An Investigation into Transport Protocols and Data Transport ...

9.2. Preferential Flow Handling Using DiffServ 215
structure of the Internet is rare. Network layer mechanisms such as ATM
assume a homogeneous infrastructure between two end-systems, which for
economic reasons may not exist. The Integrated Services (IS) Architecture
[BCS94] does not depend on a homogeneous infrastructure but often requires
that the end-systems have specialised software installed and that every network
device is capable of per flow handling of traffic - which has serious
implications upon scalability and manageability.
Differentiated services (DiffServ) [BBC + 98] defines a new packet-handling
framework that addresses the concerns of scalability. The DiffServ architecture
defines ways that a packet should be treated by interior routers based
on associated classes called per-hop-behaviour (PHB) [HFB + 99, JNP99]. It
requires that edge routers are capable of identifying flows and marking them
with this appropriate class of service whereby core routers can provide the
related PHB.
However, in contrast to previous Quality of Service (QoS) architectures
such as RSVP [BCS94], DiffServ focuses on the behaviour of aggregates
rather than individual flows. Packets are identified by simple markings that
indicate which aggregate behaviour they should be assigned to. By implementing
the complex task of identifying flows at the edges of the network,
core routers need only provide the service for each aggregate based on these
markings.
Packets are marked either by applications or by edge routers. However,
marking by applications will often require policing by the edge routers to
enforce that the correct ‘codepoints’ are marked on the packets. If edge
routers mark packets, they may choose to do so on a per-flow basis or on any
other criteria. These definitions should be agreed between different providers
through Service Level Agreements (SLAs).