Network Working Group N. Brownlee
Request for Comments: 2722 The University of Auckland
Obsoletes: 2063 C. Mills
Category: Informational GTE Laboratories, Inc
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 (C) The Internet Society (1999). All Rights Reserved.
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
Table of Contents
1 Statement of Purpose and Scope 31.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 32 Traffic Flow Measurement Architecture 52.1 Meters and Traffic Flows . . . . . . . . . . . . . . . . . 52.2 Interaction Between METER and METER READER . . . . . . . . 72.3 Interaction Between MANAGER and METER . . . . . . . . . . 72.4 Interaction Between MANAGER and METER READER . . . . . . . 82.5 Multiple METERs or METER READERs . . . . . . . . . . . . . 92.6 Interaction Between MANAGERs (MANAGER - MANAGER) . . . . . 102.7 METER READERs and APPLICATIONs . . . . . . . . . . . . . . 103 Traffic Flows and Reporting Granularity 103.1 Flows and their Attributes . . . . . . . . . . . . . . . . 103.2 Granularity of Flow Measurements . . . . . . . . . . . . . 133.3 Rolling Counters, Timestamps, Report-in-One-Bucket-Only . 15
1 Statement of Purpose and Scope
This document describes an architecture for traffic flow measurement
and reporting for data networks which has the following
- 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
- 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
- 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.
- 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
'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
The architecture specifies common metrics for measuring traffic
flows. By using the same metrics, traffic flow data can be exchanged
and compared across multiple platforms. Such data is useful for:
- Understanding the behaviour of existing networks,
- Planning for network development and expansion,
- Quantification of network performance,
- Verifying the quality of network service, and
- Attribution of network usage to users.
The traffic flow measurement architecture is deliberately structured
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].
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
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
For connectionless network protocols such as IP there is by
definition no way to tell whether a packet with a particular
source/destination combination is part of a stream of packets or not
- each packet is completely independent. A traffic meter has, as
part of its configuration, a set of 'rules' which specify the flows
of interest, in terms of the values of their attributes. It derives
attribute values from each observed packet, and uses these to decide
which flow they belong to. Classifying packets into 'flows' in this
way provides an economical and practical way to measure network
traffic and 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.
2.3 / \ 2.4
/ \ ANALYSIS
METER <-----> METER READER <-----> APPLICATION
- 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.
- METER: Meters are placed at measurement points determined by
Network Operations personnel. Each meter selectively records
network activity as directed by its configuration settings. It
can also aggregate, transform and further process the recorded
activity before the data is stored. The processed and stored
results are called the 'usage data'.
- METER READER: A meter reader transports usage data from meters so
that it is available to analysis applications.
- ANALYSIS APPLICATION: An analysis application processes the
usage data so as to provide information and reports which are
useful for network engineering and management purposes. Examples
- TRAFFIC FLOW MATRICES, showing the total flow rates for many
of the possible paths within an internet.
- FLOW RATE FREQUENCY DISTRIBUTIONS, summarizing flow rates
over a period of time.
- USAGE DATA showing the total traffic volumes sent and
received by particular hosts.
The operation of the traffic measurement system as a whole is best
understood by considering the interactions between its components.
These are described in the following sections.
2.2 Interaction Between METER and METER READER
The information which travels along this path is the usage data
itself. A meter holds usage data in an array of flow data records
known as the FLOW TABLE. A meter reader may collect the data in any
suitable manner. For example it might upload a copy of the whole
flow table using a file transfer protocol, or read the records in the
current flow set one at a time using a suitable data transfer
protocol. Note that the meter reader need not read complete flow
data records, a subset of their attribute values may well be
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:
- Flow specifications, e.g. which traffic flows are to be measured,
how they are to be aggregated, and any data the meter is required
to compute for each flow being measured.
- Meter control parameters, e.g. the 'inactivity' time for flows
(if no packets belonging to a flow are seen for this time the
flow is considered to have ended, i.e. to have become idle).
- Sampling 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:
- The meter's unique identity, i.e. its network name or address.
- How often usage data is to be collected from the meter.
- 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
- Which attribute values are to be collected for the required flow
records (e.g. all attributes, or a small subset of them)
Since redundant reporting may be used in order to increase the
reliability of usage data, exchanges among multiple entities must be
considered as well. These are discussed below.
2.5 Multiple METERs or METER READERs
-- METER READER A --
/ | \
/ | \
=====METER 1 METER 2=====METER 3 METER 4=====
\ | /
\ | /
-- METER READER B --
Several uniquely identified meters may report to one or more meter
readers. The diagram above gives an example of how multiple meters
and meter readers could be used.
In the diagram above meter 1 is read by meter reader A, and meter 4
is read by meter reader B. Meters 1 and 4 have no redundancy; if
either meter fails, usage data for their network segments will be
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
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
traffic volume was sufficient to 'roll over' counters for some flows
during the failure; this is addressed in the section on 'Rolling
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
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
3.1 Flows and their Attributes
Every traffic meter maintains a table of 'flow records' for flows
seen by the meter. A flow record holds the values of the ATTRIBUTES
of interest for its flow. These attributes might include:
- ADDRESSES for the flow's source and destination. These comprise
the protocol type, the source and destination addresses at
various network layers (extracted from the packet header), and
the number of the interface on which the packet was observed.
- First and last TIMES when packets were seen for this flow, i.e.
the 'creation' and 'last activity' times for the flow.
- COUNTS for 'forward' (source to destination) and 'backward'
(destination to source) components (e.g. packets and bytes) of
the flow's traffic. The specifying of 'source' and 'destination'
for flows is discussed in the section on packet matching below.
- OTHER attributes, e.g. 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 18.104.22.168" then only IP packets from 10.1.0.1 to
22.214.171.124 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:
- 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-
- 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.
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.
- 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
- 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
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 metered traffic group (address attributes, discussed above).
- The categorisation of packets (other attributes, discussed
- 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
must be identifiable via a RULE SET NUMBER. Granularity of metered
traffic groups is further specified by additional ATTRIBUTES. These
- Attributes which record information derived from other attribute
values. Six of these are defined (SourceClass, DestClass,
FlowClass, SourceKind, DestKind, FlowKind), and their meaning is
determined by the meter's rule set. For example, one could have
a subroutine in the rule set which determined whether a source or
destination peer address was a member of an arbitrary list of
networks, and set SourceClass/DestClass to one if the source/dest
peer address was in the list or to zero otherwise.
- Administratively specified attributes such as Quality of Service
and Priority, etc. These are not defined at this time.
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.
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.
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
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
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.
Bucket 2: Packets which were sourced by IP address a.b.c.d;
counted by rule set B.
A traffic flow meter is a device for collecting data about traffic
flows at a given point within a network; we will call this the
METERING POINT. The header of every packet passing the network
metering point is offered to the traffic meter program.
A meter could be implemented in various ways, including:
- A dedicated small host, connected to a 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.
- A multiprocessing system with one or more network interfaces,
with drivers enabling a traffic meter program to see packets. In
this case the system provides multiple metering points - traffic
flows on any subset of its network interfaces can be measured.
- A packet-forwarding device such as a router or switch. This is
similar to (b) except that every received packet should also be
forwarded, usually on a different interface.
4.1 Meter Structure
An outline of the meter's structure is given in the following
Briefly, the meter works as follows:
- Incoming packet headers arrive at the top left of the diagram and
are passed to the PACKET PROCESSOR.
- The packet processor passes them to the Packet Matching Engine
(PME) where they are classified.
- The PME is a Virtual Machine running a pattern matching program
contained in the CURRENT RULE SET. It is invoked by the Packet
Processor, 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.
- Some packets are classified as 'to be ignored'. They are
discarded by the Packet Processor.
- Other packets are matched by the PME, which returns a FLOW KEY
describing the flow to which the packet belongs.
- The flow key is used to locate the flow's entry in the FLOW
TABLE; a new entry is created when a flow is first seen. The
entry's data fields (e.g. packet and byte counters) are updated.
- A meter reader may collect data from the flow table at any time.
It may use the 'collect' index to locate the flows to be
collected within the flow table.
header | Current Rule Set |
+-------*--------+ 'match key' +------*-------+
| Packet |---------------->| Packet |
| Processor | | Matching |
| |<----------------| Engine |
+--+----------+--+ 'flow key' +--------------+
Ignore * | Count (via 'flow key')
| 'Search' index |
| Flow Table |
| 'Collect' index |
The discussion above assumes that a meter will only be running a
single rule set. A meter may, however, run several rule sets
concurrently. To do this the meter maintains a table of current
rulesets. The packet processor matches each packet against every
current ruleset, producing a single flow table 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:
- 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.
- First and last times when packets were seen for this flow.
- Counts for 'forward' (source to destination) and 'backward'
(destination to source) components of the flow's traffic.
- Other attributes, e.g. state of the flow record (discussed
The state of a flow record may be:
- INACTIVE: The flow record is not being used by the meter.
- CURRENT: The record is in use and describes a flow which belongs
to the 'current flow set', i.e. the set of flows recently seen by
- 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
4.3 Packet Handling, Packet Matching
Each packet header received by the traffic meter program is processed
- Extract attribute values from the packet header and use them to
create a MATCH KEY for the packet.
- Match the packet's key against the current rule set, as explained
in detail below.
The rule set specifies whether the packet is to be counted or
ignored. If it is to be counted the matching process produces a FLOW
KEY for the flow to which the packet belongs. This flow key is used
to find the flow's record in the flow table; if a record does not yet
exist for this flow, a new flow record may be created. The 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
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:
- The packet is recognised as one which is TO BE IGNORED.
- 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.
- 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.
--- 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) --------------+
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
'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
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.
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
+-------- test ---------+ +---- action -----+
attribute & mask = value: opcode, parameter;
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,
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.
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
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
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.
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
GotoAct: Clear the test indicator then goto the specified
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.
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
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
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
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
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
'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.
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
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.
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
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
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.
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
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
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.
A manager configures meters and controls meter readers. It does this
via the interactions described below.
6.1 Between Manager and Meter: Control Functions
- DOWNLOAD RULE SET: A meter may hold an array of rule sets. One
of these, the 'default' rule set, is built in to the meter and
cannot be changed; 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.
- 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
- 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
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
- SET FLOW TERMINATION PARAMETERS: The meter should have the good
sense in situations where lack of resources may cause data loss
to purge flow records from its tables. Such records may include:
- Flows that have already been reported to all registered meter
readers, and show no activity since the last report,
- Oldest flows, or
- Flows with the smallest number of observed packets.
- SET INACTIVITY TIMEOUT: This is a time in seconds since the last
packet was seen for a flow. Flow records may be reclaimed if
they have been idle for at least this amount of time, and have
been collected in accordance with the current collection
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
Operations personnel). At any rate, it must be understood that the
operations below do not represent the setting of independent
variables; on the contrary, each of the values set has a direct and
measurable effect on the behaviour of the other variables.
Network management operations follow:
- MANAGER and METER READER IDENTIFICATION: The manager should
ensure that meters 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.
- 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).
- 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
- Controls the amount of address information identifying each
- Determines the number of buckets into which user traffic
will be lumped together.
Since granularity is controlled by the meter's current rule set,
the manager can only change it by requesting the meter to switch
to a different rule set. The new rule set could be downloaded
when required, or it could have been downloaded as part of the
meter's initial configuration.
- FLOW LIFETIME CONTROL: Flow termination parameters include
timeout parameters for obsoleting inactive flows and removing
them from tables, and maximum flow lifetimes. This is
intertwined with reporting interval and granularity, and must be
set in accordance with the other parameters.
6.3 Exception Conditions
Exception conditions must be handled, particularly occasions when the
meter runs out of 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:
- 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.
- 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.
- MANAGER OUTAGES: If the manager fails for any reason, the meter
should continue measuring and the meter reader(s) should keep
gathering usage records.
- BUFFER PROBLEMS: The network manager may realise that there is a
'low memory' condition in the meter. This can usually be
attributed to the interaction between the following controls:
- The reporting interval is too infrequent, or
- The reporting granularity is too fine.
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
- PROTOCOL TYPE: The meter records packets by protocol type. This
will be the default rule table for Traffic Flow Meters.
- ADJACENT SYSTEMS: The meter records packets by the MAC address of
the Adjacent Systems (neighbouring originator or next-hop).
(Variants on this table are "report source" or "report sink"
only.) This strategy might be used by a regional or backbone
network which wants to know how much aggregate traffic flows to
or from its subscriber networks.
- END SYSTEMS: The meter records packets by the IP address pair
contained in the packet. (Variants on this table are "report
source" or "report sink" only.) This strategy might be used by
an End System network to get detailed host traffic matrix usage
- TRANSPORT TYPE: The meter records packets by transport address;
for IP packets this provides usage information for the various IP
- 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:
- 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
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.
- 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
- UNAUTHORIZED DISCLOSURE OF DATA: Any data that is sensitive to
disclosure can be read through active or passive attacks unless
it is suitably protected. Usage data may or may not be of this
type. Control messages, traps, etc. are not likely to be
considered sensitive to disclosure.
- UNAUTHORIZED ALTERATION, REPLACEMENT OR DESTRUCTION OF DATA:
Similarly, any data whose integrity is sensitive can be altered,
replaced/injected or deleted through active or passive attacks
unless it is suitably protected. Attackers may modify message
streams to falsify usage data or interfere with the proper
operation of the traffic flow measurement system. Therefore, all
messages, both those containing usage data and those containing
control data, should be considered vulnerable to such attacks.
The following countermeasures are recommended to address the possible
threats enumerated above:
- 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.
- UNAUTHORIZED USE OF SYSTEM RESOURCES is countered through the use
of authentication and access control services.
- UNAUTHORIZED DISCLOSURE OF DATA is countered through the use of a
confidentiality (encryption) service.
- UNAUTHORIZED ALTERATION, REPLACEMENT OR DESTRUCTION OF DATA is
countered through the use of an integrity service.
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
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
- 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'.
- Whenever there is a transfer of information its integrity should
- 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
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
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:
- tested in an IF statement
- saved by a SAVE statement or set by a STORE statement
- read from an RTFM meter
(IF, SAVE and STORE are statements in the SRL Ruleset Language
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:
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
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
END SYSTEMS, colloquially HOSTS, are SUBSCRIBERS to any of the above
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
'user identification' labels so that meters can implement proprietary
or non-standard schemes for the attribution of network traffic to
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
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
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.
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'
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
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.
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
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.
- 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.
- 'NoMatch' replaces 'Fail' as an action. This name was agreed to
at the Working Group 1996 meeting in Montreal; it better
indicates that although a particular match has failed, it may be
tried again with the packet's addresses reversed.
- The 'MatchingStoD' attribute has been added. This is a Packet
Matching Engine (PME) attribute indicating that addresses are
being matched in StoD (i.e. 'wire') order. It can be used to
perform different actions when the match is retried, thereby
simplifying some kinds of rule sets. It was discussed and agreed
to at the San Jose meeting in 1996.
- Computed attributes (Class and Kind) may now be tested within a
rule set. This lifts an unneccessary earlier restriction.
- The list of attribute numbers has been extended to define ranges
for 'basic' attributes (in this document) and 'extended'
attributes (currently being developed by the RTFM Working Group).
- The 'Security Considerations' section has been completely
rewritten. It provides an evaluation of traffic measurement
security risks and their countermeasures.
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.
[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
[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,
[IPPM-FRM] Paxson, V., Almes, G., Mahdavi, J. and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330, May
[OSI-ACT] International Standards Organisation (ISO), "Management
Framework", Part 4 of Information Processing Systems Open
Systems Interconnection Basic Reference Model, ISO 7498-4,
[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,
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