





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.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












Trammell & Boschi           Standards Track                    [Page 24]

