DiffServ interconnection classes and practice
draft-ietf-tsvwg-diffserv-intercon-01

TSVWG                                                       R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Informational                                  D. Black
Expires: September 10, 2015                              EMC Corporation
                                                           March 9, 2015

             DiffServ interconnection classes and practice
                 draft-ietf-tsvwg-diffserv-intercon-01

Abstract

   This document proposes a limited set of DiffServ PHBs and codepoints
   to be applied at (inter)connections of two separately administered
   and operated networks.  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.

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
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   This Internet-Draft will expire on September 10, 2015.

Copyright Notice

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   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Related work  . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Applicability Statement . . . . . . . . . . . . . . . . .   4
     1.3.  Document Organization . . . . . . . . . . . . . . . . . .   4
   2.  MPLS and the Short Pipe tunnel model  . . . . . . . . . . . .   4
   3.  Relationship to RFC 5127  . . . . . . . . . . . . . . . . . .   5
     3.1.  RFC 5127 Background . . . . . . . . . . . . . . . . . . .   6
     3.2.  Differences from RFC 5127 . . . . . . . . . . . . . . . .   6
   4.  The DiffServ-Intercon Interconnection Classes . . . . . . . .   7
     4.1.  End-to-end QoS: PHB and DS CodePoint Transparency . . . .  12
     4.2.  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  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Appendix A Carrier interconnection related DiffServ
                aspects  . . . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Appendix B The MPLS Short Pipe Model and IP traffic   18
   Appendix C.  Change log . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   DiffServ has been deployed in many networks.  As described by section
   2.3.4.2 of RFC 2475, remarking of packets at domain boundaries is a
   DiffServ feature [RFC2475].  This draft proposes a set of standard
   QoS classes and code points at interconnection points to which and
   from which locally used classes and code points should be mapped.

   RFC2474 specifies the DiffServ Codepoint Field [RFC2474].
   Differentiated treatment is based on the specific DSCP.  Once set, it
   may change.  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.  Many
   networks do not follow this recommendation, and instead remark
   unknown or unexpected DSCPs to the zero DSCP upon receipt for
   consistency with default (best effort) forwarding in accordance with

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   the guidance in RFC 2475 [RFC2474] to ensure that appropriate DSCPs
   are used within a DiffServ domain.

   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; further,
   limiting DiffServ to a small number of Treatment Aggregates can
   enable network traffic to leave a network with the same DSCPs that it
   was received with, 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 standard interconnection PHBs and DSCPs may
   appear to be additional effort for a network operator.  The primary
   offsetting benefit is that the 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.  Otherwise, the PHBs and
   DSCPs have to be negotiated and configured independently for each
   network interconnection, which has poor scaling properties.  Further,
   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.  This document
   envisions one-to-one DSCP remarking at network interconnections (not
   n DSCP to one DSCP remarking).

   In addition to the standard 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].  In private discussion, the authors of that RFC agree that
   the proposed interconnection class- and codepoint scheme and its
   enablement of standardised end to end classes would complement their
   own work.

   Work on signaling Class of Service at interconnection interfaces by
   BGP [I-D.knoll-idr-cos-interconnect], [ID.idr-sla] is beyond the
   scope of this draft.  When the scheme in this document is used,
   signaled access to QoS classes may be of interest.  These two BGP
   documents focus on exchanging SLA and traffic conditioning parameters

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   and assume that common PHBs identified by the signaled DSCPs have
   been established prior to BGP signaling of QoS.

1.2.  Applicability Statement

   This document is primarily applicable to use of Differentiated
   Services for interconnection traffic between networks, and in
   particular to interconnection of MPLS-based networks.  The approach
   described in this document is not intended for use within the
   interconnected (or other) networks, 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 to transport plain IP traffic terminating within or transiting
   through the receiving domain.  Transit traffic is reiceived and sent
   with the same PHB and DSCP.  Terminating traffic maintains the PHB
   with which it was received, however the DSCP may change.

1.3.  Document Organization

   This document is organized as follows: section 2 reviews the MPLS
   Short Pipe tunnel model for DiffServ Tunnels [RFC3270]; effective
   support for that model is a crucial goal of this document.  Section 3
   provides background on RFC 5127's approach to traffic class
   aggregation within a DiffServ network domain and explains why this
   document uses a somewhat different approach.  Section 4 introduces
   DiffServ interconnection Treatment Aggregates, plus the PHBs and
   DSCPs that are mapped to these Treatment Aggregates.  Further,
   section 4 discusses treatment of non-tunneled and tunneled IP traffic
   and MPLS VPN QoS aspects.  Finally Network Management PHB treatment
   is described.  Annex B describes the impact of the MPLS Short Pipe
   model (penultimate hop popping) on QoS related 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 MPLS Short Pipe model in
   order to support penultimate hop popping (PHP) of MPLS Labels,
   primarily for IP tunnels and VPNs.  The Short Pipe model and PHP have
   become popular with many network providers that operate MPLS networks
   and are now widely used to transport non-tunnelled IP traffic, not
   just traffic encapsulated in IP tunnels and VPNs.  This has important
   implications for DiffServ functionality in MPLS networks.

   RFC 2474's recommendation to forward traffic with unrecognized DSCPs
   with Default (best effort) service without rewriting the DSCP has

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   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, so a
   common practice is to use CS0 to remark 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 TC field and allow that to
   differ from the PHB indicated by the DSCP in the MPLS-encapsulated IP
   packet.  That would allow an unrecognized DSCP to be carried edge-to-
   edge over an MPLS network, because the effective DSCP used by the
   MPLS network would be encoded in the MPLS label TC field (and also
   carried edge-to-edge); this approach assumes that a provider MPLS
   label with the provider's TC field is present at all hops within the
   provider's network.

   The Short Pipe tunnel model and PHP violate that assumption because
   PHP pops and discards the MPLS provider label carrying the provider's
   TC field.  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; propagating another DSCP edge-to-
   edge requires an IP tunnel of some form.  See Annex B for a more
   detailed discussion.

   If transport of a large number (much greater than 4) DSCPs is
   required across a network that supports this DiffServ interconnection
   scheme, a tunnel or VPN can be provisioned for this purpose, so that
   the inner IP header carries the DSCP that is to be preserved not to
   be changed.  From a network operations perspective, the customer
   equipment (CE) is the preferred location for tunnel termination,
   although a receiving domains Provider Edge router is another viable
   option.

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
   some important differences caused by the characteristics of network
   interconnects.

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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 meet its
   Service Level Agreements (SLAs).  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
   because:

   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  There should be unused codes for interconnection purposes.  This
      leaves space for future standards, for private bilateral
      agreements and for local use PHBs and DSCPs.

   o  Migrations from one code point scheme to another may require spare
      QoS code points.

   RFC 5127 also follows RFC 2474 in recommending transmission of DSCPs
   through a network as they are received at the network edge.

3.2.  Differences from RFC 5127

   Like RFC 5127, this document also uses four traffic aggregates, but
   differs from RFC 5127 in three 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 when 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.
      As discussed in Section 2 above, the MPLS Short Pipe tunnel model
      effectively requires use of a DSCP that is locally valid for the
      network involved, leading to this remarking approach.

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   o  It treats network control traffic as a special case.  Within a
      network, the CS6 DSCP is used for local network control traffic
      (routing protocols and OAM traffic that is essential for 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 traffic)
      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; it is more likely to be specifically configured (e.g.,
      most networks impose on exchange of BGP for obvious reasons).  See
      Section 4.2 for further discussion.

   o  Because network control traffic is treated as a special case, a
      fourth traffic aggregate is defined for use at network
      interconnections to replace the Network Control aggregate in RFC
      5127.  Network Control traffic may still be exchanged across
      network interconnections as further discussed in Section 4.2

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 may involve remarking for the interconnection; such remarking is
   part of the DiffServ Architecture [RFC2475], at least for the network
   edge nodes involved in interconnection.  This draft proposes a
   standard interconnection set of 4 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 her internal scheme.  The set of
   DSCPs and PHBs supported across the two interconnected domains and
   the treatment of PHBs and DSCPs not recognized by the receiving
   domain should be part of the interconnect SLA.

   RFC 5127's four treatment aggregates include a Network Control
   aggregate for routing protocols and OAM traffic that is essential for
   network operation administration, control and management.  Using this
   aggregate as one of the four in RFC 5127 implicitly assumes that
   network control traffic is forwarded in potential competition with
   all other network traffic, and hence DiffServ must favor such traffic
   (e.g., via use of the CS6 codepoint) for network stability.  That is
   a reasonable assumption for IP-based networks where routing and OAM
   protocols are mixed with all other types of network traffic;
   corporate networks are an example.

   In contrast, mixing of all traffic is not a reasonable assumption for
   MPLS-based provider or carrier networks, where customer traffic is

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   usually segregated from network control (routing and OAM) traffic via
   other means, e.g., network control traffic use of separate LSPs that
   can be prioritized over customer LSPs (e.g., for VPN service) via
   other means.  This segregation of network control traffic from
   customer traffic is also used for MPLS-based network
   interconnections.  In addition, many customers of a network provider
   do not exchange Network Control traffic (e.g., routing) with the
   network provider.  For these reasons, a separate Network Control
   traffic aggregate is not important for MPLS-based carrier or provider
   networks; when such traffic is not segregated from other traffic, it
   may reasonably share the Assured Elastic treatment aggregate (as RFC
   5127 suggests for a situation in which only three treatment
   aggregates are supported).

   In contrast, VoIP is emerging as a valuable and important class of
   network traffic for which network-provided QoS is crucial, as even
   minor glitches are immediately apparent to the humans involved in the
   conversation.

   Similar approaches to 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] andMEF23.1 [MEF23.1].

   The list of the four DiffServ Interconnect traffic aggregates
   follows, highlighting differences from RFC 5127 and the specific
   traffic classes from RFC 4594 that each class aggregates.

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

   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 designed to transport the portions of
           RFC 5127's Real Time Treatment Aggregate, which consume large
           amounts of bandwidth, namely Broadcast Video, Real-Time
           Interactive and Multimedia Conferencing.  The treatment
           aggregate should be configured with a rate queue (which is in
           line with RFC 4594 for the mentioned traffic classes).  As
           compared to RFC 5127, the number of DSCPs has been reduced to
           one (initially).  The proposed queuing mechanism is in line
           with RFC4594 definitions for Broadcast Video and Real-Time

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           Interactive.  If need for three-color marked Multimedia
           Conferencing traffic arises, AF42 and AF43 PHBs may be added.

   Assured Elastic Treatment Aggregate  This Treatment Aggregate
           consists of the entire AF3 PHB group AF3, i.e., DSCPs 011
           010, 011 100 and 011 110.  As compared to RFC5127, just the
           number of DSCPs, which has been reduced.  This document
           suggests to transport signaling marked by AF31.  RFC5127
           suggests to map Network Management traffic into this
           Treatment Aggregate, if no separate Network Control Treatment
           Aggregate is supported (for a more detailed discussion of
           Network Control PHB treatment see section 3.2).  GSMA IR.34
           proposes to transport signaling traffic by AF31 too.

   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 as compared to RFC5127 is that any traffic with
           unrecognized or unsupported DSCPs may be remarked to this
           DSCP.

   RFC 4594's Multimedia Streaming class has not been mapped to the
   above scheme.  By the time of writing, the most popular streaming
   applications use TCP transport and adapt picture quality in the case
   of congestion.  These applications are proprietary and still change
   behaviour frequently.  Currently, the Bulk Real-Time Treatment
   Aggregate or the Assured Elastic Treatment Aggregate may be a
   reasonable match.  NOTE: This paragraph would benefit from WG review
   and discussion.

   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 QoS interconnection
   SLA.

   Traffic of provider O terminates within provider Ts network, while
   provider W's traffic transits through the network of provider T to
   provider F.  Assume all providers run their own internal codepoint
   schemes for a PHB group with properties of the DiffServ Intercon
   Assured Treatment Aggregate.

           Provider-O             Provider-W
           RFC5127                GSMA 34.1
               |                      |
          +----------+           +----------+
          |AF21, AF22|           | CS3, CS2 |

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          +----------+           +----------+
               |                      |
               V                      V
           +++++++++              +++++++++
           |Rtr PrO|              |Rtr PrW|  Rtr Pr:
           +++++++++              +++++++++  Router Peering
               |        DiffServ      |
          +----------+           +----------+
          |AF31, AF32|           |AF31, AF32|
          +----------+           +----------+
               |        Intercon      |
               V                      V
           +++++++++                  |
           |RtrPrTI|------------------+
           +++++++++
               |     Provider-T domain
          +-----------+
          | MPLS TC 2 |
          | DSCP rew. |           rew. -> rewrite
          | AF21, AF22|
          +-----------+
             |      |  Local DSCPs Provider-T
             |      |  +----------+   +++++++++
             V      +->|AF21, AF22|->-|RtrDstH|
             |         +----------+   +++++++++
         +----------+                 RtrDst:
         |AF21, AF22|                 Router Destination
         +----------+
             |
          +++++++++
          |RtrPrTE|
          +++++++++
             |        DiffServ
         +----------+
         |AF31, AF32|
         +----------+
             |        Intercon
          +++++++++
          |RtrPrF|
          +++++++++
             |
         +----------+
         | CS4, CS3 |
         +----------+
             |
         Provider-F
         GSM IR.34

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   DiffServ Intercon example

                                 Figure 1

   Providers only need to deploy internal DSCP to DiffServ Intercon DSCP
   mappings to exchange traffic in the desired classes.  Provider W has
   decided that the properties of his internal classes CS3 and CS2 are
   best met by the Diffserv Intercon Assured Elastic Treatment
   Aggregate, PHBs AF31 and AF32 respectively.  At the outgoing peering
   interface connecting provider W with provider T the former's peering
   router remarks CS3 traffic to AF31 and CS2 traffic to AF32.  The
   domain internal PHBs of provider T that meet the requirements of
   Diffserv Intercon Assured Elastic Treatment Aggregate are AF2x.
   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 TC value 2 for this aggregate.  Traffic received
   with AF32 is similarly remarked to AF22, but uses the same MPLS TC
   for the Treatment Aggregate, i.e. TC 2.  At the penultimate MPLS
   node, the top MPLS label is removed.  The packet should be forwarded
   as determined by the incoming MPLS TC.  The peering router connecting
   domain T with domain F classifies the packet by it's domain T
   internal DSCP AF21 for the Diffserv Intercon Assured Elastic
   Treatment Aggregate.  As it leaves domain T on the interface to
   domain F, this causes the packet to be remarked to AF31.  The peering
   router of domain F classifies the packet for domain F internal PHB
   CS4, as this is the PHB with properties matching DiffServ Intercon's
   Assured Elastic Treatment Aggregate.  Likewise, AF21 traffic is
   remarked to AF32 by the peering router od domain T when leaving it
   and from AF32 to CS3 by domain F's peering router when receiving it.

   This example can be extended.  Suppose Provider-O also supports a PHB
   marked by CS2 and this PHB is supposed to be transported by QoS
   within Provider-T domain.  Then Provider-O will remark it with a DSCP
   other than the AF31 DSCP in order to preserve the distinction from
   CS2; AF11 is one possibility that might be private to the
   interconnection between Provider-O and Provider-T; there's no
   assumption that Provider-W can also use AF11, as it may not be in the
   SLA with Provider-W.

   Now suppose Provider-W supports CS2 for internal use only.  Then no
   DiffServ Intercon DSCP mapping may be configured at the peering
   router.  Traffic, sent by Provider-W to Provider-T marked by CS2 due
   to a misconfiguration may be remarked to CS0 by Provider-T.

   See section 4.1 for further discussion of this and DSCP transparency
   in general.

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   RFC2575 states that Ingress nodes must condition all other 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 the Default PHB codepoint.  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.

   The proposed interconnect class and code point scheme is designed for
   point to point IP layer interconnections among MPLS networks.  Other
   types of interconnections are out of scope of this document.  The
   basic class and code point scheme is applicable on Ethernet layer
   too, if a provider e.g. supports Ethernet priorities like specified
   by IEEE 802.1p.

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

   This section describes how the use of a common PHB and DSCP scheme
   for interconnection can lead to end-to-end DiffServ-based QoS across
   networks that do not have common policies or practices for PHB and
   DSCP usage.  This will initially be possible for PHBs and DSCPs
   corresponding to at most 3 or 4 Treatment Aggregates due to the MPLS
   considerations discussed previously.

   Networks can be expected to differ in the number of PHBs available at
   interconnections (for terminating or transit service) and the DSCP
   values used within their domain.  At an interconnection, Treatment
   Aggregate and PHB properties are best described by SLAs and related
   explanatory material.  For the above reasons and the desire to
   support interconnection among networks with different DiffServ
   schemes, the DiffServ interconnection scheme supports a small number
   of PHBs and DSCPs; this scheme is expandable.

   The basic idea is that traffic sent with a DiffServ interconnect PHB
   and DSCP is restored to that PHB and DSCP (or a PHB and DSCP within
   the AF3 PHB group for the Assured Treatment Aggregate) at each
   network interconnection, even though a different PHB and DSCP may be
   used by each network involved.  So, Bulk Inelastic traffic could be
   sent with AF41, remarked to CS3 by the first network and back to AF41
   at the interconnection with the second network, which could mark it
   to CS5 and back to AF41 at the next interconnection, etc.  The result
   is end-to-end QoS treatment consistent with the Bulk Inelastic
   Traffic Aggregate, and that is signaled or requested by the AF41 DSCP

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   at each network interconnection in a fashion that allows each network
   operator to use their own internal PHB and DSCP scheme.

   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.

4.2.  Treatment of Network Control traffic at carrier interconnection
      interfaces

   As specified by RFC4594, section 3.2, Network Control (NC) traffic
   marked by CS6 is to be 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. if BGP is running between 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 (4.1) 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 transiting traffic, including transiting network control
   traffic.  Use of an IP tunnel is suggested in order to reduce the
   risk of CS6 markings on transiting network control traffic being
   interpreted by the network providing the transit.

   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.

   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 which isn't terminating at the
   interconnected ingress node:

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   o  classification of traffic which is network control traffic for
      both domains.  This traffic should be classified and marked for
      the NC PHB.

   o  classification of traffic which is network control traffic for the
      sending domain only.  This traffic should be classified for a PHB
      offering similar properties as the NC class (e.g.  AF31 as
      specified by this document).  As an example GSMA IR.34 proposes an
      Interactive class / AF31 to carry SIP and DIAMETER traffic.  While
      this is service control traffic of high importance to the
      interconnected Mobile Network Operators, it is certainly not
      Network Control traffic for a fixed network providing transit
      between such operators, and hence should not receive CS6 treatment
      in such a network.

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

5.  Acknowledgements

   Al Morton and Sebastien Jobert provided feedback on many aspects
   during private discussions.  Mohamed Boucadair and Thomas Knoll
   helped adding awareness of related work.  Fred Baker and Brian
   Carpenter provided intensive feedback and discussion.

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 security considerations of RFC 2475 [RFC2475]
   and RFC 4594 [RFC4594] apply.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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, December
              1998.

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

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [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, March 2002.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, April 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, May 2002.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

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

8.2.  Informative References

   [I-D.knoll-idr-cos-interconnect]
              Knoll, T., "BGP Class of Service Interconnection", draft-
              knoll-idr-cos-interconnect-13 (work in progress), November
              2014.

   [ID.idr-sla]
              IETF, "Inter-domain SLA Exchange", IETF,
              http://datatracker.ietf.org/doc/
              draft-ietf-idr-sla-exchange/, 2013.

   [IEEE802.1Q]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks - Virtual Bridged Local Area Networks", 2005.

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   [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.

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

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

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              Diffserv Service Classes", RFC 5127, February 2008.

   [RFC5160]  Levis, P. and M. Boucadair, "Considerations of Provider-
              to-Provider Agreements for Internet-Scale Quality of
              Service (QoS)", RFC 5160, March 2008.

   [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 Carrier interconnection related DiffServ aspects

   NOTE: This Appendix is likely to be deleted in the next version of
   this draft.  The authors would appreciate comments on the value (or
   lack thereof) of this text.

   This apppendix provides a general discussion of PHB and DSCP mapping
   at IP interconnection interfaces.

   The following scenarios start from a domain sending non-tunneled IP
   traffic using a PHB and a corresponding DSCP to an interconnected
   domain.  The receiving domain may:

   o  Support the PHB and offer the same corresponding DSCP.

   o  Not support the PHB and use the DSCP for a different PHB.

   o  Not support the PHB and not use the DSCP.

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   o  Support the PHB with a differing DSCP, and the DSCP of the sending
      domain is not used for another PHB

   o  Support the PHB with a differing DSCP, and the DSCP of the sending
      domain is used for another PHB.

   RFC2475 allows for local use PHBs which are only available within a
   domain.  If any such a local use PHB is present, non-tunneled IP
   traffic possibly cannot utilize 64 DSCPs end-to-end.

   If a domain receives traffic for a PHB, which it does not support,
   there are two general scenarios:

   o  The received DSCP is not available for usage within the domain.

   o  The received DSCP is available for usage within the domain.

   RFC2474 suggests transporting packets received with unrecognized
   DSCPs by the Default PHB and not changing the DSCP as received.  Also
   if a particular DSCP is unused within a domain, the network may
   subsequently change its QoS design and assign a PHB to a formerly
   unused DSCP, making transparent transport of that DSCP as an unknown
   DSCP with the Default PHB no longer possible.  Remarking to another
   DSCP apart from the Default PHBs DSCP does not seem to be a good
   option in the latter case, as it's not clear which other DSCP should
   be used.  If a domain interconnects with many other domains, the
   concerns discussed here may have to be dealt with many times.

   The scenarios above indicate, that reliably delivering a non-tunneled
   IP packet by the same PHB and DSCP unchanged end-to-end is only
   likely, if both domains support this DSCP and use the same
   corresponding DSCP.

   Limitations in the number of supported PHBs are to be expected if
   DiffServ is applied across different domains.  Unchanged end-to-end
   DSCPs should only be expected for non-tunneled IP traffic, if the PHB
   and DSCP are well specified and generally deployed.  This is true for
   Default Forwarding.  EF PHB is a candidate.  The Network Control PHB
   is a local use only example, hence end-to-end support of CS6 for non-
   tunneled IP traffic at interconnection points should only be
   expected, if the receiving domain regards this traffic as Network
   Control traffic relevant for the own domain too.

   DiffServ Intercon proposes a set of PHBs and corresponding DSCPs at
   interconnection points.  A PHB to DSCPs correspondence is specified
   for interconnection interfaces.  Supported PHBs should be available
   end-to-end, but domain internal DSCPs may change end-to-end, although
   they are restored at network interconnection points.

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Appendix B.  Appendix B 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.

   For tunneled IPv4 traffic, only the outer tunnel header is involved
   in 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.

   Non-tunneled IPv6 traffic as well as Layer 2 and Layer 3 VPN traffic
   all use an additional MPLS label; forwarding within an MPLS network
   is based on that label, as opposed to the outer IP header.

   Carriers often select QoS PHBs and DSCP without regard to
   interconnection.  As a result PHBs and DSCPs typically differ between
   network carriers.  PHBs may be mapped.  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
   mapped across the carriers involved.

   Beyond RFC3270's suggestions that the Short Pipe Model is only
   applicable to VPNs, current network structures also use it to
   transport non tunneled IPv4 traffic.  This is shown in figure 2.

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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 QoS context. The Core
       \|/                         > Router might require or enforce
        V                         |  it. 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 QoS behavior.

   Without DiffServ-Intercon treatment, the traffic is likely to leave
   each network marked with network-internal DSCP.  DSCP_send of the
   figure above is remarked to the receiving network's DiffServ scheme.

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   It leaves the domain marked by the domains 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 those 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

Appendix C.  Change log

   00 to 01  Added terminology and references.  Added details and
           information to interconnection class and codepoint scheme.
           Editorial changes.

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   01 to 02  Added some references regarding related work.  Clarified
           class definitions.  Further editorial improvements.

   02 to 03  Consistent terminology.  Discussion of Network Management
           PHB at interconnection interfaces.  Editorial review.

   03 to 04  Again improved terminology.  Better wording of Network
           Control PHB at interconnection interfaces.

   04 to 05  Large rewrite and re-ordering of contents.

   05 to 06  Description of IP and MPLS related requirements and
           constraints on DSCP rewrites.

   06 to 07  Largely rewrite, improved match and comparison with RFCs
           4594 and 5127.

   07 to 08  Added Annex A and B which where forgotten when putting
           together -07

Authors' Addresses

   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
   EMC Corporation
   176 South Street
   Hopkinton, MA
   USA

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

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