IP Flow Anonymisation Support
draft-ietf-ipfix-anon-00.txt

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Abstract

This document describes anonymisation techniques for IP flow data and the export of anonymised data using the IPFIX protocol. It provides a categorization of common anonymisation schemes and defines the parameters needed to describe them. It provides guidelines for the implementation of anonymised data export and storage over IPFIX, and describes an Options-based method for anonymisation metadata export within the IPFIX protocol, providing the basis for the definition of information models for configuring anonymisation techniques within an IPFIX Metering or Exporting Process, and for reporting the technique in use to an IPFIX Collecting Process.

Table of Contents

1.  Introduction
    1.1.  IPFIX Protocol Overview
    1.2.  IPFIX Documents Overview
    1.3.  Anonymisation within the IPFIX Architecture
2.  Terminology
3.  Categorisation of Anonymisation Techniques
4.  Anonymisation of IP Flow Data
    4.1.  IP Address Anonymisation
        4.1.1.  Truncation
        4.1.2.  Random Permutation
        4.1.3.  Prefix-preserving Pseudonymisation
    4.2.  Hardware Address Anonymisation
        4.2.1.  Random Permutation
        4.2.2.  Structured Pseudonymisation
    4.3.  Timestamp Anonymisation
        4.3.1.  Precision Degradation
        4.3.2.  Enumeration
        4.3.3.  Random Time Shifts
    4.4.  Counter Anonymisation
        4.4.1.  Precision Degradation
        4.4.2.  Binning
        4.4.3.  Random Noise Addition
    4.5.  Anonymisation of Other Flow Fields
        4.5.1.  Binning
        4.5.2.  Random Permutation
5.  Parameters for the Description of Anonymisation Techniques
    5.1.  Stability
    5.2.  Truncation Length
    5.3.  Bin Map
    5.4.  Permutation
    5.5.  Shift Amount
6.  Anonymisation Export Support in IPFIX
    6.1.  Anonymisation Options Template
    6.2.  Recommended Information Elements for Anonymisation Metadata
        6.2.1.  anonymisationStability
        6.2.2.  anonymisationTechnique
        6.2.3.  informationElementIndex
7.  Applying Anonymisation Techniques to IPFIX Export and Storage
    7.1.  Arrangement of Processes in IPFIX Anonymisation
    7.2.  IPFIX-Specific Anonymisation Guidelines
        7.2.1.  Appropriate Use of Information Elements for Anonymised Data
        7.2.2.  Anonymisation of Header Data
        7.2.3.  Anonymisation of Options Data
8.  Examples
9.  Security Considerations
10.  IANA Considerations
11.  Acknowledgments
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses

1.  Introduction

The standardisation of an IP flow information export protocol [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) and associated representations removes a technical barrier to the sharing of IP flow data across organizational boundaries and with network operations, security, and research communities for a wide variety of purposes. However, with wider dissemination comes greater risks to the privacy of the users of networks under measurement, and to the security of those networks. While it is not a complete solution to the issues posed by distribution of IP flow information, anonymisation (i.e., the deletion or transformation of information that is considered sensitive and could be used to reveal the identity of subjects involved in a communication) is an important tool for the protection of privacy within network measurement infrastructures.

This document presents a mechanism for representing anonymised data within IPFIX and guidelines for using it. It begins with a categorization of anonymisation techniques. It then describes applicability of each technique to commonly anonymisable fields of IP flow data, organized by information element data type and semantics as in [RFC5102] (Quittek, J., Bryant, S., Claise, B., Aitken, P., and J. Meyer, “Information Model for IP Flow Information Export,” January 2008.); enumerates the parameters required by each of the applicable anonymisation techniques; and provides guidelines for the use of each of these techniques in accordance with best practices in data protection. Finally, it specifies a mechanism for exporting anonymised data and binding anonymisation metadata to templates using IPFIX Options.

1.1.  IPFIX Protocol Overview

In the IPFIX protocol, { type, length, value } tuples are expressed in templates containing { type, length } pairs, specifying which { value } fields are present in data records conforming to the Template, giving great flexibility as to what data is transmitted. Since Templates are sent very infrequently compared with Data Records, this results in significant bandwidth savings. Various different data formats may be transmitted simply by sending new Templates specifying the { type, length } pairs for the new data format. See [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) for more information.

The IPFIX information model (Quittek, J., Bryant, S., Claise, B., Aitken, P., and J. Meyer, “Information Model for IP Flow Information Export,” January 2008.) [RFC5102] defines a large number of standard Information Elements which provide the necessary { type } information for Templates. The use of standard elements enables interoperability among different vendors' implementations. Additionally, non-standard enterprise-specific elements may be defined for private use.

1.2.  IPFIX Documents Overview

"Specification of the IPFIX Protocol for the Exchange of IP Traffic Flow Information" (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) [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" (Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek, “Architecture for IP Flow Information Export,” March 2009.) [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" (Quittek, J., Zseby, T., Claise, B., and S. Zander, “Requirements for IP Flow Information Export (IPFIX),” October 2004.) [RFC3917]. The IPFIX Protocol document [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) 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" (Quittek, J., Bryant, S., Claise, B., Aitken, P., and J. Meyer, “Information Model for IP Flow Information Export,” January 2008.) [RFC5102] describes the Information Elements used by IPFIX, including details on Information Element naming, numbering, and data type encoding. Finally, "IPFIX Applicability" (Zseby, T., Boschi, E., Brownlee, N., and B. Claise, “IP Flow Information Export (IPFIX) Applicability,” March 2009.) [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, the "Specification of the IPFIX File Format" (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file] describes a file format based upon the IPFIX Protocol for the storage of flow data.

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

1.3.  Anonymisation within the IPFIX Architecture

"Architecture for IP Flow Information Export" (Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek, “Architecture for IP Flow Information Export,” March 2009.) [RFC5470] defines the functions performed in sequence by the various functional blocks in an IPFIX Device as in the figure below.

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

   (*) indicates that the block is optional.

Note that, according to the original architecture specification, IPFIX Message anonymisation is optionally performed as the final operation before handing the Message to the transport protocol for export. While no provision is made in the architecture for anonymisation metadata as in Section 6 (Anonymisation Export Support in IPFIX), this arrangement does allow for the message rewriting necessary for comprehensive anonymisation of IPFIX export as in Section 7 (Applying Anonymisation Techniques to IPFIX Export and Storage). The development of the IPFIX Mediation (Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi, “IPFIX Mediation: Framework,” April 2010.) [I‑D.ietf‑ipfix‑mediators‑framework] framework and the IPFIX File Format (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file] expand upon this initial architectural allowance for anonymisation by adding to the list of places that anonymisation may be applied. The former specifies IPFIX Mediators, which rewrite existing IPFIX messages, and the latter specifies a method for storage of IPFIX data in files.

More detail on the applicable architectural arrangements of anonymisation can be found in Section 7.1 (Arrangement of Processes in IPFIX Anonymisation)

2.  Terminology

Terms used in this document that are defined in the Terminology section of the IPFIX Protocol (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) [RFC5101] document are to be interpreted as defined there.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

3.  Categorisation of Anonymisation Techniques

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

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

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

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

This classification is summarised in the table below.

4.  Anonymisation of IP Flow Data

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

While anonymisation is generally applied at the resolution of single fields within a flow record, attacks against anonymisation use entire flows and relationships between hosts and flows within a given data set. Therefore, fields which may not necessarily be identifying by themselves may be anonymised in order to increase the anonymity of the data set as a whole.

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

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

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

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

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

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

4.1.  IP Address Anonymisation

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

4.1.1.  Truncation

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

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

4.1.2.  Random Permutation

Random permutation is a direct substitution technique, replacing each IP address with an address randomly selected from the set of possible IP addresses, guaranteeing that each anonymised address represents a unique original address. The random 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 randomly permuted IP addresses.

4.1.3.  Prefix-preserving Pseudonymisation

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

4.2.  Hardware Address Anonymisation

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

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

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

4.2.1.  Random Permutation

Random permutation is a direct substitution technique, replacing each IP address with an address randomly selected from the set of possible IP addresses, guaranteeing that each anonymised address represents a unique original address. The random 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 randomly permuted IP addresses.

4.2.2.  Structured Pseudonymisation

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

4.3.  Timestamp Anonymisation

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

4.3.1.  Precision Degradation

Precision Degradation is a generalisation technique that removes the most precise components of a timestamp, accounting all events occurring in each given interval (e.g. one millisecond for millisecond level degradation) as simultaneous. This has the effect of potentially collapsing many timestamps into one. With this technique time precision is reduced, and sequencing may be lost, but the information at which time the event occurred is preserved. The anonymised data may not be generally useful for applications which require strict sequencing of flows.

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

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

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

4.3.2.  Enumeration

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

4.3.3.  Random Time Shifts

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

4.4.  Counter Anonymisation

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

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.

4.4.2.  Binning

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

Binning counters to a single bin 0-infinity, or alternately precision degradation to infinitely low precision, is equivalent to black-marker anonymisation. Removal of counter information is only recommended for analysis tasks which have no need to evaluate the removed counter, for example for counting only unique occurrences of other flow keys.

4.4.3.  Random Noise Addition

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

4.5.  Anonymisation of Other Flow Fields

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

4.5.1.  Binning

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

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

4.5.2.  Random Permutation

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

5.  Parameters for the Description of Anonymisation Techniques

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

5.1.  Stability

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

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

5.2.  Truncation Length

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

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

5.3.  Bin Map

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

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

5.4.  Permutation

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

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

5.5.  Shift Amount

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

6.  Anonymisation Export Support in IPFIX

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

Note that fields anonymised using the black-marker (removal) technique do not require any special metadata support. Black-marker anonymised fields SHOULD NOT be exported at all; the absence of the field in a given Data Set is implicitly declared by not including the corresponding Information Element in the Template describing that Data Set; exporting "empty" data elements is inefficient and in the general case impossible, as many non-counter Information Elements do not have semantically distinct null values.

6.1.  Anonymisation Options Template

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

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

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

[EDITOR'S NOTE: Multiple anon. techniques applied on an IE at the same time is indicated with multiple elements of the same type (in application order as in PSAMP). Need to verify this is actually useful given the defined techniques.]

6.2.  Recommended Information Elements for Anonymisation Metadata

6.2.1.  anonymisationStability

Description:

A description of the stability class of the anonymisation technique applied to a referenced Information Element within a referenced Template. Stability classes refer to the stability of the parameters of the anonymisation technique, and therefore the comparability of the mapping between the real and anonymised values over time. This determines which anonymised datasets may be compared with each other.

Value	Description
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. This is equivalent to 0x01 Session level stability while advising the Collecting Process that no special effort has been made to ensure stability. Collecting Processes SHOULD assume this is the case in the absence of stability class information; this is the default stability class.
1	Session: the Exporting Process will ensure that the parameters of the anonymisation technique are stable during the Transport Session. All the values of the described Information Element for each Record described by the referenced Template within the Transport Session are comparable. The Exporting Process SHOULD endeavour to ensure at least this stability class.
2	Exporter-Collector Pair: the Exporting Process will ensure that the parameters of the anonymisation technique are stable across Transport Sessions over time with the given Collecting Process, but may use different parameters for different Collecting Processes. Data exported to different Collecting Processes is not comparable.
3	Stable: the Exporting Process will ensure that the parameters of the anonymisation technique are stable across Transport Sessions over time, regardless of the Collecting Process to which it is sent.

Abstract Data Type:

unsigned8

ElementId:

TBD1

Status:

Proposed

6.2.2.  anonymisationTechnique

Description:

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

Value	Description	Applicable to	Recommended for
0	Undefined: the Exporting Process makes no representation as to whether the defined field is anonymised or not. While the Collecting Process MAY assume that the field is not anonymised, it is not guaranteed not to be. This is the default anonymisation technique.	all	all
1	None: the values exported are real.	all	all
2	Precision Degradation/Truncation: the values exported are anonymised using simple precision degradation or truncation. The new precision is implicit in the exported data, and can be deduced by the Collecting Process.	all	all
3	Binning: the values exported are anonymised into bins.	all	all
4	Enumeration: the values exported are anonymised by enumeration.	all	timestamps
5	Permutation: the values exported are anonymised by random permutation.	all	identifiers
6	Structured Permutation: the values exported are anonymised by random permutation, preserving bit-level structure as appropriate; this represents prefix-preserving IP address anonymisation or structured MAC address anonymisation.	addresses

Abstract Data Type:

unsigned8

ElementId:

TBD2

Status:

Proposed

6.2.3.  informationElementIndex

Description:

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

Abstract Data Type:

unsigned16

ElementId:

TBD3

Status:

Proposed

7.  Applying Anonymisation Techniques to IPFIX Export and Storage

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

7.1.  Arrangement of Processes in IPFIX Anonymisation

Anonymisation may be applied to IPFIX data at three stages within a the collection infrastructure: on initial export, at a mediator, or after collection, as shown in Figure 2 (Potential Anonymisation Locations). Each of these locations has specific considerations and applicability.

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

Anonymisation is generally performed before the wider dissemination or repurposing of a flow data set, e.g., adapting operational measurement data for research. Therefore, direct anonymisation of flow data on initial export is only applicable in certain restricted circumstances: when the Exporting Process is "publishing" data to a Collecting Process directly, and the Exporting Process and Collecting Process are operated by different entities. Note that certain guidelines in Section 7.2.2 (Anonymisation of Header Data) with respect to timestamp anonymisation may not apply in this case, as the Collecting Process may be able to deduce certain timing information from the time at which each Message is received.

A much more flexible arrangement is to anonymise data within a Mediator (Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi, “IPFIX Mediation: Framework,” April 2010.) [I‑D.ietf‑ipfix‑mediators‑framework]. Here, original data is sent to a Mediator, which performs the anonymisation function and re-exports the anonymised data. Such a Mediator could be located at the administrative domain boundary of the initial Exporting Process operator, exporting anonymised data to other consumers outside the organisation. In this case, the original Exporter SHOULD use TLS as specified in [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) to secure the channel to the Mediator, and the Mediator should follow the guidelines in Section 7.2 (IPFIX-Specific Anonymisation Guidelines), to mitigate the risk of original data disclosure.

When data is to be published as an anonymised data set in an IPFIX File (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file], the anonymisation may be done at the final Collecting Process before storage and dissemination, as well. In this case, the Collector should follow the guidelines in Section 7.2 (IPFIX-Specific Anonymisation Guidelines), especially as regards File-specific Options in Section 7.2.3 (Anonymisation of Options Data)

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

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

Anonymisation parameters must also be available to the Exporting Process and/or File Writer in order to ensure header data is also appropriately anonymised as in Section 7.2.2 (Anonymisation of Header Data).

Following each of the data flows through the IAP, we describe five basic types of anonymisation arrangements within this framework in Figure 4 (Possible anonymisation arrangements in the IPFIX architecture). In addition to the three arrangements described in detail above, anonymisation can also be done at a collocated Metering Process and File Writer (see section 7.3.2 of [I‑D.ietf‑ipfix‑file] (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.)), or at a file manipulator (see section 7.3.7 of [I‑D.ietf‑ipfix‑file] (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.)).

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

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

7.2.  IPFIX-Specific Anonymisation Guidelines

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

7.2.1.  Appropriate Use of Information Elements for Anonymised Data

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

When using precision degradation of timestamps, Exporting Processes SHOULD export timing information using Information Elements of an appropriate precision, as explained in Section 4.5 of [RFC5153] (Boschi, E., Mark, L., Quittek, J., Stiemerling, M., and P. Aitken, “IP Flow Information Export (IPFIX) Implementation Guidelines,” April 2008.). 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 anonymised data and anonymisation metadata, Exporting Processes SHOULD ensure that the combination of Information Element and declared anonymisation technique are compatible. Specifically, the applicable and recommended Information Element types and semantics for each technique are noted in the description of the anonymisationTechnique Information Element in Section 6.2.2 (anonymisationTechnique). In this description, a timestamp is an Information Element with the data type dateTimeSeconds, dataTimeMilliseconds, dateTimeMicroseconds, or dateTimeNanoseconds; an address is an Information Element with the data type ipv4Address, ipv6Address, or macAddress; and an identifier is an Information Element with identifier data type semantics. Exporting Process MUST NOT export Anonymisation Options records binding techniques to Information Elements to which they are not applicable, and SHOULD NOT export Anonymisation Options records binding techniques to Information Elements for which they are not recommended.

7.2.2.  Anonymisation of Header Data

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

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

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

7.2.3.  Anonymisation of Options Data

IPFIX uses the Options mechanism to export, among other things, metadata about exported flows and the flow collection infrastructure. As with the IPFIX Message Header, certain Options recommended in [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) and the IPFIX File Format (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file] containing flow timestamps and network addresses of Exporting and Collecting Processes may be used to break certain anonymisation techniques; care should be taken while using them with anonymised data export and storage.

The Exporting Process Reliability Statistics Options Template, recommended in [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.), contains an Exporting Process ID field, which may be an exportingProcessIPv4Address Information Element or an exportingProcessIPv6Address Information Element. If the Exporting Process address bears some relation to the IP addresses in the flow data (e.g., shares a network prefix with internal addresses) and the IP addresses in the flow data are anonymised in a structure-preserving way, then the Exporting Process address may be used to break the IP address anonymisation. Exporting Processes exporting anonymised data in this situation SHOULD mitigate the risk of attack either by omitting Options described by the Exporting Process Reliability Statistics Options Template, or by anonymising the Exporting Process address using a similar technique to that used to anonymise the IP addresses in the exported data.

Similarly, the Export Session Details Options Template and Message Details Options Template specified for the IPFIX File Format (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file] 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 flow data (e.g., shares a network prefix with internal addresses) and the IP addresses in the flow data are anonymised in a structure-preserving way, then the Exporting Process or Collecting Process address may be used to break the IP address anonymisation. Since these Options Templates are primarily intended for storing IPFIX Transport Session data for auditing, replay, and testing purposes, it is NOT RECOMMENDED that storage of anonymised data include these Options Templates in order to mitigate the risk of attack.

The Message Details Options Template specified for the IPFIX File Format (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file] also contains the collectionTimeMilliseconds Information Element. As with the Export Time Message Header field, if the exported flow data contains anonymised timestamp information, and the collectionTimeMilliseconds Information Element in a given Message has some relationship to the anonymised timestamp information, then this relationship can be exploited to reverse the timestamp anonymisation. Since this Options Template is primarily intended for storing IPFIX Transport Session data for auditing, replay, and testing purposes, it is NOT RECOMMENDED that storage of anonymised data include this Options Template in order to mitigate the risk of attack.

Since the Time Window Options Template specified for the IPFIX File Format (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) [I‑D.ietf‑ipfix‑file] refers to the timestamps within the flow data to provide partial table of contents information for an IPFIX File, care must be taken to ensure that Options described by this template are written using the anonymised timestamps instead of the original ones.

8.  Examples

[TODO: write this section.]

9.  Security Considerations

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

We note specifically that anonymisation is not a replacement for encryption for confidentiality. It is only appropriate for protecting identifying information in data to be used for purposes in which the protected data is irrelevant. Confidentiality in export is best served by using TLS or DTLS as in the Security Considerations section of [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.), and in long-term storage by implementation-specific protection applied as in the Security Considerations section of [I‑D.ietf‑ipfix‑file] (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.). Indeed, confidentiality and anonymisation are not mutually exclusive, as encryption for confidentiality may be applied to anonymised data export or storage, as well, when the anonymised data is not intended for public release.

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

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

Note as well that the Security Considerations section of [RFC5101] (Claise, B., “Specification of the IP Flow Information Export (IPFIX) Protocol for the Exchange of IP Traffic Flow Information,” January 2008.) applies as well to the export of anonymised data, and the Security Considerations section of [I‑D.ietf‑ipfix‑file] (Trammell, B., Boschi, E., Mark, L., Zseby, T., and A. Wagner, “Specification of the IPFIX File Format,” August 2009.) to the storage of anonymised data, or the publication of anonymised traces.

10.  IANA Considerations

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

• Information Element number TBD1 for the anonymisationStability Information Element.
• Information Element number TBD2 for the anonymisationTechnique Information Element.
• Information Element number TBD3 for the informationElementIndex Information Element.

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

11.  Acknowledgments

We thank Paul Aitken for his comments and insight, and the PRISM project for its support of this work.

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses