Information Elements which provide the necessary { type }
   information for Templates.  The use of standard elements enables
   interoperability among different vendors' implementations.
   Additionally, non-standard enterprise-specific elements may be
   defined for private use.





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1.2.  IPFIX Documents Overview

   "Specification of the IPFIX Protocol for the Exchange of IP Traffic
   Flow Information" [RFC5101] and its associated documents define the
   IPFIX Protocol, which provides network engineers and administrators
   with access to IP traffic flow information.

   "Architecture for IP Flow Information Export" [RFC5470] defines the
   architecture for the export of measured IP flow information out of an
   IPFIX Exporting Process to an IPFIX Collecting Process, and the basic
   terminology used to describe the elements of this architecture, per
   the requirements defined in "Requirements for IP Flow Information
   Export" [RFC3917].  The IPFIX Protocol document [RFC5101] then covers
   the details of the method for transporting IPFIX Data Records and
   Templates via a congestion-aware transport protocol from an IPFIX
   Exporting Process to an IPFIX Collecting Process.

   "Information Model for IP Flow Information Export" [RFC5102]
   describes the Information Elements used by IPFIX, including details
   on Information Element naming, numbering, and data type encoding.
   Finally, "IPFIX Applicability" [RFC5472] describes the various
   applications of the IPFIX protocol and their use of information
   exported via IPFIX, and relates the IPFIX architecture to other
   measurement architectures and frameworks.

   Additionally, "Specification of the IPFIX File Format" [RFC5655]
   describes a file format based upon the IPFIX Protocol for the storage
   of flow data.

   This document references the Protocol and Architecture documents for
   terminology, and extends the IPFIX Information Model to provide new
   Information Elements for anonymisation metadata.  The anonymisation
   techniques described herein are equally applicable to the IPFIX
   Protocol and data stored in IPFIX Files.

1.3.  Anonymisation within the IPFIX Architecture

   "Architecture for IP Flow Information Export" [RFC5470] defines the
   functions performed in sequence by the various functional blocks in
   an IPFIX Device as in the figure below.


                    Packet(s) coming into Observation Point(s)
                      |                                   |
                      v                                   v
     +----------------+-------------------------+   +-----+-------+
     |          Metering Process on an          |   |             |
     |             Observation Point            |   |             |



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     |                                          |   |             |
     |   packet header capturing                |   |             |
     |        |                                 |...| Metering    |
     |   timestamping                           |   | Process N   |
     |        |                                 |   |             |
     | +----->+                                 |   |             |
     | |      |                                 |   |             |
     | |   sampling Si (1:1 in case of no       |   |             |
     | |      |          sampling)              |   |             |
     | |   filtering Fi (select all when        |   |             |
     | |      |          no criteria)           |   |             |
     | +------+                                 |   |             |
     |        |                                 |   |             |
     |        |        Timing out Flows         |   |             |
     |        |    Handle resource overloads    |   |             |
     +--------|---------------------------------+   +-----|-------+
              |                                           |
      Flow Records (identified by Observation Domain)  Flow Records
              |                                           |
              +---------+---------------------------------+
                        |
   +--------------------|----------------------------------------------+
   |                    |     Exporting Process                        |
   |+-------------------|-------------------------------------------+  |
   ||                   v       IPFIX Protocol                      |  |
   ||+-----------------------------+  +----------------------------+|  |
   |||Rules for                    |  |Functions                   ||  |
   ||| Picking/sending Templates   |  |-Packetise selected Control ||  |
   ||| Picking/sending Flow Records|->|  & data Information into   ||  |
   ||| Encoding Template & data    |  |  IPFIX export packets.     ||  |
   ||| Selecting Flows to export(*)|  |-Handle export errors       ||  |
   ||+-----------------------------+  +----------------------------+|  |
   |+----------------------------+----------------------------------+  |
   |                             |                                     |
   |                    exported IPFIX Messages                        |
   |                             |                                     |
   |                +------------+-----------------+                   |
   |                |  Anonymise export packet(*)  |                   |
   |                +------------+-----------------+                   |
   |                             |                                     |
   |                +------------+-----------------+                   |
   |                |       Transport  Protocol    |                   |
   |                +------------+-----------------+                   |
   |                             |                                     |
   +-----------------------------+-------------------------------------+
                                 |
                                 v
                    IPFIX export packet to Collector



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   (*) indicates that the block is optional.


                 Figure 1: IPFIX Device functional blocks

   Note that, according to the original architecture specification,
   IPFIX Message anonymisation is optionally performed as the final
   operation before handing the Message to the transport protocol for
   export.  While no provision is made in the architecture for
   anonymisation metadata as in Section 6, this arrangement does allow
   for the message rewriting necessary for comprehensive anonymisation
   of IPFIX export as in Section 7.  The development of the IPFIX
   Mediation [I-D.ietf-ipfix-mediators-framework] framework and the
   IPFIX File Format [RFC5655] expand upon this initial architectural
   allowance for anonymisation by adding to the list of places that
   anonymisation may be applied.  The former specifies IPFIX Mediators,
   which rewrite existing IPFIX messages, and the latter specifies a
   method for storage of IPFIX data in files.

   More detail on the applicable architectural arrangements of
   anonymisation can be found in Section 7.1


2.  Terminology

   Terms used in this document that are defined in the Terminology
   section of the IPFIX Protocol [RFC5101] document are to be
   interpreted as defined there.

   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].


3.  Categorisation of Anonymisation Techniques

   Anonymisation modifies a data set in order to protect the identity of
   the people or entities described by the data set from disclosure.
   With respect to network traffic data, anonymisation generally
   attempts to preserve some set of properties of the network traffic
   useful for a given application or applications, while ensuring the
   data cannot be traced back to the specific networks, hosts, or users
   generating the traffic.

   Anonymisation may be broadly classified according to two properties:
   recoverability and countability.  All anonymisation techniques map
   the real space of identifiers or values into a separate, anonymised
   space, according to some function.  A technique is said to be



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   recoverable when the function used is invertible or can otherwise be
   reversed and a real identifier can be recovered from a given
   replacement identifier.

   Countability compares the dimension of the anonymised space (N) to
   the dimension of the real space (M), and denotes how the count of
   unique values is preserved by the anonymisation function.  If the
   anonymised space is smaller than the real space, then the function is
   said to generalise the input, mapping more than one input point to
   each anonymous value (e.g., as with aggregation).  By definition,
   generalisation is not recoverable.

   If the dimensions of the anonymised and real spaces are the same,
   such that the count of unique values is preserved, then the function
   is said to be a direct substitution function.  If the dimension of
   the anonymised space is larger, such that each real value maps to a
   set of anonymised values, then the function is said to be a set
   substitution function.  Note that with set substitution functions,
   the sets of anonymised values are not necessarily disjoint.  Either
   direct or set substitution functions are said to be one-way if there
   exists no practical method for recovering the real data point from an
   anonymised one.

   This classification is summarised in the table below.

   +------------------------+-----------------+------------------------+
   | Recoverability /       | Recoverable     | Non-recoverable        |
   | Countability           |                 |                        |
   +------------------------+-----------------+------------------------+
   | N < M                  | N.A.            | Generalisation         |
   | N = M                  | Direct          | One-way Direct         |
   |                        | Substitution    | Substitution           |
   | N > M                  | Set             | One-way Set            |
   |                        | Substitution    | Substitution           |
   +------------------------+-----------------+------------------------+


4.  Anonymisation of IP Flow Data

   Due to the restricted semantics of IP flow data, there is a
   relatively limited set of specific anonymisation techniques available
   on flow data, though each falls into the broad categories above.
   Each type of field that may commonly appear in a flow record may have
   its own applicable specific techniques.

   While anonymisation is generally applied at the resolution of single
   fields within a flow record, attacks against anonymisation use entire
   flows and relationships between hosts and flows within a given data



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   set.  Therefore, fields which may not necessarily be identifying by
   themselves may be anonymised in order to increase the anonymity of
   the data set as a whole.

   Of all the fields in an IP flow record, IP addresses are the most
   likely to be used to directly identify entities in the real world.
   Each IP address is associated with an interface on a network host,
   and can potentially be identified with a single user.  Additionally,
   IP addresses are structured identifiers; that is, partial IP address
   prefixes may be used to identify networks just as full IP addresses
   identify hosts.  This makes anonymisation of IP addresses
   particularly important.

   Hardware addresses uniquely identify devices on the network; while
   they are not often available in traffic data collected at Layer 3,
   and cannot be used to locate devices within the network, some traces
   may contain sub-IP data including hardware address data.  Hardware
   addresses may be mappable to device serial numbers, and to the
   entities or individuals who purchased the devices, when combined with
   external databases.  They may also leak via IPv6 addresses in certain
   circumstances.  Therefore, hardware address anonymisation is also
   important.

   Port numbers identify abstract entities (applications) as opposed to
   real-world entities, but they can be used to classify hosts and user
   behavior.  Passive port fingerprinting, both of well-known and
   ephemeral ports, can be used to determine the operating system
   running on a host.  Relative data volumes by port can also be used to
   determine the host's function (workstation, web server, etc.); this
   information can be used to identify hosts and users.

   While not identifiers in and of themselves, timestamps and counters
   can reveal the behavior of the hosts and users on a network.  Any
   given network activity is recognizable by a pattern of relative time
   differences and data volumes in the associated sequence of flows,
   even without host address information.  They can therefore be used to
   identify hosts and users.  Timestamps and counters are also
   vulnerable to traffic injection attacks, where traffic with a known
   pattern is injected into a network under measurement, and this
   pattern is later identified in the anonymised data set.

   The simplest and most extreme form of anonymisation, which can be
   applied to any field of a flow record, is black-marker anonymisation,
   or complete deletion of a given field.  Note that black-marker
   anonymisation is equivalent to simply not exporting the field(s) in
   question.

   While black-marker anonymisation completely protects the data in the



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   deleted fields from the risk of disclosure, it also reduces the
   utility of the anonymised data set as a whole.  Techniques that
   retain some information while reducing (though not eliminating) the
   disclosure risk will be extensively discussed in the following
   sections; note that the techniques specifically applicable to IP
   addresses, timestamps, ports, and counters will be discussed in
   separate sections.

4.1.  IP Address Anonymisation

   Since IP addresses are the most common identifiers within flow data
   that can be used to directly identify a person, organization, or
   host, most of the work on flow and trace data anonymisation has gone
   into IP address anonymisation techniques.  Indeed, the aim of most
   attacks against anonymisation is to recover the map from anonymised
   IP addresses to original IP addresses thereby identifying the
   identified hosts.  There is therefore a wide range of IP address
   anonymisation schemes that fit into the following categories.

       +------------------------------------+---------------------+
       | Scheme                             | Action              |
       +------------------------------------+---------------------+
       | Truncation                         | Generalisation      |
       | Reverse Truncation                 | Generalisation      |
       | Permutation                        | Direct Substitution |
       | Prefix-preserving Pseudonymisation | Direct Substitution |
       +------------------------------------+---------------------+

4.1.1.  Truncation

   Truncation removes "n" of the least significant bits from an IP
   address, replacing them with zeroes.  In effect, it replaces a host
   address with a network address for some fixed netblock; for IPv4
   addresses, 8-bit truncation corresponds to replacement with a /24
   network address.  Truncation is a non-reversible generalisation
   scheme.  Note that while truncation is effective for making hosts
   non-identifiable, it preserves information which can be used to
   identify an organization, a geographic region, a country, or a
   continent (or RIR region of responsibility).

   Truncation to an address length of 0 is equivalent to black-marker
   anonymisation.  Complete removal of IP address information is only
   recommended for analysis tasks which have no need to separate flow
   data by host or network; e.g. as a first stage to per-application
   (port) or time-series total volume analyses.






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4.1.2.  Reverse Truncation

   Reverse truncation removes "n" of the most significant bits from an
   IP address, replacing them with zeroes.  Reverse truncation is a non-
   reversible generalisation scheme.  Reverse truncation is effective
   for making networks unidentifiable, partially or completely removing
   information which can be used to identify an organization, a
   geographic region, a country, or a continent (or RIR region of
   responsibility).  However, it may cause ambiguity when applied to
   data collected from more than one network, since it treats all the
   hosts with the same address on different networks as if they are the
   same host.  It is not particularly useful when publishing data where
   the network of origin is known or can be easily guessed by virtue of
   the identity of the publisher.

   Like truncation, reverse truncation to an address length of 0 is
   equivalent to black-marker anonymisation.

4.1.3.  Permutation

   Permutation is a direct substitution technique, replacing each IP
   address with an address selected from the set of possible IP
   addresses, guaranteeing that each anonymised address represents a
   unique original address.  The selection function is often random,
   though it is not necessarily so.  Permutation does not preserve any
   structural information about a network, but it does preserve the
   unique count of IP addresses.  Any application that requires more
   structure than host-uniqueness will not be able to use permuted IP
   addresses.

4.1.4.  Prefix-preserving Pseudonymisation

   Prefix-preserving pseudonymisation is a direct substitution
   technique, like permutation but further restricted such that the
   structure of subnets is preserved at each level while anonymising IP
   addresses.  If two real IP addresses match on a prefix of "n" bits,
   the two anonymised IP addresses will match on a prefix of "n" bits as
   well.  This is useful when relationships among networks must be
   preserved for a given analysis task, but introduces structure into
   the anonymised data which can be exploited in attacks against the
   anonymisation technique.

   Scanning in Internet background traffic can cause particular problems
   with this technique: if a scanner uses a predictable and known
   sequence of addresses, this information can be used to reverse the
   substitution.  The low order portion of the address can be left
   unanonymized as a partial defense against this attack.




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4.2.  Hardware Address Anonymisation

   Flow data containing sub-IP information can also contain identifying
   information in the form of the hardware (MAC) address.  While
   hardware address information cannot be used to locate a node within a
   network, it can be used to directly uniquely identify a specific
   device.  Vendors or organizations within the supply chain may then
   have the information necessary to identify the entity or individual
   that purchased the device.

   Hardware address information is not as structured as IP address
   information.  EUI-48 and EUI-64 hardware addresses contain an
   Organizational Unique Identifier in the three most significant bytes
   of the address; this OUI additionally contains bits noting whether
   the address is locally or globally administered.  Beyond this, the
   address is unstructured, and there is no particular relationship
   among the OUIs assigned to a given vendor.

   Note that hardware address information also appear within IPv6
   addresses, as the EAP-64 address, or EAP-48 address encoded as an
   EAP-64 address, is used as the least significant 64 bits of the IPv6
   address in the case of link local addressing or stateless
   autoconfiguration; the considerations and techniques in this section
   may then apply to such IPv6 addresses as well.

           +-----------------------------+---------------------+
           | Scheme                      | Action              |
           +-----------------------------+---------------------+
           | Reverse Truncation          | Generalisation      |
           | Permutation                 | Direct Substitution |
           | Structured Pseudonymisation | Direct Substitution |
           +-----------------------------+---------------------+

4.2.1.  Reverse Truncation

   Reverse truncation removes "n" of the most significant bits from an
   MAC address, replacing them with zeroes.  Reverse truncation is a
   non-reversible generalisation scheme.  This has the effect of
   removing bits of the OUI, which identify manufacturers, before
   removing the least significant bits.  Reverse truncation of 24 bits
   zeroes out the OUI.

   Reverse truncation is effective for making device manufacturers
   partially or completely unidentifiable within a dataset.  However, it
   may cause ambiguity by introducing the possibility of truncated MAC
   address collision.  Also note that the utility or removing
   manufacturer information is dubious, and not particularly well-
   covered by the literature.



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   Reverse truncation to an address length of 0 is equivalent to black-
   marker anonymisation.

4.2.2.  Permutation

   Permutation is a direct substitution technique, replacing each MAC
   address with an address selected from the set of possible MAC
   addresses, guaranteeing that each anonymised address represents a
   unique original address.  The selection function is often random,
   though it is not necessarily so.  Permutation does not preserve any
   structural information about a network, but it does preserve the
   unique count of devices on the network.  Any application that
   requires more structure than host-uniqueness will not be able to use
   permuted MAC addresses.

4.2.3.  Structured Pseudonymisation

   Structured pseudonymisation for MAC addresses is a direct
   substitution technique, like permutation, but restricted such that
   the OUI (the most significant three bytes) is permuted separately
   from the node identifier, the remainder.  This is useful when the
   uniqueness of OUIs must be preserved for a given analysis task, but
   introduces structure into the anonymised data which can be exploited
   in attacks against the anonymisation technique.

4.3.  Timestamp Anonymisation

   The particular time at which a flow began or ended is not
   particularly identifiable information, but it can be used as part of
   attacks against other anonymisation techniques or for user profiling.
   Presice timestamps can be used in injected-traffic fingerprinting
   attacks as well as to identify certain activity by response delay and
   size fingerprinting.  Therefore, timestamp information may be
   anonymised in order to ensure the protection of the entire dataset.

          +-----------------------+----------------------------+
          | Scheme                | Action                     |
          +-----------------------+----------------------------+
          | Precision Degradation | Generalisation             |
          | Enumeration           | Direct or Set Substitution |
          | Random Shifts         | Direct Substitution        |
          +-----------------------+----------------------------+

4.3.1.  Precision Degradation

   Precision Degradation is a generalisation technique that removes the
   most precise components of a timestamp, accounting all events
   occurring in each given interval (e.g. one millisecond for



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   millisecond level degradation) as simultaneous.  This has the effect
   of potentially collapsing many timestamps into one.  With this
   technique time precision is reduced, and sequencing may be lost, but
   the information at which time the event occurred is preserved.  The
   anonymised data may not be generally useful for applications which
   require strict sequencing of flows.

   Note that flow meters with low time precision (e.g. second precision,
   or millisecond precision on high-capacity networks) perform the
   equivalent of precision degradation anonymisation by their design.

   Note also that degradation to a very low precision (e.g. on the order
   of minutes, hours, or days) is commonly used in analyses operating on
   time-series aggregated data, and may also be described as binning;
   though the time scales are longer and applicability more restricted,
   this is in principle the same operation.

   Precision degradation to infinitely low precision is equivalent to
   black-marker anonymisation.  Removal of timestamp information is only
   recommended for analysis tasks which have no need to separate flows
   in time, for example for counting total volumes or unique occurrences
   of other flow keys in an entire dataset.

4.3.2.  Enumeration

   Enumeration is a substitution function that retains the chronological
   order in which events occurred while eliminating time information.
   Timestamps are substituted by equidistant timestamps (or numbers)
   starting from a randomly chosen start value.  The resulting data is
   useful for applications requiring strict sequencing, but not for
   those requiring good timing information (e.g. delay- or jitter-
   measurement for QoS applications or SLA validation).

4.3.3.  Random Shifts

   Random time shifts add a random offset to every timestamp within a
   dataset.  This reversible substitution technique therefore retains
   duration and inter-event interval information as well as
   chronological order of flows.  It is primarily intended to defeat
   traffic injection fingerprinting attacks.

4.4.  Counter Anonymisation

   Counters (such as packet and octet volumes per flow) are subject to
   fingerprinting and injection attacks against anonymisation, or for
   user profiling as timestamps are.  Counter anonymisation can help
   defeat these attacks, but are only usable for analysis tasks for
   which relative or imprecise magnitudes of activity are useful.



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   Counter information can also be completely removed, but this is only
   recommended for analysis tasks which have no need to evaluate the
   removed counter, for example for counting only unique occurrences of
   other flow keys.

          +-----------------------+----------------------------+
          | Scheme                | Action                     |
          +-----------------------+----------------------------+
          | Precision Degradation | Generalisation             |
          | Binning               | Generalisation             |
          | Random noise addition | Direct or Set Substitution |
          +-----------------------+----------------------------+

4.4.1.  Precision Degradation

   As with precision degradation in timestamps, precision degradation of
   counters removes lower-order bits of the counters, treating all the
   counters in a given range as having the same value.  Depending on the
   precision reduction, this loses information about the relationships
   between sizes of similarly-sized flows, but keeps relative magnitude
   information.  Precision degradation to an infinitely low precision is
   equivalent to black-marker anonymisation.

4.4.2.  Binning

   Binning can be seen as a special case of precision degradation; the
   operation is identical, except for in precision degradation the
   counter ranges are uniform, and in binning they need not be.  For
   example, a common counter binning scheme for packet counters could be
   to bin values 1-2 together, and 3-infinity together, thereby
   separating potentially completely-opened TCP connections from
   unopened ones.  Binning schemes are generally chosen to keep
   precisely the amount of information required in a counter for a given
   analysis task.  Note that, also unlike precision degradation, the bin
   label need not be within the bin's range.  Binning counters to a
   single bin is equivalent to black-marker anonymisation.

4.4.3.  Random Noise Addition

   Random noise addition adds a random amount to a counter in each flow;
   this is used to keep relative magnitude information and minimize the
   disruption to size relationship information while avoiding
   fingerprinting attacks against anonymisation.  Note that there is no
   guarantee that random noise addition will maintain ranking order by a
   counter among members of a set.  Random noise addition is
   particularly useful when the derived analysis data will not be
   presented in such a way as to require the lower-order bits of the
   counters.



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4.5.  Anonymisation of Other Flow Fields

   Other fields, particularly port numbers and protocol numbers, can be
   used to partially identify the applications that generated the
   traffic in a a given flow trace.  This information can be used in
   fingerprinting attacks, and may be of interest on its own (e.g., to
   reveal that a certain application with suspected vulnerabilities is
   running on a given network).  These fields are generally anonymised
   using one of two techniques.

                   +-------------+---------------------+
                   | Scheme      | Action              |
                   +-------------+---------------------+
                   | Binning     | Generalisation      |
                   | Permutation | Direct Substitution |
                   +-------------+---------------------+

4.5.1.  Binning

   Binning is a generalisation technique mapping a set of potentially
   non-uniform ranges into a set of arbitrarily labeled bins.  Common
   bin arrangements depend on the field type and the analysis
   application.  For example, an IP protocol bin arrangement may
   preserve 1, 6, and 17 for ICMP, UDP, and TCP traffic, and bin all
   other protocols into a single bin, to mitigate the use of uncommon
   protocols in fingerprinting attacks.  Another example arrangement may
   bin source and destination ports into low (0-1023) and high (1024-
   65535) bins in order to tell service from ephemeral ports without
   identifying individual applications.

   Binning other flow key fields to a single bin is equivalent to black-
   marker anonymisation.  Removal of other flow key information is only
   recommended for analysis tasks which have no need to differentiate
   flows on the removed keys, for example for total traffic counts or
   unique counts of other flow keys.

4.5.2.  Permutation

   Permutation is a direct substitution technique, replacing each value
   with an value selected from the set of possible range, guaranteeing
   that each anonymised value represents a unique original value.  This
   is used to preserve the count of unique values without preserving
   information about, or the ordering of, the values themselves.


5.  Parameters for the Description of Anonymisation Techniques

   This section details the abstract parameters used to describe the



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   anonymisation techniques examined in the previous section, on a per-
   parameter basis.  These parameters and their export safety inform the
   design of the IPFIX anonymisation metadata export specified in the
   following section.

5.1.  Stability

   Any given anonymisation technique may be applied with a varying range
   of stability.  Stability is important for assessing the comparability
   of anonymised information in different data sets, or in the same data
   set over different time periods.  In general, stability ranges from
   completely stable to completely unstable; however, note that the
   completely unstable case is indistinguishable from black-marker
   anonymisation.  A completely stable anonymisation will always map a
   given value in the real space to the same value in the anonymised
   space.  In practice, an anonymisation may also be stable for every
   data set published by an a particular producer to a particular
   consumer, stable for a stated time period within a dataset or across
   datasets, or stable only for a single data set.

   If no information about stability is available, users of anonymised
   data may assume that the techniques used are stable across the entire
   dataset, but unstable across datasets.  Note that stability presents
   a risk-utility tradeoff, as completely stable anonymisation can be
   used for longer-term trend analysis tasks but also presents more risk
   of attack given the stable mapping.

5.2.  Truncation Length

   Truncation and precision degradation are described by the truncation
   length, or the amount of data still remaining in the anonymised field
   after anonymisation.

   Truncation length can be inferred from a given data set, and need not
   be specially exported or protected.

5.3.  Bin Map

   Binning is described by the specification of a bin mapping function.
   This function can be generally expressed in terms of an associative
   array that maps each point in the original space to a bin, although
   from an implementation standpoint most bin functions are much simpler
   and more efficient.

   Since knowledge of the bin mapping function can be used to partially
   deanonymise binned data, depending on the degree of generalisation,
   no information about the bin mapping function should be exported.




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5.4.  Permutation

   Like binning, permutation is described by the specification of a
   permutation function.  In the general case, this can be expressed in
   terms of an associative array that maps each point in the original
   space to a point in the anonymised space.  Unlike binning, each point
   in the anonymised space must correspond to a single, unique point in
   the original space.

   Since knowledge of the permutation function can be used to completely
   deanonymise permuted data, no information about the permutation
   function or its parameters should be exported.

5.5.  Shift Amount

   Shifting requires an amount to shift each value by.  Since the shift
   amount can be used to deanonymise data protected by shifting, no
   information about the shift amount should be exported.


6.  Anonymisation Export Support in IPFIX

   Anonymised data exported via IPFIX SHOULD be annotated with
   anonymisation metadata, which details which fields described by which
   Templates are anonymised, and provides appropriate information on the
   anonymisation techniques used.  This metadata SHOULD be exported in
   Data Records described by the recommended Options Templates described
   in this section; these Options Templates use the additional
   Information Elements described in the following subsection.

   Note that fields anonymised using the black-marker (removal)
   technique do not require any special metadata support.  Black-marker
   anonymised fields SHOULD NOT be exported at all; the absence of the
   field in a given Data Set is implicitly declared by not including the
   corresponding Information Element in the Template describing that
   Data Set.

6.1.  Anonymisation Options Template

   The Anonymisation Options Template describes anonymisation records,
   which allow anonymisation metadata to be exported inline over IPFIX
   or stored in an IPFIX File, by binding information about
   anonymisation techniques to Information Elements within defined
   Templates.  IPFIX Exporting Processes SHOULD export anonymisation
   records for any Template describing exported anonymised Data Records;
   IPFIX Collecting Processes and processes downstream from them MAY use
   anonymisation records to treat anonymised data differently depending
   on the applied technique.



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   An Exporting Process SHOULD export anonymisation records after the
   Templates they describe have been exported, and SHOULD export
   anonymisation records reliably.

   Anonymisation records, like Templates, MUST be handled by Collecting
   Processes as scoped to the Transport Session in which they are sent.
   While the Stability Class within the anonymisationFlags IE can be
   used to declare that a given anonymisation technique's mapping will
   remain stable across multiple sessions, each session MUST re-export
   the anonymisation Records along with the templates.

   +-------------------------+-----------------------------------------+
   | IE                      | Description                             |
   +-------------------------+-----------------------------------------+
   | templateId [scope]      | The Template ID of the Template         |
   |                         | containing the Information Element      |
   |                         | described by this anonymisation record. |
   |                         | This Information Element MUST be        |
   |                         | defined as a Scope Field.               |
   | informationElementId    | The Information Element identifier of   |
   | [scope]                 | the Information Element described by    |
   |                         | this anonymisation record.  This        |
   |                         | Information Element MUST be defined as  |
   |                         | a Scope Field.                          |
   | informationElementId    | The Private Enterprise Number of the    |
   | [scope] [optional]      | enterprise-specific Information Element |
   |                         | described by this anonymisation record. |
   |                         | This Information Element MUST be        |
   |                         | defined as a Scope Field if present.    |
   | informationElementIndex | The Information Element index of the    |
   | [scope] [optional]      | instance of the Information Element     |
   |                         | described by this anonymisation record  |
   |                         | identified by the informationElementId  |
   |                         | within the Template.  Optional; need    |
   |                         | only be present when describing         |
   |                         | Templates that have multiple instances  |
   |                         | of the same Information Element.  This  |
   |                         | Information Element MUST be defined as  |
   |                         | a Scope Field if present.  This         |
   |                         | Information Element is defined in       |
   |                         | Section 6.2, below.                     |
   | anonymisationFlags      | Flags describing the mapping stability  |
   |                         | and specialized modifications to the    |
   |                         | Anonymisation Technique in use.  SHOULD |
   |                         | be present.  This Information Element   |
   |                         | is defined in Section 6.2, below.       |





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   | anonymisationTechnique  | The technique used to anonymise the     |
   |                         | data.  MUST be present.  This           |
   |                         | Information Element is defined in       |
   |                         | Section 6.2, below.                     |
   +-------------------------+-----------------------------------------+

6.2.  Recommended Information Elements for Anonymisation Metadata

6.2.1.  informationElementIndex

   Description:   A zero-based index of an Information Element
      referenced by informationElementId within a Template referenced by
      templateId; used to disambiguate scope for templates containing
      multiple identical Information Elements.

   Abstract Data Type:   unsigned16

   ElementId:   TBD3

   Status:   Proposed

6.2.2.  anonymisationFlags

   Description:   A flag word describing specialized modifications to
      the anonymisation policy in effect for the anonymisation technique
      applied to a referenced Information Element within a referenced
      Template.  When flags are clear (0), the normal policy (as
      described by anonymisationTechnique) applies without modification.

      MSB   14  13  12  11  10   9   8   7   6   5   4   3   2   1  LSB
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      |                Reserved                       |LOR|PmA|   SC  |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

                            anonymisationFlags IE

   +--------+----------+-----------------------------------------------+
   | bit(s) | name     | description                                   |
   | (LSB = |          |                                               |
   | 0)     |          |                                               |
   +--------+----------+-----------------------------------------------+
   | 0-1    | SC       | Stability Class: see the Stability Class      |
   |        |          | table below, and section Section 5.1.         |








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   | 2      | PmA      | Perimeter Anonymisation: when set (1), source |
   |        |          | address Information Elements are interpreted  |
   |        |          | as external addresses, and destination        |
   |        |          | address Information Elements are interpreted  |
   |        |          | as internal addresses, for the purposes of    |
   |        |          | associating anonymisationTechnique to         |
   |        |          | Information Elements.  MUST NOT be set when   |
   |        |          | associated with a non-endpoint (i.e., source- |
   |        |          | or destination-) Information Element.  SHOULD |
   |        |          | be consistent within a record (i.e., if a     |
   |        |          | source- Information Element has this flag     |
   |        |          | set, the corresponding destination- element   |
   |        |          | SHOULD have this flag set, and vice-versa.)   |
   | 3      | LOR      | Low-Order Unchanged: when set (1), the        |
   |        |          | low-order bits of the anonymised Information  |
   |        |          | Element contain real data.  This modification |
   |        |          | is intended for the anonymisation of          |
   |        |          | network-level addresses while leaving         |
   |        |          | host-level addresses intact in order to       |
   |        |          | preserve host level-structure, which could    |
   |        |          | otherwise be used to reverse anonymisation.   |
   |        |          | MUST NOT be set when associated with a        |
   |        |          | truncation-based anonymisationTechnique.      |
   | 4-15   | Reserved | Reserved for future use: SHOULD be cleared    |
   |        |          | (0) by the Exporting Process and MUST be      |
   |        |          | ignored by the Collecting Process.            |
   +--------+----------+-----------------------------------------------+

      The Stability Class portion of this flags word describes the
      stability class of the anonymisation technique applied to a
      referenced Information Element within a referenced Template.
      Stability classes refer to the stability of the parameters of the
      anonymisation technique, and therefore the comparability of the
      mapping between the real and anonymised values over time.  This
      determines which anonymised datasets may be compared with each
      other.  Values are as follows:















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   +-----+-----+-------------------------------------------------------+
   | Bit | Bit | Description                                           |
   | 1   | 0   |                                                       |
   +-----+-----+-------------------------------------------------------+
   | 0   | 0   | Undefined: the Exporting Process makes no             |
   |     |     | representation as to how stable the mapping is, or    |
   |     |     | over what time period values of this field will       |
   |     |     | remain comparable; while the Collecting Process MAY   |
   |     |     | assume Session level stability, Session level         |
   |     |     | stability is not guaranteed.  Processes SHOULD assume |
   |     |     | this is the case in the absence of stability class    |
   |     |     | information; this is the default stability class.     |
   | 0   | 1   | Session: the Exporting Process will ensure that the   |
   |     |     | parameters of the anonymisation technique are stable  |
   |     |     | during the Transport Session.  All the values of the  |
   |     |     | described Information Element for each Record         |
   |     |     | described by the referenced Template within the       |
   |     |     | Transport Session are comparable.  The Exporting      |
   |     |     | Process SHOULD endeavour to ensure at least this      |
   |     |     | stability class.                                      |
   | 1   | 0   | Exporter-Collector Pair: the Exporting Process will   |
   |     |     | ensure that the parameters of the anonymisation       |
   |     |     | technique are stable across Transport Sessions over   |
   |     |     | time with the given Collecting Process, but may use   |
   |     |     | different parameters for different Collecting         |
   |     |     | Processes.  Data exported to different Collecting     |
   |     |     | Processes is not comparable.                          |
   | 1   | 1   | Stable: the Exporting Process will ensure that the    |
   |     |     | parameters of the anonymisation technique are stable  |
   |     |     | across Transport Sessions over time, regardless of    |
   |     |     | the Collecting Process to which it is sent.           |
   +-----+-----+-------------------------------------------------------+

   Abstract Data Type:   unsigned16

   ElementId:   TBD1

   Status:   Proposed

6.2.3.  anonymisationTechnique

   Description:   A description of the anonymisation technique applied
      to a referenced Information Element within a referenced Template.
      Each technique may be applicable only to certain Information
      Elements and recommended only for certain Infomation Elements;
      these restrictions are noted in the table below.





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   +-------+---------------------------+-----------------+-------------+
   | Value | Description               | Applicable to   | Recommended |
   |       |                           |                 | for         |
   +-------+---------------------------+-----------------+-------------+
   | 0     | Undefined: the Exporting  | all             | all         |
   |       | Process makes no          |                 |             |
   |       | representation as to      |                 |             |
   |       | whether the defined field |                 |             |
   |       | is anonymised or not.     |                 |             |
   |       | While the Collecting      |                 |             |
   |       | Process MAY assume that   |                 |             |
   |       | the field is not          |                 |             |
   |       | anonymised, it is not     |                 |             |
   |       | guaranteed not to be.     |                 |             |
   |       | This is the default       |                 |             |
   |       | anonymisation technique.  |                 |             |
   | 1     | None: the values exported | all             | all         |
   |       | are real.                 |                 |             |
   | 2     | Precision                 | all             | all         |
   |       | Degradation/Truncation:   |                 |             |
   |       | the values exported are   |                 |             |
   |       | anonymised using simple   |                 |             |
   |       | precision degradation or  |                 |             |
   |       | truncation.  The new      |                 |             |
   |       | precision or number of    |                 |             |
   |       | truncated bits is         |                 |             |
   |       | implicit in the exported  |                 |             |
   |       | data, and can be deduced  |                 |             |
   |       | by the Collecting         |                 |             |
   |       | Process.                  |                 |             |
   | 3     | Binning: the values       | all             | all         |
   |       | exported are anonymised   |                 |             |
   |       | into bins.                |                 |             |
   | 4     | Enumeration: the values   | all             | timestamps  |
   |       | exported are anonymised   |                 |             |
   |       | by enumeration.           |                 |             |
   | 5     | Permutation: the values   | all             | identifiers |
   |       | exported are anonymised   |                 |             |
   |       | by random permutation.    |                 |             |












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   | 6     | Structured Permutation:   | addresses       |             |
   |       | the values exported are   |                 |             |
   |       | anonymised by random      |                 |             |
   |       | permutation, preserving   |                 |             |
   |       | bit-level structure as    |                 |             |
   |       | appropriate; this         |                 |             |
   |       | represents                |                 |             |
   |       | prefix-preserving IP      |                 |             |
   |       | address anonymisation or  |                 |             |
   |       | structured MAC address    |                 |             |
   |       | anonymisation.            |                 |             |
   | 7     | Reverse Truncation: the   | addresses       |             |
   |       | values exported are       |                 |             |
   |       | anonymised using reverse  |                 |             |
   |       | truncation.  The number   |                 |             |
   |       | of truncated bits is      |                 |             |
   |       | implicit in the exported  |                 |             |
   |       | data, and can be deduced  |                 |             |
   |       | by the Collecting         |                 |             |
   |       | Process.                  |                 |             |
   | 8     | Noise: the values         | non-identifiers | counters    |
   |       | exported are anonymised   |                 |             |
   |       | by adding random noise to |                 |             |
   |       | each value.               |                 |             |
   | 9     | Offset: the values        | all             | timestamps  |
   |       | exported are anonymised   |                 |             |
   |       | by adding a single offset |                 |             |
   |       | to all values.            |                 |             |
   +-------+---------------------------+-----------------+-------------+

   Abstract Data Type:   unsigned16

   ElementId:   TBD2

   Status:   Proposed


7.  Applying Anonymisation Techniques to IPFIX Export and Storage

   When exporting or storing anonymised flow data using IPFIX, certain
   interactions between the IPFIX Protocol and the anonymisation
   techniques in use must be considered; these are treated in the
   subsections below.

7.1.  Arrangement of Processes in IPFIX Anonymisation

   Anonymisation may be applied to IPFIX data at three stages within the
   collection infrastructure: on initial export, at a mediator, or after



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   collection, as shown in Figure 2.  Each of these locations has
   specific considerations and applicability.

               +==========================================+
               | Exporting Process                        |
               +==========================================+
                 |                                      |
                 |    (Anonymised at Original Exporter) |
                 V                                      |
               +=============================+          |
               | Mediator                    |          |
               +=============================+          |
                 |                                      |
                 | (Anonymising Mediator)               |
                 V                                      V
               +==========================================+
               | Collecting Process                       |
               +==========================================+
                       |
                       | (Anonymising CP/File Writer)
                       V
               +--------------------+
               | IPFIX File Storage |
               +--------------------+

                Figure 2: Potential Anonymisation Locations

   Anonymisation is generally performed before the wider dissemination
   or repurposing of a flow data set, e.g., adapting operational
   measurement data for research.  Therefore, direct anonymisation of
   flow data on initial export is only applicable in certain restricted
   circumstances: when the Exporting Process is "publishing" data to a
   Collecting Process directly, and the Exporting Process and Collecting
   Process are operated by different entities.  Note that certain
   guidelines in Section 7.2.3 with respect to timestamp anonymisation
   may not apply in this case, as the Collecting Process may be able to
   deduce certain timing information from the time at which each Message
   is received.

   A much more flexible arrangement is to anonymise data within a
   Mediator [I-D.ietf-ipfix-mediators-framework].  Here, original data
   is sent to a Mediator, which performs the anonymisation function and
   re-exports the anonymised data.  Such a Mediator could be located at
   the administrative domain boundary of the initial Exporting Process
   operator, exporting anonymised data to other consumers outside the
   organisation.  In this case, the original Exporter SHOULD use TLS as
   specified in [RFC5101] to secure the channel to the Mediator, and the
   Mediator should follow the guidelines in Section 7.2, to mitigate the



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   risk of original data disclosure.

   When data is to be published as an anonymised data set in an IPFIX
   File [RFC5655], the anonymisation may be done at the final Collecting
   Process before storage and dissemination, as well.  In this case, the
   Collector should follow the guidelines in Section 7.2, especially as
   regards File-specific Options in Section 7.2.4

   In each of these data flows, the anonymisation of records is
   undertaken by an Intermediate Anonymisation Process (IAP); the data
   flows into and out of this IAP are shown in Figure 3 below.

   packets --+                     +- IPFIX Messages -+
             |                     |                  |
             V                     V                  V
   +==================+ +====================+ +=============+
   | Metering Process | | Collecting Process | | File Reader |
   +==================+ +====================+ +=============+
             |      Non-anonymised | Records          |
             V                     V                  V
   +=========================================================+
   |          Intermediate Anonymisation Process (IAP)       |
   +=========================================================+
             | Anonymised     ^            Anonymised |
             | Records        |               Records |
             V                |                       V
   +===================+    Anonymisation      +=============+
   | Exporting Process |<--- Parameters ------>| File Writer |
   +===================+                       +=============+
             |                                        |
             +------------> IPFIX Messages <----------+

          Figure 3: Data flows through the anonymisation process

   Anonymisation parameters must also be available to the Exporting
   Process and/or File Writer in order to ensure header data is also
   appropriately anonymised as in Section 7.2.3.

   Following each of the data flows through the IAP, we describe five
   basic types of anonymisation arrangements within this framework in
   Figure 4.  In addition to the three arrangements described in detail
   above, anonymisation can also be done at a collocated Metering
   Process and File Writer (see section 7.3.2 of [RFC5655]), or at a
   file manipulator (see section 7.3.7 of [RFC5655]).







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         +----+  +-----+  +----+
 pkts -> | MP |->| IAP |->| EP |-> anonymisation on Original Exporter
         +----+  +-----+  +----+
         +----+  +-----+  +----+
 pkts -> | MP |->| IAP |->| FW |-> Anonymising collocated MP/File Writer
         +----+  +-----+  +----+
         +----+  +-----+  +----+
IPFIX -> | CP |->| IAP |->| EP |-> Anonymising Mediator (Masquerading Proxy)
         +----+  +-----+  +----+
         +----+  +-----+  +----+
IPFIX -> | CP |->| IAP |->| FW |-> Anonymising collocated CP/File Writer
         +----+  +-----+  +----+
         +----+  +-----+  +----+
IPFIX -> | FR |->| IAP |->| FW |-> Anonymising file manipulator
 File    +----+  +-----+  +----+

        Figure 4: Possible anonymisation arrangements in the IPFIX
                               architecture

   Note that anonymisation may occur at more than one location within a
   given collection infrastructure, to provide varying levels of
   anonymisation, disclosure risk, or data utility for specific
   purposes.

7.2.  IPFIX-Specific Anonymisation Guidelines

   In implementing and deploying the anonymisation techniques described
   in this document, implementors should note that IPFIX already
   provides features that support anonymised data export, and use these
   where appropriate.  Care must also be taken that data structures
   supporting the operation of the protocol itself do not leak data that
   could be used to reverse the anonymisation applied to the flow data.
   Such data structures may appear in the header, or within the data
   stream itself, especially as options data.  Each of these and their
   impact on specific anonymisation techniques is noted in a separate
   subsection below.

7.2.1.  Appropriate Use of Information Elements for Anonymised Data

   Note, as in Section 6 above, that black-marker anonymised fields
   SHOULD NOT be exported at all; the absence of the field in a given
   Data Set is implicitly declared by not including the corresponding
   Information Element in the Template describing that Data Set.

   When using precision degradation of timestamps, Exporting Processes
   SHOULD export timing information using Information Elements of an
   appropriate precision, as explained in Section 4.5 of [RFC5153].  For
   example, timestamps measured in millisecond-level precision and



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   degraded to second-level precision should use flowStartSeconds and
   flowEndSeconds, not flowStartMilliseconds and flowEndMilliseconds.

   When exporting anonymised data and anonymisation metadata, Exporting
   Processes SHOULD ensure that the combination of Information Element
   and declared anonymisation technique are compatible.  Specifically,
   the applicable and recommended Information Element types and
   semantics for each technique are noted in the description of the
   anonymisationTechnique Information Element in Section 6.2.3.  In this
   description, a timestamp is an Information Element with the data type
   dateTimeSeconds, dataTimeMilliseconds, dateTimeMicroseconds, or
   dateTimeNanoseconds; an address is an Information Element with the
   data type ipv4Address, ipv6Address, or macAddress; and an identifier
   is an Information Element with identifier data type semantics.
   Exporting Process MUST NOT export Anonymisation Options records
   binding techniques to Information Elements to which they are not
   applicable, and SHOULD NOT export Anonymisation Options records
   binding techniques to Information Elements for which they are not
   recommended.

7.2.2.  Export of Perimeter-Based Anonymisation Policies

   Data collected from a single network may require different
   anonymisation policies for addresses internal and external to the
   network.  For example, internal addresses could be subject to simple
   permutation, while external addresses could be aggregated into
   networks by truncation.  When exporting anonymised perimeter
   bidirectional flow (biflow) data as in section 5.2 of [RFC5103], this
   arrangement may be easily represented by specifying one technique for
   source endpoint information (which represents the external endpoint
   in a perimeter biflow) and one technique for destination endpoint
   information (which represents the internal address in a perimeter
   biflow).

   However, it can also be useful to represent perimeter-based
   anonymisation policies with unidirectional flow (uniflow), or non-
   perimeter biflow data.  In this case, the Perimeter Anonymisation bit
   (bit 2) in the anonymisationFlags Information Element describing the
   anonymised address Information Elements can be set to change the
   meaning of "source" and "destination" of Information Elements to mean
   "external" and "internal" as with perimeter biflows, but only with
   respect to anonymisation policies.

7.2.3.  Anonymisation of Header Data

   Each IPFIX Message contains a Message Header; within this Message
   Header are contained two fields which may be used to break certain
   anonymisation techniques: the Export Time, and the Observation Domain



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   ID

   Export of IPFIX Messages containing anonymised timestamp data where
   the original Export Time Message header has some relationship to the
   anonymised timestamps SHOULD anonymise the Export Time header field
   using an equivalent technique, if possible.  Otherwise, relationships
   between export and flow time could be used to partially or totally
   reverse timestamp anonymisation.

   The similarity in size between an Observation Domain ID and an IPv4
   address (32 bits) may lead to a temptation to use an IPv4 interface
   address on the Metering or Exporting Process as the Observation
   Domain ID.  If this address bears some relation to the IP addresses
   in the flow data (e.g., shares a network prefix with internal
   addresses) and the IP addresses in the flow data are anonymised in a
   structure-preserving way, then the Observation Domain ID may be used
   to break the IP address anonymisation.  Use of an IPv4 interface
   address on the Metering or Exporting Process as the Observation
   Domain ID is NOT RECOMMENDED in this case.

7.2.4.  Anonymisation of Options Data

   IPFIX uses the Options mechanism to export, among other things,
   metadata about exported flows and the flow collection infrastructure.
   As with the IPFIX Message Header, certain Options recommended in
   [RFC5101] and [RFC5655] containing flow timestamps and network
   addresses of Exporting and Collecting Processes may be used to break
   certain anonymisation techniques; care should be taken while using
   them with anonymised data export and storage.

   The Exporting Process Reliability Statistics Options Template,
   recommended in [RFC5101], contains an Exporting Process ID field,
   which may be an exportingProcessIPv4Address Information Element or an
   exportingProcessIPv6Address Information Element.  If the Exporting
   Process address bears some relation to the IP addresses in the flow
   data (e.g., shares a network prefix with internal addresses) and the
   IP addresses in the flow data are anonymised in a structure-
   preserving way, then the Exporting Process address may be used to
   break the IP address anonymisation.  Exporting Processes exporting
   anonymised data in this situation SHOULD mitigate the risk of attack
   either by omitting Options described by the Exporting Process
   Reliability Statistics Options Template, or by anonymising the
   Exporting Process address using a similar technique to that used to
   anonymise the IP addresses in the exported data.

   Similarly, the Export Session Details Options Template and Message
   Details Options Template specified for the IPFIX File Format
   [RFC5655] may contain the exportingProcessIPv4Address Information



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   Element or the exportingProcessIPv6Address Information Element to
   identify an Exporting Process from which a flow record was received,
   and the collectingProcessIPv4Address Information Element or the
   collectingProcessIPv6Address Information Element to identify the
   Collecting Process which received it.  If the Exporting Process or
   Collecting Process address bears some relation to the IP addresses in
   the flow data (e.g., shares a network prefix with internal addresses)
   and the IP addresses in the flow data are anonymised in a structure-
   preserving way, then the Exporting Process or Collecting Process
   address may be used to break the IP address anonymisation.  Since
   these Options Templates are primarily intended for storing IPFIX
   Transport Session data for auditing, replay, and testing purposes, it
   is NOT RECOMMENDED that storage of anonymised data include these
   Options Templates in order to mitigate the risk of attack.

   The Message Details Options Template specified for the IPFIX File
   Format [RFC5655] also contains the collectionTimeMilliseconds
   Information Element.  As with the Export Time Message Header field,
   if the exported flow data contains anonymised timestamp information,
   and the collectionTimeMilliseconds Information Element in a given
   Message has some relationship to the anonymised timestamp
   information, then this relationship can be exploited to reverse the
   timestamp anonymisation.  Since this Options Template is primarily
   intended for storing IPFIX Transport Session data for auditing,
   replay, and testing purposes, it is NOT RECOMMENDED that storage of
   anonymised data include this Options Template in order to mitigate
   the risk of attack.

   Since the Time Window Options Template specified for the IPFIX File
   Format [RFC5655] refers to the timestamps within the flow data to
   provide partial table of contents information for an IPFIX File, care
   must be taken to ensure that Options described by this template are
   written using the anonymised timestamps instead of the original ones.

7.2.5.  Special-Use Address Space Considerations

   When anonymising data for transport or storage using IPFIX containing
   anonymised IP addresses, and the analysis purpose permits doing so,
   it is recommended to filter out or leave unanonymised data containing
   the special-use IPv4 addresses enumerated in [RFC3330] or the
   special-use IPv6 addresses enumerated in [RFC5153].  Data containing
   these addresses (e.g. 0.0.0.0 and 169.254.0.0/16 for link-local
   autoconfiguration in IPv4 space) are often associated with specific,
   well-known behavioral patterns.  Detection of these patterns in
   anonymised data can lead to deanonymisation of these special-use
   addresses, which increases the chance of a complete reversal of
   anonymisation by an attacker, especially of prefix-preserving
   techniques.



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8.  Examples

   In this example, consider the export or storage of an anonymised IPv4
   dataset from a single network described by a simple template
   containing a timestamp in seconds, a five-tuple, and packet and octet
   counters.  The template describing each record in this dataset is
   shown in figure Figure 5.

                            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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Set ID = 2           |          Length =  40         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Template ID = 256        |        Field Count = 8        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| flowStartSeconds        150 |       Field Length =  4       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| sourceIPv4Address         8 |       Field Length =  4       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| destinationIPv4Address   12 |       Field Length =  4       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| sourceTransportPort       7 |       Field Length =  2       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| destinationTransportPort 11 |       Field Length =  2       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| packetDeltaCount          2 |       Field Length =  4       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| octetDeltaCount           1 |       Field Length =  4       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |0| protocolIdentifier        4 |       Field Length =  1       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 5: Example Flow Template

   Suppose that this dataset is anonymised according to the following
   policy:

   o  IP addresses within the network are protected by reverse
      truncation.

   o  IP addresses outside the network are protected by prefix-
      preserving anonymisation.

   o  Octet counts are exported using degraded precision in order to
      provide minimal protection against fingerprinting attacks.

   o  All other fields are exported unanonymised.




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   In order to export anonymisation records for this template and
   policy, first, the Anonymisation Options Template shown in figure
   Figure 6 is exported.  For this example, the optional
   privateEnterpriseNumber and informationElementIndex Information
   Elements are omitted, because they are not used.

                              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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |          Set ID = 3           |          Length =  26         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |      Template ID = 257        |        Field Count = 4        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |    Scope Field Count = 2      |0| templateID              346 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Field Length = 2        |0| informationElementId    303 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Field Length = 2        |0| anonymisationFlags      339 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Field Length = 2        |0| anonymisationTechnique  344 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       Field Length = 2        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 6: Example Anonymisation Options Template

   Following the Anonymisation Options Template comes a Data Set
   containing Anonymisation Records.  This data set has an entry for
   each Information Element Specifier in Template 256 describing the
   flow records.  This Data Set is shown in figure Figure 7.  Note that
   sourceIPv4Address and destinationIPv4Address have the Perimeter
   Anonymisation (0x0004) flag set in anonymisationFlags, meaning that
   source address should be treated as network-external, and the
   destination address as network-internal.

















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                            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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Set ID = 257         |          Length =  68         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | flowStartSeconds       IE 150 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | no flags               0x0000 | Not Anonymised              1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | sourceIPv4Address        IE 8 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Perimeter, Session SC 0x0005  | Structured Permutation      6 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | destinationIPv4Address  IE 12 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Perimeter, Stable     0x0005  | Reverse Truncation          7 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | sourceTransportPort      IE 7 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | no flags               0x0000 | Not Anonymised              1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | dest.TransportPort      IE 11 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | no flags               0x0000 | Not Anonymised              1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | packetDeltaCount         IE 2 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | no flags               0x0000 | Not Anonymised              1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | octetDeltaCount          IE 1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Stable                 0x0003 | Precision Degradation       2 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Template 256         | protocolIdentifier      IE 4  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | no flags               0x0000 | Not Anonymised              1 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 7: Example Anonymisation Records

   Following the Anonymisation Records come the data sets containing the
   anonymised data, exported according to the template in figure
   Figure 5


9.  Security Considerations

   This document provides guidelines for exporting metadata about



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   anonymised data in IPFIX, or storing metadata about anonymised data
   in IPFIX Files.  It is not intended as a general statement on the
   applicability of specific flow data anonymisation techniques.
   Exporters or publishers of anonymised data must take care that the
   applied anonymisation technique is appropriate for the data source,
   the purpose, and the risk of deanonymisation of a given application.

   We note specifically that anonymisation is not a replacement for
   encryption for confidentiality.  It is only appropriate for
   protecting identifying information in data to be used for purposes in
   which the protected data is irrelevant.  Confidentiality in export is
   best served by using TLS or DTLS as in the Security Considerations
   section of [RFC5101], and in long-term storage by implementation-
   specific protection applied as in the Security Considerations section
   of [RFC5655].  Indeed, confidentiality and anonymisation are not
   mutually exclusive, as encryption for confidentiality may be applied
   to anonymised data export or storage, as well, when the anonymised
   data is not intended for public release.

   When using pseudonymisation techniques that have a mutable mapping,
   there is an inherent tradeoff in the stability of the map between
   long-term comparability and security of the dataset against
   deanonymisation.  In general, deanonymisation attacks are more
   effective given more information, so the longer a given mapping is
   valid, the more information can be applied to deanonymisation.  The
   specific details of this are technique-dependent and therefore out of
   the scope of this document.

   When releasing anonymised data, publishers need to ensure that data
   that could be used in deanonymisation is not leaked through the
   export protocol; guidelines for addressing this risk are provided in
   Section 7.2.

   Note as well that the Security Considerations section of [RFC5101]
   applies as well to the export of anonymised data, and the Security
   Considerations section of [RFC5655] to the storage of anonymised
   data, or the publication of anonymised traces.


10.  IANA Considerations

   This document specifies the creation of several new IPFIX Information
   Elements in the IPFIX Information Element registry located at
   http://www.iana.org/assignments/ipfix, as defined in Section 6.2
   above.  IANA has assigned the following Information Element numbers
   for their respective Information Elements as specified below:





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   o  Information Element number TBD1 for the anonymisationFlags
      Information Element.

   o  Information Element number TBD2 for the anonymisationTechnique
      Information Element.

   o  Information Element number TBD3 for the informationElementIndex
      Information Element.

   [NOTE for IANA: The text TBDn should be replaced with the respective
   assigned Information Element numbers where they appear in this
   document.]


11.  Acknowledgments

   We thank Paul Aitken and John McHugh for their comments and insight,
   and Carsten Schmoll for his review.  Special thanks to the ICT-PRISM
   project for its material support of this work.


12.  References

12.1.  Normative References

   [RFC5101]  Claise, B., "Specification of the IP Flow Information
              Export (IPFIX) Protocol for the Exchange of IP Traffic
              Flow Information", RFC 5101, January 2008.

   [RFC5102]  Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
              Meyer, "Information Model for IP Flow Information Export",
              RFC 5102, January 2008.

   [RFC5610]  Boschi, E., Trammell, B., Mark, L., and T. Zseby,
              "Exporting Type Information for IP Flow Information Export
              (IPFIX) Information Elements", RFC 5610, July 2009.

   [RFC5655]  Trammell, B., Boschi, E., Mark, L., Zseby, T., and A.
              Wagner, "Specification of the IP Flow Information Export
              (IPFIX) File Format", RFC 5655, October 2009.

   [RFC3330]  IANA, "Special-Use IPv4 Addresses", RFC 3330,
              September 2002.

12.2.  Informative References

   [RFC5103]  Trammell, B. and E. Boschi, "Bidirectional Flow Export
              Using IP Flow Information Export (IPFIX)", RFC 5103,



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              January 2008.

   [RFC5472]  Zseby, T., Boschi, E., Brownlee, N., and B. Claise, "IP
              Flow Information Export (IPFIX) Applicability", RFC 5472,
              March 2009.

   [RFC5470]  Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
              "Architecture for IP Flow Information Export", RFC 5470,
              March 2009.

   [I-D.ietf-ipfix-mediators-framework]
              Kobayashi, A., Claise, B., and K. Ishibashi, "IPFIX
              Mediation: Framework",
              draft-ietf-ipfix-mediators-framework-04 (work in
              progress), October 2009.

   [I-D.ietf-ipfix-mediators-problem-statement]
              Kobayashi, A., Claise, B., Nishida, H., Sommer, C.,
              Dressler, F., and E. Stephan, "IPFIX Mediation: Problem
              Statement",
              draft-ietf-ipfix-mediators-problem-statement-07 (work in
              progress), December 2009.

   [RFC5153]  Boschi, E., Mark, L., Quittek, J., Stiemerling, M., and P.
              Aitken, "IP Flow Information Export (IPFIX) Implementation
              Guidelines", RFC 5153, April 2008.

   [RFC3917]  Quittek, J., Zseby, T., Claise, B., and S. Zander,
              "Requirements for IP Flow Information Export (IPFIX)",
              RFC 3917, October 2004.

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


Authors' Addresses

   Elisa Boschi
   Hitachi Europe
   c/o ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Phone: +41 44 632 70 57
   Email: elisa.boschi@hitachi-eu.com





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   Brian Trammell
   Hitachi Europe
   c/o ETH Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Phone: +41 44 632 70 13
   Email: brian.trammell@hitachi-eu.com










































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