TSVWG R. Geib, Ed.
Internet-Draft Deutsche Telekom
Intended status: Informational February 25, 2013
Expires: August 29, 2013
DiffServ interconnection classes and practice
draft-geib-tsvwg-diffserv-intercon-02
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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. An Interconnection class and codepoint scheme . . . . . . . . 5
4. Consolidation of QoS standards by the interconnection
codepoint scheme . . . . . . . . . . . . . . . . . . . . . . . 6
5. MPLS, Ethernet and IP Precedence for aggregated classes . . . 8
6. QoS class name selection . . . . . . . . . . . . . . . . . . . 9
7. Allow for DiffServ extendability on MPLS and Ethernet level . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
This draft 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 should be mapped. Such a scheme simplifies
interconnection negotiations and ensures that end to end 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 proposes a philosophy how 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. These may
in the end be more related to transport properties than to
application requirements. Please interpret transport properties as
"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 for the
latter). An example for an internal PHB may be CS6, which some
operators to protect their ntework internal routing and / or
management traffic. This PHB may not be available to transport
customer signaling and management traffic. It IETF is interested in
this work, a later version may expand on internal PHBs and
codepoints.
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In addition to the standardisation activities which triggered this
work, other authors published RFCs or drafts which may benefit from
an interconnection class- and codepoint scheme. RFC 5160 suggests
Meta-QoS-Classes to enable deployment of standardised end to end QoS
classes [RFC5160]. The authors agree that the proposed
interconnection class- and codepoint scheme as well as the idea of
standardised end to end. Hence RFC 5160 and this work complement
each other. Work on BGP Class of Service Interconnection signaled by
BGP [I-D.knoll-idr-cos-interconnect] is beyond the the scope of this
draft. Should the basic transport and class properties of end to end
QoS utilising DiffServ based interconnection as proposed by this
draft be standardised, work on signaled access to QoS classes may be
of interest.
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
This draft tries to re-use exitsing terminology. It further tries to
indicate, where duplicate terminology may exist.
Class A class is a set of one or more PHBs. If a class consists of
a set of PHBs and these obey to an ordering constraint. In
that sense, a class is a single AF class (e.g. AF4 consisting
of AF41, AF41 and AF43) [RFC2597]. A class is a PHB group
[RFC2575] and a PHB scheduling class [RFC3260]. On IP level
all DSCPs sharing the same IP precedence value belong to a
single class. A class may consist of one or more PHBs. A
single class uses forwarding resources, which are independent
of the forwarding rseources of any other class. Different
classes must not be aggregated.
PHB A single Per Hop Behaviour [RFC2575] is identified by a
single DSCP on IP layer.
Many DiffServ related RFCs introduce new terminolgy duplicating the
existing one. The above references are incomplete and refer to the
early DiffServ RFCs only. Stopping terminology duplication may
simplify discussion.
The following current practice issues relate to the concept of the
DiffServ interconnection class propsal rather than to terminology.
They serve as additional motivation of this activity:
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o Abstract class names like "EF" are preferrential over those being
close to an application, like "Voice". Unfortunately, non QoS
experts can't handle abstract class names. Hence and usually
sooner than later, classes are named for applications or groups of
them. One consequence however is, that people tend to combine
application group class names and SLA parameters. Based on an
application specific name and some worst case performance numbers
on a paper, they often decide that their application needs a
separate new QoS class.
o Worse than that, but very present in practice, is the class
abstraction level which is preferred by those dealing with QoS (as
experts or non experts): the DSCPs or the IP precedence values.
These are the commodity abstractions applied for QoS classes.
Most of these persons have fixed class to codepoint mappings in
their minds, which they can't easily adapt on a per customer or
interconnection partner basis.
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 and comprehensible QoS interconnection class
scheme also is helpful in this area.
3. An Interconnection class and codepoint scheme
DiffServ deployments mostly follow loose class specification schemes
(often one or two AF classes, EF and Best Effort). Especially DSCP
assignment for the AF classes varies between deployments. Basic AF
class definitions are often similar however. This is in line with
the DiffServ architecture. This document doesn't propose to change
that.
Interconnecting parties 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 and IP-
Precedence values as interconnection class and codepoint scheme. A
sending party remarks DSCPs from internal schemes to the
Interconnection codepoints. The receiving party remarks IP-
Precedence and or DSCPs to their internal scheme. Thus the
interconnection codepoint scheme fully complies with the DiffServ
architecture. Such an interconnection class and codepoint scheme was
introduced by ITU-T [Y.1566] (there also includuing Ethernet). It is
specified to a higher level of detail in this document.
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At first glance, this looks like an additional effort. But 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 per
interconnection party individually. 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.
It is not necessarily resulting from individual per network mapping
negotiations.
The standards and deployments known to the author of this draft are
limited to 4 DiffServ classes at interconnection points (or
less).Draft RFC 4597 update [I-D.polk-tsvwg-rfc4594-update]doesn't
seem to generally contradict to this, as it proposes to standardise
"many services classes, not all will be used in each network at any
period of time." Some more good reasons fovour working with 4
DiffServ interconnection classes for now:
o There should be a coding reserve for interconnection classes,
leaving space for future standards, for bilateral agreements and
for carrier internal classes.
o MPLS and Ethernet support only 8 PHBs, classes or ECN indications.
Assignment of codepoints for whatever purpose must be well thought
through. Limiting interconnection QoS to four classes is MPLS and
Ethernet friendly in that sense.
o Migrations from one codepoint scheme to another may require spare
QoS codepoints.
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]. The interconnection class and
codepoint scheme may be a suitable approach to consolidate these
standards. MEF 23.1 specifies 3 aggregated classes, consuming up to
5 codepoints on Ethernet layer (EF, AF3 and AF1 and Best Effort) and
6 PHBs [MEF23.1]. MEF aggregates AF1 and Default PHB in a single
class. This is not recommended for interconnection, as it is not in
line with RFC 2597 (which requires separate forwarding resources for
each AF class and doesn't forsee aggregation of Default PHB and an AF
class).
GSMA IR.34 proposes four classes, EF, AF4, another AF class and Best
Effort with 7 PHBs in sum [IR.34]. IR.34 specifies an "Interactive"
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class consisting of 3 PHBs with fifferent drop priorities. IR.34
specfies the PHBS AF31, AF21 and AF11 for this Interactive class.
This definitely breaks RFC 2597. The interconnection class and
codepoint scheme supports the Interactive class but assigns AF3 with
PHBs AF31, AF32 and AF33.
If IETF picks up this draft, it may be a good idea to inform MEF and
GSMA about conflicts of their standards with DiffServ and suggest
joint activities to improve the situation. Information on
interworkings with MEF 23 and GSMA IR.34 with the interconnection QoS
scheme could be given by a later version of this draft.
The classes to be supported at interconnection interfaces are
specified by Y.1566 as:
Class Priority: EF, expecting the figures of merit describing the
PHB to be in the range of low single digit milliseconds. See
[RFC3246].
Bulk inelastic: Optimised for low loss, low delay, low jitter at
high bandwidth. Traffic load in this class must be
controlled, e.g. by application servers. One example could
be flow admission control. There may be infrequent
retransmissions requested by the application layer to
mitigate low levels of packet losses. Discard of packets
through active queue management should be avoided in this
class. Congestion in this class may result in bursty packet
loss. If used to carry multimedia traffic, it is recommended
to carry audio and video traffic in a single PHB. All of
these properties influence the buffer design.
Assured: This class may be optimised to transport traffic without
bandwidth requirements. It aims onVery low loss at high
bandwidths. Retransmissions after losses characterise the
class and influence the buffer design. Active queue
management with probabilistic dropping may be deployed.
Default: Default. This class may be optimised to transport traffic
without bandwidth requirements. Retransmissions after losses
characterise the class and influence the buffer design.
Active queue management with probabilistic dropping may be
deployed.
Note that other DiffServ related standards trim down class
requirements to SLA parameters. To quote e.g RFC 4594-update, "A
"service class" represents a similar set of traffic characteristics
for delay, loss, and jitter as packets traverse routers in a
network." This draft adds traffic conditioning properties
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corresponding to expected transport layer characteristics as a key
factor to a class definition: the desired class performance like
delay, jitter and worst case loss are met only if conditioning and
and transport properties meet the ones described by the class
definition. This is not to say, the other standards ignore
conditioner properties. They are e.g. a core part of RFC 4594-
update. They do not directly refer to tranport protocol properties,
as most existing QoS standards prefer the approach of assigning QoS
classes to applications or application sets. This may result in
undesirable class mappings, if an e.g. IP TV application demanding
low loss is matched to a class whose low loss guarantees depend on
AQM mechanisms.
Y.1566 does not recommend all PHBs to be supported at an
interconnection interface. This information is added by this draft.
At interconnection points, the following PHBs should be accepted
between interconnected parties:
Class: PHB (one or more)
Class Priority: EF
Bulk inelastic: AF41 (AF42 and AF43 are reserved for extension)
Assured: AF31, AF32 and AF33
Default: Default
Class names (and property specification) are picked from Y.1566.
PHBs to the level of detail introduced here are not part of Y.1566.
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:
o it allows to classify four interconnection classes based on IP
precedence.
o It supports a single PHB group (AF3), which may be mapped to up to
three different MPLS TC's or Ethernet P-Bits. Note that this
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draft doesn's favour or recommend doing that, but it is possible.
The author isn't aware of deployed service offers with 3 different
drop levels in a single class.
This is of course no requirement to depricate any DSCP to MPLS TC or
Ethernet P-Bit mapping functionality. This functionality is 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 SLA parameters like
Delay, Jitter and packet loss don't tell the reader, which
conditioning and transport properties guideed the class engineering
assumptions resulted in the conditioning 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 present (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 conditioning
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
conditioners. 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 conditioning) like bulk inelastic is
reasonable.
Active queue management may be deployed for QoS classes, which are
designed to transport traffic responding to congestion by traffic
reduction.
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The class names of this document follow Y.1566. TCP_optimised and
especially UDP_optimised are inappropriate as clas names, 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. Acknowledgements
David Black gave many helpful comments to this work. Al Morton and
Sebastien Jobert provided feedback on many aspects during private
discussions. Brian Carpenter, Mohamed Boucadair and Thomas Knoll
helped adding awareness of further potentially related work.
9. IANA Considerations
This memo includes no request to IANA.
10. Security Considerations
This document does not introduce new features, it describes how to
use existing ones. The security section of RFC 4597 [RFC4597]
applies.
11. References
11.1. Normative References
[RFC2575] Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", RFC 2575, April 1999.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
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[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.
[min_ref] authSurName, authInitials., "Minimal Reference", 2006.
11.2. Informative References
[I-D.knoll-idr-cos-interconnect]
Knoll, T., "BGP Class of Service Interconnection",
draft-knoll-idr-cos-interconnect-09 (work in progress),
November 2012.
[I-D.polk-tsvwg-diffserv-stds-problem-statement]
Polk, J., "The Problem Statement for the Standard
Configuration of DiffServ Service Classes",
draft-polk-tsvwg-diffserv-stds-problem-statement-00 (work
in progress), July 2012.
[I-D.polk-tsvwg-rfc4594-update]
Polk, J., "Standard Configuration of DiffServ Service
Classes", draft-polk-tsvwg-rfc4594-update-02 (work in
progress), 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4597] Even, R. and N. Ismail, "Conferencing Scenarios",
RFC 4597, 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-
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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. Change log
00 to 01 Added terminology and references. Added details and
information to interconnection class and codepoint scheme.
Editorial changes.
01 to 02 Added some references regarding rekated work. Clarified
class definitions. Further editorial improvements.
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
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