Keywords: psamp, selector, collector
Network Working Group N. Duffield, Ed.
Request for Comments: 5474 AT&T Labs - Research
Category: Informational D. Chiou
University of Texas
B. Claise
Cisco Systems, Inc.
A. Greenberg
Microsoft
M. Grossglauser
EPFL & Nokia
J. Rexford
Princeton University
March 2009
A Framework for Packet Selection and Reporting
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Abstract
This document specifies a framework for the PSAMP (Packet SAMPling)
protocol. The functions of this protocol are to select packets from
a stream according to a set of standardized Selectors, to form a
stream of reports on the selected packets, and to export the reports
to a Collector. This framework details the components of this
architecture, then describes some generic requirements, motivated by
the dual aims of ubiquitous deployment and utility of the reports for
applications. Detailed requirements for selection, reporting, and
exporting are described, along with configuration requirements of the
PSAMP functions.
Table of Contents
1. Introduction ....................................................4
2. PSAMP Documents Overview ........................................4
3. Elements, Terminology, and High-Level Architecture ..............5
3.1. High-Level Description of the PSAMP Architecture ...........5
3.2. Observation Points, Packet Streams, and Packet Content .....5
3.3. Selection Process ..........................................6
3.4. Reporting ..................................................7
3.5. Metering Process ...........................................8
3.6. Exporting Process ..........................................8
3.7. PSAMP Device ...............................................9
3.8. Collector ..................................................9
3.9. Possible Configurations ....................................9
4. Generic Requirements for PSAMP .................................11
4.1. Generic Selection Process Requirements ....................11
4.2. Generic Reporting Requirements ............................12
4.3. Generic Exporting Process Requirements ....................12
4.4. Generic Configuration Requirements ........................13
5. Packet Selection ...............................................13
5.1. Two Types of Selectors ....................................13
5.2. PSAMP Packet Selectors ....................................14
5.3. Selection Fraction Terminology ............................17
5.4. Input Sequence Numbers for Primitive Selectors ............18
5.5. Composite Selectors .......................................19
5.6. Constraints on the Selection Fraction .....................19
6. Reporting ......................................................19
6.1. Mandatory Contents of Packet Reports: Basic Reports .......19
6.2. Extended Packet Reports ...................................20
6.3. Extended Packet Reports in the Presence of IPFIX ..........20
6.4. Report Interpretation .....................................21
7. Parallel Metering Processes ....................................22
8. Exporting Process ..............................................22
8.1. Use of IPFIX ..............................................22
8.2. Export Packets ............................................22
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8.3. Congestion-Aware Unreliable Transport .....................22
8.4. Configurable Export Rate Limit ............................23
8.5. Limiting Delay for Export Packets .........................23
8.6. Export Packet Compression .................................24
8.7. Collector Destination .....................................25
8.8. Local Export ..............................................25
9. Configuration and Management ...................................25
10. Feasibility and Complexity ....................................26
10.1. Feasibility ..............................................26
10.1.1. Filtering .........................................26
10.1.2. Sampling ..........................................26
10.1.3. Hashing ...........................................26
10.1.4. Reporting .........................................27
10.1.5. Exporting .........................................27
10.2. Potential Hardware Complexity ............................27
11. Applications ..................................................28
11.1. Baseline Measurement and Drill Down ......................29
11.2. Trajectory Sampling ......................................29
11.3. Passive Performance Measurement ..........................30
11.4. Troubleshooting ..........................................30
12. Security Considerations .......................................31
12.1. Relation of PSAMP and IPFIX Security for
Exporting Process ........................................31
12.2. PSAMP Specific Privacy Considerations ....................31
12.3. Security Considerations for Hash-Based Selection .........32
12.3.1. Modes and Impact of Vulnerabilities ...............32
12.3.2. Use of Private Parameters in Hash Functions .......33
12.3.3. Strength of Hash Functions ........................33
12.4. Security Guidelines for Configuring PSAMP ................34
13. Contributors ..................................................34
14. Acknowledgments ...............................................34
15. References ....................................................34
15.1. Normative References .....................................34
15.2. Informative References ...................................35
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1. Introduction
This document describes the PSAMP framework for network elements to
select subsets of packets by statistical and other methods, and to
export a stream of reports on the selected packets to a Collector.
The motivation for the PSAMP standard comes from the need for
measurement-based support for network management and control across
multivendor domains. This requires domain-wide consistency in the
types of selection schemes available, and the manner in which the
resulting measurements are presented and interpreted.
The motivation for specific packet selection operations comes from
the applications that they enable. Development of the PSAMP standard
is open to influence by the requirements of standards in related IETF
Working Groups, for example, IP Performance Metrics (IPPM) [RFC2330]
and Internet Traffic Engineering (TEWG).
The name PSAMP is a contraction of the phrase "Packet Sampling". The
word "Sampling" captures the idea that only a subset of all packets
passing a network element will be selected for reporting. But PSAMP
selection operations include random selection, deterministic
selection (Filtering), and deterministic approximations to random
selection (Hash-based Selection).
2. PSAMP Documents Overview
This document is one out of a series of documents from the PSAMP
group.
RFC 5474 (this document): "A Framework for Packet Selection and
Reporting" describes the PSAMP framework for network elements to
select subsets of packets by statistical and other methods, and to
export a stream of reports on the selected packets to a Collector.
Definitions of terminology and the use of the terms "must", "should",
and "may" in this document are informational only.
[RFC5475]: "Sampling and Filtering Techniques for IP Packet
Selection" describes the set of packet selection techniques supported
by PSAMP.
[RFC5476]: "Packet Sampling (PSAMP) Protocol Specifications"
specifies the export of packet information from a PSAMP Exporting
Process to a PSAMP Collecting Process.
[RFC5477]: "Information Model for Packet Sampling Exports" defines an
information and data model for PSAMP.
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3. Elements, Terminology, and High-Level Architecture
3.1. High-Level Description of the PSAMP Architecture
Here is an informal high-level description of the PSAMP protocol
operating in a PSAMP Device (all terms will be defined presently). A
stream of packets is observed at an Observation Point. A Selection
Process inspects each packet to determine whether or not it is to be
selected for reporting. The Selection Process is part of the
Metering Process, which constructs a report on each selected packet,
using the Packet Content, and possibly other information such as the
packet treatment at the Observation Point or the arrival timestamp.
An Exporting Process sends the Packet Reports to a Collector,
together with any subsidiary information needed for their
interpretation.
The following figure indicates the sequence of the three processes
(Selection, Metering, and Exporting) within the PSAMP device.
+------------------+
| Metering Process |
| +-----------+ | +-----------+
Observed | | Selection | | | Exporting |
Packet--->| | Process |--------->| Process |--->Collector
Stream | +-----------+ | +-----------+
+------------------+
The following sections give detailed definitions of each of the
objects just named.
3.2. Observation Points, Packet Streams, and Packet Content
This section contains the definition of terms relevant to obtaining
the packet input to the Selection Process.
* Observation Point
An Observation Point is a location in the network where IP packets
can be observed. Examples include a line to which a probe is
attached, a shared medium, such as an Ethernet-based LAN, a single
port of a router, or a set of interfaces (physical or logical) of
a router.
Note that every Observation Point is associated with an
Observation Domain and that one Observation Point may be a
superset of several other Observation Points. For
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example, one Observation Point can be an entire line card. That
would be the superset of the individual Observation Points at the
line card's interfaces.
* Observed Packet Stream
The Observed Packet Stream is the set of all packets observed at
the Observation Point.
* Packet Stream
A Packet Stream denotes a set of packets from the Observed Packet
Stream that flows past some specified point within the Metering
Process. An example of a Packet Stream is the output of the
Selection Process. Note that packets selected from a stream,
e.g., by Sampling, do not necessarily possess a property by which
they can be distinguished from packets that have not been
selected. For this reason, the term "stream" is favored over
"flow", which is defined as a set of packets with common
properties [RFC3917].
* Packet Content
The Packet Content denotes the union of the packet header (which
includes link layer, network layer, and other encapsulation
headers) and the packet payload.
3.3. Selection Process
This section defines the Selection Process and related objects.
* Selection Process
A Selection Process takes the Observed Packet Stream as its input
and selects a subset of that stream as its output.
* Selection State
A Selection Process may maintain state information for use by the
Selection Process. At a given time, the Selection State may
depend on packets observed at and before that time, and other
variables. Examples include:
(i) sequence numbers of packets at the input of Selectors;
(ii) a timestamp of observation of the packet at the Observation
Point;
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(iii) iterators for pseudorandom number generators;
(iv) hash values calculated during selection;
(v) indicators of whether the packet was selected by a given
Selector.
Selection Processes may change portions of the Selection State as
a result of processing a packet. Selection State for a packet
reflects the state after processing the packet.
* Selector
A Selector defines the action of a Selection Process on a single
packet of its input. If selected, the packet becomes an element
of the output Packet Stream.
The Selector can make use of the following information in
determining whether a packet is selected:
(i) the Packet Content;
(ii) information derived from the packet's treatment at the
Observation Point;
(iii) any Selection State that may be maintained by the Selection
Process.
* Composite Selector
A Composite Selector is an ordered composition of Selectors, in
which the output Packet Stream issuing from one Selector forms the
input Packet Stream to the succeeding Selector.
* Primitive Selector
A Selector is primitive if it is not a Composite Selector.
3.4. Reporting
* Packet Reports
Packet Reports comprise a configurable subset of a packet's input
to the Selection Process, including the Packet Content,
information relating to its treatment (for example, the output
interface), and its associated Selection State (for example, a
hash of the Packet Content).
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* Report Interpretation
Report Interpretation comprises subsidiary information, relating
to one or more packets, that is used for interpretation of their
Packet Reports. Examples include configuration parameters of the
Selection Process.
* Report Stream
The Report Stream is the output of a Metering Process, comprising
two distinct types of information: Packet Reports and Report
Interpretation.
3.5. Metering Process
A Metering Process selects packets from the Observed Packet Stream
using a Selection Process, and produces as output a Report Stream
concerning the selected packets.
The PSAMP Metering Process can be viewed as analogous to the IPFIX
Metering Process [RFC5101], which produces Flow Records as its
output, with the difference that the PSAMP Metering Process always
contains a Selection Process. The relationship between PSAMP and
IPFIX is further described in [RFC5477] and [RFC5474].
3.6. Exporting Process
* Exporting Process
An Exporting Process sends, in the form of Export Packets, the
output of one or more Metering Processes to one or more
Collectors.
* Export Packets
An Export Packet is a combination of Report Interpretation(s)
and/or one or more Packet Reports that are bundled by the
Exporting Process into an Export Packet for exporting to a
Collector.
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3.7. PSAMP Device
A PSAMP Device is a device hosting at least an Observation Point, a
Metering Process (which includes a Selection Process), and an
Exporting Process. Typically, corresponding Observation Point(s),
Metering Process(es), and Exporting Process(es) are co-located at
this device, for example, at a router.
3.8. Collector
A Collector receives a Report Stream exported by one or more
Exporting Processes. In some cases, the host of the Metering and/or
Exporting Processes may also serve as the Collector.
3.9. Possible Configurations
Various possibilities for the high-level architecture of these
elements are as follows.
MP = Metering Process, EP = Exporting process
PSAMP Device
+---------------------+ +------------------+
|Observation Point(s) | | Collector(1) |
|MP(s)--->EP----------+---------------->| |
|MP(s)--->EP----------+-------+-------->| |
+---------------------+ | +------------------+
|
PSAMP Device |
+---------------------+ | +------------------+
|Observation Point(s) | +-------->| Collector(2) |
|MP(s)--->EP----------+---------------->| |
+---------------------+ +------------------+
PSAMP Device
+---------------------+
|Observation Point(s) |
|MP(s)--->EP---+ |
| | |
|Collector(3)<-+ |
+---------------------+
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The most simple Metering Process configuration is composed of:
+------------------------------------+
| +----------+ |
| |Selection | |
Observed | |Process | Packet |
Packet-->| |(Primitive|-> Stream -> |--> Report Stream
^
Stream | | Selector)| |
^
| +----------+ |
| Metering Process |
+------------------------------------+
A Metering Process with a Composite Selector is composed of:
+--------------------------------------------------...
| +-----------------------------------+
| | +----------+ +----------+ |
| | |Selection | |Selection | |
Observed | | |Process | |Process | |
Packet-->| | |(Primitive|-Packet->|(Primitive|---> Packet ...
^ ^
Stream | | |Selector1)| Stream |Selector2)| | Stream
^ ^
| | +----------+ +----------+ |
| | Composite Selector |
| +-----------------------------------+
| Metering Process
+--------------------------------------------------...
...-------------+
|
|
|
|
|---> Report Stream
|
|
|
|
|
...-------------+
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4. Generic Requirements for PSAMP
This section describes the generic requirements for the PSAMP
protocol. A number of these are realized as specific requirements in
later sections.
4.1. Generic Selection Process Requirements
(a) Ubiquity: The Selectors must be simple enough to be implemented
ubiquitously at maximal line rate.
(b) Applicability: The set of Selectors must be rich enough to
support a range of existing and emerging measurement-based
applications and protocols. This requires a workable trade-off
between the range of traffic engineering applications and
operational tasks it enables, and the complexity of the set of
capabilities.
(c) Extensibility: The protocol must be able to accommodate
additional packet Selectors not currently defined.
(d) Flexibility: The protocol must support selection of packets
using various network protocols or encapsulation layers,
including Internet Protocol Version 4 (IPv4) [RFC0791], Internet
Protocol Version 6 (IPv6) [RFC2460], and Multiprotocol Label
Switching (MPLS) [RFC3031].
(e) Robust Selection: Packet selection must be robust against
attempts to craft an Observed Packet Stream from which packets
are selected disproportionately (e.g., to evade selection or
overload measurement systems).
(f) Parallel Metering Processes: The protocol must support
simultaneous operation of multiple independent Metering
Processes at the same host.
(g) Causality: The selection decision for each packet should depend
only weakly, if at all, upon future packets' arrivals. This
promotes ubiquity by limiting the complexity of the selection
logic.
(h) Encrypted Packets: Selectors that interpret packet fields must
be configurable to ignore (i.e., not select) encrypted packets,
when they are detected.
Specific Selectors are outlined in Section 5, and described in more
detail in the companion document [RFC5475].
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4.2. Generic Reporting Requirements
(i) Self-Defining: The Report Stream must be complete in the sense
that no additional information need be retrieved from the
Observation Point in order to interpret and analyze the reports.
(j) Indication of Information Loss: The Report Stream must include
sufficient information to indicate or allow the detection of
loss occurring within the Selection, Metering, and/or Exporting
Processes, or in transport. This may be achieved by the use of
sequence numbers.
(k) Accuracy: The Report Stream must include information that
enables the accuracy of measurements to be determined.
(l) Faithfulness: All reported quantities that relate to the packet
treatment must reflect the router state and configuration
encountered by the packet at the time it is received by the
Metering Process.
(m) Privacy: Although selection of the content of Packet Reports
must be responsive to the needs of measurement applications, it
must also conform with [RFC2804]. In particular, full packet
capture of arbitrary Packet Streams is explicitly out of scope.
See Section 6 for further discussions on Reporting.
4.3. Generic Exporting Process Requirements
(n) Timeliness: Configuration must allow for limiting of buffering
delays for the formation and transmission for Export Packets.
See Section 8.5 for further details.
(o) Congestion Avoidance: Export of a Report Stream across a network
must be congestion avoiding in compliance with [RFC2914]. This
is discussed further in Section 8.3.
(p) Secure Export
(i) confidentiality: The option to encrypt exported data must
be provided.
(ii) integrity: Alterations in transit to exported data must be
detectable at the Collector.
(iii) authenticity: Authenticity of exported data must be
verifiable by the Collector in order to detect forged data.
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The motivation here is the same as for security in IPFIX export; see
Sections 6.3 and 10 of [RFC3917].
4.4. Generic Configuration Requirements
(q) Ease of Configuration: This applies to ease of configuration of
Sampling and export parameters, e.g., for automated remote
reconfiguration in response to collected reports.
(r) Secure Configuration: The option to configure via protocols that
prevent unauthorized reconfiguration or eavesdropping on
configuration communications must be available. Eavesdropping
on configuration might allow an attacker to gain knowledge that
would be helpful in crafting a Packet Stream to evade subversion
or overload the measurement infrastructure.
Configuration is discussed in Section 9.
5. Packet Selection
This section details specific requirements for the Selection Process,
motivated by the generic requirements of Section 3.3.
5.1. Two Types of Selectors
PSAMP categorizes Selectors into two types:
* Filtering: A filter is a Selector that selects a packet
deterministically based on the Packet Content, or its treatment, or
functions of these occurring in the Selection State. Two examples
are:
(i) Property Match Filtering: A packet is selected if a
specific field in the packet equals a predefined value.
(ii) Hash-based Selection: A hash function is applied to the
Packet Content, and the packet is selected if the result
falls in a specified range.
* Sampling: A Selector that is not a filter is called a Sampling
operation. This reflects the intuitive notion that if the
selection of a packet cannot be determined from its content alone,
there must be some type of Sampling taking place.
Sampling operations can be divided into two subtypes:
(i) Content-independent Sampling, which does not use Packet
Content in reaching Sampling decisions. Examples include
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systematic Sampling, and uniform pseudorandom Sampling
driven by a pseudorandom number whose generation is
independent of Packet Content. Note that in content-
independent Sampling, it is not necessary to access the
Packet Content in order to make the selection decision.
(ii) Content-dependent Sampling, in which the Packet Content is
used in reaching selection decisions. An application is
pseudorandom selection with a probability that depends on
the contents of a packet field, e.g., Sampling packets with
a probability dependent on their TCP/UDP port numbers.
Note that this is not a filter.
5.2. PSAMP Packet Selectors
A spectrum of packet Selectors is described in detail in [RFC5475].
Here we only briefly summarize the meanings for completeness.
A PSAMP Selection Process must support at least one of the following
Selectors.
* systematic count-based Sampling: Packet selection is triggered
periodically by packet count, a number of successive packets being
selected subsequent to each trigger.
* systematic time-based Sampling: This is similar to systematic
count-based Sampling except that selection is reckoned with respect
to time rather than count. Packet selection is triggered at
periodic instants separated by a time called the spacing. All
packets that arrive within a certain time of the trigger (called
the interval length) are selected.
* probabilistic n-out-of-N Sampling: From each count-based successive
block of N packets, n are selected at random.
* uniform probabilistic Sampling: Packets are selected independently
with fixed Sampling probability p.
* non-uniform probabilistic Sampling: Packets are selected
independently with probability p that depends on Packet Content.
* Property Match Filtering
With this Filtering method, a packet is selected if a specific
field within the packet and/or on properties of the router state
equal(s) a predefined value. Possible filter fields are all IPFIX
Flow attributes specified in [RFC5102]. Further fields can be
defined by vendor-specific extensions.
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A packet is selected if Field=Value. Masks and ranges are only
supported to the extent to which [RFC5102] allows them, e.g., by
providing explicit fields like the netmasks for source and
destination addresses.
AND operations are possible by concatenating filters, thus
producing a composite selection operation. In this case, the
ordering in which the Filtering happens is implicitly defined
(outer filters come after inner filters). However, as long as the
concatenation is on filters only, the result of the cascaded filter
is independent from the order, but the order may be important for
implementation purposes, as the first filter will have to work at a
higher rate. In any case, an implementation is not constrained to
respect the filter ordering, as long as the result is the same, and
it may even implement the composite Filtering in one single step.
OR operations are not supported with this basic model. More
sophisticated filters (e.g., supporting bitmasks, ranges, or OR
operations) can be realized as vendor-specific schemes.
Property match operations should be available for different
protocol portions of the packet header:
(i) IP header (excluding options in IPv4, stacked headers in
IPv6)
(ii) transport header
(iii) encapsulation headers (e.g., the MPLS label stack, if
present)
When the PSAMP Device offers Property Match Filtering, and, in its
usual capacity other than in performing PSAMP functions, identifies
or processes information from IP, transport, or encapsulation
protocols, then the information should be made available for
Filtering. For example, when a PSAMP Device is a router that
routes based on destination IP address, that field should be made
available for Filtering. Conversely, a PSAMP Device that does not
route is not expected to be able to locate an IP address within a
packet, or make it available for Filtering, although it may do so.
Since packet encryption alters the meaning of encrypted fields,
Property Match Filtering must be configurable to ignore encrypted
packets when detected.
The Selection Process may support Filtering based on the properties
of the router state:
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(i) Ingress interface at which packet arrives equals a
specified value
(ii) Egress interface to which packet is routed to equals a
specified value
(iii) Packet violated Access Control List (ACL) on the router
(iv) Failed Reverse Path Forwarding (RPF). Packets that match
the Failed Reverse Path Forwarding (RPF) condition are
packets for which ingress Filtering failed as defined in
[RFC3704].
(v) Failed Resource Reservation Protocol (RSVP). Packets that
match the Failed RSVP condition are packets that do not
fulfill the RSVP specification as defined in [RFC2205].
(vi) No route found for the packet
(vii) Origin Border Gateway Protocol (BGP) Autonomous System (AS)
[RFC4271] equals a specified value or lies within a given
range
(viii) Destination BGP AS equals a specified value or lies within
a given range
Router architectural considerations may preclude some information
concerning the packet treatment being available at line rate for
selection of packets. For example, the Selection Process may not
be implemented in the fast path that is able to access router state
at line rate. However, when Filtering follows Sampling (or some
other selection operation) in a Composite Selector, the rate of the
Packet Stream output from the sampler and input to the filter may
be sufficiently low that the filter could select based on router
state.
* Hash-based Selection:
Hash-based Selection will employ one or more hash functions to be
standardized. A hash function is applied to a subset of Packet
Content, and the packet is selected if the resulting hash falls in
a specified range. The stronger the hash function, the more
closely Hash-based Selection approximates uniform random Sampling.
Privacy of hash selection range and hash function parameters
obstructs subversion of the Selector by packets that are crafted
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either to avoid selection or to be selected. Privacy of the hash
function is not required. Robustness and security considerations
of Hash-based Selection are further discussed in [RFC5475].
Applications of hash-based Sampling are described in Section 11.
5.3. Selection Fraction Terminology
* Population:
A Population is a Packet Stream, or a subset of a Packet Stream.
A Population can be considered as a base set from which packets
are selected. An example is all packets in the Observed Packet
Stream that are observed within some specified time interval.
* Population Size
The Population Size is the number of all packets in a Population.
* Sample Size
The Sample Size is the number of packets selected from the
Population by a Selector.
* Configured Selection Fraction
The Configured Selection Fraction is the expected ratio of the
Sample Size to the Population Size, as based on the configured
selection parameters.
* Attained Selection Fraction
The Attained Selection Fraction is the ratio of the actual Sample
Size to the Population Size.
For some Sampling methods, the Attained Selection Fraction can
differ from the Configured Selection Fraction due to, for example,
the inherent statistical variability in Sampling decisions of
probabilistic Sampling and Hash-based Selection. Nevertheless,
for large Population Sizes and properly configured Selectors, the
Attained Selection Fraction usually approaches the Configured
Selection Fraction.
The notions of Configured/Attained Selection Fractions extend
beyond Selectors. An illustrative example is the Configured
Selection Fraction of the composition of the Metering Process with
the Exporting Process. Here the Population is the Observed Packet
Stream or a subset thereof. The Configured Selection Fraction is
the fraction of the Population for which Packet Reports are
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expected to reach the Collector. This quantity may reflect
additional parameters, not necessarily described in the PSAMP
protocol, that determine the degree of loss suffered by Packet
Reports en route to the Collector, e.g., the transmission
bandwidth available to the Exporting Process. In this example,
the Attained Selection Fraction is the fraction of Population
packets for which reports did actually reach the Collector, and
thus incorporates the effect of any loss of Packet Reports due,
e.g., to resource contention at the Observation Point or during
transmission.
5.4. Input Sequence Numbers for Primitive Selectors
Each instance of a Primitive Selector must maintain a count of
packets presented at its input. The counter value is to be included
as a sequence number for selected packets. The sequence numbers are
considered as part of the packet's Selection State.
Use of input sequence numbers enables applications to determine the
Attained Selection Fraction, and hence correctly normalize network
usage estimates regardless of loss of information, regardless of
whether this loss occurs because of discard of Packet Reports in the
Metering Process (e.g., due to resource contention in the host of
these processes), or loss of export packets in transmission or
collection. See [RFC3176] for further details.
As an example, consider a set of n consecutive Packet Reports r1,
r2,... , rn, selected by a Sampling operation and received at a
Collector. Let s1, s2,..., sn be the input sequence numbers reported
by the packets. The Attained Selection Fraction for the composite of
the measurement and Exporting Processes, taking into account both
packet Sampling at the Observation Point and loss in transmission, is
computed as R = (n-1)/(sn-s1). (Note that R would be 1 if all
packets were selected and there were no transmission loss.)
The Attained Selection Fraction can be used to estimate the number of
bytes present in a portion of the Observed Packet Stream. Let b1,
b2,..., bn be the number of bytes reported in each of the packets
that reached the Collector, and set B = b1+b2+...+bn. Then the total
bytes present in packets in the Observed Packet Stream whose input
sequence numbers lie between s1 and sn is estimated by B/R, i.e.,
scaling up the measured bytes through division by the Attained
Selection Fraction.
With Composite Selectors, an input sequence number must be reported
for each Selector in the composition.
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5.5. Composite Selectors
The ability to compose Selectors in a Selection Process should be
provided. The following combinations appear to be most useful for
applications:
* concatenation of Property Match Filters. This is useful for
constructing the AND of the component filters.
* Filtering followed by Sampling.
* Sampling followed by Filtering.
Composite Selectors are useful for drill-down applications. The
first component of a Composite Selector can be used to reduce the
load on the second component. In this setting, the advantage to be
gained from a given ordering can depend on the composition of the
Packet Stream.
5.6. Constraints on the Selection Fraction
Sampling at full line rate, i.e., with probability 1, is not excluded
in principle, although resource constraints may not permit it in
practice.
6. Reporting
This section details specific requirements for reporting, motivated
by the generic requirements of Section 3.4.
6.1. Mandatory Contents of Packet Reports: Basic Reports
Packet Reports must include the following:
(i) the input sequence number(s) of any Selectors that acted on
the packet in the instance of a Metering Process that
produced the report.
(ii) the identifier of the Metering Process that produced the
selected packet.
The Metering Process must support inclusion of the following in each
Packet Report, as a configurable option:
(iii) a basic report on the packet, i.e., some number of
contiguous bytes from the start of the packet, including
the packet header (which includes network layer and any
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encapsulation headers) and some subsequent bytes of the
packet payload.
Some devices may not have the resource capacity or functionality to
provide more detailed Packet Reports than those in (i), (ii), and
(iii) above. Using this minimum required reporting functionality,
the Metering Process places the burden of interpretation on the
Collector or on applications that it supplies. Some devices may have
the capability to provide extended Packet Reports, described in the
next section.
6.2. Extended Packet Reports
The Metering Process may support inclusion in Packet Reports of the
following information, inclusion of any or all being configurable as
an option.
(iv) fields relating to the following protocols used in the
packet: IPv4, IPV6, transport protocols, and encapsulation
protocols including MPLS.
(v) packet treatment, including:
- identifiers for any input and output interfaces of the
Observation Point that were traversed by the packet
- source and destination BGP AS
(vi) Selection State associated with the packet, including:
- the timestamp of observation of the packet at the
Observation Point. The timestamp should be reported to
microsecond resolution.
- hash values, where calculated.
It is envisaged that selection of fields for Extended Packet
Reporting may be used to reduce reporting bandwidth, in which case
the option to report information in (iii) may not be exercised.
6.3. Extended Packet Reports in the Presence of IPFIX
If an IPFIX Metering Process is supported at the Observation Point,
then in order to be PSAMP compliant, Extended Packet Reports must be
able to include all fields required in the IPFIX information model
[RFC5102], with modifications appropriate to reporting on single
packets rather than Flows.
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6.4. Report Interpretation
The Report Interpretation must include:
(i) configuration parameters of the Selectors of the packets
reported on;
(ii) format of the Packet Report;
(iii) indication of the inherent accuracy of the reported
quantities, e.g., of the packet timestamp.
The accuracy measure in (iii) is of fundamental importance for
estimating the likely error attached to estimates formed from the
Packet Reports by applications.
The requirements for robustness and transparency are motivations for
including Report Interpretation in the Report Stream: it makes the
Report Stream self-defining. The PSAMP framework excludes reliance
on an alternative model in which interpretation is recovered out of
band. This latter approach is not robust with respect to
undocumented changes in Selector configuration, and may give rise to
future architectural problems for network management systems to
coherently manage both configuration and data collection.
It is not envisaged that all Report Interpretation be included in
every Packet Report. Many of the quantities listed above are
expected to be relatively static; they could be communicated
periodically, and upon change.
7. Parallel Metering Processes
Because of the increasing number of distinct measurement applications
with varying requirements, it is desirable to set up parallel
Metering Processes on a given Observed Packet Stream. A device
capable of hosting a Metering Process should be able to support more
than one independently configurable Metering Process simultaneously.
Each such Metering Process should have the option of being equipped
with its own Exporting Process; otherwise, the parallel Metering
Processes may share the same Exporting Process.
Each of the parallel Metering Processes should be independent.
However, resource constraints may prevent complete reporting on a
packet selected by multiple Selection Processes. In this case,
reporting for the packet must be complete for at least one Metering
Process; other Metering Processes need only record that they selected
the packet, e.g., by incrementing a counter. The priority among
Metering Processes under resource contention should be configurable.
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It is not proposed to standardize the number of parallel Metering
Processes.
8. Exporting Process
This section details specific requirements for the Exporting Process,
motivated by the generic requirements of Section 3.6.
8.1. Use of IPFIX
PSAMP will use the IP Flow Information Export (IPFIX) protocol for
export of the Report Stream. The IPFIX protocol is well suited for
this purpose, because the IPFIX architecture matches the PSAMP
architecture very well and the means provided by the IPFIX protocol
are sufficient for PSAMP purposes. On the other hand, not all
features of the IPFIX protocol will need to be implemented by some
PSAMP Devices. For example, a device that offers only content-
independent Sampling and basic PSAMP reporting has no need to support
IPFIX capabilities based on packet fields.
8.2. Export Packets
Export Packets may contain one or more Packet Reports, and/or Report
Interpretation. Export Packets must also contain:
(i) an identifier for the Exporting Process
(ii) an Export Packet sequence number
An Export Packet sequence number enables the Collector to identify
loss of Export Packets in transit. Note that some transport
protocols, e.g., UDP, do not provide sequence numbers. Moreover,
having sequence numbers available at the application level enables
the Collector to calculate the packet loss rate for use, e.g., in
estimating original traffic volumes from Export Packets that reach
the Collector.
8.3. Congestion-Aware Unreliable Transport
The export of the Report Stream does not require reliable export.
Section 5.4 shows that the use of input sequence numbers in packet
Selectors means that the ability to estimate traffic rates is not
impaired by export loss. Export Packet loss becomes another form of
Sampling, albeit a less desirable, and less controlled, form of
Sampling.
In distinction, retransmission of lost Export Packets consumes
additional network resources. The requirement to store
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unacknowledged data is an impediment to having ubiquitous support for
PSAMP.
In order to jointly satisfy the timeliness and congestion avoidance
requirements of Section 4.3, a congestion-aware unreliable transport
protocol may be used. IPFIX is compatible with this requirement,
since it mandates support of the Stream Control Transmission Protocol
(SCTP) [RFC4960] and the SCTP Partial Reliability Extension
[RFC3758].
IPFIX also allows the use of the User Datagram Protocol (UDP)
[RFC0768], although it is not a congestion-aware protocol. However,
in this case, the Export Packets must remain wholly within the
administrative domains of the operators [RFC5101]. The PSAMP
Exporting Process is equipped with a configurable export rate limit
(see Section 8.4) that can be used to limit the export rate when a
congestion-aware transport protocol is not used. The Collector, upon
detection of Export Packet loss through missing export sequence
numbers, may reconfigure the export rate limit downwards in order to
avoid congestion.
8.4. Configurable Export Rate Limit
The Exporting Process must have an export rate limit, configurable
per Exporting Process. This is useful for two reasons:
(i) Even without network congestion, the rate of packet
selection may exceed the capacity of the Collector to
process reports, particularly when many Exporting Processes
feed a common Collector. Use of an Export Rate Limit
allows control of the global input rate to the Collector.
(ii) IPFIX provides export using UDP as the transport protocol
in some circumstances. An Export Rate Limit allows the
capping of the export rate to match both path link speeds
and the capacity of the Collector.
8.5. Limiting Delay for Export Packets
Low measurement latency allows the traffic monitoring system to be
more responsive to real-time network events, for example, in quickly
identifying sources of congestion. Timeliness is generally a good
thing for devices performing the Sampling since it minimizes the
amount of memory needed to buffer samples.
Keeping the packet dispatching delay small has other benefits besides
limiting buffer requirements. For many applications, a resolution of
1 second is sufficient. Applications in this category would include
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identifying sources associated with congestion, tracing Denial-of-
Service (DoS) attacks through the network, and constructing traffic
matrices. Furthermore, keeping dispatch delay within the resolution
required by applications eliminates the need for timestamping by
synchronized clocks at Observation Points, or for the Observation
Points and Collector to maintain bidirectional communication in order
to track clock offsets. The Collector can simply process Packet
Reports in the order that they are received, using its own clock as a
"global" time base. This avoids the complexity of buffering and
reordering samples. See [DuGeGr02] for an example.
The delay between observation of a packet and transmission of an
Export Packet containing a report on that packet has several
components. It is difficult to standardize a given numerical delay
requirement, since in practice the delay may be sensitive to
processor load at the Observation Point. Therefore, PSAMP aims to
control that portion of the delay within the Observation Point that
is due to buffering in the formation and transmission of Export
Packets.
In order to limit delay in the formation of Export Packets, the
Exporting Process must provide the ability to close out and enqueue
for transmission any Export Packet during formation as soon as it
includes one Packet Report.
In order to limit the delay in the transmission of Export Packets, a
configurable upper bound to the delay of an Export Packet prior to
transmission must be provided. If the bound is exceeded, the Export
Packet is dropped. This functionality can be provided by the timed
reliability service of the SCTP Partial Reliability Extension
[RFC3758].
The Exporting Process may enqueue the Report Stream in order to
export multiple Packet Reports in a single Export Packet. Any
consequent delay must still allow for timely availability of Packet
Reports as just described. The timed reliability service of the SCTP
Partial Reliability Extension [RFC3758] allows the dropping of
packets from the export buffer once their age in the buffer exceeds a
configurable bound. A suitable default value for the bound should be
used in order to avoid a low transmission rate due to
misconfiguration.
8.6. Export Packet Compression
To conserve network bandwidth and resources at the Collector, the
Export Packets may be compressed before export. Compression is
expected to be quite effective since the selected packets may share
many fields in common, e.g., if a filter focuses on packets with
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certain values in particular header fields. Using compression,
however, could impact the timeliness of Packet Reports. Any
consequent delay must not violate the timeliness requirement for
availability of Packet Reports at the Collector.
8.7. Collector Destination
When exporting to a remote Collector, the Collector is identified by
IP address, transport protocol, and transport port number.
8.8. Local Export
The Report Stream may be directly exported to on-board measurement-
based applications, for example, those that form composite statistics
from more than one packet. Local Export may be presented through an
interface directly to the higher-level applications, i.e., through an
API, rather than employing the transport used for off-board export.
Specification of such an API is outside the scope of the PSAMP
framework.
A possible example of Local Export could be that packets selected by
the PSAMP Metering Process serve as the input for the IPFIX protocol,
which then forms Flow Records out of the stream of selected packets.
9. Configuration and Management
A key requirement for PSAMP is the easy reconfiguration of the
parameters of the Metering Process, including those for selection and
Packet Reports, and of the Exporting Process. An important example
is to support measurement-based applications that want to adaptively
drill-down on traffic detail in real time.
To facilitate retrieval and monitoring of parameters, they are to
reside in a Management Information Base (MIB). Mandatory monitoring
objects will cover all mandatory PSAMP functionality. Alarming of
specific parameters could be triggered with thresholding mechanisms
such as the RMON (Remote Network Monitoring) event and alarm
[RFC2819] or the event MIB [RFC2981].
For configuring parameters of the Metering Process, several
alternatives are available including a MIB module with writeable
objects, as well as other configuration protocols. For configuring
parameters of the Exporting Process, the Packet Report, and the
Report Interpretation, which is an IFPIX task, the IPFIX
configuration method(s) should be used.
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Although management and configuration of Collectors is out of scope,
a PSAMP Device, to the extent that it employs IPFIX as an export
protocol, inherits from IPFIX the capability to detect and recover
from Collector failure; see Section 8.2 of [RFC5470].
10. Feasibility and Complexity
In order for PSAMP to be supported across the entire spectrum of
networking equipment, it must be simple and inexpensive to implement.
One can envision easy-to-implement instances of the mechanisms
described within this document. Thus, for that subset of instances,
it should be straightforward for virtually all system vendors to
include them within their products. Indeed, Sampling and Filtering
operations are already realized in available equipment.
Here we give some specific arguments to demonstrate feasibility and
comment on the complexity of hardware implementations. We stress
here that the point of these arguments is not to favor or recommend
any particular implementation, or to suggest a path for
standardization, but rather to demonstrate that the set of possible
implementations is not empty.
10.1. Feasibility
10.1.1. Filtering
Filtering consists of a small number of mask (bit-wise logical),
comparison, and range (greater than) operations. Implementation of
at least a small number of such operations is straightforward. For
example, filters for security Access Control Lists (ACLs) are widely
implemented. This could be as simple as an exact match on certain
fields, or involve more complex comparisons and ranges.
10.1.2. Sampling
Sampling based on either counters (counter set, decrement, test for
equal to zero) or range matching on the hash of a packet (greater
than) is possible given a small number of Selectors, although there
may be some differences in ease of implementation for hardware vs.
software platforms.
10.1.3. Hashing
Hashing functions vary greatly in complexity. Execution of a small
number of sufficiently simple hash functions is implementable at line
rate. Concerning the input to the hash function, hop-invariant IP
header fields (IP address, IP identification) and TCP/UDP header
fields (port numbers, TCP sequence number) drawn from the first 40
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bytes of the packet have been found to possess a considerable
variability; see [DuGr01].
10.1.4. Reporting
The simplest Packet Report would duplicate the first n bytes of the
packet. However, such an uncompressed format may tax the bandwidth
available to the Exporting Process for high Sampling rates; reporting
selected fields would save on this bandwidth. Thus, there is a
trade-off between simplicity and bandwidth limitations.
10.1.5. Exporting
Ease of exporting Export Packets depends on the system architecture.
Most systems should be able to support export by insertion of Export
Packets, even through the software path.
10.2. Potential Hardware Complexity
Achieving low constants for performance while minimizing hardware
resources is, of course, a challenge, especially at very high clock
frequencies. Most of the Selectors, however, are very basic and
their implementations very well understood; in fact, the average
Application-Specific Integrated Circuit (ASIC) designer simply uses
canned library instances of these operations rather than design them
from scratch. In addition, networking equipment generally does not
need to run at the fastest clock rates, further reducing the effort
required to get reasonably efficient implementations.
Simple bit-wise logical operations are easy to implement in hardware.
Such operations (NAND/NOR/XNOR) directly translate to four-transistor
gates. Each bit of a multiple-bit logical operation is completely
independent and thus can be performed in parallel incurring no
additional performance cost above a single-bit operation.
Comparisons (EQ/NEQ) take O(log(M)) stages of logic, where M is the
number of bits involved in the comparison. The log(M) is required to
accumulate the result into a single bit.
Greater-than operations, as used to determine whether a hash falls in
a selection range, are a determination of the most significant
not-equivalent bit in the two operands. The operand with that most-
significant-not-equal bit set to be one is greater than the other.
Thus, a greater-than operation is also an O(log(M)) stages-of-logic
operation. Optimized implementations of arithmetic operations are
also O(log(M)) due to propagation of the carry bit.
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Setting a counter is simply loading a register with a state. Such an
operation is simple and fast O(1). Incrementing or decrementing a
counter is a read, followed by an arithmetic operation, followed by a
store. Making the register dual-ported does take additional space,
but it is a well-understood technique. Thus, the increment/decrement
is also an O(log(M)) operation.
Hashing functions come in a variety of forms. The computation
involved in a standard Cyclic Redundancy Check (CRC), for example, is
essentially a set of XOR operations, where the intermediate result is
stored and XORed with the next chunk of data. There are only O(1)
operations and no log complexity operations. Thus, a simple hash
function, such as CRC or generalizations thereof, can be implemented
in hardware very efficiently.
At the other end of the range of complexity, the MD5 function uses a
large number of bit-wise conditional operations and arithmetic
operations. The former are O(1) operations and the latter are
O(log(M)). MD5 specifies 256 32 bit ADD operations per 16 bytes of
input processed. Consider processing 10 Gb/sec at 100 MHz (this
processing rate appears to be currently available). This requires
processing 12.5 bytes/cycle, and hence at least 200 adders, a
sizeable number. Because of data dependencies within the MD5
algorithm, the adders cannot be simply run in parallel, thus
requiring either faster clock rates and/or more advanced
architectures. Thus, selection hashing functions as complex as MD5
may be precluded for ubiquitous use at full line rate. This
motivates exploring the use of selection hash functions with
complexity somewhere between that of MD5 and CRC. In some
applications (see Section 11), a second hash may be calculated on
only selected packets; MD5 is feasible for this purpose if the rate
of production of selected packets is sufficiently low.
11. Applications
We first describe several representative operational applications
that require traffic measurements at various levels of temporal and
spatial granularity. Some of the goals here appear similar to those
of IPFIX, at least in the broad classes of applications supported.
The major benefit of PSAMP is the support of new network management
applications, specifically, those enabled by the packet Selectors
that it supports.
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11.1. Baseline Measurement and Drill Down
Packet Sampling is ideally suited to determine the composition of the
traffic across a network. The approach is to enable measurement on a
cut-set of the network links such that each packet entering the
network is seen at least once, for example, on all ingress links.
Unfiltered Sampling with a relatively low selection fraction
establishes baseline measurements of the network traffic. Packet
Reports include packet attributes of common interest: source and
destination address and port numbers, prefix, protocol number, type
of service, etc. Traffic matrices are indicated by reporting source
and destination AS matrices. Absolute traffic volumes are estimated
by renormalizing the sampled traffic volumes through division by
either the Configured Selection Fraction or the Attained Selection
Fraction (as derived from input packet counters included in the
Report Stream).
Suppose an operator or a measurement-based application detects an
interesting subset of a Packet Stream, as identified by a particular
packet attribute. Real-time drill down to that subset is achieved by
instantiating a new Metering Process on the same Observed Packet
Stream from which the subset was reported. The Selection Process of
the new Metering Process filters according to the attribute of
interest, and composes with Sampling if necessary to manage the
attained fraction of packets selected.
11.2. Trajectory Sampling
The goal of trajectory Sampling is the selection of a subset of
packets at all enabled Observation Points at which these packets are
observed in a network domain. Thus, the selection decisions are
consistent in the sense that each packet is selected either at all
enabled Observation Points or at none of them. Trajectory Sampling
is realized by Hash-based Selection if all enabled Observation Points
apply a common hash function to a portion of the Packet Content that
is invariant along the packet path. (Thus, fields such at TTL and
CRC are excluded.)
The trajectory followed by a packet is reconstructed from Packet
Reports on it that reach the Collector. Reports on a given packet
are associated by matching either a label comprising the invariant
reported Packet Content or possibly some digest of it. The
reconstruction of trajectories and methods for dealing with possible
ambiguities due to label collisions (identical labels reported by
different packets) and potential loss of reports in transmission are
dealt with in [DuGr01], [DuGeGr02], and [DuGr04].
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11.3. Passive Performance Measurement
Trajectory Sampling enables the tracking of the performance
experience by customer traffic, customers identified by a list of
source or destination prefixes, or by ingress or egress interfaces.
Operational uses include the verification of Service Level Agreements
(SLAs), and troubleshooting following a customer complaint.
In this application, trajectory Sampling is enabled at all network
ingress and egress interfaces. Rates of loss in transit between
ingress and egress are estimated from the proportion of trajectories
for which no egress report is received. Note that loss of customer
packets is distinguishable from loss of Packet Reports through use of
report sequence numbers. Assuming synchronization of clocks between
different entities, delay of customer traffic across the network may
also be measured; see [Zs02].
Extending hash selection to all interfaces in the network would
enable attribution of poor performance to individual network links.
11.4. Troubleshooting
PSAMP Packet Reports can also be used to diagnose problems whose
occurrence is evident from aggregate statistics, per interface
utilization and packet loss statistics. These statistics are
typically moving averages over relatively long time windows, e.g., 5
minutes, and serve as a coarse-grain indication of operational health
of the network. The most common method of obtaining such
measurements is through the appropriate SNMP MIBs (MIB-II [RFC1213]
and vendor-specific MIBs).
Suppose an operator detects a link that is persistently overloaded
and experiences significant packet drop rates. There is a wide range
of potential causes: routing parameters (e.g., OSPF link weights)
that are poorly adapted to the traffic matrix, e.g., because of a
shift in that matrix; a DoS attack, a flash crowd, or a routing
problem (link flapping). In most cases, aggregate link statistics
are not sufficient to distinguish between such causes and to decide
on an appropriate corrective action. For example, if routing over
two links is unstable, and the links flap between being overloaded
and inactive, this might be averaged out in a 5-minute window,
indicating moderate loads on both links.
Baseline PSAMP measurement of the congested link, as described in
Section 11.1, enables measurements that are fine grained in both
space and time. The operator has to be able to determine how many
bytes/packets are generated for each source/destination address, port
number, and prefix, or other attributes, such as protocol number,
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MPLS forwarding equivalence class (FEC), type of service, etc. This
allows the precise determination of the nature of the offending
traffic. For example, in the case of a Distributed Denial of Service
(DDoS) attack, the operator would see a significant fraction of
traffic with an identical destination address.
In certain circumstances, precise information about the spatial flow
of traffic through the network domain is required to detect and
diagnose problems and verify correct network behavior. In the case
of the overloaded link, it would be very helpful to know the precise
set of paths that packets traversing this link follow. This would
readily reveal a routing problem such as a loop, or a link with a
misconfigured weight. More generally, complex diagnosis scenarios
can benefit from measurement of traffic intensities (and other
attributes) over a set of paths that is constrained in some way. For
example, if a multihomed customer complains about performance
problems on one of the access links from a particular source address
prefix, the operator should be able to examine in detail the traffic
from that source prefix that also traverses the specified access link
towards the customer.
While it is in principle possible to obtain the spatial flow of
traffic through auxiliary network state information, e.g., by
downloading routing and forwarding tables from routers, this
information is often unreliable, outdated, voluminous, and contingent
on a network model. For operational purposes, a direct observation
of traffic flow provided by trajectory Sampling is more reliable, as
it does not depend on any such auxiliary information. For example,
if there was a bug in a router's software, direct observation would
allow the diagnosis the effect of this bug, while an indirect method
would not.
12. Security Considerations
12.1. Relation of PSAMP and IPFIX Security for Exporting Process
As detailed in Section 4.3, PSAMP shares with IPFIX security
requirements for export, namely, confidentiality, integrity, and
authenticity of the exported data; see also Sections 6.3 and 10 of
[RFC3917]. Since PSAMP will use IPFIX for export, it can employ the
IPFIX protocol [RFC5101] to meet its requirements.
12.2. PSAMP Specific Privacy Considerations
In distinction with IPFIX, a PSAMP Device may, in some
configurations, report some number of initial bytes of the packet,
which may include some part of a packet payload. This option is
conformant with the requirements of [RFC2804] since it does not
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mandate configurations that would enable capture of an entire Packet
Stream of a Flow: neither a unit Sampling rate (1 in 1 Sampling) nor
reporting a specific number of initial bytes is required by the PSAMP
protocol.
To preserve privacy of any users acting as sender or receiver of the
observed traffic, the contents of the Packet Reports must be able to
remain confidential in transit between the exporting PSAMP Device and
the Collector. PSAMP will use IPFIX as the exporting protocol, and
the IPFIX protocol must provide mechanisms to ensure confidentiality
of the Exporting Process, for example, encryption of Export Packets
[RFC5101].
12.3. Security Considerations for Hash-Based Selection
12.3.1. Modes and Impact of Vulnerabilities
A concern for Hash-based Selection is whether some large set of
related packets could be disproportionately sampled, either
(i) through unanticipated behavior in the hash function, or
(ii) because the packets had been deliberately crafted to have
this property.
As detailed below, only cryptographic hash functions (e.g., one based
on MD5) employing a private parameter are sufficiently strong to
withstand the range of conceivable attacks. However, implementation
considerations may preclude operating the strongest hash functions at
line rate. For this reason, PSAMP is not expected to standardize
around a cryptographic hash function at the present time. The
purpose of this section is to inform discussion of the
vulnerabilities and trade-offs associated with different hash
function choices. Section 6.2.2 of [RFC5475] does this in more
detail.
An attacker able to predict packet Sampling outcomes could craft a
Packet Stream that could evade selection, or another that could
overwhelm the measurement infrastructure with all its packets being
selected. An attacker may attempt to do this based on knowledge of
the hash function. An attacker could employ knowledge of selection
outcomes of a known Packet Stream to reverse engineer parameters of
the hash function. This knowledge could be gathered, e.g., from
billing information, reactions of intrusion detection systems, or
observation of a Report Stream.
Since Hash-based Selection is deterministic, it is vulnerable to
replay attacks. Repetition of a single packet may be noticeable to
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other measurement methods if employed (e.g., collection of Flow
statistics), whereas a set of distinct packets that appears
statistically similar to regular traffic may be less noticeable. The
impact of replay attacks on Hash-based Selection may be mitigated by
repeated changing of hash function parameters.
12.3.2. Use of Private Parameters in Hash Functions
Because hash functions for Hash-based Selection are to be
standardized and hence public, the packet selection decision must be
controlled by some private quantity associated with the Hash-based
Selection Selector. Making private the range of hash values for
which packets are selected is not alone sufficient to prevent an
attacker crafting a stream of distinct packets that are
disproportionately selected. A private parameter must be used within
the hash function, for example, a private modulus in a hash function,
or by concatenating the hash input with a private string prior to
hashing.
12.3.3. Strength of Hash Functions
The specific choice of hash function and its usage determines the
types of potential vulnerability:
* Cryptographic hash functions: when a private parameter is used,
future selection outcomes cannot be predicted even by an attacker
with knowledge of past selection outcomes.
* Non-cryptographic hash functions:
Using knowledge of past selection outcomes: some well-known hash
functions, e.g., CRC-32, are vulnerable to attacks, in the sense
that their private parameter can be determined with knowledge of
sufficiently many past selections, even when a private parameter is
used; see [GoRe07].
No knowledge of past selection outcomes: using a private parameter
hardened the hash function to classes of attacks that work when the
parameter is public, although vulnerability to future attacks is
not precluded.
12.4. Security Guidelines for Configuring PSAMP
Hash function parameters configured in a PSAMP Device are sensitive
information, which must be kept private. As well as using probing
techniques to discover parameters of non-cryptographic hash functions
as described above, implementation and procedural weaknesses may lead
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to attackers discovering parameters, whatever class of hash function
is used. The following measures may prevent this from occurring:
Hash function parameters must not be displayable in cleartext on
PSAMP Devices. This reduces the chance for the parameters to be
discovered by unauthorized access to the PSAMP Device.
Hash function parameters must not be remotely set in cleartext over a
channel that may be eavesdropped.
Hash function parameters must be changed regularly. Note that such
changes must be synchronized over all PSAMP Devices in a domain under
which trajectory Sampling is employed in order to maintain consistent
Sampling of packets over the domain.
Default hash function parameter values should be initialized
randomly, in order to avoid predictable values that attackers could
exploit.
13. Contributors
Sharon Goldberg contributed to Section 12.3 on security
considerations for Hash-based Selection.
Sharon Goldberg
Department of Electrical Engineering
Princeton University
F210-K EQuad
Princeton, NJ 08544
USA
EMail: goldbe@princeton.edu
14. Acknowledgments
The authors would like to thank Peram Marimuthu and Ganesh Sadasivan
for their input in early working drafts of this document.
15. References
15.1. Normative References
[RFC5476] Claise. B., Ed., "Packet Sampling (PSAMP) Protocol
Specifications", RFC 5476, March 2009.
[RFC5477] Dietz, T., Claise, B., Aitken, P., Dressler, F., and G.
Carle, "Information Model for Packet Sampling Exports",
RFC 5477, March 2009.
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[RFC5101] Claise, B., Ed., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
RFC 5102, January 2008.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758, May 2004.
[RFC5475] Zseby, T., Molina, M., Duffield, N., Niccolini, S., and F.
Raspall, " Sampling and Filtering Techniques for IP Packet
Selection", RFC 5475, March 2009.
15.2. Informative References
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[DuGeGr02] N.G. Duffield, A. Gerber, M. Grossglauser, "Trajectory
Engine: A Backend for Trajectory Sampling", IEEE Network
Operations and Management Symposium 2002, Florence, Italy,
April 15-19, 2002.
[DuGr04] N. G. Duffield and M. Grossglauser, "Trajectory Sampling
with Unreliable Reporting", Proc IEEE Infocom 2004, Hong
Kong, March 2004.
[DuGr08] N. G. Duffield and M. Grossglauser, "Trajectory Sampling
with Unreliable Reporting", IEEE/ACM Trans. on Networking,
16(1), February 2008.
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RFC 5474 Packet Selection and Reporting March 2009
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[GoRe07] S. Goldberg, J. Rexford, "Security Vulnerabilities and
Solutions for Packet Sampling", IEEE Sarnoff Symposium,
Princeton, NJ, May 2007.
[RFC2804] IAB and IESG, "IETF Policy on Wiretapping", RFC 2804, May
2000.
[RFC2981] Kavasseri, R., Ed., "Event MIB", RFC 2981, October 2000.
[RFC1213] McCloghrie, K. and M. Rose, "Management Information Base
for Network Management of TCP/IP-based internets:MIB-II",
STD 17, RFC 1213, March 1991.
[RFC3176] Phaal, P., Panchen, S., and N. McKee, "InMon Corporation's
sFlow: A Method for Monitoring Traffic in Switched and
Routed Networks", RFC 3176, September 2001.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330, May
1998.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export (IPFIX)", RFC
3917, October 2004.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
2006.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC5470] Sadasivan, G., Brownlee, N., Claise, B., and J. Quittek,
"Architecture for IP Flow Information Export", RFC 5470,
March 2009.
[RFC2819] Waldbusser, S., "Remote Network Monitoring Management
Information Base", STD 59, RFC 2819, May 2000.
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RFC 5474 Packet Selection and Reporting March 2009
[Zs02] T. Zseby, "Deployment of Sampling Methods for SLA
Validation with Non-Intrusive Measurements", Proceedings
of Passive and Active Measurement Workshop (PAM 2002),
Fort Collins, CO, USA, March 25-26, 2002.
Authors' Addresses
Derek Chiou
Department of Electrical and Computer Engineering
University of Texas at Austin
1 University Station, Stop C0803, ENS Building room 135,
Austin TX, 78712
USA
Phone: +1 512 232 7722
EMail: Derek@ece.utexas.edu
Benoit Claise
Cisco Systems
De Kleetlaan 6a b1
1831 Diegem
Belgium
Phone: +32 2 704 5622
EMail: bclaise@cisco.com
Nick Duffield, Editor
AT&T Labs - Research
Room B139
180 Park Ave
Florham Park NJ 07932
USA
Phone: +1 973-360-8726
EMail: duffield@research.att.com
Albert Greenberg
One Microsoft Way
Redmond, WA 98052-6399
USA
Phone: +1 425-722-8870
EMail: albert@microsoft.com
Duffield, et al. Informational [Page 37]
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Matthias Grossglauser
School of Computer and Communication Sciences
EPFL
1015 Lausanne
Switzerland
EMail: matthias.grossglauser@epfl.ch
Jennifer Rexford
Department of Computer Science
Princeton University
35 Olden Street
Princeton, NJ 08540-5233
USA
Phone: +1 609-258-5182
EMail: jrex@cs.princeton.edu
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