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rfc:rfc2722

Network Working Group N. Brownlee Request for Comments: 2722 The University of Auckland Obsoletes: 2063 C. Mills Category: Informational GTE Laboratories, Inc

                                                               G. Ruth
                                                   GTE Internetworking
                                                          October 1999
               Traffic Flow Measurement: Architecture

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) The Internet Society (1999).  All Rights Reserved.

Abstract

 This document provides a general framework for describing network
 traffic flows, presents an architecture for traffic flow measurement
 and reporting, discusses how this relates to an overall network
 traffic flow architecture and indicates how it can be used within the
 Internet.

Table of Contents

 1  Statement of Purpose and Scope                                   3
    1.1  Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
 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  . . . . . . . . . .  7
    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 . . . . . . . . . . . . . . . . 10
    3.2  Granularity of Flow Measurements . . . . . . . . . . . . . 13
    3.3  Rolling Counters, Timestamps, Report-in-One-Bucket-Only  . 15

Brownlee, et al. Informational [Page 1] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 4  Meters                                                          17
    4.1  Meter Structure  . . . . . . . . . . . . . . . . . . . . . 17
    4.2  Flow Table . . . . . . . . . . . . . . . . . . . . . . . . 19
    4.3  Packet Handling, Packet Matching . . . . . . . . . . . . . 20
    4.4  Rules and Rule Sets  . . . . . . . . . . . . . . . . . . . 23
    4.5  Maintaining the Flow Table . . . . . . . . . . . . . . . . 28
    4.6  Handling Increasing Traffic Levels . . . . . . . . . . . . 29
 5  Meter Readers                                                   30
    5.1  Identifying Flows in Flow Records  . . . . . . . . . . . . 30
    5.2  Usage Records, Flow Data Files . . . . . . . . . . . . . . 30
    5.3  Meter to Meter Reader:  Usage Record Transmission  . . . . 31
 6  Managers                                                        32
    6.1  Between Manager and Meter:  Control Functions  . . . . . . 32
    6.2  Between Manager and Meter Reader:  Control Functions . . . 33
    6.3  Exception Conditions . . . . . . . . . . . . . . . . . . . 35
    6.4  Standard Rule Sets . . . . . . . . . . . . . . . . . . . . 36
 7  Security Considerations                                         36
    7.1  Threat Analysis  . . . . . . . . . . . . . . . . . . . . . 36
    7.2  Countermeasures  . . . . . . . . . . . . . . . . . . . . . 37
 8  IANA Considerations                                             39
    8.1  PME Opcodes  . . . . . . . . . . . . . . . . . . . . . . . 39
    8.2  RTFM Attributes  . . . . . . . . . . . . . . . . . . . . . 39
 9  APPENDICES                                                      41
    Appendix A: Network Characterisation  . . . . . . . . . . . . . 41
    Appendix B: Recommended Traffic Flow Measurement Capabilities . 42
    Appendix C: List of Defined Flow Attributes . . . . . . . . . . 43
    Appendix D: List of Meter Control Variables . . . . . . . . . . 44
    Appendix E: Changes Introduced Since RFC 2063 . . . . . . . . . 45
 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 45
 11 References  . . . . . . . . . . . . . . . . . . . . . . . . . . 46
 12 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . 47
 13 Full Copyright Statement  . . . . . . . . . . . . . . . . . . . 48

Brownlee, et al. Informational [Page 2] RFC 2722 Traffic Flow Measurement: Architecture October 1999

1 Statement of Purpose and Scope

1.1 Introduction

 This document describes an architecture for traffic flow measurement
 and reporting for data networks which has the following
 characteristics:
  1. The traffic flow model can be consistently applied to any

protocol, using address attributes in any combination at the

     'adjacent' (see below), network and transport layers of the
     networking stack.
  1. 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.
  1. Users may specify their traffic flow measurement requirements by

writing 'rule sets', allowing them to collect the flow data they

     need while ignoring other traffic.
  1. The data reduction effort to produce requested traffic flow

information is placed as near as possible to the network

     measurement point.  This minimises the volume of data to be
     obtained (and transmitted across the network for storage), and
     reduces the amount of processing required in traffic flow
     analysis applications.
 'Adjacent' (as used above) is a layer-neutral term for the next layer
 down in a particular instantiation of protocol layering. Although
 'adjacent' will usually imply the link layer (MAC addresses), it does
 not implicitly advocate or dismiss any particular form of tunnelling
 or layering.
 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:
  1. Understanding the behaviour of existing networks,
  1. Planning for network development and expansion,
  1. Quantification of network performance,
  1. Verifying the quality of network service, and
  1. Attribution of network usage to users.

Brownlee, et al. Informational [Page 3] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 The traffic flow measurement architecture is deliberately structured
 using address attributes which are defined in a consistent way at the
 Adjacent, Network and Transport layers of the networking stack,
 allowing specific implementations of the architecture to be used
 effectively in multi-protocol environments.  Within this document the
 term 'usage data' is used as a generic term for the data obtained
 using the traffic flow measurement architecture.
 In principle one might define address attributes for higher layers,
 but it would be very difficult to do this in a general way.  However,
 if an RTFM traffic meter were implemented within an application
 server (where it had direct access to application-specific usage
 information), it would be possible to use the rest of the RTFM
 architecture to collect application-specific information.  Use of the
 same model for both network- and application-level measurement in
 this way could simplify the development of generic analysis
 applications which process and/or correlate both traffic and usage
 information.  Experimental work in this area is described in the RTFM
 'New Attributes' document [RTFM-NEW].
 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 'Internet Accounting Background' RFC [ACT-BKG].

Brownlee, et al. Informational [Page 4] RFC 2722 Traffic Flow Measurement: Architecture October 1999

2 Traffic Flow Measurement Architecture

 A traffic flow measurement system is used by Network Operations
 personnel to aid in 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 [OSI-
 ACT].

2.1 Meters and Traffic Flows

 At the heart of the traffic measurement model are network entities
 called traffic METERS.  Meters observe packets as they pass by a
 single point on their way through the network and classify them into
 certain groups.  For each such group a meter will accumulate certain
 attributes, for example the numbers of packets and bytes observed for
 the group.  These METERED TRAFFIC GROUPS may correspond to a user, a
 host system, a network, a group of networks, a particular transport
 address (e.g. an IP port number), any combination of the above, etc,
 depending on 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 'Internet Accounting
 Background' RFC [ACT-BKG]. An important aspect of meters is that they
 provide a way of succinctly aggregating traffic information.
 For the purpose of traffic flow measurement we define the concept of
 a TRAFFIC FLOW, which is like an artificial logical equivalent to a
 call or connection.  A flow is a portion of traffic, delimited by a
 start and stop time, that belongs to one of the metered traffic
 groups mentioned above.  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

Brownlee, et al. Informational [Page 5] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 which flow they belong to.  Classifying packets into 'flows' in this
 way provides an economical and practical way to measure network
 traffic and subdivide it into well-defined groups.
 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 may 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 APPLICATIONS, 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
  1. MANAGER: A traffic measurement manager is an application which

configures 'meter' entities and controls 'meter reader' entities.

     It sends configuration commands to the meters, and supervises the
     proper operation of each meter and meter reader.  It may well be
     convenient to combine the functions of meter reader and manager
     within a single network entity.
  1. 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'.
  1. METER READER: A meter reader transports usage data from meters so

that it is available to analysis applications.

Brownlee, et al. Informational [Page 6] RFC 2722 Traffic Flow Measurement: Architecture October 1999

  1. 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:
  1. TRAFFIC FLOW MATRICES, showing the total flow rates for many

of the possible paths within an internet.

  1. FLOW RATE FREQUENCY DISTRIBUTIONS, summarizing flow rates

over a period of time.

  1. 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.

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:
  1. 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.
  1. Meter control parameters, e.g. 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).

Brownlee, et al. Informational [Page 7] RFC 2722 Traffic Flow Measurement: Architecture October 1999

  1. Sampling behaviour. Normally every packet will be observed. It

may sometimes be necessary to use sampling techniques so as to

     observe only some of the packets (see following note).
 A note about sampling: Current experience with the measurement
 architecture shows that a carefully-designed and implemented meter
 compresses the data sufficiently well that in normal LANs and WANs of
 today sampling is seldom, if ever, needed.  For this reason sampling
 algorithms are not prescribed by the architecture.  If sampling is
 needed, e.g. for metering a very-high-speed network with fine-grained
 flows, the sampling technique should be carefully chosen so as not to
 bias the results.  For a good introduction to this topic see the IPPM
 Working Group's RFC "Framework for IP Performance Metrics" [IPPM-
 FRM].
 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
 after it downloads that rule set.
 One manager should be designated as the 'master' for a meter.
 Parameters such as sampling behaviour, 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:
  1. The meter's unique identity, i.e. its network name or address.
  1. How often usage data is to be collected from the meter.
  1. Which flow records are to be collected (e.g. all flows, flows for

a particular rule set, flows which have been active since a given

     time, etc.).
  1. 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.

Brownlee, et al. Informational [Page 8] RFC 2722 Traffic Flow Measurement: Architecture October 1999

2.5 Multiple METERs or METER READERs

  1. - 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 meter 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 from both meters.
 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 necesarily 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 identical usage records were required from a single meter, a
 manager could achieve this using two identical copies of a ruleset in
 that meter.  Let's call them RS1 and RS2, and assume that RS1 is
 running.  When a collection is to be made the manager switches the
 meter from RS1 to RS2, and directs the meter reader(s) to read flow
 data for RS1 from the meter.  For the next collection the manager
 switches back to RS1, and so on.  Note, however, that it is not
 possible to get identical usage records from more than one meter,
 since there is no way for a manager to switch rulesets in more than
 one meter at the same time.
 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

Brownlee, et al. Informational [Page 9] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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 a (user-specieied) METERED TRAFFIC
     GROUP."
 In practical terms, a flow is a stream of packets observed by the
 meter as they pass across a network between two end points (or from a
 single end point), which have been summarized by a traffic meter for
 analysis purposes.

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:

Brownlee, et al. Informational [Page 10] RFC 2722 Traffic Flow Measurement: Architecture October 1999

  1. 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 header), and
     the number of the interface on which the packet was observed.
  1. First and last TIMES when packets were seen for this flow, i.e.

the 'creation' and 'last activity' times for the flow.

  1. 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.
  1. OTHER attributes, e.g. the index of the flow's record in the flow

table and the rule set number 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.
 The attributes listed in this document (Appendix C) provide a basic
 (i.e. useful minimum) set; IANA considerations for allocating new
 attributes are set out in section 8 below.
 A flow's METERED TRAFFIC GROUP is specified by the values of its
 ADDRESS attributes.  For example, if a flow's address attributes were
 specified as "source address = IP address 10.1.0.1, destination
 address = IP address 26.1.0.1" then only IP packets from 10.1.0.1 to
 26.1.0.1 and back would be counted in that flow.  If a flow's address
 attributes specified only that "source address = IP address
 10.1.0.1," then all IP packets from and to 10.1.0.1 would be counted
 in that flow.
 The addresses specifying a flow's address attributes may include one
 or more of the following types:
  1. 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.
  1. The ADJACENT ADDRESS, i.e. the address in the the next layer down

from the peer address in a particular instantiation of protocol

     layering.  Although 'adjacent' will usually imply the link layer,
     it does not implicitly advocate or dismiss any particular form of
     tunnelling or layering.

Brownlee, et al. Informational [Page 11] RFC 2722 Traffic Flow Measurement: Architecture October 1999

     For example, if flow measurement is being performed using IP as
     the network layer on an Ethernet LAN [802-3], an adjacent address
     will normally be 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.
  1. The PEER ADDRESS, which identifies the source or destination of

the packet for the network layer (n) at which traffic measurement

     is being performed.  The form of a peer address will depend on
     the network-layer protocol in use, and the measurement network
     layer (n).
  1. 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.
 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 'Internet Accounting Background' RFC [ACT-BKG]. 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.

Brownlee, et al. Informational [Page 12] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 In cases where network addresses are dynamically allocated (e.g.
 dial-in subscribers), further subscriber identification will be
 necessary if flows are to ascribed to individual users.  Provision is
 made to further specify the metered traffic group 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 unspecified means within a meter.  At this time a
 subscriber ID is an arbitrary text string; later versions of the
 architecture may specify details of its contents.

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 its level of detail.  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 their
 attributes.  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.
 Time granularity may be controlled by varying the reporting interval,
 i.e. the time between meter readings.
 Flow granularity is controlled by adjusting the level of detail for
 the following:
  1. The metered traffic group (address attributes, discussed above).
  1. The categorisation of packets (other attributes, discussed

below).

  1. 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
 metered traffic group 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

Brownlee, et al. Informational [Page 13] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 must be identifiable via a RULE SET NUMBER. Granularity of metered
 traffic groups is further specified by additional ATTRIBUTES. These
 attributes include:
  1. 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.
  1. Administratively specified attributes such as Quality of Service

and Priority, etc. 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.
 A rule set can aggregate groups of addresses in two ways.  The
 simplest is to use a mask in a single rule to test for an address
 within a masked group.  The other way is to use a sequence of rules
 to test for an arbitrary group of (masked) address values, then use a
 PushRuleTo rule to set a derived attribute (e.g. FlowKind) to
 indicate the flow's group.
 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 should be sure
 that the flow's data has been collected by all meter readers which
 registered to collect it.  These two wait conditions are desired
 goals for the meter; they are not difficult to achieve in normal
 usage, however the meter cannot guarantee to fulfil them absolutely.

Brownlee, et al. Informational [Page 14] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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.

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), 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.  A flow which has sporadic
 bursts of activity interspersed with long periods of inactivity will
 produce a sequence of flow activity records, each with the same set
 of address attributes, but with increasing FLOW START times.

Brownlee, et al. Informational [Page 15] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 Each packet is counted in at most one flow for each running ruleset,
 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.
 Alternatively, multiple rulesets could be used to collect data at
 different granularities.
 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', using a
 single rule set.  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 were 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.
 Alternatively, the above could be achieved by running two rule sets
 (A and B), as follows:
         Bucket 1:  Packets which came in interface 1;
                    counted by rule set A.

Brownlee, et al. Informational [Page 16] RFC 2722 Traffic Flow Measurement: Architecture October 1999

         Bucket 2:  Packets which were sourced by IP address a.b.c.d;
                    counted by rule set B.

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:
  1. A dedicated small host, connected to a broadcast 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.
  1. 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.
  1. 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:
 Briefly, the meter works as follows:
  1. Incoming packet headers arrive at the top left of the diagram and

are passed to the PACKET PROCESSOR.

  1. The packet processor passes them to the Packet Matching Engine

(PME) where they are classified.

  1. The PME is a Virtual Machine running a pattern matching program

contained in the CURRENT RULE SET. It is invoked by the Packet

     Processor, executes the rules in the current rule set as
     described in section 4.3 below, and returns instructions on what
     to do with the packet.
  1. Some packets are classified as 'to be ignored'. They are

discarded by the Packet Processor.

Brownlee, et al. Informational [Page 17] RFC 2722 Traffic Flow Measurement: Architecture October 1999

  1. Other packets are matched by the PME, which returns a FLOW KEY

describing the flow to which the packet belongs.

  1. 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 data fields (e.g. packet and byte counters) are updated.
  1. 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.
                 packet                     +------------------+
                 header                     | Current Rule Set |
                   |                        +--------+---------+
                   |                                 |
                   |                                 |
           +-------*--------+    'match key'  +------*-------+
           |    Packet      |---------------->|    Packet    |
           |   Processor    |                 |   Matching   |
           |                |<----------------|    Engine    |
           +--+----------+--+  'flow key'     +--------------+
              |          |
              |          |
       Ignore *          | Count (via 'flow key')
                         |
                      +--*--------------+
                      | 'Search' index  |
                      +--------+--------+
                               |
                      +--------*--------+
                      |                 |
                      |   Flow Table    |
                      |                 |
                      +--------+--------+
                               |
                      +--------*--------+
                      | 'Collect' index |
                      +--------+--------+
                               |
                               *
                          Meter Reader
 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

Brownlee, et al. Informational [Page 18] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 current ruleset, producing a single flow table containing flows from
 all the rule sets.  One way to implement this is to use the Rule Set
 Number attribute in each flow as part of the flow key.
 A packet may only be counted once in a rule set (as explained in
 section 3.3 above), but it may be counted in any of the current
 rulesets.  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 'flow table', i.e. a table of TRAFFIC
 FLOW RECORDS for flows seen by the meter.  Details of how the flow
 table is maintained are given in section 4.5 below.  A flow record
 contains attribute values for its flow, including:
  1. Addresses for the flow's source and destination. These include

addresses and masks for various network layers (extracted from

     the packet header), and the identity of the interface on which
     the packet was observed.
  1. First and last times when packets were seen for this flow.
  1. Counts for 'forward' (source to destination) and 'backward'

(destination to source) components of the flow's traffic.

  1. Other attributes, e.g. state of the flow record (discussed

below).

 The state of a flow record may be:
  1. INACTIVE: The flow record is not being used by the meter.
  1. 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.
  1. 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.

Brownlee, et al. Informational [Page 19] RFC 2722 Traffic Flow Measurement: Architecture October 1999

4.3 Packet Handling, Packet Matching

 Each packet header received by the traffic meter program is processed
 as follows:
  1. Extract attribute values from the packet header and use them to

create a MATCH KEY for the packet.

  1. 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 data for
 the matching flow record can then be updated.
 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 source (S) and destination (D)
 interface identities, address values and masks.
 If measured flows were unidirectional, i.e. only counted packets
 travelling in one direction, the matching process would be simple.
 The 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.

Brownlee, et al. Informational [Page 20] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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 header, and D
 indicates its destination address.
 There are several cases to consider.  These are:
  1. The packet is recognised as one which is TO BE IGNORED.
  1. The packet would MATCH IN EITHER DIRECTION. 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.
  1. 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.
                 Ignore
 --- match(S->D) -------------------------------------------------+
      | Suc   | NoMatch                                           |
      |       |          Ignore                                   |
      |      match(D->S) -----------------------------------------+
      |       | Suc   | NoMatch                                   |
      |       |       |                                           |
      |       |       +-------------------------------------------+
      |       |                                                   |
      |       |             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) --------------+
                                                                  |
                                                                  *

Brownlee, et al. Informational [Page 21] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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.
 'NoMatch' 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).
 When writing rule sets one must remember that the meter will normally
 try to match each packet in the reverse direction if the forward
 match does not succeed.  It is particularly important that the rule
 set does not contain inconsistencies which will upset this process.
 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.  An example of such a language is
 described in the 'SRL' document [RTFM-SRL]. Another approach would be
 to write a 'rule set consistency checker' program, which could detect
 problems in hand-written rule sets.

Brownlee, et al. Informational [Page 22] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 Normally, the best way to avoid these problems is to write rule sets
 which only classify flows in the forward direction, and rely on the
 meter to handle reverse-travelling packets.
 Occasionally there can be situations when a rule set needs to know
 the direction in which a packet is being matched.  Consider, for
 example, a rule set which wants to save some attribute values (source
 and destination addresses perhaps) for any 'unusual' packets.  The
 rule set will contain a sequence of tests for all the 'usual' source
 addresses, follwed by a rule which will execute a 'NoMatch' action.
 If the match fails in the S->D direction, the NoMatch action will
 cause it to be retried.  If it fails in the D->S direction, the
 packet can be counted as an 'unusual' packet.
 To count such an 'unusual' packet we need to know the matching
 direction: the MatchingStoD attribute provides this.  To use it, one
 follows the source address tests with a rule which tests whether the
 matching direction is S->D (MatchingStoD value is 1).  If so, a
 'NoMatch' action is executed.  Otherwise, the packet has failed to
 match in both directions; we can save whatever attribute values are
 of interest and count the 'unusual' packet.

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.
 Rule set 1 (the first entry in the rule set table) 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.
 A meter also maintains an array of 'tasks', which specify what rule
 sets the meter is running.  Each task has a 'current' rule set (the
 one which it normally uses), and a 'standby' rule set (which will be
 used when the overall traffic level is unusually high).  If a task 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 an instruction for the Packet Matching
 Engine, i.e. it is an instruction for a Virtual Machine.  PME
 instructions have five component fields, forming two logical groups
 as follows:
    +-------- test ---------+    +---- action -----+
    attribute & mask = value:    opcode,  parameter;

Brownlee, et al. Informational [Page 23] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 The test group allows PME to test the value of an attribute.  This is
 done by ANDing the attribute value with the mask and comparing the
 result with the value field.  Note that there is no explicit
 provision to test a range, although this can be done where the range
 can be covered by a mask, e.g. attribute value less than 2048.
 The PME maintains a Boolean indicator called the 'test indicator',
 which determines whether or not a rule's test is performed.  The test
 indicator is initially set (true).
 The action group specifies what action may be performed when the rule
 is executed.  Opcodes contain two flags: 'goto' and 'test', as
 detailed in the table below.  Execution begins with rule 1, the first
 in the rule set.  It proceeds as follows:
    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 NoMatch.
    If the test indicator is false, or the test (above) succeeded:
       Set the test indicator to this opcode'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.  Note that when the Ignore, NoMatch, Count and CountPkt
 actions are performed, PME execution is terminated regardless of
 whether the PME is executing a subroutine ('return' stack is non-
 empty) or not.
 The second data structure, 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
 number, mask and value information from the pattern queue in the
 order it was enqueued.

Brownlee, et al. Informational [Page 24] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 An attribute number identifies the attribute actually used in a test.
 It will usually be the rule's attribute field, unless the attribute
 is a 'meter variable'.  Details of meter variables are given after
 the table of opcode actions below.
 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
      16  PopTo            1       1
      17  PopToAct         1       0
 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 number,
                 mask and value in the PME's pattern queue, then
                 construct a flow key for the flow to which this
                 packet belongs.  Return from the match() function
                 indicating success.  The meter will use the flow
                 key to search for the flow record for this
                 packet's flow.
 CountPkt:       As for Count, except that the masked value from
                 the packet header (as it would have been used in
                 the rule's test) is saved in the PME's pattern
                 queue instead of the rule's value.

Brownlee, et al. Informational [Page 25] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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
                 parameter value specified for 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.
 GotoAct:        Clear the test indicator then goto the specified
                 rule.
 PushRuleTo:     Save this rule's attribute number, 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.

Brownlee, et al. Informational [Page 26] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 PushPktTo:      Save this rule's attribute number, mask, and the
                 masked value from the packet header (as it would
                 have been used in the rule's test), in the PME's
                 pattern queue.  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 header using a specified mask.  The
                 simplest way to program this is to use a zero value
                 for the PushPktTo rule's value field, and to
                 GoToAct to the PushPktTo rule (so that it's test is
                 not executed).
 PopTo:          Delete the most recent item from the pattern
                 queue, so as to remove the information saved by
                 an earlier 'push' action.  Set the test indicator
                 then goto the specified rule.
 PopToAct:       Same as PopTo, except that the test indicator
                 is cleared before going to the specified rule.
 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 zero
                    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 number 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.

Brownlee, et al. Informational [Page 27] RFC 2722 Traffic Flow Measurement: Architecture October 1999

    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.
 The opcodes detailed above (with their above 'goto' and 'test'
 values) form a minimum set, but one which has proved very effective
 in current meter implementations.  From time to time it may be useful
 to add further opcodes; IANA considerations for allocating these are
 set out in section 8 below.

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 matched for a flow which is not in a current
 flow set a flow record is created 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 an 'inactive' record.  If no inactive
 records are available it will search for an 'idle' one instead.  Note
 that there is no particular significance in the ordering of records
 within the flow table.
 A meter's memory management routines should aim to minimise the time
 spent finding flow records for new flows, so as to minimise the setup
 overhead associated with each new flow.
 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 ReaderLastTime variables which remember the time the last
 collection was made by each reader.  A second variable,
 InactivityTime, specifies the minimum time the meter will wait before
 considering that a flow is idle.

Brownlee, et al. Informational [Page 28] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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 by all readers
 which have registered to do so.  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.
 Another recovery strategy is to leave idle flows alone as long as
 possible, which would be acceptable 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 low on '
 inactive' flow records.
 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.
 Of course a meter reader may fail, so the meter cannot wait forever
 for it.  Instead the meter must keep a table of active meter readers,
 with a timeout specified for each.  If a meter reader fails to
 collect flow data within its timeout interval, the meter should
 delete that reader from the meter's active meter reader table.

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,
 however, memory usage rises above the high-water mark the meter
 should switch to a STANDBY RULE SET so as to 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.  This would shorten the time that flows
 sit in memory waiting to be collected, allowing the meter to free
 flow memory faster.
 The meter could 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.

Brownlee, et al. Informational [Page 29] RFC 2722 Traffic Flow Measurement: Architecture October 1999

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 stable storage as 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.
 Note that the combination of start time, rule set number and flow
 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.

Brownlee, et al. Informational [Page 30] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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.  Other quantities, e.g. short-term flow
 rates, may be added later; work on such extensions is described in
 the RTFM 'New Attributes' document [RTFM-NEW].
 Each usage record contains the metered traffic group identifier of
 the meter (a set of network addresses), 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         |
 +-------------------------------------------------------------------+

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 and idle flows).  One possible
 method of achieving this transfer is to use SNMP; the 'Traffic Flow
 Measurement:  Meter MIB' RFC [RTFM-MIB] 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.

Brownlee, et al. Informational [Page 31] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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.

6.1 Between Manager and Meter: Control Functions

  1. 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; this is a diagnostic feature, ensuring that
     when a meter starts up it will be running a known ruleset.
     All other rule sets 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.
  1. SPECIFY METER TASK: Once the rule sets have been downloaded, the

manager must instruct the meter which rule sets will be the

     'current' and 'standby' ones for each task the meter is to
     perform.
  1. SET HIGH WATER MARK: A percentage of the flow table capacity,

used by the meter to determine 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

Brownlee, et al. Informational [Page 32] RFC 2722 Traffic Flow Measurement: Architecture October 1999

     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).  The meter has a separate high
     water mark value for each task it is currently running.
     If the high traffic levels persist, the meter's normal rule set
     may have to be rewritten to permanently reduce the reporting
     granularity.
  1. 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:
  1. Flows that have already been reported to all registered meter

readers, and show no activity since the last report,

  1. Oldest flows, or
  2. Flows with the smallest number of observed packets.
  1. 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.
 It might be useful if a manager could set the FLOW TERMINATION
 PARAMETERS to different values for different tasks.  Current meter
 implementations have only single ('whole meter') values for these
 parameters, and experience to date suggests that this provides an
 adequate degree of control for the tasks.

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
 collection interval must be decreased or a coarser granularity of
 aggregation must be used to reduce the number of active flows.
 Increasing the collection 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

Brownlee, et al. Informational [Page 33] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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:
  1. MANAGER and METER READER IDENTIFICATION: The manager should

ensure that meters are read by 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.
  1. 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 should 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).
  1. 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:
  1. Controls the amount of address information identifying each

flow, and

  1. 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.

Brownlee, et al. Informational [Page 34] RFC 2722 Traffic Flow Measurement: Architecture October 1999

  1. 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 space for flow data.  Since - to prevent an active
 task from counting any packet twice - packets can only be counted in
 a single flow, discarding records will result in the loss of
 information.  The mechanisms to deal with this are as follows:
  1. 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 data 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.
  1. 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.
  1. MANAGER OUTAGES: If the manager fails for any reason, the meter

should continue measuring and the meter reader(s) should keep

     gathering usage records.
  1. 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:
  1. The reporting interval is too infrequent, or
  2. The reporting granularity is too fine.

Brownlee, et al. Informational [Page 35] RFC 2722 Traffic Flow Measurement: Architecture October 1999

     Either of these may be exacerbated by low throughput or bandwidth
     of circuits carrying the usage data.  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:
  1. PROTOCOL TYPE: The meter records packets by protocol type. This

will be the default rule table for Traffic Flow Meters.

  1. 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.
  1. 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.
  1. TRANSPORT TYPE: The meter records packets by transport address;

for IP packets this provides usage information for the various IP

     services.
  1. HYBRID SYSTEMS: Combinations of the above, e.g. for one interface

report End Systems, for another interface report Adjacent

     Systems.  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 Security Considerations

7.1 Threat Analysis

 A traffic flow measurement system may be subject to the following
     kinds of attacks:
  1. ATTEMPTS TO DISABLE A TRAFFIC METER: An attacker may attempt to

disrupt traffic measurement so as to prevent users being charged

     for network usage.  For example, a network probe sending packets

Brownlee, et al. Informational [Page 36] RFC 2722 Traffic Flow Measurement: Architecture October 1999

     to a large number of destination and transport addresses could
     produce a sudden rise in the number of flows in a meter's flow
     table, thus forcing it to use its coarser standby rule set.
  1. 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.
  1. 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.
  1. 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.

7.2 Countermeasures

 The following countermeasures are recommended to address the possible
 threats enumerated above:
  1. ATTEMPTS TO DISABLE A TRAFFIC METER can't be completely

countered. In practice, flow data records from network security

     attacks have proved very useful in determining what happened.
     The most effective approach is first to configure the meter so
     that it has three or more times as much flow memory as it needs
     in normal operation, and second to collect the flow data fairly
     frequently so as to minimise the time needed to recover flow
     memory after such an attack.
  1. UNAUTHORIZED USE OF SYSTEM RESOURCES is countered through the use

of authentication and access control services.

  1. UNAUTHORIZED DISCLOSURE OF DATA is countered through the use of a

confidentiality (encryption) service.

  1. UNAUTHORIZED ALTERATION, REPLACEMENT OR DESTRUCTION OF DATA is

countered through the use of an integrity service.

Brownlee, et al. Informational [Page 37] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 A Traffic Measurement system must address all of these concerns.
 Since a high degree of protection is required, the use of strong
 cryptographic methodologies is recommended.  The security
 requirements for communication between pairs of traffic measurmement
 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    |     ##      |    ##      |
|---------+--------------+--------------+-------------+------------|
| manager |  authent     |  authent     |     ##      |  authent   |
|         |  acc ctrl    |  acc ctrl    |             |  acc ctrl  |
|         |  integrity   |  integrity   |             |  integrity |
+------------------------------------------------------------------+
   N/A = Not Applicable    ** = optional    ## = outside RTFM scope
  1. When any two elements intercommunicate they should mutually

authenticate themselves to one another. This is indicated by '

     authent' in the table.  Once authentication is complete, an
     element should check that the requested type of access is
     allowed; this is indicated on the table by 'acc ctrl'.
  1. Whenever there is a transfer of information its integrity should

be protected.

Brownlee, et al. Informational [Page 38] RFC 2722 Traffic Flow Measurement: Architecture October 1999

  1. Whenever there is a transfer of usage data it should be possible

to ensure its confidentiality if it is deemed sensitive to

     disclosure.  This is indicated by 'confid' in the table.
 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.

8 IANA Considerations

 The RTFM Architecture, as set out in this document, has two sets of
 assigned numbers.  Considerations for assigning them are discussed in
 this section, using the example policies as set out in the
 "Guidelines for IANA Considerations" document [IANA-RFC].

8.1 PME Opcodes

 The Pattern Matching Engine (PME) is a virtual machine, executing
 RTFM rules as its instructions.  The PME opcodes appear in the
 'action' field of an RTFM rule.  The current list of opcodes, and
 their values for the PME's 'goto' and 'test' flags, are set out in
 section 4.4 above ("Rules and Rulesets).
 The PME opcodes are pivotal to the RTFM architecture, since they must
 be implemented in every RTFM meter.  Any new opcodes must therefore
 be allocated through an IETF Consensus action [IANA-RFC].
 Opcodes are simply non-negative integers, but new opcodes should be
 allocated sequentially so as to keep the total opcode range as small
 as possible.

8.2 RTFM Attributes

 Attribute numbers in the range of 0-511 are globally unique and are
 allocated according to an IETF Consensus action [IANA-RFC]. Appendix
 C of this document allocates a basic (i.e. useful minimum) set of
 attribtes; they are assigned 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 64 to 127.
 Vendor-specific attribute numbers are in the range 512-1023, and will
 be allocated using the First Come FIrst Served policy [IANA-RFC].
 Vendors requiring attribute numbers should submit a request to IANA
 giving the attribute names: IANA will allocate them the next
 available numbers.

Brownlee, et al. Informational [Page 39] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 Attribute numbers 1024 and higher are Reserved for Private Use
 [IANA-RFC]. Implementors wishing to experiment with further new
 attributes should use attribute numbers in this range.
 Attribute numbers are simply non-negative integers.  When writing
 specifications for attributes, implementors must give sufficient
 detail for the new attributes to be easily added to the RTFM Meter
 MIB [RTFM-MIB]. In particular, they must indicate whether the new
 attributes may be:
  1. tested in an IF statement
  2. saved by a SAVE statement or set by a STORE statement
  3. read from an RTFM meter
 (IF, SAVE and STORE are statements in the SRL Ruleset Language
 [RTFM-SRL]).

Brownlee, et al. Informational [Page 40] RFC 2722 Traffic Flow Measurement: Architecture October 1999

9 APPENDICES

9.1 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.
 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

Brownlee, et al. Informational [Page 41] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 'user identification' labels so that meters can implement proprietary
 or non-standard schemes for the attribution of network traffic to
 responsible parties.

9.2 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 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.

Brownlee, et al. Informational [Page 42] RFC 2722 Traffic Flow Measurement: Architecture October 1999

 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.

9.3 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.
 Note that this table gives only a very brief summary.  The Meter MIB
 [RTFM-MIB] provides the definitive specification of attributes and
 their allowed values.  The MIB variables which represent flow
 attributes have 'flowData' prepended to their names to indicate that
 they belong to the MIB's flowData table.
     0  Null
     4  SourceInterface        Integer     Source Address
     5  SourceAdjacentType     Integer
     6  SourceAdjacentAddress  String
     7  SourceAdjacentMask     String
     8  SourcePeerType         Integer
     9  SourcePeerAddress      String
    10  SourcePeerMask         String
    11  SourceTransType        Integer
    12  SourceTransAddress     String
    13  SourceTransMask        String
    14  DestInterface          Integer     Destination Address
    15  DestAdjacentType       Integer
    16  DestAdjacentAddress    String
    17  DestAdjacentMask       String
    18  DestPeerType           Integer
    19  DestPeerAddress        String
    20  DestPeerMask           String
    21  DestTransType          Integer
    22  DestTransAddress       String
    23  DestTransMask          String

Brownlee, et al. Informational [Page 43] RFC 2722 Traffic Flow Measurement: Architecture October 1999

    26  RuleSet                Integer     Meter attribute
    27  ToOctets               Integer     Source-to-Dest counters
    28  ToPDUs                 Integer
    29  FromOctets             Integer     Dest-to-Source counters
    30  FromPDUs               Integer
    31  FirstTime              Timestamp   Activity times
    32  LastActiveTime         Timestamp
    33  SourceSubscriberID     String      Session attributes
    34  DestSubscriberID       String
    35  SessionID              String
    36  SourceClass            Integer     'Computed' attributes
    37  DestClass              Integer
    38  FlowClass              Integer
    39  SourceKind             Integer
    40  DestKind               Integer
    41  FlowKind               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

9.4 Appendix D: List of Meter Control Variables

    Meter variables:
       Flood Mark                    Percentage
       Inactivity Timeout (seconds)  Integer
    'per task' variables:
       Current Rule Set Number       Integer
       Standby Rule Set Number       Integer
       High Water Mark               Percentage
    'per reader' variables:
       Reader Last Time              Timestamp

Brownlee, et al. Informational [Page 44] RFC 2722 Traffic Flow Measurement: Architecture October 1999

9.5 Appendix E: 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.
  1. A Traffic Meter can now run multiple rule sets concurrently.

This makes a meter much more useful, and required only minimal

     changes to the architecture.
  1. '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.
  1. 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 some kinds of rule sets.  It was discussed and agreed
     to at the San Jose meeting in 1996.
  1. Computed attributes (Class and Kind) may now be tested within a

rule set. This lifts an unneccessary earlier restriction.

  1. 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).
  1. The 'Security Considerations' section has been completely

rewritten. It provides an evaluation of traffic measurement

     security risks and their countermeasures.

10 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.  Particular thanks are
     due to Stephen Stibler (IBM Research) for his patient and careful
     comments during the preparation of this memo.

Brownlee, et al. Informational [Page 45] RFC 2722 Traffic Flow Measurement: Architecture October 1999

11 References

 [802-3]    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.
 [ACT-BKG]  Mills, C., Hirsch, G. and G. Ruth, "Internet Accounting
            Background", RFC 1272, November 1991.
 [IANA-RFC] Alvestrand, H. and T. Narten, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.
 [IPPM-FRM] Paxson, V., Almes, G., Mahdavi, J. and M. Mathis,
            "Framework for IP Performance Metrics", RFC 2330, May
            1998.
 [OSI-ACT]  International Standards Organisation (ISO), "Management
            Framework", Part 4 of Information Processing Systems Open
            Systems Interconnection Basic Reference Model, ISO 7498-4,
            1994.
 [RTFM-MIB] Brownlee, N., "Traffic Flow Measurement: Meter MIB", RFC
            2720, October 1999.
 [RTFM-NEW] Handelman, S., Stibler, S., Brownlee, N. and G. Ruth,
            "RTFM: New Attributes for Traffic Flow Measurment", RFC
            2724, October 1999.
 [RTFM-SRL] Brownlee, N., "SRL: A Language for Describing Traffic
            Flows and Specifying Actions for Flow Groups", RFC 2723,
            October 1999.

Brownlee, et al. Informational [Page 46] RFC 2722 Traffic Flow Measurement: Architecture October 1999

12 Authors' Addresses

 Nevil Brownlee
 Information Technology Systems & Services
 The University of Auckland
 Private Bag 92-019
 Auckland, New Zealand
 Phone: +64 9 373 7599 x8941
 EMail: n.brownlee@auckland.ac.nz
 Cyndi Mills
 GTE Laboratories, Inc
 40 Sylvan Rd.
 Waltham, MA 02451, U.S.A.
 Phone: +1 781 466 4278
 EMail: cmills@gte.com
 Greg Ruth
 GTE Internetworking
 3 Van de Graaff Drive
 P.O. Box 3073
 Burlington, MA 01803, U.S.A.
 Phone: +1 781 262 4831
 EMail: gruth@bbn.com

Brownlee, et al. Informational [Page 47] RFC 2722 Traffic Flow Measurement: Architecture October 1999

13 Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Brownlee, et al. Informational [Page 48]

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