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Versions: 00                                                            
Internet Engineering Task Force                Constantinos Dovrolis
Internet Draft                                 University of Wisconsin,
Expiration Date: December, 1998                                Madison
                                               June, 1998

                        Class-Based Service Differentiation


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
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This document describes the Class-Based Service Differentiation (CBSD)
architecture and it proposes a set of Per-Hop-Behaviors that can be used
for implementing it. According to CBSD, the network traffic is mapped to
a set of classes. The differentiating factor between traffic classes is
the relative forwarding quality in a DS capable router. The Class PHBs
have quite similar semantics with the Precedence PHBs, recently proposed
by Baker et.al. in [PREC], but an attempt is made here to define them in a
more general manner, decoupling them from specific implementation
possibilities. There is also a semantical overlap between the Expedited
Forwarding PHB of [DSFIELD] and the highest Class PHB. Consequently, the
Diffserv working group may want to combine these three PHB proposals in a
single one, minimizing the number of DS Field bits used.

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1. Introduction

The IETF Diffserv Working group is working towards a differentiated
services architecture for the Internet. The ultimate goal of this
effort is to to come up with effective and scalable service discrimination
for the Internet users, without departing from the original IP network
model of stateless packet forwarding. The Diffserv approach targets
on defining Per-Hop-Behaviors (PHBs) and not end-to-end services. This is
probably a wise decision, since there is currently not much knowledge and
experiences about the types of end-to-end services that are required today,
and even more, that will be required in the future. A PHB has some
well-defined semantics about the forwarding behavior at a DS-capable router.
The set of PHBs can be viewed as a set of "legos"; as the experiences or the
requirements about differentiated services evolve, we can build a large set
of services given only a small set of PHBs.

In this document we propose a set of PHBs, called Class PHBs.
The semantics of the Class PHBs refer to the relative service level that
a packet will get at a DS capable router. In other words, a Class PHB
is meaningful only compared to another Class PHB. The Class-I PHB provides
a "better service" than any Class-J PHB, with I>J. The term "better service"
is deliberately loosely defined, and it should be interpreted only in a
statistical manner, and only locally at a DS capable router. One way to
view the "better service" differentiation is in terms of traffic load,
i.e., the Class-I PHB provides in most of the time less loaded forwarding
than any Class-J PHB (I>J). Alternatively, the term "better service"
can be interpreted based on performance metrics such as queuing delay
or drop probability, i.e., the Class-I PHB treats packets in most of the
time faster and more reliably than any Class-J PHB (I>J). The "better service"
constraint is not quantitatively defined, so that network providers can
have the flexibility to attach to it different performance ratings,
depending on the implementations they use and their traffic management
policies. A goal of this document is to decouple the relative service level
that a traffic class gets, from semantics that refer only to timely delivery
(i.e., priority), or only to relative importance (i.e., drop preference).

In Section 2, we argue that the CBSD architecture follows naturally from
the end-to-end principle. Section 3 proposes the Class PHBs. A comparison
with the Precedence PHBs of [PREC] and with the EF PHB of [DSFIELD] follows
in Section 4. Finally, a possible implementation for the Class PHBs is
sketched in Section 5. We attempt to be consistent with the terminology and
the general framework described in [DSFIELD], [DSARCH], and [DSFRMW].

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2. The End-To-End Principle in the Diffserv Context

We can design different types of network service differentiation to match
the specific needs of current or emerging applications. For example, IP
telephony applications require low jitter and low loss transfer. Packet
video applications require high throughput transfer,  but they allow higher
drop rates for the less important layers of the video stream. Some
transaction-based applications require very low end-to-end delays. Such
application requirements can be met by deploying the appropriate
network-level service differentiation mechanisms. For example, the packet
video requirements may be met by providing different levels of drop
preference. In this way, we attempt basically to make the network follow
the application requirements. It is generally accepted however, that this
approach is not flexible and extendible, as new applications with new
requirements keep emerging. For example, it is hard to predict at this
point the requirements of virtual reality applications. Besides, the lesson
learned from adaptive multimedia applications is that applications can always
be built to best match whatever service the network provides, without requiring
hard network performance guarantees.

An alternative approach would be to provide network service differentiation
in terms of network performance metrics, instead of application requirements.
In other words, in this approach it is the network that "makes the rules",
by defining a service level hierarchy that is general, simple, and
application independent. The applications and the users, then, match their
needs to specific levels of this hierarchy, based on their performance and
cost constraints. The CBSD architecture attempts to follow this approach,
as illustrated in the next examples.

An IP telephony application can select while running the service class
that best meets its jitter and loss requirements, given also some pricing
or policy constraints. Suppose, for example, that two friends that have
Internet access only up to Class-3 have an IP phone conversation. The
application starts the session at the lowest class (Class-1) and gradually
switches to higher classes until the service level is acceptable. After a
few seconds the application converges to Class-2, where it finds the service
level to be adequate. At the same time, at the same network, and using the
same application, two CEOs also have a IP phone conversation, but this time
with tighter quality requirements, and without any constraints on the cost
of the call. The application this time starts immediately from Class-4, to
save time, and finally converges to Class-6 where the performance
requirements are met.

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Similarly, consider a packet video application. The application generates
three layers of video packets, A, B, and C, that differ in their relative
importance for the final video stream (say that A is the base layer).
After a few seconds, the application converges to use Class-4 for the A
packets, Class-3 for the B packets, and Class-1 for the C packets. Again,
this selection is a function of the pricing or policy constraints. Also,
depending on the network dynamics, the application may find out later that
it can switch the A packets to Class-3, or that it has to switch them to
Class-5, instead.

Consider finally a corporate user that wants to get adequate service
from a network provider for the company's VPN. The two parts can make,
for example, a service level agreement for a certain amount of Class-5
traffic, and for unlimited Class-2 traffic.

3. Class PHBs

We are proposing a set of Class PHBs, defined in the following way:

        A Class-I PHB provides a better forwarding service than
        any Class-J PHB, for I>J.

As discussed earlier, the exact meaning of the term "better service" can
be viewed in terms of traffic load, or in terms of local performance
metrics, such as packet delay or loss. Also, in the absence of admission
control or signaling, it is likely that the term "better service" can be
only interpreted in a statistical manner, i.e., in most of the time, or for
most of the serviced packets, Class-I gets a better service than Class-J.
Network providers may want to attach to the above definition
quantitative semantics, depending on the implementation mechanisms they use
and their traffic management or marketing policies. For example, in a
hypothetical implementation the Class-I PHB may lead with a 99% probability to
at least two times smaller queuing delay and drop rate than the Class-(I-1)

A first issue is the appropriate number of classes. In this document we
propose eight classes. This is the same with the number of IPv4 precedences,
IPv6 priorities (now obsolete, [IPv6]), or with the number of priorities in
some link level technologies, such as IEEE 802.1d. Obviously, there is nothing
magic in this selection, and it may be shown in practice that this number
should be smaller or larger. Further experimentation in large networks is
required for a better decision on the number of classes.

A second issue is how much better should the service of a class be compared
to the service of the directly lower class. This is also an implementation
issue that can vary between network providers. A possible constraint, however,
can be that in the worst case, each traffic class should get on the long run
about the same level of service as if it was processed in a FIFO manner by a
non-DS capable router.

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For the eight classes architecture, we need eight PHBs. The selection of bits
in the DS-field can be arbitrary, but since legacy routers already use the
DS field bits, it is more appropriate to map the class PHBs to the IPv4
three precedence bits of the TOS byte. Since [IPv4] defines precedence seven
as the highest one, we map the PHB for the best class (Class-7) to the
precedence seven value. Specifically, the following PHBs are proposed:

        000 XXX         Class 0 (worst-service class)
        001 XXX         Class 1
        010 XXX         Class 2
        011 XXX         Class 3
        100 XXX         Class 4
        101 XXX         Class 5
        110 XXX         Class 6
        111 XXX         Class 7 (best-service class)

Other PHB selectors can be combined with the CBSD PHBs to produce further
service differentiation. Alternatively, it may be better to reserve some
"don't care" bits for defining additional service classes later.

4. Comparison with the Precedence and Expedited Forwarding PHBs

Recently, Baker et.al. proposed a set of PHBs for transferring the IPv4
Precedence semantics in the Diffserv architecture [PREC]. If we map each
class to a precedence, the Precedence PHBs seem to be the same with the
Class PHBs. In [PREC], however, or in the original specification of the
IPv4 Precedence bits in [IPv4], the semantical or "forwarding behavior"
differences that the Precedence bits specify are not clear. [PREC] states
that "In essence, a higher precedence (queue or class number) should
afford a higher probability of timely delivery than a lower precedence
packet (...)". We believe that this statement does not define the same
service differentiation as the CBSD architecture. This becomes clear
if we consider the four potential implementation mechanisms that [PREC]
suggests. We show next that neither of these mechanisms can actually
implement the Class PHB semantics.

The first possible implementation in [PREC] is a FIFO queue where the
precedence is directly mapped to the drop priority, meaning that lower
precedence packets are dropped before higher precedence packets in the
presence of congestion. This mechanism does not provide lower queuing
delays for the higher precedence packets, however, when there is persistent
queuing but not significant packet drops. Consequently, the higher
precedence packets can consistently get "worse service" than packets from
lower precedences.

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The second possible implementation in [PREC] is a set of priority queues,
where higher precedence values correspond to higher priority. This
configuration, however, allows for the lower priorities to starve and
effectively get the same low quality service. In the CBSD architecture this
is not allowed, since every class has to provide a better service than the
directly lower class.

The third possible implementation in [PREC] is a round robin scheduler,
where a certain rate is assigned to the queue servicing the traffic with
a certain precedence. However, as the authors of [PREC] also note, if a lower
precedence queue is less loaded than some higher precedence queue, the
service level of the higher precedence will be actually worse. Obviously,
this violates the CBSD architecture class semantics.

The final possible implementation in [PREC] maps each precedence to a virtual
circuit or virtual channel of an underlying link technology such as ATM or
Frame Relay. As the authors of [PREC] also mention, this mechanism is not
fundamentally different than the weighted round-robin one.

The Expedited Forwarding PHB, proposed in [DSFIELD], has also close
similarities with some of the Class PHBs. Specifically, [DSFIELD] states:
"An EF-marked packet is queued for an outgoing interface in a queue that
is expected to be relatively small and which is configured to have the
highest level of service priority (in some loose sense) within the router's
scheduling and buffer management sub-systems". However, this is basically
the same description for the highest Class-PHB, namely Class-8. If we
allow different levels of EF behavior, it may be possible that other
high classes can also have similar forwarding behaviors with the EF PHB.

5. A Possible Implementation for the Class PHBs

A possible implementation for the Class PHBs could be a CBQ scheduler [CBQ],
where the weights that are assigned to each queue are dynamically varied
depending on the load of that queue. Note that in such a CBQ implementation
it may be impossible to guarantee that a Class-I packet will always
get a lower delay compared to being serviced by the Class-J queue (I>J),
or that if a packet has to be dropped in the Class-I queue then it would also
had to be dropped in the Class-J queue (I>J). However, by applying
appropriate class-weight adjustments in the CBQ scheduler over certain
timescales, these services can be met in the statistical sense.
Other mechanisms, based on admission control, class-based routing, or other
types of buffer management and scheduling policies may also be applicable.

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6. Acknowledgements

There were two sources of motivation for writing this document. The first
was the interesting discussions at the diffserv mailing list. The second was
the presentation of the IPv6 Class field in Christian Huitema's book "IPv6:
The New Internet Protocol" (although this does not necessarily mean that
Christian agrees with what is written in this document).

7. References

[DSFIELD] K.Nichols, S.Blake,
        "Definition of the Differentiated Services Field (DS Byte)
        in the IPv4 and IPv6 Headers",
        <draft-ietf-diffserv-00.xt", May 1998

[DSARCH] D.Black, S.Blake, M.Carlson, E.Davies, Z.Wang, W.Weiss,
        "An Architecture for Differentiated Services",
        <draft-itef-diffserv-arch-00.txt>, May 1998

[DSFRMW] Y.Bernet, J.Binder, S. Blake, M.Calson, E.Davies, B.Ohlman,
        D.Verma, Z.Wang, W.Weiss,
        "A Framework for Differentiated Services",
        <draft-ietf-diffserv-framework-00.txt", May 1998

[PREC]   F.Baker, S.Brim, T.Li, F.Kastenholz, S.Jagannath, J.K.Renwick,
        "IP Precedence in Differentiated Services",
        <draft-ietf-diffserv-precedence-00.txt>, April 1998

[IPv4]   J.Postel,
        "Internet Protocol",
        RFC 791, September 1981

[IPv6]  S.Deering, R.Hinden,
        "Internet Protocol, Version 6 (IPv6) Specification",
        <draft-ietf-ipngwg-ipv6-spec-v2-01.txt>, November 1997

[CBQ]   S.Floyd, V.Jacobson,
        "Link-Sharing and Resource Management Models for Packet Networks",
        IEEE/ACM Transactions on Networking, pp.365-386, August 1995

8. Author's Address

Constantinos Dovrolis
Graduate Student
Dept. of Electrical & Computer Engineering
University of Wisconsin-Madison
1415 Engineering Drive
Madison, WI 53706
Phone: (608) 262-5130
Email: dovrolis@ece.wisc.edu

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