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Internet Engineering Task Force (IETF)
Request for Comments: 5982
Category: Informational
ISSN: 2070-1721
A. Kobayashi, Ed.
NTT PF Lab.
B. Claise, Ed.
Cisco Systems, Inc.
August 2010

IP Flow Information Export (IPFIX) Mediation: Problem Statement

Abstract

Flow-based measurement is a popular method for various network monitoring usages. The sharing of flow-based information for monitoring applications having different requirements raises some open issues in terms of measurement system scalability, flow-based measurement flexibility, and export reliability that IP Flow Information Export (IPFIX) Mediation may help resolve. This document describes some problems related to flow-based measurement that network administrators have been facing, and then it describes IPFIX Mediation applicability examples along with the problems.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5982.

Copyright Notice

Copyright © 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology and Definitions .....................................3
   3. IPFIX/PSAMP Documents Overview ..................................5
      3.1. IPFIX Documents Overview ...................................5
      3.2. PSAMP Documents Overview ...................................5
   4. Problem Statement ...............................................5
      4.1. Coping with IP Traffic Growth ..............................6
      4.2. Coping with Multipurpose Traffic Measurement ...............7
      4.3. Coping with Heterogeneous Environments .....................7
      4.4. Summary ....................................................7
   5. Mediation Applicability Examples ................................8
      5.1. Adjusting Flow Granularity .................................8
      5.2. Collecting Infrastructure ..................................8
      5.3. Correlation for Data Records ...............................9
      5.4. Time Composition ...........................................9
      5.5. Spatial Composition .......................................10
      5.6. Data Record Anonymization .................................11
      5.7. Data Retention ............................................11
      5.8. IPFIX Export from a Branch Office .........................12
      5.9. Distributing Data Record Types ............................13
      5.10. Flow-Based Sampling and Selection ........................14
      5.11. Interoperability between Legacy Protocols and IPFIX ......15
   6. IPFIX Mediators' Implementation-Specific Problems ..............15
      6.1. Loss of Original Exporter Information .....................15
      6.2. Loss of Base Time Information .............................16
      6.3. Transport Sessions Management .............................16
      6.4. Loss of Options Template Information ......................16
      6.5. Template ID Management ....................................17
      6.6. Consideration for Network Topology ........................18
      6.7. IPFIX Mediation Interpretation ............................18
      6.8. Consideration for Aggregation .............................19
   7. Summary and Conclusion .........................................20
   8. Security Considerations ........................................20
   9. Acknowledgements ...............................................21
   10. References ....................................................22
      10.1. Normative References .....................................22
      10.2. Informative References ...................................22
   Contributors ......................................................24

1. Introduction

An advantage of flow-based measurement is that it allows monitoring large amounts of traffic observed at distributed Observation Points. While flow-based measurement can be applied to one of various purposes and applications, it is difficult for flow-based measurement to apply to multiple applications with very different requirements in parallel. Network administrators need to adjust the parameters of the metering devices to fulfill the requirements of every single measurement application. Such configurations are often not supported by the metering devices, either because of functional restrictions or because of limited computational and memory resources, which inhibit the metering of large amounts of traffic with the desired setup. IP Flow Information Export (IPFIX) Mediation fills the gap between restricted metering capabilities and the requirements of measurement applications by introducing an intermediate device called the IPFIX Mediator.

The IPFIX requirements defined in [RFC3917] mention examples of intermediate devices located between Exporters and Collectors, such as IPFIX proxies or concentrators. But, there are no documents defining a generalized concept for such intermediate devices. This document addresses that issue by defining IPFIX Mediation -- a generalized intermediate device concept for IPFIX -- and examining in detail the motivations behind its application.

This document is structured as follows: Section 2 describes the terminology used in this document, Section 3 gives an IPFIX/Packet Sampling (PSAMP) document overview, Section 4 introduces general problems related to flow-based measurement, Section 5 describes some applicability examples where IPFIX Mediation would be beneficial, and, finally, Section 6 describes some problems an IPFIX Mediation implementation might face.

2. Terminology and Definitions

The IPFIX-specific and PSAMP-specific terminology used in this document is defined in [RFC5101] and [RFC5476], respectively. In this document, as in [RFC5101] and [RFC5476], the first letter of each IPFIX-specific and PSAMP-specific term is capitalized along with the IPFIX Mediation-specific terms defined here.

In this document, we call "record stream" a stream of records carrying flow- or packet-based information. The records may be encoded as IPFIX Data Records or in any other format. 

   Original Exporter

An Original Exporter is an IPFIX Device that hosts the Observation Points where the metered IP packets are observed. 

   IPFIX Mediation

IPFIX Mediation is the manipulation and conversion of a record stream for subsequent export using the IPFIX protocol.

The following terms are used in this document to describe the architectural entities used by IPFIX Mediation. 

   Intermediate Process

An Intermediate Process takes a record stream as its input from Collecting Processes, Metering Processes, IPFIX File Readers, other Intermediate Processes, or other record sources; performs some transformations on this stream, based upon the content of each record, states maintained across multiple records, or other data sources; and passes the transformed record stream as its output to Exporting Processes, IPFIX File Writers, or other Intermediate Processes, in order to perform IPFIX Mediation. Typically, an Intermediate Process is hosted by an IPFIX Mediator. Alternatively, an Intermediate Process may be hosted by an Original Exporter. 

   IPFIX Mediator

An IPFIX Mediator is an IPFIX Device that provides IPFIX Mediation by receiving a record stream from some data sources, hosting one or more Intermediate Processes to transform that stream, and exporting the transformed record stream into IPFIX Messages via an Exporting Process. In the common case, an IPFIX Mediator receives a record stream from a Collecting Process, but it could also receive a record stream from data sources not encoded using IPFIX, e.g., in the case of conversion from the NetFlow V9 protocol [RFC3954] to the IPFIX protocol.

Note that the IPFIX Mediator is a generalization of the concentrator and proxy elements envisioned in the IPFIX requirements [RFC3917]. IPFIX Mediators running appropriate Intermediate Processes provide the functionality specified therein.

3. IPFIX/PSAMP Documents Overview

IPFIX Mediation can be applied to Flow- or packet-based information. The Flow-based information is encoded as IPFIX Flow Records by the IPFIX protocol, and the packet-based information is extracted by some packet selection techniques and then encoded as PSAMP Packet Reports by the PSAMP protocol. Thus, this section describes relevant documents for both protocols.

3.1. 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 from an IPFIX Exporting Process to a Collecting Process is defined in [RFC5470], per the requirements defined in [RFC3917]. The IPFIX protocol [RFC5101] specifies how IPFIX Data Records and Templates are carried via a number of transport protocols from IPFIX Exporting Processes to IPFIX Collecting Processes. IPFIX has a formal description of IPFIX Information Elements, their names, types, and additional semantic information, as specified in [RFC5102]. [RFC5815] specifies the IPFIX Management Information Base. Finally, [RFC5472] describes what types of applications can use the IPFIX protocol and how they can use the information provided. Furthermore, it shows how the IPFIX framework relates to other architectures and frameworks. The storage of IPFIX Messages in a file is specified in [RFC5655].

3.2. PSAMP Documents Overview

The framework for packet selection and reporting [RFC5474] enables network elements to select subsets of packets by statistical and other methods and to export a stream of reports on the selected packets to a Collector. The set of packet selection techniques (Sampling and Filtering) standardized by PSAMP is described in [RFC5475]. The PSAMP protocol [RFC5476] specifies the export of packet information from a PSAMP Exporting Process to a Collector. Like IPFIX, PSAMP has a formal description of its Information Elements, their names, types, and additional semantic information. The PSAMP information model is defined in [RFC5477]. [PSAMP-MIB] describes the PSAMP Management Information Base.

4. Problem Statement

Network administrators generally face the problems of measurement system scalability, Flow-based measurement flexibility, and export reliability, even if some techniques, such as Packet Sampling, Filtering, Data Records aggregation, and export replication, have already been developed. The problems consist of adjusting some parameters of metering devices to resources of the measurement system while fulfilling appropriate conditions: data accuracy, Flow granularity, and export reliability. These conditions depend on two factors.

  • Measurement system capacity: This consists of the bandwidth of the management network, the storage capacity, and the performances of the collecting devices and exporting devices.
  • Application requirements: Different applications, such as traffic engineering, detecting traffic anomalies, and accounting, impose different Flow Record granularities, and data accuracies.

The sustained growth of IP traffic has been overwhelming the capacities of measurement systems. Furthermore, a large variety of applications (e.g., Quality-of-Service (QoS) measurement, traffic engineering, security monitoring) and the deployment of measurement systems in heterogeneous environments have been increasing the demand and complexity of IP traffic measurements.

4.1. Coping with IP Traffic Growth

Enterprise or service provider networks already have multiple 10 Gb/s links, their total traffic exceeding 100 Gb/s. In the near future, broadband users' traffic will increase by approximately 40% every year according to [TRAFGRW]. When administrators monitor IP traffic sustaining its growth at multiple Exporters, the amount of exported Flow Records from Exporters could exceed the ability of a single Collector.

To deal with this problem, current data reduction techniques (Packet Sampling and Filtering in [RFC5475], and aggregation of measurement data) have been generally implemented on Exporters. Note that Packet Sampling leads to potential loss of small Flows. With both Packet Sampling and aggregation techniques, administrators might no longer be able to detect and investigate subtle traffic changes and anomalies, as this requires detailed Flow information. With Filtering, only a subset of the Data Records are exported.

Considering the potential drawbacks of Packet Sampling, Filtering, and Data Records aggregation, there is a need for a large-scale collecting infrastructure that does not rely on data reduction techniques.

4.2. Coping with Multipurpose Traffic Measurement

Different monitoring applications impose different requirements on the monitoring infrastructure. Some of them require traffic monitoring at a Flow level while others need information about individual packets or just Flow aggregates.

To fulfill these diverse requirements, an Exporter would need to perform various complex metering tasks in parallel, which is a problem due to limited resources. Hence, it can be advantageous to run the Exporter with a much simpler setup and to perform appropriate post-processing of the exported Data Records at a later stage.

4.3. Coping with Heterogeneous Environments

Network administrators use IPFIX Devices and PSAMP Devices from various vendors, various software versions, and various device types (router, switch, or probe) in a single network domain. Even legacy flow export protocols are still deployed in current networks. This heterogeneous environment leads to differences in Metering Process capabilities, Exporting Process capacity (export rate, cache memory, etc.), and data format. For example, probes and switches cannot retrieve some derived packet properties from a routing table.

To deal with this problem, the measurement system needs to mediate the differences. However, equipping all collecting devices with this absorption function is difficult.

4.4. Summary

Due to resource limitations of the measurement system, it is important to use traffic data reduction techniques as early as possible, e.g., at the Exporter. However, this implementation is made difficult by the heterogeneous environment of exporting devices. On the other hand, keeping data accuracy and Flow granularity to meet the requirements of different monitoring applications requires a scalable and flexible collecting infrastructure.

This implies that a new Mediation function is required in typical Exporter-Collector architectures. Based on some applicability examples, the next section shows the limitation of the typical Exporter-Collector architecture model and the IPFIX Mediation benefits.

5. Mediation Applicability Examples

5.1. Adjusting Flow Granularity

The simplest set of Flow Keys is a fixed 5-tuple of protocol, source and destination IP addresses, and source and destination port numbers. A shorter set of Flow Keys, such as a triple, a double, or a single property, (for example, network prefix, peering autonomous system number, or BGP Next-Hop fields), creates more aggregated Flow Records. This is especially useful for measuring router-level traffic matrices in a core network domain and for easily adjusting the performance of Exporters and Collectors.

Implementation analysis:

Implementations for this case depend on where Flow granularity is adjusted. More suitable implementations use configurable Metering Processes in Original Exporters. The cache in the Metering Process can specify its own set of Flow Keys and extra fields. The Original Exporter thus generates Flow Records of the desired Flow granularity.

In the case where a Metering Process hosting no ability to change the Flow Keys in Original Exporters creates Flow Records, or PSAMP Packet Reports, an IPFIX Mediator can aggregate Data Records based on a new set of Flow Keys. Even in the case of a Metering Process hosting this ability, an IPFIX Mediator can further aggregate the Flow Records.

5.2. Collecting Infrastructure

Increasing numbers of IPFIX Exporters, IP traffic growth, and the variety of treatments expected to be performed on the Data Records make it more and more difficult to implement all measurement applications within a single Collector.

Implementation analysis:

To increase the collecting (e.g., the bandwidth capacity) and processing capacity, distributed Collectors close to Exporters need to be deployed. In such a case, those Collectors would become IPFIX Mediators, re-exporting Data Records on demand to centralized applications. To cope with the variety of measurement applications, one possible implementation uses an Intermediate Process deciding to which Collector(s) each record is exported. More specific cases are described in Section 5.9.

5.3. Correlation for Data Records

The correlation amongst Data Records or between Data Records and metadata provides new metrics or information, including the following.

  • One-to-one correlation between Data Records
  • One-way delay from the correlation of PSAMP Packet Reports from different Exporters along a specific path. For example, one- way delay is calculated from the correlation of two PSAMP Packet Reports, including the packet digest and the arrival time at the Observation Point. This scenario is described in Section 6.2.1.2 of [RFC5475].
  • Packet inter-arrival time from the correlation of sequential PSAMP Packet Reports from an Exporter.
  • Treatment from the correlation of Data Records with common properties, observed at incoming/outgoing interfaces. Examples are the rate-limiting ratio, the compression ratio, the optimization ratio, etc.
  • Correlation amongst Data Records

Average/maximum/minimum values from correlating multiple Data Records. Examples are the average/maximum/minimum number of packets of the measured Flows, the average/maximum/minimum one-way delay, the average/maximum/minimum number of lost packets, etc.

  • Correlation between Data Records and other metadata

Examples are some BGP attributes associated with Data Records, as determined via routing table lookup.

Implementation analysis:

One possible implementation for this case uses an Intermediate Process located between the Metering Processes and Exporting Processes on the Original Exporter, or alternatively, a separate IPFIX Mediator located between the Original Exporters and IPFIX Collectors.

5.4. Time Composition

Time composition is defined as the aggregation of consecutive Data Records with identical Flow Keys. It leads to the same output as setting a longer active timeout on Original Exporters, with one advantage: the creation of new metrics such as average, maximum, and minimum values from Flow Records with a shorter time interval enables administrators to keep track of changes that might have happened during the time interval.

Implementation analysis:

One possible implementation for this case uses an Intermediate Process located between the Metering Processes and Exporting Processes on the Original Exporter, or alternatively a separate IPFIX Mediator located between the Original Exporters and IPFIX Collectors.

5.5. Spatial Composition

Spatial composition is defined as the aggregation of Data Records in a set of Observation Points within an Observation Domain, across multiple Observation Domains from a single Exporter, or even across multiple Exporters. The spatial composition is divided into four types.

  • Case 1: Spatial composition within one Observation Domain

For example, to measure the traffic for a single logical interface in the case in which link aggregation [IEEE802.3ad] exists, Data Records metered at physical interfaces belonging to the same trunk can be merged.

  • Case 2: Spatial composition across Observation Domains, but within a single Original Exporter

For example, in the case in which link aggregation exists, Data Records metered at physical interfaces belonging to the same trunk grouping beyond the line card can be merged.

  • Case 3: Spatial composition across Exporters

Data Records metered within an administrative domain, such as the west area and east area of an ISP network, can be merged.

  • Case 4: Spatial composition across administrative domains

Data Records metered across administrative domains, such as across different customer networks or different ISP networks, can be merged. For example, a unique Collector knows in which customer network an Exporter exists, and then works out the traffic data per customer based on the Exporter IP address.

Implementation analysis:

One possible implementation for cases 1 and 2 uses an Intermediate Process located between the Metering Processes and Exporting Processes on the Original Exporter. A separate IPFIX Mediator located between the Original Exporters and IPFIX Collectors is a valid solution for cases 1, 2, 3, and 4.

5.6. Data Record Anonymization

IPFIX exports across administrative domains can be used to measure traffic for wide-area traffic engineering or to analyze Internet traffic trends, as described in the spatial composition across administrative domains in the previous subsection. In such a case, administrators need to adhere to privacy protection policies and prevent access to confidential traffic measurements by other people. Typically, anonymization techniques enable the provision of traffic data to other people without violating these policies.

Generally, anonymization modifies a data set to protect the identity of the people or entities described by the data set from being disclosed. It also attempts to preserve sets of network traffic properties useful for a given analysis while ensuring the data cannot be traced back to the specific networks, hosts, or users generating the traffic. For example, IP address anonymization is particularly important for avoiding the identification of users, hosts, and routers. As another example, when an ISP provides traffic monitoring service to end customers, network administrators take care of anonymizing interface index fields that could disclose any information about the vendor or software version of the Exporters.

Implementation analysis:

One possible implementation for this case uses an anonymization function at the Original Exporter. However, this increases the load on the Original Exporter. A more flexible implementation uses a separate IPFIX Mediator between the Original Exporter and Collector.

5.7. Data Retention

Data retention refers to the storage of traffic data by service providers and commercial organizations. Legislative regulations often require that network operators retain both IP traffic data and call detail records, in wired and wireless networks, generated by end users while using a service provider's services. The traffic data is required for the purpose of the investigation, detection, and prosecution of serious crime, if necessary. Data retention examples relevant to IP networks are the following:

  • Internet telephony (includes every multimedia session associated with IP multimedia services)
  • Internet email
  • Internet access

Data retention, for these services in particular, requires a measurement system with reliable export and huge storage, as the data must be available for a long period of time, typically at least six months.

Implementation analysis:

Regarding export reliability requirement, the most suitable implementation uses the Stream Control Transmission Protocol (SCTP) between the Original Exporter and Collector. If an unreliable transport protocol such as UDP is used, a legacy exporting device exports Data Records to a nearby IPFIX Mediator through UDP, and then an IPFIX Mediator could reliably export them to the IPFIX Collector through SCTP. If an unreliable transport protocol such as UDP is used and if there is no IPFIX Mediator, the legacy exporting device should duplicate the exports to several Collectors to lower the probability of losing Flow Records. However, it might result in network congestion, unless dedicated export links are used.

Regarding huge storage requirements, the collecting infrastructure is described in Section 5.2.

5.8. IPFIX Export from a Branch Office

Generally, in large enterprise networks, Data Records from branch offices are gathered in a central office. However, in the long- distance branch office case, the bandwidth for transporting IPFIX is limited. Therefore, even if multiple Data Record types should be of interest to the Collector (e.g., IPFIX Flow Records in both directions, IPFIX Flow Records before and after WAN optimization techniques, performance metrics associated with the IPFIX Flow Records exported at regular intervals, etc.), the export bandwidth limitation is an important factor to pay attention to.

Implementation analysis:

One possible implementation for this case uses an IPFIX Mediator located in a branch office. The IPFIX Mediator would aggregate and correlate Data Records to cope with the export bandwidth limitation.

5.9. Distributing Data Record Types

Recently, several networks have shifted towards integrated networks, such as the pure IP and MPLS networks, which include IPv4, IPv6, and VPN traffic. Data Record types (IPv4, IPv6, MPLS, and VPN) need to be analyzed separately and from different perspectives for different organizations. A single Collector handling all Data Record types might become a bottleneck in the collecting infrastructure. Data Records distributed based on their respective types can be exported to the appropriate Collector, resulting in load distribution amongst multiple Collectors.

Implementation analysis:

One possible implementation for this case uses replication of the IPFIX Message in an Original Exporter for multiple IPFIX Collectors. Each Collector then extracts the Data Record required by its own applications. However, this replication increases the load of the Exporting Process and the waste of bandwidth between the Exporter and Collector.

A more sophisticated implementation uses an Intermediate Process located between the Metering Processes and Exporting Processes in an Original Exporter. The Intermediate Process determines to which Collector a Data Record is exported, depending on certain field values. If an Original Exporter does not have this capability, it exports Data Records to a nearby separate IPFIX Mediator, and then the IPFIX Mediator could distribute them to the appropriate IPFIX Collectors.

For example, in the case of distributing a specific customer's Data Records, an IPFIX Mediator needs to identify the customer networks. The Route Distinguisher (RD), ingress interface, peering Autonomous System (AS) number, or BGP Next-Hop, or simply the network prefix may be evaluated to distinguish different customer networks. In the following figure, the IPFIX Mediator reroutes Data Records on the basis of the RD value. This system enables each customer's traffic to be inspected independently. 

                                              .---------.
                                              |Traffic  |
                                        .---->|Collector|<==>Customer#A
                                        |     |#1       |
                                        |     '---------'
                                     RD=100:1
   .----------.        .-----------.    |
   |IPFIX     |        |IPFIX      |----'     .---------.
   |Exporter#1|        |Mediator   | RD=100:2 |Traffic  |
   |          |------->|           |--------->|Collector|<==>Customer#B
   |          |        |           |          |#2       |
   |          |        |           |----.     '---------'
   '----------'        '-----------'    |
                                     RD=100:3
                                        |     .---------.
                                        |     |Traffic  |
                                        '---->|Collector|<==>Customer#C
                                              |#3       |
                                              '---------'

Figure A. Distributing Data Records to Collectors

Using IPFIX Mediator

5.10. Flow-Based Sampling and Selection

Generally, the distribution of the number of packets per Flow seems to be heavy tailed. Most types of Flow Records are likely to be small Flows consisting of a small number of packets. The measurement system is overwhelmed with a huge amount of these small Flows. If statistics information of small Flows is exported as merged data by applying a policy or threshold, the load on the Exporter is reduced. Furthermore, if the Flow distribution is known, exporting only a subset of the Data Records might be sufficient.

Implementation analysis:

One possible implementation for this case uses an Intermediate Process located between the Metering Processes and Exporting Processes on the Original Exporter, or alternatively a separate IPFIX Mediator located between the Original Exporters and IPFIX Collectors. A set of IPFIX Mediation functions, such as Filtering, selecting, and aggregation, is used in the IPFIX Mediator.

5.11. Interoperability between Legacy Protocols and IPFIX

During the migration process from a legacy protocol such as NetFlow [RFC3954] to IPFIX, both NetFlow exporting devices and IPFIX Exporters are likely to coexist in the same network. Operators need to continue measuring the traffic data from legacy exporting devices, even after introducing IPFIX Collectors.

Implementation analysis:

One possible implementation for this case uses an IPFIX Mediator that converts a legacy protocol to IPFIX.

6. IPFIX Mediators' Implementation-Specific Problems

6.1. Loss of Original Exporter Information

Both the Exporter IP address indicated by the source IP address of the IPFIX Transport Session and the Observation Domain ID included in the IPFIX Message header are likely to be lost during IPFIX Mediation. In some cases, an IPFIX Mediator might drop the information deliberately. In general, however, the Collector must recognize the origin of the measurement information, such as the IP address of the Original Exporter, the Observation Domain ID, or even the Observation Point ID. Note that, if an IPFIX Mediator cannot communicate the Original Exporter IP address, then the IPFIX Collector will wrongly deduce that the IP address of the IPFIX Mediator is that of the Original Exporter.

In the following figure, a Collector can identify two IP addresses: 192.0.2.3 (IPFIX Mediator) and 192.0.2.2 (Exporter#2), respectively. The Collector, however, needs to somehow recognize both Exporter#1 and Exporter#2, which are the Original Exporters. The IPFIX Mediator must be able to notify the Collector about the IP address of the Original Exporter. 

      .----------.          .--------.
      |IPFIX     |          |IPFIX   |
      |Exporter#1|--------->|Mediator|---+
      |          |          |        |   |
      '----------'          '--------'   |       .---------.
      IP:192.0.2.1        IP:192.0.2.3    '----->|IPFIX    |
      ODID:10             ODID:0                 |Collector|
                                         +------>|         |
      .----------.                       |       '---------'
      |IPFIX     |                       |
      |Exporter#2|-----------------------'
      |          |
      '----------'
      IP:192.0.2.2
      ODID:20

Figure B. Loss of Original Exporter Information

6.2. Loss of Base Time Information

The Export Time field included in the IPFIX Message header represents a reference timestamp for Data Records. Some IPFIX Information Elements, described in [RFC5102], carry delta timestamps that indicate the time difference from the value of the Export Time field. If the Data Records include any delta time fields and the IPFIX Mediator overwrites the Export Time field when sending IPFIX Messages, the delta time fields become meaningless and, because Collectors cannot recognize this situation, wrong time values are propagated.

6.3. Transport Sessions Management

Maintaining relationships between the incoming Transport Sessions and the outgoing ones depends on the Mediator's implementation. If an IPFIX Mediator relays multiple incoming Transport Sessions to a single outgoing Transport Session, and if the IPFIX Mediator shuts down its outgoing Transport Session, Data Records of the incoming Transport Sessions would not be relayed anymore. In the case of resetting an incoming Transport Session, the behavior of the IPFIX Mediator needs to be specified.

6.4. Loss of Options Template Information

In some cases, depending on the implementation of the IPFIX Mediators, the information reported in the Data Records defined by Options Templates could also be lost. If, for example, the Sampling rate is not communicated from the Mediator to the Collector, the Collector would miscalculate the traffic volume. This might lead to crucial problems. Even if an IPFIX Mediator were to simply relay received Data Records defined by Options Templates, the values of its scope fields could become meaningless in the content of a different Transport Session. The minimal information to be communicated by an IPFIX Mediator must be specified.

6.5. Template ID Management

The Template ID is unique on the basis of the Transport Session and Observation Domain ID. If an IPFIX Mediator is not able to manage the relationships amongst the Template IDs and the incoming Transport Session information, and if the Template ID is used in the Options Template scope, IPFIX Mediators would, for example, relay wrong values in the scope field and in the Template Withdrawal Message. The Collector would thus not be able to interpret the Template ID in the Template Withdrawal Message and in the Options Template scope. As a consequence, there is a risk that the Collector would then shut down the IPFIX Transport Session.

For example, an IPFIX Mediator must maintain the state of the incoming Transport Sessions in order to manage the Template ID on its outgoing Transport Session correctly. Even if the Exporter Transport Session re-initializes, the IPFIX Mediator must manage the association of Template IDs in a specific Transport Session. In the following figure, the IPFIX Mediator exports three Templates (256, 257, and 258), received from Exporter#3, Exporter#2, and Exporter#1, respectively. If Exporter#1 re-initializes, and the Template ID value 258 is now replaced with 256, the IPFIX Mediator must correctly manage the new mapping of (incoming Transport Session, Template ID) and (outgoing Transport Session, Template ID) without shutting down its outgoing Transport Session. 

   .----------. OLD: Template ID 258
   |IPFIX     | NEW: Template ID 256
   |Exporter#1|----+
   |          |    |
   '----------'    X
   .----------.    |           .-----------.               .----------.
   |IPFIX     |    '---------->|           |               |          |
   |Exporter#2|--------------->|IPFIX      |-------------->|IPFIX     |
   |          |Template ID 257 |Mediator   |Template ID 258| Collector|
   '----------'    +---------->|           |Template ID 257|          |
   .----------.    |           '-----------'Template ID 256'----------'
   |IPFIX     |    |
   |Exporter#3|----'
   |          | Template ID 256
   '----------'

Figure C. Relaying from Multiple Transport Sessions

to a Single Transport Session

6.6. Consideration for Network Topology

While IPFIX Mediation can be applied anywhere, caution should be taken as to how to aggregate the counters, as there is a potential risk of double counting. For example, if three Exporters export PSAMP Packet Reports related to the same flow, the one-way delay can be calculated, while summing up the number of packets and bytes does not make sense. Alternatively, if three Exporters export Flow Records entering an administrative domain, then the sum of the packets and bytes is a valid operation. Therefore, the possible function to be applied to Flow Records must take into consideration the measurement topology. The information such as the network topology, or at least the Observation Point and measurement direction, is required for IPFIX Mediation.

6.7. IPFIX Mediation Interpretation

In some cases, the IPFIX Collector needs to recognize which specific function(s) IPFIX Mediation has executed on the Data Records. The IPFIX Collector cannot distinguish between time composition and spatial composition, if the IPFIX Mediator does not export the applied function. Some parameters related to the function also would need to be exported. For example, in the case of time composition, the active timeout of original Flow Records is required to interpret the minimum/maximum counter correctly. In the case of spatial composition, spatial area information on which Data Records is aggregated is required.

6.8. Consideration for Aggregation

Whether the aggregation is based on time or spatial composition, caution should be taken regarding how to aggregate non-key fields in IPFIX Mediation. The IPFIX information model [RFC5102] specifies that the value of non-key fields, which are derived from fields of packets or from packet treatment and for which the value may change from packet to packet within a single Flow, is determined by the first packet observed for the corresponding Flow, unless the description of the Information Element explicitly specifies a different semantics.

However, this simple rule might not be appropriate when aggregating Flow Records that have different values in a non-key field. For example, if Differentiated Services Code Point (DSCP) information is to be exported, the following problem can be observed: if two Flows with identical Flow Key values are measured at different Observation Points, they may contain identical packets observed at different locations in the network and at different points in time. On their way from the first to the second Observation Point, the DSCP and potentially some other packet fields may have changed. Hence, if the Information Element ipDiffServCodePoint is included as a non-key field, it can be useful to include the DSCP value observed at either the first or the second Observation Point in the resulting Flow Record, depending on the application.

Other potential solutions include removing the Information Element ipDiffServCodePoint from the Data Record when re-exporting the aggregate Flow Record, changing the Information Element ipDiffServCodePoint from a non-key field to a Flow Key when re-exporting the aggregated Flow Record, or assigning a non-valid value for the Information Element to express to the Collector that this Information Element is meaningless.

If Packet Sampling or Filtering is applied, the IPFIX Mediator must report an adjusted PSAMP Configured Selection Fraction when aggregating IPFIX Flow Records with different Sampling rates.

Finally, special care must be taken when aggregating Flow Records resulting from different Sampling techniques such as Systematic Count-Based Sampling and Random n-out-of-N Sampling, for example.

7. Summary and Conclusion

This document describes the problems that network administrators have been facing, the applicability of IPFIX Mediation to these problems, and the problems related to the implementation of IPFIX Mediators. To assist the operations of the Exporters and Collectors, this document demonstrates that there exist various IPFIX Mediation functions from which the administrators may select.

However, there are still some open issues with the use of IPFIX Mediators. These issues stem from the fact that no standards regarding IPFIX Mediation have been set. In particular, the minimum information that should be communicated between Original Exporters and Collectors, the mapping between different IPFIX Transport Sessions, and the internal components of IPFIX Mediators should be standardized.

8. Security Considerations

A flow-based measurement system must prevent potential security threats: the disclosure of confidential traffic data, injection of incorrect data, and unauthorized access to traffic data. These security threats of the IPFIX protocol are covered by the Security Considerations section in [RFC5101] and are still valid for IPFIX Mediators.

A measurement system must also prevent the following security threats related to IPFIX Mediation:

  • Attacks against an IPFIX Mediator

IPFIX Mediators can be considered as a prime target for attacks, as an alternative to IPFIX Exporters and Collectors. IPFIX Proxies or Masquerading Proxies need to prevent unauthorized access or denial-of-service (DoS) attacks from untrusted public networks. 

   o  Man-in-the-middle attack by untrusted IPFIX Mediator

The Exporter-Mediator-Collector structure model could be misused for a man-in-the-middle attack.

  • Configuration on IPFIX Mediation

An accidental misconfiguration and unauthorized access to configuration data could lead to the crucial problem of disclosure of confidential traffic data.

  • Unintentional exposure of end-user information

The probability of collecting fine-grained information on one arbitrary end user increases with the number of Observation Points. An IPFIX Mediator facing such a situation may have to apply appropriate functions (e.g., anonymization or aggregation) to the Data Records it produces.

  • Multiple-tenancy policy on an IPFIX Mediator

An IPFIX Mediator handling traffic data from multiple tenants or customers needs to protect those tenants or customers from one another's traffic data. For example, an IPFIX Mediator needs to identify the customer's identifier, e.g., ingress interface index, network address range, VLAN ID, Media Access Control (MAC) address, etc., when feeding the customer's traffic data to a customer's own dedicated IPFIX Collector. If the IPFIX Mediator cannot identify each customer's traffic data, it may need to drop the Data Records. In addition, another technique to keep track of a customer's identifier may be required when customer sites are movable, e.g., in the case of a virtual machine moving to another physical machine.

  • Confidentiality protection via an IPFIX Mediator

To ensure security of Data Records in transit, transport of Data Records should be confidential and integrity-protected, e.g., by using Transport Layer Security (TLS) [RFC5246] or Datagram Transport Layer Security (DTLS) [RFC4347]. However, an IPFIX Collector cannot know whether received Data Records are transported as encrypted data between an Original Exporter and an IPFIX Mediator. If this information is required on the IPFIX Collector, it must be encoded in the IPFIX Mediator.

  • Certification for an Original Exporter

An IPFIX Collector communicating via an IPFIX Mediator cannot verify the identity of an Original Exporter directly. If an Original Exporter and an IPFIX Collector are located in different administrative domains, an IPFIX Collector cannot trust its Data Records. If this information is required on the IPFIX Collector, it must be encoded in the IPFIX Mediator.

9. Acknowledgements

We would like to thank the following persons: Gerhard Muenz for thorough, detailed review and significant contributions regarding the improvement of whole sections; Keisuke Ishibashi for contributions during the initial phases of the document; Brian Trammell for contributions regarding the improvement of the Terminology and Definitions section; and Nevil Brownlee, Juergen Schoenwaelder, and Motonori Shindo for their technical reviews and feedback.

10. References

10.1. 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.
   
   [RFC5476]      Claise, B., Ed., Johnson, A., and J. Quittek, "Packet
                  Sampling (PSAMP) Protocol Specifications", RFC 5476,
                  March 2009.

10.2. Informative References

   [IEEE802.3ad]  IEEE Computer Society, "Link Aggregation", IEEE
                  Std 802.3ad-2000, March 2000.
   
   [PSAMP-MIB]    Dietz, T., Ed., Claise, B., and J. Quittek,
                  "Definitions of Managed Objects for Packet Sampling",
                  Work in Progress, July 2010.
   
   [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.
   
   [RFC4347]      Rescorla, E. and N. Modadugu, "Datagram Transport
                  Layer Security", RFC 4347, April 2006.
   
   [RFC5102]      Quittek, J., Bryant, S., Claise, B., Aitken, P., and
                  J. Meyer, "Information Model for IP Flow Information
                  Export", RFC 5102, January 2008.
   
   [RFC5246]      Dierks, T. and E. Rescorla, "The Transport Layer
                  Security (TLS) Protocol Version 1.2", RFC 5246, August
                  2008.
   
   [RFC5470]      Sadasivan, G., Brownlee, N., Claise, B., and J.
                  Quittek, "Architecture for IP Flow Information
                  Export", RFC 5470, March 2009.
   
   [RFC5472]      Zseby, T., Boschi, E., Brownlee, N., and B. Claise,
                  "IP Flow Information Export (IPFIX) Applicability",
                  RFC 5472, March 2009.
   
   [RFC5474]      Duffield, N., Ed., Chiou, D., Claise, B., Greenberg,
                  A., Grossglauser, M., and J. Rexford, "A Framework for
                  Packet Selection and Reporting", RFC 5474, March 2009.
   
   [RFC5475]      Zseby, T., Molina, M., Duffield, N., Niccolini, S.,
                  and F. Raspall, "Sampling and Filtering Techniques for
                  IP Packet Selection", RFC 5475, March 2009.
   
   [RFC5477]      Dietz, T., Claise, B., Aitken, P., Dressler, F., and
                  G.  Carle, "Information Model for Packet Sampling
                  Exports", RFC 5477, March 2009.
   
   [RFC5655]      Trammell, B., Boschi, E., Mark, L., Zseby, T., and A.
                  Wagner, "Specification of the IP Flow Information
                  Export (IPFIX) File Format", RFC 5655, October 2009.
   
   [RFC5815]      Dietz, T., Ed., Kobayashi, A., Claise, B., and G.
                  Muenz, "Definitions of Managed Objects for IP Flow
                  Information Export", RFC 5815, April 2010.
   
   [TRAFGRW]      Cho, K., Fukuda, K., Esaki, H., and A. Kato, "The
                  Impact and Implications of the Growth in Residential
                  User-to-User Traffic", SIGCOMM2006, pp. 207-218, Pisa,
                  Italy, September 2006.

Contributors

   Haruhiko Nishida
   NTT Information Sharing Platform Laboratories
   3-9-11 Midori-cho
   Musashino-shi, Tokyo  180-8585
   Japan
   
   Phone: +81-422-59-3978
   EMail: nishida.haruhiko@lab.ntt.co.jp
   
   Christoph Sommer
   University of Erlangen-Nuremberg
   Department of Computer Science 7
   Martensstr. 3
   Erlangen  91058
   Germany
   
   Phone: +49 9131 85-27993
   EMail: christoph.sommer@informatik.uni-erlangen.de
   URI:   http://www7.informatik.uni-erlangen.de/~sommer/
   
   Falko Dressler
   University of Erlangen-Nuremberg
   Department of Computer Science 7
   Martensstr. 3
   Erlangen  91058
   Germany
   
   Phone: +49 9131 85-27914
   EMail: dressler@informatik.uni-erlangen.de
   URI:   http://www7.informatik.uni-erlangen.de/~dressler/

Stephan Emile
France Telecom
2 Avenue Pierre Marzin
Lannion, F-22307
France 

   Fax:   +33 2 96 05 18 52
   EMail: emile.stephan@orange-ftgroup.com

Authors' Addresses

   Atsushi Kobayashi (editor)
   NTT Information Sharing Platform Laboratories
   3-9-11 Midori-cho
   Musashino-shi, Tokyo  180-8585
   Japan
   
   Phone: +81-422-59-3978
   EMail: akoba@nttv6.net
   
   Benoit Claise (editor)
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   Diegem  1831
   Belgium
   
   Phone: +32 2 704 5622
   EMail: bclaise@cisco.com