IPFIX Working Group E. Boschi
Internet-Draft B. Trammell
Intended status: Experimental ETH Zurich
Expires: April 11, 2011 October 8, 2010
IP Flow Anonymisation Support
draft-ietf-ipfix-anon-04.txt
Abstract
This document describes anonymisation techniques for IP flow data and
the export of anonymised data using the IPFIX protocol. It
categorizes common anonymisation schemes and defines the parameters
needed to describe them. It provides guidelines for the
implementation of anonymised data export and storage over IPFIX, and
describes an information model and Options-based method for
anonymisation metadata export within the IPFIX protocol or storage in
IPFIX Files.
Status of this Memo
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This Internet-Draft will expire on April 11, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. IPFIX Protocol Overview . . . . . . . . . . . . . . . . . 4
1.2. IPFIX Documents Overview . . . . . . . . . . . . . . . . . 5
1.3. Anonymisation within the IPFIX Architecture . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Categorisation of Anonymisation Techniques . . . . . . . . . . 6
4. Anonymisation of IP Flow Data . . . . . . . . . . . . . . . . 8
4.1. IP Address Anonymisation . . . . . . . . . . . . . . . . . 9
4.1.1. Truncation . . . . . . . . . . . . . . . . . . . . . . 9
4.1.2. Reverse Truncation . . . . . . . . . . . . . . . . . . 10
4.1.3. Permutation . . . . . . . . . . . . . . . . . . . . . 10
4.1.4. Prefix-preserving Pseudonymisation . . . . . . . . . . 11
4.2. MAC Address Anonymisation . . . . . . . . . . . . . . . . 11
4.2.1. Reverse Truncation . . . . . . . . . . . . . . . . . . 12
4.2.2. Permutation . . . . . . . . . . . . . . . . . . . . . 12
4.2.3. Structured Pseudonymisation . . . . . . . . . . . . . 13
4.3. Timestamp Anonymisation . . . . . . . . . . . . . . . . . 13
4.3.1. Precision Degradation . . . . . . . . . . . . . . . . 13
4.3.2. Enumeration . . . . . . . . . . . . . . . . . . . . . 14
4.3.3. Random Shifts . . . . . . . . . . . . . . . . . . . . 14
4.4. Counter Anonymisation . . . . . . . . . . . . . . . . . . 14
4.4.1. Precision Degradation . . . . . . . . . . . . . . . . 15
4.4.2. Binning . . . . . . . . . . . . . . . . . . . . . . . 15
4.4.3. Random Noise Addition . . . . . . . . . . . . . . . . 15
4.5. Anonymisation of Other Flow Fields . . . . . . . . . . . . 15
4.5.1. Binning . . . . . . . . . . . . . . . . . . . . . . . 16
4.5.2. Permutation . . . . . . . . . . . . . . . . . . . . . 16
5. Parameters for the Description of Anonymisation Techniques . . 16
5.1. Stability . . . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Truncation Length . . . . . . . . . . . . . . . . . . . . 17
5.3. Bin Map . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4. Permutation . . . . . . . . . . . . . . . . . . . . . . . 18
5.5. Shift Amount . . . . . . . . . . . . . . . . . . . . . . . 18
6. Anonymisation Export Support in IPFIX . . . . . . . . . . . . 18
6.1. Anonymisation Records and the Anonymisation Options
Template . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2. Recommended Information Elements for Anonymisation
Metadata . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2.1. informationElementIndex . . . . . . . . . . . . . . . 21
6.2.2. anonymisationTechnique . . . . . . . . . . . . . . . . 21
6.2.3. anonymisationFlags . . . . . . . . . . . . . . . . . . 23
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7. Applying Anonymisation Techniques to IPFIX Export and
Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.1. Arrangement of Processes in IPFIX Anonymisation . . . . . 26
7.2. IPFIX-Specific Anonymisation Guidelines . . . . . . . . . 28
7.2.1. Appropriate Use of Information Elements for
Anonymised Data . . . . . . . . . . . . . . . . . . . 28
7.2.2. Export of Perimeter-Based Anonymisation Policies . . . 29
7.2.3. Anonymisation of Header Data . . . . . . . . . . . . . 29
7.2.4. Anonymisation of Options Data . . . . . . . . . . . . 30
7.2.5. Special-Use Address Space Considerations . . . . . . . 31
7.2.6. Protecting Out-of-Band Configuration and
Management Data . . . . . . . . . . . . . . . . . . . 32
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . 37
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
12.1. Normative References . . . . . . . . . . . . . . . . . . . 39
12.2. Informative References . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
The standardisation of an IP flow information export protocol
[RFC5101] and associated representations removes a technical barrier
to the sharing of IP flow data across organizational boundaries and
with network operations, security, and research communities for a
wide variety of purposes. However, with wider dissemination comes
greater risks to the privacy of the users of networks under
measurement, and to the security of those networks. While it is not
a complete solution to the issues posed by distribution of IP flow
information, anonymisation (i.e., the deletion or transformation of
information that is considered sensitive and could be used to reveal
the identity of subjects involved in a communication) is an important
tool for the protection of privacy within network measurement
infrastructures.
This document presents a mechanism for representing anonymised data
within IPFIX and guidelines for using it. It begins with a
categorization of anonymisation techniques. It then describes
applicability of each technique to commonly anonymisable fields of IP
flow data, organized by information element data type and semantics
as in [RFC5102]; enumerates the parameters required by each of the
applicable anonymisation techniques; and provides guidelines for the
use of each of these techniques in accordance with best practices in
data protection. Finally, it specifies a mechanism for exporting
anonymised data and binding anonymisation metadata to Templates and
Options Templates using IPFIX Options.
1.1. IPFIX Protocol Overview
In the IPFIX protocol, { type, length, value } tuples are expressed
in Templates containing { type, length } pairs, specifying which {
value } fields are present in data records conforming to the
Template, giving great flexibility as to what data is transmitted.
Since Templates are sent very infrequently compared with Data
Records, this results in significant bandwidth savings. Various
different data formats may be transmitted simply by sending new
Templates specifying the { type, length } pairs for the new data
format. See [RFC5101] for more information.
The IPFIX information model [RFC5102] defines a large number of
standard Information Elements which provide the necessary { type }
information for Templates. The use of standard elements enables
interoperability among different vendors' implementations.
Additionally, non-standard enterprise-specific elements may be
defined for private use.
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1.2. IPFIX Documents Overview
"Specification of the IPFIX Protocol for the Exchange of IP Traffic
Flow Information" [RFC5101] and its associated documents define the
IPFIX Protocol, which provides network engineers and administrators
with access to IP traffic flow information.
"Architecture for IP Flow Information Export" [RFC5470] defines the
architecture for the export of measured IP flow information out of an
IPFIX Exporting Process to an IPFIX Collecting Process, and the basic
terminology used to describe the elements of this architecture, per
the requirements defined in "Requirements for IP Flow Information
Export" [RFC3917]. The IPFIX Protocol document [RFC5101] then covers
the details of the method for transporting IPFIX Data Records and
Templates via a congestion-aware transport protocol from an IPFIX
Exporting Process to an IPFIX Collecting Process.
"Information Model for IP Flow Information Export" [RFC5102]
describes the Information Elements used by IPFIX, including details
on Information Element naming, numbering, and data type encoding.
Finally, "IPFIX Applicability" [RFC5472] describes the various
applications of the IPFIX protocol and their use of information
exported via IPFIX, and relates the IPFIX architecture to other
measurement architectures and frameworks.
Additionally, "Specification of the IPFIX File Format" [RFC5655]
describes a file format based upon the IPFIX Protocol for the storage
of flow data.
This document references the Protocol and Architecture documents for
terminology, and extends the IPFIX Information Model to provide new
Information Elements for anonymisation metadata. The anonymisation
techniques described herein are equally applicable to the IPFIX
Protocol and data stored in IPFIX Files.
1.3. Anonymisation within the IPFIX Architecture
According to [RFC5470], IPFIX Message anonymisation is optionally
performed as the final operation before handing the Message to the
transport protocol for export. While no provision is made in the
architecture for anonymisation metadata as in Section 6, this
arrangement does allow for the rewriting necessary for comprehensive
anonymisation of IPFIX export as in Section 7. The development of
the IPFIX Mediation [I-D.ietf-ipfix-mediators-framework] framework
and the IPFIX File Format [RFC5655] expand upon this initial
architectural allowance for anonymisation by adding to the list of
places that anonymisation may be applied. The former specifies IPFIX
Mediators, which rewrite existing IPFIX Messages, and the latter
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specifies a method for storage of IPFIX data in files.
More detail on the applicable architectural arrangements of
anonymisation can be found in Section 7.1
2. Terminology
Terms used in this document that are defined in the Terminology
section of the IPFIX Protocol [RFC5101] document are to be
interpreted as defined there. In addition, this document defines the
following terms:
Anonymisation Record: A record, defined by the Anonymisation
Options Template in section Section 6.1, that defines the
properties of the anonymisation applied to a single Information
Element within a single Template or Options Template.
Anonymised Data Record: A Data Record within a Data Set containing
at least one Information Element with anonymised values. The
Information Element(s) within the Template or Options Template
describing this Data Record SHOULD have a corresponding
Anonymisation Record.
Intermediate Anonymisation Process: An intermediate process which
takes Data Records and and transforms them into Anonymised Data
Records.
Note that there is an explicit difference in this document between a
"Data Set" (which is defined as in [RFC5101]) and a "data set". When
in lower case, this term refers to any collection of data (usually,
within the context of this document, flow or packet data) which may
contain identifying information and is therefore subject to
anonymisation.
Note also that when the term Template is used in this document,
unless otherwise noted, it applies both to Templates and Options
Templates as defined in [RFC5101]. Specifically, Anonymisation
Records may apply to both Templates and Options Templates.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Categorisation of Anonymisation Techniques
Anonymisation modifies a data set in order to protect the identity of
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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 non-brute force method for recovering the real data point
from an anonymised one in isolation (i.e., if the only way to recover
the data point is to attack the anonymised data set as a whole, e.g.
through fingerprinting or data injection).
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 |
+------------------------+-----------------+------------------------+
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4. Anonymisation of IP Flow Data
Due to the restricted semantics of IP flow data, there is a
relatively limited set of specific anonymisation techniques available
on flow data, though each falls into the broad categories above.
Each type of field that may commonly appear in a flow record may have
its own applicable specific techniques.
While anonymisation is generally applied at the resolution of single
fields within a flow record, attacks against anonymisation use entire
flows and relationships between hosts and flows within a given data
set. Therefore, fields which may not necessarily be identifying by
themselves may be anonymised in order to increase the anonymity of
the data set as a whole.
Of all the fields in an IP flow record, IP addresses are the most
likely to be used to directly identify entities in the real world.
Each IP address is associated with an interface on a network host,
and can potentially be identified with a single user. Additionally,
IP addresses are structured identifiers; that is, partial IP address
prefixes may be used to identify networks just as full IP addresses
identify hosts. This makes anonymisation of IP addresses
particularly important.
MAC addresses uniquely identify devices on the network; while they
are not often available in traffic data collected at Layer 3, and
cannot be used to locate devices within the network, some traces may
contain sub-IP data including MAC address data. Hardware addresses
may be mappable to device serial numbers, and to the entities or
individuals who purchased the devices, when combined with external
databases. MAC addresses are also often used in constructing IPv6
addresses (see section 2.5.1 of [RFC4291]), and as such may be used
to reconstruct the low-order bits of anonymised IPv6 addresses in
certain circumstances. Therefore, MAC 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,
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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.
+------------------------------------+---------------------+
| Scheme | Action |
+------------------------------------+---------------------+
| Truncation | Generalisation |
| Reverse Truncation | Generalisation |
| Permutation | Direct Substitution |
| Prefix-preserving Pseudonymisation | Direct Substitution |
+------------------------------------+---------------------+
4.1.1. Truncation
Truncation removes "n" of the least significant bits from an IP
address, replacing them with zeroes. In effect, it replaces a host
address with a network address for some fixed netblock; for IPv4
addresses, 8-bit truncation corresponds to replacement with a /24
network address. Truncation is a non-reversible generalisation
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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.
Truncation to an address length of 0 is equivalent to black-marker
anonymisation. Complete removal of IP address information is only
recommended for analysis tasks which have no need to separate flow
data by host or network; e.g. as a first stage to per-application
(port) or time-series total volume analyses.
4.1.2. Reverse Truncation
Reverse truncation removes "n" of the most significant bits from an
IP address, replacing them with zeroes. Reverse truncation is a non-
reversible generalisation scheme. Reverse truncation is effective
for making networks unidentifiable, partially or completely removing
information which can be used to identify an organization, a
geographic region, a country, or a continent (or RIR region of
responsibility). However, it may cause ambiguity when applied to
data collected from more than one network, since it treats all the
hosts with the same address on different networks as if they are the
same host. It is not particularly useful when publishing data where
the network of origin is known or can be easily guessed by virtue of
the identity of the publisher.
Like truncation, reverse truncation to an address length of 0 is
equivalent to black-marker anonymisation.
4.1.3. Permutation
Permutation is a direct substitution technique, replacing each IP
address with an address selected from the set of possible IP
addresses, such that each anonymised address represents a unique
original address. The selection function is often random, though it
is not necessarily so. Permutation does not preserve any structural
information about a network, but it does preserve the unique count of
IP addresses. Any application that requires more structure than
host-uniqueness will not be able to use permuted IP addresses.
While permutation ideally guarantees that each anonymised address
represents a unique original address, such requires significant state
in the Intermediate Anonymisation Process. Therefore, permutation
may be implemented by hashing for performance reasons, with hash
functions that may have relatively small collision probabilities.
Such techniques are still essentially direct substitution techniques,
despite the nonzero error probability.
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4.1.4. Prefix-preserving Pseudonymisation
Prefix-preserving pseudonymisation is a direct substitution
technique, like permutation but further restricted such that the
structure of subnets is preserved at each level while anonymising IP
addresses. If two real IP addresses match on a prefix of "n" bits,
the two anonymised IP addresses will match on a prefix of "n" bits as
well. This is useful when relationships among networks must be
preserved for a given analysis task, but introduces structure into
the anonymised data which can be exploited in attacks against the
anonymisation technique.
Scanning in Internet background traffic can cause particular problems
with this technique: if a scanner uses a predictable and known
sequence of addresses, this information can be used to reverse the
substitution. The low order portion of the address can be left
unanonymized as a partial defense against this attack.
4.2. MAC Address Anonymisation
Flow data containing sub-IP information can also contain identifying
information in the form of the hardware (MAC) address. While MAC
address information cannot be used to locate a node within a network,
it can be used to directly uniquely identify a specific device.
Vendors or organizations within the supply chain may then have the
information necessary to identify the entity or individual that
purchased the device.
MAC address information is not as structured as IP address
information. EUI-48 and EUI-64 MAC addresses contain an
Organizational Unique Identifier (OUI) in the three most significant
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 MAC 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.
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+-----------------------------+---------------------+
| Scheme | Action |
+-----------------------------+---------------------+
| Reverse Truncation | Generalisation |
| Permutation | Direct Substitution |
| Structured Pseudonymisation | Direct Substitution |
+-----------------------------+---------------------+
4.2.1. Reverse Truncation
Reverse truncation removes "n" of the most significant bits from an
MAC address, replacing them with zeroes. Reverse truncation is a
non-reversible generalisation scheme. This has the effect of
removing bits of the OUI, which identify manufacturers, before
removing the least significant bits. Reverse truncation of 24 bits
zeroes out the OUI.
Reverse truncation is effective for making device manufacturers
partially or completely unidentifiable within a dataset. However, it
may cause ambiguity by introducing the possibility of truncated MAC
address collision. Also note that the utility or removing
manufacturer information is dubious, and not particularly well-
covered by the literature.
Reverse truncation to an address length of 0 is equivalent to black-
marker anonymisation.
4.2.2. Permutation
Permutation is a direct substitution technique, replacing each MAC
address with an address selected from the set of possible MAC
addresses, such that each anonymised address represents a unique
original address. The selection function is often random, though it
is not necessarily so. Permutation does not preserve any structural
information about a network, but it does preserve the unique count of
devices on the network. Any application that requires more structure
than host-uniqueness will not be able to use permuted MAC addresses.
While permutation ideally guarantees that each anonymised address
represents a unique original address, such requires significant state
in the Intermediate Anonymisation Process. Therefore, permutation
may be implemented by hashing for performance reasons, with hash
functions that may have relatively small collision probabilities.
Such techniques are still essentially direct substitution techniques,
despite the nonzero error probability.
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4.2.3. Structured Pseudonymisation
Structured pseudonymisation for MAC addresses is a direct
substitution technique, like permutation, but restricted such that
the OUI (the most significant three bytes) is permuted separately
from the node identifier, the remainder. This is useful when the
uniqueness of OUIs must be preserved for a given analysis task, but
introduces structure into the anonymised data which can be exploited
in attacks against the anonymisation technique.
4.3. Timestamp Anonymisation
The particular time at which a flow began or ended is not
particularly identifiable information, but it can be used as part of
attacks against other anonymisation techniques or for user profiling.
Precise timestamps can be used in injected-traffic fingerprinting
attacks, which use known information about a set of traffic generated
or otherwise known by an attacker to recover mappings of other
anonymised fields, as well as to identify certain activity by
response delay and size fingerprinting, which compares response sizes
and inter-flow times in anonymised data to known values. Therefore,
timestamp information may be anonymised in order to ensure the
protection of the entire data set.
+-----------------------+----------------------------+
| Scheme | Action |
+-----------------------+----------------------------+
| Precision Degradation | Generalisation |
| Enumeration | Direct or Set Substitution |
| Random Shifts | Direct Substitution |
+-----------------------+----------------------------+
4.3.1. Precision Degradation
Precision Degradation is a generalisation technique that removes the
most precise components of a timestamp, accounting all events
occurring in each given interval (e.g. one millisecond for
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.
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Note also that degradation to a very low precision (e.g. on the order
of minutes, hours, or days) is commonly used in analyses operating on
time-series aggregated data, and may also be described as binning;
though the time scales are longer and applicability more restricted,
this is in principle the same operation.
Precision degradation to infinitely low precision is equivalent to
black-marker anonymisation. Removal of timestamp information is only
recommended for analysis tasks which have no need to separate flows
in time, for example for counting total volumes or unique occurrences
of other flow keys in an entire dataset.
4.3.2. Enumeration
Enumeration is a substitution function that retains the chronological
order in which events occurred while eliminating time information.
Timestamps are substituted by equidistant timestamps (or numbers)
starting from a randomly chosen start value. The resulting data is
useful for applications requiring strict sequencing, but not for
those requiring good timing information (e.g. delay- or jitter-
measurement for QoS applications or SLA validation).
4.3.3. Random Shifts
Random time shifts add a random offset to every timestamp within a
dataset. This reversible substitution technique therefore retains
duration and inter-event interval information as well as
chronological order of flows. It is primarily intended to defeat
traffic injection fingerprinting attacks.
4.4. Counter Anonymisation
Counters (such as packet and octet volumes per flow) are subject to
fingerprinting and injection attacks against anonymisation, or for
user profiling as timestamps are. Counter anonymisation can help
defeat these attacks, but are only usable for analysis tasks for
which relative or imprecise magnitudes of activity are useful.
Counter information can also be completely removed, but this is only
recommended for analysis tasks which have no need to evaluate the
removed counter, for example for counting only unique occurrences of
other flow keys.
+-----------------------+----------------------------+
| Scheme | Action |
+-----------------------+----------------------------+
| Precision Degradation | Generalisation |
| Binning | Generalisation |
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| Random noise addition | Direct or Set Substitution |
+-----------------------+----------------------------+
4.4.1. Precision Degradation
As with precision degradation in timestamps, precision degradation of
counters removes lower-order bits of the counters, treating all the
counters in a given range as having the same value. Depending on the
precision reduction, this loses information about the relationships
between sizes of similarly-sized flows, but keeps relative magnitude
information. Precision degradation to an infinitely low precision is
equivalent to black-marker anonymisation.
4.4.2. Binning
Binning can be seen as a special case of precision degradation; the
operation is identical, except for in precision degradation the
counter ranges are uniform, and in binning they need not be. For
example, a common counter binning scheme for packet counters could be
to bin values 1-2 together, and 3-infinity together, thereby
separating potentially completely-opened TCP connections from
unopened ones. Binning schemes are generally chosen to keep
precisely the amount of information required in a counter for a given
analysis task. Note that, also unlike precision degradation, the bin
label need not be within the bin's range. Binning counters to a
single bin is equivalent to black-marker anonymisation.
4.4.3. Random Noise Addition
Random noise addition adds a random amount to a counter in each flow;
this is used to keep relative magnitude information and minimize the
disruption to size relationship information while avoiding
fingerprinting attacks against anonymisation. Note that there is no
guarantee that random noise addition will maintain ranking order by a
counter among members of a set. Random noise addition is
particularly useful when the derived analysis data will not be
presented in such a way as to require the lower-order bits of the
counters.
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.
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+-------------+---------------------+
| Scheme | Action |
+-------------+---------------------+
| Binning | Generalisation |
| Permutation | Direct Substitution |
+-------------+---------------------+
4.5.1. Binning
Binning is a generalisation technique mapping a set of potentially
non-uniform ranges into a set of arbitrarily labeled bins. Common
bin arrangements depend on the field type and the analysis
application. For example, an IP protocol bin arrangement may
preserve 1, 6, and 17 for ICMP, UDP, and TCP traffic, and bin all
other protocols into a single bin, to mitigate the use of uncommon
protocols in fingerprinting attacks. Another example arrangement may
bin source and destination ports into low (0-1023) and high (1024-
65535) bins in order to tell service from ephemeral ports without
identifying individual applications.
Binning other flow key fields to a single bin is equivalent to black-
marker anonymisation. Removal of other flow key information is only
recommended for analysis tasks which have no need to differentiate
flows on the removed keys, for example for total traffic counts or
unique counts of other flow keys.
4.5.2. Permutation
Permutation is a direct substitution technique, replacing each value
with an value selected from the set of possible range, such 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.
While permutation ideally guarantees that each anonymised value
represents a unique original value, such may require significant
state in the Intermediate Anonymisation Process. Therefore,
permutation may be implemented by hashing for performance reasons,
with hash functions that may have relatively small collision
probabilities. Such techniques are still essentially direct
substitution techniques, despite the nonzero error probability.
5. Parameters for the Description of 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
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design of the IPFIX anonymisation metadata export specified in the
following section.
5.1. Stability
A stable anonymisation will always map a given value in the real
space to a single given value in the anonymised space, while an
unstable anonymisation will change this mapping over time; a
completely unstable anonymisation is essentially indistinguishable
from black-marker anonymisation. 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 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. Information about the stability
of a mapping SHOULD be exported along with the anonymised data.
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 generally be inferred from a given data set,
and need not be specially exported or protected. For bit-level
truncation, the truncated bits are generally inferable by the least
significant bit set for an instance of an Information Element
described by a given Template (or the most significant bit set, in
the case of reverse truncation). For precision degradation, the
truncation is inferable from the maximum precision given. Note that
while this inference method is generally applicable, it is data-
dependent: there is no guarantee that it will recover the exact
truncation length used to prepare the data.
In the special case of IP address export with variable (per-record)
truncation, the truncation MAY be expressed by exporting the prefix
length alongside the address.
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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 corresponds to a single, unique point in the
original space.
Since knowledge of the permutation function may, depending on the
function, 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, by omitting the
corresponding Information Elements from Template describing the Data
Set. In the case where application requirements dictate that a black-
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marker anonymised field must remain in a Template, then an Exporting
Process MAY export black-marker anonymised fields with their native
length as all-zeros, but only in cases where enough contextual
information exists within the record to differentiate a black-marker
anonymised field exported in this way from a real zero value.
6.1. Anonymisation Records and the 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 or Options 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.
Anonymisation Records contain ancillary information bound to a
Template, so many of the considerations for Templates apply to
Anonymisation Records as well. First, reliability is important: an
Exporting Process SHOULD export Anonymisation Records after the
Templates they describe have been exported, and SHOULD export
anonymisation records reliably.
Anonymisation Records MUST be handled by Collecting Processes as
scoped to the Template to which they apply within the Transport
Session in which they are sent. When a Template is withdrawn via a
Template Withdrawal Message or expires during a UDP transport
session, the accompanying Anonymisation Records are withdrawn or
expire as well, and do not apply to subsequent Templates with the
same Template ID within the Session unless re-exported.
The Stability Class within the anonymisationFlags IE can be used to
declare that a given anonymisation technique's mapping will remain
stable across multiple sessions, but this does not mean that
anonymisation technique information given in the Anonymisation
Records themselves persist across Sessions. Each new Transport
Session MUST contain new Anonymisation Records for each Template
describing anonymised Data Sets.
SCTP per-stream export [I-D.ietf-ipfix-export-per-sctp-stream] may be
used to ease management of Anonymisation Records if appropriate for
the application.
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+-------------------------+-----------------------------------------+
| IE | Description |
+-------------------------+-----------------------------------------+
| templateId [scope] | The Template ID of the Template or |
| | Options 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. Exporting Processes |
| | MUST clear then Enterprise bit of the |
| | informationElementId and Collecting |
| | Processes SHOULD ignore it; information |
| | about enterprise-specific Information |
| | Elements is exported via the |
| | privateEnterpriseNumber Information |
| | Element. |
| privateEnterpriseNumber | The Private Enterprise Number of the |
| [scope] [optional] | enterprise-specific Information Element |
| | described by this anonymisation record. |
| | This Information Element MUST be |
| | defined as a Scope Field if present. A |
| | privateEnterpriseNumber of 0 signifies |
| | that the Information Element is |
| | IANA-registered. |
| informationElementIndex | The Information Element index of the |
| [scope] [optional] | instance of the Information Element |
| | described by this anonymisation record |
| | identified by the informationElementId |
| | within the Template. Optional; need |
| | only be present when describing |
| | Templates that have multiple instances |
| | of the same Information Element. This |
| | Information Element MUST be defined as |
| | a Scope Field if present. This |
| | Information Element is defined in |
| | Section 6.2, below. |
| anonymisationFlags | Flags describing the mapping stability |
| | and specialized modifications to the |
| | Anonymisation Technique in use. SHOULD |
| | be present. This Information Element |
| | is defined in Section 6.2.3, below. |
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| anonymisationTechnique | The technique used to anonymise the |
| | data. MUST be present. This |
| | Information Element is defined in |
| | Section 6.2.2, below. |
+-------------------------+-----------------------------------------+
6.2. Recommended Information Elements for Anonymisation Metadata
6.2.1. informationElementIndex
Description: A zero-based index of an Information Element
referenced by informationElementId within a Template referenced by
templateId; used to disambiguate scope for templates containing
multiple identical Information Elements.
Abstract Data Type: unsigned16
ElementId: TBD3
Status: Proposed
6.2.2. 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 | all | all |
| | Process makes no | | |
| | representation as to | | |
| | whether the defined field | | |
| | is anonymised or not. | | |
| | While the Collecting | | |
| | Process MAY assume that | | |
| | the field is not | | |
| | anonymised, it is not | | |
| | guaranteed not to be. | | |
| | This is the default | | |
| | anonymisation technique. | | |
| 1 | None: the values exported | all | all |
| | are real. | | |
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| 2 | Precision | all | all |
| | Degradation/Truncation: | | |
| | the values exported are | | |
| | anonymised using simple | | |
| | precision degradation or | | |
| | truncation. The new | | |
| | precision or number of | | |
| | truncated bits is | | |
| | implicit in the exported | | |
| | data, and can be deduced | | |
| | by the Collecting | | |
| | Process. | | |
| 3 | Binning: the values | all | all |
| | exported are anonymised | | |
| | into bins. | | |
| 4 | Enumeration: the values | all | timestamps |
| | exported are anonymised | | |
| | by enumeration. | | |
| 5 | Permutation: the values | all | identifiers |
| | exported are anonymised | | |
| | by permutation. | | |
| 6 | Structured Permutation: | addresses | |
| | the values exported are | | |
| | anonymised by | | |
| | permutation, preserving | | |
| | bit-level structure as | | |
| | appropriate; this | | |
| | represents | | |
| | prefix-preserving IP | | |
| | address anonymisation or | | |
| | structured MAC address | | |
| | anonymisation. | | |
| 7 | Reverse Truncation: the | addresses | |
| | values exported are | | |
| | anonymised using reverse | | |
| | truncation. The number | | |
| | of truncated bits is | | |
| | implicit in the exported | | |
| | data, and can be deduced | | |
| | by the Collecting | | |
| | Process. | | |
| 8 | Noise: the values | non-identifiers | counters |
| | exported are anonymised | | |
| | by adding random noise to | | |
| | each value. | | |
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| 9 | Offset: the values | all | timestamps |
| | exported are anonymised | | |
| | by adding a single offset | | |
| | to all values. | | |
+-------+---------------------------+-----------------+-------------+
Abstract Data Type: unsigned16
ElementId: TBD2
Status: Proposed
6.2.3. anonymisationFlags
Description: A flag word describing specialized modifications to
the anonymisation policy in effect for the anonymisation technique
applied to a referenced Information Element within a referenced
Template. When flags are clear (0), the normal policy (as
described by anonymisationTechnique) applies without modification.
MSB 14 13 12 11 10 9 8 7 6 5 4 3 2 1 LSB
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Reserved |LOR|PmA| SC |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
anonymisationFlags IE
+--------+----------+-----------------------------------------------+
| bit(s) | name | description |
| (LSB = | | |
| 0) | | |
+--------+----------+-----------------------------------------------+
| 0-1 | SC | Stability Class: see the Stability Class |
| | | table below, and section Section 5.1. |
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| 2 | PmA | Perimeter Anonymisation: when set (1), |
| | | source- Information Elements as described in |
| | | [RFC5103] are interpreted as external |
| | | addresses, and destination- Information |
| | | Elements as described in [RFC5103] are |
| | | interpreted as internal addresses, for the |
| | | purposes of associating |
| | | anonymisationTechnique to Information |
| | | Elements only; see Section 7.2.2 for details. |
| | | This bit MUST NOT be set when associated with |
| | | a non-endpoint (i.e., source- or |
| | | destination-) Information Element. SHOULD be |
| | | consistent within a record (i.e., if a |
| | | source- Information Element has this flag |
| | | set, the corresponding destination- element |
| | | SHOULD have this flag set, and vice-versa.) |
| 3 | LOR | Low-Order Unchanged: when set (1), the |
| | | low-order bits of the anonymised Information |
| | | Element contain real data. This modification |
| | | is intended for the anonymisation of |
| | | network-level addresses while leaving |
| | | host-level addresses intact in order to |
| | | preserve host level-structure, which could |
| | | otherwise be used to reverse anonymisation. |
| | | MUST NOT be set when associated with a |
| | | truncation-based anonymisationTechnique. |
| 4-15 | Reserved | Reserved for future use: SHOULD be cleared |
| | | (0) by the Exporting Process and MUST be |
| | | ignored by the Collecting Process. |
+--------+----------+-----------------------------------------------+
The Stability Class portion of this flags word describes the
stability class of the anonymisation technique applied to a
referenced Information Element within a referenced Template.
Stability classes refer to the stability of the parameters of the
anonymisation technique, and therefore the comparability of the
mapping between the real and anonymised values over time. This
determines which anonymised datasets may be compared with each
other. Values are as follows:
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+-----+-----+-------------------------------------------------------+
| Bit | Bit | Description |
| 1 | 0 | |
+-----+-----+-------------------------------------------------------+
| 0 | 0 | Undefined: the Exporting Process makes no |
| | | representation as to how stable the mapping is, or |
| | | over what time period values of this field will |
| | | remain comparable; while the Collecting Process MAY |
| | | assume Session level stability, Session level |
| | | stability is not guaranteed. Processes SHOULD assume |
| | | this is the case in the absence of stability class |
| | | information; this is the default stability class. |
| 0 | 1 | Session: the Exporting Process will ensure that the |
| | | parameters of the anonymisation technique are stable |
| | | during the Transport Session. All the values of the |
| | | described Information Element for each Record |
| | | described by the referenced Template within the |
| | | Transport Session are comparable. The Exporting |
| | | Process SHOULD endeavour to ensure at least this |
| | | stability class. |
| 1 | 0 | Exporter-Collector Pair: the Exporting Process will |
| | | ensure that the parameters of the anonymisation |
| | | technique are stable across Transport Sessions over |
| | | time with the given Collecting Process, but may use |
| | | different parameters for different Collecting |
| | | Processes. Data exported to different Collecting |
| | | Processes is not comparable. |
| 1 | 1 | Stable: the Exporting Process will ensure that the |
| | | parameters of the anonymisation technique are stable |
| | | across Transport Sessions over time, regardless of |
| | | the Collecting Process to which it is sent. |
+-----+-----+-------------------------------------------------------+
Abstract Data Type: unsigned16
ElementId: TBD1
Status: Proposed
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.
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7.1. Arrangement of Processes in IPFIX Anonymisation
Anonymisation may be applied to IPFIX data at three stages within the
collection infrastructure: on initial export, at a mediator, or after
collection, as shown in Figure 1. Each of these locations has
specific considerations and applicability.
+==========================================+
| Exporting Process |
+==========================================+
| |
| (Anonymised at Original Exporter) |
V |
+=============================+ |
| Mediator | |
+=============================+ |
| |
| (Anonymising Mediator) |
V V
+==========================================+
| Collecting Process |
+==========================================+
|
| (Anonymising CP/File Writer)
V
+--------------------+
| IPFIX File Storage |
+--------------------+
Figure 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 7.2.3 with respect to timestamp anonymisation
may not apply in this case, as the Collecting Process may be able to
deduce certain timing information from the time at which each Message
is received.
A much more flexible arrangement is to anonymise data within a
Mediator [I-D.ietf-ipfix-mediators-framework]. Here, original data
is sent to a Mediator, which performs the anonymisation function and
re-exports the anonymised data. Such a Mediator could be located at
the administrative domain boundary of the initial Exporting Process
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operator, exporting anonymised data to other consumers outside the
organisation. In this case, the original Exporter SHOULD use TLS as
specified in [RFC5101] to secure the channel to the Mediator, and the
Mediator should follow the guidelines in Section 7.2, to mitigate the
risk of original data disclosure.
When data is to be published as an anonymised data set in an IPFIX
File [RFC5655], the anonymisation may be done at the final Collecting
Process before storage and dissemination, as well. In this case, the
Collector should follow the guidelines in Section 7.2, especially as
regards File-specific Options in Section 7.2.4
In each of these data flows, the anonymisation of records is
undertaken by an Intermediate Anonymisation Process (IAP); the data
flows into and out of this IAP are shown in Figure 2 below.
packets --+ +- IPFIX Messages -+
| | |
V V V
+==================+ +====================+ +=============+
| Metering Process | | Collecting Process | | File Reader |
+==================+ +====================+ +=============+
| Non-anonymised | Records |
V V V
+=========================================================+
| Intermediate Anonymisation Process (IAP) |
+=========================================================+
| Anonymised ^ Anonymised |
| Records | Records |
V | V
+===================+ Anonymisation +=============+
| Exporting Process |<--- Parameters ------>| File Writer |
+===================+ +=============+
| |
+------------> IPFIX Messages <----------+
Figure 2: Data flows through the anonymisation process
Anonymisation parameters must also be available to the Exporting
Process and/or File Writer in order to ensure header data is also
appropriately anonymised as in Section 7.2.3.
Following each of the data flows through the IAP, we describe five
basic types of anonymisation arrangements within this framework in
Figure 3. In addition to the three arrangements described in detail
above, anonymisation can also be done at a collocated Metering
Process and File Writer (see section 7.3.2 of [RFC5655]), or at a
file manipulator (see section 7.3.7 of [RFC5655]).
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+----+ +-----+ +----+
pkts -> | MP |->| IAP |->| EP |-> anonymisation on Original Exporter
+----+ +-----+ +----+
+----+ +-----+ +----+
pkts -> | MP |->| IAP |->| FW |-> Anonymising collocated MP/File Writer
+----+ +-----+ +----+
+----+ +-----+ +----+
IPFIX -> | CP |->| IAP |->| EP |-> Anonymising Mediator (Masq. Proxy)
+----+ +-----+ +----+
+----+ +-----+ +----+
IPFIX -> | CP |->| IAP |->| FW |-> Anonymising collocated CP/File Writer
+----+ +-----+ +----+
+----+ +-----+ +----+
IPFIX -> | FR |->| IAP |->| FW |-> Anonymising file manipulator
File +----+ +-----+ +----+
Figure 3: Possible anonymisation arrangements in the IPFIX
architecture
Note that anonymisation may occur at more than one location within a
given collection infrastructure, to provide varying levels of
anonymisation, disclosure risk, or data utility for specific
purposes.
7.2. IPFIX-Specific Anonymisation Guidelines
In implementing and deploying the anonymisation techniques described
in this document, implementors should note that IPFIX already
provides features that support anonymised data export, and use these
where appropriate. Care must also be taken that data structures
supporting the operation of the protocol itself do not leak data that
could be used to reverse the anonymisation applied to the flow data.
Such data structures may appear in the header, or within the data
stream itself, especially as options data. Each of these and their
impact on specific anonymisation techniques is noted in a separate
subsection below.
7.2.1. Appropriate Use of Information Elements for Anonymised Data
Note, as in Section 6 above, that black-marker anonymised fields
SHOULD NOT be exported at all; the absence of the field in a given
Data Set is implicitly declared by not including the corresponding
Information Element in the Template describing that Data Set.
When using precision degradation of timestamps, Exporting Processes
SHOULD export timing information using Information Elements of an
appropriate precision, as explained in Section 4.5 of [RFC5153]. For
example, timestamps measured in millisecond-level precision and
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degraded to second-level precision should use flowStartSeconds and
flowEndSeconds, not flowStartMilliseconds and flowEndMilliseconds.
When exporting anonymised data and anonymisation metadata, Exporting
Processes SHOULD ensure that the combination of Information Element
and declared anonymisation technique are compatible. Specifically,
the applicable and recommended Information Element types and
semantics for each technique are noted in the description of the
anonymisationTechnique Information Element in Section 6.2.2. In this
description, a timestamp is an Information Element with the data type
dateTimeSeconds, dataTimeMilliseconds, dateTimeMicroseconds, or
dateTimeNanoseconds; an address is an Information Element with the
data type ipv4Address, ipv6Address, or macAddress; and an identifier
is an Information Element with identifier data type semantics.
Exporting Process MUST NOT export Anonymisation Options records
binding techniques to Information Elements to which they are not
applicable, and SHOULD NOT export Anonymisation Options records
binding techniques to Information Elements for which they are not
recommended.
7.2.2. Export of Perimeter-Based Anonymisation Policies
Data collected from a single network may require different
anonymisation policies for addresses internal and external to the
network. For example, internal addresses could be subject to simple
permutation, while external addresses could be aggregated into
networks by truncation. When exporting anonymised perimeter
bidirectional flow (biflow) data as in section 5.2 of [RFC5103], this
arrangement may be easily represented by specifying one technique for
source endpoint information (which represents the external endpoint
in a perimeter biflow) and one technique for destination endpoint
information (which represents the internal address in a perimeter
biflow).
However, it can also be useful to represent perimeter-based
anonymisation policies with unidirectional flow (uniflow), or non-
perimeter biflow data. In this case, the Perimeter Anonymisation bit
(bit 2) in the anonymisationFlags Information Element describing the
anonymised address Information Elements can be set to change the
meaning of "source" and "destination" of Information Elements to mean
"external" and "internal" as with perimeter biflows, but only with
respect to anonymisation policies.
7.2.3. Anonymisation of Header Data
Each IPFIX Message contains a Message Header; within this Message
Header are contained two fields which may be used to break certain
anonymisation techniques: the Export Time, and the Observation Domain
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ID
Export of IPFIX Messages containing anonymised timestamp data where
the original Export Time Message header has some relationship to the
anonymised timestamps SHOULD anonymise the Export Time header field
so that the Export Time is consistent with the anonymised timestamp
data. Otherwise, relationships between export and flow time could be
used to partially or totally reverse timestamp anonymisation.
Anonymisation of timestamps and the Export Time header field should
take care to avoid times too far in the past or future; while
[RFC5101] does not make any allowance for Export Time error
detection, it is sensible that Collecting Processes may interpret
Messages with seemingly nonsensical Export Times as erroneous.
Specific limits are implementation-dependent, but this issue may
cause interoperability issues when anonymising the Export Time header
field.
The similarity in size between an Observation Domain ID and an IPv4
address (32 bits) may lead to a temptation to use an IPv4 interface
address on the Metering or Exporting Process as the Observation
Domain ID. If this address bears some relation to the IP addresses
in the flow data (e.g., shares a network prefix with internal
addresses) and the IP addresses in the flow data are anonymised in a
structure-preserving way, then the Observation Domain ID may be used
to break the IP address anonymisation. Use of an IPv4 interface
address on the Metering or Exporting Process as the Observation
Domain ID is NOT RECOMMENDED in this case.
7.2.4. Anonymisation of Options Data
IPFIX uses the Options mechanism to export, among other things,
metadata about exported flows and the flow collection infrastructure.
As with the IPFIX Message Header, certain Options recommended in
[RFC5101] and [RFC5655] containing flow timestamps and network
addresses of Exporting and Collecting Processes may be used to break
certain anonymisation techniques; care should be taken while using
them with anonymised data export and storage.
The Exporting Process Reliability Statistics Options Template,
recommended in [RFC5101], contains an Exporting Process ID field,
which may be an exportingProcessIPv4Address Information Element or an
exportingProcessIPv6Address Information Element. If the Exporting
Process address bears some relation to the IP addresses in the flow
data (e.g., shares a network prefix with internal addresses) and the
IP addresses in the flow data are anonymised in a structure-
preserving way, then the Exporting Process address may be used to
break the IP address anonymisation. Exporting Processes exporting
anonymised data in this situation SHOULD mitigate the risk of attack
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either by omitting Options described by the Exporting Process
Reliability Statistics Options Template, or by anonymising the
Exporting Process address using a similar technique to that used to
anonymise the IP addresses in the exported data.
Similarly, the Export Session Details Options Template and Message
Details Options Template specified for the IPFIX File Format
[RFC5655] may contain the exportingProcessIPv4Address Information
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 data set (e.g., shares a network prefix with internal addresses)
and the IP addresses in the data set are anonymised in a structure-
preserving way, then the Exporting Process or Collecting Process
address may be used to break the IP address anonymisation. Since
these Options Templates are primarily intended for storing IPFIX
Transport Session data for auditing, replay, and testing purposes, it
is NOT RECOMMENDED that storage of anonymised data include these
Options Templates in order to mitigate the risk of attack.
The Message Details Options Template specified for the IPFIX File
Format [RFC5655] also contains the collectionTimeMilliseconds
Information Element. As with the Export Time Message Header field,
if the exported data set contains anonymised timestamp information,
and the collectionTimeMilliseconds Information Element in a given
Message has some relationship to the anonymised timestamp
information, then this relationship can be exploited to reverse the
timestamp anonymisation. Since this Options Template is primarily
intended for storing IPFIX Transport Session data for auditing,
replay, and testing purposes, it is NOT RECOMMENDED that storage of
anonymised data include this Options Template in order to mitigate
the risk of attack.
Since the Time Window Options Template specified for the IPFIX File
Format [RFC5655] refers to the timestamps within the data set to
provide partial table of contents information for an IPFIX File, care
must be taken to ensure that Options described by this template are
written using the anonymised timestamps instead of the original ones.
7.2.5. Special-Use Address Space Considerations
When anonymising data for transport or storage using IPFIX containing
anonymised IP addresses, and the analysis purpose permits doing so,
it is recommended to filter out or leave unanonymised data containing
the special-use IPv4 addresses enumerated in [RFC5735] or the
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special-use IPv6 addresses enumerated in [RFC5156]. Data containing
these addresses (e.g. 0.0.0.0 and 169.254.0.0/16 for link-local
autoconfiguration in IPv4 space) are often associated with specific,
well-known behavioral patterns. Detection of these patterns in
anonymised data can lead to deanonymisation of these special-use
addresses, which increases the chance of a complete reversal of
anonymisation by an attacker, especially of prefix-preserving
techniques.
7.2.6. Protecting Out-of-Band Configuration and Management Data
Special care should be taken when exporting or sharing anonymised
data to avoid information leakage via the configuration or management
planes of the IPFIX Device containing the Exporting Process or the
File Writer. For example, adding noise to counters is useless if the
receiver can deduce the values in the counters from SNMP information,
and concealing the network under test is similarly useless if such
information is available in a configuration document. As the
specifics of these concerns are largely implementation- and
deployment-dependent, specific mitigation is out of scope for this
draft. The general ground rule is that information of similar type
to that anonymised should not be made available to the receiver by
any means, whether in the Data Records, in IPFIX protocol structures
such as Message Headers, or out-of-band.
8. Examples
In this example, consider the export or storage of an anonymised IPv4
data set from a single network described by a simple template
containing a timestamp in seconds, a five-tuple, and packet and octet
counters. The template describing each record in this data set is
shown in figure Figure 4.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set ID = 2 | Length = 40 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template ID = 256 | Field Count = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| flowStartSeconds 150 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| sourceIPv4Address 8 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| destinationIPv4Address 12 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| sourceTransportPort 7 | Field Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| destinationTransportPort 11 | Field Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| packetDeltaCount 2 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| octetDeltaCount 1 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| protocolIdentifier 4 | Field Length = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Example Flow Template
Suppose that this data set is anonymised according to the following
policy:
o IP addresses within the network are protected by reverse
truncation.
o IP addresses outside the network are protected by prefix-
preserving anonymisation.
o Octet counts are exported using degraded precision in order to
provide minimal protection against fingerprinting attacks.
o All other fields are exported unanonymised.
In order to export anonymisation records for this template and
policy, first, the Anonymisation Options Template shown in figure
Figure 5 is exported. For this example, the optional
privateEnterpriseNumber and informationElementIndex Information
Elements are omitted, because they are not used.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set ID = 3 | Length = 26 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template ID = 257 | Field Count = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scope Field Count = 2 |0| templateID 145 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |0| informationElementId 303 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |0| anonymisationFlags TBD1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |0| anonymisationTechnique TBD2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example Anonymisation Options Template
Following the Anonymisation Options Template comes a Data Set
containing Anonymisation Records. This data set has an entry for
each Information Element Specifier in Template 256 describing the
flow records. This Data Set is shown in figure Figure 6. Note that
sourceIPv4Address and destinationIPv4Address have the Perimeter
Anonymisation (0x0004) flag set in anonymisationFlags, meaning that
source address should be treated as network-external, and the
destination address as network-internal.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set ID = 257 | Length = 68 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | flowStartSeconds IE 150 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymised 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | sourceIPv4Address IE 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Perimeter, Session SC 0x0005 | Structured Permutation 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | destinationIPv4Address IE 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Perimeter, Stable 0x0007 | Reverse Truncation 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | sourceTransportPort IE 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymised 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | dest.TransportPort IE 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymised 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | packetDeltaCount IE 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymised 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | octetDeltaCount IE 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stable 0x0003 | Precision Degradation 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | protocolIdentifier IE 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymised 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Example Anonymisation Records
Following the Anonymisation Records come the data sets containing the
anonymised data, exported according to the template in figure
Figure 4. Bringing it all together, consider an IPFIX Message
containing three real data records and the necessary templates to
export them, shown in Figure 7. (Note that the scale of this message
is 8-bytes per line, for compactness; lines of dots '. . . . . '
represent shifting of the example bit structure for clarity.)
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1 2 3 4 5 6
0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x000a | length 135 | export time 1271227717 | msg
| sequence 0 | domain 1 | hdr
| SetID 2 | length 40 | tid 256 | fields 8 | tmpl
| IE 150 | length 4 | IE 8 | length 4 | set
| IE 12 | length 4 | IE 7 | length 2 |
| IE 11 | length 2 | IE 2 | length 4 |
| IE 1 | length 4 | IE 4 | length 1 |
| SetID 256 | length 79 | time 1271227681 | data
| sip 192.0.2.3 | dip 198.51.100.7 | set
| sp 53 | dp 53 | packets 1 |
| bytes 74 | prt 17 | . . . . . . . . . . .
| time 1271227682 | sip 198.51.100.7 |
| dip 192.0.2.88 | sp 5091 | dp 80 |
| packets 60 | bytes 2896 |
| prt 6 | . . . . . . . . . . . . . . . . . . . . . . . . . . .
| time 1271227683 | sip 198.51.100.7 |
| dip 203.0.113.9 | sp 5092 | dp 80 |
| packets 44 | bytes 2037 |
| prt 6 |
+---------+
Figure 7: Example Real Message
The corresponding anonymised message is then shown in Figure 8. The
options template set describing Anonymisation Records and the
Anonymisation Records themselves are added; IP addresses and byte
counts are anonymised as declared.
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1 2 3 4 5 6
0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x000a | length 233 | export time 1271227717 | msg
| sequence 0 | domain 1 | hdr
| SetID 2 | length 40 | tid 256 | fields 8 | tmpl
| IE 150 | length 4 | IE 8 | length 4 | set
| IE 12 | length 4 | IE 7 | length 2 |
| IE 11 | length 2 | IE 2 | length 4 |
| IE 1 | length 4 | IE 4 | length 1 |
| SetID 3 | length 30 | tid 257 | fields 4 | opt
| scope 2 | . . . . . . . . . . . . . . . . . . . . . . . . tmpl
| IE 145 | length 2 | IE 303 | length 2 | set
| IE TBD1 | length 2 | IE TBD2 | length 2 |
| SetID 257 | length 68 | . . . . . . . . . . . . . . . . anon
| tid 256 | IE 150 | flags 0 | tech 1 | recs
| tid 256 | IE 8 | flags 5 | tech 6 |
| tid 256 | IE 12 | flags 7 | tech 7 |
| tid 256 | IE 7 | flags 0 | tech 1 |
| tid 256 | IE 11 | flags 0 | tech 1 |
| tid 256 | IE 2 | flags 0 | tech 1 |
| tid 256 | IE 1 | flags 3 | tech 2 |
| tid 256 | IE41 | flags 0 | tech 1 |
| SetID 256 | length 79 | time 1271227681 | data
| sip 254.202.119.209 | dip 0.0.0.7 | set
| sp 53 | dp 53 | packets 1 |
| bytes 100 | prt 17 | . . . . . . . . . . .
| time 1271227682 | sip 0.0.0.7 |
| dip 254.202.119.6 | sp 5091 | dp 80 |
| packets 60 | bytes 2900 |
| prt 6 | . . . . . . . . . . . . . . . . . . . . . . . . . . .
| time 1271227683 | sip 0.0.0.7 |
| dip 2.19.199.176 | sp 5092 | dp 80 |
| packets 60 | bytes 2000 |
| prt 6 |
+---------+
Figure 8: Corresponding Anonymised Message
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,
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the purpose, and the risk of deanonymisation of a given application.
We note specifically that anonymisation is not a replacement for
encryption for confidentiality. It is only appropriate for
protecting identifying information in data to be used for purposes in
which the protected data is irrelevant. Confidentiality in export is
best served by using TLS or DTLS as in the Security Considerations
section of [RFC5101], and in long-term storage by implementation-
specific protection applied as in the Security Considerations section
of [RFC5655]. Indeed, confidentiality and anonymisation are not
mutually exclusive, as encryption for confidentiality may be applied
to anonymised data export or storage, as well, when the anonymised
data is not intended for public release.
When using pseudonymisation techniques that have a mutable mapping,
there is an inherent tradeoff in the stability of the map between
long-term comparability and security of the data set against
deanonymisation. In general, deanonymisation attacks are more
effective given more information, so the longer a given mapping is
valid, the more information can be applied to deanonymisation. The
specific details of this are technique-dependent and therefore out of
the scope of this document.
When releasing anonymised data, publishers need to ensure that data
that could be used in deanonymisation is not leaked through the
export protocol; guidelines for addressing this risk are provided in
Section 7.2.
Note as well that the Security Considerations section of [RFC5101]
applies as well to the export of anonymised data, and the Security
Considerations section of [RFC5655] to the storage of anonymised
data, or the publication of anonymised traces.
10. IANA Considerations
This document specifies the creation of several new IPFIX Information
Elements in the IPFIX Information Element registry located at
http://www.iana.org/assignments/ipfix, as defined in Section 6.2
above. IANA has assigned the following Information Element numbers
for their respective Information Elements as specified below:
o Information Element number TBD1 for the anonymisationFlags
Information Element.
o Information Element number TBD2 for the anonymisationTechnique
Information Element.
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o Information Element number TBD3 for the informationElementIndex
Information Element.
[NOTE for IANA: The text TBDn should be replaced with the respective
assigned Information Element numbers where they appear in this
document.]
11. Acknowledgments
We thank Paul Aitken and John McHugh for their comments and insight,
and Carsten Schmoll, Benoit Claise, and Lothar Braun for their
reviews. Special thanks to the ICT-PRISM project for its material
support of this work.
12. References
12.1. Normative References
[RFC5101] Claise, B., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008.
[RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
RFC 5102, January 2008.
[RFC5103] Trammell, B. and E. Boschi, "Bidirectional Flow Export
Using IP Flow Information Export (IPFIX)", RFC 5103,
January 2008.
[RFC5655] Trammell, B., Boschi, E., Mark, L., Zseby, T., and A.
Wagner, "Specification of the IP Flow Information Export
(IPFIX) File Format", RFC 5655, October 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC5156] Blanchet, M., "Special-Use IPv6 Addresses", RFC 5156,
April 2008.
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12.2. Informative References
[RFC5470] Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
"Architecture for IP Flow Information Export", RFC 5470,
March 2009.
[RFC5472] Zseby, T., Boschi, E., Brownlee, N., and B. Claise, "IP
Flow Information Export (IPFIX) Applicability", RFC 5472,
March 2009.
[I-D.ietf-ipfix-mediators-framework]
Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi,
"IPFIX Mediation: Framework",
draft-ietf-ipfix-mediators-framework-08 (work in
progress), August 2010.
[I-D.ietf-ipfix-export-per-sctp-stream]
Claise, B., Aitken, P., Johnson, A., and G. Muenz, "IPFIX
Export per SCTP Stream",
draft-ietf-ipfix-export-per-sctp-stream-08 (work in
progress), May 2010.
[RFC5153] Boschi, E., Mark, L., Quittek, J., Stiemerling, M., and P.
Aitken, "IP Flow Information Export (IPFIX) Implementation
Guidelines", RFC 5153, April 2008.
[RFC3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export (IPFIX)",
RFC 3917, October 2004.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
Authors' Addresses
Elisa Boschi
Swiss Federal Institute of Technology Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Email: boschie@tik.ee.ethz.ch
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Brian Trammell
Swiss Federal Institute of Technology Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Phone: +41 44 632 70 13
Email: trammell@tik.ee.ethz.ch
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