| Rfc | 7014 |
|---|
| Title | Flow Selection Techniques |
|---|
| Author | S. D'Antonio, T. Zseby, C. Henke, L.
Peluso |
|---|
| Date | September 2013 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED
STANDARD |
|---|
|
Internet Engineering Task Force (IETF) S. D'Antonio
Request for Comments: 7014 Univ. of Napoli "Parthenope"
Category: Standards Track T. Zseby
ISSN: 2070-1721 CAIDA/FhG FOKUS
C. Henke
Tektronix Communications Berlin
L. Peluso
University of Napoli
September 2013
Flow Selection Techniques
Abstract
The Intermediate Flow Selection Process is the process of selecting a
subset of Flows from all observed Flows. The Intermediate Flow
Selection Process may be located at an IP Flow Information Export
(IPFIX) Exporter or Collector, or within an IPFIX Mediator. It
reduces the effort of post-processing Flow data and transferring Flow
Records. This document describes motivations for using the
Intermediate Flow Selection process and presents Intermediate Flow
Selection techniques. It provides an information model for
configuring Intermediate Flow Selection Process techniques and
discusses what information about an Intermediate Flow Selection
Process should be exported.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/rfc7014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Difference between Intermediate Flow Selection Process and
Packet Selection . . . . . . . . . . . . . . . . . . . . . . . 7
4. Difference between Intermediate Flow Selection Process and
Intermediate Selection Process . . . . . . . . . . . . . . . . 9
5. Intermediate Flow Selection Process within the IPFIX
Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Intermediate Flow Selection Process in the Metering
Process . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Intermediate Flow Selection Process in the Exporting
Process . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Intermediate Flow Selection Process as a Function of
the IPFIX Mediator . . . . . . . . . . . . . . . . . . . . 11
6. Intermediate Flow Selection Process Techniques . . . . . . . . 12
6.1. Flow Filtering . . . . . . . . . . . . . . . . . . . . . . 12
6.1.1. Property Match Filtering . . . . . . . . . . . . . . . 12
6.1.2. Hash-Based Flow Filtering . . . . . . . . . . . . . . 13
6.2. Flow Sampling . . . . . . . . . . . . . . . . . . . . . . 13
6.2.1. Systematic Sampling . . . . . . . . . . . . . . . . . 13
6.2.2. Random Sampling . . . . . . . . . . . . . . . . . . . 14
6.3. Flow-State Dependent Intermediate Flow Selection
Process . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.4. Flow-State Dependent Packet Selection . . . . . . . . . . 15
7. Configuration of Intermediate Flow Selection Process
Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Intermediate Flow Selection Process Parameters . . . . . . 17
7.2. Description of Flow-State Dependent Packet Selection . . . 19
8. Information Model for Intermediate Flow Selection Process
Configuration and Reporting . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
9.1. Registration of Information Elements . . . . . . . . . . . 22
9.1.1. flowSelectorAlgorithm . . . . . . . . . . . . . . . . 22
9.1.2. flowSelectedOctetDeltaCount . . . . . . . . . . . . . 24
9.1.3. flowSelectedPacketDeltaCount . . . . . . . . . . . . . 24
9.1.4. flowSelectedFlowDeltaCount . . . . . . . . . . . . . . 24
9.1.5. selectorIDTotalFlowsObserved . . . . . . . . . . . . . 25
9.1.6. selectorIDTotalFlowsSelected . . . . . . . . . . . . . 25
9.1.7. samplingFlowInterval . . . . . . . . . . . . . . . . . 26
9.1.8. samplingFlowSpacing . . . . . . . . . . . . . . . . . 26
9.1.9. flowSamplingTimeInterval . . . . . . . . . . . . . . . 27
9.1.10. flowSamplingTimeSpacing . . . . . . . . . . . . . . . 27
9.1.11. hashFlowDomain . . . . . . . . . . . . . . . . . . . . 28
9.2. Registration of Object Identifier . . . . . . . . . . . . 28
10. Security and Privacy Considerations . . . . . . . . . . . . . 28
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative References . . . . . . . . . . . . . . . . . . . 30
12.2. Informative References . . . . . . . . . . . . . . . . . . 31
1. Introduction
This document describes Intermediate Flow Selection Process
techniques for network traffic measurements. A Flow is defined as a
set of packets with common properties, as described in [RFC7011]. An
Intermediate Flow Selection Process can be executed to limit the
resource demands for capturing, storing, exporting, and post-
processing 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 Intermediate Flow Selection
Process techniques and describes configuration and reporting
parameters for them.
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, these techniques were only briefly
discussed in [RFC5475]. Therefore, configuration and reporting
considerations for Flow-state dependent packet selection techniques
have been included in this document.
1.1. 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].
2. Terminology
This document is consistent with the terminology introduced in
[RFC7011], [RFC5470], [RFC5475], and [RFC3917]. As in [RFC7011] and
[RFC5476], the first letter of each IPFIX specific and Packet
Sampling (PSAMP) specific term is capitalized, along with the
Intermediate Flow Selection Process 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, Autonomous System (AS) number) make
up the Flow Key. If a Flow Record for a specific Flow Key value
already exists, the Flow Record is updated; otherwise, a new Flow
Record is created.
* Intermediate Flow Selection Process
An Intermediate Flow Selection Process is an Intermediate Process,
as defined in [RFC6183] that takes Flow Records as its input and
selects a subset of this set as its output. The Intermediate Flow
Selection Process is a more general concept than the Intermediate
Selection Process as defined in [RFC6183]. While an Intermediate
Selection Process selects Flow Records from a sequence based upon
criteria-evaluated Flow Record values and only passes on those
Flow Records that match the criteria, an Intermediate Flow
Selection Process selects Flow Records using selection criteria
applicable to a larger set of Flow characteristics and
information.
* Flow Cache
A Flow Cache is the set of Flow Records.
* Flow Selection State
An Intermediate Flow Selection Process maintains 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 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) Intermediate Flow Selection Process timeout intervals.
* Flow Selector
A Flow Selector defines the action of an Intermediate 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 or not a Flow has to be selected:
(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
Intermediate Flow Selection Process.
* Complete Flow
A Complete Flow consists of all the packets that enter the
Intermediate Flow Selection Process within the Flow timeout
interval and that belong to the same Flow, per the definition of
"Flow" in [RFC5470]. For this definition, only packets that
arrive at the Intermediate Flow Selection Process are considered.
* Flow Position
Flow Position is the position of a Flow Record within the Flow
Cache.
* 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 the packet level (e.g., Flow
Keys from packet header fields), Flow Filtering can be performed
at the 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 that are called the Hash Domain. If the hash value
falls into a predefined Hash Selection Range, the Flow is
selected.
* Flow-state Dependent Intermediate Flow Selection Process
The Flow-state dependent Intermediate Flow Selection Process 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 implement a preference for 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 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 Intermediate Flow Selection Process and Packet
Selection
The Intermediate Flow Selection Process differs from packet selection
as 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 Intermediate Flow
Selection Process operates on the Flows. The major characteristics
of the Intermediate Flow Selection Process are the following:
- The Intermediate Flow Selection Process takes Flows as basic
elements. For packet selection, packets are considered as basic
elements.
- The Intermediate Flow Selection Process typically takes place
after Packet Classification, because the classification rules
determine to which Flow a packet belongs. The Intermediate Flow
Selection Process can be performed before Packet Classification.
In that case, the Intermediate Flow Selection Process is based on
the Flow Key (and also on a hash value over the Flow Key) but not
on characteristics that are only available after Packet
Classification (e.g., Flow size, Flow duration). Packet selection
can be applied before and after Packet Classification. As an
example, packet selection before Packet Classification can be
random packet selection, whereas packet selection after Packet
Classification can be Flow-state dependent packet selection (as
described in [RFC5475]).
- The Intermediate Flow Selection Process operates on Complete
Flows. That means that after the Intermediate Flow Selection
Process, either all packets of the Flow are kept or all packets of
the Flow are discarded. That means that if the Intermediate Flow
Selection Process 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, some guidance is given
on how to categorize such techniques:
- Techniques that can be considered as both packet selection and an
Intermediate Flow Selection Process: some packet selection
techniques result in the selection of Complete Flows and therefore
can be considered as packet selection or as an Intermediate Flow
Selection Process 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 an Intermediate Flow Selection Process
and can be considered as packet selection or as an Intermediate
Flow Selection Process.
- Flow-state Dependent Packet Selection: 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 include "Sample and Hold"
[EsVa01], "Fast Filtered Sampling" [MSZC10], and 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 techniques 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 Intermediate
Flow Selection Process techniques.
4. Difference between Intermediate Flow Selection Process and
Intermediate Selection Process
The Intermediate Flow Selection Process differs from the Intermediate
Selection Process, since the Intermediate Flow Selection Process uses
selection criteria that apply to a larger set of Flow information and
properties than those used by the Intermediate Selection Process.
The typical function of an Intermediate Selection Process is Property
Match Filtering, which selects a Flow Record if the value of a
specific field in the Flow Record matches a configured value or falls
within a configured range. This means that the selection criteria
used by an Intermediate Selection Process are evaluated only on Flow
Record values. An Intermediate Flow Selection Process makes its
decision on whether a Flow has to be selected or not by taking into
account not only information related to the content of the Flow
Record but also any Flow Selection State information or variable that
can be used to select Flows in order to meet application requirements
or resource constraints (e.g., Flow Cache occupancy, export link
capacity). Examples include flow counters, Intermediate Flow
Selection Process timeout intervals, and Flow Record time
information.
5. Intermediate Flow Selection Process within the IPFIX Architecture
An Intermediate Flow Selection Process can be deployed at any of
three places within the IPFIX architecture. As shown in Figure 1,
the Intermediate Flow Selection Process can occur
1. in the Metering Process at the IPFIX Exporter
2. in the Exporting Process at the Collector
3. within a Mediator
+===========================================+
| IPFIX Exporter +----------------+ |
| | Metering Proc. | |
| +-----------------+ +----------------+ |
| | Metering | | Intermediate | |
| | Process | or | Flow Selection | |
| | | | Process | |
| +-----------------+----+----------------+ |
| | Exporting Process | |
| +----|-------------------------------|--+ |
+======|===============================|====+
| |
| |
+======|========================+ |
| | Mediator | |
| +-V-------------------+ | |
| | Collecting Process | | |
| +---------------------+ | |
| | Intermediate Flow | | |
| | Selection Process | | |
| +---------------------+ | |
| | Exporting Process | | |
| +-|-------------------+ | |
+======|========================+ |
| |
| |
+======|===============================|=====+
| | Collector | |
| +----V-------------------------------V-+ |
| | Collecting Process | |
| +--------------------------------------+ |
| | Intermediate Flow Selection Process | |
| +--------------------------------------+ |
| | Exporting Process | |
| +------------------------------|-------+ |
+================================|===========+
|
|
V
+------------------+
| IPFIX |
+------------------+
Figure 1: Potential Intermediate Flow Selection Process Locations
In contrast to packet selection, the Intermediate Flow Selection
Process is always applied after the packets are classified into
Flows.
5.1. Intermediate Flow Selection Process in the Metering Process
An Intermediate Flow Selection Process in the Metering Process uses
packet information to update the Flow Records in the Flow Cache. The
Intermediate Flow Selection Process, before Packet Classification,
can be based on the Flow Key (and also on a hash value over the Flow
Key) but not on characteristics that are only available after Packet
Classification (e.g., Flow size, Flow duration). Here, an
Intermediate Flow Selection Process is applied to reduce resources
for all subsequent processes or to select specific Flows of interest
in cases where such Flow characteristics are already observable at
the packet level (e.g., Flows to specific IP addresses). In
contrast, Flow-state dependent packet selection is a packet selection
technique, because it does not necessarily select Complete Flows.
5.2. Intermediate Flow Selection Process in the Exporting Process
An Intermediate Flow Selection Process in the Exporting Process works
on Flow Records and can therefore depend on Flow characteristics that
are only visible after the classification of packets, such as Flow
size and Flow duration. The Exporting Process may implement policies
for exporting only a subset of the Flow Records that have been stored
in the system's memory, in order to offload Flow export and Flow
post-processing. An Intermediate Flow Selection Process in the
Exporting Process may select only the subset of Flow Records that are
of interest to the user's application or select only as many Flow
Records as can be handled by the available resources (e.g., limited
export link capacity).
5.3. Intermediate Flow Selection Process as a Function of the IPFIX
Mediator
As shown in Figure 1, the Intermediate Flow Selection Process can be
performed within an IPFIX Mediator [RFC6183]. The Intermediate Flow
Selection Process takes a Flow Record stream as its input and selects
Flow Records from a sequence based upon criteria-evaluated record
values. The Intermediate Flow Selection Process can again apply an
Intermediate Flow Selection Process technique to obtain Flows of
interest to the application. Further, the Intermediate Flow
Selection Process can base its selection decision on the correlation
of data from different IPFIX Exporters, e.g., by only selecting Flows
that were recorded on two or more IPFIX Exporters.
6. Intermediate Flow Selection Process Techniques
An Intermediate Flow Selection Process technique selects either all
or none of the packets of a Flow; otherwise, the technique has to be
considered as packet selection. A difference between Flow Filtering
and Flow sampling is recognized.
6.1. Flow Filtering
Flow Filtering is a deterministic function on the IPFIX Flow Record
content. If the relevant Flow characteristics are already observable
at the packet level (e.g., Flow Keys), Flow Filtering can be applied
before aggregation at the packet level. In order to be compliant
with IPFIX, at least one of this document's Flow Filtering schemes
MUST be implemented.
6.1.1. Property Match Filtering
Property Match Filtering is performed similarly to Property Match
Filtering for packet selection as described in [RFC5475]. The
difference is that Flow Record fields are used here, instead of
packet fields, to derive the selection decision. Property Match
Filtering is 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 Flow Keys, Flow Timestamps, or Per-Flow
Counters as described in [RFC7012]. Examples include the Flow size
in bytes, the number of packets in the Flow, the observation time of
the first or last packet, and the maximum packet length. An example
of Property Match Filtering is to select Flows with more than a
threshold number of observed octets. The selection criteria can be a
specific value, a set of specific values, or an interval. For
example, a Flow is selected if destinationIPv4Address and the total
number of packets of the Flow equal two predefined values. An
Intermediate Flow Selection Process using Property Match Filtering in
the Metering Process relies on properties that are observable at the
packet level (e.g., Flow Key). For example, a Flow is selected if
sourceIPv4Address and sourceIPv4PrefixLength equal, respectively, two
specific values.
An Intermediate Flow Selection Process using Property Match Filtering
in the Exporting Process is based on properties that are only visible
after Packet Classification, such as Flow size and Flow duration. An
example is the selection of the largest Flows or a percentage of
Flows with the longest lifetime. Another example is to select and
remove from the Flow Cache the Flow Record with the lowest Flow
volume per current Flow lifetime if the Flow Cache is full.
An Intermediate Flow Selection Process using Property Match Filtering
within an IPFIX Mediator selects a Flow Record if the value of a
specific field in the Flow Record equals a configured value or falls
within a configured range [RFC6183].
6.1.2. 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 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 is in fact 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, but not at the packet level, in the Metering
Process, for example, when the size of the selection range, and
therefore the sampling probability, are dependent on the number of
observed Flows. If Hash-based Flow Filtering is used to select the
same subset of flows at different Observation Points, the Hash Domain
MUST only include parts of the Flow Record content that are invariant
on the Flow path. Refer also to the Trajectory Sampling application
example of coordinated packet selection [RFC5475], which explains the
hash-based filtering approach at the packet level.
6.2. Flow Sampling
Flow sampling operates on Flow Record sequence or arrival times. It
can use either a systematic or a random function for the Intermediate
Flow 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).
6.2.1. Systematic Sampling
Systematic sampling is a deterministic selection function. It may be
a periodic selection of the N-th Flow Record that arrives at the
Intermediate Flow Selection Process. Systematic sampling MAY be
applied in the Metering Process. An example would be to create,
besides the Flow Cache of selected Flows, an additional data
structure that saves the Flow Key values of the Flows that are not
selected. The selection of a Flow would then be based on the first
packet of a Flow. Every time a packet belonging to a new Flow (which
is not in the data structure of either the selected or non-selected
Flows) arrives at the Observation Point, a counter is increased. If
the counter is increased to a multiple of N, a new Flow Cache entry
is created; if the counter is not a multiple of N, the Flow Key value
is added to the data structure for non-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 the packet level in the Metering Process or after aggregation at
the Flow level, e.g., by selecting a Flow arriving at the Exporting
Process every n seconds.
6.2.2. Random Sampling
Random Flow sampling is based on a random process that requires the
calculation of random numbers. One can differentiate between n-out-
of-N and probabilistic Flow sampling.
6.2.2.1. n-out-of-N Flow Sampling
In n-out-of-N Sampling, n elements are selected out of the parent
population, which 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.
6.2.2.2. Probabilistic Flow Sampling
In probabilistic Sampling, the decision of 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, a difference 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) is recognized.
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 [DuLT01].
6.3. Flow-State Dependent Intermediate Flow Selection Process
The Flow-state dependent Intermediate Flow Selection Process can be a
deterministic or random Intermediate Flow Selection Process, based on
the Flow Record content and the Flow state that 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
Intermediate Flow Selection Process utilizes this information for the
selection decision. A review of Flow-state dependent Intermediate
Flow Selection Process techniques that aim at the selection of the
most frequent items by keeping additional Flow state information can
be found in [CoHa08]. The Flow-state dependent Intermediate Flow
Selection Process can only be applied after packet aggregation, when
a packet has been assigned to a Flow. The Intermediate Flow
Selection Process then decides, based on the Flow state for each
Flow, whether it is kept in the Flow Cache or not. Two Flow-state
dependent Intermediate Flow Selection Process Algorithms are
described here:
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 Flow Cache has an additional counter. The
counter is incremented each time a packet belonging to the Flow in
the Flow Cache is observed. If the observed packet does not belong
to any Flow, all counters are decremented; if any of the Flow
counters has a value of zero, the Flow is replaced with a Flow 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. Also, all Flows with a counter of zero are removed
from the Flow Cache.
6.4. Flow-State Dependent Packet Selection
Flow-state dependent packet selection is not an Intermediate Flow
Selection Process technique but a packet selection technique.
Nevertheless, configuration and reporting parameters for this
technique will be described in this document. An example is the
"Sample and Hold" algorithm [EsVa01], which tries to implement a
preference for large-volume Flows in the selection. When a packet
arrives, it is selected when a Flow Record for this packet already
exists. If there is no Flow Record, the packet is selected according
to a certain probability that is dependent on the packet size.
7. Configuration of Intermediate Flow Selection Process 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
Intermediate Flow Selection Process within an IPFIX Device. The
information model with the Information Elements (IEs) for
Intermediate Flow Selection Process configuration is described
together with the reporting IEs in Section 8. Table 1 gives an
overview of the defined Intermediate Flow Selection Process
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.
+-------------------+--------------------+--------------------------+
| Location | Selection | Selection Input |
| | Technique | |
+-------------------+--------------------+--------------------------+
| In the Metering | Flow-state | packet sampling |
| Process | Dependent Packet | probabilities, Flow |
| | Selection | Selection State, packet |
| | | properties |
| | | |
| In the Metering | Property Match | Flow Record IEs, |
| Process | Flow Filtering | Selection Interval |
| | | |
| In the Metering | Hash-based Flow | selection range, hash |
| Process | Filtering | function, Flow Key, seed |
| | | (optional) |
| | | |
| In the Metering | Time-based | Flow Position (derived |
| Process | Systematic Flow | from arrival time of |
| | sampling | packets), Flow Selection |
| | | State |
| | | |
| In the Metering | Sequence-based | Flow Position (derived |
| Process | Systematic Flow | from packet position), |
| | sampling | Flow Selection State |
| | | |
| In the Metering | Random Flow | random number generator |
| Process | sampling | or list and packet |
| | | position, Flow state |
| | | |
| In the Exporting | Property Match | Flow Record content, |
| Process/ within | Flow Filtering | filter function |
| the IPFIX | | |
| Mediator | | |
| | | |
| In the Exporting | Hash-based Flow | selection range, hash |
| Process/ within | Filtering | function, hash input |
| the IPFIX | | (Flow Keys and other |
| Mediator | | Flow properties) |
| | | |
| In the Exporting | Flow-state | Flow state parameters, |
| Process/ within | Dependent | random number generator |
| the IPFIX | Intermediate Flow | or list |
| Mediator | Selection Process | |
| | | |
| In the Exporting | Time-based | Flow arrival time, Flow |
| Process/ within | Systematic Flow | state |
| the IPFIX | sampling | |
| Mediator | | |
| | | |
| In the Exporting | Sequence-based | Flow Position, Flow |
| Process/ within | Systematic Flow | state |
| the IPFIX | sampling | |
| Mediator | | |
| | | |
| In the Exporting | Random Flow | random number generator |
| Process/ within | sampling | or list and Flow |
| the IPFIX | | Position, Flow state |
| Mediator | | |
+-------------------+--------------------+--------------------------+
Table 1: Overview of Intermediate Flow Selection Process Techniques
7.1. Intermediate Flow Selection Process Parameters
This section defines what parameters are required to describe the
most common Intermediate Flow Selection Process techniques.
Intermediate Flow Selection Process Parameters:
For Property Match Filtering:
- Information Element as specified in [IANA-IPFIX]):
Specifies the Information Element that is used as the property in
the filter expression. Section 8 specifies the Information
Elements that MUST be exported by an Intermediate Flow Selection
Process using Property Match Filtering.
- Selection Value or Value Interval:
Specifies the value or interval of the filter expression. Packets
and Flow Records 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 that 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 [RFC5475], IP
Shift-XOR (IPSX) [RFC5475], and CRC-32 [Bra75].
- 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 Intermediate Flow Selection Process:
- 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 that exceed the defined threshold
will be selected.
- Accuracy parameter:
Specifies the accuracy parameter e for techniques that deal with
the issue of mining frequent items in a dataset. 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 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):
Defines the spacing in usec between the end of one sampling
interval and the start of the next interval.
For Systematic count-based Flow sampling:
- Interval length:
Defines the number of Flows that are selected within the sampling
interval.
- Spacing:
Defines the spacing, in number of observed Flows, between the end
of one sampling interval and the start of the next interval.
For random n-out-of-N Flow sampling:
- Population Size N:
The number of all Flows in the Population from which the sample is
drawn.
- Sampling Size n:
The number of Flows that are randomly drawn from the population N.
For probabilistic Flow sampling:
- Sampling probability p:
Defines the probability by which each of the observed Flows is
selected.
7.2. 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].
8. Information Model for Intermediate Flow Selection Process
Configuration and Reporting
This section specifies the Information Elements that MUST be exported
by an Intermediate Flow Selection Process in order to support the
interpretation of measurement results from Flow measurements. 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 for providing
an accuracy statement. The IEs can provide reporting information
about Flow Records, packets, or bytes. The reported metrics are the
total number of elements and the number of selected elements. The
number of dropped elements can be derived from this information.
Table 2 shows a list of Intermediate Flow Selection Process
Information Elements:
ID Name | ID Name
----------------------------------+----------------------------------
301 selectionSequenceID | 302 selectorID
|
390 flowSelectorAlgorithm | 1 octetDeltaCount
|
391 flowSelectedOctetDeltaCount | 2 packetDeltaCount
|
392 flowSelectedPacketDeltaCount | 3 originalFlowsPresent
|
393 flowSelectedFlowDeltaCount | 394 selectorIDTotalFlowsObserved
|
395 selectorIDTotalFlowsSelected | 396 samplingFlowInterval
|
397 samplingFlowSpacing | 309 samplingSize
|
310 samplingPopulation | 311 samplingProbability
|
398 flowSamplingTimeInterval | 399 flowSamplingTimeSpacing
|
326 digestHashValue | 400 hashFlowDomain
|
329 hashOutputRangeMin | 330 hashOutputRangeMax
|
331 hashSelectedRangeMin | 332 hashSelectedRangeMax
|
333 hashDigestOutput | 334 hashInitialiserValue
|
320 absoluteError | 321 relativeError
|
336 upperCILimit | 337 lowerCILimit
|
338 confidenceLevel |
Table 2: Intermediate Flow Selection Process Information Elements
9. IANA Considerations
9.1. Registration of Information Elements
IANA has registered the following IEs in the "IPFIX Information
Elements" registry at http://www.iana.org/assignments/ipfix/.
9.1.1. flowSelectorAlgorithm
Description:
This Information Element identifies the Intermediate Flow
Selection Process technique (e.g., Filtering, Sampling) that is
applied by the Intermediate Flow Selection Process. Most of these
techniques have parameters; configuration parameter(s) MUST be
clearly specified. 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, on a First Come First Served basis
[RFC5226], subject to Expert Review [RFC5226]. Please note that
the purpose of the flow selection techniques described in this
document is the improvement of measurement functions as defined in
the Introduction (Section 1). Before adding new flow selector
algorithms, their intended purposes should be determined,
especially if those purposes contradict any policies defined in
[RFC2804]. The designated expert(s) should consult with the
community if a request that runs counter to [RFC2804] is received.
The registry can be updated when specifications of the new
method(s) and any new Information Elements are provided. The
group of experts must double-check the flowSelectorAlgorithm
definitions and Information Elements with already-defined
flowSelectorAlgorithm definitions and Information Elements for
completeness, accuracy, and redundancy. Those experts will
initially be drawn from the Working Group Chairs and document
editors of the IPFIX and PSAMP Working Groups. The following
identifiers for Intermediate Flow Selection Process Techniques are
defined here:
+----+------------------------+--------------------------+
| ID | Technique | Parameters |
+----+------------------------+--------------------------+
| 1 | Systematic count-based | flowSamplingInterval |
| | Sampling | flowSamplingSpacing |
+----+------------------------+--------------------------+
| 2 | Systematic time-based | flowSamplingTimeInterval |
| | Sampling | flowSamplingTimeSpacing |
+----+------------------------+--------------------------+
| 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 |
| | Intermediate Flow | |
| | Selection Process | |
+----+------------------------+--------------------------+
Table 3: Intermediate Flow Selection Process Techniques
Abstract Data Type: unsigned16
ElementId: 390
Data Type Semantics: identifier
Status: current
9.1.2. flowSelectedOctetDeltaCount
Description:
This Information Element specifies the volume in octets of all
Flows that are selected in the Intermediate Flow Selection Process
since the previous report.
Abstract Data Type: unsigned64
ElementId: 391
Units: octets
Status: current
9.1.3. flowSelectedPacketDeltaCount
Description:
This Information Element specifies the volume in packets of all
Flows that were selected in the Intermediate Flow Selection
Process since the previous report.
Abstract Data Type: unsigned64
ElementId: 392
Units: packets
Status: current
9.1.4. flowSelectedFlowDeltaCount
Description:
This Information Element specifies the number of Flows that were
selected in the Intermediate Flow Selection Process since the last
report.
Abstract Data Type: unsigned64
ElementId: 393
Units: flows
Status: current
9.1.5. 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 [RFC7011].
Abstract Data Type: unsigned64
ElementId: 394
Units: flows
Status: current
9.1.6. 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 [RFC7011].
Abstract Data Type: unsigned64
ElementId: 395
Units: flows
Status: current
9.1.7. 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: unsigned64
ElementId: 396
Units: flows
Status: current
9.1.8. samplingFlowSpacing
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: unsigned64
ElementId: 397
Units: flows
Status: current
9.1.9. 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: unsigned64
ElementId: 398
Units: microseconds
Status: current
9.1.10. flowSamplingTimeSpacing
Description:
This Information Element specifies the time interval in
microseconds between two "flowSamplingTimeInterval"s. A value of
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 be used to describe the configuration of a systematic
time-based Sampling Selector.
Abstract Data Type: unsigned64
ElementId: 399
Units: microseconds
Status: current
9.1.11. hashFlowDomain
Description:
This Information Element specifies the Information Elements that
are used by the Hash-based Flow Selector as the Hash Domain.
Abstract Data Type: unsigned16
ElementId: 400
Data Type Semantics: identifier
Status: Current
9.2. Registration of Object Identifier
IANA has registered the following OID in the IPFIX-SELECTOR-MIB
Functions subregistry at http://www.iana.org/assignments/smi-numbers
according to the procedures set forth in [RFC6615].
+---------+-----------------------+---------------------+-----------+
| Decimal | Name | Description | Reference |
+---------+-----------------------+---------------------+-----------+
| 8 | flowSelectorAlgorithm | This Object | [RFC7014] |
| | | Identifier | |
| | | identifies the | |
| | | Intermediate Flow | |
| | | Selection Process | |
| | | technique (e.g., | |
| | | Filtering, | |
| | | Sampling) that is | |
| | | applied by the | |
| | | Intermediate Flow | |
| | | Selection Process | |
+---------+-----------------------+---------------------+-----------+
Table 4: Object Identifiers to Be Registered
10. Security and Privacy Considerations
Flow data exported by Exporting Processes, and collected by
Collecting Processes, can be sensitive for privacy reasons and need
to be protected. Privacy considerations for collected data are
provided in [RFC7011].
Some of the described Intermediate Flow Selection Process techniques
(e.g., Flow sampling, hash-based Flow Filtering) aim at the selection
of a representative subset of flows in order to estimate parameters
of the population. An adversary may have incentives to influence the
selection of flows, for example, to circumvent accounting or to avoid
the detection of packets that are part of an attack.
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 appropriate for Hash-based Flow Selection.
[RFC5475] discusses the possibility of crafting Packet Streams that
are disproportionately selected or can be used to discover hash
function parameters. It also describes vulnerabilities of different
hash functions to these attacks and discusses practices to minimize
these vulnerabilities.
For other sampling approaches, an adversary can gain knowledge about
the start and stop triggers in time-based systematic Sampling, e.g.,
by sending test packets. This knowledge might allow adversaries to
modify their send schedule in such a way that their packets are
disproportionately selected or not selected. For random Sampling, an
input to the encryption process, like the Initialization Vector of
the CBC (Cipher Block Chaining) mode, should be used to prevent an
adversary from predicting the selection decision [Dw01].
Further security threats can occur when Intermediate Flow Selection
Process parameters are configured or communicated to other entities.
The protocol(s) for the configuration and reporting of Intermediate
Flow Selection Process parameters are out of scope for this document.
Nevertheless, a set of initial requirements for future configuration
and reporting protocols are stated below:
1. Protection against disclosure of configuration information:
Intermediate Flow Selection Process configuration information
describes the Intermediate Flow Selection Process and its
parameters. This information can be useful to attackers.
Attackers may craft packets that never fit the selection criteria
in order to prevent Flows from being seen by the Intermediate
Flow Selection Process. They can also craft a lot of packets
that fit the selection criteria and overload or bias subsequent
processes. Therefore, any transmission of configuration data
(e.g., to configure a process or to report its actual status)
should be protected by encryption.
2. Protection against modification of configuration information:
Sending incorrect configuration information to the Intermediate
Flow Selection Process can lead to a malfunction of the
Intermediate Flow Selection Process. Additionally, reporting
incorrect configuration information from the Intermediate Flow
Selection Process to other processes can lead to incorrect
estimations at subsequent processes. Therefore, any protocol
that transmits configuration information should prevent an
attacker from modifying configuration information. Data
integrity can be achieved by authenticating the data.
3. Protection against malicious nodes sending configuration
information:
The remote configuration of Intermediate Flow Selection Process
techniques should be protected against access by unauthorized
nodes. This can be achieved by access control lists at the
device that hosts the Intermediate Flow Selection Process (e.g.,
IPFIX Exporter, IPFIX Mediator, or IPFIX Collector) and by source
authentication. The reporting of configuration data from an
Intermediate Flow Selection Process has to be protected in the
same way. That means that protocols that report configuration
data from the Intermediate Flow Selection Process to other
processes also need to protect against unauthorized nodes
reporting configuration information.
The security threats that originate from communicating configuration
information to and from Intermediate Flow Selection Processes cannot
be assessed solely with the information given in this document. A
further and more detailed assessment of security threats is necessary
when a specific protocol for the configuration or reporting
configuration data is proposed.
11. 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.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC6615] Dietz, T., Kobayashi, A., Claise, B., and G. Muenz,
"Definitions of Managed Objects for IP Flow Information
Export", RFC 6615, June 2012.
[RFC7011] Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
"Specification of the IP Flow Information Export
(IPFIX) Protocol for the Exchange of Flow Information",
STD 77, RFC 7011, September 2013.
[RFC7012] Claise, B., Ed. and B. Trammell, Ed., "Information
Model for IP Flow Information Export (IPFIX)",
RFC 7012, September 2013.
12.2. Informative References
[Bra75] Brayer, K., "Evaluation of 32 Degree Polynomials in
Error Detection on the SATIN IV Autovon Error
Patterns", National Technical Information Service,
August 1975.
[CoHa08] Cormode, G. and M. Hadjieleftheriou, "Finding Frequent
Items in Data Streams", Proceedings of the 34th
International Conference on Very Large DataBases
(VLDB), Auckland, New Zealand, Volume 1, Issue 2, pages
1530-1541, August 2008.
[DuLT01] Duffield, N., Lund, C., and M. Thorup, "Charging from
Sampled Network Usage", ACM SIGCOMM Internet
Measurement Workshop (IMW) 2001, pages 245-256, San
Francisco, CA, USA, November 2001.
[Dw01] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation - Methods and Techniques", NIST Special
Publication 800-38A, December 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 (IMW) 2001, San Francisco, CA, USA,
November 2001.
[IANA-IPFIX] IANA, "IP Flow Information Export (IPFIX) Entities
Registry", <http://www.iana.org/assignments/ipfix/>.
[KaPS03] Karp, R., Papadimitriou, C., and S. Shenker, "A simple
algorithm for finding frequent elements in sets and
bags", ACM Transactions on Database Systems, Volume 28,
pages 51-55, 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, August 2010.
[MaMo02] Manku, G. and R. Motwani, "Approximate Frequency Counts
over Data Streams", Proceedings of the 28th
International Conference on Very Large DataBases
(VLDB), Hong Kong, China, pages 346-357, August 2002.
[RFC2804] IAB and IESG, "IETF Policy on Wiretapping", RFC 2804,
May 2000.
[RFC3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export (IPFIX)",
RFC 3917, October 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
[RFC5470] Sadasivan, G., Brownlee, N., Claise, B., and J.
Quittek, "Architecture for IP Flow Information Export",
RFC 5470, March 2009.
[RFC6183] Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi,
"IP Flow Information Export (IPFIX) Mediation:
Framework", RFC 6183, April 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
CAIDA/FhG FOKUS
San Diego Supercomputer Center (SDSC)
University of California, San Diego (UCSD)
9500 Gilman Drive
La Jolla, CA 92093-0505
USA
EMail: tanja.zseby@tuwien.ac.at
Christian Henke
Tektronix Communications Berlin
Wohlrabedamm 32
Berlin 13629
Germany
Phone: +49 17 2323 8717
EMail: christian.henke@tektronix.com
Lorenzo Peluso
University of Napoli
Via Claudio 21
Napoli 80125
Italy
Phone: +39 081 7683821
EMail: lorenzo.peluso@unina.it