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rfc:rfc5103

Network Working Group B. Trammell Request for Comments: 5103 CERT/NetSA Category: Standards Track E. Boschi

                                                        Hitachi Europe
                                                          January 2008
 Bidirectional Flow Export Using IP Flow Information Export (IPFIX)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document describes an efficient method for exporting
 bidirectional flow (Biflow) information using the IP Flow Information
 Export (IPFIX) protocol, representing each Biflow using a single Flow
 Record.

Trammell & Boschi Standards Track [Page 1] RFC 5103 IPFIX Biflow Export January 2008

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  IPFIX Documents Overview . . . . . . . . . . . . . . . . .  3
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
 3.  Rationale and History  . . . . . . . . . . . . . . . . . . . .  5
 4.  Biflow Semantics . . . . . . . . . . . . . . . . . . . . . . .  6
 5.  Direction Assignment . . . . . . . . . . . . . . . . . . . . .  8
   5.1.  Direction by Initiator . . . . . . . . . . . . . . . . . .  9
   5.2.  Direction by Perimeter . . . . . . . . . . . . . . . . . . 10
   5.3.  Arbitrary Direction  . . . . . . . . . . . . . . . . . . . 10
 6.  Record Representation  . . . . . . . . . . . . . . . . . . . . 11
   6.1.  Reverse Information Element Private Enterprise Number  . . 11
   6.2.  Enterprise-Specific Reverse Information Elements . . . . . 13
   6.3.  biflowDirection Information Element  . . . . . . . . . . . 13
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
 8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
 9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
   10.2. Informative References . . . . . . . . . . . . . . . . . . 15
 Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 17
 Appendix B.  XML Specification of biflowDirection Information
              Element . . . . . . . . . . . . . . . . . . . . . . . 21

Trammell & Boschi Standards Track [Page 2] RFC 5103 IPFIX Biflow Export January 2008

1. Introduction

 Many flow analysis tasks benefit from association of the upstream and
 downstream flows of a bidirectional communication, e.g., separating
 answered and unanswered TCP requests, calculating round trip times,
 etc.  Metering processes that are not part of an asymmetric routing
 infrastructure, especially those deployed at a single point through
 which bidirectional traffic flows, are well positioned to observe
 bidirectional flows (Biflows).  In such topologies, the total
 resource requirements for Biflow assembly are often lower if the
 Biflows are assembled at the measurement interface as opposed to the
 Collector.  The IPFIX Protocol requires only information model
 extensions to be complete as a solution for exporting Biflow data.
 To that end, we propose a Biflow export method using a single Flow
 Record per Biflow in this document.  We explore the semantics of
 bidirectional flow data in Section 4, "Biflow Semantics"; examine the
 various possibilities for determining the direction of Biflows in
 Section 5, "Direction Assignment"; then define the Biflow export
 method in Section 6, "Record Representation".
 This export method requires additional Information Elements to
 represent data values for the reverse direction of each Biflow, and a
 single additional Information Element to represent direction
 assignment information, as described in Sections 6.1 through 6.3.
 The selection of this method was motivated by an exploration of other
 possible methods of Biflow export using IPFIX; however, these methods
 have important drawbacks, as discussed in Section 3, "Rationale and
 History".

1.1. IPFIX Documents Overview

 "Specification of the IPFIX Protocol for the Exchange of IP Traffic
 Flow Information" [RFC5101] (informally, the IPFIX Protocol document)
 and its associated documents define the IPFIX Protocol, which
 provides network engineers and administrators with access to IP
 traffic flow information.
 "Architecture for IP Flow Information Export" [IPFIX-ARCH] (the IPFIX
 Architecture document) defines the architecture for the export of
 measured IP flow information out of an IPFIX Exporting Process to an
 IPFIX Collecting Process, and the basic terminology used to describe
 the elements of this architecture, per the requirements defined in
 "Requirements for IP Flow Information Export" [RFC3917].  The IPFIX
 Protocol document [RFC5101] then covers the details of the method for
 transporting IPFIX Data Records and Templates via a congestion-aware
 transport protocol from an IPFIX Exporting Process to an IPFIX
 Collecting Process.

Trammell & Boschi Standards Track [Page 3] RFC 5103 IPFIX Biflow Export January 2008

 "Information Model for IP Flow Information Export" [RFC5102]
 (informally, the IPFIX Information Model document) describes the
 Information Elements used by IPFIX, including details on Information
 Element naming, numbering, and data type encoding.  Finally, "IPFIX
 Applicability" [IPFIX-AS] describes the various applications of the
 IPFIX protocol and their use of information exported via IPFIX, and
 relates the IPFIX architecture to other measurement architectures and
 frameworks.
 This document references the Protocol and Architecture documents for
 terminology, uses the IPFIX Protocol to define a bidirectional flow
 export method, and proposes additions to the information model
 defined in the IPFIX Information Model document.

2. Terminology

 Capitalized terms used in this document that are defined in the
 Terminology section of the IPFIX Protocol document [RFC5101] are to
 be interpreted as defined there.  The following additional terms are
 defined in terms of the IPFIX Protocol document terminology.
 Directional Key Field:  A Directional Key Field is a single field in
    a Flow Key as defined in the IPFIX Protocol document [RFC5101]
    that is specifically associated with a single endpoint of the
    Flow. sourceIPv4Address and destinationTransportPort are example
    Directional Key Fields.
 Non-directional Key Field:  A Non-directional Key Field is a single
    field within a Flow Key as defined in the IPFIX Protocol document
    [RFC5101] that is not specifically associated with either endpoint
    of the Flow. protocolIdentifier is an example Non-directional Key
    Field.
 Uniflow (Unidirectional Flow):  A Uniflow is a Flow as defined in the
    IPFIX Protocol document [RFC5101], restricted such that the Flow
    is composed only of packets sent from a single endpoint to another
    single endpoint.
 Biflow (Bidirectional Flow):  A Biflow is a Flow as defined in the
    IPFIX Protocol document [RFC5101], composed of packets sent in
    both directions between two endpoints.  A Biflow is composed from
    two Uniflows such that:
    1.  the value of each Non-directional Key Field of each Uniflow is
        identical to its counterpart in the other, and
    2.  the value of each Directional Key Field of each Uniflow is
        identical to its reverse direction counterpart in the other.

Trammell & Boschi Standards Track [Page 4] RFC 5103 IPFIX Biflow Export January 2008

    A Biflow contains two non-key fields for each value it represents
    associated with a single direction or endpoint: one for the
    forward direction and one for the reverse direction, as defined
    below.
 Biflow Source:  The Biflow Source is the endpoint identified by the
    source Directional Key Fields in the Biflow.
 Biflow Destination:  The Biflow Destination is the endpoint
    identified by the destination Directional Key Fields in the
    Biflow.
 forward direction (of a Biflow):  The direction of a Biflow composed
    of packets sent by the Biflow Source.  Values associated with the
    forward direction of a Biflow are represented using normal
    Information Elements.  In other words, a Uniflow may be defined as
    a Biflow having only a forward direction.
 reverse direction (of a Biflow):  The direction of a Biflow composed
    of packets sent by the Biflow Destination.  Values associated with
    the reverse direction of a Biflow are represented using Reverse
    Information Elements, as defined below.
 Reverse Information Element:  An Information Element defined as
    corresponding to a normal (or forward) Information Element, but
    associated with the reverse direction of a Biflow.
 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 RFC 2119 [RFC2119].

3. Rationale and History

 In selecting the Single Record Biflow export method described in this
 document as the recommendation for bidirectional flow export using
 IPFIX, we considered several other possible methods.
 The first and most obvious would be simply to export Biflows as two
 Uniflows adjacent in the record stream; a Collecting Process could
 then reassemble them with minimal state requirements.  However, this
 has the drawbacks that it is merely an informal arrangement the
 Collecting Process cannot rely upon, and that it is not bandwidth-
 efficient, duplicating the export of Flow Key data in each Uniflow
 record.
 We then considered the method outlined in Reducing Redundancy in
 IPFIX and Packet Sampling (PSAMP) Reports [IPFIX-REDUCING] for
 reducing this bandwidth inefficiency.  This would also formally link

Trammell & Boschi Standards Track [Page 5] RFC 5103 IPFIX Biflow Export January 2008

 the two Uniflows into a single construct, by exporting the Flow Key
 as Common Properties then exporting each direction's information as
 Specific Properties.  However, it would do so at the expense of
 additional overhead to transmit the commonPropertiesId, and
 additional state management requirements at both the Collecting and
 Exporting Processes.
 A proposal was made on the IPFIX mailing list to use the Multiple
 Information Element feature of the protocol to export forward and
 reverse counters using identical Information Elements in the same
 Flow Record.  In this approach, the first instance of a counter would
 represent the forward direction, and the second instance of the same
 counter would represent the reverse.  This had the disadvantage of
 conflicting with the presently defined semantics for these counters,
 and, as such, was abandoned.

4. Biflow Semantics

 As stated in the Terminology section above, a Biflow is simply a Flow
 representing packets flowing in both directions between two endpoints
 on a network.  There are compelling reasons to treat Biflows as
 single entities (as opposed to merely ad-hoc combinations of
 Uniflows) within IPFIX.  First, as most application-layer network
 protocols are inherently bidirectional, a Biflow-based data model
 more accurately represents the behavior of the network, and enables
 easier application of flow data to answering interesting questions
 about network behavior.  Second, exporting Biflow data can result in
 improved export efficiency by eliminating the duplication of Flow Key
 data in an IPFIX message stream, and improve collection efficiency by
 removing the burden of Biflow matching from the Collecting Process
 where possible.
 Biflows are somewhat more semantically complicated than Uniflows.
 When handling Uniflows, the semantics of source and destination
 Information Elements are clearly defined by the semantics of the
 underlying packet header data: the source Information Elements
 represent the source header fields, and the destination Information
 Elements represent the destination header fields.  When representing
 Biflows with single IPFIX Data Records, the definitions of source and
 destination must be chosen more carefully.
 As in the Terminology section above, we define the Source of a Biflow
 to be that identified by the source Directional Key Field(s), and the
 Destination of the Biflow to be that identified by the destination
 Directional Key Field(s).  Note that, for IANA-registered Information
 Elements, or those defined by the IPFIX Information Model [RFC5102],
 Directional Key Fields associated with the Biflow Source are
 represented by Information Elements whose names begin with "source",

Trammell & Boschi Standards Track [Page 6] RFC 5103 IPFIX Biflow Export January 2008

 and Directional Key Fields associated with the Biflow Destination are
 represented by Information Elements whose names begin with
 "destination"; it is recommended that enterprise-specific Information
 Elements follow these conventions, as well.
 Methods for assignment of Source and Destination by the Metering and
 Exporting Processes are described in the following section.
 As the Source and Destination of a Biflow are defined in terms of its
 Directional Keys, Biflow values are also split info forward and
 reverse directions.  As in the Terminology section above, the forward
 direction of a Biflow is composed of packets sent by the Biflow
 Source, and the reverse direction of a Biflow is composed of packets
 sent by the Destination.  In other words, the two directions of a
 Biflow may be roughly thought of as the two Uniflows that were
 matched to compose the Biflow.  A Biflow record, then, contains each
 Flow Key record once, and both forward Information Elements and
 Reverse Information Elements for each non-key field.  See Figure 1
 for an illustration of the composition of Biflows from Uniflows.
            Uniflow                             Uniflow

+——-+——-+—————–+ +——-+——-+—————–+ | src A | dst B | counters/values | | src B | dst A | counters/values | +——-+——-+—————–+ +——-+——-+—————–+

      |       |          |                                   |
      V       V          V                                   V
     +-------+-------+---------------------+---------------------+
     | src A | dst B | fwd counters/values | rev counters/values |
     +-------+-------+---------------------+---------------------+
                               Biflow
            Figure 1: Bidirectional Flow Conceptual Diagram
 The reverse direction values are represented by Reverse Information
 Elements.  The representation of these Reverse Information Elements
 within Templates is detailed in Section 5.  A Flow Record may be
 considered to be a Biflow record by the Collecting Process if it
 contains at least one Reverse Information Element AND at least one
 Directional Key Field.  Flow Records containing Reverse Information
 Elements but no Directional Key Fields are illegal, MUST NOT be sent
 by the Exporting Process, and SHOULD be dropped by the Collecting
 Process.  The Collecting Process SHOULD log the receipt of such
 illegal Flow Records.
 When exporting Uniflows, Exporting Processes SHOULD use a Template
 containing no Reverse Information Elements.  Note that a Template
 whose only Reverse Information Elements are counters MAY be used to

Trammell & Boschi Standards Track [Page 7] RFC 5103 IPFIX Biflow Export January 2008

 export Uniflows, as counters with values of 0 are semantically
 equivalent to no reverse direction.  However, this approach is not
 possible for Reverse Information Elements whose zero values have a
 distinct meaning (e.g., tcpControlBits).
 Note that a Biflow traversing a middlebox [RFC3234] may show
 different flow properties on each side of the middlebox due to
 changes to the packet header or payload performed by the middlebox
 itself.  Therefore, it MUST be clear at a Collecting Process whether
 packets were observed and metered before or after modification.  The
 Observation Process SHOULD be located on one side of a middlebox, and
 the Exporting Process SHOULD communicate to the Collecting Process
 both the incoming value of the flow property changed within the
 middlebox and the changed value on the "other side".  The IPFIX
 Information Model [RFC5102] provides Information Elements with prefix
 "post" for this purpose.  The location of the Observation Point(s)
 with respect to the middlebox can be communicated using Options with
 Observation Point as Scope and elements such as lineCardID or
 samplerID.
 For further information on the effect of middleboxes within the IPFIX
 architecture, refer to Section 7 of the IPFIX Implementation
 Guidelines [IPFIX-IMPLEMENTATION].
 By the definition of Observation Domain in Section 2 of the IPFIX
 Protocol document [RFC5101], Biflows may be composed only of packets
 observed within the same Observation Domain.  This implies that
 Metering Processes that build Biflows out of Uniflows must ensure
 that the two Uniflows were observed within the same Observation
 Domain.

5. Direction Assignment

 Due to the variety of flow measurement applications and restrictions
 on Metering Process deployment, one single method of assigning the
 directions of a Biflow will not apply in all cases.  This section
 describes three methods of direction assignment, and recommends them
 based upon Metering Process position and measurement application
 requirements.  In each of the figures in this section, the "MP" box
 represents the Metering Process.
 As the method selection is dependent on Metering Process position, it
 is sufficient to configure the direction assignment method at the
 Collecting and/or the Exporting Process out-of-band.  For example, a
 Collecting Process might be configured that a specific Exporting
 Process identified by exporterIPv4Address is assigning direction by
 initiator; or both a Collecting Process and an Exporting Process
 could be simultaneously configured with a specific direction

Trammell & Boschi Standards Track [Page 8] RFC 5103 IPFIX Biflow Export January 2008

 assignment perimeter.  However, for Exporting Processes that use
 multiple direction selection methods, or for Collecting Processes
 accepting data from Exporting Processes using a variety of methods, a
 biflowDirection Information Element is provided for optional
 representation of direction assignment information.

5.1. Direction by Initiator

 If the measurement application requires the determination of the
 initiator and responder of a given communication, the Metering
 Process SHOULD define the Biflow Source to be the initiator of the
 Biflow, where possible.  This can be roughly approximated by a
 Metering Process observing packets in both directions simply assuming
 that the first packet seen in a given Biflow is the packet initiating
 the Biflow.  A Metering Process may improve upon this method by using
 knowledge of the transport or application protocols (e.g., TCP flags,
 DNS question/answer counts) to better approximate the flow-initiating
 packet.
 Note that direction assignment by initiator is most easily done by a
 single Metering Process positioned on a local link layer, as in
 Figure 2, or a single Metering Process observing bidirectional packet
 flows at a symmetric perimeter routing point, as in Figure 3.
 Note also that many Metering Processes have an "active" timeout, such
 that any flow with a duration longer than the active timeout is
 expired and any further packets belonging to that flow are accounted
 for as part of a new flow.  This mechanism may cause issues with the
 assumption that a first packet seen is from the flow initiator, if
 the "first" packet is a middle packet in a long-duration flow.
 +-------+   +-------+
 | node  |   | node  |
 +---+---+   +---+---+
     |           |       +---------+
 <===+=====+=====+======>+         +<===> Internet
           |             | router  |
       +---+---+         +---------+
       |   MP  |
       +---+---+
            Figure 2: Local Link Metering Process Position

Trammell & Boschi Standards Track [Page 9] RFC 5103 IPFIX Biflow Export January 2008

 +-------+   +-------+
 | node  |   | node  |
 +---+---+   +---+---+
     |           |       +---------+
 <===+===========+======>+         +<===> Internet
                         | router  |
                         |    +----+--+
                         +----+  MP   |
                              +-------+
      Figure 3: Symmetric Routing Point Metering Process Position

5.2. Direction by Perimeter

 If the measurement application is deployed at a network perimeter, as
 illustrated in Figure 4, such that there is a stable set of addresses
 that can be defined as "inside" that perimeter, and there is no
 measurement application requirement to determine the initiator and
 responder of a given communication, then the Metering Process SHOULD
 assign the Biflow Source to be the endpoint outside the perimeter.
 No facility is provided for exporting the address set defining the
 interior of a perimeter; this set may be deduced by the Collecting
 Process observing the set of Biflow Source and Biflow Destination
 addresses, or configured out-of-band.
               +---------+               +---------+
          ====>+ access  +====>     ====>+ access  +====>
 Internet      | router  |   Local Net   | router  |      Internet
 (link A) <====+    A    +<====     <====+    B    +<==== (link B)
               +----+----+               +---------+
                    |
                +---+---+
                |  MP   |
                +-------+
             Figure 4: Perimeter Metering Process Position

5.3. Arbitrary Direction

 If the measurement application is deployed in a network core, such
 that there is no stable set of addresses defining a perimeter (e.g.,
 due to BGP updates), as in Figure 5, and no requirement or ability to
 determine the initiator or responder of a given communication, then
 the Metering Process MAY assign the Biflow Source and Biflow
 Destination endpoints arbitrarily.

Trammell & Boschi Standards Track [Page 10] RFC 5103 IPFIX Biflow Export January 2008

 In this case, the Metering Process SHOULD be consistent in its choice
 of direction.  Once assigned, direction SHOULD be maintained for the
 lifetime of the Biflow, even in the case of active timeout of a
 long-lived Biflow.
          |
          V
     +----+----+          +---------+
 <===+ core    |          | core    +===>
     | router  +<========>+ router  |
 ===>+         |          |         +<===
     +----+----+          +----+----+
          |                    |
      +---+---+                V
      |  MP   |
      +-------+
           Figure 5: Transit/Core Metering Process Position

6. Record Representation

 As noted above, Biflows are exported using a single Flow Record, each
 of which contains the Flow Key fields once, and both forward
 Information Elements and Reverse Information Elements for each non-
 key field.  The IPFIX Information Model is extended to provide a
 Reverse Information Element counterpart to each presently defined
 forward Information Element, as required by any Information Element
 that may be a non-key field in a Biflow.

6.1. Reverse Information Element Private Enterprise Number

 Reverse Information Elements are specified as a separate "dimension"
 in the Information Element space, assigning Private Enterprise Number
 (PEN) 29305 to this document, and defining that PEN to signify "IPFIX
 Reverse Information Element" (the Reverse PEN).  This Reverse PEN
 serves as a "reverse direction flag" in the Template; each
 Information Element number within this PEN space is assigned to the
 reverse counterpart of the corresponding IANA-assigned public
 Information Element number.  In other words, to generate a Reverse
 Information Element in a Template corresponding to a given forward
 Information Element, simply set the enterprise bit and define the
 Information Element within the Reverse PEN space, as in Figure 6
 below.

Trammell & Boschi Standards Track [Page 11] RFC 5103 IPFIX Biflow Export January 2008

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| flowStartSeconds        150 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                forward           |
                                  |
                reverse           V
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |1| (rev) flowStartSeconds  150 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Reverse PEN                                      29305      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure 6: Example Mapping between Forward and Reverse IEs
 As the Reverse Information Element dimension is treated explicitly as
 such, new Information Elements can be added freely to the IANA-
 managed space without concern for whether a Reverse Information
 Element should also be added.  Aside from the initial allocation of a
 Private Enterprise Number for this purpose, there is no additional
 maintenance overhead for supporting Reverse Information Elements in
 the IPFIX Information Model.
 Note that certain Information Elements in the IPFIX Information Model
 [RFC5102] are not reversible; that is, they are semantically
 meaningless as Reverse Information Elements.  An Exporting Process
 MUST NOT export a Template containing the reverse counterpart of a
 non-reversible Information Element.  A Collecting Process receiving
 the reverse counterpart of a non-reversible Information Element MAY
 discard that Information Element from the Flow Record.  Non-
 reversible Information Elements represent properties of the Biflow
 record as a whole, or are intended for internal the use of the IPFIX
 Protocol itself.  Therefore, by definition, they cannot be associated
 with a single direction or endpoint of the Flow.
 The following specific Information Elements are not reversible:
 1.  Identifiers defined in Section 5.1 of [RFC5102] that cannot be
     associated with a single direction of Uniflow collection: flowId
     (5.1.7), templateId (5.1.8), observationDomainId (5.1.9), and
     commonPropertiesId (5.1.11).
 2.  Process configuration elements defined in Section 5.2 of
     [RFC5102].
 3.  Process statistics elements defined in Section 5.3 of [RFC5102].

Trammell & Boschi Standards Track [Page 12] RFC 5103 IPFIX Biflow Export January 2008

 4.  paddingOctets defined in Section 5.12.1 of [RFC5102].
 5.  biflowDirection (defined in Section 6.3 of this document).
 Any future addition to the Information Element Registry by IANA that
 meets the criteria defined above SHOULD also be considered to be non-
 reversible by the Collecting Process.
 Note that Information Elements commonly used as Flow Keys (e.g.,
 header fields defined in Sections 5.4 and 5.5 of the Information
 Model) are reversible, as they may be used as value fields in certain
 contexts, as when associating ICMP error messages with the flows that
 caused them.

6.2. Enterprise-Specific Reverse Information Elements

 Note that the Reverse PEN defined above is only available for
 allocating reverse counterparts of IANA-registered IPFIX Information
 Elements.  No facility is provided for allocating reverse
 counterparts of enterprise-specific Information Elements.
 The allocation of enterprise-specific Information Elements for IPFIX
 is left to the discretion of the organization allocating them.  Note
 that, as enterprise-specific Information Elements are designed for
 the internal use of private enterprises, the lack of any guidance or
 standard on Information Element allocation policies poses no
 interoperability issues.  However, if a private enterprise's own
 Information Element registry anticipates the allocation of reversible
 Information Elements, and the use of this specification for the
 export of Biflow data, that registry MAY reserve one of the fifteen
 available bits in the Information Element ID to signify the reverse
 direction.  For example, if the most significant bit were selected,
 this would reserve Information Element IDs 0x4000 to 0x7FFF for the
 reverse direction of Information Element IDs 0x0000 to 0x3FFF.

6.3. biflowDirection Information Element

 Description:   A description of the direction assignment method used
    to assign the Biflow Source and Destination.  This Information
    Element MAY be present in a Flow Record, or applied to all flows
    exported from an Exporting Process or Observation Domain using
    IPFIX Options.  If this Information Element is not present in a
    Flow Record or associated with a Biflow via scope, it is assumed
    that the configuration of the direction assignment method is done
    out-of-band.  Note that when using IPFIX Options to apply this
    Information Element to all flows within an Observation Domain or
    from an Exporting Process, the Option SHOULD be sent reliably.  If
    reliable transport is not available (i.e., when using UDP), this

Trammell & Boschi Standards Track [Page 13] RFC 5103 IPFIX Biflow Export January 2008

    Information Element SHOULD appear in each Flow Record.  This field
    may take the following values:
 +-------+------------------+----------------------------------------+
 | Value | Name             | Description                            |
 +-------+------------------+----------------------------------------+
 | 0x00  | arbitrary        | Direction was assigned arbitrarily.    |
 | 0x01  | initiator        | The Biflow Source is the flow          |
 |       |                  | initiator, as determined by the        |
 |       |                  | Metering Process' best effort to       |
 |       |                  | detect the initiator.                  |
 | 0x02  | reverseInitiator | The Biflow Destination is the flow     |
 |       |                  | initiator, as determined by the        |
 |       |                  | Metering Process' best effort to       |
 |       |                  | detect the initiator.  This value is   |
 |       |                  | provided for the convenience of        |
 |       |                  | Exporting Processes to revise an       |
 |       |                  | initiator estimate without re-encoding |
 |       |                  | the Biflow Record.                     |
 | 0x03  | perimeter        | The Biflow Source is the endpoint      |
 |       |                  | outside of a defined perimeter.  The   |
 |       |                  | perimeter's definition is implicit in  |
 |       |                  | the set of Biflow Source and Biflow    |
 |       |                  | Destination addresses exported in the  |
 |       |                  | Biflow Records.                        |
 +-------+------------------+----------------------------------------+
 Abstract Data Type:   unsigned8
 Data Type Semantics:   identifier
 ElementId:   239
 Status:   current

7. IANA Considerations

 This document specifies the creation of a new dimension in the
 Information Element space defined by the IPFIX Information Model
 [RFC5102].  This new dimension is defined by the allocation of a new
 Private Enterprise Number (PEN).  The Internet Assigned Numbers
 Authority (IANA) has assigned Private Enterprise Number 29305 to this
 document as the "IPFIX Reverse Information Element Private
 Enterprise", with this document's authors as point of contact.
 This document specifies the creation of a new IPFIX Information
 Element, biflowDirection, as defined in Section 6.3.  IANA has
 assigned Information Element number 239 in the IPFIX Information

Trammell & Boschi Standards Track [Page 14] RFC 5103 IPFIX Biflow Export January 2008

 Element registry for the biflowDirection Information Element.  The
 values defined for this Information Element are static, and as such
 do not need to be maintained by IANA in a sub-registry.

8. Security Considerations

 The same security considerations as for the IPFIX Protocol [RFC5101]
 apply.

9. Acknowledgments

 We would like to thank Lutz Mark, Juergen Quittek, Andrew Johnson,
 Paul Aitken, Benoit Claise, and Carsten Schmoll for their
 contributions and comments.  Special thanks to Michelle Cotton for
 her assistance in navigating the IANA process for Enterprise Number
 assignment, and for the IANA pre-review of the document.

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

10.2. Informative References

 [RFC2119]               Bradner, S., "Key words for use in RFCs to
                         Indicate Requirement Levels", BCP 14,
                         RFC 2119, March 1997.
 [RFC3234]               Carpenter, B. and S. Brim, "Middleboxes:
                         Taxonomy and Issues", RFC 3234,
                         February 2002.
 [RFC3917]               Quittek, J., Zseby, T., Claise, B., and S.
                         Zander, "Requirements for IP Flow Information
                         Export (IPFIX)", RFC 3917, October 2004.

Trammell & Boschi Standards Track [Page 15] RFC 5103 IPFIX Biflow Export January 2008

 [IPFIX-ARCH]            Sadasivan, G., Brownlee, N., Claise, B., and
                         J. Quittek, "Architecture for IP Flow
                         Information Export", Work in Progress,
                         September 2006.
 [IPFIX-AS]              Zseby, T., Boschi, E., Brownlee, N., and B.
                         Claise, "IPFIX Applicability", Work
                         in Progress, July 2007.
 [IPFIX-IMPLEMENTATION]  Boschi, E., Mark, L., Quittek, j.,
                         Stiemerling, M., and P. Aitken, "IPFIX
                         Implementation Guidelines", Work in Progress,
                         September 2007.
 [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.

Trammell & Boschi Standards Track [Page 16] RFC 5103 IPFIX Biflow Export January 2008

Appendix A. Examples

 The following example describes a Biflow record as specified in
 Section 6, above.  The Reverse PEN is assigned for the purpose of
 differentiating forward from Reverse Information Elements.
 The information exported in this case is:
 o  The start time of the flow: flowStartSeconds in the IPFIX
    Information Model [RFC5102], with a length of 4 octets.
 o  The reverse start time of the flow: flowStartSeconds in the IPFIX
    Information Model [RFC5102], with a length of 4 octets, and the
    enterprise bit set to 1.  The following PEN is the Reverse PEN.
 o  The IPv4 source IP address: sourceIPv4Address in the IPFIX
    Information Model [RFC5102], with a length of 4 octets.
 o  The IPv4 destination IP address: destinationIPv4Address in the
    IPFIX Information Model [RFC5102], with a length of 4 octets.
 o  The source port: sourceTransportPort in the IPFIX Information
    Model [RFC5102], with a length of 2 octets.
 o  The destination port: destinationTransportPort in the IPFIX
    Information Model [RFC5102], with a length of 2 octets.
 o  The protocol identifier: protocolIdentifier in the IPFIX
    Information Model [RFC5102], with a length of 1 octet.
 o  The number of octets of the Flow: octetTotalCount in the IPFIX
    Information Model [RFC5102], with a length of 4 octets.
 o  The reverse number of octets of the Flow: octetTotalCount in the
    IPFIX Information Model [RFC5102], with a length of 4 octets, and
    the enterprise bit set to 1.  The following PEN is the Reverse
    PEN.
 o  The number of packets of the Flow: packetTotalCount in the IPFIX
    Information Model [RFC5102], with a length of 4 octets.
 o  The reverse number of packets of the Flow: packetTotalCount in the
    IPFIX Information Model [RFC5102], with a length of 4 octets, and
    the enterprise bit set to 1.  The following PEN is the Reverse
    PEN.

Trammell & Boschi Standards Track [Page 17] RFC 5103 IPFIX Biflow Export January 2008

 and the resulting Template Set would look like the diagram below:
                       1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Set ID = 2           |          Length =  64         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      Template ID >= 256       |        Field Count = 11       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| flowStartSeconds        150 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |1| flowStartSeconds        150 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Reverse PEN                                      29305      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| sourceIPv4Address         8 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| destinationIPv4Address   12 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| sourceTransportPort       7 |       Field Length =  2       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| destinationTransportPort 11 |       Field Length =  2       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| protocolIdentifier        4 |       Field Length =  1       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| octetTotalCount          85 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |1| octetTotalCount          85 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Reverse PEN                                     29305       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |0| packetTotalCount         86 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |1| packetTotalCount         86 |       Field Length =  4       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Reverse PEN                                     29305       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 7: Single Record Biflow Template Set

Trammell & Boschi Standards Track [Page 18] RFC 5103 IPFIX Biflow Export January 2008

 The following example Data Set represents a typical HTTP transaction.
 Its format is defined by the example Template, above.
                       1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Set ID >= 256           |          Length =  41         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     2006-02-01  17:00:00                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     2006-02-01  17:00:01                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           192.0.2.2                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                           192.0.2.3                           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          32770                |               80              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       6       |                 18000                     . . .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  . . .           |                128000                     . . .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  . . .           |                  65                       . . .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  . . .           |                 110                       . . .
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  . . .           |
  +-+-+-+-+-+-+-+-+
                Figure 8: Single Record Biflow Data Set
 The following example demonstrates the use of the biflowDirection
 Information Element, as specified in Section 6.2, using the IPFIX
 Options mechanism to specify that perimeter direction selection is in
 effect for a given Observation Domain.
 The information exported in this case is:
 o  The Observation Domain: observationDomainId in the IPFIX
    Information Model [RFC5102], with a length of 4 octets.
 o  The direction assignment method: biflowDirection as defined in
    Section 6.2, above, with a length of 1 octet.

Trammell & Boschi Standards Track [Page 19] RFC 5103 IPFIX Biflow Export January 2008

 and the resulting Options Template Set would look like the diagram
 below:
                       1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          Set ID = 3           |          Length =  18         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      Template ID >= 256       |        Field Count = 2        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Scope Count = 1         |0| observationDomainId     149 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Field Length = 4        |0| biflowDirection         239 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Field Length = 1        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 9: Biflow Direction Options Template Set
 The following example Data Set would specify that perimeter direction
 selection is in effect for the Observation Domain with ID 33.  Its
 format is defined by the example Options Template, above.  Note that
 this example data set would be sent reliably, as specified in the
 description of the biflowDirection Information Element.
                       1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Set ID >= 256           |          Length =  9          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                              33                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       3       |
  +-+-+-+-+-+-+-+-+
             Figure 10: Biflow Direction Options Data Set

Trammell & Boschi Standards Track [Page 20] RFC 5103 IPFIX Biflow Export January 2008

Appendix B. XML Specification of biflowDirection Information Element

 This appendix contains a machine-readable description of the
 biflowDirection information element defined in this document, coded
 in XML.  Note that this appendix is of informational nature, while
 the text in Section 6.3 is normative.
 The format in which this specification is given is described by the
 XML Schema in Appendix B of the IPFIX Information Model [RFC5102].
 <?xml version="1.0" encoding="UTF-8"?>
 <fieldDefinitions xmlns="urn:ietf:params:xml:ns:ipfix-info"
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="urn:ietf:params:xml:ns:ipfix-info
              ipfix-info.xsd">
   <field name="biflowDirection" dataType="unsigned8"
          dataTypeSemantics="identifier" group="misc"
          elementId="239" applicability="all" status="current">
     <description>
       <paragraph>
        A description of the direction assignment method used to
        assign the Biflow Source and Destination.  This
        Information Element MAY be present in a Flow Data Record, or
        applied to all flows exported from an Exporting Process or
        Observation Domain using IPFIX Options.  If this Information
        Element is not present in a Flow Record or associated with a
        Biflow via scope, it is assumed that the configuration of
        the direction assignment method is done out-of-band.  Note
        that when using IPFIX Options to apply this Information
        Element to all flows within an Observation Domain or from an
        Exporting Process, the Option SHOULD be sent reliably.  If
        reliable transport is not available (i.e., when using UDP),
        this Information Element SHOULD appear in each Flow
        Record.  This field may take the following values:
            </paragraph>

Trammell & Boschi Standards Track [Page 21] RFC 5103 IPFIX Biflow Export January 2008

            <artwork>
 +-------+------------------+----------------------------------------+
 | Value | Name             | Description                            |
 +-------+------------------+----------------------------------------+
 | 0x00  | arbitrary        | Direction was assigned arbitrarily.    |
 | 0x01  | initiator        | The Biflow Source is the flow          |
 |       |                  | initiator, as determined by the        |
 |       |                  | Metering Process' best effort to       |
 |       |                  | detect the initiator.                  |
 | 0x02  | reverseInitiator | The Biflow Destination is the flow     |
 |       |                  | initiator, as determined by the        |
 |       |                  | Metering Process' best effort to       |
 |       |                  | detect the initiator.  This value is   |
 |       |                  | provided for the convenience of        |
 |       |                  | Exporting Processes to revise an       |
 |       |                  | initiator estimate without re-encoding |
 |       |                  | the Biflow Record.                     |
 | 0x03  | perimeter        | The Biflow Source is the endpoint      |
 |       |                  | outside of a defined perimeter.  The   |
 |       |                  | perimeter's definition is implicit in  |
 |       |                  | the set of Biflow Source and Biflow    |
 |       |                  | Destination addresses exported in the  |
 |       |                  | Biflow Records.                        |
 +-------+------------------+----------------------------------------+
            </artwork>
     </description>
   </field>
 </fieldDefinitions>

Trammell & Boschi Standards Track [Page 22] RFC 5103 IPFIX Biflow Export January 2008

Authors' Addresses

 Brian H. Trammell
 CERT Network Situational Awareness
 Software Engineering Institute
 4500 Fifth Avenue
 Pittsburgh, PA  15213
 United States
 Phone: +1 412 268 9748
 EMail: bht@cert.org
 Elisa Boschi
 Hitachi Europe
 c/o ETH Zurich
 Gloriastrasse 35
 8092 Zurich
 Switzerland
 Phone: +41 44 6327057
 EMail: elisa.boschi@hitachi-eu.com

Trammell & Boschi Standards Track [Page 23] RFC 5103 IPFIX Biflow Export January 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
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Trammell & Boschi Standards Track [Page 24]

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