TSVWG R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Informational November 06, 2012
Expires: May 10, 2013

DiffServ interconnection classes and practice
draft-geib-tsvwg-diffserv-intercon-00

Abstract

This document proposes a limited set of interconnection QoS PHBs and PHB groups. It further introduces some DiffServ deployment aspects. The proposals made here should be integrated into a revised version of RFC5127.

Status of This Memo

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Table of Contents

1. Introduction

This draft deals proposes a DiffServ interconnection class and codepoint scheme. At least one party of an interconnection often is a network provider. Aggregated DiffServ classes are often deployed within provider networks. To respect this, this draft also contains concepts and current practice relevant for a revised version of RFC5127 [RFC5127]. Its main purpose is to be considered as an input for the latter task

DiffServ sees deployment in many networks for the time being. As described in the introduction of the draft diffserv problem statement [I-D.polk-tsvwg-diffserv-stds-problem-statement], remarking of packets at domain boundaries is a DiffServ feature. This draft proposes a set of standard QoS classes and codepoints at interconnection points to which and from which locally used classes and codepoints may be mapped. Such a scheme simplifies interconnection negotiations and ensures that class properties remain roughly the same, even if codepoints change.

The proposed Interconnection class and codepoint scheme tries to reflect and consolidate related DiffServ and QoS standardisation efforts outside of the IETF, namely MEF, GSMA and ITU.

IP Precedence has been depricated when DiffServ was standardised. It is common practice today however to copy the DSCPs "IP Precedence Bits" into MPLS TC or Ethernet P-Bits, whenever possible. This is reflected by the DiffServ codepoint definitions of AF and EF. This practice and it's limits deserve to be documented and disussed briefly.

The draft further adds proposals by which philosophy to add or pick aggregated DiffServ classes. The set of available router and traffic management tools to configure and operate DiffServ classes is limited. This should be reflected by class definitions, which may in the end more strongly be related transport properties than application requirements. Please read that "congestion aware" and "not congestion aware" rather then TCP or UDP.

Finally, this draft proposes to leave some MPLS TC codepoint space to allow for future DiffServ extensions like ECN/PCN and domain internal classes (network management traffic is a good example)

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

2. Terminology

A later version of this draft needs to be clearer on that. The author prefers to talk of QoS classes. PHB or PHB groups are not commonly used, although they are better defined. An issue is, that PHB groups, which e.g. allow to offer two or more different drop levels (PHB's) within one PHB group , hardly saw commercial deployment. This may change with more Ethernet services being offered.

Some rather concept than terminology related real world issues are additional motivations of this activity:

While these issues aren't to be solved by IETF (QoS experts could and should of course teach staff to use proper Diffserv terminology and concepts), a simple QoS interconnection scheme also is helpful in this area. Those dealing with QoS on any level need to understand the interconnection classes and their codepoints, and they are able to deal with the mappings of those to their own networks class and codepoint scheme. Simplicity is important, however.

3. An Interconnection class and codepoint scheme

DiffServ deployments mostly follow loose class specification schemes (often one or two AF PHBs or PHB groups, EF and Best Effort). Especially DSCP assignment for the AF classes varies between deployment, while basic class defeinitions are often similar. This is in line with the DiffServ architecture. This document doesn't propose to change that.

Interconnecting parties usually face the problem of matching classes to be interconnected and then to agree on codepoint mapping. As stated by draft diffserv prolebm statement [I-D.polk-tsvwg-diffserv-stds-problem-statement], remarking is a standard behaviour at interconnection interfaces. This draft propses a set of 4 QoS classes with a set of well defined DSCPs as interconnection codepoint scheme. As the idea is that a sending party remarks DSCPs from internal schemes to the Interconnection codepoints and the receiving party remarks to their internal scheme, the interconnection codepoint scheme fully complies with the DiffServ architecture.

While this may look like an additional step at first, there are obvious benefits: each party sending or receiving traffic has to specify the mapping from or to the interconnection class and codepoint scheme only once. Without it, this is to be negotiated once per interconnection party. Further, end-to-end QoS in terms of traffic being classified for the same class in all passed domains is likely to result if an interconnection codepoint scheme is used, while it is not necessarily resulting from individual per network interconnection negotiations.

The Interconnection scheme is supporting aggregated DiffServ classes, as proposed by RFC 5127 [RFC5127]. Note that draft RFC 4597 update [I-D.polk-tsvwg-rfc4594-update] doesn't expect any network to deploy all possible QoS classes (and proposes to standardise DSCPs for all while maintaining deployment of classes a network specific issue). RFC2597 does not allow to aggregate separate AF classes [RFC2597]. Hence a low number of interconnection classes on aggregated level makes sense, as some codepoint space for carrier internal services and future features like ECN should be left also for aggregatd classes. MPLS and Ethernet only support up to 8 QoS classes. IP over Ethernet and / or MPLS is acommodity in todays operational environments, and inter layer QoS likely will be a commodity too.

4. Consolidation of QoS standards by the interconnection codepoint scheme

The interconnection class and codepoint scheme proposed by Y.1566 also tries to consolidate related DiffServ and QoS standardisation efforts outside of the IETF [Y.1566]. MEF 23.1 specifies 3 aggregated classes, consuming up to 5 bit codepoints (EF, two AF classes and Best Effort) and 8 DSCPs [MEF23.1]. GSMA IR.34 proposes four aggregated DiffServ classes, EF, 2 AF and Best Effort with 7 DSCPs (PHBs) in sum [IR.34]. Consolidation may be reached for EF, one AF class and Best Effort, meaning three aggregated or 5 DSCPs. Consolidation here aims on similar class definitions and DiffServ codepoints in all standards, MEF23.1, GSMA IR.34 and Y.1566. This will again simplify product design and interconnection negotiations for customers and parties following these standards.

5. MPLS, Ethernet and IP Precedence for aggregated classes

IP Precedence has been depricated when DiffServ was standardised. Ethernet and MPLS support 3 bit codepoint fields to differntiate service quality. Mapping of the IP precedence to these 3 Bit fields has been a configuration restriction in the early days of DiffServ. The concept of paying attention to the three most significant bits of a DSCP has however been part of Diffserv from start on (EF's IP Precedence is 5, that of AF4 is 4 and so on). The interconnection class and codepoint scheme respects this in different ways:

This is of course no requirement to depricate any DSCP to MPLS TC or Ethernet P-Bit mapping functionality. This functionalities are very important as well.

6. QoS class name selection

This is more of an informational discussion, proposed best practice, and mainly relates to human behviour (including QoS experts) rather than technical issues. Above the human preference for conceivable class names has been mentioned. Network engineers (including the former Diffserv WG authors) recommend to avoid application related QoS class names. Focus should be put on class properties. But these can be irritating again, as just looking at a few SLA numbers doesn't tell the reader, which engineering assumptions resulted in the scheduler configurations of a class. A router produces QoS with a scheduling mechanism, a settable queue depth and optional active queue management (including ECN), and may be a policer. Some kind of resource management may be presendt (also in Diffserv domains). It's beyond the imagination of the author how one would engineer more than half a dozen classes with distinguishable properties with this set of tools.

There's no perfect solution to the problem, as scheduler configurations are not comprehensible to most readers, even if they were communicated (they are operational secrets of course). There are (or should be) engineering assumptions, when designing QoS schedulers. But they closer relate to layer 3 or layer 4 level properties than to specific applications. In general, an application responds to congestion by reducing traffic, or it ignores congestion. Active queue management doesn't help to avoid congestion in the latter case, only resource management does. EF may be a special case. If the EF traffic is not responsive to congestion, and packets are assumed to be short, rather small jitter values can be reached if enginieering ensures that the packet arrival rate never exceeds the transmision rate of that queue (see RFC 3246 [RFC3246]). There's other non congestion-responsive traffic, for which the EF engineering assumptions may not fit. So a second class with EF like properties (but a different engineering) may be present.

Active queue management may be deployed for QoS classes, which are designed to transport traffic responding to congestion by traffic reduction.

The author couldn't yet find conceivable class names. TCP_optimised and especially UDP_optimised are inappropriate, as some UDP based application are or may be expected to become TCP friendly.

7. Allow for DiffServ extendability on MPLS and Ethernet level

Any aggregated Diffserv deployment faces codepoint depletion issues rather soon, if deployed on MPLS or Ethernet. Coding space should be left for new features, like ECN, PCN or Conex. In addition to carrying customer traffic, internal routing and network management traffic may be protected by using a separate class. offering interconnection with up to four classes and 4 - 6 MPLS TC's (or Ethernet P-bits) to that respect is probably at least a fair compromise.

8. IANA Considerations

This memo includes no request to IANA.

9. Security Considerations

This document does not introduce new features, it describes how to use existing ones. The security section of RFC 4597 [RFC4597] applies.

10. References

10.1. Normative References

[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.C.R., Benson, K., Le Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V. and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.
[min_ref] authSurName, authInitials, "Minimal Reference", 2006.

10.2. Informative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5127] Chan, K., Babiarz, J. and F. Baker, "Aggregation of DiffServ Service Classes", RFC 5127, February 2008.
[RFC4597] Even, R. and N. Ismail, "Conferencing Scenarios", RFC 4597, August 2006.
[I-D.polk-tsvwg-diffserv-stds-problem-statement] Polk, J, "The Problem Statement for the Standard Configuration of DiffServ Service Classes", Internet-Draft draft-polk-tsvwg-diffserv-stds-problem-statement-00, July 2012.
[I-D.polk-tsvwg-rfc4594-update] Polk, J, "Standard Configuration of DiffServ Service Classes", Internet-Draft draft-polk-tsvwg-rfc4594-update-02, October 2012.
[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.
[Y.1566] ITU-T, "Quality of service mapping and interconnection between Ethernet, IP and multiprotocol label switching networks ", ITU, draft Y.QoSmap (now Y.61566) https://datatracker.ietf.org/documents/LIAISON/liaison-2012-07-03-itu-t-sg-12-tsvwg-development-of-informative-codepoint-mapping-in-itu-t-study-group-12-attachment-1.zip, 2012.

Author's Address

Ruediger Geib (editor) Deutsche Telekom Heinrich Hertz Str. 3-7 Darmstdadt, 64297 Germany Phone: +49 6151 5812747 EMail: Ruediger.Geib@telekom.de