One document matched: draft-bryant-mpls-flow-ident-03.xml

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<rfc category="info" docName="draft-bryant-mpls-flow-ident-03"
     ipr="trust200902" updates="">
  <front>
    <title abbrev="MPLS FI ">MPLS Flow Identification Considerations</title>

    <author fullname="Stewart Bryant" initials="S" surname="Bryant">
      <organization>Cisco Systems</organization>

      <address>
        <email>stbryant@cisco.com</email>
      </address>
    </author>

    <author fullname="Stewart Bryant" initials="S" surname="Bryant">
      <organization>Independent</organization>

      <address>
        <postal>
          <street></street>

          <city></city>

          <region></region>

          <code></code>

          <country></country>
        </postal>

        <phone></phone>

        <facsimile></facsimile>

        <email>stewart.bryant@gmail.com</email>

        <uri></uri>
      </address>
    </author>

    <author fullname="Carlos Pignataro" initials="C" surname="Pignataro">
      <organization>Cisco Systems</organization>

      <address>
        <postal>
          <street></street>
        </postal>

        <email>cpignata@cisco.com</email>
      </address>
    </author>

    <author fullname="Mach Chen" initials="M" surname="Chen">
      <organization>Huawei</organization>

      <address>
        <postal>
          <street></street>
        </postal>

        <email>mach.chen@huawei.com</email>
      </address>
    </author>

    <author fullname="Zhenbin Li" initials="Z" surname="Li">
      <organization>Huawei</organization>

      <address>
        <postal>
          <street></street>
        </postal>

        <email>lizhenbin@huawei.com</email>
      </address>
    </author>

    <author fullname="Gregory Mirsky " initials="G" surname="Mirsky">
      <organization>Ericsson</organization>

      <address>
        <postal>
          <street></street>
        </postal>

        <email>gregory.mirsky@ericsson.com</email>
      </address>
    </author>

    <date year="2015" />

    <area>Routing</area>

    <workgroup>MPLS</workgroup>

    <keyword>OAM</keyword>

    <keyword></keyword>

    <keyword>Internet-Draft</keyword>

    <abstract>
      <t>This memo discusses the desired capabilities for MPLS flow
      identification. The key application that needs this is in-band
      performance monitoring of user data packets.</t>
    </abstract>
  </front>

  <middle>
    <section title="Introduction">
      <t>This memo discusses the desired capabilities for MPLS flow
      identification. The key application that needs this is in-band
      performance monitoring of user data packets.</t>

      <t>There is a need to identify flows in MPLS networks for applications
      such as packet loss and packet delay measurement. A method of loss and
      delay measurement in MPLS networks was defined in <xref
      target="RFC6374"></xref>. When used to measure packet loss <xref
      target="RFC6374"></xref> depends on the use of the injected Operations,
      Administration, and Maintenance (OAM) packets to designate the beginning
      and the end of the packet group over which packet loss is being
      measured. Where the misordering of packets from one group relative to
      the following group, or misordering of one of the packets being counted
      relative to the <xref target="RFC6374"></xref> packet occurs, then an
      error will occur in the packet loss measurement. In addition, this
      packet performance measurement system needs to be extended to deal with
      different granularities of flow and to address a number of the
      multi-point cases in which two or more ingress Label Switching Routers
      (LSRs) could send packets to one or more destinations.</t>

      <t>Improvements in link and transmission technologies mean that it may
      be difficult to assess packet loss using active performance measurement
      methods with synthetic traffic, due to the very low loss rate in normal
      operation. That, together with more demanding service level
      requirements, mean that network operators need to be able to measure the
      loss of the actual user data traffic by using passive performance
      measurement methods. Any technique deployed needs to be transparent to
      the end user, and it needs to be assumed that they will not take any
      active part in the measurement process. Indeed it is important that any
      flow identification technique be invisible to them and that no remnant
      of the identification of measurement process leak into their
      network.</t>

      <t>Additionally where there are multiple traffic sources, such as in
      multi-point to point and multi-point to multi-point network environments
      there needs to be a method whereby the sink can distinguish between
      packets from the various sources, that is to say, that a multi-point to
      multi-point measurement model needs to be developed.</t>
    </section>

    <section title="Requirements Language">
      <t>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 <xref
      target="RFC2119"></xref>.</t>
    </section>

    <section anchor="LM" title="Loss Measurement Considerations">
      <t>Modern networks, if not oversubscribed, normally drop very few
      packets, thus packet loss measurement is highly sensitive to counter
      errors. Without some form of coloring or batch marking such as that
      proposed in <xref target="I-D.tempia-ippm-p3m"></xref> it may not be
      possible to achieve the required accuracy in the loss measurement of
      customer data traffic. Thus where accuracy better than the data link
      loss performance of a modern optical network is required, it may be
      economically advantageous, or even a technical requirement, to include
      some form of marking in the packets to assign each packet to a
      particular counter.</t>

      <t>Where this level of accuracy is required and the traffic between a
      source-destination pair is subject to Equal-Cost Multipath (ECMP) a
      demarcation mechanism is needed to group the packets into batches. Once
      a batch is correlated at both ingress and egress, the packet accounting
      mechanism is then able to operate on the batch of packets which can be
      accounted for at both the packet ingress and the packet egress. Errors
      in the accounting are particularly acute in Label Switched Paths (LSPs)
      subjected to ECMP because the network transit time will be different for
      the various ECMP paths since:<list style="letters">
          <t>The packets may traverse different sets of LSRs.</t>

          <t>The packets may depart from different interfaces on different
          line cards on LSRs</t>

          <t>The packets may arrive at different interfaces on different line
          cards on LSRs.</t>
        </list></t>

      <t>A consideration in modifying the identity label (the MPLS label
      ordinarily used to identify the LSP, Virtual Private Network, Pseudowire
      etc) to indicate the batch is the impact that this has on the path
      chosen by the ECMP mechanism. When the member of the ECMP path set is
      chosen by deep packet inspection a change of batch represented by a
      change of identity label will have no impact on the ECMP path. Where the
      path member is chosen by reference to an entropy label <xref
      target="RFC6790"></xref> then changing the batch identifier will not
      result in a change to the chosen ECMP path. ECMP is so pervasive in
      multi-point to (multi-) point networks that some method of avoiding
      accounting errors introduced by ECMP needs to be supported.</t>
    </section>

    <section title="Delay Measurement Considerations">
      <t>Most of the existing delay measurement methods are active measurement
      that depend on the extra injected test packet to evaluate the delay of a
      path. With the active measurement method, the rate, numbers and interval
      between the injected packets may affect the accuracy of the results.
      Also, for injected test packets, these may not be co-routed with the
      data traffic due to ECMP. Thus there exists a requirement to measure the
      delay of the real traffic. </t>

      <t>For combined loss-delay measurements, both the loss and the delay
      considerations apply.</t>
    </section>

    <section anchor="UOI" title="Units of identification">
      <t>The most basic unit of identification is the identity of the node
      that processed the packet on its entry to the MPLS network. However, the
      required unit of identification may vary depending on the use case for
      accounting, performance measurement or other types of packet
      observations. In particular note that there may be a need to impose
      identify at several different layers of the MPLS label stack.</t>

      <t>This document considers following units of identifications:</t>

      <t><list style="symbols">
          <t>Per source LSR - everything from one source is aggregated.</t>

          <t>Per group of LSPs chosen by an ingress LSR - an ingress LSP
          aggregates group of LSPs (ex: all LSPs of a tunnel).</t>

          <t>Per LSP - the basic form.</t>

          <t>Per flow <xref target="RFC6790"></xref> within an LSP - fine
          graining method.</t>
        </list>Note that a fine grained identity resolution is needed when
      there is a need to perform these operations on a flow not readily
      identified by some other element in the label stack. Such fine grained
      resolution may be possible by deep packet inspection, but this may not
      always be possible, or it may be desired to minimise processing costs by
      doing this only in entry to the network, and adding a suitable
      identifier to the packet for reference by other network elements. An
      example of such a fine grained case might be traffic from a specific
      application, or from a specific application from a specific source,
      particularly if matters related to service level agreement or
      application performance were being investigated.</t>

      <t>We can thus characterize the identification requirement in the
      following broad terms:</t>

      <t><list style="symbols">
          <t>There needs to be some way for an egress LSR to identify the
          ingress LSR with an appropriate degree of scope. This concept is
          discussed further in <xref target="NS"></xref>.</t>

          <t>There needs to be a way to identify a specific LSP at the egress
          node. This allows for the case of instrumenting multiple LSPs
          operate between the same pair of nodes. In such cases the identity
          of the ingress LSR is insufficient.</t>

          <t>In order to conserve resources such as labels, counters and/or
          compute cycles it may be desirable to identify an LSP group so that
          a operation can be performed on the group as an aggregate.</t>

          <t>There needs to be a way to identify a flow within an LSP. This is
          necessary when investigating a specific flow that has been
          aggregated into an LSP.</t>
        </list></t>

      <t>The unit of identification and the method of determining which
      packets constitute a flow will be application or use-case specific and
      is out of scope of this memo.</t>
    </section>

    <section anchor="TOLSP" title="Types of LSP">
      <t>We need to consider a number of types of LSP. The two simplest types
      to monitor are point to point LSPs and point to multi-point LSPs. The
      ingress LSR for a point to point LSP, such as those created using the
      Resource Reservation Protocol - Traffic Engineering (RSVP-TE) <xref
      target="RFC5420"></xref> signalling protocol, or those that conform to
      the MPLS Transport Profile (MPLS-TP) <xref target="RFC5654"></xref> may
      be identified by inspection of the top label in the stack, since at any
      provider-edge (PE) or provider (P) router on the path this is unique to
      the ingress-egress pair at every hop at a given layer in the LSP
      hierarchy. Provided that penultimate hop popping is disabled, the
      identity of the ingress LSR of a point to point LSP is available at the
      egress LSR and thus determining the identity of the ingress LSR must be
      regarded as a solved problem. Note however that the identity of a flow
      cannot to be determined without further information being carried in the
      packet, or gleaned from some aspect of the packet payload.</t>

      <t>In the case of a point to multi-point LSP, and in the absence of
      Penultimate Hop Popping (PHP) the identity of the ingress LSR may also
      be inferred from the top label. However, it may not possible to
      adequately identify the flow from the top label alone, and thus further
      information may need to be carried in the packet, or gleaned from some
      aspect of the packet payload. In designing any solution it is desirable
      that a common flow identity solution be used for both point to point and
      point to multi-point LSP types. Similarly it is desirable that a common
      method of LSP group identification be used. In the above cases, a
      context label <xref target="RFC5331"></xref> needs to be used to provide
      the required identity information. This is widely supported MPLS
      feature.</t>

      <t>A more interesting case is the case of a multi-point to point LSP. In
      this case the same label is normally used by multiple ingress or
      upstream LSRs and hence source identification is not possible by
      inspection of the top label by the egress LSRs. It is therefore
      necessary for a packet to be able to explicitly convey any of the
      identity types described in <xref target="UOI"></xref>.</t>

      <t>Similarly, in the case of a multi-point to multi-point LSP the same
      label is normally used by multiple ingress or upstream LSRs and hence
      source identification is not possible by inspection of the top label by
      egress LSRs. The various types of identity described in <xref
      target="UOI"></xref> are again needed. Note however, that the scope of
      the identity may be constrained to be unique within the set of
      multi-point to multi-point LSPs terminating on any common node.</t>

      <t></t>
    </section>

    <section anchor="NS" title="Network Scope">
      <t>The scope of identification can be constrained to the set of flows
      that are uniquely identifiable at an ingress LSR, or some aggregation
      thereof. There is no question of an ingress LSR seeking assistance from
      outside the MPLS protocol domain.</t>

      <t>In any solution that constrains itself to carrying the required
      identity in the MPLS label stack rather than in some different
      associated data structure, constraints on the label stack size imply
      that the scope of identity reside within that MPLS domain. For similar
      reasons the identity scope of a component of an LSP should be
      constrained to the scope of that LSP.</t>
    </section>

    <section anchor="HOS" title="Backwards Compatibility">
      <t>In any network it is unlikely that all LSRs will have the same
      capability to support the methods of identification discussed in this
      memo. It is therefore an important constraint on any flow identity
      solution that it is backwards compatible with deployed MPLS equipment to
      the extent that deploying the new feature will not disable anything that
      currently works on a legacy equipment.</t>

      <t>This is particularly the case when the deployment is incremental or
      when the feature is not required for all LSRs or all LSPs. Thus in broad
      the flow identification design MUST support the co-existence of both
      LSRs that can, and cannot, identify the traffic components described in
      <xref target="UOI"></xref>. In addition the identification of the
      traffic components described in <xref target="UOI"></xref> MUST be an
      optional feature that is disabled by default. As a design
      simplification, a solution MAY require that all egress LSRs of a point
      to multipoint or a multi-point to multipoint LSP support the
      identification type in use so that a single packet can be correctly
      processed by all egress devices. The corollary of this last point is
      that either all egress LSRs are enabled to support the required identity
      type, or none of them are.</t>
    </section>

    <section anchor="DP" title="Dataplane">
      <t>There is a huge installed base of MPLS equipment, typically this type
      of equipment remains in service for an extended period of time, and in
      many cases hardware constraints mean that it is not possible to upgrade
      its dataplane functionality. Changes to the MPLS data plane are
      therefore expensive to implement, add complexity to the network, and may
      significantly impact the deployability of a solution that requires such
      changes. For these reasons, the MPLS designers have set a very high bar
      to changes to the MPLS data plane, and only a very small number have
      been adopted. Hence, it is important that the method of identification
      must minimize changes to the MPLS data plane. Ideally method(s) of
      identification that require no changes to the MPLS data plane should be
      given preferential consideration. If a method of identification makes a
      change to the data plane is chosen it will need to have a significant
      advantage over any method that makes no change, and the advantage of the
      approach will need to be carefully evaluated and documented. If a change
      is necessary to the MPLS data plane proves necessary, it should be (a)
      be as small a change as possible and (b) be a general purpose method so
      as to maximise its use for future applications. It is imperative that,
      as far as can be foreseen, any necessary change made to the MPLS data
      plane does not impose any foreseeable future limitation on the MPLS data
      plane.</t>

      <t>Stack size is an issue with many MPLS implementations both as a
      result of hardware limitations, and due to the impact on networks and
      applications where a large number of small payloads need to be
      transported In particular one MPLS payload may be carried inside
      another. For example one LSP may be carried over another LSP, or a PW or
      similar multiplexing construct may be carried over an LSP and
      identification may be required at both layers. Of particular concern is
      the implementation of low cost edge LSRs that for cost reasons have a
      significant limit on the number of Label Stack Elements (LSEs) that they
      can impose or dispose. Therefore, any method of identity MUST NOT
      consume an excessive number of unique labels, and MUST NOT result in an
      excessive increase in the size of the label stack.</t>

      <t>The MPLS data plane design provides two types of special purpose
      labels: the original 16 reserved labels and the much larger set of
      special purpose labels defined in <xref target="RFC7274"></xref>. The
      original reserved labels need one LSE, and the newer <xref
      target="RFC7274"></xref> special purpose labels need two LSEs. Given the
      tiny number of original reserved labels, it is core to the MPLS design
      philosophy that this scarce resource is only used when it is absolutely
      necessary. Using a single LSE reserved or special purpose label to
      encode flow identity thus requires two stack entries, one for the
      reserved label and one for the flow identity. The larger set of <xref
      target="RFC7274"></xref> labels requires two labels stack entries for
      the special purpose label itself and hence a total of three label stack
      entries to encode the flow identity.</t>

      <t>The use of special purpose labels (SPL) <xref
      target="RFC7274"></xref> as part of a method to encode the identity
      information therefore has a number of undesirable implications for the
      data plane and hence whilst a solution may use SPL(s), methods that do
      not require SPLs need to be carefully considered.</t>
    </section>

    <section anchor="CP" title="Control Plane">
      <t>Any flow identity design should both seek to minimise the complexity
      of the control plane and should minimise the amount of label
      co-ordination needed amongst LSRs.</t>
    </section>

    <section anchor="PC" title="Privacy Considerations">
      <t>The inclusion of originating and/or flow information in a packet
      provides more identity information and hence potentially degrades the
      privacy of the communication. Recent IETF concerns on pervasive
      monitoring <xref target="RFC7258"></xref> would lead it to prefer a
      solution that does not degrade the privacy of user traffic below that of
      an MPLS network not implementing the flow identification feature. The
      minimizing the scope of the identity indication can be useful in
      minimizing the observability of the flow characteristics.</t>
    </section>

    <section anchor="SEC" title="Security Considerations">
      <t>Any solution to the flow identification needs must not degrade the
      security of the MPLS network below that of an equivalent network not
      deploying the specified identity solution. Propagation of identification
      information outside the MPLS network imposing it must be disabled by
      default. Any solution should provide for the restriction of the identity
      information to those components of the network that need to know it. It
      is thus desirable to limit the knowledge of the identify of an endpoint
      to only those LSRs that need to participate in traffic flow.</t>
    </section>

    <section title="IANA Considerations">
      <t>This memo has no IANA considerations.</t>
    </section>

    <section title="Acknowledgements">
      <t>The authors thank Nobo Akiya (nobo@cisco.com), Nagendra Kumar Nainar
      (naikumar@cisco.com) and George Swallow (swallow@cisco.com) for their
      comments.</t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include='reference.RFC.2119'?>
    </references>

    <references title="Informative References">
      <?rfc include='reference.I-D.tempia-ippm-p3m'?>

      <?rfc include='reference.RFC.7258'?>

      <?rfc include='reference.RFC.6790'?>

      <?rfc include='reference.RFC.5420'?>

      <?rfc include='reference.RFC.7274'?>

      <?rfc include='reference.RFC.6374'?>

      <?rfc include='reference.RFC.5654'?>

      <?rfc include='reference.RFC.5331'?>
    </references>
  </back>
</rfc>