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Internet Engineering Task Force                    Brownlee, Mills, Ruth
INTERNET-DRAFT                                The University of Auckland
                                                          September 1997



                Traffic Flow Measurement:  Architecture

                 <draft-ietf-rtfm-architecture-00.txt>




Status of this Memo

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Abstract

This document describes an architecture for the measurement and
reporting of network traffic flows, discusses how this relates to an
overall network traffic flow architecture, and describes how it can be
used within the Internet.  It is intended to provide a starting point
for the Realtime Traffic Flow Measurement Working Group.




Contents

 1 Statement of Purpose and Scope                                      3
   1.1 Changes Introduced Since RFC 2063  . . . . . . . . . . . . . .  4




INTERNET-DRAFT      Traffic Flow Measurement:  Architecture     Sep 1997



 2 Traffic Flow Measurement Architecture                               5
   2.1 Meters and Traffic Flows . . . . . . . . . . . . . . . . . . .  5
   2.2 Interaction Between METER and METER READER . . . . . . . . . .  7
   2.3 Interaction Between MANAGER and METER  . . . . . . . . . . . .  8
   2.4 Interaction Between MANAGER and METER READER . . . . . . . . .  8
   2.5 Multiple METERs or METER READERs . . . . . . . . . . . . . . .  9
   2.6 Interaction Between MANAGERs (MANAGER - MANAGER) . . . . . . . 10
   2.7 METER READERs and APPLICATIONs . . . . . . . . . . . . . . . . 10

 3 Traffic Flows and Reporting Granularity                            10
   3.1 Flows and their Attributes . . . . . . . . . . . . . . . . . . 11
   3.2 Granularity of Flow Measurements . . . . . . . . . . . . . . . 13
   3.3 Rolling Counters, Timestamps, Report-in-One-Bucket-Only  . . . 15

 4 Meters                                                             16
   4.1 Meter Structure  . . . . . . . . . . . . . . . . . . . . . . . 17
   4.2 Flow Table . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.3 Packet Handling, Packet Matching . . . . . . . . . . . . . . . 19
   4.4 Rules and Rule Sets  . . . . . . . . . . . . . . . . . . . . . 22
   4.5 Maintaining the Flow Table . . . . . . . . . . . . . . . . . . 26
   4.6 Handling Increasing Traffic Levels . . . . . . . . . . . . . . 27

 5 Meter Readers                                                      27
   5.1 Identifying Flows in Flow Records  . . . . . . . . . . . . . . 27
   5.2 Usage Records, Flow Data Files . . . . . . . . . . . . . . . . 28
   5.3 Meter to Meter Reader: Usage Record Transmission . . . . . . . 28

 6 Managers                                                           29
   6.1 Between Manager and Meter: Control Functions . . . . . . . . . 29
   6.2 Between Manager and Meter Reader: Control Functions  . . . . . 30
   6.3 Exception Conditions . . . . . . . . . . . . . . . . . . . . . 32
   6.4 Standard Rule Sets . . . . . . . . . . . . . . . . . . . . . . 33

 7 APPENDICES                                                         34
   7.1 Appendix A: Network Characterisation . . . . . . . . . . . . . 34
   7.2 Appendix B: Recommended Traffic Flow Measurement Capabilities  35
   7.3 Appendix C: List of Defined Flow Attributes  . . . . . . . . . 36
   7.4 Appendix D: List of Meter Control Variables  . . . . . . . . . 37

 8 Security Considerations                                            38
   8.1 Threat Analysis  . . . . . . . . . . . . . . . . . . . . . . . 38
   8.2 Countermeasures  . . . . . . . . . . . . . . . . . . . . . . . 38

 9 Acknowledgments                                                    40

10 References                                                         40

11 Author's Addresses                                                 41


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1 Statement of Purpose and Scope


This document describes an architecture for traffic flow measurement and
reporting for data networks which has the following characteristics:



  - The traffic flow model can be consistently applied to any
    protocol/application at any network layer (e.g.  network,
    transport, application layers).

  - Traffic flow attributes are defined in such a way that they are
    valid for multiple networking protocol stacks, and that traffic
    flow measurement implementations are useful in MULTI-PROTOCOL
    environments.

  - Users may specify their traffic flow measurement requirements in a
    simple manner, allowing them to collect the flow data they need
    while ignoring other traffic.

  - The data reduction effort to produce requested traffic flow
    information is placed as near as possible to the network
    measurement point.  This reduces the volume of data to be obtained
    (and transmitted across the network for storage), and minimises the
    amount of processing required in traffic flow analysis
    applications.


The architecture specifies common metrics for measuring traffic flows.
By using the same metrics, traffic flow data can be exchanged and
compared across multiple platforms.  Such data is useful for:


  - Understanding the behaviour of existing networks,

  - Planning for network development and expansion,

  - Quantification of network performance,

  - Verifying the quality of network service, and

  - Attribution of network usage to users.


The traffic flow measurement architecture is deliberately structured so
that specific protocol implementations may extend coverage to
multi-protocol environments and to other protocol layers, such as usage
measurement for application-level services.  Use of the same model for
both network- and application-level measurement may simplify the
development of generic analysis applications which process and/or


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correlate any or all levels of traffic and usage information.  Within
this docuemt the term 'usage data' is used as a generic term for the
data obtained using the traffic flow measurement architecture.

This document is not a protocol specification.  It specifies and
structures the information that a traffic flow measurement system needs
to collect, describes requirements that such a system must meet, and
outlines tradeoffs which may be made by an implementor.

For performance reasons, it may be desirable to use traffic information
gathered through traffic flow measurement in lieu of network statistics
obtained in other ways.  Although the quantification of network
performance is not the primary purpose of this architecture, the
measured traffic flow data may be used as an indication of network
performance.

A cost recovery structure decides "who pays for what." The major issue
here is how to construct a tariff (who gets billed, how much, for which
things, based on what information, etc).  Tariff issues include
fairness, predictability (how well can subscribers forecast their
network charges), practicality (of gathering the data and administering
the tariff), incentives (e.g.  encouraging off-peak use), and cost
recovery goals (100% recovery, subsidisation, profit making).  Issues
such as these are not covered here.

Background information explaining why this approach was selected is
provided by the 'Traffic Flow Measurement:  Background' RFC [1].



1.1 Changes Introduced Since RFC 2063


The first version of the Traffic Flow Measurement Architecture was
published as RFC 2063 in January 1997.  The most significant changes
made since then are summarised below.


  - A Traffic Meter is now expected to run multiple rule sets
    concurrently.  This makes a meter much more useful, and
    required only minimal changes to the architecture.

  - 'NoMatch' replaces 'Fail' as an action.  This name was agreed to at
    the Working Group 1996 meeting in Montreal; it better indicates
    that although a particular match has failed, it may be tried again
    with the packet's addresses reversed.

  - The 'MatchingStoD' attribute has been added.  This is a Packet
    Matching Engine (PME) attribute indicating that addresses are being
    matched in StoD (i.e.  'wire') order.  It can be used to perform
    different actions when the match is retried, thereby simplifying


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    some kinds of rule sets.  It was discussed and agreed to at the San
    Jose meeting in 1996.

  - Computed attributes (Class and Kind) may now be tested within a
    rule set.  This lifts an unneccessary earlier restriction.

  - The list of attribute numbers has been extended to define ranges
    for 'basic' attributes (in this document) and 'extended' attributes
    (currently being developed by the RTFM Working Group).

  - The 'Security Considerations' section has been completely
    rewritten.  It provides an evaluation of traffic measurement
    security risks and their countermeasures.




2 Traffic Flow Measurement Architecture


A traffic flow measurement system is used by network Operations
personnel for managing and developing a network.  It provides a tool for
measuring and understanding the network's traffic flows.  This
information is useful for many purposes, as mentioned in section 1
(above).

The following sections outline a model for traffic flow measurement,
which draws from working drafts of the OSI accounting model [2].  Future
extensions are anticipated as the model is refined to address additional
protocol layers.


2.1 Meters and Traffic Flows


At the heart of the traffic measurement model are network entities
called traffic METERS. Meters count certain attributes (such as numbers
of packets and bytes) and classify them as belonging to ACCOUNTABLE
ENTITIES using other attributes (such as source and destination
addresses).  An accountable entity is someone who (or something which)
is responsible for some activity on the network.  It may be a user, a
host system, a network, a group of networks, etc, depending on the
granularity specified by the meter's configuration.

We assume that routers or traffic monitors throughout a network are
instrumented with meters to measure traffic.  Issues surrounding the
choice of meter placement are discussed in the 'Traffic Flow
Measurement:  Background' RFC [1].  An important aspect of meters is
that they provide a way of succinctly aggregating entity usage
information.



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For the purpose of traffic flow measurement we define the concept of a
TRAFFIC FLOW, which is an artificial logical equivalent to a call or
connection.  A flow is a portion of traffic, delimited by a start and
stop time, that was generated by a particular accountable entity.
Attribute values (source/destination addresses, packet counts, byte
counts, etc.)  associated with a flow are aggregate quantities
reflecting events which take place in the DURATION between the start and
stop times.  The start time of a flow is fixed for a given flow; the
stop time may increase with the age of the flow.

For connectionless network protocols such as IP there is by definition
no way to tell whether a packet with a particular source/destination
combination is part of a stream of packets or not - each packet is
completely independent.  A traffic meter has, as part of its
configuration, a set of 'rules' which specify the flows of interest, in
terms of the values of their attributes.  It derives attribute values
from each observed packet, and uses these to decide which flow they
belong to.  Classifying packets into 'flows' in this way provides an
economical and practical way to measure network traffic and ascribe it
to accountable entities.

Usage information which is not derivable from traffic flows may also be
of interest.  For example, an application may wish to record accesses to
various different information resources or a host may wish to record the
username (subscriber id) for a particular network session.  Provision is
made in the traffic flow architecture to do this.  In the future the
measurement model will be extended to gather such information from
applications and hosts so as to provide values for higher-layer flow
attributes.

As well as FLOWS and METERS, the traffic flow measurement model includes
MANAGERS, METER READERS and ANALYSIS APPLICAIONS, which are explained in
following sections.  The relationships between them are shown by the
diagram below.  Numbers on the diagram refer to sections in this
document.



                    MANAGER
                   /       \
              2.3 /         \ 2.4
                 /           \
                /             \                       ANALYSIS
           METER   <----->   METER READER  <----->   APPLICATION
                     2.2                     2.7


  - MANAGER: A traffic measurement manager is an application which
    configures 'meter' entities and controls 'meter reader' entities.
    It uses the data requirements of analysis applications to determine
    the appropriate configurations for each meter, and the proper
    operation of each meter reader.  It may well be convenient to

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    combine the functions of meter reader and manager within a single
    network entity.

  - METER: Meters are placed at measurement points determined by
    network Operations personnel.  Each meter selectively records
    network activity as directed by its configuration settings.  It can
    also aggregate, transform and further process the recorded activity
    before the data is stored.  The processed and stored results are
    called the 'usage data.'

  - METER READER: A meter reader reliably transports usage data from
    meters so that it is available to analysis applications.

  - ANALYSIS APPLICATION: An analysis application processes the usage
    data so as to provide information and reports which are useful for
    network engineering and management purposes.  Examples include:



      -  TRAFFIC FLOW MATRICES, showing the total flow rates for many of
         the possible paths within an internet.

      -  FLOW RATE FREQUENCY DISTRIBUTIONS, indicating how flow rates
         vary with time.

      -  USAGE DATA showing the total traffic volumes sent and received
         by particular hosts.


The operation of the traffic measurement system as a whole is best
understood by considering the interactions between its components.
These are described in the following sections.


2.2 Interaction Between METER and METER READER


The information which travels along this path is the usage data itself.
A meter holds usage data in an array of flow data records known as the
FLOW TABLE. A meter reader may collect the data in any suitable manner.
For example it might upload a copy of the whole flow table using a file
transfer protocol, or read the records in the current flow set one at a
time using a suitable data transfer protocol.  Note that the meter
reader need not read complete flow data records, a subset of their
attribute values may well be sufficient.

A meter reader may collect usage data from one or more meters.  Data may
be collected from the meters at any time.  There is no requirement for
collections to be synchronized in any way.




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2.3 Interaction Between MANAGER and METER


A manager is responsible for configuring and controlling one or more
meters.  Each meter's configuration includes information such as:



  - Flow specifications, e.g.  which traffic flows are to be measured,
    how they are to be aggregated, and any data the meter is required
    to compute for each flow being measured.

  - Meter control parameters, e.g.  the maximum size of its flow table,
    the 'inactivity' time for flows (if no packets belonging to a flow
    are seen for this time the flow is considered to have ended, i.e.
    to have become idle).

  - Sampling rate.  Normally every packet will be observed.  It may
    sometimes be necessary to use sampling techniques to observe only
    some of the packets.  (Sampling algorithms are not prescribed by
    the architecture; it should be noted that before using sampling one
    should verify the statistical validity of the algorithm used).
    Current experience with the measurement architecture shows that a
    carefully-designed and implemented meter compresses the data such
    that in normal LANs and WANs of today sampling is really not
    needed.


A meter may run several rule sets concurrently on behalf of one or more
managers, and any manager may download a set of flow specifications
(i.e.  a 'rule set') to a meter.  Control parameters which apply to an
individual rule set should be set by the manager when it downloads that
rule set.

One manager should be designated as the 'master' for a meter.
Parameters such as sampling rate, which affect the overall operation of
the meter, should only be set by the master manager.


2.4 Interaction Between MANAGER and METER READER


A manager is responsible for configuring and controlling one or more
meter readers.  A meter reader may only be controlled by a single
manager.  A meter reader needs to know at least the following for every
meter it is collecting usage data from:



  - The meter's unique identity, i.e.  its network name or address.

  - How often usage data is to be collected from the meter.

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  - Which flow records are to be collected (e.g.  all active flows, the
    whole flow table, flows seen since a given time, etc.).

  - Which attribute values are to be collected for the required flow
    records (e.g.  all attributes, or a small subset of them)



Since redundant reporting may be used in order to increase the
reliability of usage data, exchanges among multiple entities must be
considered as well.  These are discussed below.


2.5 Multiple METERs or METER READERs


                 -- METER READER A --
                /         |          \
               /          |           \
       =====METER 1     METER 2=====METER 3    METER 4=====
                           \           |          /
                            \          |         /
                             -- METER READER B --


Several uniquely identified meters may report to one or more meter
readers.  The diagram above gives an example of how multiple meters and
meter readers could be used.

In the diagram above meter 1 is read by meter reader A, and meter 4 is
read by meter reader B. Meters 1 and 4 have no redundancy; if either
fails, usage data for their network segments will be lost.

Meters 2 and 3, however, measure traffic on the same network segment.
One of them may fail leaving the other collecting the segment's usage
data.  Meters 2 and 3 are read by meter reader A and by meter reader B.
If one meter reader fails, the other will continue collecting usage
data.

The architecture does not require multiple meter readers to be
synchronized.  In the situation above meter readers A and B could both
collect usage data at the same intervals, but not neccesarily at the
same times.  Note that because collections are asynchronous it is
unlikely that usage records from two different meter readers will agree
exactly.

If precisely synchronized collections are required this can be achieved
by having one manager request each meter to begin collecting a new set
of flows, then allowing all meter readers to collect the usage data from
the old sets of flows.



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If there is only one meter reader and it fails, the meters continue to
run.  When the meter reader is restarted it can collect all of the
accumulated flow data.  Should this happen, time resolution will be lost
(because of the missed collections) but overall traffic flow information
will not.  The only exception to this would occur if the traffic volume
was sufficient to 'roll over' counters for some flows during the
failure; this is addressed in the section on 'Rolling Counters.'



2.6 Interaction Between MANAGERs (MANAGER - MANAGER)


Synchronization between multiple management systems is the province of
network management protocols.  This traffic flow measurement
architecture specifies only the network management controls necessary to
perform the traffic flow measurement function and does not address the
more global issues of simultaneous or interleaved (possibly conflicting)
commands from multiple network management stations or the process of
transferring control from one network management station to another.


2.7 METER READERs and APPLICATIONs


Once a collection of usage data has been assembled by a meter reader it
can be processed by an analysis application.  Details of analysis
applications - such as the reports they produce and the data they
require - are outside the scope of this architecture.

It should be noted, however, that analysis applications will often
require considerable amounts of input data.  An important part of
running a traffic flow measurement system is the storage and regular
reduction of flow data so as to produce daily, weekly or monthly summary
files for further analysis.  Again, details of such data handling are
outside the scope of this architecture.



3 Traffic Flows and Reporting Granularity


A flow was defined in section 2.1 above in abstract terms as follows:


    "A TRAFFIC FLOW is an artifical logical equivalent to a call or
    connection, belonging to an ACCOUNTABLE ENTITY."


In practical terms, a flow is a stream of packets passing across a
network between two end points (or being sent from a single end point),
which have been summarized by a traffic meter for analysis purposes.

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3.1 Flows and their Attributes


Every traffic meter maintains a table of 'flow records' for flows seen
by the meter.  A flow record holds the values of the ATTRIBUTES of
interest for its flow.  These attributes might include:


  - ADDRESSES for the flow's source and destination.  These comprise
    the protocol type, the source and destination addresses at various
    network layers (extracted from the packet), and the number of the
    interface on which the packet was observed.

  - First and last TIMES when packets were seen for this flow, i.e.
    the 'creation' and 'last activity' times for the flow.

  - COUNTS for 'forward' (source to destination) and 'backward'
    (destination to source) components (e.g.  packets and bytes) of the
    flow's traffic.  The specifying of 'source' and 'destination' for
    flows is discussed in the section on packet matching below.

  - OTHER attributes, e.g.  information computed by the meter.


The attributes listed in this document (Appendix C) provide a basic
(i.e.  useful minimum) set; they are assigned attribute numbers in the
range 0 to 63.  The RTFM working group is working on an extended set of
attributes, which will have numbers in the range 65 to 127.
Implementors wishing to experiment with further new attributes should
use attribute numbers above 128.

A flow's ACCOUNTABLE ENTITY is specified by the values of its ADDRESS
attributes.  For example, if a flow's address attributes specified only
that "source address = IP address 10.1.0.1," then all IP packets from
and to that address would be counted in that flow.  If a flow's address
list were specified as "source address = IP address 10.1.0.1,
destination address = IP address 26.1.0.1" then only IP packets between
10.1.0.1 and 26.1.0.1 would be counted in that flow.

The addresses specifying a flow's address attributes may include one or
more of the following types:



  - The INTERFACE NUMBER for the flow, i.e.  the interface on which the
    meter measured the traffic.  Together with a unique address for the
    meter this uniquely identifies a particular physical-level port.



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  - The ADJACENT ADDRESS, i.e.  the [n-1] layer address of the
    immediate source or destination on the path of the packet.  For
    example, if flow measurement is being performed at the IP layer on
    an Ethernet LAN [3], an adjacent address is a six-octet Media
    Access Control (MAC) address.  For a host connected to the same LAN
    segment as the meter the adjacent address will be the MAC address
    of that host.  For hosts on other LAN segments it will be the MAC
    address of the adjacent (upstream or downstream) router carrying
    the traffic flow.

  - The PEER ADDRESS, which identifies the source or destination of the
    PEER-LEVEL packet.  The form of a peer address will depend on the
    network-layer protocol in use, and the network layer [n] at which
    traffic measurement is being performed.

  - The TRANSPORT ADDRESS, which identifies the source or destination
    port for the packet, i.e.  its [n+1] layer address.  For example,
    if flow measurement is being performed at the IP layer a transport
    address is a two-octet UDP or TCP port number.



The four definitions above specify addresses for each of the four lowest
layers of the OSI reference model, i.e.  Physical layer, Link layer,
Network layer and Transport layer.  A FLOW RECORD stores both the VALUE
for each of its addresses (as described above) and a MASK specifying
which bits of the address value are being used and which are ignored.
Note that if address bits are being ignored the meter will set them to
zero, however their actual values are undefined.

One of the key features of the traffic measurement architecture is that
attributes have essentially the same meaning for different protocols, so
that analysis applications can use the same reporting formats for all
protocols.  This is straightforward for peer addresses; although the
form of addresses differs for the various protocols, the meaning of a
'peer address' remains the same.  It becomes harder to maintain this
correspondence at higher layers - for example, at the Network layer IP,
Novell IPX and AppleTalk all use port numbers as a 'transport address,'
but CLNP and DECnet have no notion of ports.  Further work is needed
here, particularly in selecting attributes which will be suitable for
the higher layers of the OSI reference model.

Reporting by adjacent intermediate sources and destinations or simply by
meter interface (most useful when the meter is embedded in a router)
supports hierarchical Internet reporting schemes as described in the
'Traffic Flow Measurement:  Background' RFC [1].  That is, it allows
backbone and regional networks to measure usage to just the next lower
level of granularity (i.e.  to the regional and stub/enterprise levels,
respectively), with the final breakdown according to end user (e.g.  to
source IP address) performed by the stub/enterprise networks.



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In cases where network addresses are dynamically allocated (e.g.  mobile
subscribers), further subscriber identification will be necessary if
flows are to ascribed to individual users.  Provision is made to further
specify the accountable entity through the use of an optional SUBSCRIBER
ID as part of the flow id.  A subscriber ID may be associated with a
particular flow either through the current rule set or by proprietary
means within a meter, for example via protocol exchanges with one or
more (multi-user) hosts.  At this time a subscriber ID is an arbitrary
text string; later versions of the architecture may specify its contents
in more detail.



3.2 Granularity of Flow Measurements


GRANULARITY is the 'control knob' by which an application and/or the
meter can trade off the overhead associated with performing usage
reporting against the level of detail supplied.  A coarser granularity
means a greater level of aggregation; finer granularity means a greater
level of detail.  Thus, the number of flows measured (and stored) at a
meter can be regulated by changing the granularity of the accountable
entity, the attributes, or the time intervals.  Flows are like an
adjustable pipe - many fine-granularity streams can carry the data with
each stream measured individually, or data can be bundled in one
coarse-granularity pipe.

Flow granularity is controlled by adjusting the level of detail at which
the following are reported:


  - The accountable entity (address attributes, discussed above).

  - The categorisation of packets (other attributes, discussed below).

  - The lifetime/duration of flows (the reporting interval needs to be
    short enough to measure them with sufficient precision).


The set of rules controlling the determination of each packet's
accountable entity is known as the meter's CURRENT RULE SET. As will be
shown, the meter's current rule set forms an integral part of the
reported information, i.e.  the recorded usage information cannot be
properly interpreted without a definition of the rules used to collect
that information.

Settings for these granularity factors may vary from meter to meter.
They are determined by the meter's current rule set, so they will change
if network Operations personnel reconfigure the meter to use a new rule
set.  It is expected that the collection rules will change rather
infrequently; nonetheless, the rule set in effect at any time must be


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identifiable via a RULE SET ID. Granularity of accountable entities is
further specified by additional ATTRIBUTES. These attributes include:



  - Meter variables such as the index of the flow's record in the flow
    table and the rule set id for the rules which the meter was running
    while the flow was observed.  The values of these attributes
    provide a way of distinguishing flows observed by a meter at
    different times.

  - Attributes which record information derived from other attribute
    values.  Six of these are defined (SourceClass, DestClass,
    FlowClass, SourceKind, DestKind, FlowKind), and their meaning is
    determined by the meter's rule set.  For example, one could have a
    subroutine in the rule set which determined whether a source or
    destination peer address was a member of an arbitrary list of
    networks, and set SourceClass/DestClass to one if the source/dest
    peer address was in the list or to zero otherwise.

  - Administratively specified attributes such as Quality Of Service
    and Priority, etc.  These are not defined at this time.

  - Higher-layer (especially application-level) attributes.  These are
    not defined at this time.


Settings for these granularity factors may vary from meter to meter.
They are determined by the meter's current rule set, so they will change
if network Operations personnel reconfigure the meter to use a new rule
set.

The LIFETIME of a flow is the time interval which began when the meter
observed the first packet belonging to the flow and ended when it saw
the last packet.  Flow lifetimes are very variable, but many - if not
most - are rather short.  A meter cannot measure lifetimes directly;
instead a meter reader collects usage data for flows which have been
active since the last collection, and an analysis application may
compare the data from each collection so as to determine when each flow
actually stopped.

The meter does, however, need to reclaim memory (i.e.  records in the
flow table) being held by idle flows.  The meter configuration includes
a variable called InactivityTimeout, which specifies the minimum time a
meter must wait before recovering the flow's record.  In addition,
before recovering a flow record the meter must be sure that the flow's
data has been collected by at least one meter reader.

These 'lifetime' issues are considered further in the section on meter
readers (below).  A complete list of the attributes currently defined is
given in Appendix C later in this document.


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3.3 Rolling Counters, Timestamps, Report-in-One-Bucket-Only

Once a usage record is sent, the decision needs to be made whether to
clear any existing flow records or to maintain them and add to their
counts when recording subsequent traffic on the same flow.  The second
method, called rolling counters, is recommended and has several
advantages.  Its primary advantage is that it provides greater
reliability - the system can now often survive the loss of some usage
records, such as might occur if a meter reader failed and later
restarted.  The next usage record will very often contain yet another
reading of many of the same flow buckets which were in the lost usage
record.  The 'continuity' of data provided by rolling counters can also
supply information used for "sanity" checks on the data itself, to guard
against errors in calculations.

The use of rolling counters does introduce a new problem:  how to
distinguish a follow-on flow record from a new flow record.  Consider
the following example.


                      CONTINUING FLOW        OLD FLOW, then NEW FLOW

                      start time = 1            start time = 1
Usage record N:       flow count = 2000      flow count = 2000 (done)

                      start time = 1            start time = 5
Usage record N+1:     flow count = 3000      new flow count = 1000

Total count:                 3000                    3000


In the continuing flow case, the same flow was reported when its count
was 2000, and again at 3000:  the total count to date is 3000.  In the
OLD/NEW case, the old flow had a count of 2000.  Its record was then
stopped (perhaps because of temporary idleness, or MAX LIFETIME policy),
but then more traffic with the same characteristics arrived so a new
flow record was started and it quickly reached a count of 1000.  The
total flow count from both the old and new records is 3000.

The flow START TIMESTAMP attribute is sufficient to resolve this.  In
the example above, the CONTINUING FLOW flow record in the second usage
record has an old FLOW START timestamp, while the NEW FLOW contains a
recent FLOW START timestamp.

Each packet is counted in one and only one flow, so as to avoid multiple
counting of a single packet.  The record of a single flow is informally
called a "bucket." If multiple, sometimes overlapping, records of usage
information are required (aggregate, individual, etc), the network
manager should collect the counts in sufficiently detailed granularity
so that aggregate and combination counts can be reconstructed in
post-processing of the raw usage data.

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For example, consider a meter from which it is required to record both
'total packets coming in interface #1' and 'total packets arriving from
any interface sourced by IP address = a.b.c.d.'  Although a bucket can
be declared for each case, it is not clear how to handle a packet which
satisfies both criteria.  It must only be counted once.  By default it
will be counted in the first bucket for which it qualifies, and not in
the other bucket.  Further, it is not possible to reconstruct this
information by post-processing.  The solution in this case is to define
not two, but THREE buckets, each one collecting a unique combination of
the two criteria:



        Bucket 1:  Packets which came in interface 1,
                   AND were sourced by IP address a.b.c.d

        Bucket 2:  Packets which came in interface 1,
                   AND were NOT sourced by IP address a.b.c.d

        Bucket 3:  Packets which did NOT come in interface 1,
                   AND were sourced by IP address a.b.c.d

       (Bucket 4:  Packets which did NOT come in interface 1,
                   AND NOT sourced by IP address a.b.c.d)


The desired information can now be reconstructed by post-processing.
"Total packets coming in interface 1" can be found by adding buckets 1 &
2, and "Total packets sourced by IP address a.b.c.d" can be found by
adding buckets 1 & 3.  Note that in this case bucket 4 is not explicitly
required since its information is not of interest, but it is supplied
here in parentheses for completeness.



4 Meters


A traffic flow meter is a device for collecting data about traffic flows
at a given point within a network; we will call this the METERING POINT.
The header of every packet passing the network metering point is offered
to the traffic meter program.

A meter could be implemented in various ways, including:


  - A dedicated small host, connected to a LAN (so that it can see all
    packets as they pass by) and running a 'traffic meter' program.
    The metering point is the LAN segment to which the meter is
    attached.


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  - A multiprocessing system with one or more network interfaces, with
    drivers enabling a traffic meter program to see packets.  In this
    case the system provides multiple metering points - traffic flows
    on any subset of its network interfaces can be measured.

  - A packet-forwarding device such as a router or switch.  This is
    similar to (b) except that every received packet should also be
    forwarded, usually on a different interface.



4.1 Meter Structure


An outline of the meter's structure is given in the following diagram:


                  packet                +------------------+
                  header                | Current Rule Set |
                    |                   +--------+---------+
                    |                            |
           +--------*---------+       +----------*-------------+
           | Packet Processor |<----->| Packet Matching Engine |
           +--+------------+--+       +------------------------+
              |            |
       Ignore *            | Count via flow key
                           |
                        +--*--------------+
                        | 'Search' index  |
                        +--------+--------+
                                 |
                        +--------*--------+
                        |                 |
                        |   Flow Table    |
                        |                 |
                        +--------+--------+
                                 |
                        +--------*--------+
                        | 'Collect' index |
                        +--------+--------+
                                 |
                                 *
                            Meter Reader



Briefly, the meter works as follows:


  - Incoming packet headers arrive at the top left of the diagram and
    are passed to the PACKET PROCESSOR.


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  - The packet processor passes them to the Packet Matching Engine
    (PME) where they are classified.

  - The PME is a Virtual Machine running a pattern matching program
    contained in the CURRENT RULE SET. It is invoked by the Packet
    Processor, and returns instructions on what to do with the packet.

  - Some packets are classified as 'to be ignored.'  They are discarded
    by the Packet Processor.

  - Other packets are matched by the PME, which returns a FLOW KEY
    describing the flow to which the packet belongs.

  - The flow key is used to locate the flow's entry in the FLOW TABLE;
    a new entry is created when a flow is first seen.  The entry's
    packet and byte counters are updated.

  - A meter reader may collect data from the flow table at any time.
    It may use the 'collect' index to locate the flows to be collected
    within the flow table.



The discussion above assumes that a meter will only be running a single
rule set.  A meter may, however, run several rule sets concurrently.  To
do this the meter maintains a table of current rulesets.  The packet
processor matches each packet against every current ruleset, producing a
single flow table with flows from all the rule sets.  The overall effect
of doing this is somewhat similar to running several independent meters,
one for each rule set.


4.2 Flow Table


Every traffic meter maintains a table of TRAFFIC FLOW RECORDS for flows
seen by the meter.  A flow record contains attribute values for its
flow, including:


  - Addresses for the flow's source and destination.  These include
    addresses and masks for various network layers (extracted from the
    packet), and the number of the interface on which the packet was
    observed.

  - First and last times when packets were seen for this flow.

  - Counts for 'forward' (source to destination) and 'backward'
    (destination to source) components of the flow's traffic.

  - Other attributes, e.g.  state of the flow record (discussed below).


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The state of a flow record may be:


  - INACTIVE: The flow record is not being used by the meter.

  - CURRENT: The record is in use and describes a flow which belongs to
    the 'current flow set,' i.e.  the set of flows recently seen by the
    meter.

  - IDLE: The record is in use and the flow which it describes is part
    of the current flow set.  In addition, no packets belonging to this
    flow have been seen for a period specified by the meter's
    InactivityTime variable.


4.3 Packet Handling, Packet Matching


Each packet header received by the traffic meter program is processed as
follows:



  - Extract attribute values from the packet header and use them to
    create a MATCH KEY for the packet.

  - Match the packet's key against the current rule set, as explained
    in detail below.


The rule set specifies whether the packet is to be counted or ignored.
If it is to be counted the matching process produces a FLOW KEY for the
flow to which the packet belongs.  This flow key is used to find the
flow's record in the flow table; if a record does not yet exist for this
flow, a new flow record may be created.  The counts for the matching
flow record can then be incremented.

For example, the rule set could specify that packets to or from any host
in IP network 130.216 are to be counted.  It could also specify that
flow records are to be created for every pair of 24-bit (Class C)
subnets within network 130.216.

Each packet's match key is passed to the meter's PATTERN MATCHING ENGINE
(PME) for matching.  The PME is a Virtual Machine which uses a set of
instructions called RULES, i.e.  a RULE SET is a program for the PME. A
packet's match key contains an interface number, source address (S) and
destination address (D) values.  It does not, however, contain any
attribute masks for its attributes, only their values.

If measured flows were unidirectional, i.e.  only counted packets
travelling in one direction, the matching process would be simple.  The

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PME would be called once to match the packet.  Any flow key produced by
a successful match would be used to find the flow's record in the flow
table, and that flow's counters would be updated.

Flows are, however, bidirectional, reflecting the forward and reverse
packets of a protocol interchange or 'session.'  Maintaining two sets of
counters in the meter's flow record makes the resulting flow data much
simpler to handle, since analysis programs do not have to gather
together the 'forward' and 'reverse' components of sessions.
Implementing bi-directional flows is, of course, more difficult for the
meter, since it must decide whether a packet is a 'forward' packet or a
'reverse' one.  To make this decision the meter will often need to
invoke the PME twice, once for each possible packet direction.

The diagram below describes the algorithm used by the traffic meter to
process each packet.  Flow through the diagram is from left to right and
top to bottom, i.e.  from the top left corner to the bottom right
corner.  S indicates the flow's source address (i.e.  its set of source
address attribute values) from the packet, and D indicates its
destination address.

There are several cases to consider.  These are:



  - The packet is recognised as one which is TO BE IGNORED.

  - The packet MATCHES IN BOTH DIRECTIONS. One situation in which this
    could happen would be a rule set which matches flows within network
    X (Source = X, Dest = X) but specifies that flows are to be created
    for each subnet within network X, say subnets y and z.  If, for
    example a packet is seen for y->z, the meter must check that flow
    z->y is not already current before creating y->z.

  - The packet MATCHES IN ONE DIRECTION ONLY. If its flow is already
    current, its forward or reverse counters are incremented.
    Otherwise it is added to the flow table and then counted.


The algorithm uses four functions, as follows:


match(A->B) implements the PME.  It uses the meter's current rule set
   to match the attribute values in the packet's match key.  A->B means
   that the assumed source address is A and destination address B, i.e.
   that the packet was travelling from A to B.  match() returns one of
   three results:

   'Ignore' means that the packet was matched but this flow is not
            to be counted.



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   'Fail' means that the packet did not match.  It might, however
            match with its direction reversed, i.e. from B to A.

   'Suc'  means that the packet did match, i.e. it belongs to a flow
            which is to be counted.

current(A->B) succeeds if the flow A-to-B is current - i.e. has
   a record in the flow table whose state is Current - and fails
   otherwise.

create(A->B) adds the flow A-to-B to the flow table, setting the
   value for attributes - such as addresses - which remain constant,
   and zeroing the flow's counters.

count(A->B,f) increments the 'forward' counters for flow A-to-B.
count(A->B,r) increments the 'reverse' counters for flow A-to-B.
   'Forward' here means the counters for packets travelling from
   A to B.  Note that count(A->B,f) is identical to count(B->A,r).



                    Ignore
    --- match(S->D) -------------------------------------------------+
         | Suc   | Fail                                              |
         |       |          Ignore                                   |
         |      match(D->S) -----------------------------------------+
         |       | Suc   | Fail                                      |
         |       |       |                                           |
         |       |       +-------------------------------------------+
         |       |                                                   |
         |       |             Suc                                   |
         |      current(D->S) ---------- count(D->S,r) --------------+
         |       | Fail                                              |
         |       |                                                   |
         |      create(D->S) ----------- count(D->S,r) --------------+
         |                                                           |
         |             Suc                                           |
        current(S->D) ------------------ count(S->D,f) --------------+
         | Fail                                                      |
         |             Suc                                           |
        current(D->S) ------------------ count(D->S,r) --------------+
         | Fail                                                      |
         |                                                           |
        create(S->D) ------------------- count(S->D,f) --------------+
                                                                     |
                                                                     *

When writing rule sets one must remember that the meter will normally
try to match each packet in both directions.  It is particularly
important that the rule set does not contain inconsistencies which will
upset this process.


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Consider, for example, a rule set which counts packets from source
network A to destination network B, but which ignores packets from
source network B. This is an obvious example of an inconsistent rule
set, since packets from network B should be counted as reverse packets
for the A-to-B flow.

This problem could be avoided by devising a language for specifying rule
files and writing a compiler for it, thus making it much easier to
produce correct rule sets.  Another approach would be to write a 'rule
set consistency checker' program, which could detect problems in
hand-written rule sets.

In the short term the best way to avoid these problems is to write rule
sets which only clasify flows in the forward direction, and rely on the
meter to handle reverse-travelling packets.



4.4 Rules and Rule Sets


A rule set is an array of rules.  Rule sets are held within a meter as
entries in an array of rule sets.  One member of this array is the
CURRENT RULE SET, in that it is the one which is currently being used by
the meter to classify incoming packets.

Rule set 1 is built in to the meter and cannot be changed.  It is run
when the meter is started up, and provides a very coarse reporting
granularity; it is mainly useful for verifying that the meter is
running, before a 'useful' rule set is downloaded to it.

If the meter is instructed to use rule set 0, it will cease measuring;
all packets will be ignored until another (non-zero) rule set is made
current.

Each rule in a rule set is structured as follows:


   +-------- test ---------+    +---- action -----+
   attribute & mask = value:    opcode,  parameter;


Opcodes contain two flags:  'goto' and 'test.'  The PME maintains a
Boolean indicator called the 'test indicator,' which is initially set
(true).  Execution begins with rule 1, the first in the rule set.  It
proceeds as follows:







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   If the test indicator is true:
      Perform the test, i.e. AND the attribute value with the
         mask and compare it with the value.
      If these are equal the test has succeeded; perform the
         rule's action (below).
      If the test fails execute the next rule in the rule set.
      If there are no more rules in the rule set, return from the
         match() function indicating failure.

   If the test indicator is false, or the test (above) succeeded:
      Set the test indicator to this rule's test flag value.
      Determine the next rule to execute.
         If the opcode has its goto flag set, its parameter value
            specifies the number of the next rule.
         Opcodes which don't have their goto flags set either
            determine the next rule in special ways (Return),
            or they terminate execution (Ignore, NoMatch, Count,
            CountPkt).
      Perform the action.



The PME maintains two 'history' data structures.  The first, the
'return' stack, simply records the index (i.e.  1-origin rule number) of
each Gosub rule as it is executed; Return rules pop their Gosub rule
index.  The second, the 'pattern' queue, is used to save information for
later use in building a flow key.  A flow key is built by zeroing all
its attribute values, then copying attribute and mask information from
the pattern stack in the order it was enqueued.

The opcodes are:


         opcode         goto    test

      1  Ignore           0       -
      2  NoMatch          0       -
      3  Count            0       -
      4  CountPkt         0       -
      5  Return           0       0
      6  Gosub            1       1
      7  GosubAct         1       0
      8  Assign           1       1
      9  AssignAct        1       0
     10  Goto             1       1
     11  GotoAct          1       0
     12  PushRuleTo       1       1
     13  PushRuleToAct    1       0
     14  PushPktTo        1       1
     15  PushPktToAct     1       0



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The actions they perform are:


   Ignore:         Stop matching, return from the match() function
                   indicating that the packet is to be ignored.

   NoMatch:        Stop matching, return from the match() function
                   indicating failure.

   Count:          Stop matching.  Save this rule's attribute name,
                   mask and value in the PME's pattern queue, then
                   construct a flow key for the flow to which this
                   this packet belongs.  Return from the match()
                   function indicating success.  The meter will use
                   the flow key to locate the flow record for this
                   packet's flow.

   CountPkt:       As for Count, except that the masked value from
                   the packet is saved in the PME's pattern queue
                   instead of the rule's value.

   Gosub:          Call a rule-matching subroutine.  Push the current
                   rule number on the PME's return stack, set the
                   test indicator then goto the specified rule.

   GosubAct:       Same as Gosub, except that the test indicator is
                   cleared before going to the specified rule.

   Return:         Return from a rule-matching subroutine.  Pop the
                   number of the calling gosub rule from the PME's
                   'return' stack and add this rule's parameter value
                   to it to determine the 'target' rule.  Clear the
                   test indicator then goto the target rule.

                   A subroutine call appears in a rule set as a Gosub
                   rule followed by a small group of following rules.
                   Since a Return action clears the test flag, the
                   action of one of these 'following' rules will be
                   executed; this allows the subroutine to return a
                   result (in addition to any information it may save
                   in the PME's pattern queue).

   Assign:         Set the attribute specified in this rule to the
                   value specified in this rule.  Set the test
                   indicator then goto the specified rule.

   AssignAct:      Same as Assign, except that the test indicator
                   is cleared before going to the specified rule.

   Goto:           Set the test indicator then goto the
                   specified rule.

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   GotoAct:        Clear the test indicator then goto the specified
                   rule.

   PushRuleTo:     Save this rule's attribute name, mask and value
                   in the PME's pattern queue. Set the test
                   indicator then goto the specified rule.

   PushRuleToAct:  Same as PushRuleTo, except that the test indicator
                   is cleared before going to the specified rule.

                   PushRuleTo actions may be used to save the value
                   and mask used in a test, or (if the test is not
                   performed) to save an arbitrary value and mask.

   PushPktTo:      Save this rule's attribute name, mask, and the
                   masked value from the packet, in the PME's pattern
                   SET the test indicator then goto the specified
                   rule.

   PushPktToAct:   Same as PushPktTo, except that the test indicator
                   is cleared before going to the specified rule.

                   PushPktTo actions may be used to save a value from
                   the packet using a specified mask.  The test in
                   PushPktTo rules will almost never be executed.



As well as the attributes applying directly to packets (such as
SourcePeerAddress, DestTransAddress, etc.)  the PME implements several
further attribtes.  These are:


   Null:       Tests performed on the Null attribute always succeed.

   MatchingStoD:  Indicates whether the PME is matching the packet
               with its addresses in 'wire order' or with its
               addresses reversed.  MatchingStoD's value is 1 if the
               addresses are in wire order (StoD), and != 1 otherwise.

   v1 .. v5:   v1, v2, v3, v4 and v5 are 'meter variables.'  They
               provide a way to pass parameters into rule-matching
               subroutines.  Each may hold the name of a normal
               attribute; its value is set by an Assign action.
               When a meter variable appears as the attribute of a
               rule, its value specifies the actual attribute to be
                tested.  For example, if v1 had been assigned
               SourcePeerAddress as its value, a rule with v1 as its
               attribute would actually test SourcePeerAddress.



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   SourceClass, DestClass, FlowClass,
   SourceKind, DestKind, FlowKind:
               These six attributes may be set by executing PushRuleto
               actions.  They allow the PME to save (in flow records)
               information which has been built up during matching.
               Their values may be tested in rules; this allows one
               to set them early in a rule set, and test them later.


4.5 Maintaining the Flow Table

The flow table may be thought of as a 1-origin array of flow records.
(A particular implementation may, of course, use whatever data structure
is most suitable).  When the meter starts up there are no known flows;
all the flow records are in the 'inactive' state.

Each time a packet is seen for a flow which is not in the current flow
set a flow record is set up for it; the state of such a record is
'current.'  When selecting a record for the new flow the meter searches
the flow table for a 'inactive' record - there is no particular
significance in the ordering of records within the table.

Flow data may be collected by a 'meter reader' at any time.  There is no
requirement for collections to be synchronized.  The reader may collect
the data in any suitable manner, for example it could upload a copy of
the whole flow table using a file transfer protocol, or it could read
the records in the current flow set row by row using a suitable data
transfer protocol.

The meter keeps information about collections, in particular it
maintains a LastCollectTime variable which remembers the time the last
collection was made.  A second variable, InactivityTime, specifies the
minimum time the meter will wait before considering that a flow is idle.

The meter must recover records used for idle flows, if only to prevent
it running out of flow records.  Recovered flow records are returned to
the 'inactive' state.  A variety of recovery strategies are possible,
including the following:

One possible recovery strategy is to recover idle flow records as soon
as possible after their data has been collected.  To implement this the
meter could run a background process which scans the flow table looking
for 'current' flows whose 'last packet' time is earlier than the meter's
LastCollectTime.  This would be suitable for use when one was interested
in measuring flow lifetimes.

Another recovery strategy is to leave idle flows alone as long as
possible, which would be suitable if one was only interested in
measuring total traffic volumes.  It could be implemented by having the
meter search for collected idle flows only when it ran out of 'inactive'
flow records.

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One further factor a meter should consider before recovering a flow is
the number of meter readers which have collected the flow's data.  If
there are multiple meter readers operating, each reader should collect a
flow's data before its memory is recovered.



4.6 Handling Increasing Traffic Levels


Under normal conditions the meter reader specifies which set of usage
records it wants to collect, and the meter provides them.

If memory usage rises above the high-water mark the meter should switch
to a STANDBY RULE SET so as to increase the granularity of flow
collection and decrease the rate at which new flows are created.  When
the manager, usually as part of a regular poll, becomes aware that the
meter is using its standby rule set, it could decrease the interval
between collections.  The meter should also increase its efforts to
recover flow memory so as to reduce the number of idle flows in memory.
When the situation returns to normal, the manager may request the meter
to switch back to its normal rule set.



5 Meter Readers


Usage data is accumulated by a meter (e.g.  in a router) as memory
permits.  It is collected at regular reporting intervals by meter
readers, as specified by a manager.  The collected data is recorded in a
disk file called a FLOW DATA FILE, as a sequence of USAGE RECORDS.

The following sections describe the contents of usage records and flow
data files.  Note, however, that at this stage the details of such
records and files is not specified in the architecture.  Specifying a
common format for them would be a worthwhile future development.


5.1 Identifying Flows in Flow Records


Once a packet has been classified and is ready to be counted, an
appropriate flow data record must already exist in the flow table;
otherwise one must be created.  The flow record has a flexible format
where unnecessary identification attributes may be omitted.  The
determination of which attributes of the flow record to use, and of what
values to put in them, is specified by the current rule set.



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Note that the combination of start time, rule set id and subscript (row
number in the flow table) provide a unique flow identifier, regardless
of the values of its other attributes.

The current rule set may specify additional information, e.g.  a
computed attribute value such as FlowKind, which is to be placed in the
attribute section of the usage record.  That is, if a particular flow is
matched by the rule set, then the corresponding flow record should be
marked not only with the qualifying identification attributes, but also
with the additional information.  Using this feature, several flows may
each carry the same FlowKind value, so that the resulting usage records
can be used in post-processing or between meter reader and meter as a
criterion for collection.



5.2 Usage Records, Flow Data Files


The collected usage data will be stored in flow data files on the meter
reader, one file for each meter.  As well as containing the measured
usage data, flow data files must contain information uniquely
identifiying the meter from which it was collected.

A USAGE RECORD contains the descriptions of and values for one or more
flows.  Quantities are counted in terms of number of packets and number
of bytes per flow.  Each usage record contains the entity identifier of
the meter (a network address), a time stamp and a list of reported flows
(FLOW DATA RECORDS). A meter reader will build up a file of usage
records by regularly collecting flow data from a meter, using this data
to build usage records and concatenating them to the tail of a file.
Such a file is called a FLOW DATA FILE.

A usage record contains the following information in some form:


+-------------------------------------------------------------------+
|    RECORD IDENTIFIERS:                                            |
|      Meter Id (& digital signature if required)                   |
|      Timestamp                                                    |
|      Collection Rules ID                                          |
+-------------------------------------------------------------------+
|    FLOW IDENTIFIERS:            |    COUNTERS                     |
|      Address List               |       Packet Count              |
|      Subscriber ID (Optional)   |       Byte Count                |
|      Attributes (Optional)      |    Flow Start/Stop Time         |
+-------------------------------------------------------------------+






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5.3 Meter to Meter Reader:  Usage Record Transmission


The usage record contents are the raison d'etre of the system.  The
accuracy, reliability, and security of transmission are the primary
concerns of the meter/meter reader exchange.  Since errors may occur on
networks, and Internet packets may be dropped, some mechanism for
ensuring that the usage information is transmitted intact is needed.

Flow data is moved from meter to meter reader via a series of protocol
exchanges between them.  This may be carried out in various ways, moving
individual attribute values, complete flows, or the entire flow table
(i.e.  all the active flows).  One possible method of achieving this
transfer is to use SNMP; the 'Traffic Flow Measurement:  Meter MIB'
document [4] gives details.  Note that this is simply one example; the
transfer of flow data from meter to meter reader is not specified in
this document.

The reliability of the data transfer method under light, normal, and
extreme network loads should be understood before selecting among
collection methods.

In normal operation the meter will be running a rule file which provides
the required degree of flow reporting granularity, and the meter
reader(s) will collect the flow data often enough to allow the meter's
garbage collection mechanism to maintain a stable level of memory usage.

In the worst case traffic may increase to the point where the meter is
in danger of running completely out of flow memory.  The meter
implementor must decide how to handle this, for example by switching to
a default (extremely coarse granularity) rule set, by sending a trap
message to the manager, or by attempting to dump flow data to the meter
reader.

Users of the Traffic Flow Measurement system should analyse their
requirements carefully and assess for themselves whether it is more
important to attempt to collect flow data at normal granularity
(increasing the collection frequency as needed to keep up with traffic
volumes), or to accept flow data with a coarser granularity.  Similarly,
it may be acceptable to lose flow data for a short time in return for
being sure that the meter keeps running properly, i.e.  is not
overwhelmed by rising traffic levels.



6 Managers


A manager configures meters and controls meter readers.  It does this
via the interactions described below.



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6.1 Between Manager and Meter:  Control Functions

  - DOWNLOAD RULE SET: A meter may hold an array of rule sets.  One of
    these, the 'default' rule set, is built in to the meter and cannot
    be changed; the others must be downloaded by the manager.  A
    manager may use any suitable protocol exchange to achieve this, for
    example an FTP file transfer or a series of SNMP SETs, one for each
    row of the rule set.

  - SWITCH TO SPECIFIED RULE SET: Once the rule sets have been
    downloaded, the manager must instruct the meter which rule set it
    is to actually run (i.e.  which is to be the current rule set), and
    which is to be the standby rule set.

  - SET HIGH WATER MARK: A percentage value interpreted by the meter
    which tells the meter when to switch to its standby rule set, so as
    to increase the granularity of the flows and conserve the meter's
    flow memory.  Once this has happened, the manager may also change
    the polling frequency or the meter's control parameters (so as to
    increase the rate at which the meter can recover memory from idle
    flows).

    If the high traffic levels persist, the meter's normal rule set may
    have to be rewritten to permanently reduce the reporting
    granularity.

  - SET FLOW TERMINATION PARAMETERS: The meter should have the good
    sense in situations where lack of resources may cause data loss to
    purge flow records from its tables.  Such records may include:



      -  Flows that have already been reported to at least one meter
         reader, and show no activity since the last report,

      -  Oldest flows, or

      -  Flows with the smallest number of unreported packets.


  - SET INACTIVITY TIMEOUT: This is a time in seconds since the last
    packet was seen for a flow.  Flow records may be reclaimed if they
    have been idle for at least this amount of time, and have been
    collected in accordance with the current collection criteria.








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6.2 Between Manager and Meter Reader:  Control Functions

Because there are a number of parameters that must be set for traffic
flow measurement to function properly, and viable settings may change as
a result of network traffic characteristics, it is desirable to have
dynamic network management as opposed to static meter configurations.
Many of these operations have to do with space tradeoffs - if memory at
the meter is exhausted, either the reporting interval must be decreased
or a coarser granularity of aggregation must be used so that more data
fits into less space.

Increasing the reporting interval effectively stores data in the meter;
usage data in transit is limited by the effective bandwidth of the
virtual link between the meter and the meter reader, and since these
limited network resources are usually also used to carry user data (the
purpose of the network), the level of traffic flow measurement traffic
should be kept to an affordable fraction of the bandwidth.
("Affordable" is a policy decision made by the network Operations
personnel).  At any rate, it must be understood that the operations
below do not represent the setting of independent variables; on the
contrary, each of the values set has a direct and measurable effect on
the behaviour of the other variables.

Network management operations follow:



  - MANAGER and METER READER IDENTIFICATION: The manager should ensure
    that meters report to the correct set of meter readers, and take
    steps to prevent unauthorised access to usage information.  The
    meter readers so identified should be prepared to poll if necessary
    and accept data from the appropriate meters.  Alternate meter
    readers may be identified in case both the primary manager and the
    primary meter reader are unavailable.  Similarly, alternate
    managers may be identified.

  - REPORTING INTERVAL CONTROL: The usual reporting interval should be
    selected to cope with normal traffic patterns.  However, it may be
    possible for a meter to exhaust its memory during traffic spikes
    even with a correctly set reporting interval.  Some mechanism must
    be available for the meter to tell the manager that it is in danger
    of exhausting its memory (by declaring a 'high water' condition),
    and for the manager to arbitrate (by decreasing the polling
    interval, letting nature take its course, or by telling the meter
    to ask for help sooner next time).

  - GRANULARITY CONTROL: Granularity control is a catch-all for all the
    parameters that can be tuned and traded to optimise the system's
    ability to reliably measure and store information on all the
    traffic (or as close to all the traffic as an administration
    requires).  Granularity

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      -  Controls flow-id granularities for each interface, and

      -  Determines the number of buckets into which user traffic will
         be lumped together.


    Since granularity is controlled by the meter's current rule set,
    the manager can only change it by requesting the meter to switch to
    a different rule set.  The new rule set could be downloaded when
    required, or it could have been downloaded as part of the meter's
    initial configuration.

  - FLOW LIFETIME CONTROL: Flow termination parameters include timeout
    parameters for obsoleting inactive flows and removing them from
    tables, and maximum flow lifetimes.  This is intertwined with
    reporting interval and granularity, and must be set in accordance
    with the other parameters.


6.3 Exception Conditions


Exception conditions must be handled, particularly occasions when the
meter runs out of buffer space.  Since, to prevent counting any packet
twice, packets can only be counted in a single flow at any given time,
discarding records will result in the loss of information.  The
mechanisms to deal with this are as follows:



  - METER OUTAGES: In case of impending meter outages (controlled
    restarts, etc.)  the meter could send a trap to the manager.  The
    manager could then request one or more meter readers to pick up the
    usage record from the meter.

    Following an uncontrolled meter outage such as a power failure, the
    meter could send a trap to the manager indicating that it has
    restarted.  The manager could then download the meter's correct
    rule set and advise the meter reader(s) that the meter is running
    again.  Alternatively, the meter reader may discover from its
    regular poll that a meter has failed and restarted.  It could then
    advise the manager of this, instead of relying on a trap from the
    meter.

  - METER READER OUTAGES: If the collection system is down or isolated,
    the meter should try to inform the manager of its failure to
    communicate with the collection system.  Usage data is maintained
    in the flows' rolling counters, and can be recovered when the meter
    reader is restarted.



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  - MANAGER OUTAGES: If the manager fails for any reason, the meter
    should continue measuring and the meter reader(s) should keep
    gathering usage records.

  - BUFFER PROBLEMS: The network manager may realise that there is a
    'low memory' condition in the meter.  This can usually be
    attributed to the interaction between the following controls:


      -  The reporting interval is too infrequent,

      -  The reporting granularity is too fine, or

      -  The throughput/bandwidth of circuits carrying the usage data is
         too low.


    The manager may change any of these parameters in response to the
    meter (or meter reader's) plea for help.



6.4 Standard Rule Sets


Although the rule table is a flexible tool, it can also become very
complex.  It may be helpful to develop some rule sets for common
applications:


  - PROTOCOL TYPE: The meter records packets by protocol type.  This
    will be the default rule table for Traffic Flow Meters.

  - ADJACENT SYSTEMS: The meter records packets by the MAC address of
    the Adjacent Systems (neighbouring originator or next-hop).
    (Variants on this table are "report source" or "report sink" only.)
    This strategy might be used by a regional or backbone network which
    wants to know how much aggregate traffic flows to or from its
    subscriber networks.

  - END SYSTEMS: The meter records packets by the IP address pair
    contained in the packet.  (Variants on this table are "report
    source" or "report sink" only.)  This strategy might be used by an
    End System network to get detailed host traffic matrix usage data.

  - TRANSPORT TYPE: The meter records packets by transport address; for
    IP packets this provides usage information for the various IP
    services.

  - HYBRID SYSTEMS: Combinations of the above, e.g.  for one interface
    report End Systems, for another interface report Adjacent Systems.

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    This strategy might be used by an enterprise network to learn
    detail about local usage and use an aggregate count for the shared
    regional network.




7 APPENDICES


7.1 Appendix A: Network Characterisation


Internet users have extraordinarily diverse requirements.  Networks
differ in size, speed, throughput, and processing power, among other
factors.  There is a range of traffic flow measurement capabilities and
requirements.  For traffic flow measurement purposes, the Internet may
be viewed as a continuum which changes in character as traffic passes
through the following representative levels:


        International                    |
        Backbones/National        ----------------
                                 /                \
        Regional/MidLevel     ----------   -----------
                             /     \    \ /     /     \
        Stub/Enterprise     ---   ---   ---   ----   ----
                            |||   |||   |||   ||||   ||||
        End-Systems/Hosts   xxx   xxx   xxx   xxxx   xxxx



Note that mesh architectures can also be built out of these components,
and that these are merely descriptive terms.  The nature of a single
network may encompass any or all of the descriptions below, although
some networks can be clearly identified as a single type.

BACKBONE networks are typically bulk carriers that connect other
networks.  Individual hosts (with the exception of network management
devices and backbone service hosts) typically are not directly connected
to backbones.

REGIONAL networks are closely related to backbones, and differ only in
size, the number of networks connected via each port, and geographical
coverage.  Regionals may have directly connected hosts, acting as hybrid
backbone/stub networks.  A regional network is a SUBSCRIBER to the
backbone.

STUB/ENTERPRISE networks connect hosts and local area networks.
STUB/ENTERPRISE networks are SUBSCRIBERS to regional and backbone
networks.


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END SYSTEMS, colloquially HOSTS, are SUBSCRIBERS to any of the above
networks.

Providing a uniform identification of the SUBSCRIBER in finer
granularity than that of end-system, (e.g.  user/account), is beyond the
scope of the current architecture, although an optional attribute in the
traffic flow measurement record may carry system-specific "accountable
(billable) party" labels so that meters can implement proprietary or
non-standard schemes for the attribution of network traffic to
responsible parties.


7.2 Appendix B: Recommended Traffic Flow Measurement Capabilities


Initial recommended traffic flow measurement conventions are outlined
here according to the following Internet building blocks.  It is
important to understand what complexity reporting introduces at each
network level.  Whereas the hierarchy is described top-down in the
previous section, reporting requirements are more easily addressed
bottom-up.


        End-Systems
        Stub Networks
        Enterprise Networks
        Regional Networks
        Backbone Networks


END-SYSTEMS are currently responsible for allocating network usage to
end-users, if this capability is desired.  From the Internet Protocol
perspective, end-systems are the finest granularity that can be
identified without protocol modifications.  Even if a meter violated
protocol boundaries and tracked higher-level protocols, not all packets
could be correctly allocated by user, and the definition of user itself
varies widely from operating system to operating system (e.g.  how to
trace network usage back to users from shared processes).

STUB and ENTERPRISE networks will usually collect traffic data either by
end- system network address or network address pair if detailed
reporting is required in the local area network.  If no local reporting
is required, they may record usage information in the exit router to
track external traffic only.  (These are the only networks which
routinely use attributes to perform reporting at granularities finer
than end-system or intermediate-system network address.)

REGIONAL networks are intermediate networks.  In some cases, subscribers
will be enterprise networks, in which case the intermediate system
network address is sufficient to identify the regional's immediate


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subscriber.  In other cases, individual hosts or a disjoint group of
hosts may constitute a subscriber.  Then end- system network address
pairs need to be tracked for those subscribers.  When the source may be
an aggregate entity (such as a network, or adjacent router representing
traffic from a world of hosts beyond) and the destination is a singular
entity (or vice versa), the meter is said to be operating as a HYBRID
system.

At the regional level, if the overhead is tolerable it may be
advantageous to report usage both by intermediate system network address
(e.g.  adjacent router address) and by end-system network address or
end-system network address pair.

BACKBONE networks are the highest level networks operating at higher
link speeds and traffic levels.  The high volume of traffic will in most
cases preclude detailed traffic flow measurement.  Backbone networks
will usually account for traffic by adjacent routers' network addresses.



7.3 Appendix C: List of Defined Flow Attributes


This Appendix provides a checklist of the attributes defined to date;
others will be added later as the Traffic Measurement Architecture is
further developed.


   0  Null
   1  Flow Subscript                Integer    Flow table info
   2  Flow Status                   Integer

   4  Source Interface              Integer    Source Address
   5  Source Adjacent Type          Integer
   6  Source Adjacent Address       String
   7  Source Adjacent Mask          String
   8  Source Peer Type              Integer
   9  Source Peer Address           String
  10  Source Peer Mask              String
  11  Source Trans Type             Integer
  12  Source Trans Address          String
  13  Source Trans Mask             String

  14  Destination Interface         Integer    Destination Address
  15  Destination Adjacent Type     Integer
  16  Destination Adjacent Address  String
  17  Destination AdjacentMask      String
  18  Destination PeerType          Integer
  19  Destination PeerAddress       String
  20  Destination PeerMask          String
  21  Destination TransType         Integer


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  22  Destination TransAddress      String
  23  Destination TransMask         String

  24  Packet Scale Factor           Integer    'Other' attributes
  25  Byte Scale Factor             Integer
  26  Rule Set Number               Integer

  27  Forward Bytes                 Counter    Source-to-Dest counters
  28  Forward Packets               Counter
  29  Reverse Bytes                 Counter    Dest-to-Source counters
  30  Reverse Packets               Counter
  31  First Time                    TimeTicks  Activity times
  32  Last Active Time              TimeTicks
  33  Source Subscriber ID          String     Session attributes
  34  Destination Subscriber ID     String
  35  Session ID                    String

  36  Source Class                  Integer    'Computed' attributes
  37  Destination Class             Integer
  38  Flow Class                    Integer
  39  Source Kind                   Integer
  40  Destination Kind              Integer
  41  Flow Kind                     Integer

  50  MatchingStoD                  Integer    PME variable

  51  V1                            Integer    Meter variables
  52  V2                            Integer
  53  V3                            Integer
  54  V4                            Integer
  55  V5                            Integer



  65
  ..  'Extended' attributes (to be defined by the RTFM working group)
 127



7.4 Appendix D: List of Meter Control Variables


      Current Rule Set Number       Integer
      Standby Rule Set Number       Integer
      High Water Mark               Percentage
      Flood Mark                    Percentage
      Inactivity Timeout (seconds)  Integer
      Last Collect Time             TimeTicks




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8 Security Considerations



8.1 Threat Analysis


A traffic flow measurement system may be subject to the following kinds
of attacks:


  - UNAUTHORIZED USE OF SYSTEM RESOURCES: An attacker may wish to gain
    advantage or cause mischief (e.g.  denial of service) by subverting
    any of the system elements - meters, meter readers or managers.

  - UNAUTHORIZED DISCLOSURE OF DATA: Any data that is sensitive to
    disclosure can be read through active or passive attacks unless it
    is suitably protected.  Usage data may or may not be of this type.
    Control messages, traps, etc.  are not likely to be considered
    sensitive to disclosure.

  - UNAUTHORIZED ALTERATION, REPLACEMENT OR DESTRUCTION OF DATA:
    Similarly, any data whose integrity is sensitive can be altered,
    replaced/injected or deleted through active or passive attacks
    unless it is suitably protected.  Attackers may modify message
    streams to falsify usage data or interfere with the proper
    operation of the traffic flow measurement system.  Therefore, all
    messages, both those containing usage data and those containing
    control data, should be considered vulnerable to such attacks.



8.2 Countermeasures


The following countermeasures are recommended to address the possible
threats enumerated above:


  - UNAUTHORIZED USE OF SYSTEM RESOURCES is countered through the use
    of authentication and access control services.

  - UNAUTHORIZED DISCLOSURE OF DATA is countered through the use of a
    confidentiality (encryption) service.

  - UNAUTHORIZED ALTERATION, REPLACEMENT OR DESTRUCTION OF DATA is
    countered through the use of an integrity service.


An Internet Accounting system must address all of these concerns.  Since
a high degree of protection is required, the use of strong cryptographic


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methodologies is recommended.  The security requirements for
communication between pairs of accounting system elements are summarized
in the table below.  It is assumed that meters do not communicate with
other meters, and that meter readers do not communicate directly with
other meter readers (if synchronization is required, it is handled by
the manager, see Section 2.5).  Each entry in the table indicates which
kinds of security services are required.  Basically, the requirements
are as follows:



           Security Service Requirements for RTFM elements

  +------------------------------------------------------------------+
  | from\to |    meter     | meter reader | application |  manager   |
  |---------+--------------+--------------+-------------+------------|
  | meter   |     N/A      |  authent     |     N/A     |  authent   |
  |         |              |  acc ctrl    |             |  acc ctrl  |
  |         |              |  integrity   |             |            |
  |         |              |  confid **   |             |            |
  |---------+--------------+--------------+-------------+------------|
  | meter   |   authent    |     N/A      |  authent    |  authent   |
  | reader  |   acc ctrl   |              |  acc ctrl   |  acc ctrl  |
  |         |              |              |  integrity  |            |
  |         |              |              |  confid **  |            |
  |---------+--------------+--------------+-------------+------------|
  | appl    |     N/A      |  authent     |             |            |
  |         |              |  acc ctrl    |     ##      |    N/A     |
  |---------+--------------+--------------+-------------+------------|
  | manager |  authent     |  authent     |     N/A     |  authent   |
  |         |  acc ctrl    |  acc ctrl    |             |  acc ctrl  |
  |         |  integrity   |  integrity   |             |  integrity |
  +------------------------------------------------------------------+

     N/A = Not Applicable    ** = optional    ## = outside RTFM scope


  - When any two elements intercommunicate they should mutually
    authenticate themselves to one another.

  - Whenever there is a transfer of information its integrity should be
    protected.

  - Whenever there is a transfer of usage data it should be possible to
    ensure its confidentiality if it is deemed sensitive to disclosure.


Security protocols are not specified in this document.  The system
elements' management and collection protocols are responsible for
providing sufficient data integrity, confidentiality, authentication and
access control services.


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9 Acknowledgments


An initial draft of this document was produced under the auspices of the
IETF's Internet Accounting Working Group with assistance from SNMP, RMON
and SAAG working groups.  This version documents the implementation work
done by the Internet Accounting Working Group, and is intended to
provide a starting point for the Realtime Traffic Flow Measurement
Working Group.  Particular thanks are due to Stephen Stibler (IBM
Research) for his patient and careful comments during the preparation of
this draft.



10 References


    [1] Mills, C., Hirsch, G. and Ruth, G., "Internet Accounting
    Background", RFC 1272, Bolt Beranek and Newman Inc., Meridian
    Technology Corporation, November 1991.

    [2] International Standards Organisation (ISO), "Management
    Framework," Part 4 of Information Processing Systems Open
    Systems Interconnection Basic Reference Model, ISO 7498-4,
    1994.

    [3] IEEE 802.3/ISO 8802-3 Information Processing Systems -
    Local Area Networks - Part 3:  Carrier sense multiple access
    with collision detection (CSMA/CD) access method and physical
    layer specifications, 2nd edition, September 21, 1990.

    [4] Brownlee, N., "Traffic Flow Measurement:  Meter MIB,"
    Internet Draft, 'Working draft' to become an experimental RFC.




















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11 Author's Addresses


    Nevil Brownlee
    Information Technology Systems & Services
    The University of Auckland

    Phone: +64 9 373 7599 x8941
    E-mail: n.brownlee @auckland.ac.nz


    Cyndi Mills
    BBN Systems and Technologies

    Phone: +1 617 873 4143
    E-mail: cmills@bbn.com


    Greg Ruth
    GTE Laboratories, Inc

    Phone: +1 617 466 2448
    E-mail: gruth@gte.com




























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