Network Working Group E. Boschi

Network Working Group E. Boschi
Request for Comments: 5153 Hitachi Europe
Category: Informational L. Mark
Fraunhofer FOKUS
J. Quittek
M. Stiemerling
NEC
P. Aitken
Cisco Systems, Inc.
April 2008

IP Flow Information Export (IPFIX) Implementation Guidelines

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.

Abstract

The IP Flow Information Export (IPFIX) protocol defines how IP Flow
information can be exported from routers, measurement probes, or
other devices. This document provides guidelines for the
implementation and use of the IPFIX protocol. Several sets of
guidelines address Template management, transport-specific issues,
implementation of Exporting and Collecting Processes, and IPFIX
implementation on middleboxes (such as firewalls, network address
translators, tunnel endpoints, packet classifiers, etc.).

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. IPFIX Documents Overview . . . . . . . . . . . . . . . . . 3
1.2. Overview of the IPFIX Protocol . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Template Management Guidelines . . . . . . . . . . . . . . . . 4
3.1. Template Management . . . . . . . . . . . . . . . . . . . 4
3.2. Template Records versus Options Template Records . . . . . 5
3.3. Using Scopes . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Multiple Information Elements of the Same Type . . . . . . 6
3.5. Selecting Message Size . . . . . . . . . . . . . . . . . . 6
4. Exporting Process Guidelines . . . . . . . . . . . . . . . . . 7
4.1. Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Information Element Coding . . . . . . . . . . . . . . . . 7
4.3. Using Counters . . . . . . . . . . . . . . . . . . . . . . 8
4.4. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 8

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4.4.1. Alignment of Information Elements within a Data
Record . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4.2. Alignment of Information Element Specifiers within
a Template Record . . . . . . . . . . . . . . . . . . 9
4.4.3. Alignment of Records within a Set . . . . . . . . . . 9
4.4.4. Alignment of Sets within an IPFIX Message . . . . . . 9
4.5. Time Issues . . . . . . . . . . . . . . . . . . . . . . . 10
4.6. IPFIX Message Header Export Time and Data Record Time . . 10
4.7. Devices without an Absolute Clock . . . . . . . . . . . . 11
5. Collecting Process Guidelines . . . . . . . . . . . . . . . . 11
5.1. Information Element (De)Coding . . . . . . . . . . . . . . 11
5.2. Reduced-Size Encoding of Information Elements . . . . . . 12
5.3. Template Management . . . . . . . . . . . . . . . . . . . 12
6. Transport-Specific Guidelines . . . . . . . . . . . . . . . . 12
6.1. SCTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. UDP . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.3. TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7. Guidelines for Implementation on Middleboxes . . . . . . . . . 18
7.1. Traffic Flow Scenarios at Middleboxes . . . . . . . . . . 20
7.2. Location of the Observation Point . . . . . . . . . . . . 21
7.3. Reporting Flow-Related Middlebox Internals . . . . . . . . 22
7.3.1. Packet Dropping Middleboxes . . . . . . . . . . . . . 23
7.3.2. Middleboxes Changing the DSCP . . . . . . . . . . . . 23
7.3.3. Middleboxes Changing IP Addresses and Port Numbers . . 24
8. Security Guidelines . . . . . . . . . . . . . . . . . . . . . 25
8.1. Introduction to TLS and DTLS for IPFIX Implementers . . . 25
8.2. X.509-Based Identity Verification for IPFIX over TLS
or DTLS . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.3. Implementing IPFIX over TLS over TCP . . . . . . . . . . . 26
8.4. Implementing IPFIX over DTLS over UDP . . . . . . . . . . 26
8.5. Implementing IPFIX over DTLS over SCTP . . . . . . . . . . 27
9. Extending the Information Model . . . . . . . . . . . . . . . 27
9.1. Adding New IETF-Specified Information Elements . . . . . . 27
9.2. Adding Enterprise-Specific Information Elements . . . . . 28
10. Common Implementation Mistakes . . . . . . . . . . . . . . . . 28
10.1. IPFIX and NetFlow Version 9 . . . . . . . . . . . . . . . 28
10.2. Padding of the Data Set . . . . . . . . . . . . . . . . . 29
10.3. Field ID Numbers . . . . . . . . . . . . . . . . . . . . . 30
10.4. Template ID Numbers . . . . . . . . . . . . . . . . . . . 30
11. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
13.1. Normative References . . . . . . . . . . . . . . . . . . . 31
13.2. Informative References . . . . . . . . . . . . . . . . . . 31

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1. Introduction

The IPFIX protocol [RFC5101] defines how IP Flow information can be
exported from routers, measurement probes, or other devices. In this
document, we provide guidelines for its implementation.

The guidelines are split into seven main sets. These sets address
implementation aspects for Template management, Exporting Process,
Collecting Process, transport, implementation on middleboxes,
security, and extending the information model.

Finally, this document contains a list of common mistakes related to
issues that had been misinterpreted in the first IPFIX
implementations and that created (and still might create)
interoperability problems.

1.1. IPFIX Documents Overview

The IPFIX protocol [RFC5101] provides network administrators with
access to IP Flow information. The architecture for the export of
measured IP Flow information out of an IPFIX Exporting Process to a
Collecting Process is defined in the IPFIX architecture [IPFIX-ARCH],
per the requirements defined in [RFC3917].

The IPFIX architecture [IPFIX-ARCH] specifies how IPFIX Data Records
and Templates are carried via a congestion-aware transport protocol
from IPFIX Exporting Processes to IPFIX Collecting Processes.

IPFIX has a formal description of IPFIX Information Elements, their
name, type, and additional semantic information, as specified in the
IPFIX information model [RFC5102].

Finally, the IPFIX applicability statement [IPFIX-AS] describes what
type of applications can use the IPFIX protocol and how they can use
the information provided. It furthermore shows how the IPFIX
framework relates to other architectures and frameworks.

1.2. Overview of the IPFIX Protocol

In the IPFIX protocol, { type, length, value } tuples are expressed
in Templates containing { type, length } pairs, specifying which
{ value } fields are present in Data Records conforming to the
Template, giving great flexibility as to what data is transmitted.

Since Templates are sent very infrequently compared with Data
Records, this results in significant bandwidth savings.

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Different Data Records may be transmitted simply by sending new
Templates specifying the { type, length } pairs for the new data
format. See [RFC5101] for more information.

The IPFIX information model [RFC5102] defines a large number of
standard Information Elements that provide the necessary
{ type } information for Templates.

The use of standard elements enables interoperability among different
vendors' implementations. The list of standard elements may be
extended in the future through the process defined in Section 9,
below. Additionally, non-standard enterprise-specific elements may
be defined for private use.

2. Terminology

The terminology used in this document is fully aligned with the
terminology defined in [RFC5101]. Therefore, the terms defined in
the IPFIX terminology are capitalized in this document, as in other
IPFIX documents ([RFC5101], [RFC5102], [IPFIX-ARCH]).

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

This document is Informational. It does not specify a protocol and
does not use RFC 2119 key words [RFC2119] such as "MUST" and
"SHOULD", except in quotations and restatements from the IPFIX
standards documents. The normative specification of the protocol is
given in the IPFIX protocol [RFC5101] and information model [RFC5102]
documents.

3. Template Management Guidelines

3.1. Template Management

The Exporting Process should always endeavor to send Template Records
before the related Data Records. However, since the Template Record
may not arrive before the corresponding Data Records, the Collecting
Process MAY store Data Records with an unknown Template ID pending
the arrival of the corresponding Template (see Section 9 of
[RFC5101]). If no Template becomes available, we recommend logging
the event and discarding the corresponding Data Records, and for SCTP
and TCP we recommend resetting the Transport Session. The amount of
time the Collecting Process waits for a Template before resetting
should be configurable. We recommend a default of 30 minutes. Note

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that when using UDP as the transport protocol, this delay should be
bound, when possible, by the Template Retransmit and the Template
Expiry times (see Section 6.2).

The Exporting Process must be able to resend active Templates.
Templates must be resent in the case of a Stream Control Transport
Protocol (SCTP) association restart, a User Datagram Protocol (UDP)
template refresh, or a Transmission Control Protocol (TCP) connection
restart.

The Exporting Process is responsible for the management of Template
IDs. Should an insufficient number of Template IDs be available, the
Exporting Process must send a Template Withdrawal Message in order to
free up the allocation of unused Template IDs. Note that UDP doesn't
use the Template Withdrawal Message, and the Template lifetime on the
Collecting Process relies on timeout.

3.2. Template Records versus Options Template Records

The IPFIX protocol [RFC5101] defines and specifies the use of
Templates and Options Templates. Templates define the layout of Data
Records, which represent Flow data. Options Templates additionally
specify scope Information Elements, which can be used to define
scoped Data Records. Scoped Data Records generally export control
plane data (such as metadata about processes in the IPFIX collection
architecture) or data otherwise applicable to multiple Flow Data
Records (such as common properties as in [IPFIX-REDUCING]).

Aside from Section 4 of [RFC5101], which defines specific Options
Templates to use for reporting Metering Process and Exporting Process
statistics and configuration information, the choice to use Options
Templates is left up to the implementer. Indeed, there is a trade-
off between bandwidth efficiency and complexity in the use of Options
Templates and scoped Data Records.

For example, control plane information about an Observation Point
could be exported with every Flow Record measured at that Observation
Point, or in a single Data Record described by an Options Template,
scoped to the Observation Point identifier. In the former case,
simplicity of decoding the data is gained in exchange for redundant
export of the same data with every applicable Flow Record. The
latter case is more bandwidth-efficient, but at the expense of
requiring the Collecting Process to maintain the relationship between
each applicable Flow Record and the Observation Point.

A generalized method of using Options Templates to increase bandwidth
efficiency is fully described in [IPFIX-REDUCING].

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3.3. Using Scopes

The root scope for all IPFIX Messages is the Observation Domain,
which appears in the Message Header. In other words, all Data
Records within a message implicitly belong to the Observation Domain.
All Data Records described by Options Templates (and only those) must
be restricted to an additional scope within the Observation Domain,
as defined by the scope Information Elements in the Options Template
Record.

In IPFIX, any Information Element can be used for scope. However,
Information Elements such as counters, timestamps, padding elements,
Flow properties like timeout, Flow end reason, duration, or Min/Max
Flow properties [RFC5102] may not be appropriate.

Note that it is sometimes necessary to export information about
entities that exist outside any Observation Domain, or within
multiple Observation Domains (e.g., information about Metering
Processes scoped to meteringProcessID). Such information SHOULD be
exported in an IPFIX Message with Observation Domain ID 0 (see
[RFC5101], Section 3.1).

3.4. Multiple Information Elements of the Same Type

The Exporting Process and Collecting Process MUST support the use of
multiple Information Elements of the same type in a single Template
[RFC5101]. This was first required by Packet Sampling (PSAMP)
[PSAMP-PROTO] for the export of multiple Selector IDs. Note that the
IPFIX protocol recommends that Metering Processes SHOULD use packet
treatment order when exporting multiple Information Elements of the
same type in the same record ([RFC5101] Section 8). This implies
that ordering is important, and changes to the order of multiple
identical Information Elements could cause information loss.
Therefore, we strongly recommend preservation of the order of
multiple Information Elements of the same type by Exporting and
Collecting Processes for correct processing and storage.

3.5. Selecting Message Size

Section 10.3.3 of the IPFIX protocol defines the maximum message size
for IPFIX Messages transported over UDP to be constrained by the path
MTU, or if the path MTU is not available, 512 bytes, which is the
minimum datagram size all IP implementations must support (see also
Section 8.4). However, no maximum message size is imposed on other
transport protocols, beyond the 65535-byte limit imposed by the 16-
bit Message Length field in the IPFIX Message Header specified in
Section 3.1 of [RFC5101].

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An IPFIX Exporting Process operating over SCTP or TCP may export
IPFIX Messages up to this 64-kB limit, and an IPFIX Collecting
Process must accept any IPFIX Message up to that size.

4. Exporting Process Guidelines

4.1. Sets

A Set is identified by a Set ID [RFC5101]. A Set ID has an integral
data type and its value is in the range of 0-65535. The Set ID
values of 0 and 1 are not used for historical reasons [RFC3954]. A
value of 2 identifies a Template Set. A value of 3 identifies an
Options Template Set. Values from 4 to 255 are reserved for future
use. Values above 255 are used for Data Sets. In this case, the Set
ID corresponds to the Template ID of the used Template.

A Data Set received with an unknown Set ID may be stored pending the
arrival of the corresponding Template (see Section 9 of [RFC5101]).
If no Template becomes available, we recommend logging the event and
discarding the corresponding Data Records, and for SCTP and TCP we
recommend resetting the Transport Session. The amount of time the
Collecting Process waits for a Template before resetting should be
configurable. We recommend a default of 30 minutes. Note that when
using UDP as the transport protocol, this delay should be bound, when
possible, by the Template Retransmit and the Template Expiry times
(see Section 6.2).

The arrival of a Set with a reserved Set ID should be logged, and the
Collector must ignore the Set.

4.2. Information Element Coding

[IPFIX-ARCH] does not specify which entities are responsible for the
encoding and decoding of Information Elements transferred via IPFIX.
An IPFIX device can do the encoding either within the Metering
Process or within the Exporting Process. The decoding of the
Information Elements can be done by the Collecting Process or by the
data processing application.

If an IPFIX node simply relays IPFIX Records (like a proxy), then no
decoding or encoding of Information Elements is needed. In this
case, the Exporting Process may export unknown Information Elements,
i.e., Information Elements with an unknown Information Element
identifier.

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4.3. Using Counters

IPFIX offers both Delta and Total counters (e.g., octetDeltaCount,
octetTotalCount). If information about a Flow is only ever exported
once, then it's not important whether Delta or Total counters are
used. However, if further information about additional packets in a
Flow is exported after the first export, then either:

o the metering system must reset its counters to zero after the
first export and report the new counter values using Delta
counters, or

o the metering system must carefully maintain its counters and
report the running total using Total counters.

At first, reporting the running total may seem to be the obvious
choice. However, this requires that the system accurately maintains
information about the Flow over a long time without any loss or
error, because when reported to a Collecting Process, the previous
total values will be replaced with the new information.

Delta counters offer some advantages: information about Flows doesn't
have to be permanently maintained, and any loss of information has
only a small impact on the total stored at the Collecting Process.
Finally, Deltas may be exported in fewer bytes than Total counters
using the IPFIX "Reduced Size Encoding" scheme [RFC5101].

Note that Delta counters have an origin of zero and that a Collecting
Process receiving Delta counters for a Flow that is new to the
Collecting Process must assume the Deltas are from zero.

4.4. Padding

The IPFIX information model defines an Information Element for
padding called paddingOctets [RFC5102]. It is of type octetArray,
and the IPFIX protocol allows encoding it as a fixed-length array as
well as a variable-length array.

The padding Information Element can be used to align Information
Elements within Data Records, Records within Sets, and Sets within
IPFIX Messages, as described below.

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4.4.1. Alignment of Information Elements within a Data Record

The padding Information Element gives flexible means for aligning
Information Elements within a Data Record. Aligning within a Data
Record can be useful, because internal data structures can be easily
converted into Flow Records at the Exporter and vice versa at the
Collecting Process.

Alignment of Information Elements within a Data Record is achieved by
inserting an instance of the paddingOctets Information Element with
appropriate length before each unaligned Information Element. This
insertion is explicitly specified within the Template Record or
Options Template Record, respectively, that corresponds to the Data
Record.

4.4.2. Alignment of Information Element Specifiers within a Template
Record

There is no means for aligning Information Element specifiers within
Template Records. However, there is limited need for such a method,
as Information Element specifiers are always 32-bit aligned, and 32-
bit alignment is generally sufficient.

4.4.3. Alignment of Records within a Set

There is no means for aligning Template Records within a Set.
However, there is limited need for such a method, as Information
Element specifiers are always 32-bit aligned, and 32-bit alignment is
generally sufficient.

Data Records can be aligned within a Set by appending instances of
the paddingOctets Information Element at the end of the Record.
Since all Data Records within a Set have the same structure and size,
aligning one Data Record implies aligning all the Data Records within
a single Set.

4.4.4. Alignment of Sets within an IPFIX Message

If Records are already aligned within a Set by using paddingOctets
Information Elements, then this alignment will already be achieved.
But for aligning Sets within an IPFIX Message, padding Information
Elements can be used at the end of the Set so that the subsequent Set
starts at an aligned boundary. This padding mechanism is described
in Section 3.3.1 of [RFC5101] and can be applied even if the Records
within the Set are not aligned. However, it should be noted that
this method is limited by the constraint that "the padding length
MUST be shorter than any allowable Record in the Set", to prevent the
padding from being misinterpreted as an additional Data Record.

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4.5. Time Issues

IPFIX Messages contain the export time in the Message Header. In
addition, there is a series of Information Elements defined to
transfer time values. [RFC5102] defines four abstract data types to
transfer time values in second, millisecond, microsecond, and
nanosecond resolution.

The accuracy and precision of these values depend on the accuracy and
the precision of the Metering Process clock. The accuracy and
precision of the Exporting Process clock, and the synchronization of
the Metering Process and Exporting Process clocks, are also important
when using the delta timestamp Information Elements. To ensure
accuracy, the clocks should be synchronized to a UTC time source.
Normally, it would be sufficient to derive the time from a remote
time server using the Network Time Protocol (NTP) [RFC1305]. IPFIX
Devices operating with time values of microsecond or nanosecond
resolution need direct access to a time source, for example, to a GPS
(Global Positioning System) unit.

The most important consideration in selecting timestamp Information
Elements is to use a precision appropriate for the timestamps as
generated from the Metering Process. Specifically, an IPFIX Device
should not export timestamp Information Elements of higher precision
than the timestamps used by the Metering Process (e.g., millisecond-
precision Flows should not be exported with flowStartMicroseconds and
flowEndMicroseconds).

4.6. IPFIX Message Header Export Time and Data Record Time

Section 5 of [RFC5101] defines a method for optimized export of time-
related Information Elements based upon the Export Time field of the
IPFIX Message Header. The architectural separation of the Metering
Process and Exporting Process in [IPFIX-ARCH] raises some
difficulties with this method, of which implementers should be aware.

Since the Metering Process has no information about the export time
of the IPFIX Message (that is, when the message leaves the Exporting
Process), it cannot properly use the delta time Information Elements;
it must store absolute timestamps and transmit these to the Exporting
Process. The Exporting Process must then convert these to delta
timestamps once the export time is known. This increases the
processing burden on the Exporting Process. Note also that the
absolute timestamps require more storage than their delta timestamp
counterparts. However, this method can result in reduced export
bandwidth.

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Alternatively, the Exporting Process may simply export absolute
timestamp Information Elements. This simplifies the Exporting
Process' job and reduces processing burden, but increases export
bandwidth requirements.

4.7. Devices without an Absolute Clock

Exporting just relative times in a device without an absolute clock
is often not sufficient. For instance, observed traffic could be
retained in the device's cache for some time before being exported
(e.g., if the Exporter runs once per minute), or stuck in an Inter
Process Communication (IPC) queue, or stuck in the export stack, or
delayed in the network between the Exporter and Collector.

For these reasons, it can be difficult for the Collecting Process to
convert the relative times exported using the flowStartSysUpTime and
flowEndSysUpTime Information Elements to absolute times with any sort
of accuracy without knowing the systemInitTimeMilliseconds.
Therefore, the sending of the flowStartSysUpTime and flowEndSysUpTime
Information Elements without also sending the
systemInitTimeMilliseconds Information Element is not recommended.

5. Collecting Process Guidelines

5.1. Information Element (De)Coding

Section 9 of [RFC5101] specifies: "The Collecting Process MUST note
the Information Element identifier of any Information Element that it
does not understand and MAY discard that Information Element from the
Flow Record". The Collecting Process may accept Templates with
Information Elements of unknown types. In this case, the value
received for these Information Elements should be decoded as an octet
array.

Alternatively, the Collecting Process may ignore Templates and
subsequent Data Sets that contain Information Elements of unknown
types.

It is recommended that Collecting Processes provide means to flexibly
add types of new Information Elements to their knowledge base. An
example is a configuration file that is read by the Collecting
Process and that contains a list of Information Element identifiers
and their corresponding types. Particularly for adding enterprise-
specific Information Elements, such a feature can be very useful.

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5.2. Reduced-Size Encoding of Information Elements

Since a Collector may receive data from the same device and
Observation Domain in two Templates using different reduced-size
encodings, it is recommended that the data be stored using full-size
encoding, to ensure that the values can be stored or even aggregated
together.

5.3. Template Management

Template IDs are generated dynamically by the Exporting Process.
They are unique per Transport Session and Observation Domain.

Therefore, for each Transport Session, the Collecting Process has to
maintain a list of Observation Domains. For each Observation Domain,
the Collecting Process has to maintain a list of current Template IDs
in order to decode subsequent Data Records.

Note that a restart of the Transport Session may lead to a Template
ID renumbering.

6. Transport-Specific Guidelines

IPFIX can use SCTP, TCP, or UDP as a transport protocol. IPFIX
implementations MUST support SCTP with partial reliability extensions
(PR-SCTP), and MAY support TCP and/or UDP (see [RFC5101], Section
10.1). In the IPFIX documents, the terms SCTP and PR-SCTP are often
used interchangeably to mean SCTP with partial reliability
extensions.

6.1. SCTP

PR-SCTP is the preferred transport protocol for IPFIX because it is
congestion-aware, reducing total bandwidth usage in the case of
congestion, but with a simpler state machine than TCP. This saves
resources on lightweight probes and router line cards.

SCTP, as specified in [RFC4960] with the PR-SCTP extension defined in
[RFC3758], provides several features not available in TCP or UDP.
The two of these most universally applicable to IPFIX
implementations, and which IPFIX implementers need to know about, are
multiple streams and per-message partial reliability.

An SCTP association may contain multiple streams. Streams are useful
for avoiding head-of-line blocking, thereby minimizing end-to-end
delay from the Exporting Process to the Collecting Process. Example

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applications for this feature would be using one SCTP stream per
Observation Domain, one stream per type of data (or Template ID), or
one stream for Flow data and one for metadata.

An Exporting Process may request any number of streams, and may send
IPFIX Messages containing any type of Set (Data Set, Template Set,
etc.) on any stream. A Collecting Process MUST be able to process
any Message received on any stream.

Stream negotiation is a feature of the SCTP protocol. Note, however,
that the IPFIX protocol doesn't provide any mechanism for the
Exporter to convey any information about which streams are in use to
the Collector. Therefore, stream configuration must be done out of
band.

One extra advantage of the PR-SCTP association is its ability to send
messages with different levels of reliability, selected according to
the application. For example, billing or security applications might
require reliable delivery of all their IPFIX Messages, while capacity
planning applications might be more tolerant of message loss. SCTP
allows IPFIX Messages for all these applications to be transported
over the same association with the appropriate level of reliability.

IPFIX Messages may be sent with full or partial reliability, on a
per-message basis. Fully reliable delivery guarantees that the IPFIX
Message will be received at the Collecting Process or that that SCTP
association will be reset, as with TCP. Partially reliable delivery
does not guarantee the receipt of the IPFIX Message at the Collecting
Process. This feature may be used to allow Messages to be dropped
during network congestion, i.e., while observing a Denial of Service
attack.

[RFC3758] defines the concept of a Partial Reliability policy, which
specifies the interface used to control partially reliable delivery.
It also defines a single example Partial Reliability policy called
"timed reliability", which uses a single parameter: lifetime. The
lifetime is specified per message in milliseconds, and after it
expires, no further attempt will be made to transmit the message.
Longer lifetimes specify more retransmission attempts per message and
therefore higher reliability; however, it should be noted that the
absolute reliability provided by a given lifetime is highly dependent
on network conditions, so an Exporting Process using the timed
reliability service should provide a mechanism for configuring the
lifetime of exported IPFIX Messages. Another possible Partial
Reliability policy could be limited retransmission, which guarantees
a specified number of retransmissions for each message. It is up to
the implementer to decide which Partial Reliability policy is most
appropriate for its application.

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There is an additional service provided by SCTP and useful in
conjunction with PR-SCTP: unordered delivery. This also works on a
per-message basis by declaring that a given message should be
delivered to the receiver as soon as it is queued rather than kept in
sequence; however, it should be noted that unless explicitly
requested by the sender, even messages sent partially reliably will
still be delivered in order. Unordered delivery should not be used
when the order of IPFIX Messages may matter: e.g., a Template or
Options Template. Unordered delivery should not be used when Total
counters are used, as reordering could result in the counter value
decreasing at the Collecting Process and even being left with a stale
value if the last message processed is stale.

By convention, when the IPFIX documents state a requirement for
reliable delivery (as, for example, the IPFIX protocol document does
for Template Sets, Options Template Sets, and Template Withdrawal
Messages), an IPFIX Exporting Process must not use partially reliable
delivery for those Messages. By default, and explicitly if the IPFIX
documents call for "partially reliable" or "unreliable" delivery, an
IPFIX Exporting Process may use partially reliable delivery if the
other requirements of the application allow.

The Collecting Process may check whether IPFIX Messages are lost by
checking the Sequence Number in the IPFIX header. The Collecting
Process should use the Sequence Number in the IPFIX Message Header to
determine whether any messages are lost when sent with partial
reliability. Sequence Numbers should be tracked independently for
each stream.

The following may be done to mitigate message loss:

o Increase the SCTP buffer size on the Exporter.

o Increase the bandwidth available for communicating the exported
Data Records.

o Use sampling, filtering, or aggregation in the Metering Process to
reduce the amount of exported data (see [RFC5101], Section
10.4.2.3).

o If partial reliability is used, switch to fully reliable delivery
on the Exporting Process or increase the level of partial
reliability (e.g., when using timed reliability, by specifying a
longer lifetime for exported IPFIX Messages).

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If the SCTP association is brought down because the IFPIX Messages
can't be exported reliably, the options are:

o Increase the SCTP buffer size on the Exporter.

o Increase the bandwidth available for communicating the exported
Data Records.

o Use sampling, filtering, or aggregation in the Metering Process to
reduce the amount of exported data.

Note that Templates must not be resent when using SCTP, without an
intervening Template Withdrawal or SCTP association reset. Note also
that since Template Sets and Template Withdrawal Messages may be sent
on any SCTP stream, a Template Withdrawal Message may withdraw a
Template sent on a different stream, and a Template Set may reuse a
Template ID withdrawn by a Template Withdrawal Message sent on a
different stream. Therefore, an Exporting Process sending Template
Withdrawal Messages should ensure to the extent possible that the
Template Withdrawal Messages and subsequent Template Sets reusing the
withdrawn Template IDs are received and processed at the Collecting
Process in proper order. The Exporting Process can achieve this by
one of two possible methods: 1. by sending a Template Withdrawal
Message reliably, in order, and on the same stream as the subsequent
Template Set reusing its ID; or 2. by waiting an appropriate amount
of time (on the scale of one minute) after sending a Template
Withdrawal Message before attempting to reuse the withdrawn Template
ID.

6.2. UDP

UDP is useful in simple systems where an SCTP stack is not available,
and where there is insufficient memory for TCP buffering.

However, UDP is not a reliable transport protocol, and IPFIX Messages
sent over UDP might be lost as with partially reliable SCTP streams.
UDP is not the recommended protocol for IPFIX and is intended for use
in cases in which IPFIX is replacing an existing NetFlow
infrastructure, with the following properties:

o A dedicated network,

o within a single administrative domain,

o where SCTP is not available due to implementation constraints, and

o the Collector is as topologically close as possible to the
Exporter.

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Note that because UDP itself provides no congestion control
mechanisms, it is recommended that UDP transport be used only on
managed networks, where the network path has been explicitly
provisioned for IPFIX traffic through traffic engineering mechanisms,
such as rate limiting or capacity reservations.

An important example of an explicitly provisioned, managed network
for IPFIX is the use of IPFIX to replace a functioning NetFlow
implementation on a dedicated network. In this situation, the
dedicated network should be provisioned in accordance with the
NetFlow deployment experience that Flow export traffic generated by
monitoring an interface will amount to 2-5% of the monitored
interface's bandwidth.

As recommended in [TSVWG-UDP], an application should not send UDP
messages that result in IP packets that exceed the MTU of the path to
the destination and should enable UDP checksums (see Sections 3.2 and
3.4 of [TSVWG-UDP], respectively).

Since IPFIX assumes reliable transport of Templates over SCTP, this
necessitates some changes for IPFIX Template management over UDP.
Templates sent from the Exporting Process to the Collecting Process
over UDP MUST be resent at regular time intervals; these intervals
MUST be configurable (see Section 10.3 of [RFC5101]).

We recommend a default Template-resend time of 10 minutes,
configurable between 1 minute and 1 day.

Note that this could become an interoperability problem; e.g., if an
Exporter resends Templates once per day, while a Collector expires
Templates hourly, then they may both be IPFIX-compatible, but not be
interoperable.

Retransmission time intervals that are too short waste bandwidth on
unnecessary Template retransmissions. On the other hand, time
intervals that are too long introduce additional costs or risk of
data loss by potentially requiring the Collector to cache more data
without having the Templates available to decode it.

To increase reliability and limit the amount of potentially lost
data, the Exporting Process may resend additional Templates using a
packet-based schedule. In this case, Templates are resent depending
on the number of data packets sent. Similarly to the time interval,
resending a Template every few packets introduces additional
overhead, while resending after a large amount of packets have
already been sent means high costs due to the data caching and
potential data loss.

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We recommend a default Template-resend interval of 20 packets,
configurable between 1 and 1000 data packets.

Note that a sufficiently small resend time or packet interval may
cause a system to become stuck, continually resending Templates or
Options Data. For example, if the resend packet interval is 2 (i.e.,
Templates or Options Data are to be sent in every other packet) but
more than two packets are required to send all the information, then
the resend interval will have expired by the time the information has
been sent, and Templates or Options Data will be sent continuously --
possibly preventing any data from being sent at all. Therefore, the
resend intervals should be considered from the last data packet, and
should not be tied to specific Sequence Numbers.

The Collecting Process should use the Sequence Number in the IPFIX
Message Header to determine whether any messages are lost.

The following may be done to mitigate message loss:

o Move the Collector topologically closer to the Exporter.

o Increase the bandwidth of the links through which the Data Records
are exported.

o Use sampling, filtering, or aggregation in the Metering Process to
reduce the amount of exported data.

o Increase the buffer size at the Collector and/or the Exporter.

Before using a Template for the first time, the Exporter may send it
in several different IPFIX Messages spaced out over a period of
packets in order to increase the likelihood that the Collector has
received the Template.

Template Withdrawal Messages MUST NOT be sent over UDP (per Section
10.3.6 of [RFC5101]). The Exporter must rely on expiration at the
Collector to expire old Templates or to reuse Template IDs.

We recommend that the Collector implements a Template Expiry of three
times the Exporter refresh rate.

However, since the IPFIX protocol doesn't provide any mechanism for
the Exporter to convey any information about the Template Expiry time
to the Collector, configuration must be done out of band.

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If no out-of-band configuration is made, we recommend to initially
set a Template Expiry time at the Collector of 60 minutes. The
Collecting Process may estimate each Exporting Process's resend time
and adapt the Expiry time for the corresponding Templates
accordingly.

6.3. TCP

TCP can be used as a transport protocol for IPFIX if one of the
endpoints has no support for SCTP, but a reliable transport is needed
and/or the network between the Exporter and the Collector has not
explicitly been provisioned for the IPFIX traffic. TCP is one of the
core protocols of the Internet and is widely supported.

The Exporting Process may resend Templates (per UDP, above), but it's
not required to do so, per Section 10.4.2.2 of [RFC5101]:

"A Collecting Process MUST record all Template and Options Template
Records for the duration of the connection, as an Exporting Process
is not required to re-export Template Records."

If the available bandwidth between Exporter and Collector is not
sufficient or the Metering Process generates more Data Records than
the Collector is capable of processing, then TCP congestion control
may cause the Exporter to block. Options in this case are:

o Increase the TCP buffer size on the Exporter.

o Increase the bandwidth of the links through which the Data Records
are exported.

o Use sampling, filtering, or aggregation in the Metering Process to
reduce the amount of exported data.

7. Guidelines for Implementation on Middleboxes

The term middlebox is defined in [RFC3234] as:

"any intermediary device performing functions other than the normal,
standard functions of an IP router on the datagram path between a
source host and destination host."

The list of middleboxes discussed in [RFC3234] contains:

1. Network Address Translation (NAT),

2. NAT-Protocol Translation (NAT-PT),

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3. SOCKS gateway,

4. IP tunnel endpoints,

5. packet classifiers, markers, schedulers,

6. transport relay,

7. TCP performance enhancing proxies,

8. load balancers that divert/munge packets,

9. IP firewalls,

10. application firewalls,

11. application-level gateways,

12. gatekeepers / session control boxes,

13. transcoders,

14. proxies,

15. caches,

16. modified DNS servers,

17. content and applications distribution boxes,

18. load balancers that divert/munge URLs,

19. application-level interceptors,

20. application-level multicast,

21. involuntary packet redirection,

22. anonymizers.

It is likely that since the publication of RFC 3234 new kinds of
middleboxes have been added.

While the IPFIX specifications [RFC5101] based the requirements on
the export protocol only (as the IPFIX name implies), these sections
cover the guidelines for the implementation of the Metering Process
by recommending which Information Elements to export for the
different middlebox considerations.

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7.1. Traffic Flow Scenarios at Middleboxes

Middleboxes may delay, reorder, drop, or multiply packets; they may
change packet header fields and change the payload. All these
actions have an impact on traffic Flow properties. In general, a
middlebox transforms a unidirectional original traffic Flow T that
arrives at the middlebox into a transformed traffic Flow T' that
leaves the middlebox.

+-----------+
T ---->| middlebox |----> T'
+-----------+

Figure 1: Unidirectional traffic Flow traversing a middlebox

Note that in an extreme case, T' may be an empty traffic Flow (a Flow
with no packets), for example, if the middlebox is a firewall and
blocks the Flow.

In case of a middlebox performing a multicast function, a single
original traffic Flow may be transformed into more than one
transformed traffic Flow.

+------> T'
|
+---------+-+
T ---->| middlebox |----> T''
+---------+-+
|
+------> T'''

Figure 2: Unidirectional traffic Flow traversing a middlebox with
multicast function

For bidirectional traffic Flows, we identify Flows on different sides
of the middlebox; say, T_l on the left side and T_r on the right
side.

+-----------+
T_l <--->| middlebox |<---> T_r
+-----------+

Figure 3: Bidirectional unicast traffic Flow traversing a middlebox

In case of a NAT, T_l might be a traffic Flow in a private address
realm and T_r the translated traffic Flow in the public address
realm. If the middlebox is a NAT-PT, then T_l may be an IPv4 traffic
Flow and T_r the translated IPv6 traffic Flow.

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At tunnel endpoints, Flows are multiplexed or demultiplexed. In
general, tunnel endpoints can deal with bidirectional traffic Flows.

+------> T_r1
v
+---------+-+
T_l <--->| middlebox |<---> T_r2
+---------+-+
^
+------> T_r3

Figure 4: Multiple data reduction

An example is a traffic Flow T_l of a tunnel and Flows T_rx that are
multiplexed into or demultiplexed out of a tunnel. According to the
IPFIX definition of traffic Flows in [RFC5101], T and T' or T_l and
T_rx, respectively, are different Flows in general.

However, from an application point of view, they might be considered
as closely related or even as the same Flow, for example, if the
payloads they carry are identical.

7.2. Location of the Observation Point

Middleboxes might be integrated with other devices. An example is a
router with a NAT or a firewall at a line card. If an IPFIX
Observation Point is located at the line card, then the properties of
measured traffic Flows may depend on the side of the integrated
middlebox at which packets were captured for traffic Flow
measurement.

Consequently, an Exporting Process reporting traffic Flows measured
at a device that hosts one or more middleboxes should clearly
indicate to Collecting Processes the location of the used Observation
Point(s) with respect to the middlebox(es). This can be done by
using Options with Observation Point as scope and elements like, for
instance, lineCardID or samplerID. Otherwise, processing the
measured Flow data could lead to wrong results.

At first glance, choosing an Observation Point that covers the entire
middlebox looks like an attractive choice. But this leads to
ambiguities for all kinds of middleboxes. Within the middlebox,
properties of packets are modified, and it should be clear at a
Collecting Process whether packets were observed and metered before
or after modification. For example, it must be clear whether a
reported source IP address was observed before or after a NAT changed
it or whether a reported packet count was measured before or after a

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firewall dropped packets. For this reason, [RFC5102] provides
Information Elements with prefix "post" for Flow properties that are
changed within a middlebox.

If an Observation Point is located inside a middlebox, the middlebox
must have well-defined and well-separated internal functions, for
example, a combined NAT and firewall, and the Observation Point
should be located on a boundary between middlebox functions rather
than within one of the functions.

7.3. Reporting Flow-Related Middlebox Internals

While this document recommends IPFIX implementations using
Observation Points outside of middlebox functions, there are a few
special cases where reporting Flow-related internals of a middlebox
is of interest.

For many applications that use traffic measurement results, it is
desirable to get more information than can be derived from just
observing packets on one side of a middlebox. If, for example,
packets are dropped by the middlebox acting as a firewall, NAT, or
traffic shaper, then information about how many observed packets are
dropped may be of high interest.

This section gives recommendations on middlebox internal information
that may be reported if the IPFIX Observation Point is co-located
with one or more middleboxes. Since the internal information to be
reported depends on the kind of middlebox, it is discussed per kind.

The recommendations cover middleboxes that act per packet and that do
not modify the application-level payload of the packet (except by
dropping the entire packet) and that do not insert additional packets
into an application-level or transport-level traffic stream.

Covered are the packet-level middleboxes of kinds 1, 2, 3, 5, 9, 10,
21, and 22 (according to the enumeration given at the beginning of
Section 7 of this document). Not covered are 4, 6-8 and 11-20. TCP
performance-enhancing proxies (7) are not covered because they may
add ACK packets to a TCP connection.

Still, if possible, IPFIX implementations co-located with uncovered
middleboxes (i.e., of type 7 or 11-20) should follow the
recommendations given in this section if they can be applied in a way
that reflects the intention of these recommendations.

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7.3.1. Packet Dropping Middleboxes

If an IPFIX Observation Point is co-located with one or more
middleboxes that potentially drop packets, then the corresponding
IPFIX Exporting Process should be able to report the number of
packets that were dropped per reported Flow.

Concerned kinds of middleboxes are NAT (1), NAT-PT (2), SOCKS gateway
(3), packet schedulers (5), IP firewalls (9) and application-level
firewalls (10).

7.3.2. Middleboxes Changing the DSCP

If an IPFIX Observation Point is co-located with one or more
middleboxes that potentially modify the Diffserv Code Point (DSCP,
see [RFC2474]) in the IP header, then the corresponding IPFIX
Exporting Process should be able to report both the observed incoming
DSCP value and also the DSCP value on the 'other' side of the
middlebox (if this is a constant value for the particular traffic
flow). The related Information Elements specified in [RFC5102] are:
IpClassOfService and postIpClassOfService.

Note that the current IPFIX information model only contains
Information Elements supporting packets observed before the DSCP
change, i.e. ipClassOfService and postIpClassOfService, where the
latter reports the value of the IP TOS field after the DSCP change.
We recommend, whenever possible, to move the Observation Point to the
point before the DSCP change and report the Observed and post-
values. If reporting the value of the IP TOS field before DSCP
change is required, "pre" values can be exported using enterprise-
specific Information Elements.

Note also that a classifier may change the same DSCP value of packets
from the same Flow to different values depending on the packet or
other conditions. Also, it is possible that packets of a single
unidirectional arriving Flow contain packets with different DSCP
values that are all set to the same value by the middlebox. In both
cases, there is a constant value for the DSCP field in the IP packet
header to be observed on one side of the middlebox, but on the other
side the value may vary. In such a case, reliable reporting of the
DSCP value on the 'other' side of the middlebox is not possible by
just reporting a single value. According to the IPFIX information
model [RFC5102], the first value observed for the DSCP is reported by
the IPFIX protocol in that case.

This recommendation applies to packet markers (5).

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7.3.3. Middleboxes Changing IP Addresses and Port Numbers

If an IPFIX Observation Point is co-located with one or more
middleboxes that potentially modify the:

o IP version field,

o IP source address header field,

o IP destination address header field,

o Source transport port number, or

o Destination transport port number

in one of the headers, then the corresponding IPFIX Exporting Process
should be able to report the 'translated' value of these fields, as
far as they have constant values for the particular traffic Flow, in
addition to the observed values of these fields.

If the changed values are not constant for the particular traffic
Flow but still reporting is desired, then it is recommended that the
general rule from [RFC5102] for Information Elements with changing
values is applied: the reported value is the one that applies to the
first packet observed for the reported Flow.

Note that the 'translated' value of the fields can be the values
before or after the translation depending on the Flow direction and
the location of the Observation Point with respect to the middlebox.
We always call the value that is not the one observed at the
Observation Point the translated value.

Note also that a middlebox may change the same port number value of
packets from the same Flow to different values depending on the
packet or other conditions. Also, it is possible that packets of
different unidirectional arriving Flows with different source/
destination port number pairs may be mapped to a single Flow with a
single source/destination port number pair by the middlebox. In both
cases, there is a constant value for the port number pair to be
observed on one side of the middlebox, but on the other side the
values may vary. In such a case, reliable reporting of the port
number pairs on the 'other' side of the middlebox is not possible.
According to the IPFIX information model [RFC5102], the first value
observed for each port number is reported by the IPFIX protocol in
that case.

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This recommendation applies to NAT (1), NAT-PT (2), SOCKS gateway (3)
and involuntary packet redirection (21) middleboxes. It may also be
applied to anonymizers (22), though it should be noted that this
carries the risk of losing the effect of anonymization.

8. Security Guidelines

8.1. Introduction to TLS and DTLS for IPFIX Implementers

Transport Layer Security (TLS) [RFC4346] and Datagram Transport Layer
Security (DTLS) [RFC4347] are the REQUIRED protocols for securing
network traffic exported with IPFIX (see Section 11 of [RFC5101]).
TLS requires a reliable transport channel and is selected as the
security mechanism for TCP. DTLS is a version of TLS capable of
securing datagram traffic and is selected for UDP, SCTP, and PR-SCTP.

When mapping TLS terminology used in [RFC4346] to IPFIX terminology,
keep in mind that the IPFIX Exporting Process, as it is the
connection initiator, corresponds to the TLS client, and the IPFIX
Collecting Process corresponds to the TLS server. These terms apply
only to the bidirectional TLS handshakes done at Transport Session
establishment and completion time; aside from TLS connection set up
between the Exporting Process and the Collecting Process, and
teardown at the end of the session, the unidirectional Flow of
messages from Exporting Process to Collecting Process operates over
TLS just as over any other transport layer for IPFIX.

8.2. X.509-Based Identity Verification for IPFIX over TLS or DTLS

When using TLS or DTLS to secure an IPFIX Transport Session, the
Collecting Process and Exporting Process must use strong mutual
authentication. In other words, each IPFIX endpoint must have its
own X.509 certificate [RFC3280] and private key, and the Collecting
Process, which acts as the TLS or DTLS server, must send a
Certificate Request to the Exporting Process during the TLS
handshake, and fail to establish a session if the Exporting Process
does not present a valid certificate.

Each Exporting Process and Collecting Process must verify the
identity of its peer against a set of authorized peers. This may be
done by configuring a set of authorized distinguished names and
comparing the peer certificate's subject distinguished name against
each name in the set. However, if a private certification authority
(CA) is used to sign the certificates identifying the Collecting
Processes and Exporting Processes, and the set of certificates signed
by that private CA may be restricted to those identifying peers
authorized to communicate with each other, it is sufficient to merely
verify that the peer's certificate is issued by this private CA.

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When verifying the identity of its peer, an IPFIX Exporting Process
or Collecting Process must verify that the peer certificate's subject
common name or subjectAltName extension dNSName matches the fully-
qualified domain name (FQDN) of the peer. This involves retrieving
the expected domain name from the peer certificate and the address of
the peer, then verifying that the two match via a DNS lookup. Such
verification should require both that forward lookups (FQDN to peer
address) and reverse lookups (peer address to FQDN) match. In
deployments without DNS infrastructure, it is acceptable to represent
the FQDN as an IPv4 dotted-quad or a textual IPv6 address as in
[RFC1924].

8.3. Implementing IPFIX over TLS over TCP

Of the security solutions specified for IPFIX, TLS over TCP is as of
this writing the most mature and widely implemented. Until stable
implementations of DTLS over SCTP are widely available (see
Section 8.5, below), it is recommended that applications requiring
secure transport for IPFIX Messages use TLS over TCP.

When using TLS over TCP, IPFIX Exporting Processes and Collecting
Processes should behave in all other aspects as if using TCP as the
transport protocol, especially as regards the handling of Templates
and Template withdrawals.

8.4. Implementing IPFIX over DTLS over UDP

An implementation of the DTLS protocol version 1, described in
[RFC4347] and required to secure IPFIX over UDP, is available in
OpenSSL [OPENSSL] as of version 0.9.8. However, DTLS support is as
of this writing under active development and certain implementations
might be unstable. We recommend extensive testing of DTLS-based
IPFIX implementations to build confidence in the DTLS stack over
which your implementation runs.

When using DTLS over UDP, IPFIX Exporting Processes and Collecting
Processes should behave in all other aspects as if using UDP as the
transport protocol, especially as regards the handling of Templates
and Template timeouts.

Note that the selection of IPFIX Message sizes for DTLS over UDP must
account for overhead per packet introduced by the DTLS layer.

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8.5. Implementing IPFIX over DTLS over SCTP

As of this writing, there is no publicly available implementation of
DTLS over SCTP as described in [RFC4347] and [TUEXEN].

When using DTLS over SCTP, IPFIX Exporting Processes and Collecting
Processes should behave in all other aspects as if using SCTP as the
transport protocol, especially as regards the handling of Templates
and the use of reliable transport for Template and scope information.

An implementation of the DTLS protocol version 1, described in
[RFC4347] and required to secure IPFIX over SCTP, is available in
OpenSSL [OPENSSL] as of version 0.9.8. However, DTLS support is as
of this writing under active development and certain implementations
might be unstable. We recommend extensive testing of DTLS-based
IPFIX implementations to build confidence in the DTLS stack over
which your implementation runs.

9. Extending the Information Model

IPFIX supports two sets of Information Elements: IANA-registered
Information Elements and enterprise-specific Information Elements.
New Information Elements can be added to both sets as described in
this section. If an Information Element is considered of general
interest, it should be added to the set of IETF-specified Information
Elements that is maintained by IANA.

Alternatively, private enterprises can define proprietary Information
Elements for internal purposes. There are several potential reasons
for doing so. For example, the Information Element might only relate
to proprietary features of a device or protocol of the enterprise.
Also, pre-standard product delivery or commercially sensitive product
features might cause the need for enterprise-specific Information
Elements.

The IPFIX information model [RFC5102] document contains an XML-based
specification of Template, abstract data types, and IPFIX Information
Elements, which may be used to create consistent machine-readable
extensions to the IPFIX information model. This description can be
used for automatically checking syntactic correctness of the
specification of IPFIX Information Elements and for generating code
that deals with processing IPFIX Information Elements.

9.1. Adding New IETF-Specified Information Elements

New IPFIX Information Elements that are considered to be of general
interest should be added to the set of IETF-specified Information
Elements that is maintained by IANA.

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The introduction of new Information Elements in the IANA registry is
subject to expert review. As described in Section 7.1 of [RFC5102],
an expert review is performed by one of a group of experts designated
by an IETF Operations and Management Area Director. The experts will
initially be drawn from the Working Group Chairs and document editors
of the IPFIX and PSAMP Working Groups. The group of experts must
double check the Information Elements definitions with already
defined Information Elements for completeness, accuracy, redundancy,
and correct naming following the naming conventions in [RFC5102],
Section 2.3.

The specification of new IPFIX Information Elements must use the
Template specified in [RFC5102], Section 2.1, and must be published
using a well-established and persistent publication medium.

9.2. Adding Enterprise-Specific Information Elements

Enterprises or other organizations holding a registered Structure of
Management Information (SMI) network management private enterprise
code number can specify enterprise-specific Information Elements.
Their identifiers can be chosen arbitrarily within the range of
1-32767 and have to be coupled with a Private Enterprise Identifier
[PEN]. Enterprise identifiers MUST be registered as SMI network
management private enterprise code numbers with IANA. The registry
can be found at http://www.iana.org/assignments/enterprise-numbers.

10. Common Implementation Mistakes

The issues listed in this section were identified during
implementation and interoperability testing. They do not stem from
insufficient clarity in the protocol, but each of these was an actual
mistake made in a tested IPFIX implementation. They are listed here
for the convenience of future implementers.

10.1. IPFIX and NetFlow Version 9

A large group of mistakes stems from the fact that many implementers
started implementing IPFIX from an existing version of NetFlow
version 9 [RFC3954]. Despite their similarity, the two protocols
differ in many aspects. We list here some of the most important
differences.

o Transport protocol: NetFlow version 9 initially ran over UDP,
while IPFIX must have a congestion-aware transport protocol.
IPFIX specifies PR-SCTP as its mandatory protocol, while TCP and
UDP are optional.

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o IPFIX differentiates between IANA-registered and enterprise-
specific Information Elements. Enterprise-specific Information
Elements can be specified by coupling a non-IANA-registered
Information Element identifier with an Enterprise ID
(corresponding to the vendor that defined the Information
Element).

o Options Templates: in IPFIX, an Options Template must have a
scope, and the scope is not allowed to be of length zero. The
NetFlow version 9 specifications [RFC3954] don't specify that the
scope must not be of length zero.

Message Header:

o Set ID: Even if the packet headers are different between IPFIX and
NetFlow version 9, similar fields are used in both of them. The
difference between the two protocols is in the values that these
fields can assume. A typical example is the Set ID values: the
Set ID values of 0 and 1 are used in NetFlow version 9, while they
are not used in IPFIX.

o Length field: in NetFlow version 9, this field (called count)
contains the number of Records. In IPFIX, it indicates the total
length of the IPFIX Message, measured in octets (including Message
Header and Set(s)).

o Timestamp: the NetFlow version 9 header has an additional
timestamp: sysUpTime. It indicates the time in milliseconds since
the last reboot of the Exporting Process.

o The version number is different. NetFlow version 9 uses the
version number 9, while IPFIX uses the version number 10.

10.2. Padding of the Data Set

[RFC5101] specifies that the Exporting Process MAY insert some octets
for set padding to align Data Sets within a Message. The padding
length must be shorter than any allowable Record in that set.

It is important to respect this limitation: if the padding length is
equal to or longer than the length of the shortest Record, it will be
interpreted as another Record.

An alternative is to use the paddingOctets Information Element in the
Template definition.

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10.3. Field ID Numbers

Information Element numbers in IPFIX have the range 0-32767
(0-0x7FFF). Information Element numbers outside this range (i.e.,
with the high bit set) are taken to be enterprise-specific
Information Elements, which have an additional four-byte Private
Enterprise Number following the Information Element number and
length. Inadvertently setting the high bit of the Information
Element number by selecting a number out of this range will therefore
cause Template scanning errors.

10.4. Template ID Numbers

Template IDs are generated as required by the Exporting Process.
When the same set of Information Elements is exported at different
times, the corresponding Template is usually identified by different
Template IDs. Similarly, if multiple co-existing Templates are
composed of the same set of Information Elements, they are also
identified by different Template IDs. The Collecting Process does
not know in advance which Template ID a particular Template will use.

11. Security Considerations

This document describes the implementation guidelines of IPFIX. The
security requirements for the IPFIX target applications are addressed
in the IPFIX requirements document [RFC3917]. These requirements are
considered for the specification of the IPFIX protocol [RFC5101], for
which a Security Considerations Section exists.

Section 7 of this document recommends that IPFIX Exporting Processes
report internals about middleboxes. These internals may be security-
relevant, and the reported information needs to be protected
appropriately for reasons given below.

Reporting of packets dropped by firewalls and other packet-dropping
middleboxes carries the risk that this information can be used by
attackers for analyzing the configuration of the middlebox and for
developing attacks against it. Address translation may be used for
hiding the network structure behind an address translator. If an
IPFIX Exporting Process reports the translations performed by an
address translator, then parts of the network structure may be
revealed. If an IPFIX Exporting Process reports the translations
performed by an anonymizer, the main function of the anonymizer may
be compromised.

Note that there exist vulnerabilities in DTLS over SCTP as specified
in the IPFIX protocol, such that a third party could cause messages
to be undetectably lost, or an SCTP association to shut down. These

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RFC 5153 IPFIX Implementation Guidelines April 2008

vulnerabilities are addressed by [TUEXEN]; however, it is unclear
whether initial OpenSSL-based implementations of DTLS over SCTP will
contain the required fixes. DTLS over SCTP should be used with
caution in production environments until these issues are completely
addressed.

12. Acknowledgments

We would like to thank the MoMe project for organizing two IPFIX
Interoperability Events in July 2005 and in March 2006, and
Fraunhofer Fokus for organizing the third one in November 2006. The
Interoperability Events provided us precious input for this document.
Thanks to Brian Trammell for his contributions to the SCTP section
and the security guidelines and for the multiple thorough reviews.
We would also like to thank Benoit Claise, Carsten Schmoll, and
Gerhard Muenz for the technical review and feedback, and Michael
Tuexen, Randall Stewart, and Peter Lei for reviewing the SCTP
section.

13. References

13.1. Normative References

[RFC5101] Claise, B., Ed., "Specification of the IP Flow
Information Export (IPFIX) Protocol for the
Exchange of IP Traffic Flow Information", RFC 5101,
January 2008.

[RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P.,
and J. Meyer, "Information Model for IP Flow
Information Export", RFC 5102, January 2008.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

13.2. Informative References

[IPFIX-AS] Zseby, T., Boschi, E., Brownlee, N., and B. Claise,
"IPFIX Applicability", Work in Progress, July 2007.

[IPFIX-ARCH] Sadasivan, G., Brownlee, N., Claise, B., and J.
Quittek, "Architecture for IP Flow Information
Export", Work in Progress, September 2006.

[IPFIX-REDUCING] Boschi, E., Mark, L., and B. Claise, "Reducing
Redundancy in IP Flow Information Export (IPFIX)
and Packet Sampling (PSAMP) Reports", Work
in Progress, May 2007.

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RFC 5153 IPFIX Implementation Guidelines April 2008

[PSAMP-PROTO] Claise, B., Quittek, J., and A. Johnson, "Packet
Sampling (PSAMP) Protocol Specifications", Work
in Progress, December 2007.

[TUEXEN] Tuexen, M. and E. Rescorla, "Datagram Transport
Layer Security for Stream Control Transmission
Protocol", Work in Progress, November 2007.

[TSVWG-UDP] Eggert, L. and G. Fairhurst, "UDP Usage Guidelines
for Application Designers", Work in Progress,
February 2008.

[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis",
RFC 1305, March 1992.

[RFC1924] Elz, R., "A Compact Representation of IPv6
Addresses", RFC 1924, April 1996.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field
(DS Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.

[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy
and Issues", RFC 3234, February 2002.

[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo,
"Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL)
Profile", RFC 3280, April 2002.

[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and
P. Conrad, "Stream Control Transmission Protocol
(SCTP) Partial Reliability Extension", RFC 3758,
May 2004.

[RFC3917] Quittek, J., Zseby, T., Claise, B., and S. Zander,
"Requirements for IP Flow Information Export
(IPFIX)", RFC 3917, October 2004.

[RFC3954] Claise, B., Ed., "Cisco Systems NetFlow Services
Export Version 9", RFC 3954, October 2004.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.1", RFC 4346,
April 2006.

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RFC 5153 IPFIX Implementation Guidelines April 2008

[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport
Layer Security", RFC 4347, April 2006.

[RFC4960] Stewart, R., Ed., "Stream Control Transmission
Protocol", RFC 4960, September 2007.

[OPENSSL] OpenSSL, "OpenSSL: The Open Source toolkit for SSL/
TLS", <http://www.openssl.org/>.

[PEN] IANA, "PRIVATE ENTERPRISE NUMBERS", <http://
www.iana.org/assignments/enterprise-numbers>.

Authors' Addresses

Elisa Boschi
Hitachi Europe
c/o ETH Zurich
Gloriastr. 35
8092 Zurich
Switzerland

Phone: +41 44 6327057
EMail: [email protected]

Lutz Mark
Fraunhofer FOKUS
Kaiserin Augusta Allee 31
10589 Berlin
Germany

Phone: +49 421 2246-206
EMail: [email protected]

Juergen Quittek
NEC Europe Ltd.
Kurfuersten-Anlage 36
69115 Heidelberg
Germany

Phone: +49 6221 4342-115
EMail: [email protected]

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RFC 5153 IPFIX Implementation Guidelines April 2008

Martin Stiemerling
NEC Europe Ltd.
Kurfuersten-Anlage 36
69115 Heidelberg
Germany

Phone: +49 6221 4342-113
EMail: [email protected]

Paul Aitken
Cisco Systems, Inc.
96 Commercial Quay
Edinburgh EH6 6LX
Scotland

Phone: +44 131 561 3616
EMail: [email protected]

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