TSVWG                                                       R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Informational                                  D. Black
Expires: December 22, 2016                               EMC Corporation
                                                           June 20, 2016

             Diffserv-Interconnection classes and practice


   This document defines a limited common set of Diffserv PHBs and
   codepoints (DSCPs) to be applied at (inter)connections of two
   separately administered and operated networks, and explains how this
   approach can simplify network configuration and operation.  Many
   network providers operate MPLS using Treatment Aggregates for traffic
   marked with different Diffserv PHBs, and use MPLS for interconnection
   with other networks.  This document offers a simple interconnection
   approach that may simplify operation of Diffserv for network
   interconnection among providers that use MPLS and apply the Short-
   Pipe tunnel mode.

Status of This Memo

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   This Internet-Draft will expire on December 22, 2016.

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Related work  . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Applicability Statement . . . . . . . . . . . . . . . . .   4
     1.3.  Document Organization . . . . . . . . . . . . . . . . . .   5
   2.  MPLS and the Short Pipe tunnel model  . . . . . . . . . . . .   5
   3.  Relationship to RFC 5127  . . . . . . . . . . . . . . . . . .   6
     3.1.  RFC 5127 Background . . . . . . . . . . . . . . . . . . .   6
     3.2.  Differences from RFC 5127 . . . . . . . . . . . . . . . .   7
   4.  The Diffserv-Intercon Interconnection Classes . . . . . . . .   8
     4.1.  Diffserv-Intercon Example . . . . . . . . . . . . . . . .   9
     4.2.  End-to-end QoS: PHB and DS CodePoint Transparency . . . .  12
     4.3.  Treatment of Network Control traffic at carrier
           interconnection interfaces  . . . . . . . . . . . . . . .  13
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Appendix A The MPLS Short Pipe Model and IP traffic   16
   Appendix B.  Change log (to be removed by the RFC editor) . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Diffserv has been deployed in many networks; it provides
   differentiated traffic forwarding based on the Diffserv Codepoint
   (DSCP) field [RFC2474].  This document defines a set of common
   Diffserv QoS classes (Per Hop Behaviors, PHBs)and code points at
   interconnection points to which and from which locally used classes
   and code points should be mapped.

   As described by section of RFC 2475, remarking of packets at
   domain boundaries is a Diffserv feature [RFC2475].  If traffic marked
   with unknown or unexpected DSCPs is received, RFC2474 recommends
   forwarding that traffic with default (best effort) treatment without
   changing the DSCP markings to better support incremental Diffserv
   deployment in existing networks as well as with routers that do not
   support Diffserv or are not configured to support it.  Many networks

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   do not follow this recommendation, and instead remark unknown or
   unexpected DSCPs to zero upon receipt for default (best effort)
   forwarding in accordance with the guidance in RFC 2475 [RFC2475] to
   ensure that appropriate DSCPs are used within a Diffserv domain.
   This draft assumes that latter approach by defining additional DSCPs
   that are known and expected at network interconnection interfaces.

   This document is motivated by requirements for IP network
   interconnection with Diffserv support among providers that operate
   MPLS in their backbones, but is applicable to other technologies.
   The operational simplifications and methods in this document help
   align IP Diffserv functionality with MPLS limitations resulting from
   the widely deployed Short Pipe tunnel model for operation [RFC3270].
   Further, limiting Diffserv to a small number of Treatment Aggregates
   can enable network traffic to leave a network with the DSCP value
   with which it was received, even if a different DSCP is used within
   the network, thus providing an opportunity to extend consistent QoS
   treatment across network boundaries.

   In isolation, use of a defined set of interconnection PHBs and DSCPs
   may appear to be additional effort for a network operator.  The
   primary offsetting benefit is that mapping from or to the
   interconnection PHBs and DSCPs is specified once for all of the
   interconnections to other networks that can use this approach.
   Absent this approach, the PHBs and DSCPs have to be negotiated and
   configured independently for each network interconnection, which has
   poor administrative and operational scaling properties.  Further,
   consistent end-to-end QoS treatment is more likely to result when an
   interconnection code point scheme is used because traffic is remarked
   to the same PHBs at all network interconnections.

   The interconnection approach described in this document (referred to
   as Diffserv-Intercon) uses a set of PHBs and MPLS treatment
   aggregates along with a set of interconnection DSCPs allowing
   straightforward rewriting to domain-internal DSCPs and defined DSCP
   markings for traffic forwarded to interconnected domains.  The
   solution described here can be used in other contexts benefitting
   from a defined interconnection QoS interface.

   The basic idea is that traffic sent with a Diffserv-Interconnect PHB
   and DSCP is restored to that PHB and DSCP at each network
   interconnection, even though a different PHB and DSCP may be used by
   each network involved.  The key requirement is that the network
   ingress interconnect DSCP be restored at network egress, and a key
   observation is that this is only feasible in general for a small
   number of DSCPs.  Traffic sent with other DSCPs can be remarked to an
   interconnect DSCP or dealt with via additional agreement(s) among the
   operators of the interconnected networks; remarking in the absence of

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   additional agreement(s) when the MPLS Short Pipe model is used for
   reasons explained in this document.

   In addition to the common interconnecting PHBs and DSCPs,
   interconnecting operators need to further agree on the tunneling
   technology used for interconnection (e.g., MPLS, if used) and control
   or mitigate the impacts of tunneling on reliability and MTU.

1.1.  Related work

   In addition to the activities that triggered this work, there are
   additional RFCs and Internet-drafts that may benefit from an
   interconnection PHB and DSCP scheme.  RFC 5160 suggests Meta-QoS-
   Classes to enable deployment of standardized end to end QoS classes
   [RFC5160].  The Diffserv-Intercon class- and codepoint scheme is
   intended to complement that work (e.g. by enabling a defined set of
   end-to-end QoS service classes).

   BGP signaling Class of Service at interconnection interfaces by BGP
   [I-D.knoll-idr-cos-interconnect], [ID.ietf-idr-sla] is complementary
   to Diffserv-Intercon.  These two BGP documents focus on exchanging
   SLA and traffic conditioning parameters and assume that common PHBs
   identified by the signaled DSCPs have been established (e.g., via use
   of the Diffserv-Intercon DSCPs) prior to BGP signaling of QoS.

1.2.  Applicability Statement

   This document is applicable to use of Differentiated Services for
   interconnection traffic between networks, and in particular to
   interconnection of MPLS-based networks.  This document is not
   intended for use within an individual network, where the approach
   specified in RFC 5127 [RFC5127] is among the possible alternatives;
   see Section 3 for further discussion.

   The Diffserv-Intercon approach described in this document simplifies
   IP based interconnection to domains operating the MPLS Short Pipe
   model for IP traffic, both terminating within the domain and
   transiting onward to another domain.  Transiting traffic is received
   and sent with the same PHB and DSCP.  Terminating traffic maintains
   the PHB with which it was received, however the DSCP may change.

   Diffserv-Intercon may also be applied to the Pipe tunneling model
   [RFC2983], [RFC3270], but is not applicable to the Uniform tunneling
   model [RFC2983], [RFC3270].

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1.3.  Document Organization

   This document is organized as follows: section 2 reviews the MPLS
   Short Pipe tunnel model for Diffserv Tunnels [RFC3270], because
   effective support for that model is a crucial goal of Diffserv-
   Intercon.  Section 3 provides background on RFC 5127's approach to
   traffic class aggregation within a Diffserv network domain and
   contrasts it with the Diffserv-Intercon approach.  Section 4
   introduces Diffserv-Interconnection Treatment Aggregates, along with
   the PHBs and DSCPs that they use, and explains how other PHBs (and
   associated DSCPs) may be mapped to these Treatment Aggregates.
   Section 4 also discusses treatment of non-tunneled and tunneled IP
   traffic and MPLS VPN QoS considerations and handling of high-priority
   Network Management traffic is described.  Appendix A describes how
   the MPLS Short Pipe model (penultimate hop popping) impacts QoS and
   DSCP marking for IP interconnections.

2.  MPLS and the Short Pipe tunnel model

   The Pipe and Uniform models for Differentiated Services and Tunnels
   are defined in [RFC2983].  RFC3270 adds the Short Pipe model in order
   to support MPLS penultimate hop popping (PHP) of Labels, primarily
   for MPLS-based IP tunnels and VPNs.  The Short Pipe model and PHP
   have subsequently become popular with network providers that operate
   MPLS networks and are now widely used to transport non-tunneled IP
   traffic, not just traffic encapsulated in IP tunnels and VPNs.  This
   has important implications for Diffserv functionality in MPLS

   RFC 2474's recommendation to forward traffic with unrecognized DSCPs
   with Default (best effort) service without rewriting the DSCP has
   proven to be a poor operational practice.  Network operation and
   management are simplified when there is a 1-1 match between the DSCP
   marked on the packet and the forwarding treatment (PHB) applied by
   network nodes.  When this is done, CS0 (the all-zero DSCP) is the
   only DSCP used for Default forwarding of best effort traffic, and a
   common practice is to remark to CS0 any traffic received with
   unrecognized or unsupported DSCPs at network edges.

   MPLS networks are more subtle in this regard, as it is possible to
   encode the provider's DSCP in the MPLS Traffic Class (TC) field and
   allow that to differ from the PHB indicated by the DSCP in the MPLS-
   encapsulated IP packet.  If the MPLS label with the provider's TC
   field is present at all hops within the provider network, this
   approach would allow an unrecognized DSCP to be carried edge-to-edge
   over an MPLS network, because the effective DSCP used by the
   provider's MPLS network would be encoded in the MPLS label TC field

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   (and also carried edge-to-edge).  Unfortunately this is only true for
   the Pipe tunnel model.

   The Short Pipe tunnel model and PHP behave differently because PHP
   removes and discards the MPLS provider label carrying the provider's
   TC field before the traffic exits the provider's network.  That
   discard occurs one hop upstream of the MPLS tunnel endpoint (which is
   usually at the network edge), resulting in no provider TC info being
   available at tunnel egress.  To ensure consistent handling of traffic
   at the tunnel egress, the DSCP field in the MPLS-encapsulated IP
   header has to contain a DSCP that is valid for the provider's
   network, so that IP header cannot be used to carry a different DSCP
   edge-to-edge.  See Appendix A for a more detailed discussion.

3.  Relationship to RFC 5127

   This document draws heavily upon RFC 5127's approach to aggregation
   of Diffserv traffic classes for use within a network, but there are
   important differences caused by characteristics of network
   interconnects that differ from links within a network.

3.1.  RFC 5127 Background

   Many providers operate MPLS-based backbones that employ backbone
   traffic engineering to ensure that if a major link, switch, or router
   fails, the result will be a routed network that continues to
   function.  Based on that foundation, [RFC5127] introduced the concept
   of Diffserv Treatment Aggregates, which enable traffic marked with
   multiple DSCPs to be forwarded in a single MPLS Traffic Class (TC)
   based on robust provider backbone traffic engineering.  This enables
   differentiated forwarding behaviors within a domain in a fashion that
   does not consume a large number of MPLS Traffic Classes.

   RFC 5127 provides an example aggregation of Diffserv service classes
   into 4 Treatment Aggregates.  A small number of aggregates are used

   o  The available coding space for carrying QoS information (e.g.,
      Diffserv PHB) in MPLS (and Ethernet) is only 3 bits in size, and
      is intended for more than just QoS purposes (see e.g.  [RFC5129]).

   o  The common interconnection DSCPs ought not to use all 8 possible
      values.  This leaves space for future standards, for private
      bilateral agreements and for local use PHBs and DSCPs.

   o  Migrations from one Diffserv code point scheme to a different one
      is another possible application of otherwise unused QoS code

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3.2.  Differences from RFC 5127

   Like RFC 5127, this document also uses four traffic aggregates, but
   differs from RFC 5127 in some important ways:

   o  It follows RFC 2475 in allowing the DSCPs used within a network to
      differ from those to exchange traffic with other networks (at
      network edges), but provides support to restore ingress DSCP
      values if one of the recommended interconnect DSCPs in this draft
      is used.  This results in DSCP remarking at both network ingress
      and network egress, and this draft assumes that such remarking at
      network edges is possible for all interface types.

   o  Diffserv-Intercon suggests limiting the number of interconnection
      PHBs per Treatment Aggregate to the minimum required.  As further
      discussed below, the number of PHBs per Treatment Aggreagate is no
      more than two.  When two PHBs are specified for a Diffserv-
      Intercon treatment aggregate, the expectation is that the provider
      network supports DSCPs for both PHBs, but uses a single MPLS TC
      for the Treatment Aggregate that contains the two PHBs.

   o  Diffserv-Intercon suggests mapping other PHBs and DSCPs into the
      interconnection Treatment Aggregates as further discussed below.

   o  Diffserv-Intercon treats network control traffic as a special
      case.  Within a provider's network, the CS6 DSCP is used for local
      network control traffic (routing protocols and OAM traffic that is
      essential to network operation administration, control and
      management) that may be destined for any node within the network.
      In contrast, network control traffic exchanged between networks
      (e.g., BGP) usually terminates at or close to a network edge, and
      is not forwarded through the network because it is not part of
      internal routing or OAM for the receiving network.  In addition,
      such traffic is unlikely to be covered by standard interconnection
      agreements; rather, it is more likely to be specifically
      configured (e.g., most networks impose restrictions on use of BGP
      with other networks for obvious reasons).  See Section 4.2 for
      further discussion.

   o  Because RFC 5127 used a Treatment Aggregate for network control
      traffic, Diffserv-Intercon can instead define a fourth traffic
      aggregate to be defined for use at network interconnections
      instead of the Network Control aggregate in RFC 5127.  Network
      Control traffic may still be exchanged across network
      interconnections as further discussed in Section 4.2.  Diffserv-
      Intercon uses this fourth traffic aggregate for VoIP traffic,
      where network-provided QoS is crucial, as even minor glitches are
      immediately apparent to the humans involved in the conversation.

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4.  The Diffserv-Intercon Interconnection Classes

   At an interconnection, the networks involved need to agree on the
   PHBs used for interconnection and the specific DSCP for each PHB.
   This document defines a set of 4 interconnection Treatment Aggregates
   with well-defined DSCPs to be aggregated by them.  A sending party
   remarks DSCPs from internal schemes to the interconnection code
   points.  The receiving party remarks DSCPs to their internal scheme.
   The interconnect SLA defines the set of DSCPs and PHBs supported
   across the two interconnected domains and the treatment of PHBs and
   DSCPs that are not recognized by the receiving domain.

   Similar approaches that use of a small number of traffic aggregates
   (including recognition of the importance of VoIP traffic) have been
   taken in related standards and recommendations from outside the IETF,
   e.g., Y.1566 [Y.1566], GSMA IR.34 [IR.34]and MEF23.1 [MEF23.1].

   The list of the four Diffserv-Interconnect traffic aggregates
   follows, highlighting differences from RFC 5127 and suggesting
   mappings for all RFC 4594 traffic classes to Diffserv-Intercon
   Treatment Aggregates:

    Telephony Service Treatment Aggregate:  PHB EF, DSCP 101 110 and PHB
           VOICE-ADMIT, DSCP 101100, see [RFC3246],[RFC4594]and
           [RFC5865].  This Treatment Aggregate corresponds to RFC 5127s
           real time Treatment Aggregate definition regarding the
           queuing (both delay and jitter should be minimized), but this
           aggregate is restricted to transport Telephony Service Class
           traffic in the sense of RFC 4594 [RFC4594].

   Bulk Real-Time Treatment Aggregate:  This Treatment Aggregate is
           designed to transport PHB AF41, DSCP 100 010 (the other AF4
           PHB group PHBs and DSCPs may be used for future extension of
           the set of DSCPs carried by this Treatment Aggregate).  This
           Treatment Aggregate is intended for Diffserv-Intercon network
           interconnection of the portions of RFC 5127's Real Time
           Treatment Aggregate, that consume significant bandwidth.
           This traffic is expected to consist of the RFC4594 classes
           Broadcast Video, Real-Time Interactive and Multimedia
           Conferencing.  This treatment aggregate should be configured
           with a rate queue (consistent with RFC 4594's recommendation
           for the transported traffic classes).  By comparison to RFC
           5127, the number of DSCPs has been reduced to one
           (initially).  The AF42 and AF43 PHBs could be added if there
           is a need for three-color marked Multimedia.

   Assured Elastic Treatment Aggregate  This Treatment Aggregate
           consists of PHBs AF31 and AF32 ( i.e., DSCPs 011 010 and 011

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           100).  By comparison to RFC 5127, the number of DSCPs has
           been reduced to two.  This document suggests to transport
           signaling marked by AF31 (e.g. as recommended by GSMA IR.34
           [IR.34]).  AF33 is reserved for extension of PHBs to be
           aggregated by this TA.  For Diffserv-Intercon network
           interconnection, the following RFC 4594 service classes
           should be mapped to the Assured Elastic Treatment Aggregate:
           the Signaling Service Class (being marked for lowest loss
           probability), Multimedia Streaming Service Class, the Low-
           Latency Data Service Class and the High-Throughput Data
           Service Class.

   Default / Elastic Treatment Aggregate:   transports the default PHB,
           CS0 with DSCP 000 000.  RFC 5127 example refers to this
           Treatment Aggregate as Aggregate Elastic.  An important
           difference from RFC 5127 is that any traffic with
           unrecognized or unsupported DSCPs may be remarked to this
           DSCP.  For Diffserv-Intercon network interconnection, the RFC
           4594 standard service class and Low-priority Data service
           class should be mapped to this Treatment Aggregate.  This
           document does not specify an interconnection class for RFC
           4594 Low-priority data.  This data may be forwarded by a
           Lower Effort PHB in one domain (like the PHB proposed by
           Informational [RFC3662]), but using the methods specified in
           this document will be remarked with DSCP CS0 at a Diffserv-
           Intercon network interconnection.  This has the effect that
           Low-priority data is treated the same as data sent using the
           default class.  (Note: In a network that implements RFC 2474,
           Low-priority traffic marked as CS1 would otherwise receive
           better treatment than traffic using the default class.)

   RFC2575 states that Ingress nodes must condition all inbound traffic
   to ensure that the DS codepoints are acceptable; packets found to
   have unacceptable codepoints must either be discarded or must have
   their DS codepoints modified to acceptable values before being
   forwarded.  For example, an ingress node receiving traffic from a
   domain with which no enhanced service agreement exists may reset the
   DS codepoint to CS0.  As a consequence, an interconnect SLA needs to
   specify not only the treatment of traffic that arrives with a
   supported interconnect DSCP, but also the treatment of traffic that
   arrives with unsupported or unexpected DSCPs; remarking to CS0 is a
   widely deployed behavior.

4.1.  Diffserv-Intercon Example

   The overall approach to DSCP marking at network interconnections is
   illustrated by the following example.  Provider O and provider W are

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   peered with provider T.  They have agreed upon a QoS interconnection

   Traffic of provider O terminates within provider T's network, while
   provider W's traffic transits through the network of provider T to
   provider F.  This example assumes that all providers use their own
   internal PHB and codepoint (DSCP) that correspond to the AF31 PHB in
   the Diffserv-Intercon Assured Elastic Treatment Aggregate (AF21 and
   CS2 are used in the example).

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    Provider O            Provider W
          |                      |
     +----------+           +----------+
     |  AF21    |           |    CS2    |
     +----------+           +----------+
          V                      V
      +~~~~~~~+              +~~~~~~~+
      |Rtr PrO|              |Rtr PrW|
      +~~~~~~~+              +~~~~~~~+
          |        DiffServ      |
     +----------+           +----------+
     |    AF31  |           |    AF31  |
     +----------+           +----------+
          V        Intercon      V
      +~~~~~~~+                  |
          |            Provider T domain
     |  MPLS TC 2, AF21  |
        |      |    +----------+   +~~~~~~~+
        V      `--->|   AF21   |->-|RtrDstH|
    +----------+    +----------+   +~~~~~~~+
    |   AF21   |       Local DSCPs Provider T
        |        DiffServ
    |    AF31  |
        |        Intercon
    |   AF11   |    Provider F

   Diffserv-Intercon example

                                 Figure 1

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   Providers only need to deploy mappings of internal DSCPs to/from
   Diffserv-Intercon DSCPs in order to exchange traffic using the
   desired PHBs.  In the example, provider O has decided that the
   properties of his internal class AF21 and are best met by the
   Diffserv-Intercon Assured Elastic Treatment Aggregate, PHB AF31.  At
   the outgoing peering interface connecting provider O with provider T
   the former's peering router remarks AF21 traffic to AF31.  The domain
   internal PHB of provider T meeting the requirement of Diffserv-
   Intercon Assured Elastic Treatment Aggregate are from AF2x PHB group.
   Hence AF31 traffic received at the interconnection with provider T is
   remarked to AF21 by the peering router of domain T, and domain T has
   chosen to use MPLS Traffic Class value 2 for this aggregate.  At the
   penultimate MPLS node, the top MPLS label is removed and exposes the
   IP header marked by the DSCP which has been set at the network
   ingress.  The peering router connecting domain T with domain F
   classifies the packet by its domain-T-internal DSCP AF21.  As the
   packet leaves domain T on the interface to domain F, this causes the
   packet's DSCP to be remarked to AF31.  The peering router of domain F
   classifies the packet for domain-F-internal PHB AF11, as this is the
   PHB with properties matching Diffserv-Intercon's Assured Elastic
   Treatment Aggregate.

   This example can be extended.  The figure shows Provider-W using CS2
   for traffic that corresponds to Diffserv-Intercon Assured Elastic
   Treatment Aggregate PHB AF31; that traffic is mapped to AF31 at the
   Diffserv-Intercon interconnection to Provider-T.  In addition,
   suppose that Provider-O supports a PHB marked by AF22 and this PHB is
   supposed to be transported by QoS within Provider-T domain.  Then
   Provider-O will remark it with DSCP AF32 for interconnection to

   Finally suppose that Provider-W supports CS3 for internal use only.
   Then no Diffserv- Intercon DSCP mapping needs to be configured at the
   peering router.  Traffic, sent by Provider-W to Provider-T marked by
   CS3 due to a misconfiguration may be remarked to CS0 by Provider-T.

4.2.  End-to-end QoS: PHB and DS CodePoint Transparency

   This section briefly discusses end-to-end QoS approaches related to
   the Uniform, Pipe and Short Pipe tunnel models ([RFC2983],
   [RFC3270]), when used edge-to-edge in a network.

   o  With the Uniform model, neither the DCSP nor the PHB change.  This
      implies that a network management packet received with a CS6 DSCP
      would be forwarded with an MPLS Traffic Class corresponding to
      CS6.  The uniform model is outside the scope of this document.

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   o  With the Pipe model, the inner tunnel DCSP remains unchanged, but
      an outer tunnel DSCP and the PHB could changed.  For example a
      packet received with a (network specific) CS1 DSCP would be
      transported by default PHB and if MPLS is applicable, forwarded
      with an MPLS Traffic Class corresponding to Default PHB.  The CS1
      DSCP is not rewritten.  Transport of a large variety (much greater
      than 4) DSCPs may be required across an interconnected network
      operating MPLS Short pipe transport for IP traffic.  In that case,
      a tunnel based on the Pipe model is among the possible approaches.
      The Pipe model is outside the scope of this document.

   o  With the Short Pipe model, the DCSP likely changes and the PHB
      might change.  This document describes a method to simplify QoS
      for network interconnection when a DSCP rewrite can't be avoided.

4.3.  Treatment of Network Control traffic at carrier interconnection

   As specified by RFC4594, section 3.2, Network Control (NC) traffic
   marked by CS6 is expected at some interconnection interfaces.  This
   document does not change RFC4594, but observes that network control
   traffic received at network ingress is generally different from
   network control traffic within a network that is the primary use of
   CS6 envisioned by RFC 4594.  A specific example is that some CS6
   traffic exchanged across carrier interconnections is terminated at
   the network ingress node, e.g. when BGP is used between the two
   routers on opposite ends of an interconnection link; in this case the
   operators would enter into a bilateral agreement to use CS6 for that
   BGP traffic.

   The end-to-end QoS discussion in the previous section (4.2) is
   generally inapplicable to network control traffic - network control
   traffic is generally intended to control a network, not be
   transported across it.  One exception is that network control traffic
   makes sense for a purchased transit agreement, and preservation of
   the CS6 DSCP marking for network control traffic that is transited is
   reasonable in some cases, although it is generally inappropriate to
   use CS6 for forwarding that traffic within the network that provides
   transit.  Use of an IP tunnel is suggested in order to conceal the
   CS6 markings on transiting network control traffic from the network
   that provides the transit.  In this case, Pipe model for Diffserv
   tunneling is used.

   If the MPLS Short Pipe model is deployed for non-tunneled IPv4
   traffic, an IP network provider should limit access to the CS6 and
   CS7 DSCPs so that they are only used for network control traffic for
   the provider's own network.

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   Interconnecting carriers should specify treatment of CS6 marked
   traffic received at a carrier interconnection which is to be
   forwarded beyond the ingress node.  An SLA covering the following
   cases is recommended when a provider wishes to send CS6 marked
   traffic across an interconnection link and that traffic's destination
   is beyond the interconnected ingress node:

   o  classification of traffic that is network control traffic for both
      domains.  This traffic should be classified and marked for the CS6

   o  classification of traffic that is network control traffic for the
      sending domain only.  This traffic should be forwarded with a PHB
      that is appropriate for the NC service class [RFC4594], e.g.  AF31
      as specified by this document.  As an example GSMA IR.34
      recommends an Interactive class / AF31 to carry SIP and DIAMETER
      traffic.  While this is service control traffic of high importance
      to interconnected Mobile Network Operators, it is certainly not
      Network Control traffic for a fixed network providing transit
      among such operators, and hence should not receive CS6 treatment
      in such a transit network.

   o  any other CS6 marked traffic should be remarked or dropped.

5.  Acknowledgements

   Bob Briscoe and Gorry Fairhurst reviewed the draft and provided rich
   feedback.  Fred Baker, Brian Carpenter, Al Morton and Sebastien
   Jobert discussed the draft and helped improving it.  Mohamed
   Boucadair and Thomas Knoll helped adding awareness of related work.
   James Polk's discussion during IETF 89 helped to improve the text on
   the relation of this draft to RFC 4594 and RFC 5127.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   This document does not introduce new features; it describes how to
   use existing ones.  The Diffserv security considerations in RFC 2475
   [RFC2475] and RFC 4594 [RFC4594] apply.

8.  References

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8.1.  Normative References

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <http://www.rfc-editor.org/info/rfc5129>.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, DOI 10.17487/RFC5865, May 2010,

8.2.  Informative References

              Knoll, T., "BGP Class of Service Interconnection", draft-
              knoll-idr-cos-interconnect-16 (work in progress), May

              IETF, "Inter-domain SLA Exchange", IETF,
              draft-ietf-idr-sla-exchange/, 2013.

   [IR.34]    GSMA Association, "IR.34 Inter-Service Provider IP
              Backbone Guidelines Version 7.0", GSMA,  GSMA IR.34
              ir.34.pdf, 2012.

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   [MEF23.1]  MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
              Class of Service Phase 2", MEF,  MEF23.1
              specifications/MEF_23.1.pdf, 2012.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,

   [RFC3662]  Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
              Per-Domain Behavior (PDB) for Differentiated Services",
              RFC 3662, DOI 10.17487/RFC3662, December 2003,

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
              February 2008, <http://www.rfc-editor.org/info/rfc5127>.

   [RFC5160]  Levis, P. and M. Boucadair, "Considerations of Provider-
              to-Provider Agreements for Internet-Scale Quality of
              Service (QoS)", RFC 5160, DOI 10.17487/RFC5160, March
              2008, <http://www.rfc-editor.org/info/rfc5160>.

   [Y.1566]   ITU-T, "Quality of service mapping and interconnection
              between Ethernet, IP and multiprotocol label switching
              networks", ITU,
               http://www.itu.int/rec/T-REC-Y.1566-201207-I/en, 2012.

Appendix A.  Appendix A The MPLS Short Pipe Model and IP traffic

   The MPLS Short Pipe Model (or penultimate Hop Label Popping) is
   widely deployed in carrier networks.  If non-tunneled IPv4 traffic is
   transported using MPLS Short Pipe, IP headers appear inside the last
   section of the MPLS domain.  This impacts the number of PHBs and
   DSCPs that a network provider can reasonably support.  See Figure 2
   (below) for an example.

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   For tunneled IPv4 traffic, only the outer tunnel header is relevant
   for forwarding.  If the tunnel does not terminate within the MPLS
   network section, only the outer tunnel DSCP is involved, as the inner
   DSCP does not affect forwarding behavior; in this case all DSCPs
   could be used in the inner IP header without affecting network
   behavior based on the outer MPLS header.  Here the Pipe model

   Non-tunneled IPv6 traffic as well as Layer 2 and Layer 3 VPN traffic
   all use an additional MPLS label; in this case, the MPLS tunnel
   follows the Pipe model.  Classification and queuing within an MPLS
   network is always based on an MPLS label, as opposed to the outer IP

   Carriers often select QoS PHBs and DSCP without regard to
   interconnection.  As a result PHBs and DSCPs typically differ between
   network carriers.  With the exception of best effort traffic, a DSCP
   change should be expected at an interconnection at least for plain IP
   traffic, even if the PHB is suitably mapped by the carriers involved.

   Although RFC3270 suggests that the Short Pipe Model is only
   applicable to VPNs, current networks also use it to transport non-
   tunneled IPv4 traffic.  This is shown in figure 2 where Diffserv-
   Intercon is not used, resulting in exposure of the internal DSCPs of
   the upstream network to the downstream network across the

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       \|/           IPv4, DSCP_send
   Peering Router
       \|/           IPv4, DSCP_send
   MPLS Edge Router
        |          Mark MPLS Label, TC_internal
       \|/         Remark DSCP to
        V            (Inner: IPv4, DSCP_d)
   MPLS Core Router  (penultimate hop label popping)
        |                        \
        |            IPv4, DSCP_d |  The DSCP needs to be in network-
        |                 ^^^^^^^^|  internal QoS context. The Core
       \|/                         > Router may require or enforce
        V                         |  that. The Edge Router may wrongly
        |                         |  classify, if the DSCP is not in
        |                        /   network-internal Diffserv context.
   MPLS Edge Router
        |                        \   Traffic leaves the network marked
       \|/           IPv4, DSCP_d |  with the network-internal
        V                          > DSCP_d that must be dealt with
        |                         |  by the next network (downstream).
        |                        /
   Peer Router
        |          Remark DSCP to
       \|/           IPv4, DSCP_send

   Short-Pipe / penultimate hop popping example

                                 Figure 2

   The packets IP DSCP must be in a well understood Diffserv context for
   schedulers and classifiers on the interfaces of the ultimate MPLS
   link (last link traversed before leaving the network).  The necessary
   Diffserv context is network-internal and a network operating in this
   mode enforces DSCP usage in order to obtain robust QoS behavior.

   Without Diffserv-Intercon treatment, the traffic is likely to leave
   each network marked with network-internal DSCP.  DSCP_send in the
   figure above has to be remarked into the first network's Diffserv

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   scheme at the ingress MPLS Edge Router, to DSCP_d in the example.
   For that reason, the traffic leaves this domain marked by the
   network-internal DSCP_d.  This structure requires that every carrier
   deploys per-peer PHB and DSCP mapping schemes.

   If Diffserv-Intercon is applied DSCPs for traffic transiting the
   domain can be mapped from and remapped to an original DSCP.  This is
   shown in figure 3.  Internal traffic may continue to use internal
   DSCPs (e.g, DSCP_d) and they may also be used between a carrier and
   its direct customers.

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   Internal Router
        |   Outer Header
       \|/    IPv4, DSCP_send
   Peering Router
        |  Remark DSCP to
       \|/    IPv4, DSCP_ds-int    Diffserv-Intercon DSCP and PHB
   MPLS Edge Router
        |   Mark  MPLS Label, TC_internal
       \|/  Remark DSCP to
        V     (Inner: IPv4, DSCP_d)   domain internal DSCP for
        |                             the PHB
   MPLS Core Router  (penultimate hop label popping)
        |     IPv4, DSCP_d
        |           ^^^^^^
   MPLS Edge Router--------------------+
        |                              |
       \|/  Remark DSCP to            \|/  IPv4, DSCP_d
        V     IPv4, DSCP_ds-int        V
        |                              |
        |                              |
   Peer Router              Domain internal Broadband
        |                        Access Router
       \|/  Remark DSCP to            \|/
        V     IPv4, DSCP_send          V  IPv4, DSCP_d
        |                              |

   Short-Pipe example with Diffserv-Intercon

                                 Figure 3

Appendix B.  Change log (to be removed by the RFC editor)

   00 to 01  Added an Applicability Statement.  Put the main part of the
           RFC5127 related discussion into a separate chapter.

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   01 to 02  More emphasis on the Short-Pipe tunel model as compared to
           Pipe and Uniform tunnel models.  Further editorial

   02 to 03  Suggestions how to remark all RFC4594 classes to Diffserv-
           Intercon classes at interconnection.

   03 to 04  Minor clarifications and editorial review, preparation for

Authors' Addresses

   Ruediger Geib (editor)
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   Darmstadt  64295

   Phone: +49 6151 5812747
   Email: Ruediger.Geib@telekom.de

   David L. Black
   EMC Corporation
   176 South Street
   Hopkinton, MA

   Phone: +1 (508) 293-7953
   Email: david.black@emc.com

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