Internet DRAFT - draft-gaidioz-equivalent-differentiated-services
draft-gaidioz-equivalent-differentiated-services
Network Working Group B. Gaidioz
Internet-Draft UCBL/ENS-Lyon
Expires: August 23, 2002 P. Primet
INRIA/ENS-Lyon
February 22, 2002
The Equivalent Differentiated Services Model
draft-gaidioz-equivalent-differentiated-services-00
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Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes EDS (Equivalent Differentiated Services), a
new building-block for a simple, robust, free and scalable end-to-end
service differentiation in IP networks. The EDS schema aims to
provide a spectrum of "different but equal" network services that
offer to the end-to-end flows a trade-off between delay and loss
rate. The EDS schema can be deployed incrementally in the Internet.
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1. Introduction and Requirements
With the diversification of the Internet applications, flows
sensitivity to loss or delay variations become more and more
heterogeneous. There is an important need for service
differentiation at the IP layer. Some flows require better delay,
other better loss rate than those offered by a flat best-effort
service. For example, it is well known that audio streams are
perturbed by important delay variations, but different TCP flows may
also react variously to loss rate or delay patterns according they
are long and bulk or short and carrying interactive information.
Improving the best-effort service of Internet by adding pertinent
differentiation mechanisms and simultaneously remaining close to the
IP philosophy [3], [4] is an important challenge. Any such
improvement of the IP stack has to respect the following criteria:
o the building block has to be simple, robust and scalable,
o the building block can be incrementally deployed.
o the various services must be freely usable as the unique best-
effort service provided by today's IP networks. Neither pricing
nor admission control must be performed,
Queue management systems such as RED [9] or ECN [8] mechanisms tend
to improve the best-effort service by having more control on
congestion. Even if they do respect the IP philosophy and meet the
criteria, they do not provide explicit differentiated services to
applications.
Pricing can be avoided only if the proposed services cannot be
ordered in the sense that one is better than another. Consequently,
even if flows do get different performances, when a flows gains on
one side, it has to loose something on an other side. This is the
underlying principle of "non-elevated services" [5] like ABE [1] or
BEDS [2].
Admission control can be avoided only if guarantees are relative
rather than absolute. Absolute guarantees requires resource
provisioning, admission control and traffic control to ensure traffic
does not exceed a given rate. If guarantees are relative, they can
still be ensured, whatever the network load is.
The simplicity requirement restrict the potential treatment of
individual packets. The performances of the network layer must
remain high and compatible with the actual link speeds. Today's
router are able to classify packets and to treat them differently in
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different queues efficiently. But, with the improvement of the
optical technology and the exponential increase of the network
capacity, the complexity of packet processing performed by network
routers at ultra high speed cannot increase much.
The robustness requirement implies that the network remains end-to-
end stateless. The incremental deployment means that the schema must
support service differentiation only performed partially along the
end-to-end path.
EDS proposes a new building block at the IP level that aims to
differentiate the per hop behavior (PHB) to better meet the needs of
each flows and that respect the preceding criteria. The EDS schema
aims to provide a spectrum of different but equal services, said
equivalent, that offer a trade-off between delay and loss rate.
2. Specifications
The EDS defines an arbitrary number N of equivalent service classes
(N greater or equal to 2) which are identified by numbers ranging
from `1' to `N'. The services are directly used by end-to-end
protocols or applications [4].
Each "EDS-capable router" differentiates among classes over the
queuing delay of the packets and their loss rate. A class `i' is
given two constant coefficients `d_i' (the delay coefficient) and
`l_i' (the loss rate coefficient) defined as follows. Let `i' and
`j' be two different classes: in each router, a ratio of d_i/d_j
between the queuing delays of their packet and a ratio of l_i/l_j
between their loss rates are defined. Coefficients are set so that
for each i in [1,N-1], l_i+1 is higher than l_i and d_i+1 is lower
than d_i.
Class `1' is thus the class whose packets experience the lowest loss
rate and the highest queuing delay ; packets of class `N' gets the
highest loss rate and the lowest queuing delay. Packets of classes
`i' where 1<i<N get performances in between according to the value of
their coefficients.
The EDS schema can cohabit transparently with advanced differentiated
services like premium service. In DiffServ domains, routers can
consider EDS flows as best-effort flows.
3. Class Identifier
Once the class identifier for the packet is set at the source it must
not be altered. This is different from DiffServ codepoints which can
be modified by routers along the path and therefore have no end-to-
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end guarantees.
One has to find another field in the packet to store the class
identifier. The requirements for this field are following: Its size
must be greater than one bit. Upper bound can be one byte. The size
of the class identifier field will fix the value of N, maximum number
of classes. The field has to be easily accessed by the classifier
component of routers. The class 0 corresponds systematically to the
default class that will experience at each router mean performances
in terms of delay and loss. The class identifier 0 will be
attributed to unmarked packets.
4. Router Requirements
Packet processing in routers is based on local parameters only, that
mean it does not use global criteria like flow behavior, number of
routers on the path, etc. The number N of classes is maximized by
the size of the class identifier field. Each "EDS-capable router"
knows the value of N.
Each router must provide a default class which offers mean
performances in order to approximate a best-effort service. This can
be implemented by dynamically choosing the most appropriate class or
using a specific way to schedule the best-effort packets.
EDS routers do not have to discriminate systematically each of these
N classes. Each router will map the N global classes in its local
classes. For example, let N be equal to 64 and a given router be
able to discriminate only four local classes. This router will
classify the packets identified by a global values from 1 to 16 in
the local class 1, from 17 to 32 in the local class 2, etc.
5. Coefficients setting
Coefficients setting can differ from one router to an other. The
setting has to conform to the rule that l_1<..<l_N and d_N<..<d_1.
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As an example, the following table shows three different
coefficients settings for EDS where N is equal to 8.
+-----+-----------+-----------+-----------+
| | setting 1 | setting 2 | setting 3 |
| | d / l | d / l | d / l |
+-----+-----------+-----------+-----------+
| 1 | 8.0 1.0 | 4.0 0.5 | 8.0 1.0 |
| 2 | 7.0 2.0 | 3.9 1.0 | 4.0 2.0 |
| 3 | 6.0 3.0 | 3.8 2.0 | 2.0 4.0 |
| 4 | 5.0 4.0 | 3.7 4.0 | 1.0 8.0 |
| 5 | 4.0 5.0 | 2.5 8.0 | 0.9 8.1 |
| 6 | 3.0 6.0 | 2.0 8.1 | 0.8 8.2 |
| 7 | 2.0 7.0 | 1.5 8.2 | 0.7 8.3 |
| 8=N | 1.0 8.0 | 1.0 8.3 | 0.6 8.4 |
+-----+-----------+-----------+-----------+
examples of configuration
o In setting 1, delay and loss are regularly spread on the
performance spectrum.
o In setting 2, there are mainly two groups of classes: 1 to 4 and 5
to 8. In a group of classes, there is a well defined way of
practicing differentiation. In the first group (classes 1 to 4),
there is a stronger loss differentiation than delay
differentiation. The delay differentiation is linear while loss
differentiation is quadratic. In the second group (5 to 8), both
differentiation are linear but delay differentiation is stronger.
o In setting 3, there are two groups of classes: 1 to 4 and 5 to 8.
In the first one (1 to 4) the differentiation is strong compared
to the differentiation in the second group (5 to 8).
6. Source Requirements
The source has to set the class identifier of each packet. The
successive packets of a given flow can be tagged differently. The
end-to-end transport layer can integrate some adaptation mechanisms
to contribute to end-to-end quality of service.
Existing adaptive transport protocols like TCP [6] can use the EDS
schema directly by using the default class or a "low loss" class.
But to fully use the capacities of the EDS model, transport protocol
will have to be little modified or new protocols will have to be
designed from scratch. For such adaptive protocol, the feedback
information may be a class number rather than a loss information.
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With EDS, an application that has strong delay requirements can have
some control on the latency caused by the network. It can start by
using class N and switch to the one that gives a delay close to its
threshold. It may experience a high loss if it chooses a too low
delay class. In case of strong congestion, the loss rate may be too
high and the application would have to relax its delay requirements.
In such bad case, the EDS network layer will be unable to provide a
better service. However the provided service may be better than a
flat best-effort service.
References
[1] Hurley, P., Le Boudec, J., Thiran, P. and M. Kara, "ABE:
Providing a Low-Delay Service within Best Effort", in IEEE
Network, Volume 15, Number 5, May 2001.
[2] Firoiu, V. and X. Zhang, "Best Effort Differentiated Services:
Trade-off Service Differentiation for Elastic Applications", in
Proceedings of IEEE ICT'01, June 2001.
[3] Gaidioz, B., Primet, P. and B. Tourancheau, "Differentiated
fairness: Model and implementation", in Proceedings of IEEE
HPSR'01, May 2001.
[4] Gaidioz, B. and P. Primet, "The Equivalent Differentiated
Services", INRIA/LIP Research Report RR2002-09, URL http://
www.ens-lyon.fr/~bgaidioz/RR2002-09.pdf, February 2002.
[5] Teitelbaum, B., "Future Priorities for Internet2 QoS", in
Internet2 QoS WG, see http://www.internet2.edu/qos/wg/papers/
qosFuture01.pdf, October 2001.
[6] Postel, J., "Transmission Control Protocol", RFC 793, STD 1,
September 1981.
[7] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.
Weiss, "An Architecture for Differentiated Services", RFC 2475,
December 1998.
[8] Floyd, S. and K. Ramakrishnan, "A Proposal to add Explicit
Congestion Notification (ECN) to IP", RFC 2481, January 1999.
[9] Floyd, S. and V. Jacobson, "Random Early Detection Gateways for
Congestion Avoidance", in IEEE/ACM Transactions on Networking,
Volume 1, Number 4, pages 397-413, August 1993.
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Authors' Addresses
Benjamin Gaidioz
UCBL
ENS-Lyon LR5
46, allee d'Italie
Lyon 69364
France
EMail: Benjamin.Gaidioz@ens-lyon.fr
Pascale Primet
INRIA
ENS-Lyon LR5
46, allee d'Italie
Lyon 69364
France
EMail: Pascale.Primet@ens-lyon.fr
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