IPFIX Working Group E. Boschi
Internet-Draft B. Trammell
Intended status: Experimental Hitachi Europe
Expires: October 1, 2009 March 30, 2009
IP Flow Anonymisation Support
draft-boschi-ipfix-anon-03.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
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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 anonymization 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. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. IPFIX Protocol Overview . . . . . . . . . . . . . . . . . 5
2.2. IPFIX Documents Overview . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Categorisation of Anonymisation Techniques . . . . . . . . . . 6
5. Anonymisation of IP Flow Data . . . . . . . . . . . . . . . . 7
5.1. IP Address Anonymisation . . . . . . . . . . . . . . . . . 8
5.1.1. Truncation . . . . . . . . . . . . . . . . . . . . . . 9
5.1.2. Random Permutation . . . . . . . . . . . . . . . . . . 9
5.1.3. Prefix-preserving Pseudonymisation . . . . . . . . . . 9
5.2. Timestamp Anonymisation . . . . . . . . . . . . . . . . . 10
5.2.1. Precision Degradation . . . . . . . . . . . . . . . . 10
5.2.2. Enumeration . . . . . . . . . . . . . . . . . . . . . 11
5.2.3. Random Time Shifts . . . . . . . . . . . . . . . . . . 11
5.3. Counter Anonymisation . . . . . . . . . . . . . . . . . . 11
5.3.1. Precision Degradation . . . . . . . . . . . . . . . . 11
5.3.2. Binning . . . . . . . . . . . . . . . . . . . . . . . 12
5.3.3. Random Noise Addition . . . . . . . . . . . . . . . . 12
5.4. Anonymisation of Other Flow Fields . . . . . . . . . . . . 12
5.4.1. Binning . . . . . . . . . . . . . . . . . . . . . . . 13
5.4.2. Random Permutation . . . . . . . . . . . . . . . . . . 13
6. Parameters for the Description of Anonymisation Techniques . . 13
6.1. Stability . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Truncation Length . . . . . . . . . . . . . . . . . . . . 14
6.3. Bin Map . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.4. Permutation . . . . . . . . . . . . . . . . . . . . . . . 14
6.5. Shift Amount . . . . . . . . . . . . . . . . . . . . . . . 14
7. Anonymisation Export Support in IPFIX . . . . . . . . . . . . 15
7.1. Anonymisation Options Template . . . . . . . . . . . . . . 15
7.2. Recommended Information Elements for Anonymisation
Metadata . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2.1. anonymisationStability . . . . . . . . . . . . . . . . 16
7.2.2. anonymisationTechnique . . . . . . . . . . . . . . . . 17
7.2.3. informationElementIndex . . . . . . . . . . . . . . . 18
8. Applying Anonymisation Techniques to IPFIX Export and
Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
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8.1. Arrangement of Processes in IPFIX Anonymisation . . . . . 19
8.2. IPFIX-Specific Anonymisation Guidelines . . . . . . . . . 20
8.2.1. Appropriate Use of Information Elements for
Anonymised Data . . . . . . . . . . . . . . . . . . . 20
8.2.2. Anonymisation of Header Data . . . . . . . . . . . . . 20
8.2.3. Anonymisation of Options Data . . . . . . . . . . . . 21
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Open Issues
There is not yet a mechanism for exporting information about defined-
time anonymisation stability.
The terminology section is incomplete; we should decide which of the
terms introduced in this document are to be treated as terminology.
Between "classes" of techniques and "parameters", there may be
"properties" as well; for example, binning and timestamp
anonymisation may be "ordered" or not (x>y in real --> x>y in
anonymized). We should verify that we're splitting these up
correctly.
In parallel with this, the anonymisationTechnique values might be
useful as a bitfield, with properties and classes being represented
by some set of the bits in the field. We'll have to make sure that
the properties and classes are exhaustive, if we do this.
Both anonymisationStability and anonymisationTechnique might benefit
from the creation of IANA registries; HOWEVER, in this case, it would
be very important to ensure that such a registry contains only
classes and properties of anonymised data, not information about
specific algorithms.
Certain technique/IE combinaitons (e.g. structure-preserving
counters) don't make any sense; these should be noted in "IPFIX-
Specific Anonymisation Guidelines".
Guidelines should be provided for the evaluation of _new_ IEs added
to the IANA registry after the publication of this draft for their
anonymisation potential.
This document does not cover the anonymisation of sub-IP level
information, specifically MAC addresses. It should.
2. Introduction
The standardisation of an IP flow information export protocol
[RFC5101] and associated representations removes a technical barrier
to the sharing of IP flow data across organizational boundaries and
with network operations, security, and research communities for a
wide variety of purposes. However, with wider dissemination comes
greater risks to the privacy of the users of networks under
measurement, and to the security of those networks. While it is not
a complete solution to the issues posed by distribution of IP flow
information, anonymisation is an important tool for the protection of
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privacy within network measurement infrastructures.
This document presents a mechanism for representing anonymised data
within IPFIX and guidelines for using it. It begins with a
categorization of anonymisation techniques. It then describes
applicability of each technique to commonly anonymisable fields of IP
flow data, organized by information element data type and semantics
as in [RFC5102]; enumerates the parameters required by each of the
applicable anonymisation techniques; and provides guidelines for the
use of each of these techniques in accordance with best practices in
data protection. Finally, it specifies a mechanism for exporting
anonymised data and binding anonymisation metadata to templates using
IPFIX Options.
2.1. IPFIX Protocol Overview
In the IPFIX protocol, { type, length, value } tuples are expressed
in templates containing { type, length } pairs, specifying which {
value } fields are present in data records conforming to the
Template, giving great flexibility as to what data is transmitted.
Since Templates are sent very infrequently compared with Data
Records, this results in significant bandwidth savings. Various
different data formats may be transmitted simply by sending new
Templates specifying the { type, length } pairs for the new data
format. See [RFC5101] for more information.
The IPFIX information model [RFC5102] defines a large number of
standard Information Elements which provide the necessary { type }
information for Templates. The use of standard elements enables
interoperability among different vendors' implementations.
Additionally, non-standard enterprise-specific elements may be
defined for private use.
2.2. IPFIX Documents Overview
"Specification of the IPFIX Protocol for the Exchange of IP Traffic
Flow Information" [RFC5101] and its associated documents define the
IPFIX Protocol, which provides network engineers and administrators
with access to IP traffic flow information.
"Architecture for IP Flow Information Export"
[I-D.ietf-ipfix-architecture] 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
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congestion-aware transport protocol from an IPFIX Exporting Process
to an IPFIX Collecting Process.
"Information Model for IP Flow Information Export" [RFC5102]
describes the Information Elements used by IPFIX, including details
on Information Element naming, numbering, and data type encoding.
Finally, "IPFIX Applicability" [I-D.ietf-ipfix-as] 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"
[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.
3. Terminology
Terms used in this document that are defined in the Terminology
section of the IPFIX Protocol [RFC5101] document are to be
interpreted as defined there.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
4. 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
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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.
+------------------------+-----------------+------------------------+
| Recoverability / | Recoverable | Non-recoverable |
| Countability | | |
+------------------------+-----------------+------------------------+
| N < M | N.A. | Generalisation |
| N = M | Direct | One-way Direct |
| | Substitution | Substitution |
| N > M | Set | One-way Set |
| | Substitution | Substitution |
+------------------------+-----------------+------------------------+
5. 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
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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.
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.
5.1. 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
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host, most of the work on flow and trace data anonymisation has gone
into IP address anonymisation techniques. Indeed, the aim of most
attacks against anonymisation is to recover the map from anonymised
IP addresses to original IP addresses thereby identifying the
identified hosts. There is therefore a wide range of IP address
anonymisation schemes that fit into the following categories.
+------------------------------------+---------------------+
| Scheme | Action |
+------------------------------------+---------------------+
| Truncation | Generalisation |
| Random Permutation | Direct Substitution |
| Prefix-preserving Pseudonymisation | Direct Substitution |
+------------------------------------+---------------------+
5.1.1. 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.
5.1.2. 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.
5.1.3. Prefix-preserving Pseudonymisation
Prefix-preserving pseudonymisation is a direct substitution
technique, further restricted such that the structure of subnets is
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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.
5.2. 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.
+-----------------------+----------------------------+
| Scheme | Action |
+-----------------------+----------------------------+
| Precision Degradation | Generalisation |
| Enumeration | Direct or Set Substitution |
| Random Shifts | Direct Substitution |
+-----------------------+----------------------------+
5.2.1. 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 is referred to binning; though the
time scales are longer and applicability more restricted, this is in
principle the same operation.
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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.
5.2.2. 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).
5.2.3. 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.
5.3. 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.
+-----------------------+----------------------------+
| Scheme | Action |
+-----------------------+----------------------------+
| Precision Degradation | Generalisation |
| Binning | Generalisation |
| Random noise addition | Direct or Set Substitution |
+-----------------------+----------------------------+
5.3.1. 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.
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5.3.2. 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.
5.3.3. 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.
5.4. Anonymisation of Other Flow Fields
Other fields, particularly port numbers and protocol numbers, can be
used to partially identify the applications that generated the
traffic in a a given flow trace. This information can be used in
fingerprinting attacks, and may be of interest on its own (e.g., to
reveal that a certain application with suspected vulnerabilities is
running on a given network). These fields are generally anonymised
using one of two techniques.
+--------------------+---------------------+
| Scheme | Action |
+--------------------+---------------------+
| Binning | Generalisation |
| Random Permutation | Direct Substitution |
+--------------------+---------------------+
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5.4.1. Binning
Binning is a generalisation technique mapping a set of potentially
non-uniform ranges into a set of abritrarily 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.
5.4.2. Random Permutation
Random permutation is a direct substitution technique, replacing each
key 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 flow
key values without preserving information about the keys themselves.
6. 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.
6.1. 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
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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.
6.2. 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.
6.3. 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.
6.4. 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.
6.5. Shift Amount
Shifting requires an amount to shift each value by. Since the shift
amount can be used to deanonymize data protected by shifting, no
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information about the shift amount should be exported.
7. 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.
7.1. 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
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useful given the defined techniques.]
+-------------------------+-----------------------------------------+
| IE | Description |
+-------------------------+-----------------------------------------+
| templateId [scope] | The Template ID of the Template |
| | containing the Information Element |
| | described by this anonymisation record. |
| | This Information Element MUST be |
| | defined as a Scope Field. |
| informationElementId | The Information Element identifier of |
| [scope] | the Information Element described by |
| | this anonymisation record. This |
| | Information Element MUST be defined as |
| | a Scope Field. |
| informationElementIndex | The Information Element index of the |
| [scope] [optional] | instance of the Information Element |
| | described by this anonymisation record |
| | identified by the informationElementId |
| | within the Template. Optional; need |
| | only be present when describing |
| | Templates that have multiple instances |
| | of the same Information Element. This |
| | Information Element MUST be defined as |
| | a Scope Field if present. This |
| | Information Element is defined in |
| | Section 7.2, below. |
| anonymisationStability | The stability class of the anonymised |
| | data. MUST be present. This |
| | Information Element is defined in |
| | Section 7.2, below. |
| anonymisationTechnique | The technique used to anonymise the |
| | data. MUST be present. This |
| | Information Element is defined in |
| | Section 7.2, below. |
+-------------------------+-----------------------------------------+
7.2. Recommended Information Elements for Anonymisation Metadata
7.2.1. 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.
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+-------+-----------------------------------------------------------+
| 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
7.2.2. anonymisationTechnique
Description: A description of the anonymisation technique applied
to a referenced Information Element within a referenced Template.
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+-------+-----------------------------------------------------------+
| Value | Description |
+-------+-----------------------------------------------------------+
| 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. |
| 1 | None: the values exported are real. |
| 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. |
| 3 | Binning: the values exported are anonymised into bins. |
| 4 | Enumeration: the values exported are anonymised by |
| | enumeration. |
| 5 | Permutation: the values exported are anonymised by random |
| | permutation. |
| 6 | Prefixed Permutation: the values exported are anonymised |
| | by random permutation, preserving bit-level structure; |
| | this represents prefix-preserving IP address |
| | anonymisation. |
+-------+-----------------------------------------------------------+
Abstract Data Type: unsigned8
ElementId: TBD2
Status: Proposed
7.2.3. 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
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8. 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.
8.1. 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 1. Each of these locations has
specific considerations and applicability.
+--------------------+
| IPFIX File Storage |
+--------------------+
^
| (Anonymised after collection)
|
+=======================================+
| Collecting Process |
+=======================================+
^ ^
| (Anonymised at mediator) |
| |
+=============================+ |
| Mediator | |
+=============================+ |
^ |
| (Anonymised on initial export) |
| |
+=======================================+
| Exporting Process |
+=======================================+
Figure 1: Potential Anonymisation Locations
Anonymisation is generally performed before the wider dissemination
or repurposing of a flow data set, e.g., adapting operational
measurement data for research. Therefore, direct anonymisation of
flow data on initial export is only applicable in certain restricted
circumstances: when the Exporting Process is "publishing" data to a
Collecting Process directly, and the Exporting Process and Collecting
Process are operated by different entities. Note that certain
guidelines in Section 8.2.2 with respect to timestamp anonymisation
may not apply in this case, as the Collecting Process may be able to
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deduce certain timing information from the time at which each Message
is received.
A much more flexible arrangement is to anonymise data within a
Mediator [I-D.ietf-ipfix-mediators-framework]. Here, original data
is sent to a Mediator, which performs the anonymisation function and
re-exports the anonymised data. Such a Mediator could be located at
the administrative domain boundary of the initial Exporting Process
operator, exporting anonymised data to other consumers outside the
organisation. In this case, the original Exporter SHOULD use TLS as
specified in [RFC5101] to secure the channel to the Mediator, and the
Mediator should follow the guidelines in Section 8.2, to mitigate the
risk of original data disclosure.
When data is to be published as an anonymised data set in an IPFIX
File [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 8.2, especially as regards File-specific Options in
Section 8.2.3
Note that anonymisation may occur at more than one location within a
given collection infrastructure, to provide varying levels of
anonymisation reversal risk and utility for specific purposes.
8.2. IPFIX-Specific Anonymisation Guidelines
In implementing and deploying the anonymisation techniques described
in this document, care must 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.
8.2.1. Appropriate Use of Information Elements for Anonymised Data
[TODO: reiterate black-marker guidelines here]
[TODO: note that precision degradation SHOULD use appropriately-sized
fields]
8.2.2. 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
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ID
Export of IPFIX Messages containing anonymised timestamp data where
the original Export Time Message header has some relationship to the
anonymised timestamps SHOULD anonymise the Export Time header field
using an equivalent technique, if possible. Otherwise, relationships
between export and flow time could be used to partially or totally
reverse timestamp anonymisation.
The similarity in size between an Observation Domain ID and an IPv4
address (32 bits) may lead to a temptation to use an IPv4 interface
address on the Metering or Exporting Process as the Observation
Domain ID. If this address bears some relation to the IP addresses
in the flow data (e.g., shares a network prefix with internal
addresses) and the IP addresses in the flow data are anonymised in a
structure-preserving way, then the Observation Domain ID may be used
to break the IP address anonymisation. Use of an IPv4 interface
address on the Metering or Exporting Process as the Observation
Domain ID is NOT RECOMMENDED in this case.
8.2.3. 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] and the IPFIX File Format [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], 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
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[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 [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 [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.
9. Examples
[TODO: write this section.]
10. Security Considerations
[TODO: write this section.]
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11. IANA Considerations
This document contains no actions for IANA.
[EDITOR'S NOTE: creation of anonymisationStability and
anonymisationTechnique registries may change this.]
12. Acknowledgments
We thank Paul Aitken for his comments and insight, and the PRISM
project for its support of this work.
13. References
13.1. 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.
13.2. Informative References
[I-D.ietf-ipfix-as]
Zseby, T., "IPFIX Applicability", draft-ietf-ipfix-as-12
(work in progress), July 2007.
[I-D.ietf-ipfix-architecture]
Sadasivan, G., "Architecture for IP Flow Information
Export", draft-ietf-ipfix-architecture-12 (work in
progress), September 2006.
[I-D.ietf-ipfix-file]
Trammell, B., Boschi, E., Mark, L., Zseby, T., and A.
Wagner, "Specification of the IPFIX File Format",
draft-ietf-ipfix-file-03 (work in progress), October 2008.
[I-D.ietf-ipfix-mediators-framework]
Kobayashi, A., Nishida, H., and B. Claise, "IPFIX
Mediation: Framework",
draft-ietf-ipfix-mediators-framework-02 (work in
progress), February 2009.
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[RFC3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export (IPFIX)",
RFC 3917, October 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Authors' Addresses
Elisa Boschi
Hitachi Europe
c/o ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Phone: +41 44 632 70 57
Email: elisa.boschi@hitachi-eu.com
Brian Trammell
Hitachi Europe
c/o ETH Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Phone: +41 44 632 70 13
Email: brian.trammell@hitachi-eu.com
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