Side_1_360

are defined. Each precedence level represents a
separate PHB, and thus a reserved DSCP value.

The functional blocks of a DiffServ router are
shown in Figure 2.

The functionality of the first two blocks is per-
formed at the ingress of the DiffServ domain to
enforce the SLA in question. MF-classification
typically takes place at the edge of the network,
whereas BA classification may take place in
every DiffServ router.

2.2.2 Multi-Protocol Label Switching
(MPLS) Combined with DiffServ
The router in an MPLS domain, the Label
Switched Router (LSR), works on the basis of
the MPLS header being attached to the packets
at the ingress LSR, confer Figure 3. The For-
warding Equivalent Class (FEC) of the packets
is decided on the basis of the destination address
and the DiffServ PHB or PSC classification. The
packets are encapsulated into an MPLS frame
(with MPLS overhead added) and forwarded to
the next LSR.

The route to be followed by the packets belong-
ing to a particular traffic class (a traffic trunk) or
set of traffic classes, i.e. the Label Switched Path
(LSP), has been set up in advance by ordinary
routing or constraint-based routing. The routing
process is either controlled by RSVP or LDP
(Label Distributed Path) signalling directly, or
by invoking the same functionality at a manage-
ment interface in the ingress LSR.

Each LSR contains a Label Information Base
(LIB), supporting look-up of the outgoing inter-
face as a function of the destination address.

Operations are available to merge traffic in the
transit LSRs along the route.

At the egress LSR, the MPLS header is stripped

off and further transport is again carried out on

the basis of the information in the IP header.

MPLS in isolation does not help much in provid-

ing differentiated behaviour, fair service and

quantifiable QoS. It is the combination with

DiffServ that provides the requested differentia-

tion of behaviour to traffic trunks being the basis

for quantifiable performance guarantees with the

service fairness preserved, and all this in an

environment characterized by powerful and flex-

ible means for routing traffic.

2.2.3 Integrated Services (IntServ)

An IntServ network provides specific classes of

service to individual flows or groups of flows.

In addition to Best Effort, two other classes are

available:

• Controlled Load (CL). This class delivers low
  average delay and minimum loss. It resembles
  Best Effort service as if the network were
  unloaded.


• Guaranteed Service (GS). This service offers
  quantifiable bounded queuing delay and no
  loss. It is intended for real time applications
  with strict timing requirements.

In order to establish the flow-specific state,

IntServ makes resource allocations in the routers

by means of the Resource Reservation Protocol

(RSVP). The PATH-message is transmitted

downstream from the source host to the destina-

tion carrying with it the TSpec data structure.

TSpec contains the requested delay, jitter and

bandwidth parameters including the Token

Bucket parameters. The path state established in

each router includes the address of the upstream

router. This datum is essential for the upstream

transmission of the RESV-message. When the

Figure 2 The DiffServ router

Figure 3 IP over MPLS

Classifier

Conditioner

• metering


• dropping


• marking

Forwarding

Queuing

• scheduling


• dropping

Conditioner

• shaping

Layer 3

(IP layer)

Layer 2.5

(MPLS layer)

IP router

(DiffServ

-aware)

IP router

(DiffServ

-aware)

Ingress LSR

(MPLS-aware)

LSR Egress LSR

(MPLS-aware)

LSP (carrying 1-N traffic trunks)

IP link