





Internet Engineering Task Force (IETF)                         E. Boschi
Request for Comments: 6235                                   B. Trammell
Category: Experimental                                        ETH Zurich
ISSN: 2070-1721                                                 May 2011


                     IP Flow Anonymization Support

Abstract

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6235.















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Copyright Notice

   Copyright (c) 2011 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
   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 Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
      1.1. IPFIX Protocol Overview ....................................4
      1.2. IPFIX Documents Overview ...................................5
      1.3. Anonymization within the IPFIX Architecture ................5
      1.4. Supporting Experimentation with Anonymization ..............6
   2. Terminology .....................................................6
   3. Categorization of Anonymization Techniques ......................7
   4. Anonymization of IP Flow Data ...................................8
      4.1. IP Address Anonymization ..................................10
           4.1.1. Truncation .........................................11
           4.1.2. Reverse Truncation .................................11
           4.1.3. Permutation ........................................11
           4.1.4. Prefix-Preserving Pseudonymization .................12
      4.2. MAC Address Anonymization .................................12
           4.2.1. Truncation .........................................13
           4.2.2. Reverse Truncation .................................13
           4.2.3. Permutation ........................................14
           4.2.4. Structured Pseudonymization ........................14
      4.3. Timestamp Anonymization ...................................15
           4.3.1. Precision Degradation ..............................15
           4.3.2. Enumeration ........................................16
           4.3.3. Random Shifts ......................................16
      4.4. Counter Anonymization .....................................16
           4.4.1. Precision Degradation ..............................17
           4.4.2. Binning ............................................17
           4.4.3. Random Noise Addition ..............................17
      4.5. Anonymization of Other Flow Fields ........................18
           4.5.1. Binning ............................................18
           4.5.2. Permutation ........................................18
   5. Parameters for the Description of Anonymization Techniques .....19
      5.1. Stability .................................................19



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      5.2. Truncation Length .........................................19
      5.3. Bin Map ...................................................20
      5.4. Permutation ...............................................20
      5.5. Shift Amount ..............................................20
   6. Anonymization Export Support in IPFIX ..........................20
      6.1. Anonymization Records and the Anonymization
           Options Template ..........................................21
      6.2. Recommended Information Elements for Anonymization
           Metadata ..................................................23
           6.2.1. informationElementIndex ............................23
           6.2.2. anonymizationTechnique .............................23
           6.2.3. anonymizationFlags .................................25
   7. Applying Anonymization Techniques to IPFIX Export and Storage ..27
      7.1. Arrangement of Processes in IPFIX Anonymization ...........28
      7.2. IPFIX-Specific Anonymization Guidelines ...................30
           7.2.1. Appropriate Use of Information Elements for
                  Anonymized Data ....................................30
           7.2.2. Export of Perimeter-Based Anonymization Policies ...31
           7.2.3. Anonymization of Header Data .......................32
           7.2.4. Anonymization of Options Data ......................32
           7.2.5. Special-Use Address Space Considerations ...........34
           7.2.6. Protecting Out-of-Band Configuration and
                  Management Data ....................................34
   8. Examples .......................................................34
   9. Security Considerations ........................................39
   10. IANA Considerations ...........................................41
   11. Acknowledgments ...............................................41
   12. References ....................................................41
      12.1. Normative References .....................................41
      12.2. Informative References ...................................42





















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1.  Introduction

   The standardization of an IP Flow Information Export (IPFIX) 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, anonymization (i.e., the deletion or transformation of
   information that is considered sensitive and that 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 anonymized data
   within IPFIX and guidelines for using it.  It is not intended as a
   general statement on the applicability of specific flow data
   anonymization techniques to specific situations or as a
   recommendation of any particular application of anonymization to flow
   data export.  Exporters or publishers of anonymized data must take
   care that the applied anonymization technique is appropriate for the
   data source, the purpose, and the risk of deanonymization of a given
   application.

   It begins with a categorization of anonymization techniques.  It then
   describes the applicability of each technique to commonly
   anonymizable 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 anonymization techniques; and
   provides guidelines for the use of each of these techniques in
   accordance with current best practices in data protection.  Finally,
   it specifies a mechanism for exporting anonymized data and binding
   anonymization metadata to Templates and Options 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.



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   The IPFIX information model [RFC5102] defines a large number of
   standard Information Elements (IEs) that 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.

1.2.  IPFIX Documents Overview

   "Specification of the IP Flow Information Export (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, "IP Flow Information Export (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 IP Flow Information Export
   (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 anonymization metadata.  The anonymization
   techniques described herein are equally applicable to the IPFIX
   protocol and data stored in IPFIX Files.

1.3.  Anonymization within the IPFIX Architecture

   According to [RFC5470], IPFIX Message anonymization is optionally
   performed as the final operation before handing the Message to the
   transport protocol for export.  While no provision is made in the



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   architecture for anonymization metadata as in Section 6, this
   arrangement does allow for the rewriting necessary for comprehensive
   anonymization of IPFIX export as in Section 7.  The development of
   the IPFIX Mediation [RFC6183] framework and the IPFIX File Format
   [RFC5655] expand upon this initial architectural allowance for
   anonymization by adding to the list of places that anonymization 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 for
   anonymization can be found in Section 7.1

1.4.  Supporting Experimentation with Anonymization

   The status of this document is Experimental, reflecting the
   experimental nature of anonymization export support.  Research on
   network trace anonymization techniques and attacks against them is
   ongoing.  Indeed, there is increasing evidence that anonymization
   applied to network trace or flow data on its own is insufficient for
   many data protection applications as in [Bur10].  Therefore, this
   document explicitly does not recommend any particular technique or
   implementation thereof.

   The intention of this document is to provide a common basis for
   interoperable exchange of anonymized data, furthering research in
   this area, both on anonymization techniques themselves as well as to
   the application of anonymized data to network measurement.  To that
   end, the classification in Section 3 and anonymization export support
   in Section 6 can be used to describe and export information even
   about data anonymized using techniques that are unacceptably weak for
   general application to production datasets on their own.

   While the specification herein is designed to be independent of the
   anonymization techniques applied and the implementation thereof, open
   research in this area may necessitate future updates to the
   specification.  Assuming the future successful application of this
   specification to anonymized data publication and exchange, it may be
   brought back to the IPFIX working group for further development and
   publication on the Standards Track.

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.  In addition, this document defines the
   following terms:




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   Anonymization Record:   A record, defined by the Anonymization
      Options Template in Section 6.1, that defines the properties of
      the anonymization applied to a single Information Element within a
      single Template or Options Template.

   Anonymized Data Record:   A Data Record within a Data Set containing
      at least one Information Element with anonymized values.  The
      Information Element(s) within the Template or Options Template
      describing this Data Record SHOULD have a corresponding
      Anonymization Record.

   Intermediate Anonymization Process:   An intermediate process that
      takes Data Records and transforms them into Anonymized Data
      Records.

   Note that there is an explicit difference in this document between a
   "Data Set" (which is defined as in [RFC5101]) and a "data set".  When
   in lower case, this term refers to any collection of data (usually,
   within the context of this document, flow or packet data) that may
   contain identifying information and is therefore subject to
   anonymization.

   Note also that when the term Template is used in this document,
   unless otherwise noted, it applies both to Templates and Options
   Templates as defined in [RFC5101].  Specifically, Anonymization
   Records may apply to both Templates and Options Templates.

   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.  Categorization of Anonymization Techniques

   Anonymization, as described by this document, is the modification of
   a dataset in order to protect the identity of the people or entities
   described by the dataset from disclosure.  With respect to network
   traffic data, anonymization 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.

   Anonymization may be broadly classified according to two properties:
   recoverability and countability.  All anonymization techniques map
   the real space of identifiers or values into a separate, anonymized
   space, according to some function.  A technique is said to be
   recoverable when the function used is invertible or can otherwise be
   reversed and a real identifier can be recovered from a given
   replacement identifier.  "Recoverability" as used within this



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   categorization does not refer to recoverability under attack; that
   is, techniques wherein the function used can only be reversed using
   additional information, such as an encryption key, or knowledge of
   injected traffic within the dataset, are not considered to be
   recoverable.

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

   If the dimensions of the anonymized 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 anonymized space is larger, such that each real value maps to a
   set of anonymized values, then the function is said to be a set
   substitution function.  Note that with set substitution functions,
   the sets of anonymized values are not necessarily disjoint.  Either
   direct or set substitution functions are said to be one-way if there
   exists no non-brute force method for recovering the real data point
   from an anonymized one in isolation (i.e., if the only way to recover
   the data point is to attack the anonymized data set as a whole, e.g.,
   through fingerprinting or data injection).

   This classification is summarized in the table below.

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

4.  Anonymization of IP Flow Data

   In anonymizing IP flow data as treated by this document, the goal is
   generally two-way address untraceability: to remove the ability to
   assert that endpoint X contacted endpoint Y at time T.  Address
   untraceability is important as IP addresses are the most suitable
   field in IP flow records to identify real-world entities.  Each IP
   address is associated with an interface on a network host and can



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   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 leads IP flow data anonymization to be
   concerned first and foremost with IP address anonymization.

   Any form of aggregation that combines flows from multiple endpoints
   into a single record (e.g., aggregation by subnetwork, aggregation
   removing addressing completely) may also provide address
   untraceability; however, anonymization by aggregation is out of scope
   for this document.  Additionally, of potential interest in this
   problem space but out of scope are anonymization techniques that are
   applied over multiple fields or multiple records in a way that
   introduces dependencies among anonymized fields or records.  This
   document is concerned solely with anonymization techniques applied at
   the resolution of single fields within a flow record.

   Even so, attacks against these anonymization techniques use entire
   flows and relationships between hosts and flows within a given
   dataset.  Therefore, fields that may not necessarily be identifying
   by themselves may be anonymized in order to increase the anonymity of
   the dataset as a whole.

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

   As with IP addresses, Media Access Control (MAC) 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 MAC 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.  MAC
   addresses are also often used in constructing IPv6 addresses (see
   Section 2.5.1 of [RFC4291]) and as such may be used to reconstruct
   the low-order bits of anonymized IPv6 addresses in certain
   circumstances.  Therefore, MAC address anonymization 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





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   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.  Therefore, they can 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 anonymized dataset.

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

   While black-marker anonymization completely protects the data in the
   deleted fields from the risk of disclosure, it also reduces the
   utility of the anonymized dataset 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 Anonymization

   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 anonymization has gone
   into IP address anonymization techniques.  Indeed, the aim of most
   attacks against anonymization is to recover the map from anonymized
   IP addresses to original IP addresses thereby identifying the
   identified hosts.  Therefore, there is a wide range of IP address
   anonymization schemes that fit into the following categories.

       +------------------------------------+---------------------+
       | Scheme                             | Action              |
       +------------------------------------+---------------------+
       | Truncation                         | Generalization      |
       | Reverse Truncation                 | Generalization      |
       | Permutation                        | Direct Substitution |
       | Prefix-preserving Pseudonymization | Direct Substitution |
       +------------------------------------+---------------------+



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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 generalization
   scheme.  Note that while truncation is effective for making hosts
   non-identifiable, it preserves information that can be used to
   identify an organization, a geographic region, a country, or a
   continent.

   Truncation to an address length of 0 is equivalent to black-marker
   anonymization.  Complete removal of IP address information is only
   recommended for analysis tasks that 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.

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 generalization scheme.  Reverse truncation is effective
   for making networks unidentifiable, partially or completely removing
   information that can be used to identify an organization, a
   geographic region, a country, or a continent (or Regional Internet
   Registry (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 anonymization.

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, such that each anonymized 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.





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   There are many variations of permutation functions, each of which has
   trade-offs in performance, security, and guarantees of non-collision;
   evaluating these trade-offs is implementation independent.  However,
   in general, permutation functions applied to anonymization SHOULD be
   difficult to reverse without knowing the parameters (e.g., a secret
   key for Hashed Message Authentication Code (HMAC).  Given the
   relatively small space of IPv4 addresses in particular, hash
   functions applied without additional parameters could be reversed
   through brute force if the hash function is known, and SHOULD NOT be
   used as permutation functions.  Permutation functions may guarantee
   non-collision (i.e., that each anonymized address represents a unique
   original address), but need not; however, the probability of
   collision SHOULD be low.  Nevertheless, we treat even permutations
   with low but nonzero collision probability as a direct substitution.
   Beyond these guidelines, recommendations for specific permutation
   functions are out of scope for this document.

4.1.4.  Prefix-Preserving Pseudonymization

   Prefix-preserving pseudonymization is a direct substitution
   technique, like permutation but further restricted such that the
   structure of subnets is preserved at each level while anonymizing IP
   addresses.  If two real IP addresses match on a prefix of "n" bits,
   the two anonymized 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 anonymized data that can be exploited in attacks against the
   anonymization 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.

4.2.  MAC Address Anonymization

   Flow data containing sub-IP information can also contain identifying
   information in the form of the hardware (MAC) address.  While MAC
   address information cannot be used to locate a node within a network,
   it can be used to directly and 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.

   MAC address information is not as structured as IP address
   information.  EUI-48 and EUI-64 MAC addresses contain an
   Organizational Unique Identifier (OUI) in the three most significant



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   bytes of the address; this OUI additionally contains bits noting
   whether the address is locally or globally administered.  Beyond
   this, there is no standard relationship among the OUIs assigned to a
   given vendor.

   Note that MAC address information also appears 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              |
           +-----------------------------+---------------------+
           | Truncation                  | Generalization      |
           | Reverse Truncation          | Generalization      |
           | Permutation                 | Direct Substitution |
           | Structured Pseudonymization | Direct Substitution |
           +-----------------------------+---------------------+

4.2.1.  Truncation

   Truncation removes "n" of the least significant bits from a MAC
   address, replacing them with zeroes.  In effect, it retains bits of
   OUI, which identifies the manufacturer, while removing the least
   significant bits identifying the particular device.  Truncation of 24
   bits of an EAP-48 or 40 bits of an EAP-64 address zeroes out the
   device identifier while retaining the OUI.

   Truncation is effective for making device manufacturers partially or
   completely identifiable within a dataset while deleting unique host
   identifiers; this can be used to retain and aggregate MAC-layer
   behavior by vendor.

   Truncation to an address length of 0 is equivalent to black-marker
   anonymization.

4.2.2.  Reverse Truncation

   Reverse truncation removes "n" of the most significant bits from a
   MAC address, replacing them with zeroes.  Reverse truncation is a
   non-reversible generalization 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.





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   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 of removing
   manufacturer information is not particularly well covered by the
   literature.

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

4.2.3.  Permutation

   Permutation is a direct substitution technique, replacing each MAC
   address with an address selected from the set of possible MAC
   addresses, such that each anonymized 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.

   There are many variations of permutation functions, each of which has
   trade-offs in performance, security, and guarantees of non-collision;
   evaluating these trade-offs is implementation independent.  However,
   in general, permutation functions applied to anonymization SHOULD be
   difficult to reverse without knowing the parameters (e.g., a secret
   key for HMAC).  While the EAP-48 space is larger than the IPv4
   address space, hash functions applied without additional parameters
   could be reversed through brute force if the hash function is known,
   and SHOULD NOT be used as permutation functions.  Permutation
   functions may guarantee non-collision (i.e., that each anonymized
   address represents a unique original address), but need not; however,
   the probability of collision SHOULD be low.  Nevertheless, we treat
   even permutations with low but nonzero collision probability as a
   direct substitution.  Beyond these guidelines, recommendations for
   specific permutation functions are out of scope for this document.

4.2.4.  Structured Pseudonymization

   Structured pseudonymization 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 anonymized data that can be exploited
   in attacks against the anonymization technique.





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4.3.  Timestamp Anonymization

   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 anonymization techniques or for user profiling,
   e.g., as in [Mur07].  Timestamps can be used in traffic injection
   attacks, which use known information about a set of traffic generated
   or otherwise known by an attacker to recover mappings of other
   anonymized fields, as well as to identify certain activity by
   response delay and size fingerprinting, which compares response sizes
   and inter-flow times in anonymized data to known values.  Note that
   these attacks have been shown to be relatively robust against
   timestamp anonymization techniques (see [Bur10]), so the techniques
   presented in this section are relatively weak and should be used with
   care.

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

4.3.1.  Precision Degradation

   Precision Degradation is a generalization technique that removes the
   most precise components of a timestamp, accounting for all events
   occurring in each given interval (e.g., one millisecond for
   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 regarding at which time the event occurred is
   preserved.  The anonymized data may not be generally useful for
   applications that 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 anonymization by
   their design.

   Also, note 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, in principle, this is the same operation.





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   Precision degradation to infinitely low precision is equivalent to
   black-marker anonymization.  Removal of timestamp information is only
   recommended for analysis tasks that 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 quality-of-service (QoS) applications or service-
   level agreement (SLA) validation).

   Note that enumeration is functionally equivalent to precision
   degradation in any environment into which traffic can be regularly
   injected to serve as a clock at the precision of the frequency of the
   injected flows.

4.3.3.  Random Shifts

   Random time shifts add a random offset to every timestamp within a
   dataset.  Therefore, this reversible substitution technique retains
   duration and inter-event interval information as well as the
   chronological order of flows.  Random time shifts are quite weak and
   relatively easy to reverse in the presence of external knowledge
   about traffic on the measured network.

4.4.  Counter Anonymization

   Counters (such as packet and octet volumes per flow) are subject to
   fingerprinting and injection attacks against anonymization or for
   user profiling as timestamps are.  Data sets with anonymized counters
   are useful only for analysis tasks for which relative or imprecise
   magnitudes of activity are useful.  Counter information can also be
   completely removed, but this is only recommended for analysis tasks
   that have no need to evaluate the removed counter, for example, for
   counting only unique occurrences of other flow keys.










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          +-----------------------+----------------------------+
          | Scheme                | Action                     |
          +-----------------------+----------------------------+
          | Precision Degradation | Generalization             |
          | Binning               | Generalization             |
          | 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 anonymization.

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, consider separating unopened TCP connections from
   potentially opened TCP connections.  Here, packet counters per flow
   would be binned into two bins, one for 1-2 packet flows, and one for
   flows with 3 or more packets.  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 anonymization.

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 anonymization.  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.  Anonymization 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 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 anonymized
   using one of two techniques.

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

4.5.1.  Binning

   Binning is a generalization 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 anonymization.  Removal of other flow key information is only
   recommended for analysis tasks that 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, such that each
   anonymized 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.

   While permutation ideally guarantees that each anonymized value
   represents a unique original value, such may require significant
   state in the Intermediate Anonymization Process.  Therefore,
   permutation may be implemented by hashing for performance reasons,



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   with hash functions that may have relatively small collision
   probabilities.  Such techniques are still essentially direct
   substitution techniques, despite the nonzero error probability.

5.  Parameters for the Description of Anonymization Techniques

   This section details the abstract parameters used to describe the
   anonymization techniques examined in the previous section, on a per-
   parameter basis.  These parameters and their export safety inform the
   design of the IPFIX anonymization metadata export specified in the
   following section.

5.1.  Stability

   A stable anonymization will always map a given value in the real
   space to a given value in the anonymized space, while an unstable
   anonymization will change this mapping over time; a completely
   unstable anonymization is essentially indistinguishable from black-
   marker anonymization.  Any given anonymization technique may be
   applied with a varying range of stability.  Stability is important
   for assessing the comparability of anonymized information in
   different datasets, or in the same dataset over different time
   periods.  In practice, an anonymization may also be stable for every
   dataset published by 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 dataset.

   If no information about stability is available, users of anonymized
   data MAY assume that the techniques used are stable across the entire
   dataset, but unstable across datasets.  Note that stability presents
   a risk-utility trade-off, as completely stable anonymization can be
   used for longer-term trend analysis tasks but also presents more risk
   of attack given the stable mapping.  Information about the stability
   of a mapping SHOULD be exported along with the anonymized data.

5.2.  Truncation Length

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

   Truncation length can generally be inferred from a given dataset, and
   need not be specially exported or protected.  For bit-level
   truncation, the truncated bits are generally inferable by the least
   significant bit set for an instance of an Information Element
   described by a given Template (or the most significant bit set, in
   the case of reverse truncation).  For precision degradation, the
   truncation is inferable from the maximum precision given.  Note that



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   while this inference method is generally applicable, it is data
   dependent: there is no guarantee that it will recover the exact
   truncation length used to prepare the data.

   In the special case of IP address export with variable (per-record)
   truncation, the truncation MAY be expressed by exporting the prefix
   length alongside the address.

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 the bin map for a bin mapping function is in essence the bin
   mapping key, and can be used to partially deanonymize binned data,
   depending on the degree of generalization, information about the bin
   mapping function SHOULD NOT be exported.

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 anonymized space.  Unlike binning, each point
   in the anonymized space corresponds to a single, unique point in the
   original space.

   Since the parameters of the permutation function are in essence key-
   like (indeed, for cryptographic permutation functions, they are the
   keys themselves), information about the permutation function or its
   parameters SHOULD NOT be exported.

5.5.  Shift Amount

   Shifting requires an amount by which to shift each value.  Since the
   shift amount is the only key to a shift function, and can be used to
   trivially deanonymize data protected by shifting, information about
   the shift amount SHOULD NOT be exported.

6.  Anonymization Export Support in IPFIX

   Anonymized data exported via IPFIX SHOULD be annotated with
   anonymization metadata, which details which fields described by which
   Templates are anonymized, and provides appropriate information on the
   anonymization techniques used.  This metadata SHOULD be exported in



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   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 anonymized using the black-marker (removal)
   technique do not require any special metadata support: black-marker
   anonymized fields SHOULD NOT be exported at all, by omitting the
   corresponding Information Elements from Template describing the Data
   Set.  In the case where application requirements dictate that a
   black-marker anonymized field must remain in a Template, then an
   Exporting Process MAY export black-marker anonymized fields with
   their native length as all-zeros, but only in cases where enough
   contextual information exists within the record to differentiate a
   black-marker anonymized field exported in this way from a real zero
   value.

6.1.  Anonymization Records and the Anonymization Options Template

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

   Anonymization Records contain ancillary information bound to a
   Template, so many of the considerations for Templates apply to
   Anonymization Records as well.  First, reliability is important: an
   Exporting Process SHOULD export Anonymization Records after the
   Templates they describe have been exported, and SHOULD export
   anonymization records reliably if supported by the underlying
   transport (i.e., without partial reliability when using Stream
   Control Transmission Protocol (SCTP)).

   Anonymization Records MUST be handled by Collecting Processes as
   scoped to the Template to which they apply within the Transport
   Session in which they are sent.  When a Template is withdrawn via a
   Template Withdrawal Message or expires during a UDP transport
   session, the accompanying Anonymization Records are withdrawn or
   expire as well and do not apply to subsequent Templates with the same
   Template ID within the Session unless re-exported.

   The Stability Class within the anonymizationFlags IE can be used to
   declare that a given anonymization technique's mapping will remain
   stable across multiple sessions, but this does not mean that



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   anonymization technique information given in the Anonymization
   Records themselves persist across Sessions.  Each new Transport
   Session MUST contain new Anonymization Records for each Template
   describing anonymized Data Sets.

   SCTP per-stream export [IPFIX-PERSTREAM] may be used to ease
   management of Anonymization Records if appropriate for the
   application.

   The fields of the Anonymization Options Template are as follows:

   +-------------------------+-----------------------------------------+
   | IE                      | Description                             |
   +-------------------------+-----------------------------------------+
   | templateId [scope]      | The Template ID of the Template or      |
   |                         | Options Template containing the         |
   |                         | Information Element described by this   |
   |                         | anonymization record.  This Information |
   |                         | Element MUST be defined as a Scope      |
   |                         | Field.                                  |
   | informationElementId    | The Information Element identifier of   |
   | [scope]                 | the Information Element described by    |
   |                         | this anonymization record.  This        |
   |                         | Information Element MUST be defined as  |
   |                         | a Scope Field.  Exporting Processes     |
   |                         | MUST clear then Enterprise bit of the   |
   |                         | informationElementId and Collecting     |
   |                         | Processes SHOULD ignore it; information |
   |                         | about enterprise-specific Information   |
   |                         | Elements is exported via the            |
   |                         | privateEnterpriseNumber Information     |
   |                         | Element.                                |
   | privateEnterpriseNumber | The Private Enterprise Number of the    |
   | [scope] [optional]      | enterprise-specific Information Element |
   |                         | described by this anonymization record. |
   |                         | This Information Element MUST be        |
   |                         | defined as a Scope Field if present.  A |
   |                         | privateEnterpriseNumber of 0 signifies  |
   |                         | that the Information Element is         |
   |                         | IANA-registered.                        |
   | informationElementIndex | The Information Element index of the    |
   | [scope] [optional]      | instance of the Information Element     |
   |                         | described by this anonymization 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  |



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   |                         | Information Element MUST be defined as  |
   |                         | a Scope Field if present.  This         |
   |                         | Information Element is defined in       |
   |                         | Section 6.2.                            |
   | anonymizationFlags      | Flags describing the mapping stability  |
   |                         | and specialized modifications to the    |
   |                         | Anonymization Technique in use.  SHOULD |
   |                         | be present.  This Information Element   |
   |                         | is defined in Section 6.2.3.            |
   | anonymizationTechnique  | The technique used to anonymize the     |
   |                         | data.  MUST be present.  This           |
   |                         | Information Element is defined in       |
   |                         | Section 6.2.2.                          |
   +-------------------------+-----------------------------------------+

6.2.  Recommended Information Elements for Anonymization 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

   Data Type Semantics:   identifier

   ElementId:   287

   Status:   Current

6.2.2.  anonymizationTechnique

   Description:   A description of the anonymization 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 Information 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 or not the        |                 |             |
   |       | defined field is          |                 |             |
   |       | anonymized.  While the    |                 |             |
   |       | Collecting Process MAY    |                 |             |
   |       | assume that the field is  |                 |             |
   |       | not anonymized, it is not |                 |             |
   |       | guaranteed not to be.     |                 |             |
   |       | This is the default       |                 |             |
   |       | anonymization technique.  |                 |             |
   | 1     | None: the values exported | all             | all         |
   |       | are real.                 |                 |             |
   | 2     | Precision                 | all             | all         |
   |       | Degradation/Truncation:   |                 |             |
   |       | the values exported are   |                 |             |
   |       | anonymized 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 anonymized   |                 |             |
   |       | into bins.                |                 |             |
   | 4     | Enumeration: the values   | all             | timestamps  |
   |       | exported are anonymized   |                 |             |
   |       | by enumeration.           |                 |             |
   | 5     | Permutation: the values   | all             | identifiers |
   |       | exported are anonymized   |                 |             |
   |       | by permutation.           |                 |             |
   | 6     | Structured Permutation:   | addresses       |             |
   |       | the values exported are   |                 |             |
   |       | anonymized by             |                 |             |
   |       | permutation, preserving   |                 |             |
   |       | bit-level structure as    |                 |             |
   |       | appropriate; this         |                 |             |
   |       | represents                |                 |             |
   |       | prefix-preserving IP      |                 |             |
   |       | address anonymization or  |                 |             |



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   |       | structured MAC address    |                 |             |
   |       | anonymization.            |                 |             |
   | 7     | Reverse Truncation: the   | addresses       |             |
   |       | values exported are       |                 |             |
   |       | anonymized 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 anonymized   |                 |             |
   |       | by adding random noise to |                 |             |
   |       | each value.               |                 |             |
   | 9     | Offset: the values        | all             | timestamps  |
   |       | exported are anonymized   |                 |             |
   |       | by adding a single offset |                 |             |
   |       | to all values.            |                 |             |
   +-------+---------------------------+-----------------+-------------+

   Abstract Data Type:   unsigned16

   Data Type Semantics:   identifier

   ElementId:   286

   Status:   Current

6.2.3.  anonymizationFlags

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

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

                            anonymizationFlags IE








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   +--------+----------+-----------------------------------------------+
   | bit(s) | name     | description                                   |
   | (LSB = |          |                                               |
   | 0)     |          |                                               |
   +--------+----------+-----------------------------------------------+
   | 0-1    | SC       | Stability Class: see the Stability Class      |
   |        |          | table below, and Section 5.1.                 |
   | 2      | PmA      | Perimeter Anonymization: when set (1), source |
   |        |          | Information Elements as described in          |
   |        |          | [RFC5103] are interpreted as external         |
   |        |          | addresses, and destination Information        |
   |        |          | Elements as described in [RFC5103] are        |
   |        |          | interpreted as internal addresses, for the    |
   |        |          | purposes of associating                       |
   |        |          | anonymizationTechnique to Information         |
   |        |          | Elements only; see Section 7.2.2 for details. |
   |        |          | This bit 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 anonymized Information  |
   |        |          | Element contain real data.  This modification |
   |        |          | is intended for the anonymization of          |
   |        |          | network-level addresses while leaving         |
   |        |          | host-level addresses intact in order to       |
   |        |          | preserve host level-structure, which could    |
   |        |          | otherwise be used to reverse anonymization.   |
   |        |          | MUST NOT be set when associated with a        |
   |        |          | truncation-based anonymizationTechnique.      |
   | 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 anonymization technique applied to a
      referenced Information Element within a referenced Template.
      Stability classes refer to the stability of the parameters of the
      anonymization technique, and therefore the comparability of the
      mapping between the real and anonymized values over time.  This
      determines which anonymized 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 anonymization 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 endeavor to ensure at least this       |
   |     |     | stability class.                                      |
   | 1   | 0   | Exporter-Collector Pair: the Exporting Process will   |
   |     |     | ensure that the parameters of the anonymization       |
   |     |     | 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 are not comparable.                         |
   | 1   | 1   | Stable: the Exporting Process will ensure that the    |
   |     |     | parameters of the anonymization technique are stable  |
   |     |     | across Transport Sessions over time, regardless of    |
   |     |     | the Collecting Process to which it is sent.           |
   +-----+-----+-------------------------------------------------------+

   Abstract Data Type:   unsigned16

   Data Type Semantics:   flags

   ElementId:   285

   Status:   Current

7.  Applying Anonymization Techniques to IPFIX Export and Storage

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




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7.1.  Arrangement of Processes in IPFIX Anonymization

   Anonymization may be applied to IPFIX data at three stages within the
   collection infrastructure: on initial export, at a mediator, or after
   collection, as shown in Figure 1.  Each of these locations has
   specific considerations and applicability.

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

                Figure 1: Potential Anonymization Locations

   Anonymization is generally performed before the wider dissemination
   or repurposing of a dataset, e.g., adapting operational measurement
   data for research.  Therefore, direct anonymization of flow data on
   initial export is only applicable in certain restricted
   circumstances: when the Exporting Process (EP) is "publishing" data
   to a Collecting Process (CP) 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
   anonymization 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 anonymize data within a
   Mediator [RFC6183].  Here, original data is sent to a Mediator, which
   performs the anonymization function and re-exports the anonymized
   data.  Such a Mediator could be located at the administrative domain
   boundary of the initial Exporting Process operator, exporting



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   anonymized data to other consumers outside the organization.  In this
   case, the original Exporter SHOULD use TLS [RFC5246] 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 risk of
   original data disclosure.

   When data is to be published as an anonymized dataset in an IPFIX
   File [RFC5655], the anonymization 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 anonymization of records is
   undertaken by an Intermediate Anonymization Process (IAP); the data
   flows into and out of this IAP are shown in Figure 2 below.

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

          Figure 2: Data Flows through the Anonymization Process

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

   Following each of the data flows through the IAP, we describe five
   basic types of anonymization arrangements within this framework in
   Figure 3.  In addition to the three arrangements described in detail
   above, anonymization can also be done at a collocated Metering





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   Process (MP) and File Writer (FW) (see Section 7.3.2 of [RFC5655]),
   or at a file manipulator, which combines a File Writer with a File
   Reader (FR) (see Section 7.3.7 of [RFC5655]).

         +----+  +-----+  +----+
 pkts -> | MP |->| IAP |->| EP |-> Anonymization on Original Exporter
         +----+  +-----+  +----+
         +----+  +-----+  +----+
 pkts -> | MP |->| IAP |->| FW |-> Anonymizing collocated MP/File Writer
         +----+  +-----+  +----+
         +----+  +-----+  +----+
IPFIX -> | CP |->| IAP |->| EP |-> Anonymizing Mediator (Masq. Proxy)
         +----+  +-----+  +----+
         +----+  +-----+  +----+
IPFIX -> | CP |->| IAP |->| FW |-> Anonymizing collocated CP/File Writer
         +----+  +-----+  +----+
         +----+  +-----+  +----+
IPFIX -> | FR |->| IAP |->| FW |-> Anonymizing file manipulator
 File    +----+  +-----+  +----+

        Figure 3: Possible Anonymization Arrangements in the IPFIX
                               Architecture

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

7.2.  IPFIX-Specific Anonymization Guidelines

   In implementing and deploying the anonymization techniques described
   in this document, implementors should note that IPFIX already
   provides features that support anonymized 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 anonymization 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 anonymization techniques is noted in a separate
   subsection below.

7.2.1.  Appropriate Use of Information Elements for Anonymized Data

   Note, as in Section 6 above, that black-marker anonymized 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.




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

   When exporting anonymized data and anonymization metadata, Exporting
   Processes SHOULD ensure that the combination of Information Element
   and declared anonymization technique are compatible.  Specifically,
   the applicable and recommended Information Element types and
   semantics for each technique are noted in the description of the
   anonymizationTechnique Information Element in Section 6.2.2.  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 Anonymization Options records
   binding techniques to Information Elements to which they are not
   applicable, and SHOULD NOT export Anonymization Options records
   binding techniques to Information Elements for which they are not
   recommended.

7.2.2.  Export of Perimeter-Based Anonymization Policies

   Data collected from a single network may require different
   anonymization 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 anonymized 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
   anonymization policies with unidirectional flow (uniflow), or non-
   perimeter biflow data.  In this case, the Perimeter Anonymization bit
   (bit 2) in the anonymizationFlags Information Element describing the
   anonymized 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 anonymization policies.





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7.2.3.  Anonymization 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
   anonymization techniques: the Export Time, and the Observation Domain
   ID.

   Export of IPFIX Messages containing anonymized timestamp data where
   the original Export Time Message header has some relationship to the
   anonymized timestamps SHOULD anonymize the Export Time header field
   so that the Export Time is consistent with the anonymized timestamp
   data.  Otherwise, relationships between export and flow time could be
   used to partially or totally reverse timestamp anonymization.  When
   anonymizing timestamps and the Export Time header field SHOULD avoid
   times too far in the past or future; while [RFC5101] does not make
   any allowance for Export Time error detection, it is sensible that
   Collecting Processes may interpret Messages with seemingly
   nonsensical Export Times as erroneous.  Specific limits are
   implementation dependent, but this issue may cause interoperability
   issues when anonymizing the Export Time header field.

   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 anonymized in a
   structure-preserving way, then the Observation Domain ID may be used
   to break the IP address anonymization.  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.  Anonymization 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 anonymization techniques.  When using these Options along
   anonymized data export and storage, values within the Options that
   could be used to break the anonymization SHOULD themselves be
   anonymized or omitted.

   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



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   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 anonymized in a structure-
   preserving way, then the Exporting Process address may be used to
   break the IP address anonymization.  Exporting Processes exporting
   anonymized 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 anonymizing the
   Exporting Process address using a similar technique to that used to
   anonymize 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
   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 dataset (e.g., shares a network prefix with internal addresses)
   and the IP addresses in the dataset are anonymized in a structure-
   preserving way, then the Exporting Process or Collecting Process
   address may be used to break the IP address anonymization.  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 anonymized 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 dataset contains anonymized timestamp information,
   and the collectionTimeMilliseconds Information Element in a given
   Message has some relationship to the anonymized timestamp
   information, then this relationship can be exploited to reverse the
   timestamp anonymization.  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
   anonymized 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 dataset to
   provide partial table of contents information for an IPFIX File,
   Options described by this Template SHOULD be written using the
   anonymized timestamps instead of the original ones.




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7.2.5.  Special-Use Address Space Considerations

   When anonymizing data for transport or storage using IPFIX containing
   anonymized IP addresses, and the analysis purpose permits doing so,
   it is RECOMMENDED to filter out or leave unanonymized data containing
   the special-use IPv4 addresses enumerated in [RFC5735] or the
   special-use IPv6 addresses enumerated in [RFC5156].  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
   anonymized data can lead to deanonymization of these special-use
   addresses, which increases the chance of a complete reversal of
   anonymization by an attacker, especially of prefix-preserving
   techniques.

7.2.6.  Protecting Out-of-Band Configuration and Management Data

   Special care should be taken when exporting or sharing anonymized
   data to avoid information leakage via the configuration or management
   planes of the IPFIX Device containing the Exporting Process or the
   File Writer.  For example, adding noise to counters is useless if the
   receiver can deduce the values in the counters from Simple Network
   Management Protocol (SNMP) information, and concealing the network
   under test is similarly useless if such information is available in a
   configuration document.  As the specifics of these concerns are
   largely implementation and deployment dependent, specific mitigation
   is out of scope for this document.  The general ground rule is that
   information of similar type to that anonymized SHOULD NOT be made
   available to the receiver by any means, whether in the Data Records,
   in IPFIX protocol structures such as Message Headers, or out of band.

8.  Examples

   In this example, consider the export or storage of an anonymized 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 Data Set is
   shown in Figure 4.













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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 = 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 4: Example Flow Template

   Suppose that this Data Set is anonymized 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 anonymization.

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

   o  All other fields are exported unanonymized.

   In order to export Anonymization Records for this Template and
   policy, first, the Anonymization Options Template shown in Figure 5
   is exported.  For this example, the optional privateEnterpriseNumber
   and informationElementIndex Information Elements are omitted, because
   they are not used.






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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 = 3           |          Length =  26         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Template ID = 257        |        Field Count = 4        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Scope Field Count = 2      |0| templateID              145 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Field Length = 2        |0| informationElementId    303 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Field Length = 2        |0| anonymizationFlags      285 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Field Length = 2        |0| anonymizationTechnique  286 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Field Length = 2        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: Example Anonymization Options Template

   Following the Anonymization Options Template comes a Data Set
   containing Anonymization Records.  This dataset has an entry for each
   Information Element Specifier in Template 256 describing the flow
   records.  This Data Set is shown in Figure 6.  Note that
   sourceIPv4Address and destinationIPv4Address have the Perimeter
   Anonymization (0x0004) flag set in anonymizationFlags, 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 Anonymized              1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | sourceIPv4Address        IE 8 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Perimeter, Session SC  0x0005 | Structured Permutation      6 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | destinationIPv4Address  IE 12 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Perimeter, Stable      0x0007 | Reverse Truncation          7 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | sourceTransportPort      IE 7 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | no flags               0x0000 | Not Anonymized              1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | dest.TransportPort      IE 11 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | no flags               0x0000 | Not Anonymized              1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | packetDeltaCount         IE 2 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | no flags               0x0000 | Not Anonymized              1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | octetDeltaCount          IE 1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Stable                 0x0003 | Precision Degradation       2 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Template 256         | protocolIdentifier      IE 4  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | no flags               0x0000 | Not Anonymized              1 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 6: Example Anonymization Records

   Following the Anonymization Records come the Data Sets containing the
   anonymized data, exported according to the Template in Figure 4.
   Bringing it all together, consider an IPFIX Message containing three
   real data records and the necessary templates to export them, shown
   in Figure 7.  (Note that the scale of this message is 8-bytes per
   line, for compactness; lines of dots '. . . . . ' represent shifting
   of the example bit structure for clarity.)




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             1         2         3         4         5         6
   0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | 0x000a        | length 135    | export time 1271227717        | msg
  | sequence 0                    | domain 1                      | hdr
  | SetID 2       | length 40     | tid 256       | fields 8      | tmpl
  | IE 150        | length 4      | IE 8          | length 4      | set
  | IE 12         | length 4      | IE 7          | length 2      |
  | IE 11         | length 2      | IE 2          | length 4      |
  | IE 1          | length 4      | IE 4          | length 1      |
  | SetID 256     | length 79     | time 1271227681               | data
  | sip 192.0.2.3                 | dip 198.51.100.7              | set
  | sp 53         | dp 53         | packets 1                     |
  | bytes 74                      | prt 17  | . . . . . . . . . . .
  | time 1271227682               | sip 198.51.100.7              |
  | dip 192.0.2.88                | sp 5091       | dp 80         |
  | packets 60                    | bytes 2896                    |
  | prt 6   | . . . . . . . . . . . . . . . . . . . . . . . . . . .
  | time 1271227683               | sip 198.51.100.7              |
  | dip 203.0.113.9               | sp 5092       | dp 80         |
  | packets 44                    | bytes 2037                    |
  | prt 6   |
  +---------+

                      Figure 7: Example Real Message

   The corresponding anonymized message is then shown in Figure 8.  The
   Options Template Set describing Anonymization Records and the
   Anonymization Records themselves are added; IP addresses and byte
   counts are anonymized as declared.





















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             1         2         3         4         5         6
   0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | 0x000a        | length 233    | export time 1271227717        | msg
  | sequence 0                    | domain 1                      | hdr
  | SetID 2       | length 40     | tid 256       | fields 8      | tmpl
  | IE 150        | length 4      | IE 8          | length 4      | set
  | IE 12         | length 4      | IE 7          | length 2      |
  | IE 11         | length 2      | IE 2          | length 4      |
  | IE 1          | length 4      | IE 4          | length 1      |
  | SetID 3       | length 30     | tid 257       | fields 4      | opt
  | scope 2       | . . . . . . . . . . . . . . . . . . . . . . . . tmpl
  | IE 145        | length 2      | IE 303        | length 2      | set
  | IE 285        | length 2      | IE 286        | length 2      |
  | SetID 257     | length 68     | . . . . . . . . . . . . . . . . anon
  | tid 256       | IE 150        | flags 0       | tech 1        | recs
  | tid 256       | IE 8          | flags 5       | tech 6        |
  | tid 256       | IE 12         | flags 7       | tech 7        |
  | tid 256       | IE 7          | flags 0       | tech 1        |
  | tid 256       | IE 11         | flags 0       | tech 1        |
  | tid 256       | IE 2          | flags 0       | tech 1        |
  | tid 256       | IE 1          | flags 3       | tech 2        |
  | tid 256       | IE41          | flags 0       | tech 1        |
  | SetID 256     | length 79     | time 1271227681               | data
  | sip 254.202.119.209           | dip 0.0.0.7                   | set
  | sp 53         | dp 53         | packets 1                     |
  | bytes 100                     | prt 17  | . . . . . . . . . . .
  | time 1271227682               | sip 0.0.0.7                   |
  | dip 254.202.119.6             | sp 5091       | dp 80         |
  | packets 60                    | bytes 2900                    |
  | prt 6   | . . . . . . . . . . . . . . . . . . . . . . . . . . .
  | time 1271227683               | sip 0.0.0.7                   |
  | dip 2.19.199.176              | sp 5092       | dp 80         |
  | packets 60                    | bytes 2000                    |
  | prt 6   |
  +---------+

                Figure 8: Corresponding Anonymized Message

9.  Security Considerations

   This document provides guidelines for exporting metadata about
   anonymized data in IPFIX, or storing metadata about anonymized data
   in IPFIX Files.  It is not intended as a general statement on the
   applicability of specific flow data anonymization techniques.
   Exporters or publishers of anonymized data must take care that the
   applied anonymization technique is appropriate for the data source,
   the purpose, and the risk of deanonymization of a given application.



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   Research in anonymization techniques, and techniques for
   deanonymization, is ongoing, and currently "safe" anonymization
   techniques may be rendered unsafe by future developments.

   We note specifically that anonymization 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 [RFC5246] or Datagram Transport Layer
   Security (DTLS) [RFC4347] 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 anonymization are not
   mutually exclusive, as encryption for confidentiality may be applied
   to anonymized data export or storage, as well, when the anonymized
   data is not intended for public release.

   We note as well that care should be taken even with well-anonymized
   data, and anonymized data should still be treated as privacy
   sensitive.  Anonymization reduces the risk of misuse, but is not a
   complete solution to the problem of protecting end-user privacy in
   network flow trace analysis.

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

   When releasing anonymized data, publishers need to ensure that data
   that could be used in deanonymization is not leaked through a side
   channel.  The entire workflow (hardware, software, operational
   policies and procedures, etc.) for handling anonymized data must be
   evaluated for risk of data leakage.  While most of these possible
   side channels are out of scope for this document, guidelines for
   reducing the risk of information leakage specific to the IPFIX export
   protocol are provided in Section 7.2.

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






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10.  IANA Considerations

   This document specifies the creation of several new IPFIX Information
   Elements in the IPFIX Information Element registry available from the
   IANA site (http://www.iana.org), as defined in Section 6.2.  IANA has
   assigned the following Information Element numbers for their
   respective Information Elements as specified below:

   o  Information Element number 285 for the anonymizationFlags
      Information Element.

   o  Information Element number 286 for the anonymizationTechnique
      Information Element.

   o  Information Element number 287 for the informationElementIndex
      Information Element.

11.  Acknowledgments

   We thank Paul Aitken and John McHugh for their comments and insight,
   and Carsten Schmoll, Benoit Claise, Lothar Braun, Dan Romascanu,
   Stewart Bryant, and Sean Turner for their reviews.  Special thanks to
   the FP7 PRISM and DEMONS projects for their 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.

   [RFC5103]  Trammell, B. and E. Boschi, "Bidirectional Flow Export
              Using IP Flow Information Export (IPFIX)", RFC 5103,
              January 2008.

   [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.

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



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   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

   [RFC5156]  Blanchet, M., "Special-Use IPv6 Addresses", RFC 5156,
              April 2008.

12.2.  Informative References

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

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

   [RFC6183]  Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi,
              "IP Flow Information Export (IPFIX) Mediation: Framework",
              RFC 6183, April 2011.

   [IPFIX-PERSTREAM]
              Claise, B., Aitken, P., Johnson, A., and G. Muenz, "IPFIX
              Export per SCTP Stream", Work in Progress, May 2010.

   [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.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [Bur10]    Burkhart, M., Schatzmann, D., Trammell, B., and E. Boschi,
              "The Role of Network Trace Anonymization Under Attack",
               ACM Computer Communications Review, vol. 40, no. 1, pp.
              6-11, January 2010.






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   [Mur07]    Murdoch, S. and P. Zielinski, "Sampled Traffic Analysis by
              Internet-Exchange-Level Adversaries", Proceedings of the
              7th Workshop on Privacy Enhancing Technologies, Ottawa,
              Canada, June 2007.

Authors' Addresses

   Elisa Boschi
   Swiss Federal Institute of Technology Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   EMail: boschie@tik.ee.ethz.ch


   Brian Trammell
   Swiss Federal Institute of Technology Zurich
   Gloriastrasse 35
   8092 Zurich
   Switzerland

   Phone: +41 44 632 70 13
   EMail: trammell@tik.ee.ethz.ch



























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