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IETF RFC 6235
IP Flow Anonymization Support
Last modified on Saturday, May 14th, 2011
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Internet Engineering Task Force (IETF) E. Boschi
Request for Comments: 6235 B. Trammell
Category: Experimental ETH Zurich
ISSN: 2070-1721 May 2011
IP Flow Anonymization Support
 Abstract
This document describes anonymization techniques for IP flow data and
the export of anonymized data using the IP Flow Information Export
(IPFIX) protocol. It categorizes common anonymization schemes and
defines the parameters needed to describe them. It provides
guidelines for the implementation of anonymized data export and
storage over IPFIX, and describes an information model and Options-
based method for anonymization metadata export within the IPFIX
protocol or storage in IPFIX Files.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6235.
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RFC 6235 IP Flow Anonymization Support May 2011
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
1.1. IPFIX Protocol Overview ....................................4
1.2. IPFIX Documents Overview ...................................5
1.3. Anonymization within the IPFIX Architecture ................5
1.4. Supporting Experimentation with Anonymization ..............6
2. Terminology .....................................................6
3. Categorization of Anonymization Techniques ......................7
4. Anonymization of IP Flow Data ...................................8
4.1. IP Address Anonymization ..................................10
4.1.1. Truncation .........................................11
4.1.2. Reverse Truncation .................................11
4.1.3. Permutation ........................................11
4.1.4. Prefix-Preserving Pseudonymization .................12
4.2. MAC Address Anonymization .................................12
4.2.1. Truncation .........................................13
4.2.2. Reverse Truncation .................................13
4.2.3. Permutation ........................................14
4.2.4. Structured Pseudonymization ........................14
4.3. Timestamp Anonymization ...................................15
4.3.1. Precision Degradation ..............................15
4.3.2. Enumeration ........................................16
4.3.3. Random Shifts ......................................16
4.4. Counter Anonymization .....................................16
4.4.1. Precision Degradation ..............................17
4.4.2. Binning ............................................17
4.4.3. Random Noise Addition ..............................17
4.5. Anonymization of Other Flow Fields ........................18
4.5.1. Binning ............................................18
4.5.2. Permutation ........................................18
5. Parameters for the Description of Anonymization Techniques .....19
5.1. Stability .................................................19
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5.2. Truncation Length .........................................19
5.3. Bin Map ...................................................20
5.4. Permutation ...............................................20
5.5. Shift Amount ..............................................20
6. Anonymization Export Support in IPFIX ..........................20
6.1. Anonymization Records and the Anonymization
Options Template ..........................................21
6.2. Recommended Information Elements for Anonymization
Metadata ..................................................23
6.2.1. informationElementIndex ............................23
6.2.2. anonymizationTechnique .............................23
6.2.3. anonymizationFlags .................................25
7. Applying Anonymization Techniques to IPFIX Export and Storage ..27
7.1. Arrangement of Processes in IPFIX Anonymization ...........28
7.2. IPFIX-Specific Anonymization Guidelines ...................30
7.2.1. Appropriate Use of Information Elements for
Anonymized Data ....................................30
7.2.2. Export of Perimeter-Based Anonymization Policies ...31
7.2.3. Anonymization of Header Data .......................32
7.2.4. Anonymization of Options Data ......................32
7.2.5. Special-Use Address Space Considerations ...........34
7.2.6. Protecting Out-of-Band Configuration and
Management Data ....................................34
8. Examples .......................................................34
9. Security Considerations ........................................39
10. IANA Considerations ...........................................41
11. Acknowledgments ...............................................41
12. References ....................................................41
12.1. Normative References .....................................41
12.2. Informative References ...................................42
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1. Introduction
The standardization of an IP Flow Information Export (IPFIX) protocol
[RFC 5101] and associated representations removes a technical barrier
to the sharing of IP flow data across organizational boundaries and
with network operations, security, and research communities for a
wide variety of purposes. However, with wider dissemination comes
greater risks to the privacy of the users of networks under
measurement, and to the security of those networks. While it is not
a complete solution to the issues posed by distribution of IP flow
information, anonymization (i.e., the deletion or transformation of
information that is considered sensitive and that could be used to
reveal the identity of subjects involved in a communication) is an
important tool for the protection of privacy within network
measurement infrastructures.
This document presents a mechanism for representing anonymized data
within IPFIX and guidelines for using it. It is not intended as a
general statement on the applicability of specific flow data
anonymization techniques to specific situations or as a
recommendation of any particular application of anonymization to flow
data export. Exporters or publishers of anonymized data must take
care that the applied anonymization technique is appropriate for the
data source, the purpose, and the risk of deanonymization of a given
application.
It begins with a categorization of anonymization techniques. It then
describes the applicability of each technique to commonly
anonymizable fields of IP flow data, organized by information element
data type and semantics as in [RFC 5102]; enumerates the parameters
required by each of the applicable anonymization techniques; and
provides guidelines for the use of each of these techniques in
accordance with current best practices in data protection. Finally,
it specifies a mechanism for exporting anonymized data and binding
anonymization metadata to Templates and Options Templates using IPFIX
Options.
1.1. IPFIX Protocol Overview
In the IPFIX protocol, { type, length, value } tuples are expressed
in Templates containing { type, length } pairs, specifying which
{ value } fields are present in data records conforming to the
Template, giving great flexibility as to what data is transmitted.
Since Templates are sent very infrequently compared with Data
Records, this results in significant bandwidth savings. Various
different data formats may be transmitted simply by sending new
Templates specifying the { type, length } pairs for the new data
format. See [RFC 5101] for more information.
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The IPFIX information model [RFC 5102] defines a large number of
standard Information Elements (IEs) that provide the necessary
{ type } information for Templates. The use of standard elements
enables interoperability among different vendors' implementations.
Additionally, non-standard enterprise-specific elements may be
defined for private use.
1.2. IPFIX Documents Overview
"Specification of the IP Flow Information Export (IPFIX) Protocol for
the Exchange of IP Traffic Flow Information" [RFC 5101] 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" [RFC 5470] 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" [RFC 3917]. The IPFIX Protocol document [RFC 5101] 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" [RFC 5102]
describes the Information Elements used by IPFIX, including details
on Information Element naming, numbering, and data type encoding.
Finally, "IP Flow Information Export (IPFIX) Applicability" [RFC 5472]
describes the various applications of the IPFIX protocol and their
use of information exported via IPFIX and relates the IPFIX
architecture to other measurement architectures and frameworks.
Additionally, "Specification of the IP Flow Information Export
(IPFIX) File Format" [RFC 5655] describes a file format based upon the
IPFIX protocol for the storage of flow data.
This document references the Protocol and Architecture documents for
terminology and extends the IPFIX Information Model to provide new
Information Elements for anonymization metadata. The anonymization
techniques described herein are equally applicable to the IPFIX
protocol and data stored in IPFIX Files.
1.3. Anonymization within the IPFIX Architecture
According to [RFC 5470], IPFIX Message anonymization is optionally
performed as the final operation before handing the Message to the
transport protocol for export. While no provision is made in the
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architecture for anonymization metadata as in Section 6, this
arrangement does allow for the rewriting necessary for comprehensive
anonymization of IPFIX export as in Section 7. The development of
the IPFIX Mediation [RFC 6183] framework and the IPFIX File Format
[RFC 5655] expand upon this initial architectural allowance for
anonymization by adding to the list of places that anonymization may
be applied. The former specifies IPFIX Mediators, which rewrite
existing IPFIX Messages, and the latter specifies a method for
storage of IPFIX data in files.
More detail on the applicable architectural arrangements for
anonymization can be found in Section 7.1
1.4. Supporting Experimentation with Anonymization
The status of this document is Experimental, reflecting the
experimental nature of anonymization export support. Research on
network trace anonymization techniques and attacks against them is
ongoing. Indeed, there is increasing evidence that anonymization
applied to network trace or flow data on its own is insufficient for
many data protection applications as in [Bur10]. Therefore, this
document explicitly does not recommend any particular technique or
implementation thereof.
The intention of this document is to provide a common basis for
interoperable exchange of anonymized data, furthering research in
this area, both on anonymization techniques themselves as well as to
the application of anonymized data to network measurement. To that
end, the classification in Section 3 and anonymization export support
in Section 6 can be used to describe and export information even
about data anonymized using techniques that are unacceptably weak for
general application to production datasets on their own.
While the specification herein is designed to be independent of the
anonymization techniques applied and the implementation thereof, open
research in this area may necessitate future updates to the
specification. Assuming the future successful application of this
specification to anonymized data publication and exchange, it may be
brought back to the IPFIX working group for further development and
publication on the Standards Track.
2. Terminology
Terms used in this document that are defined in the Terminology
section of the IPFIX Protocol [RFC 5101] document are to be
interpreted as defined there. In addition, this document defines the
following terms:
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Anonymization Record: A record, defined by the Anonymization
Options Template in Section 6.1, that defines the properties of
the anonymization applied to a single Information Element within a
single Template or Options Template.
Anonymized Data Record: A Data Record within a Data Set containing
at least one Information Element with anonymized values. The
Information Element(s) within the Template or Options Template
describing this Data Record SHOULD have a corresponding
Anonymization Record.
Intermediate Anonymization Process: An intermediate process that
takes Data Records and transforms them into Anonymized Data
Records.
Note that there is an explicit difference in this document between a
"Data Set" (which is defined as in [RFC 5101]) and a "data set". When
in lower case, this term refers to any collection of data (usually,
within the context of this document, flow or packet data) that may
contain identifying information and is therefore subject to
anonymization.
Note also that when the term Template is used in this document,
unless otherwise noted, it applies both to Templates and Options
Templates as defined in [RFC 5101]. Specifically, Anonymization
Records may apply to both Templates and Options Templates.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
3. Categorization of Anonymization Techniques
Anonymization, as described by this document, is the modification of
a dataset in order to protect the identity of the people or entities
described by the dataset from disclosure. With respect to network
traffic data, anonymization generally attempts to preserve some set
of properties of the network traffic useful for a given application
or applications, while ensuring the data cannot be traced back to the
specific networks, hosts, or users generating the traffic.
Anonymization may be broadly classified according to two properties:
recoverability and countability. All anonymization techniques map
the real space of identifiers or values into a separate, anonymized
space, according to some function. A technique is said to be
recoverable when the function used is invertible or can otherwise be
reversed and a real identifier can be recovered from a given
replacement identifier. "Recoverability" as used within this
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categorization does not refer to recoverability under attack; that
is, techniques wherein the function used can only be reversed using
additional information, such as an encryption key, or knowledge of
injected traffic within the dataset, are not considered to be
recoverable.
Countability compares the dimension of the anonymized space (N) to
the dimension of the real space (M), and denotes how the count of
unique values is preserved by the anonymization function. If the
anonymized space is smaller than the real space, then the function is
said to generalize the input, mapping more than one input point to
each anonymous value (e.g., as with aggregation). By definition,
generalization is not recoverable.
If the dimensions of the anonymized and real spaces are the same,
such that the count of unique values is preserved, then the function
is said to be a direct substitution function. If the dimension of
the anonymized space is larger, such that each real value maps to a
set of anonymized values, then the function is said to be a set
substitution function. Note that with set substitution functions,
the sets of anonymized values are not necessarily disjoint. Either
direct or set substitution functions are said to be one-way if there
exists no non-brute force method for recovering the real data point
from an anonymized one in isolation (i.e., if the only way to recover
the data point is to attack the anonymized data set as a whole, e.g.,
through fingerprinting or data injection).
This classification is summarized in the table below.
+------------------------+-----------------+------------------------+
| Recoverability / | Recoverable | Non-recoverable |
| Countability | | |
+------------------------+-----------------+------------------------+
| N < M | N.A. | Generalization |
| N = M | Direct | One-way Direct |
| | Substitution | Substitution |
| N > M | Set | One-way Set |
| | Substitution | Substitution |
+------------------------+-----------------+------------------------+
4. Anonymization of IP Flow Data
In anonymizing IP flow data as treated by this document, the goal is
generally two-way address untraceability: to remove the ability to
assert that endpoint X contacted endpoint Y at time T. Address
untraceability is important as IP addresses are the most suitable
field in IP flow records to identify real-world entities. Each IP
address is associated with an interface on a network host and can
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potentially be identified with a single user. Additionally, IP
addresses are structured identifiers; that is, partial IP address
prefixes may be used to identify networks just as full IP addresses
identify hosts. This leads IP flow data anonymization to be
concerned first and foremost with IP address anonymization.
Any form of aggregation that combines flows from multiple endpoints
into a single record (e.g., aggregation by subnetwork, aggregation
removing addressing completely) may also provide address
untraceability; however, anonymization by aggregation is out of scope
for this document. Additionally, of potential interest in this
problem space but out of scope are anonymization techniques that are
applied over multiple fields or multiple records in a way that
introduces dependencies among anonymized fields or records. This
document is concerned solely with anonymization techniques applied at
the resolution of single fields within a flow record.
Even so, attacks against these anonymization techniques use entire
flows and relationships between hosts and flows within a given
dataset. Therefore, fields that may not necessarily be identifying
by themselves may be anonymized in order to increase the anonymity of
the dataset as a whole.
Due to the restricted semantics of IP flow data, there is a
relatively limited set of specific anonymization techniques available
on flow data, though each falls into the broad categories discussed
in the previous section. Each type of field that may commonly appear
in a flow record may have its own applicable specific techniques.
As with IP addresses, Media Access Control (MAC) addresses uniquely
identify devices on the network; while they are not often available
in traffic data collected at Layer 3, and cannot be used to locate
devices within the network, some traces may contain sub-IP data
including MAC address data. Hardware addresses may be mappable to
device serial numbers, and to the entities or individuals who
purchased the devices, when combined with external databases. MAC
addresses are also often used in constructing IPv6 addresses (see
Section 2.5.1 of [RFC 4291]) and as such may be used to reconstruct
the low-order bits of anonymized IPv6 addresses in certain
circumstances. Therefore, MAC address anonymization is also
important.
Port numbers identify abstract entities (applications) as opposed to
real-world entities, but they can be used to classify hosts and user
behavior. Passive port fingerprinting, both of well-known and
ephemeral ports, can be used to determine the operating system
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running on a host. Relative data volumes by port can also be used to
determine the host's function (workstation, web server, etc.); this
information can be used to identify hosts and users.
While not identifiers in and of themselves, timestamps and counters
can reveal the behavior of the hosts and users on a network. Any
given network activity is recognizable by a pattern of relative time
differences and data volumes in the associated sequence of flows,
even without host address information. Therefore, they can be used
to identify hosts and users. Timestamps and counters are also
vulnerable to traffic injection attacks, where traffic with a known
pattern is injected into a network under measurement, and this
pattern is later identified in the anonymized dataset.
The simplest and most extreme form of anonymization, which can be
applied to any field of a flow record, is black-marker anonymization,
or complete deletion of a given field. Note that black-marker
anonymization is equivalent to simply not exporting the field(s) in
question.
While black-marker anonymization completely protects the data in the
deleted fields from the risk of disclosure, it also reduces the
utility of the anonymized dataset as a whole. Techniques that retain
some information while reducing (though not eliminating) the
disclosure risk will be extensively discussed in the following
sections; note that the techniques specifically applicable to IP
addresses, timestamps, ports, and counters will be discussed in
separate sections.
4.1. IP Address Anonymization
Since IP addresses are the most common identifiers within flow data
that can be used to directly identify a person, organization, or
host, most of the work on flow and trace data anonymization has gone
into IP address anonymization techniques. Indeed, the aim of most
attacks against anonymization is to recover the map from anonymized
IP addresses to original IP addresses thereby identifying the
identified hosts. Therefore, there is a wide range of IP address
anonymization schemes that fit into the following categories.
+------------------------------------+---------------------+
| Scheme | Action |
+------------------------------------+---------------------+
| Truncation | Generalization |
| Reverse Truncation | Generalization |
| Permutation | Direct Substitution |
| Prefix-preserving Pseudonymization | Direct Substitution |
+------------------------------------+---------------------+
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4.1.1. Truncation
Truncation removes "n" of the least significant bits from an IP
address, replacing them with zeroes. In effect, it replaces a host
address with a network address for some fixed netblock; for IPv4
addresses, 8-bit truncation corresponds to replacement with a /24
network address. Truncation is a non-reversible generalization
scheme. Note that while truncation is effective for making hosts
non-identifiable, it preserves information that can be used to
identify an organization, a geographic region, a country, or a
continent.
Truncation to an address length of 0 is equivalent to black-marker
anonymization. Complete removal of IP address information is only
recommended for analysis tasks that have no need to separate flow
data by host or network; e.g., as a first stage to per-application
(port) or time-series total volume analyses.
4.1.2. Reverse Truncation
Reverse truncation removes "n" of the most significant bits from an
IP address, replacing them with zeroes. Reverse truncation is a non-
reversible generalization scheme. Reverse truncation is effective
for making networks unidentifiable, partially or completely removing
information that can be used to identify an organization, a
geographic region, a country, or a continent (or Regional Internet
Registry (RIR) region of responsibility). However, it may cause
ambiguity when applied to data collected from more than one network,
since it treats all the hosts with the same address on different
networks as if they are the same host. It is not particularly useful
when publishing data where the network of origin is known or can be
easily guessed by virtue of the identity of the publisher.
Like truncation, reverse truncation to an address length of 0 is
equivalent to black-marker anonymization.
4.1.3. Permutation
Permutation is a direct substitution technique, replacing each IP
address with an address selected from the set of possible IP
addresses, such that each anonymized address represents a unique
original address. The selection function is often random, though it
is not necessarily so. Permutation does not preserve any structural
information about a network, but it does preserve the unique count of
IP addresses. Any application that requires more structure than
host-uniqueness will not be able to use permuted IP addresses.
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There are many variations of permutation functions, each of which has
trade-offs in performance, security, and guarantees of non-collision;
evaluating these trade-offs is implementation independent. However,
in general, permutation functions applied to anonymization SHOULD be
difficult to reverse without knowing the parameters (e.g., a secret
key for Hashed Message Authentication Code (HMAC). Given the
relatively small space of IPv4 addresses in particular, hash
functions applied without additional parameters could be reversed
through brute force if the hash function is known, and SHOULD NOT be
used as permutation functions. Permutation functions may guarantee
non-collision (i.e., that each anonymized address represents a unique
original address), but need not; however, the probability of
collision SHOULD be low. Nevertheless, we treat even permutations
with low but nonzero collision probability as a direct substitution.
Beyond these guidelines, recommendations for specific permutation
functions are out of scope for this document.
4.1.4. Prefix-Preserving Pseudonymization
Prefix-preserving pseudonymization is a direct substitution
technique, like permutation but further restricted such that the
structure of subnets is preserved at each level while anonymizing IP
addresses. If two real IP addresses match on a prefix of "n" bits,
the two anonymized IP addresses will match on a prefix of "n" bits as
well. This is useful when relationships among networks must be
preserved for a given analysis task, but introduces structure into
the anonymized data that can be exploited in attacks against the
anonymization technique.
Scanning in Internet background traffic can cause particular problems
with this technique: if a scanner uses a predictable and known
sequence of addresses, this information can be used to reverse the
substitution. The low-order portion of the address can be left
unanonymized as a partial defense against this attack.
4.2. MAC Address Anonymization
Flow data containing sub-IP information can also contain identifying
information in the form of the hardware (MAC) address. While MAC
address information cannot be used to locate a node within a network,
it can be used to directly and uniquely identify a specific device.
Vendors or organizations within the supply chain may then have the
information necessary to identify the entity or individual that
purchased the device.
MAC address information is not as structured as IP address
information. EUI-48 and EUI-64 MAC addresses contain an
Organizational Unique Identifier (OUI) in the three most significant
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bytes of the address; this OUI additionally contains bits noting
whether the address is locally or globally administered. Beyond
this, there is no standard relationship among the OUIs assigned to a
given vendor.
Note that MAC address information also appears within IPv6 addresses
as the EAP-64 address, or EAP-48 address encoded as an EAP-64
address, is used as the least significant 64 bits of the IPv6 address
in the case of link-local addressing or stateless autoconfiguration;
the considerations and techniques in this section may then apply to
such IPv6 addresses as well.
+-----------------------------+---------------------+
| Scheme | Action |
+-----------------------------+---------------------+
| Truncation | Generalization |
| Reverse Truncation | Generalization |
| Permutation | Direct Substitution |
| Structured Pseudonymization | Direct Substitution |
+-----------------------------+---------------------+
4.2.1. Truncation
Truncation removes "n" of the least significant bits from a MAC
address, replacing them with zeroes. In effect, it retains bits of
OUI, which identifies the manufacturer, while removing the least
significant bits identifying the particular device. Truncation of 24
bits of an EAP-48 or 40 bits of an EAP-64 address zeroes out the
device identifier while retaining the OUI.
Truncation is effective for making device manufacturers partially or
completely identifiable within a dataset while deleting unique host
identifiers; this can be used to retain and aggregate MAC-layer
behavior by vendor.
Truncation to an address length of 0 is equivalent to black-marker
anonymization.
4.2.2. Reverse Truncation
Reverse truncation removes "n" of the most significant bits from a
MAC address, replacing them with zeroes. Reverse truncation is a
non-reversible generalization scheme. This has the effect of
removing bits of the OUI, which identify manufacturers, before
removing the least significant bits. Reverse truncation of 24 bits
zeroes out the OUI.
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Reverse truncation is effective for making device manufacturers
partially or completely unidentifiable within a dataset. However, it
may cause ambiguity by introducing the possibility of truncated MAC
address collision. Also, note that the utility of removing
manufacturer information is not particularly well covered by the
literature.
Reverse truncation to an address length of 0 is equivalent to black-
marker anonymization.
4.2.3. Permutation
Permutation is a direct substitution technique, replacing each MAC
address with an address selected from the set of possible MAC
addresses, such that each anonymized address represents a unique
original address. The selection function is often random, though it
is not necessarily so. Permutation does not preserve any structural
information about a network, but it does preserve the unique count of
devices on the network. Any application that requires more structure
than host-uniqueness will not be able to use permuted MAC addresses.
There are many variations of permutation functions, each of which has
trade-offs in performance, security, and guarantees of non-collision;
evaluating these trade-offs is implementation independent. However,
in general, permutation functions applied to anonymization SHOULD be
difficult to reverse without knowing the parameters (e.g., a secret
key for HMAC). While the EAP-48 space is larger than the IPv4
address space, hash functions applied without additional parameters
could be reversed through brute force if the hash function is known,
and SHOULD NOT be used as permutation functions. Permutation
functions may guarantee non-collision (i.e., that each anonymized
address represents a unique original address), but need not; however,
the probability of collision SHOULD be low. Nevertheless, we treat
even permutations with low but nonzero collision probability as a
direct substitution. Beyond these guidelines, recommendations for
specific permutation functions are out of scope for this document.
4.2.4. Structured Pseudonymization
Structured pseudonymization for MAC addresses is a direct
substitution technique, like permutation, but restricted such that
the OUI (the most significant three bytes) is permuted separately
from the node identifier, the remainder. This is useful when the
uniqueness of OUIs must be preserved for a given analysis task, but
introduces structure into the anonymized data that can be exploited
in attacks against the anonymization technique.
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4.3. Timestamp Anonymization
The particular time at which a flow began or ended is not
particularly identifiable information, but it can be used as part of
attacks against other anonymization techniques or for user profiling,
e.g., as in [Mur07]. Timestamps can be used in traffic injection
attacks, which use known information about a set of traffic generated
or otherwise known by an attacker to recover mappings of other
anonymized fields, as well as to identify certain activity by
response delay and size fingerprinting, which compares response sizes
and inter-flow times in anonymized data to known values. Note that
these attacks have been shown to be relatively robust against
timestamp anonymization techniques (see [Bur10]), so the techniques
presented in this section are relatively weak and should be used with
care.
+-----------------------+----------------------------+
| Scheme | Action |
+-----------------------+----------------------------+
| Precision Degradation | Generalization |
| Enumeration | Direct or Set Substitution |
| Random Shifts | Direct Substitution |
+-----------------------+----------------------------+
4.3.1. Precision Degradation
Precision Degradation is a generalization technique that removes the
most precise components of a timestamp, accounting for all events
occurring in each given interval (e.g., one millisecond for
millisecond level degradation) as simultaneous. This has the effect
of potentially collapsing many timestamps into one. With this
technique, time precision is reduced and sequencing may be lost, but
the information regarding at which time the event occurred is
preserved. The anonymized data may not be generally useful for
applications that require strict sequencing of flows.
Note that flow meters with low time precision (e.g., second
precision, or millisecond precision on high-capacity networks)
perform the equivalent of precision degradation anonymization by
their design.
Also, note that degradation to a very low precision (e.g., on the
order of minutes, hours, or days) is commonly used in analyses
operating on time-series aggregated data, and may also be described
as binning; though the time scales are longer and applicability more
restricted, in principle, this is the same operation.
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RFC 6235 IP Flow Anonymization Support May 2011
Precision degradation to infinitely low precision is equivalent to
black-marker anonymization. Removal of timestamp information is only
recommended for analysis tasks that have no need to separate flows in
time, for example, for counting total volumes or unique occurrences
of other flow keys in an entire dataset.
4.3.2. Enumeration
Enumeration is a substitution function that retains the chronological
order in which events occurred while eliminating time information.
Timestamps are substituted by equidistant timestamps (or numbers)
starting from a randomly chosen start value. The resulting data is
useful for applications requiring strict sequencing, but not for
those requiring good timing information (e.g., delay- or jitter-
measurement for quality-of-service (QoS) applications or service-
level agreement (SLA) validation).
Note that enumeration is functionally equivalent to precision
degradation in any environment into which traffic can be regularly
injected to serve as a clock at the precision of the frequency of the
injected flows.
4.3.3. Random Shifts
Random time shifts add a random offset to every timestamp within a
dataset. Therefore, this reversible substitution technique retains
duration and inter-event interval information as well as the
chronological order of flows. Random time shifts are quite weak and
relatively easy to reverse in the presence of external knowledge
about traffic on the measured network.
4.4. Counter Anonymization
Counters (such as packet and octet volumes per flow) are subject to
fingerprinting and injection attacks against anonymization or for
user profiling as timestamps are. Data sets with anonymized counters
are useful only for analysis tasks for which relative or imprecise
magnitudes of activity are useful. Counter information can also be
completely removed, but this is only recommended for analysis tasks
that have no need to evaluate the removed counter, for example, for
counting only unique occurrences of other flow keys.
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RFC 6235 IP Flow Anonymization Support May 2011
+-----------------------+----------------------------+
| Scheme | Action |
+-----------------------+----------------------------+
| Precision Degradation | Generalization |
| Binning | Generalization |
| Random noise addition | Direct or Set Substitution |
+-----------------------+----------------------------+
4.4.1. Precision Degradation
As with precision degradation in timestamps, precision degradation of
counters removes lower-order bits of the counters, treating all the
counters in a given range as having the same value. Depending on the
precision reduction, this loses information about the relationships
between sizes of similarly sized flows, but keeps relative magnitude
information. Precision degradation to an infinitely low precision is
equivalent to black-marker anonymization.
4.4.2. Binning
Binning can be seen as a special case of precision degradation; the
operation is identical, except for in precision degradation the
counter ranges are uniform, and in binning, they need not be. For
example, consider separating unopened TCP connections from
potentially opened TCP connections. Here, packet counters per flow
would be binned into two bins, one for 1-2 packet flows, and one for
flows with 3 or more packets. Binning schemes are generally chosen
to keep precisely the amount of information required in a counter for
a given analysis task. Note that, also unlike precision degradation,
the bin label need not be within the bin's range. Binning counters
to a single bin is equivalent to black-marker anonymization.
4.4.3. Random Noise Addition
Random noise addition adds a random amount to a counter in each flow;
this is used to keep relative magnitude information and minimize the
disruption to size relationship information while avoiding
fingerprinting attacks against anonymization. Note that there is no
guarantee that random noise addition will maintain ranking order by a
counter among members of a set. Random noise addition is
particularly useful when the derived analysis data will not be
presented in such a way as to require the lower-order bits of the
counters.
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RFC 6235 IP Flow Anonymization Support May 2011
4.5. Anonymization of Other Flow Fields
Other fields, particularly port numbers and protocol numbers, can be
used to partially identify the applications that generated the
traffic in a given flow trace. This information can be used in
fingerprinting attacks, and may be of interest on its own (e.g., to
reveal that a certain application with suspected vulnerabilities is
running on a given network). These fields are generally anonymized
using one of two techniques.
+-------------+---------------------+
| Scheme | Action |
+-------------+---------------------+
| Binning | Generalization |
| Permutation | Direct Substitution |
+-------------+---------------------+
4.5.1. Binning
Binning is a generalization technique mapping a set of potentially
non-uniform ranges into a set of arbitrarily labeled bins. Common
bin arrangements depend on the field type and the analysis
application. For example, an IP protocol bin arrangement may
preserve 1, 6, and 17 for ICMP, UDP, and TCP traffic, and bin all
other protocols into a single bin, to mitigate the use of uncommon
protocols in fingerprinting attacks. Another example arrangement may
bin source and destination ports into low (0-1023) and high (1024-
65535) bins in order to tell service from ephemeral ports without
identifying individual applications.
Binning other flow key fields to a single bin is equivalent to black-
marker anonymization. Removal of other flow key information is only
recommended for analysis tasks that have no need to differentiate
flows on the removed keys, for example, for total traffic counts or
unique counts of other flow keys.
4.5.2. Permutation
Permutation is a direct substitution technique, replacing each value
with an value selected from the set of possible range, such that each
anonymized value represents a unique original value. This is used to
preserve the count of unique values without preserving information
about, or the ordering of, the values themselves.
While permutation ideally guarantees that each anonymized value
represents a unique original value, such may require significant
state in the Intermediate Anonymization Process. Therefore,
permutation may be implemented by hashing for performance reasons,
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RFC 6235 IP Flow Anonymization Support May 2011
with hash functions that may have relatively small collision
probabilities. Such techniques are still essentially direct
substitution techniques, despite the nonzero error probability.
5. Parameters for the Description of Anonymization Techniques
This section details the abstract parameters used to describe the
anonymization techniques examined in the previous section, on a per-
parameter basis. These parameters and their export safety inform the
design of the IPFIX anonymization metadata export specified in the
following section.
5.1. Stability
A stable anonymization will always map a given value in the real
space to a given value in the anonymized space, while an unstable
anonymization will change this mapping over time; a completely
unstable anonymization is essentially indistinguishable from black-
marker anonymization. Any given anonymization technique may be
applied with a varying range of stability. Stability is important
for assessing the comparability of anonymized information in
different datasets, or in the same dataset over different time
periods. In practice, an anonymization may also be stable for every
dataset published by a particular producer to a particular consumer,
stable for a stated time period within a dataset or across datasets,
or stable only for a single dataset.
If no information about stability is available, users of anonymized
data MAY assume that the techniques used are stable across the entire
dataset, but unstable across datasets. Note that stability presents
a risk-utility trade-off, as completely stable anonymization can be
used for longer-term trend analysis tasks but also presents more risk
of attack given the stable mapping. Information about the stability
of a mapping SHOULD be exported along with the anonymized data.
5.2. Truncation Length
Truncation and precision degradation are described by the truncation
length or the amount of data still remaining in the anonymized field
after anonymization.
Truncation length can generally be inferred from a given dataset, and
need not be specially exported or protected. For bit-level
truncation, the truncated bits are generally inferable by the least
significant bit set for an instance of an Information Element
described by a given Template (or the most significant bit set, in
the case of reverse truncation). For precision degradation, the
truncation is inferable from the maximum precision given. Note that
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RFC 6235 IP Flow Anonymization Support May 2011
while this inference method is generally applicable, it is data
dependent: there is no guarantee that it will recover the exact
truncation length used to prepare the data.
In the special case of IP address export with variable (per-record)
truncation, the truncation MAY be expressed by exporting the prefix
length alongside the address.
5.3. Bin Map
Binning is described by the specification of a bin mapping function.
This function can be generally expressed in terms of an associative
array that maps each point in the original space to a bin, although
from an implementation standpoint most bin functions are much simpler
and more efficient.
Since the bin map for a bin mapping function is in essence the bin
mapping key, and can be used to partially deanonymize binned data,
depending on the degree of generalization, information about the bin
mapping function SHOULD NOT be exported.
5.4. Permutation
Like binning, permutation is described by the specification of a
permutation function. In the general case, this can be expressed in
terms of an associative array that maps each point in the original
space to a point in the anonymized space. Unlike binning, each point
in the anonymized space corresponds to a single, unique point in the
original space.
Since the parameters of the permutation function are in essence key-
like (indeed, for cryptographic permutation functions, they are the
keys themselves), information about the permutation function or its
parameters SHOULD NOT be exported.
5.5. Shift Amount
Shifting requires an amount by which to shift each value. Since the
shift amount is the only key to a shift function, and can be used to
trivially deanonymize data protected by shifting, information about
the shift amount SHOULD NOT be exported.
6. Anonymization Export Support in IPFIX
Anonymized data exported via IPFIX SHOULD be annotated with
anonymization metadata, which details which fields described by which
Templates are anonymized, and provides appropriate information on the
anonymization techniques used. This metadata SHOULD be exported in
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RFC 6235 IP Flow Anonymization Support May 2011
Data Records described by the recommended Options Templates described
in this section; these Options Templates use the additional
Information Elements described in the following subsection.
Note that fields anonymized using the black-marker (removal)
technique do not require any special metadata support: black-marker
anonymized fields SHOULD NOT be exported at all, by omitting the
corresponding Information Elements from Template describing the Data
Set. In the case where application requirements dictate that a
black-marker anonymized field must remain in a Template, then an
Exporting Process MAY export black-marker anonymized fields with
their native length as all-zeros, but only in cases where enough
contextual information exists within the record to differentiate a
black-marker anonymized field exported in this way from a real zero
value.
6.1. Anonymization Records and the Anonymization Options Template
The Anonymization Options Template describes Anonymization Records,
which allow anonymization metadata to be exported inline over IPFIX
or stored in an IPFIX File, by binding information about
anonymization techniques to Information Elements within defined
Templates or Options Templates. IPFIX Exporting Processes SHOULD
export anonymization records for any Template describing exported
anonymized Data Records; IPFIX Collecting Processes and processes
downstream from them MAY use anonymization records to treat
anonymized data differently depending on the applied technique.
Anonymization Records contain ancillary information bound to a
Template, so many of the considerations for Templates apply to
Anonymization Records as well. First, reliability is important: an
Exporting Process SHOULD export Anonymization Records after the
Templates they describe have been exported, and SHOULD export
anonymization records reliably if supported by the underlying
transport (i.e., without partial reliability when using Stream
Control Transmission Protocol (SCTP)).
Anonymization Records MUST be handled by Collecting Processes as
scoped to the Template to which they apply within the Transport
Session in which they are sent. When a Template is withdrawn via a
Template Withdrawal Message or expires during a UDP transport
session, the accompanying Anonymization Records are withdrawn or
expire as well and do not apply to subsequent Templates with the same
Template ID within the Session unless re-exported.
The Stability Class within the anonymizationFlags IE can be used to
declare that a given anonymization technique's mapping will remain
stable across multiple sessions, but this does not mean that
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RFC 6235 IP Flow Anonymization Support May 2011
anonymization technique information given in the Anonymization
Records themselves persist across Sessions. Each new Transport
Session MUST contain new Anonymization Records for each Template
describing anonymized Data Sets.
SCTP per-stream export [IPFIX-PERSTREAM] may be used to ease
management of Anonymization Records if appropriate for the
application.
The fields of the Anonymization Options Template are as follows:
+-------------------------+-----------------------------------------+
| IE | Description |
+-------------------------+-----------------------------------------+
| templateId [scope] | The Template ID of the Template or |
| | Options Template containing the |
| | Information Element described by this |
| | anonymization record. This Information |
| | Element MUST be defined as a Scope |
| | Field. |
| informationElementId | The Information Element identifier of |
| [scope] | the Information Element described by |
| | this anonymization record. This |
| | Information Element MUST be defined as |
| | a Scope Field. Exporting Processes |
| | MUST clear then Enterprise bit of the |
| | informationElementId and Collecting |
| | Processes SHOULD ignore it; information |
| | about enterprise-specific Information |
| | Elements is exported via the |
| | privateEnterpriseNumber Information |
| | Element. |
| privateEnterpriseNumber | The Private Enterprise Number of the |
| [scope] [optional] | enterprise-specific Information Element |
| | described by this anonymization record. |
| | This Information Element MUST be |
| | defined as a Scope Field if present. A |
| | privateEnterpriseNumber of 0 signifies |
| | that the Information Element is |
| | IANA-registered. |
| informationElementIndex | The Information Element index of the |
| [scope] [optional] | instance of the Information Element |
| | described by this anonymization record |
| | identified by the informationElementId |
| | within the Template. Optional; need |
| | only be present when describing |
| | Templates that have multiple instances |
| | of the same Information Element. This |
Boschi & Trammell Experimental PAGE 22
RFC 6235 IP Flow Anonymization Support May 2011
| | Information Element MUST be defined as |
| | a Scope Field if present. This |
| | Information Element is defined in |
| | Section 6.2. |
| anonymizationFlags | Flags describing the mapping stability |
| | and specialized modifications to the |
| | Anonymization Technique in use. SHOULD |
| | be present. This Information Element |
| | is defined in Section 6.2.3. |
| anonymizationTechnique | The technique used to anonymize the |
| | data. MUST be present. This |
| | Information Element is defined in |
| | Section 6.2.2. |
+-------------------------+-----------------------------------------+
6.2. Recommended Information Elements for Anonymization Metadata
6.2.1. informationElementIndex
Description: A zero-based index of an Information Element
referenced by informationElementId within a Template referenced by
templateId; used to disambiguate scope for templates containing
multiple identical Information Elements.
Abstract Data Type: unsigned16
Data Type Semantics: identifier
ElementId: 287
Status: Current
6.2.2. anonymizationTechnique
Description: A description of the anonymization technique applied
to a referenced Information Element within a referenced Template.
Each technique may be applicable only to certain Information
Elements and recommended only for certain Information Elements;
these restrictions are noted in the table below.
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RFC 6235 IP Flow Anonymization Support May 2011
+-------+---------------------------+-----------------+-------------+
| Value | Description | Applicable to | Recommended |
| | | | for |
+-------+---------------------------+-----------------+-------------+
| 0 | Undefined: the Exporting | all | all |
| | Process makes no | | |
| | representation as to | | |
| | whether or not the | | |
| | defined field is | | |
| | anonymized. While the | | |
| | Collecting Process MAY | | |
| | assume that the field is | | |
| | not anonymized, it is not | | |
| | guaranteed not to be. | | |
| | This is the default | | |
| | anonymization technique. | | |
| 1 | None: the values exported | all | all |
| | are real. | | |
| 2 | Precision | all | all |
| | Degradation/Truncation: | | |
| | the values exported are | | |
| | anonymized using simple | | |
| | precision degradation or | | |
| | truncation. The new | | |
| | precision or number of | | |
| | truncated bits is | | |
| | implicit in the exported | | |
| | data and can be deduced | | |
| | by the Collecting | | |
| | Process. | | |
| 3 | Binning: the values | all | all |
| | exported are anonymized | | |
| | into bins. | | |
| 4 | Enumeration: the values | all | timestamps |
| | exported are anonymized | | |
| | by enumeration. | | |
| 5 | Permutation: the values | all | identifiers |
| | exported are anonymized | | |
| | by permutation. | | |
| 6 | Structured Permutation: | addresses | |
| | the values exported are | | |
| | anonymized by | | |
| | permutation, preserving | | |
| | bit-level structure as | | |
| | appropriate; this | | |
| | represents | | |
| | prefix-preserving IP | | |
| | address anonymization or | | |
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RFC 6235 IP Flow Anonymization Support May 2011
| | structured MAC address | | |
| | anonymization. | | |
| 7 | Reverse Truncation: the | addresses | |
| | values exported are | | |
| | anonymized using reverse | | |
| | truncation. The number | | |
| | of truncated bits is | | |
| | implicit in the exported | | |
| | data, and can be deduced | | |
| | by the Collecting | | |
| | Process. | | |
| 8 | Noise: the values | non-identifiers | counters |
| | exported are anonymized | | |
| | by adding random noise to | | |
| | each value. | | |
| 9 | Offset: the values | all | timestamps |
| | exported are anonymized | | |
| | by adding a single offset | | |
| | to all values. | | |
+-------+---------------------------+-----------------+-------------+
Abstract Data Type: unsigned16
Data Type Semantics: identifier
ElementId: 286
Status: Current
6.2.3. anonymizationFlags
Description: A flag word describing specialized modifications to
the anonymization policy in effect for the anonymization technique
applied to a referenced Information Element within a referenced
Template. When flags are clear (0), the normal policy (as
described by anonymizationTechnique) applies without modification.
MSB 14 13 12 11 10 9 8 7 6 5 4 3 2 1 LSB
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Reserved |LOR|PmA| SC |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
anonymizationFlags IE
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RFC 6235 IP Flow Anonymization Support May 2011
+--------+----------+-----------------------------------------------+
| bit(s) | name | description |
| (LSB = | | |
| 0) | | |
+--------+----------+-----------------------------------------------+
| 0-1 | SC | Stability Class: see the Stability Class |
| | | table below, and Section 5.1. |
| 2 | PmA | Perimeter Anonymization: when set (1), source |
| | | Information Elements as described in |
| | | [RFC 5103] are interpreted as external |
| | | addresses, and destination Information |
| | | Elements as described in [RFC 5103] are |
| | | interpreted as internal addresses, for the |
| | | purposes of associating |
| | | anonymizationTechnique to Information |
| | | Elements only; see Section 7.2.2 for details. |
| | | This bit MUST NOT be set when associated with |
| | | a non-endpoint (i.e., source or destination) |
| | | Information Element. SHOULD be consistent |
| | | within a record (i.e., if a source |
| | | Information Element has this flag set, the |
| | | corresponding destination element SHOULD have |
| | | this flag set, and vice versa.) |
| 3 | LOR | Low-Order Unchanged: when set (1), the |
| | | low-order bits of the anonymized Information |
| | | Element contain real data. This modification |
| | | is intended for the anonymization of |
| | | network-level addresses while leaving |
| | | host-level addresses intact in order to |
| | | preserve host level-structure, which could |
| | | otherwise be used to reverse anonymization. |
| | | MUST NOT be set when associated with a |
| | | truncation-based anonymizationTechnique. |
| 4-15 | Reserved | Reserved for future use: SHOULD be cleared |
| | | (0) by the Exporting Process and MUST be |
| | | ignored by the Collecting Process. |
+--------+----------+-----------------------------------------------+
The Stability Class portion of this flags word describes the
stability class of the anonymization technique applied to a
referenced Information Element within a referenced Template.
Stability classes refer to the stability of the parameters of the
anonymization technique, and therefore the comparability of the
mapping between the real and anonymized values over time. This
determines which anonymized datasets may be compared with each
other. Values are as follows:
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RFC 6235 IP Flow Anonymization Support May 2011
+-----+-----+-------------------------------------------------------+
| Bit | Bit | Description |
| 1 | 0 | |
+-----+-----+-------------------------------------------------------+
| 0 | 0 | Undefined: the Exporting Process makes no |
| | | representation as to how stable the mapping is, or |
| | | over what time period values of this field will |
| | | remain comparable; while the Collecting Process MAY |
| | | assume Session level stability, Session level |
| | | stability is not guaranteed. Processes SHOULD assume |
| | | this is the case in the absence of stability class |
| | | information; this is the default stability class. |
| 0 | 1 | Session: the Exporting Process will ensure that the |
| | | parameters of the anonymization technique are stable |
| | | during the Transport Session. All the values of the |
| | | described Information Element for each Record |
| | | described by the referenced Template within the |
| | | Transport Session are comparable. The Exporting |
| | | Process SHOULD endeavor to ensure at least this |
| | | stability class. |
| 1 | 0 | Exporter-Collector Pair: the Exporting Process will |
| | | ensure that the parameters of the anonymization |
| | | technique are stable across Transport Sessions over |
| | | time with the given Collecting Process, but may use |
| | | different parameters for different Collecting |
| | | Processes. Data exported to different Collecting |
| | | Processes are not comparable. |
| 1 | 1 | Stable: the Exporting Process will ensure that the |
| | | parameters of the anonymization technique are stable |
| | | across Transport Sessions over time, regardless of |
| | | the Collecting Process to which it is sent. |
+-----+-----+-------------------------------------------------------+
Abstract Data Type: unsigned16
Data Type Semantics: flags
ElementId: 285
Status: Current
7. Applying Anonymization Techniques to IPFIX Export and Storage
When exporting or storing anonymized flow data using IPFIX, certain
interactions between the IPFIX protocol and the anonymization
techniques in use must be considered; these are treated in the
subsections below.
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RFC 6235 IP Flow Anonymization Support May 2011
7.1. Arrangement of Processes in IPFIX Anonymization
Anonymization may be applied to IPFIX data at three stages within the
collection infrastructure: on initial export, at a mediator, or after
collection, as shown in Figure 1. Each of these locations has
specific considerations and applicability.
+==========================================+
| Exporting Process |
+==========================================+
| |
| (Anonymized at Original Exporter) |
V |
+=============================+ |
| Mediator | |
+=============================+ |
| |
| (Anonymizing Mediator) |
V V
+==========================================+
| Collecting Process |
+==========================================+
|
| (Anonymizing CP/File Writer)
V
+--------------------+
| IPFIX File Storage |
+--------------------+
Figure 1: Potential Anonymization Locations
Anonymization is generally performed before the wider dissemination
or repurposing of a dataset, e.g., adapting operational measurement
data for research. Therefore, direct anonymization of flow data on
initial export is only applicable in certain restricted
circumstances: when the Exporting Process (EP) is "publishing" data
to a Collecting Process (CP) directly, and the Exporting Process and
Collecting Process are operated by different entities. Note that
certain guidelines in Section 7.2.3 with respect to timestamp
anonymization may not apply in this case, as the Collecting Process
may be able to deduce certain timing information from the time at
which each Message is received.
A much more flexible arrangement is to anonymize data within a
Mediator [RFC 6183]. Here, original data is sent to a Mediator, which
performs the anonymization function and re-exports the anonymized
data. Such a Mediator could be located at the administrative domain
boundary of the initial Exporting Process operator, exporting
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RFC 6235 IP Flow Anonymization Support May 2011
anonymized data to other consumers outside the organization. In this
case, the original Exporter SHOULD use TLS [RFC 5246] as specified in
[RFC 5101] to secure the channel to the Mediator, and the Mediator
should follow the guidelines in Section 7.2, to mitigate the risk of
original data disclosure.
When data is to be published as an anonymized dataset in an IPFIX
File [RFC 5655], the anonymization may be done at the final Collecting
Process before storage and dissemination, as well. In this case, the
Collector should follow the guidelines in Section 7.2, especially as
regards File-specific Options in Section 7.2.4
In each of these data flows, the anonymization of records is
undertaken by an Intermediate Anonymization Process (IAP); the data
flows into and out of this IAP are shown in Figure 2 below.
packets --+ +- IPFIX Messages -+
| | |
V V V
+==================+ +====================+ +=============+
| Metering Process | | Collecting Process | | File Reader |
+==================+ +====================+ +=============+
| Non-anonymized | Records |
V V V
+=========================================================+
| Intermediate Anonymization Process (IAP) |
+=========================================================+
| Anonymized ^ Anonymized |
| Records | Records |
V | V
+===================+ Anonymization +=============+
| Exporting Process |<--- Parameters ------>| File Writer |
+===================+ +=============+
| |
+------------> IPFIX Messages <----------+
Figure 2: Data Flows through the Anonymization Process
Anonymization parameters must also be available to the Exporting
Process and/or File Writer in order to ensure header data is also
appropriately anonymized as in Section 7.2.3.
Following each of the data flows through the IAP, we describe five
basic types of anonymization arrangements within this framework in
Figure 3. In addition to the three arrangements described in detail
above, anonymization can also be done at a collocated Metering
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RFC 6235 IP Flow Anonymization Support May 2011
Process (MP) and File Writer (FW) (see Section 7.3.2 of [RFC 5655]),
or at a file manipulator, which combines a File Writer with a File
Reader (FR) (see Section 7.3.7 of [RFC 5655]).
+----+ +-----+ +----+
pkts -> | MP |->| IAP |->| EP |-> Anonymization on Original Exporter
+----+ +-----+ +----+
+----+ +-----+ +----+
pkts -> | MP |->| IAP |->| FW |-> Anonymizing collocated MP/File Writer
+----+ +-----+ +----+
+----+ +-----+ +----+
IPFIX -> | CP |->| IAP |->| EP |-> Anonymizing Mediator (Masq. Proxy)
+----+ +-----+ +----+
+----+ +-----+ +----+
IPFIX -> | CP |->| IAP |->| FW |-> Anonymizing collocated CP/File Writer
+----+ +-----+ +----+
+----+ +-----+ +----+
IPFIX -> | FR |->| IAP |->| FW |-> Anonymizing file manipulator
File +----+ +-----+ +----+
Figure 3: Possible Anonymization Arrangements in the IPFIX
Architecture
Note that anonymization may occur at more than one location within a
given collection infrastructure, to provide varying levels of
anonymization, disclosure risk, or data utility for specific
purposes.
7.2. IPFIX-Specific Anonymization Guidelines
In implementing and deploying the anonymization techniques described
in this document, implementors should note that IPFIX already
provides features that support anonymized data export, and use these
where appropriate. Care must also be taken that data structures
supporting the operation of the protocol itself do not leak data that
could be used to reverse the anonymization applied to the flow data.
Such data structures may appear in the header, or within the data
stream itself, especially as options data. Each of these and their
impact on specific anonymization techniques is noted in a separate
subsection below.
7.2.1. Appropriate Use of Information Elements for Anonymized Data
Note, as in Section 6 above, that black-marker anonymized fields
SHOULD NOT be exported at all; the absence of the field in a given
Data Set is implicitly declared by not including the corresponding
Information Element in the Template describing that Data Set.
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RFC 6235 IP Flow Anonymization Support May 2011
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 [RFC 5153]. For
example, timestamps measured in millisecond-level precision and
degraded to second-level precision should use flowStartSeconds and
flowEndSeconds, not flowStartMilliseconds and flowEndMilliseconds.
When exporting anonymized data and anonymization metadata, Exporting
Processes SHOULD ensure that the combination of Information Element
and declared anonymization technique are compatible. Specifically,
the applicable and recommended Information Element types and
semantics for each technique are noted in the description of the
anonymizationTechnique Information Element in Section 6.2.2. In this
description, a timestamp is an Information Element with the data type
dateTimeSeconds, dataTimeMilliseconds, dateTimeMicroseconds, or
dateTimeNanoseconds; an address is an Information Element with the
data type ipv4Address, ipv6Address, or macAddress; and an identifier
is an Information Element with identifier data type semantics.
Exporting Process MUST NOT export Anonymization Options records
binding techniques to Information Elements to which they are not
applicable, and SHOULD NOT export Anonymization Options records
binding techniques to Information Elements for which they are not
recommended.
7.2.2. Export of Perimeter-Based Anonymization Policies
Data collected from a single network may require different
anonymization policies for addresses internal and external to the
network. For example, internal addresses could be subject to simple
permutation, while external addresses could be aggregated into
networks by truncation. When exporting anonymized perimeter
bidirectional flow (biflow) data as in Section 5.2 of [RFC 5103], this
arrangement may be easily represented by specifying one technique for
source endpoint information (which represents the external endpoint
in a perimeter biflow) and one technique for destination endpoint
information (which represents the internal address in a perimeter
biflow).
However, it can also be useful to represent perimeter-based
anonymization policies with unidirectional flow (uniflow), or non-
perimeter biflow data. In this case, the Perimeter Anonymization bit
(bit 2) in the anonymizationFlags Information Element describing the
anonymized address Information Elements can be set to change the
meaning of "source" and "destination" of Information Elements to mean
"external" and "internal" as with perimeter biflows, but only with
respect to anonymization policies.
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7.2.3. Anonymization of Header Data
Each IPFIX Message contains a Message Header; within this Message
Header are contained two fields which may be used to break certain
anonymization techniques: the Export Time, and the Observation Domain
ID.
Export of IPFIX Messages containing anonymized timestamp data where
the original Export Time Message header has some relationship to the
anonymized timestamps SHOULD anonymize the Export Time header field
so that the Export Time is consistent with the anonymized timestamp
data. Otherwise, relationships between export and flow time could be
used to partially or totally reverse timestamp anonymization. When
anonymizing timestamps and the Export Time header field SHOULD avoid
times too far in the past or future; while [RFC 5101] does not make
any allowance for Export Time error detection, it is sensible that
Collecting Processes may interpret Messages with seemingly
nonsensical Export Times as erroneous. Specific limits are
implementation dependent, but this issue may cause interoperability
issues when anonymizing the Export Time header field.
The similarity in size between an Observation Domain ID and an IPv4
address (32 bits) may lead to a temptation to use an IPv4 interface
address on the Metering or Exporting Process as the Observation
Domain ID. If this address bears some relation to the IP addresses
in the flow data (e.g., shares a network prefix with internal
addresses) and the IP addresses in the flow data are anonymized in a
structure-preserving way, then the Observation Domain ID may be used
to break the IP address anonymization. Use of an IPv4 interface
address on the Metering or Exporting Process as the Observation
Domain ID is NOT RECOMMENDED in this case.
7.2.4. Anonymization of Options Data
IPFIX uses the Options mechanism to export, among other things,
metadata about exported flows and the flow collection infrastructure.
As with the IPFIX Message Header, certain Options recommended in
[RFC 5101] and [RFC 5655] containing flow timestamps and network
addresses of Exporting and Collecting Processes may be used to break
certain anonymization techniques. When using these Options along
anonymized data export and storage, values within the Options that
could be used to break the anonymization SHOULD themselves be
anonymized or omitted.
The Exporting Process Reliability Statistics Options Template,
recommended in [RFC 5101], contains an Exporting Process ID field,
which may be an exportingProcessIPv4Address Information Element or an
exportingProcessIPv6Address Information Element. If the Exporting
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RFC 6235 IP Flow Anonymization Support May 2011
Process address bears some relation to the IP addresses in the flow
data (e.g., shares a network prefix with internal addresses) and the
IP addresses in the flow data are anonymized in a structure-
preserving way, then the Exporting Process address may be used to
break the IP address anonymization. Exporting Processes exporting
anonymized data in this situation SHOULD mitigate the risk of attack
either by omitting Options described by the Exporting Process
Reliability Statistics Options Template or by anonymizing the
Exporting Process address using a similar technique to that used to
anonymize the IP addresses in the exported data.
Similarly, the Export Session Details Options Template and Message
Details Options Template specified for the IPFIX File Format
[RFC 5655] may contain the exportingProcessIPv4Address Information
Element or the exportingProcessIPv6Address Information Element to
identify an Exporting Process from which a flow record was received,
and the collectingProcessIPv4Address Information Element or the
collectingProcessIPv6Address Information Element to identify the
Collecting Process which received it. If the Exporting Process or
Collecting Process address bears some relation to the IP addresses in
the dataset (e.g., shares a network prefix with internal addresses)
and the IP addresses in the dataset are anonymized in a structure-
preserving way, then the Exporting Process or Collecting Process
address may be used to break the IP address anonymization. Since
these Options Templates are primarily intended for storing IPFIX
Transport Session data for auditing, replay, and testing purposes, it
is NOT RECOMMENDED that storage of anonymized data include these
Options Templates in order to mitigate the risk of attack.
The Message Details Options Template specified for the IPFIX File
Format [RFC 5655] also contains the collectionTimeMilliseconds
Information Element. As with the Export Time Message Header field,
if the exported dataset contains anonymized timestamp information,
and the collectionTimeMilliseconds Information Element in a given
Message has some relationship to the anonymized timestamp
information, then this relationship can be exploited to reverse the
timestamp anonymization. Since this Options Template is primarily
intended for storing IPFIX Transport Session data for auditing,
replay, and testing purposes, it is NOT RECOMMENDED that storage of
anonymized data include this Options Template in order to mitigate
the risk of attack.
Since the Time Window Options Template specified for the IPFIX File
Format [RFC 5655] refers to the timestamps within the dataset to
provide partial table of contents information for an IPFIX File,
Options described by this Template SHOULD be written using the
anonymized timestamps instead of the original ones.
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7.2.5. Special-Use Address Space Considerations
When anonymizing data for transport or storage using IPFIX containing
anonymized IP addresses, and the analysis purpose permits doing so,
it is RECOMMENDED to filter out or leave unanonymized data containing
the special-use IPv4 addresses enumerated in [RFC 5735] or the
special-use IPv6 addresses enumerated in [RFC 5156]. Data containing
these addresses (e.g. 0.0.0.0 and 169.254.0.0/16 for link-local
autoconfiguration in IPv4 space) are often associated with specific,
well-known behavioral patterns. Detection of these patterns in
anonymized data can lead to deanonymization of these special-use
addresses, which increases the chance of a complete reversal of
anonymization by an attacker, especially of prefix-preserving
techniques.
7.2.6. Protecting Out-of-Band Configuration and Management Data
Special care should be taken when exporting or sharing anonymized
data to avoid information leakage via the configuration or management
planes of the IPFIX Device containing the Exporting Process or the
File Writer. For example, adding noise to counters is useless if the
receiver can deduce the values in the counters from Simple Network
Management Protocol (SNMP) information, and concealing the network
under test is similarly useless if such information is available in a
configuration document. As the specifics of these concerns are
largely implementation and deployment dependent, specific mitigation
is out of scope for this document. The general ground rule is that
information of similar type to that anonymized SHOULD NOT be made
available to the receiver by any means, whether in the Data Records,
in IPFIX protocol structures such as Message Headers, or out of band.
8. Examples
In this example, consider the export or storage of an anonymized IPv4
dataset from a single network described by a simple Template
containing a timestamp in seconds, a five-tuple, and packet and octet
counters. The Template describing each record in this Data Set is
shown in Figure 4.
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RFC 6235 IP Flow Anonymization Support May 2011
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set ID = 2 | Length = 40 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template ID = 256 | Field Count = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| flowStartSeconds 150 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| sourceIPv4Address 8 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| destinationIPv4Address 12 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| sourceTransportPort 7 | Field Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| destinationTransportPort 11 | Field Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| packetDeltaCount 2 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| octetDeltaCount 1 | Field Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| protocolIdentifier 4 | Field Length = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Example Flow Template
Suppose that this Data Set is anonymized according to the following
policy:
o IP addresses within the network are protected by reverse
truncation.
o IP addresses outside the network are protected by prefix-
preserving anonymization.
o Octet counts are exported using degraded precision in order to
provide minimal protection against fingerprinting attacks.
o All other fields are exported unanonymized.
In order to export Anonymization Records for this Template and
policy, first, the Anonymization Options Template shown in Figure 5
is exported. For this example, the optional privateEnterpriseNumber
and informationElementIndex Information Elements are omitted, because
they are not used.
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RFC 6235 IP Flow Anonymization Support May 2011
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set ID = 3 | Length = 26 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template ID = 257 | Field Count = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scope Field Count = 2 |0| templateID 145 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |0| informationElementId 303 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |0| anonymizationFlags 285 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |0| anonymizationTechnique 286 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Field Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example Anonymization Options Template
Following the Anonymization Options Template comes a Data Set
containing Anonymization Records. This dataset has an entry for each
Information Element Specifier in Template 256 describing the flow
records. This Data Set is shown in Figure 6. Note that
sourceIPv4Address and destinationIPv4Address have the Perimeter
Anonymization (0x0004) flag set in anonymizationFlags, meaning that
source address should be treated as network-external, and the
destination address as network-internal.
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RFC 6235 IP Flow Anonymization Support May 2011
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set ID = 257 | Length = 68 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | flowStartSeconds IE 150 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymized 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | sourceIPv4Address IE 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Perimeter, Session SC 0x0005 | Structured Permutation 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | destinationIPv4Address IE 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Perimeter, Stable 0x0007 | Reverse Truncation 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | sourceTransportPort IE 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymized 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | dest.TransportPort IE 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymized 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | packetDeltaCount IE 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymized 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | octetDeltaCount IE 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stable 0x0003 | Precision Degradation 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Template 256 | protocolIdentifier IE 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| no flags 0x0000 | Not Anonymized 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Example Anonymization Records
Following the Anonymization Records come the Data Sets containing the
anonymized data, exported according to the Template in Figure 4.
Bringing it all together, consider an IPFIX Message containing three
real data records and the necessary templates to export them, shown
in Figure 7. (Note that the scale of this message is 8-bytes per
line, for compactness; lines of dots '. . . . . ' represent shifting
of the example bit structure for clarity.)
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RFC 6235 IP Flow Anonymization Support May 2011
1 2 3 4 5 6
0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x000a | length 135 | export time 1271227717 | msg
| sequence 0 | domain 1 | hdr
| SetID 2 | length 40 | tid 256 | fields 8 | tmpl
| IE 150 | length 4 | IE 8 | length 4 | set
| IE 12 | length 4 | IE 7 | length 2 |
| IE 11 | length 2 | IE 2 | length 4 |
| IE 1 | length 4 | IE 4 | length 1 |
| SetID 256 | length 79 | time 1271227681 | data
| sip 192.0.2.3 | dip 198.51.100.7 | set
| sp 53 | dp 53 | packets 1 |
| bytes 74 | prt 17 | . . . . . . . . . . .
| time 1271227682 | sip 198.51.100.7 |
| dip 192.0.2.88 | sp 5091 | dp 80 |
| packets 60 | bytes 2896 |
| prt 6 | . . . . . . . . . . . . . . . . . . . . . . . . . . .
| time 1271227683 | sip 198.51.100.7 |
| dip 203.0.113.9 | sp 5092 | dp 80 |
| packets 44 | bytes 2037 |
| prt 6 |
+---------+
Figure 7: Example Real Message
The corresponding anonymized message is then shown in Figure 8. The
Options Template Set describing Anonymization Records and the
Anonymization Records themselves are added; IP addresses and byte
counts are anonymized as declared.
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RFC 6235 IP Flow Anonymization Support May 2011
1 2 3 4 5 6
0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x000a | length 233 | export time 1271227717 | msg
| sequence 0 | domain 1 | hdr
| SetID 2 | length 40 | tid 256 | fields 8 | tmpl
| IE 150 | length 4 | IE 8 | length 4 | set
| IE 12 | length 4 | IE 7 | length 2 |
| IE 11 | length 2 | IE 2 | length 4 |
| IE 1 | length 4 | IE 4 | length 1 |
| SetID 3 | length 30 | tid 257 | fields 4 | opt
| scope 2 | . . . . . . . . . . . . . . . . . . . . . . . . tmpl
| IE 145 | length 2 | IE 303 | length 2 | set
| IE 285 | length 2 | IE 286 | length 2 |
| SetID 257 | length 68 | . . . . . . . . . . . . . . . . anon
| tid 256 | IE 150 | flags 0 | tech 1 | recs
| tid 256 | IE 8 | flags 5 | tech 6 |
| tid 256 | IE 12 | flags 7 | tech 7 |
| tid 256 | IE 7 | flags 0 | tech 1 |
| tid 256 | IE 11 | flags 0 | tech 1 |
| tid 256 | IE 2 | flags 0 | tech 1 |
| tid 256 | IE 1 | flags 3 | tech 2 |
| tid 256 | IE41 | flags 0 | tech 1 |
| SetID 256 | length 79 | time 1271227681 | data
| sip 254.202.119.209 | dip 0.0.0.7 | set
| sp 53 | dp 53 | packets 1 |
| bytes 100 | prt 17 | . . . . . . . . . . .
| time 1271227682 | sip 0.0.0.7 |
| dip 254.202.119.6 | sp 5091 | dp 80 |
| packets 60 | bytes 2900 |
| prt 6 | . . . . . . . . . . . . . . . . . . . . . . . . . . .
| time 1271227683 | sip 0.0.0.7 |
| dip 2.19.199.176 | sp 5092 | dp 80 |
| packets 60 | bytes 2000 |
| prt 6 |
+---------+
Figure 8: Corresponding Anonymized Message
9. Security Considerations
This document provides guidelines for exporting metadata about
anonymized data in IPFIX, or storing metadata about anonymized data
in IPFIX Files. It is not intended as a general statement on the
applicability of specific flow data anonymization techniques.
Exporters or publishers of anonymized data must take care that the
applied anonymization technique is appropriate for the data source,
the purpose, and the risk of deanonymization of a given application.
Boschi & Trammell Experimental PAGE 39
RFC 6235 IP Flow Anonymization Support May 2011
Research in anonymization techniques, and techniques for
deanonymization, is ongoing, and currently "safe" anonymization
techniques may be rendered unsafe by future developments.
We note specifically that anonymization is not a replacement for
encryption for confidentiality. It is only appropriate for
protecting identifying information in data to be used for purposes in
which the protected data is irrelevant. Confidentiality in export is
best served by using TLS [RFC 5246] or Datagram Transport Layer
Security (DTLS) [RFC 4347] as in the Security Considerations section
of [RFC 5101], and in long-term storage by implementation-specific
protection applied as in the Security Considerations section of
[RFC 5655]. Indeed, confidentiality and anonymization are not
mutually exclusive, as encryption for confidentiality may be applied
to anonymized data export or storage, as well, when the anonymized
data is not intended for public release.
We note as well that care should be taken even with well-anonymized
data, and anonymized data should still be treated as privacy
sensitive. Anonymization reduces the risk of misuse, but is not a
complete solution to the problem of protecting end-user privacy in
network flow trace analysis.
When using pseudonymization techniques that have a mutable mapping,
there is an inherent trade-off in the stability of the map between
long-term comparability and security of the dataset against
deanonymization. In general, deanonymization attacks are more
effective given more information, so the longer a given mapping is
valid, the more information can be applied to deanonymization. The
specific details of this are technique-dependent and therefore out of
the scope of this document.
When releasing anonymized data, publishers need to ensure that data
that could be used in deanonymization is not leaked through a side
channel. The entire workflow (hardware, software, operational
policies and procedures, etc.) for handling anonymized data must be
evaluated for risk of data leakage. While most of these possible
side channels are out of scope for this document, guidelines for
reducing the risk of information leakage specific to the IPFIX export
protocol are provided in Section 7.2.
Note as well that the Security Considerations section of [RFC 5101]
applies as well to the export of anonymized data, and the Security
Considerations section of [RFC 5655] to the storage of anonymized
data, or the publication of anonymized traces.
Boschi & Trammell Experimental PAGE 40
RFC 6235 IP Flow Anonymization Support May 2011
10. IANA Considerations
This document specifies the creation of several new IPFIX Information
Elements in the IPFIX Information Element registry available from the
IANA site (http://www.iana.org), as defined in Section 6.2. IANA has
assigned the following Information Element numbers for their
respective Information Elements as specified below:
o Information Element number 285 for the anonymizationFlags
Information Element.
o Information Element number 286 for the anonymizationTechnique
Information Element.
o Information Element number 287 for the informationElementIndex
Information Element.
11. Acknowledgments
We thank Paul Aitken and John McHugh for their comments and insight,
and Carsten Schmoll, Benoit Claise, Lothar Braun, Dan Romascanu,
Stewart Bryant, and Sean Turner for their reviews. Special thanks to
the FP7 PRISM and DEMONS projects for their material support of this
work.
12. References
12.1. Normative References
[RFC 5101] Claise, B., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008.
[RFC 5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
RFC 5102, January 2008.
[RFC 5103] Trammell, B. and E. Boschi, "Bidirectional Flow Export
Using IP Flow Information Export (IPFIX)", RFC 5103,
January 2008.
[RFC 5655] 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.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Boschi & Trammell Experimental PAGE 41
RFC 6235 IP Flow Anonymization Support May 2011
[RFC 5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010.
[RFC 5156] Blanchet, M., "Special-Use IPv6 Addresses", RFC 5156,
April 2008.
12.2. Informative References
[RFC 5470] Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
"Architecture for IP Flow Information Export", RFC 5470,
March 2009.
[RFC 5472] Zseby, T., Boschi, E., Brownlee, N., and B. Claise, "IP
Flow Information Export (IPFIX) Applicability", RFC 5472,
March 2009.
[RFC 6183] Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi,
"IP Flow Information Export (IPFIX) Mediation: Framework",
RFC 6183, April 2011.
[IPFIX-PERSTREAM]
Claise, B., Aitken, P., Johnson, A., and G. Muenz, "IPFIX
Export per SCTP Stream", Work in Progress, May 2010.
[RFC 5153] Boschi, E., Mark, L., Quittek, J., Stiemerling, M., and P.
Aitken, "IP Flow Information Export (IPFIX) Implementation
Guidelines", RFC 5153, April 2008.
[RFC 3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export (IPFIX)",
RFC 3917, October 2004.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC 4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[Bur10] Burkhart, M., Schatzmann, D., Trammell, B., and E. Boschi,
"The Role of Network Trace Anonymization Under Attack",
ACM Computer Communications Review, vol. 40, no. 1, pp.
6-11, January 2010.
Boschi & Trammell Experimental PAGE 42
RFC 6235 IP Flow Anonymization Support May 2011
[Mur07] Murdoch, S. and P. Zielinski, "Sampled Traffic Analysis by
Internet-Exchange-Level Adversaries", Proceedings of the
7th Workshop on Privacy Enhancing Technologies, Ottawa,
Canada, June 2007.
Authors' Addresses
Elisa Boschi
Swiss Federal Institute of Technology Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
EMail: boschie@tik.ee.ethz.ch
Brian Trammell
Swiss Federal Institute of Technology Zurich
Gloriastrasse 35
8092 Zurich
Switzerland
Phone: +41 44 632 70 13
EMail: trammell@tik.ee.ethz.ch
Boschi & Trammell Experimental PAGE 43
IP Flow Anonymization Support
RFC TOTAL SIZE: 112595 bytes
PUBLICATION DATE: Saturday, May 14th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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