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


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.

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Table of Contents

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.

As described by section 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 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.

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 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 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:

3.2. Differences from RFC5127

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

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:

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-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 the Diffserv-Intercon Assured Elastic Treatment Aggregate (AF21 and CS2 are used in the example).

 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

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.

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.

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:

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.

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.
[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, DOI 10.17487/RFC2597, 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, DOI 10.17487/RFC3246, March 2002.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P. and J. Heinanen, "Multi-Protocol Label Switching (MPLS) Support of Differentiated Services", RFC 3270, DOI 10.17487/RFC3270, May 2002.
[RFC5129] Davie, B., Briscoe, B. and J. Tay, "Explicit Congestion Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January 2008.
[RFC5865] Baker, F., Polk, J. and M. Dolly, "A Differentiated Services Code Point (DSCP) for Capacity-Admitted Traffic", RFC 5865, DOI 10.17487/RFC5865, May 2010.

8.2. Informative References

[I-D.knoll-idr-cos-interconnect] Knoll, T., "BGP Class of Service Interconnection", Internet-Draft draft-knoll-idr-cos-interconnect-17, November 2016.
[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.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983, DOI 10.17487/RFC2983, October 2000.
[RFC3662] Bless, R., Nichols, K. and K. Wehrle, "A Lower Effort Per-Domain Behavior (PDB) for Differentiated Services", RFC 3662, DOI 10.17487/RFC3662, December 2003.
[RFC4594] Babiarz, J., Chan, K. and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, DOI 10.17487/RFC4594, August 2006.
[RFC5127] Chan, K., Babiarz, J. and F. Baker, "Aggregation of Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127, February 2008.
[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.
[RFC7657] Black, D. and P. Jones, "Differentiated Services (Diffserv) and Real-Time Communication", RFC 7657, DOI 10.17487/RFC7657, November 2015.
[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.

    \|/           IPv4, DSCP_send
Peering Router         
    \|/           IPv4, DSCP_send
MPLS Edge Router
     |          Mark MPLS Label, TC_internal
    \|/         Remark DSCP to
     V            (Inner: IPv4, DSCP_d)
MPLS Core Router  (penultimate hop label popping)
     |                        \
     |            IPv4, DSCP_d |  The DSCP needs to be in network-
     |                 ^^^^^^^^|  internal 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


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 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.

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


Short-Pipe example with Diffserv-Intercon

Figure 3

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 Dell EMC 176 South Street Hopkinton, MA USA Phone: +1 (508) 293-7953 EMail: david.black@dell.com