Internet Engineering Task Force S. D'Antonio
Internet-Draft University of Napoli
Intended status: Standards Track "Parthenope"
Expires: May 17, 2012 T. Zseby
Fraunhofer Institute FOKUS
C. Henke
Technische Universitat Berlin
L. Peluso
University of Napoli
November 14, 2011
Flow Selection Techniques
draft-ietf-ipfix-flow-selection-tech-08.txt
Abstract
Flow selection is the process of selecting a subset of flows from all
observed flows. The Flow Selection Process may be located at an
observation point, or on an IPFIX Mediator. Flow selection reduces
the effort of post-processing flow data and transferring Flow
Records. This document describes motivations for flow selection and
presents flow selection techniques. It provides an information model
for configuring flow selection techniques and discusses what
information about a flow selection process should be exported.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on May 17, 2012.
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Table of Contents
1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Difference between Flow Selection and Packet Selection . . . . 7
4. Flow selection as a Function in the IPFIX Architecture . . . . 8
4.1. Flow selection during the Metering Process . . . . . . . . 10
4.2. Flow selection during the Exporting Process . . . . . . . 10
4.3. Flow selection as a function of the IPFIX Mediator . . . . 10
5. Flow Selection Techniques . . . . . . . . . . . . . . . . . . 11
5.1. Flow Filtering . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. Property Match Filtering . . . . . . . . . . . . . . . 11
5.1.2. Hash-based Flow Filtering . . . . . . . . . . . . . . 11
5.2. Flow Sampling . . . . . . . . . . . . . . . . . . . . . . 12
5.2.1. Systematic sampling . . . . . . . . . . . . . . . . . 12
5.2.2. Random Sampling . . . . . . . . . . . . . . . . . . . 12
5.3. Flow-state Dependent Flow Selection . . . . . . . . . . . 13
5.4. Flow-state Dependent Packet Selection . . . . . . . . . . 14
6. Configuration of Flow Selection Techniques . . . . . . . . . . 14
6.1. Flow Selection Parameters . . . . . . . . . . . . . . . . 16
6.2. Description of Flow-state Dependent Packet Selection . . . 18
7. Information Model for Flow Selection Configuration and
Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. FlowSelectorAlgorithm . . . . . . . . . . . . . . . . . . 19
7.2. flowSelectedOctetDeltaCount . . . . . . . . . . . . . . . 21
7.3. flowSelectedPacketDeltaCount . . . . . . . . . . . . . . . 21
7.4. flowSelectedFlowDeltaCount . . . . . . . . . . . . . . . . 21
7.5. selectorIDTotalFlowsObserved . . . . . . . . . . . . . . . 22
7.6. selectorIDTotalFlowsSelected . . . . . . . . . . . . . . . 22
7.7. samplingFlowInterval . . . . . . . . . . . . . . . . . . . 22
7.8. samplingFlowSpace . . . . . . . . . . . . . . . . . . . . 23
7.9. flowSamplingTimeInterval . . . . . . . . . . . . . . . . . 23
7.10. flowSamplingTimeSpace . . . . . . . . . . . . . . . . . . 23
7.11. hashFlowDomain . . . . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8.1. Registration of Information Elements . . . . . . . . . . . 24
8.2. Registration of Object Identifier . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . 32
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Scope
This document describes flow selection techniques for network traffic
measurements. A flow is defined as a set of packets with common
properties as described in [RFC5101]. Flow selection can be done to
limit the resource demands for capturing, storing, exporting and
post-processing of Flow Records. It also can be used to select a
particular set of flows that are of interest to a specific
application. This document provides a categorization of flow
selection techniques and describes configuration and reporting
parameters for them. In order to be compliant with this document, at
least one of the flow selection schemes MUST be implemented. That
means that the configuration parameters as well as the reporting
Information Elements for this particular scheme MUST be supported.
This document also addresses configuration and reporting parameters
for flow-state dependent packet selection as described in [RFC5475],
although this technique is categorized as packet selection. The
reason is, that flow-state dependent packet selection techniques
often aim at the reduction of resources for flow capturing and flow
processing. Furthermore, they were only briefly discussed in
[RFC5475]. Therefore we included configuration and reporting
considerations for such techniques in this document.
2. Terminology
This document is consistent with the terminology introduced in
[RFC5101], [RFC5470], [RFC5475] and [RFC3917]. As in [RFC5101] and
[RFC5476], the first letter of each IPFIX-specific and PSAMP-specific
term is capitalized along with the flow selection specific terms
defined here.
* Packet Classification
Packet Classification is a process by which packets are mapped to
specific Flow Records based on packet properties or external
properties (e.g. interface). The properties (e.g. header
information, packet content, AS number) make up the Flow Key. In
case a Flow Record for a specific Flow Key already exists the Flow
Record is updated, otherwise a new Flow Record is created.
* Packet Aggregation Process
In the IPFIX Metering Process the Packet Aggregation Process
aggregates packet data into flow data and forms the Flow Records.
After the aggregation step only the aggregated flow information is
available. Information about individual packets is lost.
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* Flow Selection Process
A Flow Selection Process takes Flow Records as its input and
selects a subset of this set as its output. A Flow Selection
Process MAY run in several places within the IPFIX architecture.
A Flow Selection Process MAY be part of an IPFIX Metering Process,
Exporting Process or as an Intermediate Selection Process as
defined for the IPFIX Mediator [RFC6183].
* Flow Selection State
A Flow Selection Process SHOULD maintain state information for use
by the Flow Selector. At a given time, the Flow Selection State
may depend on flows and packets observed at and before that time,
as well as other variables. Examples include:
(i) sequence number of packets and accounted Flow Records;
(ii) number of selected flows;
(iii) number of observed flows;
(iv) current flow cache occupancy;
(v) flow specific counters, lower and upper bounds;
(vi) flow selection timeout intervals.
* Flow Selector
A Flow Selector defines the action of a Flow Selection Process on
a single flow of its input. The Flow Selector can make use of the
following information in order to establish whether a flow has to
be selected or not:
(i) the content of the Flow Record;
(ii) any state information related to the Metering Process or
Exporting Process;
(iii) any Flow Selection State that may be maintained by the
Flow Selection Process.
* Complete Flow
A Complete Flow consists of all the packets that enter the Flow
Selection Process within the flow time-out interval, and which
belong to the same flow as defined by the flow definition in
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[RFC5470]. For this definition only packets that arrive at the
Flow Selection Process are considered. That means, packets that
are not observed at the Flow Selection Process because of prior
packet selection or packet loss are not considered as belonging to
the Complete Flow.
* Flow Filtering
Flow Filtering selects flows based on a deterministic function on
the Flow Record content, Flow Selection State, external properties
(e.g. ingress interface) or external events (e.g violated Access
Control List). If the relevant parts of the Flow Record content
can already be observed at packet level (e.g. Flow Keys from
packet header fields) Flow Filtering can be performed at packet
level by Property Match Filtering as described in [RFC5475].
* Hash-based Flow Filtering
Hash-based Flow Filtering is a deterministic flow filter function
that selects flows based on a Hash Function. The Hash Function is
calculated over parts of the Flow Record content or external
properties which are called the Hash Domain. If the hash value
falls into a predefined Hash Selection Range the flow is selected.
Hash-based Flow Filtering can already applied at packet level, in
which case the Hash Domain MUST contain the Flow Key of the
packet. In case Hash-based Flow Filtering is used to select the
same subset of flows at different observation points, the Hash
Domain MUST comprise parts of the packet or flow thar are
invariant on the packet/flow path. Also refer to the according
Trajectory Sampling Application Example on packet level in
[RFC5475]
* Flow-state Dependent Flow Selection
Flow-state Dependent Flow Selection is a selection function that
selects or drops flows based on the current Flow Selection State.
The selection can be either deterministic, random or non-uniform
random.
* Flow-state Dependent Packet Selection
Flow-state Dependent Packet Selection is a selection function that
selects or drops packets based on the current Flow Selection
State. The selection can be either deterministic, random or non-
uniform random. Flow-state Dependent Packet Selection can be used
to prefer the selection of packets belonging to specific flows.
For example the selection probability of packets belonging to
flows that are already within the Flow Cache may be higher than
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for packets that have not been recorded yet.
* Flow Sampling
Flow Sampling selects flows based on Flow Record sequence or
arrival times (e.g. entry in flow cache, arrival time at Exporter
or Mediator). The selection can be systematic (e.g. every n-th
flow) or based on a random function (e.g. select each Flow Record
with probability p, or randomly select n out of N Flow Records).
3. Difference between Flow Selection and Packet Selection
Flow selection differs from packet selection described in [RFC5475].
Packet selection techniques consider packets as the basic element and
the parent population consists of all packets observed at an
observation point. In contrast to this the basic elements in flow
selection are the flows. The parent population consists of all
observed flows and the selection process operates on the flows. The
major characteristics of flow selection are the following:
- Flow selection takes flows as basic elements. For packet
selection, packets are considered as basic elements.
- Flow selection can only take place after Packet
Classification, because the classification rules determine to
which flow a packet belongs. Packet selection can be applied
before and after Packet Classification.
- Flow selection operates on Complete Flows. That means that
after the Flow Selection Process either all packets of the
flow are kept or all packets of the flow are discarded. That
means that if the flow selection is preceded by a packet
selection process the Complete Flow consists only of the
packets that were not discarded during the packet selection.
There are some techniques that are difficult to unambiguously
categorize into one of the categories. Here we give some guidance
how to categorize such techniques:
- Techniques that can be considered as both packet and flow
selection: some packet selection techniques result in the
selection of Complete Flows and therefore can be considered
as packet or as flow selection at the same time. An example
is Property Match Filtering of all packets to a specific
destination address. If flows are defined based on
destination addresses, such a packet selection also results
in a flow selection and can be considered as packet or flow
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selection.
- Flow-state Dependent Packet Selection (as described in
[RFC5475]): there exist techniques that select packets based
on the flow state, e.g. based on the number of already
observed packets belonging to the flow. Examples of these
techniques from the literature are "Sample and Hold" [EsVa01]
"Fast Filtered Sampling" [MSZC10] or the "Sticky Sampling"
algorithm presented in [MaMo02]. Such techniques can be used
to influence which flows are captured (e.g. increase the
selection of packets belonging to large flows) and reduce the
number of flows that need to be stored in the flow cache.
Nevertheless, such techniques do not necessarily select
Complete Flows, because they do not ensure that all packets
of a selected flow are captured. Therefore Flow-state
Dependent Packet Selection methods that do not ensure that
either all or no packets of a flow are selected strictly
speaking have to be considered as packet selection techniques
and not as flow selection techniques.
4. Flow selection as a Function in the IPFIX Architecture
Figure 1 shows the IPFIX reference model as defined in [RFC5470] and
shows the Packet Classification and Packet Aggregation Process in the
Metering Process.
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Packet(s) coming in to Observation Point(s)
| |
v v
+----------------+---------------------------+ +-----+-------+
| Metering Process | | |
| | | |
| packet header capturing | | |
| | |...| Metering |
| timestamping | | Process N |
| | | | |
| packet sampling | | |
| | | | |
| (packet classification) | | |
| | | | |
| packet filtering* | | |
| | | | |
| (packet aggregation)* | | |
| | | | |
+--------|-----------------------------------+ +-----|-------+
Flow Records Flow Records
| |
+----------------------+----------------------+
|
+----------------------|-----------------+
| Exporting Process* |
+----------------------+-----------------+
| IPFIX (Flow Records)
v
+-------------------------|-----------------------+
| IPFIX Mediator | |
| v |
| Collecting Process(es) |
| | |
| Intermediate Flow Selection Process (*) |
| | |
| Exporting Process(es) (*) |
+-------------------------|-----------------------+
v
IPFIX
(*) indicates where flow selection can take place.
Figure 1: Flow selection in the IPFIX Architecture
In contrast to packet selection, flow selection is always applied
after the packets are classified into flows. Flows can be selected
at different stages of the measurement chain:
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1. during the Metering Process
2. during Exporting Process(es)
3. during an Intermediate Selection Process on a Mediator
4.1. Flow selection during the Metering Process
In the Packet Aggregation Process the packet information is used to
update the Flow Records in the flow cache. Flow selection that is
applied before aggregation equals a packet selection process. The
flow still consists of individual packets. Those are then selected
based on the classification information, i.e. based on the flow they
belong to. Flow selection before aggregation can be based on the
fields of the Flow Key (also on a hash value over these fields), but
not based on characteristics that are only available after packet
aggregation (e.g. flow size, flow duration). Flow selection during
the Metering Process is applied to reduce resources for all
succeeding processes or to select specific flows of interest in case
such flow characteristics are already observable at packet level
(e.g. flows to specific IP addresses). In contrast, Flow-state
Dependent Packet Selection is a packet selection method, because it
does not necessarily select Complete Flows.
4.2. Flow selection during the Exporting Process
The Flow Selection Process at the Exporter is similar to an
Intermediate Selection Process as described in [RFC6183] and works on
Flow records. Flow selection during the Exporting Process can
therefore also depend on flow characteristics that are only visible
after the aggregation of packets, such as flow size and flow
duration. The Exporting Process may implement policies for exporting
only a subset of the Flow Records which have been stored in the
system memory in order to unload flow export and flow post-
processing. Flow selection during the Exporting Process may select
only the subset of Flow Records which are of interest to the users
application, or select only as many Flow Records as can be handled by
the available resources (e.g. limited export link capacity).
4.3. Flow selection as a function of the IPFIX Mediator
As shown in Figure 1, flow selection can be performed as an
Intermediate Process within an IPFIX Mediator [RFC6183]. The
Intermediate Selection Process takes Flow Record stream as its input
and selects Flow Records from a sequence based upon criteria-
evaluated record values. The Intermediate Selection Process can
again apply a flow selection technique to obtain flows of interest to
the application. Further, the Intermediate Selection Process can
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base its selection decision on the correlation of data from different
observation points, e.g. by only selecting flows that were at least
recorded on two observation points.
5. Flow Selection Techniques
A flow selection technique selects either all or none of the packets
of a flow, otherwise the technique has to be considered as packet
selection. We distinguish between Flow Filtering and Flow Sampling.
5.1. Flow Filtering
Flow Filtering is a deterministic function on the IPFIX Flow Record
content. If the relevant flow characteristics are already observable
at packet level (e.g. Flow Keys), Flow Filtering can be applied
before aggregation at packet level.
5.1.1. Property Match Filtering
Flow Filtering can be performed similarly to Property Match Filtering
for packet selection described in [RFC5475]. The difference is that,
instead of packet fields, Flow Record fields are here used to derive
the selection decision. Property Match Filtering is typically used
to select a specific subset of the flows that are of interest to a
particular application (e.g. all flows to a specific destination, all
large flows, etc.). Properties on which the filtering is based can
be for example Flow Keys, the flow size in bytes, the number of
packets in the flow, the observation time of the first or last
packet, or the maximum packet length. The selection criteria can be
a specific value or an interval. Property Match Filtering can be
applied during the Metering Process if the properties are already
observable at the packet level (e.g. Flow Key fields).
There are content-based Property Match Filtering techniques that
require a computation on the current flow cache. An example is the
selection of the largest flows or a percentage of flows with the
longest lifetime. This type of Property Match Filtering is also used
in flow selection techniques that react to external events (e.g.
resource constraint). For example when the flow cache is full, the
Flow Record with the lowest flow volume per current flow life time
may be deleted.
5.1.2. Hash-based Flow Filtering
Hash-based Flow Filtering uses a Hash Function h to map the Flow Key
c onto a Hash Range R. A flow is selected if the hash value h(c) is
within the Hash Selection Range S, which is a subset of R. Hash-based
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Flow Filtering can be used to emulate a random sampling process but
still enable the correlation between selected flow subsets at
different observation points. Hash-based Flow Filtering is similar
to Hash-based Packet Selection, and in fact is identical when Hash-
based Packet Selection uses the Flow Key that defines the flow as the
hash input. Nevertheless there MAY be the incentive to apply Hash-
based Flow Filtering not on the packet level during the Metering
Process, for example when the size of the selection range and
therefore the sampling probability is dependent on the number of
observed flows.
5.2. Flow Sampling
Flow Sampling operates on Flow Record sequence or arrival times. It
can use either a systematic or a random function for the selection
process. Flow Sampling usually aims at the selection of a
representative subset of all flows in order to estimate
characteristics of the whole set (e.g. mean flow size in the
network).
5.2.1. Systematic sampling
Systematic sampling is a deterministic selection function.
Systematic sampling may be a periodic selection of the N-th Flow
Record which arrives at the Exporting or Intermediate Selection
Process. Systematic sampling MAY BE applied during the Metering
Process. An example would be to create, besides the Flow cache of
selected flows, an additional data structure that saves the Flow Keys
of the flows that are not selected. The selection of a flow would
then be based on the first packet of a flow. Everytime a packet
belonging to a new flow (which is neither in the data structure of
the selected or not selected flows) arrives at the measurement point,
a counter is increased. In case the counter is increased to a
multiple of N a new flow cache entry is created, and in case the
counter is not a multiple of N the Flow Key is added to the data
structure for not selected flows.
Systematic sampling can also be time-based. Time-based systematic
sampling is applied by only creating flows that are observed between
time-based start and stop triggers. The time interval may be applied
at packet level during the Metering Process or after aggregation on
flow level, e.g. by selecting a flow arriving at the Exporting
Process every n seconds.
5.2.2. Random Sampling
Random flow sampling is based on a random process which requires the
calculation of random numbers. One can differentiate between n-out-N
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and probabilistic flow sampling.
5.2.2.1. n-out-of-N Flow Sampling
In n-out-of-N Sampling, n elements are selected out of the parent
population that consists of N elements. One example would be to
generate n different random numbers in the range [1,N] and select all
flows that have a flow position equal to one of the random numbers.
5.2.2.2. Probabilistic Flow Sampling
In probabilistic Sampling, the decision whether or not a flow is
selected is made in accordance with a predefined selection
probability. For probabilistic Sampling, the Sample Size can vary
for different trials. The selection probability does not necessarily
have to be the same for each flow. Therefore, we distinguish between
uniform probabilistic sampling (with the same selection probability
for all flows) and non-uniform probabilistic sampling (where the
selection probability can vary for different flows). For non-uniform
probabilistic Flow Sampling the sampling probability may be adjusted
according to the Flow Record content. An example would be to
increase the selection probability of large volume flows over small
volume flows as described in the Smart Sampling technique [DuLT01].
5.3. Flow-state Dependent Flow Selection
Flow-state Dependent Flow Selection can be a deterministic or random
flow selection process based on the Flow Record content and the flow
state which may be kept additionally for each of the flows. External
processes may update counters, bounds and timers for each of the Flow
Records and the Flow Selection Process utilises this information for
the selection decision. A review of Flow-state Dependent Flow
Selection techniques that aim at the selection of the most frequent
items by keeping additional flow state information can be found in
[CoHa08]. Flow-state Dependent Flow Selection can only be applied
after packet aggregation, when a packet has been assigned to a flow.
The selection process then decides based upon the flow state for each
flow if it is kept in the flow cache or not. Two Flow State
Dependent Flow Selection Algorithms are here described:
The frequent algorithm [KaPS03] is a technique that aims at the
selection of all flows that at least exceed a 1/k fraction of the
Observed Packet Stream. The algorithm has only a flow cache of size
k-1 and each flow in the cache has an additional counter. The
counter is incremented each time a packet belonging to the flow in
the flow cache is observed. In case the observed packet does not
belong to any flow all counters are decremented and if any of the
flow counters has a value of zero the flow is replaced with a flow
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formed from the new packet.
Lossy counting is a selection technique that identifies all flows
whose packet count exceeds a certain percentage of the whole observed
packet stream (e.g. 5% of all packets) with a certain estimation
error e. Lossy counting separates the observed packet stream in
windows of size N=1/e, where N is an amount of consecutive packets.
For each observed flow an additional counter will be held in the flow
state. The counter is incremented each time a packet belonging to
the flow is observed and all counters are decremented at the end of
each window and all flows with a counter of zero are removed from the
flow cache.
5.4. Flow-state Dependent Packet Selection
Flow-state Dependent Packet Selection is not a flow selection
technique but a packet selection technique. Nevertheless we will
describe configuration and reporting parameters for this technique in
this document. An example is the "Sample and Hold" algorithm
[EsVa01] that tries to prefer large volume flows in the selection.
When a packet arrives it is selected when a Flow Record for this
packet already exists. In case there is no Flow Record, the packet
is selected by a certain probability that is dependent on the packet
size.
6. Configuration of Flow Selection Techniques
This section describes the configuration parameters of the flow
selection techniques presented above. It provides the basis for an
information model to be adopted in order to configure the Flow
Selection Process within an IPFIX Device. The actual information
model with the Information Elements (IEs) for the configuration is
described together with the reporting IEs in section 7. The
following table gives an overview of the defined selection
techniques, where they can be applied and what their input parameters
are. Depending on where the flow selection techniques are applied
different input parameters can be configured.
Overview of Flow Selection Techniques:
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+------------------+-----------------+------------------------------+
| Location | Selection | Selection Input |
| | Method | |
+------------------+-----------------+------------------------------+
| During the | Flow-state | packet sampling |
| Metering Process | Dependent | probabilities, Flow |
| based on Packets | Packet | Selection State, packet |
| | Selection | properties |
+------------------+-----------------+------------------------------+
| | Property Match | Flow record IEs, Selection |
| | Flow Filtering | Interval |
+------------------+-----------------+------------------------------+
| | Hash-based Flow | selection range, Hash |
| | Filtering | Function, Flow Key, (seed) |
+------------------+-----------------+------------------------------+
| | Time-based | flow position (derived from |
| | Systematic Flow | arrival time of packets), |
| | Sampling | flow selection state |
+------------------+-----------------+------------------------------+
| | Sequence-based | flow position (derived from |
| | Systematic Flow | packet position), flow |
| | Sampling | selection state |
+------------------+-----------------+------------------------------+
| | Random Flow | random number generator or |
| | Sampling | list and packet position, |
| | | flow state |
+------------------+-----------------+------------------------------+
| Exporting / | Property Match | Flow Record content, filter |
| Intermediate | Flow Filtering | function |
| Selection | | |
| Process | | |
+------------------+-----------------+------------------------------+
| | Hash-based Flow | selection range, Hash |
| | Filtering | Function, hash input (Flow |
| | | Keys and other flow |
| | | properties) |
+------------------+-----------------+------------------------------+
| | Flow-state | flow state parameters, |
| | Dependent Flow | random number generator or |
| | Selection | list |
+------------------+-----------------+------------------------------+
| | Time-based | flow arrival time, flow |
| | Systematic Flow | state |
| | Sampling | |
+------------------+-----------------+------------------------------+
| | Sequence-based | flow position, flow state |
| | Systematic Flow | |
| | Sampling | |
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| | Random Flow | random number generator or |
| | Sampling | list and flow position, flow |
| | | state |
+------------------+-----------------+------------------------------+
6.1. Flow Selection Parameters
In this section, we define what parameters are required to describe
the most common Flow Selection techniques.
Flow Selection Parameters:
For Property Match Filtering:
- Information Element as specified in [iana-ipfix-assignments]):
Specifies the Information Element which is used as the property
in the filter expression.
- Selection Value or Value Interval:
Specifies the value or interval of the filter expression.
Packets and Flow Record that have a value equal to the Selection
Value or within the Interval will be selected.
For Hash-based Flow Filtering:
- Hash Domain:
Specifies the bits from the packet or flow which are taken as the
hash input to the Hash Function.
- Hash Function:
Specifies the name of the Hash Function that is used to calculate
the hash value. Possible Hash Functions are BOB, IPSX, CRC-32
- Hash Selection Range:
Flows that have a hash value within the Hash Selection Range are
selected. The Hash Selection Range can be a value interval or
arbitrary hash values within the Hash Range of the Hash Function.
- Random Seed or Initializer Value:
Some Hash Functions require an initializing value. In order to
make the selection decision more secure one can choose a random
seed that configures the hash function.
For Flow-state Dependent Flow Selection:
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- frequency threshold:
Specifies the frequency threshold s for flow state dependent flow
selection techniques that try to find the most frequent items
within a dataset. All flows which exceed the defined threshold
will be selected.
- accuracy parameter:
specifies the accuracy parameter e for techniques that deal with
the frequent items problems. The accuracy parameter defines the
maximum error, i.e. no flows that have a true frequency less than
( s - e) N are selected, where s is the frequency threshold and N
is the total number of packets.
The above list of parameters for Flow-state Dependent Flow Selection
techniques is suitable for the presented frequent item and lossy
counting algorithms. Nevertheless a variety of techniques exist with
very specific parameters which are not defined here.
For Systematic time-based Flow Sampling:
- Interval length (in usec)
Defines the length of the sampling interval during which flows
are selected.
- Spacing (in usec)
The spacing parameter defines the spacing in usec between the end
of one sampling interval and the start of the next succeeding
interval.
For Systematic count-based Flow Sampling:
- Interval length
Defines the number of flows that are selected within the sampling
interval.
- Spacing
The spacing parameter defines the spacing in number of observed
flows between the end of one sampling interval and the start of
the next succeeding interval.
For random n-out-of-N Flow Sampling:
- Population Size N
The Population Size N is the number of all flows in the
Population from which the sample is drawn.
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- Sampling Size n
The sampling size n is the number of flows that are randomly
drawn from the population N.
For probabilistic Flow Sampling:
- Sampling probability p
The sampling probability p defines the probability by which each
of the observed flows is selected.
6.2. Description of Flow-state Dependent Packet Selection
The configuration of Flow-state Dependent Packet Selection has not
been described in [RFC5475] therefore the parameters are defined
here:
For Flow-state Dependent Packet Selection:
- packet selection probability per possible flow state interval
Defines multiple {flow interval, packet selection probability}
value pairs that configure the sampling probability depending on
the current flow state.
- additional parameters
For the configuration of flow state dependent packet selection
additional parameters or packet properties may be required, e.g.
the packet size ([EsVa01])
7. Information Model for Flow Selection Configuration and Reporting
In this section we describe Information Elements (IEs) that SHOULD be
exported by a flow selection process in order to support the
interpretation of measurement results from flow measurements where
only some flows are selected. The information is mainly used to
report how many packets and flows have been observed in total and how
many of them were selected. This helps for instance to calculate the
Attained Selection Fraction (see also [RFC5476]), which is an
important parameter to provide an accuracy statement. The IEs can
provide reporting information about Flow Records, packets or bytes.
The reported metrics are total number of elements and the number of
selected elements. From this the number of dropped elements can be
derived. All counters SHOULD be exported and reset when a new
measurement interval starts.
List of Flow Selection Information Elements:
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+------+-------------------------+-------+--------------------------+
| ID | Name | ID | Name |
+------+-------------------------+-------+--------------------------+
| 301 | selectionSequenceID | 302 | selectorID |
+------+-------------------------+-------+--------------------------+
| TBD1 | FlowSelectorAlgorithm | 1 | octetDeltaCount |
+------+-------------------------+-------+--------------------------+
| TBD2 | flowSelectedOctetDeltaC | 2 | PacketDeltaCount |
| | ount | | |
+------+-------------------------+-------+--------------------------+
| TBD3 | flowSelectedPacketDelta | 3 | originalFlowsPresent |
| | Count | | |
+------+-------------------------+-------+--------------------------+
| TBD4 | flowSelectedFlowDeltaCo | TBD5 | selectorIDTotalFlowsObse |
| | unt | | rved |
+------+-------------------------+-------+--------------------------+
| TBD6 | selectorIDTotalFlowsSel | TBD7 | samplingFlowInterval |
| | ected | | |
+------+-------------------------+-------+--------------------------+
| TBD8 | samplingFlowSpace | 309 | samplingSize |
+------+-------------------------+-------+--------------------------+
| 310 | samplingPopulation | 311 | samplingProbability |
+------+-------------------------+-------+--------------------------+
| TBD9 | flowSamplingTimeInterva | TBD10 | flowSamplingTimeSpace |
| | l | | |
+------+-------------------------+-------+--------------------------+
| 326 | digestHashValue | TBD11 | hashFlowOffset |
+------+-------------------------+-------+--------------------------+
| TBD1 | hashFlowSize | 329 | hashOutputRangeMin |
| 2 | | | |
+------+-------------------------+-------+--------------------------+
| 330 | hashOutputRangeMax | 331 | hashSelectedRangeMin |
+------+-------------------------+-------+--------------------------+
| 332 | hashSelectedRangeMax | 333 | hashDigestOutput |
+------+-------------------------+-------+--------------------------+
| 334 | hashInitialiserValue | 320 | absoluteError |
+------+-------------------------+-------+--------------------------+
| 321 | relativeError | 336 | upperCILimit |
+------+-------------------------+-------+--------------------------+
| 337 | lowerCILimit | 338 | confidenceLevel |
+------+-------------------------+-------+--------------------------+
7.1. FlowSelectorAlgorithm
Description:
This Information Element identifies the flow selection
method(e.g., Filtering, Sampling) that is applied by the Flow
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Selection Process. Most of these methods have parameters as
decribed in Section 6. Further Information Elements are needed to
fully specify packet selection with these methods and all their
parameters. Further method identifiers may be added to the list
below. It might be necessary to define new Information Elements
to specify their parameters. The flowSelectorAlgorithm registry
is maintained by IANA. New assignments for the registry will be
administered by IANA and are subject to Expert Review [RFC5226].
The registry can be updated when specifications of the new
method(s) and any new Information Elements are provided.
+----+------------------------+--------------------------+
| ID | Method | Parameters |
+----+------------------------+--------------------------+
| 1 | Systematic count-based | flowSamplingInterval |
| | Sampling | flowSamplingSpace |
+----+------------------------+--------------------------+
| 2 | Systematic time-based | flowSamplingTimeInterval |
| | Sampling | flowSamplingTimeSpace |
+----+------------------------+--------------------------+
| 3 | Random n-out-of-N | samplingSize |
| | Sampling | samplingPopulation |
+----+------------------------+--------------------------+
| 4 | Uniform probabilistic | samplingProbability |
| | Sampling | |
+----+------------------------+--------------------------+
| 5 | Property Match | Information Element |
| | Filtering | Value Range |
+----+------------------------+--------------------------+
| Hash-based Filtering | hashInitialiserValue |
+----+------------------------+ hashFlowDomain |
| 6 | using BOB | hashSelectedRangeMin |
+----+------------------------+ hashSelectedRangeMax |
| 7 | using IPSX | hashOutputRangeMin |
+----+------------------------+ hashOutputRangeMax |
| 8 | using CRC | |
+----+------------------------+--------------------------+
| 9 | Flow State Dependent | No agreed Parameters |
| | Flow Selection | |
+----+------------------------+--------------------------+
Abstract Data Type: unsigned16
ElementId: TBD1
Data Type Semantics: identifier
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Status: Proposed
7.2. flowSelectedOctetDeltaCount
Description:
This Information Element specifies the volume in octets of all
flows that are selected during the Flow Selection Process since
the previous report.
Abstract Data Type: unsigned64
ElementId: TBD2
Units: Octets
Status: Proposed
7.3. flowSelectedPacketDeltaCount
Description:
This Information Element specifies the volume in packets of all
flows that were selected during the Flow Selection Process since
the previous report.
Abstract Data Type: unsigned64
ElementId: TBD3
Units: Packets
Status: Proposed
7.4. flowSelectedFlowDeltaCount
Description:
This Information Element specifies the number of Flows that were
selected during the Flow Selection Process since the last report.
Abstract Data Type: unsigned64
ElementId: TBD4
Units: Flows
Status: Proposed
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7.5. selectorIDTotalFlowsObserved
Description:
This Information Element specifies the total number of flows
observed by a Selector, for a specific value of SelectorId. This
Information Element should be used in an Options Template scoped
to the observation to which it refers. See Section 3.4.2.1 of the
IPFIX protocol document [RFC5101] .
Abstract Data Type: unsigned64
ElementId: TBD5
Units: Flows
Status: Proposed
7.6. selectorIDTotalFlowsSelected
Description:
This Information Element specifies the total number of flows
selected by a Selector, for a specific value of SelectorId. This
Information Element should be used in an Options Template scoped
to the observation to which it refers. See Section 3.4.2.1 of the
IPFIX protocol document [RFC5101].
Abstract Data Type: unsigned64
ElementId: TBD6
Units: Flows
Status: Proposed
7.7. samplingFlowInterval
Description:
This Information Element specifies the number of flows that are
consecutively sampled. A value of 100 means that 100 consecutive
flows are sampled. For example, this Information Element may be
used to describe the configuration of a systematic count-based
Sampling Selector.
Abstract Data Type: unsigned32
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ElementId: TBD7
Units: Flows
Status: Proposed
7.8. samplingFlowSpace
Description:
This Information Element specifies the number of flows between two
"samplingFlowInterval"s. A value of 100 means that the next
interval starts 100 flows (which are not sampled) after the
current "samplingFlowInterval" is over. For example, this
Information Element may be used to describe the configuration of a
systematic count-based Sampling Selector.
Abstract Data Type: unsigned32
ElementId: TBD8
Units: Flows
Status: Proposed
7.9. flowSamplingTimeInterval
Description:
This Information Element specifies the time interval in
microseconds during which all arriving flows are sampled. For
example, this Information Element may be used to describe the
configuration of a systematic time-based Sampling Selector.
Abstract Data Type: unsigned32
ElementId: TBD9
Units: microseconds
Status: Proposed
7.10. flowSamplingTimeSpace
Description:
This Information Element specifies the time interval in
microseconds between two "flowSamplingTimeInterval"s. A value of
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100 means that the next interval starts 100 microseconds (during
which no flows are sampled) after the current
"flowsamplingTimeInterval" is over. For example, this Information
Element may used to describe the configuration of a systematic
time-based Sampling Selector.
Abstract Data Type: unsigned32
ElementId: TBD10
Units: microseconds
Status: Proposed
7.11. hashFlowDomain
Description:
This Information Element specifies the Information Elements that
are used by the Hash-based flow Selection Selector as the Hash
Domain.
Abstract Data Type: unsigned16
ElementId: TBD11
Data Type Semantics: identifier
Status: Proposed
8. IANA Considerations
8.1. Registration of Information Elements
IANA will register the following IEs in the IPFIX Information
Elements registry at http://www.iana.org/assignments/ipfix/ipfix.xml:
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+-----+----------------+--------+---------+-------+-----------------+
| Val | Name | Data | Data | Statu | Description |
| ue | | Type | Type | s | |
| | | | Semanti | | |
| | | | cs | | |
+-----+----------------+--------+---------+-------+-----------------+
| 1 | FlowSelectorAl | unsign | identif | Propo | This |
| | gorithm | ed16 | ier | sed | Information |
| | | | | | Element |
| | | | | | identifies the |
| | | | | | flow selection |
| | | | | | method(e.g., |
| | | | | | Filtering, |
| | | | | | Sampling) that |
| | | | | | is applied by |
| | | | | | the Flow |
| | | | | | Selection |
| | | | | | Process |
+-----+----------------+--------+---------+-------+-----------------+
| 2 | FlowSelectedOc | unsign | Octets | Propo | This |
| | tetDeltaCount | ed64 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | volume in |
| | | | | | octets of all |
| | | | | | flows that are |
| | | | | | selected during |
| | | | | | the Flow |
| | | | | | Selection |
| | | | | | Process since |
| | | | | | the previous |
| | | | | | report. |
+-----+----------------+--------+---------+-------+-----------------+
| 3 | flowSelectedPa | unsign | Packets | Propo | This |
| | cketDeltaCount | ed64 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | volume in |
| | | | | | packets of all |
| | | | | | flows that were |
| | | | | | selected during |
| | | | | | the Flow |
| | | | | | Selection |
| | | | | | Process since |
| | | | | | the previous |
| | | | | | report. |
+-----+----------------+--------+---------+-------+-----------------+
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+-----+----------------+--------+---------+-------+-----------------+
| 4 | flowSelectedFl | unsign | Flows | Propo | This |
| | owDeltaCount | ed64 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | number of Flows |
| | | | | | that were |
| | | | | | selected during |
| | | | | | the Flow |
| | | | | | Selection |
| | | | | | Process since |
| | | | | | the last |
| | | | | | report. |
+-----+----------------+--------+---------+-------+-----------------+
| 5 | selectorIDTota | unsign | Flows | Propo | This |
| | lFlowsObserved | ed64 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | total number of |
| | | | | | flows observed |
| | | | | | by a Selector, |
| | | | | | for a specific |
| | | | | | value of |
| | | | | | SelectorId. |
| | | | | | This |
| | | | | | Information |
| | | | | | Element should |
| | | | | | be used in an |
| | | | | | Options |
| | | | | | Template scoped |
| | | | | | to the |
| | | | | | observation to |
| | | | | | which it |
| | | | | | refers. See |
| | | | | | Section 3.4.2.1 |
| | | | | | of the IPFIX |
| | | | | | protocol |
| | | | | | document |
| | | | | | [RFC5101] |
+-----+----------------+--------+---------+-------+-----------------+
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+-----+----------------+--------+---------+-------+-----------------+
| 6 | selectorIDTota | unsign | Flows | Propo | This |
| | lFlowsSelected | ed64 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | total number of |
| | | | | | flows selected |
| | | | | | by a Selector, |
| | | | | | for a specific |
| | | | | | value of |
| | | | | | SelectorId. |
| | | | | | This |
| | | | | | Information |
| | | | | | Element should |
| | | | | | be used in an |
| | | | | | Options |
| | | | | | Template scoped |
| | | | | | to the |
| | | | | | observation to |
| | | | | | which it |
| | | | | | refers. See |
| | | | | | Section 3.4.2.1 |
| | | | | | of the IPFIX |
| | | | | | protocol |
| | | | | | document |
| | | | | | [RFC5101]. |
+-----+----------------+--------+---------+-------+-----------------+
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+-----+----------------+--------+---------+-------+-----------------+
| 7 | samplingFlowIn | unsign | Flows | Propo | This |
| | terval | ed32 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | number of flows |
| | | | | | that are |
| | | | | | consecutively |
| | | | | | sampled. A |
| | | | | | value of 100 |
| | | | | | means that 100 |
| | | | | | consecutive |
| | | | | | flows are |
| | | | | | sampled. For |
| | | | | | example, this |
| | | | | | Information |
| | | | | | Element may be |
| | | | | | used to |
| | | | | | describe the |
| | | | | | configuration |
| | | | | | of a systematic |
| | | | | | count-based |
| | | | | | Sampling |
| | | | | | Selector. |
+-----+----------------+--------+---------+-------+-----------------+
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+-----+----------------+--------+---------+-------+-----------------+
| 8 | samplingFlowSp | unsign | Flows | Propo | This |
| | ace | ed32 | | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | number of flows |
| | | | | | between two |
| | | | | | "samplingFlowIn |
| | | | | | terval"s. A |
| | | | | | value of 100 |
| | | | | | means that the |
| | | | | | next interval |
| | | | | | starts 100 |
| | | | | | flows (which |
| | | | | | are not |
| | | | | | sampled) after |
| | | | | | the current |
| | | | | | "samplingFlowI |
| | | | | | nterval" is ove |
| | | | | | r.For example, |
| | | | | | this |
| | | | | | Information |
| | | | | | Element may b |
| | | | | | e used to |
| | | | | | describe the |
| | | | | | configuration |
| | | | | | of a systemat |
| | | | | | iccount-based |
| | | | | | Sampling |
| | | | | | Selector. |
+-----+----------------+--------+---------+-------+-----------------+
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+-----+----------------+--------+---------+-------+-----------------+
| 9 | flowSamplingTi | unsign | microse | Propo | This |
| | meInterval | ed32 | conds | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | time interval |
| | | | | | in microseconds |
| | | | | | during which |
| | | | | | all arriving |
| | | | | | flows are |
| | | | | | sampled. For |
| | | | | | example, this |
| | | | | | Information |
| | | | | | Element may be |
| | | | | | used to |
| | | | | | describe the |
| | | | | | configuration |
| | | | | | of a systematic |
| | | | | | time-based |
| | | | | | Sampling |
| | | | | | Selector. |
+-----+----------------+--------+---------+-------+-----------------+
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+-----+----------------+--------+---------+-------+-----------------+
| 10 | flowSamplingTi | unsign | microse | Propo | This |
| | meSpace | ed32 | conds | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | time interval |
| | | | | | in microseconds |
| | | | | | between two |
| | | | | | "flowSamplingTi |
| | | | | | meInterval"s. |
| | | | | | Avalue of 100 |
| | | | | | means that the |
| | | | | | next interval |
| | | | | | starts 100 |
| | | | | | microseconds |
| | | | | | (during which |
| | | | | | no flows are |
| | | | | | sampled) after |
| | | | | | the current |
| | | | | | "flowsamplingT |
| | | | | | imeInterval" is |
| | | | | | over. For |
| | | | | | example, this |
| | | | | | Information |
| | | | | | Element may |
| | | | | | used to |
| | | | | | describe the |
| | | | | | configuration |
| | | | | | of a systemat |
| | | | | | ictime-based |
| | | | | | Sampling |
| | | | | | Selector. |
+-----+----------------+--------+---------+-------+-----------------+
| 11 | hashFlowDomain | unsign | identif | Propo | This |
| | | ed16 | ier | sed | Information |
| | | | | | Element |
| | | | | | specifies the |
| | | | | | Information |
| | | | | | Elements that |
| | | | | | are used by the |
| | | | | | Hash-based flow |
| | | | | | Selection |
| | | | | | Selector as the |
| | | | | | Hash Domain. |
+-----+----------------+--------+---------+-------+-----------------+
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8.2. Registration of Object Identifier
IANA will register the following OID in the IPFIX-SELECTOR-MIB
Functions sub-registry at http://www.iana.org/assignments/smi-numbers
according to the procedures set forth in [I-D.dkcm-ipfix-rfc5815bis]
+---------+-----------------------+---------------------+-----------+
| Decimal | Name | Description | Reference |
+---------+-----------------------+---------------------+-----------+
| 1 | FlowSelectorAlgorithm | This Object | [RFCyyyy] |
| | | Identifier | |
| | | identifies the flow | |
| | | selection | |
| | | method(e.g., | |
| | | Filtering, | |
| | | Sampling) that is | |
| | | applied by the Flow | |
| | | Selection Process | |
+---------+-----------------------+---------------------+-----------+
Editor's Note (to be removed prior to publication): the RFC editor is
asked to replace "yyyy" in this document by the number of the RFC
when the assignment has been made.
9. Security Considerations
The described flow sampling techniques and the hash-based flow
filtering technique aim at the selection of a representative subset
in order to make an accurate estimation of the population. An
adversary may have incentives to influence the selection of his
flows, for example to circumvent accounting.
Security considerations concerning the choice of a Hash Function for
Hash-based Packet Selection have been discussed in Section 6.2.3 of
[RFC5475] and are also appropiate for Hash-Based Flow Selection.
This section discussed a number of potential attacks to craft Streams
that are disproportionately detected and/or discover the Hash
Function parameters, the vulnerabilities of different Hash Functions
to these attacks, and practices to minimize these vulnerabilities.
For other sampling approaches a user can gain knowledge about the
start and stop triggers in time-based systematic Sampling, e.g., by
sending test packets. This knowledge might allow users to modify
their send schedule in a way that their packets are
disproportionately selected or not selected. For random Sampling, a
cryptographically strong random number generator should be used in
order to prevent that an advisory can predict the selection decision
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[GoRe07].
Further security threats can occur when Sampling parameters are
configured or communicated to other entities. The protocol(s) for
the configuration and reporting of Sampling parameters are out of
scope of this document. Therefore, the security threats that
originate from this kind of communication cannot be assessed with the
information given in this document. Some of these threats can
probably be addressed by keeping configuration information
confidential and by authenticating entities that configure Sampling.
Nevertheless, a full analysis and assessment of threats for
configuration and reporting has to be done if configuration or
reporting methods are proposed.
10. Acknowledgments
We would like to thank the IPFIX group, especially Brian Trammell,
Paul Aitken and Benoit Claise for fruitful discussions and for
proofreading the document.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[CoHa08] Cormode, G. and M. Hadjieleftheriou, "Finding frequent
items in data streams", Journal, Proceedings of the Very
Large DataBase Endowment VLDB Endowment, Volume 1 Issue 2,
August 2008, August 2008.
[DuLT01] Duffield, N., Lund, C., and M. Thorup, "Charging from
Sampled Network Usage", ACM Internet Measurement Workshop
IMW 2001, San Francisco, USA, November 2001.
[EsVa01] Estan, C. and G,. Varghese, "New Directions in Traffic
Measurement and Accounting: Focusing on the Elephants,
Ignoring the Mice", ACM SIGCOMM Internet Measurement
Workshop 2001, San Francisco (CA), November 2001.
[KaPS03] Karp, R., Papadimitriou, C., and S. S. Shenker, "A simple
algorithm for finding frequent elements in sets and
bags.", ACM Transactions on Database Systems, Volume 28,
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51-55, 2003, March 2003.
[MSZC10] Mai, J., Sridharan, A., Zang, H., and C. Chuah, "Fast
Filtered Sampling", Computer Networks Volume 54, Issue 11,
Pages 1885-1898, ISSN 1389-1286, January 2010.
[MaMo02] Manku, G. and R. Motwani, "Approximate Frequency Counts
over Data Streams", Proceedings of the International
Conference on Very large DataBases (VLDB) pages 346--357,
2002, Hong Kong, China, 2002.
[RFC3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export (IPFIX)",
RFC 3917, October 2004.
[RFC5101] Claise, B., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008.
[RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
RFC 5102, January 2008.
[RFC5470] Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
"Architecture for IP Flow Information Export", RFC 5470,
March 2009.
[RFC5475] Zseby, T., Molina, M., Duffield, N., Niccolini, S., and F.
Raspall, "Sampling and Filtering Techniques for IP Packet
Selection", RFC 5475, March 2009.
[RFC5476] Claise, B., Johnson, A., and J. Quittek, "Packet Sampling
(PSAMP) Protocol Specifications", RFC 5476, March 2009.
[RFC6183] Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi,
"IP Flow Information Export (IPFIX) Mediation: Framework",
RFC 6183, April 2011.
[iana-ipfix-assignments]
"IP Flow Information Export Information Elements", 2007,
<http://www.iana.org/assignments/ipfix/ipfix.xml>.
D'Antonio, et al. Expires May 17, 2012 [Page 34]
Internet-Draft Flow Selection Techniques November 2011
Authors' Addresses
Salvatore D'Antonio
University of Napoli "Parthenope"
Centro Direzionale di Napoli Is. C4
Naples 80143
Italy
Phone: +39 081 5476766
Email: salvatore.dantonio@uniparthenope.it
Tanja Zseby
Fraunhofer Institute FOKUS
Kaiserin-Augusta-Allee 31
Berlin 10589
Germany
Phone: +49 30 3463 7153
Email: tanja.zseby@fokus.fraunhofer.de
Christian Henke
Technische Universitat Berlin
Strasse des 17. Juni 135
Berlin 10623
Germany
Phone: +49 30 3463 7366
Email: c.henke@tu-berlin.de
Lorenzo Peluso
University of Napoli
Via Claudio 21
Napoli 80125
Italy
Phone: +39 081 7683821
Email: lorenzo.peluso@unina.it
D'Antonio, et al. Expires May 17, 2012 [Page 35]