A scalable architecture for end-to-end QoS provisioning
Spiridon Bakiras*, Victor O.K. Li
Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Received 23 September 2003; revised 18 March 2004; accepted 13 April 2004
Available online 4 May 2004
The Differentiated Services (DiffServ) architecture has been proposed by the Internet Engineering Task Force as a scalable solution for
providing end-to-end Quality of Service (QoS) guarantees over the Internet. While the scalability of the data plane emerges from the
definition of only a small number of different service classes, the issue of a scalable control plane is still an open research problem. The initial
proposal was to use a centralized agent, called Bandwidth Broker, to manage the resources within each DiffServ domain and make local
admission control decisions. In this article, we propose an alternative decentralized approach, which increases significantly the scalability of
both the data and control planes. We discuss in detail all the different aspects of the architecture, and indicate how to provide end-to-end QoS
support for both unicast and multicast flows. Furthermore, we introduce a simple traffic engineering mechanism, which enables the more
efficient utilization of the network resources.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Admission control; Differentiated services; Quality of service; Resource management; Traffic engineering
In the past few years, the dramatic increase in the
capacity of the Internet core, and the development of
powerful compression techniques, have allowed the
deployment of new applications such as Internet telephony,
video-conferencing, streaming audio/video, etc. These
applications are called real-time, since they require the
periodic and timely delivery of the content from the source
to the destination. Clearly, the traditional best-effort service
that is provided in the current Internet cannot offer
an acceptable level of service quality to this type of
applications. To address this problem, the Internet Engi-
neering Task Force (IETF) has proposed the Differentiated
Services (DiffServ) architecture  as a scalable solution for
providing end-to-end Quality of Service (QoS) guarantees
over the Internet. The scalability issue is of outmost
importance, since, in the future, the number of flows that
will require some QoS guarantees is expected to be very
large. Consequently, a core router should be able to
accommodate thousands of QoS-sensitive flows at any
The basic idea of the DiffServ architecture is that only
edge routers should manage traffic on a per flow basis. Core
routers should not keep any kind of per flow state, and
should process traffic on a much coarser granularity. At the
data plane this goal is achieved by specifying different Per
Hop Behaviors (PHBs), where packets belonging to the
same PHB form a Behavior Aggregate (BA) and receive
identical service at the core routers. Specifically, the edge
routers will be equipped with flow classifiers, policers, and
markers that will properly mark the incoming packets by
setting a number of bits on the DiffServ Codepoint (DSCP)
 field of the IP packet header. The DSCP value will
indicate the corresponding PHB, and the core routers will
forward the packets based on their DSCP value (by utilizing
several scheduling and buffer management techniques).
The IETF has currently specified two different PHBs.
The Expedited Forwarding (EF) PHB  offers the
equivalent of a leased line (i.e. low delay, loss, and jitter)
between a source and a destination. This is accomplished
by giving EF traffic strict priority over the traditional best-
effort traffic inside the DiffServ domain. However, each
flow has to specify in advance the required bandwidth so
that the appropriate resources may be reserved inside the
network. In addition, the maximum burst size that is allowed
is equal to two Maximum Transmission Units (MTUs).
Computer Communications 27 (2004) 1330–1340
0140-3664/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: þ852-2857-8487; fax: þ852-2559-8738.
E-mail addresses: firstname.lastname@example.org (S. Bakiras); email@example.com
The edge routers will police each flow, and the non-
conforming packets will either be dropped or shaped. The
Assured Forwarding (AF) PHB group  does not offer
hard QoS guarantees, but instead defines four different AF
classes with three different levels of drop precedence within
each class. Each AF class is assigned a certain amount of
bandwidth at each node, and when the amount of traffic
exceeds this bandwidth, packets will be dropped according
to their drop precedence value.
While the scalability of the data plane emerges from the
definition of only a small number of PHBs, the issue of a
scalable control plane is still an open research problem. The
initial proposal was to use a centralized agent, called
Bandwidth Broker (BB) , to manage the resources within
each DiffServ domain and make local admission control
decisions. The centralized approach removes the burden of
admission control from the core routers, but there might be
some scalability considerations if the BB has to process
thousands of requests per second. Moreover, this approach
has certain disadvantages that are inherent to any centra-
† The links around the BB will become very congested
when the traffic load from the signaling messages is high.
† The BB must maintain per flow information about every
flow that is currently active inside its domain.
† The BB is a single point of failure (i.e. undesirable in
In this article, we propose an alternative decentralized
architecture, where the local admission decisions are made
independently at the edge routers of each domain. The BB in
each domain is only responsible for periodically updating
the allocation of the resources inside the domain, according
to some measurements of the traffic load at the edge routers.
We discuss in detail all the aspects of the proposed
architecture (i.e. intra- and inter-domain routing, admission
control, packet forwarding, etc.), and indicate how to
provide end-to-end QoS support for both unicast and
multicast flows. Furthermore, we introduce a simple traffic
engineering mechanism, which enables the more efficient
utilization of the network resources.
The remainder of this article is organized as follows.
In Section 2 some related work on DiffServ resource
management is presented. In Section 3 we give the details of
the proposed architecture, and also discuss various
implementation issues. In Section 4 the results of the
simulation experiments are presented, while Section 5
concludes our work.
2. Related work
The standardization of the DiffServ architecture by the
IETF triggered the initiation of several projects, which aim
to provide DiffServ-based QoS guarantees over the Internet.
The largest of these projects is the Internet2 project, which
involves over 200 universities, corporations, and other
organizations worldwide. The main objective of the
Internet2 QBone initiative  is to build an experimental
testbed for providing end-to-end QoS guarantees in a
scalable manner. Their approach on resource management
follows the initial proposal of a centralized BB, which is
responsible for managing the resources within a DiffServ
domain, and performing intra-domain admission control.
For end-to-end resource reservations, inter-BB signaling is
required between the BBs of adjacent domains.
One direction towards improving the scalability of the
resource management is based on aggregated resource
reservations between DiffServ domains. The BB is still the
centralized agent responsible for resource reservation, but
the scalability is improved by reserving resources for
aggregate traffic between different domains. In Ref.  a
two-tier model is introduced, where each domain is assumed
to have long-term bilateral agreements with each of its
neighbors, specifying the amount of traffic that will be
exchanged between them. Whenever there is an increase in
the traffic between two domains, the BBs will re-negotiate
and make new agreements. In Ref.  a Clearing House
architecture is proposed, where multiple basic domains are
clustered to form a logical domain. In this way, a
hierarchical tree is created, where the BB of the logical
domain is responsible for resource reservation across the
basic domains. The BBs at the basic domains forward only
aggregation of inter-domain requests to the BB of the
logical domain, thus enhancing the scalability of this
Alternatively, an approach based on the Multiprotocol
Label Switching (MPLS)  architecture has also been
considered in Refs. [10,11]. In these two architectures,
reservations for aggregate traffic are made between pairs of
edge routers on specific Label Switched Paths (LSPs) inside
the domain. All the QoS-sensitive flows will then follow the
appropriate LSPs, in order to receive the requested QoS.
The work in Ref.  is introduced as part of the Traffic
Engineering for Quality of Service in the Internet at Large
Scale (TEQUILA) project.
3. An architecture for end-to-end QoS provisioning
In this section we introduce an architecture for DiffServ-
based networks, which enhances the scalability of both the
data and control planes. The goal is to push most of the
functionality to the edge of the network, and maintain a
simple core, which only performs a standard packet
forwarding function. Our assumption is that the Internet
consists of several independently administered DiffServ
domains that are interconnected in order to provide
global connectivity. One typical example is shown in
are interconnected through three different domains.
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