



TSVWG                                                       R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Informational                                  D. Black
Expires: June 18, 2017                                          Dell EMC
                                                       December 15, 2016


             Diffserv-Interconnection classes and practice
                 draft-ietf-tsvwg-diffserv-intercon-14

Abstract

   This document defines a limited common set of Diffserv Per Hop
   Behaviours (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 Multi
   Protocol Label Switching (MPLS) using Treatment Aggregates for
   traffic marked with different Diffserv Per Hop Behaviors, 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.  While motivated by the
   requirements of MPLS network operators that use Short-Pipe tunnels,
   this document is applicable to other networks, both MPLS and non-
   MPLS.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 18, 2017.








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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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 RFC5127 . . . . . . . . . . . . . . . . . . .   6
     3.1.  RFC5127 Background  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Differences from RFC5127  . . . . . . . . . . . . . . . .   7
   4.  The Diffserv-Intercon Interconnection Classes . . . . . . . .   8
     4.1.  Diffserv-Intercon Example . . . . . . . . . . . . . . . .  10
     4.2.  End-to-end PHB and DSCP Transparency  . . . . . . . . . .  13
     4.3.  Treatment of Network Control traffic at carrier
           interconnection interfaces  . . . . . . . . . . . . . . .  14
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Appendix A The MPLS Short Pipe Model and IP traffic   18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

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




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   As described by section 2.3.4.2 of RFC2475, 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
   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 RFC2475 [RFC2475] to
   ensure that appropriate DSCPs are used within a Diffserv domain.
   This draft is based on the latter approach, and defines additional
   DSCPs that are known and expected at network interconnection
   interfaces in order to reduce the amount of traffic whose DSCPs are
   remarked to zero.

   This document is motivated by requirements for IP network
   interconnection with Diffserv support among providers that operate
   Multi Protocol Label Switching (MPLS) in their backbones, but is also
   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 Diffserv 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 Diffserv 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 (mapped to four
   corresponding 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




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   other contexts benefitting from a defined Diffserv interconnection
   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; use of the MPLS Short Pipe
   model favors remarking unexpected DSCPs to zero in the absence of
   additional agreement(s), as explained further 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.  RFC5160 suggests Meta-QoS-
   Classes to help enabling 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 interconnection DSCPs and PHBs).

   Border Gateway Protocol (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 Service Level Agreement (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 PHB id
   codes.

1.2.  Applicability Statement

   This document is applicable to use of Differentiated Services for
   interconnection traffic between networks, and is particularly suited
   to interconnection of MPLS-based networks that use MPLS Short-pipe
   tunnels.  This document is also applicable to other network
   technologies, but it is not intended for use within an individual
   network, where the approach specified in RFC5127 [RFC5127] is among
   the possible alternatives; see Section 3 for further discussion.



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   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 is also applicable to the Pipe tunneling model
   [RFC2983], [RFC3270], but it is not applicable to the Uniform
   tunneling model [RFC2983], [RFC3270].

   The Diffserv-Intercon approach defines a set of four PHBs for support
   at interconnections (or network boundaries in general).
   Corresponding DSCPs for use at an interconnection interface are
   added.  Diffserv-intercon allows for a simple mapping of PHBs and
   DSCPs to MPLS Treatment Aggregates.  It is extensible by IETF
   standardisation and this allows additional PHBs and DSCPs to be
   specified for the Diffserv-intercon scheme.  Coding space for private
   interconnection agreements or provider internal services is left too.

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 RFC5127'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 IP traffic, MPLS VPN Diffserv
   considerations and handling of high-priority Network Management
   traffic.  Appendix A describes how the MPLS Short Pipe model
   (penultimate hop popping) impacts DSCP marking for IP
   interconnections.

2.  MPLS and the Short Pipe tunnel model

   This section provides a summary of the implications of the MPLS Short
   Pipe tunnels, and in particular their use of Penultimate Hop Popping
   (PHP, see RFC3270) on the Diffserv tunnel framework described in
   RFC2983.  The Pipe and Uniform models for Differentiated Services and
   Tunnels are defined in [RFC2983].  RFC3270 adds the Short Pipe model
   to reflect the impact of MPLS PHP, 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




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   are now widely used to transport unencapsulated IP traffic.  This has
   important implications for Diffserv functionality in MPLS networks.

   RFC2474's recommendation to forward traffic with unrecognized DSCPs
   with Default (best effort) service without rewriting the DSCP has not
   been widely deployed in 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
   (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 RFC5127

   This document draws heavily upon RFC5127'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.  RFC5127 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



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   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.

   RFC5127 provides an example aggregation of Diffserv service classes
   into 4 Treatment Aggregates.  A small number of aggregates are used
   because:

   o  The available coding space for carrying Traffic Class information
      (e.g., Diffserv PHB) in MPLS (and Ethernet) is only 3 bits in
      size, and is intended for more than just Diffserv 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 DSCPs.

3.2.  Differences from RFC5127

   Like RFC5127, this document also uses four traffic aggregates, but
   differs from RFC5127 in some important ways:

   o  It follows RFC2475 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 Aggregate 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



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      network control traffic (routing protocols and Operations,
      Administration, and Maintenance (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 RFC5127 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 RFC5127.  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 service differentiation is crucial, as even
      minor glitches are immediately apparent to the humans involved in
      the conversation.

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 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 RFC5127 and suggesting
   mappings for all RFC4594 traffic classes to Diffserv-Intercon
   Treatment Aggregates:





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    Telephony Service Treatment Aggregate:  PHB EF, DSCP 101 110 and PHB
           VOICE-ADMIT, DSCP 101 100, see [RFC3246], [RFC4594] and
           [RFC5865].  This Treatment Aggregate corresponds to RFC5127's
           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 RFC4594 [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 RFC5127'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 RFC4594's recommendation
           for the transported traffic classes).  By comparison to
           RFC5127, 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 Conferencing
           traffic.

   Assured Elastic Treatment Aggregate  This Treatment Aggregate
           consists of PHBs AF31 and AF32 ( i.e., DSCPs 011 010 and 011
           100).  By comparison to RFC5127, 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 RFC4594 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.  RFC5127 example refers to this
           Treatment Aggregate as Aggregate Elastic.  An important
           difference from RFC5127 is that any traffic with unrecognized
           or unsupported DSCPs may be remarked to this DSCP.  For
           Diffserv-Intercon network interconnection, the RFC4594
           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 RFC4594 Low-



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           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 RFC2474, Low-priority
           traffic marked as CS1 would otherwise receive better
           treatment than traffic using the default class.)

   RFC2475 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.

   During the process of setting up a Diffserv interconnection, both
   networks should define the set of acceptable and unacceptable DSCPs
   and specify the treatment of traffic marked with each DSCP.

   While Diffserv-Intercon allows modification of unacceptable DSCPs, if
   traffic using one or more of the PHBs in a PHB group (e.g., AF3x,
   consisting of AF31, AF32 and AF33) is accepted as part of a supported
   Diffserv-Intercon Treatment Aggregate, then traffic using other PHBs
   from the same PHB group should not be modified to use PHBs outside of
   that PHB group, and in particular should not be remarked to CS0
   unless the entire PHB group is remarked to CS0.  This avoids
   unexpected forwarding behavior (and potential reordering, see also
   [RFC7657]) when using Assured Forwarding (AF) PHBs [RFC2597].

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
   peered with provider T.  They have agreed upon a Diffserv
   interconnection SLA.

   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




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   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|               Rtr:   Router
      +~~~~~~~+              +~~~~~~~+             Pr[L]:   Provider[L]
          |        DiffServ      |
     +----------+           +----------+
     |    AF31  |           |    AF31  |
     +----------+           +----------+
          V        Intercon      V
      +~~~~~~~+                  |
      |RtrPrTI|------------------+            Router Provider T Ingress
      +~~~~~~~+
          |            Provider T domain
     +------------------+
     |  MPLS TC 2, AF21 |
     +------------------+
        |      |    +----------+   +~~~~~~~+
        V      `--->|   AF21   |->-|RtrDstH|    Router Destination Host
    +----------+    +----------+   +~~~~~~~+
    |   AF21   |       Local DSCPs Provider T
    +----------+
        |
     +~~~~~~~+
     |RtrPrTE|                               Router Provider T Egress
     +~~~~~~~+
        |        DiffServ
    +----------+
    |    AF31  |
    +----------+
        |        Intercon
     +~~~~~~~+
     |RtrPrF |                               Router Provider F
     +~~~~~~~+
        |
    +----------+
    |   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 so that they can exchange traffic using the
   desired PHBs.  In the example, provider O has decided that the
   properties of his internal class AF21 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 that 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 obtain Diffserv transport within Provider-T domain.  Then
   Provider-O will remark it with DSCP AF32 for interconnection to
   Provider-T.

   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 PHB and DSCP Transparency

   This section briefly discusses end-to-end Diffserv 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
      Diffserv network interconnection when a DSCP rewrite can't be
      avoided.

4.3.  Treatment of Network Control traffic at carrier interconnection
      interfaces

   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 RFC4594.  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 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 between
   networks.  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 unencapsulated 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
      DSCP.

   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.  Brian Carpenter, Fred Baker, 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 RFC4594 and RFC5127.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   The DSCP field in the IP header can expose additional traffic
   classification information at network interconnections by comparison
   to use of a zero DSCP for all interconnect traffic.  If traffic
   classification info is sensitive, the DSCP field could be remarked to
   zero to hide the classification as a countermeasure, at the cost of
   loss of Diffserv info and differentiated traffic handling on the
   interconnect and subsequent networks.  When AF PHBs are used, any
   such remarking should respect AF PHB group boundaries as further
   discussed at the end of Section 4.



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

8.  References

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,
              <http://www.rfc-editor.org/info/rfc2474>.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597,
              DOI 10.17487/RFC2597, June 1999,
              <http://www.rfc-editor.org/info/rfc2597>.

   [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,
              <http://www.rfc-editor.org/info/rfc3246>.

   [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,
              <http://www.rfc-editor.org/info/rfc3270>.

   [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,
              <http://www.rfc-editor.org/info/rfc5865>.

8.2.  Informative References

   [I-D.knoll-idr-cos-interconnect]
              Knoll, T., "BGP Class of Service Interconnection", draft-
              knoll-idr-cos-interconnect-17 (work in progress), November
              2016.





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   [ID.ietf-idr-sla]
              IETF, "Inter-domain SLA Exchange", IETF,
              http://datatracker.ietf.org/doc/
              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
              http://www.gsma.com/newsroom/wp-content/uploads/2012/03/
              ir.34.pdf, 2012.

   [MEF23.1]  MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
              Class of Service Phase 2", MEF,  MEF23.1
              http://metroethernetforum.org/PDF_Documents/technical-
              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,
              <http://www.rfc-editor.org/info/rfc2475>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <http://www.rfc-editor.org/info/rfc2983>.

   [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,
              <http://www.rfc-editor.org/info/rfc3662>.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <http://www.rfc-editor.org/info/rfc4594>.

   [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>.

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,
              <http://www.rfc-editor.org/info/rfc7657>.




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   [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 unencapsulated IP 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.

   For encapsulated IP 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
   applies.

   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 header.

   Carriers often select 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
   unencapsulated 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
   interconnection.











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        |
       \|/           IPv4, DSCP_send
        V
        |
   Peering Router
        |
       \|/           IPv4, DSCP_send
        V
        |
   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 Diffserv 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
        V
        |


   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 differentiated
   forwarding behavior.

   Without Diffserv-Intercon treatment, the traffic is likely to leave
   each network marked with network-internal DSCP.  DSCP_send in the



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   figure above has to be remarked into the first network's Diffserv
   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
        V
        |
   Peering Router
        |  Remark DSCP to
       \|/    IPv4, DSCP_ds-int    Diffserv-Intercon DSCP and PHB
        V
        |
   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
        |           ^^^^^^
       \|/
        V
        |
        |
   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

Authors' Addresses








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

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


   David L. Black
   Dell EMC
   176 South Street
   Hopkinton, MA
   USA

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

































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