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rfc:rfc5474

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.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
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 Please review these documents carefully, as they describe your rights
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 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
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 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Duffield, et al. Informational [Page 1] RFC 5474 Packet Selection and Reporting March 2009

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

Duffield, et al. Informational [Page 2] RFC 5474 Packet Selection and Reporting March 2009

    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

Duffield, et al. Informational [Page 3] RFC 5474 Packet Selection and Reporting March 2009

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.

Duffield, et al. Informational [Page 4] RFC 5474 Packet Selection and Reporting March 2009

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

Duffield, et al. Informational [Page 5] RFC 5474 Packet Selection and Reporting March 2009

    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;

Duffield, et al. Informational [Page 6] RFC 5474 Packet Selection and Reporting March 2009

     (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).

Duffield, et al. Informational [Page 7] RFC 5474 Packet Selection and Reporting March 2009

  • 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.

Duffield, et al. Informational [Page 8] RFC 5474 Packet Selection and Reporting March 2009

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)<-+      |
    +---------------------+

Duffield, et al. Informational [Page 9] RFC 5474 Packet Selection and Reporting March 2009

    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
                               |
                               |
                               |
                               |
                               |
               ...-------------+

Duffield, et al. Informational [Page 10] RFC 5474 Packet Selection and Reporting March 2009

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].

Duffield, et al. Informational [Page 11] RFC 5474 Packet Selection and Reporting March 2009

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.

Duffield, et al. Informational [Page 12] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 13] RFC 5474 Packet Selection and Reporting March 2009

           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.

Duffield, et al. Informational [Page 14] RFC 5474 Packet Selection and Reporting March 2009

   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:

Duffield, et al. Informational [Page 15] RFC 5474 Packet Selection and Reporting March 2009

       (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

Duffield, et al. Informational [Page 16] RFC 5474 Packet Selection and Reporting March 2009

   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

Duffield, et al. Informational [Page 17] RFC 5474 Packet Selection and Reporting March 2009

    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.

Duffield, et al. Informational [Page 18] RFC 5474 Packet Selection and Reporting March 2009

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

Duffield, et al. Informational [Page 19] RFC 5474 Packet Selection and Reporting March 2009

           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:
  1. identifiers for any input and output interfaces of the

Observation Point that were traversed by the packet

  1. source and destination BGP AS
      (vi) Selection State associated with the packet, including:
  1. the timestamp of observation of the packet at the

Observation Point. The timestamp should be reported to

           microsecond resolution.
  1. 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.

Duffield, et al. Informational [Page 20] RFC 5474 Packet Selection and Reporting March 2009

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.

Duffield, et al. Informational [Page 21] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 22] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 23] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 24] RFC 5474 Packet Selection and Reporting March 2009

 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.

Duffield, et al. Informational [Page 25] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 26] RFC 5474 Packet Selection and Reporting March 2009

 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.

Duffield, et al. Informational [Page 27] RFC 5474 Packet Selection and Reporting March 2009

 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.

Duffield, et al. Informational [Page 28] RFC 5474 Packet Selection and Reporting March 2009

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,

Duffield, et al. Informational [Page 30] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 31] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 32] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 33] RFC 5474 Packet Selection and Reporting March 2009

 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.

Duffield, et al. Informational [Page 34] RFC 5474 Packet Selection and Reporting March 2009

 [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.

Duffield, et al. Informational [Page 35] 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.

Duffield, et al. Informational [Page 36] 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] RFC 5474 Packet Selection and Reporting March 2009

 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

Duffield, et al. Informational [Page 38]

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